Ab initio study of electron affinity variation induced by organic molecule adsorption on the silicon (001) surface (original) (raw)
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The role of Pi-conjugation in attachment of organic molecules to the silicon (001) surface
Surface Science, 2002
We have investigated the role of conjugation in attachment of multifunctional organic molecules to the Si(0 0 1) surface. The model molecules we have chosen are acrylonitrile (CH 2 @CHCBN) and allyl cyanide (CH 2 @CHCH 2 CBN) due to the electron-withdrawing nature of the cyano group. X-ray photoelectron spectroscopy shows that significant difference in primary binding configuration exists. Infrared spectroscopy establishes that acrylonitrile attaches to the Si(0 0 1) surface primarily through a mechanism involving the cyano group, leading to a novel keteneimine structure that is evidenced by characteristic strong infrared absorbance at 1985 cm À1. Allyl cyanide adsorbs in a way that principally utilizes the vinyl portion of the molecule, leading mainly to alkane infrared absorbance below 3000 cm À1. These results show that conjugation can play an important role in controlling product distribution when attaching multifunctional organic molecules to the Si(0 0 1) surface.
The Journal of …, 2004
We use the adsorption of systematically substituted silanes with either simple alkyl or alkyl phenyl ether chains onto oxidized Si to study the electronic effects of such molecular monolayers on Si. While there is no significant effect of distance of the substituents from the surface, a strong effect of what we interpret as depolarization is found for layers made up of molecules with high (>5 D) free molecule dipole moment. This is also apparent from differences in UV-visible and Fourier transform infrared (FTIR) spectral features, suggesting changes in molecular conformation, and, especially, from the measured contact potential differences. These reflect the modified surface's electron affinity and, thus, the effective dipole moment of the monolayer. The effect is ascribed to the system's response to the energetic price of dipole-dipole repulsion.
Ab initio investigation of the adsorption of organic molecules at Si(111) and Si(100) surfaces
Surface Science, 2003
To investigate the early stages of SiC growth on silicon, we performed an ab initio study of the adsorption of C 2 H 2 and other small organic molecules on different Si surfaces. Our calculations, based both on geometry optimization and on finite-temperature molecular dynamics simulations, show that for all the molecules that we have considered the preferred adsorption sites at low temperature are confined at the surface, with no sub-surface penetration. Adsorption occurs through the formation of Si-C bonds, accompanied by a distortion of the adsorbed molecule to adapt the Si-C distance to the SiC bulk bond length. We discuss similarities and differences upon changing the organic molecule and the crystal face. To complete the study with the computation of directly measurable quantities, we analyze the optical reflectance anisotropy of one simulated structure.
Surface Science, 1998
The core-level binding energies of simple unsaturated organic molecules bonded to the Si(001) surface have been investigated using X-ray photoelectron spectroscopy (XPS). Using the Si 2p levels as an internal standard, the shifts in carbon and nitrogen levels were analyzed for a series of small unsaturated molecules, including cyclopentene, ethylene, acetylene, 3-pyrroline and pyrrolidine, adsorbed on the Si(001) surface. Alkene-like carbon atoms are found to have binding energies 0.6-0.9 eV higher than alkane-like molecules. Carbon atoms bonded directly to the silicon surface show binding energies 0.7-0.8 eV lower than those that are not bonded directly to silicon. The N 1s binding energy is decreased by 0.9 eV by bonding to silicon. The use of XPS for identification of bonding configurations of unsaturated organic molecules on the silicon (001) surface is discussed.
The Journal of Physical Chemistry C, 2011
The optimal coverage of a silicon(111) surface functionalized with small organic chains is identified by a computational approach that combines quantum mechanical calculations and classical molecular dynamics simulations. Quantum mechanics is used to evaluate the binding energy of the adsorbed organic molecules. A specific and accurate force-field, developed previously, is employed to perform all-atom molecular dynamic simulations of differently functionalized surfaces in solution. Several coverage percentages with both amine-and methyl-terminated propyl chains are compared through their free energies, pair correlation functions, solvent distributions, and internal conformations. Both types of functionalization yield a coating of ∼55%, but the alkyl-amine coverage free energy is remarkably lower in all cases due to more favorable interactions with the polar solvent.
Adsorption of small hydrocarbon molecules on Si surfaces: Ethylene on Si(001)
Physical Review B, 2008
The interaction between small unsaturated hydrocarbon molecules of C 2 H 4 with a vicinal silicon ͑001͒ surface is studied by means of reflectance anisotropy spectroscopy and analyzed with first principles calculations. Our results confirm that ethylene adsorbs without breaking the silicon dimers. Comparison of theoretical optical spectra with experimental data shows that the C 2 H 4 molecules lay on top of the silicon dimers from low to high coverage. This occurs even though, from a purely energetic point of view, a bridge configuration would be favorable at 1 monolayer coverage.
Adsorption of pentacene on a silicon surface
Surface Science, 2005
A computational description of the chemical bonding interactions of a pentacene molecule on a Si(1 0 0)-(2 · 1) surface is carried out using a combination of tight-binding and Gaussian 98 ab initio approaches. These computations identify the molecular configurations responsible for the adsorption sites parallel and perpendicular to the dimer row observed in STM studies to be a nearly flat, relatively strain-free ''tetra dimer''. The calculations confirm experimental evidence for strong monolayer adsorption dictated by the nature of the underlying reconstructed dimer row: The binding energies of the most stable perpendicular and parallel adsorption sites are 5.02 and 4.42 eV, respectively. The results are consistent with STM studies of sub-monolayer coverage of pentacene on silicon showing that the number of perpendicular adsorbed structures slightly exceeds those for parallel-adsorbed structures.
Monolayers of simple organic molecules on silicon studied by surface tools
Surface and Interface Analysis, 2002
Simple organic molecules such as 1-octadecene, 1-tetradecene, 1-dodecyne and 1,13-tetradecadiene have been grafted to hydrogenated B-doped silicon (100) surfaces. Such substrate has been obtained by etching commercial native oxide-capped flat crystal silicon wafers with procedures involving dilute HF aqueous solutions, but also by hydrogenation with a new method involving treatment of the wafer in an oven with H 2 gas. The functionalization of such a surface has been done by using described wet methodologies involving pure liquids or solutions with thermal activation of the hydride. Contact angle, ESCA, atomic force microscopy (AFM) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measurements have been carried out to characterize the above systems. Particular emphasis is devoted to the interpretation of the ToF-SIMS spectra, seldom used until now to characterize organic monolayers.
Si/C/H ReaxFF Reactive Potential for Silicon Surfaces Grafted with Organic Molecules
The Journal of Physical Chemistry C, 2018
In this work, we developed Si/C/H ReaxFF force field for the study of the functionalization and decomposition of alkyl monolayers on silicon surface. The parameterization was performed based on the main reactions involved in the decomposition of alkyl layers on small silicon clusters. The decomposition mechanisms observed in the molecular dynamics (MD) simulations were validated by the comparison of ReaxFF energy barriers for the elementary steps of the main mechanisms with density functional theory (DFT) calculations. Activation energy barriers obtained from the MD simulations from Arrhenius plots are in excellent agreement with the values calculated from DFT. The trends in the preexponential factor with the alkyl chain length follow the predictions of transition state theory. The results confirm that the main decomposition mechanism of the alkyl chains is the alkene elimination to the gas phase after a β-hydride abstraction by silyl radicals, which are formed in a previous step. The ReaxFF force field was used to comparatively investigate the alkyl surface coverage of Si(111), Si(100)−2 × 1 and "half-flat" Si(100) surfaces as a function of the alkyl chain length, showing good agreement with reported experimental values. Both the DFT and ReaxFF MD calculations predict that decyl monolayers with coverages as high as 0.8 are thermodynamically stable at moderate temperatures.