Self-powered deep ultraviolet photodetector based on p-CuI/n-ZnGa2O4 heterojunction with high sensitivity and fast speed.

Self-powered deep ultraviolet photodetectors (DUV PDs) are essential in environmental monitoring, flame detection, missile guidance, aerospace, and other fields. A heterojunction photodetector based on p-CuI/n-ZnGa2O4 has been fabricated by pulsed laser deposition combined with vacuum thermal evaporation. Under 260 nm DUV light irradiation, the photodetector exhibits apparent self-powered performance with a maximum responsivity and specific detectivity of 2.75 mA/W and 1.10 × 1011 Jones at 0 V. The photodetector exhibits high repeatability and stability under 260 nm periodic illumination. The response and recovery time are 205 ms and 133 ms, respectively. This work provides an effective strategy for fabricating high-performance self-powered DUV photodetectors.

Highly stable mixed-phase Cs-Cu-I films with tunable optoelectronic properties for UVB photodetector applications.

Ultraviolet (UV) photodetector plays an important role in military, civilian and people's daily life, and is an indispensable part of spectral detection. However, photodetectors target at the UVB region (280-320 nm) are rarely reported, and the devices detected by medium-wave UV light generally have problems such as low detection rate, low sensitivity, and poor stability, which are difficult to meet the market application needs. Herein, Cs-Cu-I films with mixed-phase have been prepared by vacuum thermal evaporation. By adjusting the proportion of evaporation sources (CsI and CuI), the optical bandgaps of mixed-phase Cs-Cu-I films can be tuned between 3.7 eV and 4.1 eV. This absorption cut-off edge is exactly at both ends of the UVB band, which indicating its potential application in the field of UVB detection. Finally, the photodetectors based on Cs-Cu-I/n-Si heterojunction are fabricated. The photodetector shows good spectral selectivity for UVB band, and has a photoresponsivity of 22 mA/W, a specific detectivity of 1.83*1011 Jones, an EQE over 8.7% and an on/off ratio above 20.

Pressure-induced distinct excitonic properties of 2D perovskites with isomeric organic molecules for spacer cations.

Spacer organic cations in two-dimensional (2D) perovskites play vital roles in inducing structural distortion of the inorganic components and dominating unique excitonic properties. However, there is still little understanding of spacer organic cations with identical chemical formulas, and different configurations have an impact on the excitonic dynamics. Herein, we investigate and compare the evolution of the structural and photoluminescence (PL) properties of [CH3(CH2)4NH3]2PbI4 ((PA)2PbI4) and [(CH3)2CH(CH2)2NH3]2PbI4 ((PNA)2PbI4) with isomeric organic molecules for spacer cations by combining steady-state absorption, PL, Raman and time-resolved PL spectra under high pressures. Intriguingly, the band gap is continuously tuned under pressure and decreased to 1.6 eV at 12.5 GPa for (PA)2PbI4 2D perovskites. Simultaneously, multiple phase transitions occur and the carrier lifetimes are prolonged. In contrast, the PL intensity of (PNA)2PbI4 2D perovskites exhibits an almost 15-fold enhancement at 1.3 GPa and an ultrabroad spectral range of up to 300 nm in the visible region at 7.48 GPa. These results indicate that the isomeric organic cations (PA+ and PNA+) with different configurations significantly mediate distinct excitonic behaviors due to different resilience to high pressures and reveal a novel interaction mechanism between organic spacer cations and inorganic layers under compression. Our findings not only shed light on the vital roles of isomeric organic molecules as organic spacer cations in 2D perovskites under pressure, but also open a route to rationally design highly efficient 2D perovskites incorporating such spacer organic molecules in optoelectronic devices.

Smart Adhesives via Magnetic Actuation.

Smart adhesives possess a wide range of applications owing to their reversibly and repeatedly switchable adhesion in transfer technology. Despite recent advances, it still remains a technical and scientific challenge to achieve strategies for rapidly tunable adhesion in a noncontact manner. In this study, a smart adhesive to achieve dynamically tunable adhesion is developed. Specifically, a mushroom-shaped adhesive with a magnetized tip is actuated to reversibly and rapidly transform the morphology via magnetic actuation. The smart adhesive has two working modes, namely, selective pickup mode and pick-and-place mode. In the selective pickup mode, the external magnetic field is applied and the tip undergoes bending deformation. Changes in tip morphology allow for a reversible switch of the adhesion between "turn on" and "turn off." In the pick-and-place mode, the external magnetic field is applied when the target object needs to be released. Upward bending deformation of the micro-beam, a part of the tip, creates an initial crack at the edge of the adhesion interface. The propagation of the edge crack modulates the adhesion from strong to weak and the target object is instantly released. The proposed smart adhesive may be of interest for practical applications demanding highly precise and swiftly controlled movements.

Realization of Ultrahigh Quality InGaN Platelets to be Used as Relaxed Templates for Red Micro-LEDs.

In this work, arrays of predominantly relaxed InGaN platelets with indium contents of up to 18%, free from dislocations and offering a smooth top c-plane, are presented. The InGaN platelets are grown by metal-organic vapor phase epitaxy on a dome-like InGaN surface formed by chemical mechanical polishing of InGaN pyramids defined by 6 equivalent {101̅1} planes. The dome-like surface is flattened during growth, through the formation of bunched steps, which are terminated when reaching the inclined {101̅1} planes. The continued growth takes place on the flattened top c-plane with single bilayer surface steps initiated at the six corners between the c-plane and the inclined {101̅1} planes, leading to the formation of high-quality InGaN layers. The top c-plane of the as-formed InGaN platelets can be used as a high-quality template for red micro light-emitting diodes.

Facile Preparation of Stable Solid-State Carbon Quantum Dots with Multi-Peak Emission.

Aggregation-caused quenching (ACQ) effect, known as the main cause to restrain solid-state luminescence of carbon quantum dots (CQDs), hinders further application of CQDs in white light-emitting diodes (WLED). Here, a complex of CQDs and phthalimide crystals (CQDs/PC) was prepared through a one-step solvothermal method. CQDs/PC prevented CQDs from touching directly by embedding the CQDs in phthalimide crystal matrix in situ, which effectively reduced the ACQ effect. Furthermore, CQDs/PC exhibited multi-peak fluorescence spectra that span the green, yellow and orange spectral regions. Finally, a WLED fabricated based on CQDs/PC achieved a color-rendering index of 82 and a correlated color temperature of 5430 K. This work provides a quick and effective strategy to apply CQDs to WLED.

Improving the internal quantum efficiency of QD/QW hybrid structures by increasing the GaN barrier thickness.

Three InGaN/GaN quantum well (QW) samples with different barrier thickness (Sample A: 15 nm, Sample B: 17.5 nm, and Sample C: 20 nm) were grown via a metal organic chemical vapor deposition (MOCVD) system. The InGaN/GaN QWs became QD/QW hybrid structures due to the high density of V-shaped pits (VPs), which cut the InGaN wells into InGaN quantum dots (QDs) and indium-rich (In-rich) QDs stemming from the indium phase separation. By increasing the thickness of GaN barriers, the interactions between InGaN wells are weakened; thus, the strain accumulation is relieved and the strain relaxation degree decreases. Abnormally, the residual internal strain first increased due to least VPs in B and then decreased for C. Lower internal strain weakens the strain-induced piezoelectric polarization effect and as a result, a higher electron-hole wave function overlap and radiative recombination efficiency are improved. Similarly, lower strain relaxation results in more homogeneous indium distribution, and accordingly, a slightly weaker carrier localization effect (CLE). The CLEs of the three samples are strong enough that carriers can be confined by localized states even at room temperature; thus, the slightly weaker CLE does not influence the internal quantum efficiency (IQE). More importantly, InGaN QDs or QWs with lower strain relaxation contain fewer stacking faults that can act as non-radiative recombination centers (NRRCs), improving the IQE. By analyzing the effects of strain-induced piezoelectric polarization, NRRCs and carrier localization on the IQE, it is found that less NRRCs are a major factor in improving the IQE of these QD/QW hybrid structures.

InGaN Platelets: Synthesis and Applications toward Green and Red Light-Emitting Diodes.

In this work, we present a method to synthesize arrays of hexagonal InGaN submicrometer platelets with a top c-plane area having an extension of a few hundred nanometers by selective area metal-organic vapor-phase epitaxy. The InGaN platelets were made by in situ annealing of InGaN pyramids, whereby InGaN from the pyramid apex was thermally etched away, leaving a c-plane surface, while the inclined {101̅1} planes of the pyramids were intact. The as-formed c-planes, which are rough with islands of a few tens of nanometers, can be flattened with InGaN regrowth, showing single bilayer steps and high-quality optical properties (full width at half-maximum of photoluminescence at room temperature: 107 meV for In0.09Ga0.91N and 151 meV for In0.18Ga0.82N). Such platelets offer surfaces having relaxed lattice constants, thus enabling shifting the quantum well emission from blue (as when grown on GaN) to green and red. For single InGaN quantum wells grown on the c-plane of such InGaN platelets, a sharp interface between the quantum well and the barriers was observed. The emission energy from the quantum well, grown under the same conditions, was shifted from 2.17 eV on In0.09Ga0.91N platelets to 1.95 eV on In0.18Ga0.82N platelets as a result of a thicker quantum well and a reduced indium pulling effect on In0.18Ga0.82N platelets. On the basis of this method, prototype light-emitting diodes were demonstrated with green emission on In0.09Ga0.91N platelets and red emission on In0.18Ga0.82N platelets.

Surface Morphology Evolution Mechanisms of InGaN/GaN Multiple Quantum Wells with Mixture N2/H2-Grown GaN Barrier.

Surface morphology evolution mechanisms of InGaN/GaN multiple quantum wells (MQWs) during GaN barrier growth with different hydrogen (H2) percentages have been systematically studied. Ga surface-diffusion rate, stress relaxation, and H2 etching effect are found to be the main affecting factors of the surface evolution. As the percentage of H2 increases from 0 to 6.25%, Ga surface-diffusion rate and the etch effect are gradually enhanced, which is beneficial to obtaining a smooth surface with low pits density. As the H2 proportion further increases, stress relaxation and H2 over- etching effect begin to be the dominant factors, which degrade surface quality. Furthermore, the effects of surface evolution on the interface and optical properties of InGaN/GaN MQWs are also profoundly discussed. The comprehensive study on the surface evolution mechanisms herein provides both technical and theoretical support for the fabrication of high-quality InGaN/GaN heterostructures.

Effect of hydrogen treatment temperature on the properties of InGaN/GaN multiple quantum wells.

InGaN/GaN multiple quantum wells (MQWs) were grown with hydrogen treatment at well/barrier upper interface under different temperatures. Hydrogen treatment temperature greatly affects the characteristics of MQWs. Hydrogen treatment conducted at 850 °C improves surface and interface qualities of MQWs, as well as significantly enhances room temperature photoluminescence (PL) intensity. In contrast, the sample with hydrogen treatment at 730 °C shows no improvement, as compared with the reference sample without hydrogen treatment. On the basis of temperature-dependent PL characteristics analysis, it is concluded that hydrogen treatment at 850 °C is more effective in reducing defect-related non-radiative recombination centers in MQWs region, yet has little impact on carrier localization. Hence, hydrogen treatment temperature is crucial to improving the quality of InGaN/GaN MQWs.

Influence of substrate misorientation on the photoluminescence and structural properties of InGaAs/GaAsP multiple quantum wells.

InGaAs/GaAsP multiple quantum wells (MQWs) were grown by metal-organic chemical vapor deposition on vicinal GaAs (001) substrates with different miscut angles of 0°, 2° and 15° towards [110]. The crystal structures of InGaAs/GaAsP were characterized by high-resolution X-ray diffraction and Raman spectroscopy. The surface morphologies of InGaAs/GaAsP MQWs were observed by atomic force microscopy. The mechanisms for step flow, step bunching and pyramid growth on 0°, 2° and 15° misoriented substrates were discussed. The results provide a comprehensive phenomenological understanding of the self-ordering mechanism of vicinal GaAs substrates, which could be harnessed for designing the quantum optical properties of low-dimensional systems. From low-temperature photoluminescence, it was observed that the luminescence from the MQWs grown on a vicinal surface exhibits a red-shift with respect to the 0° case. An extra emission was observed from the 2° and 15° off samples, indicating the characteristics of quantum wire and pyramidal self-controlled quantum-dot systems, respectively. Its absence from the PL spectrum on 0° surfaces indicates that indium segregation is modified on the surfaces. The relationship between InGaAs/GaAsP MQWs grown on vicinal substrates and their optical and structural properties was explained, which provides a technological basis for obtaining different self-controlled nanostructures.

Effect of potential barrier height on the carrier transport in InGaAs/GaAsP multi-quantum wells and photoelectric properties of laser diode.

The growth and strain-compensation behaviour of InGaAs/GaAsP multi-quantum wells, which were fabricated by metal-organic chemical vapor deposition, have been studied towards the application of these quantum wells in high-power laser diodes. The effect of the height of the potential barrier on the confined level of carrier transport was studied by incorporating different levels of phosphorus content into the GaAsP barrier. The crystal quality and interface roughness of the InGaAs/GaAsP multi-quantum wells with different phosphorus contents were evaluated by high resolution X-ray diffraction and in situ optical surface reflectivity measurements during the growth. The surface morphology and roughness were characterized by atomic force microscopy, which indicates the variation law of surface roughness, terrace width and uniformity with increasing phosphorus content, owing to strain accumulation. Moreover, the defect generation and structural disorder of the multi-quantum wells were investigated by Raman spectroscopy. The optical properties of the multi-quantum wells were characterized by photoluminescence, which shows that the spectral intensity increases as the phosphorus content increases. The results suggest that more electrons are well bound in InGaAs because of the high potential barrier. Finally, the mechanism of the effect of the height of the potential barrier on laser performance was proposed on the basis of simulation calculations and experimental results.

Investigation of temperature-dependent photoluminescence in multi-quantum wells.

Photoluminescence (PL) is a nondestructive and powerful method to investigate carrier recombination and transport characteristics in semiconductor materials. In this study, the temperature dependences of photoluminescence of GaAs-AlxGa1-xAs multi-quantum wells samples with and without p-n junction were measured under both resonant and non-resonant excitation modes. An obvious increase of photoluminescence(PL) intensity as the rising of temperature in low temperature range (T < 50 K), is observed only for GaAs-AlxGa1-xAs quantum wells sample with p-n junction under non-resonant excitation. The origin of the anomalous increase of integrated PL intensity proved to be associated with the enhancement of carrier drifting because of the increase of carrier mobility in the temperature range from 15 K to 100 K. For non-resonant excitation, carriers supplied from the barriers will influence the temperature dependence of integrated PL intensity of quantum wells, which makes the traditional methods to acquire photoluminescence characters from the temperature dependence of integrated PL intensity unavailable. For resonant excitation, carriers are generated only in the wells and the temperature dependence of integrated PL intensity is very suitable to analysis the photoluminescence characters of quantum wells.

Realization of high-luminous-efficiency InGaN light-emitting diodes in the "green gap" range.

Light-emitting diodes (LEDs) in the wavelength region of 535-570 nm are still inefficient, which is known as the "green gap" problem. Light in this range causes maximum luminous sensation in the human eye and is therefore advantageous for many potential uses. Here, we demonstrate a high-brightness InGaN LED with a normal voltage in the "green gap" range based on hybrid multi-quantum wells (MQWs). A yellow-green LED device is successfully fabricated and has a dominant wavelength, light output power, luminous efficiency and forward voltage of 560 nm, 2.14 mW, 19.58 lm/W and 3.39 V, respectively. To investigate the light emitting mechanism, a comparative analysis of the hybrid MQW LED and a conventional LED is conducted. The results show a 2.4-fold enhancement of the 540-nm light output power at a 20-mA injection current by the new structure due to the stronger localization effect, and such enhancement becomes larger at longer wavelengths. Our experimental data suggest that the hybrid MQW structure can effectively push the efficient InGaN LED emission toward longer wavelengths, connecting to the lower limit of the AlGaInP LEDs' spectral range, thus enabling completion of the LED product line covering the entire visible spectrum with sufficient luminous efficacy.

Temperature-dependent photoluminescence in light-emitting diodes.

Temperature-dependent photoluminescence (TDPL), one of the most effective and powerful optical characterisation methods, is widely used to investigate carrier transport and localized states in semiconductor materials. Resonant excitation and non-resonant excitation are the two primary methods of researching this issue. In this study, the application ranges of the different excitation modes are confirmed by analysing the TDPL characteristics of GaN-based light-emitting diodes. For resonant excitation, the carriers are generated only in the quantum wells, and the TDPL features effectively reflect the intrinsic photoluminescence characteristics within the wells and offer certain advantages in characterising localized states and the quality of the wells. For non-resonant excitation, both the wells and barriers are excited, and the carriers that drift from the barriers can contribute to the luminescence under the driving force of the built-in field, which causes the existing equations to become inapplicable. Thus, non-resonant excitation is more suitable than resonant excitation for studying carrier transport dynamics and evaluating the internal quantum efficiency. The experimental technique described herein provides fundamental new insights into the selection of the most appropriate excitation mode for the experimental analysis of carrier transport and localized states in p-n junction devices.

Effect of the thickness of undoped GaN interlayers between multiple quantum wells and the p-doped layer on the performance of GaN light-emitting diodes.

InGaN based light-emitting diodes (LEDs) with undoped GaN interlayer of variant thicknesses grown by metal-organic chemical vapor deposition technique have been investigated. It was found that the thickness of undoped GaN interlayers affected LEDs' performance greatly. The LED with 50 nm undoped GaN interlayer showed higher light output power and lower reverse-leakage current compared with the others at 20 mA. Based on electrical and optical characteristics analysis and numerical simulation, these improvements are mainly attributed to the improvement of the quality of depletion region by inserting an undoped GaN layer, as well as reduction of the Shockley-Read-Hall recombination in InGaN/GaN MQWs.

[Research on activities of drugs by use of an optical nitric oxide biosensor].

OBJECTIVE: To develop an Optical Nitric Oxide Biosensor and investigate the activities of drugs that stimulate macrophage to produce nitric oxide. METHODS: An NO biosensor was built on Cyt C which had been immobilized by Al2O3/SiO2 sol-gel procedure. Then the biosensor's characteristics in response to nitric oxide were studied by UV. The detection of NO in the culture solution that contained drugs (sulfated polysaccharides, Lentinan, Mannatide) was carried out by using interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS) as positive control. RESULTS: The standard curve was linear within the range of 20. 0-180.0 micromol/L of NO in acidified culture solution (r = 0.9959). The limit of detection was 5.0 micromol/L; within-day precision was 3.5%; day-to-day precision was 7.9%. The concentrations of NO in culture solutions were (12.24+/-3.39) micromol/L, (8.51 +/- 0.19) micromol/L and (8.23 +/- 0.26) micromol/L, which were compared with the use of Griess method, and no statistically significant difference was observed. CONCLUSION: The biosensor can be used as a semi-quantitative analysis method and be used for screening drugs preliminarily.

[Superoxide dismutase biosensor for screening substances possessing scavenging superoxide anion activity].

AIM: To establish a method for screening active substance with scavenging effects on superoxide anion in vitro by designed superoxide dismutase biosensor. METHODS: The enzyme sensor was built by connecting the immobilized CuZnSOD with optical oxygen sensor through a special way. Superoxide anions were generated by auto-oxidation of pyrogallol. The auto-oxidation speed was examined before and after adding samples into the system, and the Vit C having the scavenging radical activities was served as a positive control. RESULTS: The limit of biosensor detection was 7.0 U in activity, and lifetime of the immobilized enzyme in the reaction-cell was above 2 weeks. The scavenging effects on superoxide radicals of fifteen active substance were studied in vitro by the sensor, and some of them presented scavenging activities. CONCLUSION: The signal from biosensor is stable, easy to be determined, and the kinetic information on scavenging superoxide radicals could be obtained directly. The biosensor system can be used for screening drugs simply and rapidly.

[Optical oxygen sensor based on a sol-gel encapsulation method].

OBJECTIVE: To develop oxygen sensor by use of sol-gel encapsulation method and optical fiber technology. Method Organically modified sol-gel film was prepared by hydrolyzing tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS) together, and film-processing conditions were optimized. Overall validation of the method was carried out by building Stern-Volmer plots and determining the concentration of oxygen in samples. RESULTS: The optical fiber oxygen sensor was linear in a wide range (oxygen gas: 0%-100% and dissolved oxygen: 0.55 mg/L-33.1 mg/L 29 degrees C, r > 0.9990) and had a fast response time (T95 < 20 second). Signals were steady and not easily interfered. The results of measuring oxygen in samples by using the oxygen sensor and iodimetry respectively were compared and no significant difference was found. CONCLUSION: The sensor system is applicable to continuous measurement of gas phase oxygen and dissolved oxygen on line in a complex rigorous conditions.