Stable Ultra-thin CdTe Crystal: A Robust Direct Gap Semiconductor

Employing density functional theory based calculations, we investigate structural, vibrational and strain-dependent electronic properties of an ultra-thin CdTe crystal structure that can be de- rived from its bulk counterpart. It is found that this ultra-thin crystal has an 8-atom primitive unit cell with considerable surface reconstructions. Dynamic stability of the structure is predicted based on its calculated vibrational spectrum. Electronic band structure calculations reveal that both electrons and holes in single layer CdTe possess anisotropic in-plane masses and mobilities. Moreover, we show that the ultra-thin CdTe has some interesting electromechanical features, such as strain-dependent anisotropic variation of the band gap value, and its rapid increase under per- pendicular compression. The direct band gap semiconducting nature of the ultra-thin CdTe crystal remains unchanged under all types of applied strain. With a robust and moderate direct band gap, single-layer CdTe is a promising material for nanoscale strain dependent device applications

Formation and diffusion characteristics of Pt clusters on Graphene, 1H-MoS2 and 1T-TaS2

Many experiments have revealed that the surfaces of graphene and graphene-like structures can play an active role as a host surface for clusterization of transition metal atoms. Motivated by these observations, we investigate theoretically the adsorption, diffusion and magnetic properties of Pt clusters on three different two-dimensional atomic crystals using first principles density functional theory. We found that monolayers of graphene, molybdenum disulfide (1H-MoS2) and tantalum disulfide (1T-TaS2) provide different nucleation characteristics for Pt cluster formation. At low temperatures, while the bridge site is the most favorable site where the growth of a Pt cluster starts on graphene, top-Mo and top-Ta sites are preferred on 1H-MoS2 and 1T-TaS2, respectively. Ground state structures and magnetic properties of Ptn clusters (n = 2,3,4) on three different monolayer crystal structures are obtained. We found that the formation of Pt2 dimer and a triangle-shaped Pt3 cluster perpendicular to the surface are favored over the three different surfaces. While bent rhombus shaped Pt4 is formed on graphene, the formation of tetrahedral shaped clusters are more favorable on 1H-MoS2 and 1T-TaS2. Our study of the formation of Ptn clusters on three different monolayers provides a gateway for further exploration of nanocluster formations on various surfaces.Flemish Science Foundation (FWO-Vl); Methusalem foundation of the Flemish government; FWO Pegasus Long Marie Curie Fellowshi

Nitrogenated, phosphorated and arsenicated monolayer holey graphenes

Motivated by a recent experiment that reported the synthesis of a new 2D material nitrogenated holey graphene (C$_2$N) [Mahmood \textit{et al., Nat. Comm.}, 2015, \textbf{6}, 6486], electronic, magnetic, and mechanical properties of nitrogenated (C$_2$N), phosphorated (C$_2$P) and arsenicated (C$_2$As) monolayer holey graphene structures are investigated using first-principles calculations. Our total energy calculations indicate that, similar to the C$_2$N monolayer, the formation of the other two holey structures are also energetically feasible. Calculated cohesive energies for each monolayer show a decreasing trend going from C$_2$N to C$_2$As structure. Remarkably, all the holey monolayers are direct band gap semiconductors. Regarding the mechanical properties (in-plane stiffness and Poisson ratio), we find that C$_2$N has the highest in-plane stiffness and the largest Poisson ratio among the three monolayers. In addition, our calculations reveal that for the C$_2$N, C$_2$P and C$_2$As monolayers, creation of N and P defects changes the semiconducting behavior to a metallic ground state while the inclusion of double H impurities in all holey structures results in magnetic ground states. As an alternative to the experimentally synthesized C$_2$N, C$_2$P and C$_2$As are mechanically stable and flexible semiconductors which are important for potential applications in optoelectronics

Highly Mobile Excitons in Single Crystal Methylammonium Lead Tribromide Perovskite Microribbons

Excitons are often given negative connotation in solar energy harvesting in part due to their presumed short diffusion lengths. We investigate exciton transport in single-crystal methylammonium lead tribromide (MAPbBr3) microribbons via spectrally, spatially, and temporally resolved photocurrent and photoluminescence measurements. Distinct peaks in the photocurrent spectra unambiguously confirm exciton formation and allow for accurate extraction of the low temperature exciton binding energy (39 meV). Photocurrent decays within a few μm at room temperature, while a gate-tunable long-range photocurrent component appears at lower temperatures (about 100 μm below 140 K). Carrier lifetimes of 1.2 μs or shorter exclude the possibility of the long decay length arising from slow trapped-carrier hopping. Free carrier diffusion is also an unlikely source of the highly nonlocal photocurrent, due to their small fraction at low temperatures. We attribute the long-distance transport to high-mobility excitons, which may open up new opportunities for novel exciton-based photovoltaic applications

Nitrogenated, phosphorated and arsenicated monolayer holey graphenes

Motivated by a recent experiment that reported the synthesis of a new 2D material nitrogenated holey graphene (C$_2$N) [Mahmood \textit{et al., Nat. Comm.}, 2015, \textbf{6}, 6486], electronic, magnetic, and mechanical properties of nitrogenated (C$_2$N), phosphorated (C$_2$P) and arsenicated (C$_2$As) monolayer holey graphene structures are investigated using first-principles calculations. Our total energy calculations indicate that, similar to the C$_2$N monolayer, the formation of the other two holey structures are also energetically feasible. Calculated cohesive energies for each monolayer show a decreasing trend going from C$_2$N to C$_2$As structure. Remarkably, all the holey monolayers are direct band gap semiconductors. Regarding the mechanical properties (in-plane stiffness and Poisson ratio), we find that C$_2$N has the highest in-plane stiffness and the largest Poisson ratio among the three monolayers. In addition, our calculations reveal that for the C$_2$N, C$_2$P and C$_2$As monolayers, creation of N and P defects changes the semiconducting behavior to a metallic ground state while the inclusion of double H impurities in all holey structures results in magnetic ground states. As an alternative to the experimentally synthesized C$_2$N, C$_2$P and C$_2$As are mechanically stable and flexible semiconductors which are important for potential applications in optoelectronics