The adsorption configuration of valine on Ge(100).

The interaction between organic (bio) molecules and semiconductor surfaces has been studied by several research groups for the development of applications in a variety of fields. In particular, recent studies on the interaction between semiconductor surfaces and amino acids have laid the foundations for understanding protein adsorption on semiconductor surfaces. Understanding the behavior of amino acids on semiconductor surfaces, therefore, assists in the development of biosensors, bioartificial organs, biochips, and medical implants. Amino acids contain several groups attached to the acarbon: a carboxyl group, an amino group, a hydrogen atom, and a variable side chain (the R-group). Although the adsorption structures of glycine and alanine (in which the R-groups are hydrogen and methyl, respectively) on Group IV semiconductor surfaces have been wellcharacterized, no systematic examinations of the adsorption of valine (in which the R-group is isopropyl group, Figure 1) have been carried out. Because saturated alkyl groups (such as methyl, ethyl, and propyl groups) in various organic molecules are generally unreactive toward Group IV semiconductor surfaces, we expected that the isopropyl group of valine would also be inert on semiconductor surfaces. However, even if the isopropyl group itself is unreactive, its size may influence the adsorption structure of valine molecules on surfaces, similar to observations of the adsorption of amino acids with large saturated R-groups onto conductive surfaces. For example, the adsorption structures of glycine on Cu(111) surfaces fall into two classes (flat-lying and unidentate conformations), while the adsorption geometry of leucine (with an isobutyl R-group) converges to only the flat-lying configuration on same surface. Moreover, Wang et al. reported that two types of adsorption structures were found for valine adsorbed onto a Cu(111) electrode in aqueous solution, whereas only one adsorption geometry is observed for leucine and phenylalanine (with a benzyl R-group) adsorbed onto a Cu(111) electrode in same condition. In these studies, the authors posit that the inert side chains of leucine and phenylalanine have different interaction energies with the substrate than glycine and valine do, leading to the stabilization of the flat-lying configuration. Although these investigations examined amino acid adsorption on the Cu(111) surface, the results suggest that the inert R-groups of amino acids play a critical role in determining amino acid adsorption geometries on metal surfaces. Recently, our group reported that the adsorption geometry of glycine molecules on a Ge(100) semiconductor surface is an “intrarow O H dissociated and N dative bonded structure” based on scanning tunneling microscopy, density functional theory calculations, and high-resolution core-level photoemission spectroscopy (HRCLPES). Furthermore, as shown recently by HRCLPES experiments, the adsorption structure of alanine on the Ge(100) surface is the “intrarow O H dissociated and N dative bonded structure” at low initial coverage, while an “O H dissociation structure” also appears with this adsorption structure at higher coverage [over 0.10 monolayer (ML)] . Despite the analogous molecular structures of glycine and alanine, these dissimilar phenomena can be induced by either the side chain or the molecule’s own character. Therefore, a study of the adsorption configuration of valine on the Ge(100) surface is the next step in assessing the effects of the side chain on the adsorption geometry. Without a side chain effect in the system of valine, the adsorption structures of alanine on the Ge(100) surface are expected to result from its molecular nature. Herein, the adsorption geometry of valine molecules on the Ge(100) surface is investigated by measuring four core-level spectra (Ge3d, C1s, N1s, and O1s) using HRCLPES. To our knowledge, the adsorption structure of this system has not previously been characterized systematically. Figure 2 displays the Ge3d, C1s, N1s, and O1s core-level spectra of 0.20 ML valine on a Ge(100) surface at 300 K. After confirming the clean Ge(100) surface, which contains three well-defined features that were assigned to the bulk Ge atoms (B), the subsurface Ge atoms (S’), the up-atoms of asymmetric Ge dimers (S), as the valine molecules were deposited. As shown in Figure 2a, the adsorption of valine on the Ge(100) surface resulted in the emergence of two new peaks (marked Ge1=29.7 eV and Ge2=30.0 eV). In the HRCLPES spectrum, the spectral peaks of atoms that are more positive than other atoms of the same element are shifted to higher binding energies. The Pauling electronegativities (PEs) for nitrogen (PE= 3.1), oxygen (PE=3.6), and germanium (PE=2.0) atoms imply that Ge in the Ge OOC unit is more positive than Ge in the Ge N unit. Therefore, we assigned the two new peaks that [a] Y.-S. Youn, Prof. Dr. S. Kim Department of Chemistry Molecular-Level Interface Research Center, KAIST 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701 (Korea) Fax: (+82)42-350-2810 E-mail : [email protected] [b] Prof. Dr. H. Lee Department of Chemistry Sookmyung Women’s University 52 Hyochangwon-gil, Yongsan-gu, Seoul, 140-742 (Korea) Fax: (+82)2-2077-7321 E-mail : [email protected] Figure 1. Molecular structure of valine. Italics indicate the peak assignment labels shown in Figure 2.