Reaction Mechanism, Bonding, and Thermal Stability of 1-Alkanethiols

Apr 30, 2010 - Gillian Collins , Peter Fleming , Colm O'Dwyer , Michael A. Morris , and Justin D. Holmes. Chemistry of Materials 2011 23 (7), 1883-189...
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Reaction Mechanism, Bonding, and Thermal Stability of 1-Alkanethiols Self-Assembled on Halogenated Ge Surfaces Pendar Ardalan,† Yun Sun,‡ Piero Pianetta,‡ Charles B. Musgrave,†,§ and Stacey F. Bent*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, and Stanford Synchrotron Radiation Lightsource, Menlo Park, California 94025. §Current address: Department of Chemical & Biological Engineering, University of Colorado at Boulder, Boulder, Colorado 80309 ‡

Received December 24, 2009. Revised Manuscript Received February 10, 2010 We have employed synchrotron radiation photoemission spectroscopy to study the reaction mechanism, surface bonding, and thermal stability of 1-octadecanethiolate (ODT) self-assembled monolayers (SAMs) at Cl- and Brterminated Ge(100) surfaces. Density functional theory (DFT) calculations were also carried out for the same reactions. From DFT calculations, we have found that adsorption of 1-octadecanethiol on the halide-terminated surface via hydrohalogenic acid elimination is kinetically favorable on both Cl- and Br-terminated Ge surfaces at room temperature, but the reactions are more thermodynamically favorable at Cl-terminated Ge surfaces. After ODT SAM formation at room temperature, photoemission spectroscopy experiments show that Ge(100) and (111) surfaces contain monothiolates and possibly dithiolates together with unbound thiol and atomic sulfur. Small coverages of residual halide are also observed, consistent with predictions by DFT. Annealing studies in ultrahigh vacuum show that the Ge thiolates are thermally stable up to 150 °C. The majority of the surface thiolates are converted to sulfide and carbide upon annealing to 350 °C. By 430 °C, no sulfur remains on the surface, whereas Ge carbide is stable to above 470 °C.

1. Introduction Germanium is being investigated for integration into siliconbased electronic devices mainly due to its high bulk electron and hole mobilities and the lower thermal budget required for processing, such as lower dopant activation temperature (∼400-500 °C), that will allow formation of shallow junctions.1-3 One of the major drawbacks to the use of Ge is the difficulty in growing an insulating oxide comparable to SiO2 in Si technology. Studies of Ge oxide suggest that GeO2 is not suitable as a gate dielectric;4 that GeO, while more stable than GeO2,5 is unstable; and that GeO2 is water-soluble.6 Also, upon annealing, GeO2 transforms to GeO, which then desorbs from the surface at ∼420 °C,7 and GeO gas acts as reducing agent which can negatively affect the electrical properties of a device.3 Moreover, removal of the germanium oxide alone is not sufficient for good electrical performance in devices such as metal-oxide-semiconductor field-effect transistors (MOSFETs).8 For example, Ge surface cleaning in ultrahigh vacuum (UHV) conditions results in an oxide-free surface; however, subsequent deposition of high-κ dielectric materials was shown to result in leaky devices, and consequently, use of an appropriate *To whom correspondence should be addressed. E-mail: [email protected]. (1) Loscutoff, P. W.; Bent, S. F. Annu. Rev. Phys. Chem. 2006, 57, 467–495. (2) Misra, D.; Garg, R.; Srinivasan, P.; Rahim, N.; Chowdhury, N. A. Mater. Sci. Semicond. Process. 2006, 9, 741–748. (3) Kamata, Y. Mater. Today 2008, 11, 30–38. (4) Rivillon, S.; Chabal, Y. J.; Amy, F.; Kahn, A. Appl. Phys. Lett. 2005, 87, 253101. (5) Onsia, B.; Conard, T.; De Gendt, S.; Heyns, M.; Hoflijk, I.; Mertens, P.; Meuris, M.; Raskin, G.; Sioncke, S.; Teerlinck, I.; Theuwis, A.; Van Steenbergen, J.; Vinckier, C. Solid State Phenom. 2005, 103-104, 27–30. (6) Prabhakaran, K.; Ogino, T. Surf. Sci. 1995, 325, 263. (7) Prabhakaran, K.; Maeda, F.; Watanabe, Y.; Ogino, T. Appl. Phys. Lett. 2000, 76, 2244–2246. (8) Caymax, M.; Elshocht, S. V.; Houssa, M.; Delabie, A.; Conard, T.; Meuris, M.; Heyns, M. M.; Dimoulas, A.; Spiga, S.; Fanciulli, M.; Seo, J. W.; Goncharova, L. V. Mater. Sci. Eng., B 2006, 135, 256–260. (9) Soe, J. W.; Dieker, C.; Locquet, J.-P.; Mavrou, G.; Dimoulas, A. Appl. Phys. Lett. 2005, 87, 221906.

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passivation layer is necessary.9 Moreover, surface passivation of Ge has a direct effect on the quality and characteristics of the interfacial layer between the high-κ dielectric material and the Ge substrate.2,3,8-11 Several different passivation methods have been explored for germanium, and a number of fundamental studies investigating the passivation and organic functionalization of Ge substrates under gas phase or UHV conditions have been reported.1,5,12-21 Furthermore, various solution-based methods have been studied to achieve chemical passivation of the germanium surface prior to (10) Chui, C. O.; Kim, H.; McIntyre, P. C.; Sarasawat, C. IEEE Electron Device Lett. 2004, 25, 274–276. (11) Shang, H.; Frank, M. M.; Gusev, E. P.; Chu, J. O.; Bedell, S. W.; Guarini, K. W.; Leong, M. IBM J. Res. Dev. 2006, 4/5, 377–386. (12) Schnell, R. D.; Himpsel, F. J.; Bogen, A.; Rieger, D.; Steinmann, W. Phys. Rev. B 1985, 32, 8052–8056. (13) Roche, J.; Ryan, P.; Hughes, G. J. Appl. Surf. Sci. 2001, 174, 271–274. (14) Weser, T.; Bogen, A.; Konrad, B.; Schnell, R. D.; Schug, C. A.; Steinmann, W. Phys. Rev. B 1986, 35, 8184–8188. (15) Gothelid, M.; LeLay, G.; Wigren, C.; Bjorkqvist, M.; Karlsson, U. O. Surf. Sci. 1997, 371, 264. (16) Ardalan, P.; Davani, N.; Musgrave, C. B. J. Phys. Chem. C 2007, 111, 3692– 3699. (17) Filler, M. A.; VanDeventer, J. A.; Keung, A. J.; Bent, S. F. J. Am. Chem. Soc. 2006, 128, 770–779. (18) Cullen, G. W.; Amick, J. A.; Gerlich, D. J. Electrochem. Soc. 1962, 109, 124. (19) Fouchier, M.; McEllistrem, M. T.; Boland, J. J. Surf. Sci. 1997, 385, L905–L910. (20) Bachelet, G. B.; Schl€uter, M. Phys. Rev. B 1983, 28, 2302. (21) Cao, S.; Tang, J. C.; Shen, S. L. J. Phys.: Condens. Matter 2003, 15, 5261– 5268. (22) Sun, S.; Liu, Z.; Lee, D.-I.; Peterson, S.; Pianetta, P. Appl. Phys. Lett. 2006, 88, 021903. (23) Sun, S.; Sun, Y.; Liu, Z.; Lee, D.-I.; Pianetta, P. Appl. Phys. Lett. 2006, 89, 231925. (24) Deegan, T.; Hughes, G. Appl. Surf. Sci. 1998, 123/124, 66–70. (25) Lu, Z. H. Appl. Phys. Lett. 1996, 68, 520–522. (26) Kim, J.; McVittie, J.; Saraswat, K.; Nishi, Y. ECS Trans. 2006, 3, 1191– 1196. (27) Kim, J.; McVittie, J.; Saraswat, K.; Nishi, Y. In 8th International Symposium on Ultra Clean Processing of Silicon Surfaces, Antwerp, Belgium; Mertens, P., Meuris, M., Heyns, M., Eds.; Trans Tech Publications: Switzerland, 2006. (28) Kim, J.; Saraswat, K.; Nishi, Y. ECS Trans. 2005, 1, 214–219.

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high-κ deposition including hydrohalogenic acids,4,5,8,22-36 hydrogen peroxide,4,5,7,28,30,33,36 deionized (DI) water,4,6,28,30 and ammonium hydroxide.28,30,33,34 Deposition of oxynitride,2,8-11 oxysulfide,37 and silicon38 layers has also been utilized for Ge passivation. Our group and others have previously demonstrated that hydrohalogenic acid treatment is one of the most promising approaches to Ge surface passivation. Within this class of acids, HBr and HCl are better suited for wet functionalization than HF due to complete removal of Ge suboxides and higher stability of the passivated surfaces against reoxidation.24,26,27,29,30,35,36 Unfortunately, since even halide termination produces surfaces with limited stability, other passivation methods must be explored. One possibility is to attach an organic group to the surface. For example, deposition of organics by hydrogermylation39,40 and thiolation36,41,42 have been utilized for Ge passivation. Passivation using self-assembled monolayers (SAMs) of thiols (thiolation) is promising due to the advantage of incorporating a Ge-S bond at the interface. The understanding of the Ge-S bonding comes from a number of fundamental studies done in UHV or solution; such studies have shown promising characteristics such as high ambient stability (on order of a few days) for S-terminated Ge surfaces.13,14,31,37,43,44 Apart from passivation, an alkanethiolate SAM on Ge has potential application in area selective atomic layer deposition (ASALD),45 molecular electronics devices,46 biological sensors,47 and microelectromechanical systems (MEMS).42 In contrast to the extensive literature on the 1-alkanethiolate/ metal system,46,47 the use of 1-alkanethiolates on semiconductor surfaces such as GaAs48-52 and Ge36,41,42 has been less studied. Maboudian and co-workers demonstrated for the first time that well-packed 1-octadecanethiolate SAMs covalently bound to the surface through Ge-S bonds can be formed at the H-terminated (29) Lu, Y. F.; Mai, Z. H.; Song, W. D.; Chim, W. K. Appl. Phys. A: Mater. Sci. Process. 2000, 70, 403–406. (30) Kim, J.; Mcvittie, J.; Sarasawat, K.; Nishi, Y. Study of Germanium Surface for Wet Cleaning Applications, SRC/SEMATECH Engineering Research Center for Environmentally Benign Semiconductor Manufacturing; teleconference, 16 May 2006. (31) Bodlaki, D.; Yamamoto, H.; Waldeck, D. H.; Borguet, E. Surf. Sci. 2003, 543, 63–74. (32) Cao, S.; Tang, J. C.; Wang, L.; Zhu, P.; Shen, S. L. Surf. Sci. 2002, 505, 289. (33) Okumura, H.; Akane, T.; Matsumoto, S. Appl. Surf. Sci. 1998, 125, 125–128. (34) Hovis, J. S.; Hamers, R. J.; Greenlief, C. M. Surf. Sci. 1999, 440, L815–L819. (35) Ardalan, P.; Pickett, E. R.; Harris, J. S., Jr.; Marshall, A. F.; Bent, S. F. Appl. Phys. Lett. 2008, 92, 252902. (36) Ardalan, P.; Musgrave, C. B.; Bent, S. F. Langmuir 2009, 25, 2013–2025. (37) Frank, M. M.; Koester, S. J.; Copel, M.; Ott, J. O.; Paruchuri, V. K.; Shang, H.; Loesing, R. Appl. Phys. Lett. 2006, 89, 112905. (38) Wu, N.; Zhang, Q.; Zhu, C.; Chan, D. S. H.; Li, M. F.; Balasubramanian, N.; Chin, A.; Kwong, D.-L. Appl. Phys. Lett. 2004, 85, 4127–4129. (39) He, J.; Lu, Z. H.; Mitchell, S. A.; Wayner, D. D. M. J. Am. Chem. Soc. 1998, 120, 2660–2661. (40) Choi, K.; Buriak, J. M. Langmuir 2000, 16, 7737–7741. (41) Kosuri, M. R.; Cone, R.; Li, Q.; Han, S. M.; Bunker, B. C.; Mayer, T. M. Langmuir 2004, 20, 835–840. (42) Han, S. M.; Ashurst, W. R.; Carraro, C.; Maboudian, R. J. Am. Chem. Soc. 2001, 123, 2422–2425. (43) Maeda, T.; Takagi, S.; Ohnishi, T.; Lippmaa, M. Mater. Sci. Semicond. Process. 2006, 9, 706–710. (44) G€othelid, M.; LeLay, G.; Wigren, C.; Bj€orkqvist, M.; Rad, M.; Karlsson, U. O. Appl. Surf. Sci. 1997, 115, 87–95. (45) Chen, R. Surface Modification for Area Selective Atomic Layer Deposition on Silicon and Germanium. Ph.D. Thesis, Stanford University, 2006. (46) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1170. (47) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (48) Voznyy, O.; Dubowski, J. J. J. Phys. Chem. B 2006, 110, 23619–23622. (49) Jun, Y.; Zhu, X.-Y.; Hsu, J. W. P. Langmuir 2006, 22, 3627–3632. (50) McGuiness, C. L.; Blasini, D.; Masejewski, J. P.; Uppili, S.; Cabarcos, O. M.; Smilgies, D.; Allara, D. L. ACS Nano 2007, 1, 30–49. (51) McGuiness, C. L.; Shaporenko, A.; Mars, C. K.; Uppili, S.; Zhanikov, M.; Allara, D. L. J. Am. Chem. Soc. 2006, 128, 5231–5243. (52) McGuiness, C. L.; Shaporenko, A.; Zhanikov, M.; Walker, A. V.; Allara, D. L. J. Phys. Chem. C 2007, 111, 4226–4234.

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Ge(111) surface at room temperature and are air-stable up to 12 h.42 The adsorption kinetics of 1-alkanethiolate SAMs at Hterminated Ge(111) were also studied and indicate that the adsorption mechanism involves a two-step process that depends on the concentration and chain length of the 1-alkanethiols.41 Our group has recently demonstrated formation of 1-octadecanethiolate (ODT) SAMs on Cl- and Br-terminated Ge(100) and Ge(111) surfaces from solution.36 The advantage of this route of halogenation followed by thiolation is that the initial surface treatment (HCl or HBr) not only removes the oxide and contamination but also results in better stability, making the passivated surface suitable for wet functionalization. By employing a combination of analytical methods such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and infrared (IR) spectroscopy, we showed in our previous study that ODT SAM formation depends upon concentration of the 1-octadecanethiols, choice of solvent, crystallographic orientation of the substrate, and type of surface passivation. A comparison of the Ge(100) and Ge(111) surfaces revealed that ODT SAMs form with higher surface coverage, better packing, and better ambient stability at the halide-terminated (100) surfaces. Also, a comparison between the thiolated Ge:Br and Ge:Cl surfaces revealed thinner ODT films but with higher ambient stability for the Ge:Br samples, although this difference is more pronounced at the (111) surfaces.36 However, the previous study did not determine the bonding structures in detail nor did it elucidate the attachment mechanism. The major limitation of that study was the use of conventional XPS for surface characterization, which did not allow for analysis at sufficiently high resolution to distinguish between different structures. For example, the shape of the Ge(3d) fine scan peaks corresponding to thiolated Ge suggested a complex convolution of peaks, but due to limitations in sensitivity and resolution the peaks could not be completely resolved. Also, the low signal-tonoise ratio as well as interference of surface plasmons from the Ge(3p) core level orbital did not allow us to probe the bonding using the S(2P) peaks in the XP spectra.36 Furthermore, we were unable to quantify the residual halogen concentration which may be present at the surface even after thiolation.36 Hence, in the current work, the details of surface bonding at room temperature for the ODT SAMs formed on various halogenated surfaces were studied by employing synchrotron radiation. Compared with conventional XPS, synchrotron radiation photoemission spectroscopy (SR-PES) has much higher resolution and surface sensitivity due to its tunable photon energy and high X-ray intensity. Using SR-PES, we will show that thiolated (100) and (111) surfaces of Ge are mainly characterized by the presence of monothiolates and possibly dithiolates as well as by unbound thiol and atomic sulfur at room temperature. To complement the experimental studies, we carried out DFT calculations to probe the reaction mechanism and surface bonding of 1-alkanethiols at halogenated Ge surfaces. These calculations will show that hydrohalogenic acid elimination reactions are kinetically favorable on these surfaces at room temperature, and that the reactions are thermodynamically more favorable at Cl-terminated Ge surfaces than at Br-terminated Ge surfaces. In addition to room temperature experiments, thermal annealing studies were also performed. Such studies are important because if 1-alkanethiolate SAMs formed on Ge surfaces are to be used in electronic devices, they will be subjected to various cleaning and annealing steps. We will show that Ge-thiolates are thermally stable up to 150 °C and the majority of the surface thiolates are converted to sulfide and carbide via S-C bond scission upon annealing to 350 °C. Langmuir 2010, 26(11), 8419–8429

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2. Experimental and Computational Details Ge samples were cleaved from 2 in. Czochralski (CZ) grown n-type Ge(100) and Ge(111) wafers (Umicore, Belgium) with resistivity ranging from 1.0 to 3.0 Ω-cm. All reagents were used as purchased, including hydrobromic acid (Aldrich, reagent grade, 48 wt %), hydrochloric acid (EMD Chemicals Inc., ACS grade, 37 wt %), methanol (EMD, ACS grade), chloroform (EMD, ACS grade), hydrogen peroxide (EMD, ACS grade, 30 wt % in H2O), 2-propanol (Fisher, ACS grade), and acetone (Fisher, ACS grade). Prior to use, 1-octadecanethiol (96%, Acros Organics) was dried in a desiccator and used without further purification. Particles and adventitious organics at the as-received Ge surfaces were first removed by 10 min sonication in neat acetone followed by chloroform. The samples were subsequently blown dry with N2. For halogenation, we have employed the cyclic oxidation and etching methodology, since we have previously found that ODT SAMs formed after such treatment showed higher ambient stability than those formed on the surface after direct dipping in hydrohalogenic acids.53 Consequently, the surfaces were first oxidized by H2O2 (30 wt % in H2O) for 5 min, and after a DI water rinse1,4,6,28 Ge samples were etched in aqueous HCl solution (10 wt %) or HBr solution (10 wt %) for 10 min. These oxidation-rinsing-etching cycles were repeated three times33 to remove traces of the oxide and achieve halide termination of the Ge surfaces.24,26,27,29,30 After the final cycle, the halide-terminated samples were directly blown dry with N2.23,27 To form the SAMs, halide-passivated samples were dipped immediately after passivation into 0.1 M solutions of 1-octadecanethiol (ODT) in 2propanol (isopropanol, IPA) for 48 or 72 h.36 All of the ODT solutions were freshly prepared before the experiments. To eliminate solvent evaporation and exposure of the reaction media to ambient, the SAM containers were sealed by Parafilm and stored in an airpurged glovebox. After the ODT SAM formation, the samples were removed from the glovebox and sonicated in neat IPA for 1 min to remove possible physisorbed molecules from the Ge surface. All samples were blown dry with N2 before characterization. We have previously carried out water contact angle (WCA) measurements, ellipsometry, and Fourier transform infrared (FTIR) spectroscopy measurements on SAMs formed using the method described above.36 Those results indicate that well ordered SAMs are made by this procedure. For example, FTIR spectroscopy shows that the C-H stretching peaks observed for both Ge(100):Br and Ge(100):Cl substrates at 2918 and 2848 cm-1 coincide well with the CH2 antisymmetric and symmetric stretching vibrational modes, respectively. Moreover, the peak at 2959 cm-1 is characteristic of the CH3 antisymmetric stretching vibrational modes. These three principal peaks are in agreement with presence of ordered, crystalline-like, and well-packed monolayers on solid surfaces.54,55 For the case of Ge(111), these peak positions were slightly blue-shifted, which along with lower WCA values suggests that ODT SAMs with less surface coverage and poorer packing form on halogenated Ge(111) surfaces than on Ge(100). The photoemission experiments were conducted at beamline 10-1 (a wiggler beamline) at the Stanford Synchrotron Radiation Lightsource (SSRL). The chamber base pressure was 110-10 Torr. A PHI model 10-360 hemispherical capacitor electron energy analyzer and an Omni Focus III small area lens were mounted on the analysis chamber. To optimize the surface sensitivity and incident beam intensity, a 300 eV photon energy was selected for the S(2p), Cl(2p), and Br(3d) core levels, while the Ge(3d), C(1s), and O(1s) were monitored at 200, 350, and 620 eV, respectively. The fine scans were taken at a pass energy (PE) of 11.750 eV. The energy resolution at 200 eV is approximately 0.3 eV. For the (53) Ardalan, P.; Bent, S. F. Unpublished results. (54) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136–6144. (55) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145– 5150.

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Article annealing experiments, the samples were radiatively heated at a range of temperatures from room temperature to 470 °C for 20 min inside the UHV chamber. Next, the samples were cooled to room temperature (RT) inside the chamber for 10 min before new spectrum acquisition. The temperature was probed by using an Al-Cr thermocouple, and the temperature uncertainty is approximately 20 °C. The photoemission data was processed using a Shirley background correction56 followed by fitting to Voigt profiles such that a minimum number of meaningful peaks were employed for the peak fitting. Key fitting parameters for Ge(3d) scans are 190 meV Lorentzian width, 280 meV Gaussian width with higher values allowed for þ3 and þ4 oxidation states, 0.585 eV spin-orbit splitting,13,14,22 and 0.667 branching ratio.13,14,22 The S(2p) peaks were fitted to Voigt peaks with the same full width at halfmaximum (FWHM) using 1.2 eV spin-orbit splitting,57,58 and a branching ratio of 0.5.57,58 All peaks were adjusted using the bulk Ge(3d) peak at the same energy to correct the kinetic energies for the charge shift.59 The Cl or Br coverage at the thiolated Ge surfaces (θ) was calculated from the photoemission data based on the exponential absorption model suggested by Ranke and Jacobi60 (see the Supporting Information). The Ge9H12X2 (X = Cl or Br) one-dimer clusters were used to model the reactivity of the halide-terminated Ge(100) surfaces toward the 1-alkanethiol molecules (see Figures 2 and 3 for the structures of the clusters). These clusters consist of two Ge atoms representing the surface dimer with each dimer atom terminated by one halogen atom, and seven Ge atoms modeling three layers of subsurface bulklike atoms. The dangling bonds of the subsurface atoms are terminated by 12 hydrogen atoms to mimic the sp3 hybridization of bulk Ge. Clusters have been used previously as models of surface reactive sites to predict reaction products on both the Ge and Si surfaces and generally produce results consistent with experimental observations except in cases where significant interactions extend beyond the edge of the cluster.17,61-64 To minimize aphysical distortions of the cluster, the positions of the terminating H atoms were fixed following optimization of the unconstrained Ge9H12 cluster. Calculations were performed using density functional theory (DFT) with the Becke3 Lee-Yang-Parr (B3LYP) hybrid and gradient corrected exchange functional.65-68 The electronic structure was expanded over atomic Gaussian basis functions using a mixed basis set scheme with the polarized double-ζ 6-31G(d) basis used for chemically active atoms (Ge dimer and halogen atoms), the 6-31G basis set used for the chemically inactive terminating H atoms of the Ge9H12X2 clusters, and the LANL2 effective core potential and valence double-ζ basis set (LANL2DZ) employed for describing the chemically inactive subsurface Ge atoms. We have employed a truncated model (1-ethanethiol) to describe 1-octadecanethiol. All of the atoms were treated as chemically active atoms in this truncated model. This scheme is designed to minimize computational cost while allocating additional basis functions for describing parts of the system that undergo signifi(56) Shirley, D. A. Phys. Rev. B 1972, 5, 4709–4714. (57) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8–11. (58) Cavalleri, O.; Gonella, G.; Terreni, S.; Vignolo, M.; Pelori, P.; Floreano, L.; Morgante, A.; Canepa, M.; Rolandi, R. J. Phys.: Condens. Matter 2004, 16, S2477–S2482. (59) Miller, T.; Rosenwinkel, E.; Chiang, T.-C. Solid State Commun. 1983, 47, 935–938. (60) Ranke, W.; Jacobi, K. Surf. Sci. 1977, 63, 33–44. (61) Mui, C.; Han, J. H.; Wang, G. T.; Musgrave, C. B.; Bent, S. F. J. Am. Chem. Soc. 2002, 124, 4027. (62) Widjaja, Y.; B, M. C. Surf. Sci. 2000, 469, 9. (63) Widjaja, Y.; M, M. M.; B, M. C. J. Phys. Chem. B 2000, 104, 2527. (64) Trucks, G. W.; Raghavachari, K.; Higashi, G. S.; Chabal, Y. J. Phys. Rev. Lett. 1990, 65, 504. (65) Hohenberg, H.; Kohn, W. Phys. Rev. B 1964, 1, 36B864. (66) Kohn, W.; Sham, L. Phys. Rev. A 1965, 140, 1133. (67) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (68) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785.

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Article cant modification to the electronic structure during reaction. It is important to point out that there are two important time scales associated with the self-assembly process, namely, (1) chemisorption of the headgroup on the substrate which typically occurs on the time scale of milliseconds to minutes,46 and (2) alkyl chain reorganization (on the order of hours to days),46 which is the kinetically limiting step for SAM formation. The truncated model (1-ethanethiol) employed in these calculations serves to address only the first step, chemisorption. A frequency calculation was performed after each geometry optimization to determine zero-point energies and to verify that minima and transition states have zero and only one imaginary frequency, respectively. Intrinsic reaction coordinate (IRC) computations were carried out for key transition states to check if they connect the desired minima. Moreover, all of the transition states were visually inspected to ensure that the imaginary modes corresponded to the correct reactions. All optimization and frequency calculations were completed in the gas phase. The single-point energies were calculated using a more extensive mixed basis set by applying to the gas-phase-optimized structures in IPA media, and energies from the higher level of theory are reported. For this, the triple-ζ 6-311þþG (d, p) basis set was employed, except for the seven subsurface germanium atoms, which were described using the LANL2DZ effective core potential and basis. This approach has been found to reproduce experiments and the results of high level methods such as quadratic configuration interaction singles and doubles and connected triples (QCISD(T)) relatively accurately.69 Solvent effects were modeled using the conductor-like polarizable continuum model (CPCM), 70,71 and the atomic radii from the universal force field (UFF) were used for the solute atomic radii. The CPCM solvent parameters were selected for IPA with a dielectric constant of 18.3,72 a calculated solvent probe radius of 3.69 A˚, and a solvent density of 0.0079 molecule/A˚3.72 All relative energies reported herein are relative energies in solution phase corrected with zero-point energies obtained from the calculated gas phase frequencies. All calculations were performed using the Gaussian 03 software package.73

3. Results and Discussion 3.1. Density Functional Theory. We begin with a discussion of DFT results. In this study, we explored several different possible reactions between 1-alkanethiols and halogen-terminated Ge(100) surfaces. Figure 1 illustrates these reactions with monohalides at Ge(100) surfaces, namely, HX (X=Cl, Br) elimination, HX elimination followed by insertion, and dimer bond cleavage. Figure 2 shows the calculated HX (X=Cl, Br) elimination and HX elimination/insertion pathways for the reaction of 1-ethanethiol with Cl- and Br-terminated Ge(100) surfaces. In Figure 2, the solid lines represent two consecutive elimination pathways, (69) Wang, G. T.; Mui, C.; Tannaci, J. F.; Filler, M. A.; B, M. C.; F, B. S. J. Phys. Chem. B 2003, 107, 12256. (70) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (71) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669–681. (72) CRC Handbook of Chemistry and Physics, 81st ed.; CRC Press LLC: Boca Raton, FL, 2000-2001. (73) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Danenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Sfefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; W., C.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Pople, J. A., Gaussian 03, revision B.3; Gaussian, Inc.: Pittsburgh, PA, 2003.

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Figure 1. Possible surface reaction products of 1-alkanethiols and monohalides at Ge(100) surfaces.

Figure 2. Reactions of 1-ethanethiol on Cl- and Br-terminated Ge(100); HCl or HBr elimination and insertion pathways.

while the dashed lines represent the first elimination pathway followed by the insertion pathway. Moreover, pathways which are shown with red and black colors correspond to 1-ethanethiol reactions at Br- and Cl-terminated Ge(100), respectively. It is evident that, for all of the reactions studied, energies are above the entrance channel, which makes each of these reactions thermodynamically unfavorable. However, a constant flux of reactants and removal of the hydrohalogenic acid product may shift these reactions toward completion even at room temperature. The endothermicity of the surface elimination reactions can be explained in terms of the bond strength differences between the bonds that form (Ge-S and H-X) and the bonds that break (Ge-X and S-H) during the reaction. For example, for the HCl elimination reaction, the total energy of the Ge-Cl bond (∼88 kcal/mol) and S-H bond (∼80 kcal/mol) broken in the reaction is higher than the total energy of the Ge-S bond (∼60 kcal/mol) and H-Cl bond (∼103 kcal/mol) formed. The energetic cost to undergo the HCl elimination reaction estimated by the bond energy calculation (∼5 kcal/mol) is very close to the endoenergicity shown in the pathway in Figure 2 (4.3 kcal/mol). Although the general mechanism of the hydrohalogenic acid elimination does not depend on the surface termination, the reaction barriers and energetics of the intermediates and products show differences based on the identity of surface halides. For reactions at Cl- and Br-terminated Ge(100) surfaces, the first transition states for HCl and HBr elimination pathways are 20.1 and 21.9 kcal/mol above the entrance channel, respectively. This leads to a hydrogen-bonded intermediate before eliminating HCl or HBr and forming a surface monothiolate at 6.2 and 9.6 kcal/mol above the entrance channel on Cl- and Br-terminated Ge(100) Langmuir 2010, 26(11), 8419–8429

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surfaces, respectively. After this step, a second 1-ethanethiol molecule can react with the remaining surface halides with a similar reaction mechanism as the first elimination step, but with a slightly higher barrier from the monothiolate intermediate. Based on simple kinetics calculations, the reaction of the first thiol molecule with the unreacted halide surfaces is 10-100 times faster than the reaction on the monothiolated dimers (i.e., second elimination step). In addition, due to smaller barriers for the reverse reaction, desorption of monothiolates may compete with formation of the second thiolate, although it requires the presence of nearby HCl molecules. Because the halogen is completely removed only if thiolates form at both Ge atoms of the dimer, this suggests that some residual chlorine or bromine may remain at the surface, as will be confirmed by the SR-PES results (section 3.2). Although not calculated directly, hydrohalogenic acid elimination reactions similar to those calculated for 1-ethanethiol reactions at halogenated Ge(100) are expected to be kinetically favorable at halogenated Ge(111) surfaces. Figure 2 also shows that the insertion of the sulfur, which entails dimer bond cleavage and reconfiguration of the surface adducts, requires surmounting barriers 46.1 and 46.9 kcal/mol above the monothiolated state at Cl- and Br-terminated Ge(100) surfaces, respectively. Overall, these transition states are >50 kcal/mol above the entrance channel, which makes the insertion pathways kinetically unfavorable at room temperature. The kinetics of the alkanethiol attachment reaction (HX elimination pathway) is expected to depend upon the halogen atom involved, due to a variety of factors, including differences in electronegativity, atomic size, and bond strength. Each of these factors favors the reaction on the Cl-terminated surface, consistent with the DFT results in Figure 2, which shows more favorable energetics for each of the Cl reactions compared to the Br reactions. For example, the Pauling electronegativity difference between Ge and Br (0.95)72 is less than that between Ge and Cl (1.15),72 and thus, the Ge backbonds are more polarized when attached to Cl atoms than to Br atoms. This bond polarization is expected to help lower the activation barrier for the proposed HX elimination reactions, leading to a lower activation barrier in the case of chlorine. In addition, the larger size of the Br atom may affect the kinetics by introducing larger steric hindrance in the reaction than for Cl atoms. Also, bromine forms weaker bonds compared to chlorine in general which affects the overall energetics. Hence, a combination of sterics and bond polarization leads to kinetically and thermodynamically more favorable 1-alkanethiolate chemisorption at Cl-terminated Ge surfaces. Although these calculations shed light on the bonding of the headgroup to the surface, the second step of self-assembled monolayer formation, that is, alkyl chain reorganization, is not captured here. The dimer bond cleavage pathway illustrated in Figure 1 requires Ge-Ge bond scission with transfer of H from the thiol molecule to one of the Ge dimer atoms. The calculated barriers for this pathway on both surfaces is high (∼30 kcal/mol, data not shown), suggesting that it is not kinetically competitive at room temperature and should not contribute to the surface products after thiolation. We note that the energetics of the dimer bond cleavage pathway are nearly identical for reactions at the Cl- and Br-terminated surfaces. This similarity in energetics can be attributed to the fact that these reactions leave the surface halide species intact. The kinetics and thermodynamics of the self-assembly process become complicated when considering various solvent effects, and they are not very well understood.46 We have previously found experimentally that using polar protic solvents such as IPA results in well-packed 1-octadecanethiolate SAMs at halogenated Langmuir 2010, 26(11), 8419–8429

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Ge surfaces, whereas we were unable to generate good quality SAMs of the same molecules using nonpolar solvents such as hexane or toluene.36 Although some references suggest faster SAM formation after using nonpolar solvents such as hexane, other studies have shown that these SAMs can be less organized due to strong adsorbate-solvent interactions.46 Solvents may also interact with the substrate and possibly adsorb at the surface initially, followed by their displacement by the SAM molecules.74 Nevertheless, all of these interactions are difficult to model in detail. Consequently, in the current work, the solvent effect is captured implicitly by employing the CPCM model. An even larger energy stabilization is expected when surface adducts are allowed to interact explicitly with the solvent molecules; however, the approach employed in this work is still useful to predict the most favorable mechanism for 1-alkanethiol adsorption at the halogenated Ge(100) surfaces. The theoretical results represented in this section are intended to provide guidance for experimental studies of alkanethiol adsorption on germanium surfaces. The DFT calculations yield several key predictions: (1) residual Cl and Br will be left at the surface after the thiolation; (2) the initial thiolation reaction (attachment of the headgroup) will be faster on the Cl-terminated Ge surfaces than on the Br-terminated Ge surfaces; (3) dimer bond cleavage reactions (via insertion) will not be observed to any significant extent, and hence, the Ge-Ge dimer bonds will remain intact; (4) higher temperatures will favor the formation of thiolate products, since the reactions are endoenergetic. The experimental results described below will offer direct confirmation for the first prediction, that is, the presence of residual halide at the surface. The other predictions will require future study for substantiation. 3.2. SR-PES. 3.2.1. Bonding at Room Temperature. Figure 3 shows the Ge(3d), S(2s), and C(1s) core level spectra taken at room temperature after the Cl-terminated Ge(111), Clterminated Ge(100), and Br-terminated Ge(100) samples were dipped in the ODT solution for 72 h. These samples are labeled in the figure as “Ge(111)-Cl þ ODT”, “Ge(100)-Cl þ ODT”, and “Ge(100)-Br þ ODT”, respectively. The spectra corresponding to each SAM-coated surface are complex and can be fit to several peaks. Table 1 lists the positions and relative intensities of the fitted peaks in the Ge(3d) core level spectra from Figure 3a and the species to which they are assigned. We will discuss these peaks in detail below. For the thiolated Ge(111) surface, five components are used to fit the Ge(3d) data in Figure 3a. Apart from the Ge(3d) bulk peak (Ge0þ), four other contributions with chemical shifts of 0.33, 0.72, 1.56, and 2.49 eV are also observed (see Table 1). The latter two shifts correspond well to peaks reported in the literature for Ge bonded to O with oxidation states of þ2 (Ge2þ(O)) and þ3 (Ge3þ(O)), respectively23,75 The total oxide to bulk ratio is small (∼0.06), and hence, the Ge oxide species are minority adducts at the Ge(100) surface. The presence of a small amount of Ge oxide is further confirmed by the O(1s) core level peaks (vide infra). The analysis of the 0.33 and 0.72 eV shifted peaks is more difficult due to limitations in energy resolution at the 200 eV photon energy (∼0.3 eV). The 0.33 eV shift corresponds well with Ge1þ attached to S in the form of Ge monothiolate (Ge1þ(S)). In fact, Weser et al.14 and Roche et al.13 have reported 0.33 and 0.4 eV chemical shifts to the Ge(3d) peak per Ge-S bond, respectively. Based on this argument, a 0.66-0.8 eV shift attributed (74) Himmelhause, M.; Gauss, I.; Buck, M.; Elisert, F.; Woll, C.; Grunze, M. J. Electron Specrosc. Relat. Phenom. 1998, 92, 139–149. (75) Adhikari, H.; McIntyre, P. C.; Shiyu, S.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 2005, 87, 263109.

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Figure 3. Room temperature (a) Ge(3d) core level spectra taken at incident photon energy of 200 eV, (b) S(2p) core level spectra taken at 300 eV, and (c) C(1s) core level spectra taken at 350 eV after various halogenated Ge surfaces were dipped in ODT solution for 72 h. All the curves in (a) are normalized to the height of the bulk peak to emphasize the peak shape difference. All the curves in (b) and (c) are normalized by the incident synchrotron radiation beam flux to stress the peak intensity difference.

to Ge2þ(S) species (Ge dithiolate) can also be expected if there were such sites available at the Ge(111) surface after chlorination. However, no dichloride sites were observed on the initially Clterminated Ge(111) surface.23,36 Complicating the assignment, we have also observed residual chlorine bonded to the surface after 8424 DOI: 10.1021/la904864c

thiolation (see the Cl(2p) core level spectra supplied in the Supporting Information). From that photoemission data, the coverage of Cl at the Ge(111) surface is estimated as ∼0.2 ML after thiolation. A 0.6 eV Ge(3d) shift has been reported for ∼1 ML Ge monochloride (Ge1þ(Cl)) generated on Ge(111) after HCl Langmuir 2010, 26(11), 8419–8429

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Table 1. Positions and Relative Intensities of the Fitted Peaks in the Ge(3d) Core Level Spectra from Figure 3a and the Species to Which They Are Assigned species

kinetic energy (eV)

intensity relative to the bulk Ge

Ge(lll)-Cl þ ODT bulk Ge Ge1þ(S) Ge1þ(C1) þ Ge2þ(S) Ge2þ(O) Ge3þ(O)

166.28 165.95 165.56 164.72 163.79

1.00 0.35 0.22 0.05 0.07

Ge(100)-Cl þ ODT bulk Ge Ge1þ(S) Ge1þ(C1) þ Ge2þ(S) Ge2þ(C1) Ge2þ(O) Ge3þ(O)

166.28 165.95 165.56 165.04 164.69 163.79

1.00 0.40 0.17 0.12 0.10 0.08

Ge(100)-Br þ ODT bulk Ge Ge1þ(S) Ge1þ(Br) þ Ge2þ(S) Ge2þ(Br) Ge2þ(O) Ge3þ(O)

166.28 165.93 165.58 165.18 164.73 163.82

1.00 0.40 0.16 0.09 0.11 0.09

treatment.23,36 Hence, the 0.72 eV shifted peak observed in Figure 3a after thiolation may stem from Ge monochloride and/or Ge dithiolate. The presence of dithiolate at Ge(111) surface would be interesting, since it suggests that thiolation can change the Ge(111) surface structures such that there are sites at the surface which accommodate two sulfur atoms per Ge atom. The presence of Ge-S bonds is further confirmed by analysis of the S(2p) peak shown in Figure 3b. This peak can be fit to three components. These peaks include one from surface thiolates, which contributes to the majority of the S peak as has been reported on metals46,57,58,76-79 and other semiconductors such as GaAs(001),49,51 as well as two peaks with chemical shifts of -0.95 and þ0.60 eV in kinetic energy (KE). The peak shifted by -0.95 eV corresponds to unbound thiols.49,57,78,79 The presence of unbound thiol at room temperature indicates presence of unreacted physisorbed ODT molecules (∼ 21% of the total S peak). Furthermore, our results do not show evidence of sulfonates (-SO3-) or sulfinates (-SO2-) at lower kinetic energies, which indicates that the thiolates have not been oxidized.46,74,80,81 The peak at þ0.60 eV in KE is assigned to atomic S or other forms of S.58,78,79,82 This “atomic sulfur” peak, which here contributes to only ∼5% of the total S peak, has been previously reported for alkanethiol/Au(111) systems. However, clear assignment of this peak has been the subject of debate in the literature, and its presence appears to depend on the experimental conditions and the chain length of the thiol molecule. In such systems, this peak has been observed during the initial stages of SAM growth, (76) Yang, Y. W.; Fan, L. J. Langmuir 2002, 18, 1157–1164. (77) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799–6806. (78) Vericat, C.; Vela, M. E.; Andreasen, G.; Salvarezza, R. C. Langmuir 2001, 17, 4919–4924. (79) Vericat, C.; Vela, M. E.; Benitez, G. A.; Martin Gago, J. A.; Torrelles, X.; Salvarezza, R. C. J. Phys.: Condens. Matter 2006, 18, R867–R900. (80) Whelan, C. M.; Malcolm, R. S.; Barnes, C. J. Langmuir 1999, 15, 116–126. (81) Willey, T. M.; Vance, A. L.; van Buuren, T.; Bostedt, C.; Terminello, L. J.; Fadley, C. S. Surf. Sci. 2005, 576, 188–196. (82) Chang, Z.; Tang, W. H. Surf. Sci. 2007, 2005–2011.

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in a low coverage SAM after annealing in air, and after vacuum annealing at higher temperatures.77,79 At room temperature, C-S bond scission and formation of sulfide are quite unlikely. Nevertheless, observation of this peak at room temperature has also been attributed to dilute atomic sulfur or other thiol molecules that do not undergo molecular decomposition (e.g., sp hybridized sulfur).77,79,83 Figure 3c shows that the C(1s) core level peak is asymmetric. Besides the alkyl carbon, a peak shifted 0.80 eV toward lower KE is necessary for good peak fitting. We attribute this small peak to C-S species based on the literature.74,84-86 Using this assignment, the ratio of the C-S carbon to the alkyl C is 1/21, which is slightly lower than the 1/17 value expected for a C18H37S-Ge species, suggesting that part of the bulk C signal may arise from adventitious materials. Although the general trends are the same, some differences are observed between the Ge(100) and Ge(111) surfaces. Analysis of the Ge(3d) peak after thiolation of halogenated Ge(100) (Figure 3a) reveals differences from that of Ge(111). In the case of ODT SAM on Cl-terminated Ge(100), the spectrum shows the presence of five peaks shifted to lower KE with respect to the bulk Ge peak. These peaks are shifted by 0.33, 0.74, 1.24, 1.59, and 2.49 eV, and based on the previous arguments can be attributed to Ge monothiolate, Ge dithiolate and/or Ge monochloride, Ge dichloride (Ge2þ(Cl)),22,36 GeO, and Ge2O3, respectively (also see Table 1). As will be shown in the next sections, residual chemically bonded chlorine is present at the Ge(100) surface even after thiolation. In contrast to halogenation of Ge(111), previous studies have revealed the presence of di- and monohalides at Ge(100) after halogenation, with a higher concentration of dihalides compared to monohalides; thus, such sites are available for formation of dithiolates on Ge(100) after ODT dipping. Although, due to the limitation on the energy resolution, the 0.74 eV-shifted peak cannot be fully resolved; this peak may have contributions from both Ge monochloride and Ge dithiolate. Examination of the S(2p) peak (Figure 3b) shows the presence of similar sulfur species at the thiolated Ge(100):Cl surface as on the Ge(111):Cl surface with the following key differences. Overall, the total S and thiolate integrated S(2p) peak area is ∼2 times larger on the Ge(100) surface. In addition, the atomic sulfur contributes a larger fraction (∼12%) of the total S(2p) area. The larger thiolate signal suggests that, for the same dipping time, more thiols react at the Ge(100) surface compared to the Ge(111) surface. In fact, we have already reported that ODT SAMs formed at Ge(100) surfaces have higher water contact angles, thicknesses, and ambient stability than those formed at Ge(111) surfaces.36 The current results are consistent with those observations. This is also apparent in Figure 3c where the total C coverage is ∼1.4 times higher on Ge(100) compared to Ge(111) after thiolation. Observation of more Cl remaining after thiolation at the Ge(111):Cl surface (∼0.2 ML on Ge(111):Cl vs ∼0.1 ML on Ge(100): Cl; Supporting Information, Figures S2 and S3) further supports this argument (i.e., less Cl reacts at the Ge(111) after thiolation). Investigation of the ODT coated Ge(100):Br surface (Figure 3a) reveals a similar shaped Ge(3d) peak as for the thiolated Ge(100): Cl surface. The peak can be fit to a bulk Ge peak as well as five (83) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nakoi, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092–2096. (84) Brito, R.; Tremont, R.; Feliciano, O.; Cabrera, C. R. J. Electroanal. Chem. 2003, 540, 53–59. (85) Petrovykh, D. Y.; Kimura-Suda, H.; Opdahl, A.; Richter, L. J.; Tarlov, M. J.; Whitman, L. J. Langmuir 2006, 22, 2578–2587. (86) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp. Physical Electronics Division: Eden Prairie, MN, 1992.

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additional peaks shifted by 0.35, 0.70, 1.10, 1.55, and 2.46 eV with respect to bulk Ge (see Table 1). These shifts can be attributed to Ge monothiolate, Ge dithiolate and/or Ge monobromide (Ge1þ(Br)), Ge dibromide (Ge2þ(Br)),36,87 GeO, and Ge2O3, respectively. This is similar to ODT coated Ge(100):Cl, though the shifts due to residual bromine (∼0.3 ML is present after thiolation; see the Supporting Information, Figure S2) are expected to be smaller due to the lower electronegativity value for Br compared to Cl. Moreover, examination of the S(2p) feature (Figure 3b) for the thiolated Ge(100):Br surface shows the presence of similar species to that of thiolated Ge(100):Cl. The total integrated peak area for S(2p) is similar (only ∼1.1 times higher) to that on Ge(100):Cl. However, the ratio of unbound thiol to thiolate is ∼1.7 times lower on Ge(100):Br than on Ge(100):Cl. Also, the total amount of surface thiolates is similar at Ge(100):Cl and Ge(100):Br. The C photoemission data support the conclusion that a similar thiolated coverage is observed on Ge(100):Br as on Ge(100):Cl, with the data (Figure 3c) showing ∼1.2 times more total C on thiolated Ge(100):Cl compared to thiolated Ge(100):Br. Another interesting observation is that the C peak for the thiolated Ge(111):Cl is shifted by ∼0.3 eV to the higher KE side, with respect to the same peak that corresponds to thiolated Ge(100):Cl spectrum (Figure 3c). A peak shift, albeit smaller, is also seen for the thiolated Ge(100):Br surface. This effect has been previously attributed to differences in the SAM thickness88 (i.e., thicker SAMs are better insulators, and hence, they can better discharge the positive charge generated by the photoelectron emission). In fact, our ellipsometry measurements show that the thickness values of the ODT films formed at Ge(100):Cl, Ge(111): Cl, and Ge(100):Br surfaces are 18.6 ( 0.5, 8.3 ( 1.2, and 13.9 ( 0.9 A˚, respectively; hence, those shifts in the C(1s) peaks are consistent with the expected effect. Both the Ge(3d) photoemission data and the O(1s) photoemission data show the presence of a small amount of Ge oxides at ODT coated Ge(100) and Ge(111) surfaces formed from the halides. Although part of this can be attributed to Ge oxidation during SAM formation, we suspect that ambient exposure of the surfaces before characterization is another source of Ge oxidation. Nevertheless, both Ge halides and oxides are only minority adducts at the thiolated Ge surfaces. On the whole, as depicted in Figure 4, our data show that Ge monothiolate and possibly Ge dithiolate make up the majority of chemically bonded species on both Ge(111) and Ge(100) surfaces at room temperature. To summarize, the PES data indicate the following: after thiolation of halide-terminated germanium, both the Ge(100) and (111) surfaces contain monothiolates and possibly dithiolates. In addition, unbound thiol and atomic sulfur are also observed at the surface, together with residual halide. The observation of residual halide is consistent with the DFT results, which predict that the elimination reaction will not proceed to completion, leaving some halogen unreacted at the surface. 3.2.2. Thermal Stability. A series of studies was carried out to investigate the behavior of the thiolate SAMs upon thermal annealing. Figures 5-8 depict the results of photoemission measurements after stepwise vacuum annealing of the ODT SAMs formed on Cl-terminated Ge(100) (Ge(3d), S(2p), and C(1s) core level spectra of the ODT coated Br-terminated Ge(100) and Cl-terminated Ge(111) surfaces after vacuum annealing to 310 °C are shown in the Supporting Information, Figures S2 and (87) Sun, S. Germanium Surface Cleaning, Passivation, and Initial Oxidation. Ph.D. Thesis, Stanford University, 2007. (88) Ishida, T.; Nishida, N.; Tsuneda, S.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L1710–L1713.

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Figure 4. Molecular models representing Ge monothiolate (Ge1þ(S))

and Ge dithiolate (Ge2þ(S)) moieties after room temperature thiolation of halogenated Ge(100) and Ge(111) surfaces.

S3). The following analysis of the temperature-dependent data leads to these conclusions about the thermal stability and decomposition pathways of the thiolate SAMs at the Ge surface: (1) the thiolate SAMs are stable to nearly 150 °C; (2) decomposition of the SAMs at temperatures between 180 and 350 °C leads to formation of surface carbide and sulfide; (3) above 430 °C, the surface sulfide is completely removed but the carbide, plus a small amount of oxygen bound to carbon remains; (4) residual chlorine remaining on the surface after thiolation of the Cl-terminated surface desorbs completely by 280 °C. The data in Figures 5-7 indicate that the majority of the SAM molecules are stable to at least 150 °C. Changes in the Ge(3d) peak, as well as in the C(1s) and S(2p) peaks, are relatively minor up until this temperature. In addition, the detailed evolution of the S(2p) peak upon vacuum annealing (Figure 6) indicates that the thiolate species are stable until above 150 °C. According to Figure 6, the unbound thiol peak decreases monotonically up to 180 °C, before completely disappearing upon 280 °C anneal. At the same time, the component that corresponds to Ge thiolates increases by a factor of ∼1.4 after annealing to 150 °C. By 150 °C, the ratio of the unbound thiol to thiolates drops to 73% of that at room temperature. Consequently, we propose that the increase in the thiolate signal can be partly related to conversion of unbound thiol species to surface thiolates as the temperature is raised. At even higher temperatures (above 150 °C), the intensity of the thiolate S(2p) peak begins to decrease, but it is not completely removed until 430 °C when all other S components desorb from the Ge(100) surface. Consequently, these results show that Ge thiolates are stable at the Cl-terminated Ge(100) surface up to at least 150 °C. Minor changes can be seen in other SR-PES spectra at temperatures below 180 °C. For example, Figure 7 shows that the C(1s) peak at 61.1 eV corresponding to the alkyl chains (-C-C-) is broadened and shifted to higher KE after vacuum annealing. This peak broadening and shifting has been previously attributed to structural changes in the alkyl chains and to an electrostatic screening effect.83 Park et al. pointed out that the shift in the C peak is due to changes in the polarizability of the SAMs after structural changes.89 After the SAMs are annealed to higher temperatures, they become poorer insulators and therefore, the damaged SAMs experience less charging compared to pristine SAMs.83 Overall, this peak is shifted by ∼0.3 eV to higher KE at 280 °C compared to room temperature. More significant spectral changes are observed beginning at temperatures of 280 °C and higher. As will be described below, these changes indicate decomposition of the SAMs with concomitant formation of surface sulfides and carbides. According to Figure 5, the ratio of the S- as well as O-induced components of (89) Park, J.-S.; Nguyen, A.; Barriet, D.; Shon, Y.-S.; Randall Lee, T. Langmuir 2005, 21, 2902–2911.

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Figure 5. Ge(3d) core level spectra taken at 200 eV photon energy from a thiolated Ge(100):Cl surface after step by step vacuum annealing. The substrate was prepared by dipping Cl-terminated Ge(100) surface in ODT solution for 72 h. All the curves are normalized to the height of the bulk peak to emphasize the peak shape difference.

the Ge(3d) peak relative to bulk Ge decreases, and this occurs in conjunction with a significant decrease in the S(2p) thiolate and the O(1s) Ge oxide peaks (Figure 6; oxygen data are compiled in Supporting Information, Figure S1). A peak with -0.32 eV shift is the only feature observed on the lower KE side after annealing to 350 °C; this can be attributed to Ge with an oxidation state of 1 attached to S (i.e., Ge thiolate and/or Ge sufide). After 430 °C vacuum annealing, peak fitting shows the presence of a small feature at lower KE, which is -0.37 eV shifted with respect to the bulk Ge value. Appearance of this peak is correlated with new peaks in the C(1s) (Figure 7) spectrum, assigned to surface carbide, as will be discussed below. Vilcarromero and Marques have reported various peak shifts in the Ge(3d) orbital as a result Langmuir 2010, 26(11), 8419–8429

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Figure 6. S(2p) core level spectra taken at 300 eV photon energy from a thiolated Ge(100):Cl surface after step by step vacuum annealing. The substrate was prepared by dipping Cl-terminated Ge(100) surface in ODT solution for 72 h. All the curves are normalized by the incident synchrotron radiation beam flux to stress the peak intensity difference.

of alloying with C.90 Based on that report and the change in the C(1s) spectrum, we attribute this peak shift to Ge1þ(C) (Ge carbide). This Ge(3d) peak associated with Ge carbide remains stable at the surface after vacuum annealing to above 470 °C. Another interesting change in the Ge(3d) spectrum is the appearance of two new components on the higher KE side of the bulk peak after vacuum annealing, appearing at temperatures as low as 60 °C. One has a chemical shift of ∼þ0.3 eV and the other has a chemical shift of þ0.54 eV. Clear assignment of these surface states has been the subject of some debate in the literature; however, both of these components are generally associated with surface state peaks and grow with the annealing temperature.44,87,91 Figure 7 shows major changes in the C(1s) SR-PES peak at temperatures above 280 °C, with a third peak (shifted by þ0.7 eV in KE) becoming dominant by 430 °C. The data reveal growth of (90) Vilcarromero, J.; Marques, F. C. Appl. Phys. A: Mater. Sci. Process. 2000, 70, 581–585. (91) Cao, R.; Yang, X.; Terry, J. Phys. Rev. B 1992, 45, 13749–13752.

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Figure 8. Cl(2p) core level spectra taken at 300 eV photon energy from a thiolated Ge(100):Cl surface after step by step vacuum annealing. The substrate was prepared by dipping Cl-terminated Ge(100) surface in ODT solution for 72 h. All the curves are normalized by the incident synchrotron radiation beam flux to stress the peak intensity difference.

Figure 7. C(1s) core level spectra taken at 350 eV photon energy from a thiolated Ge(100):Cl surface after step by step vacuum annealing. The substrate was prepared by dipping Cl-terminated Ge(100) surface in ODT solution for 72 h. All the curves are normalized by the incident synchrotron radiation beam flux to stress the peak intensity difference.

this third peak beginning as low as 150 °C. This peak can be attributed to Ge carbide arising from SAM decomposition, in agreement with the literature.90 In fact, Ishida et al. reported a 0.8 eV shift for this peak with respect to the alkyl C peak after ODT SAM on Au(111) was annealed to 200 °C under vacuum for 1 h, and they attributed this new peak to SAM decomposition as well as formation of a so-called “striped phase” in which ODT molecules lie mostly parallel to the surface.83 Based on this observation, the -0.32 eV shift observed in the Ge(3d) spectrum after 350 °C vacuum annealing (Figure 5) can be assigned at least partially to Ge carbides, although we cannot distinguish the Ge carbide and Ge sulfide peaks due to energy resolution limitations. 8428 DOI: 10.1021/la904864c

In the C(1s) spectrum, carbide becomes the dominant feature after 430 °C and this peak is stable even after annealing to above 470 °C. The spectrum also shows the presence of a new C feature (-1.3 eV in KE) which contributes to only ∼2% of the total C and can be attributed to oxidized C.86 Finally, Figure 7 also shows that the (-C-S-) peak disappears after annealing to 430 °C, which is consistent with removal of S from the Ge(100) surface (Figure 6). Finally, examination of the S(2p) peak upon annealing to higher temperatures reveals significant growth of a third component at higher KE, which cannot be attributed to either unbound thiol or thiolate. Rather, this peak is assigned to surface sulfide. A higher KE (lower binding energy) peak has also been previously reported when fully covered ODT SAMs on Au(111) were annealed to 200 °C under vacuum for 1 h, and assigned to atomic sulfur as a result of C-S cleavage.83 We believe that after annealing to above room temperature this peak can be better assigned to Ge sulfide. In fact, this peak constitutes the majority of the S species on the Ge surface after annealing to above 180 °C due to cleavage of the C-S bond. Annealing to 350 °C results in a decrease in the Ge sulfide S(2p) peak, followed by complete loss by 430 °C. This is in contrast to the surface carbide, which is still present on the surface even at 430 °C.92 (92) It is also important to note that irradiated induced damage to the SAMs has been reported as the result of synchrotron radiation exposure. In fact, Ulman and coworkers reported appearance and growth of þ1.2 eV in binding energy (-1.2 in KE) shifted peak (with respect to surface thiolates) after 1-alkanethiolate SAMs on Au(111) were exposed to synchrotron radiations and S(2p) core level was measured at ∼200 eV photon energy. ( Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8–11.) Such a shift was attributed to dialkyl sulfide moieties in the form of C 3 3 3 S-C. Such an effect is not expected to be the case in our study, since neither such peak shift value nor growth of such peaks was observed in the S(2p) core level scans.

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The behavior of chlorine at the Ge(100) surface as a function of annealing temperature is also of interest. The Cl(2p) PES spectra of Figure 8 indicate that residual chlorine is clearly present on the Cl-terminated Ge(100) surface even after 72 h exposure to ODT solution and the formation of the thiolate SAM. This chlorine remains on the surface until above 180 °C. After annealing to 280 °C, the peaks corresponding to Ge chlorides disappear, in agreement with desorption of the residual Cl from the Ge(100) surface.87 Investigation of the ODT coated Ge(100):Br surface after vacuum annealing to 310 °C shows similar trends in the core level spectra as seen for ODT coated Ge(100):Cl. As evident in Figure S2 (Supporting Information), formation of Ge carbide and Ge sulfide is observed at higher temperatures, as well as desorption of the bromine. Moreover, the studies of ODT coated Ge(111):Cl after annealing to 310 °C show similar trends, too. Core level spectra of this system upon annealing (Supporting Information, Figure S3) reveal Ge sulfide and Ge carbide formation and desorption of chlorine. However, we note the following differences: Overall, the PES signal from Ge sulfide formed on Ge(111) is lower than that on Ge(100), which is consistent with lower S reaction at the Ge(111) surface at any temperature (section 3.2.1). Moreover, the surface states in the Ge(3d) core level spectra of the SAM-covered Ge(111) surface have chemical shifts of ∼0.3 and 0.65 eV to higher KE, compared to ∼0.3 and 0.54 eV on Ge(100), which suggest a complex reconstruction different at this surface than on Ge(100). However, clear assignment of these peaks has been debated.93 The results of our study reveal very good stability of the thiolate SAMS, up to temperatures as high as 150 °C. Maboudian and coworkers have studied the thermal stability of the ODT monolayer formed on H-terminated Ge(111) surfaces with high-resolution electron energy loss spectroscopy (HREELS).42 That study suggested no changes in the ODT monolayer after annealing to ∼76 °C, whereas annealing to ∼176 and ∼276 °C resulted in partial and complete desorption of ODT monolayer from Ge(111) surface, respectively. Moreover, no S was observed on the surface after ∼276 °C annealing.42 Apart from uncertainty on the exact temperature values reported herein ((20 °C), our results show clear evidence of Ge-S and S after vacuum annealing to above 310 °C.

4. Conclusions Various characterization techniques together with DFT are employed to study the reaction mechanism, surface bonding, and thermal stability of 1-octadecanethiolate self-assembled monolayers (SAMs) at Cl- and Br- terminated Ge surfaces. (93) Sieger, M. T.; Roesler, J. M.; Lin, D. S.; Miller, T.; Chiang, T.-C. Phys. Rev. Lett. 1994, 73, 3117–3120.

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Our calculations show that hydrohalogenic acid elimination reactions are kinetically favorable at these surfaces, with thermodynamically more favorable reactions occurring at Cl-terminated Ge surfaces. However, these reactions are endothermic, which suggests that removal of the products should facilitate these reactions at room temperature. The calculations also show that alternative reactions such as dimer cleavage or insertion are kinetically unfavorable and are not expected to be observed. Finally, the theoretical studies predict that residual halide concentration may be present at the surface after thiolation due to incomplete reaction. SR-PES results at room temperature indicate the presence of residual Ge halide and Ge oxide at the surface after thiolation. However, after ODT SAM formation on halogenated Ge(100) and Ge(111), both surfaces are mainly covered by monothiolates and possibly dithiolates as well as unbound thiol and a small amount of atomic sulfur or other thiols without C-S bond cleavage. Higher levels of sulfur and carbon are detected at the Ge(100) surface after thiolation, which indicates higher conversion of surface halides to surface thiolates on Ge(100). Vacuum annealing studies show that the Ge thiolates are thermally stable up to 150 °C. Furthermore, the majority of the surface thiolates are converted to surface sulfide and carbide upon annealing to 350 °C on both Ge(100) and Ge(111) surfaces. No sulfur is observed at the surface at 430 °C, whereas Ge carbide is stable to above 470 °C. Acknowledgment. The authors are indebted to Dr. S. Sun, Dr. J. S. King, Dr. A. Paul, Dr. Z. Zhang, and Dr. A. Mukhopadhyay for insightful comments. We also thank the staff of the Center for Polymer Interfaces and Macromolecular Assemblies (CPIMA) and SSRL for their support. This work was supported by the National Science Foundation [CHE 0615087 and CHE 0910717]. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. Supporting Information Available: Cl and Br coverage calculation at the thiolated Ge surfaces; O(1s) core level spectra of a thiolated Ge(100):Cl surface after step by step vacuum annealing; Ge(3d), S(2p), C(1s), Cl(2p), and Br(3d) spectra of thiolated Ge(100):Br and Ge(111):Cl surfaces at room temperature and after 310 °C vacuum annealing. This material is available free of charge via the Internet at http:// pubs.acs.org.

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