Lead Sulfide Quantum Dot Synthesis, Deposition, and Temperature Dependence Studies of the Stokes Shift
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Lead Sulfide Quantum Dot Synthesis, Deposition, and Temperature Dependence Studies of the Stokes Shift Joanna S. Wang, Bruno Ullrich, and Gail J. Brown Air Force Research Laboratory, Materials & Manufacturing Directorate, Wright Patterson AFB, OH 45433-7707, USA ABSTRACT We investigated the temperature dependence of the Stokes shift of PbS quantum dots (diameter 4.7 nm) deposited from solution on glass using a specially designed apparatus. By measuring the thermal alteration of the optical absorbance and photoluminescence in the range of 5 K – 300 K, we demonstrate that the Stokes shift shrinks from 135 meV at 5 K to 62 meV at 300 K. Extrapolation of the data presented predict an elimination temperature of the Stokes shift of about 460 K, corresponding to the thermal energy of the sum of prominent PbS phonon energies. INTRODUCTION Lead Sulfide quantum dots (QDs) have attracted considerable attention owing to the narrow direct band gap (0.41 eV) and large exciton Bohr radius, providing an excellent system for studying quantum confinement effects [1]. However, despite these extensive studies there are still opportunities to further understanding of the physics of the optical properties associated with the confined states. One such topic is the understanding of the origin of the Stokes shift between the peaks of photoluminescence (PL) and optical absorbance (OA) of PbS QDs. The PL of PbS QDs has been studied intensively during the previous years, where particular attention was paid to the thermal shift of the emission spectra [2-5]. In addition, some of the works compare both the room temperature absorption [3,5,6] and the absorption at cryogenic temperatures [5,7,8] with the PL spectra and show a redshift of the PL with respect to the absorption edge. This separation is referred to as the Stokes shift and is usually attributed to multi-phonon driven energy relaxations into the dark excition state before luminescent transitions occur [9]. The influence of the size [7,9] and stoichiometry [9] on the Stokes shift was studied for PbS QDs but not its temperature dependence S(T). The analysis of S(T), which is of considerable importance for further understanding of the PL transition and the realization of technological applications including anti-Stokes cooling [10], is presented in this paper. EXPERIMENTAL DETAILS The PbS QDs used in this study were synthesized in organic solvents, with oleic acid as a capping agent, based on a method published in the literature [11]. The synthesis starts with the reaction of PbO with oleic acid to form lead oleate by heating oleic acid with PbO in a round bottom flask under Ar atmosphere while stirring. The temperature was kept at 150 οC for 1 hr. After this, a sulfide source, bis(trimethylsilyl) sulfide, was injected into the system and the reaction mixture was allowed to ripen for 1 hr at 100 οC. The PbS sample was then washed with
methanol, which removed the excesses of oleic acid and octadecene. The QDs were dried by a stream of N2 and then dissolved in toluene. The size of the PbS
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