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Polycrystalline Cu(InGa)Se 2 (CIGS) thin film is a well-known light absorber material for photovoltaic solar cells. However, the mechanism of the benignity of its grain boundary (GB) remains in dispute. By using a combination of experimental techniques, we have systematically investigated the compositions, structures, and properties of the grain interior (GI) and GB of the polycrystalline CIGS films at nanometer scale. A definitive Cu deficient layer with thickness depending on the overall Cu content of the films was observed near the non Σ3 GBs with distinctly different characteristics from GIs. The band alignment between GB and GI was discovered to be of type II with downward offset for both conduction and valence bands at GB, well correlating to the local copper deficiency and structure. Our findings expressively support the type inversion and large hole barrier in this GB layer, and establish a comprehensive mechanism for the benignity of the CIGS grain boundaries towards carrier recombination. [ABSTRACT FROM AUTHOR]

We report the synthesis and characterization of a novel class of ionic semiconductor materials – inorganic chalcogenide perovskites. Several different compounds including BaZrS 3 , CaZrS 3 , SrTiS 3 and SrZrS 3 were synthesized by high temperature sulfurization of their oxide counterparts. Their crystal structures were identified by XRD and composition by EDX. UV–vis and photoluminescence measurements confirmed that they are direct gap semiconductors with band gap values consistent with theoretical predictions. By adopting an anion alloying approach, we demonstrate widely tunable band gap from 1.73 eV to 2.87 eV. These strongly ionic semiconductors provide a new avenue for engineering the semiconducting properties for applications such as energy harvesting, solid state lighting and sensing. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO), the functionalized graphene with oxygen-containing chemical groups, has recently attracted resurgent interests because of its superior properties such as large surface area, mechanical stability, tunable electrical and optical properties. Moreover, the surface functional groups of hydroxyl, epoxy and carboxyl make GO an excellent candidate in coordinating with other materials or molecules. Owing to the expanded structural diversity and improved overall properties, GO and its composites hold great promise for versatile applications of energy storage/conversion and environment protection, including hydrogen storage materials, photocatalyst for water splitting, removal of air pollutants and water purification, as well as electrode materials for various lithium batteries and supercapacitors. In this review, we present an overview on the current successes, as well as the challenges, of the GO-based materials for energy and environmental applications. [ABSTRACT FROM AUTHOR]

Abstract: Chemical reactions involve competing pathways. Here, two pathways for H desorption from MgH2 are identified by first-principles calculations: One involves H diffusion in bulk Mg, while the other involves H vacancy diffusion at the MgH2 surface. The latter is sensitive to the size of the reactant MgH2 and self-terminates in bulk material as dehydrogenation eventually eliminates exposed MgH2. However, this surface vacancy pathway can maximize the catalyst effect of Pd by decoupling the kinetics of H desorption from that of H diffusion. When the surface-to-bulk ratio is large as in the case of MgH2 nanostructures, H desorption will take place primarily via the low-barrier surface vacancy pathway. Our picture attributes the experimentally observed size effect of MgH2 on H desorption, beyond the quantum size regime, to a synergy between the nanosize of the reactant and the catalyst. [Copyright &y& Elsevier]

Owing to its superior visible light absorption and high chemical stability, chalcogenide perovskite barium zirconium sulfide (BaZrS 3) has attracted significant attention in the past few years as a potential alternative to hybrid halide perovskites for optoelectronics. However, the high processing temperatures of BaZrS 3 thin films at above 1000 ℃ severely limits their potential for device applications. Herein, we report the synthesis of BaZrS 3 thin films at temperatures as low as 500 ℃, by changing the chemical reaction pathway. The single phase BaZrS 3 thin film was confirmed by X-ray diffraction and Raman spectroscopy. Atomic force microscopy and scanning electron microscopy show that crystalline size and surface roughness were consistently reduced with decreasing annealing temperature. The lower temperatures further eliminate sulfur vacancies and carbon contaminations associated with high temperature processing. The ability to synthesize chalcogenide perovskite thin films at lower temperatures removes a major hurdle for their device fabrication. The photodetectors demonstrate fast response and an on/off ratio of 80. The fabricated field effect transistors show an ambipolar behavior with electron and hole mobilities of 16.8 cm2/Vs and 2.6 cm2/Vs, respectively. [Display omitted] • Lowered the growth temperature of BaZrS 3 thin films by 500 °C. • Lowered carrier density by 10 orders of magnitude by reducing vacancy formation. • Realized photodetector and FET devices. • Ambipolar behavior with electron and hole mobilities of 16.8 cm2/Vs and 2.6 cm2/Vs, respectively. [ABSTRACT FROM AUTHOR]

BaZrS 3 is a prototypical chalcogenide perovskite, an emerging class of unconventional semiconductor. Recent results on powder samples reveal that it is a material with a direct band gap of 1.7–1.8 eV, a very strong light-matter interaction, and a high chemical stability. Due to the lack of quality thin films, however, many fundamental properties of chalcogenide perovskites remain unknown, hindering their applications in optoelectronics. Here we report the fabrication of BaZrS 3 thin films, by sulfurization of oxide films deposited by pulsed laser deposition. We show that these films are n-type with carrier densities in the range of 1019-1020 cm−3. Depending on the processing temperature, the Hall mobility ranges from 2.1 to 13.7 cm2/Vs. The absorption coefficient is > 105 cm−1 at photon energy >1.97 eV. Temperature dependent conductivity measurements suggest shallow donor levels. By assuring that BaZrS 3 is a promising candidate, these results potentially unleash the family of chalcogenide perovskites for optoelectronics such as photodetectors, photovoltaics, and light emitting diodes. Image 1 • The first fabrication of high quality chalcogenide perovskite BaZrS 3 thin films, opening the door for device applications. • Exceptionally high absorption coefficient higher than 105 cm-1 at photon energy greater than 1.97 eV. • n-type doping with carrier mobility greater than 10 cm2/Vs. • Shallow defect level. [ABSTRACT FROM AUTHOR]

[GeTe] m -[Sb 2 Te 3 ] n superlattice (GST-SL) is a promising material candidate to solve the critical problem of high-power consumption for phase-change memory technology. However, the switching mechanism is still under strong debate during the last decade. A key controversial question is that whether melting in GST-SL is possible. In this work, a large-scale machine learning interatomic potential (MLIP) molecular dynamics (MD) simulation with a one-order-of-magnitude larger atom number than that of conventional density functional theory (DFT) MD captures a unique partial melting behavior in GST-SL, where a sparse-nucleation-and-growth governed melting behavior is triggered by the flipping of atoms into van der Waals (vdW) gaps and forming cation-in-vdW-gap (CiV) defect as starting point, and then is slowly developed via propagating liquid-crystalline interfaces. In great contrast, the traditional melting of rock salt (RS)-GST occurs very fast due to instant homogeneous nucleation from high-density intrinsic vacancies distributed randomly in its cubic lattice. Therefore, the melting regions in GST-SL can be spatially localized in form of partial melting and are readily controlled. Moreover, the atomic distribution after melting in GST-SL is more chemical ordering than that in RS-GST. By the large-scale MLIP-MD, the present study provides a critical atomic insight into GST-SL phase-change behaviors that conventional DFT-MDs hardly achieve. This partial-melting nature in GST-SL helps explain the long-term-confused working principle of GST-SL-based phase-change memory, which will accelerate its application in the big-data era. [Display omitted] [ABSTRACT FROM AUTHOR]

Phase-change memory (PCM) material is the promising material system for nonvolatile-memory technology. Performance optimization of PCM device urgently requires the deeper clarification of its material “Gene”. In this study, through first-principles calculations, p -orbital-aligned atom chains are identified to play important roles in governing optoelectronic reflectivity in amorphous Ge 2 Sb 2 Te 5 . These atom chains make the electronic state of the amorphous Ge 2 Sb 2 Te 5 hold strong electron-polarized components, thereby governing the optical property. The present study offers a new understanding of “Gene” for PCM materials which benefit the material design and the performance improvement of PCM devices. [ABSTRACT FROM AUTHOR]

GeSbTe alloys have the ability of rapidly transforming between amorphous and crystalline phases. Therefore, they can be used in the non-volatile phase change memory. Recently, a vacancy-ordered cubic Ge 2 Sb 2 Te 5 ( VOC GST) phase change material where the vacancies are highly ordered in the (111) plane, has been experimentally demonstrated by STEM. However, studies are mainly on the structural characterization, rather than on the phase change behavior and possible applications of the VOC GST. Here, using first-principles molecular dynamic simulations, we study the melt-quenched amorphization process and its possible applications. We find that the VOC GST exhibits a quasi-two-dimensional amorphization process that is triggered by the diffusion of Ge atoms but not others. A partial amorphous ( P-amor ) phase is obtained, which can act as an intermediate state between the pure amorphous and pure crystalline phases for possible ternary-state data storage. [ABSTRACT FROM AUTHOR]

Spontaneous construction of graphene on boron-doped diamond (111) surface has been found in first-principles calculations. Boron-doping-induced surface reconstruction is the mechanism responsible for the diamond-to-graphene phase transition. Furthermore, the effect on the surface is unexpectedly observed at any doping depth down to the 7th layer, and a low concentration of substituent boron – only ¼ – is effective for graphene formation. Amazingly, when boron atoms are incorporated into the fifth layer, the direct optimization of the 1 × 1 surface automatically gives rise to a graphene structure from the first bilayer with no energy barrier, indicating that the formation of graphene is spontaneous. These findings provide an alternative strategy for graphene synthesis on wide-gap insulators with high thermal conductivity. [ABSTRACT FROM AUTHOR]

Sulfur-doped anatase TiO 2 nanoparticles with sulfur atoms occupying oxygen sites were realized by low-temperature hydrothermal oxidation of titanium disulfide (TiS 2 ) with deionized water as the solvent. S doping can effectively narrow the bandgap of TiO 2 due to the fact that S 3 p level is higher than O 2p orbital. The large surface area caused by this doping method induces higher photocatalytic activity in the visible regime compared with S-TiO 2 oxidized at high temperature. The low-temperature hydrothermal oxidation of TiS 2 in pH neutral solvent is an environmental-friendly method to obtain S-TiO 2 with high photocatalytic activity, which can be extended to other nonmetal doping of TiO 2 or other semiconductors for bandgap engineering. [ABSTRACT FROM AUTHOR]

The repercussions of an inaccurate account of electronic states near the Fermi level by hybrid functionals in predicting several important metallic properties are investigated. The difficulties include a vanishing or severely suppressed density of states (DOS) at E F , significantly widened valence bandwidth, greatly enhanced electron–phonon (el–ph) deformation potentials, and an overestimate of magnetic moment in transition metals. The erroneously enhanced el–ph coupling calculated by hybrid functionals may lead to a false prediction of lattice instability. The main culprit of the problem comes from the simplistic treatment of the exchange functional rooted in the original Fock exchange energy. The use of a short-ranged Coulomb interaction alleviates some of the drawbacks but the fundamental issues remain unchanged. [ABSTRACT FROM AUTHOR]

The Dirac bands and flat bands are difficult to coexist because they represent two extreme ends of electronic properties. However, in this paper, we propose a carbon-Kagome-lattice (CKL) family based on first-principles calculations, and the coexistence of Dirac and flat bands are observed in this series of three-dimensional carbon structures. The flat bands are originated from the orbital interactions of the Kagome lattices, while the Dirac bands are related to the carbon zigzag chains. A tight-binding model is used to explain the various band structures in different CKLs. The coexistence of flat and Dirac bands around the Fermi level implies that CKL structures maybe can serve as superconductors. In addition, electronic properties of the thinnest CKL slabs, only consisting of benzene rings, are studied. Flat bands are found in the band spectra of the two-dimensional structures, and split into spin-up and spin-down bands because of strong correlated effect in the case of hole doping. [ABSTRACT FROM AUTHOR]

Cube-textured Ni(001) foils have been considered as a viable alternative substrate to grow high quality functional films for large area optoelectronic devices. In this work, we report the heteroepitaxial growth of CaF 2 (001) films on cube-textured Ni(001) foils at 350–600 °C with in-plane orientation of CaF 2 [110]//Ni[100] and CaF 2 [ 1 - 10 ]//Ni[010] with 45 ° rotation respect to the Ni(001) substrate. Unlike CaF 2 (111)/Ni(001) films where there exist four independent rotational domains with rotational domain boundaries, CaF 2 (001)/Ni(001) contains no rotational domains or rotational domain boundaries. This makes CaF 2 (001)/Ni(001) films better candidates as templates for the growth of high quality functional semiconductors. We also demonstrate that Ge(001) film with no rotational domains and with a grain size of ~ 50 μm similar to that of the Ni substrate can be grown on the CaF 2 (001) buffered Ni substrate. [ABSTRACT FROM AUTHOR]

Ge 1 Cu 2 Te 3 is an important candidate for high-temperature phase change memory due to the fine amorphous stability. Yet, the basic bonding chemistry for its high-temperature application is still not completely clear. In this work, a new bonding mechanism for its amorphous and crystalline phases is proposed and demonstrated by first-principles calculations. Compared to the tetrahedral environment distributed evenly in crystalline form, Cu atoms in the amorphous state tend to be accumulated as trigonal clusters. For the crystalline phase, a bonding configuration of nonequivalent sp 3 hybridization with Te lone-pair electrons is proposed without Cu d electron participation. In the amorphous phase, however, a significant bonding reconfiguration of Cu d electrons occurs due to the isolation of the Te lone-pair electrons. Therefore, the notable contrast in the Cu atomic and electronic structures between the crystalline and amorphous phases results in an obvious phase transition barrier for high-temperature storage. The mechanism presented in this study serves as a reference for other transition-metal alloyed phase-change materials. [ABSTRACT FROM AUTHOR]

SnO 2 is one of the most interesting oxide semiconductors due to its wide band gap, good transparency, high thermal/chemical resistances, and low cost. As-grown SnO 2 usually exhibits n -type conductivity with high carrier concentrations. Similar to the case of ZnO, it is difficult to dope SnO 2 into p -type. Consequently, applications of SnO 2 for optoelectronic as well as electronic devices are limited. In principle, substitution of group-III elements, including Al, Ga, and In, for Sn atom in SnO 2 could give a hole carrier resulting in a p -type conductivity. Based on the HSE functional calculations (Varley et al., 2009) [13], it has been reported that these dopants are shallow acceptor defects. However, the calculations with PBE0 functional (Scanlon and Watson, 2012) [14] showed that these dopants are deep acceptors. In this work, Ga-doped SnO 2 are revisited by using HSE functional. We found that Ga Sn defect is indeed a deep acceptor indicating that Ga Sn can serve only as a compensating acceptor defect in SnO 2 . Hoping to make the acceptor level shallower, we further study the effect of (compressive) strain on the acceptor level associated with Ga Sn defect by alloying SnO 2 with Si atoms (Si x Sn 1− x O 2 alloy where x ~ 0.17). Our results showed that even with the application of strain, the acceptor level remains too deep to be useful. [ABSTRACT FROM AUTHOR]