Semiconductor Phase Transition Research Articles

Supreme enhancement of ferromagnetism in a spontaneous-symmetry-broken 2D nanomagnet

The ability to tune and control the magnetic phases of two-dimensional (2D) nanomagnets at room temperature is indispensable for the development of future spintronics and low-dimensional spin circuits. In this work, a first-principles-based investigation combined with a Monte Carlo simulation based on a 2D Ising model is used to investigate the electronic and magnetic behaviour of a recently discovered 2D material, Cr2Ge2Se6 over a large range of strain and electric field strength. This material offers ferromagnetic antiferromagnetic and semiconductor metallic phase transitions in different regimes. In the presence of strain, a colossal enhancement of the critical temperature (T c ) is observed, from 149 K to 885 K. The application of an electric field allows a further enhancement of the T c to a value of 919 K, offering a supreme enhancement (~517%), compared to its natural condition. The origin of this behavior can be traced to a super-exchange interaction between the Cr and Se atoms and the intrinsic magnetic anisotropy of Cr2Ge2Se6. The presence of external stimuli engenders spontaneous symmetry breaking with an enhanced magnetic moment (~4.36 /Cr atom), a significant intrinsic spin polarisation (~100%) in a half-metallic regime, and a very high critical temperature. The insights of the current investigation could be useful for future developments in multi-stimuli-assisted room-temperature ferromagnetism and electronic phase control, which are of great significance for future magneto-electronic applications.

Diffraction of a Plane Electromagnetic Wave by a VO2 Microsphere in the Phase Transition Region

We present the results of diffraction studies and the electromagnetic (EM) field distribution in a microsphere of vanadium dioxide (VO2) before and after the metal–semiconductor phase transition (MSPT) is presented. As a result of the calculations, it has been shown that after the phase transition point in VO2 (T > Tc), the EM wave field practically does not pass into the microsphere, almost completely reflecting from it and scattering, which is associated with a sharp increase in the imaginary part of the dielectric constant of the microsphere, the VO2 transition into metallic state and commensurability of the EM wavelength with the radius of microsphere. Тhe radiation intensity has been found to decrease abruptly at a distance 2R from the surface of microsphere, which is associated with the interference of waves behind the microsphere (shadow region). The simulation of the diffraction and distribution of the EM field inside and outside microsphere allows us to propose a new method for studying phase transitions of the MSPT type.

Rapid thermal annealing assisted facile solution method for tungsten-doped vanadium dioxide thin films on glass substrate

Abstract Vanadium dioxide VO2 is an important thermochromic material with useful applications in smart energy devices. This paper describes the synthesis of high-quality vanadium dioxide thermochromic thin films directly on glass substrate without any barrier layer prepared using a simple solution method. We demonstrate that 50 nm thick VO2 films display excellent visible transmittance (76.9% on glass, 69.4% on fused silica) and a large NIR switching efficiency (33.8% on glass, 44.3% on fused silica) at 1600 nm. It is found that the metal to semiconductor phase transition temperature Tc of undoped films can be reduced from a theoretical value of 68 °C to 49.4 °C by rapid thermal annealing. With W doping a continuous drop in the Tc is observed and for VO2 with 2% W doping, a Tc of 25.8 °C is achieved. These good optical properties and the near room-temperature phase transition temperature suggest that solution processing with the rapid thermal annealing process is a feasible method to obtain this promising material on glass substrate for practical application in thermochromic smart windows.

Tuning Phase Transitions in Metal Oxides by Hydrogen Doping: A First-Principles Study.

Optimizing physical or chemical properties via electronic phase transitions is important for developing high-performance semiconductor materials. Using first-principles calculations, we established hydrogen doping (H-doping) as an alternative strategy for modulating phase transitions in metal oxide semiconductors. We found that H-doping can induce an insulator-to-metal phase transition in rutile TiO2 or wurtzite ZnO, a metal-to-insulator phase transition in rutile VO2, and sequential insulator-metal-insulator phase transitions in SnO2. H-doping not only creates defect-induced states inside the original bandgap but also induces new occupancy patterns in conduction band edge states. A linear relationship between occupation of the conduction band edge and H-doping concentration was found, offering the possibility of precisely tuning the material's free-carrier concentration, electrical conductance, and photoabsorption ability. We envision that this work will provide a promising way to tune electrical phase transitions in metal oxides.

Engineering the band structures of few-layer black phosphorus by adsorbed metal atoms

Few-layer black phosphorus (BP) is an important member in a family of two-dimensional materials because of its unique physical properties. But, for future applications, further tuning its electronic structure is necessary. Here we investigate, using ab initio density-function theory, the band structures of few-layer BP after the adsorption of the alkali metal atoms. It is shown that such adsorption can modify the band structures of few-layer BP in terms of the Stark effect. The charge transfer between the adsorbed atoms and few-layer BP is responsible for the internal electric field. Both the effects of the layer thickness and the concentration of adsorbed atoms on the band structures of few-layer BP are also quantitatively studied. The calculated results show that the band gap decreases with increasing the concentration of the adsorbed atoms. Further increasing the thickness of few-layer BP will close its band gap and induce the phase transition of semiconductor - Dirac metal. These findings imply that the electronic properties of few-layer BP can be modulated by adsorbing the alkali metal atoms.

The Hydrogen Evolution Reaction on Nanostructured Molybdenum Disulfide

This short review analyzes the catalytic activity of nanostructured molybdenum disulfide (MoS2). The phase transition of nanostructured MoS2 semiconductor from 2H (hexagonal) to 1T (trigonal) leads to a metal-like material through so-called surface defect engineering by chemical exfoliation. The 1T phase was found to be active for the electrocatalytic hydrogen evolution reaction (HER). Various key aspects vis-à-vis the MoS2 synthesis, characterization, support interaction, and application are, herein, highlighted.

High carrier density, electrostatic doping in organic single crystal semiconductors using electret polymers

Control of the carrier density with high doping levels is a fundamental technology in semiconductor processing. However, unavoidable structural disorder in organic semiconductors can be introduced by chemical doping methods. Here, we synthesize a novel class of ionic polymers that generate a quasi-permanent electric field, and demonstrate that organic semiconductors in proximity to the ionic polymer can be electrostatically doped with significantly high carrier densities up to 5.6 × 1013 cm−2, which results in a low sheet resistance of ca. 60 kΩ. Electret doping presented here is expected to lead to an in-depth understanding of electric phase transitions in organic semiconductors.

Interplay of orbital hopping and perpendicular magnetic field in anisotropic phase transitions for Bernal bilayer graphene and hexagonal boron-nitride.

We theoretically address the perpendicular magnetic field effects on the electronic phase of Bernal bilayer graphene and hexagonal boron-nitride (h-BN) taking into account the total and orbital-projected electronic bands using the tight-binding parameters in the Harrison model, followed by the Green's function method. First, we confirm that our model is computationally efficient and accurate for calculating the magneto-orbital electronic phase transition by reproducing the semimetallic and insulating treatments of pristine Bernal bilayer graphene and h-BN, respectively. In our model, the magnetic field couples only to the electron spin degrees of freedom (with the same contributions for spin-up and spin-down) due to the low dimension of the systems. Here, the main features of the phase transitions are characterized by the electronic density of states (DOS). We found that sp2-hybridization is destroyed when the systems are immersed in the magnetic field, leading to a phase transition to metal for both systems at strong magnetic fields. While there is no phase transition for bilayer graphene at weak magnetic fields, for the case of bilayer h-BN, an insulator to semiconductor phase transition can be viewed, making h-BN more applicable in industry. In bilayer graphene, the anisotropic phase transition appears as insulator-semiconductor, insulator-metal, and semimetal-metal for s-, {px + py}-, and pz-orbitals, respectively, whereas in the case of bilayer h-BN, one observes the same transitions for {s,pz}-orbitals but insulator-semiconductor for {px + py} orbitals. Generically, our findings highlight that the applied magnetic field manipulates the band structure of bilayer graphene and h-BN, and gives ideas to experimentalists for tuning the electro-optical properties of these materials.

β12-Borophene becomes a semiconductor and semimetal via a perpendicular electric field and dilute charged impurity.

In this paper, the possible electronic phase transitions of β12-borophene crystal are examined using a five-band tight-binding calculation. For different tight-binding models, the Green's function technique is employed for the electronic density of states (DOS). We focus on the modulation of the DOS around the Fermi level with a perpendicular electric field and the dilute charged impurity. The steps to incorporate the effects of external electric field and charged impurity are also detailed with the local Hamiltonian model and the Born approximation, respectively. Our calculations show that the inversion symmetric model is the proper model to discuss the metallic phase of the system, entailing different results compared to the homogeneous model. We find that the electric field opens a tunable band gap and a metal-to-p-doped semiconductor phase transition emerges at the strong perpendicular electric field. The influence of impurity scattering potential on the electronic phase of β12-borophene is much larger than the impurity concentration, in which a metal-to-n-doped semiconductor (metal-to-semimetal) transition takes place at high scattering potentials for the homogeneous (inversion symmetric) model, whereas there is no transition when the impurity concentration is changed. Thereby, producing semimetallic/semiconducting properties by applying an appropriate external electric field and dilute charged impurities paves the way for the realization of β12-borophene-based nano-optoelectronic devices.

Thickness-modulated thermochromism of vanadium dioxide thin films grown by magnetron sputtering

Vanadium dioxide (VO2) films were prepared on soda-lime glass by direct current magnetron sputtering at 320 °C. Effects of film thickness on the microstructure, surface morphology and thermochromic performance of VO2 films were investigated. X-ray diffraction showed that the deposited films have strong preferred orientation of VO2 (011) lattice when the film thickness higher than 102 nm. The calculated grain sizes of VO2 films increased from 16.05 nm to 34.56 nm continuously with the increasing of film thickness. UV/VIS/NIR spectrophotometer showed that the visible transmittance deceased while the infrared transmittance switching efficiency increased as the film thickness increased from 79 nm to 264 nm. Additionally, the optical band gaps of VO2 films were in a range of 1.15 eV–1.40 eV, and the thicker film exhibited the smaller value. Moreover, the results of measured temperature-dependent electrical resistivity of these VO2 films showed that the phase-transition temperature is in a range of 53–60 °C, which is much lower than that of single-crystal VO2 (68 °C). With the film thickness increasing, the metal–semiconductor phase transition becomes more obvious. Overall, films with thickness in the range of 80–100 nm showed comparatively relatively balanced combination of visible transmittance and solar switching efficiency.

Analysis the electronic properties of the zigzag and armchair single wall boron nitride nanotubes with single Li impurity in the various sites

We utilized the first-principle density functional theory (DFT) calculation to investigate the structural and electronic properties of various diameters for zigzag and armchair single wall boron nitride nanotubes (ZSWBNNTs and ASWBNNTs) with and without single Li impurity in the various sites. Using DFT method, which employed in the Quantum espresso package, to calculate the electronic band structure, band gap, density of states (DOS), total, kinetic, and electrostatic energies. In the pristine case, there are large electronic band gaps, which make all SWBNNTs have insulator behaviors. Also, total, kinetic energies are reduced with increasing the diameter of the tube, but the opposite behavior with the electrostatic energy. By substituting one B atom with single Li impurity, the electronic band gap is reduced. This impurity also changed the shape and reduced the value of the DOS, which confirmed all the obtained results in this report. Similarly, the values of all the above energies are decreased, which make the structure unstable and more reactive. The significant result in this work is that addition of only one lithium atom in the unit cell of SWBNNTs can take the Fermi level down so significantly and result in insulating to semiconducting phase transition. When we replaced one N atom with one Li impurity, all these energies are dependent on the diameter, so the values of energies are reduced with this impurity. The behaviors of these SWBNNTs are altered to P-type. This impurity is converted these SWBNNTs from an insulator to a semimetal state with zero electronic band gap and changing the shape and value of the DOS, which is very important in various applications.

Frequency electrical conductivity dependence and dielectric relaxation of ZnO–P2O5/Co composites

This work focuses on the experimental study of the conductivity and the permittivity of ZnO–P2O5/Co (ZP/Co) composites as function of frequency in the range of 100–106 Hz at room temperature. The conductivity evolution shows a high percolating jump from low to high loaded ZP matrix with Co filler, indicating an insulator–semiconductor phase transition. Above the percolation threshold, the obtained conductivity is between 10−1 and 10° S/cm and is frequency independent. While, for low filler volume fraction, it is frequency dependent and it is about 10−8 S/cm at 100 Hz. Moreover, it should be noticed that around the percolation threshold, a significant value of dielectric constant about 105 was observed at room temperature and low frequency 100 Hz. Furthermore, the distribution of the relaxation processes was also investigated below the percolation threshold. Indeed, a relaxation peak about 106 rad/s in the curves of electric modulus M″(ω) was observed. It is successively described by Davidson–Cole model and an account of the interpretation of obtained results is given.

Highly tunable charge and spin transport in silicene junctions: phase transitions and half-metallic states

We report peculiar charge and spin transport properties in S-shaped silicene junctions with the Kane–Mele tight-binding model. In this work, we investigate the effects of electric and exchange fields on the charge and spin transport properties. Our results show that by applying a perpendicular electric field, metal-semiconductor and also semimetal–semiconductor phase transitions occur in our systems. Furthermore, full spin current can be obtained in the structures, so the half-metallic states are observable. Our results enable us to control charge and spin currents and provide new opportunities and applications in silicene-based electronics, optoelectronics, and spintronics.

Synergetic photocatalytic effect between 1 T@2H-MoS2 and plasmon resonance induced by Ag quantum dots

Semiconductor phase transitions and plasma noble metal quantum dots (QDs) for visible-light-driven photocatalysts have attracted significant research interest. In this study, novel microwave hydrothermal and photo-reduction methods are proposed to synthesise a visible-light-driven plasma photocatalytic 1T@2H-MoS2/Ag composite. Photoelectrochemical results show that the introduction of the 1T phase and Ag significantly enhances the light response range and charge separation. The 1T phase can act as a co-catalyst to provide a high electron concentration. Ag QDs can effectively improve the light absorption and catalytic effect. The synergistic effect between the 1T@2H-MoS2 microspheres and localised surface plasmon resonance of the Ag QDs can effectively enhance the photocatalytic activity of 1T@2H-MoS2/Ag. The developed 1T@2H-MoS2/Ag composite is superior, not only with respect to a visible-light photocatalytic degradation of conventional dyes, but also in the photocatalytic reduction of Cr(VI). Compared with 2H-MoS2, the catalytic efficiency of 1T@2H-MoS2/Ag for Cr(VI) and MB is increased by 81% and 41%, respectively. This study demonstrates that the introduction of 1T-MoS2 and Ag QDs can significantly enhance the catalytic properties of 2H-MoS2. The microwave and photo-reduction technologies can be employed as green, safe, simple, and rapid methods for the synthesis of noble metal plasma composites.

First-principles study of phase transition, electronic, elastic and optical properties of defect chalcopyrite ZnGa2Te4 semiconductor under different pressures

The generalized gradient approximation (GGA) within the framework of density functional theory (DFT) has been used to investigate the phase transition, electronic, elastic and optical properties of ZnGa2Te4 defect chalcopyrite (DC) semiconductor at different pressures. At 18 GPa pressure, ZnGa2Te4 semiconductor has been found to undergo a structural phase transition from defect chalcopyrite (DC) to disordered rocksalt (DR) structure (phase). The calculated bandgap of DC structure at ambient pressure has been found to be 0.95 eV with direct in nature. The band structure of DR phase studied at 18 GPa pressure has been discussed through density of states. Pressure-dependent elastic stiffness coefficients (Cij), bulk modulus (B), shear modulus (G), Young modulus (E), Poisson's ratio (σ), B/G ratio and Zener anisotropy factor (A) have been calculated at 0, 10, 18 GPa pressures for DC phase and at 20, 25, 30 GPa pressures for DR phases. Further, optical properties such as dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and loss function have been estimated at 0, 10 and 18 GPa pressures for DC phase. The calculated parameters have been compared with the available experimental and theoretical values. A fairly good agreement has been obtained between them.

Switching Channel Development Dynamics in Planar Structures on the Basis of Vanadium Dioxide

The results of the experimental studies and numerical simulation of the switching channel development dynamics in planar structures on the basis of vanadium dioxide are reported. The obtained data on the variation of the temperature in the channel with time and of the current arisen after the pulsed load, and on the times of transition from the high-resistance to the low-resistance state and back are analyzed in order to determine the switching mechanism and to predict the functional characteristics of the switchable vanadium- oxide structures as promising materials for the creation of relaxation generators that can serve as prototypes of neural oscillators. It is shown that the switching behavior is associated with the metal–semiconductor phase transition in vanadium dioxide, which is stimulated by the emission of Joule heat.

Complex research of vanadium dioxide in the homoginuity field

The complex research results of vanadium dioxide VÜ2±x obtained with X-ray analysis method, a method of vacuum adiabatic calorimeter and Faraday method are discussed. The samples temperature dependences of electrical resistivity, heat capacity, and also magnetic susceptibility are presented and they prove that these characteristics jump abruptly in the phase transition area. Thermophysical, electrical, magnetic properties of vanadium dioxide are established to vary with the oxygen content in the samples. Values of Debye characteristic temperatures θD for samples VO2±x determined at temperature 150 K are presented. Values of entropy change as an amount of electronic and phonon components at phase transition were calculated. The results obtained allow to consider the phase transition of a metal semiconductor as a Mott-Peierls transition, and also to use vanadium dioxide in temperature sensors manufacture.

Investigations on Charge Transport Mechanisms in Mixed Valent La0.5Pr0.2Ba0.3–xCaxMnO3 Manganites

In this communication, we report the results of the studies on the structural and transport properties of La0.5Pr0.2Ba0.3–xCaxMnO3 (LPBCMO; x = 0.00, 0.05, 0.15, 0.20 & 0.25) manganites, synthesized by conventional solid state reaction route. All the samples possess distorted orthorhombic structure verified by Rietveld refinements. Temperature dependent resistance of LPBCMO samples shows an exhibition of metal to insulator (semiconductor) phase transition at temperature TP. To understand the charge transport mechanisms responsible for these two electronic phases, zener double exchange (ZDE) polynomial law and Mott type variable range hopping (VRH) model have been used for metallic and semiconducting regions, respectively. Possible nature of the charge carrier interactions with the lattice and activation energy have been discussed on the basis of these theoretical fittings to the resistance data.

Динамика развития канала переключения в планарных структурах на основе диоксида ванадия

AbstractThe results of the experimental studies and numerical simulation of the switching channel development dynamics in planar structures on the basis of vanadium dioxide are reported. The obtained data on the variation of the temperature in the channel with time and of the current arisen after the pulsed load, and on the times of transition from the high-resistance to the low-resistance state and back are analyzed in order to determine the switching mechanism and to predict the functional characteristics of the switchable vanadium- oxide structures as promising materials for the creation of relaxation generators that can serve as prototypes of neural oscillators. It is shown that the switching behavior is associated with the metal–semiconductor phase transition in vanadium dioxide, which is stimulated by the emission of Joule heat.

Metal Organic Framework-Templated Chemiresistor: Sensing Type Transition from P-to-N Using Hollow Metal Oxide Polyhedron via Galvanic Replacement.

Facile synthesis of porous nanobuilding blocks with high surface area and uniform catalyst functionalization has always been regarded as an essential requirement for the development of highly sensitive and selective chemical sensors. Metal-organic frameworks (MOFs) are considered as one of the most ideal templates due to their ability to encapsulate ultrasmall catalytic nanoparticles (NPs) in microporous MOF structures in addition to easy removal of the sacrificial MOF scaffold by calcination. Here, we introduce a MOFs derived n-type SnO2 (n-SnO2) sensing layer with hollow polyhedron structures, obtained from p-n transition of MOF-templated p-type Co3O4 (p-Co3O4) hollow cubes during galvanic replacement reaction (GRR). In addition, the Pd NPs encapsulated in MOF and residual Co3O4 clusters partially remained after GRR led to uniform functionalization of efficient cocatalysts (PdO NPs and p-Co3O4 islands) on the porous and hollow polyhedron SnO2 structures. Due to high gas accessibility through the meso- and macrosized pores in MOF-templated oxides and effective modulation of electron depletion layer assisted by the creation of numerous p-n junctions, the GRR-treated SnO2 structures exhibited 21.9-fold higher acetone response (Rair/Rgas = 22.8 @ 5 ppm acetone, 90%RH) compared to MOF-templated p-Co3O4 hollow structures. To the best of our knowledge, the selectivity and response amplitudes reported here for the detection of acetone are superior to those MOF derived metal oxide sensing layers reported so far. Our results demonstrate that highly active MOF-derived sensing layers can be achieved via p-n semiconducting phase transition, driven by a simple and versatile GRR process combined with MOF templating route.