Topic
Direct and indirect band gaps
About: Direct and indirect band gaps is a research topic. Over the lifetime, 10940 publications have been published within this topic receiving 327355 citations.
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TL;DR: In this article, the authors describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL) for flexible/transparent/low-energy optoelectronic devices.
Abstract: Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from their bulk layered counterpart. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). Bulk WS2 does not present PL due to its indirect band gap nature. The edges of these monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than the platelets center. The structure and composition of the platelet edges appear to be critical for the PL enhancement effect. Electron diffraction revealed that platelets present zigzag edges, while first-principles calculations indicate that sulfur-rich zigzag WS2 edges possess metallic edge states, which might tailor the optical response reported here. These novel 2D nanoscale light sources could find diverse applications including the fabrication of flexible/transparent/low-energy optoelectronic devices.
1,023 citations
TL;DR: In this article, a new three-parameter fit to the temperature dependence of semiconductor band gaps was proposed, based on the semi-empirical Varshni equation.
Abstract: In this letter we advocate the use of a new three-parameter fit to the temperature dependence of semiconductor band gaps. This fitting improves upon the semi-empirical Varshni equation* both numerically, since it gives better fits to the data, and theoretically, since the parameters of the fit may be related to an intrinsic interaction of semiconductors, namely the electron-phonon coupling. Similar expressions to ours have appeared in the literature2T3 but the practical and theoretical justification of this kind of data fit have not previously been worked out in detail. We emphasize that our approach is empirical: we aim simply to describe the data as well as possible with the minimum number of free parameters. The Varshni relation for the temperature dependence of semiconductor band gaps is Eg(T)=Eo--cYT2/(T+pA (1)
1,010 citations
TL;DR: In this paper, the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si atoms were analyzed.
Abstract: We report calculations of the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si atoms. The electric field produces a tunable band gap in the Dirac-type electronic spectrum, the gap being suppressed by a factor of about eight by the high polarizability of the system. At low electric fields, the interplay between this tunable band gap, which is specific to electrons on a honeycomb lattice, and the Kane-Mele spin-orbit coupling induces a transition from a topological to a band insulator, whereas at much higher electric fields silicene becomes a semimetal.
969 citations
TL;DR: The double perovskites Cs2AgBiBr6 and Cs 2AgBiCl6 have been synthesized from both solid state and solution routes, and X-ray diffraction measurements reveal band gaps of 2.19 eV and 2.77 eV as discussed by the authors.
Abstract: The double perovskites Cs2AgBiBr6 and Cs2AgBiCl6 have been synthesized from both solid state and solution routes. X-ray diffraction measurements show that both compounds adopt the cubic double perovskite structure, space group Fm3m, with lattice parameters of 11.2711(1) A (X = Br) and 10.7774(2) A (X = Cl). Diffuse reflectance measurements reveal band gaps of 2.19 eV (X = Br) and 2.77 eV (X = Cl) that are slightly smaller than the band gaps of the analogous lead halide perovskites, 2.26 eV for CH3NH3PbBr3 and 3.00 eV for CH3NH3PbCl3. Band structure calculations indicate that the interaction between the Ag 4d-orbitals and the 3p/4p-orbitals of the halide ion modifies the valence band leading to an indirect band gap. Both compounds are stable when exposed to air, but Cs2AgBiBr6 degrades over a period of weeks when exposed to both ambient air and light. These results show that halide double perovskite semiconductors are potentially an environmentally friendly alternative to the lead halide perovskite semico...
958 citations
TL;DR: In this work, the band structure, density of states, and effective mass of photogenerated charge carriers for anatase, rutile and brookite TiO2 are investigated by the first-principle density functional theory calculation and it is indicated that anatase appears to be an indirect band gap semiconductor, while rutiles and Brookite belong to the direct band gap chip category.
Abstract: In general, anatase TiO2 exhibits higher photocatalytic activities than rutile TiO2. However, the reasons for the differences in photocatalytic activity between anatase and rutile are still being debated. In this work, the band structure, density of states, and effective mass of photogenerated charge carriers for anatase, rutile and brookite TiO2 are investigated by the first-principle density functional theory calculation. The results indicate that anatase appears to be an indirect band gap semiconductor, while rutile and brookite belong to the direct band gap semiconductor category. Indirect band gap anatase exhibits a longer lifetime of photoexcited electrons and holes than direct band gap rutile and brookite because the direct transitions of photogenerated electrons from the conduction band (CB) to valence band (VB) of anatase TiO2 is impossible. Furthermore, anatase has the lightest average effective mass of photogenerated electrons and holes as compared to rutile and brookite. The lightest effective mass suggests the fastest migration of photogenerated electrons and holes from the interior to surface of anatase TiO2 particle, thus resulting in the lowest recombination rate of photogenerated charge carriers within anatase TiO2. Therefore, it is not surprising that anatase usually shows a higher photocatalytic activity than rutile and brookite. This investigation will provide some new insight into understanding the difference of photocatalytic activity among anatase, rutile and brookite.
920 citations