Showing papers on "Crystal published in 2016"
TL;DR: This work presents the largest database of calculated surface energies for elemental crystals to date, which contains the surface energies of more than 100 polymorphs of about 70 elements, up to a maximum Miller index of two and three for non-cubic and cubic crystals, respectively.
Abstract: The surface energy is a fundamental property of the different facets of a crystal that is crucial to the understanding of various phenomena like surface segregation, roughening, catalytic activity, and the crystal's equilibrium shape. Such surface phenomena are especially important at the nanoscale, where the large surface area to volume ratios lead to properties that are significantly different from the bulk. In this work, we present the largest database of calculated surface energies for elemental crystals to date. This database contains the surface energies of more than 100 polymorphs of about 70 elements, up to a maximum Miller index of two and three for non-cubic and cubic crystals, respectively. Well-known reconstruction schemes are also accounted for. The database is systematically improvable and has been rigorously validated against previous experimental and computational data where available. We will describe the methodology used in constructing the database, and how it can be accessed for further studies and design of materials.
537 citations
TL;DR: Wang et al. as discussed by the authors developed a general strategy to manufacture and mediate crystal defects in the host Bi2MoO6 lattice by varying the cerium dopant content, resulting in greatly improved visible-light-driven photocatalytic performance for the degradation of highly toxic nerve agent simulants (NAS) and organic dyes, as well as bacterial photoinactivation.
Abstract: Crystal defects have been extensively proved to have great influence on semiconductor photocatalysis. To optimize the reactivity of crystalline photocatalysts and achieve ideal solar energy conversion, crystal defect engineering has initiated a considerable interest in real catalysts. Herein, we develop a general strategy to manufacture and mediate crystal defects in the host Bi2MoO6 lattice by varying the cerium dopant content, resulting in greatly improved visible-light-driven photocatalytic performance for the degradation of highly toxic nerve agent simulants (NAS) and organic dyes, as well as bacterial photoinactivation. After careful examination of crystal defect structure and charge carrier dynamics, it was proved that Ce-doping-mediated crystal defects are crucial for controlling the photocatalytic efficiency of aurivillius Bi2MoO6. More importantly, the well-engineered crystal defects not only exert a beneficial influence on the electron dynamics and band structure but also facilitate the one-elec...
310 citations
TL;DR: Gallium arsenide (GaAs) nanowires during growth are image as they switch between phases as a result of varying growth conditions, finding clear differences between the growth dynamics of the phases, including differences in interface morphology, step flow and catalyst geometry.
Abstract: Controlled formation of non-equilibrium crystal structures is one of the most important challenges in crystal growth. Catalytically grown nanowires are ideal systems for studying the fundamental physics of phase selection, and could lead to new electronic applications based on the engineering of crystal phases. Here we image gallium arsenide (GaAs) nanowires during growth as they switch between phases as a result of varying growth conditions. We find clear differences between the growth dynamics of the phases, including differences in interface morphology, step flow and catalyst geometry. We explain these differences, and the phase selection, using a model that relates the catalyst volume, the contact angle at the trijunction (the point at which solid, liquid and vapour meet) and the nucleation site of each new layer of GaAs. This model allows us to predict the conditions under which each phase should be observed, and use these predictions to design GaAs heterostructures. These results could apply to phase selection in other nanowire systems.
292 citations
TL;DR: Based on first-principles phonon and finite temperature molecular dynamics calculations including spin-orbit coupling, the authors showed that buckled honeycomb and asymmetric washboard structures named as bismuthene are stable at high temperature.
Abstract: Based on first-principles phonon and finite temperature molecular dynamics calculations including spin-orbit coupling, we showed that free-standing single-layer phases of bismuth, namely buckled honeycomb and asymmetric washboard structures named as bismuthene, are stable at high temperature. We studied the atomic structure, mechanical, and electronic properties of these single-layer bismuthene phases and their bilayers. The spin-orbit coupling is found to be crucial in determining lattice constants, phonon frequencies, band gaps, and cohesion. In particular, phonons of 3D hexagonal crystal, as well as those of single-layer bismuthene phases, are softened with spin orbit coupling. By going from 3D hexagonal crystal to free-standing single-layer structures, 2D hexagonal lattice is compressed and semimetal is transformed to semiconductor as a result of confinement effect. On the contrary, by going from single-layer to bilayer bismuthenes, the lattice is slightly expanded and fundamental band gaps are narrowed. Our results reveals that interlayer coupling in multilayer and 3D Bi crystal is crucial for topologically trivial to nontrivial and semimetal to semiconductor transitions.
284 citations
TL;DR: In this paper, a photonic analogue of a three-dimensional solid-state topological insulator is proposed, where the symmetries may protect single Dirac cones on the surface of a photonics crystal.
Abstract: Crystal symmetries may protect single Dirac cones on the surface of a photonic crystal, creating a photonic analogue of a three-dimensional solid-state topological insulator.
275 citations
15 Feb 2016
TL;DR: In this article, the fundamental principles of crystal physics are discussed. But they do not discuss the relationship between crystal physics and crystal physics in terms of the properties of the crystal lattice.
Abstract: Fundamentals of crystal physics , Fundamentals of crystal physics , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی
269 citations
TL;DR: It is shown that a class of molecular compounds-known as plastic crystals-can exhibit ferroelectricity if the constituents are judiciously chosen from polar ionic molecules.
Abstract: Ferroelectrics are used in a wide range of applications, including memory elements, capacitors and sensors. Recently, molecular ferroelectric crystals have attracted interest as viable alternatives to conventional ceramic ferroelectrics because of their solution processability and lack of toxicity. Here we show that a class of molecular compounds-known as plastic crystals-can exhibit ferroelectricity if the constituents are judiciously chosen from polar ionic molecules. The intrinsic features of plastic crystals, for example, the rotational motion of molecules and phase transitions with lattice-symmetry changes, provide the crystals with unique ferroelectric properties relative to those of conventional molecular crystals. This allows a flexible alteration of the polarization axis direction in a grown crystal by applying an electric field. Owing to the tunable nature of the crystal orientation, together with mechanical deformability, this type of molecular crystal represents an attractive functional material that could find use in a diverse range of applications.
241 citations
TL;DR: In this paper, an extremely low lattice thermal conductivity of 0.5 W m−1 K−1 was achieved in SnTe-Cu2Te solid solutions, which is actually approaching the amorphous limit of SnTe.
Abstract: Due to point defect phonon scattering, formation of solid solutions has long been considered as an effective approach for enhancing thermoelectric performance through reducing the lattice thermal conductivity. The scattering of phonons by point defects mainly comes from the mass and strain fluctuations between the guest and the host atoms. Both the fluctuations can be maximized by point defects of interstitial atoms and/or vacancies in a crystal. Here, a demonstration of phonon scattering by interstitial Cu atoms is shown, leading to an extremely low lattice thermal conductivity of 0.5 W m−1 K−1 in SnTe-Cu2Te solid solutions. This is the lowest lattice thermal conductivity reported in SnTe-based materials so far, which is actually approaching the amorphous limit of SnTe. As a result, a peak thermoelectric figure of merit, zT, higher than 1 is achieved in Sn0.94Cu0.12Te at 850 K, without relying on other approaches for electrical performance enhancements. The strategy used here is believed to be equally applicable in thermoelectrics with interstitial point defects.
226 citations
TL;DR: It is found that for homogeneous excitation throughout the crystal, the charge carrier lifetime exceeds 15 μs, which means that the diffusion length in CH3NH3PbI3 can be as large as 50 μm if it is no longer limited by the dimensions of the crystallites.
Abstract: The charge carrier lifetime in organic–inorganic perovskites is one of the most important parameters for modeling and design of solar cells and other types of devices. In this work, we use CH3NH3PbI3 single crystal as a model system to study optical absorption, charge carrier generation, and recombination lifetimes. We show that commonly applied photoluminescence lifetime measurements may dramatically underestimate the intrinsic carrier lifetime in CH3NH3PbI3, which could be due to severe charge recombination at the crystal surface and/or fast electron–hole recombination close to the surface. By using the time-resolved microwave conductivity technique, we investigated the lifetime of free mobile charges inside the crystals. Most importantly, we find that for homogeneous excitation throughout the crystal, the charge carrier lifetime exceeds 15 μs. This means that the diffusion length in CH3NH3PbI3 can be as large as 50 μm if it is no longer limited by the dimensions of the crystallites.
226 citations
TL;DR: In this article, a planar planar type CH3NH3PbI3−xClx (MAPbI 3−xCLx) mixed halide perovskite solar cells were fabricated via spray coating with a controlled composition of the solvents.
Abstract: We fabricated highly efficient planar type CH3NH3PbI3−xClx (MAPbI3−xClx) mixed halide perovskite solar cells via spray coating with a controlled composition of the solvents. The cells had a power conversion efficiency of 17.8% (forward scan), 18.3% (reverse scan), and 16.08 ± 1.28% (average) for unit cells under 1 Sun conditions. We controlled the ratio of DMF (dimethylformamide), a quickly evaporating solvent, and GBL (γ-butyrolactone), a slowly evaporating solvent, to 10 : 0, 9 : 1, 8 : 2, and 7 : 3 (vol : vol). We obtained the largest MAPbI3−xClx mixed halide perovskite crystal grains in the 8 : 2 sample because the inward flux of the spray solution was balanced with the outward flux of the evaporating solvent. Consequently, the moistened underlying polycrystalline perovskite film with small crystal grains re-dissolved and merged into larger crystalline grains by re-crystallization. By controlling the re-dissolution and crystal grain growth of the MAPbI3−xClx mixed halide perovskite film via spray coating, we fabricated a sub-module (10 cm × 10 cm, active area = 40 cm2) with 10.5 V open circuit voltage, 84.15 mA short circuit current, 70.16% fill factor, and 15.5% power conversion efficiency under 1 Sun conditions.
215 citations
TL;DR: It is shown that crystal etching as the reverse process of crystal growth can directly endow nanocrystal surfaces with high-energy facets, and the etching-based strategy could be extended to the synthesis of nanocrystals of many other catalysts with more active high- energy facets.
Abstract: Creating high-energy facets on the surface of catalyst nanocrystals represents a promising method for enhancing their catalytic activity. Herein we show that crystal etching as the reverse process of crystal growth can directly endow nanocrystal surfaces with high-energy facets. The key is to avoid significant modification of the surface energies of the nanocrystal facets by capping effects from solvents, ions, and ligands. Using Cu nanocubes as the starting material, we have successfully demonstrated the creation of high-energy facets in metal nanocrystals by controlled chemical etching. The etched Cu nanocrystals with enriched high-energy {110} facets showed significantly enhanced activity toward CO2 reduction. We believe the etching-based strategy could be extended to the synthesis of nanocrystals of many other catalysts with more active high-energy facets.
TL;DR: In this article, two crystalline polymorphs of TMPE, with the space groups P21c and C2, are cultured from different solvent mixtures and display apparent blue fluorescence with the characteristic of aggregation induced emission (AIE).
Abstract: Two crystalline polymorphs of TMPE, with the space groups P21(c) and C2, are cultured from different solvent mixtures and display apparent blue fluorescence with the characteristic of aggregation induced emission (AIE). Excitedly, the P21(c) crystal exhibits easily observed mechanoluminescence (ML), while there is no mechanoluminescence for the C2 crystal. Careful investigation of their crystal structures and three analogues demonstrates that the special molecule packing of TMPE in the P21(c) crystal accounts for its exciting efficient ML performance, providing some information to understand the structure–property relationship of efficient organic ML materials.
TL;DR: In this paper, a new approach to β-Ga 2 O 3 single crystal growth was studied, using the vertical Bridgman (VB) method in ambient air, while measuring the β -Ga 2 o 3 melting temperature and investigating the effects of crucible composition and shape.
TL;DR: It is found that the addition of Cu adatoms can be used to controllably n-dope few layer black phosphorus, thereby lowering the threshold voltage for n-type conduction without degrading the transport properties.
Abstract: Few-layer black phosphorus is a monatomic two-dimensional crystal with a direct band gap that has high carrier mobility for both holes and electrons. Similarly to other layered atomic crystals, like graphene or layered transition metal dichalcogenides, the transport behavior of few-layer black phosphorus is sensitive to surface impurities, adsorbates, and adatoms. Here we study the effect of Cu adatoms onto few-layer black phosphorus by characterizing few-layer black phosphorus field effect devices and by performing first-principles calculations. We find that the addition of Cu adatoms can be used to controllably n-dope few layer black phosphorus, thereby lowering the threshold voltage for n-type conduction without degrading the transport properties. We demonstrate a scalable 2D material-based complementary inverter which utilizes a boron nitride gate dielectric, a graphite gate, and a single bP crystal for both the p- and n-channels. The inverter operates at matched input and output voltages, exhibits a ...
TL;DR: In this article, the elastic and optical properties as well as the crystal and electronic structures of two-dimensional Ti2CT2 and Ti3C2T2 (T = F, O, and OH) MXene monolayers were investigated.
Abstract: Density functional theory is used to investigate the elastic and optical properties as well as the crystal and electronic structures of two-dimensional Ti2CT2 and Ti3C2T2 (T = F, O, and OH) MXene monolayers. It is found that the elastic stiffness, optical response, crystal structure and the electronic structure show strong dependence on the surface terminated groups often formed with MXene during the etching process. The elastic stiffness maintains only with the surface termination of O atoms, but a large degradation is present in the surface terminations of F and OH atoms. The low adsorption and reflectivity in the range from infrared to ultraviolet rays account for the high transmittance of Ti3C2T2 that has been experimentally observed, and it is predicted that Ti2CT2 will have higher optical transmittance in this range. The calculations also demonstrate the presence of the optical bandgap in Ti2CO2, which renders its potential applications in optical and electronic devices.
TL;DR: The results show that seeding is a powerful technique to investigate crystal nucleation and the crystal-fluid interfacial free energy extrapolated to coexistence conditions is also in good agreement with direct calculations of such parameter.
Abstract: We present a study of homogeneous crystal nucleation from metastable fluids via the seeding technique for four different systems: mW water, Tosi-Fumi NaCl, Lennard-Jones, and Hard Spheres. Combining simulations of spherical crystal seeds embedded in the metastable fluid with classical nucleation theory, we are able to successfully describe the nucleation rate for all systems in a wide range of metastability. The crystal-fluid interfacial free energy extrapolated to coexistence conditions is also in good agreement with direct calculations of such parameter. Our results show that seeding is a powerful technique to investigate crystal nucleation.
TL;DR: The constructive nature of the SHG in this 2D crystal provides a platform to reliably develop atomically flat and controllably thin nonlinear media.
Abstract: Second-harmonic generation (SHG) has found extensive applications from hand-held laser pointers to spectroscopic and microscopic techniques. Recently, some cleavable van der Waals (vdW) crystals have shown SHG arising from a single atomic layer, where the SH light elucidated important information such as the grain boundaries and electronic structure in these ultra-thin materials. However, despite the inversion asymmetry of the single layer, the typical crystal stacking restores inversion symmetry for even numbers of layers leading to an oscillatory SH response, drastically reducing the applicability of vdW crystals such as molybdenum disulfide (MoS2). Here, we probe the SHG generated from the noncentrosymmetric 3R crystal phase of MoS2. We experimentally observed quadratic dependence of second-harmonic intensity on layer number as a result of atomically phase-matched nonlinear dipoles in layers of the 3R crystal that constructively interfere. By studying the layer evolution of the A and B excitonic transitions in 3R-MoS2 using SHG spectroscopy, we also found distinct electronic structure differences arising from the crystal structure and the dramatic effect of symmetry and layer stacking on the nonlinear properties of these atomic crystals. The constructive nature of the SHG in this 2D crystal provides a platform to reliably develop atomically flat and controllably thin nonlinear media.
TL;DR: Open circuit voltage decay and film resistance characterization revealed that the larger grain size contributed to longer carrier lifetime and smaller carrier transport resistance, both of which are beneficial for solar cell devices.
Abstract: Regulating the temperature during the direction contact and intercalation process (DCIP) for the transition from PbI2 to CH3NH3PbI3 modulated the crystallinity, crystal grain size and crystal grain orientation of the perovskite films. Higher temperatures produced perovskite films with better crystallinity, larger grain size, and better photovoltaic performance. The best cell, which had a PCE of 12.9%, was obtained on a film prepared at 200 °C. Further open circuit voltage decay and film resistance characterization revealed that the larger grain size contributed to longer carrier lifetime and smaller carrier transport resistance, both of which are beneficial for solar cell devices.
TL;DR: It is found that the OPC films spontaneously form periodic microarrays that are distinguishable from general polycrystalline perovskite materials in terms of their crystal orientation, film morphology and electronic properties.
Abstract: Controlling crystal orientations and macroscopic morphology is vital to develop the electronic properties of hybrid perovskites. Here we show that a large-area, orientationally pure crystalline (OPC) methylammonium lead iodide (MAPbI3) hybrid perovskite film can be fabricated using a thermal-gradient-assisted directional crystallization method that relies on the sharp liquid-to-solid transition of MAPbI3 from ionic liquid solution. We find that the OPC films spontaneously form periodic microarrays that are distinguishable from general polycrystalline perovskite materials in terms of their crystal orientation, film morphology and electronic properties. X-ray diffraction patterns reveal that the film is strongly oriented in the (112) and (200) planes parallel to the substrate. This film is structurally confined by directional crystal growth, inducing intense anisotropy in charge transport. In addition, the low trap-state density (7.9 × 1013 cm-3) leads to strong amplified stimulated emission. This ability to control crystal orientation and morphology could be widely adopted in optoelectronic devices.
TL;DR: An elastic organic crystal of a π-conjugated molecule has been fabricated and it was found to be a remarkably elastic crystalline material.
Abstract: An elastic organic crystal of a π-conjugated molecule has been fabricated. A large fluorescent single crystal of 1,4-bis[2-(4-methylthienyl)]-2,3,5,6-tetrafluorobenzene (over 1 cm long) exhibited a fibril lamella morphology based on slip-stacked molecular wires, and it was found to be a remarkably elastic crystalline material. The straight crystal was capable of bending more than 180° under applied stress and then quickly reverted to its original shape upon relaxation. In addition, the fluorescence quantum yield of the crystal was about twice that of the compound in THF solution. Mechanical bending-relaxation resulted in reversible change of the morphology and fluorescence. This research offers a more general approach to flexible crystals as a promising new family of organic semiconducting materials.
TL;DR: Anisotropic 2D layered material rhenium disulfide (ReS2) with high crystal quality and uniform monolayer thickness is synthesized by using tellurium-assisted epitaxial growth on mica substrate with high efficiency.
Abstract: Anisotropic 2D layered material rhenium disulfide (ReS2 ) with high crystal quality and uniform monolayer thickness is synthesized by using tellurium-assisted epitaxial growth on mica substrate. Benefit from the lower eutectic temperature of rhenium-tellurium binary eutectic, ReS2 can grow from rhenium (melting point at 3180 °C) and sulfur precursors in the temperature range of 460-900 °C with high efficiency.
TL;DR: In this article, the charge transfer characteristics of metastable-phase hexagonal molybdenum oxide (h-MoO3) and stable-phase orthorhombic MoO3 (α-MoOn3) nanocrystals have been investigated for the first time using impedance spectroscopy.
Abstract: The charge transfer characteristics of metastable-phase hexagonal molybdenum oxide (h-MoO3) and stable-phase orthorhombic MoO3 (α-MoO3) nanocrystals have been investigated for the first time using impedance spectroscopy. The results imply that the metastable phase h-MoO3 displays a 550-fold increase (at 150 °C) in the electrical conductivity relative to the stable phase α-MoO3. The conductivity also increases as the temperature increases from 130 to 170 °C, whereby analysis shows a thermal activation energy (Ea) of ∼0.42 eV. The investigation clearly identifies that the presence of intercalated ammonium ions (NH4+) and crystal water molecules (H2O) in the internal structure of h-MoO3 plays a vital role in enhancing the charge transfer characteristics and showing an ionic conductive nature. Before the impedance investigations, the h-MoO3 and α-MoO3 nanocrystals were successfully synthesized through a wet-chemical process. Here, a controlled one-step hydrothermal route was adopted to synthesize stable-phase...
TL;DR: The recent advent of liquid-phase transmission electron microscopy (TEM) and advances in cryogenic TEM are transforming our understanding of the physical and chemical mechanisms underlying the formation of materials in synthetic, biological and geochemical systems.
Abstract: The recent advent of liquid-phase transmission electron microscopy (TEM) and advances in cryogenic TEM are transforming our understanding of the physical and chemical mechanisms underlying the formation of materials in synthetic, biological and geochemical systems. These techniques have been applied to study the dynamic processes of nucleation, self-assembly, crystal growth and coarsening for metallic and semiconductor nanoparticles, (bio)minerals, electrochemical systems, macromolecular complexes, and organic and inorganic self-assembling systems. New instrumentation and methodologies that are currently on the horizon promise new opportunities for advancing the science of materials synthesis.
TL;DR: It is shown that increases in crystal plastic anisotropy enhance the probability of twin transmission by comparing the relative ease of twin Transmission in hcp materials such as Mg, Zr and Ti.
Abstract: Materials with a hexagonal close-packed (hcp) crystal structure such as Mg, Ti and Zr are being used in the transportation, aerospace and nuclear industry, respectively. Material strength and formability are critical qualities for shaping these materials into parts and a pervasive deformation mechanism that significantly affects their formability is deformation twinning. The interaction between grain boundaries and twins has an important influence on the deformation behaviour and fracture of hcp metals. Here, statistical analysis of large data sets reveals that whether twins transmit across grain boundaries depends not only on crystallography but also strongly on the anisotropy in crystallographic slip. We show that increases in crystal plastic anisotropy enhance the probability of twin transmission by comparing the relative ease of twin transmission in hcp materials such as Mg, Zr and Ti.
TL;DR: A statistical analysis shows that knowledge of trg and m alone is sufficient to predict τX* within estimated experimental errors, and surprisingly, the liquid/crystal interfacial free energy does not appear in this expression for τX*.
Abstract: The waiting time to form a crystal in a unit volume of homogeneous undercooled liquid exhibits a pronounced minimum τ_X* at a ‘nose temperature’ T* located between the glass transition temperature T_g, and the crystal melting temperature, T_L. Turnbull argued that τ_X* should increase rapidly with the dimensionless ratio t_(rg)=T_g/T_L. Angell introduced a dimensionless ‘fragility parameter’, m, to characterize the fall of atomic mobility with temperature above T_g. Both t_(rg) and m are widely thought to play a significant role in determining τ_X*. Here we survey and assess reported data for T_L, T_g, t_(rg), m and τ_X* for a broad range of metallic glasses with widely varying τ_X*. By analysing this database, we derive a simple empirical expression for τ_X*(t_(rg), m) that depends exponentially on t_(rg) and m, and two fitting parameters. A statistical analysis shows that knowledge of t_(rg) and m alone is therefore sufficient to predict τ_X* within estimated experimental errors. Surprisingly, the liquid/crystal interfacial free energy does not appear in this expression for τ_X*.
TL;DR: In this article, the authors demonstrate that PTCDA is a promising cathode for K-ion batteries with a high capacity of 122 mAh/g at 20 mA/g and moderate rate and cycling performance.
TL;DR: In this paper, the authors report a comprehensive modeling and experimental characterization of a three-dimensional phononic crystal composed of a single material, endowed with an ultra-wide complete bandgap.
Abstract: This paper reports a comprehensive modeling and experimental characterization of a three-dimensional phononic crystal composed of a single material, endowed with an ultra-wide complete bandgap. The phononic band structure shows a gap-mid gap ratio of 132% that is by far the greatest full 3D bandgap in literature for any kind of phononic crystals. A prototype of the finite crystal structure has been manufactured in polyamide by means of additive manufacturing technology and tested to assess the transmission spectrum of the crystal. The transmission spectrum has been numerically calculated taking into account a frequency-dependent elastic modulus and a Rayleigh model for damping. The measured and numerical transmission spectra are in good agreement and present up to 75 dB of attenuation for a three-layer crystal.
TL;DR: In this paper, a single-particle photoluminescence (PL) spectroscopy was used to assess the trapping, recombination, and surface reactions of photogenerated and electrically injected charges on specific facets of the promising visible active photocatalyst BiVO4.
Abstract: The performance of semiconductor materials in solar water splitting and other applications is strongly influenced by the structure-related dynamics of charge carriers in these materials. In this study, we assessed the trapping, recombination, and surface reactions of photogenerated and electrically injected charges on specific facets of the promising visible active photocatalyst BiVO4 by using single-particle photoluminescence (PL) spectroscopy. Evaluation of the electric-potential-induced PL properties and the PL response to charge scavengers revealed that the visible PL bands observed during visible laser irradiation originate from radiative recombination between holes trapped at the intraband states above the valence band and mobile (free or shallowly trapped) electrons. Furthermore, the trapped holes are preferentially located on the lateral {110} facets of the BiVO4 crystal, while the electrons are uniformly distributed over the crystal. The methodology described in this study thus provides us with a...
TL;DR: First-principles calculations demonstrate that the very low thermal expansion originates mainly from the invariability of the solid [B24 O48 ] truncated octahedra that are fixed by the [Zn4 O13 ] clusters in the ZBO structure.
Abstract: Intrinsic isotropic near-zero thermal expansion is discovered in borate crystal Zn4 B6 O13 with high transparency in the ultraviolet region. First-principles calculations demonstrate that the very low thermal expansion originates mainly from the invariability of the solid [B24 O48 ] truncated octahedra that are fixed by the [Zn4 O13 ] clusters in the ZBO structure.
TL;DR: Takahashi et al. as mentioned in this paper reported the first experimental realization of a monolayer 1T-NbSe2 crystal grown epitaxially on bilayer graphene and showed that 1T is a Mott insulator with an energy gap of 0.4
Abstract: The emergence of exotic quantum phenomena is often triggered by a subtle change in the crystal phase. Transition metal dichalcogenides (TMDs) exhibit a wide variety of novel properties, depending on their crystal phases, which can be trigonal prismatic (2H) or octahedral (1T). Bulk NbSe2 crystallizes into the 2H phase, and the charge density wave and the superconductivity emerge simultaneously and interact with each other, thereby creating various anomalous properties. However, these properties and their interplay in another polymorph, 1T-NbSe2, have remained unclear because of the difficulty of synthesizing it. Here we report the first experimental realization of a monolayer 1T-NbSe2 crystal grown epitaxially on bilayer graphene. In contrast with 2H-NbSe2, monolayer 1T-NbSe2 was found to be a Mott insulator, with an energy gap of 0.4 eV. We also found that the insulating 1T and metallic 2H phases can be selectively fabricated by simply controlling the substrate temperature during epitaxy. The present results open a path to crystal-phase engineering based on TMDs. A thin film produced by a team in Japan could lead to field-effect transistors that combine superconductors and semiconductors. When thin monolayers of niobium selenide (NbSe2) are deposited on surfaces, quantum effects that are useful for electronic devices emerge. A team led by Takashi Takahashi from Tohoku University has discovered how to fabricate NbSe2 thin films with two crystal phases that have distinct properties. By growing monolayers of NbSe2 on a bilayer graphene surface and tweaking the reaction temperature, the researchers were able to generate crystalline nanoislands that behaved as either superconductors or Mott insulators — special materials whose conductivities can be tuned by varying parameters such as pressure and disorder. Atomic-resolution imaging confirmed the different island structures, which the team sees as a path towards ultrasmall transistors and switches. We have synthesized a monolayer 1T-NbSe2 on bilayer graphene by molecular-beam-epitaxy method and investigated its electronic states by angle-resolved photoemission spectroscopy. In contrast to metallic 2H-NbSe2, monolayer 1T-NbSe2 was found to show insulating characteristics with a finite energy gap and strong modulation of density of states with periodicity. This suggests the Mott-insulating ground state of monolayer 1T-NbSe2 with the formation of ‘star of David’ clusters. We also found that 1T and 2H phases are selectively fabricated by simply controlling the substrate temperature during epitaxy. The present result opens a pathway toward the crystal-phase engineering based with transition-metal dichalcogenides.