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Bidirectional Control of Silicon’s Surface Potential by Means of Molecular Coverage Sreenivasa Reddy Puniredd,†,‡ Ilia Platzman,†,‡ Raymond T. Tung,§ and Hossam Haick*,‡ The Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, TechnionsIsrael Institute of Technology, Haifa 32000, Israel, and Department of Physics, Brooklyn College of the City UniVersity of New York, Brooklyn, New York 11210, United States ReceiVed: August 17, 2010; ReVised Manuscript ReceiVed: September 29, 2010
Here we report on a simple bidirectional strategy for increasing and decreasing the surface potential of Si, with good chemical stability. The strategy is based on a deliberate control of monolayer coverage of propenyl molecules having a steric diameter that is smaller or comparable to the distance between adjacent Si surface atoms. By modifying the coverage of the exposed functional groups, we show an ability to obtain both valleyshaped or hill-shaped modulation trends of the Si(111) surface potential. 1. Introduction
2. Experimental Section
The ability to chemically modify the surface properties of silicon (Si) is of great importance to the interfacing of molecular electronics to Si-based circuitry, in developing novel (bio)chemical sensors, in obtaining higher efficiencies from existing photovoltaic and photoelectrochemical cells, and in a plethora of other applications.1-8 However, to move forward rationally, how the chemical states of such modified surfaces correlate with resulting changes in key properties such as their electrical, physical, and electronic characteristics needs to be understood. Several studies have exploited the structure/function relationships for Si surfaces and demonstrated new and beneficial devices through modification of the Si work function by selfassembled (sub)monolayers of polar organic molecules.1-9 In those studies, it is usually observed and assumed that the work function shifts in a direction consistent with the sign of the molecular dipole, and monotonously with the coverage of the molecules. Furthermore, in order for the device to be reliable, it is desirable for the modified Si surface to be stable. Modification of Si surface dipole by a partially or fully saturated, yet loosely packed, layer of molecules is therefore not favored, because of lack of protection against oxidation.4 To tune the work function, a popular approach has been to synthesize and use derivatives of the same molecule with each having a different (electron withdrawing or accepting) functional group.1,3,5,7 Besides the synthesis challenges involved in this approach, different molecules/derivatives result in usually different molecular densities, conformations, and/or orientations and, thus, varied resistance to oxidation. Here we report on a simple strategy for both increasing and decreasing the surface potential of Si, with good chemical stability. The strategy is based on a deliberate control of monolayer coverage of short organic molecules having a steric diameter that is smaller or comparable to the distance between adjacent Si surface atoms.4 We also present evidence for a novel dependence of the work function on molecular coverage and show that this is consistent with the depolarization of molecular dipoles in densely packed layers.
2.1. Materials. Silicon wafers were obtained from singlesided, polished (111)-oriented wafers having a thickness of 525 ( 15 µm. The wafers are Sb-doped (n-type) and have a resistivity of 0.008-0.020 Ω-cm (Virginia Semiconductor Inc.). Wafers used in this study were obtained from the same cassette, and each subset of samples was obtained from a single wafer. Unless noted otherwise, all chemicals were used as received without further purification. Deionized (DI) water having a 18 MΩ cm-1 resistivity was obtained from a Millipore Nanopure system. Methanol, acetone, dichloromethane, 1,1,1-trichloroethane (all from Frutarom Ltd.), chlorobenzene, N-bromosuccinimide (NBS), dichloromethane, ammonium fluoride, phosphorus pentachloride (PCl5), benzoyl peroxide (97%), and 0.5 M 1-propenylmagnesium bromide (CH3-CHdCH-MgBr) in THF solution (all from Sigma-Aldrich) were used as received. 2.2. Sample Preparation and Functionalization. Prior to chemical modification, all Si(111) samples were cleaned by sequential rinse with DI water, methanol, acetone, dichloromethane, 1,1,1-trichloroethane, acetone, methanol, and DI water. Samples were then dried under a stream of N2(g). H-terminated Si(111) surfaces were obtained through wet chemical etching for 15 min in 40% NH4F(aq). The samples were agitated periodically to minimize the formation of etch pits. Following etching, the monohydride-terminated surfaces were rinsed by flowing DI water and dried under a stream of N2(g). Si-H samples were alkylated using the two-step chlorination/ alkylation protocol.4,10 Freshly etched Si(111) samples were first immersed in a saturated solution of PCl5 in chlorobenzene, to which a few grains of benzoyl peroxide were added. The reaction solution was then heated to 90-100 °C for 45 min. The chlorinated samples were rinsed sequentially with chlorobenzene and THF, and the samples were transferred to a N2purged glovebox. The samples were subsequently immersed for 2.5, 5, 8, 12, 16, 20, 24, and 28 h at 120-130 °C in a THF solution of 0.5 M CH3-CHdCH-MgBr to get 25%, 35%, 45%, 55%, 75%, 80%, 90%, and 100% coverage of CH3-CHd CH-Si, respectively. Alkylated Si samples were rinsed with flowing THF and then immersed in methanol. The alkylated samples were transported out of the glovebox and further rinsed with methanol, sonicated in fresh methanol, rinsed with H2O, and dried under a stream of N2(g).4,10
* To whom correspondence should be addressed. E-mail: hhossam@ technion.ac.il. † These authors contributed equally to the paper. ‡ TechnionsIsrael Institute of Technology. § Brooklyn College of the City University of New York.
10.1021/jp107806z 2010 American Chemical Society Published on Web 10/12/2010
Control of Silicon’s Surface Potential 2.3. Secondary Functionalization of Propenyl-Terminated Si(111) Samples. Si(111) samples that were fully terminated with propenyl molecules (i.e., 100% coverage; cf. ref 4) were placed in a 0.25 M solution of NBS in dry CH2Cl2 at 0 °C for different time periods. The samples were then sonicated for 2 min and rinsed vigorously with CH2Cl2 and DI water and, also, dried under a stream of N2(g). NBSsan effective reagent for bromination at an allylic positionswas used as the key compound. This functionality inserts into the C-H bonds of the terminal methyl groups through a reactive bromine.11 2.4. X-ray Photoelectron Spectroscopy (XPS). Surface analysis of molecularly terminated Si(111) surfaces was performed by XPS (Thermo VG Scientific, Sigma probe, U.K.) having a base pressure of 50% coverage the Φ of the modified Si was independent of the coverage. This difference compared to current results can be attributed to the different morphologies of adsorbed molecular layers. The partial molecular layer in previous work was arranged differently compared to that in the current work and was characterized by finite domains with characteristic diameter and separated by the pinholes. The results for the present Si surfaces covered with a homogeneous molecular layer are easy to model, because the electric potential, which is expected to be uniform laterally, arises only from the polarization of the individual molecules and the depolarization due to intermolecular interaction. In previous work involving incomplete coverage of “patchy” molecular layer, not only is the electric potential laterally inhomogeneous but the depolarization for different molecular patches may vary, depending on the layer morphology. For cases targeting an opposite “Φ versus coverage” trend, changing the coverage of a functional group, such as Br, on top of 100% propenyl-terminated Si(111) surfaces could achieve this purpose (see Figure 4). As can be seen in the figure, the Φ of the Si(111) surface increased from 4.09 ( 0.02 eV for 100% propenyl-terminated Si(111) surfaces (or 0% Br coverage) to 4.66 ( 0.02 eV at 66 ( 3% coverage, after which it decreased gradually to 4.36 ( 0.02 eV at 98 ( 2% coverage. Based on DFT calculations (see Experimental Section), the dipole moment of the Br-terminated propenyl molecules has an opposite direction to that of the propenyl molecules. Therefore, the increase in the Φ observed in Figure 4 is due to the electron transfer from the molecules to the Si surface atoms. The hill-
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like trend observed in Figure 4 can be explained in the same manner as that of Figure 3: an increase in Br-termination initially increases the work function, until at high coverages the depolarization effect surpasses the dipole effect and reverses the trend of the work function change. 4. Summary and Conclusions We have shown that homogeneous, partial, and full coverage of propenyl molecules can be achieved using a simple chlorination/alkylation process. Based on this approach, we have shown a strong effect between the molecular coverage and work function of the Si sample. Additionally, by modifying the coverage of the exposed functional groups, we were able to obtain both valley-shaped or hill-shaped modulation trends of the Si(111) surface potential. The first major implication of this work is that systematic control of semiconductor surface potential or semiconductor-containing interface, by means of organic molecules, is also possible, via a unique synthesis route, without the need to change the individual molecule’s dipole. The second major implication is that one can improve the molecular controllability and chemical stability of a given (unique) electronic device without the need to physically replace the exposed functional groups on the surface, by hard synthesis and/or self-assembly techniques. Acknowledgment. We acknowledge the U.S.-Israel Binational Science Foundation and the National Science Foundation (DMR) for financial support and Ossama Assad (Technion) and Omer Yaffe (Weizmann Institute of Science) for help. H.H. is a Knight of the Order of Academic Palms and holds the Horev Chair for Leaders in Science and Technology.
Puniredd et al. References and Notes (1) Vilan, A.; Yaffe, O.; Biller, A.; Salomon, A.; Kahn, A.; Cahen, D. AdV. Mater. 2010, 22, 140–159. (2) Magid, I.; Burstein, L.; Seitz, O.; Segev, L.; Kronik, L. Y. R. J. Phys. Chem. C 2008, 112, 7145–7150. (3) Hiremath, R. K.; Rabinal, M. K.; Mulimani, B. G.; Khazi, I. M. Langmuir 2008, 24, 11300–11306. (4) Puniredd, S. R.; Assad, O.; Haick, H. J. Am. Chem. Soc. 2008, 130, 13727–13734. (5) Gozlan, N.; Haick, H. J. Phys. Chem. C 2008, 112, 12599–12601. (6) Natan, A.; Kronik, L.; Haick, H.; Tung, R. T. AdV. Mater. 2007, 19, 4103–4117. (7) He, T.; He, J.; Lu, M.; Chen, B.; Pang, H.; Reus, W. F.; Nolte, W. M.; Nackashi, D. P.; Franzon, P. D.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 14537–14541. (8) Bashouti, M. Y.; Tung, R. T.; Haick, H. Small 2009, 5, 2761– 2769. (9) Paska, Y.; Haick, H. J. Phys. Chem. C 2009, 113, 1993–1997. (10) Puniredd, S. R.; Assad, O.; Haick, H. J. Am. Chem. Soc. 2008, 130, 9184–9185. (11) Wamser, C.; Scott, L. J. Chem. Educ. 1985, 62, 650–652. (12) Wide-scan XPS spectra for propenyl-terminated Si surfaces have not shown any residues of chlorine and/or bromine from the chlorination/ alkylation process. (13) Basu, R.; Kinser, C.; Tovar, J.; Hersam, M. Chem. Phys. 2006, 326, 144–150. (14) Jin, H.; Kinser, C.; Bertin, P.; Kramer, D.; Libera, J.; Hersam, M.; Nguyen, S.; Bedzyk, M. Langmuir 2004, 20, 6252–6258. (15) Aradi, B.; Ramos, L. E.; Deak, P.; Ohler, T.; Bechstedt, F.; Zhang, R. Q.; Frauenheim, T. Phys. ReV. B 2007, 76, 035305/1-035305/7. (16) Haick, H.; Ambrico, M.; Ligonzo, T.; Tung, R. T.; Cahen, D. J. Am. Chem. Soc. 2006, 128, 6854–6869. (17) Ishida, H.; Terakura, K. Phys. ReV. B 1987, 36, 4510–4514. (18) MacDonald, J. R.; Barlow, C. A. J. J. Phys. Chem. 1964, 68 (10), 2737–40. (19) Topping, J. Proc. R. Soc. London, Ser. A 1927, 114, 67.
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