J . Phys. Chem. 1992,96, 6529-6532 (8) Roebber, L.; Vaida, V. J . Chem. Phys. 1985,83, 2748. (9) Donaldson, D. J. J . Chem. Phys. 1989, 92, 7455. (10) Hara, K.; Phillips, D. J . Chem. SOC.,Furuduy Trans. 2 1978, 74, 1441. ( 1 1 ) Waller, 1. M.; Hepburn, J. W. J . Chem. Phys. 1987, 87, 3261.
(12) See references given in the introduction of ref 9. (13) Tzeng, W.-B.; Yin, H.-M.; Leung, W.-Y.; Luo, J.-Y.; Nourbakhsh,
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S.;Fluch, G. D.; Ng, C. Y. J . Chem. Phys. 1988,88, 1658. (14) Waller, I. M.; Davis, H. F.; Hepburn, J. W. J . Phys. Chem. 1987, 91, 506. (15) Hart, D. J.; Hepburn, J. W. J . Chem. Phys. 1987,86, 1733. (16) Starrs, C.; Jego, M.-N.; Mank, A.; Hepburn, J. W. Manuscript in
preparation. (17) Mulliken, R. S. Phys. Reu. 1941, 60, 506.
A Nanometer-Scale Galvanic Cell Wenjie Li, Jorma A. Virtanen, and Reginald M. Penner* Institute for Surface and Interface Science, Department of Chemistry, University oy California, Irvine, Irvine, California 92717 (Received: May 28, 1992)
An electrochemicalcell having a largest total dimension of 70 nm was assembled on the surface of a highly oriented pyrolytic graphite crystal by electrochemicallydepositing copper and silver pillars (diameter = 150-200 A) in close proximity to one another using the scanning tunneling microscope (STM). When the surface containing both metals is immersed in a dilute copper plating solution, copper structures spontaneouslydecrease in size and, concurrently, silver structures increase in size. The direction and magnitude of these changes are consistent with a coupled electrochemical process in which copper anodically dissolves from the copper structures and plates onto the silver structures. The spontaneityof this "discharge" reaction indicates that coexisting copper and silver deposits immersed in a dilute Cu2+-containingelectrolyte comprise a galvanic cell.
Introduction The scanning tunneling microscope (STM) is gaining in importance as a device for modifying nanometer-sized regions of conductive surfaces.' An ultimate objective of this work is the fabrication of smaller electronic devices. Very recently? we described a procedure whereby 10-30-nm-diameter silver nuclei can be electrochemically deposited onto graphite substrates using a scanning tunneling microscope (STM). In this letter, this procedure is implemented to deposit two different metals, copper and silver, in close proximity to one another on a graphite surface. This is accomplished by electrochemically depositing silver structures3from a dilute Ag+-containing solution first, exchanging this solution for a dilute Cu2+ plating solution, and then electrochemically depositing copper structures. In our initial attempts to demonstrate the deposition of several different metals by this method, we have observed that silver and copper structures, located inside the same STM image window, are not stable in the aqueous 0.5 mM CuS04 solution used for copper deposition: concurrent and spontaneous decreases in the volume of copper electrodes and increases in the volume of silver structures are reproducibly observed. These volume changes are consistent in direction and magnitude with the coupled anodic dissolution of copper from the copper nuclei and reductive deposition of copper onto the silver nuclei. The spontaneity of these reactions may derive either from the thermodynamic stabilization of the copper film on the silver surface (i.e., underpotential deposition (UPD) of copper on silver" or from a potential provided by the silver-copper c o n t a ~ tor, , ~ in principle, from a combination of these two. In either case, this system exhibits behavior analogous to that of macroscopic galvanic cells. Experimental Section A commercial airlliquid STM6 in the normal two-electrode mode was employed for the deposition and imaging experiments described here. Polymer-insulated STM tips were employed for all measurements. These were fabricated from electrochemically etched platinum wires by coating with poly(a-methylstyrene), as described previously.' The residual faradaic currents present with these tips, measured at a tip-sample separation of 5 pm, never exceeded 50 pA in these experiments. Electrochemical deposition of silver and copper were accomplished from aqueous plating Address correspondence to this author.
solutions containing 0.5 mM of either silver fluoride (Aldrich, 99.9%) or copper(I1) sulfate (Aldrich, 99%). These solutions were prepared using Nanopure water (p > 18 MO) immediately before use. Singlebias pulses were supplied by a Hewlett-Packard 214B pulse generator. The bias voltage polarity is that for the tip versus the substrate. The STM was equipped with a liquid cell, constructed of Kel-F, which enabled the immersion of the tip and sample in 100 WL of solution and which allowed for the exchange of solutions during STM imaging. Highly oriented pyrolytic graphite substrates were cleaved immediately prior to use. STM images were obtained in constant current mode and represent raw data except for the subtraction of a linear background.
Rdults Figure 1 shows a progression of STM images obtained during a single fabrication and discharge cycle for a copper-silver cell. The same 150 X 150 nm region of the graphite substrate surface is shown in each image A-D. Initially, this region of the surface was free of step edges and other gross defects, but a distinctive indentation bisected the region. Following the addition of the silver plating solution to the cell, two silver pillars were electrochemically deposited by applying two (+)a V X 50 f i s bias pulses while tunneling, as shown in Figure 1A. These silver structures have approximate diameters of 150-200 A and are 20-30 A in height. We have previously shown2that the deposition of silver on graphite occurs by a two-step mechanism in which a monolayer-deep pit in the graphite surface forms during the first 5 p s of the pulse followed by nucleation and electrochemical deposition of the silver deposit. This mechanism allows the controlled deposition of metal pillars on regions of the graphite surface where facile nucleation sites, such as step edges, are not present. The silver pillars obtained by electrochemical deposition are stable with respect to volume or geometry changes in the 0.5 mM AgF solution employed for deposition for periods of a t least 1 h, has reported previously.2 In addition, no motion of the silver pillars is apparent on the substrate surface presumably because silver pillars are firmly anchored to their respective nucleation sites on the graphite surface. Following deposition and observation of the silver pillars, the 0.5 mM AgF solution was withdrawn from the cell and replaced with aqueous 0.5 mM CuS04 without losing registry on the substrate surface. Again, the volumes and geometries of the silver pillars were monitored for approximately 30 min, during which
0022-365419212096-6529%03.00/0 0 1992 American Chemical Society
6530 The Journal of Physical Chemistry, Vol. 96, No. 16, I992
Letters
5t t
4-t
T
10 20 30 40 50 60 Time, Min. Following Silver Depositions
0
Plots of metal structure volume versus time obtained from S f M images of the cell. Ag#l and A g # 2 correspond to the silver structures at upper right and lower right in the STM images of Figure 1, respectively, and Cu # 1 is the copper structure at upper left in these images. The structure shown at lower left in Figure 1 is composed of two lobes, Cu # 2 + Cu # 3, and Cu # 3 is removed mechanically at t = 40 min leaving just C u # 2 on the surface (see text). The error bars shown estimate the uncertainty of the volume measurements.
position of the copper pillars, respectively. Volume data for a larger number of STM images obtained on this time interval are plotted as a function of time in Figure 2. The direction of the volume changes at both copper and silver structures is consistent with the anodic dissolution of copper from the copper structures and the cathodic deposition of copper (or conceivably Cu(1) as Cu20or CuOH) onto the silver structures. The dramatic decrease in volume of Cu#2 Cu#3 (i.e., the structure at lower left in Figure l), however, is due to a combination of anodic dissolution and a mechanic cleavage event apparently induced by a collision with the STM tip. At t x 40 min in Figure 2,4596 of this structure by volume (designated Cu# 3) was removed mechanically and swept out of the STM image. However, because this structure initially exhibited a two-lobed geometry (not visible in Figure 1B), it was possible to account for the volumes of each lobe (Cu#2 and Cu # 3 in Figure 2) and compensate for the mechanical loss of copper. Thus, following the cleavage event, the data clearly show that the remaining lobe of the pillar (Cu#2), having a volume of ~ 2 8 0 nm3, 0 continues to dissolve at approximately the same rate as the Cu# 1 pillar at upper left. Complete discharge, signaled by complete dissolution of the copper structures, is not observed. Instead, copper and silver structures cease to change size at approximately the same time; following 46 min of reaction. Yet in independent experiments, we have demonstrated that electrochemically generated structures of either copper or silver may be "erased" by anodic dissolution by the application of tip (-) bias pulses. The origin of incomplete discharge is closely related to the thermodynamic driving force for the cell, both of which are discussed below. An open question in all in situ STM experiments in liquid ambients is, to what extent is the surface modified by the existence of trace faradaic currents between the STM tip and sample? In the present case, we performed a number of control experiments intended to allow the effect caused by STM imaging to be observed. We have, for example, examined silver structures in both copper- and silver-containing electrolytes and copper structures in copper-containing electrolytes, and we have not been able to observe changes in the volume induced by scanning for the conditions of tip (+) bias employed in all the imaging experiments described above. In Figure 3, for example, an STM image of a large copper structure is shown immediately following its deposition from 0.5 mM CuS04 (Figure 3A) and after 70 min of intermittent STM imaging (Figure 3B). No modifications in the detailed structure of the copper deposit are evident in these data, nor are changes in the volume of the structure exceeding 1% observed. This experiment, and others involving silver, indicate the stability of copper and silver structuresunder the STM imaging conditions employed in this study. An important component of
+
Figure 1. STM images of a copper-silver nanometer-scale cell on graphite. (A) Immediately following electrochemical deposition of two silver structures from aqueous 0.5 mM AgF. (B) After exchange of 0.5 mM CuSO, for 0.5 mM AgF and deposition of two additional copper structures (C) following 46 min of reaction in 0.5 mM CuSO,. All three images were acquired using conditions of bias and current of EB= (+)30 mV, I, = 0.5 nA.
no observable changes in geometry or volume were observed. The reductive deposition of copper onto the silver structures does not occur because the standard electrode potential for Ag+/Ago (PA +/A o = 0.800 V vs NHE) is 460 mV more positive than that for 6u2'/Cuo ( ~ c u ~ + I c u=o0.340 V vs NHE). Two additional (+)6 V X 50 ps bias pulses were then applied in order to effect the electrochemical deposition of two copper structures, with diameters of =200 A and heights of 30-50 A, shown at left in Figure 1B. Immediately following the deposition of copper metal on the surface, the volumes of both copper and silver structures begin to change with time with the copper electrodes decreasing in volume and the silver electrodes increasing in volume. These changes in volume are apparent in a comparison of parts B and C of Figure 1, which were acquired at 1 and 45 min after de-
Letters
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6531 TABLE I: Discbarge Data for Copper-Silver Nanometer-Scale Cell V.initial .. Vfinal AV Electrodea (nm3)b (nm3)c (nm3)" %/ Q (fCY
r Figure 3. STM images of a single copper structure on graphite, immersed in aqueous 0.5 mM CuS04, at two times: (A) Immediately following deposition; (B) 70 min following deposition. EB= (+)30 mV, i, = 0.2 nA.
HOPC
Figure 4. Schematic diagram illustrating discharge of nanometer-scale cell by the UPD discharge mechanism (see text).
experiments such as these, however, are the insulated STM tips which suppress residual faradaic currents to almost unmeasurably small values (
