J. Phys. Chem. C 2010, 114, 17761–17767
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Pairing of (R,R)-2,3-Butanediol Molecules on a Si(001) Surface Jae Ryang Hahn,*,† Sang Hoon Jang,† and Sukmin Jeong*,‡ Department of Chemistry and Research Institute of Physics and Chemistry, Chonbuk National UniVersity, Jeonju 561-756, Korea, Department of Physics and Research Institute of Physics and Chemistry, Chonbuk National UniVersity, Jeonju 561-756, Korea ReceiVed: July 1, 2010; ReVised Manuscript ReceiVed: September 8, 2010
We used a combination of scanning tunneling microscopy and DFT calculations to determine the reaction characteristics of (R,R)-2,3-butanediol (BD) molecules on a Si(001)-2 × 1 surface. The BD molecules were adsorbed onto the surface as either single, isolated molecules or paired configurations. As single adsorbed molecules, the BD molecules preferred to bridge adjacent Si dimers through oxygen atoms while preserving their chirality. The adsorption probability depended on the molecular conformation; the CH3-gauche conformer is more stable than the CH3-anti conformer. When BD molecules were adsorbed as paired configurations, BD pairs formed between the CH3-gauche conformers with several types of configurations. Total energy calculations for the pairs revealed that they are stabilized by a direct molecular interaction and/or a Si-mediated interaction, which can be analyzed as the strain energy involved with BD adsorption and the dangling bond reactivity near the adsorbed BD molecule. These results suggest that alcohol-type chiral molecules are useful agents for homochiral modification of Si surfaces and that the surface-mediated, as well as the direct intermolecular, interactions on the surface may play an important role in enantioselective separation on a Si surface. I. Introduction Detailed understandings of enantioselective recognition are highly important for the pharmaceutical industry, agrochemicals, and food additives.1 Many studies continue to elucidate the interactions of chiral molecules in the solution phase;2 however, considerably less is known about chirality on solid surfaces. The reduced intrinsic symmetries of 2D surfaces make the study of chirality on surfaces especially interesting. Such chiral studies on surfaces are also attractive in terms of heterogeneous asymmetric catalysis and chiral separation and sensing. The chirality of organic molecules on surfaces has been successfully investigated using scanning tunneling microscopy (STM) to monitor stereoselective reaction mechanisms on the nanoscale.3-7 The depositions of organic molecules and the preservations of their structures and properties are still challenges, especially on Si surfaces. Much progress has been made in the development of metallic surface-based devices with organic molecules;8 however, the use of metallic substrates is less attractive than that of Si surfaces for potential applications. A few studies have focused on chiral adsorption on Si,7 which is important for potential applications such as silicon-based molecular devices, biosensors, and nanolithography. Molecule-molecule interactions between chiral molecules are very difficult to probe and, as a consequence, are not completely understood. Despite these difficulties, studies in this area are ongoing as chiral molecular recognition is a crucial concept in the development of chiral surfaces.2 Further understanding of chiral interactions is a key step toward the development of complex architectures that are controlled through a balance of intermolecular and molecule-substrate interactions. On metal surfaces, the molecule-surface interactions are weak and the * To whom correspondence should be addressed. E-mail:
[email protected] (J.R.H.),
[email protected] (S.J.). † Department of Chemistry and Research Institute of Physics and Chemistry. ‡ Department of Physics and Research Institute of Physics and Chemistry.
diffusion barrier remains relatively low, leading to the formation of enantiospecific molecular domains. On Si surfaces, however, it is expected that chiral characteristics will be elucidated on a molecule-by-molecule basis rather than that of domain-bydomain because of strong molecule-surface interactions and higher corrugations of surface potentials. All chiral configurations observed on the Si(100) surface have been based on [2 + 2] and [4 + 2] cycloadditions between alkenes and surface dimers.7 These adsorptions resulted in diastereomeric (R,S) and/ or enantiomeric (R,R) and (S,S) racemic configurations on the Si surface. Therefore, more variable and adaptable routes for fabricating enantiopure chiral configurations and for controlling chiral molecular interactions need to be explored to realize enantioselective processes on Si surfaces. 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; moreover, the stereochemistries of these compounds may intimately depend on that of the original butanediol.9 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 as the (R,R) configuration on the basis of chemical correlation with (S,S)-tartaric acid.10 2,3-Butanediol exists in a large variety of conformations in the gas and liquid phases because of the relatively free rotation around the C2-C3 bond. Photoelectron spectroscopy studies11 on the adsorption of 2,3-butanediol stereoisomers with Si(001) have revealed that the stereochemistries of such molecules could be identified using circular dichroism in core level photoemissions arising from the chiral carbon atoms. We previously established that the adsorbed (R,R) and (S,S) stereoisomers can be distinguished using STM images.12 In the present study, we sought to elucidate the adsorption characteristics of (R,R)-2,3-butanediol (BD) molecules on a Si surface using a combination of STM and DFT calculations. The BD molecules were adsorbed either as
10.1021/jp106099w 2010 American Chemical Society Published on Web 09/29/2010
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J. Phys. Chem. C, Vol. 114, No. 41, 2010
isolated, single molecules or as paired configurations. Single adsorbed BD molecules preferred to bridge adjacent Si dimers, while preserving their chirality. The adsorption probability depended on the molecular conformation. The paired configurations were formed between the same conformers in various configurations that were stabilized by direct and/or Si-mediated molecular interactions. II. Experimental and Calculation Methods The n-type Si(001) (P-doped) samples employed in this work were ultrasonicated with ethyl alcohol. After degassing overnight at 973 K 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 K/s to promote surface reconstruction. The cleanliness of the samples was confirmed using a homemade STM.13 The base pressure of the vacuum chamber was 2 × 10-11 Torr. Butanediol molecules 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 onto the surface at a very low coverage [
