All-inorganic perovskite CsPbBr3 microstructures growth via chemical vapor deposition for high-performance photodetectors

PbS/CsPbBr3 Heterojunction for Broadband Neuromorphic Vision Sensing

Neuromorphic light sensors with analogue-domain image processing capability hold promise for overcoming the energy efficiency limitations and latency of von Neumann architecture-based vision chips. Recently, metal halide perovskites, with strong light-matter interaction, long carrier diffusion length, and exceptional photoelectric conversion efficiencies, exhibit reconfigurable photoresponsivity due to their intrinsic ion migration effect, which is expected to advance the development of visual sensors. However, suffering from a large bandgap, it is challenging to achieve highly tunable responsivity simultaneously with a wide-spectrum response in perovskites, which will significantly enhance the image recognition accuracy through the machine learning algorithm. Herein, we demonstrate a broadband neuromorphic visual sensor from visible (Vis) to near-infrared (NIR) by coupling all-inorganic metal halide perovskites (CsPbBr3) with narrow-bandgap lead sulfide (PbS). The PbS/CsPbBr3 heterostructure is composed of high-quality single crystals of PbS and CsPbBr3. Interestingly, the ion migration of CsPbBr3 with the implementation of an electric field induces the energy band dynamic bending at the interface of the PbS/CsPbBr3 heterojunction, leading to reversible, multilevel, and linearly tunable photoresponsivity. Furthermore, the reconfigurable and broadband photoresponse in the PbS/CsPbBr3 heterojunction allows convolutional neuronal network processing for pattern recognition and edge enhancements from the Vis to the NIR waveband, suggesting the great potential of the PbS/CsPbBr3 heterostructure in artificial intelligent vision sensing.

High-sensitivity self-powered photodetector based on an in-situ prepared CsPbBr3 microwire/InGaN heterojunction

Low-dimensional CsPbBr3 perovskite materials have gained widespread attention, derived from their remarkable properties and potential for numerous optoelectronic applications. Herein, the sample of CsPbBr3 microwires were prepared horizontally onto n-type InGaN film substrate using an in-plane solution growth method. The resulting CsPbBr3 microwire/InGaN heterojunction allows for the achievement of a highly sensitive and broadband photodetector. Particularly for the implementation in a self-supplying manner, the best-performing photodetector can achieve a superior On/Off ratio of 4.6×105, the largest responsivity ∼ 800.0 mA/W, a maximum detectivity surpassing 4.6× 1012 Jones, and a high external quantum efficiency approaching 86.5% upon 405 nm light illumination. A rapid response time (∼ 4.48 ms/7.68 ms) was also achieved. The as-designed CsPbBr3 microwire/InGaN heterojunction device without any encapsulation exhibits superior comprehensive stability. Besides, the device featuring as a single pixel imaging unit can readily detect simple images under broadband light illumination with a high spatial resolution, acknowledging its outstanding imaging capability. The robust photodetection properties could be derived from the intense absorption of CsPbBr3 MWs and high-efficiency charge carriers transporting toward the in-situ formed CsPbBr3/InGaN heterointerface. The results may offer an available strategy for the in-situ construction of best-performing low-dimensional perovskite heterojunction optoelectronic devices.

High-efficiency visible-light-driven oxidation of primary C-H bonds in toluene over a CsPbBr3 perovskite supported by hierarchical TiO2 nanoflakes

Photocatalytic oxidation of toluene to valuable fine chemicals is of great significance, yet faces challenges in the development of advanced catalysts with both high activity and selectivity for the activation of inert C(sp3)-H bonds. Halide perovskites with remarkable optoelectronic properties have shown to be prospective photoactive materials, but the bulky structure with a small surface area and severe recombination of photogenerated electron-hole pairs are obstacles to application. Here, we fabricate a hierarchical nanoflower-shaped CsPbBr3/TiO2 heterojunction by assembling CsPbBr3 nanoparticles on 2D TiO2 nanoflake subunits. The design significantly downsizes the size of CsPbBr3 from micrometers to nanometers, and forms a type II heterojunction with intimate interfacial contact between CsPbBr3 and TiO2 nanoflakes, thereby accelerating the separation and transfer of photogenerated charges. Moreover, the formed hierarchical heterojunction increaseslight absorption by refraction and scattering, offers a large surface area and enhances the adsorption of toluene molecules. Consequently, the optimized CsPbBr3/TiO2 exhibits a high performance (10 200 μmol g-1 h-1) for photocatalytic toluene oxidation with high selectivity (85%) for benzaldehyde generation under visible light. The photoactivity is about 20 times higher than that of blank CsPbBr3, and is among the best photocatalytic performances reported for selective oxidation of toluene under visible light irradiation.

Controlled growth of lead-free cesium zirconium halide double perovskite nanocrystals through a microfluidic reactor

Vacancy-ordered Cs₂ZrX₆ (X = Cl, Br) double perovskite nanocrystals (NCs) have recently attracted increasing attention in optoelectronic applications due to their promising photoluminescence property, high photostability, and low toxicity. However, their ultra-fast reaction limits the growth control and kinetics study of these Cs₂ZrX₆ NCs. Here we report the synthesis of Cs₂ZrX₆ NCs through a microfluidic reactor and achievement of tunable emission wavelengths by controlling the NC size and hybrid halogen ions. Reaction kinetics study reveals that the amine ligand and reaction temperature play dominate roles in the growth and optical performance of the Cs₂ZrX₆ NCs. The effects of flow rate, precursors, and ligand ratio on the morphology and optical property of the NCs were also investigated. This study provides an insight into the growth kinetics of the Cs₂ZrX₆ perovskite NCs and their continuous production through a microfluidic reactor that could facilitate the development and optical application of lead-free vacancy-ordered double perovskite NCs.

Halide perovskite photoelectric artificial synapses: materials, devices, and applications

In recent years, there has been a research boom on halide perovskites (HPs) whose outstanding performance in photovoltaic and optoelectronic fields is obvious to all. In particular, HP materials find application in the development of artificial synapses. HP-based synapses have great potential for artificial neuromorphic systems, which is due to their outstanding optoelectronic properties, femtojoule-level energy consumption, and simple fabrication process. In this review, we present the physical properties of HPs and describe two types of synaptic devices including two-terminal (2T) memristors and three-terminal (3T) transistors. The HP layer in 2T memristors can realize the change in the device conductance through physical mechanisms dominated by ion migration. On the other hand, HPs in 3T transistors can be used as efficient light-absorbing layers and rely on some special device structures to provide reliable current changes. In the final section of the article, we discuss some of the existing applications of HP-based synapses and bottlenecks to be solved.

Miniaturized Multispectral Detector Derived from Gradient Response Units on Single MAPbX3 Microwire

Miniaturized multispectral detectors are urgently desired given the unprecedented prosperity of smart optoelectronic chips for integrated functions including communication, imaging, scientific analysis, etc. However, multispectral detectors require complicated prism optics or interference/interferometric filters for spectral recognition, which hampers the miniaturization and their subsequent integration in photonic integrated circuits. In this work, inspired by the advance of computational imaging, optical-component-free miniaturized multispectral detector on 4 mm gradient bandgap MAPbX₃ microwire with a diameter of 30 µm, is reported. With accurate composition engineering, halide ions in MAPbX₃ microwire vary from Cl to I giving in the gradual variation of optical bandgap from 2.96 to 1.68 eV along axis. The sensing units on MAPbX₃ microwire offer the response edge ranging from 450 to 790 nm with the responsivity over 20 mA W^-1 , -3dB width over 450 Hz, LDR of ≈60 dB, and a noise current less than ≈1.4 × 10^-12 A Hz^-0.5 . As a result, the derived miniaturized detector achieves the function of multispectral sensing and discrimination with spectral resolution of ≈25 nm and mismatch of ≈10 nm. Finally, the proof-of-concept colorful imaging is successfully conducted with the miniaturized multispectral detector to further confirm its application in spectral recognition.

Transfer Printing of Perovskite Whispering Gallery Mode Laser Cavities by Thermal Release Tape

The outstanding optoelectrical properties and high-quality factor of whispering gallery mode perovskite nanocavities make it attractive for applications in small lasers. However, efforts to make lasers with better performance have been hampered by the lack of efficient methods for the synthesis and transfer of perovskite nanocavities on desired substrate at quality required for applications. Here, we report transfer printing of perovskite nanocavities grown by chemical vapor deposition from mica substrate onto SiO₂ substrate. Transferred perovskite nanocavity has an RMS roughness of ~ 1.2 nm and no thermal degradation in thermal release process. We further use femtosecond laser to excite a transferred perovskite nanocavity and measures its quality factor as high as 2580 and a lasing threshold of 27.89 μJ/cm² which is almost unchanged as compared with pristine perovskite nanocavities. This method represents a significant step toward the realization of perovskite nanolasers with smaller sizes and better heat management as well as application in optoelectronic devices.

Microfluidic synthesis of optically responsive materials for nano- and biophotonics

Recently, nanomaterials demonstrating optical response under illumination, the so-called optically responsive nanoparticles (NPs), have found their broad application as optical switchers, gas adsorbents, data storage devices, and optical and biological sensors. Unique optical properties of such nanomaterials are strongly related to their chemical composition, geometrical parameters and morphology. Microfluidic approaches for NPs' synthesis allow overcoming the known critical stages in conventional synthesis of NPs due to a high rate of heat/mass transfer and precise regulation of synthesis conditions, which results in reproducible synthesis outcomes with the desired physico-chemical properties. Here, we review the recent advances in microfluidic approach for synthesis of optically responsive nanomaterials (plasmonic, photoluminescent, shape-changeable NPs), highlighting the general background of microfluidics, common considerations in the design of microfluidic chips (MFCs), and theoretical models of the NPs' formation mechanisms. Comparative analysis of microfluidic synthesis with conventional synthesis methods is provided further, along with the recent applications of optically responsive NPs in nano- and biophotonics.

Characteristics of multi-mode lasing in cesium lead bromide perovskite microwires with an isosceles right triangle cross-section

The CsPbBr₃ microwires with unique isosceles right triangle cross-sections are commonly observed via chemical vapor deposition method. In this work, we study the correlations between measured multi-mode lasing behaviors and the simulation of the mode patterns inside the triangular-rod microcavity. We confirm that lasing action with higher-order transverse modes can well sustain, even when these modes experience large optical loss due to the isosceles triangle cross-section. By comparing the experimental and simulation results, the higher-order transverse modes tend to show up prior to the fundamental transverse modes for wider microwires. We attribute this behavior to the nonuniform field distribution caused by the high absorption efficiency of CsPbBr₃. We also elaborate on the difficulties to sustain the whispering gallery mode in the CsPbBr₃ triangular-rod microcavity, which implies that the lateral dimension and geometry of the cavity should be considered carefully for the future design of low threshold wire-based laser devices.

Organic-Inorganic Perovskite Films and Efficient Planar Heterojunction Solar Cells by Magnetron Sputtering

Organic-inorganic halide perovskites have been widely used in photovoltaic technologies. Despite tremendous progress in their efficiency and stability, perovskite solar cells (PSCs) are still facing the challenges of upscaling and stability for practical applications. As a mature film preparation technology, magnetron sputtering has been widely used to prepare metals, metallic oxides, and some semiconductor films, which has great application potential in the fabrication of PSCs. Here, a unique technology where high-quality perovskite films are prepared via magnetron sputtering for controllable composition, solvent-free, large-area, and massive production, is presented. This strategy transforms the perovskite materials from powder to thin films by magnetron sputtering and post-treatment (vapor-assisted treatment with methanaminium iodide gas and methylamine gas treatment), which is greatly favorable to manufacture tandem solar cells. The power conversion efficiency (PCE) of PSCs with perovskite films fabricated by magnetron sputtering is 6.14%. After optimization, high-performance perovskite films with excellent electronic properties are obtained and stable PSCs with excellent reproducibility are realized, showing a PCE of up to 15.22%. The entirely novel synthetic approach opens up a new and promising way to achieve high-throughput magnetron sputtering for large-area production in commercial applications of planar heterojunction and tandem PSCs.