Multilayer Hybrid Films Consisting of Alternating Graphene and Titania Nanosheets with Ultrafast Electron Transfer and Photoconversion Properties

Alternating graphene (G) and titania (Ti_0.91O₂) multilayered nanosheets are fabricated using layer‐by‐layer electrostatic deposition followed by UV irradiation. Successful assemblies of graphene oxide (GO) and titania nanosheets in sequence with polyethylenimine as a linker is confirmed by UV–vis absorption and X‐ray diffraction. Photocatalytic reduction of GO into G can be achieved upon UV irradiation. Ultrafast photocatalytic electron transfer between the titania and graphene is demonstrated using femtosecond transient absorption spectroscopy. Efficient exciton dissociation at the interfaces coupled with cross‐surface charge percolation allows efficient photocurrent conversion in the multilayered Ti_0.91O₂/G films.     

Metal‐Insulator Transition in ALD VO2 Ultrathin Films and Nanoparticles: Morphological Control

Nanoscale morphology of vanadium dioxide (VO₂) films can be controlled to realize smooth ultrathin (<10 nm) crystalline films or nanoparticles with atomic layer deposition, opening doors to practical VO₂ metal‐insulator transition (MIT) nanoelectronics. The precursor combination, the valence of V, and the density for as‐deposited VO₂ films, as well as the postdeposition crystallization annealing conditions determine whether a continuous thin film or nanoparticle morphology is obtained. It is demonstrated that the films and particles possess both a structural and an electronic transition. The resistivity of ultrathin films changes by more than two orders of magnitude across the MIT, demonstrating their high quality.     

Epitaxy on Demand

Perovskite oxide heteroepitaxy is realized on the top of inorganic nanosheets that are covering the amorphous oxide surfaces of Si substrates. Utilizing pulsed laser deposition, thin films of SrRuO₃ in a (001)_pc and (110)_pc orientation on nanosheets of Ca₂Nb₃O₁₀ and Ti_0.87O₂ are grown, respectively. The two types of nanosheets are patterned to locally tailor the crystallographic orientation and properties of SrRuO₃. The success of our approach is demonstrated by electron backscatter diffraction and spatial magnetization maps. An unprecedented control of perovskite film growth on arbitrary substrates is illustrated in this work, and the methods that are developed to deposit SrRuO₃ thin films are a viable starting point for growth of artificial heteroepitaxial thin films that require a bottom electrode. Control is not just reached in the direction of film growth, as the crystal orientation and film properties are regulated laterally on the surface of micropatterned nanosheets. Local control of magnetic properties is illustrated, which holds out prospects for the fabrication of next‐generation devices like noncollinear magnetic random access memories.     

Orthorhombic Ti2O3: A Polymorph‐Dependent Narrow‐Bandgap Ferromagnetic Oxide

Magnetic semiconductors are highly sought in spintronics, which allow not only the control of charge carriers like in traditional electronics, but also the control of spin states. However, almost all known magnetic semiconductors are featured with bandgaps larger than 1 eV, which limits their applications in long‐wavelength regimes. In this work, the discovery of orthorhombic‐structured Ti₂O₃ films is reported as a unique narrow‐bandgap (≈0.1 eV) ferromagnetic oxide semiconductor. In contrast, the well‐known corundum‐structured Ti₂O₃ polymorph has an antiferromagnetic ground state. This comprehensive study on epitaxial Ti₂O₃ thin films reveals strong correlations between structure, electrical, and magnetic properties. The new orthorhombic Ti₂O₃ polymorph is found to be n‐type with a very high electron concentration, while the bulk‐type trigonal‐structured Ti₂O₃ is p‐type. More interestingly, in contrast to the antiferromagnetic ground state of trigonal bulk Ti₂O₃, unexpected ferromagnetism with a transition temperature well above room temperature is observed in the orthorhombic Ti₂O₃, which is confirmed by X‐ray magnetic circular dichroism measurements. Using first‐principles calculations, the ferromagnetism is attributed to a particular type of oxygen vacancies in the orthorhombic Ti₂O₃. The room‐temperature ferromagnetism observed in orthorhombic‐structured Ti₂O₃, demonstrates a new route toward controlling magnetism in epitaxial oxide films through selective stabilization of polymorph phases.     

Ferroelectric and Ferromagnetic Behavior of Pb(Zr0.52Ti0.48)O3–Co0.9Zn0.1Fe2O4 Multilayered Thin Films Prepared via Solution Processing

Multilayered multiferroic nanocomposite films of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and Co_0.9Zn_0.1Fe₂O₄ (CZFO) are prepared on general Pt/Ti/SiO₂/Si substrates via a simple solution‐processing method. Structural characterization by X‐ray diffraction and electron microscopy techniques reveals good surface and cross‐sectional morphologies of these multilayered thin films. In particular, at room temperature strong ferroelectric and ferromagnetic responses are simultaneously observed in the multilayered thin films, depending on the deposited sequences and volume fractions of ferroelectric PZT phase and magnetic CZFO phase.     

The Dynamic Phase Transition Modulation of Ion‐Liquid Gating VO2 Thin Film: Formation,Diffusion, and Recovery of Oxygen Vacancies

Electrolyte gating with ionic liquids (IL) on correlated vanadium dioxide (VO₂) nanowires/beams is effective to modulate the metal‐insulator transition (MIT) behavior. While for macrosize VO₂ film, the gating treatment shows different phase modulation process and the intrinsic mechanism is still not clear, though the oxygen‐vacancy diffusion channel is always adopted for the explanation. Herein, the dynamic phase modulation of electrolyte gated VO₂ films is investigated and the oxygen vacancies formation, diffusion, and recovery at the IL/oxide interface are observed. As a relatively slow electrochemical reaction, the gating effect gradually permeates from surface to the inside of VO₂ film, along with an unsynchronized changes of integral electric, optical, and structure properties. First‐principles‐based theoretical calculation reveals that the oxygen vacancies can not only cause the structural deformations in monoclinic VO_2, but also account for the MIT transition by inducing polarization charges and thereby adjusting the d‐orbital occupancy. The findings not only clarify the oxygen vacancies statement of electrolyte gated VO₂ film, but also can be extended to other ionic liquid/oxide systems for better understanding of the surface electrochemical stability and electronic properties modulation.     

Broad Range Tuning of Phase Transition Property in VO2 Through Metal‐Ceramic Nanocomposite Design

Vanadium dioxide (VO₂) is a well‐studied Mott‐insulator because of the very abrupt physical property switching during its semiconductor‐to‐metal transition (SMT) around 341 K (68 °C). In this work, through novel oxide‐metal nanocomposite designs (i.e., Au:VO₂ and Pt:VO₂), a very broad range of SMT temperature tuning from ≈ 323.5 to ≈ 366.7 K has been achieved by varying the metallic secondary phase in the nanocomposites (i.e., Au:VO₂ and Pt:VO₂ thin films, respectively). More surprisingly, the SMT T_c can be further lowered to ≈ 301.8 K (near room temperature) by reducing the Au particle size from 11.7 to 1.7 nm. All the VO₂ nanocomposite thin films maintain superior phase transition performance, i.e., large transition amplitude, very sharp transition, and narrow width of thermal hysteresis. Correspondingly, a twofold variation of the complex dielectric function has been demonstrated in these metal‐VO₂ nanocomposites. The wide range physical property tuning is attributed to the band structure reconstruction at the metal‐VO₂ phase boundaries. This demonstration paved a novel approach for tuning the phase transition property of Mott‐insulating materials to near room temperature transition, which is important for sensors, electrical switches, smart windows, and actuators.     

Synthesis of Phase‐Pure Ferromagnetic Fe3P Films from Single‐Source Molecular Precursors

A new method for the preparation of phase‐pure ferromagnetic Fe₃P films on quartz substrates is reported. This approach utilizes the thermal decomposition of the single‐source precursors H₂Fe₃(CO)₉PR (R = ^tBu or Ph) at 400 °C. The films are deposited using a simple, home‐built metal‐organic chemical vapor deposition (MOCVD) apparatus and are characterized using a variety of analytical methods. The films exhibit excellent phase purity, as evidenced by X‐ray diffraction, X‐ray photoelectron spectroscopy, and field‐dependent magnetization measurements, the results of which agree well with measurements obtained from bulk Fe₃P. Using scanning electron microscopy and atomic force microscopy techniques, the films are found to have thicknesses between 350 and 500 nm with a granular surface texture. As‐deposited Fe₃P films are amorphous, and little or no magnetic hysteresis is observed in plots of magnetization versus applied field. Annealing the Fe₃P films at 550 °C results in improved crystallinity as well as the observation of magnetic hysteresis.     

Bottom‐Up Preparation of Uniform Ultrathin Rhenium Disulfide Nanosheets for Image‐Guided Photothermal Radiotherapy

Facile preparation of multifunctional theranostic nanoplatforms with well‐controlled morphology and sizes remains an attractive in the area of nanomedicine. Here, a new kind of 2D transition metal dichalcogenide, rhenium disulfide (ReS₂) nanosheets, with uniform sizes, strong near‐infrared (NIR) light, and strong X‐ray attenuation, is successfully synthesized. After surface modification with poly(ethylene glycol) (PEG), the synthesized ReS₂‐PEG nanosheets are stable in various physiological solutions. In addition to their contrasts in photoacoustic imaging and X‐ray computed tomography imaging because of their strong NIR light and X‐ray absorptions, respectively, such ReS₂‐PEG nanosheets can also be tracked under nuclear imaging after chelator‐free labeling with radioisotope ions, ^99mTc⁴⁺. Efficient tumor accumulation of ReS₂‐PEG nanosheets is then observed after intravenous injection into tumor‐bearing mice under triple‐modal imaging. The combined in vivo photothermal radiotherapy is further conducted, achieving a remarkable synergistic tumor destruction effect. Finally, no obvious toxicity of ReS₂‐PEG nanosheets is observed from the treated mice within 30 d. This work suggests that such ultrathin ReS₂ nanosheets with well‐controlled morphology and uniform sizes may be a promising type of multifunctional theranostic agent for remotely triggered cancer combination therapy.     

ZnO–SnO2 Hollow Spheres and Hierarchical Nanosheets: Hydrothermal Preparation,Formation Mechanism,and Photocatalytic Properties

ZnO–SnO₂ hollow spheres and hierarchical nanosheets are successfully synthesized using an aqueous solution containing ZnO rods, SnCl₄, and NaOH by using a simple hydrothermal method. The effects of hydrothermal temperature and time on the morphology of ZnO–SnO₂ are investigated. The formation process of ZnO–SnO₂ hollow spheres and nanosheets is discussed. The samples are characterized using X‐ray powder diffraction, transmission electron microscopy, scanning electron microscopy, and UV‐vis absorption spectroscopy. Both hollow spheres and hierarchical nanosheets show higher photocatalytic activities in the degradation of methyl orange than that of ZnO rods or SnO₂.     

Robust Hollow Spheres Consisting of Alternating Titania Nanosheets and Graphene Nanosheets with High Photocatalytic Activity for CO2 Conversion into Renewable Fuels

Robust hollow spheres consisting of molecular‐scale alternating titania (Ti_0.91O₂) nanosheets and graphene (G) nanosheets are successfully fabricated by a layer‐by‐layer assembly technique with polymer beads as sacrificial templates using a microwave irradiation technique to simultaneously remove the template and reduce graphene oxide into graphene. The molecular scale, 2D contact of Ti_0.91O₂ nanosheets and G nanosheets in the hollow spheres is distinctly different from the prevenient G‐based TiO₂ nanocomposites prepared by simple integration of TiO₂ and G nanosheets. The nine times increase of the photocatalytic activity of G‐Ti_0.91O₂ hollow spheres relative to commercial P25 TiO₂ is confirmed with photoreduction of CO₂ into renewable fuels (CO and CH₄). The large enhancement in the photocatalytic activity benefits from: 1) the ultrathin nature of Ti_0.91O₂ nanosheets allowing charge carriers to move rapidly onto the surface to participate in the photoreduction reaction; 2) the sufficiently compact stacking of ultrathin Ti_0.91O₂ nanosheets with G nanosheets allowing the photogenerated electron to transfer fast from the Ti_0.91O₂ nanosheets to G to enhance lifetime of the charge carriers; and 3) the hollow structure potentially acting as a photon trap‐well to allow the multiscattering of incident light for the enhancement of light absorption.     

Insulator-to-metal phase transformation of VO2 films upon femtosecond laser excitation

Light-induced insulator-to-metal phase transition of vanadium dioxide films was studied by ultrafast optical pump-probe spectroscopy. The transient optical reflection measurement shows that both heating and laser illumination contribute to the phase transition of VO₂. Within 10⁻¹¹–10⁻⁹ sec, these two mechanisms are competitive. Excited-state dynamics were found to be strongly dependent on the concentration of structural defects and the pump laser power as well. Comparison of the transient reflection of VO₂ films deposited on different substrates suggests that the excitonic-controlled light-induced insulator-to-metal phase transition in VO₂ proceeded through an intermediate state. The transient reflection measurement of VO₂ in metallic phase shows a three-stage relaxation process. A polaron excitation model is introduced to describe the dynamical process for metallic VO₂.     

Thermally grown vanadium oxide films and their electrical properties

A metal–insulator transition (MIT) occurring in vanadium oxide films prepared in different ways has been widely studied in many laboratories. It consists of a resistive change of various orders of magnitude taking place while traversing a temperature close to 67 °C. In this work the properties of VO_x films synthesized by thermal treatment of vanadium films which were vacuum-evaporated on an oxidized silicon substrate are shown. Such thermal oxidizing treatment was performed under atmospheric air at different temperatures during distinct times. Ellipsometry measurements allowed determining the thickness and optical constants of the layers after the oxidation process. From XRD, Raman and FTIR measurements, several phases with distinct oxygen content, V₂O₃, V₃O₅, VO₂ and V₂O₅, were found in the films, depending on the oxidation time and temperature. Current–temperature measurements across the films were carried out by using sandwich-type metal–insulator–metal structures. Unlike former studies on similar structures, no MIT was observed from these measurements. On the other hand, from room-temperature current–voltage measurements a well defined memristive behavior was found as a regular result in most of our structures. This memristive behavior is ascribed to the complex defect structure in the films, including the variable amount of oxygen vacancies in the lattice, rather than to the above-mentioned metal–insulator transition in vanadium oxide.     

Engineering of Self‐Assembled Domain Architectures with Ultra‐high Piezoelectric Response in Epitaxial Ferroelectric Films

Substrate clamping and inter‐domain pinning limit movement of non‐180° domain walls in ferroelectric epitaxial films thereby reducing the resulting piezoelectric response of ferroelectric layers. Our theoretical calculations and experimental studies of the epitaxial PbZr_xTi_1–xO₃ films grown on single crystal SrTiO₃ demonstrate that for film compositions near the morphotropic phase boundary it is possible to obtain mobile two‐domain architectures by selecting the appropriate substrate orientation. Transmission electron microscopy, X‐ray diffraction analysis, and piezoelectric force microscopy revealed that the PbZr_0.52Ti_0.48O₃ films grown on (101) SrTiO₃ substrates feature self‐assembled two‐domain structures, consisting of two tetragonal domain variants. For these films, the low‐field piezoelectric coefficient measured in the direction normal to the film surface (d₃₃) is 200 pm V^–1, which agrees well with the theoretical predictions. Under external AC electric fields of about 30 kV cm^–1, the (101) films exhibit reversible longitudinal strains as high as 0.35 %, which correspond to the effective piezoelectric coefficients in the order of 1000 pm V^–1 and can be explained by elastic softening of the PbZr_xTi_1–xO₃ ferroelectrics near the morphotropic phase boundary.     

Electrochemically Triggered Metal–Insulator Transition between VO2 and V2O5

Distinct properties of multiple phases of vanadium oxide (VO_x) render this material family attractive for advanced electronic devices, catalysis, and energy storage. In this work, phase boundaries of VO_x are crossed and distinct electronic properties are obtained by electrochemically tuning the oxygen content of VO_x thin films under a wide range of temperatures. Reversible phase transitions between two adjacent VO_x phases, VO₂ and V₂O₅, are obtained. Cathodic biases trigger the phase transition from V₂O₅ to VO₂, accompanied by disappearance of the wide band gap. The transformed phase is stable upon removal of the bias while reversible upon reversal of the electrochemical bias. The kinetics of the phase transition is monitored by tracking the time‐dependent response of the X‐ray absorption peaks upon the application of a sinusoidal electrical bias. The electrochemically controllable phase transition between VO₂ and V₂O₅ demonstrates the ability to induce major changes in the electronic properties of VO_x by spanning multiple structural phases. This concept is transferable to other multiphase oxides for electronic, magnetic, or electrochemical applications.     

2D/2D Heterojunction of Ultrathin MXene/Bi2WO6 Nanosheets for Improved Photocatalytic CO2 Reduction

Exploring cheap and efficient cocatalysts for enhancing the performance of photocatalysts is a challenge in the energy conversion field. Herein, 2D ultrathin Ti₃C₂ nanosheets, a kind of MXenes, are prepared by etching Ti₃AlC₂ with subsequent ultrasonic exfoliation. A novel 2D/2D heterojunction of ultrathin Ti₃C₂/Bi₂WO₆ nanosheets is then successfully prepared by in situ growth of Bi₂WO₆ ultrathin nanosheets on the surface of these Ti₃C₂ ultrathin nanosheets. The resultant Ti₃C₂/Bi₂WO₆ hybrids exhibit a short charge transport distance and a large interface contact area, assuring excellent bulk‐to‐surface and interfacial charge transfer abilities. Meanwhile, the improved specific surface area and pore structure endow Ti₃C₂/Bi₂WO₆ hybrids with an enhanced CO₂ adsorption capability. As a result, the 2D/2D heterojunction of ultrathin Ti₃C₂/Bi₂WO₆ nanosheets shows significant improvement on the performance of photocatalytic CO₂ reduction under simulated solar irradiation. The total yield of CH₄ and CH₃OH obtained on the optimized Ti₃C₂/Bi₂WO₆ hybrid is 4.6 times that obtained on pristine Bi₂WO₆ ultrathin nanosheets. This work provides a new protocol for constructing 2D/2D photocatalytic systems and demonstrates Ti₃C₂ as a promising and cheap cocatalyst.     

Phase Transition VO2 Thin Films for Optical Switches

This paper presents a method to make vanadium dioxide (VO₂) crystallites on silicon substrates by reactive ion beam sputtering. The thickness of the thin film is about 100nm. The phase transition temperature of VO₂ is 65°C. The transmittance of the semiconducting phase VO₂ is about 50% and it is reduced to as low as 3% in metal phase at the infrared wavelenghth spectrum. The extinction ratio of the optical switches is 12dB. and the insertion loss is of 1-2dB. The switching time is about 1ms.     

Electromagnetic Field-Responsive and Accurate Control of Bending in VO2 Based Micro-Pillar Array

Electromagnetic field-responsive mechanical deformation enables remote control of dynamic devices including micro-robotic machines and smart surfaces. Metal–insulator transition (MIT) of vanadium dioxide (VO₂) is developed for micro devices that are electrically or optically activated, but none of these is electromagnetic field-responsive. Herein, a micro-pillar array composed of epitaxial VO₂ nanobeams that are asymmetrically coated with Cr, Au, and silicon oxide layers is demonstrated. Localized Joule heat, induced by the eddy current effect within the Cr or Au metal layer under an electromagnetic field, provides a high-sensitive thermal response that triggers MIT of VO₂, thus activates bending and relaxing of the micro-pillars. For accurate and site-specific control of the bending, layers of amorphous silicon oxide are added to the structure to endow tunable stiffness through electron beams-matter interaction. These micro-pillars are optimized to be a 2D, tractable surface on which directional transportation by remote control of the electromagnetic field in a liquid medium is realized. This discovery provides novel ideas for the design of electromagnetic field-responsive structures.     

Comparison of TiO2 and other dielectric coatings for buried‐contact solar cells: a review

This paper compares the optical, electronic, physical and chemical properties of dielectric thin films that are commonly used to enhance the performance of bulk silicon photovoltaic devices. The standard buried‐contact (BC) solar cell presents a particularly challenging set of criteria, requiring the dielectric film to act as: (i) an anti‐reflection (AR) coating; (ii) a film compatible with surface passivation; (iii) a mask for an electroless metal plating step; (iv) a diffusion barrier for achieving a selective emitter; (v) a film with excellent chemical resistance; (vi) a stable layer during high‐temperature processing. The dielectric coatings reviewed here include thermally grown silicon dioxide (SiO₂), silicon nitride deposited by plasma‐enhanced chemical vapour deposition (a‐ SiN_x :H) and low‐pressure chemical vapour deposition (Si₃N₄), silicon oxynitride (SiON), cerium dioxide (CeO₂), zinc sulphide (ZnS), and titanium dioxide (TiO₂). While TiO₂ dielectric coatings exhibit the best optical performance and a simple post‐deposition surface passivation sequence has been developed, they require an additional sacrificial diffusion barrier to survive the heavy groove diffusion step. A‐ SiN_x :H affords passivation through its high fixed positive charge density and large hydrogen concentration; however, it is difficult to retain these electronic benefits during lengthy high‐temperature processing. Therefore, for the BC solar cell, Si₃N₄ films would appear to be the best choice of dielectric films common in industrial use. Copyright © 2004 John Wiley & Sons, Ltd.     

Assessing Antisite Defect and Impurity Concentrations in Bi2Te3 Based Thin Films by High‐Accuracy Chemical Analysis

In Bi₂Te₃‐based materials charge‐carrier densities are determined by antisite defects and controlling these defects is a key issue for thermoelectric and topological insulator materials. Bi‐Te thin films with high‐quality thermoelectric properties are deposited using a nano‐alloying approach by molecular beam epitaxy (MBE) and sputtering. The in‐plane transport properties are measured at room temperature as a function of charge‐carrier density. High‐accuracy chemical analysis by wavelength‐dispersive X‐ray spectrometry (WDX) is applied for the first time to these Bi₂Te₃‐based thin films. The acquisition conditions for WDX spectrometry are established using Monte Carlo simulations for the electron trajectories, which guarantees a high lateral resolution and rules out stray radiation generated in the substrate of the films. In contrast to energy‐dispersive X‐ray spectrometry (EDX), which is usually applied, WDX offers unprecedented accuracy for measuring antisite defect concentrations and thus has a high impact on improving the quality of thin films. The charge‐carrier densities are calculated from the WDX results according to the point‐defect model of Miller and Li and the thermopower and electrical conductivity are calculated for different charge‐carrier densities by solving the linearized Boltzmann transport equation. A good quantitative agreement is found for the dependence of the thermopower on stoichiometry, whereas the electrical conductivity is sensitively affected by contaminants.