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Register a new userDirect Band Gap Semiconducting Holey Graphyne: Structure, Synthesis and Potential Applications
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Here we report two-dimensional (2D) single-crystalline holey-graphyne (HGY) created an interfacial two-solvent system through a Castro-Stephens coupling reaction from 1,3,5-tribromo-2,4,6-triethynylbenzene. HGY is a new type of 2D carbon allotrope whose structure is comprised of a pattern of six-vertex and eight-vertex rings. The carbon-carbon 2D network of HGY is alternately linked between benzene rings and sp (carbon-carbon triple bond) bonding. The ratio of the sp over sp2 bonding is 50%. It is confirmed that HGY is stable by DFT calculation. The vibrational, optic, and electric properties of HGY are investigated theoretically and experimentally. It is a p-type semiconductor that embraces a natural direct band gap (~ 1.0 eV) with high hole mobility and electron mobility at room temperature. This report is expected to help develop a new types of carbon-based semiconductor devices with high mobility.
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The monolayer Gallium sulfide (GaS) was demonstrated as a promising two-dimensional semiconductor material with considerable band gaps. The present work investigates the band gap modulation of GaS monolayer under biaxial or uniaxial strain by using Density functional theory calculation. We found that GaS monolayer shows an indirect band gap that limits its optical applications. The results show that GaS monolayer has a sizable band gap. The uniaxial strain shifts band gap from indirect to direct in Gallium monochalcogenides (GaS). This behavior, allowing applications such as electroluminescent devices and laser. The detailed reasons for the band gap modulation are also discussed by analyzing the projected density of states (PDOS). It indicates that due to the role of p$_y$ orbital through uniaxial strain become more significant than others near the Fermi level. The indirect to direct band gap transition happen at $varepsilon$=-10y$%$. Moreover, by investigating the strain energy and transverse response of structures under uniaxial strain, we show that the GaS monolayer has the Poissons ratio of 0.23 and 0.24 in the zigzag (x) and armchair (y) directions, respectively. Thus, we conclude that the isotropic nature of mechanical properties under strain.
Holey graphyne (HGY), a novel 2D single-crystalline carbon allotrope, was synthesized most recently by Castro-Stephens coupling reaction. The natural existing uniform periodic holes in the 2D carbon-carbon network demonstrate its tremendous potential application in the area of energy storage. Herein, we conducted density functional theory calculation to predict the hydrogen storage capacity of HGY sheet. Its found the Li-decorated single-layer HGY can serve as a promising candidate for hydrogen storage. Our numerical calculations demonstrate that Li atoms can bind strongly to the HGY sheet without the formation of Li clusters, and each Li atom can anchor four H2 molecules with the average adsorption energy about -0.22 eV/H2. The largest hydrogen storage capacity of the doped HGY sheet can arrive as high as 12.8 wt%, this value largely surpasses the target of the U. S. Department of Energy (9 wt%), showing the Li/HGY complex is an ideal hydrogen storage material at ambient conditions. In addition, we investigate the polarization mechanism of the storage media and and find that the polarization stemed from both the electric field induced by the ionic Li decorated on the HGY and the weak polarized hydrogen molecules dominated the H2 adsorption process.
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.
The millimeter sized monolayer and bilayer 2H-MoTe2 single crystal samples are prepared by a new mechanical exfoliation method. Based on such high-quality samples, we report the first direct electronic structure study on them, using standard high resolution angle-resolved photoemission spectroscopy (ARPES). A direct band gap of 0.924eV is found at K in the rubidium-doped monolayer MoTe2. Similar valence band alignment is also observed in bilayer MoTe2,supporting an assumption of a analogous direct gap semiconductor on it. Our measurements indicate a rather large band splitting of 212meV at the valence band maximum (VBM) in monolayer MoTe2, and the splitting is systematically enlarged with layer stacking, from monolayer to bilayer and to bulk. Meanwhile, our PBE band calculation on these materials show excellent agreement with ARPES results. Some fundamental electronic parameters are derived from the experimental and calculated electronic structures. Our findings lay a foundation for further application-related study on monolayer and bilayer MoTe2.
Lanthanum nitride (LaN) has attracted research interest in catalysis due to its ability to activate the triple bonds of N$_2$ molecules, enabling efficient and cost-effective synthesis of ammonia from N$_2$ gas. While exciting progress has been made to use LaN in functional applications, the electronic character of LaN (metallic, semi-metallic, or semiconducting) and magnitude of its band gap have so far not been conclusively determined. Here, we investigate the electronic properties of LaN with hybrid density functional theory calculations. In contrast to previous claims that LaN is semi-metallic, our calculations show that LaN is a direct-band-gap semiconductor with a band-gap value of 0.62 eV at the X point of the Brillouin zone. The dispersive character of the bands near the band edges leads to light electron and hole effective masses, making LaN promising for electronic and optoelectronic applications. Our calculations also reveal that nitrogen vacancies and substitutional oxygen atoms are two unintentional shallow donors with low formation energies that can explain the origin of the previously reported electrical conductivity. Our calculations clarify the semiconducting nature of LaN and reveal candidate unintentional point defects that are likely responsible for its measured electrical conductivity.
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