PbS Nanoparticles: Synthesis, Supercritical Fluid Deposition, and Optical Studies
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PbS Nanoparticles: Synthesis, Supercritical Fluid Deposition, and Optical Studies Joanna S. Wanga, Bruno Ullricha,b, and Gail J. Browna a
Air Force Research Laboratory, Materials & Manufacturing Directorate, Wright Patterson AFB,
OH 45433-7707, USA b
Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Cuernavaca,
Morelos, México C.P. 62210 ABSTRACT Lead sulfide (PbS) nanoparticles (NPs) of different sizes (2.0 nm - 14.4 nm) have been synthesized in our laboratory. By using those NPs, we formed colloidal films on glass and GaAs substrates employing a specialized supercritical fluid CO2 (sc-CO2) deposition method. The deposited films contain only the PbS NPs and the protecting group of oleic acids and require no polymer matrix. The NP films are solvent free, environmentally stable, and show good adhesion to the substrates. The sc-CO2 deposition process can deposit films ranging in thickness from a few monolayers, in well ordered arrays, up to 0.5 μm or greater. The photoluminescence (PL) properties of these nano-structured films were studied with Fourier transformation infrared spectroscopy from 5 K up to 300 K. INTRODUCTION The intrinsic features of PbS nanoparticles, particularly the clearly enhanced quantum confinement with respect to other attractive semiconductors such as CdTe and CdS, has boosted the research on PbS NPs during the past 15 years [1-5]. Particularly, the emission properties have attracted considerable attention [5-9]. While many of the PbS colloidal NPs studied have been embedded in glass or polymer matrices, we sought to develop a deposition process that could be compatible with standard semiconductor device processing, i.e. a cleaner process with less solvent residue and a process that enables close contact between the semiconductor surface and the NPs. We employed supercritical fluid CO2 processes due to its near zero surface tension, the ability for the removal of solvents, and the capability to arrange NPs in ordered arrays [1012] - a task difficult to achieve by traditional solvent deposition. To study the interactions of PbS NPs with semiconductor substrates colloidal PbS films of different NP diameters are highly useful. Glass substrates were used as control samples to isolate substrate charge transfer effects. For this study we developed both PbS NP synthesis and deposition processes using supercritical fluid CO2. PbS nanoparticles precipitate evenly and self-assemble to form a uniform 2-D array on TEM copper grids, glass, and GaAs substrates during the sc-CO2 deposition. EXPERIMENTAL DETAILS PbS QDs of different sizes have been selectively synthesized by the alteration of the following parameters: oleic acid/octadecene ratios, injection temperature, and growth temperature [13] during the synthesis. The TEM images of various PbS NPs synthesized with this process, ranging from 2.0 nm to 14.4 nm in diameter, are shown in Figure 1 below.
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Figure 1. TEM images of PbS QDs with different sizes. a: size = 14.4+1.6 nm, b: siz
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