Novel UV-Visible Photodetector in Photovoltaic Mode with Fast Response and Ultrahigh Photosensitivity Employing Se/TiO2 Nanotubes Heterojunction

Improved performance of UV-blue dual-band Bi2O3/TiO2 photodetectors and application of visible light communication with UV light encryption

In this paper, self-powered photodetectors (PDs) with a dual-band photoresponse and excellent photodetection capabilities in complex environments can meet the needs of diverse detection targets, complex environments and diverse tasks. Herein, Bi2O3 nanosheets were deposited on the surface of TiO2 nanorod arrays (NRs) by chemical bath deposition (CBD) to construct self-powered heterojunction PDs with a UV-blue dual-band photoresponse. The nucleation and growth of Bi2O3 nanosheets on TiO2 NRs substrates were controlled by varying the concentration of the complexing agent triethanolamine (TEA) in the precursor solution, which regulated the morphology, crystalline quality and energy band structure as well as the photoelectronic properties of Bi2O3 films. The devices fabricated at a TEA concentration of 0.3 M exhibited excellent self-powered UV-blue dual-band photoresponse characteristics, achieving a photocurrent (Iph) of 144 nA, a responsivity of 1.79 mA W-1 and a detectivity of 5.94 × 1010 Jones under 405 nm illumination at 0 V, which can be attributed to the large built-in electric field (Eb) of Bi2O3/TiO2 heterojunctions, the low interfacial transfer resistance and suitable carrier transport path. In addition, Bi2O3/TiO2 heterojunction PDs with the UV-blue dual-band photoresponse characteristics can be applied in UV-encrypted visible light communication (VLC) with a light-controlled logic gate to improve the security of information transmission.

Integrating High-Sensitivity Photodetector and High-Energy Aqueous Battery in All-in-One Triple-Twisted Fiber

Fiber-shaped photodetectors (FPDs) have attracted special attention to wearable health monitoring due to their 3D absorption capabilities. However, the practical application of traditional FPDs is severely limited by the irreversible degradation of performance caused by vulnerable interface compatibility on complex deformation and a single function. Here, an integrated photoelectrochemical FPD/battery device (FPDB) is designed, consisting of a common electrode, photoanode, anode, and sol-gel electrolyte as an isolation layer, which not only effectively avoids the short circuit problem of FPD but also endows high-efficiency energy storage capacity. As expected, the resulting all-in-one triple-twisted fiber-shaped FPDB simultaneously achieves high responsiveness of 151.45 mA W-1 and excellent volume capacity of 18.75 mAh cm-3. Such a stable architectural design and multifunctional integration of functional fibers accelerate the development of next-generation wearable fabrics.

Flame doping of indium ions into TiO2 nanorod arrays for enhanced photochemical water oxidation

Indium (In) ions were diffused into a TiO2 (In-TiO2) photoelectrode via a facile and efficient flame doping method resulting in improved photo-induced carrier separation. The dopant concentration was systematically investigated, and a volcano-type relationship between the dopant concentration and photoelectrochemical (PEC) performance was observed. The optimum incident photon-to-current efficiency and photocurrent density of In-TiO2 were 38.6% and 0.70 mA cm-2 at 1.23 V, respectively, 2.1 and 11.2 times the values of pristine TiO2, respectively. In doping resulted in improved charge separation and lower surface adsorption energies for reactant molecules, as evidenced by experimental and computational methods.

Light-enhanced electrical behavior of a Au/Al-doped ZnO/p-Si/Al heterostructure: insights from impedance and current-voltage analysis

In this study, we meticulously deposited an Al-doped ZnO nanoparticle thin film on a p-type silicon substrate using the precise sputtering method. We conducted a comprehensive exploration of the film's structure, morphology, and optical properties. X-ray diffraction (XRD) confirmed its polycrystalline wurtzite configuration with a dominant (002) orientation. High-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed a uniformly textured surface adorned with densely packed nanoparticles. Regarding optical properties, the Al-doped ZnO thin film exhibited exceptional transmittance exceeding 80% across visible and near-infrared spectra. Moving on to electrical characteristics, we assessed the Au/Al-doped ZnO/p-Si/Al heterostructure under dark and illuminated conditions. Through current-voltage (I-V) and impedance measurements, we observed significant improvements in conductivity and performance under illumination. Notably, there was an increase in current conduction and a reduction in impedance, highlighting the advantages of illumination. Collectively, these findings emphasize the promising potential of the Au/Al-doped ZnO/p-Si/Al heterostructure, particularly in the realms of optoelectronic devices and photovoltaics. With its ability to efficiently mobilize charges and adeptly assimilate light, this heterostructure stands as a frontrunner for transformative applications in these technologically vital domains.

Highly Uniform Spherical MoO2-MoO3/Polypyrrole Core-Shell Nanocomposite as an Optoelectronic Photodetector in UV, Vis, and IR Domains

A highly uniform spherical MoO2-MoO3/polypyrrole core-shell nanocomposite has been successfully synthesized as an optoelectronic photon sensing material, capable of detecting light in the UV, Vis, and IR domains. The nanocomposite is prepared through the oxidation of pyrrole using Na2MoO4, resulting in a uniform spherical morphology that has been confirmed by TEM, theoretical modeling, and SEM analyses. This morphology contributes to its promising optical behavior, characterized by a small bandgap of 1.36 eV. The optoelectronic photosensing capability of the nanocomposite has been evaluated across the UV, Vis, and IR spectra, demonstrating high efficiency. The photoresponsivity R values indicate the ability of the nanocomposite to generate hot electrons in response to incident photons. With an R value of 4.15 mA·W-1 at 440 nm, this optoelectronic device exhibits considerable promise for integration into an advanced technological apparatus. The detection (D) value of 9.30 × 108 Jones at 440 nm further confirms the high sensitivity in the Vis region. The excellent stability of the device can be attributed to the inherent MoO2-MoO3 oxide and Ppy polymer materials. This stability has been demonstrated through reproducibility studies and current-voltage measurements under various optical conditions. The combination of stability, efficiency, and sensitivity makes this optoelectronic device well suited for light sensing applications in both industrial and commercial settings. Its promising performance opens up opportunities for advancements in various fields requiring accurate and reliable light detection.

Annealing effect on the physical properties of TiO2 thin films deposited by spray pyrolysis

Titanium dioxide (TiO2) thin films were deposited on glass substrates at 350 °C using the spray pyrolysis technique. As deposited and annealed thin films were characterized by X-ray diffraction, scanning electron microscopy, UV-VIS spectroscopy, and photodetection. Unlike the as deposited samples which were amorphous, annealed samples show an anatase phase. Films were absorbent in the UV region and the band gap energy decreases from 3.78 eV to 3.4 eV with annealing. The photoresponse of TiO2 photodetectors was recorded under UV (λ1 = 365 nm, λ2 = 254 nm) and visible light illumination by reversible switching (ON/OFF) cycles using DC electrical characterization. Photosensitive properties such as reproducible photosensitivity, responsivity, and detectivity were also studied.

InGaN micro-LED array with integrated emission and detection functions for color detection application

InGaN-based micro-LEDs can detect and emit optical signals simultaneously, owing to their overlapping emission and absorption spectra, enabling color detection. In this paper, we fabricated a green InGaN-based micro-LED array with integrated emission and detection functions. On the back side of the integrated device, when the 80 μm micro-LED emitted light, the 200 μm LED could receive reflected light to accomplish color detection. The spacing between the 80 μm and the 200 μm micro-LEDs was optimized to be 1 mm to reduce the effect of the direct light transmitted through the n-GaN layer without reflection. The integrated device shows good detection performance for different colors and skin colors, even in a dark environment. In addition, light can be emitted from the top side of the device. Utilization of light from both sides of the integrated device provides the possibility of its application in display, communication, and detection on the different sides.

Photodetection Enhancement via Graphene Oxide Deposition on Poly 3-Methyl Aniline

A graphene oxide (GO)/poly 3-methyl aniline (P3MA) photodetector has been developed for light detection in a broad optical region: UV, Vis, and IR. The 3-methyl aniline was initially synthesized via radical polymerization using an acid medium, i.e., K₂S₂O₈ oxidant. Consequently, the GO/P3MA composite was obtained through the adsorption of GO into the surface of P3MA. The chemical structure and optical properties of the prepared materials have been illustrated via XRD, FTIR, SEM, and TEM analysis. The absorbance measurements demonstrate good optical properties in the UV, Vis, and near-IR regions, although a decrease in the bandgap from 2.4 to 1.6 eV after the composite formation was located. The current density (J_ph) varies between 0.29 and 0.68 mA·cm^-2 (at 2.0 V) under dark and light, respectively. The photodetector has been tested using on/off chopped light at a low potential, in which the produced J_ph values decrease from 0.14 to 0.04 µA·cm^-2, respectively. The GO/P3MA photodetector exhibits excellent R (and D) values of 4 and 2.7 mA·W^-1 (0.90 × 10⁹ and 0.60 × 10⁹ Jones) in the UV (340 nm) and IR (730 nm) regions, respectively. The R and D values obtained here make the prepared photodetector a promising candidate for future light detection instruments.

Fast all-fiber ultraviolet photodetector based on an Ag-decorated ZnO micro-pillar

There are urgent demands of ultraviolet (UV) photodetectors with high sensitivity and fast response due to the wide application of ultraviolet light in the fields of medical treatment, space exploration, optical communication and semiconductor industry. The response speed of traditional ZnO-based UV photodetectors is always limited by the carrier mobility and electrical resistance caused by the external circuits. Utilizing the all-optical detection method may replace the complex circuit structure and effectively improve the response speed of photodetectors. Here, a fast-response fiber-optic UV photodetector is proposed, where a ZnO micro-pillar is fixed on the end face of a fiber-tip and acts as a Fabry-Pérot interferometer (FPI). Under the irradiation of UV light, the photo-generated carriers change the refractive index of the ZnO micro-pillar, leading to a redshift of the interference wavelengths of the ZnO FPI. To enhance this effect, a discontinuous Ag film with an island-like structure is coated on the surface of ZnO micro-pillars through magnetron sputtering, and therefore the sensitivity of the proposed device achieves to 1.13 nm/(W·cm^-2), which is 3.9 times higher than that of without Ag-decoration, due to the intensification of photo-carrier change with the help of the Schottky junction formed between Ag film and ZnO micro-pillar. Meanwhile, since the response speed of the proposed device is mainly determined by the temporal RI change of ZnO micro-pillar, the fiber-optic UV photodetector also shows very fast response with a rise time of 35 ns and a decay time of 40 µs. The demonstrated structure takes full advantage of optical fiber devices, exhibiting compactness, flexibility, fast response and immune to electromagnetic interference, which paves a new way for the next generation of photodetection devices.

Synergistic effects of nano-structured WO3-Se heterojunction decorated by organic Nafion layer on improving photoelectrochemical performance

Exploration of high-performance photoanodes is considered as an essential challenge in photoelectrochemical (PEC) water splitting due to the complex four-electron reaction in water oxidation. Herein, the nano-structured WO₃-Se heterojunction decorated by organic Nafion layer is designed. The optimized WO₃-Se200-0.05Nafion photoanode shows a remarkable photocurrent of 1.40 mA cm^-2at 1.23 V versus reversible hydrogen electrode, which is 2.5-fold higher than that of pure WO₃nanosheets (WO₃NS) photoelectrode. Remarkably, the photocurrent increments of WO₃-Se200-0.05Nafion is larger than the increment sum of WO₃-Se200 and WO₃-0.05Nafion, which affirming the synergistic effect of Se nanospheres and Nafion layer. The improved PEC performances are attributed to the quick charge separation and transfer, the increased electric conductivity, and the excellent kinetics of oxygen evolution, which is derived from the strong interaction among WO₃, Se and Nafion. Meanwhile, the better visible-light harvesting from Se nanospheres as photosensitizer and the induction of transparent Nafion as a passivation layer can explain this synergy. It hopes this heterostructure design with organic Nafion decoration can inspire to exploit outstanding performance photoanodes for PEC water splitting.

Self-powered p-CuI/n-GaN heterojunction UV photodetector based on thermal evaporated high quality CuI thin film

With vacuum thermal evaporation, the CuI film was deposited on quartz and n-GaN substrates, and the morphology, crystalline structure and optical properties of the CuI films were investigated. According to the XRD results, the CuI film preferentially grew along [111] crystal orientation on the GaN epilayer. With Au and Ni/Au ohmic contact electrodes fabricated on CuI and n-GaN, a prototype p-CuI/n-GaN heterojunction UV photodetector strong UV spectral selectivity was created. At 0 V and 360 nm front illumination (0.32 mW/cm²), the heterojunction photodetector displayed outstanding self-powered detection performance with the responsivity (R), specific detectivity (D*), and on/off ratio up to 75.5 mA/W, 1.27×10¹² Jones, and ∼2320, respectively. Meanwhile, the p-CuI/n-GaN heterojunction photodetector had excellent atmosphere stability.

Integration of H2V3O8 nanowires and a GaN thin film for self-powered UV photodetectors

H₂V₃O₈/GaN n-n heterojunction ultraviolet photodetectors are fabricated via a facile dip-coating method. The Schottky junction between the GaN and H₂V₃O₈ builds a built-in electric field to achieve the self-powered phenomenon. The photodetector presents a high photocurrent (0.23 μA) and a fast response speed (less than 0.3 s) at 0 V bias and under 365 nm light illumination (24.50 mW cm^-2). Furthermore, the photocurrent increases steadily as the light intensity increases from 0.53 to 24.50 mW cm^-2. The H₂V₃O₈/GaN heterojunction holds great potential to realize high-performance hybrid PDs.

Self-powered UV photodetectors and imaging arrays based on NiO/IGZO heterojunctions fabricated at room temperature

Self-powered UV photodetectors and imaging arrays based on p-type NiO/n-type InGaZnO (IGZO) heterojunctions are fabricated at room temperature by using ratio-frequency magnetron sputtering. The p-n heterojunction exhibits typical rectifying characteristics with a rectification ratio of 7.4×10⁴ at a ±4 V applied bias. A high photo-responsivity of 28.8 mA/W is observed under zero bias at a wavelength of 365 nm. The photodetector possesses a fast response time of 15 ms which is among the best in reported oxide-based p-n junction-based UV photodetectors. Finally, recognition of an "H" pattern is demonstrated by a 10×10 photodetector array at zero bias. The results indicate that the NiO/IGZO based photodetectors may have a great potential in constructing large-scale self-powered UV imaging systems.

Application of Nanostructured TiO2 in UV Photodetectors: A Review

As a wide-bandgap semiconductor material, titanium dioxide (TiO₂ ), which possesses three crystal polymorphs (i.e., rutile, anatase, and brookite), has gained tremendous attention as a cutting-edge material for application in the environment and energy fields. Based on the strong attractiveness from its advantages such as high stability, excellent photoelectric properties, and low-cost fabrication, the construction of high-performance photodetectors (PDs) based on TiO₂ nanostructures is being extensively developed. An elaborate microtopography and device configuration is the most widely used strategy to achieve efficient TiO₂ -based PDs with high photoelectric performances; however, a deep understanding of all the key parameters that influence the behavior of photon-generated carriers, is also highly required to achieve improved photoelectric performances, as well as their ultimate functional applications. Herein, an in-depth illustration of the electrical and optical properties of TiO₂ nanostructures in addition to the advances in the technological issues such as preparation, microdefects, p-type doping, bandgap engineering, heterojunctions, and functional applications are presented. Finally, a future outlook for TiO₂ -based PDs, particularly that of further functional applications is provided. This work will systematically illustrate the fundamentals of TiO₂ and shed light on the preparation of more efficient TiO₂ nanostructures and heterojunctions for future photoelectric applications.

Constructing a High-Quality t-Se/Si Interface Via In Situ Light Annealing of a-Se for a Self-Powered Image Sensor

The quality of the interface, e.g., the semiconductor-semiconductor or metal-semiconductor interface, is the main factor restricting the photodetection performance of a heterojunction. In this study, a high-quality Se/Si interface is constructed via in situ directional transformation of amorphous Se (a-Se) into crystalline Se (t-Se) on a Si substrate via light annealing. Benefitting from the high-quality interface and appropriate energy band between Si and Se, the t-Se/Si heterojunction exhibits an extremely high responsivity and detectivity of 583.33 mA W^-1 and 8.52 × 10¹² Jones at 760 nm, respectively. In addition, the device exhibits an ultrafast rise time of 183 µs and a decay time of 405 µs. Furthermore, an image sensor fabricated via local light annealing successfully recognizes patterns of "N," "P," and "U." This study provides valuable guidance for the construction of high-quality interfaces and the design of self-powered image sensors.

Engineering Plasmonic Environments for 2D Materials and 2D-Based Photodetectors

Two-dimensional layered materials are considered ideal platforms to study novel small-scale optoelectronic devices due to their unique electronic structures and fantastic physical properties. However, it is urgent to further improve the light–matter interaction in these materials because their light absorption efficiency is limited by the atomically thin thickness. One of the promising approaches is to engineer the plasmonic environment around 2D materials for modulating light–matter interaction in 2D materials. This method greatly benefits from the advances in the development of nanofabrication and out-plane van der Waals interaction of 2D materials. In this paper, we review a series of recent works on 2D materials integrated with plasmonic environments, including the plasmonic-enhanced photoluminescence quantum yield, strong coupling between plasmons and excitons, nonlinear optics in plasmonic nanocavities, manipulation of chiral optical signals in hybrid nanostructures, and the improvement of the performance of optoelectronic devices based on composite systems.

A transparent, self-powered photodetector based on p-CuI/n-TiO2heterojunction film with high on-off ratio

Ultraviolet(UV) photodetectors(PDs) can monitor UV radiation, enabling it to be effective for many applications, such as communication, imaging and sensing. The rapid progress on portable and wearable optoelectronic devices places a great demand on self-powered PDs. However, high-performance self-powered PDs are still limited. Herein we display a transparent and self-powered PD based on a p-CuI/n-TiO₂heterojunction, which exhibits a high on-off ratio (∼10⁴at 310 nm) and a fast response speed (rise time/decay time = 0.11 ms/0.72 ms) without bias. Moreover, the device shows an excellent UV-selective sensitivity as a solar-blind UV PD with a high UV/visible rejection ratio (R_{300 nm}/R_{400 nm}= 5.3 × 10²), which can be ascribed to the wide bandgaps of CuI and TiO₂. This work provides a feasible route for the construction of transparent, self-powered PDs based on p-n heterojunctions.

High-responsivity photodetector based on scrolling monolayer MoS2hybridized with carbon quantum dots

The monolayer MoS₂based photodetectors have been widely investigated, which show limited photoelectric performances due to its low light absorption and uncontrollable adsorbates. In this paper, we present a MoS₂-based hybrid nanoscrolls device, in which one-dimensional nanoscrollsof MoS₂is hybridized with carbon quantum dots (CQDs). This device architecture effectively enhanced the photodetection performance. The photoresponsivity and detectivity values of MoS₂/CQDs-NS photodetectors are respectively 1793 A W^-1and 5.97 × 10¹²Jones, which are 830-fold and 268-fold higher than those of pristine MoS₂under 300 nm illumination atV_ds = 5 V. This research indicates a significant progress in fabricating high-performance MoS₂photodetectors.

Operando Raman spectroscopic evidence of electron-phonon interactions in NiO/TiO2 pn junction photodetectors

The pn junctions significantly affect the responsivity of photodetectors (PDs). However, the enhancement mechanism of the pn junction is still unclear. Herein, operando Raman spectroscopy was employed to study PDs with NiO/TiO₂ pn junctions composed of p-NiO nanoparticles (NPs) and n-TiO₂ nanotube arrays (TNAs). The results suggest that the built-in potential field of the NiO/TiO₂ interface decreases the charge transfer resistance and changes the vibrational frequency of the phonon modes of TiO₂, which is attributed to the electron-phonon coupling effect. Operando Raman spectroscopy is proved to be a powerful tool for manufacturing highly responsive PDs.

Preparation of hexagonal nanoporous Al2O3/TiO2/TiN as a novel photodetector with high efficiency

The unique optical properties of metal nitrides enhance many photoelectrical applications. In this work, a novel photodetector based on TiO2/TiN nanotubes was deposited on a porous aluminum oxide template (PAOT) for light power intensity and wavelength detection. The PAOT was fabricated by the Ni-imprinting technique through a two-step anodization method. The TiO2/TiN layers were deposited by using atomic layer deposition and magnetron sputtering, respectively. The PAOT and PAOT/TiO2/TiN were characterized by several techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive X-ray (EDX). The PAOT has high-ordered hexagonal nanopores with dimensions ~ 320 nm pore diameter and ~ 61 nm interpore distance. The bandgap of PAOT/TiO2 decreased from 3.1 to 2.2 eV with enhancing absorption of visible light after deposition of TiN on the PAOT/TiO2. The PAOT/TiO2/TiN as photodetector has a responsivity (R) and detectivity (D) of 450 mAW-1 and 8.0 × 1012 Jones, respectively. Moreover, the external quantum efficiency (EQE) was 9.64% at 62.5 mW.cm-2 and 400 nm. Hence, the fabricated photodetector (PD) has a very high photoelectrical response due to hot electrons from the TiN layer, which makes it very hopeful as a broadband photodetector.

Mixed-dimensional Te/CdS van der Waals heterojunction for self-powered broadband photodetector

The 2D layered crystals can physically integrate with other non-2D components through van der Waals (vdW) interaction, forming mixed-dimensional heterostructures. As a new elemental 2D material, tellurium (Te) has attracted intense recent interest for high room-temperature mobility, excellent air-stability, and the easiness of scalable synthesis. To date, the Te is still in its research infancy, and optoelectronics with low-power consumption are less reported. Motivated by this, we report the fabrication of a mixed-dimensional vdW photodiode using 2D Te and 1D CdS nanobelt in this study. The heterojunction exhibits excellent self-powered photosensing performance and a broad response spectrum up to short-wave infrared. Under 520 nm wavelength, a high responsivity of 98 mA W^-1is obtained at zero bias with an external quantum efficiency of 23%. Accordingly, the photo-to-dark current ratio and specific detectivity reach 9.2 × 10³and 1.9 × 10¹¹Jones due to the suppressed dark current. This study demonstrates the promising applications of Te/CdS vdW heterostructure in high-performance photodetectors. Besides, such a mixed-dimensional integration strategy paves a new way for device design, thus expanding the research scope for 2D Te-based optoelectronics.

Improved responsivity of MgZnO film ultraviolet photodetectors modified with vertical arrays ZnO nanowires by light trapping effect

The light trapping effect of ZnO nanowires (NWs) is attracting increasing attention as it effectively enhances the photoelectric effect. In this paper, high-density ZnO NWs are grown on a metal-semiconductor-metal structure MgZnO film UV photodetector (PD) as a light trapping unit. The photogenerated carriers diffuse along the longitudinal axis of the ZnO NWs, then diffuse onto the thin film and are collected by an applied bias electrode. When the device is connected to the NWs, the responsivity is about 12 times higher than that of the pure MgZnO film UV PD with a large light-dark current ratio (4.93 × 10⁴). The array structure of the ZnO NWs enhances the number of photogenerated carriers at the top interface and provides a longer optical path length and a larger surface area. The resulting light trapping effect endows the device with excellent photoelectric properties. In this work, the introduction of NWs not only fundamentally improves the performance of the MgZnO thin film UV PD, but the resulting photodetector also demonstrates a sharp contrast between light trapping UV PD and the MgZnO thin film UV PD.

High-Performance and Self-Powered Alternating Current Ultraviolet Photodetector for Digital Communication

Self-powered ultraviolet photodetectors offer great potential in the field of optical communication, smart security, space exploration, and others; however, achieving high sensitivity with maintaining fast response speed has remained a daunting challenge. Here, we develop a titanium dioxide-based self-powered ultraviolet photodetector with high detectivity (≈1.8 × 10¹⁰ jones) and a good photoresponsivity of 0.32 mA W^-1 under pulsed illumination (λ = 365 nm, 4 mW cm^-2), which demonstrate an enhancement of 114 and 2017%, respectively, due to the alternating current photovoltaic effect compared to the conventional direct current photovoltaic effect. Further, the photodetector demonstrated a high on/off ratio (≈10³), an ultrafast rise/decay time of 112/63 μs, and a noise equivalent power of 5.01 × 10^-11 W/Hz^1/2 under self-biased conditions. Photoconductive atomic force microscopy revealed the nanoscale charge transport and offered the possibility to scale down the device size to a sub-10-nanometer (∼35 nm). Moreover, as one of the practical applications, the device was successfully utilized to interpret the digital codes. The presented results enlighten a new path to design energy-efficient ultrafast photodetectors not only for the application of optical communication but also for other advanced optoelectronic applications such as digital display, sensing, and others.

Recent progress, challenges, and prospects in emerging group-VIA Xenes: synthesis, properties and novel applications

The discovery of graphene (G) attracted considerable attention to the study of other novel two-dimensional materials (2DMs), which is identified as modern day "alchemy" since researchers are converting the majority of promising periodic table elements into 2DMs. Among the family of 2DMs, the newly invented monoelemental, atomically thin 2DMs of groups IIIA-VIA, called "Xenes" (where, X = IIIA-VIA group elements, and "ene" is the Latin word for nanosheets (NSs)), are a very active area of research for the fabrication of future nanodevices with high speed, low cost and elevated efficiency. Currently, any novel structure of 2DMs from the typical Xenes will probably be applicable in electronic technology. Analysis of their possible highly sensitive synthesis and characterization present opportunities for theoretically examining proposed 2D-Xenes with atomic precision in ideal circumstances, thus providing theoretical predictions for experimental support. Several theoretically predicted and experimentally synthesized 2D-Xene materials have been investigated for the group-VIA elements (tellurene (2D-Te), and selenene (2D-Se)), which are similar to topological insulators (TIs), thus potentially rendering them suitable materials for application in upcoming nanodevices. Although the investigation and device application of these materials are still in their infancy, theoretical studies and a few experiment-based investigations have proven that they are complementary to conventional (i.e., layered bulk-derived) 2DMs. This review focuses on the synthesis of novel group-VIA Xenes (2D-Te and 2D-Se) and summarizes the current development in understanding their basic properties, with the current advancement in signifying device applications. Lastly, the future research prospects, further advanced applications and associated shortcomings of the group-VIA Xenes are summarized and highlighted.

Crystallization of ZnO thin films via thermal dissipation annealing method for high-performance UV photodetector with ultrahigh response speed

ZnO-based ultraviolet (UV) photodetector can be easily fabricated by using sol-gel spin-coating method, however, the crystallization of amorphous state ZnO thin films is necessary to fabricate high performance UV photodetector. Thus, we devised a thermal dissipation annealing (TDA) method in which the heat transfer to the ZnO thin films can be synchronized with the heat release from the substrate. It was found that sol-gel spin-coated ZnO thin films can be crystallized through the mobility difference of ZnO molecules placed at the surface of ZnO thin films. Also, UV photodetector based on ZnO thin films annealed with the TDA method exhibited faster rise and decay time constant (τ_r = 35 ms and τ_d = 73 ms, respectively), a higher on/off current ratio, and reproducible photocurrent characteristics compared to that of the ZnO thin films annealed by using furnace and IR lamp. Therefore, these results indicated that the TDA method is a feasible alternative route for the fabrication of ZnO based high performance optoelectronic devices.

Graphdiyne-modified TiO2 nanofibers with osteoinductive and enhanced photocatalytic antibacterial activities to prevent implant infection

Titanium implants have been widely used in bone tissue engineering for decades. However, orthopedic implant-associated infections increase the risk of implant failure and even lead to amputation in severe cases. Although TiO2 has photocatalytic activity to produce reactive oxygen species (ROS), the recombination of generated electrons and holes limits its antibacterial ability. Here, we describe a graphdiyne (GDY) composite TiO2 nanofiber that combats implant infections through enhanced photocatalysis and prolonged antibacterial ability. In addition, GDY-modified TiO2 nanofibers exert superior biocompatibility and osteoinductive abilities for cell adhesion and differentiation, thus contributing to the bone tissue regeneration process in drug-resistant bacteria-induced implant infection.

Vertically aligned GaN nanorod arrays/p-Si heterojunction self-powered UV photodetector with ultrahigh photoresponsivity

Self-powered photodetectors have demonstrated potential for developing future wireless and implantable devices. Herein, we present a self-powered UV photodetector with an ultrahigh photoresponse based on vertically oriented and high crystalline quality n-type GaN nanorod arrays: poly(methyl methacrylate)/p-Si heterojunction. Benefiting from the highly efficient separation and transport of photoexcited electron-hole pairs, significant improvements in photoresponsivity are experimentally obtained. In a zero-biased self-powered detection mode, a 6.7AW^-1 responsivity and 2.68×10¹³ Jones detectivity are achieved under 355 nm light illumination, and the response time is as low as 0.29/3.07 ms (rise/fall times).

Recent Advances of Spatial Self-Phase Modulation in 2D Materials and Passive Photonic Device Applications

Optical nonlinearity in 2D materials excited by spatial Gaussian laser beam is a novel and peculiar optical phenomenon, which exhibits many novel and interesting applications in optical nonlinear devices. Passive photonic devices, such as optical switches, optical logical gates, photonic diodes, and optical modulators, are the key compositions in the future all-optical signal-processing technologies. Passive photonic devices using 2D materials to achieve the device functionality have attracted widespread concern in the past decade. In this Review, an overview of the spatial self-phase modulation (SSPM) in 2D materials is summarized, including the operating mechanism, optical parameter measurement, and tuning for 2D materials, and applications in photonic devices. Moreover, some current challenges are also proposed to solve, and some possible applications of SSPM method are predicted for the future. Therefore, it is anticipated that this summary can contribute to the application of 2D material-based spatial effect in all-optical signal-processing technologies.

Solution-Processed Sb2Se3 on TiO2 Thin Films Toward Oxidation- and Moisture-Resistant, Self-Powered Photodetectors

Semiconductor-sensitized TiO₂ thin films with long-term air stability are attractive for optoelectronic devices and applications. Herein, we demonstrate the potential of the TiO₂ thin film (∼800 nm in thickness) sensitized with a Sb₂Se₃ layer (∼350 nm) grown from solution spin coating and processed by annealing recrystallization at 300 °C for high-performance optical detection. The type-II band alignment, p-Sb₂Se₃/n-TiO₂ heterojunction, and narrow band gap of Sb₂Se₃ (∼1.25 eV) endow the film photodetector with a large photocurrent, high switching stability and on/off ratio (>10³), and fast response speeds (<20 ms) under the broadband visible-near-infrared irradiation in a zero-bias self-powered photovoltaic mode. In particular, the photodetector shows notable resistance to oxidation and moisture for long-term operation, which is linked to the modest surface oxidation (Sb-O) of Sb₂Se₃, as verified by X-ray photoelectron spectroscopy. The first-principles calculations show that a low and medium concentration of oxygen substitution for Se (O_Se) and oxygen interstitial (O_i) with negative formation energies can lead to such a moderate surface oxidation but do not generate impurity states or just introduce a shallow-level acceptor state in the electronic structures of Sb₂Se₃ without degrading its optoelectronic performance. Our theoretical results offer a rational explanation for the air-stable and oxidation/moisture-resistant characteristics in moderately oxidized Sb₂Se₃ and may shed light on the surface oxidation-property relationship studies of other nonoxide semiconductor-sensitized devices.

Void-free 3D Bioprinting for In-situ Endothelialization and Microfluidic Perfusion

Two major challenges of 3D bioprinting are the retention of structural fidelity and efficient endothelialization for tissue vascularization. We address both of these issues by introducing a versatile 3D bioprinting strategy, in which a templating bioink is deposited layer-by-layer alongside a matrix bioink to establish void-free multimaterial structures. After crosslinking the matrix phase, the templating phase is sacrificed to create a well-defined 3D network of interconnected tubular channels. This void-free 3D printing (VF-3DP) approach circumvents the traditional concerns of structural collapse, deformation and oxygen inhibition, moreover, it can be readily used to print materials that are widely considered "unprintable". By pre-loading endothelial cells into the templating bioink, the inner surface of the channels can be efficiently cellularized with a confluent endothelial layer. This in-situ endothelialization method can be used to produce endothelium with a far greater uniformity than can be achieved using the conventional post-seeding approach. This VF-3DP approach can also be extended beyond tissue fabrication and towards customized hydrogel-based microfluidics and self-supported perfusable hydrogel constructs.

The recent progress of triboelectric nanogenerator-assisted photodetectors

Since 2012, triboelectric nanogenerator (TENG) has attracted significant interest from researchers in the field of energy conversion due to its unique output characteristics of high voltage, pulse and low current. In addition, recent advancements have demonstrated that photodetection platforms based on TENG exhibit great advantages such as being simple, low-cost, portable, with high sensitivity, high response, etc, and are environment friendly. Here, this article provides a comprehensive review on the state-of-the-art photodetectors based on TENG in recent years, and a detailed introduction to the structural design and potential mechanisms. It mainly focuses on self-powered photodetectors (including photodetectors as a load resistance of a TENG and photosensitive materials such as tribo-layer of TENG) and the modulation of photodetectors based on TENG (including utilizing the voltage of TENG as well as triboelectric microplasma). Finally, we put forward some perspectives and outlook, including structure engineering and mechanism guidance, for the future development of simple, high-performance and portable photodetectors based on TENG.

In Situ Raman Investigation of TiO2 Nanotube Array-Based Ultraviolet Photodetectors: Effects of Nanotube Length

TiO₂ nanotube arrays (TNAs) with tube lengths of 4, 6, and 7 μm were prepared via two-step anodization. Thereafter, ultraviolet (UV) photodetectors (PDs) with Au/TiO₂/Au structures were prepared using these TNAs with different tube lengths. The effects of TNA length and device area on the performance of the device were investigated using in situ Raman spectroscopy. The maximum laser/dark current ratio was achieved by using a TNA with a size of 1 × 1 cm² and a length of 7 μm, under a 532 nm laser. In addition, when the device was irradiated with a higher energy laser (325 nm), the UV Raman spectrum was found to be more sensitive than the visible Raman spectrum. At 325 nm, the laser/dark current ratio was nearly 24 times higher than that under a 532 nm laser. Six phonon modes of anatase TNAs were observed, at 144, 199, 395, 514, and 635 cm⁻¹, which were assigned to the E_g(1), E_g(2), B_1g(1), A_1g/B_1g(2), and E_g(3) modes, respectively. The strong low-frequency band at 144 cm⁻¹ was caused by the O-Ti-O bending vibration and is a characteristic band of anatase. The results show that the performance of TNA-based PDs is length-dependent. Surface-enhanced Raman scattering signals of 4-mercaptobenzoic acid (4-MBA) molecules were also observed on the TNA surface. This result indicates that the length-dependent performance may be derived from an increase in the specific surface area of the TNA. In addition, the strong absorption of UV light by the TNAs caused a blueshift of the E_g(1) mode.

Effect of Al thickness on the structural and ethanol vapor sensing performance of ZnO porous nanostructures prepared by microwave-assisted hydrothermal method

Highly porous ZnO nanoflakes were successfully prepared using a microwave-assisted hydrothermal method. The presence of aluminum changes the environment of the preparation reaction, which controlled the crystallographic orientation. The unique morphology and properties of ZnO nanoflakes may be due to the effect of microwave irradiation and the ambient condition. The approach is very simple and there is rapid growth of around 3 μm ZnO within 30 min. The mechanism of the construction of unique ZnO nanoflake growth using the present approach is proposed. Hence, the prospective performance of ethanol vapor sensing for the rapid growth of ZnO porous nanostructures was investigated.

A mixed-dimensional 1D Se-2D InSe van der Waals heterojunction for high responsivity self-powered photodetectors

Mixed-dimension van der Waals (vdW) p-n heterojunction photodiodes have inspired worldwide efforts to combine the excellent properties of 2D materials and traditional semiconductors without consideration of lattice mismatch. However, owing to the scarcity of intrinsic p-type semiconductors and insufficient optical absorption of the few layer 2D materials, a high performance photovoltaic device based on a vdW heterojunction is still lacking. Here, a novel mixed-dimension vdW heterojunction consisting of 1D p-type Se nanotubes and a 2D flexible n-type InSe nanosheet is proposed by a facile method, and the device shows excellent photovoltaic characteristics. Due to the superior properties of the hybrid p-n junction, the mix-dimensional van der Waals heterojunction exhibited high on/off ratios (103) at a relatively weak light intensity of 3 mW cm-2. And a broadband self-powered photodetector ranging from the UV to visible region is achieved. The highest responsivity of the device could reach up to 110 mA W-1 without an external energy supply. This value is comparable to that of the pristine Se device at 5 V and InSe device at 0.1 V, respectively. Furthermore, the response speed is enhanced by one order of magnitude over the single Se or InSe device even at a bias voltage. This work paves a new way for the further development of high performance, low cost, and energy-efficient photodetectors by using mixed-dimensional vdW heterostructures.

Recent advances in solution-processed photodetectors based on inorganic and hybrid photo-active materials

Due to their excellent and tailorable optoelectronic performance, low cost, facile fabrication, and compatibility with flexible substrates, solution-processed inorganic and hybrid photo-active materials have attracted extensive interest for next-generation photodetector applications. This review gives a comprehensive compilation of solution-processed photodetectors. The basic structures of the device and important parameters of photodetectors will be firstly summarized. Then the development of various solution processing technologies containing solution synthesis and liquid phase film-forming processes for the preparation of semiconductor films is described. From the materials science point of view, we give a comprehensive overview about the current status of solution processed semiconductor materials including inorganic and hybrid photo-active materials for the application of photodetectors. Moreover, challenges and future trends in the field of solution-processed photodetectors are proposed.

CsPbI3 Nanotube Photodetectors with High Detectivity

In the present work, the exploration of photodetectors (PDs) based on CsPbI₃ nanotubes are reported. The as-prepared CsPbI₃ nanotubes can be stable for more than 2 months under air conditions. It is found that, in comparison to the nanowires, nanobelts, and nanosheets, the nanotubes can be advantageous to be used as the functional units for PDs, which is mainly attributed to the enhanced light absorption ability induced by the light trapping effect within the tube cavity. As a proof of concept, the as-constructed PDs based on CsPbI₃ nanotube present an overall excellent performance with a responsivity (R_λ ), external quantum efficiency (EQE) and detectivity of 1.84 × 10³ A W^-1 , 5.65 × 10⁵ % and 9.99 × 10¹³ Jones, respectively, which are all comparable to state-of-the-art ones for all-inorganic perovskite PDs.

Van der Waals Integration of Bismuth Quantum Dots-Decorated Tellurium Nanotubes (Te@Bi) Heterojunctions and Plasma-Enhanced Optoelectronic Applications

Van der Waals (vdW)-integrated heterojunctions have been widely investigated in optoelectronics due to their superior photoelectric conversion capability. In this work, 0D bismuth quantum dots (Bi QDs)-decorated 1D tellurium nanotubes (Te NTs) vdW heterojunctions (Te@Bi vdWHs) are constructed by a facile bottom-up assembly process. Transient absorption spectroscopy suggests that Te@Bi vdWH is a promising candidate for new-generation optoelectronic devices with fast response properties. The subsequent experiments and density functional theory calculations demonstrate the vdW interaction between Te NTs and Bi QDs, as well as the enhanced optoelectronic characteristics owing to the plasma effects at the interface between Te NTs and Bi QDs. Moreover, Te@Bi vdWHs-based photodetectors show significantly improved photoresponse behavior in the ultraviolet region compared to pristine Te NTs or Bi QDs-based photodetectors. The proposed integration of vdWHs is expected to pave the way for constructing new nanoscale heterodevices.

Recent progress in black phosphorus and black-phosphorus-analogue materials: properties, synthesis and applications

Black phosphorus (BP), a novel two-dimensional (2D) layered semiconductor material, has attracted tremendous attention since 2014 due to its prominent carrier mobility, thickness-dependent direct bandgap and in-plane anisotropic physical properties. BP has been considered as a promising material for many applications, such as in transistors, photonics, optoelectronics, sensors, batteries and catalysis. However, the development of BP was hampered by its instability under ambient conditions, as well as by the lack of methods to synthesize large-area and high quality 2D nanofilms. Recently, some BP-analogue materials such as binary phosphides (MPx), transition metal phosphorus trichalcogenides (MPX3), and 2D group V (pnictogens) and 2D group VI materials have attracted increasing interest for their unique and stable structures, and excellent physical and chemical properties. This article, which focuses on BP and BP-analogue materials, will present their crystal structure, properties, synthesis methods and applications. Also the similarity and difference between BP and BP-analogue materials will be discussed, and the presentation of the future opportunities and challenges of the materials are included at the end.

Improved responsivity performance of ZnO film ultraviolet photodetectors by vertical arrays ZnO nanowires with light trapping effect

The light trapping effect of zinc oxide (ZnO) ultraviolet photodetectors (UV PDs) has been established as a promising way to optimize the performance of optoelectronic devices. In this paper, we report a light trapping fabricated metal-semiconductor-metal structure, consisting of a ZnO nanowire array as a top layer light absorber supported by a ZnO film. The ZnO film is bridged between two interdigitated metal electrodes for collecting photo-generated carriers. In this connection, high-dense ZnO nanowires can be used as a light trapping unit and to transmit the photogenerated carriers towards the ZnO film. The photogenerated carriers diffuse along the longitudinal direction of the ZnO nanowire and then to the ZnO film and are collected by the applied bias electrode. Compared to present ZnO thin film UV PDs, our device has an effective light trapping effect and the enhancement of photo-generated carriers at the top interface by a ZnO nanowire array structure are highly beneficial to UV light detection as they can provide a long optical path and more surface area. In addition, when the device was connected with nanowires, a 10 times augment of responsivity appeared accompanied by a giant photo-to-dark current ratio (1.6 × 10³). This novel work not only enhanced fundamental improvement of nanowires to ZnO film UV PDs, but also provided a distinct contrast between light trapping UV PDs and ZnO film UV PDs.

Built-in electric field promotes photoexcitation separation and depletion of most carriers in TiO2:C UV detectors

TiO₂ has been widely used in ultraviolet (UV) photodetectors, but due to the large number of structural defects and strong band-to-band recombination of the exciton in TiO₂, the devices usually have large dark current (I _d) and low light current (I _l), which seriously reduces the sensitivity and responsivity (R) of the TiO₂ based devices. In this work, carbon (C) quantum dots (QDs) are introduced into TiO₂ film to ameliorate these issues. Due to the difference of work function between TiO₂ nanoparticles and C QDs, the built-in electric field (E _bi) can be formed, which effectively facilitates the photogenerated exciton dissociation in the TiO₂ film under UV illumination. Meanwhile, the constructed depletion region in dark reduces the majority carrier density, thus decreasing the I _d of the photodetector. Moreover, the E _bi and depletion region will also contribute to the faster charge collection under UV illumination and recombination of the electron in dark, which is beneficial for the improved response/recovery speed of the device.

An Important Factor Affecting the Supercapacitive Properties of Hydrogenated TiO2 Nanotube Arrays: Crystal Structure

Employing a suitable crystal structure can significantly modify the electrochemical performances of materials. Herein, hydrogenated TiO₂ nanotube arrays with <001> orientation and different rutile/anatase ratio were fabricated via anodisation, high-temperature annealing and electrochemical hydrogenation. The crystal structure was determined by TEM and X-ray diffraction pattern refinement of whole powder pattern fitting. Combined with the model of anatase to rutile transformation and the characterisation of crystal structure, the effect of phase transition on the super capacitive properties of <001> oriented hydrogenated TiO₂ nanotube arrays was discussed. The results suggested that the anatase grains were characterised by orientation in <001> direction with plate crystallite and stacking vertically to the substrate resulting in excellent properties of electron/ion transport within hydrogenated TiO₂ nanotube arrays. In addition, the specific capacitance of <001> oriented hydrogenated TiO₂ could be further improved from 20.86 to 24.99 mF cm^-2 by the partial rutile/anatase transformation due to the comprehensive effects of lattice disorders and rutile, while the good rate performance and cyclic stability also retained.

Enhanced Photodetection Properties of Tellurium@Selenium Roll-to-Roll Nanotube Heterojunctions

Non-layered tellurium (Te) is a promising material for applications in transistor and optoelectronic devices for its advantages in excellent intrinsic electronic and optoelectronic properties. However, the poor photodetection performance and relatively uncertain stability of tellurene under ambient conditions greatly limit the practical applications. In order to improve the performance of tellurene-based materials, Te@Se roll-to-roll nanotubes with different selenium (Se) contents synthesized by epitaxial growth of Se on Te nanotubes are, for the first time, employed to fabricate working electrodes for photoelectrochemical (PEC)-type broadband photodetection. They exhibit not only a preferably enhanced capacity for self-powered broadband photodetection but also significantly improved photocurrent density and stability in various aqueous environments (HCl, NaCl, and KOH solutions), compared to tellurene-based photodetectors. It is anticipated that the present work can open up new possibilities for high-performance tellurene-based optoelectronic devices.

Hydrogenated TiO2 Nanorod Arrays Decorated with Carbon Quantum Dots toward Efficient Photoelectrochemical Water Splitting

Limited light harvesting and charge collection are recognized as grand challenges for the exploration of highly efficient TiO₂ photoanodes. To overcome these intrinsic shortcomings, we reported the designed photoanode based on TiO₂ nanoarrays with both hydrogenation treatment and surface decoration of carbon quantum dots (CQDs) toward efficient photoelectrochemical water splitting. The results revealed that hydrogenation treatment could cause the formation of oxygen vacancies to suppress the recombination of photoinduced carriers. Meanwhile, the decorated CQDs could not only play as the electron reservoirs to trap photoinduced electrons but also remarkably enhance the solar light harvesting due to their upconversion effect. The as-fabricated photoanodes exhibited a large photocurrent density of ∼3.0 mA/cm² at 1.23 V versus reversible hydrogen electrode under simulated sunlight, which was the highest one among hydrogenated TiO₂ photoanodes ever reported and was ∼6 times that of pristine analogues.

Materials and Designs for Wearable Photodetectors

Photodetectors (PDs), as an indispensable component in electronics, are highly desired to be flexible to meet the trend of next-generation wearable electronics. Unfortunately, no in-depth reviews on the design strategies, material exploration, and potential applications of wearable photodetectors are found in literature to date. Thus, this progress report first summarizes the fundamental design principles of turning "hard" photodetectors "soft," including 2D (polymer and paper substrate-based devices) and 1D PDs (fiber shaped devices). In short, the flexibility of PDs is realized through elaborate substrate modification, material selection, and device layout. More importantly, this report presents the current progress and specific requirements for wearable PDs according to the application: monitoring, imaging, and optical communication. Challenges and future research directions in these fields are proposed at the end. The purpose of this progress report is not only to shed light on the basic design principles of wearable PDs, but also serve as the roadmap for future exploration in wearable PDs in various applications, including health monitoring and Internet of Things.

Self-Powered, Broad Band, and Ultrafast InGaN-Based Photodetector

A self-powered, broad band and ultrafast photodetector based on n⁺-InGaN/AlN/n-Si(111) heterostructure is demonstrated. Si-doped (n⁺ type) InGaN epilayer was grown by plasma-assisted molecular beam epitaxy on a 100 nm thick AlN template on an n-type Si(111) substrate. The n⁺-InGaN/AlN/n-Si(111) devices exhibit excellent self-powered photoresponse under UV-visible (300-800 nm) light illumination. The maximum response of this self-powered photodetector is observed at 580 nm for low-intensity irradiance (0.1 mW/cm²), owing to the deep donor states present near the InGaN/AlN interface. It shows a responsivity of 9.64 A/W with rise and fall times of 19.9 and 21.4 μs, respectively. A relation between the open circuit voltage and the responsivity has been realized.