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Probing Potential-Tuned Resonant Tunneling through Redox Molecules

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VOLUME 76, NUMBER 21 PHYSICAL REVIEW LETTERS 20MAY 1996Probing Potential-Tuned Resonant Tunneling through Redox Molecules withScanning Tunneling MicroscopyN.J. TaoDepartment of Physics, Florida International University, Miami, Florida 33199(Received 28 August 1995)We have studied electron transfer through redox molecules adsorbed on a conductive substrate withscanning tunneling microscopy (STM) in aqueous solution. By adjusting the substrate potential, theFermi levels of the substrate and tip can be easily shifted relative to the energy levels of the molecules.Aligning the Fermi levels to an energy level, a nearly tenfold increase in the tunneling current that flowsbetween the substrate and the STM tip via the molecules, due to resonant tunneling is observed. Thisexperiment also shows that structurally similar molecules may be identified with STM based on theirdifferent redox properties. [S0031-9007(96)00248-7]PACS numbers: 82.45.+z, 73.40.Gk, 85.65.+h, 87.64.DzThe rapid progress in electronics over the last sev-eral decades relies critically on miniaturization which hasbeen realized by carving smaller and smaller features onsemiconductor chips. Another approach towards ultimateminiaturization that has attracted much interest is to builda circuit with single molecules [1]. These two approachesbegin to overlap as semiconductor devices shrink towardsthe nanometer scale. On such a small scale, the discretenature of charge and energy becomes important, and de-vices such as quantum dots begin to exhibit atomic ormolecular properties [2]. An important task in develop-ing molecular devices is to understand electron transferbetween molecules and electrodes [1]. Using a scanningtunneling microscope, Joachim et al. [3] have recentlystudied electron tunneling through single C60molecules inultrahigh vacuum as a function of tip displacement. In thiswork, we attach redox molecules to a conductive substratein aqueous solution and study the electron transfer throughthe molecules between the substrate and a scanning tunnel-ing microscopy (STM) tip [Fig. 1(a)]. In contrast to fab-ricated quantum dot systems, the discrete nature of chargeand energy in the present system is important even at roomtemperature because of the small size and the large separa-tion between the energy levels of the molecules. Anotherdistinctive feature of the present system is that the substrateand tip Fermi levels can be flexibly adjusted relative to themolecular orbitals by controlling the substrate potential vsa reference electrode inserted in the solution (Fig. 1). Bytuning the substrate Fermi level to an unoccupied molec-ular orbital, we have observed a nearly tenfold increase inthe tunneling current due to a resonant enhancement.Studying electron transfer reactions of molecules inaqueous solution is of great importance also because ofits central role in a variety of processes ranging fromelectrodeposition in electrochemistry to photosynthesis inbiology [5]. To date, experimental information aboutthe phenomenon has been provided primarily by spectro-scopic and electrochemical techniques that measure vari-ous quantities averaged over a large number of molecules.The present work demonstrates a method for following theelectron transfer reactions of individual molecules whichmay, therefore, provide new information (e.g., the depen-dence of electron transfer on the adsorption sites of theredox molecules) that is buried in the statistical averagingof conventional techniques.The possibility of studying electron transfer of redoxmolecules in aqueous solution with STM was first pro-posed by Schmickler et al. [6] by performing tunnelingFIG. 1. (A) Schematic of a sample molecule coadsorbedwith reference molecules on a substrate as probed by anSTM tip. RE and CE represent the reference and counterelectrodes, respectively. Vsuband Vbiasare the substratepotential (with respect to the reference electrode) and thetip-substrate bias voltage, respectively, which are controlledindependently by a bipotentiostat. (B) Energy diagram ofFe(III)-protoporphyrin IX in relation to the Fermi levels of thesubstrate and tip (Ref. [4]). PP LUMO and PP HOMO are thelowest unoccupied and highest occupied molecular orbitals ofprotoporphyrin IX, respectively, and Fe LUMO is the LUMOof Fe(III) in Fe(III)-protoporphyrin IX. Note that the additionof Fe(III) in protoporphyrin IX causes little change in the PPLUMO and PP HOMO.4066 0031-9007y96y76(21)y4066(4)$10.00 © 1996 The American Physical SocietyVOLUME 76, NUMBER 21 PHYSICAL REVIEW LETTERS 20MAY 1996spectroscopy. Although tunneling spectroscopy has beensuccessfully used to obtain local electronic information ofvarious samples in ultrahigh vacuum [7], the method isdifficult to apply to molecules in aqueous solution due toa polarization current induced by the sweeping of the tip-substrate bias voltage. In the present work, we coadsorba sample species with a reference species on a substrateand study the tunneling via the sample molecule, in rela-tion to that via the reference molecules, as a function ofthe substrate potential with a small fixed tip-substrate biasvoltage. The reference species is chosen so that the en-ergy levels of its molecular orbitals are far away from thesubstrate and tip Fermi levels, therefore the tunneling cur-rent via it is not sensitive to the substrate potential. Thisapproach does not have the problem of polarization cur-rent because no rapid sweeping of the tip-substrate bias isinvolved. It also reduces errors due to possible changesin the tip states during the measurement because the tun-neling current via the sample molecules is measured inrelation to that via the reference molecules.The STM experiment was performed on a Pico-STM system (Molecular Imaging Co.) controlled by aNanoscope III controller (Digital Instruments Inc.) with ahome-made solution cell. The system can be operated at atunneling current as small as 1 pA and has a typical driftof ,0.1 Åys. STM tips were etched electrochemicallyfrom Pt0.8Ir0.2and W wires which were then coated withapiezon wax [8]. The etching conditions were optimizedto achieve a desirable tip profile for the coating. Al-though the tips were not the sharpest, they had a verysmall leakage current (,1 pA) over a broad potentialrange. In order to minimize the mechanical disturbanceof the scanning tip on the molecules, the tunneling currentwas set to 30 pA or less with a tip-substrate bias of 0.1 V.Highly oriented pyrolytic graphite (Union


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