Multiredox tetrathiafulvalene-modified oxide-free hydrogen-terminated Si(100) surfaces

Tetrathiafulvalene (TTF) monolayers covalently bound to oxide-free hydrogen-terminated Si(100) surfaces have been prepared from the hydrosilylation reaction involving a TTF-terminated ethyne derivative. FTIR spectroscopy characterization using similarly modified porous Si(100) substrates revealed the presence of vibration bands assigned to the immobilized TTF rings and the Si-C═C- interfacial bonds. Cyclic voltammetry measurements showed the presence of two reversible one-electron systems ascribed to TTF/TTF(.+) and TTF(.+)/TTF(2+) redox couples at ca. 0.40 and 0.75 V vs SCE, respectively, which compare well with the values determined for the electroactive molecule in solution. The amount of immobilized TTF units could be varied in the range from 1.7 × 10(-10) to 5.2 × 10(-10) mol cm(-2) by diluting the TTF-terminated chains with inert n-decenyl chains. The highest coverage obtained for the single-component monolayer is consistent with a densely packed TTF monolayer.     

Branched fluoropolymer-Si hybrids via surface-initiated ATRP of pentafluorostyrene on hydrogen-terminated Si(100) surfaces

Linear, branched, and arborescent fluoropolymer-Si hybrids were prepared via surface-initiated atom transfer radical polymerization (ATRP) from the 4-vinylbenzyl chloride (VBC) inimer and ClSO(3)H-modified VBC that were immobilized on hydrogen-terminated Si(100), or Si-H, surfaces. The simple approach of UV-induced coupling of VBC with the Si-H surface provided a stable, Si-C bonded monolayer of "monofunctional" ATRP initiators (the Si-VBC surface). The aromatic rings of the Si-VBC surface were then sulfonated by ClSO(3)H to introduce sulfonyl chloride (-SO(2)Cl) groups and to give rise to a monolayer of "bifunctional" ATRP initiators. Kinetics study indicated that the chain growth of poly(pentafluorostyrene) from the functionalized silicon surfaces was consistent with a "controlled" or "living" process. The chemical composition and functionality of the silicon surface were tailored by the well-defined linear and branched fluoropolymer brushes. Atomic force microscopy images revealed that the surface-initiated ATRP of pentafluorostyrene (PFS) had proceeded uniformly on the Si-VBC surface to give rise to a dense and molecularly flat surface coverage of the linear brushes. The uniformity of surfaces with branched brushes was controlled by varying the feed ratio of the monomer and inimer (VBC in the present case). The living chain ends on the functionalized silicon surfaces were used as the macroinitiators for the synthesis of diblock copolymer brushes, consisting of the PFS and methyl methacrylate polymer blocks.     

Bonding structure of phenylacetylene on hydrogen-terminated Si(111) and Si(100): surface photoelectron spectroscopy analysis and ab initio calculations

Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100), are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA-Si(100) systems: a broad π-π* shakeup peak at 292 eV (XPS), a broad first ionization peak at 3.8 eV (UPS), and a low-energy C 1s → π* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like π-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its π-conjugation character on the hydrogen-terminated Si(100) [H:Si(100)] surface composed of the dihydride (SiH(2)) groups as well as on hydrogen-terminated Si(111) having the monohydride (SiH) group. The formation mechanism of the PA-Si(100) interface is investigated within cluster ab initio calculations, and the possible structure of the H:Si(100) surface is discussed based on available data.     

Morphology, structure, and magnetism of FeCo thin films electrodeposited on hydrogen-terminated Si(111) surfaces

In this work we describe the fabrication of FeCo alloy (less than 10 at% Co) thin films from aqueous ammonium sulfate solutions onto n-type Si(111) substrates using potentiostatic electrodeposition at room temperature. The incorporation of Co into the deposits tends to inhibit Fe silicide formation and to protect deposits against oxidation under air exposure. As the incorporation of Co was progressively increased, the sizes of nuclei consisting of FeCo alloy increased, leading to films with a highly oriented body-centered cubic structure with crystalline texture, where (110) planes remain preferentially oriented parallel to the film surface.     

Molecular modeling of alkyl and alkenyl monolayers on hydrogen-terminated Si(111)

On H-Si(111) surfaces monolayer formation with 1-alkenes results in alkyl monolayers with a Si-C-C linkage, while 1-alkynes yield alkenyl monolayers with a Si-C═C linkage. Recently, considerable structural differences between both types of monolayers were observed, including an increased thickness, improved packing, and higher surface coverage for the alkenyl monolayers. The precise origin thereof could experimentally not be clarified yet. Therefore, octadecyl and octadecenyl monolayers on Si(111) were studied in detail by molecular modeling via PCFF molecular mechanics calculations on periodically repeated slabs of modified surfaces. After energy minimization the packing energies, structural properties, close contacts, and deformations of the Si surfaces of monolayers structures with various substitution percentages and substitution patterns were analyzed. For the octadecyl monolayers all data pointed to a substitution percentage close to 50-55%, which is due the size of the CH(2) groups near the Si surface. This agrees with literature and the experimentally determined coverage of octadecyl monolayers. For the octadecenyl monolayers the minimum in packing energy per chain is calculated around 60% coverage, i.e., close to the experimentally observed value of 65% [Scheres et al. Langmuir 2010, 26, 4790], and this packing energy is less dependent on the substitution percentage than calculated for alkyl layers. Analysis of the chain conformations, close contacts, and Si surface deformation clarifies this, since even at coverages above 60% a relatively low number of close contacts and a negligible deformation of the Si was observed. In order to evaluate the thermodynamic feasibility of the monolayer structures, we estimated the binding energies of 1-alkenes and 1-alkynes to the hydrogen-terminated Si surface at a range of surface coverages by composite high-quality G3 calculations and determined the total energy of monolayer formation by adding the packing energies and the binding energies. It was shown that due to the significantly larger reaction exothermicity of the 1-alkynes, thermodynamically even a substitution percentage as high as 75% is possible for octadecenyl chains. However, because sterically (based on the van der Waals footprint) a coverage of 69% is the maximum for alkyl and alkenyl monolayers, the optimal substitution percentage of octadecenyl monolayers will be presumably close to this latter value, and the experimentally observed 65% is likely close to what is experimentally maximally obtainable with alkenyl monolayers.     

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.     

Thin films derived from giant, tripod-shaped oligophenylenes end-capped with triallylsilyl groups on hydrogen-terminated Si(111) surfaces

Monolayers of giant, tripod-shaped molecules 1 with each tripod leg composed of seven phenylene units end-capped with a triallylsilyl group were prepared on hydrogen-terminated silicon surfaces (H-Si(111)) via thermally induced surface hydrosilylation. The films were characterized by ellipsometry, contact-angle goniometry, and X-ray photoelectron spectroscopy (XPS). The measured ellipsometric thickness of 24 Angstrom of the films suggests anchoring of 1 on the substrate surface with a tripod orientation of high coverage. By measuring the contact angle hysteresis of a series of probe liquids with systematically varied sizes, the molecular pores present on the films consisting of the intercalated molecules of 1 are similar to the cross sectional areas of glycerol and decalin of 0.32-0.49 nm(2). Finally, as evidenced by XPS, excellent yields ( approximately 90%) of Suzuki coupling reactions with arylboronic acid derivatives on the films was achieved, suggesting that the desired tripod orientation of such giant molecules as 1 helps to eliminate the steric hindrance for the reaction.     

Mild methods to assemble and pattern organic monolayers on hydrogen-terminated Si(111)

Mild methods to assemble well-ordered organic monolayers of olefins on Si(111) using 2,2,6,6-tetramethyl-1-piperidinyloxy and to pattern these monolayers on the micrometer-size scale using soft lithography are reported.     

Highly ordered chevron-shaped arrays of continuous copper nano-dot lines formed by electroless deposition on hydrogen-terminated Si(111) surfaces

We have succeeded in forming highly ordered chevron-shaped arrays of continuous copper nano-dot lines by electroless deposition on hydrogen-terminated Si(111) (H-Si(111)) surfaces. Detailed investigations have shown that tiny Cu clusters are preferentially formed at step edges when the electroless deposition is carried out in a deoxygenated neutral aqueous solution of a low Cu2+ concentration (less than 10 microM) with pH approximately = 7. This finding was combined with highly ordered step-edge lines on H-Si(111) prepared by the previously reported method of Teflon scratching and NH4F etching, which has led to the above success. The present result indicates that designed ordered metal nanowires can be produced by the electroless deposition method, using H-Si(111) surfaces with well-regulated step lines as a substrate.     

Activation of surface hydroxyl groups by modification of H-terminated Si(111) surfaces

Chemical functionalization of semiconductor surfaces, particularly silicon oxide, has enabled many technologically important applications (e.g., sensing, photovoltaics, and catalysis). For such processes, hydroxyl groups terminating the oxide surface constitute the primary reaction sites. However, their reactivity is often poor, hindering technologically important processes, such as surface phosphonation requiring a lengthy postprocessing annealing step at 140 °C with poor control of the bonding geometry. Using a novel oxide-free surface featuring a well-defined nanopatterned OH coverage, we demonstrate that hydroxyl groups on oxide-free silicon are more reactive than on silicon oxide. On this model surface, we show that a perfectly ordered layer of monodentate phosphonic acid molecules is chemically grafted at room temperature, and explain why it remains completely stable in aqueous environments, in contrast to phosphonates grafted on silicon oxides. This fundamental understanding of chemical activity and surface stability suggests new directions to functionalize silicon for sensors, photovoltaic devices, and nanoelectronics.     

Electron correlation effects at semiconductor surfaces and interfaces: Si(111)-5x5, Si(111)-7x7 and Sn/Ge(111)

Electron correlation effects associated with the dangling bond surface states of Si(111)-5×5, Si(111)-7×7 and Sn/Ge(111)-3×3 are analyzed. In all the cases, extensive LDA-calculations are performed and effective two-dimensional Hamiltonians are deduced. Our analysis of these Hamiltonians shows that: (a) the Si(111)-5×5 surface states exhibits a metal-insulator transition; (b) the Si(111)-7×7 surface shows important similarities with the Si(111)-5×5 case, but it has a dangling bond surface band having a metallic character; (c) finally, the Sn/Ge(111)-3×3 dangling bond surface bands also shows important correlation effects that are found, however, not to affect the metallic character of the surface bands.     

Photochemical desorption from chlorinated Si(100) and Si(111) surfaces — Mechanisms and models

Understanding the mechanisms controlling the anisotropy of microetching is particularly critical as the scale of semiconductor devices shrink. Defining complex, dynamic chemical systems such as halogen etching require microscopic measurements combining kinetics, dynamics, surface layer composition and micromorphology on prototypical surfaces. This study is concerned with two important variables in addition to spontaneous chemical etching, the role of electronic defects induced by high level doping in producing site-specefic reaction and the enhancement of etching by irradiation at low fluences.

Substitutional defects introduced by selective doping significantly influence the rate of chlorine etching by forming shallow electronic states that are ionized at room temperature¹. We have shown that chlorine sticking coeficients as well as laser-assisted etching are significantly affected by doping at very high dopant levels. Enhancement for n-type doping is consistent with the simple assumption that holes at the surface should enhance Si-Si surface bond breaking and in disagreement with the fact that heavily p-doped silicon has a higher chlorine sticking coefficient than n-doped material².

Carrier effects generated by photoirradiation with above bandgap photons are considerably more complex than simple doping. A depletion layer and associated electric field are set up at the surface and minority carriers are preferentially swept to the surface. The type of photocarrier present at the surface is determined by both the doping and the photoirradiation.

Using photoinduced etching of heavily doped Si(100) and Si(111) by chlorine at low laser fluences, we studied the mechanism of photostimulated desorption using core-level photoemission and time-of-flight measurements of the photoproducts². These results will be interpreted in terms of field-modified electron-hole transport together with carrier-modified chlorine adsorption and desorption.     

Reactive coupling of 4-vinylaniline with hydrogen-terminated Si(100) surfaces for electroless metal and "synthetic metal" deposition

Pristine and resist-patterned Si(100) substrates were etched by aqueous HF to produce hydrogen-terminated silicon (H-Si(100)) surfaces. The H-Si(100) surface was then subjected to UV-induced reactive coupling of 4-vinylaniline (VAn) to produce the VAn monolayer-modified silicon (VAn-Si) surface. The VAn-Si surface was first functionalized with a "synthetic metal" by oxidative graft polymerization of aniline with the aniline moieties of the coupled VAn molecules. The composition and topography of the VAn-Si and polyaniline (PAn)-grafted VAn-Si (PAn-VAn-Si) surfaces were characterized by X-ray photoelectron spectroscopy and atomic force microscopy, respectively. The doping-undoping (protonation-deprotonation) and redox-coupling (metal reduction) behavior, as well as the electrical conductivity, of the surface-grafted PAn were found to be similar to those of the aniline homopolymer. The VAn-Si surface was also funtionalized by the electroless plating of copper. Not only did the VAn layer provide chemisorption sites for the palladium catalyst, in the absence of prior sensitization by SnCl2, during the electroless plating process, it also served as an adhesion promotion layer and a low-temperature diffusion barrier for the electrolessly deposited copper. Finally, micropatterning of the grafted PAn and of the electrolessly deposited copper were demonstrated on the resist-patterned VAn-Si surfaces.     

Competition and selectivity of organic reactions on semiconductor surfaces: reaction of unsaturated ketones on Si(100)-2 x 1 and Ge(100)-2 x 1

A combined experimental and theoretical study of a model system of multifunctional unsaturated ketones, including ethyl vinyl ketone (EVK), 2-cyclohexen-1-one, and 5-hexen-2-one, on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces was performed in order to probe the factors controlling the competition and selectivity of organic reactions on clean semiconductor surfaces. Multiple internal reflection infrared spectroscopy data and density functional theory calculations indicate that EVK and 2-cyclohexen-1-one undergo selective [4 + 2] hetero-Diels-Alder and [4 + 2] trans cycloaddition reactions on the Ge(100)-2 x 1 surface at room temperature. In contrast, on the Si(100)-2 x 1 surface, evidence is seen for significant ene and possibly [2 + 2] C=O cycloaddition side products. The greater selectivity of these compounds on Ge(100) versus Si(100) is explained by differences between the two surfaces in both thermodynamic factors and kinetic factors. With 5-hexen-2-one, for which [4 + 2] cycloaddition is not possible, a small [2 + 2] C=C cycloaddition product is observed on Ge(100) and possibly Si(100), even though the [2 + 2] C=C transition state is calculated to be the highest barrier reaction by several kilocalories per mole. The results suggest that, due to the high reactivity of clean semiconductor surfaces, thermodynamic selectivity and control will play important roles in their selective functionalization, favoring the use of Ge for selective attachment of multifunctional organics.     

Mild and efficient functionalization of hydrogen-terminated Si(111) via sonochemical activated hydrosilylation

Efficient chemical functionalization of hydrogen-terminated Si(111) with simple and bifunctional 1-alkenes was achieved via novel sonochemical activated hydrosilylation, utilizing just a simple ultrasonic bath. It is an extremely mild method that allows the specific attachment of unprotected bifunctional alkenes such as undecenol, undecylenic acid, and even a heat/UV-sensitive alkene, bearing an activated leaving group (N-succinimidyl undecylenate), without suffering any degradation.     

Collective vibrations in cluster models for semiconductor surfaces: vibrational spectra of acetylenyl and methylacetylenyl functionalized Si(111)

The geometries and harmonic vibrational frequencies of the acetylenyl and methylacetylenyl functionalized Si(111) surfaces are investigated using quantum chemical calculations. The vibrational spectra are computed using a previously introduced method whereby the collective vibrational modes that correspond to the vibrations of the infinite periodic system are derived from modest sized cluster models. Our predictions should be useful for the interpretation of the experimental spectra when they become available. The symmetry elements of the methylacetylenyl Si(111) surface that are derived from the space group of the optimized structure and a vibrational mode resulting from photon-adsorbate coupling are explored.     

Competing mechanisms in the optically activated functionalization of the hydrogen-terminated Si(111) surface

Recent experiments have shown that organic monolayers on silicon surfaces can be formed through the optically activated surface reaction of H-terminated Si surfaces with terminally unsaturated organic molecules (Eves et al. J. Am. Chem. Soc. 2004, 126, 14318; Sun et al. J. Am. Chem. Soc. 2005, 127, 2514). Possible mechanisms for the formation of this monolayer involve the abstraction of a H atom either at the same attachment site of the molecule (Path A) or from a neighboring site (Path B). Using periodic Density Functional Theory calculations together with an efficient method for finding reaction pathways, we examine both optically activated reaction mechanisms for an alkene and an aldehyde reacting with H-Si(111). Our results show that while Path A is energetically more favorable its significant barrier is likely to limit its viability. Path B on the other hand encounters a much lower H atom abstraction barrier and appears to be more viable.     

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.     

Facet-dependent lithium intercalation into Si crystals: Si(100) vs. Si(111)

Fast Li transport in battery electrodes is essential to meeting the demanding requirements for a high-rate capability anode. We studied the intercalation of a Li atom into the surface and subsurface layers of Si(100) and Si(111) using density functional calculations with a slab representation of the surfaces. We suggest that the Li atom migrates on the Si surfaces and is subsequently inserted into the inside for both Si(100) and Si(111). The rate-determining steps are the surface incorporation and subsurface diffusion in Si(100) and Si(111), respectively. Our diffusion rate calculations reveal that, once the Li atom is incorporated into the Si surface, Li diffuses faster by at least two orders of magnitude along the <100> direction than along the <111> direction. The importance of careful treatment of the slab thickness for the study of impurity insertion into subsurface layers is also stressed.