Chemical and electrical passivation of silicon (111) surfaces through functionalization with sterically hindered alkyl groups

Crystalline Si(111) surfaces have been alkylated in a two-step chlorination/alkylation process using sterically bulky alkyl groups such as (CH3)2CH- (iso-propyl), (CH3)3C- (tert-butyl), and C6H5- (phenyl) moieties. X-ray photoelectron spectroscopic (XPS) data in the C 1s region of such surfaces exhibited a low energy emission at 283.9 binding eV, consistent with carbon bonded to Si. The C 1s XPS data indicated that the alkyls were present at lower coverages than methyl groups on CH(3)-terminated Si(111) surfaces. Despite the lower alkyl group coverage, no Cl was detected after alkylation. Functionalization with the bulky alkyl groups effectively inhibited the oxidation of Si(111) surfaces in air and produced low (<100 cm s(-1)) surface recombination velocities. Transmission infrared spectroscopy indicated that the surfaces were partially H-terminated after the functionalization reaction. Application of a reducing potential, -2.5 V vs Ag+/Ag, to Cl-terminated Si(111) electrodes in tetrahydrofuran resulted in the complete elimination of Cl, as measured by XPS. The data are consistent with a mechanism in which the reaction of alkyl Grignard reagents with the Cl-terminated Si(111) surfaces involves electron transfer from the Grignard reagent to the Si, loss of chloride to solution, and subsequent reaction between the resultant silicon radical and alkyl radical to form a silicon-carbon bond. Sites sterically hindered by neighboring alkyl groups abstract a H atom to produce Si-H bonds on the surface.     

Scanning tunneling microscopy of ethylated Si(111) surfaces prepared by a chlorination/alkylation process

Scanning tunneling microscopy (STM) and computational modeling have been used to study the structure of ethyl-terminated Si(111) surfaces. The ethyl-terminated surface was prepared by treating the H-terminated Si(111) surface with PCl5 to form a Cl-terminated Si(111) surface with subsequent exposure to C(2)H(5)MgCl in tetrahydrofuran to produce an alkylated Si(111) surface. The STM data at 77 K revealed local, close-packed, and relatively ordered regions with a nearest-neighbor spacing of 0.38 nm as well as disordered regions. The average spot density corresponded to approximately 85% of the density of Si atop sites on an unreconstructed Si(111) surface. Molecular dynamics simulations of a Si(111) surface randomly populated with ethyl groups to a total coverage of approximately 80% confirmed that the ethyl-terminated Si(111) surface, in theory, can assume reasonable packing arrangements to accommodate such a high surface coverage, which could be produced by an exoergic surface functionalization route such as the two-step chlorination/alkylation process. Hence, it is possible to consistently interpret the STM data within a model suggested by recent X-ray photoelectron spectroscopic data and infrared absorption data, which indicate that the two-step halogenation/alkylation method can provide a relatively high coverage of ethyl groups on Si(111) surfaces.     

Lead underpotential deposition on gold single-crystal surfaces: The (100) face and its vicinal faces

Results concerning underpotential deposition of lead on gold single-crystal faces are presented. The (100) face and its vicinal faces are studied by cyclic voltammograms, potential step measurements and phase shift measurements. The explanation of the complex CV of the (100) face is attempted; adsorption, reconstruction and nucleation processes are discussed. Some details concerning the single-crystal techniques are given in the Appendix.     

Chemical Reconstruction of Pt-Rh(100) Alloy and Pt/Rh(100), Rh/Pt(100) Bimetallic Surfaces

When Cu(110), Ni(l 10), Ag(110) surfaces are exposed to O₂ at room temperature, one dimensional metal-oxygen strings grow in the < 001 > direction of the (110) surfaces. A similar phenomenon occurs in the adsorption of H₂ on Ni( 110) surface at room temperature, where the one dimensional strings grow along the < 110 > direction. These phenomena are undoubtedly different from the adsorption induced reconstruction but are explained by the chemical reconstruction involving the formation of quasi-compounds and their self-ordering on the metal surfaces. The chemical reconstruction is indispensablly important to understand the structure and catalysis of alloy and bimetallic surfaces. Pt_0.25Rh_0.75(100) alloy surface being active for the reaction of NO with H₂ is an interesting example. When the Pt-Rh(100) alloy surface is exposed to NO or O₂ at arround 500 K, a p(3 × 1) ordered Rh-O over-layer is obtained on a Pt-enriched 2nd layer by the chemical reconstruction. Ordering of Rh-0 in the p(3 × 1) structure on the Pt(100) surface was reproduced by heating a Rh/Pt(100) bimetallic surface in O₂, and the chemical reconstruction making the p(3 × 1) Rh-O overlayer on a Pt enriched 2nd layer was also proved by heating a Pt/Rh(100) bimetallic surface in O₂ or NO. The activation mechanism of the Pt-Rh alloy and the Pt/Rh bimetallic surfaces by the chemical reconstruction was evidently shown by using a Pt deposited Rh(100), Pt/Rh(100), surface. That is, the Pt/Rh(100) is not so active for the reaction of NO with H₂, but the reconstructed p(3 × 1)Rh-O/Pt-layer/Rh(100) surface is very active for the reaction. Therefore, it was concluded that the chemical reconstruction of the Pt-Rh catalyst makes the active surface which is composed of Rh-O and a Pt layer.     

Surface passivation of luminescent colloidal quantum dots with poly(dimethylaminoethyl methacrylate) through a ligand exchange process

Polydimethylaminoethyl methacrylate (PDMAEMA) was used as a multidentate ligand to modify the surface of CdSe/ZnS core-shell colloidal quantum dots in toluene with trioctylphosphine oxide (TOPO) as the surface ligand. Adsorption of PDMAEMA was accompanied by release of TOPO. The process is free of agglomeration, and the modified nanocrystals become soluble in methanol. The photoluminescence properties are well-preserved in either toluene or methanol.     

Azidation of silicon(111) surfaces

A two-step chlorination/azidation process was reported to prepare azide-modified silicon(111) surfaces. XPS and IR analyses show the covalent bonding of azide with silicon. In combination with scanning tunneling microscopy and spectroscopy analyses, different kinetic rates, azide coverages, and surface-area distributions were derived depending on the azidation solvent.     

Facile interfacial electron transfer through n-alkyl monolayers formed on silicon (111) surfaces

Electron transfer (ET) kinetics through alkyl monolayers, formed on n-type Si(111) surface by the direct reaction of alkylmagnesium bromide (n-C_nH_2n+1MgBr, n=2, 6, 10, and 15) with hydrogen-terminated Si(111), was investigated in acetonitrile (MeCN) with anthraquinone (AQ) as the electrochemical probe. Cyclic voltammetric measurements indicate that the ability of the monolayer to block interfacial electron transfer increases with increasing alkyl chain length. In particular, the voltammetric behavior changes from non-rectifying (i.e., chemically reversible redox couple), to rectifying (i.e., diode-like when the reverse wave is pushed into the gap) with increasing chain length. The dependence of the logarithm of the electron transfer rate constant as a function on the number of carbons in the alkyl chain is not consistent with electron tunneling through the full thickness of the film. In fact, the measured constant, 0.05 ± 0.03 per methylene, is much smaller than the well-established tunneling constant, ∼1.0/CH₂ in the closely packed alkanethiol monolayers on gold suggesting permeation of the AQ into the film.     

Structure effects of benzene hydrogenation studied with sum frequency generation vibrational spectroscopy and kinetics on Pt(111) and Pt(100) single-crystal surfaces

Sum frequency generation (SFG) surface vibrational spectroscopy and kinetic measurements using gas chromatography have identified at least two reaction pathways for benzene hydrogenation on the Pt(100) and Pt(111) single-crystal surfaces at Torr pressures. Kinetic studies at low temperatures (310-370 K) show that benzene hydrogenation does not proceed through cyclohexene. A Langmuir-Hinshelwood-type rate law for the low-temperature reaction pathway is identified. The rate-determining step for this pathway is the addition of the first hydrogen atom to adsorbed benzene for both single-crystal surfaces, which is verified by the spectroscopic observation of adsorbed benzene at low temperatures on both the Pt(100) and Pt(111) crystal faces. Low-temperature SFG studies reveal chemisorbed and physisorbed benzene on both surfaces. At higher temperatures (370-440 K), hydrogenation of benzene to pi-allyl c-C(6)H(9) is observed only on the Pt(100) surface. Previous single-crystal studies have identified pi-allyl c-C(6)H(9) as the rate-determining step for cyclohexene hydrogenation to cyclohexane.     

Theoretical study of NO adsorption/dissociation on Pd (100) and (111) surfaces

Both adsorption and dissociation of the diatomic molecular NO on Pd (100) and (111) surfaces are studied using the extended London‐Eyring‐Polyani‐Sato (LEPS) method constructed by means of 5‐MP (the 5‐parameter Morse potential). All critical characteristics of the system that we obtain, such as adsorption geometry, binding energy, eigenvalues for vibration, are in good agreement with the experimental results. On Pd (100) surface, NO prefers to adsorb in fourfold hollow site (H) uprightly at low coverage. With increase in the coverage NO gradually tilts in fourfold hollow and bridge sites. For NO? Pd (111) system, two adsorption states are found at low coverage, of which one adsorption state is the B(tilt) state that the centroid of NO projects at bridge site, another (H? B? H state) that NO almost parallels to the (111) surface with the vibration frequency of 610 cm^?1, but the frequency is near to that of the atoms, which is easy to be ignored in experiments. At high coverage, two transitional states (BH and HT) are found. NO is difficult to dissociate on Pd (100) and (111) surfaces. Especially for NO? Pd (111) system, the three‐well‐potential dissociation mode is initially put forward to show the remarkable dissociation process with two dissociation transitional states of NO on Pd (111). Copyright © 2008 John Wiley & Sons, Ltd.     

The electrocatalytical influence of underpotential lead and thallium adsorbates on the cathodic reduction of oxygen on (111), (100) and (110) silver single-crystal surfaces

The influence of underpotential Pb and Tl adsorbates on the electrochemical reduction of oxygen on rotating-disc Ag(111), (100), and (110) single-crystal surfaces has been studied in aerated 0.5 M HClO₄ solutions at various concentrations of Cl^?. On the bare silver substrates oxygen is reduced completely to H₂O. Depending on the degree of coverage and the structural arrangements of Pb and Tl adsorbates on the different crystal planes, a partial inhibition of the oxygen reduction is obtained predominatly leading to the formation of the stable intermediate, H₂O₂. In the presence of Cl^? ions in solution, the overvoltage for charge-transfer controlled oxygen reduction increases according to (?E/? logc_Cl^?)_{i_}=?60 mV, due to a specific adsorption of chloride on the silver substrate. In 0.5 M HCl solutions a stimulating effect on the oxygen reduction induced by the underpotential deposition of Pb has been found, which can be interpreted in terms of a competitive adsorption-desorption mechanism involving a replacement of chloride by lead.     

单晶Au表面和Au/TiO2（110）模型催化剂表面CO氧化反应机理和活性位

在分子尺度上介绍了Au/TiO2(110)模型催化剂表面和单晶Au表面CO氧化反应机理和活性位、以及H2O的作用.在低温(<320 K), H2O起着促进CO氧化的作用, CO氧化的活性位位于金纳米颗粒与TiO2载体界面(Auδ+–Oδ––Ti)的周边. O2和H2O在金纳米颗粒与TiO2载体界面边缘处反应形成OOH,而形成的OOH使O–O键活化,随后OOH与CO反应生成CO2.300 K时CO2的形成速率受限于O2压力与该反应机理相印证.相反,在高温(>320 K)下,因暴露于CO中而导致催化剂表面重组,在表面形成低配位金原子.低配位的金原子吸附O2,随后O2解离,并在金属金表面氧化CO.     

Direct attachment of well-aligned single-walled carbon nanotube architectures to silicon (100) surfaces: a simple approach for device assembly

A new approach for the attachment of vertically-aligned shortened carbon nanotube architectures to a silicon (100) substrate by chemical anchoring directly to the surface has been demonstrated for the first time. The ordered assembly of single-walled carbon nanotubes (SWCNTs) was accomplished by hydroxylating the silicon surface followed by a condensation reaction with carboxylic acid functionalised SWCNTs. This new nanostructure has been characterised by X-ray photoelectron, Raman and Fourier transform infrared (FTIR) spectroscopy as well as scanning electron and atomic force microscopy. The assembly behaviour of SWCNTs onto the silicon surface shows a fast initial step producing isolated functionalised carbon nanotubes or nanotube bundles anchored to the silicon surface followed by a slower step where the adsorbed nanotubes grow into larger aggregates via van der Waals interactions between adsorbed and solvated nanotubes. The electrochemical and optical properties of the SWCNTs directly attached to silicon have also been investigated. These new nanostructures are excellent electrochemical electrodes. They also fluoresce in the wavelength range 650-800 nm. The successful attachment of the SWCNTs directly to silicon provides a simple, new avenue for fabrication and development of silicon-based nanoelectronic, nano-optoelectronic and sensing devices. Compared to existing techniques, this new approach has several advantages including low operating temperature, low cost and the possibility of further modification.     

Chiral templating of surfaces: adsorption of (S)-2-methylbutanoic acid on Pt(111) single-crystal surfaces

The adsorption and thermal chemistry of (S)-(+)-2-methylbutanoic acid ((S)-2MBA) on Pt(111) single-crystal surfaces was characterized by using temperature programmed desorption (TPD) and reflection-adsorption infrared (RAIRS) spectroscopies. Particular emphasis was placed on the characterization of the chiral superstructures formed upon the deposition of the submonolayer coverages of enantiopure (S)-2-methylbutanoate species that are produced by thermal dehydrogenation of the (S)-2MBA. The enantioselectivity of the empty platinum sites left open on those structures were identified by their difference in behavior toward the adsorption of the two enantiomers of propylene oxide. It was found that a significant enhancement in adsorption is possible on surfaces with the same chirality of the probe molecule, specifically that the uptake of (S)-propylene oxide is larger than that of (R)-propylene oxide on (S)-2-methylbutanoate adsorbed layers. This contrasts with the lack of enantioselectivity previously reported for the same adsorbate on Pd(111). Detectable differences in adsorption energetics of (R)- vs (S)-propylene oxide on the (S)-2-methylbutanoate/Pt(111) overlayers were measured but deemed not to be the controlling factor in the enantioselectivity reported in this system.     

The interaction of hydrogen and carbon monoxide on Cu-Ni (110) single-crystal surfaces

The co-adsorption of H₂ and CO on a Cu-Ni (110) surface was studied by thermal desorption spectroscopy (TDS) and ultraviolet photoemission spectroscopy (UPS). Strong interactions between adsorbed CO and hydrogen observed in the CO desorption spectrum and CO valence emissions were attributed to a blockage of certain CO adsorption sites by hydrogen.     

Molecular adsorption on ZnO(1010) single-crystal surfaces: morphology and charge transfer

While ZnO has excellent electrical properties, it has not been widely used for dye-sensitized solar cells, in part because ZnO is chemically less stable than widely used TiO(2). The functional groups typically used for surface passivation and for attaching dye molecules either bind weakly or etch the ZnO surface. We have compared the formation of molecular layers from alkane molecules with terminal carboxylic acid, alcohol, amine, phosphonic acid, or thiol functional groups on single-crystal zinc oxide (1010) surfaces. Atomic force microscopy (AFM) images show that alkyl carboxylic acids etch the surface whereas alkyl amine and alkyl alcohols bind only weakly on the ZnO(1010) surface. Phosphonic acid-terminated molecules were found to bind to the surface in a heterogeneous manner, forming clusters of molecules. Alkanethiols were found to bind to the surface, forming highly uniform monolayers with some etching detected after long immersion times in an alkanethiol solution. Monolayers of hexadecylphosphonic acid and octadecanethiol were further analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. AFM scratching shows that thiols were bound strongly to the ZnO surface, suggesting the formation of strong Zn-S covalent bonds. Surprisingly, the tridentate phosphonic acids adhered much more weakly than the monodentate thiol. The influence of organic grafting on the charge transfer to ZnO was studied by time-resolved surface photovoltage measurements and electrochemical impedance measurements. Our results show that the grafting of thiols to ZnO leads to robust surfaces and reduces the surface band bending due to midgap surface states.     

Diffusion of buffer layer assisted grown gold nanoclusters on Ru(100) and p(1 x 2)-O/Ru(100) surfaces

Patterning of metallic clusters on surfaces is demonstrated by utilizing a buffer layer assisted laser patterning technique (BLALP). This method has been employed in order to measure the diffusion of AFM and STM characterized size selected gold nanoclusters (5-10 nm diameter), over Ru(100) and p(1 x 2)-O/Ru(100) surfaces. Optical linear diffraction from gold cluster coverage gratings was utilized for the macroscopic diffusion measurements. The clusters were found to diffuse on the surface intact without significant coalescence or sintering. The barrier for metastable gold nanocluster diffusion on the surface is thought to be lower than the energy required for surface wetting. The apparent activation energy for diffusion was found to depend on the cluster size, increasing from 6.2 +/- 0.4 kcal/mol for 5 nm clusters to 10.6 +/- 0.5 kcal/mol for 9 nm clusters. The macroscopic diffusion of gold nanoclusters has been studied on the p(1 x 2)-O/Ru(100) surface as well, where surface diffusion was found to be rather insensitive to the clusters size with activation energy of 5.5 +/- 1 kcal/mol. The difference between the two surfaces is discussed in terms of a better commensurability (higher level of friction) of the gold facets at the contact area with the clean Ru(100) than in the case of the oxidized surface.     

N-Chlorophthalimide as a mild and efficient chlorination reagent in the Gassman ortho alkylation of aromatic amines. Synthesis of 3-(methylthio)oxindoles

A practical modification of the Gassman 3-(methylthio)oxindole synthesis is reported. In our method, substituted anilines and 2-(methylthio)acetamide were reacted under mild reaction conditions, in the presence of N-chlorophthalimide as a chlorinating agent to give α-amidosulfides, which, in the next step of the process, were cyclized to give 3-(methylthio)oxindoles. The method was successfully applied for the synthesis of the key intermediate, 2-(2-amino-3-benzoylphenyl)-2-(methylthio)acetamide, in the process of the preparation of nepafenac, a commonly used ophthalmic drug.     

Organic functionalization of the Si (100) and Ge (100) surfaces by cycloadditions of carbenes and nitrenes: a theoretical prediction

By means of density functional theory (B3LYP/6-31G*) coupled with effective cluster models, we predict that the well-known cycloaddition reactions of carbenes and nitrenes to alkenes in organic chemistry can be employed as a new type of surface reaction to organically functionalize the Si (100) and Ge (100) surfaces at low temperature. The well-established abundance of carbenes and nitrenes addition chemistry in organic chemistry provides versatile flexibility of functionalizing the surfaces of Si (100) and Ge (100), which can potentially impart new organic functionalities to the semiconductors surface for novel applications in a diversity of fields. Our predictions strongly advance the concept of using organic reactions to modify the solid surface in a controlled manner and quite intriguing chemistry can lie in the material featuring the analogous bonding motif. In further perspective, implications for other theoretical work, regarding disilenes, digermenes, silenes, and germenes that all feature the bonding motif similar to alkenes, are also discussed.     

Oxidation of the (100) face of crystalline silicon

The density functional theory was used to calculate the equilibrium structures of clusters serving as models of the sequence of reaction steps in the oxidation of the (100) face of crystalline silicon and their relative heat effects. The formation of all the intermediates on the (100) face proceeds without activation energy, suggesting the feasibility of avalanche-like formation of an SiO₂ film. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 4, pp. 257–261, July–August, 2007.