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J. Phys. Chem. C 2010, 114, 9988–9996
Influence of Atmospheric Gases on the Electrical Properties of PbSe Quantum-Dot Films Kurtis S. Leschkies, Moon Sung Kang, Eray S. Aydil,* and David J. Norris* Department of Chemical Engineering & Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455 ReceiVed: February 24, 2010; ReVised Manuscript ReceiVed: April 21, 2010
We report the effects of N2 and O2 on the electrical properties of PbSe quantum-dot (QD) films treated with 1,2-ethanedithiol (EDT) by measuring the changes in the current-voltage characteristics of QD field-effect transistors (FETs). EDT-treated PbSe QD films at a base pressure of ∼10-5 Torr exhibit ambipolar transport. Exposing these films to N2 shifts the transfer characteristics toward negative gate-voltage values and increases the electron mobility. These changes could be reversed entirely by removing the N2 gas over the FET and returning to base pressure. Oxygen exposure shifts the transfer characteristics in the opposite direction toward positive gate-voltage values. Moreover, oxygen exposure reduces charge mobility but increases film conductivity. For exposures up to ∼108 langmuir, these O2-induced changes could be reversed completely by removing the O2 gas over the sample and returning to base pressure. However, after ∼1010 langmuir of O2 exposure, the changes are irreversible. The QD films then permanently become p-type and the decrease in charge mobility remains even after returning to base pressure. Introduction Due to the unique electronic and optical properties of colloidal semiconductor quantum dots (QDs),1 they have potential applications in field-effect transistors (FETs),2 light-emitting diodes,3,4 photodetectors,5 and solar cells.6-9 In particular, researchers hope to exploit the size-dependent shifts in electronicenergy levels that occur in these materials due to quantum confinement. While the size-dependent optical properties of individual colloidal QDs can be easily studied by dispersing them in various solvents, characterization of electronic and optoelectronic devices requires one to deposit thin films of QDs. This is typically achieved by casting colloidal QDs onto a substrate. However, because the QDs are coated with bulky surface ligands (e.g., oleic acid) that inhibit charge transport, the resulting films are electrically insulating. To increase electronic coupling between the QDs and improve conductivity, these ligands are removed or replaced with shorter molecules.2 Unfortunately, this process also increases the sensitivity of the QDs to air exposure, which can affect the stability of electrical transport through the films. For example, lead chalcogenide semiconductors have become the prototypical QD material for electronic and optoelectronic devices. These QDs exhibit superior transport properties compared to those made from other II-VI semiconductors, such as CdSe or ZnSe.2,10 However, the optical and electrical properties of lead chalcogenide semiconductors, both as QDs and in the bulk, are very sensitive to surface states and adsorbates.11-21 In bulk lead chalcogenide films, this leads to the formation of a native oxide layer upon exposure to air.20 Once the leadchalcogenide-oxide interface is present, carrier radiative recombination becomes more efficient at this bulk interface and the photoluminescence is enhanced. In contrast, a 20% reduction in the luminescence of PbSe QDs has been reported when they are exposed to oxygen.19 In addition to these effects, bulk n-type lead chalcogenides have been shown to demonstrate p-type * To whom correspondence should be addressed. E-mail: aydil@ umn.edu;
[email protected].
electrical transport characteristics when exposed to air.16,22 Such reports suggest that oxygen in the air causes the appearance of p-type charge carriers in the film. These changes can have adverse effects on the performance of QD devices assembled with lead chalcogenides, and consequently, much effort has gone into protecting these devices from air exposure. For example, solar cells assembled with PbSe QDs are unstable in the presence of air, converting from powergenerating diodes to ohmic resistors within minutes of exposure.23,24 Similarly, PbSe QD-based thermoelectric devices experience thermopower losses when exposed to air.25 On the other hand, PbSe QD FETs can still show ambipolar gating if they are exposed to air for less than a few seconds. However, they are irreversibly affected if the air exposure lasts longer than this.21 While most of the authors of these studies surmised that oxygen in the air is the culprit in device degradation, experiments that test the influence of individual atmospheric gases on electrical properties have not been reported. Here we provide this information by studying the impact of dinitrogen and dioxygen exposure on the electrical properties of PbSe QD films. Surprisingly, we see a strong influence of not only oxygen but also nitrogen. Experimental Section Chemicals and Substrates. Anhydrous 200 proof ethanol (g99.5%), anhydrous methanol (99.8%), anhydrous butanol (99.8%), anhydrous octane (g99%), anhydrous hexane (g99%), anhydrous acetonitrile (99.8%), 1,2-ethanedithiol (EDT, g98%), tri-n-octylphosphine (TOP, technical grade, 90%), oleic acid (technical grade, 90%), 1-octadecene (ODE, 90%), and lead(II) oxide (PbO, 99.999%) were purchased from Aldrich. Selenium shot (Se, 99.999%) was obtained from Alfa Aesar. Acetone (ACS grade) and isopropyl alcohol (IPA, ACS grade) were purchased from Mallinckrodt Chemicals. All chemicals were used as delivered. 〈100〉-oriented, boron-doped 100 mm silicon (Si) wafers (F ) 0.005-0.01 Ω cm, thickness )525 ( 25 µm) coated with 300 nm of thermal oxide (SiO2) were acquired from Silicon Valley Microelectronics. Ultrapure grades of nitrogen
10.1021/jp101695s 2010 American Chemical Society Published on Web 05/06/2010
Electrical Properties of PbSe QD Films gas (N2, 99.999%), oxygen gas (O2, 99.998%), and argon gas (Ar, 99.999%) were purchased from Minneapolis Oxygen, Matheson Gas Company, and Airgas, respectively. The Certificate of Analysis for these gases are listed in Table S1 in the Supporting Information. Patterning of Si/SiO2 for Field-Effect Transistors. Si wafers covered with a 300 nm thick thermally grown SiO2 were patterned using standard liftoff techniques and used as substrates for Au-coated back-gated field-effect transistors, as detailed elsewhere.9 Source and drain contacts used in this work consisted of 2.5 nm Cr/22.5 nm Au. A gate electrode was deposited on the backside of the Si/SiO2 wafer and consisted of 10 nm Al/75 nm Au. Multiple source and drain electrodes were patterned onto a single substrate to allow for electrical measurements using various channel lengths (5-100 µm). Synthesis of Colloidal PbSe Quantum Dots. Colloidal PbSe QDs were prepared according to the approach of ref 26. Our exact procedure can be found in ref 9, which we followed here with one exception. Diphenylphosphine was not added to the Se precursor solution. It is also important to note that, as in ref 9, the QDs were removed from the reaction vessel via cannula and transferred to a Schlenk flask to avoid exposure to ambient. This flask was then moved immediately to a nitrogen glovebox, where all subsequent processing steps (precipitation, centrifugation, redispersion, filtration, etc.) were carried out. Deposition and Chemical Treatment of PbSe QuantumDot Films. Colloidal PbSe QDs were deposited as films onto the patterned Si/SiO2 substrates by spin-casting from a 20 mg/ mL dispersion in anhydrous octane under nitrogen. Once dry, the films were dipped into a 0.08 M EDT solution in anhydrous acetonitrile for 5 s and removed at a rate of 0.5 cm/s. When treated with EDT, QD films tend to crack due to reduced interparticle spacing. To fill the cracks and voids, a second coating of QDs was cast from a 5 mg/mL dispersion. Following a second EDT exposure, PbSe QD films were approximately 70 nm thick as measured by atomic-force microscopy (AFM). Characterization of PbSe Quantum Dots and QuantumDot Films. Optical absorption spectra of the PbSe QDs were obtained with a Cary 5E spectrophotometer. The size of the QDs was determined by comparing the position of the lowest energy absorption feature with a published calibration.27 Similarly, QD-size distributions were estimated from the full-widthat-half-maximum (fwhm) of this feature in comparison to literature values.26 Fourier-transform-infrared (FTIR) absorption spectra of QD films obtained using attenuated-total-internalreflection (ATR) were collected in a nitrogen glovebox using a Nicolet 6700 FTIR-NIR spectrometer with an InGaAs detector and an electronically temperature-controlled EverGlo IR source.10 The IR beam was focused normal onto a 45° beveled edge of a trapezoidal Si ATR crystal. Spectra were obtained by averaging 250 interferograms at 2 cm-1 resolution. AFM images were obtained using a Nanoscope IIIA multimode microscope (Veeco Metrology) located in a nitrogen glovebox with tapping mode tips having a spring constant of 42 N/m, resonance frequency of ∼285 kHz, and a radius of curvature
