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

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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
Page 1: Formation of atomic point contacts
Page 5 and 6: can flow between the electrodes. Th

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