Article 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 CaliforniaDavis, 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 1,2ethanedithiol 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.
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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 © 2013 American Chemical Society
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.
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EXPERIMENTAL DETAILS PbSe QDs were synthesized using standard air-free techniques as described elsewhere.14 The QDs were transferred and stored in a N2 glovebox (O2 < 10 ppm, H2O < 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 SiO2 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 N2 without rinsing. Small angle X-ray scattering (SAXS) and X-ray diffraction measurements were performed with a Bruker D8 Discover Xray 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, 2013 3713
dx.doi.org/10.1021/jp306893e | J. Phys. Chem. C 2013, 117, 3713−3717
The Journal of Physical Chemistry C
Article
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.
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 9.4 × 1012 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.17 2. 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
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.
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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 (
