Stretchable electronic materials and devices have important applications in flexible electronic systems including wearable electronics and bioelectronics. Convenient electricity generation such as thermoelectric conversion is required for the flexible electronic systems. Hence, it is development of high‐performance thermoelectric materials with high mechanical stretchability would be highly desirable. Here, stretchable and transparent ionogels with high thermoelectric properties are demonstrated. The ionogels made of elastomeric waterborne polyurethane and 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA, an ionic liquid) are prepared by solution processing. Their mechanical and electrical properties depend on the loading of EMIM:DCA. The ionogels with 40 wt% EMIM:DCA can have a high mechanical stretchability of up to 156%, low tensile strength of 0.6 MPa, and low Young's modulus of 0.6 MPa. They also exhibit a high ionic thermovoltage of 34.5 mV K−1, high ionic conductivity of 8.4 mS cm−1 and low thermal conductivity of 0.23 W m−1 K−1 at a relative humidity of 90%. As a result, it can have a high ionic figure of merit (ZTi) of 1.3 ± 0.2. Both the thermovoltage and the ZTi value are the highest for stretchable thermoelectric materials. They can be used in ionic thermoelectric capacitors to convert heat into electricity.

Transition metal oxides offer functional properties beyond conventional semiconductors. Bridging the gap between the fundamental research frontier in oxide electronics and their realization in commercial devices demands a wafer-scale growth approach for high-quality transition metal oxide thin films. Such a method requires excellent control over the transition metal valence state to avoid performance deterioration, which has been proved challenging. Here we present a scalable growth approach that enables a precise valence state control. By creating an oxygen activity gradient across the wafer, a continuous valence state library is established to directly identify the optimal growth condition. Single-crystalline VO2 thin films have been grown on wafer scale, exhibiting more than four orders of magnitude change in resistivity across the metal-to-insulator transition. It is demonstrated that ‘electronic grade' transition metal oxide films can be realized on a large scale using a combinatorial growth approach, which can be extended to other multivalent oxide systems.

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This paper reviews the recent progress on electronic and optoelectronic devices based on 2D black phosphorus (BP). First, the crystal structure, band structure, and optical properties of BP, as well as some currently-known passivation methods used for making BP stable in ambient conditions are briefly summarized. Device architectures and operating principles of the state-of-the-art few-layer BP based electronic and optoelectronic devices will then be discussed in detail, with a focus on field-effect transistors, heterojunction diodes, and photodetectors. Next, solution-based exfoliation methods aimed for addressing the scalability challenge faced by BP are briefly discussed, followed by their potential applications in gas sensors and biomedicine. By reviewing recent process and discussing remaining challenges faced by BP, this paper aims to provide perspectives on opportunities and future research directions for utilizing BP and moving towards practical applications.