Metal-Insulator Transition of Single-Crystal V2O3 through van der Waals Interface Engineering.

Strongly correlated electron materials harbor interesting materials physics, such as high-Tc superconductivity, colossal magnetoresistance, and metal-insulator transition. These physical properties can be greatly influenced by the dimensionality and geometry of the hosting materials and their interaction strengths with underlying substrates. In a classic strongly correlated oxide vanadium sesquioxide (V2O3), the coexistence of a metal-insulator and paramagnetic-antiferromagnetic transitions at ∼150 K makes this material an excellent platform for exploring basic physics and developing future devices. So far, most studies have been focused on epitaxial thin films in which the strongly coupled substrate has a pronounced effect on V2O3, leading to the observations of intriguing phenomena and physics. In this work, we unveil the kinetics of a metal-insulator transition of V2O3 single-crystal sheets at nano and micro scales. We show the presence of triangle-like alternating metal/insulator phase patterns during phase transition, which is drastically different from the epitaxial film. The observation of single-stage metal-insulator transition in V2O3/graphene compared to the multistage in V2O3/SiO2 evidence the importance of sheet-substrate coupling. Harnessing the freestanding form of the V2O3 sheet, we show that the phase transition of V2O3 sheet can generate a large dynamic strain to monolayer MoS2 and tune its optical property based on the MoS2/V2O3 hybrid structure. The demonstration of the capability in tuning phase transition kinetics and phase patterns using designed hybrid structure of varied sheet-substrate coupling strengths suggests an effective knob in the design and operation of emerging Mott devices.

Liquid-Phase van der Waals Epitaxy of a Few-Layer and Unit-Cell Thick Ruddlesden-Popper Halide Perovskite.

2D Ruddlesden-Popper (RP) halide perovskites with natural multiple quantum well structures are an ideal platform to integrate into vertical heterostructures, which may introduce plentiful intriguing optoelectronic properties that are not accessible in a single bulk crystal. Here, we report liquid-phase van der Waals epitaxy of a 2D RP hybrid perovskite (4,4-DFPD)2PbI4 (4,4-DFPD is 4,4-difluoropiperidinium) on muscovite mica and fabricate a series of perovskite-perovskite vertical heterostructures by integrating it with a second 2D RP perovskite R-NPB [NPB = 1-(1-naphthyl)ethylammonium lead bromide] sheets. The grown (4,4-DFPD)2PbI4 nanobelt array can be multiple layers to unit-cell thin and are crystallographically aligned on the mica substrate. An interlayer photo emission in this R-NPB/(4,4-DFPD)2PbI4 heterostructure with a lifetime of about 25 ns at 120 K has been revealed. Our demonstration of epitaxial (4,4-DFPD)2PbI4 array grown on mica via liquid-phase van der Waals epitaxy provides a paradigm to prepare orderly distributed 2D RP hybrid perovskites for further integration into multiple heterostructures. The discovery of a new interlayer emission in the R-NPB/(4,4-DFPD)2PbI4 heterostructure enriches the basic understanding of interlayer charge transition in halide perovskite systems.

Giant pyroelectricity in nanomembranes.

Pyroelectricity describes the generation of electricity by temporal temperature change in polar materials1-3. When free-standing pyroelectric materials approach the 2D crystalline limit, how pyroelectricity behaves remained largely unknown. Here, using three model pyroelectric materials whose bonding characters along the out-of-plane direction vary from van der Waals (In2Se3), quasi-van der Waals (CsBiNb2O7) to ionic/covalent (ZnO), we experimentally show the dimensionality effect on pyroelectricity and the relation between lattice dynamics and pyroelectricity. We find that, for all three materials, when the thickness of free-standing sheets becomes small, their pyroelectric coefficients increase rapidly. We show that the material with chemical bonds along the out-of-plane direction exhibits the greatest dimensionality effect. Experimental observations evidence the possible influence of changed phonon dynamics in crystals with reduced thickness on their pyroelectricity. Our findings should stimulate fundamental study on pyroelectricity in ultra-thin materials and inspire technological development for potential pyroelectric applications in thermal imaging and energy harvesting.

Doping-Enabled Reconfigurable Strongly Correlated Phase in a Quasi-2D Perovskite.

Highlighted by the discovery of high-temperature superconductivity, strongly correlated oxides with highly distorted perovskite structures serve as intriguing model systems for pursuing emerging materials physics and testing technological concepts. While 3d correlated oxides with a distorted perovskite structure are not uncommon, their 4d counterparts are unfortunately rare. In this work, we report the tuning of the electrical and optical properties of a quasi-2D perovskite niobate CsBiNb2O7 via hydrogenation. It is observed that hydrogenation induces drastic changes of lattice dynamics, optical transmission, and conductance. It is suggested that changing the orbital occupancy of Nb d orbitals could trigger the on-site Coulomb interaction in the NbO6 octahedron. The observed hydrogen doping-induced electrical plasticity is implemented for simulating neural synaptic activity. Our finding sheds light on the role of hydrogen in 4d transition metal oxides and suggests a new avenue for the design and development of novel electronic phases.

A Reconfigurable Remotely Epitaxial VO2 Electrical Heterostructure.

The reconfigurability of the electrical heterostructure featured with external variables, such as temperature, voltage, and strain, enabled electronic/optical phase transition in functional layers has great potential for future photonics, computing, and adaptive circuits. VO2 has been regarded as an archetypal phase transition building block with superior metal-insulator transition characteristics. However, the reconfigurable VO2-based heterostructure and the associated devices are rare due to the fundamental challenge in integrating high-quality VO2 in technologically important substrates. In this report, for the first time, we show the remote epitaxy of VO2 and the demonstration of a vertical diode device in a graphene/epitaxial VO2/single-crystalline BN/graphite structure with VO2 as a reconfigurable phase-change material and hexagonal boron nitride (h-BN) as an insulating layer. By diffraction and electrical transport studies, we show that the remote epitaxial VO2 films exhibit higher structural and electrical quality than direct epitaxial ones. By high-resolution transmission electron microscopy and Cs-corrected scanning transmission electron microscopy, we show that a graphene buffered substrate leads to a less strained VO2 film than the bare substrate. In the reconfigurable diode, we find that the Fermi level change and spectral weight shift along with the metal-insulator transition of VO2 could modify the transport characteristics. The work suggests the feasibility of developing a single-crystalline VO2-based reconfigurable heterostructure with arbitrary substrates and sheds light on designing novel adaptive photonics and electrical devices and circuits.