Recently, graphene and other graphene-based materials have become an essential part of composite science and technology. Their unique properties are not only restricted to graphene but also shared with derivative compounds like graphene oxide, reduced graphene oxide, functionalized graphene, and so forth. One of the most structurally important materials, graphene oxide (GO), is prepared by the oxidation of graphite. Though removal of the oxide groups can create vacancies and structural defects, reduced graphene oxide (rGO) is used in composites as effective filler similar to GO. Authors developed a new polyurethane nanocomposite using a derivative of grapheme, thermally reduced graphene oxide (rGO), to modify the matrix of polyurethane elastomers, by rGO.
We present a systematic investigation of the electronic properties of bulk and few layer ReS2 van der Waals crystals using low temperature optical spectroscopy. Weak photoluminescence emission is observed from two non-degenerate band edge excitonic transitions separated by ∼ 20 meV. The comparable emission intensity of both excitonic transitions is incompatible with a fully thermalized (Boltzmann) distribution of excitons, indicating the hot nature of the emission. While DFT calculations predict bilayer ReS2 to have a direct fundamental band gap, our optical data suggests that the fundamental gap is indirect in all cases.Emerging transition metal dichalcogenides (TMDs) such as MoS 2 , MoSe 2 , MoTe 2 , WS 2 and WSe 2 are attracting great attention due to their remarkable electronic properties. In particular, the energy and the character of the band gap can be easily tuned by varying the number of atomic layers in the crystal. 1-5 The two dimensional confinement and reduced dielectric screening in the single and few layer limit result in significantly enhanced exciton and trion binding energies, 6-9 while the lack of inversion symmetry in the monolayer leads to valley-selective optical selection rules. [10][11][12][13] Recently, layered semiconductors with in-plane anisotropy such as black phosphorus, 14,15 and rhenium dichalcogenides (ReX 2 , where X stands for Se or S atoms) 16 have joined the family of intensively investigated van der Waals crystals. The sizable in-plane crystal asymmetry results in anisotropic optical, 3,5,[17][18][19][20][21][22][23][24][25] and electrical 17,23,[26][27][28] properties, which can be employed in field effect transistors, 29-32 polarization sensitive photodetectors, 33 and new plasmonic devices. 34 The major advantage of Re dichalcogenides over black phosphorus is their stability under ambient conditions, 35 making them potentially interesting for applications.Unlike the more extensively studied Mo and W based dichalcogenides, Re based TMDs crystallize in the distorted 1T' structure (schematically shown in Fig. 1(a)) of lower triclinic symmetry. 4,[36][37][38] In rhenium dichalcogenides, two non-degenerate direct excitons couple to light, as observed in the reflectivity contrast and photoluminescence (PL) spectra. 3,5,39 The strong linear polarization of the excitonic transitions provides a new degree of freedom to control the optical response 40,41 of this material. Despite the flurry of recent investigations of the ReS 2 and ReSe 2 , 16 knowledge about their fundamental electronic properties is extremely limited. For example, the nature of the fundamental band gap remains controversial.Existing band structure calculations provide no consensus concerning the nature of the fundamental band gap. 4,5,38,[42][43][44] Studies regarding the absorption edge indicate that both materials have an indirect band gap in the bulk form. [18][19][20][21]45 This assignment is supported by recent angle-resolved photoemission spectroscopy (ARPES) 44 and photoemission electron microscopy (PEEM) 4...
We report detailed optical studies of BaWO4:Ce and BaWO4:Ce,Na single crystals. The material does not emit any luminescence at ambient pressure under near-UV (325 nm) excitation. Efficient green light is emitted only at high pressure (HP) and low temperature (LT). The luminescence is of excitonic character, since the lowest Ce3+ 5d level is degenerate with the conduction band also under hydrostatic pressures. To explain these phenomena, absorption measurements were made together with powder X-ray diffraction (XRD) and confocal micro-Raman and Fourier transform infrared (FTIR) spectroscopy. Raman experiments confirm the existence of a metastable phase, induced by certain nonhydrostatic conditions, before the reversible transition at a high-pressure range above 9 GPa, where efficient photoluminescence (PL) occurs. Although the phase transition is reversible, it proceeds with a prominent hysteresis observed in luminescence and Raman experiments. FTIR focuses on the existence of Ce3+ multisites observed during LT measurements.
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