Clear evidence for two emitting states in PbSe nanocrystals (NCs) has been observed. The flow of population between these two states as temperature increases is interrupted by the presence of nonradiative trap states correlated with the exposure of the NC film to air. Quenching of the higher-energy emission begins after only seconds of exposure, with the effect saturating after several days. Unlike short-term oxygen-related effects in solution, the emission quenching appears to be irreversible, signaling a distinction between surface reactivity in NCs in films and that in solution. The origin of the two emissive centers and the impact of trapping on other NC film properties (e.g., electron/hole mobilities) remain important issues to be resolved.
L ead chalcogenide (PbX) quantum dots (QDs) are promising candidates for high efficiency, low cost photovoltaics (PV) due to their size-tunable band gap and the recent observation of efficient multiple exciton generation (MEG). 1,2 The successful integration of lead chalcogenide QDs into PV requires a detailed understanding of the energy relaxation pathways that control charge carrier populations following photoexcitation. Electron-phonon coupling can be extremely efficient in such highly quantum-confined systems, and electron-phonon interactions likely affect many dynamic processes such as intraband relaxation of excited states and radiative recombination. 3-6 It is therefore crucial to understand the detailed phonon spectra for QDs to appreciate the relevant energy scales that influence these electron-phonon interactions. In addition, QD films to be utilized in solar cells may be processed in the presence or absence of oxygen. Recent studies have suggested remarkably different electrical and optical properties of QD films depending on the degree of air exposure, but in most cases the detailed mechanisms underlying these observations are not known. 7-10 Thus, there is currently not a full understanding of the primary oxidation and photooxidation products, and how these species affect fundamental photophysical events such as photoluminescence.Lead chalcogenides possess the centro-symmetric rock salt crystal structure, which belongs to the m3m space group or O h point group. First-order Raman scattering from an ideal rocksalt lattice is forbidden due to the center of inversion symmetry, although some theoretical predictions suggest that higher order scattering should be weakly allowed. 11 Despite this restriction, several Raman studies of lead chalcogenide QDs have yielded strong peaks that are often attributed to first-order Raman modes. 12,13 However, little attention is typically given to handling the sample in the absence of oxygen, an important consideration for a material set that is severely prone to oxidation. 14-18 Thus, as some recent reports have pointed out, many oxide-related peaks may be mistakenly attributed to intrinsic PbX phonons. 17,19 A systematic study of the effects of oxidation on a series of lead chalcogenide QDs is warranted in order to better understand the intrinsic QD vibrational spectra and how these spectra are used to interpret processes such as radiative recombination. 12 In this report, we carefully study films of three lead chalcogenide QD samples (PbS, PbSe, and PbTe) in both ambient conditions and in rigorously air-free conditions, which allows us to unambiguously identify oxide-related Raman peaks. We find a number of Raman peaks in air-exposed PbX QD samples that can be attributed to lead oxide (PbO) and chalcogenate peaks (PbXO 4 or PbO 3 PbXO 4 ), some of which have been assigned to intrinsic PbX phonon modes in previous reports. 12,13 In air-free PbX QD samples, these oxide-related peaks are completely absent, and the Raman spectra contain no peaks that could be assigned to...
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