Formation of atomic point contacts and molecular junctions with a ...

Formation of atomic point contacts and molecularjunctions with a combined mechanical breakjunction and electrodeposition methodX.L. Li, S.Z. Hua, H.D. Chopra and N.J. TaoAbstract: A method to create atomic point contacts and molecular junctions by combiningmechanically controlled break junction and electrochemical deposition/etching techniques isdescribed. This approach provides both stability and flexibility, and is suitable for studying electrontransport properties of single molecule junctions or atomic point contacts in aqueous solutions andunder potential control. Using the approach, the electron transport properties of 4,4 0 -bipyridine as afunction of electrochemical gate and Ni point contacts have been studied.1 IntroductionIn order to determine the electron transport properties of asingle molecule, one must first connect the molecule to atleast two external electrodes. One way to achieve thisrequirement is to fabricate a pair of facing electrodes on asolid substrate and then bridge the electrodes with the moleculeterminated with proper anchoring groups that can bindto the electrodes. A key task of this method is to fabricateelectrodes separated with a molecular scale gap, whichhas been a difficult task. This is because most moleculesare a few nanometres or less, and the fabrication of such asmall gap between electrodes is beyond the reach of conventionalmicro- and nanofabrication facilities [1–11].A closely related system is atomic point contacts in whichtwo electrodes are connected by a single or a smallnumber of metal atoms [12–16]. For many metals, suchas Au, the conductance of an atomic point contact isquantised, approximately given by an integer multiple ofG 0 ¼ 2e 2 /h ¼ 77.4 mS. Such a system is also interestingfor its rich magneto-transport properties [17–19].One method that has been used to create a molecularjunction and atomic point contact is mechanically controllablebreak junction (MCBJ) [20–22]. The basic principle ofMCBJ is to break a nanobridge between two electrodesfixed on a substrate fabricated with techniques, such as electronbeam lithography. The breakdown process is controlledby bending the substrate with a mechanical actuator (e.g.piezoelectric transducer (PZT) and a stepping motor)(Fig. 1). Bending the substrate elongates the narrowbridge, which reduces the diameter of the thinnest portionof the bridge. When the thinnest portion is decreased tothe atomic scale, an atomic point contact is eventuallyformed. If one chooses to further bend the substrate, the# The Institution of Engineering and Technology 2006Micro & Nano Letters online no. 20065049doi:10.1049/mnl:20065049Paper first received 8th September 2006X.L. Li and N.J. Tao are with the Department of Electrical Engineering, ArizonaState University, Tempe, AZ 85287, USAS.Z. Hua and H.D. Chopra are with the Materials Program, Mechanical andAerospace Engineering Department, State University of New York at Buffalo,Buffalo, NY 14260, USAE-mail: xiulan@asu.edubridge breaks and results in a gap between the two facingelectrodes. By connecting the gap with a molecule of interest,a molecular junction is formed. An important advantageof MCBJ is that the formation of an atomic point contact, ora molecular scale gap can be continuously adjusted.However, the mechanical actuation makes it complicatedand difficult for many potential device applications and itis also limited to contacts between the same metal.Another technique to fabricate atomic point contactsand molecular junctions is the electrochemical method [1, 2,4, 23–25]. One strategy is to electrochemically etch thenarrow portion of a metal wire fixed on a solid substrateuntil it decreases to the atomic scale to form an atomicpoint contact, or a molecular scale gap for molecular junctions[1]. An alternative approach is to start with a pair of electrodesseparated initially with a large gap, and then electrochemicallydeposit atoms onto the electrodes to decrease the gap to themolecular scale, or all the way, until an atomic pointcontact is created [1, 2, 4, 23]. Since electrochemicaletching (deposition) is reversible, the electrochemical fabricationmethods allow one to control the fabrication processuntil a desired gap or contact is reached. In comparison tothe mechanical method, the electrochemical method doesnot require mechanical actuation, can fabricate the electrodesor contacts with different metals [26], and is amenable todevice applications.Combination of MCBJ and electromigration to study thesingle-molecule transistor with solid gate was demonstratedby Ralph and co-workers [27]. We report here a combinedmechanical and electrochemical method to fabricateatomic point contacts and molecular junctions in aqueoussolutions. The technique combines the advantages of theelectrochemical method along with the ability to makeadjustable gaps afforded by the mechanical method. Theaqueous environment is required for studying biologicallyrelevant molecules, molecular binding process and is convenientfor one to introduce sample molecules into themeasurement. In addition, it allows one to control the electrontransport processes through the atomic point contacts ormolecular junctions using an electrochemical gate. Usingthis method, we have measured the conductance of4,4 0 -bipyridine and studied the electrochemical gate effecton the conductance of the molecule. We have alsofabricated and studied charge transport across Ni atomicpoint contacts.Micro & Nano Letters, Volume 1, Issue 2 83

Fig. 1 Schematic drawing of a MCBJ setupAu wire with nanobridge in the middle is fixed on a substrateThe device is mounted by a three-point setupBy bending up the substrate with a mechanical actuator (e.g. piezoelectrictransducer and a stepping motor), the nanobridge is elongated2 Experimental method and resultsWe first fabricated a pair of Au electrodes separated with1 mm gap on an Si substrate using photolithography. Toreduce ionic leakage current, the Au electrodes werecovered with Si 3 N 4 insulating layer except for a smallwindow to expose the gap. We then etched a trench in theSi underneath the Au electrodes to suspend the electrodes.The last step was to reduce the separation of the two suspendedelectrodes to a molecular gap or to form an atomicpoint contact by electrochemically depositing Au onto theelectrodes. A mechanical actuation in an MCBJ configurationallows one to further adjust the gap or contact between theelectrodes. We provide more fabrication details and discussexperimental results subsequently.2.1 Fabrication of the deviceWe used a 360-mm thick Si (1 0 0) wafer (7–13 V cmresistivity) covered with a 50 nm thermal SiO 2 insulationlayer. We then patterned the Si substrate with an array ofAu electrode pairs using standard photolithography tools.The separation between the two electrodes in each pairwas 1 mm. The Au electrodes were thermally evaporatedwith a thickness of 60–100 nm on top of 8-nm Cradhesion layer. Thicker Au electrodes were not attemptedto avoid hard lift-off. A 400-nm thick Si 3 N 4 layer wasdeposited by chemical vapour deposition (CVD) on theentire wafer except for a 10-mm wide strip that exposesthe gap regions of all the electrode pairs. The Si 3 N 4 layerserved as an insulation layer to minimise leakage currentdue to ionic conduction which is important for carryingout electrical measurement in electrolytes. The leakagecurrent was typically a few pA, but it increased afterwet-etching Si 3 N 4 , SiO 2 and Si. The Si 3 N 4 also functionedas a mask needed to etch the 10-mm wide strip into a trench.This was carried out by first removing the Si 3 N 4 and SiO 2using 20:1 buffered oxide etchant (BOE). The etch ratewas calibrated on Si wafer with thermal SiO 2 or CVDSi 3 N 4 in advance. It was followed by etching away theexposed Si in the strip in 0.44 g/ml KOH. This wetetching was anisotropic and it created a triangular trenchin the Si substrate (Fig. 2a). We covered the backside ofthe wafer by another layer of 400 nm CVD Si 3 N 4 beforewet etching Si, in order to prevent the backside frometching during the last etching process. Before electrochemicaldeposition of Au in the next step, we usedoxygen plasma to remove the organic contaminationduring the process, followed by rinsing with acetone,ethanol, and 18-MV pure water for 5 min each.84Fig. 2 SEM images of two pairs of Au electrodesa Without Au deposition, Au electrodes are separated with 1 mm gapb After Au deposition, an Au nanojunction is formed. Note that theL-shape electrode in the upper left of a is a marker for alignmentduring the microfabrication processThe inset is a typical current plot during electrodeposition processshowing that a rather stable molecular gap can be obtained by controllingthe potentialThe bias was 0.01 V during the electrodepositionThe two Au electrodes in a pair were then bridged byelectrochemically depositing Au onto the electrodes. Thedeposition was controlled by a homemade bipotentiostatusing a Pt coil as the counter electrode and an Ag wire asthe quasi-reference electrode. The quasi-reference electrodewas calibrated against the more commonly used Ag/AgCl(in 3.5 M KCl). Au material was deposited onto the Au electrodesat constant potential of 20.15 V against Ag/AgClwith 8–10 mM sodium gold thiolsulfate in 0.3 M citricacid (pH 3.5). During the electrodeposition process, thecurrent between the two electrodes was monitored continuously.Initially, the current was small and entirely due toionic leakage current, but it increased sharply when amolecular gap or atomic point contact was formed. The conductanceof the junction was used as a feedback for us tocontrol the deposition process (Fig. 2b). The inset inFig. 2 shows a typical current plot during the electrodepositionprocess. The tunneling current showed a big jump whenMicro & Nano Letters, Volume 1, Issue 2

the gap width was decreased to the molecular scale. Bycontrolling the electrodeposition (solid grey line), a stablemolecular scale gap can be formed, as indicated by asteady tunneling current (solid black line). After completingthe fabrication, we thoroughly rinsed the chip with 18 MVpure water to remove Au deposition solution and dried it outwith N 2 gas. We then mounted it on a MCBJ setup to furthercontrol the gap or point contact mechanically.2.2 CalibrationSince the mechanical control of the gap width or pointcontact size is achieved by bending the chip with a PZT(AE0203D08 piezoelectric stack, THORLAB Inc.), it isnecessary to calibrate the actual separation of the electrodesfor a given amount of PZT movement. Displacement ratio r,defined as the ratio of the elongation of the PZT over theinduced electrode separation, serves this calibrationpurpose. This ratio is given by r ¼ L 2 /3 mH, where L isthe distance between the two counter supports, m is thewidth of the trench and H is the thickness of the substrate(Fig. 1). In our setup, L ¼ 12 mm, m ¼ 10 mm andH ¼ 360 mm, the calculated ratio is 1.3 10 4 . To determinethe displacement ratio experimentally, we recordedthe tunneling current while applying a triangular wave tothe PZT driver. Similar displacement ratio was obtainedfrom three different devices, of which a typical one isshown in Fig. 3. From the tunneling current, we estimatedthe gap width using relation, I ¼ (2e 2 /h)V * exp(2bd),where I is the tunneling current, e is the electron charge,h is the Planck constant, V is the applied bias voltageacross the gap, d is the gap width and b, the tunnelingdecay constant in solution, is in the order of 10 nm 21[28]. According to the high and low stable tunnelingcurrent in Fig. 3, at a fixed bias voltage of 0.01 V, the correspondingelectrode separation was varied by 0.075 nm.Since the sensitivity of the PZT is 0.06 mm/V and thevoltage change in the PZT drive was 11.5 V, the elongationof PZT was determined to be 0.69 mm. Using these parameters,the measured displacement ratio was found to be1 10 4 , which is close to the theoretical estimate. Thisratio is large, so a large PZT movement is demagnifiedinto a small change in the gap width. This is the reasonfor the good stability of the setup, but it also limits thetotal adjustable range of the gap to 1 nm, with themaximum elongation of 9 mm of PZT in our setup. Theproblem associated with this limited mechanicallyFig. 3 Calibration of the displacement ratio, rThe PZT driver was modulated with a triangular wave (grey colour)and the associated tunneling current is recorded vs time (black colour)The bias in the calibration was 0.01 Vadjustable range is solved by combining it with electrochemicaletching and deposition, which provides an additionalcontrol of the size of the metal junctions. We note that sinceonly a small amount of mechanical adjustment is used in oursetup, it reduces the likelihood of breaking the Si substrate.We found that if we deposit a large amount of metal tobridge the gap, it will require a large mechanical movementto re-open the gap, which often causes the Si substrate tobreak. The mechanical breakdown of the Si substrate wasthe reason that the yield of the entire process was around50%, over a total of 200 devices. Once a device wasfabricated successfully, it could be used repeatedly. But inorder to minimise possible contamination, we use newdevices daily.2.3 Electron transport in 4,4 0 -bipyridineWe have used this method to study electron transport in4,4 0 -bipyridine. The molecule is terminated with two pyridinegroups which can bind to Au electrodes. It is expectedto be relatively conducting due to its conjugated structure.Finally, 4,4 0 -bipyridine is soluble in aqueous solution,which allows us to study the electrochemical gate effect.We performed the conductance measurement by breakingan Au contact in 1 mM 4,4 0 -bipyridine aqueous solution(with 0.05 M NaClO 4 as supporting electrolyte for potentialcontrol). During the breaking process, we first observedconductance quantisation associated with breakup of thegold contact (Fig. 4a). Clean quantisation steps show thatthe Au junction formed by electrodeposition was continuousand fresh. When the gold contact was completely broken,steps with smaller conductance showed up (Fig. 4b). Ourresults indicate that these steps are due to the formation ofAu–4,4 0 -bipyridine–Au junction with different number ofmolecules trapped between the two Au electrodes. Fig. 4cis a conductance histogram constructed from 180 individualtraces such as those shown in Fig. 4b. A pronouncedconductance peak located at 0.01G 0 was revealed, whichis consistent with the previously published STM breakjunction result [29]. Broad width of conductance peakreflects the variable molecule–electrode contact geometriesin the formation of molecular junctions.To measure the I–V curves of the molecular junctions,we adjusted the electrode separation by controlling thePZT until the conductance locked on a stable plateau, correspondingto the formation of a single molecule junction, andthen froze the PZT and measured the current while sweepingthe bias voltage. The bias range studied here wasbetween 20.4 V and þ0.4 V, beyond which the junctionbecame increasingly unstable. This instability is likely dueto the fact that the N–Au binding is relatively weak and4,4 0 -bipyridine desorbs from Au surfaces at negative potentials.We found that if thiol anchoring groups are used tobridge molecules to Au electrodes, the bias voltage rangecould be expanded to +1.2 V, due to stronger S–Aubinding. For comparison, the I–V data previously obtainedwith an STM setup [29] is also shown as grey solid squares,which shows that the two methods are consistent with eachother (Fig. 5a).We have studied the electrochemical gate effect on theconductance of single 4,4 0 -bipyridine with the setup. Theelectrochemical gate was controlled by a homemadebipotentiostat using a Pt wire as the counter electrode andan Ag wire as the quasi-reference electrode. The quasi referenceelectrode was calibrated against the more commonlyused Ag/AgCl (in 3.5 M KCl) reference electrode. Tomeasure the gate effect, we created a stable 4,4 0 -bipyridinejunction and then monitored the current through theMicro & Nano Letters, Volume 1, Issue 2 85

Fig. 4 Conductance quantisation of Aua Au quantisation steps near integer multiples of G 0 ¼ 2e 2 /h as the Aujunction was elongated into an atomic point contactb After the Au atomic point contact was broken, a new set of conductancesteps showed up in presence of 4,4 0 -bipyridineThe steps are due to the formation of Au–4,4 0 -bipyridine–Aujunctionsc Conductance histogram of 4,4 0 -bipyridine constructed from 180individual curves displays conductance peak at 0.01G 0 and 0.02G 0Grey solid lines are the Gaussian fittingBias was 0.1 V in the measurementmolecule while sweeping the electrochemical gate voltage,during which the bias voltage between two Au electrodeswas fixed at 0.1 V. A typical source-drain current (I sd )against gate voltage (V g ) curve is shown in Fig. 5b.Source-drain current increased 25%, as the gate voltagewas swept negatively. There was a run-to-run variation,but statistical analysis based on 17 different junctionsshowed only a small electrochemical gate effect (Fig. 5c).To verify the weak gate effect on 4,4 0 -bipyridine, wecarried out a series of control experiments: (1) Wechanged the sweeping rate of the electrochemical gatevoltage and found no obvious dependence of the gateinducedconductance change on the sweep rate, which indicatesthat the gate effect is not due to a capacitive chargingprocess. (2) We also measured the tunneling current across a86Fig. 5 Electrical characteristics of 4,4 0 -bipyridineaI–V curves of single 4,4 0 -bipyridine obtained by MCBJ method bysweeping bias voltage (black solid lines) and from the positions ofthe peaks in the conductance histograms by Scanning TunnelingMicroscope (STM) break junctions (grey solid squares)b A typical source-drain current (I sd ) as a function of electrochemicalgate voltage (V g ) of a 4,4 0 -bipyridine molecular junction in 0.05 MNaClO 4The gate voltage was swept from 20.2 V to þ0.6 V against Ag/AgClCorresponding ionic leakage current is shown as grey solid line forcomparisonc Statistic of the current change of 17 individual curves shown in bBias was fixed at 0.1 V in the measurementpair of bare electrodes (without the presence of molecules)as a function of electrochemical gate voltage and observedno significant change in the tunneling current. (3) To eliminateionic leakage current as a possible cause for the gateeffect, we measured the current between the two electrodesby separating them far apart to ensure no tunneling electronsMicro & Nano Letters, Volume 1, Issue 2

can flow between the electrodes. The current was typicallywell below 0.2 nA in the electrolyte shown as the grey solidline in Fig. 5b, much smaller than the conduction currentthrough the molecular junction. The electrochemical gateeffect observed here is rather weak, which could be partiallydue to the screening of the gate field by the proximity of thetwo electrodes (serving as source and drain).3 Ni atomic point contactThe same setup can create atomic point contacts by eithermechanically controlling the elongation of the Au electrodesor by electrodeposition/etching process (Fig. 4a). Theuse of electrodeposition makes it easy to fabricate atomicpoint contacts of different metals, and even a pointcontact between two different metals. To demonstrate thisapplication, we have fabricated Ni atomic point contactsusing this combined approach. The first step is to electrochemicallydeposit Ni onto Au electrodes by holding thepotentials of the two electrodes at 21.1 V against Ag/AgCl with 0.1 M nickel sulphamate in 0.5 M boric acid(pH 3.5). We can also choose to deposit Ni onto only oneof the two electrodes to fabricate an Au–Ni contact. Aftera point contact is formed, we can further control the sizeof the atomic point contact mechanically. Since Ni is aferromagnetic material with partial occupied d-orbital, ithas been argued that the strong exchange splitting of theelectron bands may lift the spin degeneracy. The up- anddown-spin electrons contribute independently to the electrictransport, which results in the conductance of atomic pointcontact located at integer multiples of e 2 /h. In the preliminaryexperiments, we have performed the measurementwith a magnetic field (0.4 T). A typical conductancetrace is shown in Fig. 6a. Unlike the results without a magneticfield (result not shown), steps near or at integer multiplesof e 2 /h have rather frequently been observed in Nipoint contacts. The conductance histogram (Fig. 6b) constructedfrom 18 individual curves from two differentdevices shows some evidence of conductance peakslocated at integer multiples of e 2 /h [30–32]. Furtherstudies are in progress to verify and understand the preliminaryfindings.4 SummaryWe have described a combined mechanical and electrochemicaldeposition method to fabricate atomic point contactsand molecular junctions. Using the method, we have createdatomic point contacts and studied electron transport through4,4 0 -bipyridine. The setup is particularly suitable for studyingatomic point contacts and molecular junctions inaqueous solutions, which is required for biologically relevantmolecules, molecular binding events and electrochemicalgate-control of single molecule conductance.5 AcknowledgmentsWe would like to thank Bingqian Xu and Joshua Hihath forthe help and acknowledge the financial support fromNSF-DMR-03-05242.6 ReferencesFig. 6 Conductance quantisation of Nia Conductance quantisation steps of a Ni atomic point contact. Thequantised conductance steps are located at integer multiples of e 2 /hb Conductance histogram constructed from 18 individual curves asshown in a1 Li, C.Z., Bogozi, A., Huang, W., and Tao, N.J.: ‘Fabrication of stablemetallic nanowires with quantized conductance’, Nanotechnology,1999, 10, pp. 221–2232 Morpurgo, A.F., Marcus, C.M., and Robinson, D.B.: ‘Controlledfabrication of metallic electrodes with atomic separation’, Appl.Phys. Lett., 1999, 14, p. 20823 Kervennic, Y.V., Thijssen, J.M., Vanmaekelbergh, D., Dabirian, R.,Jenneskens, L.W., van Walree, C.A., and van der Zant, H.S.J.:‘Charge transport in three-terminal molecular junctions incorporatingsulfur-end-functionalized tercyclohexylidene spacers’, Angew. Chem.Int. Ed. Engl., 2006, 45, pp. 2540–25424 Li, X.L., He, H.X., Xu, B.Q., Xiao, X., Tsui, R., Nagahara, L.A.,Amlani, I., and Tao, N.J.: ‘Electron transport properties ofelectrochemically and mechanically formed molecular junctions’,Surf. Sci., 2004, 573, pp. 1–105 Park, J., Pasupathy, A.N., Goldsmith, J.L., Chang, C., Yaish, Y., Petta,J.R., Rinkoski, M., Sethna, J.P., Abruna, H.D., McEuen, P.L., andRalph, D.C.: ‘Coulomb blockade and the Kondo effect insingle-atom transistors’, Nature, 2002, 417, pp. 722–7256 Liang, W.J., Shores, M., Bockrath, M., Long, J.R., and Park, H.:‘Kondo resonance in a single-molecule transistor’, Nature, 2002,417, pp. 725–7297 Lee, J.-O., Lientschnig, G., Wiertz, F., Struijk, M., Janssen, R.A.J.,Egberink, R., Reinhoudt, D.N., Hadley, P., and Dekker, C.:‘Absence of strong gate effects in electrical measurements onphenylene-based conjugated molecules’, Nano Lett., 2003, 3,pp. 113–1178 Luber, S.M., Strobel, S., Tranitz, H.P., Wegscheider, W., Schuh, D.,and Tornow, M.: ‘Nanometre spaced electrodes on a cleavedAlGaAs surface’, Nanotechnology, 2005, 16, pp. 1182–11859 Kubatkin, S., Danilov, A., Hjort, M., Cornil, J., Bredas, J.-L.,Stuhr-Hansen, N., Hedegard, P., and Bjornholm, T.: ‘Single-electrontransistor of a single organic molecule with access to several redoxstates’, Nature, 2003, 425, pp. 698–70110 Zhitenev, N.B., Meng, H., and Bao, Z.: ‘Conductance of smallmolecular junctions’, Phys. Rev. Lett., 2002, 92, pp. 186805/186801–186805/18680411 Ghosh, S., Halimun, H., Mahapatro, A.K., Choi, J., Lodha, S., andJanes, D.: ‘Device structure for electronic transport throughindividual molecules using nanoelectrodes’, Appl. Phys. Lett., 2005,87, p. 233509Micro & Nano Letters, Volume 1, Issue 2 87

12 Gimzewski, J.K., and Moller, R.: ‘Transition from the tunnelingregime to point contact studied using scanning tunnelingmicroscopy’, Phys. Rev. B, 1987, 36, pp. 1284–128713 Pascual, J.I., Mendez, J., Gomez-Herrero, J., Baro, A.M., Garcia, N.,and Binh, V.T.: ‘Quantum contact in gold nanostructures by scanningtunneling microscopy’, Phys. Rev. Lett., 1993, 71, pp. 1852–185514 Krans, J.M., Ruitenbeek, J.M.v., Fisun, V.V., Yanson, I.K., and Jongh,L.J.d.: ‘The signature of condutance quantization in metallic pointcontacts’, Nature, 1995, 375, pp. 767–76915 Agrait, N., Rodrigo, J.G., and Vieira, S.: ‘Conductance steps andquantization in atomic-size contacts’, Phys. Rev. B, 1993, 52,pp. 12345–1234816 Yanson, A.I., Rubio, Bollinger G., van den Brom, H.E., Agrait, N.,and van Ruitenbeek, J.M.: ‘Formation and manipulation of ametallic wire of single gold atoms’, Nature, 1998, 395, pp. 783–78517 Garcia, N., Munoz, M., and Zhao, Y.-W.: ‘Magnetoresistance inexcess of 200% in ballistic Ni nanocontacts at room temperatureand 100 Oe’, Phys. Rev. Lett., 1999, 82, pp. 2923–292618 Chopra, H.D., and Hua, S.Z.: ‘Ballistic magnetoresistance over3000% in Ni nanocontacts at room temperature’, Phys. Rev. B,2002, 66, p. 02040319 Chopra, H.D., Sullivan, M.R., Armstrong, J.N., and Hua, S.Z.: ‘Thequantum spin-valve in cobalt atomic point contacts’, Nature Mater.,2005, 4, pp. 832–83720 Moreland, J., and Ekin, J.W.: ‘Electron tunneling experimentsusing Nb–Sn break junctions’, J. Appl. Phys., 1985, 58,pp. 3888–389521 Muller, C.J., van Ruitenbeek, J.M., and de Jongh, L.J.: ‘Conductanceand supercurrent discontinuities in atomic-scale metallic constrictionsof variable width’, Phys. Rev. Lett., 1992, 69, pp. 140–14322 Ruitenbeek, J.M.v., Allvarez, A., Pineyro, I., Grahmann, C., Joyez, P.,Devoret, M.H., Esteve, E., and Urbina, C.: ‘Adjustable nanofabricatedatomic size contacts’, Rev. Sci. Instrum., 1996, 67, pp. 108–11123 Li, C.Z., and Tao, N.J.: ‘Quantum transport in metallic nanowiresfabricated by electrochemical deposition/dissolution’, Appl. Phys.Lett., 1998, 72, pp. 894–89724 Xiang, J., Liu, B., Wu, S.T., Ren, B., Yang, F.Z., Mao, B.W., Chow,Y.L., and Tian, Z.Q.: ‘A controllable electrochemical fabrication ofmetallic electrodes with a nanometer/angstrom-sized gap using anelectric double layer as feedback’, Angew. Chem. Int. Ed., 2005, 44,pp. 1265–126825 Qing, Q., Chen, F., Li, P.G., Tang, W.H., Wu, Z.Y., and Liu, Z.F.:‘Finely tuning metallic nanogap size with electrodeposition byutilizing high-frequency impedance in feedback’, Angew. Chem. Int.Ed., 2005, 44, pp. 7771–777526 Deshmukh, M.M., Prieto, A.L., Gu, Q., and Park, H.: ‘Fabrication ofasymmetric electrode pairs with nanometer separation made of twodistinct metals’, Nano Lett., 2003, 3, pp. 1383–138527 Champagne, A.R., Pasupathy, A.N., and Ralph, D.C.: ‘Mechanicallyadjustable and electrically gated single-molecule transistors’, NanoLett., 2005, 5, pp. 305–30828 Vaught, A., Jing, T.W., and Lindsay, S.M.: ‘Nonexponential tunnelingin water near an electrode’, Chem. Phys. Lett., 1995, 236, pp. 306–31029 Xu, B.Q., and Tao, N.J.: ‘Measurement of single moleculeconductance by repeated formation of molecular junctions’, Science,2003, 301, pp. 1221–122330 Elhoussine, F., Matefi-Tempfli, S., Encinas, A., and Piraux, L.:‘Conductance quantization in magnetic nanowires electrodepositedin nanopores’, Appl. Phys. Lett., 2002, 81, pp. 1681–168331 Ono, T., Ooka, Y., Miyajima, H., and Otani, Y.: ‘2e(2)/h to e(2)/hswitching of quantum conductance associated with a change innanoscale ferromagnetic domain structure’, Appl. Phys. Lett., 1999,75, pp. 1622–162432 Shimizu, M., Saitoh, E., Miyajima, H., and Otani, Y.: ‘Conductancequantization in ferromagnetic Ni nano-constriction’, J. Magn. Magn.Mater., 2002, 239, pp. 243–24588Micro & Nano Letters, Volume 1, Issue 2