Self-directed growth of benzonitrile line on H-terminated Si(001) surface

Using first-principles density-functional calculations we predict a self-directed growth of benzonitrile molecular line on a H-terminated Si(001) surface. The C[triple bond]N bond of benzonitrile reacts with a single Si dangling bond which can be generated by the removal of a H atom, forming one Si-N bond and one C radical. Subsequently, the produced C radical can be stabilized by abstracting a H atom from a neighboring Si dimer, creating another H-empty site. This H-abstraction process whose activation barrier is 0.65 eV sets off a chain reaction to grow one-dimensional benzonitrile line along the Si dimer row. Our calculated energy profile for formation of the benzonitrile line shows its relatively easier formation compared with previously reported styrene and vinylferrocene lines.     

Density functional theory study of one-dimensional growth of styrene on the hydrogen-terminated Si(001)-(3 x 1) surface

Recent experimental work has shown that the addition of styrene molecules to hydrogen-terminated Si(001) surfaces leads to the formation of one-dimensional molecular structures through a radical-initiated surface chain reaction mechanism. These nanometric structures are observed to be directed parallel to the dimer rows on the H-Si(001)-(2 x 1) surface and perpendicular to the same rows on H-Si(001)-(3 x 1). Using periodic density functional theory (DFT) calculations, we have studied the initial steps of the radical chain mechanism on the H-Si(001)-(3 x 1) surface and compared them to analogous results for H-Si(001)-(2 x 1). On the H-Si(001)-(3 x 1) surface, one of the crucial steps of the surface chain reaction, namely, the abstraction of a H atom from a nearby surface hydride unit, is found to have a somewhat smaller activation energy in the direction perpendicular to the dimer rows (H abstraction from the nearest dihydride site) than along the rows (H abstraction from a neighboring dimer). Additionally, due to the steric repulsion between the styrene molecules and the SiH2 subunits, growth along the dimer rows is not thermodynamically favorable on the (3 x 1) surface. On the other hand, due to the absence of the SiH2 subunits, growth parallel to the Si dimer rows becomes favored on the H-Si(001)-(2 x 1) surface.     

Surface reaction of alkynes and alkenes with H-Si(111): a density functional theory study

Recent experiments on the addition of alkene and alkyne molecules to H-terminated silicon surfaces have provided evidence for a surface chain reaction initiated at isolated Si dangling bonds and involving an intermediate carbon radical state, which, after abstraction of a hydrogen atom from a neighboring Si-H unit, transforms into a stable adsorbed species plus a new Si dangling bond. Using periodic density functional theory (DFT) calculations, together with an efficient method for determining reaction pathways, we have studied the initial steps of this chain reaction for a few different terminal alkynes and alkenes interacting with an isolated Si dangling bond on an otherwise H-saturated Si(111) surface. Calculated minimum energy pathways (MEPs) indicate that the chain mechanism is viable in the case of C(2)H(2), whereas for C(2)H(4) the stabilization of the intermediate state is so small and the barrier for H-abstraction so (relatively) large that the molecule is more likely to desorb than to form a stable adsorbed species. For phenylacetylene and styrene, stabilization of the intermediate state and decrease of the H-abstraction barrier take place. While a stable adsorbed species exists in both cases, the overall heat of adsorption is larger for the alkyne molecules.     

Electronic effects induced by single hydrogen atoms on the Ge(001) surface

The properties of an isolated dangling bond formed by the chemisorption of a single hydrogen atom on a dimer of the Ge(001) surface are investigated by first-principles density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements. Two stable atomic configurations of the Ge-Ge-H hemihydride with respect to the neighboring bare Ge-Ge dimers are predicted by DFT. For both configurations, the unpaired electron of the HGe(001) system is found to be delocalized over the surface, rendering the isolated dangling bond of the hemihydride unoccupied. However, local surface charge accumulation, such as may occur during STM imaging, leads to the localization of two electrons onto the hemihydride dangling bond. The calculated surface densities of states for one of the charged Ge-Ge-H hemihydride configurations are found to be in good agreement with atomic-resolution STM measurements on n-type Ge(001). Comparison with a Si-Si-H hemihydride of the Si(001) surface shows similarities in structural properties, but substantial differences in electronic properties.     

Formation of organic monolayers on silicon via gas-phase photochemical reactions

A new method for the formation of molecular monolayers on silicon surfaces utilizing gas-phase photochemical reactions is reported. Hydrogen-terminated Si(111) surfaces were exposed to various gas-phase molecules (hexene, benzaldehyde, and allylamine) and irradiated with ultraviolet light from a mercury lamp. The surfaces were studied with in situ Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and scanning tunneling microscopy. The generation of gas-phase radicals was found to be the initiator for organic monolayer formation via the abstraction of hydrogen from the H/Si(111) surface. Monolayer growth can occur through either a radical chain reaction mechanism or through direct radical attachment to the silicon dangling bonds.     

Self-directed chain reaction by small ketones with the dangling bond site on the Si(100)-(2 x 1)-H surface: acetophenone, a unique example

Using scanning tunneling microscope (STM) at 300 K, we studied the growth of one-dimensional molecular assemblies (molecular lines) on the Si(100)-(2 x 1)-H surface through the chain reaction of small ketone (CH 3COCH 3, PhCOPh, and PhCOCH 3) molecules with dangling bond (DB) sites of the substrate. Acetone and benzophenone show the growth of molecular lines exclusively parallel to the dimer row direction. In contrast, acetophenone molecules show some molecular lines perpendicular, in addition to parallel, to the dimer row direction. Most of the molecular lines perpendicular to the dimer row direction were grown by self-turning the propagation direction of a chain reaction from parallel to perpendicular directions relative to the dimer row. A chiral center created upon adsorption of an acetophenone molecule allows the adsorbed molecules to align with identical as well as alternate enantiomeric forms along the dimer row direction, whereas such variations in molecular arrangement are not observed in the case of acetone and benzophenone molecules. The observed molecular lines growth both parallel and perpendicular to dimer row directions appears to be unique to acetophenone among all the molecules studied to date. Hence, the present study opens new possibility for fabricating one-dimensional molecular assemblies of various compositions in both high-symmetry directions on the Si(100)-(2 x 1)-H surface.     

Linear nanostructure formation of aldehydes by self-directed growth on hydrogen-terminated silicon(100)

The self-directed growth of organic molecules on silicon surfaces allows for the rapid, parallel production of hybrid organic-silicon nanostructures. In this work, the formation of benzaldehyde- and acetaldehyde-derived nanostructures on hydrogen-terminated H-Si(100)-2x1 surface is studied by scanning tunneling microscopy in ultrahigh vacuum and by quantum mechanical methods. The reaction is a radical-mediated process that binds the aldehydes, through a strong Si-O covalent bond, to the surface. The aldehyde nanostructures are generally composed of double lines of molecules. Two mechanisms that lead to double line growth are elucidated.     

Mechanism of the hydrosilylation reaction of alkenes at porous silicon: experimental and computational deuterium labeling studies

The mechanism of the formation of Si-C bonded monolayers on silicon by reaction of 1-alkenes with hydrogen-terminated porous silicon surfaces has been studied by both experimental and computational means. We propose that monolayer formation occurs via the same radical chain process as at single-crystal surfaces: a silyl radical attacks the 1-alkene to form both the Si-C bond and a radical center on the beta-carbon atom. This carbon radical may then abstract a hydrogen atom from a neighboring Si-H bond to propagate the chain. Highly deuterated porous silicon and FTIR spectroscopy were used to provide evidence for this mechanism by identifying the IR bands associated with the C-D bond formed in the proposed propagation step. Deuterated porous silicon surfaces formed by galvanostatic etching in 48% DF/D2O:EtOD (1:1) electrolytes showed a 30% greater density of Si-D sites on the surface than Si-H sites on hydrogen-terminated porous silicon surfaces prepared in the equivalent H-electrolyte. The thermal reaction of undec-1-ene and the Lewis acid catalyzed reaction of styrene on a deuterated surface both resulted in alkylated surfaces with the same C-C and C-H vibrational features as formed in the corresponding reactions at a hydrogen-terminated surface. However, a broad band around 2100 cm(-1) was observed upon alkylating the deuterated surfaces. Ab initio and density functional theory calculations on small molecule models showed that the integrated absorbance of this band was comparable to the intensity expected for the C-D stretches predicted by the chain mechanism. The calculations also indicate that there is substantial interaction between the hydrogen atoms on the beta-carbons and the hydrogen atoms on the Si(111)-H surface. These broad 2100 cm(-1) features are therefore assigned to C-D bands arising from the involvement of surface D atoms in the hydrosilylation reactions, while the line broadening can be explained partly by interaction with neighboring surface atoms/groups.     

Studies of carbon incorporation on the diamond [100] surface during chemical vapor deposition using density functional theory

Accurate potential energy surface calculations are presented for many of the key steps involved in diamond chemical vapor deposition on the [100] surface (in its 2 x 1 reconstructed and hydrogenated form). The growing diamond surface was described by using a large (approximately 1500 atoms) cluster model, with the key atoms involved in chemical steps being described by using a quantum mechanical (QM, density functional theory, DFT) method and the bulk of the atoms being described by molecular mechanics (MM). The resulting hybrid QM/MM calculations are more systematic and/or at a higher level of theory than previous work on this growth process. The dominant process for carbon addition, in the form of methyl radicals, is predicted to be addition to a surface radical site, opening of the adjacent C-C dimer bond, insertion, and ultimate ring closure. Other steps such as insertion across the trough between rows of dimer bonds or addition to a neighboring dimer leading to formation of a reconstruction on the next layer may also contribute. Etching of carbon can also occur; the most likely mechanism involves loss of a two-carbon moiety in the form of ethene. The present higher-level calculations confirm that migration of inserted carbon along both dimer rows and chains should be relatively facile, with barriers of approximately 150 kJ mol (-1) when starting from suitable diradical species, and that this step should play an important role in establishing growth of smooth surfaces.     

Inhomogeneous decomposition of ultrathin oxide films on Si(100): application of Avrami kinetics to thermal desorption spectra

Thermal decomposition of ultrathin oxide layers on silicon surface was investigated with temperature programed desorption. Oxide layers were formed on Si(100) at 400 degrees C by exposure to O(2) molecular beam. Desorption spectrum for oxygen coverages between 1.7 and 2.6 ML exhibits a single dominant peak with an additional broad peak at lower temperatures. The former peak corresponds to stable binding states of O atoms at dimer bridge sites and dimer backbond sites. The high peak intensity indicates that most O atoms are at stable states. The latter peak corresponds to an unstable binding state, where O atoms are presumably trapped at dangling bonds. The SiO desorption rate from the stable binding states is well described by Avrami kinetics, suggesting that the decomposition process is spatially inhomogeneous with void formation and growth. The rate-determining step is the reaction at void perimeter even if the overlap between voids becomes quite large. The Avrami exponents determined from our experiment indicate that the increase in the initial coverage makes the oxide layer more stable and suppresses the rate of void formation at the potential nucleation sites.     

Thermally activated mechanisms of hydrogen abstraction by growth precursors during plasma deposition of silicon thin films

Hydrogen abstraction by growth precursors is the dominant process responsible for reducing the hydrogen content of amorphous silicon thin films grown from SiH(4) discharges at low temperatures. Besides direct (Eley-Rideal) abstraction, gas-phase radicals may first adsorb on the growth surface and abstract hydrogen in a subsequent process, giving rise to thermally activated precursor-mediated (PM) and Langmuir-Hinshelwood (LH) abstraction mechanisms. Using results of first-principles density functional theory (DFT) calculations on the interaction of SiH(3) radicals with the hydrogen-terminated Si(001)-(2x1) surface, we show that precursor-mediated abstraction mechanisms can be described by a chemisorbed SiH(3) radical hopping between overcoordinated surface Si atoms while being weakly bonded to the surface before encountering a favorable site for hydrogen abstraction. The calculated energy barrier of 0.39 eV for the PM abstraction reaction is commensurate with the calculated barrier of 0.43-0.47 eV for diffusion of SiH(3) on the hydrogen-terminated Si(001)-(2x1) surface, which allows the radical to sample the entire surface for hydrogen atoms to abstract. In addition, using the same type of DFT analysis we have found that LH reaction pathways involve bond breaking between the silicon atoms of the chemisorbed SiH(3) radical and the film prior to hydrogen abstraction. The LH reaction pathways exhibit energy barriers of 0.76 eV or higher, confining the abstraction only to nearest-neighbor hydrogens. Furthermore, we have found that LH processes compete with radical desorption from the hydrogen-terminated Si(001)-(2x1) surface and may be suppressed by the dissociation of chemisorbed SiH(3) radicals into lower surface hydrides. Analysis of molecular-dynamics simulations of the growth process of plasma deposited silicon films have revealed that qualitatively similar pathways for thermally activated hydrogen abstraction also occur commonly on the amorphous silicon growth surface.     

Selective attachment of 1,4-benzenedimethanethiol on the copper mediated Si(111)-(7 x 7) surface through S-Cu linkage

The well-defined and patterned copper clusters formed on the Si(111)-(7 x 7) surface have been employed as a template for selective binding of 1,4-benzenedimethanethiol (HS-CH2-C6H4-CH2-SH, 1,4-BDMT), to form ordered molecular nanostructures. Scanning tunneling microscopic (STM) studies showed that each 1,4-BDMT molecule preferentially binds to two neighboring copper atoms within one copper cluster through the S-Cu interaction with its molecular plane parallel to the surface, whereas some 1,4-BDMT bond to individually adsorbed copper atoms, resulting in an upright configuration. Large-scale two-dimensional molecular nanostructures can be obtained using this patterned assembly technique. Our experiments demonstrate the feasibility for controllable growth of ordered molecular nanostructures on the Si(111)-(7 x 7) surface.     

Excitation mechanisms and photochemistry of adsorbates with spherical symmetry

By comparing the photo-stimulated desorption of Xe from an oxidized Si(100) surface with the photochemistry of methane on metal surfaces, we try to deduce a common concept in the excited state and the excitation mechanism responsible for the photo-induced processes. Xe atoms are desorbed from the oxidized Si(100) surface by the irradiation of photons in the range 1.16-6.43 eV. Two velocity components with average kinetic energy 0.85 and 0.25 eV are observed in the time-of-flight distributions. The fast component appears only if the photon energy exceeds approximately 3 eV, but the slow component is present over the entire photon energy range. By analogy with the photochemistry of methane on metal surfaces, the excitation mechanism responsible for the fast component is postulated to be a transition from the 5p state of Xe to the excited state originating from strong hybridization between the 6s state of Xe and the dangling bond at a surface silicon atom bonded to oxygen inserted in the dimer bond. In this scheme an excited electron is transferred from the adsorbate to the substrate, which is the reverse direction to the substrate-mediated excitation frequently assumed in surface photochemistry.     

Organic nanostructures on hydrogen-terminated silicon report on electric field modulation of dangling bond charge state

We pursue dynamic charge and occupancy modulation of silicon dangling bond sites on H-Si(100)-2 × 1 with a biased scanning tunneling microscope tip and demonstrate that the reactivity and mechanism of product formation of cyclobutylmethylketone (CBMK) on the surface at the active sites may be thus spatially regulated. Reactivity is observed to be dependent on the polarity between tip and surface while the area over which reactivity modulation is established scales according to the dopant concentration in the sample. We account for these observations with examination of the competition kinetics applicable to the CBMK/H-Si reaction and determine how said kinetics are affected by the charge state of DB sites associated with reaction initiation and propagation. Our experiments demonstrate a new paradigm in lithographic control of a self-assembly process on H-Si and reveal a variant to the well-known radical mediated chain reaction chemistry applicable to the H-Si surface where self-assembly is initiated with dative bond formation between the molecule and a DB site.     

烯烃分子在氢终止Si(100)-2×1表面的自由基链反应

应用密度泛函理论(DFT)和从头计算分子动力学方法(ab initio MD)研究了不饱和烯烃在氢终止的Si(100)-2×1表面的自由基链反应. 计算表明, 自由基链反应中重要的一步是烷烃链自由基的氢抽提过程, 硅表面上邻近位置(the nearest neighbor, NN)的氢抽提比次邻近位置(the next-nearest neighbor, NNN)的氢抽提有一稍低的能垒. 从头算分子动力学显示, 过渡态的烷烃自由基与氢终止Si(100)-2×1表面上的氢原子能够很容易形成C—H键, 完成一个氢抽提过程, 同时在硅表面产生下一个孤电子, 继续引发链反应.     

ANISOTROPIC STRAIN EFFECT ON MORPHOLOGY AND ATOMIC STRUCTURE OF VICINAL Si(001) SURFACE

On vicinal Si(001) surfaces, dependence of growth morphology on the applied strain direction and formation of vacancy lines from Ag-induced missing dimer vacancies are studied. Both phenomena are intimately related to the anisotropic nature of the strain field which originates from the surface dimerization. Strain relief mechanism, reflecting on the surface morphology, is shown to be different in two orthogonal directions. Normal to the steps, step-pair bunching and waving lead to formation of hillocks and pits. Along the step direction, bending of step pairs forms a cusp which later develops into a deep groove. Toward the atomic scale, the formation of the vacancy lines is driven by the short-range attractive interaction between the vacancies in adjacent dimer rows and the long-range repulsive interaction between them in the same dimer row. A full form and magnitudes of the interactions are derived from the thermally-excited wandering of the vacancy lines formed by a nominal amount of Ag depositing onto the surface.     

X-ray studies of self-assembled organic monolayers grown on hydrogen-terminated Si(111)

The structure of self-assembled monolayers (SAMs) of undecylenic acid methyl ester (SAM-1) and undec-10-enoic acid 2-bromo-ethyl ester (SAM-2) grown on hydrogen-passivated Si(111) were studied by X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), atomic force microscopy, and X-ray photoelectron spectroscopy (XPS). The two different SAMs were grown by immersion of H-Si(111) substrates into the two different concentrated esters. UV irradiation during immersion was used to create Si dangling bond sites that act as initiators of the surface free-radical addition process that leads to film growth. The XRR structural analysis reveals that the molecules of SAM-1 and SAM-2 respectively have area densities corresponding to 50% and 57% of the density of Si(111) surface dangling bonds and produce films with less than 4 angstroms root-mean-square roughness that have layer thicknesses of 12.2 and 13.2 angstroms. Considering the molecular lengths, these thicknesses correspond to a 38 degrees and 23 degrees tilt angle for the respective molecules. For SAM-2/Si(111) samples, XRF analysis reveals a 0.58 monolayer (ML) Br total coverage. Single-crystal Bragg diffraction XSW analysis reveals (unexpectedly) that 0.48 ML of these Br atoms are at a Si(111) lattice position height that is identical to the T1 site that was previously found by XSW analysis for Br adsorbed onto Si(111) from a methanol solution and from ultrahigh vacuum. From the combined XPS, XRR, XRF, and XSW evidence, it is concluded that Br abstraction by reactive surface dangling bonds competes with olefin addition to the surface.     

Solid Phase Supported “Click”-Chemistry Approach for the Preparation of Water Soluble Gold Nanoparticle Dimers

We investigated the potential of the Cu(I) catalyzed azide-alkyne cycloaddition between water soluble azide and alkyne functionalized gold nanoparticles in terms of dimer formation via a solid phase support. Alkyne and azide lipoic acid derivatives are prepared and utilized as stabilizing ligands for 15?nm gold colloids. For the solid phase supported click reaction first citrate stabilized gold nanoparticles are immobilized on amine terminated silicon wafers. In the following step the citrate ligands of the upper free accessible nanoparticle surface are exchanged against a mixture of the alkyne derivative of lipoic acid and lipoic acid. Upon addition of lipoic acid/lipoic acid azide derivative stabilized 15?nm gold nanoparticles and the Cu(I) catalyst solution covalent interparticle coupling between immobilized and gold nanoparticles added is achieved. The formed structures are analyzed by scanning electron microscopy directly on the solid support. It is demonstrated that the yield of dimeric structures on the solid phase support increases with increased molar ratio of the catalyst, thus indicating that dimers are indeed formed by covalent bond formation. Upon treatment with ultrasound the formed structures could be released and detected with transmission electron microscopy measurements.     

Protection-deprotection chemistry to control styrene self-directed line growth on hydrogen-terminated Si(100)

TEMPO, 2,2,6,6-tetramethylpiperidinyloxy, was used in a series of protection-deprotection chemical reactions in order to gain single molecule-level control over the extent of styrene line growth on hydrogen-terminated Si(100). The mechanism involves the reaction of TEMPO with the dangling bond at the end of individual styrene lines. The TEMPO cap protects the dangling bond from further reaction with styrene resulting in the termination of line growth. TEMPO is then selectively removed from desired lines, deprotecting the dangling bond, using the scanning tunneling microscope. Further exposure of the surface to styrene ensures that only the deprotected areas continue to grow while the protected lines do not. All lines can then be capped with TEMPO, and this allows for the generation of stable styrene lines of varying lengths.     

General solution route for nanoplates of hexagonal oxide or hydroxide

A citric acid (CA)-assisted hydrothermal process was used to prepare Fe2O3 hexagonal nanoplates with a lateral size of about 100 nm. In addition, the hexagonal nanoplates of Co(OH)2, MnCO3, and Ni(OH)2 were also synthesized by this route, indicative of the universality of the solution route presented herein. The morphologies and structures of the synthesized platelike nanostructures have been characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). Furthermore, the mechanism for the formation of the platelike nanostructures has been preliminarily discussed. It is believed that the capping molecule of CA, which inhibits crystal growth along the <001> direction due to its chelating effect, plays a critical role in the hydrothermal formation of the nanoplates.