Pulsed laser deposition of a Ga2O3 thin film for an optoelectronic synaptic device.

With the rapid development of information era, the traditional von Neumann architecture faces the computing bottleneck, and integration of memory and perception is regarded as a potential solution. Herein, a Ga2O3/Si heterojunction based multi-modulated optoelectronic synaptic device is fabricated and demonstrated. As stimulated by ultraviolet (UV) optical spikes, the heterojunction device reveals typical synaptic functions of excitatory-postsynaptic current (EPSC), paired-pulse facilitation (PPF), spike-timing-dependent plasticity (STDP), and switch between short-term memory (STM) and long-term memory (LTM). In addition, stronger stimulations like higher reading voltage, stronger optical stimulated intensity, and longer pulse duration time can significantly prolong the attenuation of EPSC, which contributes to the improvement of the forgetting process. Our work provides a potential strategy for future neuromorphic computation through a UV light driven stimulation.

Study of resistive switching behavior in HfO2nanocrystals synthesized via a low temperature hydrothermal method.

The resistive switching property in HfO2have attracted increasing interest in recent years. In this work, amorphous HfO2nanocrystals are synthesized by a facile hydrothermal method. Then, the as-synthesized nanocrystals are rapid thermal annealed in different atmospheres for improving the crystal quality, and monoclinic phase is determined as the main crystal structure of the annealed HfO2. Subsequently, metal-insulator-metal structure devices based on HfO2samples are fabricated. Electrical measurement indicates that 700 °C annealing processes in Air and Ar environments can slightly improve the bipolar resistive switching and retention behaviors. Higher annealed temperature (900 °C) will further improve the crystal quality of HfO2, while the resistive switching and retention behaviors of the devices continuously attenuate, which can be ascribed to the reduction of the conductive filaments induced by defects.

Room temperature amplified spontaneous emissions in a sub-centimeter sized CsPbBr3 bulk single crystal.

All inorganic perovskite CsPbBr3 shows great potential in laser device because of its excellent luminescence characteristics, while the room temperature amplified spontaneous emission (ASE) in a large size CsPbBr3 bulk single crystal is still quite difficult. Herein, we have obtained the room temperature ASE in a sub-centimeter size CsPbBr3 bulk single crystal pumped with the single-photon excitation. Based on the reproducible light path within the CsPbBr3 bulk single crystal, the photonic feedback between the bottom and top facets naturally enhances the population inversion, which exhibits an amplified spontaneous emission threshold of ∼320 µJ/cm2. The blue shift of the ASE peak along with the increased pumping intensity is also observed and ascribed to the reduction of the refractive index and the energy band filling effect. These findings demonstrate the sub-centimeter size CsPbBr3 bulk single crystal to be an excellent candidate as an optical gain media for crystal lasers.

A photodetector fabricated from 2H-PbI2 micro-crystals recycled from waste lead acid batteries.

Recycling Pb from lead acid batteries is rather important in environmental protection, but current strategies need a high temperature or produce secondary pollution. Herein, we present a green reactant recycling method to synthesize PbI2 micro-crystals by extracting the Pb from waste lead acid batteries. Systematical characterizations indicate that the as-prepared PbI2 micro-crystals show high purity, high crystal quality with a 2H-hexagonal crystal structure, and excellent optical properties with a bandgap of 2.3 eV. Based on the recycled 2H-PbI2 micro-crystals, a symmetrically structured ITO/PbI2/ITO photodetector is fabricated. Under 10 V bias voltage, the device reveals a distinct photo-response to UV-visible light and superior performance, with a dark current of 1.06 nA, an on-off ratio of 103, a responsivity of 15.5 mA/W, and a detectivity of 4.7 × 1010 Hz1/2 W-1. In addition, the photodetector also exhibits relatively rapid response speeds of 69 ms (rise time) and 64 ms (decay time). Our study provides an innovative and green strategy for producing a UV-visible photodetector based on recycled lead acid batteries, which is significant in environmental protection and the recycling economy.

A vertical CsPbBr3/ZnO heterojunction for photo-sensing lights from UV to green band.

In this work, we have reported a vertical CsPbBr3/ZnO heterojunction photodetector for photo-sensing lights from UV to visible band. The ZnO thin film is deposited on the c-sapphire substrate through a molecular beam epitaxy (MBE) technique, and then the CsPbBr3 thin film is synthesized on the as-prepared ZnO film layer by using a solution processing method. The as-prepared CsPbBr3/ZnO heterostructure presents type-II energy band structure induced by the energy band offset effect, which can promote the separation and extraction efficiencies of the photo-generated electron-hole pairs. Compared with the CsPbBr3 based metal-semiconductor-metal (MSM) structure photodetector, the heterojunction photodetector presents higher responsivity and detectivity of 630 µA/W and 7 × 109 Jones. While compared with the ZnO based MSM structure photodetector, the heterojunction device reveals much faster response speeds of 61 µs (rise time) and 1.4 ms (decay time). These findings demonstrate that the CsPbBr3/ZnO heterojunction photodetector is promising for constructing next generation perovskite based optoelectronic devices.

Insights into the Solid-State Synthesis of Defect-Rich Zr-UiO-66.

Metal-organic frameworks (MOFs), a new type of porous material, have shown many possible applications in gas storage and separation, biomedicine, catalysis, and so on. While most MOFs are synthesized through solvothermal synthesis where a large quantity of organic solvent is used, the green synthetic approach using a minimized amount of solvent is important to prevent irreversible environmental compacts. In this study, we successfully synthesized Zr-MOFs with SBUs (e.g., UiO-66 and MIL-140A) using a simple metal source and investigated the role of organic modulators in modulating the MOF structures during solid-state synthesis. Meanwhile, UiO-66 rich in defects synthesized via a solid-state conversion strategy shows good catalytic performance for the ring-opening of epoxides with alcohols. This work contributes to the understanding of the role of organic modulators in the solid-state synthesis of MOFs.

Synthesis of a Spatially Confined, Highly Durable, and Fully Exposed Pd Cluster Catalyst via Sequential Site-Selective Atomic Layer Deposition.

Bottom-up synthesis based on site-selective atomic layer deposition is a powerful atomic-scale processing approach to fabricate materials with desired functionalities. Typical selective atomic layer deposition (ALD) can be achieved using selective activation of a growth area or selective deactivation of a protected area. In this work, we explored the site selectivity based on the difference of the inherent surface reactivity between different materials and within the same materials. By sequentially applying two site-selective atomic layer deposition, the ALD Pd catalyst is spatially confined on ALD SnO2 modified h-BN substrate Pd/SnO2/h-BN shows improved catalytic activity and stability due to strong metal-support interactions and spatial confinement. The results reveal that sequential site-selective ALD is a feasible and effective synthesis strategy that provides an attractive path toward designing and developing highly stable catalysts.

An Ultrathin Functional Layer Based on Porous Organic Cages for Selective Ion Sieving and Lithium-Sulfur Batteries.

Thin films with effective ion sieving ability are highly desired in energy storage and conversion devices, including batteries and fuel cells. However, it remains challenging to design and fabricate cost-effective and easy-to-process ultrathin films for this purpose. Here, we report a 300 nm-thick functional layer based on porous organic cages (POCs), a new class of porous molecular materials, for fast and selective ion transport. This solution processable material allows for the design of thin films with controllable thickness and tunable porosity by tailoring cage chemistry for selective ion separation. In the prototype, the functional layer assembled by CC3 can selectively sieve Li+ ions and efficiently suppress undesired polysulfides with minimal sacrifice for the system's total energy density. Separators modified with POC thin films enable batteries with good cycle performance and rate capability and offer an attractive path toward the development of future high-energy-density energy storage devices.

Schottky-type GaN-based UV photodetector with atomic-layer-deposited TiN thin film as electrodes.

In this work, a GaN-based UV photodetector with an asymmetric electrode structure was fabricated by atomic layer deposition (ALD) of TiN layers. The thickness of the TiN can be monitored in situ by a quartz crystal microbalance (QCM) and precisely controlled through the modulation of deposition cycles. During the ALD process, periodic variation in the QCM frequency was observed and correlated to the physical adsorption, chemical bonding, and the excessive precursor exhaust, which included tetrakis(dimethylamino)titanium (TDMAT) and N sources. The asymmetric TiN/GaN/TiN photodetector showed excellent photosensing performance, with a UV-visible rejection ratio of 173, a responsivity of 4.25 A/W, a detectivity of 1.1×1013 Jones, and fast response speeds (a rise time of 69 µs and a decay time of 560 µs). Moreover, the device exhibits high stability, with an attenuation of only approximately 0.5% after 360 nm light irradiation for 157 min. This result indicates the potential of TiN as a transparent contact electrode for GaN-based optoelectronic devices.

A direct solvent-free conversion approach to prepare mixed-metal metal-organic frameworks from doped metal oxides.

We propose a novel strategy to introduce platinum into the metal nodes of ZIF-8 by preloading Pt as a dopant in ZnO (Pt-ZnO) and then convert it to Pt doped ZIF-8 (Pt-ZIF-8) through a chemical vapor deposition (CVD) approach. The solvent-free conversion of Pt-ZnO to Pt-ZIF-8 allows the Pt dopant in ZnO to coordinate with organic linkers directly without the formation of Pt nanoparticles, which is a general issue of many methods. This general synthesis strategy may facilitate the discovery of MMOFs that have not been reported previously.

Simultaneous Enhancement of Interfacial Stability and Kinetics of Single-Crystal LiNi0.6Mn0.2Co0.2O2 through Optimized Surface Coating and Doping.

Balancing interfacial stability and Li+ transfer kinetics through surface engineering is a key challenge in developing high-performance battery materials. Although conformal coating enabled by atomic layer deposition (ALD) has shown great promise in controlling impedance increase upon cycling by minimizing side reactions at the electrode-electrolyte interface, the coating layer itself usually exhibits poor Li+ conductivity and impedes surface charge transfer. In this work, we have shown that by carefully controlling postannealing temperature of an ultrathin ZrO2 film prepared by ALD, Zr4+ surface doping could be achieved for Ni-rich layered oxides to accelerate the charge transfer yet provide sufficient protection. Using single-crystal LiNi0.6Mn0.2Co0.2O2 as a model material, we have shown that surface Zr4+ doping combined with ZrO2 coating can enhance both the cycle performance and rate capability during high-voltage operation. Surface doping via controllable postannealing of ALD surface coating layer reveals an attractive path toward developing stable and Li+-conductive interfaces for single-crystal battery materials.

An ultrahigh responsivity self-powered solar-blind photodetector based on a centimeter-sized ß-Ga2O3/polyaniline heterojunction.

Wide band gap semiconductors are promising UV photodetector materials due to their suitable bandgap, high crystal quality, strong absorption and large carrier mobility. Up to now, deep UV photodetectors are mainly based on epitaxial thin films, which have some undesired properties such as p-type doping difficulty. Lattice mismatch hinders the further development of these devices. Here, a high performance self-powered solar-blind UV photodetector was realized by a facile combination of a centimeter-sized single crystal ß-Ga2O3 microwire and polyaniline. Owing to the excellent organic/inorganic hybrid p-n junction, the device shows an ultrahigh responsivity of 21 mA W-1 at 246 nm with a sharp cut-off wavelength of 272 nm without an external power supply. Moreover, the dark current is 0.08 pA, which is smaller than those of almost all the previous metallic oxide based solar-blind UV photodetectors. The photodetector also shows a high UV/visible rejection ratio (102) at zero bias voltage. Finally, a physical model of the self-powered photodetector is also proposed. This work provides a simple, low-cost, and effective method for preparing high performance self-powered solar-blind UV photodetectors based on organic/inorganic heterojunctions.

High Responsivity and High Rejection Ratio of Self-Powered Solar-Blind Ultraviolet Photodetector Based on PEDOT:PSS/ß-Ga2O3 Organic/Inorganic p-n Junction.

A high responsivity self-powered solar-blind deep UV (DUV) photodetector with high rejection ratio was proposed based on inorganic/organic hybrid p-n junction. Owing to the high crystallized ß-Ga2O3 and excellent transparent conductive polymer PEDOT:PSS, the device exhibited ultrahigh responsivity of 2.6 A/W at 245 nm with a sharp cutoff wavelength at 255 nm without any power supply. The responsivity is much larger than that of previous solar-blind DUV photodetectors. Moreover, the device exhibited an ultrahigh solar-blind/UV rejection ratio (R245 nm/R280 nm) of 103, which is two orders of magnitude larger than the average value reported in Ga2O3-based solar-blind photodetectors. In addition, the photodetector shows a narrow bandpass response of only 17 nm in width. This work might be of great value in developing a high wavelength selective DUV photodetector with respect to low cost for future energy-efficient photoelectric devices.

UV Photodetectors Based on BiOCl Nanosheet Arrays: The Effects of Morphologies and Electrode Configurations.

A facile chemical bath method is adopted to grow bismuth oxychloride (BiOCl) nanosheet arrays on a piece of Cu foil (denoted as BiOCl-Cu) and isolated BiOCl nanosheets are collected by ultrasonication. A self-supporting BiOCl film is obtained by the removal of Cu foil. Photodetectors (PDs) based on these BiOCl materials are assembled and the effects of morphologies and electrode configurations on the photoelectric performance of these PDs are examined. The BiOCl nanosheet PD achieves high responsivities in the spectral range from 250 to 350 nm, while it presents quite a small photocurrent and slow response speed. The BiOCl film PD yields low photocurrents and near-unity on-off ratios, demonstrating poor photoelectric performance. The photocurrent of the BiOCl-Cu PD with both electrodes on the BiOCl film is much higher than those of these above-mentioned PDs, and the response times are fast. Meanwhile, the BiOCl-Cu PD with separate electrodes on the BiOCl film and Cu foil achieves even higher photocurrents and presents a self-powering characteristic, depicting the improved photodetecting performances induced by the specific morphology and distinct electrode configuration. These results would promote the applications of BiOCl nanostructures in the photoelectric devices.

A Real-Time Wearable UV-Radiation Monitor based on a High-Performance p-CuZnS/n-TiO2 Photodetector.

Solar radiation, especially ultraviolet (UV) light, is a major hazard for most skin-related cancers. The growing needs for wearable health monitoring systems call for a high-performance real-time UV sensor to prevent skin diseases caused by excess UV exposure. To this end, here a novel self-powered p-CuZnS/n-TiO2 UV photodetector (PD) with high performance is successfully developed (responsivity of 2.54 mA W-1 at 0 V toward 300 nm). Moreover, by effectively replacing the Ti foil with a thin Ti wire for the anodization process, the conventional planar rigid device is artfully turned into a fiber-shaped flexible and wearable one. The fiber-shaped device shows an outstanding responsivity of 640 A W-1 , external quantum efficiency of 2.3 × 105 %, and photocurrent of ≈4 mA at 3 V, exceeding those of most current UV PDs. Its ultrahigh photocurrent enables it to be easily integrated with commercial electronics to function as a real-time monitor system. Thus, the first real-time wearable UV radiation sensor that reads out ambient UV power density and transmits data to smart phones via wifi is demonstrated. This work not only presents a promising wearable health monitor, but also provides a general strategy for designing and fabricating smart wearable electronic devices.

Self-Powered Ultraviolet Photodetectors Driven by Built-In Electric Field.

Self-powered ultraviolet (UV) photodetectors, which have vast applications in the military and for civilian purposes, have become particularly attractive in recent years due to their advantages of high sensitivity, ultrasmall size, and low power consumption. In particular, self-powered UV photodetectors driven by a built-in electric field cannot only detect UV signals but also be powered by the incident signals instead of external power. In this concept, the key issues and most recent developments on photovoltaic type UV photodetectors driven by p-n homojunction, heterojunction, and Schottky junction are surveyed. This should generate extensive interest in this field and encourage more researchers to engage in and tackle the scientific challenges.

Electrically driven deep ultraviolet MgZnO lasers at room temperature.

Semiconductor lasers in the deep ultraviolet (UV) range have numerous potential applications ranging from water purification and medical diagnosis to high-density data storage and flexible displays. Nevertheless, very little success was achieved in the realization of electrically driven deep UV semiconductor lasers to date. In this paper, we report the fabrication and characterization of deep UV MgZnO semiconductor lasers. These lasers are operated with continuous current mode at room temperature and the shortest wavelength reaches 284 nm. The wide bandgap MgZnO thin films with various Mg mole fractions were grown on c-sapphire substrate using radio-frequency plasma assisted molecular beam epitaxy. Metal-semiconductor-metal (MSM) random laser devices were fabricated using lithography and metallization processes. Besides the demonstration of scalable emission wavelength, very low threshold current densities of 29~33 A/cm2 are achieved. Numerical modeling reveals that impact ionization process is responsible for the generation of hole carriers in the MgZnO MSM devices. The interaction of electrons and holes leads to radiative excitonic recombination and subsequent coherent random lasing.

Enhanced Exciton Binding Energy of ZnO by Long-Distance Perturbation of Doped Be Atoms.

The excitonic effect in semiconductors is sensitive to dopants. Origins of dopant-induced large variation in the exciton binding energy (E(b)) is not well understood and has never been systematically studied. We choose ZnO as a typical high-E(b) material, which is very promising in low-threshold lasing. To the best of our knowledge, its shortest wavelength electroluminescence lasing was realized by ZnO/BeZnO multiple quantum wells (MQWs). However, this exciting result is shadowed by a controversial E(b) enhancement claimed. In this Letter, we reveal that the claimed E(b) is sensible if we take Be-induced E(b) variation into account. Detailed first-principle investigation of the interaction between dopant atoms and the lattice shows that the enhancement mainly comes from the long-distance perturbation of doped Be atoms rather than the local effect of doping atoms. This is a joint work of experiment and calculation, which from the angle of methology paves the way for understanding and predicting the E(b) variation induced by doping.

Understanding the origin of phase segregation of nano-crystalline in a Be(x)Zn(1-x)O random alloy: a novel phase of Be(1/3)Zn(2/3)O.

The usage of a BexZn1-xO alloy in ultraviolet (UV)-region optoelectronic devices is largely hindered by its intricate phase segregation of crystallites of different sizes. To understand the physical origin of this phase segregation phenomenon on the atomistic scale, we have undertaken an extensive study of the structural evolution of the segregation phases in the BexZn1-xO alloy at finite temperatures by using first-principles calculations combined with the cluster expansion approach. We find that a random alloy of BexZn1-xO tends to segregate into a mix-ordered phase below a critical temperature, by the growth of prototype and nano-sized structures. The segregated phases in BexZn1-xO entail not only ZnO or BeO crystallites, but also two as yet unreported phases with beryllium concentration of 1/3 and 2/3. Both new phases of BexZn1-xO are direct wide-gap semiconductors with band gap values of 4.88 eV and 6.78 eV respectively. We envisioned that the novel Be1/3Zn2/3O crystal is highly promising for solar-blind device applications.

Wide range bandgap modulation based on ZnO-based alloys and fabrication of solar blind UV detectors with high rejection ratio.

Theoretical calculations on formation energies of MgZnO, BeZnO and BeMgZnO alloys are presented. The ternary alloy MgZnO (BeZnO) is found to be unstable with high Mg (Be) contents. However, the quaternary system BeMgZnO is predicted to be stable with small Be/Mg atom ratio. Subsequently, a wurtzite Be0.17Mg0.54Zn0.29O alloy with a bandgap of 5.15 eV has been acquired experimentally. Its bandgap is in the middle of solar blind region and thus it is an ideal material for realizing a high rejection ratio solar blind ultraviolet (UV) detector, which has long been a problem. A metal-semiconductor-metal (MSM) structured solar blind UV detector based on this material is then fabricated, realizing a much higher rejection ratio than reported MgZnO-based detectors. One more interesting thing is, as a complicated quaternary system, BeMgZnO can maintain its crystal quality in a wide compositional range, which is not happening in MgZnO and BeZnO. To get some microscopic insight into the Be-Mg mutual stabilizing mechanism, more calculations on the lattice constants of BeZnO and MgZnO alloys, and the coordination preference of Be ions in alloy were conducted. The a-axis lattice compensation and 4-fold coordination preference of Be atom are confirmed the major origins for Be-Mg mutual stabilizing in ZnO lattice.