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Coverage Dependence of the Adsorption Structure of Alanine on Ge(100) Hangil Lee,*,† Young-Sang Youn,‡ and Sehun Kim*,‡ †
Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Republic of Korea, and ‡ Department of Chemistry, Molecular-Level Interface Research Center, KAIST, Daejeon 305-701, Republic of Korea Received May 29, 2009. Revised Manuscript Received August 17, 2009
The variations with coverage and annealing temperature in the adsorption structure of alanine on Ge(100) have been investigated using high-resolution core-level photoemission spectroscopy (HRCLPES). The C 1s, N 1s, and O 1s corelevel spectra at a low initial coverage show that both the carboxyl and amine groups of the alanine molecules participate in bonding with the Ge(100) surface in an “intrarow O-H dissociated and N dative bonded structure”. However, at higher coverage we found that in addition to this structure an “O-H dissociation structure” is present. Moreover, we systematically monitored the variation of the bonding features of alanine adsorbed on Ge(100) with annealing temperature and thus were able to track the desorption processes. By analyzing the C 1s, N 1s, and O 1s spectra at 420 K, we conclude that the principal adsorption structure at this temperature is the “O-H dissociation structure” because of the disconnection of Ge-N dative bonding. At higher temperatures, the “O-H dissociation structure” is converted into various fragments such as Ge oxide (or Ge-CO), Ge nitride (Ge cyanide), and Ge carbide.
I. Introduction In recent decades, the research into the adsorption structures of amino acids on metal and semiconductor surfaces has intensified not only because of possible industrial applications in technologies such as biosensors and electronic devices but also because of academic interest in the multifunctional groups of amino acids.1-5 Moreover, the multifunctionalization of biomolecules with amino acids is a growing field in fusion technology since the manipulation of the interfaces between biological molecules and semiconductor (or metal) surfaces at the molecular level is important to potential applications in nanobiotechnology and biomedical sciences.6,7 An amino acid consists basically of a carboxyl group (-COOH), an amino group (-NH2), a hydrogen atom (-H) and a side chain (the R-group) attached to the R-carbon. Moreover, because amino acids contain two or more functional groups, various adsorption structures can arise as a result of the competition of the intra- or intermolecular interactions between the its functional groups and the semiconductor (or metal) surfaces. Youn et al. recently reported that glycine, which is the simplest amino acid, adsorbs on Ge(100) via an “intrarow O-H dissociated and N dative bonded structure” based on scanning tunneling microscopy (STM), density functional theory (DFT) calculation, and high-resolution core-level photoemission spectroscopy (HRCLPES).8 Alanine is a simple amino acid, whose inert methyl side chain makes it slightly larger than glycine (see Figure 1). This side chain of alanine is unreactive toward
the Ge(100) surface. Moreover, the properties of this system should also be useful to prediction of the properties of more complex systems containing valine, leucine, and histidine. Recently, Ardalan et al. concluded from theoretical calculations that in the adsorption of alanine on Ge(100) the major product is expected to arise from “O-H dissociation structure” through the carbonyl dative bonded state because it is both kinetically and thermodynamically favorable.9 It is noted that there are some discrepancies between experimental results of glycine on Ge(100)8 and theoretical results of alanine on Ge(100),9 even though molecular structure of glycine is analogous to that of alanine. To converge these experimental and theoretical conclusions, we systematically performed HRCLPES measurements at various coverage and annealing temperatures. In this study, we observed two different adsorption structures as a function of coverage in alanine on Ge(100), monitoring three core-level spectra (C 1s, N 1s, and O 1s). Through the analysis of the binding energies, we confirmed that “intrarow O-H dissociated and N dative bonded structure” emerges at low coverage (0.05 ML), which is the same to the result of glycine on Ge(100) system, whereas “O-H dissociation structure” also appears at higher coverage. Therefore, our research makes the discrepant results of experimental and theoretical studies the convergence. To our knowledge, no previous experimental results for the adsorption structures of alanine on Ge(100) have been reported.
*Corresponding authors: Tel þ82-2-710-9409, Fax þ82-2-2077-7321, e-mail
[email protected] (H.L.); Tel þ82-42-350-2831, Fax þ8242-350-2810, e-mail
[email protected] (S.K.).
The Ge(100) surface (p-type, R = 0.10-0.39 Ω) was cleaned by carrying out several cycles of sputtering with 1 keV Arþ ions for 20 min at 700 K, followed by annealing at 900 K for 10 min. The cleanness of the Ge(100) surface was checked by using low-energy electron diffraction (LEED). L-Alanine (CH3CH(NH2)COOH, 99% purity) was purchased from Aldrich and further purified with several sublimation and pumping cycles to remove any dissolved gases prior to exposure to the Ge(100) surface. We performed HRCLPES at the 7B1 beamline of the Pohang Accelerator Laboratory. The C 1s, N 1s, and O 1s core-level
(1) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature 2000, 405, 665–668. (2) Goede, K.; Busch, P.; Grundmann, M. Nano Lett. 2004, 4, 2115–2120. (3) Peelle, B. R.; Krauland, E. M.; Wittrup, K. D.; Belcher, A. M. Langmuir 2005, 21, 6929–6933. (4) Schiffrin, A.; Riemann, A.; Auw€arter, W.; Pennec, Y.; Weber-Bargioni, A.; Cvetko, D.; Cossaro, A.; Morgante, A.; Barth, J. V. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 5279–5284. (5) Estephan, E.; Larroque, C.; Cuisinier, F. J. G.; Balint, Z.; Gergely, C. J. Phys. Chem. B 2008, 112, 8799–8805. (6) Kasemo, B. Curr. Opin. Solid State Mater. Sci. 1998, 3, 451–459. (7) Kasemo, B. Surf. Sci. 2002, 500, 656–677. (8) Youn, Y.-S.; Jung, S. J.; Lee, H.; Kim, S. Langmuir 2009, 25, 7438–7442.
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II. Experimental Section
(9) Ardalan, P.; Davani, N.; Musgrave, C. B. J. Phys. Chem. C 2007, 111, 3692– 3699.
Published on Web 10/07/2009
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Figure 1. Structure of alanine molecule.
Figure 2. C 1s, N 1s, and O 1s core-level spectra for various amounts of alanine adsorbed on Ge(100) at 300 K: (a, e, i) clean; (b, f, j) 0.05 ML; (c, g, k) 0.10 ML; (d, h, l) 0.15 ML. The points are the experimental values, and the solid lines are the results of peak fitting. spectra were obtained with a PHOIBOS 150 electron energy analyzer equipped with a two-dimensional charge-coupled device (2D CCD) detector (Specs GmbH); photon energies of 340, 460, and 588 eV were used to enhance the surface sensitivity. The binding energies of three core-level spectra were calibrated with respect to that of the clean Au 4f core-level spectrum (84.0 eV) for the same photon energy. The base pressure of the chamber was maintained below 9.5 10-11 Torr. All of the spectra were recorded in the normal emission mode. The photoemission spectra were carefully analyzed by using a standard nonlinear least-squares fitting procedure with Voigt functions.10
III. Results and Discussion Figure 2 show a series of C 1s, N 1s, and O 1s core-level spectra as a function of alanine coverage. We first determined the C 1s core-level spectra for three different alanine coverage (0.05, 0.10, and 0.15 monolayer (ML)) after checking the cleanness of the substrate (Figure 2a). Figure 2b shows the C 1s core-level spectrum obtained after exposure of 0.05 ML alanine at 300 K. This spectrum contains three distinct bonding features due to three carbons in alanine molecule, which confirms that alanine is well adsorbed on the Ge(100) surface without bond breakage. Taking account of the relationship between Pauling electronegativity and binding energy, we assigned the bonding features designated the C1, C2, and C3 to a methyl carbon (284.0 eV), an R-carbon (285.9 eV), and a deprotonated carbonyl carbon (288.3 eV), respectively. Increasing the coverage of alanine to 0.10 ML (Figure 2c), we found apparent variations in these features compared to those found at 0.05 ML. Note in particular the emergence of the new peak (marked C4) at 285.0 eV, which indicates the appearance of the fresh bonding feature caused by (10) Schreier, F. J. Quant. Spectrosc. Radiat. Transfer 1992, 48, 743–762.
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the formation of a new adsorption structure. In this process, we can suggest that the new bonding is induced by the change of electronic state (or electric charge) of the amine group from positive to neutral. If the electric charge of the amine group alters into neutral, the electric charge of the adjacent R-carbon will also become less positive due to the reduced withdrawing ability of the neutral nitrogen atom. Therefore, the binding energy of the R-carbon shifts to the lower energy region, and this shift gives rise to a new peak (C4). Figure 2d displays the C 1s core-level spectrum obtained for 0.15 ML alanine. This spectrum is similar to the data obtained for an exposure of 0.10 ML. Thus, the adsorption structures of alanine on Ge(100) at a coverage of 0.15 ML are the same as that at 0.10 ML. By analyzing the variations in the N and O 1s core-level spectra with coverage, we explain this interesting phenomenon. With the same procedure as described above, we obtained the N 1s corelevel spectra at various coverages. After checking the cleanness of the substrate (Figure 2e), we first obtained N 1s core-level spectra for 0.05 ML of alanine (Figure 2f) adsorbed on Ge(100) at 300 K. This spectrum clearly contains a single N 1s peak (400.1 eV), which corresponds to the one N atom in the alanine molecule. Through the peak assignments of molecules adsorbed on the Ge(100) surface in previous studies, we found that at low initial coverage the nitrogen atom in the amine group of alanine is adsorbed on Ge(100) surfaces through a dative bonding configuration.8,11 Parts g and h of Figure 2 show the N 1s core-level spectra obtained for coverages of 0.10 and 0.15 ML alanine on the Ge(100) surface, respectively. In these figures, note the appearance of a new peak at 397.4 eV (marked N2), which is critical proof that a new adsorption structure occurs at these coverages. As mentioned in the interpretation of the C 1s spectra, the new N 1s peak (N2) arises from the change in the charge of the amine group from positive to neutral. Therefore, we conclude that the N1 peak is induced by an “O-H dissociated and N dative bonded structure” and that the N2 peak is caused by an “O-H dissociation structure”. Thus, in addition to the O-H dissociated and N dative bonded adsorption structure of alanine on the Ge(100) surface, which was not found in a previous theoretical study, an “O-H dissociation structure”, which matches the result of the previous theoretical calculation, also is present.9 The emergence of new adsorption configuration in the N 1s core-level spectra is in good agreement with the results obtained from the C 1s core-level spectra. Finally, we examined the variations in the O 1s peaks as a function of exposure to alanine molecules. Figure 2j shows the O 1s core-level spectrum obtained after the deposition of 0.05 ML alanine. The O 1s peak in this figure is divided into two components due to two types of chemically inequivalent oxygen atoms. As in a previous study,8,12,13 we assign peaks O1 and O2 to Ge-O (531.0 eV) and -CdO (532.0 eV) in the carboxyl group of alanine molecules adsorbed on Ge(100), which indicates that a typical O-H dissociation has occurred. Interestingly, although the coverage was increased from 0.10 to 0.15 ML, there were no changes in the O 1s peaks. In other words, at higher coverage both adsorption structures undergo O-H dissociation. In Table 1, we have summarized the binding energies obtained from the three core-level spectra for various coverages of alanine on the Ge(100) surface at 300 K. As a result, with the analysis of (11) Chen, X. H.; Ranke, W. Surf. Sci. 1992, 262, 294–306. (12) Huang, J. Y.; Huang, H. G.; Lin, K. Y.; Liu, Q. P.; Sun, Y. M.; Xu, G. Q. Surf. Sci. 2004, 549, 255–264. (13) 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.
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Table 1. Binding Energies Obtained from the Core-Level Spectra as a Function of Coverage after Deposition of Alanine on the Ge(100) Surface at 300 K
coverage (ML) 0.05 0.10 0.15
C 1s (eV)
N 1s (eV)
O 1s (eV)
C1/C2/C3/C4 284.0/285.9/288.3/(N/A) 284.0/285.9/288.3/285.0 284.0/285.9/288.3/285.0
N1/N2 400.1/(N/A) 400.1/397.4 400.1/397.4
O1/O2 531.0/532.0 531.0/532.0 531.0/532.0
Figure 3. Schematic diagrams of the adsorption structures of alanine on Ge(100): (a) 0.05 ML of alanine and (b) 0.10 and 0.15 ML of alanine.
Figure 4. C 1s, N 1s, and O 1s core-level spectra for 0.15 ML of alanine adsorbed on Ge(100): (a, e, i) alanine deposited on the Ge(100) surface (as shown in Figure 3d,h,l); (b, f, j) annealed at 420 K for 5 min; (c, g, k) annealed at 500 K for 5 min; (d, h, l) annealed at 570 K for 5 min. The points are the experimental values, and the solid lines are the results of peak fitting.
the C 1s, N 1s, and O 1s spectra, we suggest that the “intrarow O-H dissociated and N dative bonded structure” is predominant at low initial coverage (0.05 ML), which is the same to the result of glycine on Ge(100) system, whereas “O-H dissociation structure” emerges at higher coverage (over 0.10 ML). Figure 3 shows our proposed adsorption structures for alanine on the Ge(100) surface. Next, we performed HRCLPES measurements for various annealing temperatures in order to determine the temperature dependence of the adsorption structure of alanine on Ge(100) surface. We recorded a series of C 1s, N 1s, and O 1s core-level spectra for alanine adsorbed on a Ge(100) surface for various annealing temperatures at 420, 500, and 570 K for 5 min, after exposing the substrate to 0.15 ML of alanine at 300 K (see Figure 4a for C 1s, Figure 4e for N 1s, and Figure 4i for O 1s). With this spectral analysis of the temperature dependence, we obtained useful information about the desorption process. Figure 4b shows the C 1s core-level spectrum obtained after annealing at 420 K for 5 min. Comparison of this spectrum with 12576 DOI: 10.1021/la901914n
that obtained before annealing (Figure 4a) reveals some interesting results. In particular, the only C2 peak related to -C-NH2þbonding has completely disappeared at 420 K, which means that the “O-H dissociated and N dative bonded structure” has vanished. From this result, we infer that the “O-H dissociated and N dative bonded structure” is converted to the “O-H dissociation structure” due to the disconnection of relatively weak dative bonding (-NH2þ-Ge) by surface heating. Furthermore, we note that the “O-H dissociation structure” is somewhat stable with respect to surface heating at this temperature, which implies that the Ge-O dissociated bonding is stronger than the Ge-N dative bonding. We determined the C 1s core-level spectrum after annealing at 500 K for 5 min (Figure 4c). As shown in this figure, there are significant differences between this adsorption structure and that obtained after annealing at 420 K (Figure 4b). First, the C3 peak due to the carboxyl group has completely disappeared, which indicates that the alanine molecule is fully dissociated. Second, the binding energies of the C1 and C4 peaks have shifted toward the lower energy region from 284.0 and 285.0 eV to 283.6 (C10 ) and 284.8 eV (C40 ), respectively. The chemical shifts of these bonding features show that Ge nitride and Ge carbide are present on the Ge(100) surface at this annealing temperature. After annealing at 570 K for 5 min (Figure 4d), we also observed another change in the adsorption structure: the C40 (Ge nitride) features have completely disappeared, and the intensity of the C10 peak has decreased. These findings display that at the annealing temperature of 570 K the alanine molecules adsorbed on the Ge(100) surface have fully dissociated with only carbon fragments remaining on the surface, and Ge carbide has formed. We also checked the variation in the desorption process with the annealing temperature by examining the N 1s and O 1s corelevel spectra. Figure 4f shows the N 1s core-level spectrum recorded after annealing at 420 K for 5 min. Comparison of this spectrum at 420 K with that acquired before annealing reveals that the positively charged nitrogen atom in the alanine molecule (marked N1) in the “O-H dissociated and N dative bonded structure” disappears completely after annealing, whereas the N2 peak due to the “O-H dissociation structure” remains with reduced intensity. This change in the N 1s peak is consistent with the results for the C 1s peaks annealed at 420 K, which indicates that the disappearance of the C4 and N2 peaks occurs simultaneously. Hence, we clearly confirm the structural conversion from the “O-H dissociated and N dative bonded structure” to the “O-H dissociation structure” at 420 K. We then increased the annealing temperature to 500 K and checked for changes in electronic structure. Figure 4g shows the N 1s core-level spectrum recorded after annealing at 500 K for 5 min. When this spectrum is compared to the 420 K spectrum, we see that there is a new peak (marked N3) with a binding energy of 398.6 eV due to Ge nitride or Ge cyanide at 500 K. At this annealing temperature, the desorption of alanine is at an advanced stage. Finally, we carried out annealing at 570 K for 5 min. As shown in Figure 4h, the N3 feature has disappeared, which implies that the Ge nitride (or Ge cyanide) species have also desorbed. Finally, we obtained O 1s core-level spectra for various annealing temperatures. Figure 4j shows the O 1s core-level spectrum recorded after annealing at 420 K for 5 min. When this spectrum is compared to that obtained before annealing, it can be seen that there is no chemical shift. On increasing the annealing temperature to 500 K for 5 min (Figure 4k), we found a new peak at 530.6 eV (marked O3), which is a typical Ge oxide feature. Therefore, at this annealing temperature, the “O-H dissociation structure” is fully dissociated and only Ge oxide feature (or Ge-CO) is present on the Ge(100) surface. Finally, when annealing was carried out at 570 K Langmuir 2009, 25(21), 12574–12577
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Table 2. Binding Energies Obtained from the Core-Level Spectra as a Function of Annealing Temperature after Deposition of 0.15 ML of Alanine on the Ge(100) Surface at 300 K
annealing temperature before annealing 420 K 500 K 570 K
C 1s (eV)
N 1s (eV)
O 1s (eV)
C1(C10 )/C2/C3/C4(C40 ) 284.0/285.9/288.3/285.0 284.0/(N/A)/288.3/285.0 (283.6)/(N/A)/(N/A)/(284.8) (283.6)/(N/A)/(N/A)/(N/A)
N1/N2/N3 400.1/397.4/(N/A) (N/A)/397.4/(N/A) (N/A)/(N/A)/398.6 (N/A)
O1/O2/O3 531.0/532.0/(N/A) 531.0/532.0/(N/A) (N/A)/(N/A)/530.6 (N/A)
for 5 min (Figure 4l), no oxygen peak was found, which indicates the complete desorption of oxygen from the Ge(100) surface. In Table 2, we have summarized the binding energies obtained from the core-level spectra as a function of annealing temperature after deposition of 0.15 ML of alanine on the Ge(100) surface at 300 K.
IV. Conclusions We have investigated the adsorption and desorption structures of alanine adsorbed on a Ge(100) surface by carrying out HRCLPES experiments. On the basis of the results obtained at a low initial coverage (0.05 ML), we have confirmed that both the amine and carboxyl groups of alanine participate in its adsorption onto the Ge(100) surface. At higher coverage of alanine (0.10 and 0.15 ML), the “intrarow O-H dissociated and N dative bonded structure” and the “O-H dissociation structure” are both present on the Ge(100) surface. Furthermore, our analysis of the
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HRCLPES spectra of alanine adsorbed on a Ge(100) surface shows that the adsorption structure of alanine on Ge(100) changes from the “intrarow O-H dissociated and N dative bonded structure” to the “O-H dissociation structure” at 420 K. At 500 and 570 K, the “O-H dissociation structure” is converted to various fragments such as Ge oxide (or Ge-CO), Ge nitride (or Ge cyanide), and Ge carbide bonding features. Acknowledgment. We thank Dr. Chan-Cuk Hwang and HanNa Hwang for assisting the HRCLPES experiments at the 7B1 beamline at the Pohang Accelerator Laboratory. This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (KRF-2008-314-C00169). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0083525).
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