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Register a new userVariation of the fundamental band gap nature in curved two-dimensional WS2
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We report a strain-induced direct-to-indirect band gap transition in mechanically deformed WS2 monolayers (MLs). The necessary amount of strain is attained by proton irradiation of bulk WS2 and the ensuing formation of one-ML-thick, H2-filled domes. The electronic properties of the curved MLs are mapped by spatially- and time-resolved micro-photoluminescence revealing the mechanical stress conditions that trigger the variation of the band gap character. This general phenomenon, also observed in MoS2 and WSe2, further increases our understanding of the electronic structure of transition metal dichalcogenide MLs and holds a great relevance for their optoelectronic applications.
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In atomically thin transition metal dichalcogenide semiconductors, there is a crossover from indirect to direct bandgap as the thickness drops to one monolayer, which comes with a fast increase of the photoluminescence signal. Here, we show that for different alloy compositions of WS2(1-x)Se2x this trend may be significantly affected by the alloy content and we demonstrate that the sample with the highest Se ratio presents a strongly reduced effect. The highest micro-PL intensity is found for bilayer WS2(1-x)Se2x (x = 0.8) with a decrease of its maximum value by only a factor of 2 when passing from mono- to bi-layer. To better understand this factor and explore the layer-dependent band structure evolution of WS2(1-x)Se2x, we performed a nano-angle resolved photoemission spectroscopy study coupled with first-principles calculations. We find that the high micro-PL value for bilayer WS2(1-x)Se2x (x = 0.8) is due to the overlay of direct and indirect optical transitions. This peculiar high PL intensity in WS2(1-x)Se2x opens the way for spectrally tunable light-emitting devices.
We have predicted a new phase of nitrogen with octagon structure in our previous study, which we referred to as octa-nitrogene (ON). In this work, we make further investigation on its electronic structure. The phonon band structure has no imaginary phonon modes, which indicates that ON is dynamically stable. Using ab initio molecular dynamic simulations, the structure is found to stable up to 100K, and ripples that are similar to that of graphene is formed on the ON sheet. Based on DFT calculation on its band structure, single layer ON is a 2D large-gap semiconductor with a band gap of 4.7eV. Because of inter-layer interaction, stackings can decrease the band gap. Biaxial tensile strain and perpendicular electric field can greatly influence the band structure of ON, in which the gap decreases and eventually closes as the biaxial tensile strain or the perpendicular electric field increases. In other words, both biaxial tensile strain and perpendicular electric field can drive the insulator-to-metal transition, and thus can be used to engineer the band gap of ON. From our results, ON has potential applications in the electronics, semiconductors, optics and spintronics, and so on.
An analysis of the optical response of a triangular-shaped photonic band-gap prism is presented. Numerical simulations have been performed in the framework of multiple-scattering theory, which is applied considering spot illumination to avoid diffraction effects. First of all, refractive properties in the frequency range below the first TM band-gap are analyzed and compared with the available experimental data. It validates the approach employed and supports the predictions obtained in the frequency range above the gap. At these high frequencies we found an unusual superprism effect characterized by an angle- and frequency-sensitivity of the intensity of outgoing beams. We report several representative examples that could be used in device applications. The results are interpreted in terms of the corresponding semi-infinite photonic crystal, through the analysis of the coupling between external radiation and bulk eigenmodes, using the 2D Layer- Korringa-Kohn-Rostoker method. The procedure presented here constitutes a simple but functional alternative to the methods used until now with the same purpose.
We demonstrate the crossover from indirect- to direct band gap in tensile-strained germanium by temperature-dependent photoluminescence. The samples are strained microbridges that enhance a biaxial strain of 0.16% up to 3.6% uniaxial tensile strain. Cooling the bridges to 20 K increases the uniaxial strain up to a maximum of 5.4%. Temperature-dependent photoluminescence reveals the crossover to a fundamental direct band gap to occur between 4.0% and 4.5%. Our data are in good agreement with new theoretical computations that predict a strong bowing of the band parameters with strain.
By means of ab initio calculations we investigate the possibility of existence of a boron nitride (BN) porous two-dimensional nanosheet which is geometrically similar to the carbon allotrope known as biphenylene carbon. The proposed structure, which we called Inorganic Graphenylene (IGP), is formed spontaneously after selective dehydrogenation of the porous Boron Nitride (BN) structure proposed by Ding et al. We study the structural and electronic properties of both porous BN and IGP and it is shown that, by selective substitution of B and N atoms with carbon atoms in these structures, the band gap can be significantly reduced, changing their behavior from insulators to semiconductors, thus opening the possibility of band gap engineering for this class of two-dimensional materials.
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