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J. Phys. Chem. C 2007, 111, 340-344
Adsorption Structures of 2,3-Butanediol on Si(001) Sang Hoon Jang,† Sukmin Jeong,‡ and Jae Ryang Hahn*,† Department of Chemistry and Research Institute of Physics and Chemistry, Chonbuk National UniVersity, Jeonju 561-756, Korea, and Department of Physics and Research Institute of Physics and Chemistry, Chonbuk National UniVersity, Jeonju 561-756, Korea ReceiVed: June 29, 2006; In Final Form: October 11, 2006
In the present study we used a combination of scanning tunneling microscopy and density functional theory calculations to examine the structures of 2,3-butanediol stereoisomers adsorbed on a Si(001)-2 × 1 surface at room temperature. We found that most molecules prefer to adsorb on the bridge between the ends of two adjacent dimers within the dimer row; this adsorption preference differs from that of alkene-type molecules, which undergo a cycloaddition reaction with a Si dimer on Si(001) surfaces. Determination of the absolute chirality of (R,R)- and (S,S)-2,3-butanediol adsorbates on Si(001) indicated that each molecule preserves its chirality during bonding to Si atoms. In addition, 2,3-butanediol preferred to adsorb in the CH3-gauche conformation rather than the CH3-anti conformation, which can be explained by steric considerations. These results suggest that alcohol-type organic molecules are useful agents for producing chirally modified Si surfaces.
The attachment of organic molecules to Si surfaces1-11 has been studied by using both ultrahigh vacuum and wet chemical approaches, and has attracted considerable interest from both fundamental and technological standpoints. The organic functionalization of a silicon surface, i.e., the selective attachment of organic molecules retaining a functional group, is emerging as an important area in the construction of organic-silicon hybrid nanostructures suitable for use in advanced microelectronics, biosensors, and optical devices. For example, organically modified surfaces can potentially be used to develop sensors capable of complex molecular recognition tasks such as discriminating enantiomers. Progress in this area will require the design of enantioselective reactions with silicon surfaces. One possible approach for inducing such enantioselectivity is to adsorb chiral molecules onto the Si surface. It is therefore necessary to develop methods for probing and controlling not only the placement of molecules on a Si surface, but also their orientation and conformation. In particular, researchers should seek to identify molecules that retain their geometric configurations after reaction with the surface. Recently Kim et al.12,13 studied the reaction of 2,3-butanediol stereoisomers with the Si(001) surface using photoelectron spectroscopy. They suggested that the stereochemistry of such molecules can be identified through circular dichroism in corelevel photoemission arising from the chiral carbon atoms. Specifically, the asymmetry in the carbon (chiral center) 1s intensity excited by right and left circularly polarized light may enable determination of the molecular chirality of the adsorbed phase. However, the absolute chirality of 2,3-butanediol stereoisomers and their adsorption configurations on Si(001) are not yet understood. In the gas and liquid phases, the configuration of chiral 2,3-butanediol has been determined by chemical correlation, with (+)-2,3-butanediol being assigned to an (S,S) configuration on the basis of chemical correlation with (R,R)* Address correspondence to this author. E-mail:
[email protected]. † Department of Chemistry and Research Institute of Physics and Chemistry. ‡ Department of Physics and Research Institute of Physics and Chemistry.
tartaric acid.14 2,3-Butanediol exists in a large variety of conformations in the gas and liquid phases because rotation around a single C-C bond is relatively free. The absolute configuration and predominant conformations of 2,3-butanediol stereoisomers in various environments are particularly important because a large number of compounds are synthesized from 2,3-butanediol and its derivatives, and the stereochemistry of these compounds may depend on that of the original butanediol.15 In the present study we used a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations to examine the structures of 2,3-butanediol stereoisomers adsorbed on a Si(001)-2 × 1 surface at room temperature. We found that 2,3-butanediol molecules predominantly adsorb on the bridge between the ends of two adjacent dimers within the dimer row with preservation of their chirality. In addition, we established that the adsorption probability depends on the molecular conformation. To our knowledge, this is the first atomic-scale study of alcohol-type molecules adsorbed on Si(001)-2 × 1. The n-type Si(001) (P doped) samples employed in this work were ultrasonicated with ethyl alcohol. After degassing overnight at 700 °C in an ultrahigh vacuum system with a pressure lower than 1 × 10-10 Torr, clean Si samples were prepared by applying repeated cycles of 1 keV Ne+ ion sputtering and flashing at 1500 K. After flashing, the sample was rapidly cooled to 1200 K, and then cooled to room temperature at a rate of 2 deg/s to promote surface reconstruction. The cleanliness of the samples was confirmed by using STM.16 The base pressure of the vacuum chamber was 2 × 10-11 Torr. 2,3-Butanediol stereoisomers (Aldrich) were further purified through several cycles of freeze-pump-thaw before being dosed onto the clean Si(001)-2 × 1 surface. Dosing was carried out at room temperature through a leak valve equipped with a microcapillary-array filled tube for uniform adsorption on the surface. Electrochemically etched tungsten tips were prepared by using repeated cycles of self-sputtering by field emission in a Ne atmosphere and by heating in a strong electric field.
10.1021/jp064078z CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2006
Adsorption Structures of 2,3-Butanediol on Si(001)
Figure 1. (a) Filled state STM image of a Si(001)-2 × 1 surface with a very low coverage of 2,3-butanediol stereoisomers. Protruding features are due to molecular adsorption. Features a and b were observed after the exposure of the surface to (R,R)-2,3-butanediol and feature c was observed following additional exposure of (S,S)-2,3-butanediol. The image was obtained at a tunneling current of 0.5 nA and a sample bias voltage of -2.0 V at room temperature. The scan area is 6 × 6 nm2. (b) Cross-sectional cuts for features a and c along the direction of the dimer row. The arrow in panel a indicates the direction of cross-sectional cuts.
To calculate atomic and electronic structures, we employed the Vienna ab initio simulation package (VASP),17 which incorporates ultrasoft pseudopotentials18 and the generalizedgradient approximation of Perdew and Wang19 for the exchangecorrelation energy. The surface was simulated by a repeated slab model with a 4 × 4 lateral periodicity in which six Si layers and a 14 Å vacuum layer were included. We used a cutoff energy of 25 Ry for the plane-wave basis and a 2 × 2 k-point mesh for the surface Brillouin zone sampling, which produce well-converged results.20 We considered the c(4 × 2) and p(2 × 2) reconstructions because both of these types of reconstructions have nearly the same surface energies [within 0.002 eV/ (4 × 4)]. The STM images were simulated by using the Tersoff-Hamann theory, in which the tunneling current is proportional to the local density-of-states integrated between the bias voltage and Fermi level.21 The morphology of the Si(001)-2 × 1 surface onto which 2,3-butanediol stereoisomers had been adsorbed at room temperature was investigated with STM. Figure 1a shows a representative topographical image (filled-state) obtained after molecular adsorption at a very low coverage (
