Fast Current Blinking in Individual PbS and CdSe Quantum Dots

Mar 8, 2013 - single dot switches on and off at time scales ranging from microseconds to seconds with power-law distributions for both the on and off ...
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Fast Current Blinking in Individual PbS and CdSe Quantum Dots Klara Maturova, Sanjini U. Nanayakkara, Joseph M. Luther, and Jao van de Lagemaat* Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States ABSTRACT: Fast current intermittency of the tunneling current through single semiconductor quantum dots was observed through time-resolved intermittent contact conductive atomic force microscopy in the dark and under illumination at room temperature. The current through a single dot switches on and off at time scales ranging from microseconds to seconds with power-law distributions for both the on and off times. On states are attributed to the resonant tunneling of charges from the electrically conductive AFM tip to the quantum dot, followed by transfer to the substrate, whereas off states are attributed to a Coulomb blockade effect in the quantum dots that shifts the energy levels out of resonance conditions due to the presence of the trapped charge, while at the same bias. The observation of current intermittency due to Coulomb blockade effects has important implications for the understanding of carrier transport through arrays of quantum dots. KEYWORDS: quantum dots, nanocrystals, intermittent current, conductive atomic force microscopy, tuning fork atomic force microscopy, lead sulfide

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measured by c-AFM on single QDs. In this Letter, we show results of time-resolved current measurements on single QDs using c-AFM in the tapping mode and describe observations of intermittency in the current signal on multiple time scales. The mechanism of this switching behavior is, in accordance with the most common explanation for PL blinking, ascribed to surface trapping4 of charges that halts the flow of tunneling current by a Coulomb blockade effect. Emptying of this surface trap reactivates the tunneling current by lifting the Coulomb blockade. Great insight into the local transport of charges in single and arrays of nanoparticles has been obtained using conventional STS and c-AFM. However conventional scanning probe techniques might influence the surface of soft samples or move around low-dimensional nanostructures like QDs and nanotubes. Scanning tunneling microscopy (STM), the basis for STS, utilizes a constant current feedback, which might result in crashing an atomically sharp metallic tip into the sample if the sample conductance changes dramatically when the tip scans over a QD. Alternatively, in c-AFM, topography information is obtained from monitoring interatomic forces between the tip and the sample in constant height or force mode, while simultaneously monitoring the current. Because of the force-based feedback of c-AFM, it is possible to characterize samples containing regions of rapidly changing conductivity. Conventional AFM utilizes a laser that is reflected from the top of the cantilever into an array of photodiodes to measure the force that is bending the cantilever. Background laser light can result in unwanted photoexcitation of the sample and thus

luorescence intermittency or blinking of single molecules and single semiconductor quantum dots (QDs) has been extensively studied for over a decade.1,2 In single semiconductor QDs, switching between on and off states in fluorescence under constant illumination remains poorly understood. Generally, it is theorized that the off periods are caused by the presence of an additional charge in the QD in either a trap state or in a bulk energy level that extinguishes luminescence (through Auger processes).1−4 Although a large number of studies of photoluminescence (PL) blinking on single QDs has been published, similar effects in the single particle conductance have not been studied. If luminescence blinking is caused by charge trapping and subsequent exciton neutralization, similar behavior might be observed in the current signal in scanning tunneling experiments or conductive atomic force microscopy (c-AFM). Additional trapped charge inside the QD can give rise to a Coulomb blockade and thus disable current injection. Scanning tunneling spectroscopy (STS) on a single QD has been performed by multiple researchers on many different types of QDs showing that energy levels, trap energies, charging energies, and other fundamental parameters can be obtained.5−17 A c-AFM study on isolated QDs showed that the conductance spectrum can be measured and the energy gap Eg obtained at room temperature.18 It has also been shown that electrical current switching can be observed in electrical measurements on single CdSe nanorods suspended between electrodes.19 In the latter case, an exponential distribution of on and off times was observed that could be explained by a single trap state on the quantum rod. Lastly, it has been demonstrated that the edge of the conduction band of single QDs dynamically shifts due to long-lived charge trapping.17,20 There is no published study about intermittency of the current © XXXX American Chemical Society

Received: September 27, 2012 Revised: February 28, 2013

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Figure 1. (Top) Schematic representation of the tuning-fork based AFM electric circuit. Both prongs of the quartz tuning fork are connected to the AFM controller. The metallic tip is connected to a current controller circuit through a transimpedance amplifier (TIA). (Bottom) The black line shows the raw electrical current signal acquired by the current circuit on a Au(111) sample.31 The gray line shows the signal after removal of the sine wave caused by parasitic coupling between the “current” and “AFM” circuits.

performed in helium atmosphere gloveboxes. PbS QDs with a first exciton at 1855 nm were synthesized by combining 5.94 g of 1-octadecene, 0.52 g of PbO, and 16.43 g of oleic acid in a round-bottom three-neck flask to make Pb-oleate. The contents were heated to 105 °C for an hour under vacuum and then heated to 160 °C under nitrogen flow. A syringe was loaded in a glovebox with 315 μL of bis(trimethylsilyl)sulfide diluted in 8 mL of 1-octadecene and was rapidly added to the flask containing the Pb-oleate. After 2 min, the heat was removed and the flask was placed in a water bath to rapidly cool to room temperature. The QDs were precipitated and redispersed twice using anhydrous ethanol and hexane. Three-nm diameter CdSe QDs were prepared by adding 3.0 g of trioctylphosphine oxide, 0.28 g octadecylphosphonic acid, and 60 mg of CdO to a flask and heating to 105 °C for an hour under vacuum and then heating to 300 °C under flowing nitrogen. A 1.5 g sample of trioctylphosphine (TOP) was added and the temperature was then raised to 370 °C. One milliliter of 1 M TOP/Se was rapidly injected and the heat was removed after 40 s. Once the solution has cooled to 150 °C, the flask was placed in a water bath to cool to room temperature. The QDs were washed several times using methanol and toluene. We used Au(111) on mica and hydrogen-annealed Au(111) on glass as a substrate for the AFM measurements. QDs were bound to the substrate through a hexanedithiol self-assembled monolayer (SAM) on the Au surface. To prepare the SAM, the substrate was immersed in a 1 mM 1,6-hexanedithiol solution in ethanol for 2 h and then simply dipped into the QD solution.

change the measured current. To obtain true dark measurements or to ensure photoexcitation at only a single wavelength the experiments described in the current Letter use a tuningfork based AFM.21−23 In these measurements, an atomically sharp metallic tip is mounted on the end of a tuning fork and connected to a current-detecting circuit. Feedback is obtained by monitoring the change of the resonant frequency of the tuning fork when the tip interacts with the surface. The tip oscillates above the sample surface and on the downward tap, when the tip is in contact with (or within tunneling distance to) the sample for a brief moment, electrons are injected. This allows for the observation of time-resolved injection of the current with submicrosecond resolution. In this Letter, we demonstrate multiple time-scale intermittency of the current signal as well as a power-law time distribution of on and off states. We show that current switching can be explained by a model similar to that for PL blinking where a trapped carrier causes a Coulomb blockade of the current and that current blinking should be correlated to photoluminescence blinking as the same trapped charges should extinguish the QD’s PL. Blinking has been described as having an on and off state when steady state PL of a single QD is monitored whereby the QD exhibits time periods of constant illumination (on) and time period of dark (off). We will refer to the equivalent phenomena in tunneling current as on and off states as well. Methods. QD synthesis was performed using standard airfree techniques on a Schlenk line while processing was B

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Figure 2. (Top) Absolute value of the current versus applied bias for an individual PbS QD. The colored lines are I−V curves collected on a single PbS QD. The thick black line is the average of multiple I−V curves. Inset: Conductive AFM current map of the PbS QD, over which the I−V data were acquired. (Bottom) Energy level diagrams illustrating charge injection at different applied biases. (a) Zero applied bias. (b) Negative sample bias at V = Eg. (c) Positive sample bias. (d) For Vapplied < Eg, electrons can be injected from the sample to the QD leading to charging of the QD and shift of the energy levels, this shift will subsequently allow injection of holes from the tip. This will result in apparent energy levels inside the band gap.

All processing and film formation was performed using anhydrous solvents in oxygen-free conditions. The sample, however, was very briefly exposed to ambient during transfer into the STM vacuum chamber. The experiments in this Letter were performed using a Nanonics CryoView 2000, which uses an oscillating quartz tuning fork as a force detector. Sharp metallic tips were mounted to commercially available tuning forks. The tips were prepared by electrochemical etching of either Au or Pt/Ir wire and sharpened by focused ion beam milling to obtain an atomically sharp apex. Our homemade tuning forks for c-AFM have resonant frequencies f = 31−35 kHz. The conductive tip was connected to a current−voltage amplifier and data acquisition card with a temporal resolution of 4 MHz. The Nanonics microscope does not scan over the sample in a continuous sweep but moves from pixel to pixel with the tip staying over a single spot for about 8 μs, resulting in ∼150 taps at a single location. All the experiments were performed in high vacuum (10−7 mbar). We have performed experiments in the dark and under illumination using a green (530 nm) collimated 5 mW LED. In our experiments, we apply bias to the sample while keeping the tip grounded. Since positive substrate biases are known to sometimes break the gold−sulfur bond between the QDs and substrate leading to transferring of the QD to the tip, all experiments were performed at a small negative bias (−1 to −0.5 V).24 We also performed several experiments with various feedback conditions and have measured different control samples (Au, ITO, HOPG, organic layers, selfassembled monolayers (SAMs) of hexanedithiol (HDT) on Au) to ensure that the observed secondary peaks (explained

below) and off states are not the result of tip−sample interaction or unstable feedback. Figure 1 (top panel) shows a schematic representation of the tuning-fork based AFM electrical circuit. The AFM controller is connected to both prongs of the tuning fork. A quartz tuning fork simultaneously serves as an actuator and a sensor of the tip−sample interaction, allowing regulation of the tip−sample distance without the use of a laser and photosensitive diode. An electrochemically etched metallic wire is mounted on the end of the tuning fork and connected to a transimpedance amplifier (TIA). An electrical bias is applied between the sample and the tip. The obtained signal shown in the bottom panel of Figure 1 (black line) shows periodic current spikes on the top of the ac signal. Capacitive coupling between the two circuits causes a parasitic sine wave signal from the AFM circuit to occur in the current measurement. This effect can be diminished by complete electrical separation of these circuits from each other. However, the presence of the sine-wave signal enables precise timing of the experiment, does not impact the detection of the current spikes and is easily removed by software subtraction (gray line) and therefore is beneficial to the experiment. Results and Discussion. In all the experiments discussed below, isolated QDs were bound to gold substrates using selfassembled monolayers of hexanedithiol molecules. The combination of such a sample and the sharp conductive tip placed above the ligand shell of the QD forms a double-barrier tunneling junction (DBTJ).18,25−27 One tunneling barrier is located between the tip and the QD and the second between the QD and the substrate. C

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Figure 3. (Top) Current amplitudes measured on single PbS QD for 30 s at −0.5 V. Bottom (a−c) Proposed mechanism for current blinking in the c-AFM experiment. (a) Uncharged dot. Tip and substrate are resonant with the 1Sh level and current can flow. (b) Electron trapped at a surface trap causing a Coulomb blockade of the current. (c) Similar situation as in (b) but with a trapped hole.

the “tap”, the distribution of applied bias varies with the distance between the tip and QD. Figure 2 (top panel) shows the current versus voltage for an individual PbS QD measured at room temperature and in the dark using tapping-mode conductive AFM. The current shown is the average of the maximum current during one tap averaged over many tapping events in a 10 ms time interval at a single bias on a spot of the isolated QD. The black line shows the average from six measurements over one single QD. Figure 2 (top panel) shows that the 1Se state is observed already at 0 V, which can be attributed to the use of Au (or Pt/Ir) tips that have the same work function (5.1 eV) as the ionization potential of PbS QDs (5.12 eV see ref 30). This means that if no dipole layers, trapped charges, and so forth are present, the conduction level is resonant with the tip at 0 V when a gold tip and substrate are used. The same behavior was observed in I−V STS for PbSe using tungsten tips.13 Outside of the current-free region, a rapid rise of the current is observed, correlating with current injection from the tip into the QD or vice versa. The variability observed in the spectra is not experimental noise but is caused by intermittent conduction through the QD as demonstrated below in Figures 3 and 4. Figure 2 (bottom panel) shows various energy level alignments to aid in understanding current versus voltage spectroscopy. The presence of the band gap edge at 0 V indicates that the Fermi level of both electrodes is close to the first conduction band level of the QD. At a negative bias of V = −Eg, the Fermi level of the tip is aligned with the top valence energy levels of the QD, causing holes to be injected from the tip into the QD, followed by hole tunneling to the substrate or electron tunneling from the substrate to the first conduction level. In this condition, it is sometimes possible to observe

Similar to other single QD and single-molecule experiments, current−voltage spectroscopy on isolated quantum dots displays a current gap (also called zero conductance gap) which is defined as ΔV = V+ − V−, where V+ is the onset of the current at the positive bias, and V− is the onset of the current at negative bias. The zero conductance gap can be related to the single-particle energy gap. To relate the zero conductance band gap to the Eg of the QD and the bias voltage at resonance (peaks in the tunneling spectra) to the energy levels of the QD, the distribution of the bias voltage over the DBTJ must be known. One can assume that the applied bias drops only across the tunneling barriers and that the potential drop across the much higher dielectric constant QD is close to constant.7,27−29 In this case, the distribution of the applied bias between the tip and the substrate is characterized by the parameter η = (Vdot − Vtip)/Vbias, where η is the fraction of the bias that drops between the tip and the QD, Vdot is the potential at the center of the QD, Vtip is the electrochemical potential of the tip, and Vbias is the applied bias between the tip and the substrate (potential difference over the entire DBTJ). Since the radius of the tip is much smaller than the curvature of the substrate, STM experiments tend to run in the η > 0.5 regime. The electrons or holes can tunnel on both sides of the zero conductivity gap for a symmetrical distribution of the potential across the DBTJ (η ∼ 0.5). However for asymmetrical systems where η ∼ 1 (or η ∼ 0) electrons (or holes) can tunnel only on one side of the zero-conductance gap and holes (or electrons) on the other side. In our experiments, the current magnitude is generally in the picoampere range which indicates that this system operates close to asymmetric case η ∼ 1 (potentials drops cross the QD-tip junction of the DBTJ) when the tip is at the closest approach to the dot. During the rest of D

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tip continuously, in c-AFM in tapping mode current flows only when the tip is in close proximity to the sample. On conductive samples such as bare gold, the tapping mode c-AFM current increases with decreasing distance between the tip and the sample, reaches the maximum when the tip is closest to the surface, and decreases with the tip withdrawing from the surface, resulting in single peak occurrence during one tap and no other intermittency is observed. When the tip is over a QD, the current signal has multiple peaks and lies below the envelope created by the signal obtained from a gold surface. The second, third, and fifth current injection events consist of three peaks, the major one with the highest amplitude of the current and two satellites; all three peaks are separated by a gap of ∼2 μs. The rest of the injection events have one major peak and one satellite. The multitude of peaks during a single tapping event results from intermittent current injection occurring at a very fast (submicroseconds) time scale and it is most likely caused by the same mechanism as current blinking on a longer time scale. Each peak represents current flowing between the tip and the sample with periods in between where no current flows through the QD. The secondary, tertiary, and so forth peaks are lower in the amplitude because of the larger distance between the tip and sample. We have additionally verified for a variety of samples such as ITO, gold, HOPG, and SAM layers on gold that the blinking of the current can only be observed when a QD is present. The histograms of both off and on times in single dot PL studies generally show a temperature-independent power-law 32,33 distribution of the form ρ(τoff/on) ∝ τ−αoff/on The off and off/on . on times follow a power law distribution over five32,34 and three decades,34,4 respectively. The αoff and αon exponents are in the range of 1 to 2. In most experiments reported in the literature, the typical time-trace duration is ∼1000 s and the experiments use a time bin size of around Δτ ≈ 10 ms. The exponents reported for PbS are 1.22 for off states and 1.36 for on states.35 In order to further characterize the current intermittency behavior and compare to single dot photoluminescence, Figure 5 shows histograms of the on and off times of the tunneling current. The QD is considered to be off when the current amplitude of the highest peak in one tuning fork oscillation period drops to the noise level ( −Eg (−0.5 V) only one resonance occurs. For V ≤ −Eg, one resonance occurs close to 0.2 pA and several others are observed at higher current levels.

Figure 7. Histogram of maximal current in single tap data obtained under illumination (green lines) and in the dark (black lines) at −0.5 V obtained for three different isolated QD.

Figure 7 shows three traces. The black and dark gray histograms were obtained under illumination (bright green LED) and the light gray curve was obtained in the dark at the same sample bias −0.5 V. The dark measurements exhibit only a single peak, whereas the measurements on illuminated QDs show multiple peaks suggesting that a multitude of resonant states is possible due to perhaps multiple photo excitations or multiple possible states of the QD. The proposed mechanism of current intermittency in Figure 2 (bottom panel), immediately shows that current blinking should be accompanied with PL blinking. In the current-on state, the dot should also be luminescent, while in the currentoff state, the dot should be dark. Experiments are underway to investigate this correlation between PL and current blinking. Also, from previous experiments performed under similar conditions, it was found that there is a strong coupling between the metallic substrate and excitons on the QDs.31 We found that the luminescence of the dot can be induced by generating surface plasmons on the substrate of sufficient energy to directly excite the QD, leading to luminescence. On the other hand, when the voltage applied is such that tunneling is occurring resonantly through the dot, the surface plasmon emission is extinguished.44 This suggests that also in the c-AFM experiment there can be a strong involvement of surface plasmons on the substrate when the applied bias is high enough. Considering the influence of the current intermittency of the QDs observed here, it is clear that there could be a very strong influence of the Coulomb blockade caused by surface-trapped charges on individual dots. Such influence of Coulomb blockade has been observed in PbSe QD arrays before45 although other reports found no evidence for such a blockade,46 indicating that it is likely that the specific surface chemistry has a large influence. This is consistent with our hypothesis of surface traps being involved in the current blinking. In conclusion, current intermittency was observed in conductive AFM experiments on individual PbS and CdSe QDs on gold substrates. The observed behavior is consistent with a model that is similar to that proposed for PL blinking of G

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single QDs in which a surface-trapped carrier causes a Coulomb blockade for current flow. It is predicted therefore that the current blinking correlates with PL blinking of the QDs which will be the subject of future investigations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The microscopy parts of this work were funded (K.M. and J.v.d.L.) by the Solar Photochemistry Program of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy. The particle synthesis and sample preparation (J.M.L. and S.U.N.) was funded through the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES). Funding was provided to the National Renewable Energy Laboratory (NREL) through contract DE-AC36-08GO28308.

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