Dynamic Refractive Index-Matching for Adaptive Thermoresponsive Smart Windows.

Thermoresponsive smart windows (TRSWs) take great advantages in energy-efficient buildings and on-demand devices owing to their self-adaptiveness and external energy consumption-free nature. Currently used TRSWs largely rely on thermal-induced phase transitions in single-material systems, however, the intrinsic characteristics of which may not be suited for practical window utilization, such as poor luminous transparency and fixed critical temperature (Tc ). Herein, an adaptive TRSW based on dynamic refractive index (RI) matching between two phases is demonstrated, which is facilely fabricated by embedding ethylene glycol solution microdroplets into polydimethylsiloxane (PDMS) via a one-step emulsification approach, realizing a smart temperature response in PDMS. The TRSW presents high transparency (≈92%) and bidirectional transparency-temperature response (≈20% at 73 °C, ≈40% at 8 °C). Moreover, the RI dispersion generates a unique effect of wavelength selectivity with temperature. Notably, the effective optical-temperature response with variable Tc could be tuned over a wide range of 13-68 °C by adjusting the EGS concentration. The proposed strategy with dynamic RI matching allows TRSW construction to extend beyond phase transitional materials and greatly broadens the applicable scope of TRSWs, which is promising in the fields of smart optical devices such as smart windows, anti-counterfeiting, optical switches, and optical selection.

Cobalt vacancies assisted ion diffusion in Co2AlO4 carbon nanofibers for enhancing lithium battery performance.

The rational design of one-dimensional nanofibers, concentrating on the compositions, morphology, structure and defects, has emerging importance in the preparation of anode materials with desired performance for lithium-ion batteries. In the present work, we prepared cobalt vacancies enriched Co2AlO4/carbon nanofibers coated with Co2AlO4 nanosheets by using electrospinning and multi-step sintering processes. As the anode of the lithium-ion battery, the as-prepared nanofibers show excellent cycling stability, and particularly the discharge capacity can remain at 627.4 mA h g-1 after 500 cycles under 500 mA g-1. We contributed the improved performances to the carbon-based networks, the presence of cobalt vacancy on Co2AlO4 and the larger specific surface area of the present species. Moreover, density functional theory (DFT) calculations have implied that introducing Co vacancies could reduce the energy barrier of ion diffusion, leading to a faster diffusion rate of lithium ions during cycling. Apparently, the present approach could afford many essential advantages for anode material preparation, such as carbon-based matrix, larger specific surface area and cation vacancy, and more importantly, it can be extended to other spinel mixed transition metal oxides.

Thickness-Tunable Synthesis of Ultrathin Type-II Dirac Semimetal PtTe2 Single Crystals and Their Thickness-Dependent Electronic Properties.

The recent discovery of topological semimetals has stimulated extensive research interest due to their unique electronic properties and novel transport properties related to a chiral anomaly. However, the studies to date are largely limited to bulk crystals and exfoliated flakes. Here, we report the controllable synthesis of ultrathin two-dimensional (2D) platinum telluride (PtTe2) nanosheets with tunable thickness and investigate the thickness-dependent electronic properties. We show that PtTe2 nanosheets can be readily grown, using a chemical vapor deposition approach, with a hexagonal or triangular geometry and a lateral dimension of up to 80 µm, and the thickness of the nanosheets can be systematically tailored from over 20 to 1.8 nm by reducing the growth temperature or increasing the flow rate of the carrier gas. X-ray-diffraction, transmission-electron microscopy, and electron-diffraction studies confirm that the resulting 2D nanosheets are high-quality single crystals. Raman spectroscopic studies show characteristics Eg and A1g vibration modes at ∼109 and ∼155 cm-1, with a systematic red shift with increasing nanosheet thickness. Electrical transport studies show the 2D PtTe2 nanosheets display an excellent conductivity up to 2.5 × 106 S m-1 and show strong thickness-tunable electrical properties, with both the conductivity and its temperature dependence varying considerably with the thickness. Moreover, 2D PtTe2 nanosheets show an extraordinary breakdown current density up to 5.7 × 107 A/cm2, the highest breakdown current density achieved in 2D metallic transition-metal dichalcogenides to date.

Synthesis of 2D Layered BiI3 Nanoplates, BiI3 /WSe2 van der Waals Heterostructures and Their Electronic, Optoelectronic Properties.

Two-dimensional layered materials (2DLMs) have attracted considerable recent interest as a new material platform for fundamental materials science and potential new technologies. Here we report the growth of layered metal halide materials and their optoelectronic properties. BiI3 nanoplates can be readily grown on SiO2 /Si substrates with a hexagonal geometry, with a thickness in the range of 10-120 nm and a lateral dimension of 3-10 µm. Transmission electron microscopy and electron diffraction studies demonstrate that the individual nanoplates are high quality single crystals. Micro-Raman studies show characteristic Ag band at ≈115 cm-1 with slight red-shift with decreasing thickness, and micro-photoluminescence studies show uniform emission around 690 nm with blue-shift with decreasing thickness. Electrical transport studies of individual nanoplates show n-type semiconductor characteristics with clear photoresponse. Further, the BiI3 can be readily grown on other 2DLMs (e.g., WSe2 ) to form van der Waals heterostructures. Electrical transport measurements of BiI3 /WSe2 vertical heterojunctions demonstrate p-n diode characteristics with gate-tunable rectification behavior and distinct photovoltaic effect. The synthesis of the BiI3 nanoplates can expand the library of 2DLMs and enable a wider range of van der Waals heterostructures.

Growth of Single-Crystalline Cadmium Iodide Nanoplates, CdI2/MoS2 (WS2, WSe2) van der Waals Heterostructures, and Patterned Arrays.

Two-dimensional layered materials (2DLMs) have attracted considerable recent interest for their layer-number-dependent physical and chemical properties, as well as potential technological opportunities. Here we report the synthesis of two-dimensional layered cadmium iodide (CdI2) nanoplates using a vapor transport and deposition approach. Optical microscopy and scanning electron microscopy studies show that the resulting CdI2 nanoplates predominantly adopt hexagonal and triangular morphologies with a lateral dimension of ∼2-10 µm. Atomic force microscopy studies show that the resulting nanoplates exhibit a thickness in the range of 5-220 nm with a relatively smooth surface. X-ray diffraction studies reveal highly crystalline CdI2 in hexagonal phase, which is also confirmed by the characteristic Raman Ag mode at 110 cm-1. High-resolution transmission electron microscopy and selected area electron diffraction reveal that the resulting CdI2 nanoplates are single crystals. Taking a step further, we show the CdI2 nanoplates were readily grown on other 2DLMs (e.g., WS2, WSe2, MoS2), forming diverse van der Waals heterostructures. Using prepatterned WS2 monolayer square arrays as the nucleation and growth templates, we also show that regular arrays of CdI2/WS2 vertical heterostructures can be prepared. The synthesis of the CdI2 nanoplates, heterostructures, and heterostructure arrays offers a valuable material system for 2D materials science and technology.