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Importance of Monolayer Quality for Interpreting Current Transport through Organic Molecules: Alkyls on Oxide-Free Si Oliver Seitz,†,⊥ Till Bo¨cking,‡,⊥ Adi Salomon,† J. Justin Gooding,§ and David Cahen*,† Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot, Israel, School of Physics, UniVersity of New South Wales, Sydney, NSW, Australia, and School of Chemistry, UniVersity of New South Wales, Sydney, NSW, Australia ReceiVed March 16, 2006. In Final Form: June 1, 2006 We study the effect of monolayer quality on the electrical transport through n-Si/CnH2n+1/Hg junctions (n ) 12, 14, and 18) and find that truly high quality layers and only they, yield the type of data, reported by us in Phys. ReV. Lett. 2005, 95, 266807, data that are consistent with the theoretically predicted behavior of a Schottky barrier coupled to a tunnel barrier. By using that agreement as our starting point, we can assess the effects of changing the quality of the alkyl monolayers, as judged from ellipsometer, contact angle, XPS, and ATR-FTIR measurements, on the electrical transport. Although low monolayer quality layers are easily identified by one or more of those characterization tools, as well as from the current-voltage measurements, even a combination of characterization techniques may not suffice to distinguish between monolayers with minor differences in quality, which, nevertheless, are evident in the transport measurement. The thermionic emission mechanism, which in these systems dominates at low forward bias, is the one that is most sensitive to monolayer quality. It serves thus as the best quality control. This is important because, even where tunneling characteristics appear rather insensitive to slightly diminished quality, their correct analysis will be affected, especially if layers of different lengths are also of different quality.
Introduction Self-assembled monolayers (SAMs) of organic molecules present one of the main systems to study electron transport through molecules. Electrical transport measurements through the wellorganized array of molecules formed on the substrate by selfassembly can be carried out after adding an electrical contact to the top of the monolayer. Alternatively, electron transport measurements can be carried out on single molecules, whereby one molecule is arranged between two macroscopically accessible contacts. The approach using SAMs has the advantage that the chemical and structural properties of the molecules in the monolayer can be characterized using a wide range of surface sensitive techniques. However, measuring electrical current through an array, separating two electrodes, is highly sensitive to defects in the array, which present paths of lower resistance than the molecules of the array. Indeed, electrical transport measurements on SAMs are probably one of the most sensitive ways to detect defects in monolayers, especially those that are not suitable for STM investigations. Thus, the question arises as to how to assess if the transport measurements on SAMs reflect the intrinsic electrical transport properties of the molecules or if they are dominated by defects in the monolayer. The question is all the more vexing because different electrical transport results are reported in the literature for what should be very similar or even identical device structures. We consider this question using alkyl SAMs on oxide-free silicon, a system that has been studied extensively since such monolayers were first reported.1,2 These highly robust mono* To whom correspondence should be addressed. E-mail: david.cahen@ weizmann.ac.il. † Weizmann Institute of Science. ‡ School of Physics, University of New South Wales. § School of Chemistry, University of New South Wales. ⊥ These authors contributed equally to this work. (1) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 1263112632. (2) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155.
layers are bound to the substrate via covalent Si-C bonds and withstand treatment with acids, bases, organic and aqueous solvents,2 as well as high temperatures.3 One of the major differences between these SAMs and silane SAMs assembled on silicon oxide is the absence of the oxide interlayer. Although many reports have appeared on the properties (electrical and nonelectrical) of silane SAMs on oxidized silicon,4-13 the advantage of silicon bound SAMs is that eliminating the additional oxide component (whose thickness and quality is not always easy to control) simplifies the interpretation of electronic transport measurements. By making electrical contact with a metal onto the SAM in a nondestructive fashion, a semiconductor/SAM/metal structure is created that allows for the desired electronic transport measurements. There are several advantages of using a semiconductor/SAM/metal instead of a metal/SAM/metal device structure. First, without changing the chemical properties of the substrate, electrical transport can be probed on electronically different substrates by changing doping type and level of the semiconductor. Second, SAM polarity and molecules/electrode interactions can be estimated from the Schottky barrier present inside the semiconductor. In the context of this study, an additional (3) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164-6168. (4) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly. Academic Press: New York, 1991. (5) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (6) Fontaine, P.; Goguenheim, D.; Deresmes, D.; Vuillaume, D.; Garet, M.; Rondelez, F. Appl. Phys. Lett. 1993, 62, 2256-2258. (7) Boulas, C.; Davidovits, J. V.; Rondelez, F.; Vuillaume, D. Phys. ReV. Lett. 1996, 76, 4797-4800. (8) Vuillaume, D.; Boulas, C.; Collet, J.; Davidovits, J. V.; Rondelez, F. Appl. Phys. Lett. 1996, 69, 1646-1648. (9) Collet, J.; Lenfant, S.; Vuillaume, D.; Bouloussa, O.; Rondelez, F.; Gay, J. M.; Kham, K.; Chevrot, C. Appl. Phys. Lett. 2000, 76, 1339-1341. (10) Cohen, R.; Zenou, N.; Cahen, D.; Yitzchaik, S. Chem. Phys. Lett. 1997, 279, 270-274. (11) Chai, L.; Cahen, D. Mater. Sci., Eng. C 2002, 19, 339-343. (12) Selzer, Y.; Salomon, A.; Cahen, D. J. Am. Chem. Soc. 2002, 124, 28862887. (13) Selzer, Y.; Salomon, A.; Cahen, D. J. Phys. Chem. B 2002, 106, 1043210439.
10.1021/la060718d CCC: $33.50 © 2006 American Chemical Society Published on Web 07/07/2006
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important reason for choosing silicon/SAM/metal structures is that we can use well-defined substrates with near-atomically flat surfaces allowing reproducible high-quality SAM formation.14 The use of mercury to form a metal contact to the top of the monolayer is a convenient and reproducible method for forming semiconductor/SAM/metal junctions and has been used in a number of recent studies on oxide-free alkyl monolayers on silicon.15-17 Liu and Yu15 prepared a series of alkyl layers with different chain lengths by reaction of Grignard reagents on H-terminated n-Si(111) and reported a dependence of the electron transport through the n-Si/CnH2n+1/Hg junctions (n ) 6, 8, 10, and 12) on the chain length of the molecules. Faber et al.17 studied a similar system using a mercury electrode on a series of SAMs prepared by hydrosilylation of alkenes in refluxing mesitylene on H-terminated n- and p-Si(100) substrates. For n-Si their results showed an unexpected trend in the current density, measured below 0.4 V, with chain length, whereby the order of the current density is C10 > C22 > C16 > C12. Recently, we reported results of electrical transport measurements (cf. the Supporting Information for details on reproducibility) through such monolayers prepared by thermal reaction of neat alkenes on H-terminated n-Si(111) and found that the electron transport at low bias potentials (
