Thiol-Terminated Monolayers on Oxide-Free Si: Assembly of

Feb 1, 2007 - Self-assembled monolayers formed by thermal hydrosilylation of a trifluoroacetyl-protected alkenylthiol on Si−H surfaces, followed by ...
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Langmuir 2007, 23, 3236-3241

Thiol-Terminated Monolayers on Oxide-Free Si: Assembly of Semiconductor-Alkyl-S-Metal Junctions Till Bo¨cking,§,¶ Adi Salomon,‡ David Cahen,‡ and J. Justin Gooding*,¶ School of Physics, UniVersity of New South Wales, Sydney, New South Wales 2052, Australia, School of Chemistry, UniVersity of New South Wales, Sydney, New South Wales 2052, Australia, and Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100 Israel ReceiVed October 16, 2006. In Final Form: December 15, 2006 Self-assembled monolayers formed by thermal hydrosilylation of a trifluoroacetyl-protected alkenylthiol on Si-H surfaces, followed by removal of the protecting groups, yield essentially oxide-free monolayers suitable for the formation of Si-C11H22-S-Hg and Si-C11H22-S-Au junctions in which the alkyl chains are chemically bound to the silicon surface (via Si-C bonds) and the metal electrode (via Hg-S or Au-S bonds). Two barriers to charge transport are present in the system: at low bias the current is temperature activated and hence limited by thermionic emission over the Schottky barrier in the silicon, whereas as at high bias transport is limited by tunneling through the organic monolayer. The thiol-terminated monolayer on oxide-free silicon provides a well-characterized system allowing a careful study of the importance of the interfacial bond to the metal electrode for current transport through saturated molecules.

Introduction The reactivity of thiol-terminated monolayers has been exploited for the fabrication of microelectronic devices, e.g., for the deposition of metal films on semiconductors,1-4 and with a view to (bio-)sensor development5 for the immobilization of biomolecules via stable thioether6-8 or reversible disulfide linkages.9-13 The former application of using sulfhydryl groups as “glue” for thin metal films can greatly improve the adhesion of the metal layer to the substrate such that it is unaffected by subsequent chemical processing steps.1 Patterns of gold2,3 or copper4 can be transferred with high fidelity and resolution from the relief features of a stamp to substrates modified with a thiol-terminated monolayer in a process called nanotransfer printing. Importantly, the metal layer or pattern formed by evaporation14 or nanotransfer printing15 can be used to form * To whom correspondence should be addressed. E-mail: Justin.Gooding@ unsw.edu.au. § School of Physics, University of New South Wales. ¶ School of Chemistry, University of New South Wales. ‡ Weizmann Institute. (1) Mahapatro, A. K.; Scott, A.; Manning, A.; Janes, D. B. Appl. Phys. Lett. 2006, 88, 151917/151911-15191.7/151913. (2) Loo, Y.-L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654-7655. (3) Loo, Y.-L.; Hsu, J. W. P.; Willett, R. L.; Baldwin, K. W.; West, K. W.; Rogers, J. A. J. Vac. Sci. Technol. B 2002, 20, 2853-2856. (4) Felmet, K.; Loo, Y.-L.; Sun, Y. Appl. Phys. Lett. 2004, 85, 3316-3318. (5) McGovern, M. E.; Thompson, M. Can. J. Chem. 1999, 77, 1678-1689. (6) Wang, Y.; Yao, X.; Wang, J.; Zhou, F. Electroanalysis 2004, 16, 17551761. (7) Wang, X.; Hu, W.; Ramasubramaniam, R.; Bernstein, G. H.; Snider, G.; Lieberman, M. Langmuir 2003, 19, 9748-9758. (8) Smith, E. A.; Wanat, M. J.; Cheng, Y.; Barreira, S. V. P.; Frutos, A. G.; Corn, R. M. Langmuir 2001, 17, 2502-2507. (9) Rogers, Y.-H.; Jiang-Baucom, P.; Huang, Z.-J.; Bogdanov, V.; Anderson, S.; Boyce-Jacino, M. T. Anal. Biochem. 1999, 266, 23-30. (10) Lenigk, R.; Carles, M.; Ip, N. Y.; Sucher, N. J. Langmuir 2001, 17, 2497-2501. (11) Trau, D.; Lee, T. M. H.; Lao, A. I. K.; Lenigk, R.; Hsing, I. M.; Ip, N. Y.; Carles, M. C.; Sucher, N. J. Anal. Chem. 2002, 74, 3168-3173. (12) Pavlovic, E.; Quist, A. P.; Nyholm, L.; Pallin, A.; Gelius, U.; Oscarsson, S. Langmuir 2003, 19, 10267-10270. (13) Pavlovic, E.; Quist, A. P.; Gelius, U.; Nyholm, L.; Oscarsson, S. Langmuir 2003, 19, 4217-4221. (14) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Small 2005, 1, 725-729.

electrical contacts to the top of thiolated monolayers whereby the strong chemical driving force of Au-S bond formation16 can help prevent the formation of shorts through the monolayer. As an alternative to depositing thin metal films, nanoparticles (e.g., Au or Pt) can be self-assembled into patterns on thiolated surfaces to serve as electrodes or electrically conducting nanowires.17-19 Molecular monolayers on semiconductors, contacted on the top with a metal electrode, represent an important experimental system to study the electrical transport properties of organic molecules. In recent years Si-molecule-metal junctions prepared on the hydride-terminated Si surface with molecules directly bound to Si via Si-C bonds (i.e., without intervening oxide layer between the Si and the molecular monolayer) emerged as an attractive experimental system. The system is of interest not only for characterizing fundamental electron transport properties (molecular electronics) but also with a view to evaluating the utility of these Si-bound monolayers as ultrathin insulators in electronicdevicessuchasSi-organicinsulator-metalcapacitors.20-26 So far molecular junctions based on Si-C-linked monolayers have been largely limited to simple alkyl monolayers. The next step is the introduction of functional groups in the molecular monolayer to exert chemical influence on the charge transport through these junctions whereby the functional group can affect the dielectric properties of the insulator or the energy band alignment. Of particular interest are terminal functions that can (15) Loo, Y.-L.; Lang, D. V.; Rogers, J. A.; Hsu, J. W. P. Nano Lett. 2003, 3, 913-917. (16) Beebe, J. M.; Kim, B.; Gadzuk, J. W.; Frisbie, C. D.; Kushmerick, J. G. Phys. ReV. Lett. 2006, 97, 026801(026801)-026801(026804). (17) Fresco, Z. M.; Frechet, J. M. J. J. Am. Chem. Soc. 2005, 127, 8302-8303. (18) Liu, J.-F.; Zhang, L.-G.; Gu, N.; Ren, J.-Y.; Wu, Y.-P.; Lu, Z.-H.; Mao, P.-S.; Chen, D.-Y. Thin Solid Films 1998, 327-329, 176-179. (19) Liu, J.; Zhang, L.; Mao, P.; Chen, D.; Gu, N.; Ren, J.; Wu, Y.; Lu, Z. Chem. Lett. 1997, 1147-1148. (20) Liu, Y.-J.; Yu, H.-Z. ChemPhysChem 2002, 3, 799-802. (21) Liu, Y.-J.; Yu, H.-Z. J. Phys. Chem. B 2003, 107, 7803-7811. (22) Liu, Y.-J.; Yu, H.-Z. ChemPhysChem 2003, 4, 335-342. (23) Miramond, C.; Vuillaume, D. J. Appl. Phys. 2004, 96, 1529-1536. (24) Faber, E. J.; de Smet, L. C. P. M.; Olthuis, W.; Zuilhof, H.; Sudhoelter, E. J. R.; Bergveld, P.; van den Berg, A. ChemPhysChem 2005, 6, 2153-2166. (25) Salomon, A.; Boecking, T.; Chan, C. K.; Amy, F.; Girshevitz, O.; Cahen, D.; Kahn, A. Phys. ReV. Lett. 2005, 95, 266807/266801-26680.7/266804. (26) Seitz, O.; Bo¨cking, T.; Salomon, A.; Gooding, J. J.; Cahen, D. Langmuir 2006, 22, 6915-6922.

10.1021/la063034e CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007

Thiol-Terminated Monolayers on Oxide-Free Si

bond chemically to the other (metal) electrode that is brought in contact with the top of the monolayer to form the junction. In other systems, such as metal/SAM/metal (comparing thiol and dithiol SAMs)27,28 and silicon-SiO2/SAM/metal14,29 structures, the introduction of a second chemical bond to the top electrode via metal-sulfur linkages has resulted in large increases in the current through the alkyl chain monolayer but the underlying causes for this are not obvious (possibly due to better adhesion of the gold contact to the surface). The formation of thiolterminated monolayers is readily available on silicon-SiO2 surfaces via self-assembly from commercially available thiolated silanes.14,30 However, on hydride-terminated silicon surfaces the assembly of thiol-terminated monolayers presents challenges because the sulfhydryl group may cross-react with the surface.31,32 Because of our ability to make highly reproducible junctions based on the Si-C bonds,26 we designed, prepared, characterized, and measured a system of thiol-terminated Si-C-linked monolayers on Si(111) forming Hg-S bonds to a mercury and Au-S ones to an Au electrode. The purpose of this paper is to demonstrate the utility of our strategy that allows the assembly of thiol-terminated monolayers on hydride-terminated silicon surfaces via hydrosilylation of protected alkenylthiols, followed by removal of the protecting group. The chemical transformations at the Si surface in the assembly process are investigated in detail by X-ray photoelectron spectroscopy. A thorough understanding of the chemical composition of the surface, with respect to the organic monolayer and its terminal functionality, as well as the Si substrate, is crucial for the interpretation of the electrical transport measurements because any changes or contamination at the interface, such as silicon oxide formation, can alter the electronic properties of the junction and hence lead to changes in current transport by several orders of magnitude.26 The utility of these monolayers for the formation of molecular junctions is then demonstrated by contacting the monolayer surface with a Hg drop electrode or by indirect evaporation of Au. The electrical transport33,34 and electronic properties of these monolayer systems were characterized using temperature-dependent I-V and contact potential difference (CPD) measurements. Experimental Methods Materials. Ethanol, ethyl acetate, light petroleum, and dichloromethane were redistilled. Milli-Q water (18 MΩ cm) was used for rinsing of samples and preparation of solutions. Semiconductorgrade chemicals were used for cleaning (30% H2O2, 98% H2SO4) and etching (40% NH4F solution) pieces of silicon wafer. Silicon wafers (n-type Si(111), P-doped, with a nominal resistivity of 1-10 Ω‚cm and p-type Si(111), B-doped, with a nominal resistivity of 1-10 Ω‚cm) were purchased from Virginia Semiconductor. S-Undec10-enyl-2,2,2-trifluoroethanethioate was prepared as described in the Supporting Information. (27) Cui, X. D.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Primak, A.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Nanotechnology 2002, 13, 5-14. (28) Engelkes, V. B.; Beebe, J. M.; Frisbie, C. D. J. Am. Chem. Soc. 2004, 126, 14287-14296. (29) Selzer, Y.; Salomon, A.; Cahen, D. J. Phys. Chem. B 2002, 106, 1043210439. (30) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Nanotechnology 2005, 16, 3064-3068. (31) Coulter, S. K.; Schwartz, M. P.; Hamers, R. J. J. Phys. Chem. B 2001, 105, 3079-3087. (32) Lai, Y.-H.; Yeh, C.-T.; Yeh, C.-C.; Hung, W.-H. J. Phys. Chem. B 2003, 107, 9351-9356. (33) More detailed information on the transport behavior of Si-C11H22-SHg junctions can be found in ref 34. (34) Salomon, A.; Bo¨cking, T.; Gooding, J. J.; Cahen, D. Nano Lett. 2006, 6, 2873-2876.

Langmuir, Vol. 23, No. 6, 2007 3237 Preparation of Si-Bound Monolayers. Si(111) wafers were cleaved into pieces (∼10 mm × 15 mm), rinsed with solvents (ethanol, CH2Cl2, ethyl acetate), and blown dry under a stream of argon. The pieces of silicon wafer were then thoroughly cleaned in Piranha solution (concentrated H2SO4/30% H2O2 ) 3:1, v/v) at 90 °C for 30 min followed by copious rinsing with MilliQ water. The silicon sample was etched in deoxygenated 40% NH4F solution for 10-15 min to remove the native oxide layer and produce a H-terminated surface. The NH4F solution was deoxygenated by bubbling with argon for at least 45 min just prior to the etching step. The Piranha treatment and NH4F solution etching steps were repeated once. The sample emerged dry from the NH4F solution and was immediately transferred under argon to a Schlenk flask containing the neat alkene, which had been deoxygenated by 4-5 freeze-pump-thaw cycles. The flask was immersed in an oil bath at 200 °C for 3-4 h. Then, the reaction the flask was removed from the oil bath and opened to the atmosphere and the sample was removed. The sample was cleaned by rinsing with solvents (CH2Cl2, ethyl acetate) followed by immersion in boiling CH2Cl2 to remove physisorbed material and finally blown dry under a stream of argon. Preparation of Thiol-Terminated Si-Bound Monolayers: Removal of the Trifluoroacetyl Protecting Group. Silicon surfaces derivatized by reaction of Si-H with S-undec-10-enyl-2,2,2trifluoroethanethioate were immersed in 10% aqueous ammonium hydroxide solution for 2-7 min to remove the trifluoroacetyl protecting group and expose the free sulfhydryl group at the monolayer surface. The sample was subsequently rinsed with MilliQ water, ethanol, and CH2Cl2 and blown dry under a stream of inert gas. Alternatively, the sample was immersed in a 1 M solution of triethylamine in CH2Cl2/95% ethanol (1:1, v/v) for 20 min to achieve removal of the TFA group. Subsequently, the sample was rinsed with ethanol and CH2Cl2 and blown dry under a stream of inert gas. XPS Measurements. XP spectra were obtained using an EscaLab 220-IXL spectrometer with a monochromated Al KR source (1486.6 eV), hemispherical analyzer, and multichannel detector. The spectra were accumulated at a takeoff angle of 90° with a 0.79 mm2 spot size at a pressure of less than 10-8 mbar. Contact Angle Goniometry. Contact angle measurements were determined using a Rame-Hart contact angle goniometer. Static contact angles were determined on several spots on the sample by depositing a sessile drop onto the surface (needle withdrawn). The experimental error of the contact angle measurements is estimated to be (2°. Contact Potential Difference (CPD) Measurements. CPD measurements were performed in order to verify the sample’s work function before and after removing the protection group. Dark CPD measurements were monitored in air at room temperature, using a commercial Kelvin probe unit (Besocke Delta Phi) with a sensitivity of ∼5 mV. Junction Formation and Current Voltage Measurements. Junctions were formed by contacting the silicon-bound thiolterminated monolayer with the mercury drop or by indirect evaporation of Au.35 Current-voltage (I-V) characteristics were collected in the voltage scan mode using an HP 4155 semiconductor parameter analyzer. At forward bias, the Hg was positive relative to n-Si. The measurements were carried out at 20 mV/s and 10 mV steps. Cyclic scans were used to ensure junction stability. Temperature-dependent I-V measurements were collected in a Faraday cage apparatus (MMR technologies). An additional cooling finger was placed in the apparatus to prevent water condensation onto the sample during the cooling process. Measurements performed at g5 different temperatures showed a consistent trend in the temperature dependence of the current. Cyclic measurements of heating and cooling were performed to ensure junction stability during the measurements.

Results Preparation of Sulfhydryl-Terminated Si-C-Linked Monolayers with Trifluoroacetyl Protection. For its intended use in (35) Haick, H.; Ambrico, M.; Ghabboun, J.; Ligonzo, T.; Cahen, D. Phys. Chem. Chem. Phys. 2004, 6, 4538-4541.

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Figure 1. Preparation of thiol-terminated monolayers on Si(111).

molecular junctions the self-assembling functionalized molecule should ideally allow the formation of densely packed siliconbound monolayers without the formation of silicon oxide and without degradation of the thiol function at the surface, as these factors have an influence on the electron transport properties of the system. The approach for modifying Si with thiol-terminated monolayers fulfilling these criteria is shown in Figure 1. First, Si(111)-H was derivatized with organic monolayers terminated with trifluoroacetyl (TFA)-protected sulfhydryl groups by thermal hydrosilylation of S-undec-10-enyl-2,2,2-trifluoroethanethioate (C11-S-TFA). The protecting group was necessary not only to avoid side reactions of the sulfhydryl group with the hydrideterminated silicon surface but also to avoid degradation and contamination of the “sticky” monolayer surface during storage. The small TFA group was chosen over bulkier protecting groups such as the dimethoxytrityl group to avoid steric interference with the packing of the alkyl chains in the monolayer and because it could be removed rapidly (within minutes) at room temperature36 just prior to junction formation. An additional advantage of the TFA group (containing three fluorine atoms) is that its removal from the surface could be monitored with high sensitivity by X-ray photoelectron spectroscopy. First, it was confirmed that the trifluoroacetyl protecting group of C11-S-TFA remained intact under the conditions used in the thermal hydrosilylation reaction. The alkene was heated under argon at 200 °C for 210 min and then analyzed by NMR spectroscopy. No degradation of the TFA-protected alkene was evident in the NMR spectrum whereby special attention was paid to detecting possible decomposition products (free thiol, disulfides). The XP survey spectrum (Figure 2) of the monolayer formed by reaction of the Si(111)-H surface with C11-S-TFA showed the Si 2s and Si 2p peaks from the silicon substrate, as well as the 1s signals for carbon, oxygen, and fluorine, expected for the organic monolayer. The S associated with the monolayer was detectable by a small 2s peak in the survey spectrum, and its presence was confirmed in the S 2s narrow scan (Figure 3a) showing a peak at 229.0 eV consistent with the presence of the protected sulfhydryl group. The 2p peak of S was obscured by the presence of the silicon loss peak at ∼167 eV and hence was not observed in the survey spectrum. In a narrow scan of the S 2p region the signal could be detected at 164.9 eV consistent with literature data for TFA-protected thiols.5 Because of the difficulty in clearly discerning the S 2p peak on these samples, only the S 2s scan was acquired for deprotected monolayers (see below).

The C1s narrow scan (Figure 3b) was deconvoluted into four peaks, assigned to the C-C-linked carbons of the alkyl chain (285.0 eV), the C-S-linked carbon (286.3 eV), the carbonyl carbon (289.1 eV), and the trifluoromethyl (CF3) carbon (293.5 eV). The peak area ratio between these peaks was approximately 11.5:1.3:0.7:1.0, in reasonable agreement with the expected ratio of 10:1:1:1. The binding energies of the peaks associated with the TFA group were consistent with those observed for trifluoroacetyl ester-terminated SAMs on silicon37 and gold,38

(36) A precursor in the preparation of C11-S-TFA is the acetyl-protected derivative (see Supporting Information). While this compound could also be used for monolayer preparation, removal of the acetyl group would require harsher conditions such as heating at reflux for several hours in acidic solutions.

(37) The C 1s binding energies for the trifluoromethyl carbon and the carbonyl carbon of trifluoroacetyl ester-terminated SAMs on silicon were 293.7 and 290.4 eV, respectively. (38) Pan, S.; Castner, D. G.; Ratner, B. D. Langmuir 1998, 14, 3545-3550.

Figure 2. XP survey scan of the TFA-protected monolayer on Si(111).

Figure 3. XP narrow scans of the (a) S 2s, (b) C 1s, (c) O 1s, and (d) F 1s regions of the TFA-protected monolayer.

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Figure 4. XP survey scan of the deprotected thiol-terminated monolayer on Si(111).

as well as those observed for TFA ester-containing polymers.39 The ratio of the CF3 carbon to S was 1:1, as expected for the TFA protected sulfhydryl group. The O 1s envelope (Figure 3c) could be deconvoluted into a peak at 533.1 eV tentatively assigned to the carbonyl oxygen of the TFA group and an additional peak at 532.0 eV (possibly associated with adventitious physisorbed O2, some S oxidation to S-O, and/or very low levels of oxidized Si). Two peaks were detectable in the F 1s region (Figure 3d). The large peak at 689.0 eV (fwhm 1.6 eV) was due to the CF3 group of the organic monolayer, and the small broad peak at ∼686.3-686.9 eV (fwhm 2.1-2.5 eV) was assigned to residual fluoride ions remaining on the surface after the etching procedure in ammonium fluoride solution.40 Varying levels of contamination with fluoride ions have been observed previously for Si-Clinked monolayers.40 Deprotection of the Monolayer. The sample was immersed in 10% aqueous NH4OH solution for 2-7 min to remove the trifluoroacetate protecting group, rinsed with water and solvents, and blown dry under a stream of inert gas. The XP survey scan after this treatment is shown in Figure 4. The spectrum showed the presence of the peaks arising from the underlying silicon substrate, as well as a large C 1s peak and a small S 2s peak expected for the deprotected thiol-terminated monolayer. The F 1s peak was greatly diminished compared to that observed for the protected monolayer, which was consistent with removal of the majority of the TFA protecting groups. A relatively low level of oxygen was also associated with the sample. The peak in the S 2s region (Figure 5a) was shifted by 0.8 eV to lower binding energies (228.2 eV) compared to that prior to deprotection (cf. Figure 3a). This binding energy is consistent with the formation of free thiols12,41,42 Importantly, the binding energy of the S 2s peak is considerably lower than that expected for sulfur oxidized to a higher oxidation state, which occurs at binding energies g231.0 eV.12 The C 1s scan (Figure 5b) showed the disappearance of the peaks at higher binding energies associated with the carbonyl and CF3 carbons. The peak from the C-S bonded carbons of the deprotected monolayer were shifted to a lower binding energy compared to the corresponding peak of the TFA-protected monolayer (Figure 3b) such that this peak could no longer be resolved from the main C-C peak, arising from the alkyl chains. Consequently, the C 1s envelope was fitted with a single peak (39) Beamson, G.; Briggs, D. High resolution XPS of organic polymers; Wiley: New York, 1992. (40) Bo¨cking, T.; James, M.; Coster, H. G. L.; Chilcott, T. C.; Barrow, K. D. Langmuir 2004, 20, 9227-9235. (41) Bensebaa, F.; Yu, Z.; Deslandes, Y.; Kruus, E.; Ellis, T. H. Surf. Sci. 1998, 405, L472-L476. (42) It cannot be ruled out that disulfides form under the basic conditions used for removal of the TFA group, as the binding energy of sulfur in disulfides is not sufficiently different from that of thiols.

Figure 5. XP narrow scans of the (a) S 2s, (b) C 1s, (c) O 1s, and (d) F 1s regions of the thiol-terminated monolayer after deprotection. The line in the O 1s narrow scan at 533.1 eV denotes the position where the peak corresponding to the carbonyl oxygen of the protecting group was present before deprotection (cf. Figure 3c).

at 285.0 eV with a slightly broadened full-width at half-maximum of ∼1.3 eV. The C 1s peaks, associated with the TFA group, that are present before deprotection (Figure 3b), were not detectable even when the C1s region was scanned many times to improve the signal-to-noise ratio of the data. The O 1s shown in Figure 5c was difficult to deconvolute into its component peaks, but the entire peak was shifted toward lower binding energies relative to the O 1s peak of the protected monolayer (cf. 533.1 eV in Figure 3c), as expected for the removal of the carbonyl oxygen, associated with the TFA group. The residual oxygen peak centered at ∼532 eV could be attributed to physisorbed oxygen or traces of silicon oxide.43 The large F 1s peak associated with the CF3 group predominantly disappears after deprotection. The residual F 1s signal was partly due to contamination with fluoride ions (∼686 eV) remaining on the silicon surface after etching in NH4F solution. However, the presence of two peaks in the F 1s narrow scan (Figure 5d) indicates that there is not complete removal of the TFA group. Several different deprotection treatments using 10% aqueous ammonium hydroxide solution (2 and 7 min exposure) or a 1 M solution of the organic base triethylamine in CH2Cl2/ 95% ethanol (1:1, v/v) (20 min exposure) gave comparable levels of residual fluorine on the sample surface, suggesting that not all accessible TFA groups were removed during deprotection (see Supporting Information for further details). Therefore, we attribute the remaining CF3 signal to some protecting groups being immersed in the monolayer and inaccessible to deprotecting agents. On the basis of peak areas, it was estimated that g90% of the TFA groups were removed from the sample. This observation was similar to that for TFA-protected thiol-terminated monolayers formed by silanization on silicon-SiO2 substrates where residual fluorine remained on the surface despite prolonged treatment with base.5 Oxide Levels Associated with the Modified Silicon Substrate. The Si 2p narrow scan (Figure 6) of the C11-S-TFAmodified sample showed the Si2p3/2-2p1/2 doublet (99.5 and 100.1 eV) arising from the silicon substrate. There was no clear (43) Howard, J. M.; Craciun, V.; Essary, C.; Singh, R. K. Appl. Phys. Lett. 2002, 81, 3431-3433.

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Figure 6. XP narrow scan of the Si 2p region for Si(111) (a) derivatized with C11-S-TFA and (b) for the same sample after removal of the trifluoroacetyl protecting group with 10% aqueous NH4OH. The expansions (insets) show that a peak for oxidized silicon (102-104 eV) is not detectable.

(Si+-Si4+),

evidence for the formation of silicon oxides which would appear as a broad peak between 100-104 eV. It was more difficult to prepare oxide-free thiol-terminated monolayers (as judged from the Si 2p narrow scan) using C11-S-TFA relative to methyl-terminated monolayers using simple C18 alkenes. While it was possible to prepare samples with undetectable oxide levels (Figure 6a), similar to those observed for high-quality methyl-terminated monolayers,26 other samples had very low but detectable levels of silicon oxide. It can therefore not be excluded that these slight variations in oxide content affect the Schottky barrier inside the silicon substrate by altering the electrostatic landscape of the junction (whereby higher oxide levels yield a lower Schottky barrier26). A reduction in the Schottky barrier height would result in an increase in the current through the junction in the bias regime where transport is limited by thermionic emission over the Schottky barrier (i.e., reverse and low forward bias for n-type Si). Figure 6b shows that there was no apparent change in the Si 2p spectrum after treatment of the C11-S-TFA-modified silicon sample with aqueous NH4OH solution to remove the TFA group. No obvious peak was detected in the region where oxidized silicon is expected (Figure 6b, inset), indicating that oxidation of the silicon substrate in the presence of base was negligible. Changes of the Surface Properties of the Deprotected Monolayer Over Time. Contact angle goniometry was used to follow the deprotection reaction and to monitor changes in the wettability of the thiol-terminated surface over time. The static water contact angle of the trifluoroacetyl-protected monolayer was 83°, and this value was stable over time. The contact angle measured immediately after deprotection was 71-73°, in good agreement with that reported for thiol-terminated silane monolayers on glass.44 Upon exposure of the sample to the atmosphere and water (e.g., the water drops used for the contact angle measurements), the contact angle increased and was no longer uniform across the sample surface. This gradual change in contact angle started within ∼15 min after deprotection, indicating that the surface of the thiol-terminated monolayer is prone to changes, presumably due to contamination and/or reaction of the thiols (e.g., to form disulfides). Characterization with CPD to Verify the Changes in the Samples’ Work Function. The CPD values (work function) for n- and p-type Si modified with silicon-bound monolayers were measured (Table 1). For samples modified with C11H22-STFA, the CPDs differed by about 0.4 eV compared to the samples without the protecting group. After removal of the protecting group, the Si work function was ∼4.6-4.7 eV both for n- and p-Si. This indicates that the Fermi level was pinned at about the (44) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621-1627.

Figure 7. Temperature dependence of I-V curves for an n-SiC11H22-S-Hg junction. The inset shows the temperature dependence of I-V curves for an n-Si-C12H25//Hg junction at forward bias for comparison.48 Table 1. Contact Potential Difference (CPD) Measurements (Dark Values) CPD (dark) (0.01 eV p-Si-CnH2n+1 n-Si-CnH2n+1 p-Si-C11H22-S-TFA p-Si-C11H22-SH n-Si-C11H22-S-TFA n-Si-C11H22-SH

-0.65 eV -0.65 eV 0.01 eV -0.35 eV 0.14 eV -0.29 eV

with respect to Au (5 eV) (0.1 eV 4.3 eV 4.3 eV 5 eV 4.6 eV 5.1 eV 4.7 eV

same position in the Si gap at the Si/alkyl chain interface, similar to the observation for samples modified with simple alkyl chains. Formation of Si-C11H22-S-Hg Junctions: Temperature Dependence of I-V Curves. Semiconductor-alkyl monolayermetal junctions were formed by contacting the Si-bound thiolterminated monolayer with a mercury drop electrode. In contrast to methyl-terminated monolayers, it was observed that the mercury drop adhered to the monolayer against gravity presumably because of Hg-S bond formation at the mercury-monolayer interface. The junction showed stable I-V characteristics over repeated cyclic voltage scans, even at elevated temperature (330 K). Current transport through this junction is characterized by two coupled transport mechanisms which dominate in different bias regimes as previously observed for n-Si-CnH2n+1//Hg junctions.25 At reverse bias and low forward bias the current is limited by thermionic emission over the Schottky barrier inside the silicon (which arises from band bending close to the interface), whereas at high forward bias the Schottky barrier becomes negligible and the current is dominated by tunneling through the organic monolayer. The temperature dependence of the I-V characteristics of the n-Si-C11H22-S-Hg junction shown in Figure 7 is consistent with these two transport mechanisms. At reverse and low forward bias the current transport through the junction is temperature activated, i.e., a current increase of up to 1 order of magnitude was observed when the temperature was raised from 260 to 330 K, consistent with the notion that the current in this bias regime is limited by thermionic emission over the Schottky barrier. The Schottky barrier decreases with increasing forward bias until it becomes negligible such that the current through the junction is no longer temperature activated. For the n-Si-C11H22S-Hg junctions the potential at which transport is no longer temperature activated (i.e., the potential at which the I-V curves recorded at 260 and 330 K coincide) and becomes limited by tunneling through the organic monolayer is ∼0.5 V. The temperature dependence of the n-Si-C11H22-S-Hg junction was consistent with that observed for the n-Si-C12H25//

Thiol-Terminated Monolayers on Oxide-Free Si

Hg junction25 (Figure 7, inset) with the tunnel current (measured at forward bias >0.5 V) decreasing slightly with an increase in the temperature. We have earlier suggested that this observation can be due to slight changes in monolayer structure with temperature. Thus, an expansion of the organic layer45,46 or an increase in gauche defects in the alkyl chains47 with temperature may lead to a reduction in the current. The main difference between the junctions with and without S-termination is that the switch from thermionic emission to tunneling dominated transport occurs at a lower forward bias for the junction with thiol groups at the monolayer-Hg interface (∼0.5 V) compared to the junction prepared from the monolayer without terminal functional group (∼0.7 V).48 This difference between the junctions can be explained by a lowering of the Schottky barrier in the n-Si-C11H22-SHg junction due to the presence of sulfur at the monolayer surface making the molecular dipole more positive.34 Since this result was obtained consistently and reproducibly, it is unlikely that such a barrier lowering is primarily due to Si oxidation. Temperature-dependent I-V measurements were also carried out on samples contacted on the monolayer surface by indirect evaporation of Au. The Si-C11H22-S-Au junctions showed the same behavior and stability (up to 330 K), as observed for the Si-C11H22-S-Hg junctions.

Discussion and Conclusions Self-assembled monolayer systems need to be sufficiently well defined and well characterized to allow the effects of the molecular architecture on current transport to be studied because defects in the monolayer or contamination at the substrate-monolayer interface can govern the current flowing through the molecular junction. We have recently shown that silicon-bound monolayers prepared by reaction of simple alkenes with the hydride-terminated silicon surface and contacted by Hg on the top fulfill these criteria and yield extremely robust and reproducible molecular junctions.26,49 One important question in molecular electronics concerns the effect of the molecule-electrode interface (physical contact or chemical bond) on the current transport through the junction. In this context it is of great interest to introduce thiol groups onto the monolayer surface, which can form chemical contacts to the metal electrode. To obtain a system that is comparable to the monolayers prepared from simple alkenes it is essential to control the chemical composition (quality) of the silicon-monolayer and monolayer-S-Hg interfaces. Here we have designed self-assembling molecules with thiol function for formation of well-controlled silicon-bound monolayers and carefully characterized these interfaces because semiconductor/ monolayer/metal devices are extremely sensitive to changes at the interfaces. Preparation of thiol-terminated monolayers on oxide-free silicon via thermal hydrosilylation of functionalized alkenes requires the use of a protection group to avoid side reaction of the thiol with the Si-H surface. Furthermore, the protected monolayer can be stored and transported without degradation, whereas the monolayer surface with free thiol groups can change (45) Feng, D. Q.; Wisbey, D.; Tai, Y.; Losovyj, Y. B.; Zharnikov, M.; Dowben, P. A. J. Phys. Chem. B 2006, 110, 1095-1098. (46) Zhang, L. Z.; Wesley, K.; Jiang, S. Y. Langmuir 2001, 17, 6275-6281. (47) Haran, A.; Waldeck, D. H.; Naaman, R.; Moons, E.; Cahen, D. Science 1994, 263, 948-950. (48) The difference in currents between the two junctions is due to different contact areas between the mercury drop and the monolayer surface (i.e, in this case the contact area for the n-Si-C12H25//Hg junction is considerably smaller than that for the n-Si-C11H22-S-Hg junction). The contact area could not be determined accurately in the set-up for temperature-dependent I-V measurements such that the current density could not be determined. (49) Salomon, A.; Arad-Yellin, R.; Shanzer, A.; Karton, A.; Cahen, D. J. Am. Chem. Soc. 2004, 126, 11648-11657.

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gradually when exposed to the laboratory atmosphere as evidenced by contact angle measurements. The protection strategy developed here was evaluated by XPS with respect to its suitability for the formation molecular junctions. The first aspect concerns the deprotection to form free thiols at the monolayer surface. Under the basic conditions used here, the sulfur fortunately did not oxidize to the state of sulfinic or sulfonic acids, as these oxidized functions would not form strong chemical linkages to the Hg surface. While disulfide formation as observed in air on other thiol-terminated surfaces13,44,50 cannot be ruled out here, it is not expected to interfere with the ability of the monolayer surface to form chemical bonds with the Hg drop.51 It is well known that disulfides dissociate on adsorption to metal surfaces (such as gold) to give metal thiolates and thus form similar monolayers on metals as those derived from thiols.52,53 Indeed, monolayers of disulfides bound to gold and silver,54 as well as to Hg,49,55 have been used for the formation of molecular junctions previously. The second aspect with a view to molecular junction formation concerns the silicon-monolayer interface. It has been shown that low levels of oxide have a huge impact on the current observed through molecular junctions assembled with silicon-bound monolayers.26 The observation that no oxide was detectable on the samples after treatment with aqueous NH4OH solution during deprotection indicates that the packing of the alkyl chains in the monolayer was sufficiently dense to prevent oxidation of the silicon substrate in the presence of base. Contacting the Si-C11H22-SH monolayers with a Hg drop electrode resulted in the formation of stable molecular junctions. This system is well-suited for experimental investigation of the fundamental properties governing current transport in semiconductor/monolayer/metal junctions. Moreover, the observation that deposition of Au contacts by indirect evaporation gave stable junctions (up to 330 K) is promising for device development and fabrication. Although the thiol end group results in a reduction of the Schottky barrier relative to the methyl-terminated monolayer, the device still shows a high rectification ratio, especially at low temperature. Acknowledgment. J.J.G. and T.B. thank the Australian Research Council for support. D.C. thanks the Israel Science and Bikura foundations and the Kimmel Nanoscale Science Centre for partial support. D.C. holds the Schaefer Energy Research chair. A.S. acknowledges a Clore fellowship. Supporting Information Available: Synthetic methods for preparation of the functionalized alkenes, additional XPS data, and additional temperature-dependent I-V curves. This material is available free of charge via the Internet at http://pubs.acs.org. LA063034E (50) Carot, M. L.; Esplandiu, M. J.; Cometto, F. P.; Patrito, E. M.; Macagno, V. A. J. Electroanal. Chem. 2005, 579, 13-23. (51) If disulfide linkages are formed on the surface but not desired for a particular application, then these could be reduced readily to the free thiol using dithiothreitol. (52) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766-1770. (53) Rousseau, R.; De Renzi, V.; Mazzarello, R.; Marchetto, D.; Biagi, R.; Scandolo, S.; del Pennino, U. J. Phys. Chem. B 2006, 110, 10862-10872. (54) Chabinyc, M. L.; Chen, X.; Holmlin, R. E.; Jacobs, H.; Skulason, H.; Frisbie, C. D.; Mujica, V.; Ratner, M. A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 11730-11736. (55) Selzer, Y.; Salomon, A.; Ghabboun, J.; Cahen, D. Angew. Chem., Int. Ed. 2002, 41, 827-830.