Step terrace tuned anisotropic transport properties of highly epitaxial LaBaCo2O5.5+δ thin films on vicinal SrTiO3 substrates

A time-shared switching scheme designed for multi-probe scanning tunneling microscope

We report the design of a time-shared switching scheme, aiming to realize the manipulation and working modes (imaging mode and transport measurement mode) switching between multiple scanning tunneling microscope (STM) probes one by one with a shared STM control system (STM CS) and an electrical transport characterization system. This scheme comprises three types of switch units, switchable preamplifiers (SWPAs), high voltage amplifiers, and a main control unit. Together with the home-made software kit providing the graphical user interface, this scheme achieves a seamless switching process between different STM probes. Compared with the conventional scheme using multiple independent STM CSs, this scheme possesses more compatibility, flexibility, and expansibility for lower cost. The overall architecture and technique issues are discussed in detail. The performances of the system are demonstrated, including the millimeter scale moving range and atomic scale resolution of a single STM probe, safely approached multiple STM probes beyond the resolution of the optical microscope (1.1 µm), qualified STM imaging, and accurate electrical transport characterization. The combinational technique of imaging and transport characterization is also shown, which is supported by SWPA switches with ultra-high open circuit resistance (909 TΩ). These successful experiments prove the effectiveness and the usefulness of the scheme. In addition, the scheme can be easily upgraded with more different functions and numbers of probe arrays, thus opening a new way to build an extremely integrated and high throughput characterization platform.

Breaking Lattice Symmetry in Highly Strained Epitaxial VO2 Films on Faceted Nanosurface

The lattice symmetry of strongly correlated oxide heterostructures determines their exotic physical properties by coupling the degrees of freedom between lattices and electrons, orbitals, and spin states. Systematic studies on VO₂, a Mott insulator, have previously revealed that lattice distortion can be manipulated by the interfacial strain and electronic phase separation can emerge. However, typical epitaxial film-substrate interface strain provides a very limited range for exploring such interface-engineered phenomena. Herein, epitaxially grown VO₂ thin films on asymmetrically faceted m-plane sapphire substrates with the hill-and-valley type surfaces have been demonstrated. Interestingly, lattice symmetry breaking has been proven based on the large residual strain from the different faceted planes. By this lattice symmetry breaking, electronic phase separation and metal-insulator transition in the VO₂ films are modulated, and anisotropy in optical responses is exhibited. These results on asymmetrical interfacial engineering in oxide heterostructures open up new routes for novel functional materials design and functional electro/optic device nanofabrication.

Electric field control of resistive switching and magnetization in epitaxial LaBaCo2O5+δ thin films

The low operating temperature and volatile characteristics of the magnetization change are the main obstacles for the practical applications of spintronic and magnetic memories. In this work, both the resistive switching and magnetization switching are realized in Pt/LaBaCo2O5+δ (LBCO)/Nb-doped SrTiO3 (Nb-STO) devices at room temperature through an electric field. Unlike the traditional approach of an external stress inducing a volatile magnetization change, the magnetization in the Pt/LBCO/Nb-STO device is modulated by an electrical field, along with the resistive switching. The resistive and magnetization switching can be attributed to the variation of the depletion layer width at the LBCO/Nb-STO interface via oxygen vacancy migration and the increase/decrease of the Co-O-Co bond length, respectively. The present device with the synchronous manipulation of both resistance and magnetization at room temperature can be applied in nonvolatile resistive memories and novel magnetic multifunctional devices.

Structure and magnetic properties of highly oriented LaBaCo2O5+δ films deposited on Si wafers with Pt/Ti buffer layer

High-quality crystalline LaBaCo₂O_5+δ films are successfully deposited on Si wafers with Pt/Ti buffer layer, and tunable electrical and magnetic properties are achieved.

Role of temperature and oxygen content on structural and electrical properties of LaBaCo2O5+δ thin films

The role of temperature and the oxygen content in the structural transformation and electrical conductivity of epitaxial double perovskite LaBaCo2O5+δ (0≤ δ ≤ 1) thin films was systematically investigated. Reciprocal space mapping and ω-2θ x-ray diffraction performed at different temperatures in vacuum indicate that oxygen vacancies in the films become ordered at high temperature in a reducing environment. The changes of the oxygen content and the degree of oxygen vacancy ordering in the films result in a strong in-plane anisotropic lattice deformation and a large thermal expansion coefficient along the c-axis direction. The electrical conductivity measurements reveal that these behaviors are related to the degree of oxygen vacancy formation and lattice deformation in the films.

Upgrade of a commercial four-probe scanning tunneling microscopy system

Upgrade of a commercial ultra-high vacuum four-probe scanning tunneling microscopy system for atomic resolution capability and thermal stability is reported. To improve the mechanical and thermal performance of the system, we introduced extra vibration isolation, magnetic damping, and double thermal shielding, and we redesigned the scanning structure and thermal links. The success of the upgrade is characterized by its atomically resolved imaging, steady cooling down cycles with high efficiency, and standard transport measurement capability. Our design may provide a feasible way for the upgrade of similar commercial systems.

Influence of the vicinal surface on the anisotropic dielectric properties of highly epitaxial Ba0.7Sr0.3TiO3 thin films

Epitaxial thin films of Ba_0.7Sr_0.3TiO₃ (BST) were grown on the designed vicinal single-crystal LaAlO₃ (001) substrates to systematically investigate the evolution of microstructures and in-plane dielectric properties of the as-grown films under the strains induced by surface step terraces. Anisotropic dielectric properties were observed, which can be attributed to different tetragonalities induced by vicinal LaAlO₃ substrates with miscut orientations along the [100] and [110] directions with different miscut angles of 1.0°, 2.75° and 5.0°. A terrace geometric model with both compressive and tensile strained domains in the BST film was established, which is in good agreement with the experimental results. Our experimental studies not only shed new light on the heteroepitaxial growth mechanism, but also provide a promising platform for the design and integration of high performance device applications.

Manipulation of Optical Transmittance by Ordered-Oxygen-Vacancy in Epitaxial LaBaCo2O5.5+δ Thin Films

Giant optical transmittance changes of over 300% in wide wavelength range from 500 nm to 2500 nm were observed in LaBaCo₂O_5.5+δ thin films annealed in air and ethanol ambient, respectively. The reduction process induces high density of ordered oxygen vacancies and the formation of LaBaCo₂O_5.5 (δ = 0) structure evidenced by aberration-corrected transmission electron microscopy. Moreover, the first-principles calculations reveal the origin and mechanism of optical transmittance enhancement in LaBaCo₂O_5.5 (δ = 0), which exhibits quite different energy band structure compared to that of LaBaCo₂O₆ (δ = 0.5). The discrepancy of energy band structure was thought to be the direct reason for the enhancement of optical transmission in reducing ambient. Hence, LaBaCo₂O_5.5+δ thin films show great prospect for applications on optical gas sensors in reducing/oxidizing atmosphere.

Surface step terrace tuned microstructures and dielectric properties of highly epitaxial CaCu3Ti4O12 thin films on vicinal LaAlO3 substrates

Controllable interfacial strain can manipulate the physical properties of epitaxial films and help understand the physical nature of the correlation between the properties and the atomic microstructures. By using a proper design of vicinal single-crystal substrate, the interface strain in epitaxial thin films can be well controlled by adjusting the miscut angle via a surface-step-terrace matching growth mode. Here, we demonstrate that LaAlO₃ (LAO) substrates with various miscut angles of 1.0°, 2.75°, and 5.0° were used to tune the dielectric properties of epitaxial CaCu₃Ti₄O₁₂ (CCTO) thin films. A model of coexistent compressive and tensile strained domains is proposed to understand the epitaxial nature. Our findings on the self-tuning of the compressive and tensile strained domain ratio along the interface depending on the miscut angle and the stress relaxation mechanism under this growth mode will open a new avenue to achieve CCTO films with high dielectric constant and low dielectric loss, which is critical for the design and integration of advanced heterostructures for high performance capacitance device applications.

Gas Sensing Properties of Epitaxial LaBaCo2O5.5+δ Thin Films

Chemical reactivity and stability of highly epitaxial mixed-conductive LaBaCo₂O_5.5+δ (LBCO) thin films on (001) LaAlO₃ (LAO) single-crystalline substrates, fabricated by using pulsed laser deposition system, were systematically investigated. Microstructure studies from x-ray diffraction indicate that the films are c-axis oriented with the interface relationship of [100]_LBCO//[100]_LAO and (001)_LBCO//(001)_LAO. LBCO thin films can detect the ethanol vapor concentration as low as 10ppm and the response of LBCO thin film to various ethanol vapor concentrations is very reliable and reproducible with the switch between air and ethanol vapor. Moreover, the fast response of the LBCO thin film, as the p-type gas sensor, is better than some n-type oxide semiconductor thin films and comparable with some nanorods and nanowires. These findings indicate that the LBCO thin films have great potential for the development of gas sensors in reducing/oxidizing environments.

Dislocation-induced nanoparticle decoration on a GaN nanowire

GaN nanowires with homoepitaxial decorated GaN nanoparticles on their surface along the radial direction have been synthesized by means of a chemical vapor deposition method. The growth of GaN nanowires is catalyzed by Au particles via the vapor-liquid-solid (VLS) mechanism. Screw dislocations are generated along the radial direction of the nanowires under slight Zn doping. In contrast to the metal-catalyst-assisted VLS growth, GaN nanoparticles are found to prefer to nucleate and grow at these dislocation sites. High-resolution transmission electron microscopy (HRTEM) analysis demonstrates that the GaN nanoparticles possess two types of epitaxial orientation with respect to the corresponding GaN nanowire: (I) [1̅21̅0]np//[1̅21̅0]nw, (0001)np//(0001)nw; (II) [1̅21̅3]np//[12̅10]nw, (101̅0)np//(101̅0)nw. An increased Ga signal in the energy-dispersive spectroscopy (EDS) profile lines of the nanowires suggests GaN nanoparticle growth at the edge surface of the wires. All the crystallographic results confirm the importance of the dislocations with respect to the homoepitaxial growth of the GaN nanoparticles. Here, screw dislocations situated on the (0001) plane provide the self-step source to enable nucleation of the GaN nanoparticles.

Strain-induced anisotropic transport properties of LaBaCo₂O₅.₅+δ thin films on NdGaO₃ substrates

Thin films of double-perovskite structural LaBaCo2O5.5+δ were epitaxially grown on (110) NdGaO3 substrates by pulsed laser deposition. Microstructural studies by high-resolution X-ray diffraction and transmission electron microscopy revealed that the films have an excellent quality epitaxial structure. In addition, strong in-plane anisotropic strains were measured. Electrical transport properties of the films were characterized by an ultra-high-vacuum four-probe scanning tunneling microscopy system at different temperatures. It was found that the anisotropic in-plane strain can significantly tune the values of film resistance up to 590%.