Hai-Tian Zhang

Correlated metals as transparent conductors

The fundamental challenge for designing transparent conductors used in photovoltaics, displays and solid-state lighting is the ideal combination of high optical transparency and high electrical conductivity. Satisfying these competing demands is commonly achieved by increasing carrier concentration in a wide-bandgap semiconductor with low effective carrier mass through heavy doping, as in the case of tin-doped indium oxide (ITO). Here, an alternative design strategy for identifying high-conductivity, high-transparency metals is proposed, which relies on strong electron-electron interactions resulting in an enhancement in the carrier effective mass. This approach is experimentally verified using the correlated metals SrVO3 and CaVO3, which, despite their high carrier concentration (>2.2 × 10(22) cm(-3)), have low screened plasma energies (<1.33 eV), and demonstrate excellent performance when benchmarked against ITO. A method is outlined to rapidly identify other candidates among correlated metals, and strategies are proposed to further enhance their performance, thereby opening up new avenues to develop transparent conductors.

Controllably Manipulating Three-Dimensional Hybrid Nanostructures for Bulk Nanocomposites with Large Energy Products

Hybrid nanostructures that comprise two or more nanoscale functional components are fascinating for applications in electronics, energy conversion devices, and biotechnologies. Their performances are strongly dependent on the characteristics of the individual components including the size, morphology, orientation, and distribution. However, it remains challenging to simultaneously control these structural properties in a three-dimensional (3D) hybrid nanostructure. Here, we introduce a robust strategy for concurrently manipulating these characteristics in a bulk SmCo/Fe(Co) nanocomposite. This method can tune nanocrystals in size (down to sub-10 nm), morphology (sphere, rod, or disc), and crystallographic orientation (isotropic or anisotropic). We have therefore achieved the desired nanostructures: oriented hard magnetic SmCo grains and homogeneously distributed soft magnetic Fe(Co) grains with high fractions (∼26 wt %) and small sizes (∼12.5 nm). The resulting anisotropic nanocomposite exhibits an energy product that is approximately 50% greater than that of its corresponding pure SmCo magnet and 35% higher than the reported largest value in isotropic SmCo/Fe(Co) systems. Our findings pave a new way to manipulating 3D hybrid nanostructures in a controllable manner.

Novel Bimorphological Anisotropic Bulk Nanocomposite Materials with High Energy Products

Nanostructuring of magnetically hard and soft materials is fascinating for exploring next-generation ultrastrong permanent magnets with less expensive rare-earth elements. However, the resulting hard/soft nanocomposites often exhibit random crystallographic orientations and monomorphological equiaxed grains, leading to inferior magnetic performances compared to corresponding pure rare-earth magnets. This study describes the first fabrication of a novel bimorphological anisotropic bulk nanocomposite using a multistep deformation approach, which consists of oriented hard-phase SmCo rod-shaped grains and soft-phase Fe(Co) equiaxed grains with a high fraction (≈28 wt%) and small size (≈10 nm). The nanocomposite exhibits a record-high energy product (28 MGOe) for this class of bulk materials with less rare-earth elements and outperforms, for the first time, the corresponding pure rare-earth magnet with 58% enhancement in energy product. These findings open up the door to moving from a pure permanent-magnet system to a stronger nanocomposite system at lower costs.

Self-regulated growth of LaVO3 thin films by hybrid molecular beam epitaxy

LaVO3 thin films were grown on SrTiO3 (001) by hybrid molecular beam epitaxy. A volatile metalorganic precursor, vanadium oxytriisopropoxide (VTIP), and elemental La were co-supplied in the presence of a molecular oxygen flux. By keeping the La flux fixed and varying the VTIP flux, stoichiometric LaVO3 films were obtained for a range of cation flux ratios, indicating the presence of a self-regulated growth window. Films grown under stoichiometric conditions were found to have the largest lattice parameter, which decreased monotonically with increasing amounts of excess La or V. Energy dispersive X-ray spectroscopy and Rutherford backscattering measurements were carried out to confirm film compositions. Stoichiometric growth of complex vanadate thin films independent of cation flux ratios expands upon the previously reported self-regulated growth of perovskite titanates using hybrid molecular beam epitaxy, thus demonstrating the general applicability of this growth approach to other complex oxide materials, where a precise control over film stoichiometry is demanded by the application.

Accessing a growth window for SrVO3 thin films

Stoichiometric SrVO3 thin films were grown over a range of cation fluxes on (001) (La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates using hybrid molecular beam epitaxy, where a thermal effusion cell was employed to generate a Sr flux and V was supplied using the metal-organic precursor vanadium oxytriisopropoxide (VTIP). By systematically varying the VTIP flux while keeping the Sr flux constant, a range of flux ratios were discovered in which the structural and electronic properties of the SrVO3 films remained unaltered. The intrinsic film lattice parameter and residual resistivity were found to be the smallest inside the growth window, indicating the lowest defect concentration of the films, and rapidly increased for cation flux ratios deviating from ideal growth condition. Reflection high-energy electron diffraction showed that films grown within this range had smooth surfaces and diffraction patterns were free of additional spots, while otherwise the growing surface was rough and contained additional crystalline phases. Results show the existence of a SrVO3 growth window at sufficiently high growth temperature, in which high-quality, stoichiometric films can be grown in a robust, highly reproducible manner that is invulnerable to unintentional flux variation.

Phase stabilization of VO2 thin films in high vacuum

A new growth approach to stabilize VO2 on Al2O3 in high vacuum is reported by reducing vanadium oxytriisopropoxide (VTIP) with vanadium metal. Phase stabilization and surface wetting behavior were studied as a function of growth parameters. The flux balance of VTIP to V in combination with growth temperature was identified to be critical for the growth of high quality VO2 thin films. High V fluxes were required to suppress the island formation and to ensure a coalesced film, while too high V fluxes ultimately favored the formation of the undesired, epitaxially stabilized V2O3 phase. Careful optimization of growth temperature, VTIP to V ratio, and growth rate led to high quality single phase VO2 thin films with >3.5 orders of magnitude change in resistivity across the metal-to-insulator transition. This approach opens up another synthesis avenue to stabilize oxide thin films into desired phases.

Engineering Bulk, Layered, Multicomponent Nanostructures with High Energy Density.

The precise control of individual components in multicomponent nanostructures is crucial to realizing their fascinating functionalities for applications in electronics, energy-conversion devices, and biotechnologies. However, this control remains particularly challenging for bulk, multicomponent nanomaterials because the desired structures of the constitute components often conflict. Herein, a strategy is reported for simultaneously controlling the structural properties of the constituent components in bulk multicomponent nanostructures through layered structural design. The power of this approach is illustrated by generating the desired structures of each constituent in a bulk multicomponent nanomaterial (SmCo + FeCo)/NdFeB, which cannot be attained with existing methods. The resulting nanostructure exhibits a record high energy density (31 MGOe) for this class of bulk nanocomposites composed of both hard and soft magnetic materials, with the soft magnetic fraction exceeding 20 wt%. It is anticipated that other properties beyond magnetism, such as the thermoelectric and mechanical properties, can also be tuned by engineering such layered architectures.

High-quality LaVO3 films as solar energy conversion material

Mott insulating oxides and their heterostructures have recently been identified as potential photovoltaic materials with favorable absorption properties and an intrinsic built-in electric field that can efficiently separate excited electron-hole pairs. At the same time, they are predicted to overcome the Shockley-Queisser limit due to strong electron-electron interaction present. Despite these premises a high concentration of defects commonly observed in Mott insulating films acting as recombination centers can derogate the photovoltaic conversion efficiency. With use of the self-regulated growth kinetics in hybrid molecular beam epitaxy, this obstacle can be overcome. High-quality, stoichiometric LaVO3 films were grown with defect densities of in-gap states up to 2 orders of magnitude lower compared to the films in the literature, and a factor of 3 lower than LaVO3 bulk single crystals. Photoconductivity measurements revealed a significant photoresponsivity increase as high as tenfold of stoichiometric LaVO3 films compared to their nonstoichiometric counterparts. This work marks a critical step toward the realization of high-performance Mott insulator solar cells beyond conventional semiconductors.

Photoluminescence of monolayer transition metal dichalcogenides integrated with VO2

Integrating a phase transition material with two-dimensional semiconductors can provide a route towards tunable opto-electronic metamaterials. Here, we integrate monolayer transition metal dichalcogenides with vanadium dioxide (VO2) thin films grown via molecular beam epitaxy to form a 2D/3D heterostructure. Vanadium dioxide undergoes an insulator-to-metal transition at 60-70 °C, which changes the band alignment between MoS2 and VO2 from a semiconductor-insulator junction to a semiconductor-metal junction. By switching VO2 between insulating and metallic phases, the modulation of photoluminescence emission in the 2D semiconductors was observed. This study demonstrates the feasibility to combine TMDs and functional oxides to create unconventional hybrid optoelectronic properties derived from 2D semiconductors that are linked to functional properties of oxides through proximity coupling.

Bulk anisotropic nanocomposite magnets prepared by the thermo-mechanical processing of Nd3.6Pr5.4Fe83Co3B5 with different microstructures

Bulk nanocomposite magnets with enhanced magnetic properties and (0 0 l) crystallographic texture for the (Nd,Pr)2Fe14B hard magnetic phase were prepared by thermo-mechanical process of Nd3.6Pr5.4Fe83Co3B5 ribbons. Ribbons with different microstructures, i.e. an amorphous state and amorphous matrix with a few α-(Fe,Co) nanocrystals, were employed in this study. Optimum magnetic properties were obtained in the magnets made by the hot deformation of the ribbons with nanocrystals in the amorphous matrix; the maximum energy product (BH)max = 118.6 kJ m−3. The magnets made from ribbons with a fully amorphous structure showed the largest magnetic anisotropy, and the values of (BH)max measured parallel (∥) and perpendicular (⊥) to the stress direction were ~102.7 kJ m−3 and 79.6 kJ m−3, respectively. This study is of importance for the development of bulk anisotropic nanocomposite magnets with enhanced magnetic properties.