Article pubs.acs.org/JPCC
Effect of Ligand Structure on the Optical and Electronic Properties of Nanocrystalline PbSe Films Anthony R. Smith,*,§,∥ Woojun Yoon,§,∥ William B. Heuer,†,⊥ Sophie I. M. Baril,‡ Janice E. Boercker,§ Joseph G. Tischler,§ and Edward E. Foos*,§ †
Chemistry Department, U.S. Naval Academy, 572 Holloway Road, Annapolis, Maryland 21402 United States School of Engineering and Applied Science, The George Washington University, 725 23rd Street NW, Washington, DC 20052 United States § U.S. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375 United States ‡
S Supporting Information *
ABSTRACT: Films of nanocrystalline PbSe were fabricated with a set of structurally varied short-chain dicarboxylic acids. Oxidation rates were studied via NIR spectroscopy to determine the effect of the structure of the diacid ligands on film stability under ambient conditions. Ligands favoring a non-bridging bonding mode were found to provide the best protection against oxidation, while among ligands expected to bridge between adjacent nanocrystals in the films, those with shorter chain lengths conferred better oxidative stability. Electronic coupling was observed as a red shift in the optical data of the ground excitonic peak of the PbSe films and found to be strongly influenced by the structure of the ligand. Transport measurements were made in air using thin-film transistors that were treated with a thin Al2O3 coating via remote plasma ALD. Films prepared using fumaric, maleic, and oxalic acids yielded mobility numbers of 2.5 × 10−5, 3.7 × 10−5, and 1.6 × 10−3 cm2/ V·s, respectively. Results suggest that the internanocrystal distance is the major contributor to electron mobility through the nanocrystalline films, while the electronic coupling is heavily influenced by multiple factors related to the structure of the surface ligands in addition to the internanocrystal distance.
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INTRODUCTION Thin films of solution-synthesized semiconducting nanocrystals are anticipated to impact several areas of current technology. Through entirely solution-based methods, such as spin-coating, inkjet printing, and layer-by-layer (LbL) dip-coating,1,2 devices composed of these materials can potentially be fabricated over large areas, keeping processing costs in check. Applications using semiconducting nanomaterials have been found in lightemitting diodes,3 lasers,4,5 detectors,6 and photovoltaics.7−9 Films of lead chalcogenide nanoparticles in particular have received considerable attention for photovoltaics due to their absorbance into the infrared, relatively high mobility,1 and efficient multiexciton generation,10 properties that result from the narrow band gap, light electron and hole masses, large dielectric constant, and high degeneracy of the ground electronic states in these materials. However, because these systems are also sensitive to surface states9 and junction formation on the nanoscale,11,12 understanding their stability under ambient conditions remains an important issue for device fabrication. Oxidation in air has detrimental effects that must be suppressed in order to obtain useful devices, and strategies to protect films from oxidation must preserve the optical and electrical properties of the nanocrystals. Strongly binding bifunctional ligands may provide a path toward passivation © 2012 American Chemical Society
through inherent oxidation suppression when used to synthesize functional devices. Films of unmodified (as-synthesized) lead chalcogenide nanocrystals are insulated electronically with a surface monolayer of hydrocarbons, typically oleic acid.1,13 These long-chain ligands provide good stability and solubility for the inorganic core in solution, yet hinder electron transport in films due primarily to their length and inability to tether to more than one surface, which isolates the inorganic cores both physically and electronically from each other. To produce functional devices, these ligands must be replaced with smaller species that can bind adjacent nanocrystals together and permit closer contact, thereby increasing the electronic interactions between particles. An ideal ligand would, following this exchange, produce a film that exhibits stability in air and increased electronic transport while maintaining the quantum effects of the size-confined particles. Short-chain bifunctional molecules have been used to exchange the native oleic acid on PbSe nanocrystals1,2,14−18 but have some undesirable traits, including increasing the film’s sensitivity to oxidation. Carboxylic acid functional groups have Received: November 17, 2011 Revised: February 17, 2012 Published: February 17, 2012 6031
dx.doi.org/10.1021/jp2111023 | J. Phys. Chem. C 2012, 116, 6031−6037
The Journal of Physical Chemistry C
Article
been shown to provide some stability against oxidation;19 however, the ligands examined previously were primarily short, monofunctional acids. In terms of coupling the nanocrystals together electronically, bifunctional ligands would seem to be more advantageous. The experiments detailed here explore the use of bifunctional ligand molecules to both tether PbSe nanocrystals to increase electronic coupling as well as limit film oxidation upon exposure to air. Five different short-chain bifunctional ligands were used to fabricate the nanocrystalline films using an LbL dip-coating technique. Length, π-bonding, and configuration of the ligands were varied. Through transmission FTIR spectroscopy, the shift in the ground excitonic absorption band (1Sh− 1Se) for PbSe nanocrystalline films was followed over time as a means to measure both coupling and oxidation of the material. PbSe nanocrystal thin-film transistors (TFTs) were also utilized to enable measurement of the charge carrier mobility of the films. The oxidation, electronic coupling, and transport properties are all highly dependent on the structure of the ligand used for film fabrication.
suppress H2O and CO2 absorption features in the attenuance spectra as well as to reduce oxidation/photooxidation processes during the optical measurements. Scanning electron microscopy (SEM) was performed using a Carl Zeiss SMT Supra 55 field emitting microscope operating at 15 kV. Both dip-cast and drop-cast samples were imaged on Si and glass substrates. Ligand exchange was characterized by FTIR using a Nicolet Magna-IR 750 spectrometer using either KBr pellets for the free acids or films prepared on NaCl. Electrical Characterization. PbSe nanocrystal TFTs were prepared using 10 dip-casting cycles, producing films of roughly 35 nm in thickness onto the prepatterned substrates. For the bottom-contacted transistors, a heavily doped n-type silicon substrate was used as the gate electrode and the substrate. For the gate dielectric, HfO2 films (∼34 nm) were deposited on silicon substrates by remote plasma atomic layer deposition (ALD). The source and drain contacts were formed by standard photolithography and lift-off of Au (100 nm) with a 15 nm Ti adhesive layer, defining a channel length (L) of 10 μm and width (W) of 1000 μm atop the HfO2 layer. To protect the films from atmospheric exposure, a thin coating of Al2O3 (∼8 nm) was deposited onto the PbSe nanocrystals by remote plasma ALD at 150 °C. Electrical characterization of the devices was performed with a semiconductor parameter analyzer (Agilent 4145B) at room temperature in darkness under ambient atmosphere. The mobility was extracted from the linear region of the output characteristics of the devices. Additional measurement details are provided in the Supporting Information.
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EXPERIMENTAL SECTION Materials. Fumaric acid, lead oxide (99.9+%), maleic acid, malonic acid, 1-octadecene (90%), oleic acid (90%), oxalic acid, selenium powder (99.5+%), succinic acid, and trioctylphosphine (90%) were purchased from Sigma-Aldrich. Chloroform, ethanol (200 proof), heptane, and hexanes were acquired from commercial sources. Acetonitrile and chloroform were degassed by the freeze−pump−thaw method and dried over molecular sieves for air-free fabrication procedures. All other chemicals were used as received without further purification. Film Formation. PbSe nanocrystalline films were deposited through an LbL dip-coating procedure following reported methods.2 Substrates were cleaned using a Samco UV & Ozone dry stripper under a 0.5 L/min ozone flow rate at 150 °C for 12 min, followed by an N2 purge. The substrates were then briefly soaked (
