Spin and valley degrees of freedom in materials without inversion symmetry promise previously unknown device functionalities, such as spin-valleytronics. Control of material symmetry with electric fields (ferroelectricity), while breaking additional symmetries, including mirror symmetry, could yield phenomena where chirality, spin, valley, and crystal potential are strongly coupled. Here we report the synthesis of a halide perovskite semiconductor that is simultaneously photoferroelectricity switchable and chiral. Spectroscopic and structural analysis, and first-principles calculations, determine the material to be a previously unknown low-dimensional hybrid perovskite (R)-(−)-1-cyclohexylethylammonium/(S)-(+)-1 cyclohexylethylammonium) PbI3. Optical and electrical measurements characterize its semiconducting, ferroelectric, switchable pyroelectricity and switchable photoferroelectric properties. Temperature dependent structural, dielectric and transport measurements reveal a ferroelectric-paraelectric phase transition. Circular dichroism spectroscopy confirms its chirality. The development of a material with such a combination of these properties will facilitate the exploration of phenomena such as electric field and chiral enantiomer–dependent Rashba-Dresselhaus splitting and circular photogalvanic effects.

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Crystallographic dislocation has been well-known to be one of the major causes responsible for the unfavorable carrier dynamics in conventional semiconductor devices. Halide perovskite has exhibited promising applications in optoelectronic devices. However, how dislocation impacts its carrier dynamics in the ‘defects-tolerant’ halide perovskite is largely unknown. Here, via a remote epitaxy approach using polar substrates coated with graphene, we synthesize epitaxial halide perovskite with controlled dislocation density. First-principle calculations and molecular-dynamics simulations reveal weak film-substrate interaction and low density dislocation mechanism in remote epitaxy, respectively. High-resolution transmission electron microscopy, high-resolution atomic force microscopy and Cs-corrected scanning transmission electron microscopy unveil the lattice/atomic and dislocation structure of the remote epitaxial film. The controlling of dislocation density enables the unveiling of the dislocation-carrier dynamic relation in halide perovskite. The study provides an avenue to develop free-standing halide perovskite film with low dislocation density and improved carried dynamics.

ZnO films became ferromagnetic when defects were introduced by thermal-annealing in flowing argon. This ferromagnetism, as shown by the photoluminescence measurement and positron annihilation analysis, was induced by the singly occupied oxygen vacancy with a saturated magnetization dependent positively on the amount of this vacancy. This study clarified the origin of the ferromagnetism of un-doped ZnO thin films and provides possibly an alternative way to prepare ferromagnetic ZnO films.

Among different structures, two extreme cases are of special interest: (1) when m goes to infinity, a complete 3D structure is formed due to the absence of larger alkyl amine ligands, e.g., CH 3 NH 3 PbI 3 ; [4] (2) when m goes to 1, smaller alkyl groups are absent and a complete 2D structure is formed where each single layer of lead halide octahedrons is separated by large alkyl groups, i.e., (RNH 3 ) 2 PbX 4 . [5] A finite m value which exceeds unity will lead to multilayers of lead halide octahedrons in each individual inorganic slab, e.g., (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 . [8] At present, most of the successful devices are based on 3D perovskites, e.g., CH 3 NH 3 PbX 3 , whose long carrier recombination lifetime, long diffusion length, and high carrier mobility make them promising for photovoltaic applications. [3] In comparison, 2D perovskites will not only inherit most of the aforementioned advantages of 3D perovskites but also possess the superiorities of other 2D materials (transitional metal dichalcogenides (TMDCs) and graphene), e.g., high in-plane carrier mobility [9] and easiness in exfoliation or transfer. [10,11] These advantages make them competitive candidates for high-speed transistors, heterostructures, or planar photodetectors. Figure 1b showed the polyhedron model of a typical 2D perovskite (C 4 H 9 NH 3 ) 2 PbI 4 where each layer of inorganic octahedrons (drawn grey) is sandwiched between two layers of organic butylamine ligands. As illustrated in Figure 1c, this unique AB stacking of organic-inorganic layers provides a natural quantum well structure where charge carriers are localized within inorganic layers due to the insulating organic ligands. The dielectric confinement and quantum confinement effect both enhance the charge carrier localization such that exciton binding energy amounts to several hundred meV, leading to observable excitonic effect even at room temperature. [10,[12][13][14] In addition, when integrated with 3D perovskites, 2D perovskites serve as protection layers and enhances the moisture stability of the system. [8] These promising properties have attracted intensive investigations into (RNH 3 ) 2 PbX 4 with various organic ligands (R ranging from C 4 H 9  to C 12 H 25  and other aromatic compounds) [13,[15][16][17] and halide elements (usually Br and I). [15,17] The great potential of (RNH 3 ) 2 PbX 4 is being rapidly revealed, as witnessed by its recent success in solar cells, light-emitting diodes (LEDs), and photodetectors. [8,9,18,19] 2D hybrid perovksite (RNH 3 ) 2 PbX 4 materials not only serve as ideal platforms to study fundamental physics such as polariton dynamics but also show promise for optoelectronic and electro-optic applications. However, for the preparation of high optical quality crystals, mechanical exfoliation has to be applied in the past. In this work, the vapor phase growth of single crystalline (C 4 H 9 NH 3 ) 2 PbI 4 flakes with high optical quality is reported. Individual single crystalline domains show lateral size about 5-10 µm w...

Abstract

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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.