Charge Transport and Rectification in Arrays of SAM-Based Tunneling Junctions Christian A. Nijhuis, William F. Reus, Jabulani R. Barber, Michael D. Dickey, and George M. Whitesides* Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138 ABSTRACT This paper describes a method of fabrication that generates small arrays of tunneling junctions based on self-assembled monolayers (SAMs); these junctions have liquid-metal top-electrodes stabilized in microchannels and ultraflat (template-stripped) bottom-electrodes. The yield of junctions generated using this method is high (70-90%). The junctions examined incorporated SAMs of alkanethiolates having ferrocene termini (11-(ferrocenyl)-1-undecanethiol, SC 11 Fc); these junctions rectify currents with large rectification ratios (R), the majority of which fall within the range of 90-180. These values are larger than expected (theory predicts R e 20) and are larger than previous experimental measurements. SAMs of n-alkanethiolates without the Fc groups (SC n-1 CH 3 , with n ) 12, 14, 16, or 18) do not rectify (R ranged from 1.0 to 5.0). These arrays enable the measurement of the electrical characteristics of the junctions as a function of chemical structure, voltage, and temperature over the range of 110-293 K, with statistically large numbers of data (N ) 300-800). The mechanism of rectification with Fc-terminated SAMs seems to be charge transport processes that change with the polarity of bias: from tunneling (at one bias) to hopping combined with tunneling (at the opposite bias). KEYWORDS Nanoelectronics, molecular electronics, charge transport, self-assembled monolayers, rectification, charge transfer M olecular electronics 1 originally promised that mol- ecule(s) bridging two or more electrodes would generate electronic function and overcome the scaling limits of conventional semiconductor technology. 2-4 Although, so far, there have been no commercially successful molecular electronic devices, the subject of charge transport in molecularly scaled systems is unquestionably fundamen- tally interesting. Much of the work on the physical and physical-organic mechanisms of charge-transport has used self-assembled monolayers (SAMs) of alkanethiolates on Au and Ag sub- strates, with these metals as the bottom-electrode, but a variety of top-electrodes. We also use SAMs of organic thiolates on Ag in our studies. Fabricating even simple molecular circuits using SAMs of alkanethiolates chemi- sorbed on Au or Ag substrates and contacted by metal top- electrodes has been a challenge: most fabrication techniques produce junctions in low yields. 5 These junctions have been dominated by artifacts 6 (especially conducting filaments 7-10 ) and generate too few reliable data for statistical analysis (the work of Lee et al. provides an exception 11 ). Physical-organic studies connecting molecular structure and electrical proper- ties have been difficult or impossible to carry out with most junctions based on SAMs of Au or Ag, and studies that include measurements as a function of temperatures measurements necessary to determine the mechanism(s) of charge transport across SAM-based junctionsshave also been difficult (the work of Allara et al. 12 and Tao et al. 13 provide examples of successful studies). An alternative approach to studies of charge transport through organic monolayers is that of Kahn et al., Cahen et al., and Boecking, who covalently attach monolayers of alkyl chains to a doped Si substrate via hydrosilylation of alkenes with a H-passivated Si surface, 14 and contact these mono- layers using a Hg top-elecrode. 15 These Si-alkyl//Hg junctions are unquestionably useful for studying charge transport and are more stable than have been most junctions in which SAMs of alkanethiolates chemisorbed on Au or Ag are directly contacted by a metal top-electrode, but like other techniques, Si-alkyl//Hg junctions have both advantages and disadvantages. The primary advantage is that contacting the monolayer with a Hg drop does not damage the monolayer (by penetration through pinholes or diffusion of mercury vapor through the SAM). As a result, measurements using these junctions appear to be quite reproducible across the set of junctions reported (although the interpretation of these measurements would benefit from more information con- cerning the statistical distribution of data, and yields of reported junctions, compared to unreported junctions or shorts). Another advantage of these junctions is that they support measurements of charge transport over a limited range of temperatures 16 (T ) 250-330 K), measurements which have yielded valuable information on the mechanism of charge transport in these junctions. Among the disadvan- tages of organic monolayers covalently bound to Si is that they are less well ordered than SAMs of alkanethiolates on Au or Ag, and it is difficult synthetically to achieve large variations in the structure of the organic R group. With respect to charge transport, these junctions are mechanisti- * To whom correspondence should be addressed, gwhitesides@ gmwgroup.harvard.edu. Received for review: 05/31/2010 Published on Web: 08/18/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 3611 DOI: 10.1021/nl101918m | Nano Lett. 2010, 10, 3611–3619