Tellurium: Fast Electrical and Atomic Transport along the Weak Interaction Direction

In anisotropic materials, the electrical and atomic transport along the weak interaction direction is usually much slower than that along the chemical bond direction. However, Te, an important semiconductor composed of helical atomic chains, exhibits nearly isotropic electrical transport between intrachain and interchain directions. Using first-principles calculations to study bulk and few-layer Te, we show that this isotropy is related to similar effective masses and potentials for charge carriers along different transport directions, benefiting from the delocalization of the lone-pair electrons. This delocalization also enhances the interchain binding, and thus facilitates diffusion of vacancies and interstitial atoms across the chains, which together with the fast intrachain diffusion enable rapid self-healing of these defects at low temperature. Interestingly, the interstitial atoms diffuse along the chain via a concerted rotation mechanism. Our work reveals the unconventional properties underlying the superior performance of Te while providing insight into the transport in anisotropic materials

Solution-Derived ZnO Homojunction Nanowire Films on Wearable Substrates for Energy Conversion and Self-Powered Gesture Recognition

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. <i>Angew. Chem., Int. Ed.</i> <b>2012</b>, <i>51</i>, 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. <i>Nature</i> <b>2007</b>, <i>449</i>, 885. Javey, A.; et al. <i>Nature</i> <b>2003</b>, <i>424</i>, 654. Cui, Y.; et al. <i>Science</i> <b>2001</b>, <i>291</i>, 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

Hybridizing Triboelectrification and Electromagnetic Induction Effects for High-Efficient Mechanical Energy Harvesting

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

Dual-Mode Triboelectric Nanogenerator for Harvesting Water Energy and as a Self-Powered Ethanol Nanosensor

When water is passing through the air or an insulating tube, it will contain not only the mechanical energy but also the electrostatic energy due to the existence of triboelectric charges on its surface as a result of contact with the air/solid surface. In this paper, a hybrid triboelectric nanogenerator (TENG) is designed to simultaneously harvest the electrostatic and mechanical energies of flowing water. Water-TENG, mainly constructed by a super­hydro­phobic TiO₂ layer with hierarchical micro/nanostructures, is used to collect the electrostatic energy of water (Output 1). Contact-TENG, composed by a poly­tetra­fluoro­ethylene film and a layer of assembled SiO₂ nanoparticles, is used to collect the mechanical energy of water (Output 1 and Output 2). Using TiO₂ nanomaterials in water-TENG provides the advantages of photocatalytic activity and antibacterial property for water purification. Under the impact of a water stream from a household faucet at a flowing rate of 40 mL s^–1, the generated short-circuit current from Output 1 and Output 2 of dual-mode TENG can reach 43 and 18 μA, respectively. The instantaneous output power densities from Output 1 and Output 2 of dual-mode TENG are 1.31 and 0.38 W m^–2, respectively, when connecting to a load resistor of 44 MΩ. The rectified outputs have been applied to drive light-emitting diodes and charge commercial capacitors. Besides, the water-TENG has also been demonstrated as a self-powered nanosensor for ethanol detection

Triboelectric Nanogenerator Built on Suspended 3D Spiral Structure as Vibration and Positioning Sensor and Wave Energy Harvester

An unstable mechanical structure that can self-balance when perturbed is a superior choice for vibration energy harvesting and vibration detection. In this work, a suspended 3D spiral structure is integrated with a triboelectric nanogenerator (TENG) for energy harvesting and sensor applications. The newly designed vertical contact–separation mode TENG has a wide working bandwidth of 30 Hz in low-frequency range with a maximum output power density of 2.76 W/m² on a load of 6 MΩ. The position of an in-plane vibration source was identified by placing TENGs at multiple positions as multichannel, self-powered active sensors, and the location of the vibration source was determined with an error less than 6%. The magnitude of the vibration is also measured by the output voltage and current signal of the TENG. By integrating the TENG inside a buoy ball, wave energy harvesting at water surface has been demonstrated and used for lighting illumination light, which shows great potential applications in marine science and environmental/infrastructure monitoring

Optoelectronic Properties of Solution Grown ZnO n‑p or p‑n Core–Shell Nanowire Arrays

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

Clear Experimental Demonstration of Hole Gas Accumulation in Ge/Si Core–Shell Nanowires

Selective doping and band-offset in germanium (Ge)/silicon (Si) core–shell nanowire (NW) structures can realize a type of high electron mobility transistor structure in one-dimensional NWs by separating the carrier transport region from the impurity-doped region. Precise analysis, using Raman spectroscopy of the Ge optical phonon peak, can distinguish three effects: the phonon confinement effect, the stress effect due to the heterostructures, and the Fano effect. The Fano effect is the most important to demonstrate hole gas accumulation in Ge/Si core–shell NWs. Using these techniques, we obtained conclusive evidence of the hole gas accumulation in Ge/Si core–shell NWs. The control of hole gas concentration can be realized by changing the B-doping concentration in the Si shell

Triboelectric Active Sensor Array for Self-Powered Static and Dynamic Pressure Detection and Tactile Imaging

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

Piezotronic Effect in Solution-Grown p‑Type ZnO Nanowires and Films

Investigating the piezotronic effect in p-type piezoelectric semiconductor is critical for developing a complete piezotronic theory and designing/fabricating novel piezotronic applications with more complex functionality. Using a low temperature solution method, we were able to produce ultralong (up to 60 μm in length) Sb doped p-type ZnO nanowires on both rigid and flexible substrates. For the p-type nanowire field effect transistor, the on/off ratio, threshold voltage, mobility, and carrier concentration of 0.2% Sb-doped sample are found to be 10⁵, 2.1 V, 0.82 cm²·V^–1·s^–1, and 2.6 × 10¹⁷ cm^–3, respectively, and the corresponding values for 1% Sb doped samples are 10⁴, 2.0 V, 1.24 cm²·V^–1·s^–1, and 3.8 × 10¹⁷ cm^–3. We further investigated the universality of piezotronic effect in the as-synthesized Sb-doped p-type ZnO NWs and reported for the first time strain-gated piezotronic transistors as well as piezopotential-driven mechanical energy harvesting based on solution-grown p-type ZnO NWs. The results presented here broaden the scope of piezotronics and extend the framework for its potential applications in electronics, optoelectronics, smart MEMS/NEMS, and human-machine interfacing

Transparent Triboelectric Nanogenerators and Self-Powered Pressure Sensors Based on Micropatterned Plastic Films

Transparent, flexible and high efficient power sources are important components of organic electronic and optoelectronic devices. In this work, based on the principle of the previously demonstrated triboelectric generator, we demonstrate a new high-output, flexible and transparent nanogenerator by using transparent polymer materials. We have fabricated three types of regular and uniform polymer patterned arrays (line, cube, and pyramid) to improve the efficiency of the nanogenerator. The power generation of the pyramid-featured device far surpassed that exhibited by the unstructured films and gave an output voltage of up to 18 V at a current density of ∼0.13 μA/cm². Furthermore, the as-prepared nanogenerator can be applied as a self-powered pressure sensor for sensing a water droplet (8 mg, ∼3.6 Pa in contact pressure) and a falling feather (20 mg, ∼0.4 Pa in contact pressure) with a low-end detection limit of ∼13 mPa