Relaxation Processes and Exciton-Phonon Interactions in Nanocomposites Based on CdSe/ZnS Semiconductor Quantum Dots and
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RELAXATION PROCESSES AND EXCITON-PHONON INTERACTIONS IN NANOCOMPOSITES BASED ON CdSe/ZnS SEMICONDUCTOR QUANTUM DOTS AND PORPHYRIN MOLECULES E. I. Zenkevich,1 C. Von Borczyskowski,2 D. R. T. Zahn2
UDC 535.373 + 539.2 + 541.14
Using spectral-kinetic data obtained for nanoassemblies based on CdSe/ZnS semiconductor quantum dots and tetrapyridylporphyrin molecules in methylcyclohexane-toluene mixture 6:1, it was justified that the temperature lowering is accompanied by conformational transformation of the surface ligand (trioctylphosphine oxide or amine) layer in individual quantum dots, which is accelerated in nanoassemblies. It has been shown for the CdSe/ZnS quantum dots that he states determining the absorption band of the first exciton transition, on the one hand, and the photoluminescence band, on the other hand, have different natures. In the frames of existing models for exciton-phonon coupling in semiconductor nanostructures, an analysis has been carried out for the temperature dependence (77–293 K) of quantum dot absorption and photoluminescence properties taking into account conformational effects. It has been established that the formation of the absorption band for the first exciton transition takes place with participation of CdSe core LO phonons, while photoluminescence properties reflect also additional interactions with ZnS shell LO phonons. Keywords: semiconductor quantum dots, porphyrin, nanoassemblies, temperature dependences of the absorption and photoluminescence spectra, exciton-phonon interactions, phase reorganization of a stabilizing ligand layer.
INTRODUCTION Nowadays inhomogeneous nanoasseblies based on colloid semiconductor quantum dots (QDs) of different nature and morphology in combination with functional molecules of organic dyes are widely investigated and considered to be nanostructures promising for their possible applications in nanosensorics, photovoltaics, and biomedicine [1–3]. At the same time, despite considerable successes in this area, the main problem in the development of effective nanomaterials based on semiconductor QDs and organic dyes is lacking of detailed information on the properties of the QD surface, specifics of its interactions with functional ligands in solutions, and the influence of these interactions on the structure of energy levels and processes of exciton relaxation [2, 4, 5]. In addition, for the QDs under conditions of three-dimensional quantum limitations, not only electronic energy levels, but also vibrational modes of the lattice (phonons) are discrete [6–9]. Accordingly, a study of exciton-phonon interactions under conditions of quantum limitations is obviously important both for solving fundamental (a study of the structure of electronic states, dynamics of exciton relaxation, etc.) and applied problems. As a rule, these problems are experimentally solved for QDs by two methods: 1) Raman spectroscopy, that is, measuring the ratio of intensities of the phononless lines and the phonon wings [10, 11], and 2) investigation of the temperature dependence
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