Semiconductor-metal Phase Transition Research Articles

In-situ stirring assisted hydrothermal synthesis of W-doped VO2 (M) nanorods with improved doping efficiency and mid-infrared switching property

Vanadium dioxide (VO2) is considered to be one of the most competitive thermochromic materials, due to its semiconductor-metal phase transition characteristics. And the doping of tungsten (W) into the lattice of VO2 can significantly reduce the phase transition temperature to near room temperature. However, the reduction efficiency dependent on doping amount is quite different in previous reports. In this work, we proposed an in-situ stirring assisted hydrothermal to synthesize W-doped VO2 nanorods with improved doping efficiency. The effects of in-situ stirring during the hydrothermal synthesis on phase composition, sample morphology, and phase-transition temperature of VO2 with different W doping amounts were studied systematically. The phase transition temperature reduction efficiency of W-doped VO2 prepared by hydrothermal synthesis without in-situ stirring was at % W. By contrast, the phase transition temperature of W-doped VO2 prepared by in-situ stirring hydrothermal synthesis was depressed as much as 24.55 °C/at % W when W/V ranged in 0.75–2.0 at %. Further research found that the phase transition temperature of W-doped VO2 (M) could be simply tuned by changing the in-situ stirring time. Then, the 1 at% W-doped VO2 (M) (with phase transition temperature of around 41.5 °C) nanorods and acrylic resin were mixed to prepare VO2 composite film. It exhibited suitable infrared transmission (>60%) and excellent mid-infrared transmission switching performance (23.77%). This work proposed an in-situ stirring hydrothermal method for high efficiency synthesis of W-doped VO2 (M) and provided significant insights for its application in thermochromic windows.

Strain effects on conductivity and charge transport in La-doped BaTiO3 thin films

We investigate how epitaxial strain affects the conductivity and charge transport of Ba0.6La0.4TiO3 (BLTO) films deposited by pulsed laser deposition. Depositing BLTO films with thicknesses varying from 14.1 to 95.9 nm gives differing epitaxial strains, with a maximum strain up to ~6% for the thinnest film of 14.1 nm. Transport measurements demonstrate that film thickness (i.e. epitaxial strain) affects the conductivity, carrier mobility, and concentration. Measurements of resistivity versus temperature demonstrate that all the BLTO films undergo a distinct semiconductor-metal phase transition, and the transition temperature TSM also depends clearly on strain. For all the films, the charge transport follows the small-polaron hopping mechanism below TSM and the thermal phonon scattering mechanism above TSM. These results prove that strain in epitaxial films strongly affects the conductivity and charge transport of traditional insulating ferroelectric oxide films.

Electronic, optical and photocatalytic properties of fully hydrogenated GeC monolayer

In this work, we study the electronic, optical, and photocatalytic properties of fully hydrogenated GeC monolayer under strain engineering and external electric field using first-principles investigations. Our calculations demonstrate that at the equilibrium state, fully hydrogenated GeC monolayer is a indirect-semiconductor with band gap of 3.493 eV and it possesses photocatalytic characteristics for water splitting and in particular, photocatalytic activities can be enhanced by a negative electric field under ultraviolet light. We can control the band gap of fully hydrogenated GeC monolayer by biaxial strain or external electric field and semiconductor–metal phase transition happens at certain elongation of biaxial strain. Compared to pure monolayer GeC, the fully hydrogenation causes optical absorption peaks of GeC shifting to a higher energy region. While the optical spectra of the fully hydrogenated GeC monolayer are strongly dependent on the strain, the effect of the electric field on them is negligible. Our findings can provide useful information for the applicability of fully hydrogenated GeC monolayer in nanoelectronic devices and photocatalytic water splitting. • Fully hydrogenated GeC monolayer is a semiconductor with indirect bandgap of 3.493 eV. • Semiconductor–metal phase transition happens in H–GeC–H at certain elongation of biaxial strain. • Full hydrogenation causes optical absorption peaks of GeC shifting to a higher energy region. • H–GeC–H possesses photocatalytic characteristics for water splitting at equilibrium. • Photocatalytic activities in H–GeC–H can be enhanced by a negative electric field under ultraviolet light.

Influence of copper on electrical conductivity of glass-ceramics based on vanadium dioxide

The results of studying the electrical conductivity σ for the glass-ceramic systems VO2 ‒vanadium-phosphate glass (VPG) and VO2‒VPG‒SnO2 with the additions of Cu and Cu2O are presented. It is found that the jump of conductivity at the temperature of metal-semiconductor phase transition (MSPT) in VO2 (Tt = 341K) takes place only for glass-ceramics containing not more than 5 wt% of these additives. When their content exceeds 5 wt%, the VO2 content decreases sharply according to the data of differential thermal analysis. The reason for this is the oxidation-reduction reactions in the liquid phase between Cu and VO2 at ceramics synthesis. These reactions result in the appearance of Magneli phases in glass‒ceramics composition. The phase transitions in Magneli phases V4O7, V5O9, and V6O13 are indicated by the temperature dependence of σ, as bends of the straight lines in coordinates log10σ ~ 1/T at the phase transition temperatures Tt of these phases: 240K, 150K, and 130K. The activation energy of σ for T> Tt is lower than for T <Tt, which indicates a change in the structure of energy zones inherent to MSPT.

Strain-tunable electronic and optical properties of monolayer GeSe: Promising for photocatalytic water splitting applications

In this work, we investigate the electronic and optical properties of monolayer GeSe and the possibility of enhancement the photocatalytic activities for the water splitting of monolayer GeSe through strain engineering using first-principles calculations. Our calculations indicate that monolayer GeSe is a semiconductor with a moderate indirect gap of 1.13 eV at equilibrium and we can control its band gap by biaxial strain. In the presence of biaxial strain εb, the semiconductor-metal phase transition happens at large compressive strain of -10% and the indirect-direct gap transition occurs at εb=4%. The optical spectrum of monolayer GeSe are highly anisotropic and biaxial strain can increase the absorption coefficient of monolayer GeSe up to about 6×105 cm−1. Our calculations demonstrate that monolayer GeSe possesses photocatalytic properties for water splitting at εb=5% and we can enhance its photocatalytic activity by strain.

Characteristics of the Phase Transition in Vanadium Dioxide Films Obtained via Chemical Vapor Deposition

Changes in the characteristics of the semiconductor–metal phase transition in VO2 polycrystalline films obtained via chemical vapor deposition from vanadyl acetylacetonate vapor and oxygen with the temperature of synthesis varying from 373 to 473°C and the oxygen pressure varying from 25 to 90 sccm were studied. The critical temperature of the phase transition and the width of the hysteresis are calculated from the temperature dependences of the films’ coefficients of reflection. These dependences are determined using a special spectrophotometric system during the thermal cycling of the samples from room temperature to 100°C. The relationship between the phase transition characteristics and the phase composition and texture of the films is established. It is found that the critical temperature and the narrowest hysteresis correspond to films containing single monoclinic phase VO2(M1) with the preferential orientation (texture) of crystallites in the (011) plane.

Semiconductor–Metal Phase Transition and “Tristable” Electrical Switching in Nanocrystalline Vanadium Oxide Films on Silicon

Temperature dependences of the ac conductivity of nanocrystalline mixed vanadium oxide films on silicon revealed a multistep shape of the hysteresis loop observed during the semiconductor–metal phase transition in VO2, which was not manifested in the case of dc measurements. These peculiarities are related to the size effect, heterophase character of vanadium oxide films, and different types of charge carriers in the bulk of nanocrystallites and on their surfaces. The appearance of steps is explained by the phase transition taking place in separate groups of crystallites with close dimensions. The phenomenon of “tristable” electrical switching in these vanadium oxide films was observed for the first time.

Investigation of electrical properties in AB-Stacked bilayer Graphene-DNA nanostructures

Abstract In this paper, we study electrical properties of AB-stacked bilayer graphene-DNA hybrid nanostructure connected to two semi-infinite monolayer graphene nanoribbon leads. We investigated two types of devices with both zigzag and armchair edges and the results are evaluated by replacing the junction position. Our calculations are based on the tight-binding approximation and the Green's function method in which the electrical transmission, the density of states (DOS) and the local density of states (LDOS) are calculated numerically. In the metallic bilayer graphene device via creating a nanopore and translocating the DNA molecule, the electron transmission reduces and an energy gap opens, so that, the metal-semiconductor phase transition occurs. Also, our results indicate that the model AC leads to a more considerable energy gap than model AB. Our results have important applications in DNA sensing.

Band gap tuning of defective silicon carbide nanotubes under external electric field: Density functional theory

We have theoretically investigated the effect of applying longitudinal and transverse electric field on silicon carbide nanotubes with different orientations of Stone Wales defects. We found that each type of Stone Wales defects maintained different formation energy. We have also successfully proved that the orientation of Stone Wales defects in silicon carbide nanotubes response quite differently upon applying external electric field, whereas, two important and interesting phenomena were observed. First, the semiconductor-metal phase transition occurred in silicon carbide nanotubes as well as the three types of Stone Wales defects. However, clear band gap variations were observed in all silicon carbide nanotubes under study. Second, the band gap variations in pristine silicon carbide nanotubes and nanotubes with different orientations of Stone Wales defects have the same trend, even though all silicon carbide nanotubes have clear band gap values under different strengths of the applied external electric field. However, band gap tuning under longitudinal electric field is less significant compared to band gap tuning under the transverse electric field.

Metal Insulator Transition in Vanadium Dioxide Hydrated by Means of the Plasma-Immersion Ion Implantation Method

In the work, we explore the modification of the structure of vanadium dioxide films, as well as the metal-semiconductor phase transition in them, when hydrated by the method of plasma-immersion ion implantation. Based on a detailed X-ray analysis and the Raman spectra of the initial and hydrated films, it is shown that the plasma-ion modification of the transition and the metallization of vanadium dioxide (at a hydrogen concentration above 10at. %) are most likely associated with electron-correlation effects amplified by hydrogen implantation and an increase in the free charge carrier density in the material.

Highly anisotropic thermoelectric properties of carbon sulfide monolayers

Strain engineering applied to carbon monosulphide monolayers allows to control the bandgap, controlling electronic and thermoelectric responses. Herein, we study the semiconductor–metal phase transition of this layered material driven by strain control on the basis of first-principles calculations. We consider uniaxial and biaxial tensile strain and we find a highly anisotropic electronic and thermoelectonic responses depending on the direction of the applied strain. Our results indicate that strain-induced response could be an effective method to control the electronic response and the thermoelectric performance.

Peculiar optical properties of bilayer silicene under the influence of external electric and magnetic fields

We conduct a comprehensive investigation of the effect of an applied electric field on the optical and magneto-optical absorption spectra for AB-bt (bottom-top) bilayer silicene. The generalized tight-binding model in conjunction with the Kubo formula is efficiently employed in the numerical calculations. The electronic and optical properties are greatly diversified by the buckled lattice structure, stacking configuration, intralayer and interlayer hopping interactions, spin-orbital couplings, as well as the electric and magnetic fields ({E}_{z}hat{z}& {B}_{z}hat{z}). An electric field induces spin-split electronic states, a semiconductor-metal phase transitions and the Dirac cone formations in different valleys, leading to the special absorption features. The Ez-dependent low-lying Landau levels possess lower degeneracy, valley-created localization centers, peculiar distributions of quantum numbers, well-behaved and abnormal energy spectra in Bz-dependencies, and the absence of anti-crossing behavior. Consequently, the specific magneto-optical selection rules exist for diverse excitation categories under certain critical electric fields. The optical gaps are reduced as Ez is increased, but enhanced by Bz, in which the threshold channel might dramatically change in the former case. These characteristics are in sharp contrast with those for layered graphene.

High-Pressure Magnetic and Transport Properties of Zn0.1Cd0.9GeAs2 + n wt % MnAs (n = 10 or 15) Nanocomposites

The magnetic susceptibility χ(T) of Zn0.1Cd0.9GeAs2 + n wt % MnAs (n = 10 or 15) ferromagnetic nanocomposites with a Curie temperature TC = 310 K has been measured as a function of temperature in the temperature range 270–350 K, and their resistivity ρ(p), Hall coefficient RH(p), and magnetization M(p) have been measured as functions of pressure near room temperature. The materials have been shown to undergo a pressure-induced ferromagnetic-to-paramagnetic phase transition, accompanied by a semiconductor–metal phase transition, at a hydrostatic pressure p ≈ 3.2 GPa and room temperature.

Фазовый переход полупроводник-металл и "тристабильное" электрическое переключение в нанокристаллических пленках оксида ванадия на кремнии

Abstract. It has been found at measurements on alternating current that the loop of a thermal hysteresis of semiconductor–metal phase transition in vanadium dioxide VO2 has the asymmetrical multistage form. On a direct current the complex shape of a hysteresis loop wasn't shown. Such behavior for VO2 films can be defined by heterophase nanocrystal structure of vanadium oxide and different type of charge carriers in grains volume and surfaces. The phenomenon is connected with consecutive phase transition in groups of nanocrystallites of the close size. In such films electrical switching with the "tristate" mode for the first time is revealed.

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.

Electronic properties of WS2 and WSe2 monolayers with biaxial strain: A first-principles study

In the present work, we consider electronic properties of WX2 (X = S, Se) monolayers under a biaxial strain εb using the first principles study. Our calculations indicate that, at equilibrium, the WS2 and WSe2 monolayers are semiconductors with a direct band gap of respectively 1.800 eV and 1.566 eV while their bulk structures are indirect semiconductors. The electronic properties of the WX2 monolayers are very sensitive with the biaxial strain, especially compression strain. The biaxial strain εb is the cause of the band gap of the WX2 monolayers and especially the semiconductor-metal phase transition has occurred in the WS2 monolayer at εb=-10%. In addition, the direct-indirect band gap transition was observed in both WS2 and WSe2 monolayers at a certain elongation of biaxial strain εb. The phase transitions in these monolayers can be very useful for their applications in nanoelectromechanical devices.

Effect of strains on electronic and optical properties of monolayer SnS: Ab-initio study

In this work, we consider the effect of biaxial ɛb and uniaxial ɛac∕zz strains on electronic properties and optical parameters of monolayer SnS using first-principles calculations. Our calculations show that the monolayer SnS is a semiconductor with an indirect energy gap of 1.63 eV at the equilibrium state. While an effect of tensile strains on bandgap is quite small, the bandgap of monolayer SnS depends strongly on the compressive strains, especially a semiconductor-metal phase transition is occurred due to the uniform compressive biaxial strain at −14% elongation. The optical spectra of the monolayer are high anisotropic, and the absorption coefficient of monolayer SnS tends to increase in the presence of compression strains, while the tensile strains reduce the absorption coefficient of the monolayer SnS. We believe that the phase transition and extraordinary optical properties of the strained monolayer SnS will make it become a useful material in nanoelectromechanical devices and optoelectronic applications.

Linear giant Stark effect in [formula omitted] nanoribbons

In this paper the electronic properties of WSe2 nanoribbons is theoretically and numerically investigated by using six-band tight-binding (TB) model. Zigzag and armchair models of WSe2 nanoribbon show semiconducting and metallic phases, respectively. Effect of external electric field on the electronic properties of WSe2 via solving the Laplace's equation is also studied. A semiconductor-metal phase transition is observed for armchair structure by 2.1Vw of applied transverse electric field, where w is the width of the ribbon. A linear giant Stark effect is also occurred in both armchair and zigzag WSe2 nanoribbons by variations of applied transverse electric field. Moreover, we investigate electronic transport of armchair WSe2 nanoribbon putting on a ferromagnetic EuS substrate using nonequilibrium Green's function method. Spin polarization is occurred by effect of an exchange field of 0.2eV, induced by a ferromagnetic EuS substrate.

Ab-initio study of electronic and optical properties of biaxially deformed single-layer GeS

In the present work, using density functional theory (DFT), we investigate the influence of biaxial strain εb on electronic and optical properties of single-layer GeS. Our DFT calculations show that single-layer GeS is a semiconducting material at equilibrium and semiconductor-metal phase transition may occur at large compression biaxial strain. The optical absorption of single-layer GeS is high in the range of the middle ultraviolet lights. Besides, the biaxial has a great impact on the optical spectra of single-layer GeS in the high energy domains. The semiconductor-metal phase transition and the computational results of the absorption of GeS can provide more useful information for applications in nanoelectromechanical and optoelectronic devices.

Anisotropic thermal transport in van der Waals layered alloys WSe2(1-x)Te2x

Transition metal dichalcogenide (TMD) alloys have attracted great interest in recent years due to their tunable electronic properties and the semiconductor-metal phase transition along with their potential applications in solid-state memories and thermoelectrics among others. However, the thermal conductivity of layered TMD alloys remains largely unexplored despite that it plays a critical role in the reliability and functionality of TMD-enabled devices. In this work, we study the composition- and temperature-dependent anisotropic thermal conductivity of the van der Waals layered TMD alloys WSe2(1-x)Te2x in both the in-plane direction (parallel to the basal planes) and the cross-plane direction (along the c-axis) using time-domain thermoreflectance measurements. In the WSe2(1-x)Te2x alloys, the cross-plane thermal conductivity is observed to be dependent on the heating frequency (modulation frequency of the pump laser) due to the non-equilibrium transport between different phonon modes. Using a two-channel heat conduction model, we extracted the anisotropic thermal conductivity at the equilibrium limit. A clear discontinuity in both the cross-plane and the in-plane thermal conductivity is observed as x increases from 0.4 to 0.6 due to the phase transition from the 2H to the Td phase in the layered alloys. The temperature dependence of thermal conductivity for the TMD alloys was found to become weaker compared with the pristine 2H WSe2 and Td WTe2 due to the atomic disorder. This work serves as an important starting point for exploring phonon transport in layered alloys.