Chemical Principles of Materials Production

Over the past century, experimental physicists and engineers have developed sophisticated methods to create semiconducting silicon-based devices, such as diodes, transistors and memory elements, of increas-ingly small dimensions. Meanwhile, chemists have acquired a detailed understanding of the relationship between the chemical structure and the electronic properties of a molecule through reaction chemistry, advanced physical chemistry and theoretical methods. These multidisciplinary efforts have converged in the field of single-molecule electronics (SMEs), in which the ultimate goal is to use molecules as active elements in electronic circuitry1,2. A rich knowledge base detail-ing the electronic properties of molecules already exists in the context of chemical reactivity; reimagining such properties in the framework of SMEs might then inspire tremendous advancements in the field.Powerful methods have been developed to charac-terize and manipulate the conductance properties of single molecules3–6. Molecular conductance has been measured using techniques based on scanning tunnel-ling microscopy (STM)7, mechanically controlled break junctions8–11, STM break junctions12,13, conductive atomic force microscopy14, electromigration15, nanoparticle arrays16,17 and other approaches18–21. The sophistication of these techniques provides the opportunity for chemists to collaborate with physicists and engineers to incorporate well-understood chemical principles into the study of structure–conductivity relationships in molecular wires.Reviews on SMEs are usually written from the per-spective of those who build devices and measure their properties — the link between chemistry and SME devices is rarely the focus. Filling this gap will address the questions that arise from the expanding structural diversity of molecular wires in the SME literature. For example, how will the single-molecule conduct-ance paradigm, which was first developed for simple molecular structures, shift as molecular wires grow in structural complexity and diversity? Which lessons can be drawn from reaction chemistry to guide the design of molecular electronics? How can chemical expertise be used to engineer new functions into single-molecule wires?In this Review, we integrate the languages of chemis-try and device physics to explain the chemical concepts that underlie single-molecule conductance. We discuss the structure–property relationships of single-molecule junctions by deconstructing the junction into three distinct components: the anchor, the electrode and the bridge (FIG. 1). We survey the modularity of each component and describe how tuning the structure of each part affects the charge-transport properties of the junction, primarily in the context of break-junction experiments. Finally, we examine emerging areas in SME research, such as single-molecule conductance switches and quantum interference (QI), and discuss how these are fundamentally related to well-established chemical principles.1Department of Chemistry, Columbia University.2Department of Physics and Applied Math, Columbia University, New York,New York 10027, USA. Correspondence to M.L.S., L.V. and C.N.mls2064@;lv2117@;cn37@Arricle number: 16002doi:10.1038/natrevmats.2016.2 Published online 23 Feb 2016Chemical principles of single-molecule electronicsTimothy A. Su1, Madhav Neupane1, Michael L. Steigerwald1, Latha Venkataraman1,2and Colin Nuckolls1Abstract | The field of single-molecule electronics harnesses expertise from engineering, physics and chemistry to realize circuit elements at the limit of miniaturization; it is a subfield of nanoelectronics in which the electronic components are single molecules. In this Review, we survey the field from a chemical perspective and discuss the structure–property relationships of the three components that form a single-molecule junction: the anchor, the electrode and the molecular bridge. The spatial orientation and electronic coupling between each component profoundly affect the conductance properties and functions of thesingle-molecule device. We describe the design principles of the anchor group, the influence of the electronic configuration of the electrode and the effect of manipulating the structure of the molecular backbone and of its substituent groups. We discuss single-molecule conductance switches as well as the phenomenon of quantum interference and then trace their fundamental roots back to chemical principles.REVIEWSAnchor groupThe anchor group (also known as the linker or contact group) connects the molecular wire to the electrodes both mechanically and electronically. Usually, a single anchoring group terminates each end of the molecule to form the metal–molecule–metal junction; however, including more anchoring units along the molecular bridge can offer additional handles for tuning con-ductance, depending on which two anchors form the most conductive pathway 22–27. Anchoring groups typi-cally bind to electrodes either through donor–acceptor (dative) interactions or through covalent bonding. Prototypical anchor groups for each type of electrode–linker interaction are shown in FIG. 2a . Because gold is the most used electrode material in SME studies, we focus primarily on the interaction of anchor groups with gold electrodes.Dative interactions involve the electron donation from a π donor or a lone pair donor to a Lewis acidic Au atom. Common π donors include fullerenes 28,29 and other π-conjugated hydrocarbons 19,26,30,31. Many lone pair anchoring groups are common σ-donor ligands familiar from coordination chemistry 32. Dative con-tacts, such as amines, are advantageous because they bind selectively to undercoordinated adatoms on the electrode surface; this narrows the conductance distri-bution because it limits the Au–linker contact geom-etry 33. Covalent contacts between the metal and the molecule result from covalent bonding between mole-cular radicals and metallic electrode surfaces. Covalent contacts are valuable because they are physically robust linkages that strongly couple the electronics of the molecule and the metal.Conductance depends not only on the class of the anchoring group but also on the nature of the inter-action between the anchor group and the other com-ponents of the junction — that is, the bridge and the electrode. In the following sections, we explore how the spatial overlap between the orbitals of these three components affects the charge-transport properties of the junction.Before we continue, we must clarify the meaning of the conductance values that are discussed. There is significant measurement-to-measurement variability in single-molecule experiments that is due to fluctuations in the molecular conformation, the electrode–anchor contact geometry and the electrode surface geometry. To account for this variability and to better under-stand the nature of conductance in a single molecule,researchers analyse hundreds to thousands of measure-ment traces together by compiling them into conduct-ance histograms to obtain a distribution of all measured conductance values. The conductance values that we report here refer to the conductance peak values from such histograms reported in units of G 0. G 0 is the con-ductance quantum and is defined as G 0 = 2e 2/h = 77.5 μS, where e is the charge of an electron and h is Planck’s constant; it is the preferred unit used to describe the conductance between metal point contacts as well as molecular conductance.Effect of anchor–bridge orbital overlap on conductance. The anchor group often dictates whether the mole c ular wire transports holes (highest occupied molecular orbital (HOMO)-dominated conductance) or elec-trons (lowest unoccupied molecular orbital (LUMO)-dominated conductance). The dominant conducting molecular orbital is typically the orbital that is closest in energy to the electrode Fermi level, E F . Conductance depends on the energy offset, ΔE , between E F and the conducting orbital, and on the strength of the elec-trode–molecule hybridization, Γ (BOX 1). The nature of the conducting orbital can be determined experimen-tally through thermopower measurements 34,35 or com-putationally by transmission calculations 36. However, in simple structures, we can predict the nature of charge carriers from basic chemical principles by considering the nature of the molecular backbone and the geome-try of the lone pair relative to the conjugated orbitals of the molecule (FIG. 2b). To illustrate this method, we use a phenyl ring terminated at the para positions by the dative linker groups from FIG. 2a . This analysis can be applied to other basic aromatic wires as well. For ben-zene rings terminated with linkers such as –SR, –NH 2, –PR 2 or –SeR at the 1,4-positions, the lone pair orbitals are included in the HOMO because they are coplanar with and energetically destabilized by the filled π-con-jugated bridge orbitals. Thus, owing to the alignment of the Au–lone pair bonds with the π system of the bridge, conductance occurs strongly through the HOMO when such anchor groups are used.By contrast, conductance in phenyl rings terminated by pyridine 37, isocyanide 38 and cyanide 39 linkers occurs primarily through the LUMO. For these anchor groups, the lone pair lies in the σ plane of the molecule, rigidly orthogonal to the π channel of the wire. Conduction through the lone pair orbitals is weak because the car-bon sp 2 σ orbitals are poorly conjugated 40. Moreover, the lone pair orbital is generally quite low in energy because it is part of the σ system; thus, transport through this orbital has a marginal contribution to conductance (ΔE is large). Just as importantly, the elec-tron-withdrawing nature of these linker groups facili-tates LUMO-dominated conductance by lowering the energy of the π*-antibonding orbitals towards the E F (and the HOMO-conducting π-bonding orbitals away from the E F ). The conductance is then controlled by the coupling between the electrodes and the π*-antibond-ing orbitals of the LUMO. When molecular bridges are very electron-deficient structures, such as thiophene| Materials Figure 1 | A schematic of a single-molecule junction with electrode, anchor and bridge components. The bridge unit can be further deconstructed into backbone (blue block) and substituent (red circles) subunits. I , current.dioxides 41 and porphyrins 42, in which the HOMO and LUMO energies are substantially lowered, conductance through the LUMO can dominate regardless of the type of linker used.HOMO and LUMO conduction can also be understood from the perspective of coordination chemistry and of the different modes of interaction between ligands and transition metals. Hole trans-port or HOMO-dominated conduction occurs when the metal–molecule bond, formed using the σ-donor orbital of the molecule (which in most cases is the HOMO of the isolated molecule) and the σ-accepting orbital of the metal, can become coplanar with the con-jugated bridge orbitals. This coplanarity ensures that the metal–molecule bond, the ‘gateway’, can mix with the delocalized, conjugated bridge orbitals and estab-lish the conductivity path. Conversely, if the metal- to-molecule σ bond cannot mix with the conjugated pathway, the charge carriers cannot use the molecular HOMO; it is geometrically unavailable and energet-ically distant from the E F . However, it is well known from coordination and organometallic chemistry that although ligand-to-metal σ donation usually dominates metal–ligand bonding, it is often supplemented by d metal to π*ligand ‘back-bonding’, in which occupied orbit-als of the metal mix with unoccupied orbitals of theligand. In the case of molecular conduction, when the σ-donation (HOMO-transporting) pathway is un a vail-able, this π* back-bonding (LUMO-transporting) path-way may be available if the π* orbital, into which the metal back- d onates, is a conjugated orbital that spans the molecule and is connected to both electrodes.The stereochemistry of the Au–anchor bond with respect to the conductive channel of the molecular bridge determines the strength of the electronic cou-pling between the metal and the molecule; manipulating this stereochemistry is a powerful handle for controlling conductance. The charge flow between the electrodes increases if the Au–anchor bond is aligned with the con-jugated orbitals of the molecule. The position of sulfur lone pairs can be locked into alignment with the molec-ular π backbone using a dihydrobenzothiophene (BT) thioether linker 43. The frustrated rotation of the S lone pair results in increased conductance and in a sharper conductance peak compared with analogous aromatic wires with thiomethyl linkers that can freely rotate. The BT linker has been incorporated into several dif-ferent molecular wires to strengthen the anchor–bridge coupling 27,44–46.Poor coupling between the electrode and the mole c ule can also be a desirable quality. Electrode– molecule coupling can be disrupted by insertingNature Reviews | Materials abH 2N-Ph-NH 2(HOMO)NC-Ph-CN (HOMO-2)NC-Ph-CN (LUMO)Figure 2 | Anchor group archetypes and the nature of charge carriers for common dative anchors. a | Molecularstructures of common anchors. Dative anchors can be classified as π donating or lone pair donating. For lone pair donors, the anchors shown in the left and in the right columns impart lowest unoccupied molecular orbital (LUMO)- and highest occupied molecular orbital (HOMO)-dominated conductance, respectively, in simple π-conjugated systems. Covalent anchors commonly used to generate direct Au–S and Au–C contacts are shown in the last column. These contacts can be generated from thiol oxidation, Au–Sn transmetalation, fluoride-initiated desilylation and diazonium electroreduction reactions. b | The highest energy molecular orbital surfaces that feature strong lone pair character are depicted for 1,4-diaminobenzene (left panel) and 1,4-dicyanobenzene (middle panel) (B3LYP/6-31G**). In 1,4-diaminobenzene, the N-centred lone pairs occupy p orbitals that, like the benzene π orbitals, are perpendicular to the plane of the ring. In 1,4-dicyanobenzene, these N-centred lone pair orbitals are orthogonal to the π channel of the benzene ring; conductance in this system is dominated instead by transport through the LUMO (right panel).R E V I E W Smethylene (CH2) units between the S atom and the phenyl ring. The methylene spacers allow gating effects, such as Coulomb blockade, to occur in three-electrode systems because they decouple the molecule from the source and drain the electrodes47(FIG. 3a). However, the reduction in molecule–electrode coupling has been shown to decrease the junction conductance by three orders of magnitude in π-conjugated molecules47. Interesting functions can be implemented into molec-ular electronic devices by synthetically engineering anchor groups with both strong and weak electrode–molecule coupling character. A bulky –SPh anchor group can be used to misalign the S–Au bond with the molecular bridge, decreasing the molecule–electrode coupling45. This enables the creation of a single-mole-cule rectifier, whereby the molecule is strongly coupled to the electrode by a covalent Au–C bond at one end and weakly coupled by a Au–SPhR bond at the other end. A class of oligosilanes and oligogermanes that switch between different conductance values depending on the strength of the coupling between the electrode and the molecule has recently been described48,49. Junction elongation stretches the terminal ends of the molecule into a geometry (ortho–ortho (O–O)) that is optimal for conductance and electrode–molecule cou-pling; junction compression relaxes the molecule into dihedral geometries (anti–anti (A–A) or ortho–anti (O–A)) with diminished electrode–molecule coupling and lower conductance (FIG. 3b).In situ chemical reactions to produce covalent contacts. Au–S linkages formed from the reduction of thiol (–SH) linkers on Au surfaces are the most widely studied form of covalent contact. Strong Au–S linkages ena-ble the molecular junction to withstand harsh external conditions such as mechanical stress50 and high-bias voltages51. Thiols can oxidize to disulfides rather easily under ambient conditions; this is problematic because dithiolated molecular wires can polymerize and form insoluble polydisulfides52. Although S–S bonds can be reduced on gold surfaces53, there is no guarantee that the reduction will be exhaustive and that only theR E V I E W Smonomers will contribute to conduction measure-ments. A common approach for increasing the ambi-ent stability of thiol-based wires is to functionalize the thiols with thioacetate-protecting groups that can cleave on the electrode surface to form covalent Au–S contacts 52,54. Thiol-based junctions tend to show broad conductance features owing to the large variability in the anchor–electrode contact geometry. Several groups have studied the effect of binding geometry on conduct-ance 55–59, but understanding and thus gaining control over the variability of contact geometry is an ongoing challenge in the field 42,60–62.Au–C contacts are particularly promising covalent anchors because they give well-defined conductance peaks owing to the selective binding to undercoordi-nated gold 63. Furthermore, molecular wires with Au–C contacts generally demonstrate higher conductance peak values than structurally analogous wires with Au–S contacts. For example, the Au–benzenedithiol–Au junction shows a broad conductance, with reported conductance peak values ranging from 10−2 to 10−4 G 0 (REFS 11,64–66). By contrast, the structurally similar Au–xylyl–Au junction conducts at 0.9 G 0 (REF . 67). The difference in conductance arises from two factors. First,| MaterialsadceC u r r e n t (p A )C u r r e n t (μA )Differential conductance (1/GΩ)Differential conductance (1/MΩ)Bias (mV)Bias (mV)60bSiSiSiSiS AuAu b | Newman projections depicting the anti (A) and G state), with weakly coupled A–A and O–Ac | Density of states around d -orbital character of Au compared with Ag near the d | The Hammett σpara in substituted benzene rings. e | Conductance decreases with an increasing twist angle between biphenyl rings. G H , conductance of unsubstituted molecule; G X , conductance of substituted molecule; N , number of substituents. Panel a is adapted with permission from REF . 47, American Chemical Society. Panel b is from REF . 48, Nature Publishing Group. Panel c is adapted with permission from REF . 92, American Chemical Society. Panel d is reproduced with permission from REF . 103, American Chemical Society. Panel e is from REF . 13, Nature Publishing Group.R E V I E W Sthe C–Au bond is more strongly coupled to the π system than the S–Au bond because it is much shorter in bond length. Second, ΔE is much smaller for the Au–C gate-way states that describe the covalent metal–molecule hybridization. For example, the Au–S gateway state is centred at E – E F = −1.4 eV for alkane dithiols 59, whereas the Au–C gateway state is centred at E – E F = −0.8 eV for bis(trimethylstannyl)alkanes 67. This difference in energy alignment with the E F contributes significantly to the difference in conductance.Three methods have been developed for the in situ generation of direct Au–C covalent contacts. The first method involves the transmetalation of C–SnR 3 bonds on gold surfaces to generate C–Au bonds (and tin oxide by-products under ambient conditions 68). This method was first used to obtain self-assembled alkane monolay-ers on gold surfaces from organotin species 69, and it was later used for the in situ cleavage of terminal C–SnMe 3 bonds to obtain covalent Au–arene and Au–alkane con-tacts in single-molecule junctions 67. Alkane 67 and para-phenylene 70 wires terminated with Au–CH 2R contacts demonstrate a 10- to 100-fold increase in conductance compared with the analogous bridges terminated with dative Au–NH 2R contacts. The applicability of this method was recently expanded to include Au–acetylene contacts 71. A potential shortcoming of this approach is that it both uses and produces toxic and volatile trimeth-yltin species. Furthermore, this reaction does not occur universally for all organotin molecules; for example, this manner of cleavage does not occur in perfluorinated benzene backbones 70. The most important molecu-lar design rule for creating C–Au contacts via C–SnR 3 transmetallation on gold is that the bond between the molecular bridge and the tin atom must be the most reactive of the four organotin bonds. For example, n‑al-kane backbones with SnBu 3 end groups do not show clean conductance features in the STM break junction 67, presumably because the cleavage of the four C–Sn bonds is not selective. By contrast, Au–(CH 2)n –Au junctions form cleanly for trimethyltin-terminated alkane wires because of the greater stability of RH 2C • radicals com-pared with H 3C •, which enables the preferential cleav-age of the RH 2C–Sn bond. Similarly, benzyltrimethyltin molecules cleave instantaneously at the Sn–benzyl bond, even at −110 ˚C (REF . 63), whereas Sn–aryl bonds cleave slowly, with Au–aryl–Au junctions appearing only after 2.5 hours at room temperature 67. This design strategy allows the programming of the junction that will form on Sn–C cleavage.The second method for obtaining covalent Au–C contacts involves a fluoride-initiated desilylation of oligo(phenylene ethynylene) wires terminated with trimethylsilyl (TMS) end groups 72. Addition of tetrabutylammonium fluoride to a solution of the TMS-protected target molecules selectively cleaves the termi-nal ethynyl–Si bonds. This approach is inspired from a classic synthetic chemistry method that exploits the strong affinity between silicon and fluorine to unmask acetylene groups 73. The applicability of this method is hindered by the NBu 4+ electrolytes that participate in ionic conductance between the electrodes; theseelectrolytes give rise to significant conductance noise that may cover the signal of low-conductance mole-cules. However, ionic conductance can be reduced by coating the electrodes with an insulating layer 74.The third method to produce covalent electrode anchors involves the electroreduction of diazonium salts on gold surfaces 75. It was first used in the context of break-junction experiments by electrochemically reducing the terminal diazonium end groups on a biphenyl ring to generate covalent Au–biphenyl–Au junctions 76. This approach is attractive because cova-lent Au–C contacts can be generated on demand by increasing the reduction potential via a gate electrode to irreversibly cleave the aryl–N bond. However, diazonium salts are known to be thermally unstable and, in many cases, explosive 77. In particular, alkyl diazonium salts are especially unstable, which limits the range of diazonium-functionalized structures that can be easily measured in single-molecule junctions.There are still many unsolved issues in the imple-mentation of single-molecule devices with covalent contacts. The choice of precursors for the desilylation and diazonium reduction methods has been limited thus far to those that place the Au–C bonds in the σ plane of the molecular bridge. This is an important con-sideration because maintaining coplanarity between the metal–carbon bond and the bridge π system is essential for optimizing the coupling between molecule and elec-trode. Moreover, molecular wires with Au–C contacts are not particularly robust, as they tend to oligomerize in situ during break-junction experiments. This may be unavoidable under ambient conditions, as Au–C bonds are inherently sensitive to oxidation and dimerization pathways. Tuning the structure of the wire to make the metal–carbon bond more stable is a possible solution to avoid device failure. Using electrode materials that form more stable electrode–carbon bonds is another possible route for enhancing the stability of the device; we discuss this topic in the following section.ElectrodeThe electrode as a chemical reagent. Using the elec-trode as a reagent in synthetic reactions is a promising and underexplored route for the development of SME devices with desirable properties. Concepts in organo-metallic chemistry describe how the electronic struc-ture of different metals affects their chemical reactivity. Inorganic chemical principles, such as the hard–soft acid–base concept and ligand field theory, provide a general roadmap for understanding the chemical groups that can be used to functionalize electrode surfaces; ‘soft’ metals that are commonly used as elec-trode materials interact strongly with ‘soft’ and high-field ligands 32,78, such as the ones depicted in FIG. 2a . Several classic organometallic and organic reactions have already been transposed from reaction flasks to electrode surfaces. For example, the Ullmann coupling reaction, which uses the metal-mediated homocou-pling of halobenzenes to fuse aryl rings together, was discovered 79 in 1901 — more than a century later it was reimagined on a gold surface to synthesize grapheneR E V I E W Snanoribbons 80,81. The coupling reactions between an amine and a carboxylic acid, which are fundamental for peptide chemistry, have been used to produce covalent electrode–molecule–electrode junctions by reacting amine-terminated molecules with carboxylate point defects in carbon-nanotube electrodes 18. The ruthe-nium alkylidene chemistry familiar to olefin metathesis reactions 82 has been used for functionalizing ruthe-nium electrodes with alkylidenes that are well coupled, which is relevant for charge transport, and catalytically active, so that longer wires can be grown 83. As the field of SMEs develops, more examples will arise in which the reactivity profile of specific metals is exploited to functionalize electrode surfaces.Electrode materials. Gold is the most common elec-trode material in break-junction experiments because of its inertness, which enables the measurement of single-molecule junctions with consistency and repro-ducibility under ambient conditions. Other metals have interesting electronic properties, but many of them quickly oxidize in air, creating oxide layers on the electrode surface that prevent the clean formation of metal–molecule–metal junctions. Measuring in air-free or ultrahigh vacuum conditions can help to cir-cumvent this problem but adds a significant degree of complexity to the experiment. Electrode materials that have been used for SME devices include metals such as Ag 84,85, Pd 86 and Pt 87–89, and graphitic nanostructures such as graphene 19 and carbon nanotubes 18.The density of states of a metal at the E F strongly influences the conductance of the single-molecule junction. This was demonstrated in a study on isothi-ocyanate-terminated alkanes, in which the observed conductance was two- to threefold higher in Pd and Pt junctions than in Au junctions 90. The metal d band possesses the appropriate symmetry to couple with the isothiocyanate π orbitals near the E F ; thus, the increased d character at the E F of Pd and Pt relative to Au enhances the metal–molecule d –π interaction 91. In another study, it was found that the conductance peak value in 4,4ʹ-bi-pyridine is more than an order of magnitude lower when it is measured with Ag electrodes rather than Au electrodes. This difference in conductance arises from the weaker d yz -orbital character of the Ag density of states at the E F that results in reduced d –π* hybrid-ization and metal–molecule coupling 92 (FIG. 3c). Metals with unpaired spins can also confer interesting magnetic properties to single-molecule junctions 93–95. Harnessing the unique chemical and physical properties of different electrode materials will allow a greater degree of control over charge transport in single-molecule junctions and will enable the design of new SME devices.Molecular bridgeThe idea that a molecule can function as an active com-ponent in an electrical circuit was first formulated in the context of the design of a theoretical rectifier with an asymmetric molecular bridge structure, in which charge would flow preferentially from electron-rich to elec-tron-deficient regions 1. Out of the three modules of themolecular junction, the bridge has the greatest potential for manipulation with synthetic chemistry: any chemi-cally reasonable structure can be prepared and can serve as a molecular bridge as long as it contains two anchor-ing groups. There are two distinct subcomponents in the bridge: the backbone and the substituents. The backbone is the main pathway through which charge flows — such as the π bonds in phenyl rings or the Si–Si σ bonds in oligosilanes. The substituents are the chemical groups attached to the main backbone chain, and they can alter both the electronic structure and the conformation of the molecule.β values and electronic coupling in the molecular back-bone. The ability of different oligomeric backbones to transport charge can be evaluated by comparing how their conductance decays with increasing oligomer length. This is quantitatively described by their β values, which are given in units of inverse length. The β value is derived by plotting conductance on a semi-logarithmic scale against the molecular length (L ) of the oligomer. The β value is then extracted using the formula G = A e −βL . β values depend on the coupling strength between repeat units: backbones that are strongly conjugated and effec-tive at transporting charge have a shallow conductance decay and, consequently, a low β value. Here, we limit the discussion to β values obtained from measurements of single molecules (rather than molecular assemblies), in which conductance is dominated by coherent tunnelling mechanisms. Excellent reviews have been written that discuss β values obtained from a wider range of meas-urement techniques, wire structures and conductance mechanisms 96,97.Representative β values for several oligomeric mate-rials that conduct via coherent tunnelling are listed in TABLE 1. Alkanes are characterized by high β values (0.84 Å−1) because they do not have strongly conjugated bonds that can carry charge 12,57. Permethyloligosilanes ([SiMe 2]n ) terminated with methylthiomethyl electrode linkers have a β value (0.39 Å−1) comparable to that of aromatic π conductors 48. Despite their structural sim-ilarity to alkanes, oligosilanes transport charge more effectively because Si–Si σ bonds are more strongly con-jugated than C–C σ bonds, as their bonding orbitals are much larger in size and much higher in energy 98. From the perspective of the transmission function (BOX 1), the nearest neighbour coupling τ in silanes is stronger than in alkanes. The low β value in silanes also opens up the possibility of observing QI effects in σ systems, as discussed further below. Isostructural permethyloligo-germanes also have a low β value (0.36 Å−1), which is slightly lower than that of permethyloligosilanes 49.π-conjugated backbones tend to be associated with low β values. Conjugated but non-aromatic systems typically have a lower β value than purely aromatic species. Mapping the conductance of molecular wires against their degree of aromaticity suggests that the conductance is inversely proportional to the resonance stabilization energy 99. In the transmission picture, this aromatic stabilization energy decreases the on-site energies (ε), thereby lowering ΔE (BOX 1). The higherR E V I E W S。

Advanced Materials CharacterizationAdvanced materials characterization plays a crucial role in various scientific and engineering fields, providing valuable insights into the properties and behavior of materials at the atomic and molecular levels. This multidisciplinary field encompasses a wide range of techniques and methods aimed at understanding the structure, composition, and performance of advanced materials. From nanomaterials to biomaterials, the characterization of these advanced materials is essential for the development of innovative technologies and applications. In this discussion, we will explore the significance of advanced materialscharacterization from different perspectives, highlighting its impact on research, industry, and society. From a scientific standpoint, advanced materials characterization enables researchers to gain a deeper understanding of the fundamental properties of materials. Techniques such as X-ray diffraction, electron microscopy, and spectroscopy provide detailed information about the crystal structure, morphology, and chemical composition of materials. This knowledge is invaluable for elucidating the underlying principles governing the behavior of materials, paving the way for the design and synthesis of new materials with tailored properties. Moreover, advanced materials characterization plays a pivotal role in advancing our knowledge of nanomaterials, metamaterials, and other cutting-edge materials, driving the frontiers of scientific discovery. In the realm of engineering, advanced materials characterization is indispensable for the development and optimization of materials for various applications. Whether in the aerospace, automotive, or electronics industry, the ability to characterize materials at the micro and nanoscale is essential for ensuring their reliability and performance. For instance, the characterization of composite materials used in aircraft structures allows engineers to assess their mechanical properties, fatigue resistance, and damage tolerance. This, in turn, contributes to the enhancement of safety standards and the development of lightweight, high-strength materials for advanced aerospace applications. Furthermore, advanced materials characterization has significant implications for the medical and healthcare sectors. Biomaterials play a critical role in various medical devices, implants, and tissue engineering applications. Characterization techniques such asscanning electron microscopy and atomic force microscopy are employed to analyzethe surface topography and mechanical properties of biomaterials, therebyinfluencing their biocompatibility and functionality. Additionally, the characterization of drug delivery systems and nanomedicines is essential for understanding their release kinetics, stability, and interactions with biological systems, ultimately impacting the efficacy and safety of therapeutic interventions. In the context of environmental and energy-related applications, advancedmaterials characterization contributes to the development of sustainable technologies. The characterization of materials for energy storage and conversion, such as batteries, fuel cells, and photovoltaic devices, is essential forimproving their efficiency, durability, and cost-effectiveness. Moreover, thestudy of advanced materials for environmental remediation, pollution control, and sustainable infrastructure relies on advanced characterization techniques toassess their performance under different environmental conditions. By gaining insights into the degradation mechanisms and long-term behavior of materials, researchers and engineers can contribute to the development of eco-friendly and resilient materials for addressing global challenges. From a societal perspective, the impact of advanced materials characterization is far-reaching. The advancements in materials science and characterization techniques have led to the development of innovative consumer products, from high-performance electronics to advanced medical devices, improving the quality of life for individuals worldwide. Moreover, the integration of advanced materials in various industries has the potential to drive economic growth and create new opportunities for employment and entrepreneurship. By fostering collaboration between academia, industry, and government agencies, the field of advanced materials characterization plays apivotal role in translating research discoveries into practical solutions that benefit society as a whole. In conclusion, advanced materials characterization isa multidisciplinary field that holds immense significance for scientific research, technological innovation, and societal advancement. By enabling a comprehensive understanding of the structure-property relationships of materials, advanced characterization techniques empower researchers, engineers, and industries to develop materials with tailored functionalities and enhanced performance. As thefrontiers of materials science continue to expand, the continued advancement of advanced materials characterization will undoubtedly shape the future of diverse fields, from healthcare and energy to environmental sustainability and beyond.。

理化检验的基本原理Physical and chemical testing is a fundamental aspect of scientific research and industrial applications. It involves the analysis of the physical and chemical properties of materials, substances, and compounds to determine their composition, purity, and quality. By understanding the basic principles of physical and chemical testing, researchers can ensure the accuracy and reliability of their data.理化检验是科学研究和工业应用的基础。

它涉及对材料、物质和化合物的物理和化学性质进行分析，以确定它们的成分、纯度和质量。

通过了解理化检验的基本原理，研究人员可以确保其数据的准确性和可靠性。

One of the key principles of physical and chemical testing is accuracy. This involves ensuring that the measurements and data obtained are precise and reliable. To achieve this, scientists must adhere to strict protocols and standards, using calibrated equipment and performing tests under controlled conditions.理化检验的一个关键原则是准确性。

CHAPTER 1INTRODUCTION1.1MATERIALS PROCESSINGChemically reactive plasma discharges are widely used to modify the surface prop-erties of materials.Plasma processing technology is vitally important to several of the largest manufacturing industries in the world.Plasma-based surface processes are indispensable for manufacturing the very large scale integrated circuits (ICs)used by the electronics industry.Such processes are also critical for the aerospace,automotive,steel,biomedical,and toxic waste management industries.Materials and surface structures can be fabricated that are not attainable by any other commer-cial method,and the surface properties of materials can be modified in unique ways.For example,0.2-m m-wide,4-m m-deep trenches can be etched into silicon films or substrates (Fig.1.1).A human hair is 50–100m m in diameter,so hundreds of these trenches would fit endwise within a human hair.Unique materials such as diamond films and amorphous silicon for solar cells have also been produced,and plasma-based hardening of surgically implanted hip joints and machine tools have extended their working lifetimes manyfold.It is instructive to look closer at integrated circuit fabrication,which is the key application that we describe in this book.As a very incomplete list of plasma pro-cesses,argon or oxygen discharges are used to sputter-deposit aluminum,tungsten,or high-temperature superconducting films;oxygen discharges can be used to grow SiO 2films on silicon;SiH 2Cl 2=NH 3and Si(OC 2H 5)4=O 2discharges are used for the plasma-enhanced chemical vapor deposition (PECVD)of Si 3N 4and SiO 2films,1Principles of Plasma Discharges and Materials Processing ,by M.A.Lieberman and A.J.Lichtenberg.ISBN 0-471-72001-1Copyright #2005John Wiley &Sons,Inc.respectively;BF 3discharges can be used to implant dopant (B)atoms into silicon;CF 4=Cl 2=O 2discharges are used to selectively remove silicon films;and oxygen dis-charges are used to remove photoresist or polymer films.These types of steps (deposit or grow,dope or modify,etch or remove)are repeated again and again in the manufacture of a modern IC.They are the equivalent,on a micrometer-size scale,of centimeter-size manufacture using metal and components,bolts and solder,and drill press and lathe.For microfabrication of an IC,one-third of the tens to hundreds of fabrication steps are typically plasma based.Figure 1.2shows a typical set of steps to create a metal film patterned with sub-micrometer features on a large area (300mm diameter)wafer substrate.In (a ),the film is deposited;in (b ),a photoresist layer is deposited over the film;in (c ),the resist is selectively exposed to light through a pattern;and in (d ),the resist is developed,removing the exposed resist regions and leaving behind a patterned resist mask.In (e ),this pattern is transferred into the film by an etch process;the mask protects the underlying film from being etched.In (f ),the remaining resist mask is removed.Of these six steps,plasma processing is generally used for film deposition (a )and etch (e ),and may also be used for resist development (d )and removal (f ).The etch process in (e )is illustrated as leading to vertical sidewalls aligned with the resist mask;that is,the mask pattern has been faithfully transferred into the metal film.This can be accomplished by an etch process that removes material in the vertical direction only.The horizontal etch rate is zero.Such anisotropic etches are easily produced by plasma processing.On the other hand,one mightimagineFIGURE 1.1.Trench etch (0.2m m wide by 4m m deep)in single-crystal silicon,showing the extraordinary capabilities of plasma processing;such trenches are used for device isolation and charge storage capacitors in integrated circuits.2INTRODUCTIONthat exposing the masked film (d )to a liquid (or vapor phase)etchant will lead to the undercut isotropic profile shown in Figure 1.3a (compare to Fig.1.2e ),which is produced by equal vertical and horizontal etch rates.Many years ago,feature spa-cings (e.g.,between trenches)were tens of micrometers,much exceeding required film thicknesses.Undercutting was then acceptable.This is no longer true with submicrometer feature spacings.The reduction in feature sizes and spacings makes anisotropic etch processes essential.In fact,strictly vertical etches are some-times not desired;one wants controlled sidewall angles.Plasma processing is the only commercial technology capable of such control.Anisotropy is a critical process parameter in IC manufacture and has been a major force in driving the development of plasma processing technology.The etch process applied to remove the film in Figure 1.2d is shown in Figure 1.2e as not removing,either the photoresist or the underlying substrate.This selectivity is another critical process parameter for IC manufacture.Whereas FIGURE 1.2.Deposition and pattern transfer in manufacturing an integrated circuit:(a )metal deposition;(b )photoresist deposition;(c )optical exposure through a pattern;(d )photoresist development;(e )anisotropic plasma etch;(f )remaining photoresist removal.1.1MATERIALS PROCESSING 3wet etches have been developed having essentially infinite selectivity,highly selec-tive plasma etch processes are not easily designed.Selectivity and anisotropy often compete in the design of a plasma etch process,with results as shown in Figure 1.3b .Compare this to the idealized result shown in Figure 1.2e .Assuming that film-to-substrate selectivity is a critical issue,one might imagine simply turning off the plasma after the film has been etched through.This requires a good endpoint detection system.Even then,variations in film thickness and etch rate across the area of the wafer imply that the etch cannot be stopped at the right moment every-where.Hence,depending on the process uniformity ,there is a need for some selectivity.These issues are considered further in Chapter 15.Here is a simple recipe for etching silicon using a plasma discharge.Start with an inert molecular gas,such as CF 4.Excite the discharge to sustain a plasma by electron–neutral dissociative ionization,e þCF 4À!2e þCF þ3þFand to create reactive species by electron–neutral dissociation,e þCF 4À!e þF þCF 3À!e þ2F þCF2FIGURE 1.3.Plasma etching in integrated circuit manufacture:(a )example of isotropic etch;(b )sidewall etching of the resist mask leads to a loss of anisotropy in film etch;(c )illustrating the role of bombarding ions in anisotropic etch;(d )illustrating the role of sidewall passivating films in anisotropic etch.4INTRODUCTIONThe etchant F atoms react with the silicon substrate,yielding the volatile etch product SiF4:Si(s)þ4F(g)À!SiF4(g)Here,s and g indicate solid and gaseous forms,respectively.Finally,the product is pumped away.It is important that CF4does not react with silicon,and that the etch product SiF4is volatile,so that it can be removed.This process etches siliconisotropically.For an anisotropic etch,there must be high-energy ion(CFþ3)bombard-ment of the substrate.As illustrated in Figures1.3c and d,energetic ions leaving the discharge during the etch bombard the bottom of the trench but do not bombard the sidewalls,leading to anisotropic etching by one of two mechanisms.Either the ion bombardment increases the reaction rate at the surface(Fig.1.3c),or it exposes the surface to the etchant by removing passivatingfilms that cover the surface(Fig.1.3d).Similarly,Cl and Br atoms created by dissociation in a discharge are good etch-ants for silicon,F atoms and CF2molecules for SiO2,O atoms for photoresist,and Cl atoms for aluminum.In all cases,a volatile etch product is formed.However,F atoms do not etch aluminum,and there is no known etchant for copper,because the etch products are not volatile at reasonable substrate temperatures.We see the importance of the basic physics and chemistry topics treated in this book:(1)plasma physics(Chapters2,4–6,and18),to determine the electron and ion densities,temperatures,and ion bombardment energies andfluxes for a given dis-charge configuration;and(2)gas-phase chemistry and(3)surface physics and chem-istry(Chapters7and9),to determine the etchant densities andfluxes and the etch rates with and without ion bombardment.The data base for thesefields of science is provided by(4)atomic and molecular physics,which we discuss in Chapters3 and8.We also discuss applications of equilibrium thermodynamics(Chapter7)to plasma processing.The measurement and experimental control of plasma and chemical properties in reactive discharges is itself a vast subject.We provide brief introductions to some simple plasma diagnostic techniques throughout the text.We have motivated the study of the fundamentals of plasma processing by exam-ining isotropic and anisotropic etches for IC manufacture.These are discussed in Chapter15.Other characteristics motivate its use for deposition and surface modi-fication.For example,a central feature of the low-pressure processing discharges that we consider in this book is that the plasma itself,as well as the plasma–substrate system,is not in thermal equilibrium.This enables substrate temperatures to be relatively low,compared to those required in conventional thermal processes, while maintaining adequate deposition or etch rates.Putting it another way,plasma processing rates are greatly enhanced over thermal processing rates at the same sub-strate temperature.For example,Si3N4films can be deposited over aluminumfilms by PECVD,whereas adequate deposition rates cannot be achieved by conventional chemical vapor deposition(CVD)without melting the aluminumfilm.Chapter16 gives further details.Particulates or“dust”can be a significant component in processing discharges and can be a source of substrate-level contamination in etch and deposition1.1MATERIALS PROCESSING56INTRODUCTIONprocesses.One can also control dust formation in useful ways,for example,to produce powders of various sizes or to incorporate nanoparticles during deposition to modifyfilm properties.Dusty plasmas are described in Chapter17.The nonequilibrium nature of plasma processing has been known for many years, as illustrated by the laboratory data in Figure1.4.In time sequence,this showsfirst, the equilibrium chemical etch rate of silicon in the XeF2etchant gas;next,the tenfold increase in etch rate with the addition of argon ion bombardment of the sub-strate,simulating plasma-assisted etching;andfinally,the very low“etch rate”due to the physical sputtering of silicon by the ion bombardment alone.A more recent application is the use of plasma-immersion ion implantation(PIII)to implant ions into materials at dose rates that are tens to hundreds of times larger than those achievable with conventional(beam based)ion implantation systems.In PIII,a series of negative high-voltage pulses are applied to a substrate that is immersed directly into a discharge,thus accelerating plasma ions into the substrate.The devel-opment of PIII has opened a new implantation regime characterized by very high dose rates,even at very low energies,and by the capability to implant both large area and irregularly shaped substrates,such asflat panel displays or machine tools and dies. This is illustrated in Figure1.5.Further details are given in Chapter16.1.2PLASMAS AND SHEATHSPlasmasA plasma is a collection of free charged particles moving in random directions that is,on the average,electrically neutral(see Fig.1.6a).This book deals withweakly Array FIGURE1.4.Experimental demonstration of ion-enhanced plasma etching.(Coburn and Winters,1979.)ionized plasma discharges,which are plasmas having the following features:(1)they are driven electrically;(2)charged particle collisions with neutral gas mol-ecules are important;(3)there are boundaries at which surface losses are important;(4)ionization of neutrals sustains the plasma in the steady state;and (5)the electrons are not in thermal equilibrium with the ions.A simple discharge is shown schematically in Figure 1.6b .It consists of a voltage source that drives current through a low-pressure gas between two parallel conduct-ing plates or electrodes.The gas “breaks down”to form a plasma,usually weakly ionized,that is,the plasma density is only a small fraction of the neutral gas density.We describe some qualitative features of plasmas in this section;discharges are described in the following section.Plasmas are often called a fourth state of matter.As we know,a solid substance in thermal equilibrium generally passes into a liquid state as the temperature is increased at a fixed pressure.The liquid passes into a gas as the temperature is further increased.At a sufficiently high temperature,the molecules in the gas decompose to form a gas of atoms that move freely in random directions,except for infrequent collisions between atoms.If the temperature is furtherincreased,FIGURE 1.5.Illustrating ion implantation of an irregular object:(a )In a conventional ion beam implanter,the beam is electrically scanned and the target object is mechanically rotated and tilted to achieve uniform implantation;(b )in plasma-immersion ion implantation (PIII),the target is immersed in a plasma,and ions from the plasma are implanted with a relatively uniform spatialdistribution.VFIGURE 1.6.Schematic view of (a )a plasma and (b )a discharge.1.2PLASMAS AND SHEATHS 7then the atoms decompose into freely moving charged particles(electrons and positive ions),and the substance enters the plasma state.This state is characterized by a common charged particle density n e%n i%n particles/m3and,in equilibrium, a temperature T e¼T i¼T.The temperatures required to form plasmas from puresubstances in thermal equilibrium range from roughly4000K for easy-to-ionize elements like cesium to20,000K for hard-to-ionize elements like helium.The fractional ionization of a plasma isx iz¼n i n gþn iwhere n g is the neutral gas density.x iz is near unity for fully ionized plasmas,and x iz(1for weakly ionized plasmas.Much of the matter in the universe is in the plasma state.This is true because stars,as well as most interstellar matter,are plasmas.Although stars are plasmas in thermal equilibrium,the light and heavy charged particles in low-pressure proces-sing discharges are almost never in thermal equilibrium,either between themselves or with their surroundings.Because these discharges are electrically driven and are weakly ionized,the applied power preferentially heats the mobile electrons,while the heavy ions efficiently exchange energy by collisions with the background gas. Hence,T e)T i for these plasmas.Figure1.7identifies different kinds of plasmas on a log n versus log T e diagram. There is an enormous range of densities and temperatures for both laboratory and space plasmas.Two important types of processing discharges are indicated on the figure.Low-pressure discharges are characterized by T e%1–10V,T i(T e,and n%108–1013cm23.These discharges are used as miniature chemical factories in which feedstock gases are broken into positive ions and chemically reactive etch-ants,deposition precursors,and so on,which thenflow to and physically or chemi-cally react at the substrate surface.While energy is delivered to the substrate also, for example,in the form of bombarding ions,the energyflux is there to promote the chemistry at the substrate,and not to heat the substrate.The gas pressures for these discharges are low:p%1mTorr–1Torr.These discharges and their use for processing are the principal subject of this book.We give the quantitative frame-work for their analysis in Chapter10.High-pressure arc discharges are also used for processing.These discharges have T e%0.1–2V and n%1014–1019cm23,and the light and heavy particles are more nearly in thermal equilibrium,with T i.T e.These discharges are used mainly to deliver heat to the substrate,for example,to increase surface reaction rates, to melt,sinter,or evaporate materials,or to weld or cut refractory materials.Opera-ting pressures are typically near atmospheric pressure(760Torr).High-pressure discharges of this type are beyond the scope of this book.Figure1.8shows the densities and temperatures(or average energies)for various species in a typical rf-driven capacitively coupled low-pressure discharge;for example,for silicon etching using CF4,as described in Section1.1.We see that the feedstock gas,etchant atoms,etch product gas,and plasma ions have roughly 8INTRODUCTIONthe same temperature,which does not exceed a few times room temperature (0.026V).The etchant F and product SiF 4densities are significant fractions of the CF 4density,but the fractional ionization is very low:n i 10À5n g .The electron temperature T e is two orders of magnitude larger than the ion temperature T i .However,we note that the energy of ions bombarding the substrate can be 100–1000V,much exceeding T e .The acceleration of low-temperature ions510152025log 10 T (V)el o g 10n (c m –3)FIGURE 1.7.Space and laboratory plasmas on a log n versus log T e diagram (after Book,1987).l De is defined in Section 2.4.1.2PLASMAS AND SHEATHS 9across a thin sheath region where the plasma and substrate meet is central to all pro-cessing discharges.We describe this qualitatively below and quantitatively in later chapters.Although n i and n e may be five orders of magnitude lower that n g ,the charged particles play central roles in sustaining the discharge and in processing.Because T e )T i ,it is the electrons that dissociate the feedstock gas to create the free radicals,etchant atoms,and deposition precursors,required for the chemistry at the substrate.Electrons also ionize the gas to create the positive ions that sub-sequently bombard the substrate.As we have seen,energetic ion bombardment can increase chemical reaction rates at the surface,clear inhibitor films from the surface,and physically sputter materials from or implant ions into the surface.T e is generally less than the threshold energies E diss or E iz for dissociation and ionization of the feedstock gas molecules.Nevertheless,dissociation and ionization occur because electrons have a distribution of energies.Letting g e (E )d E be the number of electrons per unit volume with energies lying between E and E þd E ,then the distribution function g e (E )is sketched in Figure 1.9.Electrons having ener-gies below E diss or E iz cannot dissociate or ionize the gas.We see that dissociation and ionization are produced by the high-energy tail of the distribution.Although the distribution is sketched in the figure as if it were Maxwellian at the bulk electron temperature T e ,this may not be the case.The tail distribution might be depressed below or enhanced above a Maxwellian by electron heating and electron–neutral collision processes.Two temperature distributions are sometimes observed,with Te 1081010n (c m –3)T or ·Ò (V)FIGURE 1.8.Densities and energies for various species in a low-pressure capacitive rf discharge.10INTRODUCTIONfor the bulk electrons lower than T h for the energetic electron tail.Non-Maxwellian distributions can only be described using the kinetic theory of discharges,which we introduce in Chapter 18.SheathsPlasmas,which are quasi-neutral (n i %n e ),are joined to wall surfaces across thin positively charged layers called sheaths .To see why,first note that the electron thermal velocity (e T e =m )1=2is at least 100times the ion thermal velocity (e T i =M )1=2because m =M (1and T e &T i .(Here,T e and T i are given in units of volts.)Consider a plasma of width l with n e ¼n i initially confined between two grounded (F ¼0)absorbing walls (Fig.1.10a ).Because the net charge density r ¼e (n i Àn e )is zero,the electric potential F and the electric field E x is zero every-where.Hence,the fast-moving electrons are not confined and will rapidly be lost to the walls.On a very short timescale,however,some electrons near the walls are lost,leading to the situation shown in Figure 1.10b .Thin (s (l )positive ion sheaths form near each wall in which n i )n e .The net positive r within the sheaths leads to a potential profile F (x )that is positive within the plasma and falls sharply to zero near both walls.This acts as a confining potential “valley”for electrons and a “hill”for ions because the electric fields within the sheaths point from the plasma to the wall.Thus the force ÀeE x acting on electrons is directed into the plasma;this reflects electrons traveling toward the walls back into the plasma.Conversely,ions from the plasma that enter the sheaths are accel-erated into the walls.If the plasma potential (with respect to the walls)is V p ,then we expect that V p a few T e in order to confine most of the electrons.The energy of ions bombarding the walls is then E i a few T e .Charge uncovering is treated quan-titatively in Chapter 2,and sheaths in Chapter 6.Figure 1.11shows sheath formation as obtained from a particle-in-cell (PIC)plasma simulation.We use PIC results throughout this book to illustrate various dis-charge phenomena.In this simulation,the left wall is grounded,the right wall is floating (zero net current),and the positive ion density is uniform and constant in time.The electrons are modeled as N sheets having charge-to-mass ratio Àe =mFIGURE 1.9.Electron distribution function in a weakly ionized discharge.1.2PLASMAS AND SHEATHS 1112INTRODUCTION(b)FIGURE1.10.The formation of plasma sheaths:(a)initial ion and electron densities andsheath.potential;(b)densities,electricfield,and potential after formation of the Array FIGURE1.11.PIC simulation of positive ion sheath formation:(a)v x–x electron phase space,with horizontal scale in meters;(b)electron density n e;(c)electricfield E x;(d)potential F;(e)electron number N versus time t in seconds;(f)right hand potential V r versus time t.that move in one dimension (along x )under the action of the time-varying fields pro-duced by all the other sheets,the fixed ion charge density,and the charges on the walls.Electrons do not collide with other electrons,ions,or neutrals in this simu-lation.Four thousand sheets were used with T e ¼1V and n i ¼n e ¼1013m À3at time t ¼0.In (a ),(b ),(c ),and (d ),we,respectively,see the v x –x electron phase space,electron density,electric field,and potential after the sheath has formed,at t ¼0.77m s.The time history of N is shown in (e );40sheets have been lost to form the sheaths.Figures 1.11a –d show the absence of electrons near each wall over a sheath width s %6mm.Except for fluctuations due to the finite N ,the field in the bulk plasma is near zero,and the fields in the sheaths are large and point from the plasma to the walls.(E x is negative at the left wall and positive at the right wall to repel plasma electrons.)The potential in the center of the discharge is V p %2:5V and falls to zero at the left wall (this wall is grounded by definition).The potential at the right wall is also low,but we see in (f )that it oscillates in time.We will see in Chapter 4that these are plasma oscillations .We would not see them if the initial sheet positions and velocities were chosen exactly symmetrically about the midplane,or if many more sheets were used in the simulation.If the ions were also modeled as moving sheets,then on a longer timescale we would see ion acceleration within the sheaths,and a consequent drop in ion density near the walls,as sketched in Figure 1.10b .We return to this in Chapter 6.The separation of discharges into bulk plasma and sheath regions is an important paradigm that applies to all discharges.The bulk region is quasi-neutral,and both instantaneous and time-averaged fields are low.The bulk plasma dynamicsare FIGURE 1.11.(Continued ).1.2PLASMAS AND SHEATHS 1314INTRODUCTIONdescribed by diffusive ion loss at high pressures and by free-fall ion loss at low pressures.In the positive space charge sheaths,highfields exist,leading to dynamics that are described by various ion space charge sheath laws,including low-voltage sheaths and various high-voltage sheath models,such as collisionless and collisional Child laws and their modifications.The plasma and sheath dynamics must be joined at their interface.As will be seen in Chapter6,the usual joining condition is to require that the mean ion velocity at the plasma-sheath edge be equal to the ion-sound(Bohm)velocity:u B¼(e T e=M)1=2,where e and M are the charge and mass of the ion,respectively,and T e is the electron temperature in volts.1.3DISCHARGESRadio Frequency DiodesCapacitively driven radio frequency(rf)discharges—so-called rf diodes—are commonly used for materials processing.An idealized discharge in plane parallel geometry,shown in Figure1.12a,consists of a vacuum chamber containing two planar electrodes separated by a spacing l and driven by an rf power source.The sub-strates are placed on one electrode,feedstock gases are admitted toflow through the discharge,and effluent gases are removed by the vacuum pump.Coaxial discharge geometries,such as the“hexode”shown in Figure1.12b,are also in widespread use. Typical parameters are shown in Table1.1.The typical rf driving voltage is V rf¼100–1000V,and the plate separation is l¼2–10cm.When operated at low pressure,with the wafer mounted on the powered electrode,and used to remove substrate material,such reactors are commonly called reactive ion etchers (RIEs)—a misnomer,since the etching is a chemical process enhanced by energetic ion bombardment of the substrate,rather than a removal process due to reactive ions alone.For anisotropic etching,typically pressures are in the range10–100mTorr, power densities are0.1–1W/cm2,the driving frequency is13.56MHz,and mul-tiple wafer systems are common.Typical plasma densities are relatively low, 109–1011cm23,and the electron temperature is of order3V.Ion acceleration ener-gies(sheath voltages)are high,greater than200V,and fractional ionization is low. The degree of dissociation of the molecules into reactive species is seldom measured but can range widely from less than0.1percent to nearly100percent depending on gas composition and plasma conditions.For deposition and isotropic etch appli-cations,pressures tend to be higher,ion bombarding energies are lower,and fre-quencies can be lower than the commonly used standard of13.56MHz.The operation of capacitively driven discharges is reasonably well understood. As shown in Figure1.13for a symmetrically driven discharge,the mobile plasma electrons,responding to the instantaneous electricfields produced by the rf driving voltage,oscillate back and forth within the positive space charge cloud of the ions.The massive ions respond only to the time-averaged electricfields.Oscil-lation of the electron cloud creates sheath regions near each electrode that containnet positive charge when averaged over an oscillation period;that is,the positive charge exceeds the negative charge in the system,with the excess appearing within the sheaths.This excess produces a strong time-averaged electric field within each sheath directed from the plasma to the electrode.Ions flowing out of the bulk plasma near the center of the discharge can be accelerated by the sheath fields to high energies as they flow to the substrate,leading to energetic-ion enhanced processes.Typical ion-bombarding energies E i can be as high as V rf =2for symmetric systems (Fig.1.13)and as high as V rf at the powered electrode for asymmetric systems (Fig.1.12).A quantitative description of capacitive discharges is given in Chapter11.FIGURE 1.12.Capacitive rf discharges in (a )plane parallel geometry and (b )coaxial “hexode”geometry (after Lieberman and Gottscho,1994).1.3DISCHARGES 15。

化学与STSESTSE是由科学、技术、社会和环境的首字母缩写而成的词，强调了它们之间的相互关系和影响。

2.STSE创新题通常与最新科技成果、与社会、生活联系紧密的材料有关，是高起点、低落点的题目。

这类题目的出题特点常以选择题或简答题等形式出现，但都是用化学知识解决问题。

3.STSE的分类包括化学与生活以及化学与环境保护。

化学与生活涉及的内容有食品、烟酒、油漆、涂料、化妆品、药品、毒品、饮用水、维生素、氟和钙等。

化学与环境保护的重点关注工业“三废”和生活垃圾处理不当对空气、水体和土壤造成的污染，以及绿色化学新理念在工、农业生产中的应用。

绿色化学观点包括开发"原子经济"反应、采用无毒、无害的原料、催化剂和溶剂、利用可再生的资源合成化学品以及环境友好产品。

富营养化是水体污染的一个重要问题，其中含磷洗衣粉的使用是造成水体富营养化的主要原因之一。

这种现象在江、河、湖泊中出现称为“水华”，在海湾中出现叫做“赤潮”。

为了解决这一问题，我们需要采取有效的措施，比如限制含磷洗衣粉的使用。

绿色食品是指无污染、无公害、安全且有营养价值的卫生食品。

这种食品对人们的健康非常有益，因此我们需要加强对绿色食品的生产和推广，让更多的人受益。

绿色化学是一种能够从根本上消灭污染，能彻底防止污染产生的科学。

它包括“原料绿色化”、“化学反应绿色化”、“产物绿色化”等内容。

我们需要加强对绿色化学的研究和应用，以减少对环境的污染。

白色污染是指各种塑料垃圾对环境所造成的污染。

因为它们很难降解，会破坏土壤结构。

为了减少白色污染，我们需要加强对塑料垃圾的回收和处理。

光化学烟雾是指汽车、工厂等污染源排入大气的碳氢化合物和氮氧化合物等一次污染物，在阳光（紫外线）作用下会发生光化学反应生成二次污染物，参与光化学反应过程的一次污染物和二次污染物的混合物所形成的烟雾污染现象。

我们需要采取有效措施减少这种污染，比如加强对污染源的管理和控制。

CHAPTER8MOLECULAR COLLISIONS8.1INTRODUCTIONBasic concepts of gas-phase collisions were introduced in Chapter3,where we described only those processes needed to model the simplest noble gas discharges: electron–atom ionization,excitation,and elastic scattering;and ion–atom elastic scattering and resonant charge transfer.In this chapter we introduce other collisional processes that are central to the description of chemically reactive discharges.These include the dissociation of molecules,the generation and destruction of negative ions,and gas-phase chemical reactions.Whereas the cross sections have been measured reasonably well for the noble gases,with measurements in reasonable agreement with theory,this is not the case for collisions in molecular gases.Hundreds of potentially significant collisional reactions must be examined in simple diatomic gas discharges such as oxygen.For feedstocks such as CF4/O2,SiH4/O2,etc.,the complexity can be overwhelming.Furthermore,even when the significant processes have been identified,most of the cross sections have been neither measured nor calculated. Hence,one must often rely on estimates based on semiempirical or semiclassical methods,or on measurements made on molecules analogous to those of interest. As might be expected,data are most readily available for simple diatomic and polyatomic gases.Principles of Plasma Discharges and Materials Processing,by M.A.Lieberman and A.J.Lichtenberg. ISBN0-471-72001-1Copyright#2005John Wiley&Sons,Inc.235236MOLECULAR COLLISIONS8.2MOLECULAR STRUCTUREThe energy levels for the electronic states of a single atom were described in Chapter3.The energy levels of molecules are more complicated for two reasons. First,molecules have additional vibrational and rotational degrees of freedom due to the motions of their nuclei,with corresponding quantized energies E v and E J. Second,the energy E e of each electronic state depends on the instantaneous con-figuration of the nuclei.For a diatomic molecule,E e depends on a single coordinate R,the spacing between the two nuclei.Since the nuclear motions are slow compared to the electronic motions,the electronic state can be determined for anyfixed spacing.We can therefore represent each quantized electronic level for a frozen set of nuclear positions as a graph of E e versus R,as shown in Figure8.1.For a mole-cule to be stable,the ground(minimum energy)electronic state must have a minimum at some value R1corresponding to the mean intermolecular separation (curve1).In this case,energy must be supplied in order to separate the atoms (R!1).An excited electronic state can either have a minimum( R2for curve2) or not(curve3).Note that R2and R1do not generally coincide.As for atoms, excited states may be short lived(unstable to electric dipole radiation)or may be metastable.Various electronic levels may tend to the same energy in the unbound (R!1)limit. Array FIGURE8.1.Potential energy curves for the electronic states of a diatomic molecule.For diatomic molecules,the electronic states are specifiedfirst by the component (in units of hÀ)L of the total orbital angular momentum along the internuclear axis, with the symbols S,P,D,and F corresponding to L¼0,+1,+2,and+3,in analogy with atomic nomenclature.All but the S states are doubly degenerate in L.For S states,þandÀsuperscripts are often used to denote whether the wave function is symmetric or antisymmetric with respect to reflection at any plane through the internuclear axis.The total electron spin angular momentum S (in units of hÀ)is also specified,with the multiplicity2Sþ1written as a prefixed superscript,as for atomic states.Finally,for homonuclear molecules(H2,N2,O2, etc.)the subscripts g or u are written to denote whether the wave function is sym-metric or antisymmetric with respect to interchange of the nuclei.In this notation, the ground states of H2and N2are both singlets,1Sþg,and that of O2is a triplet,3SÀg .For polyatomic molecules,the electronic energy levels depend on more thanone nuclear coordinate,so Figure8.1must be generalized.Furthermore,since there is generally no axis of symmetry,the states cannot be characterized by the quantum number L,and other naming conventions are used.Such states are often specified empirically through characterization of measured optical emission spectra.Typical spacings of low-lying electronic energy levels range from a few to tens of volts,as for atoms.Vibrational and Rotational MotionsUnfreezing the nuclear vibrational and rotational motions leads to additional quan-tized structure on smaller energy scales,as illustrated in Figure8.2.The simplest (harmonic oscillator)model for the vibration of diatomic molecules leads to equally spaced quantized,nondegenerate energy levelse E v¼hÀv vib vþ1 2(8:2:1)where v¼0,1,2,...is the vibrational quantum number and v vib is the linearized vibration frequency.Fitting a quadratic functione E v¼12k vib(RÀ R)2(8:2:2)near the minimum of a stable energy level curve such as those shown in Figure8.1, we can estimatev vib%k vibm Rmol1=2(8:2:3)where k vib is the“spring constant”and m Rmol is the reduced mass of the AB molecule.The spacing hÀv vib between vibrational energy levels for a low-lying8.2MOLECULAR STRUCTURE237stable electronic state is typically a few tenths of a volt.Hence for molecules in equi-librium at room temperature (0.026V),only the v ¼0level is significantly popula-ted.However,collisional processes can excite strongly nonequilibrium vibrational energy levels.We indicate by the short horizontal line segments in Figure 8.1a few of the vibrational energy levels for the stable electronic states.The length of each segment gives the range of classically allowed vibrational motions.Note that even the ground state (v ¼0)has a finite width D R 1as shown,because from(8.2.1),the v ¼0state has a nonzero vibrational energy 1h Àv vib .The actual separ-ation D R about Rfor the ground state has a Gaussian distribution,and tends toward a distribution peaked at the classical turning points for the vibrational motion as v !1.The vibrational motion becomes anharmonic and the level spa-cings tend to zero as the unbound vibrational energy is approached (E v !D E 1).FIGURE 8.2.Vibrational and rotational levels of two electronic states A and B of a molecule;the three double arrows indicate examples of transitions in the pure rotation spectrum,the rotation–vibration spectrum,and the electronic spectrum (after Herzberg,1971).238MOLECULAR COLLISIONSFor E v.D E1,the vibrational states form a continuum,corresponding to unbound classical motion of the nuclei(breakup of the molecule).For a polyatomic molecule there are many degrees of freedom for vibrational motion,leading to a very compli-cated structure for the vibrational levels.The simplest(dumbbell)model for the rotation of diatomic molecules leads to the nonuniform quantized energy levelse E J¼hÀ22I molJ(Jþ1)(8:2:4)where I mol¼m Rmol R2is the moment of inertia and J¼0,1,2,...is the rotational quantum number.The levels are degenerate,with2Jþ1states for the J th level. The spacing between rotational levels increases with J(see Figure8.2).The spacing between the lowest(J¼0to J¼1)levels typically corresponds to an energy of0.001–0.01V;hence,many low-lying levels are populated in thermal equilibrium at room temperature.Optical EmissionAn excited molecular state can decay to a lower energy state by emission of a photon or by breakup of the molecule.As shown in Figure8.2,the radiation can be emitted by a transition between electronic levels,between vibrational levels of the same electronic state,or between rotational levels of the same electronic and vibrational state;the radiation typically lies within the optical,infrared,or microwave frequency range,respectively.Electric dipole radiation is the strongest mechanism for photon emission,having typical transition times of t rad 10À9s,as obtained in (3.4.13).The selection rules for electric dipole radiation areDL¼0,+1(8:2:5a)D S¼0(8:2:5b) In addition,for transitions between S states the only allowed transitions areSþÀ!Sþand SÀÀ!SÀ(8:2:6) and for homonuclear molecules,the only allowed transitions aregÀ!u and uÀ!g(8:2:7) Hence homonuclear diatomic molecules do not have a pure vibrational or rotational spectrum.Radiative transitions between electronic levels having many different vibrational and rotational initial andfinal states give rise to a structure of emission and absorption bands within which a set of closely spaced frequencies appear.These give rise to characteristic molecular emission and absorption bands when observed8.2MOLECULAR STRUCTURE239using low-resolution optical spectrometers.As for atoms,metastable molecular states having no electric dipole transitions to lower levels also exist.These have life-times much exceeding10À6s;they can give rise to weak optical band structures due to magnetic dipole or electric quadrupole radiation.Electric dipole radiation between vibrational levels of the same electronic state is permitted for molecules having permanent dipole moments.In the harmonic oscillator approximation,the selection rule is D v¼+1;weaker transitions D v¼+2,+3,...are permitted for anharmonic vibrational motion.The preceding description of molecular structure applies to molecules having arbi-trary electronic charge.This includes neutral molecules AB,positive molecular ions ABþ,AB2þ,etc.and negative molecular ions ABÀ.The potential energy curves for the various electronic states,regardless of molecular charge,are commonly plotted on the same diagram.Figures8.3and8.4give these for some important electronic statesof HÀ2,H2,and Hþ2,and of OÀ2,O2,and Oþ2,respectively.Examples of both attractive(having a potential energy minimum)and repulsive(having no minimum)states can be seen.The vibrational levels are labeled with the quantum number v for the attrac-tive levels.The ground states of both Hþ2and Oþ2are attractive;hence these molecular ions are stable against autodissociation(ABþ!AþBþor AþþB).Similarly,the ground states of H2and O2are attractive and lie below those of Hþ2and Oþ2;hence they are stable against autodissociation and autoionization(AB!ABþþe).For some molecules,for example,diatomic argon,the ABþion is stable but the AB neutral is not stable.For all molecules,the AB ground state lies below the ABþground state and is stable against autoionization.Excited states can be attractive or repulsive.A few of the attractive states may be metastable;some examples are the 3P u state of H2and the1D g,1Sþgand3D u states of O2.Negative IonsRecall from Section7.2that many neutral atoms have a positive electron affinity E aff;that is,the reactionAþeÀ!AÀis exothermic with energy E aff(in volts).If E aff is negative,then AÀis unstable to autodetachment,AÀ!Aþe.A similar phenomenon is found for negative molecular ions.A stable ABÀion exists if its ground(lowest energy)state has a potential minimum that lies below the ground state of AB.This is generally true only for strongly electronegative gases having large electron affinities,such as O2 (E aff%1:463V for O atoms)and the halogens(E aff.3V for the atoms).For example,Figure8.4shows that the2P g ground state of OÀ2is stable,with E aff% 0:43V for O2.For weakly electronegative or for electropositive gases,the minimum of the ground state of ABÀgenerally lies above the ground state of AB,and ABÀis unstable to autodetachment.An example is hydrogen,which is weakly electronegative(E aff%0:754V for H atoms).Figure8.3shows that the2Sþu ground state of HÀ2is unstable,although the HÀion itself is stable.In an elec-tropositive gas such as N2(E aff.0),both NÀ2and NÀare unstable. 240MOLECULAR COLLISIONS8.3ELECTRON COLLISIONS WITH MOLECULESThe interaction time for the collision of a typical (1–10V)electron with a molecule is short,t c 2a 0=v e 10À16–10À15s,compared to the typical time for a molecule to vibrate,t vib 10À14–10À13s.Hence for electron collisional excitation of a mole-cule to an excited electronic state,the new vibrational (and rotational)state canbeFIGURE 8.3.Potential energy curves for H À2,H 2,and H þ2.(From Jeffery I.Steinfeld,Molecules and Radiation:An Introduction to Modern Molecular Spectroscopy ,2d ed.#MIT Press,1985.)8.3ELECTRON COLLISIONS WITH MOLECULES 241FIGURE 8.4.Potential energy curves for O À2,O 2,and O þ2.(From Jeffery I.Steinfeld,Molecules and Radiation:An Introduction to Modern Molecular Spectroscopy ,2d ed.#MIT Press,1985.)242MOLECULAR COLLISIONS8.3ELECTRON COLLISIONS WITH MOLECULES243 determined by freezing the nuclear motions during the collision.This is known as the Franck–Condon principle and is illustrated in Figure8.1by the vertical line a,showing the collisional excitation atfixed R to a high quantum number bound vibrational state and by the vertical line b,showing excitation atfixed R to a vibra-tionally unbound state,in which breakup of the molecule is energetically permitted. Since the typical transition time for electric dipole radiation(t rad 10À9–10À8s)is long compared to the dissociation( vibrational)time t diss,excitation to an excited state will generally lead to dissociation when it is energetically permitted.Finally, we note that the time between collisions t c)t rad in typical low-pressure processing discharges.Summarizing the ordering of timescales for electron–molecule collisions,we havet at t c(t vib t diss(t rad(t cDissociationElectron impact dissociation,eþABÀ!AþBþeof feedstock gases plays a central role in the chemistry of low-pressure reactive discharges.The variety of possible dissociation processes is illustrated in Figure8.5.In collisions a or a0,the v¼0ground state of AB is excited to a repulsive state of AB.The required threshold energy E thr is E a for collision a and E a0for Array FIGURE8.5.Illustrating the variety of dissociation processes for electron collisions with molecules.collision a0,and it leads to an energy after dissociation lying between E aÀE diss and E a0ÀE diss that is shared among the dissociation products(here,A and B). Typically,E aÀE diss few volts;consequently,hot neutral fragments are typically generated by dissociation processes.If these hot fragments hit the substrate surface, they can profoundly affect the process chemistry.In collision b,the ground state AB is excited to an attractive state of AB at an energy E b that exceeds the binding energy E diss of the AB molecule,resulting in dissociation of AB with frag-ment energy E bÀE diss.In collision b0,the excitation energy E b0¼E diss,and the fragments have low energies;hence this process creates fragments having energies ranging from essentially thermal energies up to E bÀE diss few volts.In collision c,the AB atom is excited to the bound excited state ABÃ(labeled5),which sub-sequently radiates to the unbound AB state(labeled3),which then dissociates.The threshold energy required is large,and the fragments are hot.Collision c can also lead to dissociation of an excited state by a radiationless transfer from state5to state4near the point where the two states cross:ABÃðboundÞÀ!ABÃðunboundÞÀ!AþBÃThe fragments can be both hot and in excited states.We discuss such radiationless electronic transitions in the next section.This phenomenon is known as predisso-ciation.Finally,a collision(not labeled in thefigure)to state4can lead to dis-sociation of ABÃ,again resulting in hot excited fragments.The process of electron impact excitation of a molecule is similar to that of an atom,and,consequently,the cross sections have a similar form.A simple classical estimate of the dissociation cross section for a level having excitation energy U1can be found by requiring that an incident electron having energy W transfer an energy W L lying between U1and U2to a valence electron.Here,U2is the energy of the next higher level.Then integrating the differential cross section d s[given in(3.4.20)and repeated here],d s¼pe24021Wd W LW2L(3:4:20)over W L,we obtains diss¼0W,U1pe24pe021W1U1À1WU1,W,U2pe24021W1U1À1U2W.U28>>>>>><>>>>>>:(8:3:1)244MOLECULAR COLLISIONSLetting U2ÀU1(U1and introducing voltage units W¼e E,U1¼e E1and U2¼e E2,we haves diss¼0E,E1s0EÀE11E1,E,E2s0E2ÀE1EE.E28>>>><>>>>:(8:3:2)wheres0¼pe4pe0E12(8:3:3)We see that the dissociation cross section rises linearly from the threshold energy E thr%E1to a maximum value s0(E2ÀE1)=E thr at E2and then falls off as1=E. Actually,E1and E2can depend on the nuclear separation R.In this case,(8.3.2) should be averaged over the range of R s corresponding to the ground-state vibrational energy,leading to a broadened dependence of the average cross section on energy E.The maximum cross section is typically of order10À15cm2. Typical rate constants for a single dissociation process with E thr&T e have an Arrhenius formK diss/K diss0expÀE thr T e(8:3:4)where K diss0 10À7cm3=s.However,in some cases E thr.T e.For excitation to an attractive state,an appropriate average over the fraction of the ground-state vibration that leads to dissociation must be taken.Dissociative IonizationIn addition to normal ionization,eþABÀ!ABþþ2eelectron–molecule collisions can lead to dissociative ionizationeþABÀ!AþBþþ2eThese processes,common for polyatomic molecules,are illustrated in Figure8.6.In collision a having threshold energy E iz,the molecular ion ABþis formed.Collisionsb andc occur at higher threshold energies E diz and result in dissociative ionization,8.3ELECTRON COLLISIONS WITH MOLECULES245leading to the formation of fast,positively charged ions and neutrals.These cross sections have a similar form to the Thompson ionization cross section for atoms.Dissociative RecombinationThe electron collision,e þAB þÀ!A þB Ãillustrated as d and d 0in Figure 8.6,destroys an electron–ion pair and leads to the production of fast excited neutral fragments.Since the electron is captured,it is not available to carry away a part of the reaction energy.Consequently,the collision cross section has a resonant character,falling to very low values for E ,E d and E .E d 0.However,a large number of excited states A Ãand B Ãhaving increasing principal quantum numbers n and energies can be among the reaction products.Consequently,the rate constants can be large,of order 10À7–10À6cm 3=s.Dissocia-tive recombination to the ground states of A and B cannot occur because the potential energy curve for AB þis always greater than the potential energycurveFIGURE 8.6.Illustration of dissociative ionization and dissociative recombination for electron collisions with molecules.246MOLECULAR COLLISIONSfor the repulsive state of AB.Two-body recombination for atomic ions or for mol-ecular ions that do not subsequently dissociate can only occur with emission of a photon:eþAþÀ!Aþh n:As shown in Section9.2,the rate constants are typically three tofive orders of magnitude lower than for dissociative recombination.Example of HydrogenThe example of H2illustrates some of the inelastic electron collision phenomena we have discussed.In order of increasing electron impact energy,at a threshold energy of 8:8V,there is excitation to the repulsive3Sþu state followed by dissociation into two fast H fragments carrying 2:2V/atom.At11.5V,the1Sþu bound state is excited,with subsequent electric dipole radiation in the ultraviolet region to the1Sþg ground state.At11.8V,there is excitation to the3Sþg bound state,followedby electric dipole radiation to the3Sþu repulsive state,followed by dissociation with 2:2V/atom.At12.6V,the1P u bound state is excited,with UV emission tothe ground state.At15.4V,the2Sþg ground state of Hþ2is excited,leading to the pro-duction of Hþ2ions.At28V,excitation of the repulsive2Sþu state of Hþ2leads to thedissociative ionization of H2,with 5V each for the H and Hþfragments.Dissociative Electron AttachmentThe processes,eþABÀ!AþBÀproduce negative ion fragments as well as neutrals.They are important in discharges containing atoms having positive electron affinities,not only because of the pro-duction of negative ions,but because the threshold energy for production of negative ion fragments is usually lower than for pure dissociation processes.A variety of pro-cesses are possible,as shown in Figure8.7.Since the impacting electron is captured and is not available to carry excess collision energy away,dissociative attachment is a resonant process that is important only within a narrow energy range.The maximum cross sections are generally much smaller than the hard-sphere cross section of the molecule.Attachment generally proceeds by collisional excitation from the ground AB state to a repulsive ABÀstate,which subsequently either auto-detaches or dissociates.The attachment cross section is determined by the balance between these processes.For most molecules,the dissociation energy E diss of AB is greater than the electron affinity E affB of B,leading to the potential energy curves shown in Figure8.7a.In this case,the cross section is large only for impact energies lying between a minimum value E thr,for collision a,and a maximum value E0thr for8.3ELECTRON COLLISIONS WITH MOLECULES247FIGURE 8.7.Illustration of a variety of electron attachment processes for electron collisions with molecules:(a )capture into a repulsive state;(b )capture into an attractive state;(c )capture of slow electrons into a repulsive state;(d )polar dissociation.248MOLECULAR COLLISIONScollision a 0.The fragments are hot,having energies lying between minimum and maximum values E min ¼E thr þE affB ÀE diss and E max ¼E 0thr þE af fB ÀE diss .Since the AB Àstate lies above the AB state for R ,R x ,autodetachment can occur as the mol-ecules begin to separate:AB À!AB þe.Hence the cross section for production of negative ions can be much smaller than that for excitation of the AB Àrepulsive state.As a crude estimate,for the same energy,the autodetachment rate is ffiffiffiffiffiffiffiffiffiffiffiffiffiM R =m p 100times the dissociation rate of the repulsive AB Àmolecule,where M R is the reduced mass.Hence only one out of 100excitations lead to dissociative attachment.Excitation to the AB Àbound state can also lead to dissociative attachment,as shown in Figure 8.7b .Here the cross section is significant only for E thr ,E ,E 0thr ,but the fragments can have low energies,with a minimum energy of zero and a maximum energy of E 0thr þE affB ÀE diss .Collision b,e þAB À!AB ÀÃdoes not lead to production of AB Àions because energy and momentum are not gen-erally conserved when two bodies collide elastically to form one body (see Problem3.12).Hence the excited AB ÀÃion separates,AB ÀÃÀ!e þABunless vibrational radiation or collision with a third body carries off the excess energy.These processes are both slow in low-pressure discharges (see Section 9.2).At high pressures (say,atmospheric),three-body attachment to form AB Àcan be very important.For a few molecules,such as some halogens,the electron affinity of the atom exceeds the dissociation energy of the neutral molecule,leading to the potential energy curves shown in Figure 8.7c .In this case the range of electron impact ener-gies E for excitation of the AB Àrepulsive state includes E ¼0.Consequently,there is no threshold energy,and very slow electrons can produce dissociative attachment,resulting in hot neutral and negative ion fragments.The range of R s over which auto-detachment can occur is small;hence the maximum cross sections for dissociative attachment can be as high as 10À16cm 2.A simple classical estimate of electron capture can be made using the differential scattering cross section for energy loss (3.4.20),in a manner similar to that done for dissociation.For electron capture to an energy level E 1that is unstable to autode-tachment,and with the additional constraint for capture that the incident electron energy lie within E 1and E 2¼E 1þD E ,where D E is a small energy difference characteristic of the dissociative attachment timescale,we obtain,in place of (8.3.2),s att¼0E ,E 1s 0E ÀE 1E 1E 1,E ,E 20E .E 28>><>>:(8:3:5)8.3ELECTRON COLLISIONS WITH MOLECULES 249wheres 0%p m M R 1=2e 4pe 0E 1 2(8:3:6)The factor of (m =M R )1=2roughly gives the fraction of excited states that do not auto-detach.We see that the dissociative attachment cross section rises linearly at E 1to a maximum value s 0D E =E 1and then falls abruptly to zero.As for dissociation,E 1can depend strongly on the nuclear separation R ,and (8.3.5)must be averaged over the range of E 1s corresponding to the ground state vibrational motion;e.g.,from E thr to E 0thr in Figure 8.7a .Because generally D E (E 0thr ÀE thr ,we can write (8.3.5)in the forms att %p m M R 1=2e 4pe 0 2(D E )22E 1d (E ÀE 1)(8:3:7)where d is the Dirac delta ing (8.3.7),the average over the vibrational motion can be performed,leading to a cross section that is strongly peaked lying between E thr and E 0thr .We leave the details of the calculation to a problem.Polar DissociationThe process,e þAB À!A þþB Àþeproduces negative ions without electron capture.As shown in Figure 8.7d ,the process proceeds by excitation of a polar state A þand B Àof AB Ãthat has a separ-ated atom limit of A þand B À.Hence at large R ,this state lies above the A þB ground state by the difference between the ionization potential of A and the electron affinity of B.The polar state is weakly bound at large R by the Coulomb attraction force,but is repulsive at small R .The maximum cross section and the dependence of the cross section on electron impact energy are similar to that of pure dissociation.The threshold energy E thr for polar dissociation is generally large.The measured cross section for negative ion production by electron impact in O 2is shown in Figure 8.8.The sharp peak at 6.5V is due to dissociative attachment.The variation of the cross section with energy is typical of a resonant capture process.The maximum cross section of 10À18cm 2is quite low because autode-tachment from the repulsive O À2state is strong,inhibiting dissociative attachment.The second gradual maximum near 35V is due to polar dissociation;the variation of the cross section with energy is typical of a nonresonant process.250MOLECULAR COLLISIONS。

The Chemical Properties of SubstancesThe chemical properties of a substance are those properties that relate to its participation in chemical reactions.Chemical reactions are the processes that convert substances into other substances。

Thus sodium chloride·has the property of changing into a soft metal，sodium，and a greenish-yellow gas，chlorine, when it is decomposed by passage of an electriccurrent through it. It also has the property, when it is dissolved in water，of produ-cing a white precipitate when a solution of silver nitrate is added to it，and it hasmany other chemical properties.Iron has the property of combining readily with the oxygen in moist air to form iron rust; whereas an alloy of iron with chromium and nickel(stainless steel)isfound to resist this process of rusting. It is evident from this example that the chemi-cal properties of materials are important in engineering.Many chemical reactions take place in the kitchen. When biscuits are made with use of sour milk and baking soda there is a chemical reaction between the baking sodaand a substance in the sour milk，lactic acid，to produce the gas carbon dioxide，which leavens the dough by forming small bubbles in it. And, of course，a greatmany chemical reactions take place in the human body. Foods that we eat are digestedin the stomach and intestines. Oxygen in the inhaled air combines with a substance，hemoglobin, in the red cells of the blood, and then is released in the tissues, whereit takes part in many different reactions. Many biochemists and physiologists are en-gaged in the study of the chemical reactions that take place in the human body.Most substances have the power to enter into many chemical reactions. The study of these reactions constitutes a large part of the study of chemistry. Chemistrymay be defined as the science of substances-their structure, their properties，and thereactions that change them into other substances.2.2 Chemical Changes and Physical ChangesDifferent kinds of matter have different physical and chemical properties. The properties of a substance are its characteristics. We know one substance from anotherby their physical and chemical properties. In a physical change the composition of asubstance is not changed. Ice can be changed into water. This is a physical changebecause the composition of water is not changed. In a chemical change the composi-tion of a substance is changed. One or more new substances are formed.Iron rusts in moist air. When iron rusts，it unites with the oxygen from the air.A new substance is formed. It is iron oxide. It has other different properties. Woodwill burn if it is heated in air. When wood burns，it reacts with the oxygen from theair. New substances are formed. They are carbon dioxide and water. Carbon dioxideand water have different properties. Heat is given off if the combustion of any fueltakes place.The above two cases are chemical changes.Chemical changes are very common. They are going on around us all the time.Whenever anything burns，there is a chemical change. When iron rusts，the changeis a chemical change. A chemical change goes on when things decay.Physical changes are very common, too. Tearing a piece of paper in two is aphysical change. The paper is still paper.We all know that this is not a chemical change. But we do not always know with ease whether a change is a chemical change or a physical change.If you dissolve sugar in water，the sugar disappears. You may think that a new material has been formed. But really there is no new material. The sugar is still sug- You can still taste it. Dissolving anything is a physical change.When water freezes，the change is a physical change. The water changes from a liquid to a solid. Its chemical formula is still H20. The freezing of any liquid is a physical change.In a word，any change in state is a physical change. When anything melts，it changes from a solid to a liquid. When it evaporates，it changes from a solid or a liq- uid to a gas. When it condenses，it changes from a gas to a liquid or a solid. But it is the same material still.Now we see that a chemical change is different from a physical change in that the chemical change causes a change of matter in chemical composition，but the physical change does not.。

Chemical Engineering PrinciplesIntroductionChemical engineering principles form the foundation of designing and operating various chemical processes in industries. These principles encompass a wide range of essential concepts and theories that allow engineers to understand and manipulate the behavior of different substances and materials. In this document, we will explore the fundamental principles of chemical engineering and their applications in various industrial processes.Mass and Energy BalancesMass and energy balances are fundamental concepts in chemical engineering. Mass balance involves the conservation of mass during a chemical process. It states that the mass entering a system must be equal to the mass leaving the system, taking into account any accumulation or depletion of mass within the system. Energy balance, on the other hand, deals with the conservation of energy. It states that the energy entering a system must be equal to the energy leaving the system, considering energy transformations and transfers within the system.ThermodynamicsThermodynamics plays a vital role in chemical engineering as it deals with the study of energy transformations and the relationships between various forms of energy. It provides a framework for understanding the behavior of substances and their ability to undergo chemical reactions. The laws of thermodynamics, namely the first and second laws, are essential principles in chemical engineering. The first law states that energy cannot be created or destroyed, but it can be transformed from one form to another. The second law states that the entropy of a system tends to increase over time, leading to a spontaneous direction for chemical reactions.Fluid MechanicsFluid mechanics is another crucial aspect of chemical engineering that deals with the behavior of fluids (liquids and gases) in motion. It involves the study of fluid properties and their flow characteristics in pipes, channels, and other equipment. Fluid mechanics principles are applied in designing efficient transportation systems for fluids, such as pumps and pipelines. Additionally, the analysis of fluid flow patterns and pressure drops is essential in optimizing process efficiency and preventing equipment failures.Heat TransferHeat transfer is the study of how thermal energy is transferred between different substances. In chemical engineering, an understanding of heat transfer is essential for designing and operating heat exchangers, reactors, and other process equipment. There are three main modes of heat transfer: conduction, convection, and radiation. Conduction involves heat transfer through direct contact between substances, convection involves heat transfer due to fluid motion, and radiation involves heat transfer through electromagnetic waves.Separation ProcessesSeparation processes are vital in chemical engineering to isolate and purify desired products from mixtures. These processes involve the separation of different components based on their physical or chemical properties. Some common separation processes include distillation, absorption, extraction, filtration, and crystallization. The selection and design of appropriate separation processes depend on the composition of the mixture and the desired product specifications.Reaction EngineeringReaction engineering focuses on the study of chemical reactions and their kinetics. Chemical reactions are at the core of many industrial processes, and understanding their behavior is crucial for optimizing reaction conditions and maximizing product yields. Reaction engineering involves the design and operation of reactors, including considerations such as reaction rates, temperature, pressure, and catalysts. Engineers use reaction kinetics and thermodynamics principles to determine the optimal reaction conditions for desired product formation.Process ControlProcess control is an essential aspect of chemical engineering that involves maintaining optimal operating conditions for chemical processes. It aims to ensure consistent product quality and maximize process efficiency. Process control utilizes various techniques, such as feedback control systems, instrumentation, and process optimization algorithms, to monitor and adjust process variables like temperature, pressure, flow rate, and composition.ConclusionChemical engineering principles form the backbone of various industrial processes. Understanding these principles enables engineers to design, optimize, and operate chemical processes efficiently. This document has provided an overview of some fundamental principles, including mass and energy balances, thermodynamics, fluid mechanics, heat transfer, separation processes, reaction engineering, and process control. These principles are essential for any aspiringchemical engineer to master in order to contribute effectively to the field and tackle real-world challenges.。

化学结构英文The Chemical StructureChemistry is a fundamental branch of science that delves into the composition, properties, and behavior of matter. At the core of this field lies the concept of chemical structure, which is the arrangement of atoms and their connections within a molecule. Understanding the chemical structure is crucial for comprehending the properties, reactivity, and applications of various substances.One of the fundamental aspects of chemical structure is the arrangement of atoms. Atoms are the basic building blocks of matter, and they consist of a nucleus surrounded by a cloud of electrons. The number and arrangement of protons and neutrons in the nucleus, as well as the number and distribution of electrons, determine the unique characteristics of each element. When atomsof different elements combine, they form molecules, and the specific way in which these atoms are arranged within the molecule is known as the chemical structure.The chemical structure of a molecule can be represented using various models, such as the ball-and-stick model, the space-fillingmodel, and the line-angle model. The ball-and-stick model depicts atoms as spheres connected by sticks, representing the chemical bonds. The space-filling model, on the other hand, shows the relative size and shape of the atoms in the molecule, providing a more accurate representation of the overall structure. The line-angle model is a simplified representation where atoms are shown as points, and the bonds between them are represented by lines or angles.The arrangement of atoms within a molecule is governed by the principles of chemical bonding. There are several types of chemical bonds, including covalent bonds, ionic bonds, and hydrogen bonds. Covalent bonds are formed by the sharing of electrons between atoms, and they are the most common type of bond in organic chemistry. Ionic bonds are formed by the transfer of electrons from one atom to another, resulting in the formation of positively and negatively charged ions. Hydrogen bonds, on the other hand, are a special type of intermolecular interaction that occurs between a hydrogen atom and a highly electronegative atom, such as oxygen or nitrogen.The chemical structure of a molecule also determines its overall shape and geometry. The shape of a molecule is influenced by the number and type of bonds, as well as the arrangement of the atoms around the central atom. The most common molecular geometriesinclude linear, trigonal planar, tetrahedral, and octahedral, among others. These geometric arrangements have a significant impact on the physical and chemical properties of the molecule, such as its polarity, reactivity, and intermolecular interactions.The understanding of chemical structure is crucial for many practical applications in various fields, including medicine, material science, and environmental science. In the field of medicine, the chemical structure of drugs plays a crucial role in determining their effectiveness and potential side effects. Researchers and pharmaceutical companies use this knowledge to design and develop new drugs that target specific biological processes. In material science, the chemical structure of materials is essential for understanding and manipulating their properties, such as strength, flexibility, and conductivity, which are crucial for the development of new technologies.Moreover, the chemical structure of molecules is also important in environmental science, as it can help in understanding and addressing environmental issues. For example, the chemical structure of pollutants can provide insights into their behavior, fate, and potential impact on the environment, which is crucial for developing effective strategies for environmental remediation and protection.In conclusion, the chemical structure is a fundamental concept inchemistry that underpins our understanding of the composition, properties, and behavior of matter. By studying the arrangement of atoms and their connections within molecules, scientists can gain valuable insights into the natural world and develop innovative solutions to various challenges facing humanity.。

The chemistry of materials and theirproperties材料的化学与性质从我们身边的各种物品，到制造技术的迅速发展，材料化学在当今社会中扮演着极其重要的角色。

材料化学英文中也叫Materials Chemistry，是木材，塑料，陶瓷，玻璃，合金等物质领域的研究。

通过这种化学，人们可以设计和发现新的物质，借此来提升性能、增强耐用性以及降低成本。

材料的化学与性质紧紧相连，即材料的化学构成决定了其物理性质，材料的性质又反过来影响了它的更广泛应用。

这种相互关联的特性至关重要，证明了科学与工程之间的紧密合作，推动了各种新技术的开发。

一、基本原理设计一种材料需要考虑的主要因素是其一个化学分子的构成和物理结构。

例如，玻璃是由非晶硅氧化物结构组成的，它们并没有任何定型的模式。

此外，多种硅氧化合物会被添加到玻璃中，用于控制矽氧结构的特性，以及透明度和硬度等的器重要物理特征。

对于一种金属合金，其熔点，干燥时间，形状回复以及其他特性很大程度是取决于各种化学元素之间的相互作用。

因此，对于材料化学家来说，了解材料的基本原理，探究材料的物理结构和有机化学组成，是他们工作的起点。

二、催化剂和新的制造技术材料化学广泛应用于催化领域，这涉及到提高化学反应的效率。

催化剂是一种在化学反应中引起或加速反应的物质，它们能够通过提高反应的速度，增加反应的产物收率和纯度，来推动一种新材料的开发。

举例来说，石墨烯就是一种材料，它是由碳元素组成的单层物质层，其拥有超高的导电性和柔韧性。

发展石墨烯的制造技术需要高温、高压等复杂技术，然而，相应的催化剂技术的发展，却能够极大地提供这种材料制造的效率和稳定性。

材料化学家通过精确控制催化剂的结构和化学元素组成，有效提升了制造技术的效率和质量。

三、新材料的开发材料化学也是发现新材料的一个重要领域。

例如，材料化学家可以使用新的化学实验设计和建造研究大厅，在这里，他们可以经过数百次化学试验，发现一个有重大应用价值的新材料。

Chemical Principles教学设计1. 简介《Chemical Principles》是一门通识课程，旨在介绍化学基本概念和化学过程的原理。

本教学设计主要面向大学本科化学专业和非化学专业的学生。

通过本门课程的学习，学生将会掌握化学原理和实验技能，提高对化学科学的兴趣和认识。

2. 教学目标•理解化学物质的本质、化学反应的基本原理；•掌握量子力学基础及其在化学中的应用；•掌握原子结构、化学键理论、化学键的形成和断裂；•理解化学平衡的基本概念、掌握酸碱平衡常数和消解平衡的计算方法；•掌握溶液浓度及其计算方法、理解溶剂与溶质的相互作用等；•理解气体原理、掌握理想气体状态方程、实际气体状态方程、气体分压定律等；•掌握化学反应热力学和化学动力学等方面的理论及其应用。

3. 教学内容第一章概论•化学与人类的关系•化学的基本领域和研究对象•化学的基本概念及其应用第二章原子和原子结构•原子性质概述•原子的基本结构•原子核与同位素•电子结构与元素周期表第三章化学键•价键•金属键•离子键•内壳层键•无定形物质与分子间力•钢铁的变形及其mechanism第四章气体•气体的物理性质•理想气体状态方程•实际气体状态方程•气体分压定律第五章溶液•浓度概念及计量单位•溶液的化学计量•溶液中化学反应•溶液的溶解度和溶度积第六章化学反应平衡•相变平衡•水的自解离平衡•酸碱平衡及酸碱平衡常数•氧化还原反应平衡及氧化还原电极电位•消解平衡及其计算方法第七章化学热力学•热力学基本概念•内能与焓•化学反应焓变、熵变及自由能•使反应非正常进行的因素第八章化学动力学•化学反应的速率•反应级数与速率定律•化学反应速率与温度的关系•反应机理及其研究方法4. 教学方法•讲授式教学•课堂讨论与探究•实验教学•课外阅读、讲座及报告5. 教学评估与考核•平时成绩: 出勤、课堂讨论、阅读报告等 20%•实验成绩: 实验记录、实验报告等 20%•期末考试: 60%6. 教学资源•《Chemical Principles》（第七版），Peter Atkins and Loretta Jones，2012， Freeman&Company•电子课件：原子的基本结构、化学键、化学反应平衡、化学热力学、化学动力学等方面的内容；•化学实验室。

小学上册英语第1单元综合卷英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.I like to ______ puzzles on rainy days. (solve)2.What do we call a place where you can buy books?A. LibraryB. BookstoreC. MarketD. SchoolB3. A compound's properties may differ from those of its ______.4. A __________ is a natural feature formed by erosion.5.The ______ (自然灾害) can damage plant life.6.The __________ (可持续发展) promotes responsible use of resources.7.The classroom is ______ (full) of books.8.My dad brings home ____.9.My dad is a __________ (修车工).10.What is the boiling point of water?A. 50°CB. 100°CC. 150°CD. 200°C11.The capital of Guyana is __________.12.I love to ______ (与他人分享) my knowledge.13.What is the capital of Gabon?A. LibrevilleB. Port-GentilC. FrancevilleD. MoandaA14.The __________ is a region known for its rich biodiversity.15. A ________ is a large area of land that is raised above the surrounding land.16.What is the main ingredient in pancakes?A. FlourB. RiceC. SugarD. ButterA17.The __________ (历史的背景故事) enrich our understanding.18.I want to _______ a superhero when I grow up.19.Leaves collect ______ (阳光).20.I like to collect ________ shells.21.The ______ helps protect the body from injury.22.What is the name of the plant that produces coffee?A. CocoaB. TeaC. Coffee beanD. WheatC Coffee bean23.The country with the highest population density is ________ (人口密度最高的国家是________).24.The ______ is very inspiring.25.The state of matter that has no definite shape is _______.26.How many planets are in our solar system?A. 8B. 9C. 10D. 1127.The bird is perched on a ______.28.The chemical formula for chromium(II) sulfate is _____.29.The chemical formula for potassium sulfate is _____.30.The Earth's crust is constantly undergoing changes due to natural ______.31.My ________ (玩具名称) helps me create wonderful stories.32.I enjoy ________ (听音乐) while studying.33.The _______ (Korean War) began in 1950 between North and South Korea.34.His favorite book is about a ________.35.Which fairy tale features a girl with long hair?A. Snow WhiteB. CinderellaC. RapunzelD. Sleeping BeautyC36.Erosion can reshape the landscape and create new __________.37.What do we call the process of water vapor turning into liquid?A. EvaporationB. CondensationC. PrecipitationD. SublimationB38.I think kindness is the best _______ (礼物) we can give. It costs nothing but means everything.39.I like to _____ (pick) fruits from the trees.40.The ________ likes to swim in the pool.41.She _____ (likes/love) ice cream.42.My favorite animal is a ________ that loves to eat.43.What do you call a long sandwich?A. BurgerB. SubC. WrapD. SaladB44.What do we call the three primary colors?A. Red, Blue, YellowB. Green, Orange, PurpleC. Pink, Brown, BlackD. Gray, White, GoldA45.She is _____ (riding) a bike.46.My favorite ________ is pink.47.What is the main ingredient in ketchup?A. TomatoesB. OnionsC. PeppersD. Garlic48.Stars are born in ______ clouds of gas and dust.49.He is wearing ______ shoes. (red)50.What do you call the sound a cat makes?A. BarkB. PurrC. MeowD. Roar51.The monkey swings from _________ (树) to tree.52.The _______ of a substance is the amount of mass per unit volume. (密度)53.Chemical engineering involves applying principles of chemistry to design processes for producing _____.54.The French Revolution began in the year _______.55.The _____ (青蛙) jumps from lily pad to lily pad. It is green and slimy. 青蛙从睡莲叶跳到睡莲叶。

学化学的好处英语作文The Benefits of Studying Chemistry。

Chemistry is a fascinating subject that offers numerous benefits to those who study it. From understanding the composition of matter to exploring the interactions between different substances, chemistry provides valuable insights into the world around us. In this essay, we will explorethe advantages of studying chemistry and how it can positively impact our lives.First and foremost, studying chemistry helps us tobetter understand the world. By learning about thestructure of atoms, molecules, and compounds, we gain a deeper appreciation for the complexity of the natural world. This knowledge allows us to make sense of the chemical processes that occur in everyday life, from the food we eat to the air we breathe. With a solid understanding of chemistry, we can better appreciate the beauty andintricacy of the world around us.Furthermore, studying chemistry can lead to exciting career opportunities. The principles of chemistry are fundamental to many industries, including pharmaceuticals, environmental science, and materials engineering. By gaining a strong foundation in chemistry, students can pursue careers in research, development, and innovation. Whether it's creating new drugs to combat diseases or developing sustainable technologies, the skills and knowledge gained from studying chemistry can open doors to a wide range of fulfilling and impactful careers.In addition to providing a deeper understanding of the natural world and offering career opportunities, studying chemistry can also have practical benefits in our daily lives. For example, knowledge of chemistry can help us make informed decisions about the products we use and the foods we consume. By understanding chemical reactions and properties, we can better evaluate the safety and effectiveness of household products, cosmetics, and medications. This knowledge empowers us to make choicesthat promote our health and well-being.Moreover, studying chemistry can foster critical thinking and problem-solving skills. The process of conducting experiments, analyzing data, and drawing conclusions in the laboratory encourages students to think critically and logically. These skills are not only valuable in the field of chemistry but also in many other areas of life. By honing their analytical and problem-solving abilities, students can become better equipped to tackle challenges and make informed decisions in a variety of contexts.Finally, studying chemistry can inspire a sense of wonder and curiosity about the world. The study of chemistry often involves exploring the unknown and uncovering new phenomena. This sense of discovery can be incredibly rewarding and can inspire a lifelong passion for learning. By delving into the mysteries of chemistry, students can cultivate a sense of curiosity and a desire to explore the world around them.In conclusion, studying chemistry offers a wide rangeof benefits, from gaining a deeper understanding of the natural world to opening up exciting career opportunities. Additionally, the practical benefits of chemistry knowledge in everyday life, the development of critical thinking and problem-solving skills, and the inspiration of curiosity and wonder are all valuable outcomes of studying chemistry. Whether pursuing a career in the sciences or simply seeking to better understand the world, studying chemistry can have a positive and lasting impact on our lives.。

The Chemistry of Materials by Design材料设计化学材料是我们生活中不可或缺的一部分，从衣服到建筑、从电子设备到运输工具，都需要材料的支持。

过去，人们直接采用已有的材料来制造产品，但现今，科学家们已经较为深入地研究了运用材料设计化学的方法来创造出更加优良的材料。

本文将介绍材料设计化学，包括化学的基本概念和一些具体的实用案例。

一、材料设计化学基本概念材料设计化学是近年来材料科学中的重要领域，其主要目的是单指通过合理使用化学原理，运用科学的方法和手段，提高材料的性能和质量，以满足特定的需求及应用。

通俗地讲，材料设计化学就是把化学与材料科学结合起来的一种方法。

化学作为一门科学，其基础概念在材料设计化学中也不可忽略。

例如，化学元素的性质、化学反应中的能量变化和化学结构的稳定性，都与材料性质密切相关。

在材料设计化学研究中，化学思维被广泛应用，以设计出新材料。

实际上，设计新材料旨在改进舊材料的性能或製造全新的材料。

材料对于各行各业的科技发展都有着关键作用，因此，材料设计化学成为了一个重要的领域。

二、材料设计化学案例1. 电池材料化学设计锂离子电池在当今的电池市场上拥有广泛的应用。

2001年，尤利乌斯·杨和斯坦利·沃莱特提出了第一个锂离子电池模型，该模型使用了锂离子在图层状氧化物中的插层反应。

此后，许多材料科学家致力于研究和发展材料，以改进锂离子电池的能源密度、稳定性和寿命。

例如，2004年，日本学者Akira Yoshino设计出了一种锂离子电池，该电池使用碳材料充当负极，而正极则使用了锂离子插层的若干种过渡金属氧化物。

这个设计后来成为了电动汽车及智能手机等各类电子设备上标配的能源来源。

2. 人工酶化学设计酶是许多生命过程的必要组成部分。

由于这是生物自然产生的，其在化学反应速度、特异度和选择性方面相对于人工催化剂明显更高。

然而，工业应用上，原本最理想的酶反应一半都无法直接变质，无法在批量生产作为实际商用。

化学分子英文Chemical MoleculesThe world we live in is a complex and intricate tapestry, woven together by the intricate dance of atoms and molecules. These fundamental building blocks of matter are the foundation upon which our entire universe is constructed, from the smallest living organism to the grandest celestial bodies. Among these myriad molecules, chemical molecules stand out as the most fundamental and essential components, shaping the very fabric of our existence.At the heart of every chemical molecule lies a delicate balance of atoms, held together by the powerful forces of attraction and repulsion. These atoms, each with its own unique properties and characteristics, come together in a myriad of combinations to form the diverse array of chemical molecules that we encounter in our daily lives. From the simple water molecule, composed of two hydrogen atoms and one oxygen atom, to the complex structures of proteins and DNA, the world of chemical molecules is a veritable playground for the curious and the inquisitive.One of the most fascinating aspects of chemical molecules is theirability to undergo a wide range of transformations and reactions. When two or more molecules interact, they can form new compounds, breaking and reforming the bonds between atoms in a dance of chemical change. This process is the foundation of countless chemical processes, from the combustion of fuels to the metabolic reactions that power the cells of living organisms.At the heart of these chemical reactions are the fundamental principles of chemistry, which govern the behavior of molecules and the way they interact with one another. From the laws of thermodynamics, which describe the flow of energy in chemical systems, to the principles of kinetics, which explain the rates and mechanisms of chemical reactions, the world of chemical molecules is a rich and complex tapestry of scientific understanding.Yet, despite the depth and breadth of our knowledge about chemical molecules, there is still much to be explored and discovered. With each new breakthrough in scientific research, our understanding of the fundamental nature of matter continues to evolve, revealing new insights and opening up new avenues of exploration.One of the most exciting frontiers in the world of chemical molecules is the field of nanotechnology. By manipulating and engineering molecules at the nanoscale, scientists are able to create new materials and devices with unprecedented properties and capabilities.From the development of advanced drug delivery systems to the creation of ultra-strong and lightweight materials, the potential of chemical molecules at the nanoscale is truly boundless.Another exciting area of research in the world of chemical molecules is the study of the role of these molecules in living organisms. From the complex biochemical pathways that power the cells of the human body to the intricate chemical communication systems that govern the behavior of entire ecosystems, the role of chemical molecules in the natural world is a topic of intense study and fascination.As we continue to delve deeper into the mysteries of the chemical world, it is clear that the study of chemical molecules will remain a central and essential component of our scientific understanding. Whether we are exploring the fundamental principles of chemistry or pushing the boundaries of what is possible with the manipulation of matter at the nanoscale, the world of chemical molecules holds the key to unlocking some of the greatest mysteries of our universe.In the end, the beauty and complexity of chemical molecules lies not only in their scientific importance, but also in their ability to inspire wonder and curiosity in the human mind. From the elegant simplicity of the water molecule to the breathtaking complexity of the human genome, the world of chemical molecules is a testament to theincredible ingenuity and creativity of the natural world. As we continue to explore and unravel the secrets of these fundamental building blocks of matter, we can only imagine the wonders that await us in the endless frontiers of chemical discovery.。

科研女神之——鲍哲南｜人工电子皮肤领军者简介鲍哲南，现任斯坦福大学化学工程院院长，K.K. Lee衔称化学工程教授，同时还是化学教授及材料科学与工程教授。

此外，鲍教授还是研究学院、森林研究所和先进分子光电研究中心等机构的教授。

她是国家工程院和国家发明家学院的院士，还被选为MRS, ACS, AAAS, SPIE, ACS PMSE和ACS POLY等协会的会员，以及David Filo and Jerry Yang衔称研究员。

鲍哲南，1987年毕业于南京大学化学系，获得化学学士学位，随后到美国芝加哥大学深造，分别于1991年获化学硕士、1995年获化学博士学位。

1995-2004年，鲍哲南是贝尔实验室朗讯技术的杰出技术人员，2004年加入斯坦福大学。

2016年，鲍创立了斯坦福可穿戴电子公司(eWEAR)，并担任教务主任。

鲍哲南教授还是C3 Nano和PyrAmes公司的联合创始人和董事会成员，这两家公司都是硅谷的创业公司，她还担任Fusion Venture Capital的顾问合伙人。

鲍哲南教授的研究领域包括：功能有机和高分子材料的合成、有机电子器件的设计与制造、有机电子的应用开发等。

因此，她的团队中不仅有化学工程师、化学家，还有材料科学家、物理科学家和电气工程师。

他们的研究方法是多学科的，涉及化学、化学工程、生物医学工程、材料科学和工程、物理和电气工程的概念和专业知识鲍哲南教授带领团队，利用化学、物理和材料科学的基本原理，研究和开发柔性、可拉伸的电子和能源器件。

目前，她们感兴趣的器件有有机和碳纳米管薄膜晶体管、有机光电池、化学/生物传感器和分子开关。

这些装置被用作表征工具，用于基本电荷输运和光物理研究。

在纳米电子学、替代能源、低成本和大面积柔性塑料电路、显示器和一次性传感器等领域，它们也具有实用价值。

她的研究成果显赫，不仅是基础研究，而且具有重要应用价值，获得了众多科研机构、公司的资助：目前，鲍哲南教授有500多篇论文和超过65项美国专利，H-Index>155。