Fourier Transform Infrared Absorbance and Photoluminescence Spectroscopy Studies of CdSe Colloidal Quantum Dot/Conductin
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Fourier Transform Infrared Absorbance and Photoluminescence Spectroscopy Studies of CdSe Colloidal Quantum Dot/Conducting Polymer Nanocomposites for Application to Infrared Photodetectors Kevin R. Lantz and Adrienne D. Stiff-Roberts Electrical and Computer Engineering, Duke University, Box 90291, Durham, NC, 27708-0291
ABSTRACT In this paper we investigate the optical properties of four CdSe colloidal quantum dot/conducting polymer nanocomposites deposited on GaAs substrates using photoluminescence and Fourier transform infrared absorbance. The purpose of this investigation is to find an appropriate electron-conducting polymer for use in a photoconductor that utilizes intraband transitions in the conduction band to detect mid- to long-wave-infrared radiation. As a feasibility demonstration, we fabricate a two-terminal photoconductor and characterize its dark current and spectral responsivity (at 125 K), demonstrating intraband peaks at 0.465 and 0.527 eV, which correspond to 2.67 and 2.35 µm. INTRODUCTION The use of hybrid, colloidal quantum dot (CQD)/conducting polymer nanocomposites for infrared (IR) photodetection has become an area of great interest recently due to the promise of room temperature operation. Previous work in this field has demonstrated IR photodetection in hybrid nanocomposites featuring PbS or PbSe CQDs embedded in hole-conducting polymers (poly[2-methoxy-5-(2í-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV)1 and poly-N-vinyl carbazole2), or in spin-coated, dispersed CQD thin films.3 These devices use interband transitions in the near-IR regime (1-3µm). Other works have demonstrated near-IR sensitivity in CQDs by intraband absorption from the 1Se to 1Pe electronic levels of excitonic transitions.4,5 These intraband excitonic transitions are typically observed by pump-probe, photo-induced (visible) absorption measurements in dispersed CQDs or CQD solids in which no additional quantum confinement is present. The distinct motivation of this work is to use electron intraband transitions within the conduction band of CQDs to push the photodetector spectral response into the mid- and long-wave IR regimes (3-5µm and 8-14µm, respectively). These IR windows are of great interest for medical diagnosis, atmospheric monitoring, and thermal imaging. Such intraband transitions in hybrid nanocomposites require that a quantum confinement potential be provided by the conducting polymer, as has been demonstrated theoretically.6 We have experimentally observed mid-IR, intraband absorption in CdSe CQD/MEH-PPV nanocomposites deposited on GaAs substrates.7 Our proposed mechanism for IR, intraband absorption in this material system is as follows: i) incident near-IR light is absorbed by the GaAs substrate,
generating electron-hole pairs that are injected into the CdSe/MEH-PPV nanocomposite; ii) due to the difference in electron affinities between CdSe and MEH-PPV, a spherical confinement potential is formed in the conduction band, providing discrete CQD electron energy levels; iii) holes are blocked fro
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