A Mechanism for Conductance Switching in Carbon-Based Molecular Electronic Junctions Ali Osman Solak, a Srikanth Ranganathan, b Takashi Itoh, c, * and Richard L. McCreery* ,z Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 A molecular junction formed by a 10-15 Å organic monolayer between carbon and mercury contacts exhibited conductance switching for several monolayer structures. When the carbon potential was scanned to a sufficiently negative voltage relative to the mercury, the junction resistance suddenly decreased by at least an order of magnitude, and high resistance could be restored by a positive voltage scan. The high and low conductance states were persistent, and conductance switching was repeatable at least 100 cycles for the case of a terphenyl junction. The switching behavior is consistent with phenyl ring rotation and formation of a planar, quinoid structure as a consequence of electron injection into the monolayer. A unique feature of the junction structure is the strong electronic coupling between the monolayer  system and the graphitic carbon through a quinoid double bond. Not only does this interaction lead to high conductivity and possible practical applications as a molecular switch, it also combines the electronic properties of the conjugated monolayer with those of the graphitic substrate. The switching mechanism reported here is an example of ‘‘dry electrochemistry’’in which a redox process appears to occur under the influence of a high electric field in the absence of solvent or electrolyte. © 2002 The Electrochemical Society. DOI: 10.1149/1.1490716 Manuscript submitted March 28, 2002; revised manuscript received April 26, 2002. Available electronically June 10, 2002. Conductance switching is the basis of many potential molecular electronic devices, and has been the focus of numerous research efforts in recent years. 1-8 If a single molecule or an assembly of molecules can be switched between high and low conductance states by an electrical or optical stimulus, molecular scale memory and logic elements become possible, and molecular electronic compo- nents may be integrated with conventional microelectronics or as- sembled into true molecular circuits. 9-12 A range of molecular struc- tures which exhibit conductance switching has been reported, including self-assembled monolayers SAMs of phenylethynyl oligomers 2,7,13,14 and rotaxanes oriented between two conductors by Langmuir-Blodgett assembly. 5,6 The phenomenon of negative differ- ential resistance NDR is notable as an example of a small collec- tion of molecules changing from high to low resistance state in an applied electric field, then back to a high resistance state as the field increased further. 2,7,13 In several cases the states of the molecule are persistent and controllable, and may be repeatedly cycled between ‘‘On’’ low resistance and ‘‘Off’’ high resistance states. Although several examples of conductance switching have been investigated extensively, the switching mechanism in most cases is unknown and is the subject of some controversy. NDR was origi- nally attributed to redox reactions involving a nitro and/or amino group in the monolayer molecule, 2 but switching was subsequently observed without such groups present. 4,14 NDR was also attributed to shifts in molecular orbital energies in response to an applied electric field, resulting in resonant tunneling at certain applied field magnitudes. 15 Switching by isolated phenylethynyl molecules sur- rounded by aliphatic SAMs was observed to be stochastic by scan- ning tunneling microscopy STM, and was attributed to conforma- tion changes, possibly involving interactions with the monolayer surrounding the active molecules. 4 Of course, any mechanism estab- lished to explain conductance switching also bears directly on the broader issue of which factors control electronic conductivity in organic molecules, a topic of wide interest in the areas of conducting polymers, optoelectronic materials, and energy conversion, in addi- tion to molecular electronics. 9,10,12 We report here a completely new approach to fabricating mo- lecular junctions which exhibit conductance switching and which provide critical insights into the switching mechanism. Shown sche- matically in Fig. 1, the carbon-based molecular junction 8 consists of a covalently bonded monolayer on a very flat rms roughness 5Å graphitic carbon substrate made by pyrolysis of a photoresist film PPF. 16,17 Spectroscopy of the monolayer/PPF interface has estab- lished that the bonding is covalent and stable to at least 400°C in vacuum, 18 and that the phenyl rings of the monolayer are rotation- ally disordered relative to the graphitic aromatic rings. 19,20 The junc- tion is completed by a suspended Hg drop placed on the monolayer which defines a junction area of approximately 0.78 mm 2 . Conduc- tance switching of nitrobiphenyl and biphenyl molecular junctions is apparent in the current/voltage i/V curves of Fig. 2. The initial junction resistance varies for different junctions, but in the case shown was 25 k for nitrobiphenyl and 13 k for biphenyl ( V =50 mV). If the voltage applied to the PPF relative to the Hg is scanned in a range of approximately 0.5 V, the i/V curve is nonlinear, invari- ant with scan rate for 0.01 to 1000 V/s, shows no observable hysteresis, and exhibits weak rectification. When the voltage was scanned more negative than approximately -0.8 V, a sudden in- crease in current was observed, which occasionally occurred in sev- eral steps. Upon the return scan near zero voltage, the junction re- sistance had decreased to 1k, and this high conductance state persisted for at least several minutes with V = 0. Upon scanning to a positive voltage of greater than approximately +0.9 V, the junc- tion resistance returned to its initial value. The complete cycle of turning the junction On at negative voltage and Off at positive volt- age was repeatable many times before apparent degradation resulted in a nonswitching junction resistance between the On and Off val- ues. The number of switching repetitions before failure varies with the sample and molecular structure, but several examples are listed in Table I. Eight monolayer structures were investigated to probe the switching mechanism, including a polynuclear aromatic hydrocar- bon chrysene and a molecule which includes an aliphatic spacer 4-phenylmethylenephenyl. Repetitive conductance switching was observed for the first four molecules listed in Table I, with the observed Off/On resistance ratio indicated. Not all junctions of a given molecule exhibited switching, and the percentage of active junctions i.e., those exhibiting at least three successive switching cycles is listed as well. Junctions made with the second group of four molecules occasionally exhibited conductance switching, but with a shorter lifetime. A few junctions which exhibited conductance switching were examined over a range of temperatures, and switch- * Electrochemical Society Active Member. a Permanent address: Department of Chemistry, University of Ankara, Ankara, Tur- key. b Permanent address: General Electric Corporation, John F. Welch Technology Cen- ter, Bangalore 560 066, India. c Permanent address: Department of Applied Chemistry, Tohoku University, Sendai 980-8579, Japan. z E-mail: mccreery.2@osu.edu Electrochemical and Solid-State Letters, 5 8 E43-E46 2002 0013-4651/2002/58/E43/4/$7.00 © The Electrochemical Society, Inc. E43