NANO LETTERS
Observation of Current Reversal in the Scanning Tunneling Spectra of Fullerene-like WS2 Nanoparticles
2006 Vol. 6, No. 4 760-764
Doron Azulay,‡ Frieda Kopnov,§ Reshef Tenne,†,§ Isaac Balberg,‡ and Oded Millo*,‡ Racah Institute of Physics, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel, and Department of Materials and Interfaces, Weizmann Institute, RehoVot 76100, Israel Received January 9, 2006; Revised Manuscript Received February 23, 2006
ABSTRACT Current−voltage characteristics measured using STM on fullerene-like WS2 nanoparticles show zero-bias current and contain segments in which the tunneling current flows opposite to the applied bias voltage. In addition, negative differential conductance peaks emerge in these reversed current segments, and the characteristics are hysteretic with respect to the change in the voltage sweep direction. Such unusual features resemble those appearing in cyclic voltammograms, but are uniquely observed here in tunneling spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. This behavior is attributed to tunneling-driven electrochemical processes.
The inorganic fullerene-like (IF) metal dichalcogenide MX2 materials, where M ) W or Mo (in particular) and X ) S or Se, have been studied thoroughly using various techniques since their discovery1,2 in the early 1990s. Special interest was devoted to their intriguing mechanical and tribological properties that yield many potential applications for reduced friction and wear. In addition, they also show interesting electronic and optical properties, for example, the dependence of the excitonic band-gap on size and on the number of nested MX2 layers, which were studied using optical techniques.3 However, systematic tunneling spectroscopy measurements, which may yield information on the quasiparticle band-gap and on other electronic and transport properties of these materials, are still lacking. This is in contrast to the case of WS2 nanotubes, for which scanning tunneling microscopy (STM) was employed to monitor the diameter dependence of the (quasi-particle) band-gap.4 The I-V curves showed a featureless gap structure, and the inferred band-gap was found to reduce with decreasing diameter, in contrast to the case of semiconductor nanocrystal quantum-dots5 and nanorods.6 This behavior was attributed4 to the effect of size-dependent curvature of the WS2 layers. STM measurements of iodine-doped individual single-wall MoS2 nanotubes, showing metallic behavior in the axial direction, were also reported recently.7-9 * Corresponding author. E-mail:
[email protected]. † E-mail:
[email protected]. ‡ The Hebrew University of Jerusalem. § Weizmann Institute. 10.1021/nl060044r CCC: $33.50 Published on Web 03/16/2006
© 2006 American Chemical Society
In this paper we report an STM investigation of pellets consisting of IF-WS2 particles having diameters in the range of 50-70 nm. The tunneling current-voltage (I-V) characteristics acquired here with (relatively small, see below) voltage ranges such as those typically applied in previous experiments4,10 exhibit a regular semiconductor gap structure. Surprisingly, however, those acquired with a larger bias voltage range show unique anomalous features. These include zero-bias current, reversed current (flowing in opposite direction to the applied bias voltage) and hysteresis with respect to the reversal of the bias sweep direction. In addition, negative differential conductance-like (NDC-like) peaks appear in the reversed current regions of the I-V curves. These features are reminiscent of those appearing in cyclic voltammograms measured in electrochemical cells,11 but are observed here, for the first time, in STM spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. We attribute this behavior to electrochemical reactions involving the W atoms and water molecules embedded in the sample (see below), processes that are triggered by the large tip-sample voltages applied in our experiments. Our IF-WS2 nanoparticles were produced via a hightemperature solid-gas reaction of precursor WO3 nanoparticles with H2 and H2S, as detailed elsewhere.12,13 The resulting IF particles, as shown in Figure 1a, consist of nested WS2 layers (typically 30 in our case) encapsulating a hollow core. This structure is relatively defect free, except at loci where sharp folding of the WS2 layers take place, near which
Figure 1. (a) TEM micrograph of a typical IF-WS2 nanoparticle, showing the closed nested WS2 layers and the hollow central core. (b) SEM image of the surface of an IF-WS2 pellet (scale bar is 1 µm).
nanovoids as large as 0.5 nm and other defects may develop. Pellets, 5 mm in diameter, were then prepared by pressing 50 mg of IF-WS2 nanoparticles dye under a pressure of ∼2.5 × 103 kg/cm2. The SEM images of such pellets, presented in Figure 1b, show that they are highly disordered and contain an intricate network of nanopores. Recently, it was shown14 (using nuclear magnetic resonance measurements) that water molecules, one of the products of the above reaction, occupy the aforementioned voids (in the central part of the IF particles or near the defect sites) and/or the interparticle nanopores. These water molecules can be released by vacuum annealing at elevated temperatures. Four gold contacts were deposited onto the sides of the pellets (for macroscopic transport measurements in the van der Pauw geometry, not shown here), and one of them served as a counter electrode to the STM tip (made of Pt-Ir). The STM measurements were performed at room temperature at different environmental conditions: in vacuum, in air, and in dry overpressured He atmosphere, and using different STM systems. It is important to note here that the results hardly depended on these experimental conditions. Tunneling current-voltage (I-V) characteristics were acquired while momentarily disconnecting the STM feedback loop. Typically, four I-V curves were acquired consecutively at each tip location, to confirm data reproducibility. Figure 2a presents an STM topographic image focusing on a single IF-WS2 particle. This image shows an overall structure that resembles the one depicted in Figure 1a. An
atomically resolved image measured on the same particle is presented in Figure 2b. The image shows a hexagonal structure with a lattice parameter somewhat larger than 3 Å, consistent with the basal plane structure of the corresponding bulk material.15 We note here that atomic resolution was not achieved yet for any IF-MX2 material (in contrast to the case of the corresponding bulk single crystals, e.g., refs 16 and 17). This was attributed10 to the inherently loose adhesion of such particles to the surface, a problem that does not exist in the pellets studied here. However, atomically resolved images similar to ours were obtained on long MoS2 nanotubes,4 where the van der Waals forces were strong enough to provide sufficient stabilization of the nanotubes on the substrate. Our main spectroscopic results are summarized in Figure 3. I-V curves acquired with a relatively small tip-bias voltage range, typically -0.8 < Vt < 0.8 V, and on particles that were not previously subjected to a large bias voltage, are regularly behaved, as demonstrated by Figure 3a. They exhibit a (slightly asymmetric) gap structure similar to those observed in previous works on MX2 nanoparticles,4,10 as expected for tunneling into a semiconductor. We note in passing that the particles are too large and well connected to each other for single electron charging effects (e.g., the Coulomb blockade)18 to be observed at room temperature. Figure 3a also demonstrates that the characteristics do not depend on the sweep direction of the bias voltage. This regular behavior does not persist when the tip voltage extends to values larger than (typically) 1.5 V. A common scenario is presented by Figure 3b, showing a set of four I-V curves acquired consecutively at one tip location with Vt sweeping from negative to positive values (only part of the total -2.22.2 V voltage range is shown, for clarity). The first curve (black, marked by arrows) exhibits an erratic behavior, where the current increases rapidly to the saturation value of the preamplifier, and then sharply drops (around Vt ) 0.8 V) and continues to increase smoothly with Vt. The next three curves evolve smoothly with Vt but exhibit a unique feature that, to the best of our knowledge, was not observed previously in STM spectroscopy (on any material). In the
Figure 2. (a) 50 × 50 nm2 STM topographic image focusing on a single IF-WS2 nanoparticle in the pellet. (b) 2 × 2 nm2 image acquired on the same particle, showing atomic resolution. The images were measured with tip bias of Vt ) 1 V and I ) 0.1 nA. Nano Lett., Vol. 6, No. 4, 2006
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Figure 4. (a) Series of tunneling I-V curves measured (in ambient) at the same location by sweeping the bias from negative to positive values. The pre-acquisition time delay (with the bias set at -2 V) was changed from 0.1 ms (left most curve), through 0.5, 1, 5, 10, 50, 100, 200, and 300 ms, up to 400 ms. The position of the NDC peak shifts monotonically toward more positive tip-bias values, as shown in the inset.
Figure 3. (a) Five I-V curves acquired in one location with a “small” bias voltage range. Four of the curves were taken consecutively with the tip voltage sweeping from negative to positive values and one (magenta) with the opposite sweep direction, showing good reproducibility. (b) A series of four tunneling I-V curves acquired consecutively in one location, with time intervals (pre-acquisition delay time for each curve) of 10 ms. The first curve (black curve, marked by arrows) exhibits an erratic behavior and triggers the appearance of reversed current and a negative differential conductance peak around -0.4 V in the next curves. The tip bias was swept from negative to positive values, as indicated by the thick arrow. A blow-up of the reversed current region is shown in the inset. (c) I-V curve acquired by sweeping the tip bias from positive to negative, showing reversed current for positive tip-bias voltage smaller than 0.75 V. The data presented here were acquired in ambient conditions with a voltage sweep rate of 350 V/s.
voltage range of -0.5 < Vt < -0.25 V, the current reverses its sign and flows opposite to the direction of the applied bias. In addition, a negative deferential conductance-like (NDC-like) peak appears at around -0.4 V. We emphasize that NDC peaks in STM spectra were observed by various groups and on different materials,19-21 including MoS2,17 but never showing “negatiVe” (reVersed) current at the same time. The I-V curves taken at the same location, but with sweeping the bias from positive to negative values, also depict a region of negative tunneling current (0 < Vt < 0.75 762
V in Figure 2c) and significant zero-bias current, but with no apparent NDC-like peak. Such a peak is observed here at Vt ≈ -0.6 V, where the current flows in the expected direction. The main features described above, namely, the reversedcurrent, NDC-like peaks, zero bias current (for at least one sweep direction) and the hysteresis with respect to the voltage sweep direction were observed in practically all of the tens of IF-WS2 nanoparticles that we have measured, on three different pellets. Moreover, the measurement environment, vacuum, ambient, or dry He atmosphere, as well as the STM setting (determining the tip-sample distance), had no qualitative effect on the results. It should be noted, however, that for many nanoparticles, the above-described anomalous I-V characteristics appeared only after a few sets of regularly behaved characteristics were measured. Yet, from there on the anomalous features were observed repeatedly. However, the height of the NDC-like peak decreased with repetitive data acquisition (by a factor of ∼5 after acquiring 200 I-V curves), and the erratic curve (marked by arrows in Figure 3b), which seems to trigger the anomalous features, was absent in some of the sets of curves that followed. The positions of the onset of reversed current regions and the positions of the corresponding NDC peaks (when sweeping the bias from negative to positive values) varied from one nanoparticle to another. Usually, the NDC peaks are found to be positioned either around -0.35 or -0.25 V for curves taken with minimal pre-acquisition delay time values (the delay time between setting the initial bias voltage and the onset of data acquisition), ∼10 µs. The NDC peak is found to shift toward more positive values upon increasing this time delay (with Vt set at -2 V), as shown in Figure 4. In fact, its position could thus be quite accurately and reproducibly controlled. In addition, its magnitude increased in a rather nonlinear fashion. In contrast, the peak position did not change (for a given pre-sample delay time) with the Nano Lett., Vol. 6, No. 4, 2006
Figure 5. Two sets of I-V curves, taken on different IF-WS2 particles (bias sweep from negative to positive values). All curves exhibit two NDC-like peaks, as well as a reversed-current region. The peak positions differ from one group to another, but their separation is nearly the same, ∼0.45 V. Set 1 was measured in a dry He atmosphere, and set 2 was measured in vacuum.
bias sweep rate in the (experimentally available) range of 100-700 V/s. On about 15% of the measured WS2 nanoparticles, a second peak, but neVer more, was clearly observed. As shown in Figure 5, this second NDC peak is located about 0.45 V above the first, irrespective of the position of the latter (∼ -0.35 or ∼ -0.25 V). We could find no correlation between the local topography (e.g., the shape and size of the nanoparticle, local defects) and the emergence of this second peak or the position of the first one. It is worth noting here that multiple NDC-like peak structures were observed previously in voltammograms measured in conventional electrochemical systems on ensembles of well separated metallic nanoparticles (∼2 nm in diameter) and C60 fullerenes.22,23 Those curves consisted of 3-8 nearly evenly spaced peaks, where the spacing corresponded well with expectation for the single electron charging energy. In our case, however, where the particles are an order of magnitude larger and are well attached to one another, the peak spacing cannot be accounted for by an analogous single electron charging model. There are two main relevant effects that can yield reversed and zero-bias currents, as manifested in our spectra. One is the photovoltaic effect, observed in many STM experiments on semiconductor surfaces, including24 WSe2, and the other is an electrochemical (‘battery’) effect. The first effect should not play any role in our experiments because the STM measurements were all performed in the dark. Moreover, it cannot naturally explain the NDC-like features and the dependence of the spectra on the voltage range, that is, the fact that regularly behaved characteristics are observed when measuring with small bias range values. The second, and much more probable scenario, is that electrochemical processes, taking place in the pellet and triggered by the large applied tip voltage, give rise to the above anomalous features in our I-V curves. Indeed, these features resemble those appearing in typical cyclic voltammograms measured in Nano Lett., Vol. 6, No. 4, 2006
electrochemical cells, also with an STM.25,26 The specific chemical processes are not yet clear to us, and following our findings we can only speculate that they involve some (irreversible, see below) redox processes associated with the W atoms and the aforementioned absorbed water molecules. These can reside in the individual IF-WS2 particles (near kink-related defect sites or in the central hollow core) or in the interparticle nanoporous region. To check this conjecture, we have measured a pellet that was heated for 21 days at 425 °C in vacuum (10-7 Torr). The atmosphere of the chamber was monitored during the vacuum annealing using a residual gas analyzer. The loss of sulfur was found to be insignificant, and the integrity of the treated sample was verified by electron microscopy. These annealing conditions were found14 sufficient to practically completely disrobe the above water molecules. Indeed, only regular and nonhysteretic I-V curves were observed all over the annealed pellet, even for tip voltages reaching values as large as (()6 V. Moreover, the anomalous features were not recovered after exposing the sample to ambient or even to water vapor, suggesting that the relevant electrochemical processes involve the water molecules that are embedded inside the IF-WS2 nanoparticles or in voids between nanoparticles deeper in the sample. The “as prepared” and annealed pellets also differed in their “macroscopic” transport properties, an issue that will be discussed in more detail elsewhere. We shall only mention here that the measured resistance of the “as prepared” pellets did not stabilize (either increased or decreased) for periods of hours, probably because of the longterm relaxation processes, a phenomenon that was not observed for the annealed samples. We shall now discuss in more detail the possible chemical processes that may take place during, and be triggered by, the acquisition of our tunneling I-V curves. We recall that the synthesis of the IF-WS2 nanoparticles involves the reduction of the tungsten in the precursor WO3 nanoparitcles from +6 to +4 valance states, and the formation of the aforementioned water molecules. Noting that we plot the I-V curves as a function of tip bias (the less common way), positive current refers here to electron tunneling form the sample to the tip. Therefore, the NDC peaks observed in the I-V curves that were acquired with Vt sweeping from negative to positive values may be associated with the oxidation of W atoms (possibly by forming an oxisulfide compound). In particular, the two (but never more) peaks appearing in some of our curves (Figure 5) may reflect the oxidation stages of W4+ back to its highest valance state, W6+. Alternatively, one of the peaks could stem from the oxidation of occluded hydrogen molecules (also a product of the reaction forming the IF-WS2 particles), which were unable to escape from the IF-nanoparticle matrix during the fast potential scan. This process can be catalyzed by tungsten atoms in point defects formed in the IF structure. The above oxidation reactions are probably accompanied by the reduction of the absorbed water molecules, taking place while the tip is negatively biased (electrons tunneling into the sample) at voltages larger than the required threshold voltage of ∼1.5 V. In this latter process, hydrogen gas is released and thus 763
is not available for oxidation during the positive potential scan. This irreversible electrochemical process11 manifests itself in our spectra by the asymmetry between I-V curves acquired with opposite sweep directions (Figure 3), by the results shown in Figure 4 and by the fact that the NDC-like peak decayed upon repeated data acquisition (as described above). Regarding the latter effect, we note that integration under the peak (taking into account the sweep rate) typically yields a charge of ∼105 electrons, suggesting (because each particle consists of ∼106 tungsten atoms) that more than one IF-WS2 nanoparticle takes part in the tunneling-driven electrochemical processes. One troubling question that remains regarding the electrochemical picture given above, is why such anomalous I-V curves were not observed in any of the previous STM studies of MX2 materials.4,10,16,17,24 There may be a few explanations for this. For the bulk materials,16,17,24 it is very likely that they are relatively free of defect sites and no water molecules are expected to be absorbed inside them. The IF-MoS2 particles studied in ref 10 were prepared by a different method than the one used here. There, the particles were synthesized by applying a short electrical pulse from the STM tip to amorphous MoS3 nanocrystals, a process that is not expected to yield embedded water molecules. One can argue here that because this experiment was performed in ambient conditions water molecules are expected to be adsorbed on the surface. However, as discussed above, these (possibly existing) surface molecules do not give rise to the anomalous features observed in our I-V curves. Finally, the measurements reported in ref 4 were performed on WS2 nanotubes, which contain less defect sites compared to the corresponding IF materials (because the layers there are more rounded and evenly folded). In addition, the I-V characteristics were acquired (in that case, as well as in most of the abovementioned previous studies) in smaller voltage ranges compared to those used here, possibly not sufficient to trigger the redox reactions and consequently the anomalous I-V curves (Figure 3). In summary, I-V characteristics measured using STM on pellets consisting of IF-WS2 nanoparticles exhibit features that are typical for (irreversible) cyclic voltammograms. This is in spite of the fact that the measurements were performed in conditions remote from those in electrochemical cells: in vacuum, ambient conditions, and in overpressured dry He atmosphere. The I-V curves show current at zero bias as well as regions of reversed current accompanied by NDClike peaks. The characteristics are also hysteretic with respect to reversal of the voltage sweep direction. We attribute these anomalous features to (irreversible) redox processes, triggered by the STM measurement, taking place within the pellet. These processes involve, probably, the tungsten atoms and water molecules, attached to defect sites in the IF-WS2 particles or residing in their central hollow cores, or within
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the interparticle nanopores. Our spectra thus manifest an intriguing way by which the electronic properties of semiconductor samples can be altered in the course of the STM measurement, beyond the more trivial effects, such as charging and band-bending. Our results also suggest that the IF-MX2 family of materials exhibit, in addition to their unique tribological and mechanical properties, intricate electrical properties that may lead to further applications. Acknowledgment. We are indebted to “NanoMaterials” for providing the IF-WS2 nanoparticles for these measurements. This research was supported by the Israel Science Foundation (HUJ and WIS groups); the DIP and EU SANANO programs (HUJ group); the German Israeli Foundation (WIS group); the Minerva Foundation and the G.M.J. Schmidt Minerva Center for Supramolecular Chemistry (WIS group). References (1) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. (2) Margulis, L.; Salitra, G.; Tenne, R.; Talianker, M. Nature 1993, 365, 113. (3) Frey, G. L.; Elani, S.; Homyonfer, M.; Feldman, Y.; Tenne, R. Phys. ReV. B 1998, 57, 6666. (4) Scheffer, L.; Rosentzveig, R.; Margolin, A.; Popovitz-Biro, R.; Seifert, G.; Cohen, S. R.; Tenne, R. Phys. Chem. Chem. Phys. 2002, 4, 2095. (5) For a review on STM measurement on semiconductor quantum-dots, see Banin, U.; Millo, O. Annu. ReV. Phys. Chem. 2003, 54, 465. (6) Katz, D.; Wizansky, T.; Millo, O.; Rothenberg, E.; Mokari, T.; Banin, U.; Phys. ReV. Lett. 2002, 89, 86801. (7) Hassaniena, A.; Tokumotoa, M.; Mrzel, A.; Mihailovic, D.; Kataura, H. Physica E 2005, 29, 684. (8) Remskar, M.; Mrzel, A.; Sanjines, R.; Cohen, H.; Levy, F. AdV. Mater. 2003, 15, 237. (9) Remskar, M. AdV. Mater. 2004, 16, 1497. (10) Homyonfer, M.; Matsai, Y.; Hershfinkel, M.; Volterra, V.; Hutchison, J. L.; Tenne, R. J. Am. Phys. Soc. 1996, 118, 7804. (11) Bard, A. J.; Faulkner L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980; pp 230231. (12) Feldman, Y.; Lyakhovitskaya, V.; Tenne, R. J. Am. Chem. Soc. 1998, 120, 4176. (13) Feldman, Y.; Zak, Z.; Popovitz-Biro, R.; Tenne, R. Solid State Sci. 2000, 2, 663. (14) Panich, A. M.; Kopnov, F.; Tenne, R. J. Nanosci. Nanotechnol., in press. (15) Albeand, K.; Klein, A. Phys. ReV. B 2002, 66, 073413. (16) Sarid, D.; Henson, T. D.; Armstron, N. R.; Bell, L. S. Appl. Phys. Lett. 1966, 52, 2252. (17) Youngquist, M. G.; Baldeschwieler, J. D. J. Vac. Sci. Technol., B 1991, 9, 1083. (18) Hanna, A. E.; Tinkham, M. Phys. ReV. B 1991, 44, R5919. (19) Lyo, I. W.; Avouris, P. Science 1989, 245, 1369. (20) Steiner, D.; Mokari, T.; Banin, U.; Millo, O. Phys. ReV. Lett. 2005, 95, 056805. (21) Alperson, B.; Rubinstain, I.; Hodes, G. Phys. ReV. B 2001, 63, 81303. (22) Chen, S.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996. (23) Deng, F.; Yang, Y.; Hwang, S.; Shon, Y.-S.; Chen, S. Anal. Chem. 2004, 76, 6102. (24) Hiesgen, R.; Meissner, D. J. Phys. Chem. B 1998, 102, 6549. (25) Bard, A. J.; Fan, F.-R. F.; Lev, O. Anal. Chem. 1989, 61, 132. (26) Tao, N. J. Phys. ReV. Lett. 1996, 76, 4066.
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