Planar Nanoactuators Based on VO2 Phase Transition.

Actuation at micro- and nanoscale often requires large displacements and applied forces. The high work energy density that lies inside many phase transitions is an appealing feature for developing new actuating schemes, especially if the transition is reversible and scalable into small actuating domains. Here, we show the fabrication of a planar nanomechanical actuator having chevron-type geometry and based on the phase transition of VO2. This device is thermally activated through heating just above room temperature to trigger the VO2 crystalline symmetry change associated with the metal-insulator transition. The large lattice expansion of VO2 phase transition, compared to standard materials, is further amplified by the chevron-type geometry. DC and AC operation of the device are discussed.

Long-range propagation of protons in single-crystal VO2 involving structural transformation to HVO2.

Vanadium dioxide (VO2) is a strongly correlated electronic material with a metal-insulator transition (MIT) near room temperature. Ion-doping to VO2 dramatically alters its transport properties and the MIT temperature. Recently, insulating hydrogenated VO2 (HVO2) accompanied by a crystal structure transformation from VO2 was experimentally observed. Despite the important steps taken towards realizing novel applications, essential physics such as the diffusion constant of intercalated protons and the crystal transformation energy between VO2 and HVO2 are still lacking. In this work, we investigated the physical parameters of proton diffusion constants accompanied by VO2 to HVO2 crystal transformation with temperature variation and their transformation energies. It was found that protons could propagate several micrometers with a crystal transformation between VO2 and HVO2. The proton diffusion speed from HVO2 to VO2 was approximately two orders higher than that from VO2 to HVO2. The long-range propagation of protons leads to the possibility of realizing novel iontronic applications and energy devices.

Barrier Formation at the Contacts of Vanadium Dioxide and Transition-Metal Dichalcogenides.

Phase-transition field-effect transistors (FETs) are a class of steep-slope devices that show abrupt on/off switching owing to the metal-insulator transition (MIT) induced in the contacting materials. An important avenue to develop phase-transition FETs is to understand the charge injection mechanism at the junction of the contacting MIT materials and semiconductor channels. Here, toward the realization of high-performance phase-transition FETs, we investigate the contact properties of heterojunctions between semiconducting transition-metal dichalcogenides (TMDCs) and vanadium dioxide (VO2) that undergoes a MIT at a critical temperature (Tc) of approximately 340 K. We fabricated transistors based on molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) in contact with the VO2 source/drain electrodes. The VO2-contacted MoS2 transistor exhibited n-type transport both below and above Tc. Across the MIT, the on-current was observed to increase only by a factor of 5, in contrast to the order-of-magnitude change in the resistance of the VO2 electrodes, suggesting the existence of high contact resistance. The Arrhenius analyses of the gate-dependent drain current confirmed the formation of the interfacial barrier at the VO2/MoS2 contacts, irrespective of the phase state of VO2. The VO2-contacted WSe2 transistor showed ambipolar transport, indicating that the Fermi level lies near the mid gap of WSe2. These observations provide insights into the contact properties of phase-transition FETs based on VO2 and TMDCs and suggest the need for contact engineering for high-performance operations.

Growth of vanadium dioxide thin films on hexagonal boron nitride flakes as transferrable substrates.

Vanadium dioxide (VO2) is an archetypal metal-insulator transition (MIT) material, which has been known for decades to show an orders-of-magnitude change in resistivity across the critical temperature of approximately 340 K. In recent years, VO2 has attracted increasing interest for electronic and photonic applications, along with advancement in thin film growth techniques. Previously, thin films of VO2 were commonly grown on rigid substrates such as crystalline oxides and bulk semiconductors, but the use of transferrable materials as the growth substrates can provide versatility in applications, including transparent and flexible devices. Here, we employ single-crystalline hexagonal boron nitride (hBN), which is an insulating layered material, as a substrate for VO2 thin film growth. VO2 thin films in the polycrystalline form are grown onto hBN thin flakes exfoliated onto silicon (Si) with a thermal oxide, with grains reaching up-to a micrometer in size. The VO2 grains on hBN are orientated preferentially with the (110) surface of the rutile structure, which is the most energetically favorable. The VO2 film on hBN shows a MIT at approximately 340 K, across which the resistivity changes by nearly three orders of magnitude, comparable to VO2 films grown on common substrates such as sapphire and titanium dioxide. The VO2/hBN stack can be picked up from the supporting Si and transferred onto arbitrary substrates, onto which VO2 thin films cannot be grown directly. Our results pave the way for new possibilities for practical and versatile applications of VO2 thin films in electronics and photonics.

Gate-Tunable Thermal Metal-Insulator Transition in VO2 Monolithically Integrated into a WSe2 Field-Effect Transistor.

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.

Enhanced electronic-transport modulation in single-crystalline VO2 nanowire-based solid-state field-effect transistors.

Field-effect transistors using correlated electron materials with an electronic phase transition pave a new avenue to realize steep slope switching, to overcome device size limitations and to investigate fundamental science. Here, we present a new finding in gate-bias-induced electronic transport switching in a correlated electron material, i.e., a VO2 nanowire channel through a hybrid gate, which showed an enhancement in the resistive modulation efficiency accompanied by expansion of metallic nano-domains in an insulating matrix by applying gate biases near the metal-insulator transition temperature. Our results offer an understanding of the innate ability of coexistence state of metallic and insulating domains in correlated materials through carrier tuning and serve as a valuable reference for further research into the development of correlated materials and their devices.

Selective High-Frequency Mechanical Actuation Driven by the VO2 Electronic Instability.

Relaxation oscillators consist of periodic variations of a physical quantity triggered by a static excitation. They are a typical consequence of nonlinear dynamics and can be observed in a variety of systems. VO2 is a correlated oxide with a solid-state phase transition above room temperature, where both electrical resistance and lattice parameters undergo a drastic change in a narrow temperature range. This strong nonlinear response allows to realize spontaneous electrical oscillations in the megahertz range under a DC voltage bias. These electrical oscillations are employed to set into mechanical resonance a microstructure without the need of any active electronics, with small power consumption and with the possibility to selectively excite specific flexural modes by tuning the value of the DC electrical bias in a range of few hundreds of millivolts. This actuation method is robust and flexible and can be implemented in a variety of autonomous DC-powered devices.

Electrochemical gating-induced reversible and drastic resistance switching in VO2 nanowires.

Reversible and drastic modulation of the transport properties in vanadium dioxide (VO2) nanowires by electric field-induced hydrogenation at room temperature was demonstrated using the nanogaps separated by humid air in field-effect transistors with planer-type gates (PG-FET). These PG-FETs allowed us to investigate behavior of revealed hydrogen intercalation and diffusion aspects with time and spatial evolutions in nanowires. These results show that air nanogaps can operate as an electrochemical reaction field, even in a gaseous atmosphere, and offer new directions to explore emerging functions for electronic and energy devices in oxides.

Fractal Nature of Metallic and Insulating Domain Configurations in a VO2 Thin Film Revealed by Kelvin Probe Force Microscopy.

We investigated the surface work function (WS) and its spatial distribution for epitaxial VO2/TiO2 thin films using Kelvin probe force microscopy (KPFM). Nearly grain-boundary-free samples allowed observation of metallic and insulating domains with distinct WS values, throughout the metal-insulator transition. The metallic fraction, estimated from WS maps, describes the evolution of the resistance based on a two-dimensional percolation model. The KPFM measurements also revealed the fractal nature of the domain configuration.

Programmable mechanical resonances in MEMS by localized joule heating of phase change materials.

A programmable micromechanical resonator based on a VO2 thin film is reported. Multiple mechanical eigenfrequency states are programmed using Joule heating as local power source, gradually driving the phase transition of VO2 around its Metal-Insulator transition temperature. Phase coexistence of domains is used to tune the stiffness of the device via local control of internal stresses and mechanical properties. This study opens perspectives for developing mechanically configurable nanostructure arrays.

Controlled fabrication of artificial ferromagnetic (Fe,Mn)3O4 nanowall-wires by a three-dimensional nanotemplate pulsed laser deposition method.

We have developed a new method to fabricate extremely small transition-metal oxide nanowires. Using a combination of nanoimprint template patterning and inclined substrate pulsed laser deposition, we successfully fabricated magnetic oxide Fe(2.5)Mn(0.5)O(4) nanowall-wires, and controlled the width in a range from 120 nm down to about 20 nm by varying deposition parameters. Magnetoresistance measurements revealed ferromagnetic properties of the Fe(2.5)Mn(0.5)O(4) nanowall-wire. This method enables the study of mesoscopic transport properties of transition-metal oxides towards the development of oxide-based nanodevices.

Position-controlled functional oxide lateral heterostructures consisting of artificially aligned (Fe,Zn)3O4 nanodots and BiFeO3 matrix.

We demonstrate an advanced fabrication method for perfectly position-controlled ferromagnetic semiconductor (Fe,Zn)(3)O(4) nanodot arrays down to several hundred nanometers in size surrounded by a ferroelectric BiFeO(3) matrix. By performing position-selective crystal growth of perovskite BiFeO(3) on the position-controlled epitaxial spinel (Fe,Zn)(3)O(4) nanodot-seeding template, which is prepared using a hollow molybdenum mask lift-off nanoimprint lithography process on a perovskite La-doped SrTiO(3)(001) substrate, we produce functional oxide three-dimensional lateral heterojunctions. The position-selectivity can be explained based on standard surface diffusion theory with a critical nucleation point. Establishing this fabrication process could lead to innovative nanointegration techniques for spintronic oxide materials.

Multistate memory devices based on free-standing VO2/TiO2 microstructures driven by Joule self-heating.

Two-terminal multistate memory elements based on VO(2)/TiO(2) thin film microcantilevers are reported. Volatile and non-volatile multiple resistance states are programmed by current pulses at temperatures within the hysteretic region of the metal-insulator transition of VO(2). The memory mechanism is based on current-induced creation of metallic clusters by self-heating of micrometric suspended regions and resistive reading via percolation.

Direct fabrication of integrated 3D epitaxial functional transition metal oxide nanostructures using extremely small hollow nanopillar nano-imprint metal masks.

A novel nanofabrication technique is developed for functional oxides. Combining nano-imprint lithography, sidewall-etching and sidewall-deposition processes enables us to prepare Mo hollow nanopillar masks with 100 and 60 nm window sizes, which is smaller than the original nano-imprint mold size of 250 nm. Using this Mo nanomask, extremely small epitaxial ferromagnetic oxide (Fe(2.5)Mn(0.5)O(4)) nanostructures can be directly grown on sapphire substrates at the deposition temperature of 350 °C in a pulsed laser deposition (PLD) process.

Enhancement of spin polarization in a transition metal oxide ferromagnetic nanodot diode.

Enhancement of spin polarization was observed in a transition metal oxide (Fe,Zn)(3)O(4)/Nb-SrTiO(3) ferromagnetic nanodot Schottky diode. The highly integrated oxide nanodot diodes were constructed using nanoimprint lithography based on a Mo lift-off method in combination with a pulsed laser deposition technique. The junction magnetoresistance of diodes increased as diode size increased. The spin polarization estimated from the thermionic emission model is enhanced from P = 0.74 in a conventional film to P = 0.89 in a nanodot diode whose size is 300 × 300 nm(2). The nanofabrication technique used here will enable us to construct superior transition metal oxide spintronic nanomaterial and nanodevices.

Giant magnetoresistance observed in (Fe,Mn)3O4 artificial nanoconstrained structures at room temperature.

We fabricated ferromagnetic oxide (Fe,Mn)(3)O(4) (FMO) nanoconstrained structures using two different steps involving atomic force microscope lithography in combination with a pulsed laser deposition technique. The widths of three FMO nanoconstrained structures were 150, 100, and 50 nm. The I-V characteristic of the 50 nm width FMO nanoconstrained structure changed dramatically from linear to nonlinear after forming the nanoconstrained structure. The magnetoresistance also increased drastically from -0.2% to 150% at room temperature. These results indicate that well-defined complicated artificial transition metal oxide nanostructures allow the creation of huge spin response devices even at room temperature.

Direct fabrication of integrated 3D Au nanobox arrays by sidewall deposition with controllable heights and thicknesses.

This paper provides a unique strategy for controlling integrated hollow nanostructure arrays such as boxes or pillars at the nanometer scale. The key merit of this technique is that it can overcome resolution limits by sidewall deposition and deposit various materials using a sputtering method. The sputtering method can be replaced by other dry deposition techniques such as pulsed laser deposition (PLD) for complex functional materials. Furthermore, it can produce low-cost large-area fabrication and high reproducibility using the NIL (nanoimprint lithograph) process. The fabrication method consists of a sequence of bilayer spin-coating, UV-NIL, RIE (reactive ion etching), sputtering, ion milling and piranha cleaning processes. By changing the deposition time and molds, various thicknesses and shapes can be fabricated, respectively. Furthermore, the fabricated Au box nanostructure has a bending zone of the top layer and a approximately 17 nm undercut of the bottom layer as observed by SEM (scanning electron microscope). The sidewall thickness was changed from 12 to 61 nm by controlling the deposition time, and was investigated to understand the relationship with blanket thicknesses and geometric effects. The calculated sidewall thickness matched well with experimental results. Using smaller hole-patterned molds, integrated nanobox arrays, with inner squares measuring approximately 160 nm, and nanopillar arrays, with inside pores measuring approximately 65 nm, were fabricated under the same conditions.

Noise-driven attractor switching device.

Problems with artificial neural networks originate from their deterministic nature and inevitable prior learnings, resulting in inadequate adaptability against unpredictable, abrupt environmental change. Here we show that a stochastically excitable threshold unit can be utilized by these systems to partially overcome the environmental change. Using an excitable threshold system, attractors were created that represent quasiequilibrium states into which a system settles until disrupted by environmental change. Furthermore, noise-driven attractor stabilization and switching were embodied by inhibitory connections. Noise works as a power source to stabilize and switch attractors, and endows the system with hysteresis behavior that resembles that of stereopsis and binocular rivalry in the human visual cortex. A canonical model of the ring network with inhibitory connections composed of class 1 neurons also shows properties that are similar to the simple threshold system.

Ordered nano-lslands on (La,Ba)MnO3 thin film surface by self-organization.

Self-organized nanostructures composed of discrete coherent islands were found on La0.67Ba0.33 MnO3 (LBMO) film surfaces. Our results revealed that rippled surface structures on LBMO films appeared during post-annealing at high temperature, and that the step edges of the rippled surfaces offer energetically favorable positions for the nucleation of coherent islands. The rippled surface serves as a template for the arrangement of coherent islands. This provides a new way to prepare ordered surface nanostructures in perovskite oxides without the necessity to employ any preprocessing techniques prior to the growth.