Synthesis of InAl-alloyed Ga2O3 nanowires for self-powered ultraviolet detectors by a CVD method.

Ga2O3 is a kind of wide-band gap semiconductor, which has great potential in deep ultraviolet detection because of its high efficiency and fast response. Doping can improve the photoelectric properties of Ga2O3 materials. In this paper, In and Al elements alloyed Ga2O3 nanowires (InAl-Ga2O3 NWs) were successfully grown on p-GaN using a cost-effective chemical vapor deposition method and a vertical structure. The GaN/InAl-Ga2O3 NWs p-n self-powered wide-gap UV photodetector (PD) was constructed based on sputtered gold film as the bottom and top electrodes, and spin coated with polymethyl methacrylate as the insulating layer in the vertical direction. The GaN/InAl-Ga2O3 UV PD exhibits excellent performances, including an extremely low dark current of 0.015 nA, a maximum photocurrent of about 16 nA at zero-bias voltage under 265 nm illumination, and a light-to-dark current ratio greater than 103. The responsivity is 0.94 mA W-1, the specific detectivity is 9.63 × 109 jones, and the good fast response/attenuation time is 31.2/69.6 ms. The self-powered characteristics are derived from the internal electric field formed between p-type GaN and n-type InAl-Ga2O3 NWs, which is conducive to the rapid separation and transfer of photogenerated carriers. This work provides an innovative mechanism of high-performance metal oxide nanowires for the application of p-n junction photodetectors, which can operate without any external bias.

Ultrahigh Bipolar Photoresponse in a Self-Powered Ultraviolet Photodetector Based on GaN and In/Sn-Doped Ga2O3 Nanowires pn junction.

Self-powered ultraviolet photodetectors with bipolar photoresponse have great potential in the fields of ultraviolet optical communication, all-optical controlled artificial synapses, high-resolution ultraviolet imaging equipment, and multiband photoelectric detection. However, the current low optoelectronic performance limits the development of such polar switching devices. Here, we construct a self-powered ultraviolet photodetector based on GaN and In/Sn-doped Ga2O3 (IGTO) nanowires (NWs) pn junction structure. This unique nanowire/thin film structure allows GaN and IGTO to dominate the absorption of light at different wavelengths, resulting in a highly bipolar photoresponse. The device has a responsivity of 2.04 A/W and a normalized detectivity of 7.18 × 1013 Jones at 254 nm and a responsivity of -2.09 A/W and a normalized detectivity of -7 × 1013 Jones at 365 nm, both at zero bias. In addition, it has an extremely high Ilight/Idark ratio of 1.05 × 105 and ultrafast response times of 2.4/1.9 ms (at 254 nm) and 5.7/5.2 ms (at 365 nm). These excellent properties are attributed to the high specific surface area of the one-dimensional nanowire structure and the abundant voids generated by the nanowire network to enhance the absorption of light, and the p-n junction structure enables the rapid separation and transfer of photogenerated electron-hole pairs. Our findings provide a feasible strategy for high-performance wavelength-controlled polarity switching devices.

Nanoscale detection of carbon dots-induced changes in actin skeleton of neural cells.

Understanding the cytotoxicity of fluorescent carbon dots (CDs) is crucial for their applications, and various biochemical assays have been used to study the effects of CDs on cells. Knowledge on the effects of CDs from a biophysical perspective is integral to the recognition of their cytotoxicity, however the related information is very limited. Here, we report that atomic force microscopy (AFM) can be used as an effective tool for studying the effects of CDs on cells from the biophysical perspective. We achieve this by integrating AFM-based nanomechanics with AFM-based imaging. We demonstrate the performance of this method by measuring the influence of CDs on living human neuroblastoma (SH-SY5Y) cells at the single-cell level. We find that high-dose CDs can mechanically induce elevated normalized hysteresis (energy dissipation during the cell deformation) and structurally impair actin skeleton. The nanomechanical change highly correlates with the alteration of actin filaments, indicating that CDs-induced changes in SH-SY5Y cells are revealed in-depth from the AFM-based biophysical aspect. We validate the reliability of the biophysical observations using conventional biological methods including cell viability test, fluorescent microscopy, and western blot assay. Our work contributes new and significant information on the cytotoxicity of CDs from the biophysical perspective.

Schottky infrared detectors with optically tunable barriers beyond the internal photoemission limit.

Internal photoemission is a prominent branch of the photoelectric effect and has emerged as a viable method for detecting photons with energies below the semiconductor bandgap. This breakthrough has played a significant role in accelerating the development of infrared imaging in one chip with state-of-the-art silicon techniques. However, the performance of these Schottky infrared detectors is currently hindered by the limit of internal photoemission; specifically, a low Schottky barrier height is inevitable for the detection of low-energy infrared photons. Herein, a distinct paradigm of Schottky infrared detectors is proposed to overcome the internal photoemission limit by introducing an optically tunable barrier. This device uses an infrared absorbing material-sensitized Schottky diode, assisted by the highly adjustable Fermi level of graphene, which subtly decouples the photon energy from the Schottky barrier height. Correspondingly, a broadband photoresponse spanning from ultraviolet to mid-wave infrared is achieved, with a high specific detectivity of 9.83 × 1010 cm Hz1/2 W-1 at 2,700 nm and an excellent specific detectivity of 7.2 × 109 cm Hz1/2 W-1 at room temperature under blackbody radiation. These results address a key challenge in internal photoemission and hold great promise for the development of the Schottky infrared detector with high sensitivity and room temperature operation.

Polarity-Tunable Field Effect Phototransistors.

Field-effect phototransistors feature gate voltage modulation, allowing dynamic performance control and significant signal amplification. A field-effect phototransistor can be designed to be inherently either unipolar or ambipolar in its response. However, conventionally, once a field-effect phototransistor has been fabricated, its polarity cannot be changed. Herein, a polarity-tunable field-effect phototransistor based on a graphene/ultrathin Al2O3/Si structure is demonstrated. Light can modulate the gating effect of the device and change the transfer characteristic curve from unipolar to ambipolar. This photoswitching in turn produces a significantly improved photocurrent signal. The introduction of an ultrathin Al2O3 interlayer also enables the phototransistor to achieve a responsivity in excess of 105 A/W, a 3 dB bandwidth of 100 kHz, a gain-bandwidth product of 9.14 × 1010 s-1, and a specific detectivity of 1.91 × 1013 Jones. This device architecture enables the gain-bandwidth trade-off in current field-effect phototransistors to be overcome, demonstrating the feasibility of simultaneous high-gain and fast-response photodetection.

Electrochemical epitaxial PbTe nanowires photodetector for NIR response.

Lead telluride nanowires deposited by electrochemical atomic layers have broad application prospects in the field of photodetectors. In this work, using the method of electrochemical atomic layer deposition, we obtained different morphologies of lead telluride materials by controlling the deposition parameters, such as deposition time, temperature, and potential, and characterized them using SEM, TEM, XPS, and other techniques. A lead telluride nanowire detector with good performance was prepared. The photoresponsivity of the detector is 102 mA W-1, the detectivity is 2.1 × 108Jones, and the response time and recovery time are 0.52 s and 0.54 s respectively at 2.7µm wavelength laser irradiation.

Ultrahigh Photogain Short-Wave Infrared Detectors Enabled by Integrating Graphene and Hyperdoped Silicon.

Highly sensitive short-wave infrared (SWIR) detectors, compatible with the silicon-based complementary metal oxide semiconductor (CMOS) process, are regarded as the key enabling components in the miniaturized system for weak signal detection. To date, the high photogain devices are greatly limited by a large bias voltage, low-temperature refrigeration, narrow response band, and complex fabrication processes. Here, we demonstrate high photogain detectors working in the SWIR region at room temperature, which use graphene for charge transport and Te-hyperdoped silicon (Te-Si) for infrared absorption. The prolonged lifetime of carriers, combined with the built-in potential generated at the interface between the graphene and the Te-Si, leads to an ultrahigh photogain of 109 at room temperature (300 K) for 1.55 µm light. The gain can be improved to 1012, accompanied by a noise equivalent power (NEP) of 0.08 pW Hz-1/2 at 80 K. Moreover, the proposed device exhibits a NEP of 4.36 pW Hz-1/2 at 300 K at the wavelength of 2.7 µm, which is exceeding the working region of InGaAs detectors. This research shows that graphene can be used as an efficient platform for silicon-based SWIR detection and provides a strategy for the low-power, uncooled, high-gain infrared detectors compatible with the CMOS process.

High-efficiency photoreduction of CO2 in a low vacuum.

Photoreduction of CO2 into CO, CH4 or hydrocarbons is attractive, due to environmental compatibility and economic feasibility. Optimizing the reaction engineering of CO2 reduction is an effective and general strategy that should be given special consideration. In this article, the photocatalytic CO2 reduction performances are originally investigated in a low vacuum in both dilute (10%) and pure CO2. We discover that the CH4 yield increased above one hundred times as the vacuum degree increased from barometric pressure to -80 kPa in dilute CO2. It also reveals long-term stability and good cycling performance in a low vacuum. The enhanced CO2 photoreduction performance in a low vacuum comes from better accumulation of photogenerated electrons, less intense Brownian movement of gas molecules in the environment and hindrance of the active site-blocking of gas molecules in the environment. Improved photocatalytic CO2 reduction in a low vacuum is further verified by Pt-TiO2 catalysts. This research presents a general route for producing clean fuels by photocatalytic CO2 reduction in a more effective way.

Lateral Structured Phototransistor Based on Mesoscopic Graphene/Perovskite Heterojunctions.

Due to their outstanding optical properties and superior charge carrier mobilities, organometal halide perovskites have been widely investigated in photodetection and solar cell areas. In perovskites photodetection devices, their high optical absorption and excellent quantum efficiency contribute to the responsivity, even the specific detectivity. In this work, we developed a lateral phototransistor based on mesoscopic graphene/perovskite heterojunctions. Graphene nanowall shows a porous structure, and the spaces between graphene nanowall are much appropriated for perovskite crystalline to mount in. Hot carriers are excited in perovskite, which is followed by the holes' transfer to the graphene layer through the interfacial efficiently. Therefore, graphene plays the role of holes' collecting material and carriers' transporting channel. This charge transfer process is also verified by the luminescence spectra. We used the hybrid film to build phototransistor, which performed a high responsivity and specific detectivity of 2.0 × 103 A/W and 7.2 × 1010 Jones, respectively. To understand the photoconductive mechanism, the perovskite's passivation and the graphene photogating effect are proposed to contribute to the device's performance. This study provides new routes for the application of perovskite film in photodetection.

Interface Engineering of a Silicon/Graphene Heterojunction Photodetector via a Diamond-Like Carbon Interlayer.

Silicon/graphene nanowalls (Si/GNWs) heterojunctions with excellent integrability and sensitivity show an increasing potential in optoelectronic devices. However, the performance is greatly limited by inferior interfacial adhesion and week electronic transport caused by the horizontal buffer layer. Herein, a diamond-like carbon (DLC) interlayer is first introduced to construct Si/DLC/GNWs heterojunctions, which can significantly change the growth behavior of the GNWs film, avoiding the formation of horizontal buffer layers. Accordingly, a robust diamond-like covalent bond with a remarkable enhancement of the interfacial adhesion is yielded, which notably improves the complementary metal oxide semiconductor compatibility for photodetector fabrication. Importantly, the DLC interlayer is verified to undergo a graphitization transition during the high-temperature growth process, which is beneficial to pave a vertical conductive path and facilitate the transport of photogenerated carriers in the visible and near-infrared regions. As a result, the Si/DLC/GNWs heterojunction detectors can simultaneously exhibit improved photoresponsivity and response speed, compared with the counterparts without DLC interlayers. The introduction of the DLC interlayer might provide a universal strategy to construct hybrid interfaces with high performance in next-generation optoelectronic devices.

Hybrid structure of PbS QDs and vertically-few-layer MoS2 nanosheets array for broadband photodetector.

A novel three-dimensional (3D) vertically-few-layer MoS2 (V-MoS2) nanosheets- zero-dimensional PbS quantum dots (QDs) hybrid structure based broadband photodetector was fabricated, and its photoelectric performance was investigated in detail. We synthesized the V-MoS2 nanosheets by chemical vapor deposition, using the TiO2 layer as the induced layer, and proposed a possible growth mechanism. The use of the TiO2 induction layer successfully changed the growth direction of MoS2 from parallel to vertical. The prepared V-MoS2 nanosheets have a large specific surface area, abundantly exposed edges and excellent light absorption capacity. The V-MoS2 nanosheets detector was then fabricated and investigated, which exhibits a high sensitivity for 635 nm light, a fast response time and an excellent photoelectric response. The V-MoS2 nanosheets with a height of approximately 1 µm successfully broke the light absorption limit caused by the atomic thickness. Finally, we fabricated the PbS QDs/V-MoS2 nanosheets hybrid detector and demonstrated their potential for high-performance broadband photodetectors. The response wavelength of the hybrid detector extends from the visible band to the near-infrared band. The responsivity of the hybrid detector reaches 1.46 A W-1 under 1450 nm illumination. The combination of 3D MoS2 nanosheets and QDs further improves the performance of MoS2-based photodetector devices. We believe that the proposed zero-dimensional QDs and 3D vertical nanosheets hybrid structure broadband photodetector provides a promising way for the next-generation optoelectronic devices.

Self-catalyst ß-Ga2O3 semiconductor lateral nanowire networks synthesis on the insulating substrate for deep ultraviolet photodetectors.

Monoclinic gallium oxide (ß-Ga2O3) is a super-wide bandgap semiconductor with excellent chemical and thermal stability, which is an ideal candidate for detecting deep ultraviolet (DUV) radiation (100-280 nm). The growth of ß-Ga2O3 is challenging and most methods require Au as the catalyst and a long reacting time (more than 1 hour). In this work, the self-catalyst ß-Ga2O3 lateral nanowire networks were synthesized on an insulating substrate rapidly by a simple low-cost Chemical Vapor Deposition (CVD) method. A thin film of ß-Ga2O3 nanowire networks was synthesized within a reacting time of 15 minutes, which possesses a huge possibility for the rapid growth of ß-Ga2O3 metal oxide nanowires networks and application in the future solar-blind photodetector. MSM (metal-semiconductor-metal) photodetectors based on the ß-Ga2O3 nanowire networks revealed fast response (on-off ratios is about 103), which is attributed to the unique cross-junction barrier-dominated conductance of the nanowire networks. In addition, the self-catalyst ß-Ga2O3 nanowires grown on insulating SiO2 are achieved and could be expected to find important applications in a bottom-up way of fabricating the next generation semiconductor nanoelectronics.

Role of manganese superoxide dismutase (Mn-SOD) against Cr(III)-induced toxicity in bacteria.

The toxicity of Cr(VI) was widely investigated, but the defense mechanism against Cr(III) in bacteria are seldom reported. Here, we found that Cr(III) inhibited bacterial growth and induced reactive oxygen species (ROS). After exposure to Cr(III), loss of sodA not only led to the excessive generation of ROS, but also enhanced the level of lipid peroxidation and reduced the GSH level, indicating that the deficiency of Mn-SOD decreased the bacterial resistance ability against Cr(III). The adverse effects of oxidative stress caused by Cr(III) could be recovered by the rescue of Mn-SOD in the sodA-deficient strain. Besides the oxidative stress, Cr(III) could cause the bacterial morphology variation, which was distinct between the wild-type and the sodA-deficient strains due to the differential expressions of Z-ring division genes. Moreover, Mn-SOD might prevent Cr(III) from oxidation on the bacterial surface by combining with Cr(III). Taken together, our results indicated that the Mn-SOD played a vital role in regulating the stress resistance, expression of cell division-related genes, bacterial morphology, and chemistry valence state of Cr. Our findings firstly provided a more in-depth understanding of Cr(III) toxicity and bacterial defense mechanism against Cr(III).

Enhancement of the Photoresponse of Monolayer MoS2 Photodetectors Induced by a Nanoparticle Grating.

Photodetectors based on two-dimensional (2D) materials such as monolayer MoS2 are attractive because they can be directly integrated into the current metal-oxide semiconductor (CMOS) structures. Unfortunately, such devices suffer from low responsivity due to low absorption by the monolayer MoS2. Combining MoS2 with plasmonic nanostructures is an alternative solution for enhancing the absorption of the 2D semiconductor, and this can drastically increase the photoresponsivity of the corresponding photodetector. Herein, a device incorporating a grating-patterned nanoparticle structure is fabricated using traditional photolithography together with an annealing step. We demonstrate that this new structure leads to a strong enhancement in the photocurrent due to the coupling of the MoS2 to localized surface plasmons in the nanoparticle grating. Compared to a simple Au nanoparticle array, the nanoparticle grating structure generates a 100% increase in optical absorption. Thus, under 532 nm illumination, the composite nanoparticle grating/monolayer MoS2 integrated photodetector shows a 111-fold increase in the photocurrent compared to the same device in the absence of nanoparticles. The gateless responsivity can be up to 38.57 A/W and a specific detectivity of 9.89 × 109 Jones is realized. Moreover, photothermal flux derivations indicate that, in addition to the expected increase due to light-generated carrier multiplication, the thermal effects of plasmons provide a significant contribution to the photocurrent enhancement.

Microwave plasma assisted reduction synthesis of hexagonal cobalt nanosheets with enhanced electromagnetic performances.

In this study, we employed a microwave plasma assisted reduction (MPAR) method to prepare metallic nanoparticles with desirable morphology. Compared with the hydrogen thermal reduction technique, the MPAR technique could greatly maintain the original morphology of self-sacrificing precursors, as well as proving to be highly efficient, energy-saving and pollution-free. Taking ferromagnetic metallic Co as a forerunner, Co nanosheets with inerratic hexagonal morphology were successfully synthesized on a large scale uniformly. The lateral dimension of the achieved Co nanosheets is in the range of 3∼5 µm with tens of nanometers in thickness. The intact hexagonal flaky shape of Co nanosheets is beneficial for improving dielectric loss by increasing electric channels and interfacial polarization. Consequently, the minimum reflection loss could reach up to -71 dB at a thin thickness of 1.2 mm. Furthermore, the effective bandwidth (RL < -10 dB) could be achieved in a wide range of 2.8∼18 GHz by integrating the thickness from 5.0∼1.0 mm, which provides the possibility for applications in electromagnetic shielding and radar stealth fields. It is believed that the MPAR technique is suitable for designing and preparing novel microwave absorbers on the basis of appropriate precursors, providing new opportunities to acquire high-performance microwave absorbers in the future.

Light Trapping in Conformal Graphene/Silicon Nanoholes for High-Performance Photodetectors.

Hybrid graphene/silicon heterojunctions have been widely utilized in photodetectors because of their unique characteristics of high sensitivity, fast response, and CMOS compatibility. However, the photoresponse is restricted by the high reflectance of planar silicon (up to 50%). Herein, an improved graphene/Si detector with excellent light absorption performance is proposed and demonstrated by directly growing graphene on the surface of silicon nanoholes (SiNHs). It is shown that the combination of SiNHs with conformal graphene provides superior interfaces for efficient light trapping and transport of the photoexcited carriers. A high absorption of up to 90% was achieved, and the conformal graphene/SiNH-based photodetectors exhibited a higher photoresponsivity (2720 A/W) and faster response (∼6.2 µs), compared with the counterpart of the planar graphene/Si, for which the corresponding values are 850 A/W and 51.3 µs. These results showcase the vital role of the material morphology in optoelectronic conversion and pave the way to explore novel high-performance heterojunction photodetectors.

Facile and Controllable Synthesis of Large-Area Monolayer WS2 Flakes Based on WO3 Precursor Drop-Casted Substrates by Chemical Vapor Deposition.

Monolayer WS2 (Tungsten Disulfide) with a direct-energy gap and excellent photoluminescence quantum yield at room temperature shows potential applications in optoelectronics. However, controllable synthesis of large-area monolayer WS2 is still challenging because of the difficulty in controlling the interrelated growth parameters. Herein, we report a facile and controllable method for synthesis of large-area monolayer WS2 flakes by direct sulfurization of powdered WO3 (Tungsten Trioxide) drop-casted on SiO2/Si substrates in a one-end sealed quartz tube. The samples were thoroughly characterized by an optical microscope, atomic force microscope, transmission electron microscope, fluorescence microscope, photoluminescence spectrometer, and Raman spectrometer. The obtained results indicate that large triangular monolayer WS2 flakes with an edge length up to 250 to 370 µm and homogeneous crystallinity were readily synthesized within 5 min of growth. We demonstrate that the as-grown monolayer WS2 flakes show distinctly size-dependent fluorescence emission, which is mainly attributed to the heterogeneous release of intrinsic tensile strain after growth.

Hybrid graphene heterojunction photodetector with high infrared responsivity through barrier tailoring.

The graphene/Si heterojunction is attractive for high gain and broadband photodetection through photogating effect. However, the photoresponsivity in these devices are still limited to under 1 A W-1 if no narrowband absorption-enhanced nanostructures were used. In this paper, the effects of barriers on photoresponse are systematically studied at 1550 nm wavelength. Different barrier heights are obtained through selection of substrates, graphene doping and electrical tuning. Lower barrier height for graphene side and higher barrier height for silicon side are found to be beneficial for better infrared photoresponse. Through Polyetherimide doping of graphene and back-gated electrical modulation, the responsivity finally reached 5.71 A W-1, which to our knowledge is among the best results for graphene-based infrared photodetectors with graphene adopted as a light-absorption material. It is found that the thermionic emission efficiency of indirect transition in graphene is related to the difference in emissioin barrier height, and the lifetime of photoinduced carriers in the channel can be enhanced by built-in potential. These results lay the foundation for the photodetection applicatioins of graphene/Si heterojunction in the longer-wavelength infrared region.

Fabrication of PbS QDs/Graphene Heterostructure Photoelectrochemical Cell by Electrochemical Atomic Layer Epitaxy Method.

In this work, PbS quantum dots were for the first time directly deposited on graphene nanowalls substrate by electrochemical atomic layer epitaxy (ECALE) deposition method. The morphology, composition and 3D structure of PbS/graphene were characterized by field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD) and Raman spectrum, respectively. The sensitized three-dimensional graphene electrode with PbS quantum dots had great improvement in the performance of photo-electrochemical (PEC), with photocurrent almost four times that of pure graphene film, which presents its great application potential in the fields of photodetector or solar cell.

Synthesis of ternary oxide Zn2GeO4 nanowire networks and their deep ultraviolet detection properties.

Ternary oxide Zn2GeO4 with a wide bandgap of 4.84 eV, as a candidate for fourth generation semiconductors, has attracted a great deal of attention for deep ultraviolet (DUV) photodetector applications, because it is expected to be blind to the UV-A/B band (290-400 nm) and only responsive to the UV-C band (200-290 nm). Here, we report on the synthesis of Zn2GeO4 nanowire (NW) networks by lower pressure chemical vapor deposition and investigate their corresponding DUV detection properties. We find that pure Zn2GeO4 NWs could be obtained at a growth pressure of 1 kPa. The DUV detection tests reveal that growth pressure exerts a significant effect on DUV detection performance. The Zn2GeO4 NW networks produced under 1 kPa show an excellent solar-blind photoresponsivity with fast rise and decay times (t rise ≈ 0.17 s and t decay ≈ 0.14 s).