light, these piezoelectric charges have been utilized to enhance the conversion effi ciency of light-emitting diodes [ 10 ] and solar cells [ 11 ] in addition to the sensitivity of photodetectors [ 12 ] and active imaging pressure sensors [ 13 ] via the piezo-phototronic effect. The piezotronic and piezo-phototronic effects expand the applications of piezoelectric semiconductor materials.For developing high-output-power NGs, the geometrical design and inner resistance of semiconductor nanomaterials have been shown to be paramount, both in experiments [ 14,15 ] and calculations. [16][17][18][19] The room-temperature carrier concentration in semiconductors is often overlooked in the design of NGs, even though it is one of the most fundamental characteristics and can be easily tuned through doping and photoexcitation. For instance, it has been demonstrated that the output performance of n-type ZnO-based NGs can be enhanced by p-type doping, [ 20 ] but decreased with higher hole concentration achieved with antimony (Sb). [ 21 ] Surface passivation improves the output voltage of NGs under UV illumination because the carrier screening effect is minimized. [ 22 ] This implies that the doping concentration greatly affects NG performance and should thus be studied in depth. However, its effect has rarely been studied due to the lack of an effi cient method of controlling the defect levels inside intrinsic ZnO.GaN, which has a low intrinsic electron concentration and is insensitive to the ambient, is an ideal model material for the investigation of the carrier concentration effect on the output performance of NGs. In this work, a series of Si-doped GaN nanowire (NW) arrays was employed to study the carrier screening effect on piezopotential, and thus the output power of NGs.The basic semiconductor and crystal properties of the GaN NW arrays synthesized by plasma-assisted molecular beam epitaxial (PA-MBE) system were investigated since they enormously infl uence the power generation and the mechanisms of electricity generation of NGs. The typical diameter and density of the as-grown vertically aligned n-GaN NW arrays are ≈50 nm and 7.95 × 10 10 cm −2 , respectively, as shown in Figure 1 a, while the length is ≈500 nm after 2 h growth. Figure 1 b shows a typical high-resolution TEM (HR-TEM) image of a NW. The GaN NWs exhibit high crystallinity without noticeable linear or planar defects. Moreover, the spacing of the lattice fringes is measured to be ≈0.251 nm, which corresponds to the c -plane interplanar distance of a GaN crystal. The polarity of the GaN NWs was found to be -c polar with respect to the growth direction. A detailed crystal polarity characterization is presented in the Supporting Information.As fossil fuels are consumed at an extremely fast rate, various types of renewable energy technologies have been developed to either replace or work with traditional energy power plants. Recently, piezoelectric nanogenerators (NGs) have been proposed for scavenging mechanical energy from movements or vibrations. Both dire...

This paper proposes an obliquely aligned InN nanorod array to maximize nanorod deformation in the application of nanopiezotronics. The surface-dependent piezotronic I-V characteristics of the InN nanorod array with exposed polar (0002) and semipolar ( ̅1102) planes were studied by conductive atomic force microscopy. The effects of the piezopotential, created in the InN under straining, and the surface quantum states on the transport behavior of charge carriers in different crystal planes of the InN nanorod were investigated. The crystal plane-dependent electron density in the electron surface accumulation layer and the strain-dependent piezopotential distribution modulate the interfacial contact of the Schottky characteristics for the (0002) plane and the quasi-ohmic behavior for the ( ̅1102) plane. Regarding the piezotronic properties under applied forces, the Schottky barrier height increases in conjunction with the deflection force with high current density at large biases because of tunneling. The strain-induced piezopotential can thus tune the transport process of the charge carriers inside the InN nanorod over a larger range than in ZnO. The quantized surface electron accumulation layer is demonstrated to modulate the piezopotential-dependent carrier transport at the metal/InN interfaces and become an important factor in the design of InN-based piezotronic devices and nanogenerators.

This study investigates the role of carrier concentration in semiconducting piezoelectric single-nanowire nanogenerators (SNWNGs) and piezotronic devices. Unintentionally doped and Si-doped GaN nanowire arrays with various carrier concentrations, ranging from 10(17) (unintentionally doped) to 10(19) cm(-3) (heavily doped), are synthesized. For SNWNGs, the output current of individual nanowires starts from a negligible level and rises to the maximum of ≈50 nA at a doping concentration of 5.63 × 10(18) cm(-3) and then falls off with further increase in carrier concentration, due to the competition between the reduction of inner resistance and the screening effect on piezoelectric potential. For piezotronic applications, the force sensitivity based on the change of the Schottky barrier height works best for unintentionally doped nanowires, reaching 26.20 ± 1.82 meV nN(-1) and then decreasing with carrier concentration. Although both types of devices share the same Schottky diode, they involve different characteristics in that the slope of the current-voltage characteristics governs SNWNG devices, while the turn-on voltage determines piezotronic devices. It is demonstrated that free carriers in piezotronic materials can influence the slope and turn-on voltage of the diode characteristics concurrently when subjected to strain. This work offers a design guideline for the optimum doping concentration in semiconductors for obtaining the best performance in piezotronic devices and SNWNGs.

Piezoelectric materials such as ZnO and III-nitride are gaining increasing attention for their energy-related applications, including high-brightness light-emitting diodes (LEDs), fullspectrum solar cells, and nanogenerators. Because of the inconvenience of using chemical batteries to power wireless sensors, harvesting energy from ambient mechanical movements in variable and uncontrollable environments is an effective method of powering wireless mobile electronics for a wide range of applications in everyday life. The operating principle of a nanowire-based nanogenerator involves the unique coupling of the piezoelectric and semiconducting properties and the gating effect of the Schottky barrier formed between metal tips and semiconductor nanomaterials. Consequently, nanogenerators convert mechanical energy from ambient movement into electricity that can be used to power nanodevices without batteries. Researchers have made great progress in developing piezoelectric nanogenerators based on II-VI compound semiconductor nanomaterials such as ZnO, [ 1 , 2 ] ZnS, [ 3 ] and CdS, [ 4 ] which have great potential for the integration of piezotronics and nanogenerators. Recent efforts to enhance nanogenerator effi ciency have focused on materials with higher piezoelectric coeffi cients, including poly(vinylidene fl uoride) (PVDF) nanofi bers, [ 5 ] BaTiO 3 thin fi lms, [ 6 ] and lead zirconate titanate (PZT) nanofi bers.

Piezoelectric nanogenerators have been investigated to generate electricity from environmental vibrations due to their energy conversion capabilities. In this study, we demonstrate an optimal geometrical design of inertial vibration direct-current piezoelectric nanogenerators based on obliquely aligned InN nanowire (NW) arrays with an optimized oblique angle of ∼58°, and driven by the inertial force of their own weight, using a mechanical shaker without any AC/DC converters. The nanogenerator device manifests potential applications not only as a unique energy harvesting device capable of scavenging energy from weak mechanical vibrations, but also as a sensitive strain sensor. The maximum output power density of the nanogenerator is estimated to be 2.9 nW cm, leading to an improvement of about 3-12 times that of vertically aligned ZnO NW DC nanogenerators. Integration of two nanogenerators also exhibits a linear increase in the output power, offering an enormous potential for the creation of self-powered sustainable nanosystems utilizing incessantly natural ambient energy sources.