苯在Si(111)-7*7表面化学吸附的理论研究

采用两种大小不同的原子簇模型Si_(30)H_(28)和Si_(13)H_(16),分别用两层 ONIOM方法（对较大原子簇）和普通量子化学方法（对较小原子族）考察了苯分子 在Si(111)-7 * 7表面的化学吸附。对三种可能的吸附物种分别用DFT或HF方法进行 了计算。通过大小原子簇吸附物种的吸附能以及几何构型优化参数的比较发现,对 于稳定的吸附物种,较小的原子簇基本上可以代替较大的原子簇进行计算,而对于 不太稳定的吸附物种,就不得不考虑周边原子的影响。计算结果表明苯在Si(111)- 7 * 7表面的主要吸附种是双σ成键的1,4加成产物,不稳定的单吸附物种可能是 1,4加成物种的前驱态。     

Interaction of Hg Atom with Bare Si（111） Surface

1INTRODUCTION In the past decades,mercury has been a very use-ful electrode material in the fabrication and electrical measurement of molecule modified metal-metal and metal-semiconductor junctions.Majda et al.[1,2]constructed a symmetric Hg-SCn-CnS-Hg junction to study the electron tunneling properties of alkanethio-late bilayers.Whitesides et al.[3～5]fabricated Hg-SAM/SAM-Metal(Ag,Au,Cu)junctions to investi-gate the electrical breakdown voltage of self-assem-bled monolayers(SAMs…     

From pure C(60) to silicon carbon fullerene-based nanotube: an ab initio study

The energetics, geometrical, and electronic properties of the silicon carbon fullerene-based materials, obtained from C(60) by replacing 12 carbon atoms of the C(60) cage with silicon atoms, are studied based on ab initio calculations. We have found that, of the two C(48)Si(12) isomers obtained, the one with the carbon atoms and the silicon atoms located in separated region, i.e., with a phase-separated structure is more stable. Fullerene-based C(36)Si(24) cluster, C(36)Si(24)-C(36)Si(24) dimer, and the nanotube constructed from the clusters are then studied. The calculations on the electronic properties of these silicon carbon fullerene-based nanomaterials demonstrate that the energy gaps are greatly modified and show a decreasing trend with increasing the size of the clusters. The silicon carbon fullerene-based nanotube has a narrow and direct energy band gap, implying that it is a narrow gap semiconductor and may be a promising candidate for optoelectronic devices.     

ATR-FTIR Investigation of Hydrogen-terminated Si(111) Surface in Various NH4F-Based Solutions

Wet chemical cleaning of silicon is a critical step in the semiconductor manufacturing. Particles, contaminants, metallic impurities, roughness and native oxide on silicon surface after wet chemical cleaning deteriorate the reliability of transistor performance in integrated circuits^[1]. Wet chemical etching of Si(111) and Si(100) in fluoride and alkaline solutions has been extensively studied in the past few years by using scanning tunneling microscopy (STM) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR)^[2-11]. In the present work, we extend our study to Si(111) surface after treating with NH₄F/HCl mixtures. STM, X-ray photo spectroscopy (XPS), and ATR-FTIR are used to determine surface roughness, contamination and bond information on Si(111) surface after wet chemical cleaning with various NH₄F/HCl mixtures. The results are discussed in details by comparison to those treated with RCA and HF solutions, indicating that ultra-clean and flat Si(111) surface is obtained by treatment with NH₄F/HCl mixture.     

Silicon surfaces as electron acceptors: dative bonding of amines with Si(001) and Si(111) surfaces.

The bonding of the trimethylamine (TMA) and dimethylamine (DMA) with crystalline silicon surfaces has been investigated using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and density-functional computational methods. XPS spectra show that TMA forms stable dative-bonded adducts on both Si(001) and Si(111) surfaces that are characterized by very high N(1s) binding energies of 402.2 eV on Si(001) and 402.4 eV on Si(111). The highly ionic nature of these adducts is further evidenced by comparison with other charge-transfer complexes and through computational chemistry studies. The ability to form these highly ionic charge-transfer complexes between TMA and silicon surfaces stems from the ability to delocalize the donated electron density between different types of chemically distinct atoms within the surface unit cells. Corresponding studies of DMA on Si(001) show only dissociative adsorption via cleavage of the N-H bond. These results show that the unique geometric structures present on silicon surfaces permit silicon atoms to act as excellent electron acceptors.     

Electronic structure of methoxy-, bromo-, and nitrobenzene grafted onto Si(111)

The properties of Si(111) surfaces grafted with benzene derivatives were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The investigated materials were nitro-, bromo-, and methoxybenzene layers (-C(6)H(4)-X, with X = NO(2), Br, O-CH(3)) deposited from diazonium salt solutions in a potentiostatic electrochemical process. The UPS spectra of the valence band region are governed by the molecular orbital density of states of the adsorbates, which is modified from the isolated state in the gas phase due to molecule-molecule and molecule-substrate interaction. Depending on the adsorbate, clearly different emission features are observed. The analysis of XPS intensities clearly proves multilayer formation for bromo- and nitrobenzene in agreement with the amount of charge transferred during the grafting process. Methoxybenzene forms only a sub-monolayer coverage. The detailed analysis of binding energy shifts of the XPS emissions for determining the band bending and the secondary electron onset in UPS spectra for determining the work function allow one to discriminate between surface dipole layers--changing the electron affinity--and band bending, affecting only the work function. Thus, complete energy band diagrams of the grafted Si(111) surfaces can be constructed. It was found that silicon surface engineering can be accomplished by the electrochemical grafting process using nitrobenzene and bromobenzene: silicon-derived interface gap states are chemically passivated, and the adsorbate-related surface dipole effects an increase of the electron affinity.     

Covalent attachment of acetylene and methylacetylene functionality to Si(111) surfaces: scaffolds for organic surface functionalization while retaining Si-C passivation of Si(111) surface sites

Si(111) surfaces have been functionalized with Si-CC-R species, where R = H or -CH3, using a two-step reaction sequence involving chlorination of H-Si(111) followed by treatment with Na-CC-H or CH3-CC-Na reagents. The resulting surfaces showed no detectable oxidation as evidenced by X-ray photoelectron spectroscopic (XPS) data in the Si 2p region, electrochemical measurements of Si-H oxidation, or infrared spectroscopy. The Si-CC-R-terminated surfaces exhibited a characteristic CC stretch in the infrared at 2179 cm-1, which was strongly polarized perpendicular to the Si(111) surface plane. XPS measurements in the C 1s region showed a low binding energy peak indicative of Si-C bonding, with a coverage that was, within experimental error, identical to that of the CH3-terminated Si(111) surface, which has been shown to fully terminate the Si atop sites on an unreconstructed Si(111) surface. The Si-CC-H-terminated surfaces were further functionalized by exposure to n-C4H9Li followed by exposure to para Br-C6H5-CF3, allowing for introduction of para -C6H5CF3 groups while maintaining the desirable chemical and electrical properties that accompany complete Si-C termination of the atop sites on the Si(111) surface.     

A comparative first-principles study of orbital hybridization in two-dimensional C, Si, and Ge

Information on orbital hybridization is very important to understand the structural, physical, and chemical properties of a material. Results of a comparative first-principles study on the behaviours of orbital hybridization in the two-dimensional single-element phases by carbon, silicon, and germanium are presented. From the well-known three-dimensional hexagonal lonsdaleite structure, in which the atoms are in ideal sp(3)-bonding, the layer spacing along c-axis is gradually stretched to simulate the evolutions of structural and electronic properties from three-dimensional to two-dimensional lattice configurations in the three materials. A turning point of the total system energy due to the sp(3) to sp(2) transition is observed during this process in carbon. In contrast, no such phenomenon is found in silicon and germanium. The differences in electronic structure and bonding behaviour are further examined through comparative investigation of atomic angular-momentum projected density of states and electronic energy band spectrums of these materials. We demonstrate that the valence electronic orbital in the two-dimensional hexagonal crystals of Si and Ge shows sp(3)-like behaviour for the partial hybridization of s and p(z), which leads to their different lattice configurations to graphene. The role of π-bonds in stabilizing the flat configuration of graphene is also discussed.     

High-resolution soft X-ray photoelectron spectroscopic studies and scanning auger microscopy studies of the air oxidation of alkylated silicon(111) surfaces

High-resolution soft X-ray photoelectron spectroscopy was used to investigate the oxidation of alkylated silicon(111) surfaces under ambient conditions. Silicon(111) surfaces were functionalized through a two-step route involving radical chlorination of the H-terminated surface followed by alkylation with alkylmagnesium halide reagents. After 24 h in air, surface species representing Si(+), Si(2+), Si(3+), and Si(4+) were detected on the Cl-terminated surface, with the highest oxidation state (Si(4+)) oxide signal appearing at +3.79 eV higher in energy than the bulk Si 2p(3/2) peak. The growth of silicon oxide was accompanied by a reduction in the surface-bound Cl signal. After 48 h of exposure to air, the Cl-terminated Si(111) surface exhibited 3.63 equivalent monoleyers (ML) of silicon oxides. In contrast, after exposure to air for 48 h, CH(3)-, C(2)H(5)-, or C(6)H(5)CH(2)-terminated Si surfaces displayed <0.4 ML of surface oxide, and in most cases only displayed approximately 0.20 ML of oxide. This oxide was principally composed of Si(+) and Si(3+) species with peaks centered at +0.8 and +3.2 eV above the bulk Si 2p(3/2) peak, respectively. The silicon 2p SXPS peaks that have previously been assigned to surface Si-C bonds did not change significantly, either in binding energy or in relative intensity, during such air exposure. Use of a high miscut-angle surface (7 degrees vs < or =0.5 degrees off of the (111) surface orientation) yielded no increase in the rate of oxidation nor change in binding energy of the resultant oxide that formed on the alkylated Si surfaces. Scanning Auger microscopy indicated that the alkylated surfaces formed oxide in isolated, inhomogeneous patches on the surface.     

Grafting Poly(3‐hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3‐hexylthiophene) (SiNC@P3HT) by applying surface‐initiated Kumada catalyst transfer polycondensation (SI‐KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor–acceptor interfaces. Systematic characterization provides evidence of a core–shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type‐II heterostructure.     

Attachment of styrene and phenylacetylene on Si(111)-7 x 7: the influence of substitution groups on the reaction mechanism and formation of pi-conjugated skeletons

The interactions of styrene and phenylacetylene and their isotope substitutions with a Si(111)-7 x 7 surface have been studied as model systems to mechanistically understand the chemical binding of conjugated pi-electron systems to di-radical-like silicon dangling bonds of the adjacent adatom-rest atom pair. Vibrational studies show that styrene mainly binds to the surface through a diradical reaction involving both the external C=C and its conjugated internal C=C of the phenyl ring with an adjacent adatom-rest atom pair, forming a 5-ethylidene-1,3-cyclohexadiene-like skeleton. On the other hand, phenylacetylene was shown to be covalently attached to Si(111)-7 x 7 through the external C[triple bond]C, forming a styrene-like conjugation system. These experimental results are consistent with density functional theory calculations. The different binding mechanisms for styrene and phenylacetylene clearly demonstrate that reaction channels for multifunctional organic molecules are strongly dependent on the chemical and physical properties of the functional groups. The resulting pi-electron conjugation structures may possibly be employed as intermediates for further organic syntheses and fabrication of multilayer organic films on semiconductor surfaces.     

Ethanol adsorption on the Si (111) surface: first principles study

Equilibrium atomic configurations and electron energy structure of ethanol adsorbed on the Si (111) surface are studied by the first principles density functional theory. Geometry optimization is performed by the total energy minimization method. Equilibrium atomic geometries of ethanol, both undissociated and dissociated, on the Si (111) surface are found and analysed. Reaction pathways and predicted transition states are discussed in comparison with available experimental data in terms of the feasibility of the reactions occurring. Analysis of atom and orbital resolved projected density of states indicates substantial modifications of the Si surface valence and conduction electron bands due to the adsorption of ethanol affecting the electronic properties of the surface.     

Ultra-clean Si(111) and Si(001) Surfaces Prepared by Advanced Wet Chemical Cleaning

Wet chemical cleaning of silicon is a critical step, e.g., pre-gate clean, in the semiconductor manufacturing^[1]. For example, pre-gate oxide cleaning demands ultra-clean silicon surface with least surface roughness. It is well known that metallic infinities and roughness cause the lower breakdown voltage in gate dielectric^[2]. It has stringent requirements for ultra-clean and atomically flat silicon surface as the thickness of gate oxide is decreasing. In the present work, we have extended our study on Si(100) surface13] and extensively investigated wet chemical cleaning of Si(111) and Si(100) surfaces in NH₄F-based solutions by using scanning tunneling microscopy (STM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and total reflection X-ray fluorescence spectrometry (TXRF). Surface roughness, organic contamination, metallic impurities and surface termination on the silicon surfaces after wet chemical cleaning with various NH₄F-based solutions have been determined and compared with those treated with RCA cleans, HF solutions and other industrially used solutions. Our results indicate that ultra-clean and smooth Si(111) and Si(001) surfaces are obtained by treatment with NH4F-based solutions.     

Selective bonding of pyrazine to silicon(100)-2x1 surfaces: the role of nitrogen atoms

The covalent binding of pyrazine on Si(100) have been investigated using high-resolution electron energy loss spectroscopy (HREELS) and x-ray photoelectron spectroscopy. Experimental results clearly suggest that the attachment occurs exclusively through the bonding of the two para-nitrogen atoms with the surface without the involvement of the carbon atoms, as evidenced from the retention of the (sp2) C-H stretching mode in HREELS and a significant down shift of 1.6 eV in the binding energy of N 1s. The binding mechanism for pyrazine on Si(100) demonstrates that reaction channels for heteroatomic aromatic molecules are strongly dependent on the electronic properties of the constituent atoms.     

Surface,Size and Thermal Effects in Alkali Metal with Core-Electron Binding-Energy Shifts

Consistency between density functional theory calculations and X-ray photoelectron spectroscopy measurements confirms our predications on the undercoordination-induced local bond relaxation and core level shift of alkali metal, which determine the surface, size and thermal properties of materials. Zone-resolved photoelectron spectroscopy analysis method and bond order-length-strength theory can be utilized to quantify the physical parameters regarding bonding identities and electronic property of metal surfaces, which allows for the study of the core-electron binding-energy shifts in alkali metals. By employing these methods and first principle calculation in this work, we can obtain the information of bond and atomic cohesive energy of under-coordinated atoms at the alkali metal surface. In addition, the effect of size and temperature towards the binding-energy in the surface region can be seen from the view point of Hamiltonian perturbation by atomic relaxation with atomic bonding.     

Phenanthrenequinone adsorbed on Si(001): geometries, electronic properties, and optical response

As a prototypical case of a pi-conjugated organic overlayer on a semiconductor surface the adsorption of phenanthrenequinone (C14O2H8) on the Si(001) surface is studied by means of first principles calculations, using gradient-corrected density functional theory together with ultrasoft pseudopotentials and the projector augmented wave method. A thermodynamic phase diagram gives adsorption geometries depending on experimental conditions, the microscopically most favorable bonding configuration representing a "[4+2]-cycloaddition product". The surface electronic structure depends strongly on the respective adsorption configuration. Calculations of the surface optical signature show its sensitivity to molecular adsorption and are in agreement with experimental results. A detailed analysis illustrates that the bonding to the surface has to be taken into account accurately to unveil the molecule's contribution to the surface optical response.     

Dehydrative cyclocondensation reactions on hydrogen-terminated Si(100) and Si(111): an ex situ tool for the modification of semiconductor surfaces

Dehydrative cyclocondensation processes for semiconductor surface modification can be generally suggested on the basis of well-known condensation schemes; however, in practice this approach for organic functionalization of semiconductors has never been investigated. Here we report the modification of hydrogen-terminated silicon surfaces by cyclocondensation. The cyclocondensation reactions of nitrobenzene with hydrogen-terminated Si(100) and Si(111) surfaces are investigated and paralleled with selected cycloaddition reactions of nitro- and nitrosobenzene with Si(100)-2x1. Infrared spectroscopy is used to confirm the reactions and verify an intact phenyl ring and C-N bond in the reaction products as well as the depletion of surface hydrogen. High resolution N 1s X-ray photoelectron spectroscopy (XPS) suggests that the major product for both cyclocondensation reactions investigated is a nitrosobenzene adduct that can only be formed following water elimination. Both IR and XPS are augmented by density functional theory (DFT) calculations that are also used to investigate the feasibility of several surface reaction pathways, which are insightful in understanding the relative distribution of products found experimentally. This novel surface modification approach will be generally applicable for semiconductor functionalization in a highly selective and easily controlled manner.     

Thinking Like a Chemist: Intuition in Thermoelectric Materials

The coupled transport properties required to create an efficient thermoelectric material necessitates a thorough understanding of the relationship between the chemistry and physics in a solid. We approach thermoelectric material design using the chemical intuition provided by molecular orbital diagrams, tight binding theory, and a classic understanding of bond strength. Concepts such as electronegativity, band width, orbital overlap, bond energy, and bond length are used to explain trends in electronic properties such as the magnitude and temperature dependence of band gap, carrier effective mass, and band degeneracy and convergence. The lattice thermal conductivity is discussed in relation to the crystal structure and bond strength, with emphasis on the importance of bond length. We provide an overview of how symmetry and bonding strength affect electron and phonon transport in solids, and how altering these properties may be used in strategies to improve thermoelectric performance.     

Organic electronics—Silicon device design without semiconductor band theory

Before the invention of the transistor in the 1940s, semiconductors were used as detectors in radios in a device called a “cat’s whisker”. At that time their operation was completely mysterious. Only after the introduction of semiconductor band theory did it become clear that the “cat’s whisker” is a primitive example of a metal-semiconductor Schottky diode. Today organic materials are being investigated for their electronic properties. Such materials are especially attractive for lightweight, flexible, and low-cost solar cells and light emitting devices, as well as transistors and electrophotographic photoreceptors. Yet, even after 40 years of work and a large database, the physics and chemistry that determines the electronic properties of organic materials are not well understood. Practicing organic electronics is like attempting to do silicon device design without semiconductor band theory. It is the purpose of this paper to briefly summarize what is known about the electronic properties of organic materials from charge transport data. It will be shown that our understanding of the charge transport mechanism and the electronic properties of organic materials is at a rudimentary phase which is a limiting factor in applying these materials to practical devices, very similar to the “cat’s whisker” phase of inorganic semiconductor research.     

Theoretical Investigation on the Adsorption of Ag^＋ and Hydrated Ag^＋ Cations on Clean Si（111） Surface

In this paper, the adsorption of Ag^＋ and hydrated Ag^＋ cations on clean Si（111） surface were investigated by using cluster （Gaussian 03） and periodic （DMol^3） ab initio calculations. Si（111） surface was described with cluster models （Si14H17 and Si22H21） and a four-silicon layer slab with periodic boundary conditions. The effect of basis set superposition error （BSSE） was taken into account by applying the counterpoise correction. The calculated results indicated that the binding energies between hydrated Ag^＋ cations and clean Si（111） surface are large, suggesting a strong interaction between hydrated Ag^＋ cations and the semiconductor surface. With the increase of number, water molecules form hydrogen bond network with one another and only one water molecule binds directly to the Ag^＋ cation. The Ag^＋ cation in aqueous solution will safely attach to the clean Si（111） surface.