Piezotronic effect in solution-grown p-type ZnO nanowires and films

Vertically Well-Aligned ZnO Nanoscintillator Arrays with Improved Photoluminescence and Scintillation Properties

ZnO nanoarrays were grown via a low-temperature hydrothermal method. Solutions, each with different additive combinations, were prepared and evaluated. The effects of the additives involved in the growth procedure, i.e., ammonium hydroxide and sodium citrate, were studied in terms of the morphological, optical and scintillation properties of the ZnO nanostructures. Measurement of the nanorod (NR) length, corresponding photoluminescence (PL) and scintillation spectra and their dependence on the additives present in the solution are discussed. ZnO NRs grown on a silica substrate, whose UV transmission was found to be better than glass, showed high-quality structural and optical properties. It was found that the addition of sodium citrate significantly reduced defects and correspondingly increased the intrinsic near-band-edge (NBE) UV emission intensity at ~380 nm. To obtain high-quality nanostructures, samples were annealed in a 10% H2 + 90% N2 atmosphere. The anneal in the forming gas atmosphere enhanced the emission of the UV peak by reducing defects in the nanostructure. NRs are highly tapered towards the end of the structure. The tapering process was monitored using time growth studies, and its effect on PL and reflectance spectra are discussed. A good alpha particle response was obtained for the grown ZnO NRs, confirming its potential to be used as an alpha particle scintillator. After optimizing the reaction parameters, it was concluded that when ammonium hydroxide and sodium citrate were used, vertically well-aligned and long ZnO nanoarrays with highly improved optical and scintillation properties were obtained.

Piezotronic Antimony Sulphoiodide/Polyvinylidene Composite for Strain-Sensing and Energy-Harvesting Applications

This study investigates the piezoelectric and piezotronic properties of a novel composite material comprising polyvinylidene fluoride (PVDF) and antimony sulphoiodide (SbSI) nanowires. The material preparation method is detailed, showcasing its simplicity and reproducibility. The material's electrical resistivity, piezoelectric response, and energy-harvesting capabilities are systematically analyzed under various deflection conditions and excitation frequencies. The piezoelectric response is characterized by the generation of charge carriers in the material due to mechanical strain, resulting in voltage output. The fundamental phenomena of charge generation, along with their influence on the material's resistivity, are proposed. Dynamic strain testing reveals the composite's potential as a piezoelectric nanogenerator (PENG), converting mechanical energy into electrical energy. Comparative analyses highlight the composite's power density advantages, thereby demonstrating its potential for energy-harvesting applications. This research provides insights into the interplay between piezoelectric and piezotronic phenomena in nanocomposites and their applicability in energy-harvesting devices.

Fundamentals and Applications of ZnO-Nanowire-Based Piezotronics and Piezo-Phototronics

The piezotronic effect is a coupling effect of semiconductor and piezoelectric properties. The piezoelectric potential is used to adjust the p-n junction barrier width and Schottky barrier height to control carrier transportation. At present, it has been applied in the fields of sensors, human-machine interaction, and active flexible electronic devices. The piezo-phototronic effect is a three-field coupling effect of semiconductor, photoexcitation, and piezoelectric properties. The piezoelectric potential generated by the applied strain in the piezoelectric semiconductor controls the generation, transport, separation, and recombination of carriers at the metal-semiconductor contact or p-n junction interface, thereby improving optoelectronic devices performance, such as photodetectors, solar cells, and light-emitting diodes (LED). Since then, the piezotronics and piezo-phototronic effects have attracted vast research interest due to their ability to remarkably enhance the performance of electronic and optoelectronic devices. Meanwhile, ZnO has become an ideal material for studying the piezotronic and piezo-phototronic effects due to its simple preparation process and better biocompatibility. In this review, first, the preparation methods and structural characteristics of ZnO nanowires (NWs) with different doping types were summarized. Then, the theoretical basis of the piezotronic effect and its application in the fields of sensors, biochemistry, energy harvesting, and logic operations (based on piezoelectric transistors) were reviewed. Next, the piezo-phototronic effect in the performance of photodetectors, solar cells, and LEDs was also summarized and analyzed. In addition, modulation of the piezotronic and piezo-phototronic effects was compared and summarized for different materials, structural designs, performance characteristics, and working mechanisms' analysis. This comprehensive review provides fundamental theoretical and applied guidance for future research directions in piezotronics and piezo-phototronics for optoelectronic devices and energy harvesting.

Boost piezocatalytic activity of BaSO4 by coupling it with BaTiO3, Cu:BaTiO3, Fe:BaTiO3, S:BaTiO3 and modify them by sucrose for water purification

The purpose of this study is to improve the efficiency of decontamination using BaSO₄ as a piezocatalyst. Three techniques are employed in this study to enhance the piezocatalytic activity of BaSO₄. The first method involves coupling BaSO₄ with BaTiO₃. The acid red 151 and acid blue 113 decontamination rates improved from 56.7% and 60.9% to 61.3% and 64.4%, respectively, as a result of this strategy. Additionally, the composite of BaSO₄ and BaTiO₃ was doped with copper, iron, sulfur, and nitrogen. By doping BaTiO₃, acid red 151 and acid blue 113 achieved 86.7% and 89.2% efficiency, respectively. Finally, the nanostructures were modified with sucrose. These strategies improved degradation efficiency for acid red 151 and acid blue 113 to 92.9% and 93.3%, respectively. The reusability results showed that the piezo-catalytic activity of the m-S-BaSO₄-BaTiO₃ catalyst did not show a significant loss after five recycles for the degradation of AB113.

Preparation and characterization of self-assembled ZnO nanowire devices: nanowire strain sensor and homogeneous p-n junction

In this work, intrinsic and p-type ZnO nanowires (NWs) have been synthesized. Pure intrinsic ZnO nanowires have been fabricated by direct oxidation method and their aspect ratio reached up to 271.3. Sb-doped ZnO nanowires were uniformly grown on Si substrates by chemical vapor deposition with diameters ranging from 0.5 to 5μm and lengths ranging from 100μm to 3 mm. Directional arrangement of nanowires has been realized by two self-assembly methods, pulling method and water flow method, and two kinds of ZnO nanodevices (strain sensor and homogenous p-n junction) were prepared and characterized based on the directional arranged nanowires. According to the current response of ZnO nanowire strain sensor, the deformation quantities of elastic plate under the action of external forces in orthogonalXandYdirection were calculated respectively. The ZnO nanowire homogenous p-n junction was made of two vertical Sb-doped and intrinsic ZnO nanowires. TheI-Vcharacteristic curve showed good rectification characteristics, and the forward turn-on voltage was about 10 V. However, since the current was too small due to the small carrier concentration in the ZnO single crystal, it is difficult to achieve electroluminescence at present.

Progress in ZnO Nanosensors

Developing various nanosensors with superior performance for accurate and sensitive detection of some physical signals is essential for advances in electronic systems. Zinc oxide (ZnO) is a unique semiconductor material with wide bandgap (3.37 eV) and high exciton binding energy (60 meV) at room temperature. ZnO nanostructures have been investigated extensively for possible use as high-performance sensors, due to their excellent optical, piezoelectric and electrochemical properties, as well as the large surface area. In this review, we primarily introduce the morphology and major synthetic methods of ZnO nanomaterials, with a brief discussion of the advantages and weaknesses of each method. Then, we mainly focus on the recent progress in ZnO nanosensors according to the functional classification, including pressure sensor, gas sensor, photoelectric sensor, biosensor and temperature sensor. We provide a comprehensive analysis of the research status and constraints for the development of ZnO nanosensor in each category. Finally, the challenges and future research directions of nanosensors based on ZnO are prospected and summarized. It is of profound significance to research ZnO nanosensors in depth, which will promote the development of artificial intelligence, medical and health, as well as industrial, production.

Liquid metal-based synthesis of high performance monolayer SnS piezoelectric nanogenerators

The predicted strong piezoelectricity for monolayers of group IV monochalcogenides, together with their inherent flexibility, makes them likely candidates for developing flexible nanogenerators. Within this group, SnS is a potential choice for such nanogenerators due to its favourable semiconducting properties. To date, access to large-area and highly crystalline monolayer SnS has been challenging due to the presence of strong inter-layer interactions by the lone-pair electrons of S. Here we report single crystal across-the-plane and large-area monolayer SnS synthesis using a liquid metal-based technique. The characterisations confirm the formation of atomically thin SnS with a remarkable carrier mobility of ~35 cm² V⁻¹ s⁻¹ and piezoelectric coefficient of ~26 pm V⁻¹. Piezoelectric nanogenerators fabricated using the SnS monolayers demonstrate a peak output voltage of ~150 mV at 0.7% strain. The stable and flexible monolayer SnS can be implemented into a variety of systems for efficient energy harvesting.

The presence of strong inter-layer interactions has hindered the synthesis efforts towards large-area and highly crystalline monolayer SnS. Here, the authors report synthesis of large-area monolayer SnS using a liquid metal-based technique, and fabricate piezoelectric nano-generators with average peak output voltage of 150 mV at 0.7% strain.

A high performance flexible two dimensional vertically aligned ZnO nanodisc based piezoelectric nanogenerator via surface passivation

Herein, we present the growth of pristine vertically aligned flexible two dimensional (2D) pure ZnO nanodiscs via a simple seed assisted solution route and their use in the fabrication of a piezoelectric nanogenerator. The preferred growth direction and morphology of wurtzite ZnO nanodiscs were investigated using X-ray diffraction and field emission scanning electron microscopy (FESEM) studies. A flexible piezoelectric nanogenerator was fabricated using the vertically aligned ZnO nanodiscs as the active piezoelectric material and a carbon nanotube-polydimethylsiloxane (CNT : PDMS) film as the top electrode. This unique 2D-type ZnO nanodisc-based nanogenerator generated a direct current (DC) type output voltage and current density of about 2.5 V and 30 nA cm^-2 under compressive vertical strain, respectively. Significant enhancement of the piezoelectric output voltage from the flexible nanogenerator based on the vertically aligned two dimensional (2D) zinc oxide (ZnO) nanodiscs was achieved via thermal annealing. An output voltage and current density of 17 V and 150 nA cm^-2 were detected from the thermally annealed 2D ZnO nanodisc based nanogenerator which is approximately 8 times higher (voltage) than that from the pristine nanogenerator. It is proposed that the output performance of the vertically aligned ZnO nanodisc based nanogenerators increases due to surface passivation and reduction of oxygen vacancies.

Piezotronics and Piezo-phototronics of Third Generation Semiconductor Nanowires

With the fast development of nanoscience and nanotechnology in the last 30 years, semiconductor nanowires have been widely investigated in the areas of both electronics and optoelectronics. Among them, representatives of third generation semiconductors, such as ZnO and GaN, have relatively large spontaneous polarization along their longitudinal direction of the nanowires due to the asymmetric structure in their c-axis direction. Two-way or multiway couplings of piezoelectric, photoexcitation, and semiconductor properties have generated new research areas, such as piezotronics and piezo-phototronics. In this review, an in-depth discussion of the mechanisms and applications of nanowire-based piezotronics and piezo-phototronics is presented. Research on piezotronics and piezo-phototronics has drawn much attention since the effective manipulation of carrier transport, photoelectric properties, etc. through the application of simple mechanical stimuli and, conversely, since the design of new strain sensors based on the strain-induced change in semiconductor properties.

Dramatically Enhanced Broadband Photodetection by Dual Inversion Layers and Fowler-Nordheim Tunneling

Silicon photonics is now widely accepted as a key technology in a variety of systems. But owing to material limitations, now it is challenging to greatly improve the performance after decades of development. Here, we show a high-performance broadband photodetector with significantly enhanced sensitivity and responsivity operating over a wide wavelength range of light from near-ultraviolet to near-infrared at low power consumption. The specially designed textured top ceiling electrode works effectively as an antireflection layer to greatly improve the absorption of near-infrared light, thereby overcoming the absorption limitation of near-infrared light. Instead of the conventional p-n junction and p-intrinsic-n junction, we introduce a ∼15 nm thick alumina insulator layer between a p-type Si substrate and n-type ZnO nanowire (NW) arrays, which significantly enhances the charge carrier separation and collection efficiency. The photosensing responsivity and sensitivity are found to be nearly 1 order of magnitude higher than that of a reference device of p-Si/n-ZnO NW arrays, significantly higher than the commercial silicon photodiodes as well. The light-induced charge carriers flow across the appropriate thickness of insulator layer via the quantum mechanical Fowler-Nordheim tunneling mechanism. By virtue of the piezo-phototronic effect, the charge density at the interfaces can be tuned to alter the energy bands and the potential barrier distance for tunneling. Additionally, along with the use of incident light of different wavelengths, the influence of the insulator layer on the transport of electrons and holes separately is further investigated. The demonstrated concepts and study would lead to sensitivity improvement, quality enhancement of data transfer, decrease of power consumption, and cost reduction of silicon photonics.

Investigation of nanoscale voids in Sb-doped p-type ZnO nanowires

While it has multiple advantageous optoelectronic and piezoelectric properties, the application of zinc oxide has been limited by the lack of a stable p-type dopant. Recently, it was discovered that antimony doping can lead to stable p-type doping in ZnO, but one curious side effect of the doping process is the formation of voids inside the nanowire. While previously used as a signifier of successful doping, up until now, little research has been performed on these structures themselves. In this work, the effect of annealing on the size and microstructure of the voids was investigated using TEM and XRD, finding that the voids form around a region of Zn₇Sb₂O₁₂. Furthermore, using Raman spectroscopy, a new peak associated with successful doping was identified. The most surprising finding, however, was the presence of water trapped inside the nanowire, showing that this is actually a composite structure. Water was initially discovered in the nanowires using atom probe tomography, and verified using Raman spectroscopy.

1D Piezoelectric Material Based Nanogenerators: Methods, Materials and Property Optimization

Due to the enhanced piezoelectric properties, excellent mechanical properties and tunable electric properties, one-dimensional (1D) piezoelectric materials have shown their promising applications in nanogenerators (NG), sensors, actuators, electronic devices etc. To present a clear view about 1D piezoelectric materials, this review mainly focuses on the characterization and optimization of the piezoelectric properties of 1D nanomaterials, including semiconducting nanowires (NWs) with wurtzite and/or zinc blend phases, perovskite NWs and 1D polymers. Specifically, the piezoelectric coefficients, performance of single NW-based NG and structure-dependent electromechanical properties of 1D nanostructured materials can be respectively investigated through piezoresponse force microscopy, atomic force microscopy and the in-situ scanning/transmission electron microcopy. Along with the introduction of the mechanism and piezoelectric properties of 1D semiconductor, perovskite materials and polymers, their performance improvement strategies are summarized from the view of microstructures, including size-effect, crystal structure, orientation and defects. Finally, the extension of 1D piezoelectric materials in field effect transistors and optoelectronic devices are simply introduced.

Field effect transistors and phototransistors based upon p-type solution-processed PbS nanowires

We demonstrate the fabrication of solution processed highly crystalline p-type PbS nanowires via the oriented attachment of nanoparticles. The analysis of single nanowire field effect transistor (FET) devices revealed a hole conduction behaviour with average mobilities greater than 30 cm² V^-1 s^-1, which is an order of magnitude higher than that reported to date for p-type PbS colloidal nanowires. We have investigated the response of the FETs to near-infrared light excitation and show herein that the nanowires exhibited gate-dependent photo-conductivities, enabling us to tune the device performances. The responsivity was found to be greater than 10⁴ A W^-1 together with a detectivity of 10¹³ Jones, which benefits from a photogating effect occurring at negative gate voltages. These encouraging detection parameters are accompanied by relatively short switching times of 15 ms at positive gate voltages, resulting from a combination of the standard photoconduction and the high crystallinity of the nanowires. Collectively, these results indicate that solution-processed PbS nanowires are promising nanomaterials for infrared photodetectors as well as p-type nanowire FETs.

Piezo-Phototronic Matrix via a Nanowire Array

Piezoelectric semiconductors, such as ZnO and GaN, demonstrate multiproperty coupling effects toward various aspects of mechanical, electrical, and optical excitation. In particular, the three-way coupling among semiconducting, photoexcitation, and piezoelectric characteristics in wurtzite-structured semiconductors is established as a new field, which was first coined as piezo-phototronics by Wang in 2010. The piezo-phototronic effect can controllably modulate the charge-carrier generation, separation, transport, and/or recombination in optical-electronic processes by modifying the band structure at the metal-semiconductor or semiconductor-semiconductor heterojunction/interface. Here, the progress made in using the piezo-phototronic effect for enhancing photodetectors, pressure sensors, light-emitting diodes, and solar cells is reviewed. In comparison with previous works on a single piezoelectric semiconducting nanowire, piezo-phototronic nanodevices built using nanowire arrays provide a promising platform for fabricating integrated optoelectronics with the realization of high-spatial-resolution imaging and fast responsivity.

Low-temperature hydrothermally grown 100 μm vertically well-aligned ultralong and ultradense ZnO nanorod arrays with improved PL property

The hydrothermal synthesis of ZnO nanorods (NRs) has been investigated using ammonium hydroxide and polyethyleneimine as additives to the conventional nitrate based synthesis route, to obtain thin-films of well-aligned, ultradense and ultralong nanostructures. ZnO NRs longer than 60 μm were obtained in a one-cycle growth run and rod lengths ~ 100 μm by a two-cycle growth. The lengths of the rods were distributed uniformly across the substrate in all samples and highly dense NR arrays were observed. These conditions were obtained by a careful review of the nucleation and growth kinetics for this material system, such that the supersaturation of the solution was only relieved by precipitation on and in the presence of crystalline ZnO, and by the exploitation of a second growth phase due to the chelating behave of PEI and the products of HMTA. Also, the growth behavior was correlated to the solution pH values. The structural and optical data were found to be supportive of the growth conditions. The photoluminescence (PL) spectra from as-grown ultralong ZnO NRs exhibited a strong broad (580-625 nm) visible emission peak. However, annealing in a forming gas atmosphere at 623K (350°C) revealed a PL spectrum with a significantly decreased visible emission and an increased near band gap UV emission at 379 nm. Thus, the mechanisms associated with ammonium hydroxide and PEI addition provide a simple route for synthesizing ultralong and dense arrays of ZnO NRs at low temperature i.e. 368K (95°C).

Lead-free piezoelectrics: V3+ to V5+ ion conversion promoting the performances of V-doped Zinc Oxide

Vanadium doped ZnO (VZO) thin films were grown by RF magnetron sputtering, starting from a ZnO:V ceramic target. The crystal structure, chemical composition, electric and piezoelectric properties of the films were investigated either on the as-grown thin films or after a post-deposition rapid thermal annealing (RTA) treatment performed at 600 °C for different lengths of time (1 and 5 min) in an oxygen atmosphere. Substitutional doping of Zn²⁺ with V³⁺ and V⁵⁺ ions strongly deteriorated the hexagonal wurtzite ZnO structure of the as-grown thin films due to lattice distortion. The resulting slight amorphization led to a poor piezoelectric response and higher resistivity. After the RTA treatment, strong c-axis oriented VZO thin films were obtained, together with a partial conversion of the starting V³⁺ ions into V⁵⁺. The improvement of the crystal structure and the stronger polarity of both V³⁺ – O and V⁵⁺ – O chemical bonds, together with the corresponding easier rotation under the application of an external electric field, positively affected the piezoelectric response and increased conductivity. This was confirmed by closed-loop butterfly piezoelectric curves, by a maximum d₃₃ piezoelectric coefficient of 85 pm·V⁻¹, and also by ferroelectric switching domains with a well-defined polarization hysteresis curve, featuring a residual polarization of 12.5 μC∙cm⁻².

A retrospect on the role of piezoelectric nanogenerators in the development of the green world

This paper gives a detailed report of the evolution and potential applications of piezoelectric nanogenerators (PENGs).

Hydrothermal Synthesis of ZnO Structures Formed by High-Aspect-Ratio Nanowires for Acetone Detection

Snowflake-like ZnO structures originating from self-assembled nanowires were prepared by a low-temperature aqueous solution method. The as-grown hierarchical ZnO structures were investigated by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The results showed that the snowflake-like ZnO structures were composed of high-aspect-ratio nanowires. Furthermore, gas-sensing properties to various testing gases of 10 and 50 ppm were measured, which confirms that the ZnO structures were of good selectivity and response to acetone and could serve for acetone sensor to detect low-concentration acetone.

Piezo-phototronic effect enhanced UV photodetector based on CuI/ZnO double-shell grown on flexible copper microwire

In this work, we present a facile, low-cost, and effective approach to fabricate the UV photodetector with a CuI/ZnO double-shell nanostructure which was grown on common copper microwire. The enhanced performances of Cu/CuI/ZnO core/double-shell microwire photodetector resulted from the formation of heterojunction. Benefiting from the piezo-phototronic effect, the presentation of piezocharges can lower the barrier height and facilitate the charge transport across heterojunction. The photosensing abilities of the Cu/CuI/ZnO core/double-shell microwire detector are investigated under different UV light densities and strain conditions. We demonstrate the I-V characteristic of the as-prepared core/double-shell device; it is quite sensitive to applied strain, which indicates that the piezo-phototronic effect plays an essential role in facilitating charge carrier transport across the CuI/ZnO heterojunction, then the performance of the device is further boosted under external strain.

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.

Lattice Strain Induced Remarkable Enhancement in Piezoelectric Performance of ZnO-Based Flexible Nanogenerators

In this work, by employing halogen elements (fluorine, chlorine, bromine, and iodine) as dopant we demonstrate a unique strategy to enhance the output performance of ZnO-based flexible piezoelectric nanogenerators. For a halogen-doped ZnO nanowire film, dopants and doping concentration dependent lattice strain along the ZnO c-axis are established and confirmed by the EDS, XRD, and HRTEM analysis. Although lattice strain induced charge separation was theoretically proposed, it has not been experimentally investigated for wurtzite structured ZnO nanomaterials. Tuning the lattice strain from compressive to tensile state along the ZnO c-axis can be achieved by a substitution of halogen dopant from fluorine to other halogen elements due to the ionic size difference between dopants and oxygen. With its focus on a group of nonmetal element induced lattice strain in ZnO-based nanomaterials, this work paves the way for enhancing the performance of wurtzite-type piezoelectric semiconductor nanomaterials via lattice strain strategy which can be employed to construct piezoelectric nanodevices with higher efficiency in a cost-effective manner.

Fabrication of flexible self-powered UV detectors based on ZnO nanowires and the enhancement by the decoration of Ag nanoparticles

The flexible self-powered UV detector based on ZnO NWs shows good performance both in flat and bending conditions.

Low-temperature-grown p–n ZnO nanojunction arrays as rapid and self-driven UV photodetectors

The rapid and self-driven photodetectors have been demonstrated by using low-temperature-grown p–n ZnO nanorod arrays.

The large response current of a vacuum pressure sensor based on a vertically-aligned ZnO nanowires array

A vertically-aligned ZnO nanowires (VA-ZnO-NWs) array was prepared via chemical vapor deposition, which was used to fabricate a vacuum pressure sensor and its sensitive characteristics were measured using a semiconductor parameter tester.

Fabrication of high performance field-effect transistors and practical Schottky contacts using hydrothermal ZnO nanowires

The production of large quantities of single crystalline semiconducting ZnO nanowires (NWs) at low cost can offer practical solutions to realizing several novel electronic/optoelectronic and sensor applications on an industrial scale. The present work demonstrates high-density single crystalline NWs synthesized by a multiple cycle hydrothermal process at ∼100 °C. The high carrier concentration in such ZnO NWs is greatly suppressed by a simple low cost thermal annealing step in ambient air at ∼450 °C. Single ZnO NW FETs incorporating these modified NWs are characterized, revealing strong metal work function-dependent charge transport, unobtainable with as-grown hydrothermal ZnO NWs. Single ZnO NW FETs with Al as source and drain (s/d) contacts show excellent performance metrics, including low off-state currents (fA range), high on/off ratio (10(5)-10(7)), steep subthreshold slope (<600 mV/dec) and excellent field-effect carrier mobility (5-11 cm(2)/V-s). Modified ZnO NWs with platinum s/d contacts demonstrate excellent Schottky transport characteristics, markedly different from a reference ZnO NW device with Al contacts. This included abrupt reverse bias current-voltage saturation characteristics and positive temperature coefficient (∼0.18 eV to 0.13 eV). This work is envisaged to benefit many areas of hydrothermal ZnO NW research, such as NW FETs, piezoelectric energy recovery, piezotronics and Schottky diodes.

Enhancing Light Emission of ZnO-Nanofilm/Si-Micropillar Heterostructure Arrays by Piezo-Phototronic Effect

n-ZnO nanofilm/p-Si micropillar heterostructure light-emitting diode (LED) arrays for white light emissions are achieved and the light emission intensity of LED array is enhanced by 120% under -0.05% compressive strains. These results indicate a promising approach to fabricate Si-based light-emitting components with high performances enhanced by the piezo-phototronic effect, with potential applications in touchpad technology, personalized signatures, smart skin, and silicon-based photonic integrated circuits.

One-Dimensional Au-ZnO Heteronanostructures for Ultraviolet Light Detectors by a Two-Step Dielectrophoretic Assembly Method

One-dimensional ZnO decorated with metal nanoparticles has received much attention in the field of ultraviolet light detection because of its high photosensitivity and fast response, while how to form effective metal-ZnO heterostructures cost efficiently is still in development. We report an efficient and well-controlled method to form Au-ZnO heterostructures by two-step dielectrophoretic assembly. First, ZnO nanowires dispersed in deionized water were assembled dielectrophoretically in a planar microelectrode system. To control the number and position of assembled ZnO nanowires, a planar triangle-shaped microelectrode pair was imposed with a high-frequency ac voltage signal in this assembly process. Then a droplet of Au nanoparticle suspension was applied to decorate the preformed ZnO nanowire by another dielectrophoretic assembly process. The near-field dielectrophoretic force induced by the existence of ZnO nanowire spanning the electrode gap attracts Au nanoparticles onto the surface of ZnO nanowires and forms effective Au-ZnO heterostructures. After the adsorption of Au nanoparticles, the performances of Au-ZnO heteronanostructures in UV detection were studied. Experimental results indicate that the ratio of the photo-to-dark current of the Au-ZnO heteronanostucture-based detector was improved significantly, and the photoresponse was accelerated considerably. This kind of enhancement in performance can be attributed to the localized Schottky junctions on the surface of ZnO nanowire which improves the surface band bending.

Focused energy field method for the localized synthesis and direct integration of 1D nanomaterials on microelectronic devices

In the focused energy field method, localized heating, and convective mass transfer in a liquid precursor realizes selective synthesis and direct integration of 1D nanomaterials as well as their surface functionalization, all in a low-temperature, liquid environment. This allows facile fabrication of 1D nanomaterial-based nanoelectronic devices.

ZnO homojunction UV photodetector based on solution-grown Sb-doped p-type ZnO nanorods and pure n-type ZnO nanorods

The p-type Sb-doped ZnO NRs act as one of promising candidates for electronic and optoelectronic devices in the future.

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. 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.

Sequential growth of zinc oxide nanorod arrays at room temperature via a corrosion process: application in visible light photocatalysis

Many photocatalyst systems catalyze chemical reactions under ultraviolet (UV) illumination, because of its high photon energies. Activating inexpensive, widely available materials as photocatalyst using the intense visible part of the solar spectrum is more challenging. Here, nanorod arrays of the wide-band-gap semiconductor zinc oxide have been shown to act as photocatalysts for the aerobic photo-oxidation of organic dye Methyl Orange under illumination with red light, which is normally accessible only to narrow-band semiconductors. The homogeneous, 800-1000-nm-thick ZnO nanorod arrays show substantial light absorption (absorbances >1) throughout the visible spectral range. This absorption is caused by defect levels inside the band gap. Multiple scattering processes by the rods make the nanorods appear black. The dominantly crystalline ZnO nanorod structures grow in the (0001) direction, i.e., with the c-axis perpendicular to the surface of polycrystalline zinc. The room-temperature preparation route relies on controlled cathodic delamination of a weakly bound polymer coating from metallic zinc, an industrially produced and cheaply available substrate. Cathodic delamination is a sequential synthesis process, because it involves the propagation of a delamination front over the base material. Consequently, arbitrarily large sample surfaces can be nanostructured using this approach.

Doped ZnO 1D nanostructures: synthesis, properties, and photodetector application

In the past decades, the doping of ZnO one-dimensional nanostructures has attracted a great deal of attention due to the variety of possible morphologies, large surface-to-volume ratios, simple and low cost processing, and excellent physical properties for fabricating high-performance electronic, magnetic, and optoelectronic devices. This article mainly concentrates on recent advances regarding the doping of ZnO one-dimensional nanostructures, including a brief overview of the vapor phase transport method and hydrothermal method, as well as the fabrication process for photodetectors. The dopant elements include B, Al, Ga, In, N, P, As, Sb, Ag, Cu, Ti, Na, K, Li, La, C, F, Cl, H, Mg, Mn, S, and Sn. The various dopants which act as acceptors or donors to realize either p-type or n-type are discussed. Doping to alter optical properties is also considered. Lastly, the perspectives and future research outlook of doped ZnO nanostructures are summarized.

3D fiber-based hybrid nanogenerator for energy harvesting and as a self-powered pressure sensor

In the past years, scientists have shown that development of a power suit is no longer a dream by integrating the piezoelectric nanogenerator (PENG) or triboelectric nanogenerator (TENG) with commercial carbon fiber cloth. However, there is still no design applying those two kinds of NG together to collect the mechanical energy more efficiently. In this paper, we demonstrate a fiber-based hybrid nanogenerator (FBHNG) composed of TENG and PENG to collect the mechanical energy in the environment. The FBHNG is three-dimensional and can harvest the energy from all directions. The TENG is positioned in the core and covered with PENG as a coaxial core/shell structure. The PENG design here not only enhances the collection efficiency of mechanical energy by a single carbon fiber but also generates electric output when the TENG is not working. We also show the potential that the FBHNG can be weaved into a smart cloth to harvest the mechanical energy from human motions and act as a self-powered strain sensor. The instantaneous output power density of TENG and PENG can achieve 42.6 and 10.2 mW/m(2), respectively. And the rectified output of FBHNG has been applied to charge the commercial capacitor and drive light-emitting diodes, which are also designed as a self-powered alert system.

Lithium-doped zinc oxide nanowires-polymer composite for high performance flexible piezoelectric nanogenerator

We present a method to develop high performance flexible piezoelectric nanogenerators (NGs) by employing Li-doped ZnO nanowires (NWs). We synthesized Li-doped ZnO NWs and adopted them to replace intrinsic ZnO NWs with a relatively low piezoelectric coefficient. When we exploited the ferroelectric phase transition induced in Li-doped ZnO NWs, the performance of the NGs was significantly improved and the NG fabrication process was greatly simplified. In addition, our approach can be easily expanded for large-scale NG fabrication. Consequently, the NGs fabricated by our simple method exhibit the excelling output voltage and current, which are stable and reproducible during periodic bending/releasing measurement over extended cycles. In addition, output voltage and current up to ∼ 180 V and ∼ 50 μA, respectively, were obtained in the large-scale NG. The approach introduced here extends the performance limits of ZnO-based NGs and their potentials in practical applications.

Separation of the piezotronic and piezoresistive effects in a zinc oxide nanowire

The strain-induced band structure change in a semiconductor can change its resistivity, known as the piezoresistive effect. If the semiconductor is also a piezoelectric material, strain-induced polarization charge can control the current transport at the metal-semiconductor contact, which is called a 'piezotronic effect'. Piezotronic effect is intertwined with piezoresistive effect in the study of present piezotronic nanowire devices. Decoupling those effects will facilitate the fundamental study on the piezotronic devices and simplify the data analysis in real applications. Here, we report a general method to separate the piezotronic and piezoresistive effects in the same nanowire, based on modified four-point measurements. Current transport characteristics of each contact was extracted and showed different responses to the strain. The piezoresistive effect was measured in zinc oxide nanowires for the first time, and the result confirmed the dominant role of piezotronic effect in the strain-induced change of transport characteristics in a piezoelectric semiconductor. This study validates the assumption made in present piezotronic devices and provides a guideline for further investigation.

A low temperature process for phosphorous doped ZnO nanorods via a combination of hydrothermal and spin-on dopant methods

We demonstrate the fabrication of solution based low temperature-processed p-type ZnO NRs doped with phosphorous by using a spin-on-dopant method coupled with a hydrothermal process. We confirmed the incorporation of phosphorous dopants into a ZnO crystal by analyzing SIMS profiles, together with the evolution of the photoluminescence spectra. It is further revealed that the electrical properties of the p-type ZnO/n-type Si heterojunction diode exhibited good rectifying behavior, confirming that p-type ZnO NRs were successfully formed. In addition, we demonstrate that a piezoelectric nanogenerator with p-type ZnO NRs made on a glass substrate shows large enough power to drive polymer dispersed liquid crystal displays.

Cl-doped ZnO nanowires with metallic conductivity and their application for high-performance photoelectrochemical electrodes

Doping semiconductor nanowires (NWs) for altering their electrical and optical properties is a critical strategy for tailoring the performance of nanodevices. ZnO NWs grown by hydrothermal method are pervasively used in optoelectronic, photovoltaic, and piezoelectric energy-harvesting devices. We synthesized in situ Cl-doped ZnO NWs with metallic conductivity that would fit seamlessly with these devices and improve their performance. Possible Cl doping mechanisms were discussed. UV-visible absorption spectroscopy confirmed the visible light transparency of Cl-doped ZnO NWs. Cl-doped ZnO NW/TiO2 core/shell-structured photoelectrochemical (PEC) anode was fabricated to demonstrate the application potential of highly conductive ZnO NWs. Higher photocurrent density and overall PEC efficiency compared with the undoped ZnO NW-based device were achieved. The successful doping and low resistivity of ZnO could unlock the potential of ZnO NWs for applications in low-cost flexible transparent electrodes.

Controllable fabrication of one-dimensional ZnO nanoarrays and their application in constructing silver trap structures

1-D ZnO NAs with controllable density and diameter have successfully been synthesized and found potential applications in silver trap construction.

Piezotronics and piezo-phototronics: fundamentals and applications

Technology advancement that can provide new solutions and enable augmented capabilities to complementary metal–oxide–semiconductor (CMOS)-based technology, such as active and adaptive interaction between machine and human/ambient, is highly desired. Piezotronic nanodevices and integrated systems exhibit potential in achieving these application goals. Utilizing the gating effect of piezopotential over carrier behaviors in piezoelectric semiconductor materials under externally applied deformation, the piezoelectric and semiconducting properties together with optoelectronic excitation processes can be coupled in these materials for the investigation of novel fundamental physics and the implementation of unprecedented applications. Piezopotential is created by the strain-induced ionic polarization in the piezoelectric semiconducting crystal. Piezotronics deal with the devices fabricated using the piezopotential as a ‘gate’ voltage to tune/control charge-carrier transport across the metal–semiconductor contact or the p–n junction. Piezo-phototronics is to use the piezopotential for controlling the carrier generation, transport, separation and/or recombination for improving the performance of optoelectronic devices. This review intends to provide an overview of the rapid progress in the emerging fields of piezotronics and piezo-phototronics. The concepts and results presented in this review show promises for implementing novel nano-electromechanical devices and integrating with micro/nano-electromechanical system technology to achieve augmented functionalities to the state-of-the-art CMOS technology that may find applications in the human–machine interfacing, active flexible/stretchable electronics, sensing, energy harvesting, biomedical diagnosis/therapy, and prosthetics.

GaN nanobelt-based strain-gated piezotronic logic devices and computation

Using the piezoelectric polarization charges created at the metal-GaN nanobelt (NB) interface under strain to modulate transport of local charge carriers across the Schottky barrier, the piezotronic effect is utilized to convert mechanical stimuli applied on the wurtzite-structured GaN NB into electronic controlling signals, based on which the GaN NB strain-gated transistors (SGTs) have been fabricated. By further assembling and integrating GaN NB SGTs, universal logic devices such as NOT, AND, OR, NAND, NOR, and XOR gates have been demonstrated for performing mechanical-electrical coupled piezotronic logic operations. Moreover, basic piezotronic computation such as one-bit binary addition over the input mechanical strains with corresponding computation results in an electrical domain by half-adder has been implemented. The strain-gated piezotronic logic devices may find applications in human-machine interfacing, active flexible/stretchable electronics, MEMS, biomedical diagnosis/therapy, and prosthetics.