Abstract
Insertion of halogens such as bromine or iodine affects the electronic polarizability of ions and the local field inside the medium, and thus modifies the refractive index. Acquiring precise knowledge of the dispersion of refractive index and ultimately tailoring conventional semiconductors for wide-range refractive index control have been a vital issue to resolve before realizing advanced organic optoelectronic devices. In this report, dispersions of the refractive index of a single crystal tetramethyltetraselenafulvalene [C10H12Se4] (TMTSF) are thoroughly studied from broadband interference modulations of photoluminescence (PL) spectra at various temperatures and doping levels. A large enhancement of the refractive index, more than 20% of the intrinsic value, is achieved with inclusion of a small composition of iodide ions, while the structural and optical properties remain mostly intact. Nearly temperature independent dispersion of the refractive index suggests that, unlike most polymers in which the thermal expansion coefficient dominates over the change of polarizability with temperature, the latter enhances significantly and may become more or less comparable to the thermal expansion coefficient given by 1.71 × 10−4/K, when single crystal TMTSF is doped by iodine.
Highlights
Organic crystals of small conjugated molecules, together with conjugated polymers, have attracted much attention because of their fundamental scientific interest and impressive improvements in performance in wide range of organic material based devices, such as organic field effect transistors, organic solar cells, organic light emitting diode displays, organic single crystal lasers[1,2,3,4]
The dispersions of the refractive index of a small molecular semiconductor tetramethyltetraselenafulvalene [C10H12Se4] (TMTSF) [C10H12Se4] and their temperature variations are thoroughly studied from the broadband interference modulation of PL spectra in which a pair of the parallel crystal facets takes a role as a cavity resonator of the Farby-Perot type
The dispersion of the refractive index and its temperature dependences of organic single crystal tetramethyltetraselenafulvalene (TMTSF) are systematically studied from the broadband interference modulation of photoluminescence (PL) spectra, which is essentially the emission spectra coupled to the longitudinal cavity multimode formed naturally between a pair of the parallel crystal facets
Summary
Data from the thick sample with a better resolution are displayed in the main panel in which an average uncertainty ±0.023 is given in the estimate of the refractive indices It is not clear at the moment whether the dispersions of the refractive index shift downward with increasing temperature, as expected for most polymers. It must be associated with the enhancement of the refractive index which would effectively prevent the emissions from leaking out through the surface due to smaller critical angle for the total internal reflection. With the added means to tune and monitor the optical characteristics that we have shown, we expect that the TMTSF single crystal should serve as a new benchmark system succeeding pentacene and rubrene semiconductors, both of which have provided significant insights into the fundamental understanding of organic semiconductors
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