Catalytic Hydrogen Doping of NdNiO3 Thin Films under Electric Fields.

The electric-field-assisted hydrogenation and corresponding resistance modulation of NdNiO3 (NNO) thin-film resistors were systematically studied as a function of temperature and dc electric bias. Catalytic Pt electrodes serve as triple-phase boundaries for hydrogen incorporation into a perovskite lattice. A kinetic model describing the relationship between resistance modulation and proton diffusion was proposed by considering the effect of the electric field during hydrogenation. An electric field, in addition to thermal activation, is demonstrated to effectively control the proton distribution along its gradient with an efficiency of ∼22% at 2 × 105 V/m. The combination of an electric field and gas-phase annealing is shown to enable the elegant control of the diffusional doping of complex oxides.

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.

Three-Dimensional Nanoconfinement Supports Verwey Transition in Fe3O4 Nanowire at 10 nm Length Scale.

Herein, we construct three-dimensional (3D) Fe3O4 epitaxial nanowires at a 10 nm length scale on a 3D MgO nanotemplate using an original nanofabrication technique that mainly comprises nanoimprint lithography and inclined thin-film deposition. Despite the high density of inevitable nanoscale defects, the ultrasmall Fe3O4 nanowires exhibit a prominent Verwey transition at about 112 K with a maximum relative change in resistance of 9.5, which is 6 times larger than that of the thin-film configuration. Numerous measurements on a large number of Fe3O4 nanowires grown concurrently on the same 3D MgO nanotemplate reveal a dramatic difference in their electrical transport property with the presence/absence of the Verwey transition. A comparative study of Fe3O4 wires of increasing volume and a thin film reveals that a profound change in the Verwey transition is observed only for wires with a volume on the order of 10 nm3. Moreover, a significant decrease in the sharpness of the resistance jump and the transition temperature of the Verwey response is noticed with an increasing volume of Fe3O4. This indicates the potency of the 3D nanofabrication technique in controlling nanoscale defects, which is further reconfirmed through magnetoresistance measurement. A feature of the magnetoresistance curve identifies the antiphase boundaries as a major source of defects. The occurrence of the smallest magnetoresistance in the ultrasmall nanowire with the highest Verwey transition temperature and resistance change ratio proves that 3D isotropic spatial confinement into a length scale comparable to the average spacing between two antiphase boundaries enables the favorable control over nanoscale defects. A simple statistical model satisfactorily illustrates the dependence of electrical transport properties on the volume of Fe3O4 from the macroscale down to the nanoscale. Finally, an ultrasmall nanowire with a low defect concentration allows the estimation of the true coherence length of the fundamental quasiparticle, the trimeron, responsible for the Verwey transition.

Non-contact detection of nanoscale structures using optical nanofiber.

The detection of nanoscale structure/material property in a wide observation area is becoming very important in various application fields. However, it is difficult to utilize current optical technologies. Toward the realization of novel alternative, we have investigated a new optical sensing method using an optical nanofiber. When the nanofiber vertically approached a glass prism with a partial gold film, the material differences between the glass and the gold were detected as a transmittance difference of 6% with a vertical resolution of 9.6 nm. The nanofiber was also scanned 100 nm above an artificial small protruding object with a width of 240 nm. The object was detected with a horizontal resolution of 630 nm, which was less than the wavelength of the probe light.

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.

Strongly correlated perovskite lithium ion shuttles.

Solid-state ion shuttles are of broad interest in electrochemical devices, nonvolatile memory, neuromorphic computing, and biomimicry utilizing synthetic membranes. Traditional design approaches are primarily based on substitutional doping of dissimilar valent cations in a solid lattice, which has inherent limits on dopant concentration and thereby ionic conductivity. Here, we demonstrate perovskite nickelates as Li-ion shuttles with simultaneous suppression of electronic transport via Mott transition. Electrochemically lithiated SmNiO3 (Li-SNO) contains a large amount of mobile Li+ located in interstitial sites of the perovskite approaching one dopant ion per unit cell. A significant lattice expansion associated with interstitial doping allows for fast Li+ conduction with reduced activation energy. We further present a generalization of this approach with results on other rare-earth perovskite nickelates as well as dopants such as Na+ The results highlight the potential of quantum materials and emergent physics in design of ion conductors.

Identification of Giant Mott Phase Transition of Single Electric Nanodomain in Manganite Nanowall Wire.

In the scaling down of electronic devices, functional oxides with strongly correlated electron system provide advantages to conventional semiconductors, namely, huge switching owing to their phase transition and high carrier density, which guarantee their rich functionalities even at the 10 nm scale. However, understanding how their functionalities behave at a scale of 10 nm order is still a challenging issue. Here, we report the construction of the well-defined (La,Pr,Ca)MnO3 epitaxial oxide nanowall wire by combination of nanolithography and subsequent thin-film growth, which allows the direct investigation of its insulator-metal transition (IMT) at the single domain scale. We show that the width of a (La,Pr,Ca)MnO3 nanowall sample can be reduced to 50 nm, which is smaller than the observed 70-200 nm-size electronic domains, and that a single electronic nanodomain in (La,Pr,Ca)MnO3 exhibited an intrinsic first-order IMT with an unusually steep single-step change in its magnetoresistance and temperature-induced resistance due to the domains arrangement in series. A simple model of the first-order transition for single electric domains satisfactorily illustrates the IMT behavior from macroscale down to the nanoscale.

Selection of di(meth)acrylate monomers for low pollution of fluorinated mold surfaces in ultraviolet nanoimprint lithography.

We used fluorescence microscopy to show that low adsorption of resin components by a mold surface was necessary for continuous ultraviolet (UV) nanoimprinting, as well as generation of a low release energy on detachment of a cured resin from a template mold. This is because with low mold pollution, fracture on demolding occurred at the interface between the mold and cured resin surfaces rather than at the outermost part of the cured resin. To achieve low mold pollution, we investigated the radical photopolymerization behaviors of fluorescent UV-curable resins and the mechanical properties (fracture toughness, surface hardness, and release energy) of the cured resin films for six types of di(meth)acrylate-based monomers with similar chemical structures, in which polar hydroxy and aromatic bulky bisphenol moieties and methacryloyl or acryloyl reactive groups were present or absent. As a result, we selected bisphenol A glycerolate dimethacrylate (BPAGDM), which contains hydroxy, bisphenol, and methacryloyl moieties, which give good mechanical properties, monomer bulkiness, and mild reactivity, respectively, as a suitable base monomer for UV nanoimprinting under an easily condensable alternative chlorofluorocarbon (HFC-245fa) atmosphere. The fluorescent UV-curable BPAGDM resin was used for UV nanoimprinting and lithographic reactive ion etching of a silicon surface with 32 nm line-and-space patterns without a hard metal layer.

MoS2 nanocube structures as catalysts for electrochemical H2 evolution from acidic aqueous solutions.

Core-shell PMMA-Au nanocube structures made by a combination of nanoimprint lithography and sidewall deposition were used as template for electrodeposition of MoS2, Ni, and Pt. Linear sweep voltammetry experiments obtained in an aqueous solution containing 0.29 M H2SO4 (pH 0.24) showed that the onset potential of the core-shell-shell PMMA-Au-MoS2 nanocube electrode for the hydrogen evolution reaction (HER) was shifted to the positive direction (i.e., requiring a lower overpotential) by 20-40 mV compared to planar MoS2 films. This indicates that the nanocube electrodes have a significantly increased HER activity, which is probably because of a higher density of catalytically active edge sites available at the nanocube surface. It was also found that the HER activity initially increased with increasing MoS2 deposition time, but decreased after deposition for 60 min because the edges of the nanocubes became rounded, thereby decreasing the number of active edge sites. By depositing Ni and Pt on top of PMMA-Au nanocubes, it was shown that this method can also be used for the synthesis of nanocube structures with varying compositions.

Ni and p-Cu2O nanocubes with a small size distribution by templated electrodeposition and their characterization by photocurrent measurement.

A method for the reproducible formation of Ni and Cu2O nanocubes with dimensions of 200-500 nm and a small size distribution is introduced. For this, the well-known templated electrodeposition technique was extended to cubic PMMA templates made by nanoimprint lithography. When making cubic templates in larger quantities, this method has the potential to become simple and cost-effective. This method was successfully used for the formation of Ni and p-Cu2O nanocubes as well as for the formation of segmented nanobars containing both phases. The lateral dimensions of the nanocubes exactly resembled the dimensions of the template, and the height could be varied by adjusting the deposition time. Nanocubes formed via this method can remain attached to the substrate or can be dispersed in solution. p-Cu2O is considered to be one of the most promising photocathode materials for solar water splitting. It is demonstrated that the activity of the p-Cu2O nanocubes for photocatalytic water splitting can be measured, and it was found that the nanocube morphology enhances the photocatalytic activity compared to thin films.

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.

Position-, size-, and shape-controlled highly crystalline ZnO nanostructures.

Highly ordered ZnO nanoboxes and nanowire structures with a width of ∼ 20 nm have been successfully fabricated by the combination of nanoimprint lithography and pulsed laser deposition utilizing a glancing angle deposition (GLAD) technique. The periodicity, size, and shape of the ZnO nanoboxes and nanobelts can be easily controlled over a large area by changing the molds and deposition conditions. At the initial stage of growth by GLAD, nanonucleation led to nanopillar structures, which agglomerated to form nanobox and nanobelt structures at room temperature (RT). The ZnO nanostructures have a c-axis orientation along the nanopillar direction after postannealing and exhibit an intense cathodoluminescence peak around 380 nm at RT.

Improved optical and electrical characteristics of free-standing GaN substrates by chemical polishing utilizing photo-electrochemical method.

The optical and electrical properties of GaN(0001) surfaces treated by a novel chemical polishing method are described. Scanning microscopic photoluminescence images reveal that the polished GaN surface shows improved luminescence properties compared to the untreated surface. Current-voltage measurements of Schottky barriers formed using the GaN substrates show that the polished GaN surface has a lower reverse leakage current, and that the barrier height and ideality factor are improved after the polishing treatment.

Structure and magnetic properties of mono- and bi-layer graphene films on ultraprecision figured 4H-SiC(0001) surfaces.

Monolayer and bilayer graphene films with a few hundred nm domain size were grown on ultraprecision figured 4H-SiC(0001) on-axis and 8 degrees -off surfaces by annealing in ultra-high vacuum. Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, reflection high-energy electron diffraction, low-energy electron diffraction (LEED), Raman spectroscopy, and scanning tunneling microscopy, we investigated the structure, number of graphene layers, and chemical bonding of the graphene surfaces. Moreover, the magnetic property of the monolayer graphene was studied using in-situ surface magneto-optic Kerr effect at 40 K. LEED spots intensity distribution and XPS spectra for monolayer and bilayer graphene films could become an obvious and accurate fingerprint for the determination of graphene film thickness on SiC surface.

Dependence of process characteristics on atomic-step density in catalyst-referred etching of 4H-SiC(0001) surface.

Catalyst-referred etching (CARE) is a novel abrasive-free planarization method. CARE-processed 4H-SiC(0001) surfaces are extremely flat and undamaged over the whole wafer. They consist of single-bilayer-height atomic steps and atomically flat terraces. This suggests that the etching properties depend principally on the atomic-step density of the substrate surface. We used on-axis and 8 degrees off-axis substrates to investigate the processing characteristics that affect the atomic-step density of these substrates. We found a strong correlation between the removal rate and the atomic-step density of the two substrates. For the on-axis substrate, the removal rate increased with increasing surface roughness, which increases with an increasing atomic-step density. The removal rate ratio is approximately the same as the atomic-step density ratio of the two substrates.

Efficient wet etching of GaN (0001) substrate with subsurface damage layer.

Photoenhanced chemical (PEC) etching is applicable for processing an n-GaN (0001) surface rapidly. In this process, the surface oxidation is enhanced by photo-generated holes and the resulting oxide can dissolve into solutions. In current work, we conduct bias-assisted PEC etching in a KOH solution with a positively biased wafer, to remove the crystallographically highly damaged layer. The employed substrate was mechanically polished with diamond slurry of sub-micrometer particle size. Without the positive bias, the rate of PEC etching was quite low because the photogenerated holes were quickly depleted by the recombination process at the crystallographic defects and they could not contribute to the oxidation. On the other hand, in the case where the bias was applied, the photogenerated holes and electrons are separated forcibly in the band-bended surface, which effectively contributed to surface oxidation. As a result, a high removal rate was realized even on the damaged surface.