Positive Temperature Coefficient of Resistance and Bistable Conduction in Lead Selenide Quantum Dot Thin Films

Transcription

1 pubs.acs.org/jpcc Positive Temperature Coefficient of Resistance and Bistable Conduction in Lead Selenide Quantum Dot Thin Films Tyler N. Otto and Dong Yu* Department of Physics, University of California Davis, 1 Shields Ave, Davis, California 95616, United States ABSTRACT: We report the observation of a positive temperature coefficient of resistance in strongly coupled quantum dot (QD) arrays. Conductivities of lead selenide (PbSe) QD thin films treated with 1,2- ethanedithiol increase when cooled from room temperature to 78 K, consistent with bulk PbSe crystals and indicating bandlike transport. Small angle X-ray scattering and infrared absorption spectroscopy results confirm a very strong electronic coupling among QDs. These QD thin films also exhibit bistable conduction above 160 K. As the electric field reaches a threshold, on the order of 0.1 V/μm, the film conductance increases abruptly by up to 10 orders of magnitude and remains high until the bias voltage is reduced to about zero volts. This bistability is likely induced either by filament formation or by a traprelated electronic switching. INTRODUCTION Colloidal semiconductor quantum dots (QDs) have optoelectronic properties that greatly differ from their bulk counterparts. Both n-type and p-type bulk lead chalcogenide crystals exhibit unusual positive temperature coefficients of resistance (resistance increases with increasing temperature). 1 On the other hand, colloidal lead selenide (PbSe) QDs show negative temperature coefficients of resistance in almost all reports to date. 2 5 This difference is likely caused by a weak electronic coupling among QDs, where the low temperature conduction behaviors can be described very well by the variable range hopping mechanism. 5 7 Improving the coupling, while maintaining quantum confinement, can lead to more efficient carrier extraction and better electronic devices based upon QDs. QDs have also recently been studied as potential building blocks for electrically driven memory devices, 8 12 which take advantage of the low fabrication cost and tunable electronic properties of QDs. The investigation of the memory effect can also shed light on the charge transport mechanism in QD thin films. Bistable conduction of QD thin films is typically achieved by embedding the QDs in a polymer matrix, where the application of a voltage pulse changes the conductivity of the system between two metastable conductive states. 8 10,13 The bistability in the QD/polymer system has been attributed to an electric field induced charge transfer from QDs to the conducting polymers. Recently, there have been reports of bistable conduction of pure QD thin films. 9,12,13 However, the memory switching mechanism of devices composed solely of QDs is not well understood. Here, we report a positive temperature coefficient of resistance in strongly coupled PbSe QD thin films. While the QDs in the films treated with 1,2-ethanedithiol (EDT) still show clear quantum confined absorption peaks, conductivity increases with decreasing temperature, as in the bulk PbSe. In addition, we have observed a drastic, repeatable, and rapid increase in the conductivity of QDs by up to 10 orders of magnitude within 1 ms, under a small threshold field on the order of 0.1 V/μm. EXPERIMENTAL DETAILS PbSe QDs were synthesized using standard air-free techniques as described elsewhere. 14 The QDs were transferred and stored in a N 2 glovebox (O 2 < 10 ppm, H 2 O < 0.1 ppm) where all EDT treatment and subsequent electrical measurements were performed. Planar devices with bottom contact to QDs were made on 300 nm SiO 2 covered Si wafer previously patterned with interdigitated Cr/Au (5/25 nm) electrodes by photolithography (Figure 1a). The electrodes consisted of 350 pairs of 10 μm wide, 7 mm long Au pads with 10 μm gap. The QDs were dispersed in a 9:1 mixture of hexane/octane and passed through a 0.2 μm PTFE filter before being drop cast on the prepatterned substrate. The film was then placed in 0.1 M EDT in anhydrous acetonitrile for 2 3 min and was subsequently dried in N 2 without rinsing. Small angle X-ray scattering (SAXS) and X-ray diffraction measurements were performed with a Bruker D8 Discover X- ray diffractometer. Absorption spectra were taken with a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer. Atomic force microscopic (AFM) images were measured with a Veeco Dimension 3100 system. Electrical characterization was made with a NI data acquisition system and a Stanford Research Systems model SR570 current preamplifier. The QD devices were transferred into a cryostat without exposure to air and placed under vacuum with a base pressure of 20 mtorr Received: July 11, 2012 Revised: January 22, 2013 Published: January 25, American Chemical Society 3713

2 Figure 1. (a) Schematic of a planar device with a QD thin film on top of prepatterned interdigitated electrodes. (b) Absorption spectra of PbSe QDs with different diameters. (c) X-ray diffraction spectrum of PbSe QDs. for temperature dependent measurements. To ensure an inert and water free environment, the device was transferred into the cryostat inside the glovebox. The cryostat was then sealed and transferred out of the glovebox and placed under vacuum for characterization. RESULTS AND DISCUSSION 1. X-ray Diffraction and Spectroscopy Characterization. The QD sizes were determined from the first exciton peak positions following the literature (Figure 1b). 15 The first exciton absorption peaks of the PbSe QDs were narrow (<40 mev), corresponding to a size distribution of <10% for all three samples (Figure 1b). XRD measurements confirmed the rock salt structure of the PbSe QDs (Figure 1c). The final film thickness after EDT treatment was typically around 40 nm as measured by AFM. The entire film had a RMS roughness of about 15 nm, corresponding to 2 3 monolayers of QDs. In order to determine the interdot spacing, we performed SAXS measurements before and after EDT treatment. The QD samples were exposed to air during the SAXS measurements, and we observed that the results slowly changed over hours due to oxidation. The following data was taken immediately after the samples were removed from the glovebox, and before a significant change occurred. SAXS measurements of the QD thin films before EDT treatment exhibited clear superlattice peaks indicating the film had a long-range order. After EDT treatment, these peaks were greatly diminished and broadened, signaling that the film transitioned to a more disordered structure. The scattering intensity for spherical particles of uniform radius R is proportional to the form factor F(q) = (4/ 3)πR 3 {3[sin(qR) qr cos(qr)]/(qr) 3 }, where q =(4πsin θ)/ λ. We extracted R by setting the derivative of F(q) to zero and plugging in θ at maximum scattering intensity. By analyzing the SAXS data this way, the interdot spacing was found to decrease by approximately 1.6 nm (Figure 2a), bringing the interdot Figure 2. (a) SAXS data showing a 1.6 nm reduction in interdot spacing. (b) Spectral shift of 3.9 nm diameter PbSe QDs after EDT treatment. Upper blue line, EDT treated thin film; middle black line, as deposited thin film; bottom red line, QD solution. spacing within a couple angstroms after treatment, consistent with the length of an EDT molecule. 16 The SAXS result confirms that the compact EDT ligands have successfully replaced the bulky oleic acid ligands and the QDs are in near direct contact with each other after EDT treatment. We then took the absorption spectra of the QD thin films before and after EDT treatment. After EDT treatment, QDs maintained the discrete peaks indicating the QDs were still quantum confined. The slight increase in the peak width can be attributed to the electron enhanced coupling. 17 The first exciton absorption peak of the QD thin films red-shifted by 61 mev after EDT treatment, while maintaining discrete energy levels (Figure 2b). The red-shift in our films was significantly larger than that of thicker spin-coated films (27 32 mev) or films created by the layer-by-layer method (23 mev). 3,16 The red-shift is caused by the increase in electronic coupling between QDs or by other effects, such as the change in the dielectric environment. Liu et al. has observed a 22 mev redshift due to noncoupling effect, 16 from which we can estimate our coupling energy, hγ, is approximately 39 mev, corresponding a charge transfer rate, Γ, of s 1. This coupling energy is significantly higher than the previously reported 10 mev, 16 indicating that our QDs are very strongly coupled. This large shift in the absorption spectrum is consistent with the small interdot distance indicated by the large SAXS shift. A large red-shift in the absorption spectrum has also been observed previously in thiocyanate treated CdSe QDs exhibiting bandlike transport Charge Transport. We now present the electronic characterization of the PbSe QD thin films. The QD thin films exhibited a bistable conduction detailed in the later section and here we first focus on their charge transport behavior in the conductive state. The as-deposited films were completely insulating beyond the detection limit of our setup (σ <10 11 S/cm). The EDT treatment led to an increase in film conductivity of at least 10 orders of magnitude (σ 10 1 S/ cm) after the film conductance was turned on by applying an overthreshold bias voltage. Our film conductivity was 1 2 orders of magnitude higher than that in earlier reports, 3,16 indicating a stronger electronic coupling between neighboring QDs and consistent with our larger absorption peak shift and small interdot spacing. We prepared our QD films by drop casting instead of a layer-by-layer method or spin-casting as in previous reports. It should be noted that the electrical properties of QD thin films are sensitive to the details of the film preparation, as the charge transfer rate exponentially depends on the interdot distance and is strongly influenced by the surface traps. This sensitivity has been noted by other groups as well. 5,14 We want to make two notes regarding to the electrical measurements. (1) The interdigitated electrode structure 3714

3 allowed us to greatly increase the electrode area and thus overall conductance; however, it prevented us from making 4- point measurements. Though the measurements were made in 2-point devices, our linear I V curves (even at low temperatures) and high conductivity in the conductive state strongly indicate ignorable contact barriers. (2) These films only showed temporary gate response as in earlier reports. 3 In our films, the gate response decayed on the order of seconds, preventing us from separating the current generated by the gate pulse from the temporary increase in carrier concentration. This decay is not understood, but it has been attributed to a slow screening of the applied gate field by trapped carriers. 3 The instability in gate response prohibits us from extracting the field effect mobilities. To better understand the charge transport mechanism, we cooled a device incorporating a QD thin film to approximately 78 K and slowly warmed, while the conductance was measured at 1 V bias so that the device stayed in its conductive state. The conductance decreased by approximately 40% as the temperature increased from 78 to 300 K (Figure 3), showing a positive CdSe QDs. 17 The bandlike transport in our QD films is consistent with our other observations: near direct contact between neighboring QDs confirmed by SAXS and strong electronic coupling indicated by the absorption peak red-shift. The observed positive temperature coefficient of resistance further suggests that strong electronic coupling lead to extended electronic states and electronic band formation. The 40% resistivity increase in our PbSe QD thin films from 78 to 300 K is significantly smaller than the 10-fold increase in bulk PbSe, indicating the carrier scattering at the boundary of the QDs still play an important role in the QD thin film. The other possibility is that the films have already undergone a metal insulator transition at room temperature. 18 This would explain the absence of a persistent gate response. However, given the positive temperature coefficient of bulk PbSe, along with the observations of bandlike transport in QD thin films by other groups, 17,19 we believe it is more likely that the observed positive temperature coefficient in PbSe QD thin films is caused by a bandlike transport. 3. Bistable Conduction. At room temperature, the EDT treated films showed a strong hysteresis in their I V curves (Figure 4). Upon reaching a threshold voltage (V th ), the film Figure 3. On-state conductance extracted at different temperatures for QDs with a diameter of 5.8 nm. Black line is a linear fit of the data. temperature coefficient of resistance. We measured the temperature dependence of four devices, and all exhibited a positive temperature coefficient of resistance. This positive temperature coefficient is the first such observation for chemically treated QDs. Previously a positive temperature coefficient has been seen only below 110 K in laser ablated and ligand free PbSe QDs, 18 which has been attributed to an insulator-to-metal transition at low temperature. On the other hand, bulk PbSe crystals also exhibit a positive temperature coefficient, with a resistivity increase by a factor of 10 from 78 to 300 K. The resistivity change of bulk crystals is mainly caused by the drastic increase in carrier mobility at low temperature because of reduced carrier-phonon scattering. 1 As mentioned previously, our devices do not show a persistent gate response, even at low temperature, preventing us from directly measuring the mobility, but we can deduce the sign of dμ/dt from the negative sign of dσ/dt. The conductivity, σ, is determined by the product of the carrier mobility μ and the carrier density n (σ = neμ). As temperature increases, σ decreases and n is expected to increase or remain constant if acceptor states are completely ionized at given temperature. Thus, μ must decrease with temperature, suggesting bandlike transport through extended electronic states. A negative dμ/dt has been previously observed, as a signature of bandlike transport, in CdSe QDs capped with molecular metal chalcogenide complexes (MCCs) 19 and thiocyanate treated Figure 4. (a) Room temperature I V curve of a QD thin film device showing precipitous change in conductance upon bias sweeping. (b) Semilog plot of the I V curve in (a). (QD diameter = 5.8 nm.) conductivity increased abruptly from to 10 1 S/cm (Figure 4b). This increased conductivity was maintained even after the bias was reduced below V th. The film returned to its insulating state once the voltage was reduced below a holding voltage V hold. V th and V hold could have the same or opposite polarity depending on sample, but V hold was always much smaller than V th and near 0 V. The I V curves were usually symmetric and the film switched to the conductive state as the voltage reached a more negative bias (V th ). The time required for switching was measured to be less than 1 ms, limited by the temporal resolution of the preamplifier. This memory switching was robust, and a device could be switched on and off many times. For a given device, V th remained constant upon repeated switching (Figure 5). While V th usually varies from 0.2 to 2 V in the more than 21 devices measured, a couple of devices showed V th as high as 10 V. This variation was observed even when the films were composed of the same batch of QDs and fabricated with the same procedure. This V th variation made it difficult to identify any size dependence. Out of the 21 devices measured, 16 devices showed conductance change over 10 orders of magnitude, while other devices exhibited smaller on/off ratios. Hysteresis in chemically treated CdSe QD thin films has been seen in the past, 9 but not with such drastic on/off ratios and with such a rapid transition. The direction of hysteresis observed in CdSe QDs was clockwise in contrast to our counterclockwise 3715

4 Figure 5. Room temperature I V curves of a different device in 8 consecutive bias scans, demonstrating reproducibility in threshold voltage and asymmetry. (QD diameter = 5.8 nm.) hysteresis. These observations indicate that the mechanism for the bistable conduction in EDT treated PbSe thin films must be different from that of CdSe QDs. Upon exposure to air, these devices no longer showed hysteresis and remained in the conductive state with a slight increase in conductance, consistent with previous reports. 3,20 Most interestingly, the bistable conduction exhibited a strong temperature dependence (Figure 6). After cooling to 175 K, the hysteresis became Figure 6. I V curves taken at various temperatures showing the disappearance of bistability at low temperatures. (QD diameter = 6.2 nm.) weaker. When the temperature was below 160 K, the hysteresis completely disappeared and the device remained in the conductive state over the entire bias range. The hysteresis was recovered upon warming above 160 K. Two possible mechanisms may account for the observed bistability as described below. 1. Filament Formation. under an applied bias of about 1 V, a reversible redox reaction may happen in the QD thin film. This would result in a conductive filament bridging the gap between the electrodes. Given the rapid switching and the relatively large gap (10 μm), it is more likely and energetically favorable that metallic filaments are formed at the necking between neighboring QDs, instead of having a metallic filament bridging the entire gap. It is most likely that any filament formed would be composed of lead, as the QDs are terminated by lead atoms. 15 The nearly direct contact between neighboring QDs could be the cause of such a reaction. Because lead only acts as short connection between QDs, the conductance is expected to be dominated by the PbSe QDs and our early discussion on the positive temperature coefficient of resistivity still holds. At room temperature, such metallic filaments are not stable at zero-bias and will likely react with Se, returning the film to the insulating state. At low temperature (<160 K), the 3716 redox reaction significantly slows down and the metallic filaments remain even at zero bias. After exposure to air, the PbSe QD thin film remains conductive likely because the redox reaction stops at the oxidized surface. Though the above filament mechanism appears reasonable, there are also a couple of difficulties: there has been no report on lead filament formation though PbSe QDs have been studied for more than one decade, and it is unclear why the elements stay in the metallic state at low temperature. 2. Electronic Switching. We propose a possible mechanism following the three steps labeled in Figure 4b: (1) When V b is below V th, the QD array is insulating, likely because conduction is prohibited by the Coulomb repulsion of the charges trapped at the QD surface. EDT treatment will likely leave the surface poorly passivated, allowing for the trapping of several charges. As V b increases, charges injected from the contacts into the QDs are efficiently trapped at the QD surface. By trapping multiple charges, the total charging energy can be increased over k B T and thus prohibit conduction. 16 The observed small bias threshold (V th 1 V) indicates that the electric field is inhomogeneous. 2 The inhomogeneous electric field can be caused by the nonuniform charging of QDs along the film. The potential drop is mainly distributed between charged and uncharged QDs and this charge front propagates through the QD arrays as bias increases. 21 (2) When V b is increased above V th, the charge front finally propagates through the entire film and creates a conductive pathway, leading to an abrupt transition to the conductive state. (3) After the transition, the electric field becomes uniformly distributed and the charge carrier wave function becomes extended in the strongly coupled QD thin films. The charging energy is greatly reduced, since it is proportional to the inverse of the localization length of the charge wave function. Thus, when we reduce V b below V th, the conductive state will persist until the bias is sufficiently lowered below the reduced charging energy. Then the charges are detrapped and the QD arrays return to the insulating state. The above hypothesis is consistent with our experimental observations, as well as the conclusion that the films are behaving with bandlike properties. The disappearance of hysteresis at low temperature can be understood as surface states are frozen at low temperature. There is likely a potential barrier between the QD conduction states and the surface states. At low temperature, the charges do not gain sufficient thermal energy to overcome this barrier and remain trapped at the surface states, even when the bias is reduced. Thus, the carriers will maintain the extended wave function, and in-turn, high conductance, over the entire bias range. At higher temperature, the trapped charges can be released at low bias causing switching to the insulating state. Furthermore, oxidized samples are conductive and do not show any hysteresis, since oxidation leads to a high hole density 3 and thus no external electric field is required to turn on conduction. We think the trap-related electronic switching more likely accounts for the bistable conduction behavior in QD thin films, as the bistability is sensitive to the chemical treatment and electronic coupling between QDs. Further work is necessary to clarify on the origin of the bistable conduction. CONCLUSION In summary, we have observed a positive temperature coefficient of resistance in strongly coupled PbSe QD thin films. The large red shift of the absorption peak, high film conductivity, and SAXS results are all consistent with a strong

5 electronic coupling between QDs. This finding demonstrates semiconductor nanocrystal solids can show bandlike transport properties when the neighboring QDs are strongly coupled, which can lead to improved charge extraction and possibly higher mobilities in QD devices. EDT treated PbSe QD thin films also exhibit bistable conduction. The conductivity increases abruptly by up to 10 orders of magnitude when the bias reaches a threshold, and remains high until the bias is reduced much lower than the threshold. We attribute the insulating state to the Coulomb repulsion by the trapped charges, and the conductive state to the reduced charging energy due to extended charge carrier wave function. Alternatively, we acknowledge that the bistability could be due to the formation of conductive filaments in the QD thin films. The drastic and fast memory switching may find applications in memory devices. AUTHOR INFORMATION Corresponding Author * yu@physics.ucdavis.edu. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Startup fund by University of California at Davis and the Hellman fellowship. We thank Prof. P. Guyot-Sionnest for helpful discussions. We also acknowledge Prof. Y. Takamura and Peter Greene for their assistance with the SAXS measurements. REFERENCES (1) Dalven, R. A Review of Semiconductor Properties of PbTe, PbSe, PbS and PbO. Infrared Phys. 1969, 9, 141. (2) Hu, Z.; Fischbein, M. D.; Drndic, M. Local Charge Transport in Two-Dimensional PbSe Nanocrystal Arrays Studied by Electrostatic Force Microscopy. Nano Lett. 2005, 5, (3) Luther, J. M.; Law, M.; Song, Q.; Perkins, C. L.; Beard, M. C.; Nozik, A. J. Structural, Optical, and Electrical Properties of Self- Assembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol. ACS Nano 2008, 2, (4) Mentzel, T. S.; Porter, V. J.; Geyer, S.; MacLean, K.; Bawendi, M. G.; Kastner, M. A. Charge Transport in PbSe Nanocrystal Arrays. 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