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A material with reversible temperature change capability under an external electric field, known as the electrocaloric effect (ECE), has long been considered as a promising solid-state cooling solution. However, electrocaloric (EC) performance of EC materials generally is not sufficiently high for real cooling applications. As a result, exploring EC materials with high performance is of great interest and importance. Here, we report on the ECE of ferroelectric materials with van der Waals layered structure (CuInP2S6 or CIPS in this work in particular). Over 60% polarization charge change is observed within a temperature change of only 10 K at Curie temperature. Large adiabatic temperature change (|{\Delta}T|) of 3.3 K, isothermal entropy change (|{\Delta}S|) of 5.8 J kg-1 K-1 at |{\Delta}E|=142.0 kV cm-1 at 315 K (above and near room temperature) are achieved, with a large EC strength (|{\Delta}T|/|{\Delta}E|) of 29.5 mK cm kV-1. The ECE of CIPS is also investigated theoretically by numerical simulation and a further EC performance projection is provided., Comment: 32 pages, 10 figures

The mid-infrared spectral band (2–20 μm) is of significant technological importance for thermal imaging, spectroscopic sensing, and free-space communications. Lack of optical materials compatible w...

Sb doped ZnO nanowires grown using the low-temperature hydrothermal method have the longest reported p-type stability of over 18 months. Using this growth system, bulk homojunction films of core-shell ZnO nanowires were synthesized with either n or p-type cores and the oppositely doped shell. Extensive transmission electron microscopy (TEM) characterization showed that the nanowires remain single crystalline, and the previously reported signs of doping remain intact. The electronic properties of these films were measured, and ultraviolet photodetection was observed. This growth technique could serve as the basis for other optoelectronic devices based on ZnO such as light emitting diodes and photovoltaics.

Due to the non-centrosymmetric crystal structures, wurtzite family semiconducting materials possess piezoelectric properties and exhibit polarizations along certain directions upon straining. Utilizing strain-induced piezoelectric polarization charges to modulate the energy band structures and thus to tune/control the transport processes of charge carriers is referred to as the piezotronic effect. Distinct from the previous studies of c-axis GaN nanowires, here we systematically study the piezotronic-effect-induced modifications of energy band structures and the corresponding influence on electronic transport properties of a-axis GaN nanobelts. The physical mechanism is carefully illustrated and further confirmed by theoretical simulations via finite element analysis. The spatial distributions of local carrier concentration and the energy band diagrams of a-axis GaN under various straining conditions are calculated. This work provides a thorough understanding of strain-gated transport properties of a-axis GaN piezotronic transistors and its future applications in semiconductor devices.

Emerging applications in wearable technology, pervasive computing, human-machine interfacing, and implantable biomedical devices demand an appropriate power source that can sustainably operate for extended periods of time with minimal intervention (Wang, Z. L.; et al. Angew. Chem., Int. Ed. 2012, 51, 11700). Self-powered nanosystems, which harvest operating energy from its host (i.e., the human body), may be feasible due to their extremely low power consumption (Tian, B. Z.; et al. Nature 2007, 449, 885. Javey, A.; et al. Nature 2003, 424, 654. Cui, Y.; et al. Science 2001, 291, 851). Here we report materials and designs for wearable-on-skin piezoelectric devices based on ultrathin (2 μm) solution-derived ZnO p-n homojunction films for the first time. The depletion region formed at the p-n homojunction effectively reduces internal screening of strain-induced polarization charges by free carriers in both n-ZnO and Sb-doped p-ZnO, resulting in significantly enhanced piezoelectric output compared to a single layer device. The p-n structure can be further grown on polymeric substrates conformable to a human wrist and used to convert movement of the flexor tendons into distinguishable electrical signals for gesture recognition. The ZnO homojunction piezoelectric devices may have applications in powering nanodevices, bioprobes, and self-powered human-machine interfacing.

The recently introduced triboelectric nanogenerator (TENG) and the traditional electromagnetic induction generator (EMIG) are coherently integrated in one structure for energy harvesting and vibration sensing/isolation. The suspended structure is based on two oppositely oriented magnets that are enclosed by hollow cubes surrounded with coils, which oscillates in response to external disturbance and harvests mechanical energy simultaneously from triboelectrification and electromagnetic induction. It extends the previous definition of hybrid cell to harvest the same type of energy with multiple approaches. Both the sliding-mode TENG and contact-mode TENG can be achieved in the same structure. In order to make the TENG and EMIG work together, transformers are used to match the output impedance between these two power sources with very different characteristics. The maximum output power of 7.7 and 1.9 mW on the same load of 5 kΩ was obtained for the TENG and EMIG, respectively, after impedance matching. Benefiting from the rational design, the output signal from the TENG and the EMIG are in phase. They can be added up directly to get an output voltage of 4.6 V and an output current of 2.2 mA in parallel connection. A power management circuit was connected to the hybrid cell, and a regulated voltage of 3.3 V with constant current was achieved. For the first time, a logic operation was carried out on a half-adder circuit by using the hybrid cell working as both the power source and the input digit signals. We also demonstrated that the hybrid cell can serve as a vibration isolator. Further applications as vibration dampers, triggers, and sensors are all promising.

We report an innovative, large-area, and self-powered pressure mapping approach based on the triboelectric effect, which converts the mechanical stimuli into electrical output signals. The working mechanism of the triboelectric active sensor (TEAS) was theoretically studied by both analytical method and numerical calculation to gain an intuitive understanding of the relationship between the applied pressure and the responsive signals. Relying on the unique pressure response characteristics of the open-circuit voltage and short-circuit current, we realize both static and dynamic pressure sensing on a single device for the first time. A series of comprehensive investigations were carried out to characterize the performance of the TEAS, and high sensitivity (0.31 kPa(-1)), ultrafast response time (5 ms), long-term stability (30,000 cycles), as well as low detection limit (2.1 Pa) were achieved. The pressure measurement range of the TEAS was adjustable, which means both gentle pressure detection and large-scale pressure sensing were enabled. Through integrating multiple TEAS units into a sensor array, the as-fabricated TEAS matrix was capable of monitoring and mapping the local pressure distribution applied on the device with distinguishable spatial profiles. This work presents a technique for tactile imaging and progress toward practical applications of nanogenerators, providing potential solutions for accomplishment of artificial skin, human-electronic interfacing, and self-powered systems.

We have developed a one-step method to fabricate large-scale polymer nanowire (PNW) arrays of any organic materials, with a good control over the density and length. The PNWs are formed by ion etching a polymer film that is covered with a thin layer of metal nanoparticles, which serves as the “mask” for creating the surface roughness required for creating the PNWs. Our study demonstrates an effective approach for creating functional organic NW arrays for applications in sensors, electronics, biomaterials, and energy materials.