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Hydrogermylation of Alkenes and Alkynes on Hydride-Terminated Ge(100) Surfaces Kwangwook Choi and Jillian M. Buriak* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received March 17, 2000. In Final Form: June 20, 2000 Covalently bonded organic monolayers on semiconductor surfaces are potentially important for the fabrication of novel electronic devices and sensors. In contrast to the recent interest in and development of methods for monolayer formation on silicon surfaces, only one wet chemical route has been previously published for germanium surfaces. We present novel strategies to prepare organic monolayers on hydrideterminated Ge(100) surfaces utilizing the available Ge-H bonds as chemical handles. The new hydrideterminated Ge(100) surface was prepared by utilizing an efficient preparation method involving soaking in an aqueous 10% HF solution. Lewis acid mediated hydrogermylation of alkynes and alkenes on the hydride-terminated Ge(100) surfaces results in alkenyl and alkyl surfaces, respectively, bound through Ge-C bonds. Thermal treatment of hydride-terminated Ge surface with neat alkenes and alkynes, or solutions in mesitylene, also results in similar organic monolayers. Finally, these two approaches were contrasted with UV photoinduced hydrogermylation of alkenes. The resulting organic monolayers were characterized by infrared spectroscopy (ATR-FTIR), stability studies, and contact angle measurements.
Introduction The preparation of organic monolayers on semiconductor surfaces has recently become an area of intense investigation because of their myriad of potential applications.1 While the chemistry of silicon surfaces, both porous and single crystal, is presently being developed, little is known about its congener germanium. Because monolayers on silicon bound through Si-C bonds are of special interest due to their stability and ability to be tailored for specific applications, we decided to extend this approach to germanium. While our interest is fundamental, with the desire to illuminate the surface chemistry of this poorly studied material, the results outlined here are useful for future devices employing germanium. In contrast to the recent interest in and development of methods for monolayer formation on Si surfaces, only one route has been previously published for preparation of organic monolayers on Ge surfaces through wet chemistry. This approach, reported by Cullen et. al in 1962, results in apparent Ge-C bond formation on crystalline germanium surfaces.2 By chlorination of the surface to form Ge-Cl bonds and then reaction with an ethyl Grignard, the resulting surface was decorated with ethyl groups. This approach has been reinvestigated recently using a modified chlorination strategy by Wayner and co-workers.3 The resulting organic monolayers consist of stable, alkyl chains covalently bounded to a Ge(111) surface. The reaction conditions require elevated temperatures (60-80 °C) and extended reaction times (6 h-7 (1) (a) Ulman, A. Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (b) Sze, S. M. The Physics of Semiconductor Devices, 2nd ed.; Wiley: New York, 1981. (c) Buczkowski, A.; Radzimski, Z. J.; Rozgonyi, G. A.; Shimura, F. J. Appl. Phys. 1991, 69, 6495. (d) Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1998, 102, 1067-1070. (e) Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1998, 102, 4058-4060. (2) Cullen, G. W.; Amick, J. A.; Gerlich, D. J. Electrochem. Soc. 1962, 109, 124. Organic monolayers have been prepared recently on Ge surface under UHV conditions: (a) Teplyakov, A. V.; Noah, P.; Lal, Y. A.; Bent, S. F. J. Am. Chem. Soc. 1998, 120, 7377. (b) Hamers, R. J.; Hovis, J. S.; Greenlief, C. M.; Padowitz, D. F. Jpn. J. Appl. Phys. 1999, 38, 3879. (c) Lee, S. W.; Nelen, L. N.; Ihm, H.; Scoggins, T.; Greenlief, C. M. Surf. Sci. 1998, 410, L773. (3) He, J.; Lu, Z.-H.; Mitchell, S. A.; Wayner, D. D. M. J. Am. Chem. Soc. 1998, 120, 2660-2661.
days). In addition, the use of highly reactive organomagnesium or organolithium compounds for alkylation on semiconductor surfaces limits the array of functional groups that can be incorporated into the surface monolayer. Surface contamination by residual Mg or Li ions might also be problematic due to their potential influence on the performance of the modified semiconductor. We and others have found hydrosilylation of carboncarbon unsaturated bonds on hydride-terminated silicon surfaces to be a very useful functionalization strategy, allowing access to a broad range of monolayers.4,5 In this paper, we describe hydrogermylation chemistry on newly prepared GeHx-terminated flat (100) surfaces. Lewis acid mediated and thermally induced hydrogermylation of alkynes and alkenes on the hydride-terminated Ge(100) surfaces result in alkenyl and alkyl surfaces, respectively, as outlined in Scheme 1. On the basis of the stability of these monolayers and FTIR studies of the results of alkyne hydrogermylation, we conclude that the reactions result in the formation of Ge-C bonds. (4) For a review of Si-C bond formation on Si surfaces, see: Buriak, J. M. J. Chem. Soc., Chem. Commun. 1999, 1051. Leading references for hydrosilylation on flat single-crystal Si surfaces: (a) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1999, 15, 82888291. (b) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (c) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759-1768. (d) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164-6168. (e) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem Soc. 1993, 115, 12631-12632. (f) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831-3835. (g) Zazzera, L. A.; Evans, J. F.; Deruelle, M.; Tirrell, M.; Kessel, C. R.; Mckeown, P. J. Electrochem. Soc. 1997, 144, 2184-2189. (h) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. Engl. 1998, 37, 2462-2464. (i) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056. (j) Terry, J.; Mo, R.; Wigren, C.; Cao, R.; Mount, G.; Pianetta, P.; Linford, M. R.; Chidsey, C. E. D. Nucl. Instrum. Methods. Phys. Res. Sect. B 1997, 133, 94. (k) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (5) Hydrosilylation on porous silicon surfaces: (a) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 1339-1340. (b) Buriak, J. M.; Stewart, M. J.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491-11502. (c) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 3257. (d) Bateman, J. E., Eagling, R. D., Worrall, B. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2683.
10.1021/la000413d CCC: $19.00 © 2000 American Chemical Society Published on Web 08/24/2000
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Scheme 1
Figure 1. ATR-FTIR spectra of the ν(GeHx) stretching region of Ge(100) hydride terminated surface prepared by etching with an aqueous 10% HF solution: (a) after 2 min; (b) after 5 min; (c) after 10 min; (d) after 15 min.
Results and Discussion Hydride Termination. Hydride-terminated germanium surfaces were prepared by etching shards of (100) germanium wafers or a (100) parallelopiped Ge ATR crystal in an aqueous 10% HF solution for 1-10 min. The crystalline surfaces had been pretreated by immersion in 30% H2O2 for 10 s to remove trace organics, rendering a clean oxidized Ge surface before fluoride etching (see Scheme 1). The resulting GeHx surface (Ge-ATR crystal) was characterized by ATR-FTIR spectroscopy. The IR absorption spectrum, shown in Figure 1, of the hydrideterminated Ge sample showed the characteristic stretching vibration mode band (ν(GeHx)) at 2010 cm-1. The relatively broad vibration is most likely due to the presence of mono-, di-, and trihydride species. To the best of our knowledge, this is the only known report of a hydrideterminated flat germanium surface prepared via chemical etching; a GeHx-capped porous germanium sample was reported in 1995,6 and mono- (1990-1960 cm-1) and dihydride (2020 cm-1) terminated Ge surfaces were observed during an in-situ IR study of germanium electrodes in acidic electrolytes.7 The bond strength of the Ge-F bond (485 kJ/mol) is stronger than the Ge-H bond (322 kJ/mol),8 and thus on the basis of thermodynamic considerations, a fluoride terminated surface would be expected; like etching of Si surfaces with fluoride, however, the observation of atomic H on the surface indicates that termination of the Ge surface is largely determined by reaction kinetics rather than thermodynamics.9 The use of lower (2-5%) concentrations of aqueous HF resulted in only a very weak GeHx stretching intensity, while higher concentrations (25%) produced an intense GeHx vibration but with a roughened surface, as observed visually and by AFM. Optimum peak intensity is reached in 10 min with a 10% aqueous HF etching solution (Figure 1c). Prolonged etching with this HF solution resulted in a decreased intensity of the GeHx peak (Figure 1d). This observation indicates that the hydride-terminated Ge surface is not kinetically stable in under these etching (6) Miyazaki, S.; Sakamoto, K.; Shiba, K.; Horose, M. Thin Solid Films 1995, 255, 99-102. (7) Maroun, F.; Ozanam, F.; Chazalviel, J.-N. J. Phys. Chem. B 1999, 103, 5280-5288. (8) CRC Handbook of Chemistry and Physics, 74th ed.; Lied, D. R., Ed.; CRC Press: Boca Raton, FL, 1993. (9) Sailor, H. J.; Lee, E. J. Adv. Mater. 1997, 783.
conditions. The hydride terminated surface (10 min of etching with 10% HF) has a surface roughness on the order of 8-10 nm, as determined by AFM.10 The resulting hydride terminated surface is stable in air up to 1 h and can be easily manipulated in inert atmosphere for further functionalization. Lewis Acid Mediated Hydrogermylation. Since the Lewis acid EtAlCl2 induces room-temperature hydrosilylation of alkynes and alkenes on porous silicon surfaces,5a,b identical conditions were attempted on hydrideterminated flat germanium. With employment of a commercial hexanes solution of EtAlCl2, hydrogermylation of alkynes and alkenes takes place at room temperature in 1 and 12 h, respectively. It is of interest to note that Lewis acid mediated hydrosilylation of olefins on hydrideterminated Si(111) surfaces has been reported to require elevated temperatures and longer contact times.4f The ATR-FTIR spectrum (Figure 2a,b) of the resulting Ge surface formed through aliphatic alkyne hydrogermylation supports the formation of alkenyl layers through Ge-C bonds as evidenced by the loss of the ν(GeHx) (2010 cm-1) and the alkyne ν(CtC) stretch and appearance of new peaks which correspond to the alkenyl derivatization. Figure 2a,b shows the ATR-FTIR spectrum of 1-dodecenyl and 1-pentenyl Ge surfaces, respectively. In both spectra, the ν(CdC) stretching frequency is observed at 1594 cm-1 and methylene and methyl bending modes, δ(CH2) and δ(CH3), are observed at 1465 and 1379 cm-1, respectively. The relative intensity of ν(CHx) stretches clearly depends on the chain length of alkyl substituent on the alkyne. The observation of this relatively low energy ν(CdC) stretching frequency mode strongly suggests a surface bound vinyl group, tGesCdCR, as has been previously reported on porous silicon surface.5a-c,11 Further hydrogermylation of surface vinyl group may also occur, resulting in a bis(germyl)-substituted alkyl moiety.12 That the 1594 cm-1 vibration corresponds to a vinyl group was confirmed through hydroboration reduction (Scheme 2) with 1.0 M BH3‚THF in THF solution under inert atmosphere, followed by quenching in air.5a,b,13 As shown in Figure 3b, the disappearance of the olefinic stretch (1594 (10) Contact mode AFM studies indicate that the original commercial oxide terminated Ge crystal has a surface roughness on the order of 2-3 Å. (11) Holland, J. M.; Stewart, M. P.; Allen, M. J.; Buriak, J. M. J. Solid State Chem. 1999, 147, 251. (12) Robins, E. G.; Stewart, M. P.; Buriak, J. M. J. Chem. Soc., Chem. Commun. 1999, 2479. (13) Peter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic Press: San Diego, CA, 1988.
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Figure 3. ATR-FTIR spectra of (a) 1-dodecenyl terminated Ge(100) prepared by Lewis acid mediated hydrogermylation on the Ge(100)-H surface with 1-dodecyne and (b) hydroboration of the surface-bound vinyl group of the dodecenyl moiety with BH3‚THF. The insert shows the difference spectrum corresponding to before and after hydroboration.
cm-1) occurs with concomitant appearance of a new peak at 1337 cm-1 which corresponds to the B-O stretching frequency.14 Alkyl-terminated Ge surfaces may also be prepared through a similar route, but Lewis acid mediated alkene hydrogermylation requires longer reaction times (12 h at room temperature) due to their lower reactivity as compared to alkynes. Figure 2c,d shows the ATR-FTIR spectra of the resulting alkyl terminated surfaces. The dodecyl-terminated surface (Figure 2c) is similar to that of the dodecenyl surface (Figure 2a), with the exception of the absence of the vinyl stretch. The asymmetric and symmetric methylene stretching vibrations νas(CH2) and νs(CH2) appear at 2923 and 2853 cm-1, respectively. The
ν(CH2) vibrations are diagnostic of the packing of the long alkyl chains on the surface, with the methylene stretching vibration dropping from 2928 to 2920 cm-1 and from 2856 to 2850 cm-1 going from liquid to solid alkane.15 The ν(CH2) vibrations of the 1-dodecyl Ge surfaces appear at slightly higher energy than those reported for alkyl monolayers on an Si(100) surface (2921 and 2852 cm-1),4c which suggests somewhat less ordered alkyl layers on the Ge surface. As shown in Figure 2d, hydrogermylation of styrene yields a distinct IR spectrum with ν(Ar CH) at 3081, 3060, and 3025 cm-1. Other features in this ATRFTIR include ν(CH2) at 2923 and 2850 cm-1 and aromatic ring modes ν(Ar CdC) at 1600, 1492, and 1451 cm-1. Similar vibration frequencies for hydrosilylation of styrene on porous silicon surfaces, which also result in surfacebound phenethyl groups, have been previously reported.5c Thermally Induced Hydrogermylation. Olefin and alkyl monolayers on GeHx-terminated surfaces were prepared through the reaction of corresponding alkynes or alkenes at 200-220 °C. Thermally induced alkene hydrosilylation on flat single-crystal hydride-terminated Si surfaces has been demonstrated to be one of the most effective methods for formation of organic monolayers on this substrate.4a-e In some cases, even dilute solutions of 1-alkenes in various aromatic solvents such as mesitylene yield well-ordered monolayers of alkyl on Si surface.4a Thermally induced hydrogermylation of 1-dodecyne, 1-dodecene, and 1-hexadecene on hydride-terminated Ge surfaces was carried out both with neat alkyne/alkene and in a diluted solution in mesitylene. Both neat 1-dodecyne and a 25% (v/v) solution in mesitylene resulted in a dodecenyl monolayer on the Ge surface at 220 °C for 2 h. The features of ATR-FTIR spectra of thermally induced monolayers are very similar to the Lewis acid mediated monolayers (vide supra), with the exception of slightly lower intensities of ν(CHx) vibrations. In comparison, dilute solutions (5 and 33%, v/v in mesitylene) of 1-dodecene yield only trace incorporation (by ATR-FTIR) of organic layers on surface. Under these conditions with 1-dodecene, not only is an absence of ν(CHx) peaks observed but also the disappearance of ν(GeHx) vibrations, implying that higher temperatures and longer reaction times will
(14) Bellamy, L. J. The Infra-Red Spectra of Complex Molecules; Methuen and Co.: New York, 1960.
(15) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150.
Figure 2. ATR-FTIR spectra of alkenyl and alkyl monolayers on Ge(100) prepared by Lewis acid mediated hydrogermylation with (a) 1-dodecyne, (b) 1-pentyne, (c) 1-dodecene, and (d) styrene. Scheme 2
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Table 1. Water Contact Angle (in deg) Measurements of Organic Monolayers on Ge(100), Formed through Lewis Acid Mediated, Thermal or UV-Induced Hydrogermylation Reactions with Alkynes and Alkenes alkene/alkyne 1-hexadecene 1-hexadecene 1-hexadecene 1-dodecene 1-dodecene 1-dodecyne 1-dodecyne
hydrogermylation method Lewis acid thermal UV Lewis acid thermal Lewis acid thermal
Θa
Θr
95 106 105 95 102 97 95
87 97 92 84 91 88 86
not assist in the formation of alkyl monolayers because of the apparent instability of the Ge-H bonds at elevated temperature. The combination of lower concentrations of the less reactive alkene (as compared to alkyne) and the instable hydride-terminated Ge surfaces at elevated temperatures could be responsible for the failure of organic monolayer formation under these conditions. Well-ordered alkyl monolayers could be prepared, however, through use of neat 1-dodecene and 1-hexadecene at 220 °C for 2 h. The asymmetric and symmetric methylene stretching vibrations of surface-bound dodecyl groups appear at 2925 and 2855 cm-1 for the dodecyl surface and 2923 and 2853 cm-1 for the hexadecyl surface, respectively, again indicating moderate ordering of alkyl chain packing within the monolayer. Control experiments were performed on hydrideterminated Ge surfaces with neat alkynes and alkenes in the absence of EtAlCl2 at room temperature. Reaction times are 1 and 12 h for alkynes and alkenes, respectively. Low incorporation was observed as indicated by the appearance of weak ν(CHx) vibrations above 2800 cm-1. In addition, no Ge-H stretching peaks were observed after control experiments due to their in-situ degradation, perhaps resulting from reaction with trace oxygen, water, or other impurities and handling. Considering the close thermodynamic properties of Si-H (299 kJ/mol) and Ge-H bonds (322 kJ/mol), these observations indicate that Ge-H bonds are more kinetically labile than Si-H bonds.8 Comparisons Using Contact Angle Measurements. Measurements of water contact angles were carried out to assess the quality of monolayers (Table 1). Thermally induced hydrogermylation of 1-hexadecene yielded contact angles of 106°/97° (Θa/Θr), which are indicative of wellordered monolayers. These values are comparable to previous reports on Si surfaces.4a-d,h In comparison, the Lewis acid mediated route resulted in lower contact angles of 95°/87° (Θa/Θr). In general, thermally induced hydrogermylation of olefins resulted in higher contact angles than Lewis acid route which conflicts with the ATR FTIR data indicating more ordered monolayers via the Lewis acid route (lower νas(CH2)). The significantly lower contact angle achieved through Lewis acid mediated hydrogermylation suggests that the resulting surface might be contaminated by residual aluminum oxide due to hydrolysis of the EtAlCl2 which renders the surface more hydrophilic. Indeed, a broad vibration, albeit of low intensity, is often observed around 3300 cm-1, presumably due to ν(OH) from hydrated aluminum oxide. It is of interest that Lewis acid mediated hydrogermylation of 1-dodecene and 1-hexadecene gave similar contact angles, whereas longer hydrocarbon layers resulted in more ordered organic surface via the thermal route. The Lewis acid mediated and thermal hydrogermylations of 1-dodecyne gave contact angles of 97°/88 ° (Θa/Θr) and 95°/86° (Θa/Θr), respectively. Both advancing and receding angles are somewhat lower than the reported
Figure 4. ATR-FTIR spectra of the alkyl ν(CHx) stretching region of hexadecyl-terminated Ge(100) prepared by thermally induced hydrogermylation on the GeHx surface with 1-hexadecene: (a) as-prepared followed by rinsing with various organic solvents; (b) followed by rinsing with 1:1 EtOH/49% aqueous HF solution; (c) after step (b) followed by sonication in chloroform for 5 min at room temperature; (d) after step (c) followed by immersion in boiling water for 20 min; (e) after step (d) followed by immersion in boiling chloroform for 20 min.
literature values on alkyl Si surfaces4a-d and alkyl Ge surfaces mentioned above. Compared to oxide-coated Ge or a hydride-terminated Ge surface, however, the significant increase of contact angle indicates the formation of hydrophobic organic layers. Lower contact angles of alkenyl surfaces are understandable in light of the probable (Z) conformation of the surface vinyl group, formed through trans addition of the Ge-H moiety in an anti-Markovnikov fashion, as is observed on porous silicon, and with molecular silanes in solution.5b,16 The formation of alkyl monolayers by UV photoinduced hydrogermylation was also examined by contact angle measurement. UV photoinduced hydrogermylation of 1-hexadecene with irradiation of mercury lamp (254 nm) for 1 and 2 h resulted in contact angle of 101°/90° and 105°/92° (Θa/Θr), respectively. However, higher wavelengths (350 nm, 1 h) gave significantly lower contact angles, 87°/75° (Θa/Θr). UV induced hydrosilylation is known on Si(111) and Si(100) surfaces.4h-k Chemical Stability. Chemical stability of the resulting organic monolayers has been tested by IR studies. A thermally induced 1-hexadecyl surface was employed for stability tests. Figure 4 shows the series of ATR-FTIR spectra concentrating on the alkyl C-H stretching region. After completion of functionalization, the surfaces were intensively washed with dichloromethane, hexane, ethanol, and pentane before taking the spectra. As shown in Figure 4a,b, washing with 25% HF solution resulted in 20% decrease of the IR intensities. However, subsequent sonication in chloroform for 5 min (Figure 4c), followed by immersion in boiling water for 20 min (Figure 4d), and then by immersion in boiling chloroform for 20 min (Figure 4e) did not change the position and intensities of methylene stretching peaks. Therefore, the initial decrease of peak intensity is most likely due to removal of physisorbed hydrocarbon. The observed decrease may also be due to dissolution of spurious oxide, resulting in removal of neighboring alkyl-terminated areas. These stability test results show the significant chemical stability of alkyl monolayers prepared through the thermal route. In comparison, a Lewis acid mediated 1-dodecenyl surface is observed to undergo a 20% and 30% decrease of the IR (16) (a) Asao, N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61, 7654. (b) Sudo, T.; Asao, N.; Gevorgyan, V.; Yamamoto, Y. J. Org. Chem. 1999, 64, 2494.
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intensity of methylene stretches upon immersion in boiling water for 20 min and sonication in chloroform for 5 min, respectively. Complete removal of the organic layers occurs when the modified surfaces were immersed in 30% H2O2 in 4 min, in accordance with previous observations.3 Conclusions Novel strategies to prepare organic monolayers through hydrogermylation approaches on hydride-terminated Ge(100) surfaces via the hydride-terminated Ge surface have been described. Hydride-terminated Ge surfaces serve as a flexible intermediate for different hydrogermylation reaction conditions. Lewis acid mediated and thermally induced as well as UV promoted hydrogermylation of olefins and alkynes on hydride-terminated Ge(100) surfaces results in alkyl and alkenyl surfaces, respectively. Resulting alkyl monolayers showed significant chemical stability. Experimental Section Germanium wafers (Ge(100), n-type, 200 µm thickness, 0.0010.02 Ω‚cm) were purchased from Waferworld. Ge(100) parallelepiped plates (45°, 50 mm × 10 mm × 3 mm and 50 mm × 10 mm × 2 mm) were purchased from Harrick. All chemicals were reagent grade from Acros. All alkenes, alkynes, and mesitylene (Acros, 99%) were distilled at reduced pressure and stored in inert atmosphere. Germanium surfaces were oxidized using 30% H2O2 for 10 s and followed by a water rinse, followed by placing the samples in HF solution (10% in water) for 1-10 min, and finally followed by an ethanol/pentane rinse to produce the hydride termination. ATR-FTIR spectroscopy was carried out using a Nicolet Nexus 670 spectrometer with MCT detector at 4 cm-1 resolution. The infrared light was incident to one of the 45° bevels of the ATR-plate. Background spectra were obtained using a freshly oxidized Ge surface. Water contact angle measurements were performed using a Rame-Hart 100 goniometer. Atomic force microscopy was carried out using a Digital Instruments Nanoscope IIIa in contact mode with silicon nitride tips. Lewis Acid Mediated Hydrogermylation. A freshly prepared hydride-terminated Ge(100) surface was brought into a nitrogen filled glovebox, and a 1.0 M hexane solution of EtAlCl2 (Aldrich) was dropped onto the wafer or ATR crystal surface
Langmuir, Vol. 16, No. 20, 2000 7741 with a microliter syringe, followed by addition of the alkyne or alkene at room temperature. The actual amounts of the Lewis acid and substrates utilized was dependent on the surface size: 50 µL of the 1.0 M hexane solution of EtAlCl2 and 100 µL of the alkyne or alkene were used for Ge wafer shards (
