Ligong Zhang

Fast Photoconductive Responses in Organometal Halide Perovskite Photodetectors

Inorganic semiconductor-based photodetectors have been suffering from slow response speeds, which are caused by the persistent photoconductivity of semiconductor materials. For realizing high speed optoelectronic devices, the organometal halide perovskite thin films were applied onto the interdigitated (IDT) patterned Au electrodes, and symmetrical structured photoconductive detectors were achieved. The detectors were sensitive to the incident light signals, and the photocurrents of the devices were 2-3 orders of magnitude higher than dark currents. The responsivities of the devices could reach up to 55 mA W(1-). Most importantly, the detectors have a fast response time of less than 20 μs. The light and bias induced dipole rearrangement in organometal perovskite thin films has resulted in the instability of photocurrents, and Ag nanowires could quicken the process of dipole alignment and stabilize the photocurrents of the devices.

Direct colorimetric study on the interaction of Escherichia coli with mannose in polydiacetylene Langmuir-Blodgett films

Abstract The membranes of polydiacetylene backbone decorated with mannose assembled by Langmuir–Blodgett technology can interact with Escherichia coli . The interactions lead to the color transition of the membranes which was readily visible to the naked eyes and could be quantified by visible absorption spectroscopy. To understand the mechanism of the chromatic transition, the affinochromism properties of polydiacetylene were examined by resonance Raman spectroscopy. The results demonstrated that the side chains of polymer backbone performed rearrangement, and the electronic structure in the polymer backbone changed from acetylene to butatriene form when the chromatic transformation from blue to red. The direct colorimetric detection by polydiacetylene membranes not only opens a new path for the use of these membranes in the area of biosensor development but also offers new possibilities for diagnostic applications and screening for binding ligand.

Influence of Exciton Localization on the Emission and Ultraviolet Photoresponse of ZnO/ZnS Core-Shell Nanowires.

The structural and optical properties of ZnO and ZnO/ZnS core-shell nanowires grown by a wet chemical method are investigated. The near-bandgap ultraviolet (UV) emission of the ZnO nanowires was enhanced by four times after coating with ZnS. The enhanced emission was attributed to surface passivation of the ZnO nanowires and localized states introduced during ZnS growth. The emission of the ZnO and ZnO/ZnS core-shell nanowires was attributed to neutral donor-bound excitons and localized excitons, respectively. Localized states prevented excitons from diffusing to nonradiative recombination centers, so therefore contributed to the enhanced emission. Emission from the localized exciton was not sensitive to temperature, so emission from the ZnO/ZnS core-shell nanowires was more stable at higher temperature. UV photodetectors based on the ZnO and ZnO/ZnS core-shell nanowires were fabricated. Under UV excitation, the device based on the ZnO/ZnS core-shell nanowires exhibited a photocurrent approximately 40 times higher than that of the device based on the ZnO nanowires. The differing photoresponse of the detectors was consistent with the existence of surface passivation and localized states. This study provides a means for modifying the optical properties of ZnO materials, and demonstrates the potential of ZnO/ZnS core-shell nanowires in UV excitonic emission and detection.

Influencing Mechanism of the Selenization Temperature and Time on the Power Conversion Efficiency of Cu2ZnSn(S,Se)4-Based Solar Cells.

Cu2ZnSn(S,Se)4 (CZTSSe) films were deposited on the Mo-coated glass substrates, and the CZTSSe-based solar cells were successfully fabricated by a facile solution method and postselenization technique. The influencing mechanisms of the selenization temperature and time on the power conversion efficiency (PCE), short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) of the solar cell are systematically investigated by studying the change of the shunt conductance (Gsh), series resistance (Rs), diode ideal factor (n), and reversion saturation current density (J0) with structure and crystal quality of the CZTSSe film and CZTSSe/Mo interface selenized at various temperatures and times. It is found that a Mo(S1-x,Sex)2 (MSSe) layer with hexagonal structure exists at the CZTSSe/Mo interface at the temperature of 500 °C, and its thickness increases with increasing selenization temperature and time. The MSSe has a smaller effect on the Rs, but it has a larger influence on the Gsh, n, and J0. The PCE, Voc, and FF change dominantly with Gsh, n, and J0, while Jsc changes with Rs and Gsh, but not Rs. These results suggest that the effect of the selenization temperature and time on the PCE is dominantly contributed to the change of the CZTSSe/CdS p-n junction and CZTSSe/MSSe interface induced by variation of the quality of the CZTSSe film and thickness of MSSe in the selenization process. By optimizing the selenization temperature and time, the highest PCE of 7.48% is obtained.

Gas sensing properties of asymmetrically substituted phthalocyanines bearing one crown ether ring

The gas sensing-properties of phthalocyanine bearing one crown ether ring are investigated using different thin film deposition methods. Those well ordered and tightly packed LB films display slower response and reversal time while an incompact spin-coated film reveals that both response and reversal are much faster and complete with good reproducibility and high sensitivity. These results indicate that films prepared by different fabrication techniques show different response and reversal kinetics.

A novel amphiphilic zinc phthalocyanine LB flms as gas sensor material and its interaction with NH3

Abstract Two kinds of amphiphilic phthalocyanines(AmPc,AmPcZn) have been deposited as smooth and well ordered LB films. Absorption spectra show that AmPc films are consistent with a stacked cofacial columnar structure while the AmPcZn films consist of aggregates with weakly interacting molecules. These two films exhibit different behavior toward NH3 gas-sensing. AmPcZn films gave strong and fast response on exposure to NH3 whereas AmPc films showed very weak response. It is proposed that coordination of NH3 by AmPcZn results in lowering the energy gap and hence the activation energy of AmpcZn that account for the larger increase in conductance of AmpcZn films compared with AmPc films.

Identification of degradation mechanisms of blue InGaN/GaN laser diodes

A comprehensive analysis of the degradation mechanism of blue InGaN/GaN laser diodes (LDs) is carried out by investigating the electrical and optical characteristics. The increase in the leakage current as well as decrease in the slope efficiency is observed. The luminescence properties of the active region at different aging stages are studied by means of cathodoluminescence. Significant degradation of the active region is observed on the room temperature cathodoluminescence while the low temperature cathodoluminescence shows almost no degradation, indicating that the degradation of the LDs is due to generation of low temperature frozen point defects. Furthermore, the generation of the defects follows a kinetic mechanism enhanced by electron-hole non-radiative recombination which explains the acceleration of time degradation in our LDs.

Effect of annealing on photoluminescence properties of neon implanted GaN

The effect of thermal annealing on the luminescence properties of neon implanted GaN thin films was studied. Low temperature photoluminescence (PL) measurements were carried out on the samples implanted with different doses ranging from 10(14) to 9 x 10(15) cm(-2) and annealed isochronally at 800 and 900 degrees C. We observed a new peak appearing at 3.44 eV in the low temperative PL spectra of all the implanted samples after annealing at 900 degrees C. This peak has not been observed in the PL spectra of implanted samples annealed at 800 degrees C except for the samples implanted with the highest dose. The intensity of the yellow luminescence (YL) band noticed in the PL spectra measured after annealing was observed to decrease with the increase in dose until it was completely suppressed at a dose of 5 x 10(15) cm(-2). The appearance of a new peak at 3.44 eV and dose dependent suppression of the YL band are attributed to the dissociation of VGaON complexes caused by high energy ion implantation.

Shallow Acceptor State in Mg-Doped CuAlO2 and Its Effect on Electrical and Optical Properties: An Experimental and First-Principles Study

Shallow acceptor states in Mg-doped CuAlO2 and their effect on structural, electrical, and optical properties are investigated by combining first-principles calculations and experiments. First-principles calculations demonstrate that Mg substituting at the Al site in CuAlO2 plays the role of shallow acceptor and has a low formation energy, suggesting that Mg doping can increase hole concentration and improve the conductivity of CuAlO2. Hall effect measurements indicate that the hole concentration of the Mg-doped CuAlO2 thin film is 2 orders of magnitude higher than that of undoped CuAlO2. The best room temperature conductivity of 8.0 × 10-2 S/cm is obtained. A band gap widening is observed in the optical absorption spectra of Mg-doped CuAlO2, which is well supported by the results from first-principles electronic structure calculations.

Investigation of Interface Effect on the Performance of CH3NH3PbCl3/ZnO UV Photodetectors

Recent investigations indicate that the performance of organic-inorganic perovskite optoelectronic devices can be improved by combining the perovskites and the inorganic materials. However, very few studies have focused on the investigation of perovskites/inorganic semiconductor hybrid UV photodetectors and their detailed performance-enhancement mechanism is still not very clear. In this work, a CH3NH3PbCl3/ZnO UV photodetector has been first demonstrated and investigated. Both the photoresponsivity and response speed of the hybrid device are higher than those of pure CH3NH3PbCl3 and ZnO devices. The photoluminescence and transient absorption spectra indicate that the photoinduced electron transfer between CH3NH3PbCl3 and ZnO should be responsible for the performance enhancement of the hybrid device. In addition, the high crystal quality of CH3NH3PbCl3 on ZnO film is another important reason for the excellent UV detection performance. Our findings in this work provide new insights into the intrinsic photophysics essential for perovskite optoelectronic devices.