Metal-insulator Transition Behavior Research Articles

Optimization of electrical transport and magnetoresistance of La0.72Ca0.28-xPbxMnO3 ceramics by Pb2+ doping

Here, La0.72Ca0.28-xPbxMnO3 (LCPMO, 0 ≤ x ≤ 0.06) polycrystalline ceramics are prepared by the sol-gel method and the influence of Pb2+ doping on microstructure, chemical composition, and electrical transport properties of LCPMO ceramics is systematically investigated. The study was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy-Dispersive Spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and four-probe method. The four-probe test shows that all samples have MIT (metal-insulator transition) behavior, and the peak resistance gradually decreases with the increase of Pb2+ doping amount. With the increase of Pb2+ doping amount, the temperature coefficient of resistance (TCR) and magnetoresistance (MR) first increased and then decreased, the TCR increased from 24.6 % to 36.5 %·K−1, and the MR value increased from 47 % to 71.5 %, and both corresponded to high temperature direction. Referring to previous studies, higher TCR and MR values near room temperature are what researchers have been pursuing for a long time. it can be seen that the LCPMO ceramics prepared in this article can have good application prospects in infrared detection and magnetic storage.

Solvent-free microfabrication of thin film device using the focused ion beam

We introduce a novel fabrication technique for thin film devices, utilizing Focused Ion Beam micro-machining. Unlike conventional methods requiring post processes such as etching or solvent treatment, this method involves creating a pre-patterning step-like structure on the substrate before growing the thin film, achieving the desired device pattern right after thin film deposition. As an example, we fabricated devices with VO2 thin film of which electrical transport properties are examined. The VO2 devices exhibit robust metal-insulator transition behavior and V-I characteristics, indicating the high quality of the patterned devices. The results indicate that our approach enables intricate patterning, comparable to conventional e-beam lithography, without the risk of sample degradation. The simplicity, stability, and versatility of this technique suggest new possibilities for thin film device fabrication.

Effects of oxygen vacancies and interfacial strain on the metal-insulator transition of VO2 nanobeams.

The role of oxygen vacancies and interfacial strain on the metal-insulator transition (MIT) behavior of high-quality VO2 nanobeams (NBs) synthesized on SiO2/Si substrates employing V2O5 as a precursor has been investigated in this research. Selective oxygen vacancies have been generated by argon plasma irradiation. The MIT is progressively suppressed as the duration of plasma processing increases; in addition, the temperature of MIT (TMIT) drops by up to 95 K relative to the pristine VO2 NBs. Incorporating oxygen vacancies into VO2 may increase its electron concentration, which might shift the Fermi levels upward, strengthen the electronic orbital overlap of the V-V chains, and further stabilize the metallic phase at lower temperatures, based on first-principles calculations. Furthermore, in order to evaluate the influence of substrate-induced strain in our situation, the MIT in two distinct types of VO2 NB samples is examined without metal contacts by using the distinctive light scattering characteristics of the metal (M) and insulator (I) phases (i.e., M/I domains) by optical microscopy. It is found that the domain structures in the "clamped" NBs persisted up to ∼453 K, while the "released" NBs (transferred to a new substrate) did not exhibit any domain structures and turned into an entirely M phase with a dark contrast above ∼348 K. When combined with first-principles calculations, the electronic orbital occupancy in the rutile phase contributes to explaining the interfacial strain-induced modulation of MIT. The current findings shed light on how interfacial strain and oxygen vacancies impact MIT behavior. It also suggests several types of control strategies for MIT in VO2 NBs, which are essential for a broader spectrum of VO2 NB applications.

Strong Substrate Influence on Atomic Structure and Properties of Epitaxial VO2 Thin Films

AbstractThe metal–insulator transition (MIT) observed in vanadium dioxide has been a topic of great research interest for past decades, with the underlying physics yet not fully understood due to the complex electron interactions and structures involved. The ability to understand and tune the MIT behavior is of vital importance from the perspective of both underlying fundamental science as well as potential applications. In this work, scanning transmission electron microscopy (STEM) is used to investigate cross‐section lamella of the VO2 films deposited using pulsed laser deposition on three substrates: c‐cut sapphire, TiO2(101) and TiO2(001). Advanced STEM imaging is performed in which also the oxygen atom columns are resolved. The overall film quality and structures on atomic and nanoscale are linked to the electrical transition characteristics. Relatively poor MIT characteristics are observed on c‐sapphire due to the presence of very small domains with six orientation variants, and on TiO2 (001) due to the presence of cracks induced by stress relaxation. However, the MIT on TiO2 (101) behaves favorably, despite similar stress relaxation which, however, only leads to domain boundaries but no cracks.

Continuously tunable the phase transition behavior of VO2 by T-doping and the formation of T-T chain (T = Ta, Nb)

Vanadium dioxide (VO2), a typical strongly correlated transition metal oxide, undergoes a fascinating reversible metal-insulator transition (MIT) at 340 K, attracting increasing interest from both scientific and technological perspectives. Nevertheless, achieving a rational regulation of the phase transition temperature (Tc) and gaining a deeper understanding of the modulation mechanism, as well as clarifying the relationship between the dopants and Tc in VO2, remain significant ongoing challenges. In this work, we present a density functional theoretical study on the structural and electronic properties of T-doped (T = Ta, Nb) VO2. The results demonstrate that the introduction of T (T = Ta, Nb) atoms tends to drive the R phase structure towards a dimerized monoclinic-like structure, while the uniformly aligned V–V chains exhibit alternating short and long V–V dimerization feature. Notably, the R phase VO2 appears to be more susceptible to structural perturbations. Moreover, T (T = Ta, Nb) doping concentration, compressive strain and the numbers of T-T chain cause curious changes in the structural evolution and electronic structures of VO2, acting as effective modulation methods for tuning the phase transition temperature. In addition, the Tc of VO2 could be significantly reduced when the concentration of T (T = Ta, Nb) dopant is less than 6.3%. The current results offer valuable insights into the mechanism of the doping driven MIT in T-doped (T = Ta, Nb) VO2, and contribute to a deeper understanding of the relationship between the dopants and phase transition temperature, further providing valuable insights into its metal-insulator transition behavior.

Printed and Room Temperature Processed Nanoparticulate VO2 Thin Films Toward Memristive Device Applications

Vanadium dioxide (VO2) has received tremendous research interest in recent years for its versatile electronic application possibilities based on its reversible metal-insulator transition (MIT) behavior, which can be triggered by a large set of stimuli, such as current, electric field, or light. VO2 undergoes a phase transition from a semi-insulating monoclinic to metallic rutile phase at a low critical temperature of 68 °C, which is very close to room temperature, and compatible with any inexpensive flexible substrate. However, owing to the existence of various vanadium oxide polymorphs, the synthesis of the pure monoclinic VO2 (M) phase is a challenge. In fact, the difficulty level increases further when it is to be solution-processed. However, synthesis protocol for phase-pure VO2 (M) following wet chemical routes is essential to obtain high throughput, cost-efficient, and substrate-independent fabrication routine for the large volume of devices. In this regard, in the present study, we report a solution-based synthesis of ultra-small VO2 (M) nanoparticles that are phase-pure and the small nanoparticles may easily be used to formulate inkjet printable VO2 nanoparticulate. The printed VO2 (M) films have been found to be homogeneous, crack-free and the memristor device thus fabricated has demonstrated temperature-dependent phase transition and a resistance change of more than three orders of magnitude around 68 °C; on the other hand, retention of the metallic and insulating state has also been demonstrated for 180 minutes.

Metal-insulator transition tuned by valence variation of Nb dopants in Nb-doped VO2 films

High-valence cation doping was proved to be an effective method in reducing the high metal–insulator transition (MIT) temperature (θc). However, the mechanism of cation-dopants reducing θc is not quite clear and the reported data about the θc reduction induced by Nb dopants are dispersive. In this work, we prepared Nb-doped VO2 films by hydrothermal process and investigated the effects of Nb dopants on the MIT behavior. Valence variation of Nb dopants was observed in the Nb-doped VO2. The Nb5+ dopants elevate the valence band top to higher level than the Nb4+-containing sample. Correspondingly, the Nb5+ doping reduces θc in a higher rate of ∼21 ℃/at%Nb than the Nb4+-containing sample which has the decline rate of ∼18 ℃/at%Nb. The results imply Nb5+ dopants contributing excess electrons modify the band structure and consequently promoting the MIT transition. In addition, the Nb doping realizes the θc reduction almost without detriment to the optical properties of VO2, suggesting the promising applications in smart window and other optical switching devices.

Dynamic Modulation of Phase Transition by External Strain Engineering in Quasi‐van der Waals Epitaxial VO2 Films on Fluorophlogopite

AbstractUnique metal–insulator transition behaviors of strongly correlated electronic materials, vanadium dioxide (VO2), and their wide potential applications have gained much attention for investigation. In this research, high‐quality epitaxial films of VO2 (020) are directly grown on fluorophlogopite (001) substrates, and the relationship between phase‐transition temperature and external strains is revealed. After verifying the like‐freestanding property and low intrinsic resistance changing of VO2/fluorophlogopite, variable phase‐transition temperatures under different external strains with a tuning rate of 5.37 K per 0.1% strain are obtained. Based on experimental results and theoretical calculation, it is speculated that lattice constant and bandgap between d// and π * are strongly affected by the external strains, which allow for more effective dynamic modulation of phase‐transition process. This research provides a comprehensive understanding of strain engineering on phase‐transition properties and also broadens the possibility of potential applications of certain optoelectronic devices for strain modulation.

Nonvolatile Control of Metal-Insulator Transition in VO2 by Ferroelectric Gating.

Controlling phase transitions in correlated materials yields emergent functional properties, providing new aspects to future electronics and a fundamental understanding of condensed matter systems. With vanadium dioxide (VO2 ), a representative correlated material, an approach to control a metal-insulator transition (MIT) behavior is developed by employing a heteroepitaxial structure with a ferroelectric BiFeO3 (BFO) layer to modulate the interaction of correlated electrons. Owing to the defect-alleviated interfaces, the enhanced coupling between the correlated electrons and ferroelectric polarization is successfully demonstrated by showing a nonvolatile control of MIT of VO2 at room temperature. The ferroelectrically-tunable MIT can be realized through the Mott transistor (VO2 /BFO/SrRuO3 ) with a remanent polarization of 80 µC cm-2 , leading to a nonvolatile MIT behavior through the reversible electrical conductance with a large on/off ratio (≈102 ), long retention time (≈104 s), and high endurance (≈103 cycles). Furthermore, the structural phase transition of VO2 is corroborated by ferroelectric polarization through in situ Raman mapping analysis. This study provides novel design principles for heteroepitaxial correlated materials and innovative insight to modulate multifunctional properties.

Cu doping effects on the electronic structure of Fe1−xCuxSe

Using angle-resolved photoemission spectroscopy, we investigated the evolution of the electronic structure of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{Se}$ from $x=0$ to 0.10. We found that the substitution of Fe by Cu introduces extra electron carriers. The hole bands near the $\mathrm{\ensuremath{\Gamma}}$ point were observed to shift downward with increasing doping $x$ and completely sank down below the Fermi level (${E}_{F}$) for $x\ensuremath{\ge}0.05$. Meanwhile, the electron pockets near the M point became larger but lost the spectral weight near ${E}_{F}$. Concomitantly, the effective mass of the electron bands increased with doping. Our results show how a metal-insulator transition behavior occurs upon Cu doping in view of the electronic structure and provide a platform to further investigation on the origin of emergent magnetic fluctuation in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{Se}$.

Electrical conductivity properties of porous SmBaCo2O5+d and SmBa0.5Sr0.5Co2O5+d layered perovskite oxide systems for solid oxide fuel cell

In this study, the electrical conductivity characteristics of SmBaCo2O5+d (SBCO) and SmBa0.5Sr0.5Co2O5+d (SBSCO) were measured and analyzed by changing the characteristics of the microstructure from dense microstructure to porous microstructure for the cathode application in solid oxide fuel cells. SBCO and SBSCO comprised of the dense microstructure showed metal insulator transition (MIT) and metallic behavior, respectively. In SBCO, when the oxygen partial pressure is reduced, the conductivity value decreases, and the conductivity behavior changes to the behavior of a semiconductor. However, the electrical conductivity behavior of SBSCO did not change even when the oxygen partial pressure was decreased. The electrical conductivities of the porous cathodes were lower than those of the dense cathodes due to the discontinuous electric path, but all porous cathodes showed semiconductor behavior. The conductivity value decreases when the oxygen partial pressure decreases, but the general conductivity behavior of the samples with a porous microstructure does not change under N2 atmosphere. The porous cathode showed the highest electrical conductivity when Pt lines were led to the top of the cathode. In this case, a relatively high electrical conductivity was measured using the method of measuring multiple conductivities at different temperatures while decreasing the measurement temperature starting from a high temperature rather than the method of measuring while raising the temperature starting from a low temperature. In the dense cathode, higher electrical conductivities were measured when a low current was applied, but in the porous cathode, the same electrical conductivity values were measured regardless of the applied current values.

Coherent epitaxy of trilayer nickelate (Nd0.8Sr0.2)4Ni3O10 films by high-pressure magnetron sputtering

Rare-earth nickelates (such as perovskite RNiO3, trilayer R4Ni3O10, and infinite layer RNiO2) have attracted tremendous interest very recently. However, unlike the widely studied RNiO3 and RNiO2 films, the synthesis of trilayer nickelate R4Ni3O10 films is rarely reported. Here, single-crystalline (Nd0.8Sr0.2)4Ni3O10 epitaxial films were coherently grown on SrTiO3 substrates by high-pressure magnetron sputtering. The crystal and electronic structures of (Nd0.8Sr0.2)4Ni3O10 films with oxygen ligand holes were characterized by high-resolution x-ray diffraction, x-ray photoemission spectroscopy, and resonant soft x-ray absorption spectroscopy. The electrical transport measurements reveal a metal–insulator transition behavior near 82 K and negative magnetoresistance in (Nd0.8Sr0.2)4Ni3O10 films. Our work provides a novel route to synthesize high-quality trilayer nickelate R4Ni3O10 films with RNiO3 targets by high-pressure magnetron sputtering.

Tuning the metal insulator transition of vanadium dioxide on oxide nanosheets

For practical applications, tuning the metal-insulator transition (MIT) behavior of high-quality vanadium dioxide (VO2) on arbitrary substrates, such as Si and glass, is desirable. Here, we demonstrate the ability to tune the MIT temperature (TMIT) of VO2 films by growing them on NbWO6 (NWO) nanosheets on arbitrary substrates and varying the film thicknesses. The oxidation and crystal structure of VO2 films are determined by x-ray photoelectron spectroscopy and temperature-dependent x-ray diffraction, respectively. It is observed that as the film thickness increases, the TMIT also increases to the bulk value, 341 K, because of the increase in the rutile c-axis of VO2. The strain effect accompanying with the film thickness variation on NWO nanosheets contribute to the shortening of the rutile cR axis in thin films and, hence, the lowering of TMIT of VO2. Furthermore, the arbitrary underlying substrates have negligible influence on the MIT behavior of VO2 on NWO nanosheets. These results open up the possibility to more freely choose a technical substrate material for functional VO2 films and tune its MIT.

Electrical transport properties of Sm-doped La0.7Ca0.3MnO3 polycrystalline ceramics

Improving the magnetoresistance effect of perovskite ceramic materials under a low applied magnetic field to expand its application range is one of the main research directions of this type of material. In this study, La0.7Ca0.3MnO3 was doped with different levels of Sm by the sol-gel method to yield a series of La0.7-xSmxCa0.3MnO3 (LSCMO) polycrystalline ceramics. X-ray diffraction (XRD) results revealed that LSCMO ceramics possessed standard perovskite structures. Scanning electron microscopy (SEM) showed grains closely connected without obvious holes. In addition, the grain size gradually decreased with the increase in Sm doping content. The resistivity temperature curves displayed a clear metal-insulator transition behavior of LSCMO accompanied by a steep change from ferromagnetic to paramagnetic behavior (FM-PM). The metal-insulator transition temperature (Tp) values of the as-obtained LSCMO gradually shifted toward lower temperatures with increase in Sm content. Moreover, resistivity temperature coefficient (TCR) and magnetoresistance (MR) values also gradually increased with Sm doping content. The transport properties in polycrystalline ceramics could be adequately explained by the double exchange model, which would be useful for interpreting the CMR effects when used in magnetic devices.

Modulating the metal-insulator transition in VO2/Al2O3 (001) thin films by grain size and lattice strain

A series of high quality VO2/Al2O3 thin films were prepared by DC sputtering, and the effects of surface topology change on their metal-insulator transition (MIT) behaviors have been discussion in this paper. A ~103 nm-thick VO2/Al2O3 thin film shows extremely large order of magnitude change (~4.2) and low TMIT (56 ℃), and both grain size and lattice strain have been determined as the main causes responsible for the improvement of metal-insulator transition. Specifically, the increase of crystalline size that results in enlarging the grain size, has been identified significantly improves the sheet conductivity of VO2 thin films in rutile phase, which indeed also implements large change in order of magnitude. Besides, the lattice strain has been proven strongly affects the electronic orbitals (π* and d||) occupancy in rutile phase, which has been identified as the one of significant signatures in indicating TMIT change, and thus the increase of lattice strain is believed efficient to modulate TMIT. The close relationships between the surface grain size, lattice strain, TMIT and sheet conductivity in this study for sure will open the door to the fabrication and research on highly quality VO2/Al2O3 thin films.

SmBa1-xCaxCo2O5+d Layered Perovskite Cathodes for Intermediate Temperature-operating Solid Oxide Fuel Cells.

In SmBa1-xCaxCo2O5+d (x = 0.01, 0.03, 0.1, and 0.2, SBCCO) oxide systems calcined at 1100°C for 8 h, the XRD patterns of the SBCCO single phase were maintained in the cases of SmBa0.97Ca0.03Co2O5+d (SBCCO-0.97) and SmBa0.99Ca0.01Co2O5+d (SBCCO-0.99) compositions. In SmBa0.8Ca0.2Co2O5+d (SBCCO-0.8) and SmBa0.9Ca0.1Co2O5+d (SBCCO-0.9), CaCoSmO4 existed with the pattern SBCCO. SBCCO structures were identified as orthorhombic crystal structures because they showed splitting of the X-ray diffraction (XRD) peaks at 23.4°, 47.9°, and 59.1°.Typical metallic conduction behaviors were found in all measured compositions except SBCCO-0.8, which showed a metal-insulator transition (MIT) behavior. Compared to other SmBa1-xCaxCo2O5+d compositions, SBCCO-0.8 showed the highest electrical conductivity of 460 S/cm at 500°C. In particular, SBCCO-0.9 was found to have an excellent ASR characteristic of about 0.077 Ωcm2 at 700°C. The activation energy of SBCCO-0.9 was the lowest among SBCCO oxide systems with a value of 0.77 eV.

Synthesis of vanadium dioxide thin films and nanostructures

Vanadium oxides (VxOy) are classic “smart functional materials” used in a wide array of thermochromic, electronic, and catalytic applications. Specifically, vanadium dioxide (VO2) class nanomaterials are of enormous interest due to their unique first order reversible metal-insulator phase transition (MIT) behavior accompanied by a structural phase transition, inducing dramatic changes in electrical and optical properties with large lattice deformation. To date, a plethora of reports exemplifying the MIT characteristics of VO2, synthetic methods of VO2, and modulating VO2 phase transition temperatures (Tc) have been published. In this Tutorial Review, we present an overview on the fundamentals of the VO2 band structure and principles of MIT and outline various reported synthetic approaches for VO2 thin films, including dimensionally oriented VO2 nanostructures. Discussion on recent trends in VO2 applications, challenges in VO2 synthesis, and future perspectives are also elaborated in detail.

Versatile Tunability of the Metal Insulator Transition in (TiO2)m/(VO2)m Superlattices

AbstractIn contrast to perovskites that share only common corners of cation‐occupied octahedra, binary‐oxides in addition share edges and faces increasing the versatility for tuning the properties and functionality of reduced dimensionality systems of strongly correlated oxides. This approach for tuning the electronic structure is based on the ability of X‐ray spectroscopy methods to monitor the creation and transformation of occupied and unoccupied electronic states produced by interface coupling and lattice distortions. X‐ray diffraction reveals a new range of structural metastability in (TiO2)m/(VO2)m/TiO2(001) superlattices with m = 1, 3, 5, 20, 40, and electrical transport measurements show metal insulator transition (MIT) behavior typically associated with presence of high oxygen vacancy concentrations. However, X‐ray absorption spectroscopy (XAS) at the Ti and V L3,2‐edge and resonant inelastic X‐ray scattering (RIXS) at the Ti and V L3‐edge show no excitations characteristic of oxygen vacancy induced valance change in V and negligible intensities in Ti RIXS. The unexpected absence of oxygen vacancy related states in the X‐ray spectroscopy data suggests that superlattice fabrication is capable of suppressing oxygen vacancy formation while still affording a wide tunability range of the MIT. Achieving a wide range of MIT tunability while reducing or eliminating oxygen vacancies that are detrimental to electrical properties is highly desirable for technological applications of strongly correlated oxides.

Ionic Liquid Gating and Phase Transition Induced Semiconducting to Metallic Transition in La0.67Sr0.33MnO3/BaTiO3 Heterostructures.

The strongly correlated La0.67Sr0.33MnO3 (LSMO) thin films were deposited on the BaTiO3 (BTO) substrates to study the influence of temperature-dependent phase transition of BTO on the magnetic and electric transport properties of the LSMO films. Because of the large lattice mismatch between the LSMO and BTO substrates, the ultrathin LSMO film exhibits semiconducting transport behavior rather than the normal metal-insulator transition behavior. A Mn2+-O2--Mn3+ dominated double exchange interaction, which leads to a semiconducting to metallic transition behavior, was formed by the joint effect of the ion-liquid gating and phase transition of the BTO substrates. Our work shows a special way to modulate the conduction mechanism, which opens up a new field for further study of the magnetoelectric coupling of the LSMO/BTO heterostructures.

Thermochromatic vanadium dioxide (VO2) thin films synthesized by atomic layer deposition and post-treatments

As a thermochromatic material showing metal–insulator transition behavior below 70 ℃, crystalline films of vanadium dioxide (VO2) have potential applications in many fields. In this work, amorphous vanadium oxide (VxOy) films were deposited on glass substrates by atomic layer deposition (ALD). The film growth mechanism and growth rate were studied by quartz crystal micro balance, and a constant growth rate of about 0.5 Å/cycle was obtained. X-ray diffraction and X-ray photoelectron spectroscopy characterizations demonstrated that the as-deposited ALD VxOy film was amorphous with mostly V4+ oxidation state. Crystalline V2O5 films were prepared by annealing the as-deposited ALD VxOy films at 300 ℃ under O2 flow. Subsequently, different annealing methods and conditions were tested for transforming V2O5 films to VO2 films. It turned out that high purity crystalline VO2 films could be obtained by heating the V2O5 films in an oxidizing environment at a reduced pressure. Results of transmittance measurements revealed that these crystalline VO2 films had excellent thermochromatic properties with a reversible change in transmittance at about 65 ℃.