Dezhong Zhang

Visible-light photodetector with enhanced performance based on a ZnO@CdS heterostructure

In this article, the heterostructure of ZnO particles on single-crystal CdS nanowires (ZnO@CdS) has been successfully synthesized via a facile two-step solvothermal process. The appealing application of the ZnO@CdS heterostructure as visible-light photodetector (PD) is presented. Photocurrent illuminated with light (shorter than 510 nm) to dark-current ratio of structurally-optimized ZnO@CdS nanomaterials based photon detector was enhanced significantly compared to the value of the pristine CdS nanowires based one. The corresponding mechanism for the phenomenon was discussed. Additionally, measurements of time resolved responses were conducted. The ZnO@CdS heterostructure based device kept a fast rise (5 ms) and decay (10 ms) speed to irradiation. This work demonstrates a promising application of ZnO@CdS heterostructure based visible-light detectors with high photocurrent/dark-current ratio, ultrafast time response and very good stability.

Gas Sensors Based on Metal Sulfide Zn1–xCdxS Nanowires with Excellent Performance

Metal sulfide Zn1-xCdxS nanowires (NWs) covering the entire compositional range prepared by one step solvothermal method were used to fabricate gas sensors. This is the first time for ternary metal sulfide nanostructures to be used in the field of gas sensing. Surprisingly, the sensors based on Zn1-xCdxS nanowires were found to exhibit enhanced response to ethanol compared to those of binary CdS and ZnS NWs. Especially for the sensor based on the Zn1-xCdxS (x = 0.4) NWs, a large sensor response (s = 12.8) and a quick rise time (2 s) and recovery time (1 s) were observed at 206 °C toward 20 ppm ethanol, showing preferred selectivity. A dynamic equilibrium mechanism of oxygen molecules absorption process and carrier intensity change in the NWs was used to explain the higher response of Zn1-xCdxS. The reason for the much quicker response and recovery speed of the Zn1-xCdxS NWs than those of the binary ZnS NWs was also discussed. These results demonstrated that the growth of metal sulfide Zn1-xCdxS nanostructures can be utilized to develop gas sensors with high performance.

Xylene gas sensor based on Ni doped TiO2 bowl-like submicron particles with enhanced sensing performance

In this work, novel bowl-like TiO2 submicron scale particles were prepared via a simple electrospray technique combined with high-temperature calcination. The morphologies of the particles are easily controlled by changing the TBT content (16 wt%, 23 wt%, 30 wt%, 37 wt%) in the precursor solutions. To improve the xylene sensing properties of the TiO2, appropriate Ni amounts (0, 2, 4, and 6 mol% doping) were doped into the bowl-like particles. Among them, the 2 mol% Ni doped TiO2 bowl-like particles show the lowest optimum working temperature and highest response while demonstrating a fast response (9 s) and recovery speed (1.2 s) to 100 ppm xylene gas.

Fast Decay Time and Low Dark Current Mechanism in TiO 2 Ultraviolet Detector

In this letter, TiO2 thin films were prepared via sol-gel method and metal/semiconductor/metal ultraviolet (UV) detectors with Pt Schottky contact were fabricated. At 5 V bias, the dark current of the device was only 80 pA. Diffusion theory was adopted to analyze the low dark current mechanism, which is consistent with the experimental results. The device shows a remarkably reduced decay time of 41.53 ms. The low dark current and improved time response performance may be attributed to the high effective Schottky barrier between Pt and TiO2 film. High responsivity of 34.5 A/W was achieved at 300 nm UV light and the ratio of photocurrent to dark current is about five orders of magnitude, which is much larger than that of other semiconductor photodetectors.

Enhanced performance of a TiO2 ultraviolet detector modified with graphene oxide

The performance of a Schottky metal–semiconductor–metal (MSM) ultraviolet (UV) photodetector is limited by the insufficient gain and the uncontrollable noise current. A remarkable detectivity UV detector is demonstrated based on graphene oxide (GO) modified TiO2 with high gain and low noise. The GO layer completely prevents the flow of electrons forming a hole-only device thus decreasing the dark current and noise current. Furthermore, gain of the holes is promoted under UV illumination. Moreover, the GO layer efficiently extracts the holes therefore reducing the fall time. Under a bias of 6 V, the responsivity value reaches 826.8 A W−1 and the noise current is only 1.8 pA, thereby, our device provides a detectivity of 2.82 × 1013 cm Hz1/2 W−1 at 280 nm. The results offer an effective approach to enhance the performance of a UV detector.

Fluorinated photopolymer thermo-optic switch arrays with dielectric-loaded surface plasmon polariton waveguide structure

Novel polymer thermo-optic switch arrays were successfully designed and fabricated based on dielectric-loaded surface plasmon polariton waveguide (DLSPPW) structure. Highly fluorinated low-loss photopolymers and organic–inorganic grafting materials were used as the waveguide core and cladding, respectively. The low absorption loss characteristics and excellent thermal stabilities of the core and cladding materials were obtained. The proposed DLSPPW model was included of fluorinated polymer ridge with 4 × 4 μm2 size loaded on 60-nm thin gold stripe electrode heaters, organic-inorganic grafting material cladding and PMMA substrate. The operation of the device at signal wavelengths is controlled via the thermo-optic effect by electrically heating the gold stripes of dielectric-loaded surface plasmon polariton waveguides. The optimized structural properties of dielectric-loaded surface plasmon polariton waveguides were provided. The propagation loss of a 4-μm wide straight DLSPPW was measured as 0.55 dB∕cm at 1550 nm wavelength. The insertion loss of the device was measured to be about 4.5 dB. The switching rise and fall time of the device applied by 200 Hz square-wave voltage were obtained as 287.5 μs and 370.2 μs, respectively. The switching power was about 5.6 mW, and the extinction ratio was about 13.5 dB. The flexible low-loss multi-functional waveguide switch arrays are suitable for realizing large-scale optoelectronic integrated circuits.

Fast polymer thermo-optic switch with silica under-cladding

A polymer/silica hybrid 2 × 2 multimode-interference Mach-Zehnder interferometer thermo-optic (TO) switch is designed and fabricated. Silica is adopted as the under-cladding to accelerate heat release for its large thermal conductivity. The fabricated switch exhibits low power consumption of 6.2 mW, low crosstalk of -28 dB, and fast speed. The rise and fall times of this hybrid switch are 103 and 91 μs, respectively. Comparing with the fabricated TO switch (174 and 191 μs) using polymer as both upper-cladding and under-cladding, response times of this hybrid one are shortened by 40.8% and 52.4%, respectively.

Facilitated extrinsic majority carrier depletion and photogenerated exciton dissociation in an annealing-free ZnO:C photodetector

Applications of ZnO in photodetectors are limited by the great quantity of extrinsic majority carriers due to structural defects and difficult exciton dissociation due to the large exciton binding energy; these generally lead to a higher dark current (Id) and lower light current (Il), severely degrading the responsivity and detectivity. C dots are incorporated into an annealing-free ZnO layer to innovatively construct a local built-in electric field (Ebi) using the difference in the work functions; this simultaneously overcomes the drawbacks of the pristine ZnO photosensitive layer. In dark, the extrinsic majority carrier of ZnO is depleted around the incorporated C dots due to the self-depleting effect; thus, the Id decreases. Under UV illumination, the photogenerated exciton driven by the local Ebi is easily dissociated into a free charge carrier, contributing to the improved Il. This study paves a universal way to effectively improve the detection characteristics of photoconductive devices by incorporating the local Ebi.

Decreased Charge Transport Barrier and Recombination of Organic Solar Cells by Constructing Interfacial Nanojunction with Annealing-Free ZnO and Al Layers

To overcome drawbacks of the electron transport layer, such as complex surface defects and unmatched energy levels, we successfully employed a smart semiconductor-metal interfacial nanojunciton in organic solar cells by evaporating an ultrathin Al interlayer onto annealing-free ZnO electron transport layer, resulting in a high fill factor of 73.68% and power conversion efficiency of 9.81%. The construction of ZnO-Al nanojunction could effectively fill the surface defects of ZnO and reduce its work function because of the electron transfer from Al to ZnO by Fermi level equilibrium. The filling of surface defects decreased the interfacial carrier recombination in midgap trap states. The reduced surface work function of ZnO-Al remodulated the interfacial characteristics between ZnO and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM), decreasing or even eliminating the interfacial barrier against the electron transport, which is beneficial to improve the electron extraction capacity. The filled surface defects and reduced interfacial barrier were realistically observed by photoluminescence measurements of ZnO film and the performance of electron injection devices, respectively. This work provides a simple and effective method to simultaneously solve the problems of surface defects and unmatched energy level for the annealing-free ZnO or other metal oxide semiconductors, paving a way for the future popularization in photovoltaic devices.

41-Channel arrayed waveguide grating multiplexer using UV curable polymer operating around 1550nm

In this paper, a polymeric 41-channel AWG multiplexer was fabricated by simple direct ultraviolet photolithography process. We try to use a cross-linkable negative photoresist as core material and an optically clear UV curable epoxy as cladding material. Once exposed to ultraviolet light through a photomask, the polymeric waveguide stripes were obtained upon development. The propagation loss is low as 2.03 dB/cm at 1550 nm wavelength. The AWG multiplexer has channel spacing of 0.789 nm and the center wavelength is close to 1550 nm. The new technique will be well suited for fabricating optical integrated circuits (OICS).