pubs.acs.org/NanoLett
Electrical Current Switching in Single CdSe Nanorods Hadar Steinberg,†,§ Omri Wolf,‡,§ Adam Faust,†,§ Asaf Salant,†,§ Yigal Lilach,§ Oded Millo,*,‡,§ and Uri Banin*,†,§ †
Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel, ‡ Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel, and § The Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem 91904, Israel ABSTRACT Electrical current measurements through individually wired colloidal CdSe nanorods exhibit pronounced multistability. This current switching is analogous to the widely observed fluorescence intermittency in similar systems and may be associated with surface charge dynamics. Such association is quantitatively established for the case when the current is bistable, where the probability of the sojourn time t at the high or low current state follows an exponential dependence. Remarkably, this behavior can be modeled by charging dynamics of a single surface trap, whose position could be estimated from the intermittent current-voltage characteristics. The methodology presented here provides a unique route for charge dynamic sensing at the nanoscale, where the nanorod senses its own surface charge. KEYWORDS Semiconductor nanorods, nanoelectronics, electrical transport, charging effects
T
Charging of quantum dots (QDs) defined in a twodimensional electron gas or localized traps was elegantly probed previously by single-electron transistors (SETs)18 or by quantum point contacts (QPCs),19-23 and in metaloxide-silicon transistor structures,24 all of which are sensitive to fine changes in their electrostatic environment. In colloidal nanocrystals, electrostatic force microscopy was used to study light-induced charging processes.25 Here we present a novel approach to the study of local charging dynamics on the surface of a colloidal NR based on tracing the time domain electrical transport through individual NRs. The mechanism underlying the current switching in our case, as explained below, is the modification of the NR conduction potential by a nearby trapped charge. It expands the conceptual use of charge sensors to strongly coupled devices, unlike SETs,18 and does not require the NR to support a ballistic channel, like a QPC.19-23 Furthermore, it allows in some cases the extraction of the charging site position with relatively high precision. Samples are fabricated using the method introduced in ref 5. Briefly, CdSe NRs are synthesized using colloidal techniques26 and are contacted by tungsten lines fabricated using electron-beam-induced deposition (EBID). Over 60 NRs were connected, almost all of which exhibited current multistability. We focus on data taken from two samples, multistable “Sample A” and bistable “Sample B”, both of which are 30 × 5 nm CdSe NRs. A SEM image of sample A is presented in the inset to Figure 1a. A typical example of a multistable I(V) curve, taken on sample A at T ) 4.2K, is presented in Figure 1a. At source-drain voltages VSD > 0.25 V, the current exhibits large fluctuations. Valuable information can be obtained by fixing the voltage and recording current vs time traces,
he prospect of nanoelectronics based on colloidal semiconducting nanocrystals (NCs) has prompted attempts to integrate NCs into electronic circuitry.1-4 Recently, it was demonstrated that strongly coupled electrical contacts could be established, either by electron beam induced deposition of metallic leads5 or by the growth of Au at the tips of nanorods (NRs)6 forming Au-CdSe nanodumbbells.7 Nevertheless, despite the remarkable control achieved in the size, shape, and composition of the NCs, there is less control and understanding of their surface properties. The latter are revealed in optical studies, where spectral meandering is interpreted as due to surface charge dynamics8-10 and sudden spectral shifts were assigned to charging and discharging of surface states.10-12 Surface charge dynamics can also give rise to the anomalous statistics found in fluorescence intermittency, also known as optical blinking, which is ubiquitous in NCs.13,14 Here we study an analogous problem, that of the temporal behavior of current-voltage characteristics through a single colloidal NR. Most NRs exhibit current switching, which we associate with charging and discharging of traps at their vicinity. Similar to the discovery of fluorescence intermittency, also here, the current intermittency could be detected only on the single particle level as the effect is washed out in the ensemble.15-17 Moreover, the study of current intermittency provides important information on the local charge distribution and its dynamics, since the electrical transport through nano-objects is highly sensitive to the nearby fluctuating charges.
* To whom correspondence should be addressed,
[email protected] and
[email protected]. Received for review: 02/16/2010 Published on Web: 05/27/2010 © 2010 American Chemical Society
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FIGURE 2. Current statistics taken on a 30 × 5 nm NR (sample B) at T ) 4.2 K and VSD ) 0.2 V. Sampling time is 55 ms. (a) I(t) during part of the 33 min long scan. (b) Current-histogram of the data set showing two distinct current levels. The probabilities of the current staying for duration t at the high (H) and low (L) current states are presented in (c) and (d), respectively, along with fits to exp(-t/τH,L), presented by the green lines.
In Figure 2 we present the statistics of the switching times measured on bistable sample B. Panel a shows the I(t) signal trace, taken over 60 s (a partial scan of a 33 min long data set). Throughout this long measurement time only two current values were observed, and no other currents nor any drift were detected, as seen in the histogram in Figure 2b. This is remarkable and suggests that indeed only one trap site was active throughout this entire time. Switching time statistics are calculated by setting a demarcation value, and noting the duration of “low” and “high” signal events over the entire measurement. The normalized probabilities PH(L)(t) of finding events of duration t in the high (low) current states are presented in parts c and d of Figure 2, respectively. We find that both probabilities follow an exponential dependence on time, PH,L(t) ∼ exp(-t/τH,L), where the fits presented in the figure yield τH ) 0.55 s and τL ) 0.7 s. In Figure 3a we present the current histogram vs VSD of sample B (at a different cool-down from Figure 2). Two main branches are observed at the positive bias, whereas in the negative bias only one branch is seen. We note that the appearance of bistability is not limited to one polarity, and in some samples there exists significant bistability in both polarities (see Figure S3 in the Supporting Information). Generally, the histogram pattern was stable with time, as long as the sample was kept at cryogenic temperatures, but couldchangeuponthermalcyclingthroughroomtemperature. We can reasonably estimate the trap position and energy based on the VSD data in Figure 3 and temperature-dependent data presented in Figure 4. We begin by noting that as the bias voltage increases in the positive direction, the current gradually shifts from the high current state to the low current state. This clear dependence of charging statistics on VSD suggests that the trap resides at close proximity to the NR, where the effect of the field induced between the source and drain electrodes is significant. Further insight into the microscopic charging-dynamics processes can be gleaned from the dependence of current statistics on temperature, presented in Figure 4. Here we
FIGURE 1. Current multistability in a CdSe NR. (a) I(V) curve of a 30 × 5 nm NR (sample A) at T ) 4.2 K. The current is multistable at the positive onset of conductance. The dotted vertical line marks the voltage where the data in the inset to (b) is taken. Inset: Image of the wired NR, visible between the EBID W contacts. The bright region on the left is the Au electrode. The bar size is 20 nm. (b) Grayscale coded normalized current histogram vs VSD, extracted at each voltage from a 45 s long time-domain signal with sampling resolution of 23 ms, revealing the multistable behavior. Inset: Timedomain signal at VSD ) 0.36 V (marked by a vertical dashed line), exhibiting current switching between several levels.
analogous to fluorescence intensity time traces measured to follow the photoluminescence blinking.14,27 An example of such a time-domain I(t) trace taken at VSD ) 0.36 V is presented in the inset to Figure 1b, demonstrating that the current switches between several distinct levels. In Figure 1b we present the current histogram taken for a range of source-drain voltages VSD. The histograms are gray-scale coded with the most frequently detected current levels in black and the least frequent in white. This technique has the advantage of resolving from the noisy signal several branches of simultaneously occurring I(V) characteristics. This intricate behavior, including many current levels, is most likely due to charging/discharging dynamics of several traps, which is difficult to follow. Therefore, we will focus now on special simpler cases, that are nevertheless quite frequently encountered, in which the sample is bistable and only two current branches are detected, tentatively associated with the presence of a single trap. In such samples, quantitative timedomain analysis is made possible. © 2010 American Chemical Society
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repeat the time-domain measurement over a range of temperatures, keeping VSD ) 0.5 V. The histograms are presented in color code in panel a. The current appears to increase exponentially with temperature in the range T > 40 K (panel b), in agreement with ref 5, manifesting the effect of band gap reduction with temperature on the FowlerNordheim tunneling probability between the leads and the NR.5 This agreement suggests, in turn, that the trap is probably neutral (thus the conduction band remains unperturbed) at the high-T regime. At lower T, the current exhibits a set of intricate transitions: (i) A transition between a dominating high to low states at T ) 15-19 K. (ii) At 19 K < T < 40 K, a transition back to a stronger high-current state takes place (panel c). The high-current branch appears to persist at T > 40, and following the above considerations, it can be reasonably associated with the neutral trap state and, consequently, the lower branch is related to the charged trap state (see details below). The latter transition is smooth and allows the study of temperature-dependent time scales. We note in passing that each branch portrays also a small secondary split, which may be related to finer NR level structure such as vibrational states. Therefore, for the sake of extracting τH (associated now with the residence time in the uncharged state, or the time for the charging process) and τL (discharging process), we consider each split set of peaks as corresponding to a single charging state. The figure shows that the sharpness of the transition at T > 19 K arises from the surprisingly strong temperature dependence of τL, which changes by 4 orders of magnitude over 15 K, while τH changes by very little, reflecting a pronounced asymmetry betweenthebarrierheightsassociatedwiththetwoprocesses. The temperature dependence of τL at T > 19 K (and VSD ) 0.5 V) can be modeled by a thermally activated behavior
FIGURE 3. NR as a surface charge sensor. (a) Current histogram vs VSD, of a 30 × 5 nm NR (sample B) at T ) 25 K. The experimental histograms are extracted from 1000 data point I(t) scans at sampling rate of 20 ms, taken for each voltage beginning at VSD ) 0.5 V. The maximum of each histogram is presented as a blue dot (vertically shifted for clarity). The red line is the simulated I(V) characteristic corresponding to the discharged state, calculated with L ) 30 nm and U ) 64 meV. The I(V) of the charged state, presented in green, is calculated with a trap site at distance x ) 18 nm from the source electrode and R ) 4.0 nm from the axis of the NR. Inset: Schematic of the potential landscape at VSD ) 0.35 V considered in the I(V) calculation: blue, pristine (discharged site); red, energy barrier modified by the charged site (exaggerated for clarity); black lines, chemical potentials at the source and drain. The red dot marks the position and energy of the trap. (b) Lower part: Simulated current ISIM vs trap position x calculated for a range of distances R from the NR axis. The horizontal line corresponds to the measured current at the charged state at VSD ) 0.5 V. Top part: R vs x for values agreeing with the data. The blue curve is extracted from the condition ISIM ) Ic (+) and the red curve from the condition ISIM ) Ic (-) (not shown). The x,R values of the curve intersection were used for the simulation (green curve) in panel a.
τL ) AL exp(EXD /KBT) -9
where AL ) 6 × 10 s and EXD ≈ 48 meV is the barrier facing the discharging process. EXD most likely stands for the trap-drain energy barrier. And τH, which corresponds to the charging process that is assumed to be dominated by the source-trap transition rate, can also be fit in a similar way with τH ) AH exp(ESX/KBT), ESX ∼ 6.9 meV, and AH ) 0.08 s. These temperature dependencies were repeatable and stable throughout the entire cool-down of the sample, for the duration of days. The data reported in Figure 4 were measured twice, several hours apart, and were found the same to the finest detail. The difference between ESX and EXD may arise from anisotropy in the trap environment or slight differences between the source and drain electrodes. The anomalously large difference between the prefactors AH and AL is, however, not understood. Nevertheless, the general behavior of the temperature dependence points to an activated mechanism responsible for charging and discharging the trap. Having identified the charging dynamics with a thermally activated mechanism, we now have a better understanding of the effect of the source-drain potential, VSD, which modifies the charging and discharging rates by deforming
FIGURE 4. Current histogram vs T at VSD ) 0.5 V (sample B). (a) Colorplot compiled histograms of a series of time-domain scans taken at T ) 4-78 K, with histograms normalized to unity. (b) Peak current (blue dot) of the histograms in (a) vs T, plotted on a semilog scale. The line is a fit to I ∼ log(T/T0), in agreement with ref 5. (c) Partial set of the histograms at the range T ) 19-37 K, vertically displaced for clarity. A transition from the low to high current branch occurs at this range. (d) τL,H vs 1/T corresponding to the data sets of panel c, extracted using the method described in Figure 2. Continuous lines: Fits to τL,H ∼ exp(EXD,SX/KT). © 2010 American Chemical Society
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the potential barriers around the trap. Such pronounced deformation requires the trap to reside at close proximity to the NR. We can make precise assessments of the trap position on the NR surface by applying the Fowler-Nordheim model5 to simulate the currents. The large-current branch, associated with a discharged neutral NR, is modeled using the formula
( )
J ) sgn(VSD)
VSD L
2
(
exp -
4√2 √mSC U3/2L 3 pe |VSD |
)
in the charged state at the negative bias, the same calculation can be repeated for the current at VSD ) -0.5. Comparing with the experimental Ic(-) ≡ I(charged state, VSD ) -0.5 V), one arrives at a different trajectory, marked by the red line. Elegantly, the intersection of the two trajectories, at x ) 18 nm and R ) 4 nm, yields the position of the trap, allowing then to calculate the full I(V) curve of a charged trap. This curve is presented by the green line in Figure 3a and is seen to fit the data at both bias polarities remarkably well. The same calculation can be repeated assuming the NR is discharged at negative bias (Figure S2 in the Supporting Information). We also present another example of the same analysis on a different sample, which is bistable at both polarities (Figure S3 in the Supporting Information). One important consequence of this analysis is that in this particular bistable case, indeed only one trap site can account for the data while surface charge meandering between different sites along the NR is unlikely. This is because Figure 3b suggests that such charge meandering would modify the current (for each hopping event) by an amount comparable to its amplitude, an effect which is absent here. This also implies that the more common multistable case involves several trap sites. It should be noted, however, that we could account for data with other trap (x and R) positions (albeit in the NR vicinity) and even assuming a positively charged trap, but these would require more assumptions and fitting parameters. Lastly, it is obviously of interest to compare the current switching reported here, with the widely investigated fluorescence “blinking” process in semiconductor NCs.13 The latter bistable on-off fluorescence behavior typically exhibits power-law statistics of the form P(t) ∼ t-R with 1 < R < 2 for the on and off times distribution, for CdSe/ZnS core/shell quantum dots. This power law behavior is significantly different from the observed well-defined time scales, τL,H, for the current switching in the present work. However, recent work on CdSe/CdS NCs reported the appearance of an intermediate gray state with low emission intensity.28 Additional work on such NCs with three emitting levels showed exponential dependence with well-defined time scales for the residence time in the highly emitting state.29 This points to a possible connection between the trapping mechanism underlying the two phenomena, which calls for further study. In particular, the power-law statistics of off times found ubiquitously in optical blinking is attributed by some models to the charge meandering in the vicinity of a photoionized NC13 or to the presence of a number of surface traps.30 Although this seems to be incompatible with our results for the bistable current case as discussed above, charge meandering and multiple-trap dynamics may be the underlying mechanism yielding multistable I(t) traces. Using our technique to address the origin of optical blinking requires performing a similar study on an illuminated sample. This could also provide important insight into the problem of surface-charge mobility along the NR. We conclude that
(1)
where mSC is the effective electron mass in the semiconductor, L is the effective NR length (spanned between the metallic leads), and U is the displacement of the conduction band from the metallic-lead chemical potential. In the simulation L ) 30 nm, which yields (for best fit to the experimental data, presented by the blue dots) U ) 64 meV (red line in Figure 3a). This value is well within the reported range for the offset between tungsten and CdSe.5 To model the current at the charged trap state, we note that our NRs are strongly coupled to the metallic leads, as reflected by the smooth rise of current without a steplike structure, and already detailed in our previous paper.5 In this strong coupling case, single electron tunneling effects are suppressed and the potential induced by the trapped charge affects the system by spatially modulating the conductionband edge and thus modifying the Fowler-Nordheim tunneling barrier encountered by an incoming electron. This is in contrast to the weakly coupled limit, where the energies of the localized states are shifted due to capacitive coupling to the trap. The induced potential can be quantitatively evaluated by applying the image-charge method (see Figure S1 in the Supporting Information), simulating the metallic lead surface as a sphere of radius 15 nm. The modified shape of the potential barrier is then used in a WKB calculation of the current, similar to the Fowler-Nordheim model described above. This is illustrated in the inset to Figure 3a: The presence of the charge increases the potential locally (red line Figure 3a) with respect to the discharged state (blue line) which effectively broadens the tunneling barrier and reduces the current. This local potential modulation can be appreciable when the charging site is sufficiently close to the NR. The high sensitivity of the NR as a charge sensor is demonstrated at the bottom part of Figure 3b, where the current ISIM(x,R) at VSD ) 0.5 V and in the presence of an electron at position x along the NR and at distance R from the NR axis is calculated (each curve in the figure is calculated for a different R). The condition that the simulated current value, ISIM, matches Ic(+), the measured current at VSD ) 0.5 V for the charged state, is met at a range of charge locations in the x-R plane, depicted by the blue trajectory portrayed at the top part of Figure 3b. Assuming the NR is © 2010 American Chemical Society
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(5) (6)
the technique presented here could be utilized to monitor charge dynamics at the nanoscale in various systems and, consequently, contribute to better understanding of the physics governing nanocrystal based nanoelectronic and optoelectronic devices.
(7) (8) (9) (10)
Acknowledgment. The authors wish to acknowledge the help given by the staff of the Unit for Nanofabrication of the Center for Nanoscience and Nanotechnology in the Hebrew University, Jerusalem, headed by Dr. S. Eliav. We further acknowledge the help of E. Elmelem in synthesis. This research was supported by a Grant from the G.I.F., the German-Israeli Foundation for Scientific Research and Development (UB). O.M. acknowledges support from the Harry de Jur Chair in Applied Science. U.B. thanks the Alfred and Erica Larisch Memorial Chair.
(11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
Supporting Information Available. Calculation of potential profile, trap-position analysis under different assumptions, and another example of trap-position analysis. This material is available free of charge via the Internet at http:// pubs.acs.org.
(21) (22) (23) (24) (25) (26) (27)
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DOI: 10.1021/nl100567v | Nano Lett. 2010, 10, 2416-–2420
