Erasable memory properties of spectral selectivity modulated by temperature and bias in an individual CdS nanobelt-based photodetector

Negative Photoconductivity Transistors for Visuomorphic Computing

Visuomorphic computing aims to simulate and potentially surpass the human retina by mimicking biological visual perception with an artificial retina. Despite significant progress, challenges persist in perceiving complex interactive environments. Negative photoconductivity transistors (NPTs) mimic synaptic behavior by achieving adjustable positive photoconductivity (PPC) and negative photoconductivity (NPC), simulating "excitation" and "inhibition" akin to sensory cell signals. In complex interactive environments, NPTs are desired for visuomorphic computing that can achieve a better sense of information, lower power consumption, and reduce hardware complexity. In this review, it is started by introducing the development process of NPTs, while placing a strong emphasis on the device structures, working mechanisms, and key performance parameters. The common material systems employed in NPTs based on their functions are then summarized. Moreover, it is proceeded to summarize the noteworthy applications of NPTs in optoelectronic devices, including advanced multibit nonvolatile memory, optoelectronic logic gates, optical encryption, and visual perception. Finally, the challenges and prospects that lie ahead in the ongoing development of NPTs are addressed, offering valuable insights into their applications in optoelectronics and a comprehensive understanding of their significance.

Control of positive and negative photo- and thermal-responses in a single PbI2@CH3NH3PbI3 micro/nanowire-based device for real-time sensing, nonvolatile memory, and logic operation

CH3NH3PbI3 has shown great potential for photodetectors and photovoltaic devices due to its excellent positive response to visible light. However, its real-time response characteristics hinder its application in optical memory and logic operation; moreover, the presence of excessive PbI2 is a double-edged sword. Herein, we constructed a dual-terminal device using a single CH3NH3PbI3 micro/nanowire with two Ag electrodes, and then in situ introduced PbI2 quantum dots (QDs) as hole trap centres by thermal decomposition at 160 °C. An anomalous negative photoconductivity (NPC) effect for sub-bandgap light below the PbI2 bandgap is obtained. Importantly, an electrically erasable nonvolatile photomemory can be realized. Furthermore, the device also exhibits an abnormal positive thermal resistance (PTR)-related thermomemory effect, and the thermal-induced high-resistance state (HRS) can be erased by a large bias or an illumination of 365 nm super-bandgap UV light. Additionally, logical "OR" gate operations are achieved through a combination of 650 nm sub-bandgap light and a 70 °C temperature-induced HRS, as well as a large bias and 365 nm super-bandgap light-triggered low-resistance state. These effects are attributed to the excitation and injection of holes in QDs and structural defect traps. This multifunctional device, integrating real-time sensing, nonvolatile memory, and logical operation, holds significant potential for novel electronic and optoelectronic applications.

Controllably modulated asymmetrical photoresponse with a nonvolatile memory effect in a single CH3NH3PbI3 micro/nanowire for photorectifiers and photomemory

Nanostructured hybrid organic-inorganic perovskites exhibit remarkable photodetection performance due to their abundant surface states and high responsivity to visible light. However, in traditional photodetectors with a symmetrical configuration of two-terminal electrodes, the photoresponse is independent of bias polarity. Moreover, for self-powered photodetectors, an asymmetric structure of the chemical composition, such as p-n and Schottky junctions, and two different electrodes are necessary. Herein, we demonstrate a modulable asymmetrical photoresponse by packing only one electrode end in a single CH3NH3PbI3 micro/nanowire with two symmetrical Ag electrodes. This not only enables the high performance of light- and bias-modulated multifunctional photorectifiers and self-powered photodetectors, but also allows controllable implementation of nonvolatile photomemory with a tunable spectral responsivity and range. At an unpacked electrode interface, trace moisture in the environment promotes a good bonding of Ag+ and I-, substantially decreasing the interface barrier. Conversely, at a packed electrode interface, abundant surface states can be well preserved, leading to a high interface barrier. Notably, under a large voltage and strong light, the redox of Ag/AgI at the unpacked electrode interface and the injection and ejection of holes at the packed electrode interface can be reversibly conducted by inverting the voltage polarity, enabling a controllable nonvolatile modulation. Therefore, by clarifying the actual origin of the photoelectrical response of CH3NH3PbI3 micro/nanowires at electrode interfaces, high-performance multifunctional photorectifiers and self-powered photodetectors based on asymmetrical interface photovoltaic effects with two symmetrical electrodes can be controllably realized. Furthermore, by precise cooperative modulation of two electrode interface states with a large voltage and strong illumination, nonvolatile photomemory with a tunable spectral responsivity and range can be implemented.

Giant Piezoresistive Effect of CdS@C Hybrid Nanobelts for Volatile Real-Time Sensor and Erasable Nonvolatile Memory to Stress

Here, CdS@C nanohybrid composites, where CdS quantum dots (QDs) are uniformly embedded in carbon micro-/nanobelt matrixes, are synthesized via a combustion synthesis followed by a postvulcanization. In the nanohybrids, trap centers are effectively created by the introduction of QDs and moreover their barrier height and filling level can be effectively modulated through a coupling of externally loaded strain and bias. Thus, a single CdS@C micro-/nanobelt-based two-terminal device can exhibit an ultrahigh real-time response to compressive and tensile strains with a tremendous gauge factor of above 10⁴, high sensitivity, and fast response and recovery. More importantly, the trapped charges can be mechanically excited by stress, and furthermore, the stress-triggered high-resistance state can be well-maintained at room temperature and a relatively low operation bias. However, it can be back to its initial low resistance state by loading a relatively large bias, showing a superior erasable stress memory function with a window of about 10³. By an effective construction of trap centers in hybrid composites, not only can an ultrahigh performance of volatile real-time stress sensor be obtained under the synergism of external stress and electric field but also can an outstanding erasable nonvolatile stress memory be successfully realized.

Stabilizing CuGaS2 by crystalline CdS through an interfacial Z-scheme charge transfer for enhanced photocatalytic CO2 reduction under visible light

CuGaS2 is one of the most excellent visible-light-active photocatalysts for CO2 reduction and water splitting. However, CuGaS2 suffers from serious deactivation in photocatalytic reactions, which is mainly due to the photo-oxidation induced self-corrosion (Cu+ to Cu2+). Here, we constructed a CuGaS2/CdS hybrid photocatalyst dominated by a Z-scheme charge transfer mechanism. The transfer of photo-generated electrons from excited nanocrystalline CdS to CuGaS2 across the coherent interface reduces Cu2+ formation and favors Cu+ regeneration. This process suppresses the deactivation of CuGaS2 and maintains high performance. Both the activity and stability of photocatalytic CO2 reduction to produce CO over the CuGaS2/CdS hybrid were remarkably improved, which was approximately 4-fold higher than CuGaS2 and 3-fold higher than CdS in converting CO2 into CO. Our study demonstrates that even using the semiconductors prone to photo-corrosion, it is possible to obtain satisfactory catalytic activity and stability by designing efficient Z-scheme-charge-transfer-type photocatalysts.

Zinc oxide ultraviolet photodetectors: rapid progress from conventional to self-powered photodetectors

Currently, the development of ultraviolet (UV) photodetectors (PDs) has attracted the attention of the research community because of the vast range of applications of photodetectors in modern society. A variety of wide-band gap nanomaterials have been utilized for UV detection to achieve higher photosensitivity. Specifically, zinc oxide (ZnO) nanomaterials have attracted significant attention primarily due to their additional properties such as piezo-phototronic and pyro-phototronic effects, which allow the fabrication of high-performance and low power consumption-based UV PDs. This article primarily focuses on the recent development of ZnO nanostructure-based UV PDs ranging from nanomaterials to architectural device design. A brief overview of the photoresponse characteristics of UV PDs and potential ZnO nanostructures is presented. Moreover, the recent development in self-powered PDs and implementation of the piezo-phototronic effect, plasmonic effect and pyro-phototronic effect for performance enhancement is highlighted. Finally, the research perspectives and future research direction related to ZnO nanostructures for next-generation UV PDs are summarized.

Bias-Controlled Tunable Electronic Transport with Memory Characteristics in an Individual ZnO Nanowire for Realization of a Self-Driven UV Photodetector with Two Symmetrical Electrodes

ZnO nanostructures are exceedingly important building blocks for nanodevices due to their wide band gap and large exciton binding energy. However, their electronic transport characteristics are unstable and unrepeatable with external environment variation. Here, we demonstrate that electron transport of an individual ZnO nanowire-based device with the two same electrodes can be controllably modulated by applying a relatively large uni-/bidirectional bias. After being modulated, moreover, their electrical properties can well be maintained at relatively low operation bias and room temperature, demonstrating a memory behavior. The presence of surface states related to lattice periodicity breaking and traps associated with oxygen vacancy (V_o) and zinc interstitial (Zn_i) deep-level defects plays a crucial role in tunable electron transport with a memory feature. For the single nanowire-based two-terminal device, two back-to-back connected surface barrier diodes with series resistance are formed. The filling and emptying of traps near two end electrodes can remarkably adjust the width and height of the surface barrier. At a relatively low bias, the unmodulated conductance is governed by the electron hopping of bulk traps since the height of emptied traps is higher than that of the surface barrier, whereas at a relatively large bias, it is dominated by thermion emission due to a dramatic decrease of the surface barrier width resulting from the electron injection into traps from a negative electrode. Moreover, it will be beneficial for a thin surface barrier to penetrate UV light and separate photoexcited electron-hole pairs. After being asymmetrically modulated by a unidirectional injection, it can be successfully applied to realize a self-driven UV photodetector based on a photovoltaic effect in the symmetrical two-electrode structure. Our work provides a new route to tune electrical properties of nanostructures, which may inspire the development of novel electronic and optoelectronic devices.

Construction, characterization, and growth mechanism of high-density jellyfish-like GaN/SiOxNy nanomaterials on p-Si substrate by Au-assisted chemical vapor deposition approach

High-density GaN/SiO_xN_y jellyfish-like nanomaterials are synthesized on Au-coated p-type Si substrates by a chemical vapor deposition approach.