Nanowire piezo-phototronic photodetector: theory and experimental design

Light-regulated pyro-phototronic effects in a perovskite Cs2SnI6-reinforced ferroelectric polymer hybrid nanostructure

The 'pyro-phototronic effect' plays a nontrivial role in advancing ferroelectric (FE) devices of light detectors, light-emitting diodes, and other smart technologies. In this work, a premier FE copolymer, poly(vinylidene fluoride-co-trifluoro ethylene) (P(VDF-TrFE)), is reinforced with a lead-free double perovskite, Cs2SnI6, to render profound properties in a hybrid nanostructure. It presents a unique example of the coupling of ferro-, pyro- and piezo-electrics to the 'photoexcitation' of exotic charges that actively empower the synergetic features. Cs2SnI6 embodied in small crystallites therein is distorted in a non-centrosymmetric class of a rhomboid crystal structure (a new phase) rather than a well-known centrosymmetric face-centred cubic (fcc) phase. It boosts the emerging phototronic properties. A systematic study of the bulk heterojunction reveals the four-stage pyro-phototronic response of transient photocurrent under visible light illumination of a solar simulator (intensity ∼100 mW cm-2). Illumination at a frequency of 0.025 Hz induces a temporal temperature change, ΔT → 3.1 K, in the system, leading to induced pyroelectricity in an integrated circuit. The rise time and response time for the heterojunction are observed as ∼326 ms and ∼225 ms, respectively. The output pyro-phototronic current increases as ΔT increases in an on-off cycle. As a result, the integrated pyro-phototronic effect can be utilized to empower optoelectronic devices and harvest stray 'thermal energy' for running small energy devices.

Emerging Trends in 2D TMDs Photodetectors and Piezo-Phototronic Devices

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field. Mechanical exfoliation and chemical vapor deposition (CVD) influence the piezo-phototronic effect on a transparent, ultrasensitive, and flexible van der Waals (vdW) heterostructure, which allows the use of intrinsic semiconductors, such as 2D transition metal dichalcogenides (TMD). The latest and most promising 2D TMD-based photodetectors and piezo-phototronic devices are discussed in this review article. As a result, it is possible to make flexible piezo-phototronic photodetectors, self-powered sensors, and higher strain tolerance wearable and implantable electronics for health monitoring and generation of piezoelectricity using just a single semiconductor or vdW heterostructures of various nanomaterials. A comparison is also made between the functionality and distinctive properties of 2D flexible electronic devices with a range of applications made from 2D TMDs materials. The current state of the research about 2D TMDs can be applied in a variety of ways in order to aid in the development of new types of nanoscale optoelectronic devices. Last, it summarizes the problems that are currently being faced, along with potential solutions and future prospects.

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.

Piezo-phototronic effect on photocatalysis, solar cells, photodetectors and light-emitting diodes

The piezo-phototronic effect (a coupling effect of piezoelectric, photoexcitation and semiconducting properties, coined in 2010) has been demonstrated to be an ingenious and robust strategy to manipulate optoelectronic processes by tuning the energy band structure and photoinduced carrier behavior. The piezo-phototronic effect exhibits great potential in improving the quantum yield efficiencies of optoelectronic materials and devices and thus could help increase the energy conversion efficiency, thus alleviating the energy shortage crisis. In this review, the fundamental principles and challenges of representative optoelectronic materials and devices are presented, including photocatalysts (converting solar energy into chemical energy), solar cells (generating electricity directly under light illumination), photodetectors (converting light into electrical signals) and light-emitting diodes (LEDs, converting electric current into emitted light signals). Importantly, the mechanisms of how the piezo-phototronic effect controls the optoelectronic processes and the recent progress and applications in the above-mentioned materials and devices are highlighted and summarized. Only photocatalysts, solar cells, photodetectors, and LEDs that display piezo-phototronic behavior are reviewed. Material and structural design, property characterization, theoretical simulation calculations, and mechanism analysis are then examined as strategies to further enhance the quantum yield efficiency of optoelectronic devices via the piezo-phototronic effect. This comprehensive overview will guide future fundamental and applied studies that capitalize on the piezo-phototronic effect for energy conversion and storage.

Economic Friendly ZnO-Based UV Sensors Using Hydrothermal Growth: A Review

Ultraviolet (UV) sensors offer significant advantages in human health protection and environmental pollution monitoring. Amongst various materials for UV sensors, the zinc oxide (ZnO) nanostructure is considered as one of the most promising candidates due to its incredible electrical, optical, biomedical, energetic and preparing properties. Compared to other fabricating techniques, hydrothermal synthesis has been proven to show special advantages such as economic cost, low-temperature process and excellent and high-yield production. Here, we summarize the latest progress in research about the hydrothermal synthesis of ZnO nanostructures for UV sensing. We particularly focus on the selective hydrothermal processes and reveal the effect of key factors/parameters on ZnO architectures, such as the laser power source, temperature, growth time, precursor, seeding solution and bases. Furthermore, ZnO hydrothermal nanostructures for UV applications as well as their mechanisms are also summarized. This review will therefore enlighten future ideas of low-temperature and low-cost ZnO-based UV sensors.

Low-Temperature Induced Enhancement of Photoelectric Performance in Semiconducting Nanomaterials

The development of light-electricity conversion in nanomaterials has drawn intensive attention to the topic of achieving high efficiency and environmentally adaptive photoelectric technologies. Besides traditional improving methods, we noted that low-temperature cooling possesses advantages in applicability, stability and nondamaging characteristics. Because of the temperature-related physical properties of nanoscale materials, the working mechanism of cooling originates from intrinsic characteristics, such as crystal structure, carrier motion and carrier or trap density. Here, emerging advances in cooling-enhanced photoelectric performance are reviewed, including aspects of materials, performance and mechanisms. Finally, potential applications and existing issues are also summarized. These investigations on low-temperature cooling unveil it as an innovative strategy to further realize improvement to photoelectric conversion without damaging intrinsic components and foresee high-performance applications in extreme conditions.

Photoflexoelectric effect in halide perovskites

Harvesting environmental energy to generate electricity is a key scientific and technological endeavour of our time. Photovoltaic conversion and electromechanical transduction are two common energy-harvesting mechanisms based on, respectively, semiconducting junctions and piezoelectric insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here we demonstrate that halide perovskites, a family of highly efficient photovoltaic materials^1-3, display a photoflexoelectric effect whereby, under a combination of illumination and oscillation driven by a piezoelectric actuator, they generate orders of magnitude higher flexoelectricity than in the dark. We also show that photoflexoelectricity is not exclusive to halides but a general property of semiconductors that potentially enables simultaneous electromechanical and photovoltaic transduction and harvesting in unison from multiple energy inputs.

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.

Robust Piezo-Phototronic Effect in Multilayer γ-InSe for High-Performance Self-Powered Flexible Photodetectors

The piezo-phototronic effect has been promising as an effective means to improve the performance of two-dimensional (2D) semiconductor based optoelectronic devices. However, the current reported monolayer 2D semiconductors are not regarded as suitable for actual flexible piezotronic photodetectors due to their insufficient optical absorption and mechanical durability, although they possess strong piezoelectricity. In this work, we demonstrate that, unlike 2H-phase transition-metal dichalcogenides, γ-phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers and has a strong second-harmonic generation effect. Moreover, driven by the piezo-phototronic effect, a flexible self-powered photodetector based on multilayer γ-InSe, which can work without any energy supply, is proposed. The device exhibited ultrahigh photon responsivity of 824 mA/W under light illuminations of 400 nm (0.368 mW/cm²). Moreover, the responsivity and response speed of this photodetector were enhanced further by as much as 696% and 1010%, respectively, when a 0.62% uniaxial tensile strain was applied. Our devices exhibit high reliability and stability during a 6 month test time. These significant findings offer a promising pathway to construct high-performance flexible piezo-phototronic photodetectors based on multilayer 2D semiconductors.

Crystal-Structure-Dependent Piezotronic and Piezo-Phototronic Effects of ZnO/ZnS Core/Shell Nanowires for Enhanced Electrical Transport and Photosensing Performance

We report the crystal-structure-dependent piezotronic and piezo-phototronic effects of ZnO/ZnS core/shell nanowires (CS NWs) having different shell layer crystalline structures. The wurtzite (WZ) ZnO/WZ ZnS CS NWs showed higher electrical transport and photosensing properties under external strain than the WZ ZnO/zinc blende (ZB) ZnS CS NWs. The WZ ZnO/WZ ZnS CS NWs under a compressive strain of -0.24% showed 4.4 and 8.67 times larger increase in the output current (1.93 × 10^-4 A) and photoresponsivity (8.76 × 10^-1 A/W) than those under no strain. However, the WZ ZnO/ZB ZnS CS NWs under the same strain condition showed 3.2 and 2.16 times larger increase in the output current (1.13 × 10^-4 A) and photoresponsivity (2.16 × 10^-1 A/W) than those under no strain. This improvement is ascribed to strain-induced piezopolarization charges at both the WZ ZnO NWs and the grains of the WZ ZnS shell layer in WZ ZnO/WZ ZnS CS NWs, whereas piezopolarization charges are induced only in the ZnO core region of the WZ ZnO/ZB ZnS CS NWs. These charges can change the type-II band alignment in the ZnO and ZnS interfacial region as well as the Schottky barrier height at the junction between the semiconductor and the metal, thus facilitating electrical transport and reducing the recombination probability of charge carriers under UV irradiation.

Construction of self-powered cytosensing device based on ZnO nanodisks@g-C3N4 quantum dots and application in the detection of CCRF-CEM cells

We herein report a self-powered and renewable cytosensing device based on ZnO nanodisks(NDs)@g-C3N4 quantum dots. The device features enhanced photoelectrochemical (PEC) activity compared to ZnO NDs or g-C3N4 QDs alone. The enhanced PEC ability is attributed to the synergistic effect of the high visible light sensitivity of g-C3N4 QDs and the staggered band alignment heterojunction structure with suitable band offset, which affords higher photoelectron transfer and separation efficiency. In addition, the hybridization of g-C3N4 QDs further accelerates interfacial electron transfer and blocks recombination between electron donors and photo-generated holes. The device was applied to the detection of CCRF-CEM cells. By conjugation to Sgc8c aptamer, which preferentially interacts with membrane-bound PTK7 on CCRF-CEM membranes, capture of target CCRF-CEM cells resulted in a decrease in apparent power output, which was then exploited for the ultrasensitive detection of the target cells. This decrease in power output can be recovered by simply increasing the temperature to release the cells, thus recycling the cytosensing performance. The device displayed a linear relationship between the change of power output and the logarithm of the cell concentration from 20 to 20,000 cell/mL (R2 = 0.9837) and a detection limit down to 20 cell/mL, as well as excellent selectivity and reproducibility. Thus, this ZnO NDs@g-C3N4 QDs-based device exhibits high potential for the detection of CCRF-CEM cells.

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.

Flexible Photodetectors Based on Novel Functional Materials

Flexible photodetectors have attracted a great deal of research interest in recent years due to their great possibilities for application in a variety of emerging areas such as flexible, stretchable, implantable, portable, wearable and printed electronics and optoelectronics. Novel functional materials, including materials with zero-dimensional (0D) and one-dimensional (1D) inorganic nanostructures, two-dimensional (2D) layered materials, organic semiconductors and perovskite materials, exhibit appealing electrical and optoelectrical properties, as well as outstanding mechanical flexibility, and have been widely studied as building blocks in cost-effective flexible photodetection. Here, we comprehensively review the outstanding performance of flexible photodetectors made from these novel functional materials reported in recent years. The photoresponse characteristics and flexibility of the devices will be discussed systematically. Summaries and challenges are provided to guide future directions of this vital research field.

Simultaneously Enhancing Light Emission and Suppressing Efficiency Droop in GaN Microwire-Based Ultraviolet Light-Emitting Diode by the Piezo-Phototronic Effect

Achievement of p-n homojuncted GaN enables the birth of III-nitride light emitters. Owing to the wurtzite-structure of GaN, piezoelectric polarization charges present at the interface can effectively control/tune the optoelectric behaviors of local charge-carriers (i.e., the piezo-phototronic effect). Here, we demonstrate the significantly enhanced light-output efficiency and suppressed efficiency droop in GaN microwire (MW)-based p-n junction ultraviolet light-emitting diode (UV LED) by the piezo-phototronic effect. By applying a -0.12% static compressive strain perpendicular to the p-n junction interface, the relative external quantum efficiency of the LED is enhanced by over 600%. Furthermore, efficiency droop is markedly reduced from 46.6% to 7.5% and corresponding droop onset current density shifts from 10 to 26.7 A cm^-2. Enhanced electrons confinement and improved holes injection efficiency by the piezo-phototronic effect are revealed and theoretically confirmed as the physical mechanisms. This study offers an unconventional path to develop high efficiency, strong brightness and high power III-nitride light sources.

Enhanced Performance of a Self-Powered Organic/Inorganic Photodetector by Pyro-Phototronic and Piezo-Phototronic Effects

Self-powered photodetectors (PDs) have long been realized by utilizing photovoltaic effect and their performances can be effectively enhanced by introducing the piezo-phototronic effect. Recently, a novel pyro-phototronic effect is invented as an alternative approach for performance enhancement of self-powered PDs. Here, a self-powered organic/inorganic PD is demonstrated and the influences of externally applied strain on the pyro-phototronic and the photovoltaic effects are thoroughly investigated. Under 325 nm 2.30 mW cm^-2 UV illumination and at a -0.45% compressive strain, the PD's photocurrent is dramatically enhanced from ≈14.5 to ≈103 nA by combining the pyro-phototronic and piezo-phototronic effects together, showing a significant improvement of over 600%. Theoretical simulations have been carried out via the finite element method to propose the underlying working mechanism. Moreover, the pyro-phototronic effect can be introduced by applying a -0.45% compressive strain to greatly enhance the PD's response to 442 nm illumination, including photocurrent, rise time, and fall time. This work provides in-depth understandings about the pyro-phototronic and the piezo-phototronic effects on the performances of self-powered PD to light sources with different wavelengths and indicates huge potential of these two effects in optoelectronic devices.

Piezophototronic-Effect-Enhanced Electrically Pumped Lasing

This study demonstrates significant improvements of ZnO nanowire lasers by the piezophototronic effect. The laser output power can be enhanced by a factor of 4.96, and the threshold voltage can be decreased from 48 to 20 V by applying pressure. The mechanism of the improved performance can be attributed to the enhanced carrier injection and recombination due to the piezophototronic effect.

Strain Gradient Modulated Exciton Evolution and Emission in ZnO Fibers

One-dimensional semiconductor can undergo large deformation including stretching and bending. This homogeneous strain and strain gradient are an easy and effective way to tune the light emission properties and the performance of piezo-phototronic devices. Here, we report that with large strain gradients from 2.1–3.5% μm⁻¹, free-exciton emission was intensified, and the free-exciton interaction (FXI) emission became a prominent FXI-band at the tensile side of the ZnO fiber. These led to an asymmetric variation in energy and intensity along the cross-section as well as a redshift of the total near-band-edge (NBE) emission. This evolution of the exciton emission was directly demonstrated using spatially resolved CL spectrometry combined with an in situ tensile-bending approach at liquid nitrogen temperature for individual fibers and nanowires. A distinctive mechanism of the evolution of exciton emission is proposed: the enhancement of the free-exciton-related emission is attributed to the aggregated free excitons and their interaction in the narrow bandgap in the presence of high bandgap gradients and a transverse piezoelectric field. These results might facilitate new approaches for energy conversion and sensing applications via strained nanowires and fibers.

Tuning carrier lifetime in InGaN/GaN LEDs via strain compensation for high-speed visible light communication

In recent years, visible light communication (VLC) technology has attracted intensive attention due to its huge potential in superior processing ability and fast data transmission. The transmission rate relies on the modulation bandwidth, which is predominantly determined by the minority-carrier lifetime in III-group nitride semiconductors. In this paper, the carrier dynamic process under a stress field was studied for the first time, and the carrier recombination lifetime was calculated within the framework of quantum perturbation theory. Owing to the intrinsic strain due to the lattice mismatch between InGaN and GaN, the wave functions for the holes and electrons are misaligned in an InGaN/GaN device. By applying an external strain that "cancels" the internal strain, the overlap between the wave functions can be maximized so that the lifetime of the carrier is greatly reduced. As a result, the maximum speed of a single chip was increased from 54 MHz up to 117 MHz in a blue LED chip under 0.14% compressive strain. Finally, a bandwidth contour plot depending on the stress and operating wavelength was calculated to guide VLC chip design and stress optimization.

Progress in piezo-phototronic effect modulated photovoltaics

Wurtzite structured materials, like ZnO, GaN, CdS, and InN, simultaneously possess semiconductor and piezoelectric properties. The inner-crystal piezopotential induced by external strain can effectively tune/control the carrier generation, transport and separation/combination processes at the metal-semiconductor contact or p-n junction, which is called the piezo-phototronic effect. This effect can efficiently enhance the performance of photovoltaic devices based on piezoelectric semiconductor materials by utilizing the piezo-polarization charges at the junction induced by straining, which can modulate the energy band of the piezoelectric material and then accelerate or prevent the separation process of the photon-generated electrons and vacancies. This paper introduces the fundamental physics principles of the piezo-phototronic effect, and reviews recent progress in piezo-phototronic effect enhanced solar cells, including solar cells based on semiconductor nanowire, organic/inorganic materials, quantum dots, and perovskite. The piezo-phototronic effect is suggested as a suitable basis for the development of an innovative method to enhance the performance of solar cells based on piezoelectric semiconductors by applied extrinsic strains, which might be appropriate for fundamental research and potential applications in various areas of optoelectronics.

Enhancing Photoresponsivity of Self-Aligned MoS2 Field-Effect Transistors by Piezo-Phototronic Effect from GaN Nanowires

We report high-performance self-aligned MoS2 field-effect transistors (FETs) with enhanced photoresponsivity by the piezo-phototronic effect. The FETs are fabricated based on monolayer MoS2 with a piezoelectric GaN nanowire (NW) as the local gate, and a self-aligned process is employed to define the source/drain electrodes. The fabrication method allows the preservation of the intrinsic property of MoS2 and suppresses the scattering center density in the MoS2/GaN interface, which results in high electrical and photoelectric performances. MoS2 FETs with channel lengths of ∼200 nm have been fabricated with a small subthreshold slope of 64 mV/dec. The photoresponsivity is 443.3 A·W(-1), with a fast response and recovery time of ∼5 ms under 550 nm light illumination. When strain is introduced into the GaN NW, the photoresponsivity is further enhanced to 734.5 A·W(-1) and maintains consistent response and recovery time, which is comparable with that of the mechanical exfoliation of MoS2 transistors. The approach presented here opens an avenue to high-performance top-gated piezo-enhanced MoS2 photodetectors.

Flexible Photodetectors Based on 1D Inorganic Nanostructures

Flexible photodetectors with excellent flexibility, high mechanical stability and good detectivity, have attracted great research interest in recent years. 1D inorganic nanostructures provide a number of opportunities and capabilities for use in flexible photodetectors as they have unique geometry, good transparency, outstanding mechanical flexibility, and excellent electronic/optoelectronic properties. This article offers a comprehensive review of several types of flexible photodetectors based on 1D nanostructures from the past ten years, including flexible ultraviolet, visible, and infrared photodetectors. High-performance organic-inorganic hybrid photodetectors, as well as devices with 1D nanowire (NW) arrays, are also reviewed. Finally, new concepts of flexible photodetectors including piezophototronic, stretchable and self-powered photodetectors are examined to showcase the future research in this exciting field.

Progress in Piezo-Phototronic-Effect-Enhanced Light-Emitting Diodes and Pressure Imaging

Wurtzite materials exhibit both semiconductor and piezoelectric properties under strains due to the non-central symmetric crystal structures. The three-way coupling of semiconductor properties, piezoelectric polarization and optical excitation in ZnO, GaN, CdS and other piezoelectric semiconductors leads to the emerging field of piezo-phototronics. This effect can efficiently manipulate the emission intensity of light-emitting diodes (LEDs) by utilizing the piezo-polarization charges created at the junction upon straining to modulate the energy band diagrams and the optoelectronic processes, such as generation, separation, recombination and/or transport of charge carriers. Starting from fundamental physics principles, recent progress in piezo-phototronic-effect-enhanced LEDs is reviewed; following their development from single-nanowire pressure-sensitive devices to high-resolution array matrices for pressure-distribution mapping applications. The piezo-phototronic effect provides a promising method to enhance the light emission of LEDs based on piezoelectric semiconductors through applying static strains, and may find perspective applications in various optoelectronic devices and integrated systems.

Piezo-Phototronic Enhanced UV Sensing Based on a Nanowire Photodetector Array

A large array of Schottky UV photodetectors (PDs) based on vertical aligned ZnO nanowires is achieved. By introducing the piezo-phototronic effect, the performance of the PD array is enhanced up to seven times in photoreponsivity, six times in sensitivity, and 2.8 times in detection limit. The UV PD array may have applications in optoelectronic systems, adaptive optical computing, and communication.

Fabrication and Optimization of Vertically Aligned ZnO Nanorod Array-Based UV Photodetectors via Selective Hydrothermal Synthesis

Vertically aligned ZnO nanorod array (NRA)-based ultraviolet (UV) photodetectors (PDs) were successfully fabricated and optimized via a facile hydrothermal process. Using a shadow mask technique, the thin ZnO seed layer was deposited between the patterned Au/Ti electrodes to bridge the electrodes. Thus, both the Au electrodes could be connected by the ZnO seed layer. As the sample was immersed into growth solution and heated at 90 °C, the ZnO NRAs were crystallized and vertically grown on the ZnO seed layer, thus creating a metal-semiconductor-metal PD structure. To investigate the size effect of ZnO NRAs on photocurrent, the PDs were readily prepared with different concentrations of growth solution. For the ZnO NRAs grown at 25 mM of concentration, the PD with 10 μm of channel width (i.e., gap distance between two electrodes) exhibited a high photocurrent of 1.91 × 10(-4) A at an applied bias of 10 V under 365 nm of UV light illumination. The PD was optimized by adjusting the channel width. For 15 μm of channel width, a relatively high photocurrent on-off ratio of 37.4 and good current transient characteristics were observed at the same applied bias. These results are expected to be useful for cost-effective and practical UV PD applications.

Enhanced ultraviolet-visible light responses of phototransistors based on single and a few ZrS₃ nanobelts

Phototransistors based on single and three ZrS3 nanobelts were fabricated on SiO2/Si wafers by photolithography and the lift-off technique, respectively, and their light-induced electric properties were investigated in detail. Both the devices demonstrate a remarkable photoresponse from ultraviolet to near infrared light. The photoswitch current ratio (PCR) of the single-nanobelt phototransistor is 13 under the illumination of 405 nm light with an optical power of 10.5 mW cm(-2) at a bias of 5 V, while the PCR of the three-nanobelt device is 210 under the illumination of 405 nm light with an optical power of 5.57 mW cm(-2) at a bias of 1 V. On comparison of the photoresponses under the same conditions, the latter is found to be superior to the former, and both the devices show a much better photoresponse than the reported flexible ZrS3-nanobelt-film photodetector.

Piezo-phototronic Effect Enhanced UV/Visible Photodetector Based on Fully Wide Band Gap Type-II ZnO/ZnS Core/Shell Nanowire Array

A high-performance broad band UV/visible photodetector has been successfully fabricated on a fully wide bandgap ZnO/ZnS type-II heterojunction core/shell nanowire array. The device can detect photons with energies significantly smaller (2.2 eV) than the band gap of ZnO (3.2 eV) and ZnS (3.7 eV), which is mainly attributed to spatially indirect type-II transition facilitated by the abrupt interface between the ZnO core and ZnS shell. The performance of the device was further enhanced through the piezo-phototronic effect induced lowering of the barrier height to allow charge carrier transport across the ZnO/ZnS interface, resulting in three orders of relative responsivity change measured at three different excitation wavelengths (385, 465, and 520 nm). This work demonstrates a prototype UV/visible photodetector based on the truly wide band gap semiconducting 3D core/shell nanowire array with enhanced performance through the piezo-phototronic effect.

Detecting Liquefied Petroleum Gas (LPG) at Room Temperature Using ZnSnO3/ZnO Nanowire Piezo-Nanogenerator as Self-Powered Gas Sensor

High sensitivity, selectivity, and reliability have been achieved from ZnSnO3/ZnO nanowire (NW) piezo-nanogenerator (NG) as self-powered gas sensor (SPGS) for detecting liquefied petroleum gas (LPG) at room temperature (RT). After being exposed to 8000 ppm LPG, the output piezo-voltage of ZnSnO3/ZnO NW SPGS under compressive deformation is 0.089 V, much smaller than that in air ambience (0.533 V). The sensitivity of the SPGS against 8000 ppm LPG is up to 83.23, and the low limit of detection is 600 ppm. The SPGS has lower sensitivity against H2S, H2, ethanol, methanol and saturated water vapor than LPG, indicating good selectivity for detecting LPG. After two months, the decline of the sensing performance is less than 6%. Such piezo-LPG sensing at RT can be ascribed to the new piezo-surface coupling effect of ZnSnO3/ZnO nanocomposites. The practical application of the device driven by human motion has also been simply demonstrated. This work provides a novel approach to fabricate RT-LPG sensors and promotes the development of self-powered sensing system.

Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors

A critical challenge for the integration of optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. Generally, a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spatially resolved photocurrent investigation, we find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors based on layered GaSe is mainly generated from the region close to the metal-GaSe interface with higher electrical potential. The photoresponsivity monotonically increases with shrinking the spacing distance before the direct tunneling happens, which was significantly enhanced up to 5,000 AW⁻¹ for the bottom Ti/Au contacted device. It is more than 1,700-fold improvement over the previously reported results. The response time of the Ti/Au contacted devices is about 10–20 ms and reduced down to 270 μs for the devices with single layer graphene as metallic electrodes. A theoretical model has been developed to well explain the photoresponsivity for these two types of device configurations. Our findings realize reducing the size and improving the performance of 2D semiconductor based MSM photodetectors simultaneously, which could pave the way for future high density integration of optoelectronics with high performances.

Flexible visible-light photodetectors with broad photoresponse based on ZrS3 nanobelt films

Two new flexible visible-light photodetectors based on ZrS3 nanobelts films are fabricated on a polypropylene (PP) film and printing paper, respectively, by an adhesive-tape transfer method, and their light-induced electric properties are investigated in detail. The devices demonstrate a remarkable response to 405 to 780 nm light, a photocurrent that depends on the optical power and light wavelength, and an excellent photoswitching effect and stability. This implies that ZrS3 nanobelts are prospective candidates for high-performance nanoscale optoelectronic devices that may be practically applied in photodetection of visible to near infrared light. The facile fabrication method is extendable to flexible nanodevices with different nanostructures.

Ag nanoparticles@ZnO nanowire composite arrays: an absorption enhanced UV photodetector

A novel heterojunction ultraviolet (UV) photodetector of assembling Ag nanoparticles (NPs) onto ZnO nanowire (NW) arrays was fabricated via combination of chemical vapor deposition and thermal evaporation route. The fabricated composite Ag@ZnO NW arrays show blue-shift of UV peaks, suppression of the visible peaks, and obvious enhancements in absorption from ultraviolet to infrared region and photoluminescence (PL) emission at room-temperature. These phenomena are attributed to the Localized Surface Plasmon Resonance (LSPR) effect. Benefiting from absorption enhancement and surface heterojunctions, Ag@ZnO heterostructures show a photocurrent increment by 117%, a short response time of 80 ms and a recovery time of 3.27 s under 365 nm UV illumination of 0.24 mW/cm². This research presented a simple route to obtain high performance UV photodetectors and would be of some benefit in optical-electron devices manufacture.

Theoretical study of piezo-phototronic nano-LEDs

Two-dimensional finite-element simulation of the piezo-phototronic effect in p-n-junction-based devices is carried out for the first time. A charge channel can be induced at the p-n junction interface when strain is applied, given the n-side is a piezoelectric semiconductor and the p-type side is non-piezoelectric semiconductor. This provides the first simulated evidence supporting the previously suggested mechanism responsible for the experimentally observed gigantic change of light-emission efficiency in piezo-phototronic light-emitting devices.

Flexible ultraviolet photodetectors with broad photoresponse based on branched ZnS-ZnO heterostructure nanofilms

The application of nanofilm networks made of branched ZnS-ZnO nanostructures as a flexible UV photodetector is demonstrated. The fabricated devices show excellent operational characteristics: tunable spectral selectivity, widerange photoresponse, fast response speed, and excellent environmental stability.

CdS nanoscale photodetectors

CdS nanostructures have received much attention in recent years as building blocks for optoelectronic devices due to their unique physical and chemical properties. This progress report provides an overview of recent research about rational design of CdS nanoscale photodetectors. Three kinds of photodetectors according to the metal-semiconductor contact types are discussed in detail: Ohmic contact, Schottky contact, and field enhanced transistor configuration. The focus is on the tuning of optical and electrical properties CdS nanostructures by element doping, composition and bandgap engineering, and heterojunction integration, along with thus modified device performances generated during these tuning processes. Latest concepts of photodetector design such as flexible, self-powered, plasmonic, and piezophototronic photodetectors with novel properties are introduced to demonstrate the future directions of such an exciting research field.

Single CuTCNQ charge transfer complex nanowire as ultra high responsivity photo-detector

We report ultra large photo responsivity ℜ (ratio of photo-generated current to absorbed power) in a single nanowire (NW) device made from a single strand of a nanowire (diameter ~30nm and length ~200nm) of an organomettalic semiconducting charge transfer complex material of CuTCNQ. The device shows responsivity of 8x10(4) A/Watt at 1 volt applied bias with an enhancement over the dark current exceeding 10(5) at zero bias. The observed photo current has a spectral dependence that strongly follows the main absorption peak (close to 405 nm) showing the primary role of absorbed photo-generated carriers.

Synthesis of CdS nanorod arrays and their applications in flexible piezo-driven active H2S sensors

A flexible piezo-driven active H2S sensor has been fabricated from CdS nanorod arrays. By coupling the piezoelectric and gas sensing properties of CdS nanorods, the piezoelectric output generated by CdS nanorod arrays acts not only as a power source, but also as a response signal to H2S. Under externally applied compressive force, the piezoelectric output of CdS nanorod arrays is very sensitive to H2S. Upon exposure to 600 ppm H2S, the piezoelectric output of the device decreased from 0.32 V (in air) to 0.12 V. Such a flexible device can be driven by the tiny mechanical energy in our living environment, such as human finger pinching. Our research can stimulate a research trend on designing new material systems and device structures for high-performance piezo-driven active gas sensors.

Features of the piezo-phototronic effect on optoelectronic devices based on wurtzite semiconductor nanowires

The piezo-phototronic effect, a three way coupling effect of piezoelectric, semiconductor and photonic properties in non-central symmetric semiconductor materials, utilizing the piezo-potential as a "gate" voltage to tune the charge transport/generation/recombination and modulate the performance of optoelectronic devices, has formed a new field and attracted lots of interest recently. The mechanism was verified in various optoelectronic devices such as light emitting diodes (LEDs), photodetectors and solar cells etc. The fast development and dramatic increasing interest in the piezo-phototronic field not only demonstrate the way the piezo-phototronic effects work, but also indicate the strong need for further research in the physical mechanism and potential applications. Furthermore, it is important to distinguish the contribution of the piezo-phototronic effect from other factors induced by external strain such as piezoresistance, band shifting or contact area change, which also affect the carrier behaviour and device performance. In this perspective, we review our recent progress on piezo-phototronics and especially focus on pointing out the features of piezo-phototronic effect in four aspects: I-V characteristics; c-axis orientation; influence of illumination; and modulation of carrier behaviour. Finally we proposed several criteria for describing the contribution made by the piezo-phototronic effect to the performance of optoelectronic devices. This systematic analysis and comparison will not only help give an in-depth understanding of the piezo-phototronic effect, but also work as guide for the design of devices in related areas.

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.

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 10(5), 2.1 V, 0.82 cm(2)·V(-1)·s(-1), and 2.6 × 10(17) cm(-3), respectively, and the corresponding values for 1% Sb doped samples are 10(4), 2.0 V, 1.24 cm(2)·V(-1)·s(-1), and 3.8 × 10(17) 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.

Piezo-phototronic effect enhanced visible/UV photodetector of a carbon-fiber/ZnO-CdS double-shell microwire

A branched ZnO-CdS double-shell NW array on the surface of a carbon fiber (CF/ZnO-CdS) was successfully synthesized via a facile two-step hydrothermal method. Based on a single CF/ZnO-CdS wire on a polymer substrate, a flexible photodetector was fabricated, which exhibited ultrahigh photon responsivity under illuminations of blue light (1.11 × 10(5) A/W, 8.99 × 10(-8) W/cm(2), 480 nm), green light (3.83 × 10(4) A/W, 4.48 × 10(-8) W/cm(2), 548 nm), and UV light (1.94 × 10(5) A/W, 1.59 × 10(-8) W/cm(2), 372 nm), respectively. The responsivity of this broadband photon sensor was enhanced further by as much as 60% when the device was subjected to a -0.38% compressive strain. This is because the strain induced a piezopotential in ZnO, which tunes the barrier height at the ZnO-CdS heterojunction interface, leading to an optimized optoelectronic performance. This work demonstrates a promising application of piezo-phototronic effect in nanoheterojunction array based photon detectors.

Piezotronic effect on the sensitivity and signal level of Schottky contacted proactive micro/nanowire nanosensors

We demonstrated the first piezoelectric effect on the performance of a pH sensor using an MSM back-to-back Schottky contacted ZnO micro/nanowire device. When the device is subjected to an external strain, a piezopotential is created in the micro/nanowire, which tunes the effective heights of the Schottky barriers at the local contacts, consequently increasing the sensitivity and signal level of the sensors. Furthermore, the strain-produced piezopotential along the ZnO micro/nanowire will lead to a nonuniform distribution of the target molecules near the micro/nanowire surface owing to electrostatic interaction, which will make the sensor proactive to detect the target molecules even at extremely low overall concentration, which naturally improves the sensitivity and lowers the detection limit. A theoretical model is proposed to explain the observed performance of the sensor using the energy band diagram. This prototype device offers a new concept for designing supersensitive and fast-response micro/nanowire sensors by introducing an external strain and piezotronic effect, which may have great applications in building sensors with fast response and reset time, high selectivity, high sensitivity, and good signal-to-noise ratio for chemical, biochemical, and gas sensing.

Largely enhanced efficiency in ZnO nanowire/p-polymer hybridized inorganic/organic ultraviolet light-emitting diode by piezo-phototronic effect

ZnO nanowire inorganic/organic hybrid ultraviolet (UV) light-emitting diodes (LEDs) have attracted considerable attention as they not only combine the high flexibility of polymers with the structural and chemical stability of inorganic nanostructures but also have a higher light extraction efficiency than thin film structures. However, up to date, the external quantum efficiency of UV LED based on ZnO nanostructures has been limited by a lack of efficient methods to achieve a balance between electron contributed current and hole contributed current that reduces the nonradiative recombination at interface. Here we demonstrate that the piezo-phototronic effect can largely enhance the efficiency of a hybridized inorganic/organic LED made of a ZnO nanowire/p-polymer structure, by trimming the electron current to match the hole current and increasing the localized hole density near the interface through a carrier channel created by piezoelectric polarization charges on the ZnO side. The external efficiency of the hybrid LED was enhanced by at least a factor of 2 after applying a proper strain, reaching 5.92%. This study offers a new concept for increasing organic LED efficiency and has a great potential for a wide variety of high-performance flexible optoelectronic devices.

Silica-coated and annealed CdS nanowires with enhanced photoluminescence

The CdS/SiO(2) core/shell nanowires (NWs) with controlled shell thickness were successfully synthesized and subsequently heat-treated at 500 °C. The influences of silica shell coating and annealing processes on their optical properties have been investigated. Compared with original CdS NWs, the annealed CdS/SiO(2) NWs exhibited an enhanced band-edge emission with slowed photoluminescence lifetime, while the intensity of defect emission decreased. The results were ascribed to the surface passivation and recrystallization by shell coating and annealing. We believe our finding would help improving the optical properties of semiconductor NWs, and facilitate its applications in various realms, such as nanoscale emitter, sensor, and photoelectric device.

Piezo-phototronic effect enhanced visible and ultraviolet photodetection using a ZnO-CdS core-shell micro/nanowire

The piezo-phototronic effect is about the use of the piezoelectric potential created inside some materials for enhancing the charge carrier generation or separation at the metal-semiconductor contact or pn junction. In this paper, we demonstrate the impact of the piezo-phototronic effect on the photon sensitivity for a ZnO-CdS core-shell micro/nanowire based visible and UV sensor. CdS nanowire arrays were grown on the surface of a ZnO micro/nanowire to form a ZnO-CdS core-shell nanostructure by a facile hydrothermal method. With the two ends of a ZnO-CdS wire bonded on a polymer substrate, a flexible photodetector was fabricated, which is sensitive simultaneously to both green light (548 nm) and UV light (372 nm). Furthermore, the performance of the photon sensor is much enhanced by the strain-induced piezopotential in the ZnO core through modulation of the Schottky barrier heights at the source and drain contacts. This work demonstrates a new application of the piezotronic effect in photon detectors.