CdS nanoscale photodetectors

Zinc and cadmium thioamidate complexes: rational design of single-source precursors for the AACVD of ZnS

A series of zinc(II) thioamidate complexes [Zn{SC(iPr)NR}2]n for R = iPr (n = 2) (2), tBu (3) (n = 1), Ph (4) (n = 2) and Cy (5) (n = 2) and one cadmium(II) thioamidate complex [Cd{SC(iPr)NtBu}2]3, (6), were designed and synthesised as single-source precursors for AACVD ZnS and CdS. Solid-state structures of all four zinc(II) compounds revealed distorted tetrahedral or trigonal bipyramidal geometries, with varying tendencies for dimeric association, mediated by {Zn-S} bridging bonds. The thermogravimetric analysis identified the {tBu} derivertive, 3, as the most promising precursor based on its low decomposition onset (118 °C) and clean conversion to ZnS. This was attributed to the greater availability of β-hydrogen atoms promoting the pyrolysis mechanism. The corresponding cadmium thioamide 6 was found to crystallise as a trimetallic molecule which lacked the thermal stability to be considered viable for AACVD. Hence, 3 was used to deposit ZnS thin films by AACVD at 200-300 °C. Powder X-ray diffraction confirmed phase-pure growth of hexagonal wurtzite ZnS, with approximate crystallite sizes of 15-20 nm. Scanning electron microscopy revealed densely packed spherical nanoclusters. The morphology and crystallinity were most consistent for depositions between 250-300 °C. Energy dispersive X-ray spectroscopy indicated slightly sulfur-deficient stoichiometries.

Experimental and DFT study of photocatalytic activity of reduced graphene oxide/copper sulfide composite for removal of organic dyes from water

In this work, successful nanocomposites composed of different ratios of reduced graphene oxide and copper sulphide (xCuS-rGO) were fabricated to aid in treating water contaminated with organic dyes. XRD, TEM, SEM, XPS, IR, EDX and BET were applied for the characterization of (CuS-rGO). The photocatalytic strength of the prepared nanocomposites was evaluated using artificial sunlight irradiation. The nanocomposites were tested for their ability to degrade both anionic and cationic organic dyes, including amaranth and rhodamine B (RhB). The excellent photocatalytic strength of our composites, relative to pristine CuS and rGO, was interpreted as rGO sheets being very porous. In addition, the charge moved efficiently from rGO to CuS. The combined properties enhanced the efficiency of photodegradation of CuS-rGO composite across the dyes under the illumination of simulated sunlight. The electron transportation from rGO sheets to the CuS conduction band enhances the charge separation and transportation. The role of superoxide radicals in photocatalytic degradation was unveiled and the interactions between the studied dyes and our catalysts were investigated by density functional theory study and scavenging investigation. This work gives new ideas about the preparation and properties of (CuS-rGO) composites and their broad application in solving environmental problems.

Investigating various metal contacts for p-type delafossite α-CuGaO2 to fabricate ultraviolet photodetector

Delafossite semiconductors have attracted substantial attention in the field of electro-optics owing to their unique properties and availability of p-type materials that are applicable for solar cells, photocatalysts, photodetectors (PDs) and p-type transparent conductive oxides (TCOs). The CuGaO₂ (CGO), as one of the most promising p-type delafossite materials, has appealing electrical and optical properties. In this work, we are able to synthesize CGO with different phases by adopting solid-state reaction route using sputtering followed by heat treatment at different temperatures. By examining the structural properties of CGO thin films, we found that the pure delafossite phase appears at the annealing temperature of 900 °C. While at lower temperatures, delafossite phase can be observed, but along with spinel phase. Furthermore, their structural and physical characterizations indicate an improvement of material-quality at temperatures higher than 600 °C. Thereafter, we fabricated a CGO-based ultraviolet-PD (UV-PD) with a metal-semiconductor-metal (MSM) configuration which exhibits a remarkable performance compared to the other CGO-based UV-PDs and have also investigated the effect of metal contacts on the device performance. We demonstrate that UV-PD with the employment of Cu as the electrical contact shows a Schottky behavior with a responsivity of 29 mA/W with a short response time of 1.8 and 5.9 s for rise and decay times, respectively. In contrast, the UV-PD with Ag electrode has shown an improved responsivity of about 85 mA/W with a slower rise/decay time of 12.2/12.8 s. Our work sheds light on the development of p-type delafossite semiconductor for possible optoelectronics application of the future.

Broadband Photoresponses from Ultraviolet to Near-Infrared (II) Region through Light-induced Pyroelectric Effects in a Hybrid Perovskite

Broadband photodetection has shown a great promise for diverse applications, while the realization of plateau photoresponse from ultraviolet (UV) to near-infrared (NIR) spectral region is very challenging. Herein, we exploit photoexcited pyroelectric effect in a chiral hybrid perovskite, (N, N-dimethylcyclohexylammonium)PbBr₃ (1), serving as a new pathway to drive broadband photoactivities. It is a room-temperature pyroelectric with large polarization of ≈6.4 μC cm^-2 and high pyroelectric figure-of-merits (F_V =1.0×10^-2  cm²   μC^-1 and F_D =7.1×10^-5  Pa^-1/2 ). Strikingly, light-induced pyroelectric effect arising from spontaneous polarization is observed in 1, which cover UV (266 nm) to NIR-II (1950 nm) full spectral region. The broadband photoresponses actualized by pyroelectricity break the limit of optical band gap. As the first demonstration of photo-pyroelectricity covering UV-to-NIR spectral region in hybrid perovskites, this work paves a pathway to assemble high-performance smart devices.

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.

High-Performance and Broadband Flexible Photodetectors Employing Multicomponent Alloyed 1D CdSxSe1-x Micro-Nanostructures

Low-cost multicomponent alloyed one-dimensional (1D) semiconductors exhibit broadband absorption from the ultraviolet to the near-infrared regime, which has attracted a great deal of interest in high-performance flexible optoelectronic devices. Here, we report the facile one-step fabrication of high-performance broadband rigid and flexible photodevices based on multicomponent alloyed 1D cadmium-sulfur-selenide (CdS_xSe_1-x) micro-nanostructures obtained via a vapor transport route. Photoresponse measurements have demonstrated their superior spectral photoresponsivity (5.8 × 10⁴ A/W), several orders of magnitude higher than the pristine CdSe nanobelt photodevice, high specific detectivity (2 × 10¹⁵ Jones), photogain (1.2 × 10⁵), external quantum efficiency (EQE, 1.4 × 10⁷%), rapid response speed (13 ms), and excellent long-term environmental stability. The multicomponent alloyed CdS_xSe_1-x nanobelt photodevice demonstrated about three times higher photocurrent as well as can operate under multiple color illuminations (200-800 nm) and at a high applied bias of 10 V with the photoresponsivity and EQE being boosted to 4.34 × 10⁵ A/W and 8.96 × 10⁷%, respectively. Furthermore, multicomponent alloyed CdS_xSe_1-x nanobelt flexible photodevices show excellent mechanical and flexural photostabilities with identical photoresponse as rigid nanodevices. The improvement mechanism found in the present research can be exploited to lead to the design of high-performance flexible photodevices comprising other multicomponent nanomaterials.

Probing ultrafast hot charge carrier migration in MoS2 embedded CdS nanorods

Efficient utilization of hot charge carriers is of utmost benefit for a semiconductor-based optoelectronic device. Herein, a one-dimensional (1D)/two-dimensional (2D) heterojunction was fabricated in the form of CdS/MoS₂ nanorod/nanosheet composite and migration of hot charge carriers was being investigated with the help of transient absorption (TA) spectroscopy. The band alignment was such that both the electrons and holes in the CdS region tend to migrate into the MoS₂ region following photoexcitation. The composite system is composed of optical signatures of both CdS and MoS₂, with the dominance of CdS nanorods. In addition, the TA signal of MoS₂ is substantially enhanced in the heterosystem at the cost of the diminished CdS signal, confirming the migration of charge carrier population from CdS to MoS₂. This migration phenomenon was dominated by the hot carrier transfer. The hot carriers in the high energy states of CdS are preferentially migrated into the MoS₂ states rather than being cooled to the band edge. The hot carrier transfer time for a 400 nm pump excitation was calculated to be 0.21 ps. This is much faster than the band edge electron transfer process, occurring at 2.0 ps time scale. We found that these migration processes are very much dependent on the applied pump photon energy. Higher energy pump photons are more efficient in the hot carrier transfer process and place these hot carriers in the higher energy states of MoS₂, further extending charge carrier separation. This detailed spectroscopic investigation would help in the fabrication of better 1D/2D heterojunctions and advance the optoelectronic field.

Fabrication of graphitic carbon nitride functionalized P-CoFe2O4 for the removal of tetracycline under visible light: Optimization, degradation pathways and mechanism evaluation

In this study, nano-sized CoFe₂O₄ composites were prepared through co-precipitation process. Then the phosphorus-doped strong magnetic graphitic carbon nitride hybrids composites (P-CoFe₂O₄@GCN) was stemmed from the CoFe₂O₄ composites via the thermal polymerization method. The TEM results show that the CoFe₂O₄ nanoparticles have been successfully embedded into the graphitic carbon nitride (GCN). The BET specific surface area of P-CoFe₂O₄@GCN-1 could reach 36.91 m²/g, which was 5.38 times higher than that of GCN. Thus, it provided sufficient reaction active sites to enhance the photocatalytic activity for tetracycline (TC) decomposition. The results from the photocatalytic experiments showed that the degradation efficiency of TC by P-CoFe₂O₄@GCN-1 could reach 96.2% within 60 min, which is 3.19 times higher than that of GCN. The h⁺, O₂•^- and •OH radicals detected by the electron spin resonance (ESR) were responsible for the TC decomposition in the photocatalytic reaction system. Persulfate (PS) can further activate the hybrid mixture system, and the fitting model predicted by the response surface methodology (RSM) indicated that the maximum tetracycline removal could reach 99.6% within 30 min. In addition, the degradation intermediates of TC were detected by HPLC-MS and the photodegradation mechanism was discussed.

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.

A reasonably designed 2D WS2 and CdS microwire heterojunction for high performance photoresponse

Heterojunctions based on low-dimensional materials can combine the superiorities of each component and realize novel properties. Herein, a mixed-dimensional heterojunction comprising multilayer WS₂, CdS microwire, and few-layer WS₂ has been demonstrated. The working mechanism and its application in a photodetector are investigated. The multilayer WS₂ and CdS microwire are utilized to provide efficient light absorption, while the few-layer WS₂ is utilized to passivate interfacial impurity scattering. In addition, based on the reasonable band alignment of the components, three built-in electric fields are formed, which efficiently separate the photo-generated carriers and enhance the photocurrent. In particular, the photo-generated electrons are trapped in CdS, while the photo-generated holes circulate in the external circuit, leading to a high photoconductivity gain. Motivated by these, we constructed a device that exhibits a photoresponsivity of ∼4.7 A W^-1, a response/recovery time of 13.7/15.8 ms, and a detectivity of 3.4 × 10¹² Jones, which are much better than the counterparts. All of these clearly demonstrate the importance of advanced device designs for realizing high performance optoelectronic devices.

A bioinspired Au-Cu1.97S/Cu2S film with efficient low-angle-dependent and thermal-assisted photodetection properties

Inspired by the geological processes, this study develops an innovative low-concentration-ratio H₂ reduction method to reduce the stoichiometric Au-CuS nanoparticles to produce completely reduced stoichiometric Cu₂S with “invisible” Au achieved for solid solution Au enhancement. A stable Au-Cu_1.97S/Cu₂S micro/nano-composite is then formed by spontaneous oxidation. From this composite, in combination with biomimetic technology, an omnidirectional photoabsorption and thermoregulated film (Au-Cu_1.97S/Cu₂S-C-T_FW) is designed and fabricated as a photothermal-assisted and temperature-autoregulated photodetector for broadband and low-angle-dependent photodetection that presents good performance with high responsivity (26.37 mA/W), detectivity (1.25×10⁸ Jones), and good stability at low bias (0.5 V). Solid solution Au exhibits significantly enhanced photodetection (1,000 times). This study offers a new concept for improving the stability and photoelectric properties of copper chalcogenides. Moreover, it opens up a new avenue toward enhancing the performance of optoelectronic and photovoltaic devices using solid solution metal atoms and thermal-assisted, anti-overheating temperature autoregulation.

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Inspired by geological process, solid solution-atom enhancement in photoresponse

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A photothermal-assist-enhanced and temperature-autoregulated photodetector

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The photodetector exhibited excellent low-angle-dependent photodetection

Scalable Synthesis of a Sub-10 nm Chalcopyrite (CuFeS2) Nanocrystal by the Microwave-Assisted Synthesis Technique and Its Application in a Heavy-Metal-Free Broad-Band Photodetector

A heavy-metal-free chalcopyrite (CuFeS₂) nanocrystal has been synthesized via microwave-assisted growth. Large-scale nanocrystals with an average particle size of 5 nm are fabricated by this technique within a very short period of time without any need for organic ligands. Scanning electron microscopy study (SEM) of individual synthesis steps indicates that aggregates of nanocrystals are formed as flakes during microwave-assisted synthesis. The colloidal solution of the CuFeS₂ nanocrystal was prepared by sonicating these flakes. Transmission electron microscopy (TEM) study reveals the growth of sub-10 nm CuFeS₂ nanocrystals that are further characterized by X-ray diffraction. UV-visible absorption spectroscopic study shows that the band gap of this nanocrystal is ∼1.3 eV. To investigate the photosensitive nature of this nanocrystal, a bilayer p-n heterojunction photodetector has been fabricated using this nontoxic CuFeS₂ nanocrystal as a photoactive material and n-type ZnO as a charge-transport layer. The detectivity of this photodetector reaches above 10¹² Jones in visible and near-infrared (NIR) regions under 10 V external bias, which is significantly high for a nontoxic nanocrystal-based photodetector.

The Evolution of Flexible Electronics: From Nature, Beyond Nature, and To Nature

The flourishing development of multifunctional flexible electronics cannot leave the beneficial role of nature, which provides continuous inspiration in their material, structural, and functional designs. During the evolution of flexible electronics, some originated from nature, some were even beyond nature, and others were implantable or biodegradable eventually to nature. Therefore, the relationship between flexible electronics and nature is undoubtedly vital since harmony between nature and technology evolution would promote the sustainable development. Herein, materials selection and functionality design for flexible electronics that are mostly inspired from nature are first introduced with certain functionality even beyond nature. Then, frontier advances on flexible electronics including the main individual components (i.e., energy (the power source) and the sensor (the electric load)) are presented from nature, beyond nature, and to nature with the aim of enlightening the harmonious relationship between the modern electronics technology and nature. Finally, critical issues in next-generation flexible electronics are discussed to provide possible solutions and new insights in prospective exploration directions.

A microwave-assisted template-free route for large-scale synthesis of photoluminescent single crystal CsPbI3 nanotubes

High-quality single crystalline CsPbI₃ nanotubes featuring highly uniform sizes and stable and bright photoluminescence were synthesized through a microwave-assisted synthetic approach.

Phonon-Assisted Electro-Optical Switches and Logic Gates Based on Semiconductor Nanostructures

High-performance nanostructured electro-optical switches and logic gates are highly desirable as essential building blocks in integrated photonics. In contrast to silicon-based optoelectronic devices, with their inherent indirect optical bandgap, weak light-modulation mechanism, and sophisticated device configuration, direct-bandgap-semiconductor nanostructures with attractive electro-optical properties are promising candidates for the construction of nanoscale optical switches for on-chip photonic integrations. However, previously reported semiconductor-nanostructure optical switches suffer from serious drawbacks such as high drive voltage, limited operation spectral range, and low modulation depth. High-efficiency electro-optical switches based on single CdS nanobelts with low drive voltage, ultra-high on/off ratio, and broad operation wavelength range, properties resulting from unique electric-field-dependent phonon-assisted optical transitions, are demonstrated. Furthermore, functional NOT, NOR, and NAND optical logic gates are demonstrated based on these switches. These switches and optical logic gates represent an important step toward integrated photonic circuits.

Constructing a Green Light Photodetector on Inorganic/Organic Semiconductor Homogeneous Hybrid Nanowire Arrays with Remarkably Enhanced Photoelectric Response

We demonstrate that a novel photodetector is constructed by CdS/poly( p-phenylene vinylene) (PPV) homogeneous hybrid nanowire arrays via a simple template-assisted electrochemical codeposition approach. Owing to the well-matched energy levels between CdS and PPV, the recombination of photogenerated electrons and holes in CdS/PPV hybrid nanowire arrays is greatly inhibited. It is found that the homogeneous hybrid nanowire arrays exhibit remarkably enhanced photoelectric response and the ON/OFF ratio by 17 times compared to the individual CdS component. More importantly, the CdS/PPV hybrid nanowire arrays are observed with significant spectral selectivity especially for green light under 545 nm. In addition, a straight linear relationship is obtained between the ON/OFF ratios and the illumination intensities, implying that the quantitative detection of illumination intensity can be achieved. The new as-prepared homogeneous hybrid organic/inorganic semiconductor nanowire arrays have a bright prospect for applications in high-sensitivity and high-speed green photodetectors.

Properties of iron vanadate over CdS nanorods for efficient photocatalytic hydrogen production

In this study, V³⁺/VO²⁺/VO₂⁺ was successfully employed as a redox mediator to modify CdS nanorods for the first time, and remarkable enhancement of efficient hydrogen evolution was obtained.

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

Single CdS nanobelt-based photodetectors are strongly dependent on bias and temperature. They not only show a strong photoresponse to close bandgap energy light with ultrahigh responsivity of approximately 10⁷ A W^-1, large photo-to-dark current ratio of 10⁴, photoconductive gain of 10⁷, and fast response and recovery speed at a large bias of 20 V, but can also show a weak photoresponse to above- and below-bandgap energy light. Moreover, their spectral response range can show tunable selectivity to above- and below-bandgap light, which can be accurately controlled by temperature and bias. More importantly, the modulated spectral response characteristics show excellent memory behaviour after reversible writing and erasing by using temperature and bias. In nanostructures, abundant surface states and stacking fault-related traps play a vital role in the ultrahigh photoresponse to bandgap light and the erasable memory effect on spectral response range selectivity. Given the erasable memory of the spectral response selectivity with excellent photoconduction performance, the CdS NBs possess important applications in new-generation photodetection and photomemory devices.

Fast response CdS-CdSxTe1-x-CdTe core-shell nanobelt photodetector

Quasi-one-dimensional semiconductor nanostructure-based photodetectors show high sensitivity but suffer from slow response speed due to surface reaction. Here, we report a fast-response CdS-CdS_xTe_1-x-CdTe core-shell nanobelt photodetector with a rise time of 11 μs, which is the fastest among CdS based photodetectors reported previously. The improved response speed is ascribed to the suppressed possibilities of surface reaction resulting from the core-shell structure and heterojunction among the CdS, CdS_xTe_1-x and CdTe. The measured response spectrum of CdS-CdS_xTe_1-x-CdTe core-shell nanobelt photodetector covers a wide range from 355 to 785 nm. Moreover, high responsivity (1,520 A/W) and large 3 dB bandwidth (∼22.9 kHz) are obtained along with the fast response. The high performance in responsivity, sensitivity, spectral response and photoresponse speed makes this device a promising candidate for practical application in optical sensing, communication and imaging.

High-Performance Visible-Blind Ultraviolet Photodetector Based on IGZO TFT Coupled with p-n Heterojunction

A visible-blind ultraviolet (UV) photodetector was designed based on a three-terminal electronic device of thin-film transistor (TFT) coupled with two-terminal p-n junction optoelectronic device, in hope of combining the beauties of both of the devices together. Upon the uncovered back-channel surface of amorphous indium-gallium-zinc-oxide (IGZO) TFT, we fabricated PEDOT:PSS/SnO _x/IGZO heterojunction structure, through which the formation of a p-n junction and directional carrier transfer of photogenerated carriers were experimentally validated. As expected, the photoresponse characteristics of the newly designed photodetector, with a photoresponsivity of 984 A/W at a wavelength of 320 nm, a UV-visible rejection ratio up to 3.5 × 10⁷, and a specific detectivity up to 3.3 × 10¹⁴ Jones, are not only competitive compared to the previous reports but also better than those of the pristine IGZO phototransistor. The hybrid photodetector could be operated in the off-current region with low supply voltages (<0.1 V) and ultralow power dissipation (<10 nW under illumination and ∼0.2 pW in the dark). Moreover, by applying a short positive gate pulse onto the gate, the annoying persistent photoconductivity presented in the wide band gap oxide-based devices could be suppressed conveniently, in hope of improving the response rate. With the terrific photoresponsivity along with the advantages of photodetecting pixel integration, the proposed phototransistor could be potentially used in high-performance visible-blind UV photodetector pixel arrays.

Self-Powered Si/CdS Flexible Photodetector with Broadband Response from 325 to 1550 nm Based on Pyro-phototronic Effect: An Approach for Photosensing below Bandgap Energy

Cadmium sulfide (CdS) has received widespread attention as the building block of optoelectronic devices due to its extraordinary optoelectronic properties, low work function, and excellent thermal and chemical stability. Here, a self-powered flexible photodetector (PD) based on p-Si/n-CdS nanowires heterostructure is fabricated. By introducing the pyro-phototronic effect derived from wurtzite structured CdS, the self-powered PD shows a broadband response range, even beyond the bandgap limitation, from UV (325 nm) to near infrared (1550 nm) under zero bias with fast response speed. The light-induced pyroelectric potential is utilized to modulate the optoelectronic processes and thus improve the photoresponse performance. Lasers with different wavelengths have different effects on the self-powered PDs and corresponding working mechanisms are carefully investigated. Upon 325 nm laser illumination, the rise time and fall time of the self-powered PD are 245 and 277 µs, respectively, which are faster than those of most previously reported CdS-based nanostructure PDs. Meanwhile, the photoresponsivity R and specific detectivity D* regarding to the relative peak-to-peak current are both enhanced by 67.8 times, compared with those only based on the photovoltaic effect-induced photocurrent. The self-powered flexible PD with fast speed, stable, and broadband response is expected to have extensive applications in various environments.

Controllable Vapor Growth of Large-Area Aligned CdS x Se1-x Nanowires for Visible Range Integratable Photodetectors

The controllable growth of large area band gap engineered-semiconductor nanowires (NWs) with precise orientation and position is of immense significance in the development of integrated optoelectronic devices. In this study, we have achieved large area in-plane-aligned CdS _x Se_1-x nanowires via chemical vapor deposition method. The orientation and position of the alloyed CdS _x Se_1-x NWs could be controlled well by the graphoepitaxial effect and the patterns of Au catalyst. Microstructure characterizations of these as-grown samples reveal that the aligned CdS _x Se_1-x NWs possess smooth surface and uniform diameter. The aligned CdS _x Se_1-x NWs have strong photoluminescence and high-quality optical waveguide emission covering almost the entire visible wavelength range. Furthermore, photodetectors were constructed based on individual alloyed CdS _x Se_1-x NWs. These devices exhibit high performance and fast response speed with photoresponsivity ~ 670 A W^-1 and photoresponse time ~ 76 ms. Present work provides a straightforward way to realize in-plane aligned bandgap engineering in semiconductor NWs for the development of large area NW arrays, which exhibit promising applications in future optoelectronic integrated circuits.

High-Sensitive Ultraviolet Photodetectors Based on ZnO Nanorods/CdS Heterostructures

The ultraviolet (UV) photodetectors with ZnO nanorods (NRs)/CdS thin film heterostructures on glass substrates have been fabricated and characterized. It can be seen that the UV photoresponsivity of such a device became higher as the ZnO NR length was increased in the investigation. With an incident wavelength of 350 nm and 5 V applied bias, the responsivity of photodetectors based on ZnO NR/CdS heterostructures with the ZnO NR length at 500, 350, and 200 nm and traditional CdS film were at 12.86, 3.83, 0.91, and 0.75 A/W, respectively. The measurement results of the fabricated photodetectors based on ZnO nanorods (NRs)/CdS heterostructures have shown a significant high sensitivity in the range of UV light, which can be useful for the application of UV detection.

Self-Powered Nanoscale Photodetectors

Novel self-powered nanoscale photodetectors that can work without an external power source, which have great application potential in next-generation nanodevices that operate wirelessly and independently, are being widely studied. This review aims to give a comprehensive summary of the state-of-the-art research results on self-powered nanoscale photodetectors. An introduction of recent progress on Schottky junction photodetectors is provided. Two types of Schottky junctions are discussed in detail: metal-semiconductor and semiconductor-graphene junctions. Next, recent developments of p-n junction photodetectors are highlighted, including homojunction and heterojunction photodetectors. Then, piezo-phototronic effect enhanced photodetection performances of Schottky junctions and p-n junctions are discussed. Then, significant results on the photoelectrochemical-cell-based photodetector and integrated self-powered nanosystem are presented, followed by a systematic comparison of different types of photodetectors. Finally, a summary of the previous results is given, and the major challenges that need to be addressed in the future are outlined. The hope is that this review can provide valuable insights into the current status of self-powered photodetectors and spur new structure and device designs to further enhance photodetection performance.

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.

Ultrahigh Detectivity and Wide Dynamic Range Ultraviolet Photodetectors Based on BixSn1-xO2 Intermediate Band Semiconductor

The ultraviolet (UV) photodetectors have significant applications different fields. High detectivity, high responsivity and wide active area are required to probe a weak UV light in actual ambient. Unfortunately, most practical UV photoconductors based on wide bandgap semiconductor films can hardly have both a high responsivity and a low dark current density. In this study, the intermediate band engineering in semiconductor has been proposed try to solve this problem. The intermediate band UV photodetectors based on Bi_xSn_1-xO₂ (0.017 < x < 0.041) films show a detectivity of 6.1 × 10¹⁵ Jones at 280 nm and a quantum efficiency of 2.9 × 10⁴ %. The dynamic range is 195 dB, which is much higher than other UV photodetector. The recovery time is about 1 s after exposing device into ethanol steam. Our results demonstrate that the intermediate band semiconductor Bi_xSn_1-xO₂ films can serve as a high performance UV photodetector.

Ultrasensitive and Highly Selective Photodetections of UV-A Rays Based on Individual Bicrystalline GaN Nanowire

The detection of UV-A rays (wavelength of 320-400 nm) using functional semiconductor nanostructures is of great importance in either fundamental research or technological applications. In this work, we report the catalytic synthesis of peculiar bicrystalline GaN nanowires and their utilization for building high-performance optoelectronic nanodevices. The as-prepared UV-A photodetector based on individual bicrystalline GaN nanowire demonstrates a fast photoresponse time (144 ms), a high wavelength selectivity (UV-A light response only), an ultrahigh photoresponsivity of 1.74 × 10⁷ A/W and EQE of 6.08 × 10⁹%, a sensitivity of 2 × 10⁴%, and a very large on/off ratio of more than two orders, as well as robust photocurrent stability (photocurrent fluctuation of less than 7% among 4000 s), showing predominant advantages in comparison with other peer semiconductor photodetectors. The outstanding optoelectronic performance of the bicrystalline GaN nanowire UV-A photodetector is further analyzed based on a detailed high-resolution transmission electron microscope (HRTEM) study, and the two separated crystal domains within the GaN nanowires are believed to provide separated and rapid carrier transfer channels. This work paves a solid way toward the integration of high-performance optoelectronic nanodevices based on bicrystalline or horizontally aligned one-dimensional semiconductor nanostructures.

Quantitative measurement of transport properties: Ag-doped nanocrystalline CdS thin films

This work highlights the transport properties of undoped and Ag doped nc-CdS thin films for optoelectronic devices.

High performance, self-powered ultraviolet photodetector based on a ZnO nanoarrays/GaN structure with a CdS insert layer

A self-powered photodetector based on a ZnO nanoarrays/CdS/GaN structure with a responsivity as high as 176 mA W⁻¹ at 300 nm.

Adhesive nanocomposites of hypergravity induced Co3O4nanoparticles and natural gels as Li-ion battery anode materials with high capacitance and low resistance

This communication reports adhesive nanocomposites of hypergravity Co₃O₄/XG gel as Li-ion battery anodes, which exhibit enhanced electrochemical performance and low resistance.

Surface Charge Transfer Doping of Low-Dimensional Nanostructures toward High-Performance Nanodevices

Device applications of low-dimensional semiconductor nanostructures rely on the ability to rationally tune their electronic properties. However, the conventional doping method by introducing impurities into the nanostructures suffers from the low efficiency, poor reliability, and damage to the host lattices. Alternatively, surface charge transfer doping (SCTD) is emerging as a simple yet efficient technique to achieve reliable doping in a nondestructive manner, which can modulate the carrier concentration by injecting or extracting the carrier charges between the surface dopant and semiconductor due to the work-function difference. SCTD is particularly useful for low-dimensional nanostructures that possess high surface area and single-crystalline structure. The high reproducibility, as well as the high spatial selectivity, makes SCTD a promising technique to construct high-performance nanodevices based on low-dimensional nanostructures. Here, recent advances of SCTD are summarized systematically and critically, focusing on its potential applications in one- and two-dimensional nanostructures. Mechanisms as well as characterization techniques for the surface charge transfer are analyzed. We also highlight the progress in the construction of novel nanoelectronic and nano-optoelectronic devices via SCTD. Finally, the challenges and future research opportunities of the SCTD method are prospected.

On-Nanowire Axial Heterojunction Design for High-Performance Photodetectors

We report the growth of high-quality CdS/CdSxSe1-x axial heterostructure nanowires (NWHs) via a temperature-controlled chemical vapor deposition method. Microstructural characterizations revealed that these NWHs have a single-crystalline structure with abrupt heterojunctions. Local photoluminescence and mapping near the heterojunctions show only two separated narrow band-edge emission bands from the two different adjacent semiconductors, further demonstrating the high-quality of these heterostructures. Moreover, the photodetector based on the single NWH shows a performance (higher responsivity (1.18 × 10(2) A/W), faster response speed (rise ∼68 μs, decay ∼137 μs), higher Ion/Ioff ratio (10(5)), higher EQE (3.1 × 10(4) %), and broader detection range (350-650 nm)) at room temperature superior to that of photodetectors based on single band gap nanostructures. This work suggests a much simpler route to achieve superior NWHs for applications in optoelectronic devices.

Flexible, transparent and ultra-broadband photodetector based on large-area WSe2 film for wearable devices

Although two-dimensional (2D) materials have attracted considerable research interest for use in the development of innovative wearable optoelectronic systems, the integrated optoelectronic performance of 2D materials photodetectors, including flexibility, transparency, broadband response and stability in air, remains quite low to date. Here, we demonstrate a flexible, transparent, high-stability and ultra-broadband photodetector made using large-area and highly-crystalline WSe2 films that were prepared by pulsed-laser deposition (PLD). Benefiting from the 2D physics of WSe2 films, this device exhibits excellent average transparency of 72% in the visible range and superior photoresponse characteristics, including an ultra-broadband detection spectral range from 370 to 1064 nm, reversible photoresponsivity approaching 0.92 A W(-1), external quantum efficiency of up to 180% and a relatively fast response time of 0.9 s. The fabricated photodetector also demonstrates outstanding mechanical flexibility and durability in air. Also, because of the wide compatibility of the PLD-grown WSe2 film, we can fabricate various photodetectors on multiple flexible or rigid substrates, and all these devices will exhibit distinctive switching behavior and superior responsivity. These indicate a possible new strategy for the design and integration of flexible, transparent and broadband photodetectors based on large-area WSe2 films, with great potential for practical applications in the wearable optoelectronic devices.

Hot electron induced NIR detection in CdS films

We report the use of random Au nanoislands to enhance the absorption of CdS photodetectors at wavelengths beyond its intrinsic absorption properties from visible to NIR spectrum enabling a high performance visible-NIR photodetector. The temperature dependent annealing method was employed to form random sized Au nanoparticles on CdS films. The hot electron induced NIR photo-detection shows high responsivity of ~780 mA/W for an area of ~57 μm². The simulated optical response (absorption and responsivity) of Au nanoislands integrated in CdS films confirms the strong dependence of NIR sensitivity on the size and shape of Au nanoislands. The demonstration of plasmon enhanced IR sensitivity along with the cost-effective device fabrication method using CdS film enables the possibility of economical light harvesting applications which can be implemented in future technological applications.

Ultraviolet/visible photodetectors with ultrafast, high photosensitivity based on 1D ZnS/CdS heterostructures

One-dimensional (1D) semiconducting heterostructures have been widely studied for optoelectronics applications because of their unique geometry and attractive physical properties. In this study, we successfully synthesized 1D ZnS/CdS heterostructures, which can be used to fabricate high performance ultraviolet/visible photodetectors. Due to the separation of photo-generated electron-hole pairs, the resultant photodetector showed excellent photoresponse properties, including ultrahigh Ion/Ioff ratios (up to 10(5)) and specific detectivity (2.23 × 10(14) Jones), relatively fast response speed (5 ms), good stability and reproducibility. Moreover, the as-fabricated flexible photodetectors showed great mechanical stability under different bending conditions. Our results revealed the possibility of 1D ZnS/CdS heterostructures for application in the detection of UV and visible light. The main advantages of the heterostructures have great potential application for future optoelectronic devices.

CdS-Nanowires Flexible Photo-detector with Ag-Nanowires Electrode Based on Non-transfer Process

In this study, UV-visible flexible resistivity-type photo-detectors were demonstrated with CdS-nanowires (NWs) percolation network channel and Ag-NWs percolation network electrode. The devices were fabricated on Mixed Cellulose Esters (MCE) membrane using a lithographic filtration method combined with a facile non-transfer process. The photo-detectors demonstrated strong adhesion, fast response time, fast decay time, and high photo sensitivity. The high performance could be attributed to the high quality single crystalline CdS-NWs, encapsulation of NWs in MCE matrix and excellent interconnection of the NWs. Furthermore, the sensing performance was maintained even the device was bent at an angle of 90°. This research may pave the way for the facile fabrication of flexible photo-detectors with high performances.

Nanostructured Photodetectors: From Ultraviolet to Terahertz

Inspired by nanoscience and nanoengineering, numerous nanostructured materials developed by multidisciplinary approaches exhibit excellent photoelectronic properties ranging from ultraviolet to terahertz frequencies. As a new class of building block, nanoscale elements in terms of quantum dots, nanowires, and nanolayers can be used for fabricating photodetectors with high performance. Moreover, in conjunction with traditional photodetectors, they exhibit appealing performance for practical applications including high density of integration, high sensitivity, fast response, and multifunction. Therefore, with the perspective of photodetectors constructed by diverse low-dimensional nanostructured materials, recent advances in nanoscale photodetectors are discussed here; meanwhile, challenges and promising future directions in this research field are proposed.

High performance broadband photodetector using fabricated nanowires of bismuth selenide

Recently, very exciting optoelectronic properties of Topological insulators (TIs) such as strong light absorption, photocurrent sensitivity to the polarization of light, layer thickness and size dependent band gap tuning have been demonstrated experimentally. Strong interaction of light with TIs has been shown theoretically along with a proposal for a TIs based broad spectral photodetector having potential to perform at the same level as that of a graphene based photodetector. Here we demonstrate that focused ion beam (FIB) fabricated nanowires of TIs could be used as ultrasensitive visible-NIR nanowire photodetector based on TIs. We have observed efficient electron hole pair generation in the studied Bi₂Se₃ nanowire under the illumination of visible (532 nm) and IR light (1064 nm). The observed photo-responsivity of ~300 A/W is four orders of magnitude larger than the earlier reported results on this material. Even though the role of 2D surface states responsible for high reponsivity is unclear, the novel and simple micromechanical cleavage (exfoliation) technique for the deposition of Bi₂Se₃ flakes followed by nanowire fabrication using FIB milling enables the construction and designing of ultrasensitive broad spectral TIs based nanowire photodetector which can be exploited further as a promising material for optoelectronic devices.

Dual-ligand mediated one-pot self-assembly of Cu/ZnO core/shell structures for enhanced microwave absorption

A facile one-pot method has developed to assemble Cu/ZnO core/shell nanocrystals with different aspect ratios for enhanced microwave absorption. Besides, the one-pot method has shown the appreciable yields and no cumbersome multistep operations.