Metal Phase Transition Research Articles

Photoinduced multistage phase transitions in Ta2NiSe5

Ultrafast control of material physical properties represents a rapidly developing field in condensed matter physics. Yet, accessing the long-lived photoinduced electronic states is still in its early stages, especially with respect to an insulator to metal phase transition. Here, by combining transport measurement with ultrashort photoexcitation and coherent phonon spectroscopy, we report on photoinduced multistage phase transitions in Ta2NiSe5. Upon excitation by weak pulse intensity, the system is triggered to a short-lived state accompanied by a structural change. Further increasing the excitation intensity beyond a threshold, a photoinduced steady new state is achieved where the resistivity drops by more than four orders at temperature 50 K. This new state is thermally stable up to at least 350 K and exhibits a lattice structure different from any of the thermally accessible equilibrium states. Transmission electron microscopy reveals an in-chain Ta atom displacement in the photoinduced new structure phase. We also found that nano-sheet samples with the thickness less than the optical penetration depth are required for attaining a complete transition.

Hopping conduction of non metallic heavily doped delta-layers in CVD diamond

It is shown that the experimental temperature dependence of the sheet resistance of non metallic (just below the insulator–metal phase transition) heavily boron doped delta-layers in chemically vapor deposited (CVD) diamond in a wide temperature range from 30 to 450 K can be explained by hole nearest-neighbor hopping between localized states formed probably by clusters of many negatively charged boron ions. The electrostatic potentials of these states are likely of a short-range type, which can be explained by the screening effect due to holes in neighbor clusters. The suggested theoretical temperature dependence of the sheet resistance by the choice of only two fitting parameters can be well fit to the experimental one in the whole above mentioned temperature range.

Effects of pressure on narrowing the band gap, visible light absorption, and semi-metallic transition of lead-free perovskite CsSnBr3 for optoelectronic applications

Lead free (non-toxic) metal halide perovskites have achieved the demanding position for solar cells and optoelectronic devices due to their environmentally friendly nature and great optoelectronic properties. The present density functional theory (DFT) based investigations have been performed to enhance the optoelectronic properties under hydrostatic pressure. The pressure applied method as a green and facile method to enhance the behavior of a material. In this study, the structural, optical, electronic and elastic properties of CsSnBr3 have been studied by using DFT. The investigated optical nature exhibits the enhanced absorbance in the visible region as well as the ultraviolet region under pressure effect than the ambient condition. The absorption edge shifts towards the lower energy region (red-shift) under pressure compared to the ambient pressure. High-intensity peak values of conductivity, dielectric constant and less reflectivity have also been observed in visible regions. The pressure effect on band structure shows that the conduction band and valence band overlapping each other, as a result, exciting photoelectron easily moves towards the conduction band, hence enhancing the absorptive, conductive and other optical nature. The elastic constants suggest the mechanical stability and highly ductile in nature of CsSnBr3. Semiconducting to metallic phase transition has been observed between 2.5 and 3 GPa. A combined observation and analysis of optical, electronic and elastic properties suggest that the narrowing band gap and enhanced properties of CsSnBr3 under low hydrostatic pressure increase the possibility to use in a solar cell (PV applications) and optoelectronic devices with compared to ambient pressure.

Resistance fluctuations in superconducting K x Fe2 – y Se2 single crystals studied by low-frequency noise spectroscopy* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774303 and 11574373) and Joint Fund of Yunnan Provincial Science and Technology Department, China (Grant No. 2019FY003008).

Low-frequency resistance noise spectroscopy is applied to investigate bulk single crystals of the intercalated iron-selenide K x Fe2 – y Se2 superconductors with different iron vacancy orders. Based on a generalized fluctuation model, the well-observed resistance hump above 100 K is interpreted as an insulator–metal phase transition with a characteristic transition energy of 0.1–0.6 eV, indicating a highly inhomogeneous energy distribution configuration. In the superconducting transition regime, we find that the normalized resistance noise scales with resistance R excellently as SR /R 2 ∝ R l rs with the noise exponent l rs ≈ 1.4. With reduced iron vacancy disordering in enhanced superconductivity K x Fe2 – y Se2 crystals, the level of resistance fluctuations is greatly suppressed, suggesting a geometrical phase transition for conduction channel, which is directly related to the microstructure of the crystals.

Photodriven Transient Picosecond Top-Layer Semiconductor to Metal Phase-Transition in p-Doped Molybdenum Disulfide.

Visible light is shown to create a transient metallic S-Mo-S surface layer on bulk semiconducting p-doped indirect-bandgap 2H-MoS2 . Optically created electron-hole pairs separate in the surface band bending region of the p-doped semiconducting crystal causing a transient accumulation of electrons in the surface region. This triggers a reversible 2H-semiconductor to 1T-metal phase-transition of the surface layer. Electron-phonon coupling of the indirect-bandgap p-doped 2H-MoS2 enables this efficient pathway even at a low density of excited electrons with a distinct optical excitation threshold and saturation behavior. This mechanism needs to be taken into consideration when describing the surface properties of illuminated p-doped 2H-MoS2 . In particular, light-induced increased charge mobility and surface activation can cause and enhance the photocatalytic and photoassisted electrochemical hydrogen evolution reaction of water on 2H-MoS2 . Generally, it opens up for a way to control not only the surface of p-doped 2H-MoS2 but also related dichalcogenides and layered systems. The findings are based on the sensitivity of time-resolved electron spectroscopy for chemical analysis with photon-energy-tuneable synchrotronradiation.

The Structure of Defects, the Electron Energy-Band Structure, and the Semiconductor–Metal Transition in PrBaCo2O5.5 Cobaltite: Ab Initio PAW Approach

The electronic energy-band structure of the PrBaCo2O5 + δ cobaltite at the oxygen content close to 5.5 are calculated by the first-principle PAW methods. The semiconductor–metal phase transition at 5 + δ = 5.5 is shown to be a result of the transition of cobalt atoms in the octahedral environment from the high-spin to low-spin state. The cause of the appearance of the metallic conduction is an increase in the energy of antibonding eg states of pyramidal cobalt atoms, and, as a result, they are at the Fermi level, thereby determining the metallic character of the system. The effect of a deviation of the oxygen content from 5.5 on the energy-band structure and the conductivity is studied. The semiconductor–metal transition is shown can be observed only in a narrow range of the values of 5 + δ lower 5.5.

Electron injection induced phase transition of 2H to 1T MoS2 by cobalt and nickel substitutional doping

The semiconducting to metallic phase transition mechanism of MoS2 has gained immense attention due to the excellent electrocatalytic activity seen in catalysts and energy storage devices. However, the harsh conditions required for effective phase transition limits the applications of these materials. Compared with using flammable reductants, obtain metallic phase MoS2 by a chimie-douce route is widely expected. Herein, we synthesized high-1T phase-ratio MoS2 through a mild hydrothermal method with assistance of cobalt and nickel. The microstructure and composition of the Co,Ni-MoS2 material were detailed explored by cs-corrector transmission electron microscopy and synchrotron radiation spectrum. The process of phase transition was simulated by density functional theory calculation. We found that the phase transition was induced by electron injection during cobalt and nickel co-doping. Owing to abundant 1T-phase and smaller hydrogen adsorption/desorption energy, the prepared Co,Ni-MoS2 shows excellent hydrogen evolution reaction performance in alkaline solution.

Computational study on strain and electric field tunable electronic and optical properties of InTe monolayer

Abstract We comprehensively study the electronic and optical properties of InTe monolayer in the presence of biaxial strain and electric field using density functional theory. Via analysis phonon spectrum, InTe monolayer is confirmed to be dynamically stable. InTe monolayer has an indirect semiconducting characteristic with a band gap of 1.25 eV at equilibrium and can be controlled by strain or electric field. The semiconductor–metal phase transition has been found in InTe monolayer at a large electric field of E = 5 V/nm. The optical attributes of InTe monolayer depend strongly on the polarization direction of the incident light. The optical absorption coefficient is activated in the infrared region and increases rapidly in the visible light region. While optical properties are independent of the electric field, strain engineering alters not only the intensity of optical peaks but also their position.

Improving phase transition temperature of VO2 via Ge doping: a combined experimental and theoretical study

Enhancing the semiconductor–metal phase transition temperature (TSMT) of VO2 is of great consequence for further exploring the potential applications of VO2 at elevated temperatures. In this study, Ge4+-doped VO2 (GexV1−xO2) samples were prepared by the hydrothermal and annealing approach. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), differential scanning calorimetry (DSC) and resistivity–temperature (R-T) analyses were used to investigate the influence of Ge doping on the lattice structures and phase transition properties of GexV1–xO2 samples. We found that the lattice parameter of GexV1−xO2 decreased with the Ge concentration increasing from 2 at% to 18 at%, which was further supported by density functional theory (DFT)-based first-principle simulations. TSMT firstly increased from 64.5 to 73.0 °C at 8 at% Ge and then decreased to 71.5 °C at higher Ge concentration. Furthermore, DFT analysis revealed that the impact of lattice distortion induced by Ge doping rather than the changes in electronic structure is more pronounced on modulating TSMT of GexV1−xO2. The present work has pointed out the direction that the TSMT could be enhanced and illustrated the physical reason behind the regulation of TSMT in ions-doped VO2 systems. The d (logρ)/dT vs T curves are plotted for GexV1−xO2 (0≤x≤0.18) samples (a) un-doped VO2 ; (b) 2%; (c) 8%; (d) 18%, the transition temperatures upon heating, Th, and cooling, Tc. The difference between Th and Tc gives the hysteresis width, ΔTt, while the FWHM determines the sharpness of the semiconductor-to-metal transition

Controlling phase transition in WSe2 towards ideal n-type transistor

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been rapidly established as promising building blocks for versatile atomic scale circuits and multifunctional devices. However, the high contact resistance in TMDs based transistors seriously hinders their applications in complementary electronics. In this work, we show that an Ohmic homojunction n-type tungsten diselenide (WSe2) transistor is realized through spatially controlling cesium (Cs) doping region near the contacts. We find that the remarkable electron doping effect of Cs stimulates a semiconductor to metal (2H to 1T’) phase transition in WSe2, and hence the formation of 2H-1T’ hetero-phase contact. Our method significantly optimizes the WSe2 transport behavior with a perfect low subthreshold swing of ∼ 61 mV/dec and ultrahigh current on/off ratio exceeding ∼ 109. Meanwhile, the electron mobility is enhanced by nearly 50 times. We elucidate that the ideal n-type behavior originates from the negligible Schottky barrier height of ∼ 19 meV and low contact resistance of ∼ 0.9 kΩ·µm in the 2H-1T’ homojunction device. Moreover, based on the Ohmic hetero-phase configuration, a WSe2 inverter is achieved with a high gain of ∼ 270 and low power consumption of ∼ 28 pW. Our findings envision Cs functionalization as an effective method to realize ideal Ohmic contact in 2D WSe2 transistors towards high performance complementary electronic devices.

Guest-mediated phase transitions in a flexible pillared-layered metal-organic framework under high-pressure.

The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn2bdc2dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction. A pressure-induced structural phase transition is found for the EtOH- and DMF-included frameworks during compression in a hydrostatic medium of the guest species, which is dependent upon the nature and quantity of the guest in the channels. The EtOH-included material undergoes a phase transition from P4/mmm to C2/m at 0.69 GPa, which is accompanied by a change in the pore shape from square to rhombus via super-filling of the pores. The DMF-included material undergoes a guest-mediated phase transition from I4/mcm to P4/mmm at 0.33 GPa via disordering of the DMF guest. In contrast, the benzene-included framework features a structure with rhombus-shaped channels at ambient pressure and shows direct compression under hydrostatic pressure. These results demonstrate the large influence of guest molecules on the high-pressure phase behavior of flexible MOFs. Guest-mediated framework flexibility is useful for engineering MOFs with bespoke pore shapes and compressibility.

Recent developments in 2D transition metal dichalcogenides: phase transition and applications of the (quasi-)metallic phases.

The advent of two-dimensional transition metal dichalcogenides (2D-TMDs) has led to an extensive amount of interest amongst scientists and engineers alike and an intensive amount of research has brought about major breakthroughs in the electronic and optical properties of 2D materials. This in turn has generated considerable interest in novel device applications. With the polymorphic structural features of 2D-TMDs, this class of materials can exhibit both semiconducting and metallic (quasi-metallic) properties in their respective phases. This polymorphic property further increases the interest in 2D-TMDs both in fundamental research and for their potential utilization in novel high-performance device applications. In this review, we highlight the unique structural properties of few-layer and monolayer TMDs in the metallic 1T- and quasi-metallic 1T'-phases, and how these phases dictate their electronic and optical properties. An overview of the semiconducting-to-(quasi)-metallic phase transition of 2D-TMD systems will be covered along with a discussion on the phase transition mechanisms. The current development in the applications of (quasi)-metallic 2D-TMDs will be presented ranging from high-performance electronic and optoelectronic devices to energy storage, catalysis, piezoelectric and thermoelectric devices, and topological insulator and neuromorphic computing applications. We conclude our review by highlighting the challenges confronting the utilization of TMD-based systems and projecting the future developmental trends with an outlook of the progress needed to propel this exciting field forward.

Stable and reversible phase change performance of TiO2 coated VO2 nano-columns: Experiments and theoretical analysis

We have fabricated VO2 nano-columns (NCs) on the aluminized quartz glass substrates using glancing angle deposition (GLAD) technique inside the electron beam (e-beam) evaporator chamber. An e-beam deposited TiO2 thin cap-layer has been introduced on the top of the VO2 NCs to protect VO2 phase which is usually unstable and turns to stable higher oxidation state, i.e., V2O5. The X-ray diffraction (XRD) analysis of TiO2 thin film (TF) coated VO2 NCs shows the presence of VO2 phases along with the other oxidation states. The Fourier-Transform-Infrared-Spectroscopy (FTIR) confirms the presence of Ti–O–V, V–O vibrations bands in VO2 NCs/TiO2 TF samples. The VO2 NCs/TiO2 TF sample possesses a sharp drop (~103 order) in resistance at the transition temperature (Tc) ~66 °C. This sample also shows a stable semiconducting to metallic phase (vice versa as well) transition temperature (PTT) hysteresis even after one month which ensures the stable performance in terms of phase transition characteristic. Small-radius polaron model (SRPM) is used to calculate the sheet resistance of the VO2 NCs, where we consider the effect of thermal lattice vibrations on the overlap integral of the crystal atoms. The temperature dependency of constants A and ε have been observed uniquely. The Fermi-Dirac (F-D), Migdal distribution (MD) model have been taken into account for the first time to calculate the conduction band carrier density of VO2 NCs and ratio of the resistance in the semiconducting (RS), metallic phase (RM) is also calculated. The calculated electron concentration (nc) is 3.1 × 1019 cm-3 at the point of PTT (66 °C).

Hybrid Electrothermal Model for Insulator-to- Metal Transition in VO2 Thin Films

Comprehensive model for the insulator to metal phase transition in vanadium dioxide thin film in the presence of external electrical stimulus is proposed. This model divides the film layer into several segments and solves the Joule heating equations due to current flow and heat exchange with the environment and Poole–Frankel equation to obtain carrier concentration in response to the applied voltage, and transition is checked for each cell separately. The model follows the experimental results and exhibits both switching and hysteresis effect in planar structures and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}$ </tex-math></inline-formula> -shaped current–voltage characteristics in sandwich metal–insulator–metal structure based on the vanadium dioxide thin film. This model shows that by reducing the vanadium dioxide layer thickness, the electric field-assisted transition plays the dominant role in the transition, and therefore, it is a promising tool for the design and analysis of vanadium dioxide structures with various electrode distances.

Tunable electronic properties and electric-field-induced phase transition in phosphorene/graphene heterostructures

The shortcomings of mono-component systems e.g. the gapless nature of graphene, the lack of air-stability in phosphorene, etc, have drawn great attention toward stacked materials that are expected to show interesting electronic and optical properties. Using a tight-binding approach and a Green’s function method, we investigate the electronic properties of armchair-edged lateral phosphorene–graphene heterostructures, which are either semiconductor–semiconductor or semiconductor–metal heterostructures, depending on the width of the graphene ribbon. It is found that the system is narrow-gapped, and that the bandgap can be modulated by tuning the sizes of the domains. Besides, an analysis of the bandgap variation versus the width of the component phosphorene ribbon indicates that, in a semiconductor–metal heterostructure, a phosphorene ribbon does not induce any electronic state near the Fermi level, suggesting that the suppressed electron transport should be attributed to hole transfer across the interface. Furthermore, we show that a transverse electric field can significantly diversify the electronic behavior of a heterostructure i.e. the heterostructure undergoes a semiconductor–metal phase transition. Moreover, tuning the transverse electric field yields the intriguing possibility that the system can undergo a topological phase transition from a band insulator to a topological insulator.

Inside Back Cover: Configurational Entropy Driven High‐Pressure Behaviour of a Flexible Metal–Organic Framework (MOF) (Angew. Chem. Int. Ed. 2/2021)

Configurational entropy is revealed to play an important role in the pressure-induced phase transition of a pillared-layer metal–organic framework (MOF), as reported by Rochus Schmid, Gregor Kieslich, and co-workers in their Research Article on page 787. The cover image illustrates the wine-rack-type unit cell of the MOF, with focus on the spatial degrees of freedom of the dibutoxy side chains that lead to large contributions of configurational entropy in the large pore phase.

Topological Strain-Induced Regioselective Linker Elimination in a Chiral Zr(IV)-Based Metal-Organic Framework

Summary Zr-carboxylate metal-organic frameworks (MOFs) are structurally robust materials, in part due to their strong coordination bonds. The regioselective Zr–O bond cleavage and formation between 3D architectures are thus challenging and are heretofore unexplored. In this work, by introducing highly flexible 18-crown-6-ether functionalities into a homochiral Zr-MOF, we report an unprecedented topology transition in which a 4,10-connected framework undergoes a rapid solid-state transition into a thermodynamically more stable 4,8-connected analog by a regioselective-linker-elimination under ambient conditions. The transition process was unambiguously unraveled by single-crystal and powder X-ray diffraction studies, and we proposed a possible transition mechanism based on various control experiments and theoretical calculations. The excellent chemical stability and substantially expanded porosity and pore apertures endowed the transformed chiral MOF with an exceptional capacity for the enantioadsorptive and solid-phase extractive separation of the racemic drug molecule of lansoprazole with 98% ee and 93% ee, respectively.

Contrasting physical properties of the trilayer nickelates Nd4Ni3O10 and Nd4Ni3O8

We report the crystal structures and physical properties of trilayer nickelates Nd$_4$Ni$_3$O$_{10}$ and Nd$_4$Ni$_3$O$_8$. Measurements of magnetization and electrical resistivity display contrasting behaviors in the two compounds. Nd$_4$Ni$_3$O$_{10}$ shows a paramagnetic metallic behavior with a metal to metal phase transition($T^{\ast}$) at about 162 K, as revealed by both magnetic susceptibility and resistivity. Further magnetoresistance and Hall coefficient results show a negative magnetoresistance at low temperatures and the carrier type of Nd$_4$Ni$_3$O$_{10}$ is dominated by hole-type charge carriers. The significant enhancement of Hall coefficient and resistivity below $T^{\ast}$ suggest that effective charge carrier density decreases when cooling through the transition temperature. In contrast, Nd$_4$Ni$_3$O$_8$ shows an insulating behavior despite small value of resistivity at room temperature. The compound shows paramagnetic behavior, with the similar magnetic moments as in Nd$_4$Ni$_3$O$_{10}$ derived from the Curie-Weiss fitting. This may suggest that the magnetic moments in both systems are contributed by the Nd ions. By applying pressures up to about 49 GPa, the insulating behavior is still present and becomes even stronger. Our results suggest that the different Ni configurations ($Ni^{1+/2+}$ or $Ni^{2+/3+}$) and competition between localized and itinerant electrons may account for the contrasting behaviors in trilayer nickelates Nd$_4$Ni$_3$O$_{10}$ and Nd$_4$Ni$_3$O$_8$.

A Phase Transition Oxide/Graphene Interface for Incident‐Angle‐Agile, Ultrabroadband, and Deep THz Modulation

AbstractAlthough plenty of functional materials have been explored to dynamically control the terahertz (THz) transmission, the limits in modulation depth, operation bandwidth, or angle‐agile characteristics hinder their applications in highly active THz devices. Here, a 2D/3D (graphene/phase transition oxide VO2) interface is demonstrated for unprecedented high‐efficiency modulation of THz waves with incident‐angle‐agile and ultrabroadband. An impedance matching state can be met at the graphene/VO2 interface by adjusting the stacked graphene layers, which leads to a minimal terahertz reflection amplitude. Then the interfacial impedance can be changed continuously via the semiconductor–metal phase transition of VO2 and achieve a THz switching. Based on this mechanism, this heterostructure demonstrates giant THz average amplitude modulation of 93%, 94%, and 96% in 0.3–1.5 THz at the incident angle of 35°, 48°, and 53°, respectively. Moreover, the corresponding insertion loss is only −7.4, −4.1, and −6.5, which is much lower than most of the present THz modulation. These coupling phenomena and functions arising from the integration of 2D material and 3D phase transition oxide film broaden the previous materials system for dynamic controlling of THz wave, and would promote advanced THz smart devices for practical applications.

Electrical-mechanical coupling performance of gallium arsenide under nanoindentation

In this research, the electrical-mechanical coupling performance of gallium arsenide (GaAs) crystalline has been investigated by carrying out the in situ electrical measurement experiments which enabled us to record precisely both the mechanical behavior of the nanodeformed GaAs and the accompanying electrical response during nanoindentation. An effective and precise In situ electrical measurement system has been set up by introducing a conductive diamond indenter and both traditional and cyclic nanoindentation experiments have been performed. The experimental results showed that the application of indentation force caused a significant increase in the current flow due to the pressure-induced metallic phase transition of GaAs, and the magnitude of current flow was increased with increasing either the applied force or voltage. Furthermore, the current flow showed symmetric changes with the indentation loading and unloading, indicating a fully reversible metallic phase transformation of GaAs in indentation. On the other hand, the externally applied voltage (electrical field) was observed to barely affect the indentation force-depth curve (mechanical performance) of GaAs in respect of Young’s modulus and hardness.