Jingxian Wang

Large-Area Synthesis of Monolayer and Few-Layer MoSe2 Films on SiO2 Substrates

We present successful synthesis of large area atomically thin MoSe2 films by selenization of MoO3 in a vapor transport chemical vapor deposition (CVD) system. The homogeneous thin film can reach an area of 1 × 1 cm(2) consisting primarily of monolayer and bilayer MoSe2 film. Scanning transmission electron microscopy (STEM) images reveal the highly crystalline nature of the thin film and the atomic structure of grain boundaries in monolayers. Raman and photoluminescence spectroscopy confirm the high quality of as-grown MoSe2 in optics, and electronic transport measurements highlight the potential applications of the sample in nanoelectronics.

Intrinsically patterned two-dimensional materials for selective adsorption of molecules and nanoclusters

Two-dimensional (2D) materials have been studied extensively as monolayers, vertical or lateral heterostructures. To achieve functionalization, monolayers are often patterned using soft lithography and selectively decorated with molecules. Here we demonstrate the growth of a family of 2D materials that are intrinsically patterned. We demonstrate that a monolayer of PtSe2 can be grown on a Pt substrate in the form of a triangular pattern of alternating 1T and 1H phases. Moreover, we show that, in a monolayer of CuSe grown on a Cu substrate, strain relaxation leads to periodic patterns of triangular nanopores with uniform size. Adsorption of different species at preferred pattern sites is also achieved, demonstrating that these materials can serve as templates for selective self-assembly of molecules or nanoclusters, as well as for the functionalization of the same substrate with two different species.

Facile low temperature solid state synthesis of iodoapatite by high-energy ball milling

The apatite structure-type has been proposed as a potential waste form for the immobilization of long-lived fission products, such as I-129; however, it is difficult to synthesize iodoapatite without significant iodine loss due to its high volatility. In this study, we report a facile low temperature (∼50 °C) solid-state method for successfully synthesizing lead-vanadate iodoapatite by high-energy ball milling (HEBM) of constituent compounds: PbI2, PbO and V2O5. As-milled iodoapatite is in the form of an amorphous matrix embedded with nanocrystals and can be readily crystallized by subsequent thermal annealing at a low temperature of 200 °C with minimal iodine loss. Rietveld refinement of the X-ray diffraction patterns indicates that the thermally-annealed iodoapatite is iodine deficient with an iodine concentration of ∼4.2 at%. Thermal gravimetric analysis (TGA) indicates that low temperature annealing greatly improves the thermal stability and iodine confinement. This novel approach, using HEBM and thermal annealing, is a very promising method for synthesizing advanced materials that can confine highly volatile radionuclides, such as I-129, which pose significant challenges for the successful disposal of high-level nuclear waste.

Ferroelectricity and ferroelectric resistive switching in sputtered Hf0.5Zr0.5O2 thin films

Ferroelectric properties and ferroelectric resistive switching (FE-RS) of sputtered Hf0.5Zr0.5O2 (HZO) thin films were investigated. The HZO films with the orthorhombic phase were obtained without capping or post-deposition annealing. Ferroelectricity was demonstrated by polarization-voltage (P-V) hysteresis loops measured in a positive-up negative-down manner and piezoresponse force microscopy. However, defects such as oxygen vacancies caused the films to become leaky. The observed ferroelectricity and semiconducting characteristics led to the FE-RS effect. The FE-RS effect may be explained by a polarization modulated trap-assisted tunneling model. Our study not only provides a facile route to develop ferroelectric HfO2-based thin films but also explores their potential applications in FE-RS memories.

Ferroelectricity emerging in strained (111)-textured ZrO2 thin films

(Anti-)ferroelectricity in complementary metal-oxide-semiconductor (CMOS)-compatible binary oxides have attracted considerable research interest recently. Here, we show that by using substrate-induced strain, the orthorhombic phase and the desired ferroelectricity could be achieved in ZrO2 thin films. Our theoretical analyses suggest that the strain imposed on the ZrO2 (111) film by the TiN/MgO (001) substrate would energetically favor the tetragonal (t) and orthorhombic (o) phases over the monoclinic (m) phase of ZrO2, and the compressive strain along certain ⟨11-2⟩ directions may further stabilize the o-phase. Experimentally ZrO2 thin films are sputtered onto the MgO (001) substrates buffered by epitaxial TiN layers. ZrO2 thin films exhibit t- and o-phases, which are highly (111)-textured and strained, as evidenced by X-ray diffraction and transmission electron microscopy. Both polarization-electric field (P-E) loops and corresponding current responses to voltage stimulations measured with appropriate a...

Structure and Thermal Expansion of Calcium-Thorium Apatite, [Ca4]F[Ca2Th4]T[(SiO4)6]O2.

Thorium silicate apatite with the formula [Ca3.84Th0.16]F[Ca2.79Th3.21]T(SiO4)6O2 · x(H) was synthesized by solid-state reaction, and its structure refined in P63/m from powder X-ray diffraction (XRD) data using the Rietveld method (a = 9.50172(9) Å, c = 6.98302(8) Å, V = 545.98(1) Å(3); R-Bragg = 2.102%). It was found that thorium partitions strongly to the tunnel (T) 6h position rather than the framework (F) 4f site. Fourier transform infrared spectroscopy revealed only SiO4 tetrahedron, with SiO5 and SiO6 groups, sometimes observed in siliceous apatites absent, at least to the limit of detection of this technique. Thermal expansion of the thorium apatite determined by high-temperature XRD from 298-1173 K found Δa (0.87%) dilation to exceed Δc (0.73%) with increasing temperature consistent with other silicate apatites.

Understanding Fundamentals and Reaction Mechanisms of Electrode Materials for Na-Ion Batteries

Development of efficient, affordable, and sustainable energy storage technologies has become an area of interest due to the worsening environmental issues and rising technological dependence on Li-ion batteries. Na-ion batteries (NIBs) have been receiving intensive research efforts during the last few years. Owing to their potentially low cost and relatively high energy density, NIBs are promising energy storage devices, especially for stationary applications. A fundamental understanding of electrode properties during electrochemical reactions is important for the development of low cost, high-energy density, and long shelf life NIBs. This Review aims to summarize and discuss reaction mechanisms of the major types of NIB electrode materials reported. By appreciating how the material works and the fundamental flaws it possesses, it is hoped that this Review will assist readers in coming up with innovative solutions for designing better materials for NIBs.

Nonlinear Optical Behavior of Transparent Nanohybrids of Nanocrystalline TiO2 in Poly(methyl methacrylate) Prepared by In Situ Sol-Gel Polymerization Technique

Nanohybrid thin films consisting of nanocrystalline TiO2 particles in poly(methyl methacrylate)(PMMA) were successfully synthesized by in-situ sol-gel and polymerization assisted by spin coating. Using titanium isoproproxide (Ti-iP), methyl methacrylate (MMA) and 3-(trimethoxysily)propyl methacrylate (MSMA) as the starting materials, nanohybrids containing up to 60% Ti-iP in PMMA were realized. The resulting nanohybrid thin films coated on quartz substrates are optically transparent and demonstrate nonlinear optical behaviour, where their nonlinear absorption increases with the loading of Ti-iP in PMMA, as confirmed by Z-scan measurements. Using pump-probe technique, these thin films are shown to exhibit an ultrafast relaxation time of ~1.5 picosecond.