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Disorder Control in Crystalline GeSb <sub/>2</sub> Te <sub/>4</sub> and its Impact on Characteristic Length Scales
摘要： Crystalline GeSb2Te4 (GST) is a remarkable material, as it allows to continuously tune the electrical resistance by orders of magnitude without involving a structural phase transition or stoichiometric changes. While well-ordered specimen are metallic, increasing amounts of disorder eventually lead to an insulating state with vanishing conductivity in the 0 K limit, but a similar number of charge carriers. Hence, GST provides ideal grounds to explore the impact of disorder on transport properties. Here, a sputter-deposition process is employed that enables growing biaxially textured GST films with large grain sizes on mica substrates. The resulting films exhibit a systematic variation between metallic and truly insulating specimen upon varying deposition temperature. Transport measurements reveal that their electron mean free path can be altered by a factor of 20, while always remaining more than an order of magnitude smaller than the lateral grain size. This proves unequivocally that grain boundaries play a negligible role for electron scattering, while intra-grain scattering, presumably by disordered vacancies, dominates. These findings underline that the insulating state and the system’s evolution toward metallic conductivity are intrinsic properties of the material.
关键词： metal–insulator transition,electrical properties,phase-change materials,disorder,structural properties
更新于2025-09-23 15:22:29
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Gate-Tunable Thermal Metal–Insulator Transition in VO <sub/>2</sub> Monolithically Integrated into a WSe <sub/>2</sub> Field-Effect Transistor
摘要： Vanadium dioxide (VO2) shows promise as a building block of switching and sensing devices because it undergoes an abrupt metal-insulator transition (MIT) near room temperature, where the electrical resistivity changes by orders of magnitude. A challenge for versatile applications of VO2 is to control the MIT by gating in the field-effect device geometry. Here, we demonstrate a gate-tunable abrupt switching device based on a VO2 microwire that is monolithically integrated with a two-dimensional (2D) tungsten diselenide (WSe2) semiconductor by van der Waals stacking. We fabricated the WSe2 transistor using the VO2 wire as the drain contact, titanium as the source contact, and hexagonal boron nitride as the gate dielectric. The WSe2 transistor was observed to show ambipolar transport, with higher conductivity in the electron branch. The electron current increases continuously with gate voltage below the critical temperature of the MIT of VO2. Near the critical temperature, the current shows an abrupt and discontinuous jump at a given gate voltage, indicating that the MIT in the contacting VO2 is thermally induced by gate-mediated self-heating. Our results have paved the way for the development of VO2-based gate-tunable devices by the van der Waals stacking of 2D semiconductors, with great potential for electronic and photonic applications.
关键词： 2D materials,field-effect transistor,tungsten diselenide,phase-change materials,vanadium dioxide,metal-insulator transition,van der Waals heterostructures
更新于2025-09-23 15:22:29
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Gram scale synthesis of monoclinic VO2 microcrystals by hydrothermal and argon annealing treatment
摘要： We report gram scale synthesis of 100 % phase fraction of VO2 (M) monoclinic in powder form with reversible phase transition by combining hydrothermal method and Ar annealing at high temperature. Optimization of single phase VO2 (M) growth and its phase transition characteristics have been analysed systematically by varying growth parameters such as time, synthesis temperature and post growth annealing conditions. Argon annealing of hydrothermally grown VO2 powders at 800°C found to play key role in obtaining VO2 (M) phase in gram scale with characteristic phase transition temperature of 68°C. In-situ TEM has been performed to investigate the microstructure and phase change across the annealing temperature. Detailed characterizations have been carried out to correlate the phases, microstructure and transition temperature of VO2 with respect to growth parameters.
关键词： metal-insulator transition,annealing treatment,monoclinic VO2,in-situ TEM,microcrystals,Scalable synthesis
更新于2025-09-23 15:21:21
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The role of ionic liquid breakdown in the electrochemical metallization of VO <sub/>2</sub> : An NMR study of gating mechanisms and VO <sub/>2</sub> reduction
摘要： Metallization of initially insulating VO2 via ionic liquid electrolytes, otherwise known as electrolyte gating, has recently been a topic of much interest for possible applications such as Mott transistors and memory devices. It is clear that the metallization takes place electrochemically and, in particular, there has previously been extensive evidence for the removal of small amounts of oxygen during ionic liquid gating. Hydrogen intercalation has also been proposed, but the source of the hydrogen has remained unclear. In this work, solid-state magic angle spinning NMR spectroscopy (1H, 2H, 17O and 51V) is used to investigate the thermal metal-insulator transition in VO2, before progressing to catalytically hydrogenated VO2 and electrochemically metallized VO2. In these experiments electrochemical metallization of bulk VO2 particles is shown to be associated with intercalation of hydrogen, the degree of which can be measured with quantitative 1H NMR spectroscopy. Possible sources of the hydrogen are explored, and by using a selectively deuterated ionic liquid, it is revealed that the hydrogenation is due to deprotonation of the ionic liquid; specifically, for the commonly used dialkyl-imidazolium based ionic liquids, it is the “carbene” proton which is responsible. Increasing the temperature of the electrochemistry is shown to increase the degree of hydrogenation, forming first a less hydrogenated metallic orthorhombic phase then a more hydrogenated insulating Curie-Weiss paramagnetic orthorhombic phase, both of which were also observed for catalytically hydrogenated VO2. The NMR results are supported by magnetic susceptibility measurements, which corroborate the degree of Pauli and Curie-Weiss paramagnetism. Finally, NMR spectroscopy is used to identify the presence of hydrogen in an electrolyte gated thin film of VO2, suggesting that electrolyte breakdown, proton intercalation and reactions with decomposition products within the electrolyte should not be ignored when interpreting the electronic and structural changes observed in electrochemical gating experiments.
关键词： metal-insulator transition,hydrogen intercalation,VO2,NMR spectroscopy,electrolyte gating,ionic liquid
更新于2025-09-23 15:21:21
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Effect of photo-irradiation on metal insulator transition in vanadium dioxide
摘要： We report a large transition temperature (TC) decrease in a VO2 thin film device under photo-irradiation. With the increasing photo-irradiation intensity (PIntensity) with a wavelength of 405 nm, the TC of the VO2 device decreased at a rate of (cid:2)3:2 (cid:3) 10(cid:4)2 W=cm2 and reached as low as 40.0 (cid:5)C at a PIntensity of 8:4 (cid:3) 102 W=cm2. While the change in TC is primarily due to the photothermal effect when the PIntensity is below 3:6 (cid:3) 102 W=cm2, both the photothermal and photo-induced carrier density effects contribute to the change in TC when the PIntensity is above 6:4 (cid:3) 102 W=cm2.
关键词： metal-insulator transition,vanadium dioxide,photothermal effect,photo-induced carrier density,photo-irradiation
更新于2025-09-23 15:21:21
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Isostructural metal-insulator transition in VO <sub/>2</sub>
摘要： The metal-insulator transition in correlated materials is usually coupled to a symmetry-lowering structural phase transition. This coupling not only complicates the understanding of the basic mechanism of this phenomenon but also limits the speed and endurance of prospective electronic devices. We demonstrate an isostructural, purely electronically driven metal-insulator transition in epitaxial heterostructures of an archetypal correlated material, vanadium dioxide. A combination of thin-film synthesis, structural and electrical characterizations, and theoretical modeling reveals that an interface interaction suppresses the electronic correlations without changing the crystal structure in this otherwise correlated insulator. This interaction stabilizes a nonequilibrium metallic phase and leads to an isostructural metal-insulator transition. This discovery will provide insights into phase transitions of correlated materials and may aid the design of device functionalities.
关键词： electronic phase transition,vanadium dioxide,metal-insulator transition,correlated materials,heterostructures
更新于2025-09-23 15:21:01
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A laser-ARPES study of LaNiO <sub/>3</sub> thin films grown by sputter deposition
摘要： Thin films of the correlated transition-metal oxide LaNiO3 undergo a metal–insulator transition when their thickness is reduced to a few unit cells. Here, we use angle-resolved photoemission spectroscopy to study the evolution of the electronic structure across this transition in a series of epitaxial LaNiO3 films of thicknesses ranging from 19 u.c. to 2 u.c. grown in situ by RF magnetron sputtering. Our data show a strong reduction in the electronic mean free path as the thickness is reduced below 5 u.c. This prevents the system from becoming electronically two-dimensional, as confirmed by the largely unchanged Fermi surface seen in our experiments. In the insulating state, we observe a strong suppression of the coherent quasiparticle peak, but no clear gap. These features resemble previous observations of the insulating state of NdNiO3.
关键词： LaNiO3,angle-resolved photoemission spectroscopy,electronic structure,metal–insulator transition,thin films
更新于2025-09-23 15:21:01
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by angle-resolved photoelectron spectroscopy, scanning tunneling spectroscopy, and density functional theory
摘要： The electronic structure of 1T -TaS2 showing a metal-insulator transition and a sequence of different charge density wave (CDW) transformations was discussed in the frame of variable temperature angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. For the commensurate charge density wave phase (CCDW) the Mott gap was estimated to be 0.4 eV and energy gaps (cid:2)CCDW,1, (cid:2)CCDW,2, (cid:2)B3-HHB, (cid:2)B4-B3 were observed. For the nearly commensurate charge density wave phase (NCCDW), the reminiscent of higher and lower Hubbard bands and a very pronounced electronic state associated with the parabolic band at the ˉ(cid:3) point in the Brillouin zone were identi?ed. The incommensurate charge density wave phase (ICCDW) showed a high value of local density of states at the Fermi level and a very pronounced edge of the metallic surface state located in the range of 0.15–0.20 eV above the Fermi level. The obtained STS and ARPES results were consistent with our theoretical calculations performed within DFT formalism including spin-orbit coupling.
关键词： STS,1T -TaS2,charge density wave,ARPES,DFT,metal-insulator transition
更新于2025-09-23 15:21:01
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Selective growth of monolayer semiconductors for diverse synaptic junctions
摘要： The information computation through synapse networks in the brain plays a vital role for cognitive behaviors such as image/video recognition, self-learning, and decision-making. Achieving proper synaptic networks by conventional semiconductor and memristive devices has encountered critical issues such as the spatial density requiring a number of transistors for one synapse, reliable filament formation in memristors, or emulating diverse excitatory and inhibitory synaptic plasticity with two-terminal device geometry. Here, we report selective growth of variously doped MoS2 with controllable conductance plasticity, which can be used for emulating diverse synaptic junctions. The conductance plasticity in the monolayer MoS2 was found to originate from resistive-heating near the junctions with electrodes in the two-terminal device geometry and the carrier-concentration-dependent metal-insulator transition in the MoS2 channel. A spatiotemporal synaptic summation is demonstrated where the firing of a proper postsynaptic membrane potential can be designed for cognitive processes. Compared with previously reported three terminal synaptic devices with atomically thin materials, our two-terminal devices with flexible synaptic strengths have advantages for integrating three-dimensional neuronal networks. This provides a new insight on two-dimensional materials as a promising arena for integrated synaptic functionalities in artificial neural networks.
关键词： chemical vapor deposition,metal-insulator transition,defects engineering,two dimensional materials,synaptic junction
更新于2025-09-23 15:21:01
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Development of a Physical Model of Thermovoltaic Effects in the Thin Films of Zinc Oxide Doped with Transition Metals
摘要： A model of the thermovoltaic effect emergence in ZnO/ZnO<Me> (Me = Cu, Fe), sandwich structures has been developed in the article. The samples were made by the sol-gel method. When they were uniformly heated in a laboratory furnace in the temperature range of 200–300 ?C, there an electromotive force (EMF) of ?7~10 mV, not associated with the Seebeck effect, emerged. The developed physical mechanisms of the effect emergence consist of the following well-known fact: iron and copper coexist in zinc oxide in two states, namely, Fe2+ and Fe3+ (donor), and Cu2+ and Cu+ (acceptor). During the heating of the ZnO/ZnO–Me system, the concentration of charge carriers in the layers will increase, while in the upper layer its value will be larger because of the presence of electrically active impurities. At room temperatures, Coulomb forces retain an electron that is located on the Fe2+ ion, as well as a hole on Cu2+ ion, and the main states undergo ionization. However, as the temperature increases, the carrier concentration can reach a critical level, when they can screen the ion charge (the Debye screening radius decreases to the Bohr radius of the impurity). In this case, an abrupt collective endothermic process of ionization of multivalent impurities takes place, accompanied by the appearance of a concentration gradient of free carriers in the sample, and accordingly, the emergence of an electromotive force. Quantitative calculations of the critical temperature, at which the onset of EMF generation is observed, performed within the framework of the developed models.
关键词： spontaneous voltage generation,metal-insulator transition,thermovoltaic effect,zinc oxide
更新于2025-09-19 17:15:36