Corrosion Resistance of Thiol- and Alkene-Passivated Germanium

May 18, 2010 - Xiaotang Lu , Yang He , Scott X. Mao , Chong-min Wang , and Brian A. Korgel. The Journal of Physical Chemistry C 2016 120 (50), 28825- ...
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3698 Chem. Mater. 2010, 22, 3698–3703 DOI:10.1021/cm1005696

Corrosion Resistance of Thiol- and Alkene-Passivated Germanium Nanowires Vincent C. Holmberg and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712 Received February 24, 2010. Revised Manuscript Received April 25, 2010

The corrosion resistance of organic monolayer-coated germanium (Ge) nanowires was tested by exposure to various harsh chemical treatments, including boiling water, concentrated sulfuric acid, and hydrogen peroxide. The Ge nanowires were grown by the supercritical fluid-liquid-solid (SFLS) process with in situ thermal hydrogermylation or thiolation, with alkyl monolayer coatings of varying chain length, from 6 to 18 C atoms. All of these nanowires exhibited almost-imperceptible surface oxidation in air or water, and degraded only when exposed to the harshest oxidative conditions. Nanowires with monolayers attached through Ge-C and Ge-S bonds exhibited similar corrosion resistance, except when exposed to peroxide, in which case the alkylated nanowires were more resistant to oxidation. Introduction Semiconductor nanowires have enormous surface area, which leads to significantly enhanced surface reactivity, compared to bulk crystals.1-4 Therefore, surface passivation is vital for most nanowire applications, particularly in cases when nanowires are exposed to corrosive environments. Obviously, nanowire degradation must be prevented, but even the slightest surface oxidation can also cause problems, as in the case of electronic nanowire devices such as field effect transistors (FETs).5-7 The study herein focuses on organic monolayer passivated germanium (Ge) nanowires. Ge nanowires can be made in industrially relevant quantities via chemical methods such as the supercritical fluid-liquid-solid (SFLS) process, and dispersed in solvents as an “ink”; these nanowire dispersions represent a semiconductor material that can be printed onto a wide variety of supports, including mechanically flexible lightweight plastics using continuous, low-cost, roll-to-rolltype processes. Therefore, chemically made semiconductor nanowires have been proposed for use in many electronic and optoelectronic devices, including thin-film transistors *Author to whom correspondence should be addressed. Tel.: 512-4715633. Fax: 512-471-7060. E-mail: [email protected].

(1) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2004, 126, 15466. (2) Wang, D.; Chang, Y.-L.; Liu, Z.; Dai, H. J. Am. Chem. Soc. 2005, 127, 11871. (3) Adhikari, H.; McIntyre, P. C.; Sun, S.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 2005, 87, 263109. (4) Bashouti, M. Y.; Stelzner, T.; Berger, A.; Christiansen, S.; Haick, H. J. Phys. Chem. C 2008, 112, 19168. (5) Wang, D.; Chang, Y.-L.; Wang, Q.; Cao, J.; Farmer, D. B.; Gordon, R. G.; Dai, H. J. Am. Chem. Soc. 2004, 126, 11602. (6) Hanrath, T.; Korgel, B. A. J. Phys. Chem. B 2005, 109, 5518. (7) Haick, H.; Hurley, P. T.; Hochbaum, A. I.; Yang, P.; Lewis, N. S. J. Am. Chem. Soc. 2006, 128, 8890. (8) Cui, Y.; Zhong, Z.; Wang, D.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 149. (9) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (10) Koto, M.; Leu, P. W.; McIntyre, P. C. J. Electrochem. Soc. 2009, 156, K11.

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(TFTs), chemical sensors, photovoltaics (PVs), and optical detectors.5-13 One problem with germanium is that it is very susceptible to oxidative corrosion; for example, Ge nanowires with bare surfaces dissolve in water within only a couple of hours.1 Only brief exposure of Ge nanowires to air leads to an electrically defective suboxide that significantly degrades nanowire performance.5-8 To prevent surface oxidation, Ge nanowires can be coated with organic monolayers and improve device (transistor) performance and enhance corrosion resistance.2,3,5-8 Self-assembled monolayer chemistry has been extensively studied on well-defined monolithic bulk crystal surfaces; however, there have been few systematic studies of surface oxidation and corrosion resistance of organic monolayercoated semiconductor nanowires. On bulk silicon and germanium surfaces, hydrosilylation and hydrogermylation reactions are well-understood,14-20 and thiol passivation of germanium has been accomplished.20-23 These passivation schemes commonly rely on an initial chemical (11) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885. (12) Kelzenberg, M. D.; Turner-Evans, D. B.; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Lewis, N. S.; Atwater, H. A. Nano Lett. 2008, 8, 710. (13) Cao, L.; White, J. S.; Park, J.-S.; Schuller, J. A.; Clemens, B. M.; Brongersma, M. L. Nature Mat. 2009, 8, 643. (14) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (15) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (16) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164. (17) Choi, K.; Buriak, J. M. Langmuir 2000, 16, 7737. (18) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudh€ olter, E. J. R. Adv. Mater. 2000, 12, 1457. (19) Buriak, J. M. Chem. Rev. 2002, 102, 1271. (20) Loscutoff, P. W.; Bent, S. F. Annu. Rev. Phys. Chem. 2006, 57, 467. (21) Han, S. M.; Ashurst, W. R.; Carraro, C.; Maboudian, R. J. Am. Chem. Soc. 2001, 123, 2422. (22) Kosuri, M. R.; Cone, R.; Li, Q.; Han, S. M.; Bunker, B. C.; Mayer, T. M. Langmuir 2004, 20, 835. (23) Ardalan, P.; Musgrave, C. B.; Bent, S. F. Langmuir 2009, 25, 2013.

Published on Web 05/18/2010

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Figure 1. Schematic of the supercritical fluid-liquid-solid (SFLS) nanowire growth reactor system. A high-performance liquid chromatography (HPLC) pump is used to pressurize the system and control the flow rate. The pressure is manually fine-tuned to achieve a steady-state condition by adjusting the micrometering valve. Precursors and other reagents are introduced via the ports on the six-way valve. The reactor temperature is maintained using a heating block connected to a variable voltage source coupled with a temperature controller.

priming of the semiconductor surface with an etchant, such as HF, NH4F,19,20 HCl, or HBr,19,20,23 to remove surface oxide and provide either a hydrogen-terminated surface that can be reacted via Lewis-acid-mediated, photochemical, or thermally activated hydrogermylation reactions,19,20 or a halogen-terminated surface that reacts with alkyl Grignard reagents.23,24 For nanowires, surface passivation chemistry requiring multiple steps is not desirable, because it adds processing steps and cost to the materials. Furthermore, surface etching chemistry is not easily applied to nanowires, because the corrosive chemicals involved can rapidly etch the semiconductor surface and degrade the nanowires; they also generate a lot of waste. We have shown that Ge nanowires grown via SFLS can be passivated directly in the reactor by thermal hydrogermylation or thiolation, yielding monolayerterminated Ge nanowires with uniform, complete surface coverage, and very good oxidation resistance with exposure to air and water.1 Herein, we report the chemical stability of hydrogermylated and thiolated Ge nanowires passivated with monolayers of varying chain length, subjected to a wide range of harsh chemical environments.

Materials. Anhydrous benzene (99.8%) was purchased from Sigma-Aldrich, and diphenylgermane (DPG) was purchased from Gelest. Gold (Au) nanocrystals 2 nm in diameter, capped with 1-dodecanethiol (Aldrich, g98%), were prepared in deionized water and toluene (Sigma-Aldrich, g99.5%), using the method of Brust et al.25 Hydrogen tetrachloroaurate(III)

trihydrate (g99.9%), tetraoctylammonium bromide (98%), and sodium borohydride (g98.0%) were purchased from Aldrich. 1-Hexene (g99%), 1-dodecene (95%), 1-octadecene (90%), 1-hexanethiol (95%), 1-dodecanethiol (g98%), and 1-octadecanethiol (98%) were also purchased from Aldrich. Synthesis of Germanium Nanowires. Ge nanowires were synthesized by the supercritical fluid-liquid-solid (SFLS) method using 2-nm-diameter Au nanocrystals and DPG in supercritical benzene.26,27 Typically, a 30 mL solution that contained 16 mg/L Au nanocrystals and 34 mM DPG in anhydrous benzene was prepared in a glovebox. Meanwhile, a 10-mL titanium tubular reactor sealed in a nitrogen atmosphere was heated to 380 °C and pressurized to 6.2 MPa with anhydrous benzene using a high-performance liquid chromatography (HPLC) pump (see Figure 1).26 After the system had equilibrated, the solution of DPG and Au nanocrystals was injected into the reactor at a rate of 0.5 mL/min for 40 min. During this injection period, the exit stream was continually adjusted to maintain a reactor pressure of 6.2 MPa. [Careful control of the pressure is extremely important to avoid overpressurizing the reactor. The system was fitted with a rupture disk as an additional safety precaution.] Immediately after injection, the reactor was sealed and isochorically cooled to the desired temperature for surface passivation. Nanowires were exposed to alkenes at 220 °C and thiols at 80 °C. During the cooling process, a solution of 8 mL of anhydrous benzene and 4 mL of alkene (or thiol) was prepared in a glovebox. After the temperature stabilized, 10 mL of alkene or thiol solution was injected into the reactor with the exit valve closed until the pressure was increased back to 6.2 MPa. After complete injection of the reactant, the reactor was sealed by closing the inlet and exit valves and held at 220 °C (for alkene passivation) or 80 °C (for thiol passivation) for 2 h. The 6.2 MPa overpressure helps to ensure that no oxygen leaks into the

(24) Webb, L. J.; Lewis, N. S. J. Phys. Chem. B 2003, 107, 5404. (25) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Am. Chem. Soc., Chem. Commun. 1994, 7, 801.

(26) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2002, 124, 1424. (27) Hanrath, T.; Korgel, B. A. Adv. Mater. 2003, 15, 437.

Experimental Details

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reactor during the hydrogermylation/thiolation process. The reactor was then allowed to cool to room temperature. The contents of the reactor were transferred to a vial using a glass Pasteur pipet, and the nanowires were then isolated by decanting off the reactant solution. The nanowire product was purified by redispersion in a 2:1:1 volume mixture of chloroform, toluene, and ethanol, followed by centrifugation at 8000 rpm for 5 min. This washing procedure was repeated two more times to remove excess passivating ligand and unreacted phenylgermane. The nanowires were then dried under vacuum and stored in air under ambient conditions. Ge nanowires hydrogermylated with 1-hexene, 1-dodecene, and 1-octadecene and thiolated with 1-hexanethiol, 1-dodecanethiol, and 1-octadecanethiol were prepared. As a control, an unpassivated sample was also prepared by injecting a solution of anhydrous benzene that contained no alkene or thiol during the passivation step. Materials Characterization. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos AXIS Ultra DLD spectrometer. Spectra were recorded using a monochromatic aluminum source, in conjunction with a charge neutralizer, to mitigate sample charging. Substrates were prepared by depositing a 5-nm adhesion layer of chromium, followed by a 60-nmthick layer of gold on top of silicon substrates. Roughly, 1 mg of nanowires was placed on a gold-coated silicon wafer and the oxidation studies were conducted directly on the XPS substrates. Scanning electron microscopy (SEM) images were captured with a Zeiss Model SUPRA 40 VP SEM system operated at 5.0 kV. Transmission electron microscopy (TEM) images were captured using a JEOL Model 2010F TEM system operated at 200 kV. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were recorded using a Thermo Mattson Infinity Gold FTIR spectrometer. Dispersions of Ge nanowires in chloroform were drop cast onto the surface of a zinc selenide ATR crystal in a Spectra-Tech Thermal ARK module and allowed to dry, forming a thin nanowire film. The spectrometer chamber was purged with nitrogen, and IR spectra were recorded from the thin films.

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Figure 2. (A) SEM and (B) TEM images of SFLS-grown Ge nanowires without surface passivation, along with XPS of the nanowires measured (C) immediately after the reaction and (D) after exposure to air for one week. Data points represented by black circles were fit (black line) by adding separate peak contributions from the Geo 3d5/2, Geo 3d3/2, Ge1þ, Ge2þ, Ge3þ, and Ge4þ signals shown in color. The 3d5/2 and 3d3/2 peaks were fit with Voigt profiles centered at 29.4 and 30.0 eV, respectively, and the intensity ratio was held at 3:2, corresponding to the spin-orbit splitting ratio for d-orbitals.28 The Ge1þ, Ge2þ, Ge3þ, and Ge4þ peak contributions were fit using Voigt profiles, which include contributions from both the 3d5/2 and 3d3/2 energy states, centered at 30.25, 31.1, 31.95, and 32.8 eV, respectively.1

Results and Discussion Germanium Nanowires without Surface Passivation. Ge nanowires made without a surface passivation layer are very susceptible to oxidation. Figure 2 shows SEM and TEM images and XPS data of Ge nanowires synthesized without surface passivation. Initially, the nanowires exhibit only a minute oxide signal, because of the presence of a thin native oxide layer that forms during sample transfer. This thin subnanometer native oxide layer is visible by TEM (Figure 2B). After one week of air exposure, a large oxide signal has appeared in the XPS data. The large shoulder on the Ge 3d peak at higher binding energy in Figure 2D corresponds to oxidized Ge species. Thiol- and Alkene-Passivated Germanium Nanowires. Figures 3 and 4 show SEM images of Ge nanowires synthesized by SFLS growth and passivated with alkenes and thiols with alkyl chain lengths of either 6, 12, or 18. Figure 5 illustrates the thermally induced hydrogermylation and thiolation reactions used to passivate the Ge nanowires in this study. We are able to achieve chemically stable alkyl- and thiol-passivated Ge surfaces in situ, using a single one-component injection step, without halogenating

Figure 3. SEM images of Ge nanowires passivated with thiols exposed to (A-C) air (1 week), (D-F) boiling H2O (1 h), (G-I) 2.5 M H2SO4 in dioxane (1 h), and (J-L) 30% H2O2 (1 h).

the surface. This process minimizes exposure of the nanowires to air prior to surface passivation, and, as shown in Figures 3 and 4, the surface passivation step does not affect the nanowire morphology.

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Figure 6. ATR-FTIR spectra of SFLS-grown Ge nanowires passivated with thiols (red traces) or alkenes (black traces). The four vertical lines indicate the peak positions for liquid-like (dashed) and crystalline (solid) packing. Table 1. Symmetric and Asymmetric CH2 Stretching Peak Positions

Figure 4. SEM images of Ge nanowires passivated with alkenes exposed to (A-C) air (1 week), (D-F) boiling H2O (1 h), (G-I) 2.5 M H2SO4 in dioxane (1 h), and (J-L) 30% H2O2 (1 h).

Figure 5. (A) Thermal hydrogermylation reaction scheme. (B) Thiolation passivation scheme.

ATR-FTIR spectroscopy (Figure 6) confirmed that the nanowires were coated with alkyl ligands. Symmetric and asymmetric alkane CH2 stretches are visible at 2850 and 2920 cm-1, respectively, and the asymmetric methyl CH3 stretch is present at 2960 cm-1.24 The symmetric methyl CH3 stretch appears at ∼2870 cm-1 as a weak shoulder on the symmetric CH2 stretching peak, although it is less intense than the asymmetric CH3 stretch. Noticeably absent is the alkene =C-H stretch at 3080 cm-1 in the hydrogermylated samples, indicating that the double bond has reacted with the Ge surface and no longer exists. The spectra also confirm that all unreacted alkene has been washed away from the sample, leaving only the covalently bound monolayer. S-H stretches were not observed in the FTIR spectra of the thiolated nanowires. It is unlikely that the thiols remain protonated after adsorption to the Ge surface; however, the lack of thiol (28) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992.

1-octadecanethiol 1-dodecanethiol 1-hexanethiol 1-octadecene 1-dodecene 1-hexene

νa(CH2) (cm-1)

νs(CH2) (cm-1)

2918.0 2923.0 2923.5 2924.0 2922.5 2921.5

2849.0 2852.0 2853.0 2853.5 2853.0 2852.5

IR peaks is not definitive proof that this is the case, because C-S and C-S-H stretches have extremely weak absorption in IR spectra.29 As a result, thiol stretches were not observed and only the CH2 and CH3 stretches from the alkane chain were detected. The positions of the symmetric and asymmetric alkane stretches shown in Figure 6 and summarized in Table 1 varied slightly, indicating slight differences in molecular packing in the organic layer on the nanowires. For disordered liquid-like alkane chains, the symmetric and asymmetric CH2 stretches occur at 2855 and 2924 cm-1, respectively.30 Alkane chains with more ordered packing give rise to signals at lower vibrational frequency. In Figure 6, the symmetric and asymmetric CH2 stretches for the 1-octadecanethiol monolayer appear in positions corresponding to a fully crystalline alkane, with peaks occurring at 2849 and 2918 cm-1, respectively. This is because, of the eight different monolayers investigated, 1-octadecanethiol is the only monolayer that solidifies at room temperature. The difference between vibrational frequency of the liquid state and the ordered crystalline state is only ∼6 cm-1; nevertheless, slight shifts in the methylene peaks provide insight into the local environment within the monolayer. Examining the methylene shifts in the thiolated nanowire samples indicates that adsorbed 1-hexanethiol forms the least-ordered monolayer, with increasing packing order as the alkane chain length is increased. This trend is consistent with studies of alkanethiol monolayers on gold substrates, where decreasing the chain length increases the disorder of the (29) Coates, J. Interpretation of Infrared Spectra: A Practical Approach. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; Wiley: Chichester, U.K., 2000. (30) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.

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monolayer.30 It has been hypothesized that short chains adopt a less-extended, more-disordered conformation, because of weaker interchain interactions. Conversely, the hydrogermylated samples seem to follow the opposite trend, with the long-chain alkenes showing significantly more disorder than the shorter chains. Thiols may be able to rearrange and pack more easily on the germanium surface during the passivation step, because of a more labile Ge-S bond, whereas a stronger Ge-C bond may inhibit rearrangements on the surface. As a result, the steric hindrance from fixed long-chain alkenes may prevent high packing densities for strong interchain interactions to lead to increased order. Consequently, shortchain alkenes may be able to pack more tightly on the surface, leading to the observed trend in vibrational frequency. Even for a perfectly packed crystalline monolayer, it is impossible to passivate all of the Ge surface atoms, because of steric hindrance between adjacent molecules. Corrosion Resistance of Organic Monolayer-Coated Germanium Nanowires. The nanowires that were passivated with thiols and alkenes with C6, C12, and C18 chain lengths were subjected to various oxidizing and corrosive conditions, and the extent of surface oxidation was monitored by XPS (see Figure 7). The integrity of the nanowires was also checked by SEM (Figures 3 and 4). In contrast to the unpassivated Ge nanowires (Figure 2), the thiolated and hydrogermylated Ge nanowires exhibited no observable oxidation signal in the XPS after exposure to air for 1 week (see Figures 7A and 7B). The XPS profiles of both the thiol- and alkene-treated Ge nanowires remained unchanged after more than a month of air exposure. Both thiol- and alkene-treated Ge nanowires exhibited no detectable oxidation signal by XPS when soaked in deionized water for 1 h. This starkly contrasts the unpassivated Ge nanowires, which completely dissolve in water over the course of only a couple of hours.1 In fact, the passivated nanowires that were subjected to boiling water for 1 h also did not show any sign of oxidation in the XPS signal (see Figures 7C and 7D), with the exception of the shortest-chain alkene, 1-hexene, which showed a slight GeO2 peak. SEM examination (Figures 3D-3F and 4D-4F) of the nanowires confirmed that there was no change in nanowire morphology after immersion in boiling water. Ge nanowires without surface passivation dissolved in boiling water within a matter of minutes. The robust stability of organic monolayer-passivated Ge nanowires, when exposed to water, is explained in part by the hydrophobicity of the passivation layer, which helps prevent water from accessing the germanium surface. Passivated nanowires soaked in 2.5 M sulfuric acid for 1 h also showed no signs of surface oxidation. Again, the hydrophobicity of the capping layer prevents penetration of the aqueous sulfuric acid solution into the mesh of nanowires and helps prevent surface oxidation. When the aqueous sulfuric acid solution was replaced with an organic solution of 2.5 M sulfuric acid in p-dioxane, significant oxidation was observed after soaking for 1 h. GeO2 peaks

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Figure 7. XPS of Ge nanowires passivated with thiols (red) or alkenes (black) exposed to (A, B) air (1 week), (C, D) boiling H2O (1 h), (E, F) 2.5 M H2SO4 in dioxane (1 h), and (G, H) 30% H2O2 (1 h). Data points are shown for thiol and alkene ligands with 6, 12, and 18 C atoms.

become clearly visible in the XPS scans of all of the passivated nanowire samples (see Figures 7E and 7F) and SEM examination (see Figures 3G-3I and Figures 4G-4I) revealed extensive nanowire corrosion; the nanowires were largely destroyed and transformed into germanium particulates. Nanowires passivated with longer-chain alkenes and thiols were slightly more stable; however, all of the passivating ligands were ultimately ineffective with regard to protection against concentrated acid. Nanowires subjected to highly oxidative conditions by immersion in 30% hydrogen peroxide (H2O2) for 1 h also exhibited significant deterioration. H2O2 etches a bare germanium surface at a rate of 40 nm/min.23,31 XPS (Figures 7G and 7H) and SEM (Figures 3J-3L, 4J-4L) examination showed that all nanowire samples were significantly oxidized and corroded. Virtually all of the thiolated Ge nanowires converted to germanium oxide, while the hydrogermylated samples had only a minute amount of elemental germanium (Ge0) remaining in the sample. In the SEM images of the thiolated nanowires, only piles of GeO2 particles were observed on the XPS substrate. The hydrogermylated nanowires were slightly more resistant to peroxide oxidation and some remaining nanowires could be observed by SEM. (31) Onsia, B.; Conrad, 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, 27.

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Figure 8. XPS analysis of passivated Ge nanowires: (A) S 2p XPS of Ge nanowires passivated with 1-octadecanethiol before (top) and after (bottom) exposure to 30% H2O2 (1 h), and (B) C 1s XPS of Ge nanowires passivated with 1-octadecene before (top) and after (bottom) exposure to 30% H2O2 (1 h). (C) Schematic illustrating the successive oxidation of a thiol-passivated germanium surface in the presence of peroxide.

Oxidative Resistance: Differences between Thiol and Alkene Passivation. The difference in resiliency to peroxide exposure of the thiol- and alkene-passivated nanowires indicates that oxidative attack occurs via different mechanisms. In the presence of strong oxidizing agents, thiols progressively oxidize from sulfide to sulfoxide to sulfone to sulfonic acid.32 A similar oxidative pathway exists on thiolated Ge nanowires, with chemisorbed thiol being fully oxidized to a sulfonic acid by peroxide (see Figure 8). Direct oxidation of the thiol ligands was confirmed by monitoring the S 2p XPS signal before and after peroxide exposure. Prior to peroxide exposure, a S 2p3/2 peak is observed at 162.7 eV with a S 2p1/2 shoulder at 163.9 eV, corresponding to the expected binding energies for mercaptans (see Figure 8).28 After peroxide exposure, a peak develops at 167.7 eV, corresponding to the binding energy for sulfone molecules.28 In contrast, the C 1s XPS signal of the alkene-passivated nanowires remains largely unchanged before and after peroxide exposure (see Figure 8). The oxidation and removal of the thiol passivation layer exposes the germanium surface, which is vulnerable to direct oxidation and etching. Since carbon cannot access d-orbitals to expand its octet like sulfur can, an analogous ligand oxidation and removal pathway is not possible for purely alkylated surfaces. This, combined with the fact that Ge-C has stronger bond energy than (32) Jones, C. W. Applications of Hydrogen Peroxide and Derivatives; The Royal Society of Chemistry: Cambridge, U.K., 1999.

Ge-S, results in faster oxidation of the thiolated surface in the presence of peroxide. Conclusions Ge nanowires passivated with alkyl and alkanethiol layers are resistant to reasonably harsh oxidizing conditions. For example, with the exception of the shortestchain alkene, passivated Ge nanowires did not exhibit observable oxidation when exposed to boiling water or sulfuric acid. This is due in part to the hydrophobicity of the protective monolayer that prevents water penetration through the passivation layer. Stronger oxidizing agents, such as hydrogen peroxide and sulfuric acid in dioxane did corrode the nanowires. Alkyl- and thiol-bonded monolayers exhibited nearly equivalent stability, with the exception of the case of peroxide exposure, in which case Ge-C bonded monolayers were slightly more resistant to oxidative attack than Ge-S bonded monolayers. These results confirm that alkyl and alkanethiol passivation layers provide an adequate oxidation barrier for most applications, and that direct in situ thermal hydrogermylation and thiolation provide simple and attractive ways of achieving chemically stable germanium surfaces. Acknowledgment. This work was funded in part by grants from the Air Force Research Laboratory (FA8650-07-2-5061) and the Robert A. Welch Foundation (Grant No. F-1464). V.C.H. also acknowledges the Fannie and John Hertz Foundation and the National Science Foundation Graduate Research Fellowship Program for financial support.