UHV STM I(V) and XPS Studies of Aryl Diazonium Molecules

Molecular layers formed from 4-trifluoromethylbenzenediazonium tetrafluoroborate and 4-Methylbenzenediazonium tetrafluoroborate have been assembled on...
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Langmuir 2007, 23, 4700-4708

UHV STM I(V) and XPS Studies of Aryl Diazonium Molecules Assembled on Si(111) Deepak Pandey,*,†,‡ Dmitry Y. Zemlyanov,‡ Kirk Bevan,§ Ronald G. Reifenberger,†,‡ Shawn M. Dirk,| Steve W. Howell,| and D. R. Wheeler| Department of Physics, Birck Nanotechnology Center, and School of Electrical and Computer Engineering, Purdue UniVersity, West Lafayette, Indiana 47907, and Sandia National Laboratory, Micro Analytical Systems, P.O. Box 5800, Albuquerque, New Mexico 87185 ReceiVed NoVember 5, 2006. In Final Form: February 22, 2007 Molecular layers formed from 4-trifluoromethylbenzenediazonium tetrafluoroborate and 4-Methylbenzenediazonium tetrafluoroborate have been assembled on H-passivated Si(111) and studied by UHV STM and XPS. STM imaging shows well-developed Si(111) step edges and terraces both on Si(111):H and Si(111) substrates covered with a molecular layer. STM I(V) data acquired at different tip-substrate separations reveals a factor of ∼10 enhancement in current for positive bias voltage when current flows through the 4-trifluoromethyl molecule when compared to the 4-methyl variant. The observed current enhancement in I(V) can be understood by comparing the projected density of states of the two molecule-Si systems calculated using a density functional theory local density approximation after geometry optimization was performed via the conjugate gradient method. XPS data independently confirm that H-passivated Si(111) remains oxygen free for short exposures to ambient conditions and provide evidence that the molecules chemically react with the silicon surface.

Introduction During the past 10 years, there have been an increasing number of studies on the I(V) characteristics of molecular layers using a wide variety of different experimental techniques. While the reliability and reproducibility of sample preparation from laboratory to laboratory is still an issue, considerable progress has resulted, and numerous techniques for testing future hybrid organic devices have been developed.1-3 In parallel with the rapid increase in molecular conduction experiments, there also has been growing interest in moving from close-packed metal surfaces (like Au(111)) to surfaces of greater technological interest. As a result, a number of studies reporting height and current-voltage measurements on a broad range of hydrocarbons such as C2H4, C2H2, styrene, benzene, diphenyl, and cyclopentene on Si(100) have appeared.4-14 In * Corresponding author. E-mail: [email protected] or rr@ physics.purdue.edu. Phone: (765)494-3032. Fax: (765)494-0706. † Department of Physics, Purdue University. ‡ Birck Nanotechnology Center, Purdue University. § School of Electrical and Computer Engineering, Purdue University. | Sandia National Laboratory. (1) Hersam, M. C.; Reifenberger, R. MRS Bull. 2004, 29, 385-90. (2) James, D. K.; Tour, J. M. Aldrichimica Acta 2006, 39, 47-56. (3) James, D. K.; Tour, J. M. Chem. Mater. 2004, 16, 4423-4435. (4) Piva, P. G.; DiLabio, G. A.; Pitters, J. L.; Zikovsky, J.; Rezeq, M. D.; Dogel, S.; Hofer, W. A.; Wolkow, R. A. Nature 2005, 435, 658-661. (5) Lastapis, M.; Martin, M.; Riedel, D.; Hellner, L.; Comtet, G.; Dujardin, G. Science 2005, 308, 1000-1003. (6) Guisinger, N. P.; Yoder, N. L.; Hersam, M. C. Proc. Natl. Acad. Sci. 2005, 102, 8838-8843. (7) Mayne, A. J.; Avery, A. R.; Knall, J.; Jones, T. S.; Briggs, G. A. D.; Weinberg, W. H. Surf. Sci. 1993, 284, 247-56. (8) Hofer, W. A.; Fisher, A. J.; Wolkow, R. A. Surf. Sci. 2001, 475, 83-88. (9) Kim, W.; Kim, H.; Lee, G.; Hong, Y.-K.; Lee, K.; Hwang, C.; Kim, D.-H.; Koo, J.-Y. Phys. ReV. B 2001, 64, 193313-317. (10) Hofer, W. A.; Fisher, A. J.; Lopinski, G. P.; Wolkow, R. A. Chem. Phys. Lett. 2002, 365, 129-134. (11) Kirczenow, G.; Piva, P. G.; Wolkow, R. A. Phys. ReV. B 2005, 72, 245306-323. (12) Lopinski, G. P.; Fortier, T. M.; Moffatt, D. J.; Wolkow, R. A. J. Vac. Sci. Technol. A 1998, 16, 1037-1042. (13) Sloan, P. A.; Palmer, R. E. Nature 2005, 434, 367-371. (14) Rignanese, G. M.; Blase, X.; Louie, S. G. Phys. ReV. Lett. 2001, 86, 2110-2113.

addition, studies of alkyl molecules on Si(111) have also been reported.15,16 The possibility of adding molecular monolayers onto silicon to add new functionality to silicon-based microelectronics has thus become an emerging theme. The possibility of fabricating molecular samples on silicon from wet chemistry conditions, rather than relying on UHV deposition techniques, might open doors for the future device applications. The utilization of aryl diazonium salts to assemble a covalently bound molecular layer on oxide-free silicon is a promising technique that has emerged.17-20 Using this approach, I(V) measurements of prototypical devices fabricated from lithographically formed structures of micron dimensions have been reported. A novel hybrid organic/nanoparticle array structure utilizing one-dimensional 7 nm gaps in silicon has been fabricated by initially treating the silicon with butoxycarbonylsulfanylbenzenediazonium, unmasking the protecting group and stitching Au nanoparticles together with octanedithiol characterized by I(V).21 A major limitation of all the above studies is the large number of benzenediazonium molecules contained in any given test structure, leading to uncertainties in the uniformity of the molecular layers and their attachment to a substrate. There appear to be no studies of a single benzenediazonium molecule reported to date. In addition, since a demonstration of a device structure is often the main goal, most experiments on molecular layers are conducted under less than optimal conditions on devices that (15) Seitz, O.; Boecking, T.; Salomon, A.; Gooding, J. J.; Cahen, D. Langmuir 2006, 22, 6915-6922. (16) Segev, L.; Salomon, A.; Natan, A.; Cahen, D.; Kronik, L.; Amy, F.; Chan, C. K.; Kahn, A. Phys. ReV. B 2006, 74, 165323-329. (17) Hartig, P.; Rappich, J.; Dittrich, T. Appl. Phys. Lett. 2002, 80, 67-69. (18) Wang, W.; Lee, T.; Reed, M. A.; Stewart, M. P.; Hwang, J.-J.; Tour, J. M. Superlattices Microstruct. 2003, 33, 217-226. (19) Wang, W.; Lee, T.; Kamdar, M.; Reed, M. A.; Stewart, M. P.; Hwang, J.-J.; Tour, J. M. N.Y. Acad. Sci. 2003, 1006, 36-47. (20) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370-378. (21) Howell, S. W.; Dirk, S. M.; Childs, K.; Pang, H.; Blain, M.; Simonson, R. J.; Tour, J. M.; Wheeler, D. R. Nanotechnology 2005, 16, 754-758.

10.1021/la063235i CCC: $37.00 © 2007 American Chemical Society Published on Web 03/29/2007

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Scheme 1. Synthesis of I and II Were Initiated from the Corresponding Aryl Analinesa

Figure 1. Two molecules 4-trifluoromethylbenzenediazonium tetrafluoroborate (I) and 4-methylbenzenediazonium tetrafluoroborate (II) synthesized for this study.

a

Molecules under study are expected to attach to H-terminated silicon as shown schematically in Figure 2.

have been subjected to multiple processing steps, leading to uncertainties in the chemical state of the molecular film under study. To address these issues, we describe the results of a series of measurements to both characterize and measure the electronic properties of two benzenediazonium molecules chemically assembled to single-crystal Si(111) substrates. Both UHV STM and XPS techniques were employed to both measure and characterize the molecular layers under study. The molecules were chosen so that one has a trifluoride end group and a large dipole moment while the other has a methyl end group and a correspondingly smaller dipole moment. STM and XPS experimental techniques were chosen because, when combined, they allow reliable topographic, electronic, and structural information with high spatial resolution on the molecular layers. The substrates required for this work rely on the chemical passivation of Si(111) with hydrogen, followed by diazonium chemistry to covalently bond molecules to the H-terminated Si(111) surface, procedures that require wet chemistry and brief exposure to ambient conditions. A secondary goal of our study was to generally assess the reliability of molecular layers prepared in this way. This study has been greatly facilitated by the ability to reliably produce hydrogen-terminated silicon using techniques that are particularly well-established and documented in the literature.22-28

Sample Preparation Molecular Synthesis. Compound I was synthesized using the method of Doyle and Bryker29 starting from 4-trifluoroaniline (see Scheme 1). The synthesis was achieved in 61% yield as a white powder after precipitation from ether. Compound II was synthesized from p-toluidine by the method of Kosynkin et al.30 quantitatively as a pale brown solid. Figure 1 shows the structures of both compounds I and II. 4-Trifluoromethylbenzenediazonium Tetrafluoroborate (I). Boron trifluoride etherate (3.3 mL, 13 mmol) and THF (2.0 mL) were added to a 100 mL round-bottom flask equipped with a stir bar and cooled to -30 °C. To the THF solution was added dropwise a solution of 4-trifluomethylaniline (0.77 g, 6.6 mmol) dissolved in THF (2.0 mL). t-Butylnitrite (1.2 mL, 9.9 mmol) (22) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656. (23) Jakob, P.; Dumas, P.; Chabal, Y. J. Appl. Phys. Lett. 1991, 59, 2968. (24) Jakob, P.; Chabal, Y. J. J. Chem. Phys. 1991, 95, 2897-909. (25) Ramonda, M.; Dumas, P.; Salvan, F. Surf. Sci. Lett. 1998, 411, L839. (26) Wade, C. P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1679. (27) Allongue, P.; de Villeneuve, C. H.; Morin, S.; Boukherroub, R.; Wayner, D. D. M. Electrochimica Acta 2000, 45, 4591-98. (28) Wayner, D. M.; Wolkow, R. A. J. Chem. Soc. Perkin Trans. 2002, 2, 23-34. (29) Doyle, M. P.; Bryker, W. J. J. Org. Chem. 1979, 44, 1572-4. (30) Kosynkin, D.; Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc. 1997, 119, 4846-4855.

dissolved in THF (1.0 mL) was added dropwise. After 30 min at -30 °C, the solution was allowed to warm to 0 °C and ether (50 mL) was added. The precipitate was collected to yield 0.83 g (61%) of the desired product. 1H NMR (400 MHz, CD3CN, δ): 8.70 (dd, J ) 11.2, 2.4 Hz, 2 H), 8.21 (dd, J ) 11.2, 2.0 Hz, 2H). 4-Methylbenzenediazonium Tetrafluoroborate (II). Nitrosonium tetrafluoroborate (1.2 g, 10.3 mmol) was added to a roundbottom flask equipped with a stir bar and cooled to -30 °C. Acetonitrile (3 mL) was added to the nitrosonium tetrafluoroborate containing flask. p-Toluidine (1.0 g, 9.3 mmol) was added to a second round-bottom flask, and acetonitrile (7 mL) was added. The contents of the p-toluidine-containing flask were added dropwise to the nitrosonium-containing flask. After 30 min at -30 °C, the solution was allowed to warm to 0 °C, and ether (50 mL) was added. The precipitate was collected to yield 1.9 g (100%) of the desired product. 1H NMR (400 MHz, CD3CN, δ): 8.34 (dt, J ) 8.8, 2.0 Hz, 2 H), 7.72 (m, 2H), 2.60 (s, 3H). The molecules under study are expected to attach to Hterminated silicon as shown schematically in Figure 2.31 Preparation of Hydrogen-Terminated Silicon. It is important to produce atomically flat and oxide free substrates in a reliable way before the attachment of any molecular layer. In this study, we employ a well-documented chemical etching technique using 40% NH4F to produce atomically flat Si(111) surfaces. Among the many parameters influencing the atomic flatness of the hydrogen-terminated silicon surface are pH of the of the fluoride solution,22,23 miscut angle of the Si(111) substrates,24 doping level of the substrate,25 and use of an oxygen-free etching solution.26,27 The substrate used in this study was a double-sided polished n-doped Si(111) wafer (resistively of 0.1-0.3 Ω‚cm-1) purchased from Montco Silicon Technologies, Inc. Approximately 10 mm × 10 mm pieces were cut from the as-supplied wafer, which had a typical miscut angle of 0.5°. Prior to chemical processing, one side of the sample was slightly roughened using sand paper. Prior to hydrogen termination, the Si samples were first rinsed in a sonicator using the sequence of high-purity solvents: acetone, methanol, 2-propanol, and ethanol. After solvent cleaning, the wafers were further cleaned in hot piranha (3:1 H2SO4:H2O2) for 30 min. After cleaning in hot piranha, the substrate was rinsed under running high-quality Millipore water and dried under a stream of dry nitrogen. For hydrogen termination of the silicon surface we used 40% ammonium fluoride solution (finyte grade from J.T. Baker). Before etching the silicon substrate, 40% NH4F solution and high-quality Millipore water were degassed using an Ar bubbler for 45 min to ensure that the etching solution is oxygen free. After NH4F solution degassing, a clean silicon substrate was held in the etching solution at an angle, rough side facing upward,27 for 8-10 min. After this treatment, the substrate was washed using degassed high-quality Millipore water and dried under a stream of dry nitrogen. (31) The compounds were named using the AutoNom algorithm from Beilstein included in ChemDraw Ultra, version 7.0.1.

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Figure 2. Attachment of I and II to Si:H substrates. For the studies reported, end groups are CF3 for I or CH3 for II.

Preparation of Molecular Samples. Prior to execution of the above substrate procedure, powders of the molecule 4-trifluoromethylbenzenediazonium tetrafluoroborate (I) or 4-methylbenzenediazonium tetrafluoroborate (II) were removed from a covered vial stored at -10 °C. Acetonitrile (Mallinckrodt Baker) was degassed with a N2 bubbler for 45 min before dissolving ∼0.8 mg of the molecular powder in 3 mL of the solvent. In this way, a 1 mM molecular solution was prepared in acetonitrile. After preparing the molecular solution, a hydrogen-terminated silicon substrate was placed in a clean capped glass vial containing the molecular solution for 1 h. After this treatment, the substrate was washed in degassed acetonitrile, dried under a stream of dry nitrogen, and stored in a vacuum desiccator until inserted into either the UHV STM or the XPS sample insertion load-lock chamber. It required approximately 6 min exposure to ambient conditions to transfer the prepared sample from the desiccator into a loadlock chamber mounted on the STM apparatus. For the UHV STM chamber, the pump downtime to achieve a pressure of 10-8 Torr was approximately 45 min. At 10-8 Torr base pressure in the STM sample preparation chamber, the silicon substrate was heated to ∼150 °C (estimate) for approximately 60 min. This procedure was adopted to desorb hydrocarbon contaminants from the substrate’s surface before insertion into the UHV STM chamber for further analysis. For the XPS analysis, the sample was transported in a vacuum desiccator to the XPS equipment, followed by insertion and pump down. In this case, exposure to ambient was less than 10 min. Experimental Section Two separate experimental setups were used in this study. The STM experiments were performed using an Omicron UHV STM located inside an apparatus with three compartmentalized chambers serving as a sample insertion chamber, a sample processing chamber, and an STM chamber. The STM is housed in an ion-pump vacuum chamber with a base pressure in the low 10-10 Torr range. The entire apparatus is mounted on a floating table to achieve vibration isolation. Electrochemically etched tungsten tips were used in the STM study.32,33 Tip preparation was carried out in an aqueous solution of 2 M NaOH. The etching voltage was 12 V. In this setup, the cathode was a carbon rod, while the anode was 0.15 mm diameter W wire. I(V) data were acquired from regions of the substrate deemed to be representative as judged by STM images. The tip was typically positioned in the middle of a flat terrace, and a sequence of approximately 2000 I(V) spectra were obtained. Between each I(V) curve, the set-point conditions were reestablished to correct for small drifts in the tip position after the feedback circuit was disabled. One I(V) curve was obtained by averaging the approximately 2000 individual I(V) curves acquired from the same place on the substrate. (32) Mendez, J.; Luna, M.; Baro, A. M. Surf. Sci. 1992, 266, 294. (33) Melmed, A. J. J. Vac. Sci. Technol. B. 1991, 9, 601-08.

When the STM electronics apply a negative bias voltage, electrons flow from the sample to the tip. To assess the coupling of the tip states to the molecular layer, I(V) spectra were acquired by selecting a fixed set-point tunnel current (300 pA) at a specified bias voltage, turning off the feedback, and ramping the applied bias while digitizing the tunnel current. After sweeping the bias, the bias is returned to the original value, and the feedback circuit is again enabled before another I(V) data set is acquired. When implementing this procedure, the initial set-point current must be chosen carefully since too high of a value can cause damage or desorption to the molecular layer. After acquiring a number of I(V) data sets, the tip is controllably positioned closer to the molecular layer by systematically reducing the bias. This technique is ideal for assessing changes in the I(V) characteristics related to tip-molecule separation while keeping the molecule-substrate coupling fixed. The XPS data were obtained using a Kratos Ultra DLD spectrometer using monochromatic Al KR radiation (hν ) 1486.58 eV) and fixed analyzer pass energy of 20 eV. The atomic concentrations of the chemical elements in the near-surface region were estimated after the subtraction of a Shirley-type background, taking into account the corresponding Scofield atomic sensitivity factors and empirically chosen attenuation function to compensate the different attenuation length for the photoelectron emitted from the different electron levels. The binding energy (BE) values referred to the Fermi level were calibrated using Cu 2p and Au 4f lines. Typical resolution measured as a full width at half-maximum (fwhm) of the Si 2p peaks was 0.5 eV. X-ray generated photoelectrons originate from the topmost ∼5-10 nm of a substrate, providing useful information about surface chemical composition. Furthermore, by changing the emission angle, the spectrometer can operate in a surface sensitive mode (so-called angle resolved XPS). The XPS spectra were fitted by CasaXPS software34 assuming Gaussian-Lorentzian line shape. It is useful to note that I has two bonding configurations for carbon atoms: carbon atom in the CF3 functional group and carbon atoms in the benzene ring. Therefore, a constraint used when fitting the C 1s XPS data from I was to specify two components representing CF3 and C-C contribution with a 1:6 ratio. Three additional components were required to model residual hydrocarbon contamination.

Results Using XPS techniques, it is possible to critically assess the quality of the samples prepared for this study. Important questions that we addressed are (i) the extent of oxidation of the H-terminated silicon when exposed to ambient conditions, (ii) the evidence for any remaining NH4+ or F- left over from etching, (iii) the extent of hydrocarbon contamination of the hydrogen terminated surfacesboth from solvent cleaning as well as exposure to ambient, (iv) confirmatory evidence that molecular binding has been achieved, (v) the effect of temperature on the molecular layer, and (vi) the composition (in atomic %) of the molecular layer on the surface of Si(111). (34) Fairley, N. CasaXPS, Software Version 2.3.12.

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Figure 3. Panel a: Si 2p core level spectra obtained from the Si:H. Panel b: Si 2p core level spectra obtained from I sample. The spectra collected in the normal direction, θ ) 0°.

In these initial XPS studies, parallel samples of Si(111) coated with I were prepared and studied using STM. This particular molecule is convenient for XPS study because the fluorine atoms in the trifluoride end group provide a clear XPS signal. Identical surface preparation techniques were used for II, but these molecules lack a clear signature end group like fluorine. For this reason, no XPS study of the methylbenzenediazonium-covered surfaces was attempted. XPS Study of Si:H Sample. XPS was used to analyze typical hydrogen-terminated Si(111) samples before and after molecular adsorption. The Si 2p spectrum obtained from the hydrogenterminated Si(111) surface (see Figure 3a) revealed the characteristic doublet structure, Si 2p3/2 and Si 2p1/2. The BE of the Si 2p3/2 peak was 99.3 eV. No peak near a binding energy of ∼103 eV indicates the absence oxidation of the hydrogenterminated Si(111) substrate during the transport and insertion into the XPS system. The main conclusion is that the proposed procedure for hydrogen termination of Si(111) samples works and that a sample can be transferred through atmosphere without severe contamination. In addition, a C 1s peak centered at 284.8 eV (Figure 4a) was found and is characteristic of hydrocarbons that adsorb onto the surface of a solvent-cleaned sample after exposure to ambient conditions. The C 1s spectrum can be fitted with three components: C-C, C-O, and OdC-OH, as shown in Figure 4a. XPS Study of 4-Trifluoromethylbenzenediazonium Tetrafluoroborate on Si(111). XPS was used to search for evidence that the chemical bonding outlined in Figure 2 had been successfully implemented. From an analysis of the XPS spectra, we conclude that the top ∼5 nm of the Si(111):I molecular sample after transfer to the analysis chamber was found to contain the following: fluorine (6.4 atom %), oxygen (20.3 atom %), nitrogen (1.3 atom %), carbon (13.4 atom %), and silicon (58.5 atom %).

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Figure 4. Panel a: C 1s spectra obtained from the Si:H. Panel b: C 1s spectra obtained from I sample when bonded to Si(111). The constraint (see text) for I is also shown. The spectra are collected in the normal direction, θ ) 0°.

The absence of other elements, in particular boron, provides a validation of the mechanism shown in Figure 2. A representative Si 2p spectrum is shown in Figure 3b and reveals a small peak at 103.2 eV, which can be assigned to SiO2 formed during the formation of the molecular layer.35 The peak at 103.2 eV could also assigned to a Si-F bond.36 This later assignment suggests an excess of surface fluorine (left after the chemical reaction) with respect to carbon. A typical C 1s spectrum obtained from the I sample is shown in Figure 4b. Close examination reveals a new peak at 292.3 eV, which is characteristic of the CF3 group.37,38 A careful curvefitting analysis revealed five Gaussian-Lorentzian components, three of which (284.8, 286.3, and 289.0 eV) corresponded to residual hydrocarbons similar to the Si:H sample (Figure 4a). The components at 284.8 and 292.3 eV originate from 4-trifluoromethylbenzenediazonium. The peak at 284.8 eV is assigned to carbon atoms of benzene ring, whereas the peak at 292.3 eV is associated with the CF3 group. In order to verify the surface nature of the adsorbed species, angle resolved XPS (ARXPS) experiments were performed. By varying the detection angle higher, surface sensitivity can be achieved. As shown in Figure 5, the carbon, oxygen, and fluorine signals all tend to increase with angle, whereas the silicon concentration decreases. The angle is measured from the surface normal. This result is expected because the effective concentration of those elements situated at the substrate’s surface increase (35) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, NJ, 1992. (36) Gray, R. C.; Hercules, D. M. Inorg. Chem. 1977, 16, 1426-7. (37) Beamson, G., Briggs, D., Eds.; The XPS of Polymers Database; SurfaceSpectra Ltd.: Manchester, U.K., 2000 (ISBN 0-9537848-4-3; CD version). (38) Ameen, A. P.; Ward, R. J.; Short, R. D.; Beamson, G.; Briggs, D. Polymer 1993, 34, 1795-9.

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Figure 5. Atomic concentration as a function of the cosine of the emission angle for C, F, N, O, and Si. The emission angle θ is provided for convenience on a nonlinear scale across the top of the graph.

with angle while silicon, lying under the adsorption layer, can be considered as a bulk compound. The bulk contribution should decrease with the angle. It is notable that the silicon concentration is a linear function of cos θ. According to the analysis of photoelectron peak intensities reported by Fadley,39 the peak of the k subshell from a semi-infinite substrate with a non-attenuating overlayer at fractional monolayer coverage with Ekin ≡ Ek is given by

Nk(θ) ) I0 × S0(Ek) × F ×

dσk × Λe(Ek) cos θ dΩ

(1)

where I0 is the photon flux; S0(Ek) is the spectrometer response function (including the instrument detection efficiency, acceptance angle dependence, and analyzed-area dependence); F is the substrate density (per cm-3); dσk/dΩ is the differential ionization cross-section for the k subshell; and Λe(Ek) is the electron attenuation depth. The details of the analysis based on Fadley’s forumla39 are provided in the Supporting Information. The linear dependence of the Si 2p peak on cos θ suggests that the nonattenuating approximation is valid. The analysis of the angular dependence of the Si0 and SiO2 peaks revealed that SiO2 likely forms small particles on the surface and does not cover the entire surface with a thin film. The coverage of I was estimated using the equation for a semi-infinite substrate with a non-attenuating overlayer at fractional monolayer coverage:39

dσk subst Nl(θ) S0(Ek) × dΩ × Λe (Ek) cos θ coverage ) (2) dσl Nk(θ) S0(El) × ×d dΩ where d is the interlayer distance for Si(111) to be 3.14 Å. The coverage of 4-trifluoromethylbenzenediazonium was 0.10 ( 0.02 mL. Tests to better understand the nature of the adsorbed hydrocarbon contamination were also performed. By heating the sample to 200 °C under UHV conditions, a 2-fold decrease of carbon concentration was observed, but the ratio between the (39) Fadley, C. S. Electron Spectrosc.: Theory, Tech., Appl. 1978, 2, 1-156.

various C 1s components did not significantly change. This result is interpreted as desorption due to heating of adsorbed species without noticeable decomposition as judged from the XPS spectra. Independent confirmation of this result is also supported by parallel STM studies (see below). Further studies indicated that heating to higher temperatures near 400 °C results in decomposition and desorption of the molecular species I. In summary, the picture of the surface obtained from XPS studies is evidence for CF3 and C bonding in a benzene ring, strongly indicating that the 4-trifluoromethylbenzenediazonium molecule is chemically attached to the Si(111) substrate. Heating to 200 °C indicates the desorption of hydrocarbon contaminations resulting from residual solvent cleaning or exposure to ambient conditions. Substrate Characterization of Si:H and Si:4-Trifluoromethylbenzenediazonium Substrates from STM Topography. Extensive STM studies were performed on different hydrogenterminated and molecular substrates. The studies ranged from routine STM imaging to detailed measurements of I(V). Typical STM images revealed well-formed atomic steps for molecular as well as hydrogen-terminated silicon surface as shown in Figure 6. These images show clear evidence of well-formed step edges as well as occasional triangular etch pits. Evidence for the presence of a molecular layer can be obtained from careful studies of the STM images. While clear images of individual molecules bonded to the silicon substrate have not yet been obtained, surface roughness analysis of 20 nm × 20 nm regions on a flat terrace indicates that the typical surface roughness observed for Si:H increases by roughly a factor of 2 when the molecular layer I is added to the Si(111) substrate. This is consistent with the coverage of 0.10 ( 0.02 mL estimated from analysis of the XPS data above. We find evidence that the apparent surface roughness increases near the bottom of a step edge when compared to the middle of a flat terrace. In addition, STM images of the molecular layer I reveal evidence for localized bright (high) regions of ∼5 nm diameter on otherwise flat terraces (not shown). These bright regions have been observed to remain fixed in position for a few scans (lasting typically 10-20 min) and then disappear. There is evidence that these bright spots increase with the time of the sample in the UHV chamber and that they seem more likely to appear near step edges. At this time, it is not clear whether these fluctuating bright regions are related to weakly chemisorbed gases present in the STM chamber or to local fluctuations in the benzene ring orientation that persist over extended periods of time. Every once in a while, due to the 0.1 mL coverage of I inferred from the XPS studies, it is reasonable that the I(V) data might resemble Si(111):H more than I on Si(111). This is in fact the case, and we find that roughly 1 in 5 times, when the tip is randomly positioned over a flat region of the Si substrate, the resulting I(V) data are more similar to Si(111):H than to I(V) data typically obtained from a Si(111):I region. Electrical Characterization from STM I(V) Studies. The majority of our effort was to establish that reproducible I(V) spectra could be acquired from both hydrogen-terminated and molecular-coated silicon substrates. The data in Figure 7 taken for I show representative I(V) data from three different samples. Our studies demonstrate that reproducible and reliable I(V) data can be acquired. Figure 8 summarizes the important trends we have found between hydrogen-terminated Si(111):H, Si(111):(I), and Si(111):(II). Only a limited number of I(V) results are displayed so that the overall trend in the data can be clearly observed. As

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Figure 6. Typical STM images of (a) Si(111):H and (b) Si(111) covered with I. Clear evidence for flat terraces and step edges is present. Scan size is 200 nm × 200 nm.

Figure 7. I(V) data illustrating the typical reproducibility from three different Si(111) substrates covered with the 4-trifluoromethylbenzenediazonium (I) molecule.

Figure 9. Ratio of the current through I to the current through II for different applied bias. The data near zero bias are suppressed due to excess noise in the current ratio when the tunnel current goes to zero. The horizontal dashed line indicates the behavior expected if no current enhancement is observed. The solid lines through the data points are guides to the eye.

to II. In Figure 9, the ratio of the current from I with respect to II is plotted for different set-point voltages (i.e., for different tip-molecule separations). All three data sets of data show a similar trend with a peak enhancement of approximately 12 ( 3 occurring at a positive bias of 0.4 ( 0.2 V. Since the data were acquired at different tip-molecule separations, the data indicate that the current enhancement does not critically depend on the tip position above the molecule. Because of the close similarities in the binding of the two molecules to Si(111), the enhancement is most likely due to a shift in the position of a molecular state, causing an approximate factor of 10 enhancement in current through I with respect to II.

Theoretical Analysis Figure 8. Representative I(V) data for Si(111):H, Si(111):(I), and Si(111):(II). Data for two different tip-molecule separations are plotted.

the set point voltage is increased, the tip-molecule distance becomes larger in order to keep the set-point tunnel current constant. The data clearly indicate that I produces an enhancement in the current flow as the substrate is biased positive with respect to the tip (electron flow from tip to substrate) when compared

A quantitative prediction of current flow through a molecule is difficult and requires close attention to a number of issues which include (i) the substrate DOS; (ii) the movement of molecular states (levels) under an applied bias; (iii) the reduction of the vacuum barrier as a tunneling contact is made; (iv) an estimate of the fraction of applied bias dropped across the molecule; (v) the atom positions forming both contacts; and (vi) the atomic and/or molecular deformation due to large surface forces. A theory that quantitatively includes these effects is very computationally demanding, requiring a full atomistic nonequi-

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Figure 10. Conjugate gradient relaxed geometry obtained via the DFT local density approximation. (a) Trifluoromethylbenzene on an eight-layer Si(111) slab. (b) Methylbenzene on an eight-layer Si(111) slab. (c) Free trifluoromethylbenzene molecule. (d) Free methylbenzene molecule. The shaded regions indicate the atoms from which PDOS information is extracted in the transport analysis.

librium analysis of the combined tip and substrate system.40 To make progress experimentally, it becomes important to carefully choose molecular systems that can be meaningfully compared to one another. In this way, a few of the issues raised above can be specifically targeted while many of the uncertainties can at best be controlled. We can gain a very good qualitative understanding of the underlying physics from a detailed electronic structure analysis of the substrate alone, ignoring tip-sample interactions and nonequilibrium effects. Therefore, we present a detailed theoretical analysis of the relaxed atomic structure of I and II covalently bonded to Si(111). Geometry optimization of the two molecules bonded to the Si(111) substrate was performed via the conjugate gradient method. We applied the density functional theory (DFT) local density approximation (LDA)41 and converged each geometry to 0.01 eV/Å on a real-space grid corresponding to an energy cutoff of 300 Ry. A double-ζ polarized local atomic orbital basis set was employed, and each unit cell was periodically repeated in all calculations to form a super cell with 66 k-points in the (x, y) plane. The relaxed atomic structure for I on an Si(111) slab eight layers deep is shown in Figure 10a. The relaxed atomic structure for II is nearly identical (Figure 10b), with the orientation of the benzene ring positioned upward in both cases by the C-Si bond in agreement with the structure shown in Figure 2. The projected density of states (PDOS) of I and II on Si(111) show a strong asymmetry about -4.5 eV or mid-gap. The eigenstates of I have an overall shift of 0.5 eV downward (Figure 11a). This shift is not due to a charge screening interaction between the strong dipole42,43 in I with the Si(111) substrate and is found to be present in the isolated DOS of I and II as shown in Figure 11b (see also Figure 10, panels c and d). To a first-order approximation, we can assume electron transmission through (40) Bevan, K. H.; Ferdows, Z.; Kienle, D.; Guo, H. Phys. ReV. B. Submitted for publication. (41) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcı´a, A.; Junquera, J.; Ordejo´n, P.; Sa´nchez-Portal, D. J. Phys.: Cond. Matt. 2002, 14, 2745-2779.

the molecule is proportional to the PDOS44 and place our Fermi level at approximately -4.0 eV.45 Note that at reverse bias a drop in transmission occurs if one were to interpret the bare PDOS plots (see dashed blue and solid black curves in Figure 11). This reverse bias drop in transmission is not however reflected in the experimental I(V) curves. In the experimental curves plotted in Figure 8, forward bias gain is present, but no such reverse bias drop in current exists because the same set-point current (-0.3 nA) is used in each measurement. A slight movement of the tip toward I, on the order of 0.05 nm,40 will likely increase the reverse bias transmission through I to reach equivalence with the reverse bias transmission through II. Hence, to ensure the same reverse bias set current in our PDOS/transmission, we normalize the curves at -1.5 V bias and obtain an increased PDOS/transmission profile for I shown as a red dotted curve in Figure 11a. The normalized curve for I clearly shows that the transmission at forward bias should be near an order of magnitude greater than that of II. This qualitatively confirms the near order of magnitude experimental current ratio observed at forward bias in Figure 9. As a further confirmation, we find that the ratio (42) 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.; Lyengar, 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.; Bakken, V.; 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.; Dannenberg, 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.; Stefanov, 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.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford CT, 2004. (43) In Gaussian03 we have found the dipole moment of trifluoromethylbenzene to be 3.00 D and that of methylbenzene to be 0.45 D. (44) Tersoff, J.; Hamann, D. R. Phys. ReV. B. 1985, 31, 805-813. (45) CRC Handbook of Chemistry and Physics; CRC Press: New York, 2003.

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Figure 11. (a) PDOS for trifluoromethylbenzene and methylbenzene adsorbed on Si(111). The black solid curve displays the PDOS for methylbenzene. The dashed blue curve displays the PDOS for trifluoromethylbenzene. The dotted red curve displays the normalized PDOS/ transmission for trifluoromethylbenzene. This is done to match the experimental conditions and insures the same set current for methylbenzene and trifluoromethylbenzene at -1.5 V. (b) DOS for the isolated trifluoromethylbenzene (blue dashed) and methylbenzene (solid black) molecules. This shows that the shift in electronic levels is not due to an interaction between the strong trifluoromethylbenzene dipole and the Si(111) substrate.

between currents over I at +1.5 V and -1.5 V is found to be 8.11 (Ef ) -4.0 eV). Likewise the ratio between currents over II at +1.5 V and -1.5 V is found to be 0.69 (Ef ) -4.0 eV). Both ratios are close to the experimental values given in Figure 8. These calculations confirm that the experimental I(V) data are indicative of conduction through I and II covalently bonded to Si(111). We have neglected band bending and molecular level motion under bias in our substrate electron transmission model. However, first-order band bending calculations46 (not shown here) have shown that for both molecules, the degree of silicon band bending due to an applied bias is nearly equivalent and will cancel when analyzing the trends in the ratio of the I(V) data. Yet the presence

of a strong dipole in I, which would likely influence level motion under bias, cannot be entirely neglected. For this reason it seems worthwhile to perform a full nonequilibrium transport analysis40,47 to obtain a quantitative understanding of the I(V) characteristics. Such a detailed study would conclusively determine what fraction of the current through I and II are due to the tip position and how much is due to the position of molecular states under bias.

(46) TMA Medici, Two-Dimensional DeVice Simulation Program Version 4.0 Users Manual, 4.0; Technology Modeling Associates Inc.: Sunnyvale, CA, 1997.

(47) Hofer, W. A.; Fisher, A. J.; Lopinski, G. P.; Wolkow, R. A. Chem. Phys. Lett. 2002, 365, 129-134.

Conclusions In general, the ability to produce high-quality molecular interfacial layers of nanometer thickness with a specified polarity at the interface of a silicon surface provides a useful approach

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for molecularly tailoring future Si devices. Using molecular layers to locate dipoles at an interface is a way to enhance current flow by adjusting the polarity of end groups while maintaining a welldefined chemistry for the attachment of the molecules to the interface. To address these important issues, UHV STM studies have been completed on molecular layers formed from I and II. The molecules chosen for this study rely on a Si-C bond using diazonium chemistry to form a molecular layer on H-passivated Si(111). We find that the H-passivation is effective in maintaining the integrity of the Si(111) surface even while molecular bonding is achieved under wet chemical conditions. XPS data independently confirm that H-passivated Si(111) remains oxygen-free for short exposures to ambient conditions and provide evidence that the diazonium molecules used in this study chemically react with the silicon surface, forming a Si-C bond. STM I(V) studies at different tip-substrate separations reveal asymmetric I(V) data for the two molecules studied. The magnitude of the measured asymmetry is consistent with the projected density of states calculations found from a DFT calculation after geometry

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optimization of the two molecules on Si(111) was performed via the conjugate gradient method. Acknowledgment. This work benefited from partial support from Sandia National Laboratory. Sandia is a multiprogram laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC0494AL85000. We acknowledge Dr. J. Simonson who partially funded the work through Sandia’s DARPA MGA Program. The theoretical analysis was supported by an ARO-DURINT contract. The invaluable advice provided by J. Pitters and R. Wolkow in the early stages of this study on the preparation of high-quality H-passivated Si(111) is most gratefully acknowledged. Supporting Information Available: A summary of the formalism found in ref 39 with small modifications. This material is available free of charge via the Internet at http://pubs.acs.org. LA063235I