Nondestructive Contact Deposition for Molecular Electronics: Si-Alkyl

Jul 7, 2010 - of our earlier lift-off, float-on (LOFO) method, using as a basis its offspring, the polymer-assisted lift-off. (PALO) method, where a b...
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J. Phys. Chem. C 2010, 114, 12769–12776

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Nondestructive Contact Deposition for Molecular Electronics: Si-Alkyl//Au Junctions Nir Stein, Roman Korobko, Omer Yaffe, Rotem Har Lavan, Hagay Shpaisman, Einat Tirosh, Ayelet Vilan,* and David Cahen* Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: May 6, 2010; ReVised Manuscript ReceiVed: June 15, 2010

One of the major problems in molecular electronics is how to make electronically conducting contact to the “soft” organic and biomolecules without altering the molecules. As a result, only a small number of metals can be applied, mostly by special deposition methods with severe limitations. Transferring a predefined thin metal leaf onto a molecular layer provides a nondestructive, noninvasive contacting method that is, in principle, applicable to many types of metal and a variety of metal/molecules combinations. Here we report a modification of our earlier lift-off, float-on (LOFO) method, using as a basis its offspring, the polymer-assisted lift-off (PALO) method, where a backing polymer enables simultaneous deposition of multiple contacts as well as reduces wrinkles in the thin metal leaf. The modified PALO (MoPALO) method, reported here, adds lithography steps to obviate the need to punch through the polymer, as is done to complete PALO contacts. Morphological characterization of the electrodes indicates highly uniform, wrinkle-free contacts of negligible roughness. The good electrical performance of the MoPALO contacts was proven with metal/organic-monolayer/ semiconductor (MOMS) junctions, which are known to be very sensitive to molecular degradation and metal penetration. We also show how MoPALO contacts enabled us to compare the effect of varying the metal work function and contact area on the current-voltage characteristics of MOMS devices. 1. Introduction Research on electronic charge transport through molecules uses both isolated single (or few) molecules and monomolecular layers (of at least thousands of molecules) as platforms for fundamental studies,1-6 with the latter more likely to be relevant for possible future applications. One of the main challenges is how to connect organic molecules electrically to the outside world in a permanent, yet nondestructive manner, because commonly used methods, such as vacuum deposition or sputtering, can easily damage the molecules. Among the methods used or developed for making macroscopic top contacts with minimum damage to the molecules1-3 are nanotransfer printing (nTP),7,8 Hg drop,9,10 indirect evaporation (IE)11,12 and LOFO13,14 (lift-off, float-on). However, none of these is generally applicable to molecular electronics. For example, reliable nTP contacts require a chemical bond between the molecules and the contact, which limits the molecules that can be used. It has been argued that metal penetration is driven by the tendency of the top and bottom electrodes to chemically interact.15 We have previously presented simple ways to identify metal penetration from electrical characteristics of MOMS junctions16 and showed that even with a “soft” deposition method as IE the evaporated metal can penetrate into the monolayer (in the case of large area Au contacts).16 Hg, a liquid metal under normal ambient conditions, allows highly reproducible and reliable current-voltage (I-V) measurements, which is why it is a popular top contact in research today. Nevertheless, the substrate materials that can be contacted by Hg are limited because of amalgamation with the underlying conducting substrate (e.g., Au) or with (parts of) the sample (e.g., CdSe). In addition, liquid Hg as well as other mechanically * Corresponding authors. Tel: +972 8 9342246. Fax: +972 8 9344138. E-mail: [email protected] (D. C.); [email protected] (A. V.).

driven contacts (e.g., break junctions, tips) are temporal by nature and therefore unlikely to find practical use in devices. The use of conductive polymers for contacts17,18 is promising but suffers from limited electronic charge carrier mobility and possible mismatch in hydrophilicity with the molecules that are to be contacted.18 Alternatively, both electrodes can be formed prior to molecular adsorption (cf. refs 19 and 20). However, in such configuration, molecular transport is likely to be bypassed by the inorganic insulator, supporting the nanogapped electrodes, unless highly conductive molecules that are long enough to provide a suitable electrode separation are used. Therefore, methods are still sought to overcome as many of these disadvantages as possible to provide reliable contacts with welldefined structure. Such contacts should allow reproducible and reliable electrical transport measurements of molecular junctions with a high yield. Only with a critical mass of such results can we hope to make progress toward understanding, and subsequently controlling, charge transport through molecules. The LOFO method13,14 requires considerable skill to deposit the pad without wrinkling. It is limited to contacts that can be manipulated manually (g0.5 mm diameter), and pads are deposited one by one. Polymer-assisted lift-off (PALO), which was introduced21 to overcome some of LOFO’s problems, uses a polymer backing layer made of poly(methyl methacrylate) (PMMA) that is spin-coated onto long, thin Au stripes (electrodes). Then, the PMMA + electrodes layer is lifted off from the sacrificial substrate and, as in LOFO, using a liquid medium, floated onto a molecular layer that in the case of PALO was previously self-assembled onto a different set of long, thin Al electrodes. The result is a cross-bar configuration that allows contacting each electrode far from the junction itself and, by that, prevents mechanical damage to the junction. PALO contacts yielded good electrical transport characteristics for metal/organic molecule/metal (MOMM) junctions with a wide

10.1021/jp104130w  2010 American Chemical Society Published on Web 07/07/2010

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Figure 1. Modified polymer-assisted lift-off (MoPALO) process (not to scale); see text for details: (a) metal evaporation; (b) polyimide (PI) spin-coating; photolithography to make holes through the PI; (c) PI + electrodes film is detached from the sacrificial substrate; (d) floating PI/metal film onto DI water; (e) float-on of PI + electrodes film onto target substrate; and (f) final device structure, ready for electrical measurement.

range of junction contact areas without the wrinkling of the electrodes that LOFO pads frequently undergo. However, the need to punch through the PMMA backing layer physically with a probe poses a problem. Unless this is done far away from the active junction, it can damage the molecules or cause electronic shorts. Therefore, PALO is not suitable for the macroscopic nonpatterned substrates that allow monolayer quality to be examined directly by complementary methods such as FTIR, ellipsometry, XPS, contact angle, and CPD, which typically require at least millimeter-wide samples. In addition, to be able to check the effects of any patterning of a surface, comparison with otherwise identical but nonpatterned surfaces is critical. For example, etching can increase surface roughness, and rougher substrates are known to affect the quality of the adsorbed monolayer.10,22 To overcome this PALO limitation, we replace the PMMA backing layer with a photoresist and use photolithography to make the necessary holes above the electrodes to yield “modified polymer-assisted lift-off” (MoPALO) noninvasive contacts to the molecules. To assess how far the molecules are damaged by MoPALO, we use a semiconductor instead of a metal as the substrate to absorb the molecules onto because of the enhanced sensitivity to molecule damage of the former.16 We find that MoPALO contacts are smooth and wrinkle-free and show good electrical behavior in actual metal/organic-monolayer/semiconductor (MOMS) junctions, that is, the molecules under the MoPALO contacts, as well as under the Hg contact, are not damaged/ changed as far as can be judged from the electrical transport characteristics, which, in our experience, present the most sensitive way to assess this. We used MoPALO also toward the goal of permanent metal contacts for actual molecular electronic devices by adding a third processing layer, where a metal is evaporated also on top of the polymer to form leads and wire-bonding pads away from the delicate molecules.23 Obviously, possible future practical ME devices that can complement or compete with current microtechnologies will require evolutions in technologies (cf. refs 24 and 25). Electrical characterization of Au MoPALO contacts to alkyl monolayers on Si also can help to study effects of metal work function, parasitic series resistance (RS), and contact area of the molecular junctions. (See the Supporting Information.) To this end, we use permanent MoPALO (PeMoPALO) contacts because those can have areas that are significantly smaller than the ones that can be achieved with a Hg drop or with LOFO, areas that are similar to those used in “large-area” ME devices.7,8,12,15,17,18,21,26

2. Experimental Methods Contact Preparation Procedures. Here we describe how MoPALO and PeMoPALO contacts are made. Further experimental details are given in section I of the Supporting Information. 2.1. Modified Polymer-Assisted Lift-Off Process. MoPALO contact preparation is illustrated in Figure 1: (1) evaporation of metal pads onto a sacrificial solid substrate through a shadow mask; (2) spin-coating of polyimide (PI) photoresist onto these metal pads, followed by photolithography to make holes in the PI above the center of the metal pads; (3) lift-off: detaching the PI + electrodes film from the sacrificial solid substrate by acid solution and then lifting it off onto deionized (DI) water, onto which it floats; and (4) float-on: depositing the floating PI + electrodes film onto a previously prepared substrate. The first step in MoPALO is evaporation of the metal pads (mostly Au) that will become the electrodes onto a sacrificial solid substrate, usually mica, through a shadow mask (Figure 1a). The pad diameter ranges from 0.15 to 2 mm (smaller contacts are possible with PeMoPALO; see below). The pad has to be sufficiently thick to be mechanically stable and without holes but not so thick that it becomes too rigid for lift-off. For Au, we found a 60 nm thickness to be suitable. Mica as sacrificial substrate provides both minimal roughness and ease of detachment. We tested several alternatives to mica, including very smooth Si (hard to detach the PI + electrodes film) and oxidized Si or cover slide glass (easy to detach, but rougher, (1.1 and (0.35 nm, respectively, than mica, (0.2 nm, on a 0.5 × 0.5 µm2 area). Mostly Au was used as electrode material, but Ni and V were also found to be suitable, whereas Ag and Cu were not because of poor adhesion between those metals and the PI. A plausible solution for such adhesion problems will be to add another metal between the bottom electrode and the PI, an approach that was not yet tried. The second step in the process is to spin-coat the hydrophobic backing polymer, durimide 7505 negative photoresist, onto the substrate + electrodes. As in PALO,21 the key requirement for the backing polymer is that it needs to be hydrophobic to repel water during the float-on step. PMMA, used in PALO, is an e-beam resist, whereas PI is a photoresist, allowing the use of photolithography, which is suitable for large area writing. Apart from being hydrophobic and a photoresist, the new polymer should also (1) have better adhesion to the metal than to the sacrificial substrate; (2) be able to float on water or other solvents; (3) be mechanically strong, stable, and flexible; and

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Figure 2. General concept (not to scale) of permanent MoPALO (PeMoPALO): (a) evaporation of ∼60 nm thick bottom contacts; (b) polyimide photolithography; and (c) metal evaporation (300 nm thick) to fill the wells and form connections to external contact pads.

(4) not degrade upon immersion in the acid, used to detach the electrodes + PI film from the substrate. Durimide 7505 was found to fulfill all of these requirements. The next step is to pattern the PI with holes above the center of the metal pads (Figure 1b). In the present process, the holes in the PI should be, on the one hand, large enough for a wire to enter and make good Ohmic contact to the metal pad while, on the other hand, small enough so as not to weaken the PI as a backing layer. The photolithography process, used to make these holes, includes a necessary postbake step of the PI, where the substrate + electrodes + PI film is heated for 1 h at 350 °C in an inert gas (Ar) flow. This annealing is also beneficial for the metal pads because it makes them smoother. The (PI + metal pads + mica) film can be stored in a desiccator (40 Torr) for up to ∼12 weeks. Longer storage led to some observable degradation in I-V measurements. In the subsequent lift-off stage, the (PI + metal pads) film is detached from the substrate by immersing the (mica + electrodes + PI) film in a 2% HF acid solution in a Teflon beaker for 4.5 eV (i.e., for Au and Hg) for C12 monolayers are explained by inversion of the Si surface.31 The electrostatic balance among the metal work function, the molecular dipole, and the Si work function leads to such band bending at the Si surface so that the dominant type of charge carriers changes; the region is now type-“inverted” compared with the bulk of the samples. As a result, over the bias range

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Figure 4. AFM images of the (a,b) Au and (c,d) polyimide (PI) on the sides of MoPALO electrodes that will contact the molecules: (a) Au 3.5 × 3.5 µm2 area with 3 Å rms roughness for the whole image; (b) cross section of image a; (c) PI, 4.5 × 4.5 µm2 area with 1 Å rms roughness for the whole image; and (d) cross section of image c.

Figure 5. SEM image of permanent MoPALO contact (top side) showing (a) the outer Au over the 3 µm deep well (I) and a spurious hole in the PI (II) and (b) a magnification of (I). The PeMoPALO has the following dimensions: bottom electrode, 40 µm L and 60 nm thick; PI well, 25 µm L and 3 µm deep; top leads, 600 µm L and 300 nm thick.

where they are inverted, these MOMS junctions are now minority rather than majority carrier-dominated ones (in the case of n-Si, holes near the surface and electrons in the bulk).6,31 In contrast with transport by majority-carriers, where variations in band bending affect the semiconductor current exponentially, transport by minority-carriers, as is the case under inversion, depends only on intrinsic properties of the semiconductor,30 which, therefore, is independent of insulator thickness (molecular length) or metal work function.31 Inversion of the Si is biasdependent, decreases with increasing forward bias, and disappears above ∼+0.5 V (vertical line in Figure 6a), a range that will be discussed later. The overlap of the measured J-V curves, for monolayers of different lengths, at reverse and low forward bias (Figure 6) indicates the presence of a highly covering molecular monolayer with minimal surface oxidation under the contact.16 The reason is that achieving Si inversion is extremely sensitive to interface

chemistry.6 Both Si-H and Si-Ox terminations, which are likely to form if Si-C bonds are broken, drive the Si out of inversion, leading to orders of magnitude higher current at reverse bias.6,10,31 Therefore, evidence of inversion is a strong indication that the MoPALO contacts do not affect the molecules during or after the deposition. Moreover, the overlap between the Hg, Au MoPALO, and Au PeMoPALO curves implies that metal penetration is negligible because a minimal area of direct metalSi interface would also drive the surface out of inversion.16,31 Photovoltage measurements are even more sensitive to the band bending in the Si than the reverse bias current. We compared16 the three different contacting modes, Hg, AuMoPALO, and Au, by IE by measuring their open-circuit voltage, VOC, for Si-C18 and Si-H junctions. The presence of the C18 monolayer drastically changed the VOC of junctions with Hg or Au-MoPALO contacts; however, with Au-IE contacts, the Si-H and Si-C18 junctions gave identical VOC values.16 This constant VOC is explained by direct metal-semiconductor contacts due to metal penetration, which limits the molecular dipole effect that pushes the Si into strong inversion. 3.2.2. Molecular Length Dependence. A further indication that the molecular layer is intact is that its contribution to net transport is exponentially attenuated according to its width6,9,30,31 at forward bias >0.5 V (cf. C12 and C16 alkyls in Figure 6b). Above that bias, semiconductor band-bending decreased sufficiently so that transport is no longer semiconductor-limited. The transition from semiconductor-limited to tunneling-limited can be identified by a dual change in the slope of the J-V curve, which we term “inflection point”.10,31 The inflection point is clearest for the C16 junctions with Au PeMoPALO contact, less obvious in the C16 one with Hg, and not observed with a MoPALO contact (Figure 6).

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Figure 6. Log(J)-V curves of n-Si-CnH2n+1/metal junctions with n ) 12, 16 (a-c) or n ) 16 (d). (a) Hg with 0.3 mm2 contact area. (b) Au permanent MoPALO with 0.018 mm2 contact area. (c) Au MoPALO with 0.28 mm2 contact area. (d) Zoom in on semiconductor-limited voltage bias range of Si-C16 alkyl junctions, comparing the three contacting modes: large area Au MoPALO (black triangles, A ) 0.28 mm2), large area Hg (red dashed line, A ) 0.3 mm2), and small area permanent MoPALO (blue circles, A ) 0.018 mm2). Bias is applied to the metal electrode, and the Si is grounded. Results are the averages of at least 10 different junctions (except for C16/Au PeMoPALO, which are the average of 4 different junctions) with a scan rate of 20 mV/s, and the error bars represent standard deviations.

Looking at Figure 6a (Hg contacts), we see an inflection point for C16 but not for C12. In addition, the current is lengthattenuated across only part of the bias range. In contrast, with the Au PeMoPALO contacts (Figure 6b) a clear tunneling regime is seen up to maximal bias and for junctions with both of the molecular lengths. Finally, for the larger area, Au MoPALO (Figure 6c) contacts showed neither inflection nor length-dependence across the full bias range, even though the contact areas here are similar to those with Hg (Figure 6a). The variations in the expression of tunneling characteristics are explained by the effect of a parasitic series resistance, RS, which has a larger effect, the larger is the current.6,31 For a given RS, the expected, typical tunneling J-V behavior (inflection point and length dependence) will show up by reducing the net current, either by increasing the insulator width (via the molecular length; this increases junction resistance exponentially) or by decreasing the contact area of the junctions. Therefore, smaller contacts (Figure 6b) express tunneling characteristics better than larger ones (Figure 6a,c), and for a junction with thicker insulator (C16) the inflection point is clearer than that for junctions with a thinner insulator (C12), as in Figure 6a. On the basis of calculations and experiments with sets of Hg and Au (PeMoPALO) contacts of different areas (see RS discussion in section II of the Supporting Information), we find that a resistive loss (I × RS) of 5 mV suffices to hide the tunneling characteristics, whereas a 0.5 mV resistive loss has a negligible effect. The resistance of our setup was ∼10-40 Ω, implying that the net current should not exceed the low tens of microamperes at the transition bias. 3.3. Semiconductor-tunneling Transition. 3.3.1. Semiconductor-tunneling Transition Bias As Indication for Residual SolWents. With MoPALO contacts we can, in principle, study the effect of metal work function on MIS transport. The J-V curves of inorganic MIS junctions were predicted30 and found32 to be independent of metal work function as long as the

semiconductor is inverted; that is, varying the metal’s work function in the direction of stronger inversion is not expressed directly in the magnitude of the semiconductor-limited current.6,31 The exact forward bias of the transition from semiconductorto tunneling-limited current (i.e., the inflection point) increases (for n-type) with increasing work function.32 Au has a 0.2 to 0.6 eV higher work function than Hg (depending on how clean the Au is),33,34 so that we would expect its inflection point to appear at higher positive bias than for Hg. The inflection points for C16 junctions are at ∼0.6 V for Hg (Figure 6a, see also Figure S2 in the Supporting Information). The absence of inflection in the J-V curves of junctions with large Au MoPALO contacts (Figure 6c) is attributed to the expected higher theoretical inflection voltage (>0.6 V). At such high voltage, the current is already series-resistance limited. For the smaller Au contacts, however, the inflection bias is ∼0.4 V, suggesting that another factor is involved, which we identify with residual solvent between the molecules and the contact. (See section III of the Supporting Information.) 3.3.2. Reproducibility and DeWiations in the Shape of the I-V CurWe. MOMS are unique in molecular electronics because of the excellent reproducibility of the measured currents. (See ref 22 for statistics on large area MIM and ref 35 for variation in single molecules studies.) Figure 7 shows experimental I-V curves of three different, small contact area (A ) 0.0013 mm2), n-Si-C12/Au PeMoPALO junctions on the same sample. Differences between different junctions of identical area are observed in the bias range, where the J-V curves change from exponential to linear, whereas the curves overlap in all other bias ranges. The overlaps in the semiconductor-limited regime and at higher forward bias suggest that the alkyl monolayer remained functional and intact in these junctions; that is, the contact deposition did not damage the monolayer. The transition range is where the Si drastically changes its dominant charge carriers from minority (inversion) to majority

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Figure 7. Log(I)-V for three different n-Si-C12//40 µm Ø Au junctions, representing extreme and median cases of measured curves.

ones (depletion). The shape of the J-V curve in this region reflects the rate of this charging, which depends on the presence of interface states. (See Figure 9a in ref 23.)30,36 These details are important only around the transition bias (the inflection point observed in the J-V curves). At lower biases (semiconductorlimited transport) the semiconductor is inverted and the intrinsic semiconductor properties dictate the transport. At biases above the transition range, tunneling from/to main semiconductor bands limits the current with a negligible contribution from interface states. Therefore, interface states are important only in the transition voltage range, which is when they are partially filled. Therefore, we regard Figure 7 as additional confirmation for the applicability of MIS theory30,32,36 to MOMS. Moreover, Figure 7 suggests that the excellent reproducibility observed in MOMS at low and high forward bias might originate in their transport physics and does not imply perfect monolayers because there are variations in the monolayer, but these can be expressed only in the transition window. Naturally, MOMS are in good company because most unconventional metal/semiconductor junctions will have some degree of heterogeneity.37 These deviations around the transition region were not observed for Hg contacts. The reason can be either that Hg does not induce interface states in the Si or, more likely, that as a contact it is more homogeneous than ready-made Au PeMoPALO ones. In principle, Au could also induce morphological changes in the monolayer, either because of its rigidity or by electromigration of Au filaments into the monolayer, induced by the electric field. Au migration was observed in some highcurrent MOMS (p++-Si),28 but it is highly unlikely here because we did not observe any hysteresis in cyclic J-V measurements (between -1 and +2 V). Furthermore, protrusion-limited current is expected to affect the transport at any bias, larger than the inflection bias, in contrast with the observed narrow range of poor reproducibility, which points to an electrostatic inhomogeneity rather than a morphological one. 4. Conclusions We demonstrated a nondestructive method for solid metal contact deposition onto molecular monolayers and a variant that allows making small permanent contact, MoPALO and PeMoPALO respectively, which extend the LOFO13,14 and PALO21 processes. The method can be used to create many junctions simultaneously, so as to facilitate collecting meaningful statistics for I-V measurements on molecular junctions. AFM, SEM, and optical microscopy show that MoPALO and PeMoPALO provide wrinkle-free, well-defined contacts with a surface roughness of a few angstroms. J-V measurements show that depositing such contacts onto a high-quality molecular monolayer does not damage monolayer quality, similar to what is

Acknowledgment. We thank the Israel Science Foundation (Jerusalem) via its Centre for Excellence programs, the Nancy and Stephen Grand Center for Sensors and Security, and the Gerhard Schmidt Minerva Centre for Supramolecular Chemistry, for partial support. We thank Marc Altman, Aviram Feingold, Arnon Arbel, and Daniel Majer for practical help and advice with the PI photolithography process. D.C. holds the Sylvia and Rowland Schaefer chair in Energy Research. Supporting Information Available: Additional experimental details, series resistance consideration, and comments on the variation in inflection bias. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Haick, H.; Cahen, D. Prog. Surf. Sci. 2008, 83, 217. (2) Haick, H.; Cahen, D. Acc. Chem. Res. 2008, 41, 359. (3) Akkerman, H. B.; de Boer, B. J. Phys.: Condens. Matter 2008, 20, 013001. (4) Salomon, A.; Cahen, D.; Lindsay, S.; Tomfohr, J.; Engelkes, V. B.; Frisbie, C. D. AdV. Mater. 2003, 15, 1881. (5) McCreery, L. R.; Bergren, J. A. AdV. Mater. 2009, 21, 4303. (6) Vilan, A.; Yaffe, O.; Biller, A.; Salomon, A.; Kahn, A.; Cahen, D. AdV. Mater. 2009, 22, 140. (7) Loo, Y. L.; Lang, D. V.; Rogers, J. A.; Hsu, J. W. P. Nano Lett. 2003, 3, 913. (8) Coll, M.; Miller, L. H.; Richter, L. J.; Hines, D. R.; Jurchescu, O. D.; Gergel-Hackett, N.; Richter, C. A.; Hacker, C. A. J. Am. Chem. Soc. 2009, 131, 12451. (9) Salomon, A.; Boecking, T.; Chan, C. K.; Amy, F.; Girshevitz, O.; Cahen, D.; Kahn, A. Phys. ReV. Lett. 2005, 95, 266807. (10) Seitz, O.; Bo¨cking, T.; Salomon, A.; Gooding, J. J.; Cahen, D. Langmuir 2006, 22, 6915. (11) Haick, H.; Ambrico, M.; Ghabboun, J.; Ligonzo, T.; Cahen, D. Phys. Chem. Chem. Phys. 2004, 6, 4538. (12) Scott, A.; Risko, C.; Valley, N.; Ratner, M. A.; Janes, D. B. J. Appl. Phys. 2010, 107, 024505. (13) Vilan, A.; Shanzer, A.; Cahen, D. Nature 2000, 404, 166. (14) Vilan, A.; Cahen, D. AdV. Funct. Mater. 2002, 12, 795. (15) Hacker, C. A.; Richter, C. A.; Gergel-Hackett, N.; Richter, L. J. J. Phys. Chem. C 2007, 111, 9384. (16) Shpaisman, H.; Har-Lavan, R.; Stein, N.; Yaffe, O.; Korobko, R.; Seitz, O.; Vilan, A.; Cahen, D. AdV. Funct. Mater. 2010, 20, 1–8. (17) Akkerman, H. B.; Blom, P. W. M.; de Leeuw, D. M.; de Boer, B. Nature 2006, 441, 69. (18) Van Hal, P. A.; Smits, E. C. P.; Geuns, T. C. T.; Akkerman, H. B.; De Brito, B. C.; Perissinotto, S.; Lanzani, G.; Kronemeijer, A. J.; Geskin, V.; Cornil, J.; Blom, P. W. M.; De Boer, B.; De Leeuw, D. M. Nat. Nanotechnol. 2008, 3, 749. (19) Cerofolini, G. F.; Arena, G.; Camalleri, C. M.; Galati, C.; Reina, S.; Renna, L.; Mascolo, D. Nanotechnology 2005, 16, 1040. (20) Sondergaard, R.; Strobel, S.; Bundgaard, E.; Norrman, K.; Hansen, A. G.; Albert, E.; Csaba, G.; Lugli, P.; Tornow, M.; Krebs, F. C. J. Mater. Chem. 2009, 19, 3899. (21) Shimizu, K. T.; Fabbri, J. D.; Jelincic, J. J.; Melosh, N. A. AdV. Mater. 2006, 18, 1499. (22) Weiss, E. A.; Chiechi, R. C.; Kaufman, G. K.; Kriebel, J. K.; Li, Z.; Duati, M.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2007, 129, 4336. (23) Korobko, R. Permanent Metallic Solid Contacts to Molecules Based on MoPALO, M. Sc. Thesis, Weizmann Institute of Science, 2008. (24) Cerofolini, G. F.; Romano, E. Appl. Phys. A: Mater. Sci. Process. 2008, 91, 181. (25) Green, J. E.; Choi, J. W.; Boukai, A.; Bunimovich, Y.; JohnstonHalperin, E.; DeIonno, E.; Luo, Y.; Sheriff, B. A.; Xu, K.; Shin, Y. S.; Tseng, H.-R.; Stoddart, J. F.; Heath, J. R. Nature 2007, 445, 414. (26) Miramond, C.; Vuillaume, D. J. Appl. Phys. 2004, 96, 1529. (27) Golan, Y.; Margulis, L.; Rubinstein, I. Surf. Sci. 1992, 264, 312.

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(28) Korobko, R.; Vilan, A.; Har Lavan, R.; Stein, N.; Cahen, D., to be published. (29) Senden, T. J.; Ducker, W. A. Langmuir 1992, 8, 733. (30) Green, M. A.; King, F. D.; Shewchun, J. Solid-State Electron. 1974, 17, 551. (31) Yaffe, O.; Scheres, L.; Puniredd, S. R.; Stein, N.; Biller, A.; HarLavan, R.; Shpaisman, H.; Zuilhof, H.; Haick, H.; Cahen, D. Nano Lett. 2009, 9, 2390. (32) Camporese, D. S.; Pulfrey, D. L. J. Appl. Phys. 1985, 57, 373. (33) Wan, A.; Hwang, J.; Amy, F.; Kahn, A. Org. Electron. 2005, 6, 47.

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