Multistate Memory Enabled by Interface Engineering Based on Multilayer Tungsten Diselenide.

The diversification of data types and the explosive increase of data size in the information era continuously required to miniaturize the memory devices with high data storage capability. Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for flexible and transparent electronic and optoelectronic devices with high integration density. Multistate memory devices based on TMDs could possess high data storage capability with a large integration density and thus exhibit great potential applications in the field of data storage. Here, we report the multistate data storage based on multilayer tungsten diselenide (WSe2) transistors by interface engineering. The multiple resistance states of the WSe2 transistors are achieved by applying different gate voltage pulses, and the switching ratio of the memory can be as large as 105 with high cycling endurance. The water and oxygen molecules (H2O/O2) trapped at the interface between the SiO2 substrate and WSe2 introduce the trap states and thus the large hysteresis of the transfer curves, which leads to the multistate data storage. In addition, the laminated Au thin film electrodes make the contact interface between the electrodes and WSe2 free of dangling bond and Fermi level pinning, thus giving rise to the excellent performance of memory devices. Importantly, the interface trap states can be easily controlled by a simple oxygen plasma treatment of the SiO2 substrate, and subsequently, the performance of the multistate memory devices can be manipulated. Our findings provide a simple and efficient strategy to engineer the interface states for the multistate data storage applications and would motivate more investigations on the trap state-associated applications.

Controllable Growth of Centimeter-Sized 2D Perovskite Heterostructures for Highly Narrow Dual-Band Photodetectors.

Heterostructures consisting of 2D layered perovskites are expected to exhibit interesting physical phenomena inaccessible to the single 2D perovskites and can greatly extend their functionalities for electronic and optoelectronic applications. Herein, we develop a solution method to synthesize (C4H9NH3)2PbI4/(C4H9NH3)2(CH3NH3)Pb2I7 heterostructures with centimeter size, high phase purity, controllable thickness and junction depth, high crystalline quality, and great stability for highly narrow dual-band photodetectors. On the basis of the different lattice constant, solubility, and growth rate between (C4H9NH3)2PbI4 and (C4H9NH3)2(CH3NH3)Pb2I7, the designed synthetic method allows to first grow the (C4H9NH3)2PbI4 at the water-air interface and subsequently the (C4H9NH3)2(CH3NH3)Pb2I7 layer is formed via a diffusion process. Such a growth process provides an efficient way for us to readily obtain heterostructures with various thickness and junction depth by controlling the concentration, reaction temperature, and time. The formation of heterostructures has been verified by X-ray diffraction, cross-section photoluminescence, and reflection spectroscopy with the estimated junction width below 100 nm. Photodetectors based on such heterostructures exhibit low dark current (∼10-12 A), high on-off current ratio (∼103), and highly narrow dual-band spectral response with a full-width at half-maximum (fwhm) of 20 nm at 540 nm and 34 nm at 610 nm. The high performance can be attributed to the high crystalline quality of the heterostructures and the extremely large resistance in the out-of-plane direction. The synthetic strategy is versatile for other 2D perovskites, and the narrow dual-band spectral response with all fwhm < 40 nm can be continuously tuned from red to blue by properly changing the halide compositions.

A Strategy toward New Low-Dimensional Hybrid Halide Perovskites with Anionic Spacers.

The low-dimensional halide perovskites have received enormous attention due to their unique photovoltaic and optoelectronic performances. Periodic spacers are used to inhibit the growth of 3D perovskite and fabricate a 2D counterpart with layered structure, mostly based on organic/inorganic cations. Herein, by introducing organic anions (e.g., pentanedioic acid (PDA) and hexanedioic acid (HDA) simultaneously), leaf-shaped (Cs3 Pb2 Br5 )2 (PDA-HDA) microplates with low-dimensional structure are synthesized. They also exhibit significant photoluminescence (PL) centered at 540 nm with a narrow emission peak. The synthesis of single crystals of Pb(PDA) and Pb(HDA) allows to further clarify the crystal structure of (Cs3 Pb2 Br5 )2 (PDA-HDA) perovskite and its structural evolution mechanism. Moreover, the cooperative introduction of dicarboxylic acid pairs with appropriate lengths is thermodynamically favored for the low-dimensional perovskite crystallization. The temperature-dependent PL indicates a V-shaped Stokes shift with elevated temperature that could be associated with the localization of excitons in the inorganic layers between organic dicarboxylic acid molecules. This work demonstrates low-dimensional halide perovskite with anionic spacers, which also opens up a new approach to the growth of low-dimensional organic-inorganic hybrid perovskite crystals.

Two-Step Growth of 2D Organic-Inorganic Perovskite Microplates and Arrays for Functional Optoelectronics.

Two-dimensional (2D) perovskites have recently attracted intensive interest for their great stability against moisture, oxygen, and illumination compared with their three-dimensional (3D) counterparts. However, their incompatibility with a typical lithography process makes it difficult to fabricate integrated device arrays and extract basic optical and electronic parameters from individual devices. Here, we develop a combination of solution synthesis and a gas-solid-phase intercalation strategy to achieve hexagonal-shaped 2D perovskite microplates and arrays for functional optoelectronics. The 2D perovskite microplates were achieved by first synthesizing the lead iodide (PbI2) microplates from an aqueous solution and then following with intercalation via the vapor transport method. This method further allows us to synthesize arrays of 2D perovskite microplates and create individual 2D perovskite microplate-based photodetectors. In particular, chlorine (Cl) can be efficiently incorporated into the microplates, resulting in significantly improved performance of the 2D perovskite microplate-based photodetectors.