Effect of Metal−Molecule Contact Roughness on Electronic Transport

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Effect of Metal-Molecule Contact Roughness on Electronic Transport: Bacteriorhodopsin-Based, Metal–Insulator–Metal Planar Junctions Yongdong Jin,*,† Noga Friedman,† Mordechai Sheves,† and David Cahen*,‡ Departments of Organic Chemistry and Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed December 12, 2007. ReVised Manuscript ReceiVed February 20, 2008 Molecular electronics is very much about contacts, and thus understanding of any generic contact effect is essential to its advance. For example, it is still not obvious in how far variations in electrode roughness of macroscopic contacts can lead to rectification. Here we report an investigation of this contact effect on electronic transport properties using metal–insulator–metal planar junctions with a 5 nm thick bacteriorhodopsin-based insulator as model system. We demonstrate that the experimentally observed rectifying behavior is not an intrinsic property of the molecules used, but rather of the local contact quality. Even a slight increase in surface roughness of the bottom electrode gives rise to distinct rectifying behavior in these and, by extrapolation, possibly other molecular junctions.

Introduction Electronic junctions consisting of a molecular monolayer, sandwiched between two electrodes, hold promise for a bottomup approach to nanometer-scale electronic devices with possible unique electrical characteristics.1–3 However, experimental problems of producing junctions reproducibly with high yield, which give reproducible measurement results, still trouble the field.4–6 Indeed questions as to whether results are due to molecules or to extrinsic, even uncontrolled factors abound.5,6 One property that has been at the center of attention is that of rectification.7 Rectification has been observed not only for systems with built-in asymmetry in the structure of the molecule,8,9 but, at times, also on symmetric molecules.10–16 In other cases very little or no rectification was found for systems that contained symmetric molecules.1,17 * Corresponding authors. E-mail: [email protected] (Y.J.); [email protected] (D.C.). † Department of Organic Chemistry. ‡ Department of Materials & Interfaces. (1) Reed, M. A.; Zhou, C.; Miller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (2) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddard, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (3) Chen, Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Williams, R. S.; Jeppesen, J. O.; Nielsen, K. A.; Stoddard, J. F.; Olynick, D. L.; Anderson, E. Appl. Phys. Lett. 2003, 82, 1610. (4) Ratner, M. Nature 2000, 404, 137. (5) Hipps, K. W. Science 2001, 294, 536. (6) Zhirnov, V. V.; Cavin, R. K. Nat. Mater. 2006, 5, 11. (7) Metzger, R. M. Chem. Rec. 2004, 4, 291. (8) See, for example: Chabinyc, M. L.; Chen, X. 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. (9) Elbing, M.; Ochs, R.; Koentopp, M.; Fischer, M.; von Hanisch, C.; Weigend, F.; Evers, F.; Weber, H. B.; Mayor, M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8815. (10) Dhirani, A.; Lin, P. H.; Guyot-Sionnest, P.; Zehner, R.; Sita, L. S. J. Chem. Phys. 1997, 106, 5249. (11) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones, L.; Tour, J. M. Appl. Phys. Lett. 1997, 71, 611. (12) Kergueris, C.; Bourgoin, J.-P.; Palacin, S.; Esteve, D.; Urbina, C.; Magoga, M.; Joachim, C. Phys. ReV. B 1999, 59, 12505. (13) Ottaviano, L.; Santucci, S.; Di Nardo, S.; Lozzi, L.; Passacantando, M.; Picozzi, P. J. Vac. Sci. Technol. 1997, A15, 1014. (14) Pomerantz, M.; Aviram, A.; McCorkle, R. A.; Li, L.; Schrott, A. G. Science 1992, 255, 1115. (15) Taylor, J.; Brandbyge, M.; Stokbro, K. Phys. ReV. Lett. 2002, 89, 138301-1. (16) Reichert, J.; Ochs, R.; Beckmann, D.; Weber, H. B.; Mayor, M.; Löhneysen, H. v. Phys. ReV. Lett. 2002, 88, 176804-1.

It is by now well recognized that molecular electronics is very much about the contacts between the molecule(s) and the external world.5,18–20 Molecule-electrode contacts can be the extrinsic cause for rectification, for example by asymmetric coupling to electrodes,15 contact realization,16 and contact-induced molecule modification.21 Ideally, the metal/molecule contacts in metalmolecule-metal junctions should be ohmic so that any nonlinearity in transport across the junction can be attributed to the molecule(s) and can, thus, be studied.5 However, current–voltage characteristics of molecular devices can be very sensitive to the microscopic details of the contact.16 The double challenge is, thus, to identify causes for contact-induced effects and to find sufficiently ideal contacts.18,22 Rectification, due to asymmetric contacts of molecular junctions, including differences in the nature and the area of the contact, is a well-documented phenomenon.15,16,23,24 Most rectifying behavior was observed with small (conducting) molecules (typically 3 h to ensure vesicle fusion. Because the vesicles are negatively charged, due to the bR in them, their adsorption onto the surface is aided by electrostatics, as the APTMS-modified surface is positively charged, after treatment with 0.1 N HCl. After cleaning by sonication for ∼30 s, the sample was, after drying in a flow of N2, ready for characterization and measurements. Two different types of Al substrates, with root-meansquare (rms) surface roughness, after modification with APTMS, of ∼0.6 and ∼0.9 nm were used in this study. The planar junction structures were then completed by depositing a second electrode onto the membrane, adsorbed on the aluminum surface, without damaging the molecules. We deposited Au dots, 60 nm thick and 0.5 mm in diameter, as top contacts on the membrane surface in a “soft” manner using the “lift-off, float-on” technique (LOFO).32 To this end Au dots were evaporated onto clean glass slides, from which they were allowed to peel by dipping the slide at an angle in 2 vol % solution of HF in water. The slide was then dipped into pure water, containing the modified Al substrates, to allow the Au leaves to float. The bR-modified Al substrates were then lifted out of the solution, with the Au pads on top of the substrate, and the samples were dried at room temperature under a very mild N2 stream. Current-voltage (I-V) characteristics were measured using a W needle, connected to a micromanipulator to contact the Au pad. An InGa drop on the Au pad minimizes mechanical (pressureinduced) damage to the film. An HP 4155 semiconductor parameter analyzer, in the voltage scan mode with a sweep rate of 0.1–0.2 V/s, (25) Salomon, A.; Boecking, T.; Seitz, O.; Markus, T.; Amy, F.; Chan, C.; Zhao, W.; Cahen, D.; Kahn, A. AdV. Mater. 2007, 19, 445. (26) Nesher, G.; Vilan, A.; Cohen, H.; Cahen, D.; Amy, F.; Chan, C.; Hwang, J.; Kahn, A. J. Phys. Chem. B 2006, 110, 14363. (27) Segev, L.; Salomon, A.; Natan, A.; Cahen, D.; Kronik, L.; Amy, F.; Chan, C. K.; Kahn, A. Phys. ReV. B 2006, 74, 165323. (28) Kogut, L.; Komvopoulos, K. J. Appl. Phys. 2004, 95, 576. (29) Oesterhelt, D.; Stoeckenius, W. Methods Enzymol. 1974, 31, 667. (30) Racker, E. Biochem. Biophys. Res. Commun. 1973, 55, 224. (31) Jin, Y. D.; Kang, X. F.; Song, Y. H.; Zhang, B. L.; Cheng, G. J.; Dong, S. J. Anal. Chem. 2001, 73, 2843. (32) Vilan, A.; Shanzer, A.; Cahen, D. Nature 2000, 404, 166.

Langmuir, Vol. 24, No. 10, 2008 5623 was used for the electrical transport measurements. The voltage was applied to the Au top electrode with the Al bottom electrode grounded. All transport measurements were performed at ambient conditions and in the dark. AFM topographic images were acquired in the tapping mode under ambient conditions with a Nanoscope IIIa (Digital Instruments, Inc.), using a standard silicon nitride cantilever.

Results and Discussion Preparing a thick molecular film for the purpose stated in the Introduction is problematic. Thus, we find that at ambient condition pure lipid bilayers are not very stable. As a pragmatic solution we use the much more stable bR-containing membranes. The improved stability is attributed to the high negative charges on bR surfaces, which can interact strongly with the positively charged substrate surface and act as a scaffold to strengthen also the lipid bilayers between the bR parts. This property makes them sufficiently robust to allow reliable electron transport studies under ambient conditions. The layer is one bR molecule thick, where bR, arranged in the trimer form, is embedded in a lipid bilayer. We then use the ∼5 nm thick bacteriorhodopsin in lipid layer adsorbed onto the Al/AlOx with the LOFO-deposited top metal contact to obtain a junction structure that we have characterized earlier.33,34 With this system we demonstrate that the experimentally observed rectifying behavior is not an intrinsic property of the sandwiched molecules, but rather of the local contact quality as we compare cases with different electrode roughness. With working junctions, even a slight increase of surface roughness of one of the contacts leads to distinct rectifying behavior in these planar junctions. Figure 1a shows a representative atomic force microscopy (AFM) image of a substrate, covered by bR-containing, vesiclefused membranes, prepared by a 10-min adsorption on the substrate. Section analysis shows the average membrane thickness to be ∼5 nm. AFM images confirm the above-stated problem; i.e., at ambient condition the pure lipid bilayers are much less stable than the bR-containing ones. The advantages of the junction configuration that we use here are as follows: (1) The molecular layers formed by vesicle fusion over selfassembled monolayers (SAMs) can span flexibly across pits and holes (from a few to even tens of nanometers diameter), also on a less than ideally flat, bottom electrode surface. This method yields a molecular film that is macroscopically still flat enough to allow top contact deposition and electrical transport measurements. The situation here can be compared to that of the usual SAMs, which, if deposited on rough surfaces, are prone to yield short circuits between bottom and top metal contacts. It is this property of the vesicle fusion-formed molecular layers that makes it possible to study the effect of macroscopic contact roughness on electronic transport properties. (2) The top “LOFO”-prepared contact of preformed metal patches can gently float on and span small pinholes and cracks (typically up to tens of nanometers, as revealed by AFM) in the molecule layers. (3) These aspects, coupled to the ∼5 nm width of the organic film that is used, further minimize direct contact between the bottom and top electrodes during electronic transport measurements. In our study we kept all experimental conditions identical, except for the surface roughness of the bottom Al electrode. (33) Jin, Y. D.; Friedman, N.; Sheves, M.; He, T.; Cahen, D. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 8601. (34) Jin, Y. D.; Friedman, N.; Sheves, M.; Cahen, D. AdV. Funct. Mater. 2007, 17, 1417.

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Figure 1. (a) Representative AFM image of an APTMS-modified Al/AlOx substrate, covered by bR-containing, vesicle-fused membranes, prepared by a 10-min electrostatic adsorption on the substrate. Section analysis shows the averaged membrane thickness is about 5 nm. (b, c) Typical AFM images of the two batches of Al substrates with rms surface roughness of ∼0.6 and ∼0.9 nm (denoted as Al(1) and Al(2)), respectively, after modification of APTMS monolayers. Section analysis shows that the typical maximum peak-to-valley values are ∼1–2 and 2–4 nm, for Al(1) and Al(2), respectively.

Therefore, differences in the electrical characteristics of these devices reflect differences in the surface roughness of the bottom electrode. We used two different types of Al substrates, with rms surface roughness, after modification with APTMS, of ∼0.6 and ∼0.9 nm, denoted as Al(1) and Al(2), respectively. Figure 1b,c shows typical AFM morphologies of the two types of Al substrates, modified with APTMS.35 Section analysis of Al(2) substrates shows that the typical peak-to-valley value of the substrate surface is ∼2-4 nm, i.e., less than the membrane thickness. Significantly, in light of the three above-noted advantages, it was not possible to prepare junctions on substrates with higher rms surface roughness because of electrical shorts.

Current-voltage (I-V) characteristics were measured in ambient at room temperature in the voltage scan mode. The voltage was applied to the Au top electrode with the Al bottom electrode grounded. To simplify data analyses, all transport measurements were performed in the dark, thereby excluding possible photoeffects. Parts a and b of Figure 2show typical steady-state I-V characteristics of an Al/(bR in lipid bilayer)/ Au junction prepared with Al(1) and Al(2) as bottom electrodes, respectively, over a (1 V bias range.36 The insets of Figure 2 show how the surface roughness and rectification behavior of the I-V curves can be related. The arrows show current flow directions and magnitudes. The Al(1)/insulator/Au junction has

(35) The APTMS monolayer is homogeneous since there is no big polymer island formation and no pronounced difference in rms surface roughness of the substrate before and after silanization.

(36) Often transient behavior of I-V characteristics is confused with nonlinear characteristics, as shown in: Ma, Y. F.; Seminario, J. M. J. Phys. Chem. B 2006, 110, 9708.

bR-Based Metal-Insulator-Metal Planar Junctions

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Figure 3. Plots of rectification ratios, RR ) I(at –1 V)/I(at 1 V), vs eight junctions for junctions Al(1)/insulator/Au (bottom) and junctions Al(2)/ insulator/Au (top).

Figure 2. (a, b) Typical steady state I-V characteristics of a resulting Al/insulator/Au planar junction prepared by using Al(1) and Al(2), respectively, as bottom electrode, over a (1 V bias range. The insets show the schematic junction structures and highlight the membrane/ bottom Al electrode contacts. The arrows show current flow directions and magnitude. The data are plotted in a direction opposite the conventional one, as can be seen clearly in that they appear to show an inverted diode in (b). This is done to keep them consistent with the curves shown in refs 33 and 34. Negative bias corresponds to electron injection from the Au top contact into the molecular layer, i.e., electron flow from Au to Al. Au pad area: 2 × 10-3 cm2.

only a very slightly asymmetric I-V characteristic, while a strong rectification effect is obvious for the Al(2)/insulator/Au junction. As shown in Figure 2b, the current at -1 V is much higher than at +1 V bias. Negative bias corresponds to electron injection from the Au top contact into the molecular layer, i.e., electron flow from Au to Al. The data are very reproducible for as long as the junction remains stable, i.e., does not short-circuit, which can occur due to the mechanical pressure of the W tip, during the measurements. Most devices will continue to yield reproducible measurements for days. Between devices the current magnitude varies slightly due to variation in real device contact area, but the pronounced rectifying behavior of Figure 2b is always observed for Al(2)/insulator/Au junctions, while at best a very slight rectification is observed for some Al(1)/insulator/Au junctions. Figure 3 shows plots of the rectification ratios, RR ) I(at -1 V)/I(at +1 V), for eight junctions of both types of junctions. The rectification ratios are in the range of ∼10-35 for the Al(2)/insulator/Au junctions and 0.8 -1.5 for the Al(1)/insulator/Au junctions. Among the possible reasons for the current–voltage asymmetry of these junctions, we note that we use two electrodes with different work functions (WF), 4.2 eV for Al and ∼5 eV for Au. Indeed, higher currents are expected if a positive bias is applied to the electrode with the smallest work function,37 as was observed for metal/SAM/metal junctions with Au (∼5 eV) and Ti (4.3 eV) electrodes.11 The reason is that connecting the two electrodes (37) Simmons, J. G. J. Appl. Phys. 1963, 34, 2581.

will create an interface dipole with its positive pole near the low-WF electrode and its negative pole near the high-WF electrode. Then, applying a positive voltage to the low-WF electrode will add to, rather than oppose, the electrostatic potential profile, due to the built-in interface dipole (or, in the case of a metal/insulator/metal junction, it will add to, rather than oppose, the electrostatic potential drop over the insulator). However, since no or only very slight rectification was observed for the Al(1)/insulator/Au junctions, the difference in the work functions of the two dissimilar electrodes cannot explain the result of Figure 2b. Another reason can be the formation of oxide on one and not the other electrode,16,38 but we would, again, expect that effect to be similar for the Al(1) and Al(2) junctions, in contrast to what is observed. Rather, the only difference between the two types of junction devices is the surface roughness of the bottom Al electrodes as all other experimental conditions, including surface modification of the bottom electrode, membrane deposition, top contact preparation, and molecular layer quality (checked by AFM before top contact deposition), were the same. In order to get as smooth and as good a mechanical contact, molecule-metal top contacts were obtained by “LOFO” deposition of Au pads, using the “fast LOFO” (cf. ref 39) modification. This approach was used earlier to prepare top contacts for junction structures with ∼1-1.5 nm thick molecular layers,39,40 in addition to its application to structures of the type used here.33 Control devices with only an APTMS monolayer incorporated between Al(1) or Al(2) substrates and the Au contact always gave electrical shorts, indicating that the thin Al oxide and APTMS layers cannot be the origin of the rectifying behavior observed here. A molecular origin of such behavior can also be safely ruled out because the same molecular membrane layers were used in all experiments. As mentioned above, since no or only very small rectification was observed for the Al(1)/insulator/Au junctions, as was the case for earlier reported Al(1)/ gold nanoparticle monolayer/ Au junctions,41 the cause for the strong rectification effect observed with Al(2) samples cannot be the work function difference between the two dissimilar electrodes. Therefore, we ascribe the rectification observed in the Al(2) junctions to the increased surface roughness of the bottom Al contacts. Our experimental results confirm recent related findings, showing that a nonlinear (38) Ho, G.; Heath, J. R.; Kondratenko, M.; Perepichka, D. F.; Arseneault, K.; Pézolet, M.; Bryce, M. R. Chem.sEur. J. 2005, 11, 2914. (39) Vilan, A.; Cahen, D. AdV. Funct. Mater. 2002, 12, 795. (40) (a) Selzer, Y.; Cahen, D. AdV. Mater. 2001, 13, 508. (b) Haick, H.; Ambrico, M.; Ligonzo, T.; Tung, R. T.; Cahen, D. J. Am. Chem. Soc. 2006, 128, 6854. (41) Jin, Y. D.; Friedman, N. J. Am. Chem. Soc. 2005, 127, 11902.

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Figure 4. Varied I-V plots of a junction with in-between rms roughness (higher than that of Al(1), but lower than that of Al(2)) during running at successive three potentials (from curves 1 to 3).

I-V characteristic can be traced to the nature of the contacts rather than being an intrinsic molecular property. The chances of finding nonlinear I-V characteristics increase as the roughness of the surfaces increases, as was shown, for example, in the case of negative differential resistance.42,43 How then can we understand the difference between the Al(1) and Al(2) junctions? Among the various possible explanations we suggest, using Ockam’s razor, attributing the rectifying behavior tentatively to poorer electrical contact between the bottom Al electrode and the 5-nm-thick membrane. Such contact can contain for example some air gaps, thereby making the effective contact area smaller and leading to different potential drop contours for electrodes with different roughnesses. This explanation is reasonable because roughness of the Al(2) electrode surface is appreciably higher than that of the molecular membrane (g0.4 nm), as measured by AFM.44 Therefore, gaps can exist unless the vesicle-fused membrane is able to follow all the surface contours. The smoother surface of the Al(1) electrode can make a mechanically better contact with the membrane surface than the Al(2) one. Such asymmetric electrode coupling results in an asymmetric potential profile along the molecules.17 If the potential profile is asymmetric, the way the molecular electronic energy levels align with the electrode energy levels will be different for positive bias and negative bias,15 resulting in rectification. We also used Al bottom electrodes with rms roughness in between that of Al(1) and Al(2). With such electrodes the rectification ratio is in between that shown in Figure 2a and that shown in Figure 2b. In addition, we find that at times the rectifying behavior becomes less pronounced during successive potential scans. Figure 4 shows three successive I-V curves of such a junction. This observation is consistent with the idea that the (42) Yan, L. M.; Seminario, J. M. Int. J. Quantum Chem. 2007, 107, 440. (43) Seminario, J. M.; Ma, Y. F.; Agapito, L. A.; Yan, L. M.; Araujo, R. A.; Bingi, S.; Vadlamani, N. S.; Chagarlamudi, K.; Sudarshan, T. S.; Myrick, M. L.; Colavita, P. E.; Franzon, P. D.; Nackashi, D. P.; Cheng, L.; Yao, Y. X.; Tour, J. M. J. Nanosci. Nanotechnol. 2004, 4, 907. (44) Muller, D. J.; Heymann, J. B.; Oesterhelt, F.; Moller, C.; Gaub, H.; Buldt, G.; Engel, A. Biochim. Biophys. Acta 2000, 1460, 27.

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observed rectifying behavior is caused by contact roughness, because as current passes we can expect local changes in the interface, both due to electrostriction and due to heating, which can result in smoother contact.45 Further work is needed to arrive at a clear physical mode, beyond what we noted above, possibly by taking a fresh look at earlier reports.15,17,19 That earlier work comprised a firstprinciples study of electronic transport and rectification, especially the role of electrode coupling, with molecular wires attached to gold electrodes;15 a comparison of theoretical and experimental current–voltage characteristics of self-assembled monolayers on a gold substrate measured with a scanning tunneling microscopy probe;17 and the influence of contact defects on the electrical characteristics of mercury drop junctions.19 Brinkman et al.46 showed how tunneling through asymmetric barriers leads to asymmetric I-V characteristics. As noted above, we suggest that the potential distribution at the rough and smooth surfaces is different, because of issues such as current crowding and edge leakage, leading to an asymmetric barrier, which then allows use of the Brinkman et al. model. In addition, we can expect different electron and hole trapping at rugged (roughened) and smooth structures,47 which also can give rise to distortion of the barrier from symmetrical to asymmetrical, and thus result in rectification.

Conclusions In summary, the electrical transport behavior in molecular junctions can depend critically on the contact quality and rectifying behavior can be due to a local contact effect. A small increase in surface roughness can lead to distinct rectification behavior in our planar molecular junctions. Because, indeed, the contacts are one of the most problematic parts of conductor/molecule/ conductor systems, our result suggests that similar effects can be at work in other molecular junctions and that care needs to be exercised before rectifying behavior in such junctions is unequivocally ascribed to the molecules themselves. Acknowledgment. We thank the reviewers for helpful comments. The authors acknowledge support from the Ilse Katz Centre for Materials, the Kimmel Centre for Nanoscale Science, and the Nancy & Stephen Grand Center for Sensors & Security. Y.J. thanks the Norman Sosnow foundation for a postdoctoral fellowship. D.C. is incumbent of the Schaefer Chair in Energy Research. LA703859A (45) To further check if the observed rectifying behavior can be related to the roughness of the Al electrode, we looked at an extreme change in electrode/ sample contact. An Al(1)/insulator/Au junction was measured and then the mechanical contact between the bottom Al electrode and the electrode clip, connected to the outside world, was adjusted slightly. Starting from an almost symmetric curve, we carefully decreased the mechanical contact pressure to get a “poor” electrical contact and observed rectifying behavior, similar to what was seen in Figure 2b, but with (at -1 V) nearly 3 orders of magnitude lower current than that see in Figure 2b (data not shown). (46) Brinkman, W. F.; Dynes, R. C.; Rowell, J. M. J. Appl. Phys. 1970, 41, 1915. (47) Chan, H. C.; Mathews, V.; Fazan, P. C. IEEE Electron DeVice Lett. 1991, 12, 468.