pubs.acs.org/Langmuir © 2009 American Chemical Society
Intrarow Adsorption Structure of Glycine on Ge(100) Young-Sang Youn,† Soon Jung Jung,† Hangil Lee,*,‡ and Sehun Kim*,† †
Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea, and ‡Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Republic of Korea Received January 28, 2009. Revised Manuscript Received March 24, 2009
The adsorption structure of glycine on Ge(100) was investigated using scanning tunneling microscopy (STM), density functional theory (DFT) calculations, and high-resolution core-level photoemission spectroscopy (HRCLPES). We found a major adsorption feature of glycine on Ge(100) in the STM images. This feature appeared as a bright protrusion between two dimer rows with a dark adjacent dimer. The position of the bright protrusion located in the middle of the two dimer rows indicates a multibonding adsorption structure. The results of the theoretical calculations confirm that the adsorption structure of glycine on Ge(100) (between two possible multibonding adsorption structures) is an “intrarow O-H dissociated and N dative bonded structure”. In the HRCLPES experiments, we found an N 1s peak (at 399.5 eV) and two O 1s peaks (at 531.1 and 532.0 eV), which represent strong evidence that the adsorption configuration of glycine on Ge(100) is composed of both O-H dissociation and N dative bonding. All our STM, DFT, and HRCLPES results suggest that the adsorption structure of glycine molecules on Ge(100) is an “intrarow O-H dissociated and N dative bonded structure”.
Introduction Studies focusing on the interaction between biomolecules and semiconductor surfaces are interesting for the development of devices such as biosensors, artificial organs, biochips, and medical implants.1 In particular, as the sizes of biodevices decrease, studies of the interactions between biomolecules and surfaces at the atomic level become much more important. Understanding the reactions of amino acids, the building blocks of peptides, on semiconductor surfaces at the atomic scale may contribute critical information for many bioapplications.2-4 Glycine;the simplest amino acid;contains a carboxylic acid group (-COOH) and an amino group (-NH2). While many studies have examined the adsorption properties of glycine on metal surfaces, only a few reports focus on the adsorption structure of glycine on semiconductor surfaces.5-9 Unfortunately, the studies of the adsorption of glycine on metal surfaces do not give sufficient information to infer the behavior of glycine on semiconductors because the properties of metal surfaces are totally different from those of semiconductor surfaces. Therefore, studies of the adsorption of glycine on semiconductor surfaces are strongly required to obtain general and fundamental information on semiconductor-based bioapplications. In the case of a group IV semiconductor surface, a dimer of the (100) surface can react with an organic molecule by cycloaddition *Corresponding authors: Tel +82-2-710-9409, Fax +82-2-2077-7321, e-mail
[email protected] (H.L.); Tel +82-42-350-2831, Fax +82-42-350-2810, e-mail
[email protected] (S.K.). (1) Kasemo, B. Surf. Sci. 2002, 500, 656–677. (2) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature (London) 2000, 405, 665–668. (3) Goede, K.; Busch, P.; Grundmann, M. Nano Lett. 2004, 4, 2115–2120. (4) Estephan, E.; Larroque, C.; Cuisinier, F. J. G.; Balint, Z.; Gergely, C. J. Phys. Chem. B 2008, 112, 8799–8805. (5) L€ofgren, P.; Krozer, A.; Lausmaa, J.; Kasemo, B. Surf. Sci. 1997, 370, 277– 292. (6) Gao, F.; Li, Z.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Phys. Chem. C 2007, 111, 9981–9991. (7) Nyberg, M.; Odelius, M.; Nilsson, A.; Pettersson, L. G. M. J. Chem. Phys. 2003, 119, 12577–12585. (8) Nyberg, M.; Hasselstrom, J.; Karis, O.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Pettersson, L. G. M. J. Chem. Phys. 2000, 112, 5420–5427. (9) Zhao, X.; Yan, H.; Zhao, R. G.; Yang, W. S. Langmuir 2003, 19, 809–813.
7438 DOI: 10.1021/la9003565
and/or by nucleophilic reactions; this is because the dimer coincidently has a weak π-bonding and a zwitterionic character.10-12 Hence, we should consider several possibilities for the adsorption structures of glycine (including two functional groups) on the (100) surface;even if other research groups have previously reported that glycine adsorbs on Si(100) via an O-H dissociation [these results were obtained from high-resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) studies].13,14 Recently, Huang et al. reported that the adsorption structure of glycine on Si(111) is not only Si-OOCCH2NH2 (through O-H dissociation) but also Si-OOCCH2HN-Si (via O-H and N-H dissociation).15 Although the researchers used a Si(111) surface, we can see a strong possibility of multibonding adsorption structures of glycine on (100) surfaces. Additionally, Jung et al. in our group proposed that there is a possibility of the adsorption structure of glycine on the Ge(100) surface via O-H dissociation and N dative bonding using infrared spectroscopy and DFT calculations.16 Regarding these studies, we consider various multibonding adsorption structures when we approach the research of the adsorption structure of glycine on a semiconductor (100) surface. In this paper, we clearly delineate the adsorption structure of glycine molecules on Ge(100) using scanning tunneling microscopy (STM), DFT calculations, and high-resolution core-level photoemission spectroscopy (HRCLPES).
Experimental and Computational Details The Ge(100) surface (p-type, R = 0.10-0.39 Ω) was cleaned by several sputtering cycles using 1 keV Ar+ ions (20 min, 700 K), followed by annealing at 900 K for 10 min. The cleanliness of the Ge(100) surface was checked with STM and low-energy electron (10) Kubby, J. A.; Boland, J. J. Surf. Sci. Rep. 1996, 26, 61–204. (11) Waltenburg, H. N.; Yates, J. T. Chem. Rev. 1995, 95, 1589–1673. (12) Filler, M. A.; Bent, S. F. Prog. Surf. Sci. 2003, 73, 1–56. (13) Lopez, A.; Heller, T.; Bitzer, T.; Richardson, N. V. Chem. Phys. 2002, 277, 1–8. (14) Qu, Y.-Q.; Wang, Y.; Li, J.; Han, K.-L. Surf. Sci. 2004, 569, 12–22. (15) Huang, J. Y.; Ning, Y. S.; Yong, K. S.; Cai, Y. H.; Tang, H. H.; Shao, Y. X.; Alshahateet, S. F.; Sun, Y. M.; Xu, G. Q. Langmuir 2007, 23, 6218–6226. (16) Jung, S. J.; Kachian, J. S.; Kim, S.; Bent, S. F. Manuscript in preparation.
Published on Web 06/10/2009
Langmuir 2009, 25(13), 7438–7442
Youn et al.
Article
diffraction (LEED). Glycine (NH2CH2COOH, 98.5% purity) was purchased from Aldrich and purified through several sublimation and pumping cycles to remove all the dissolved gases prior to dosing. To obtain an appropriate vapor pressure for dosing, the dosing line was heated during glycine deposition. STM observations were performed in an ultrahigh-vacuum (UHV) chamber equipped with an OMICRON VT-STM instrument at a base pressure below 1.2 10-10 Torr. All STM images were recorded with electrochemically etched W-tips at bias voltages of Vs = -2.0 V with a tunneling current of It = 0.1 nA. All DFT calculations of the adsorption energies were performed employing the JAGUAR 4.2 software package, which applies the hybrid density functional method, and including Becke’s three-parameter nonlocal-exchange functional with the correlation functional of Lee-Yang-Parr (B3LYP).17 In these calculations, we considered four-dimer (Ge35H32) cluster models. The geometries of important local minima on the potential energy surface were determined at the B3LYP/LACVP** level of theory. The LACVP** basis set is a mixed basis set that uses the LACVP basis set to describe the Ge atoms and the 6-31G basis set for the remaining atoms. Moreover, the LACVP basis set describes atoms beyond Ar in the periodic table using the Los Alamos effective core potentials developed by Hay and Wadt.18 All structures were fully optimized without any geometrical constraints on the clusters. The HRCLPES results were obtained using the 7B1 beamline at the Pohang Accelerator Laboratory. The Ge 3d, C 1s, N 1s, and O 1s core-level spectra, which are included in the system consisting of glycine adsorbed on Ge(100), were obtained with a PHOIBOS 150 electron energy analyzer equipped with a two-dimensional charge-coupled device (2D CCD) detector (Specs GmbH) using photon energies of 80, 340, 460, and 588 eV to enhance the surface sensitivity. Four binding energies of the core-level spectra were determined relative to the clean Au valence band (Fermi energy) for the same photon energy. The base pressure of the chamber was maintained below 2.0 10-10 Torr. All the spectra were recorded in the normal-emission mode. The photoemission spectra were carefully analyzed using a standard nonlinear least-squares fitting procedure with Voigt functions.19
Results A. Possible Adsorption Structures of Glycine on Ge (100). Glycine has three reaction sites, namely, two oxygen atoms in the carboxyl group and one nitrogen atom in the amine group. Therefore, we expect various adsorption structures of glycine on a Ge(100) surface. On the basis of previous studies focusing on the interaction between organic molecules and surface dimers of Ge(100), we consider eight possible adsorption geometries for glycine on the semiconductor surface: the “hydroxyl oxygen dative bonded structure” (Figure 1a),20 the “carbonyl oxygen dative bonded structure” (Figure 1b),21,22 the “O-H dissociation structure” (Figure 1c),20-23 the “N dative bonded structure” (Figure 1d),24,25 the “N-H dissociation structure” (Figure 1e),25 the “O-H dissociated and CdO dative bonded structure” (Figure 1f),21,22 the “intrarow O-H dissociated and N dative bonded structure” (Figure 1g), and the “interrow O-H dissociated and N dative bonded structure” (Figure 1h). Among the various possible adsorption mechanisms for organic molecules on Ge(100), the N dative and OH dissociation (17) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (19) Schreier, F. J. Quant. Spectrosc. Radiat. Transfer 1992, 48, 743–762. (20) Jung, S. J.; Lee, J. Y.; Hong, S.; Kim, S. J. Phys. Chem. B 2005, 109, 24445– 24449. (21) Hwang, E.; Kim, D. H.; Hwang, Y. J.; Kim, A.; Hong, S.; Kim, S. J. Phys. Chem. C 2007, 111, 5941–5945. (22) Filler, M. A.; Deventer, J. A. V.; Keung, A. J.; Bent, S. F. J. Am. Chem. Soc. 2006, 128, 770–779.
Langmuir 2009, 25(13), 7438–7442
reactions are quite well documented and studied.20-25 However, the multibonding structures of complex molecules on semiconductor surfaces have not been well researched. In this paper, we have considered several possible multiple interactions, such as “O-H dissociated and CdO dative bonded structure”, “intrarow O-H dissociated and N dative bonded structures”, and “interrow O-H dissociated and N dative bonded structures”, as shown in Figure 1f-h. B. STM Experiments and DFT Calculations. Figure 2 shows a filled-state STM image taken after exposure of the Ge (100) surface to 0.05 monolayer (ML) of glycine at 300 K. We found a major adsorption feature with a dominant population of 92% in STM image of Figure 2. As shown in the magnified STM image at the right upper panel of Figure 2, it is shown as a bright protrusion between two dimer rows with a dark adjacent dimer. To determine the adsorption structure of this feature, we concentrate on the origin of a dark site beside a bright protrusion. According to previous results on Ge(100) and Si(100) surfaces, the adsorption sites attributed to the O-H and N-H dissociation reactions are shown as depressed features in the STM image.20,23,26 However, no N-H dissociation is observed at 300 K when the N atom of the primary amine group is attached on Ge(100) via N dative bonding.24 Furthermore, DFT calculations performed to determine the reaction pathway of the N-H dissociation of dimethylamine on Ge(100) indicate that the activation barrier for this dissociation reaction is 24.3 kcal/mol, contrary to the 3.2 kcal/mol determined for the O-H dissociation reaction via carboxyl oxygen dative bond in acetic acid on Ge(100).22,24 On the basis of these previous theoretical results, we expect that O-H dissociation may take place whenever an N-H dissociation reaction of glycine on Ge(100) occurs; this is due to the lower activation barrier of the former reaction. Since there, however, is only one dark site in the major feature, we confirm that the dark site is due to an O-H dissociation. As a result, we explain that the dark dimer (marked as a dashed circle in the right lower panel of Figure 2) is induced by the O-H dissociation reaction, whereas the bright protrusion (marked as a solid box in the right lower panel of Figure 2) originates from the other functional group of the adsorbed glycine molecule. If the adsorption of glycine on Ge(100) would take place only via an O-H dissociation, the bright protrusion would be shown on the dark site. However, since the bright protrusion is observed not on the dark site but on the adjacent dimer of the dark site, we conclude that the glycine molecule additionally interacts with the adjacent dimer of the dark site through its other functional group. Furthermore, as shown in Figure 3, one side of the dark site of glycine adsorbed on the Ge dimer is represented by asymmetric c(4 2) dimers (indicated as a green arrow), whereas the other side is represented by symmetric 2 1 dimers (indicated as a yellow arrow). Although both sides of the dark site of glycine adsorbed on the Ge dimer are mostly composed of asymmetric c(4 2) Ge dimers, the existence of symmetric 2 1 dimers strongly indicates that the 2 1 dimers are converted into c(4 2) Ge dimers due to the adsorption of the glycine molecule. In other words, as a result of the adsorption of the glycine molecule leading to the stop of the flip-flop of the symmetric 2 1 Ge dimers, which continuously flip between two asymmetric configurations at room (23) Bae, S.-S.; Kim, D. H.; Kim, A.; Jung, S. J.; Hong, S.; Kim, S. J. Phys. Chem. C 2007, 111, 15013–15019. (24) Mui, C.; Han, J. H.; Wang, G. T.; Musgrave, C. B.; Bent, S. F. J. Am. Chem. Soc. 2002, 124, 4027–4038. (25) Kim, A.; Filler, M. A.; Kim, S.; Bent, S. F. J. Phys. Chem. B 2005, 109, 19817–19822. (26) Chung, O. N.; Kim, H.; Chung, S.; Koo, J.-Y. Phys. Rev. B 2006, 73, 033303-1–033303-4.
DOI: 10.1021/la9003565
7439
Article
Youn et al.
Figure 1. Possible adsorption structures of glycine on Ge(100): (a) hydroxyl oxygen dative bonded structure, (b) carbonyl oxygen dative bonded structure, (c) O-H dissociation structure, (d) N dative bonded structure, (e) N-H dissociation structure, (f) O-H dissociated and CdO dative bonded structure, (g) intrarow O-H dissociated and N dative bonded structure, and (h) interrow O-H dissociated and N dative bonded structure.
Figure 3. (a) Filled-state STM image (7.0 7.0 nm2, Vs = -2.0 V,
Figure 2. Filled-state STM image (18.6 18.6 nm2, Vs = -2.0 V, It = 0.1 nA) of glycine adsorbed on Ge(100) at 300 K (after exposure to 0.05 ML glycine). The enlarged STM image (3.0 3.0 nm2) of the major feature is shown in the right upper panel. The schematic model shown in the right lower panel depicts a bright protrusion position (marked as a solid box) and a dark site (represented by a dashed circle) of the glycine molecule adsorbed on the Ge(100) surface. The bright protrusions of the Ge atoms (represented by small and large yellow spots) are shown as red lines.
temperature,10 the symmetric 2 1 Ge dimers are changed into the asymmetric c(4 2) Ge dimers. Therefore, we also confirm that the glycine molecule additionally interacts with adjacent dimers except for O-H dissociation, thereby inducing asymmetrical dimers;behavior that is similar to the well-known structural conversion induced by pyridine molecules adsorbed on Ge (100) via N dative bonding.27 On the basis of these results, we (27) Cho, Y. E.; Maeng, J. Y.; Kim, S. J. Am. Chem. Soc. 2003, 125, 7514–7515.
7440 DOI: 10.1021/la9003565
It = 0.1 nA) of the main feature at 300 K. (b) The schematic model of (a) depicts a bright protrusion position (marked as a solid box) and a dark site (described by a dashed circle) of the glycine molecule adsorbed on the Ge(100) surface. The bright protrusions of the Ge atoms (represented by small and large yellow spots) are shown as red lines. The green and yellow arrows indicate asymmetric c(4 2) and symmetric 2 1 Ge dimers, respectively.
conclude that the adsorption structure of glycine on Ge(100) is either an “intrarow O-H dissociated and N dative bonded structure” (see Figure 1g) or an “O-H dissociated and CdO dative bonded structure” (see Figure 1f). We performed DFT calculations to compare the stabilities of the “intrarow O-H dissociated and N dative bonded structure” and the “O-H dissociated and CdO dative bonded structure”. As shown in Figure 4a,b, we found that the former structure (with Eads = -64.5 kcal/mol) is more stable than the latter one (with Eads = -49.5 kcal/mol). According to the DFT calculations, the stabilized glycine structure itself has O1-O2, N-O1, and N-O2 bond distances of 2.23, 2.83, and 3.73 A˚, respectively, as shown in Figure 4c. In this stabilized structure, we consider that the relative positions of the C1, C2, O1, and O2 atoms are fixed to maintain a trigonal plane because the C2 atom has three sp2 hybrid orbitals Langmuir 2009, 25(13), 7438–7442
Youn et al.
Figure 4. (a, b) Local minima for the “intrarow O-H dissociated and N dative bonded structure” and the “O-H dissociated and CdO dative bonded structure” on a Ge35H32 cluster predicted at the B3LYP/LACVP** level of theory. The adsorption energies (Eads) are shown in both cases; these were calculated using the following equation: Eads = E(glycine on Ge35H32) - E(glycine) E(Ge35H32) (in kcal/mol). (c) Distances between the two oxygen atoms and the nitrogen atom in the local minima for a calculated glycine molecule. (d) Length and interval of the Ge dimers at the Ge(100) surface (obtained in a previous study).28
and one 2p orbital. This means that the O1-O2 atomic distance (of 2.23 A˚) is fixed. The position of the N atom, on the other hand, can be changed. Considering the distance between two dimers, namely, 4.00 A˚,28 the “O-H dissociated and CdO dative bonded structure” is expected to have a higher steric hindrance because the O1-O2 distance (2.23 A˚) is much smaller than the interval between two dimers (4.00 A˚). This could be the reason for the instability of the “O-H dissociated and CdO dative bonded structure”. With these evidence, we clearly demonstrate that the major product is induced by an “intrarow O-H dissociated and N dative bonded structure”. For a more specific study of a dark site in the main feature, we carried out a line profile analysis. Figure 5b shows the line profile of the dark site in Figure 5a. The height of the dark dimer (reacted dimer; AA0 red solid line) is below that of the ordinary dimer (unreacted dimer; BB0 blue dashed line) in the line profile of AA0 in Figure 5b. Furthermore, the dark site of the major feature is distinguished by a dark deep site and a bright shallow site, which means that there are structural differences between two Ge atoms at the dark site. In a previous study, the adsorption structure of methanol on Ge(100) via O-H dissociation was shown as a dark site consisting of a dark deep site due to Ge-H bond formation and a bright shallow site due to the charge density of GeOCH3.23 Compared with this previous research, we assign the dark deep site to the Ge-H feature and the bright shallow site to the other part of the glycine molecule, as shown in Figure 5c. Through STM experiments and DFT calculations we clearly demonstrated that the adsorption structure of glycine on Ge(100) is the “intrarow O-H dissociated and N dative bonded structure”, contrary to what was reported in previous studies for glycine adsorbed on Si(100).13,14 C. HRCLPES Experiments. To confirm the electronic structure of adsorbed glycine on Ge(100), we also carried out HRCLPES experiments. After checking the cleanness of the Ge (100) surface (see Figure 6a), which can be resolved into three well-defined features,29 we deposited 0.20 ML of glycine at 300 K. (28) Zandvliet, H. J. W. Phys. Rep. 2003, 388, 1–40. (29) Landmark, E.; Karlsson, C. J.; Johansson, L. S. O.; Uhrberg, R. I. G. Phys. Rev. B 1994, 49, 16523–16533.
Langmuir 2009, 25(13), 7438–7442
Article
Figure 5. (a) Filled-state STM image (5.0 5.0 nm2, Vs = -2.0 V, It = 0.1 nA) of the major product. (b) Line profile analysis of a dark site. The AA0 red solid and BB0 blue dashed lines correspond respectively to the AA0 red solid line of a dark site and the BB0 blue dashed line of an ordinary Ge dimer in (a). (c) Schematic model of the “intrarow O-H dissociated and N dative bonded structure”. The solid circles and solid triangles in (b) and (c) have the same meaning.
As shown in Figure 6b, the adsorption of glycine molecules on the Ge(100) surface leads to an attenuation of the surface-state (S) and subsurface (S0 ) peaks. Moreover, we found that new peaks, assigned to Ge1 and Ge2, appear 0.5 and 0.8 eV above the bulk Ge 3d peak (29.2 eV). Recently, we reported that the binding energy of the Ge 3d peak related to the Ge-Cl bond moves to higher values as compared to the Ge 3d peak associated with the Ge-Al bond for AlCl3 on Ge(100).30 This is because the Pauling electronegativity (PE) value of the Cl atom is higher than that of the Al atom. Therefore, considering Pauling electronegativity for the nitrogen (PE = 3.1) and oxygen (PE = 3.6) atoms,31 we assign the peaks at 29.7 and 30.0 eV to Ge-N and Ge-OOC, respectively. As shown in Figure 6c, we resolve the C 1s core-level spectrum as two different bonding features (marked as C1 and C2). On the basis of a prior study of the adsorption of acetic acid on Ge (100),22 we assign the C1 and C2 peaks to an R-carbon (284.6 eV) and a deprotonated carbonyl carbon (286.3 eV), respectively. Moreover, these assignments are consistent with the C 1s peaks obtained for glycine adsorbed on Pt(111) and Si(111).5,15 Through this spectrum, we also confirm that glycine is well adsorbed on Ge (100) without bond breaking. When an ammonia molecule is adsorbed on a Ge(100) surface via a N dative bond, the binding energy changes to 399.7 eV at low coverage,32 which is similar to our value of N 1s (399.5 eV) displayed in Figure 6d. Therefore, we demonstrate that the amine group of the glycine molecule adsorbs on the Ge(100) surface via N dative bonding. Finally, as shown in the O 1s core-level spectrum of Figure 6e, the O 1s peak is clearly divided into two components, which correspond to two different kinds of (chemically nonequivalent) oxygen atoms. On the basis of this result, we confirm that the adsorption structure of glycine on Ge(100) is not an “O-H dissociated and CdO dative bonded structure” containing two (30) Jung, S. J.; Youn, Y.-S.; Lee, H.; Kim, K.-J.; Kim, B.; Kim, S. J. Am. Chem. Soc. 2008, 130, 3288–3289. (31) Allen, L. C. J. Am. Chem. Soc. 1989, 111, 9003–9014. (32) Chen, X. H.; Ranke, W. Surf. Sci. 1992, 262, 294–306.
DOI: 10.1021/la9003565
7441
Article
Youn et al.
fore, in our system, the neutral glycine molecules adsorbed on Ge (100) do not change into the zwitterionic form via hydrogen migration. Furthermore, considering the “intrarow O-H dissociated and N dative bonded structure” (Ge-OOCCH2H2N+Ge), the additional adsorption of a hydrogen atom on the N atom is unlikely because the N atom already has a positive character. Additionally, we did not find two types of N 1s peaks in our HRCLPES data, but only one, which means that no glycine molecules interact with each other to change from the neutral structure to the zwitterionic configuration. Conclusively, we assume that the adsorbed glycine molecules on the Ge(100) surface are neutral.
Figure 6. Ge 3d core-level spectra of (a) a clean Ge(100) surface and (b) Ge 3d, (c) C 1s, (d) N 1s, and (e) O 1s core-level spectra of glycine (0.20 ML) adsorbed on Ge(100) at 300 K. The dots are experimental values, and the solid lines represent the results of peak fitting.
equivalent O atoms, but an “intrarow O-H dissociated and N dative bonded structure” including nonequivalent oxygen atoms. Moreover, from the systems of glycine on Si(111)15 and formic acid on Si(100),33 we exactly assign the O1 and O2 peaks to the Ge-O1 (531.1 eV) and -CdO2 (532.0 eV) bonding features in the carboxyl group of glycine adsorbed on Ge(100). On the basis of our HRCLPES results, we conclude that both the carboxyl and the amine groups of glycine concurrently participate in the adsorption on the Ge(100) surface. Therefore, we strongly insist that the adsorption structure of glycine on Ge(100) is the “intrarow O-H dissociated and N dative bonded adsorption structure”, which agrees well with our STM and DFT results.
Discussion Generally, there are two possible mechanisms for converting the neutral form of glycine in the gas phase to the zwitterionic form on a surface.6 First, when the neutral glycine molecules react on a surface through O-H dissociation, the adsorbed hydrogen atom can move to cause a change from the amine group (-NH2) of the adsorbed neutral glycine into the zwitterionic (-NH3+) form. Second, the zwitterionic configuration is formed by the interaction between glycine molecules because this may help to stabilize them. In this case, both neutral and zwitterionic glycine molecules will coexist at low coverage. It has been reported that unpaired H atoms adsorbed on the Ge dimers of a Ge(100) surface are converted into paired H atoms when the temperature of the Ge(100) surface increases from 300 to 500 K.34 From this previous study, we assume that no hydrogen atoms adsorbed on Ge(100) will easily migrate at 300 K. There(33) Huang, J. Y.; Huang, H. G.; Lin, K. Y.; Liu, Q. P.; Sun, Y. M.; Xu, G. Q. Surf. Sci. 2004, 549, 255–264. (34) Lee, J. Y.; Maeng, J. Y.; Kim, A.; Cho, Y. E.; Kim, S. J. Chem. Phys. 2003, 118, 1929–1936.
7442 DOI: 10.1021/la9003565
Conclusion We have investigated the adsorption structure of glycine adsorbed on a Ge(100) surface using STM, DFT calculations, and HRCLPES experiments. The STM images show a major adsorption feature, which is imaged as a bright protrusion between two dimer rows with a dark adjacent dimer. The observation of this bright protrusion adjacent to rather than above a dark dimer provides evidence that glycine forms multibonding adsorption structures. Therefore, this product corresponds to either an “intrarow O-H dissociated and N dative bonded structure” or an “O-H dissociated and CdO dative bonded structure”. We performed DFT calculations to compare the stabilities of these two structures and found that the former structure (Eads = -64.5 kcal/mol) is more stable than the latter one (Eads = -49.5 kcal/ mol). As a result, we confirm that the main product is induced by an “intrarow O-H dissociated and N dative bonded structure”. Furthermore, we carried out HRCLPES experiments to confirm the electronic structure of the adsorption configuration of glycine on Ge(100). The binding energy of the N 1s peak (399.5 eV) is similar to that of ammonia (399.7 eV), which is adsorbed on Ge(100) via an N dative bond. In addition to the O 1s peak being divided into two components, we also confirm that the adsorption feature of glycine on Ge(100) is not an “O-H dissociated and CdO dative bonded structure” containing two equivalent O atoms, but rather an “intrarow O-H dissociated and N dative bonded structure” including nonequivalent oxygen atoms. Conclusively, we clearly demonstrate that the adsorption structure of glycine molecules on Ge(100) is an “intrarow O-H dissociated and N dative bonded structure”. Acknowledgment. We thank Dr. Chan-Cuk Hwang and HanNa Hwang for supporting our HRCLPES experiments at the 7B1 beamline in the Pohang Accelerator Laboratory. This work was supported by High Risk High Return Project of KAIST and the Korea Research Foundation (Grant KRF-2005-070-C00063). H. L. was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008-314-C00169). The experiments at PAL were supported in part by the Korean Ministry of Science and Technology (MOST) and POSTECH.
Langmuir 2009, 25(13), 7438–7442
