Disorder-Induced Metal–Insulator Transition in Conducting Polymers

The metal–insulator (M-I) transition in conducting polymers is particularly interesting; critical behavior has been observed over a relatively wide temperature range in a number of systems, including polyacetylene, polypyrrole, poly (p-phenylene vinylene), and polyaniline. In each case, metallic, critical, and insulating regimes have been identified through Zabrodskii plots of the logarithmic derivative of the conductivity. The critical regime (in which the conductivity varies as T, where 1/3) is tunable by varying the extent of disorder and by applying external pressure and/or an external magnetic field. The transitions from metallic to critical behavior and from critical to insulating behavior have been induced with a magnetic field and from insulating to metallic behavior with applied pressure.     

A Thermal Radiation Modulation Platform by Emissivity Engineering with Graded Metal–Insulator Transition

Thermal radiation from a black body increases with the fourth power of absolute temperature (T⁴), an effect known as the Stefan–Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (ε_int), is insensitive to temperature (|dε_int/dT| ≈ 10⁻⁴ °C^–1). The resultant radiance bound by the T⁴ law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which ε_int can be engineered to decrease in an arbitrary manner near room temperature (|dε_int/dT| ≈ 8 × 10⁻³ °C^–1), enabling unprecedented manipulation of infrared radiation. As an example, ε_int is programmed to vary with temperature as the inverse of T⁴, precisely counteracting the T⁴ dependence; hence, thermal radiance from the surface becomes temperature-independent, allowing the fabrication of flexible and power-free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten-doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.     

Anomalous Domain Wall Magnetoresistance in Ultrathin Manganite Films Near M–I Transition Boundary

Resistance and magnetoresistance in compressively strained epitaxial ultrathin films have been studied. The samples were first demagnetized in different ways so that different magnetic structures were created, such as random domain and single domain states. Very large difference in resistance in zero applied magnetic field was observed between different states. The large change of resistance between states is attributed to spin-dependent scattering at the domain walls. We have shown for the first time that large domain wall resistance can be obtained in strained ultrathin manganite films and the result cannot be explained by the double-exchange model.     

Chemical Modulation of Metal–Insulator Transition toward Multifunctional Applications in Vanadium Dioxide Nanostructures

The metal–insulator transition (MIT) of vanadium dioxide (VO₂) has been of great interest in materials science for both fundamental understanding of strongly correlated physics and a wide range of applications in optics, thermotics, spintronics, and electronics. Due to the merits of chemical interaction with accessibility, versatility, and tunability, chemical modification provides a new perspective to regulate the MIT of VO₂, endowing VO₂ with exciting properties and improved functionalities. In the past few years, plenty of efforts have been devoted to exploring innovative chemical approaches for the synthesis and MIT modulation of VO₂ nanostructures, greatly contributing to the understanding of electronic correlations and development of MIT-driven functionalities. Here, this comprehensive review summarizes the recent achievements in chemical synthesis of VO₂ and its MIT modulation involving hydrogen incorporation, composition engineering, surface modification, and electrochemical gating. The newly appearing phenomena, mechanism of electronic correlation, and structural instability are discussed. Furthermore, progresses related to MIT-driven applications are presented, such as the smart window, optoelectronic detector, thermal microactuator, thermal radiation coating, spintronic device, memristive, and neuromorphic device. Finally, the challenges and prospects in future research of chemical modulation and functional applications of VO₂ MIT are also provided.     

Static and Dynamic Properties of the Insulator–Metal Transition in Magnetic Semiconductors, Including the Perovskites

All magnetic semiconductors experience an insulator–metal transition with decreasing temperature near and below the magnetic ordering temperature. T_c, as long as the carrier concentration, whether electrons or holes, is low enough. For somewhat higher carrier concentrations, a resistivity anomaly ordinarily occurs near T_c, which cannot be explained in terms of scattering from the magnetic fluctuations alone. The thrust of this paper is to review the magnetotransport properties of various magnetic semiconductors and to present arguments that magnetic polarons, a many body state involving charge carriers and the localized magnetic spins in their immediate neighborhoods, are involved in the physical processes leading to the insulator–metal transition. Magnetic polarons have been observed directly by a variety of physical techniques including, for example, magnetic measurements, neutron diffraction, scanning tunneling microscopy, and, most recently, by noise measurements. This article will review these physical manifestations of the existence of magnetic polarons and relate them to the transition.     

Electrical Conductivity of Composite Materials Based on Fine-Particle Silicon near the Metal–Insulator Transition

Si/SiO₂ composites containing 17, 19, and 21 wt % crystalline B-doped Si particles (near the insulator–metal transition) are studied by impedance spectroscopy. The results indicate that, at low frequencies, the nonlinear variation of the real part of electrical conductivity, Re, with applied dc voltage V for the composite containing 17 wt % Si (dielectric properties) is due to the current being limited by the space charge buildup at traps in the silica layers between Si particles. At high frequencies, the nonlinear Re(V) behavior is due to tunneling through the contacts between the particles.     

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

Technical Physics Letters - Temperature dependences of the ac conductivity of nanocrystalline mixed vanadium oxide films on silicon revealed a multistep shape of the hysteresis loop observed during...     

Dynamic Electronic Structure in Transition Metal Oxides

The isomer shift of La1-xCaxFeO3 decreases gradually from the value for trivalent iron to that for tetravalent iron, as the Ca content x increases from 0 to 1. This indicates that iron in La1.x-CaxFeO3 has an intermediate valence state. The intermediate valence state of iron increases gradually from trivalence to tetravalence. This can be interpreted as being due to electron delocalization among iron ions on identical octahedral sites. The Mossbauer spectra at various temperatures of La0.5Ca0.5FeO3 and their theoretical treatment show that electron delocalization slows down with decreasing temperature.     

Surface Modification of α-Fe Metal Particles by Chemical Surface Coating

The structure of α-Fe metal magnetic recording particles coated with silane coupling agents have been studied by TEM, FT-IR, EXAFS, Mossbauer. The results show that a close, uniform, firm and ultra thin layer, which is beneficial to the magnetic and chemical stability, has been formed by the cross-linked chemical bond Si-O-Si. And the organic molecule has chemically bonded to the particle surface, which has greatly affected the surface Fe atom electronic structure. Furthermore, the covalent bond between metal particle surface and organic molecule has obvious effect on the near edge structure of the surface Fe atoms.     

Dynamic Scattering of Light in Liquid-Crystal Cells with Metal–Insulator–Semiconductor Microstructures Containing Gold Nanoparticles

Technical Physics Letters - The possibility of reducing the threshold of the effect of dynamic light scattering in liquid-crystal cells has been demonstrated. The threshold lowering is favored by...     

Electrical and Thermal Characteristics of the Insulator–Metal Transition in Crystalline $$\hbox {V}_{2}\hbox {O}_{5}$$ Films

The electrical and thermal properties with respect to the crystallization in \(\hbox {V}_{2}\hbox {O}_{5}\) thin films were investigated by measuring the resistance at different temperatures and applied voltages. The changes in the crystal structure of the films at different temperatures were also explored using Raman measurements. The thermal diffusivity of the crystalline \(\hbox {V}_{2}\hbox {O}_{5}\) film was measured by the nanosecond thermoreflectance method. The microstructures of amorphous and crystalline \(\hbox {V}_{2}\hbox {O}_{5}\) were observed by SEM and XRD measurements. The temperature-dependent Raman spectra revealed that a structural phase transition does not occur in the crystalline film. The resistance measurements of an amorphous film indicated semiconducting behavior, whereas the resistance of the crystalline film revealed a substantial change near \(250\,{^{\circ }}\hbox {C}\), and Ohmic behavior was observed above \(380\,{^{\circ }}\hbox {C}\). This result was due to the metal–insulator transition induced by lattice distortion in the crystalline film, for which \(T_{\mathrm{c}}\) was \(260\,{^{\circ }}\hbox {C}\). \(T_{\mathrm{c}}\) of the film decreased from 260 \({^{\circ }}\hbox {C}\) to \(230\,{^{\circ }}\hbox {C}\) with increasing applied voltage from 0 V to 10 V. Furthermore, the thermal diffusivity of the crystalline film was \(1.67\times 10^{-7}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}\) according to the nanosecond thermoreflectance measurements.     

Metal–Nonmetal Transition in Rubidium Tungsten Bronze RbxWOy

An oxygen concentration dependent metal–nonmetal (MN) transition was observed for Rb_0.23WO _y with 2.80 < y < 3.08. As 2.80 < y < 3.0, the room temperature resistivity (_RT) of the Rb_0.23WO _y is about 5 × 10^–4 cm. While in the case of y > 3.04, the _RT of the Rb_0.23WO _y exhibits a four orders of magnitude increase with a value of 5 cm. Correspondingly, the lattice constant along c-direction slightly shortens as oxygen concentration increases from 2.80 to 3.08. The observed results suggest that the hybridization between W 5d (t_2g) and O 2p orbitals might be responsible for the MN transition. In addition, similar measurements were performed for Rb _x WO_3.04 and Rb _x WO_2.85 with 0.19 < x < 0.27. No rubidium concentration dependent MN transition was observed, indicating the electronic structure of the host WO _y is not modified significantly by varying the soluble rubidium concentration.     

Study of the Electron–Phonon Relaxation in Thin Metal Films Using Transient Thermoreflectance Technique

Electron–phonon (e–ph) relaxation in thin metal films is an important consideration in many ultra-small and ultra-fast applications. In this work, e–ph relaxation in thin gold and aluminum films has been studied using the transient thermoreflectance technique which is demonstrated sensitive enough to study the relaxation process. The optical properties of the thin metal films are different from those of bulk metal and have been measured. Based on confirmation of the measurements, the effects of metal type, film thickness, and interface on e–ph relaxation have been experimentally studied. The thermoreflectance traces of gold and aluminum films have been compared. The results show that the e–ph relaxation and the effect of electron and lattice temperatures on the thermoreflectance of gold and aluminum are quite different. The e–ph relaxation is independent of film thickness and interface.     

Radiative and Optical Properties of La1−xSrxMnO3 (0≤x≤0.4) in the Vicinity of Metal–Insulator Transition Temperatures from 173 to 413K

Radiative and optical properties of polycrystalline La_1–x Sr _x MnO₃ (0x0.4) in the vicinity of the metal–insulator transition are presented. The temperature dependence of the total hemispherical emittance _H of La_1–x Sr _x MnO₃ was measured by the calorimetric method in the temperature range from 173 to 413K. It was confirmed that _H showed unexpected variation as a result of changes in the hole concentration (x). Especially in the case of La_0.825Sr_0.175MnO₃, _H remains high above the transition temperature T _C due to insulator-like behavior; on the other hand, it decreases sharply below T _C because of metallic behavior. The spectral reflectance was measured by FT-IR in the wavelength range of 0.25 to 100 m at room temperature. The optical constants were calculated by Kramers–Kronig analysis of the spectral reflectance data. An insulator-like character of the optical properties appears at lower Sr²⁺ doping levels while a metallic one exists at higher Sr²⁺ doping levels.     

Ultrathin 2D Metal–Organic Framework Nanosheets In situ Interpenetrated by Functional CNTs for Hybrid Energy Storage Device

The controllable construction of two-dimensional(2D)metal–organic framework(MOF)nanosheets with favorable electrochemical performances is greatly challenging for energy storage.Here,we design an in situ induced growth strategy to construct the ultrathin carboxylated carbon nanotubes(C-CNTs)interpenetrated nickel MOF(Ni-MOF/C-CNTs)nanosheets.The deliberate thickness and specific surface area of novel 2D hybrid nanosheets can be effectively tuned via finely controlling C-CNTs involvement.Due to the unique microstructure,the integrated 2D hybrid nanosheets are endowed with plentiful electroactive sites to promote the electrochemical performances greatly.The prepared Ni-MOF/C-CNTs nanosheets exhibit superior specific capacity of 680 C g^−1 at 1 A g^−1 and good capacity retention.The assembled hybrid device demonstrated the maximum energy density of 44.4 Wh kg^−1 at a power density of 440 W kg^−1.Our novel strategy to construct ultrathin 2D MOF with unique properties can be extended to synthesize various MOF-based functional materials for diverse applications.     

Magnetic Field Effects on Surface Morphology and Magnetic Properties of Co–Ni–N Thin Films Prepared by Electrodeposition

The surface morphology and magnetic properties of Co?CNi?CN thin films electrodeposited under an external magnetic field were investigated. The films were electroplated on Al substrates using the same electrodeposition parameters (temperature and pH) for all experiments, with an external magnetic field of 107?Oe applied to the cathode surface. The films were compared with similar samples obtained in the absences of magnetic field. The magneto-induced modifications in the Co?CNi?CN morphology can be explained by the specific local convection of ions at the interface cathode-electrolyte, which promotes changes both in the electrical charge of the double layer and in the thickness of the diffusion layer. From the magnetic measurements, we found that the coercivity varied between H _c =(14÷27)?kA/m depending on the direction of the applied magnetic field and on the sodium nitrate content in the plating bath. It was observed that an induced anisotropy appeared in the Co?CNi?CN films due to the preferential orientation of the easy axis of magnetization in the magnetic field direction. In addition, the Co?CNi?CN alloy films showed good magnetic property, which is considered that not only the smaller grain size of the films, but also more uniform surface of the films than that deposited in absence of applied magnetic field.     

Preparation of Thick LaCu1 – x Ni x O2.5 + δ Films Exhibiting a High-Temperature Metal–Semiconductor Transition

The conditions for producing thick LaCu_{1 – x} Ni _x O_{2.5 +} films on different substrates were optimized. The effects of the heat-treatment conditions, substrate material, and the nature of the liquid organic binder on the composition, structure, and properties of the films were studied. Single-phase coatings obtained on MgO, ZrO₂, BaSO₄, and Ni substrates 50 to 200 m in thickness were close in properties to bulk LaCu_{1 – x} Ni _x O_{2.5 +} and exhibited a metal–semiconductor transition at about 500°C.