Recent Advances in Structure Design and Application of Metal Halide Perovskite-Based Gas Sensor.

Metal halide perovskites (MHPs) are emerging gas-sensing materials and have attracted considerable attention in gas sensors due to their unique bandgap structure and tunable optoelectronic properties. The past decade has witnessed significant developments in the gas-sensing field; however, their intrinsic structural instability and ambiguous gas-sensing mechanisms hamper their practical applications. Herein, we summarize the recent advances in MHP-based gas sensors. The physicochemical properties of MHPs are discussed at first. The structure design, including dimension design and engineering design, is overviewed as well as their fabrication methods, and we put forward our insights into the gas-sensing mechanism of MHPs. It is believed that enhanced understanding of gas-sensing mechanisms of MHPs are helpful for their application as gas-sensing materials, and structure design can enhance their stability, sensing sensitivity, and selectivity to target gases as gas sensors. Subsequently, the latest developments in MHP-based gas sensors are summarized according to their different application scenarios. Finally, we conclude with the current status and challenges in this field and propose future perspectives.

Toward Thin-Film Laser Diodes with Metal Halide Perovskites.

Metal halide perovskite semiconductors hold a strong promise for enabling thin-film laser diodes. Perovskites distinguish themselves from other non-epitaxial media primarily through their ability to maintain performance at high current densities, which is a critical requirement for achieving injection lasing. Coming in a wide range of varieties, numerous perovskites delivered low-threshold optical amplified spontaneous emission and optically pumped lasing when combined with a suitable optical cavity. A progression toward electrically pumped lasing requires the development of efficient light-emitting structures with reduced optical losses and high radiative efficiency at lasing-level current densities. This involves a set of important trade-offs in terms of material choice, stack and waveguide design, as well as resonator integration. In this Perspective, the key milestones are highlighted that have been achieved in the study of passive optical waveguides and light-emitting diodes, and these learnings are translated toward more complex laser diode architectures. Finally, a novel resonator integration route is proposed that is capable of relaxing optical and electrical design constraints.

Investigation of the Octahedral Network Structure in Formamidinium Lead Bromide Nanocrystals by Low-Dose Scanning Transmission Electron Microscopy.

Metal halide perovskites (MHP) are highly promising semiconductors. In this study, we focus on FAPbBr3 nanocrystals, which are of great interest for green light-emitting diodes. Structural parameters significantly impact the properties of MHPs and are linked to phase instability, which hampers long-term applications. Clearly, there is a need for local and precise characterization techniques at the atomic scale, such as transmission electron microscopy. Because of the high electron beam sensitivity of MHPs, these investigations are extremely challenging. Here, we applied a low-dose method based on four-dimensional scanning transmission electron microscopy. We quantified the observed elongation of the projections of the Br atomic columns, suggesting an alternation in the position of the Br atoms perpendicular to the Pb-Br-Pb bonds. Together with molecular dynamics simulations, these results remarkably reveal local distortions in an on-average cubic structure. Additionally, this study provides an approach to prospectively investigating the fundamental degradation mechanisms of MHPs.

Defects of Metal Halide Perovskites in Photocatalytic Energy Conversion: Friend or Foe?

Photocatalytic solar-to-fuel conversion over metal halide perovskites (MHPs) has recently attracted much attention, while the roles of defects in MHPs are still under debate. Specifically, the mainstream viewpoint is that the defects are detrimental to photocatalytic performance, while some recent studies show that certain types of defects contribute to photoactivity enhancement. However, a systematic summary of why it is contradictory and how the defects in MHPs affect photocatalytic performance is still lacking. In this review, the innovative roles of defects in MHP photocatalysts are highlighted. First, the origins of defects in MHPs are elaborated, followed by clarifying certain benefits of defects in photocatalysts including optical absorption, charge dynamics, and surface reaction. Afterward, the recent progress on defect-related MHP photocatalysis, i.e., CO2 reduction, H2 generation, pollutant degradation, and organic synthesis is systematically discussed and critically appraised, putting emphasis on their beneficial effects. With defects offering peculiar sets of merits and demerits, the personal opinion on the ongoing challenges is concluded and outlining potentially promising opportunities for engineering defects on MHP photocatalysts. This critical review is anticipated to offer a better understanding of the MHP defects and spur some inspiration for designing efficient MHP photocatalysts.

Cold-Sintered All-Inorganic Perovskite Bulk Composite Scintillators for Efficient X-ray Imaging.

Cost-effective bulk scintillators with high density, large-area, and long-term stability are desirable for high-energy radiation detections. Conventional bulk polycrystalline or single-crystal scintillators are generally synthesized by high-temperature approaches, and it is challenging to realize simultaneously high detectivity/responsivity, spatial resolution, and rapid time response. Here, we report the cold sintering of bulk scintillators (at 90 °C) based on an "emitter-in-matrix" principle, in which emissive CsPbBr3 nanocrystals are embedded in a durable and transparent Cs4PbBr6 matrix. These bulk scintillators exhibit high light yield (33,800 photons MeV-1), low detection limit (79 nGyair s-1), fast decay time (9.8 ns), and outstanding spatial resolution of 8.9 lp mm-1 to X-ray radiation and an energy resolution of 19.3% for γ-ray (59.6 keV) detection. The composite scintillator also shows exceptional stability against environmental degradation and cyclic X-ray radiation. Our results demonstrate a cost-effective strategy for developing perovskite-based bulk transparent scintillators with exceptional performance and high radioluminescence stability for high-energy radiation detection and imaging.

Tuning Electronic and Structural Properties of Lead-Free Metal Halide Perovskites: A Comparative Study of 2D Ruddlesden-Popper and 3D Compositions.

In recent decades, two-dimensional (2D) perovskites have emerged as promising semiconductors for next-generation photovoltaics, showing notable advancements in solar energy conversion. Herein, we explore the impact of alternative inorganic lattice BX-based compositions (B=Ge or Sn, X=Br or I) on the energy gap and stability. Our investigation encompasses BA2Man-1BnX3n+1 2D Ruddlesden-Popper perovskites (for n=1-5 layers) and 3D bulk (MA)BX3 systems, employing first-principles calculations with spin-orbit coupling (SOC), DFT-1/2 quasiparticle, and D3 dispersion corrections. The study unveils how atoms with smaller ionic radii induce anisotropic internal and external distortions within the inorganic and organic lattices. Introducing the spacers in the low-layer regime reduces local distortions but widens band gaps. Our calculation protocol provides deeper insights into the physics and chemistry underlying 2D perovskite materials, paving the way for optimizing environmentally friendly alternatives that can efficiently replace with sustainable materials.

Electron Spectroscopy and Microscopy: A Window into the Surface Electronic Properties of Polycrystalline Metal Halide Perovskites.

In the past years, an increasing number of experimental techniques have emerged to address the need to unveil the chemical, structural, and electronic properties of perovskite thin films with high vertical and lateral spatial resolutions. One of these is angle-resolved photoemission electron spectroscopy which can provide direct access to the electronic band structure of perovskites, with the aim of overcoming elusive and controversial information due to the complex data interpretation of purely optical spectroscopic techniques. This perspective looks at the information that can be gleaned from the direct measurement of the electronic band structure of single crystal perovskites and the challenges that remain to be overcame to extend this technique to heterogeneous polycrystalline metal halide perovskites.

Spatially Resolved Anion Diffusion and Tunable Waveguides in Bismuth Halide Perovskites.

Bismuth halide perovskites are widely regarded as nontoxic alternatives to lead halide perovskites for optoelectronics and solar energy harvesting applications. With a tailorable composition and intriguing optical properties, bismuth halide perovskites are also promising candidates for tunable photonic devices. However, robust control of the anion composition in bismuth halide perovskites remains elusive. Here, we established chemical vapor deposition and anion exchange protocols to synthesize bismuth halide perovskite nanoflakes with controlled dimensions and variable compositions. In particular, we demonstrated the gradient bromide distribution by controlling the anion exchange and diffusion processes, which is spatially resolved by time-of-flight secondary ion mass spectrometry. Moreover, the optical waveguiding properties of bismuth halide perovskites can be modulated by flake thicknesses and anion compositions. With a unique gradient anion distribution and controllable optical properties, bismuth halide perovskites provide new possibilities for applications in optoelectronic devices and integrated photonics.

Room-Temperature Ferroelectric Epitaxial Nanowire Arrays with Photoluminescence.

The development of large-scale, high-quality ferroelectric semiconductor nanowire arrays with interesting light-emitting properties can address limitations in traditional wide-bandgap ferroelectrics, thus serving as building blocks for innovative device architectures and next-generation high-density optoelectronics. Here, we investigate the optical properties of ferroelectric CsGeX3 (X = Br, I) halide perovskite nanowires that are epitaxially grown on muscovite mica substrates by vapor phase deposition. Detailed structural characterizations reveal an incommensurate heteroepitaxial relationship with the mica substrate. Furthermore, photoluminescence that can be tuned from yellow-green to red emissions by varying the halide composition demonstrates that these nanowire networks can serve as platforms for future optoelectronic applications. In addition, the room-temperature ferroelectricity and ferroelectric domain structures of these nanowires are characterized using second harmonic generation (SHG) polarimetry. The combination of room-temperature ferroelectricity with photoluminescence in these nanowire arrays unlocks new avenues for the design of novel multifunctional materials.

Surface Passivation to Enhance the Interfacial Pyro-Phototronic Effect for Self-Powered Photodetection Based on Perovskite Single Crystals.

The interfacial pyro-phototronic effect (IPPE) presents a novel approach for improving the performance of self-powered photodetectors (PDs) based on metal halide perovskites (MHPs). The interfacial contact conditions within the Schottky junctions are crucial in facilitating the IPPE phenomenon. However, the fabrication of an ideal Schottky junction utilizing MHPs is a challenging endeavor. In this study, we present a surface passivation method aimed at enhancing the performance of self-powered photodetectors based on inverted planar perovskite structures in micro- and nanoscale metal-halide perovskite SCs. Our findings demonstrate that the incorporation of a lead halide salt with a benzene ring moiety for surface passivation leads to a substantial improvement in photoresponses by means of the IPPE. Conversely, the inclusion of an alkane chain in the salt impedes the IPPE. The underlying mechanism can be elucidated through an examination of the band structure, particularly the work function (WF) modulated by surface passivation. Consequently, this alteration affects the band bending and the built-in field (VBi) at the interface. This strategy presents a feasible and effective method for producing interfacial pyroelectricity in MHPs, thus facilitating its potential application in practical contexts such as energy conversion and infrared sensors.

Electrical Doping of Metal Halide Perovskites by Co-Evaporation and Application in PN Junctions.

Electrical doping of semiconductors is a revolutionary development that enabled many electronic and optoelectronic technologies. While doping of many inorganic and organic semiconductors is well-established, controlled electrical doping of metal halide perovskites (MHPs) is yet to be demonstrated. In this work, efficient n- and p-type electrical doping of MHPs by co-evaporating the perovskite precursors alongside organic dopant molecules is achieved. It is demonstrated that the Fermi level can be shifted by up to 500 meV toward the conduction band and by up to 400 meV toward the valence band by n- and p-doping, respectively, which increases the conductivity of the films. The doped layers are employed in PN and NP diodes, showing opposing trends in rectification. Demonstrating controlled electrical doping by a scalable, industrially relevant deposition method opens the route to developing perovskite devices beyond solar cells, such as thermoelectrics or complementary logic.

Localized Temperature Engineering Enables Writing of Heterostructures in Glass for Polarized Photoluminescence of Perovskites.

Metal halide perovskite (MHP) structures that exhibit polarized photoluminescence (PL) have attracted significant interest in fabricating light field regulation elements for display, imaging, and information storage applications. We report a three-dimensional direct lithography of heterostructures for controllable polarized PL inside glass by laser-induced localized temperature engineering. The heterostructures consisted of oriented periodic structures (OPSs) and MHP nanocrystals, and the mechanism for hierarchical distribution of heterostructures was illustrated. The patterning of heterostructures for manipulable polarized PL can be used for information encryption, wave-plate, and polarized micro-LEDs.

Micro/Nano Perovskite Materials for Advanced X-ray Detection and Imaging.

Halide perovskites have emerged as highly promising materials for ionizing radiation detection due to their exceptional characteristics, including a large mobility-lifetime product, strong stopping power, tunable band gap, and cost-effective crystal growth via solution processes. Semiconductor-type X-ray detectors employing various micro/nano perovskite materials have shown impressive progress in achieving heightened sensitivity and lower detection limits. Here, we present a comprehensive review of the applications of micro/nano perovskite materials for direct type X-ray detection, with a focus on the requirements for micro/nano crystal assembly and device properties in advanced X-ray detectors. We explore diverse processing techniques and optoelectronic considerations applied to perovskite X-ray detectors. Additionally, this review highlights the challenges and promising opportunities for perovskite X-ray detector arrays in real-world applications, potentially necessitating further research efforts.

Ligand-Induced Chirality in ClMBA2 SnI4 2D Perovskite.

Chiral perovskites possess a huge applicative potential in several areas of optoelectronics and spintronics. The development of novel lead-free perovskites with tunable properties is a key topic of current research. Herein, we report a novel lead-free chiral perovskite, namely (R/S-)ClMBA2 SnI4 (ClMBA=1-(4-chlorophenyl)ethanamine) and the corresponding racemic system. ClMBA2 SnI4 samples exhibit a low band gap (2.12 eV) together with broad emission extending in the red region of the spectrum (∼1.7 eV). Chirality transfer from the organic ligand induces chiroptical activity in the 465-530 nm range. Density functional theory calculations show a Rashba type band splitting for the chiral samples and no band splitting for the racemic isomer. Self-trapped exciton formation is at the origin of the large Stokes shift in the emission. Careful correlation with analogous lead and lead-free 2D chiral perovskites confirms the role of the symmetry-breaking distortions in the inorganic layers associated with the ligands as the source of the observed chiroptical properties providing also preliminary structure-property correlation in 2D chiral perovskites.

Designing Yolk-Shell Nanostructures for Reversible Water-Vapor-Responsive Dual-Mode Switching of Fluorescence and Structural Color.

Metal halide perovskites offer ample opportunities to develop advanced optoelectronic devices. This work showcases that the integration of metal halide perovskites into metal oxide nanoshells with controllable interior cavities can enable water-vapor-responsive dual-mode switching of fluorescence and structural color. Through a ship-in-a-bottle method to introduce a controlled amount of CsPbBr3 into MnO2 nanoshells, we have designed CsPbBr3@MnO2 yolk-shell nanostructures, which can uptake a defined amount of water to exhibit rapid (less than 1 s) and reversible (≥100 cycles) responses in both fluorescence on-off and color change when exposed to dynamic water vapor. These responses originate from the water-triggered phase transformation of CsPbBr3 to CsPb2Br5 and the structural color change of the MnO2 shell. The altered electronic and bonding structure at the oxide-halide interface, rapid water accumulation in the yolk-shell cavity, and protective effect of the oxide shell facilitate the reversible transformations. The response characteristics of the yolk-shell nanostructures have been further demonstrated in fabricating patterned films capable of multiple fluorescence/structural color responses, highlighting their potential for applications in advanced anticounterfeiting and encryption.

How Photogenerated I2 Induces I-Rich Phase Formation in Lead Mixed Halide Perovskites.

Bandgap tunability of lead mixed halide perovskites (LMHPs) is a crucial characteristic for versatile optoelectronic applications. Nevertheless, LMHPs show the formation of iodide-rich (I-rich) phase under illumination, which destabilizes the semiconductor bandgap and impedes their exploitation. Here, it is shown that how I2 , photogenerated upon charge carrier trapping at iodine interstitials in LMHPs, can promote the formation of I-rich phase. I2 can react with bromide (Br- ) in the perovskite to form a trihalide ion I2 Br- (Iδ- -Iδ+ -Brδ- ), whose negatively charged iodide (Iδ- ) can further exchange with another lattice Br- to form the I-rich phase. Importantly, it is observed that the effectiveness of the process is dependent on the overall stability of the crystalline perovskite structure. Therefore, the bandgap instability in LMHPs is governed by two factors, i.e., the density of native defects leading to I2 production and the Br- binding strength within the crystalline unit. Eventually, this study provides rules for the design of chemical composition in LMHPs to reach their full potential for optoelectronic devices.

Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains.

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behavior. In the higher temperature, higher symmetry phases of these materials, several complex structural features are observed. The local structure can differ greatly from the average structure and there is evidence that dynamic 2D structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI3 is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, it is revealed that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and the length and timescales of the motion are reported. Finally, the spatial arrangement of these features and their interactions are investigated and visualized, providing a comprehensive picture of local structure in the higher symmetry phases.

A Comprehensive Review on Perovskite Solar Cells Integrated Photo-supercapacitors and Perovskites-Based Electrochemical Supercapacitors.

Perovskite solar cells (PSCs) have rapidly become a prevalent photovoltaic technology owing to their simple structure, low processing cost, and remarkable increase in solar-to-electric power conversion efficiency (PCE). However, the intermittent nature of solar radiation induces some technical and financial challenges for its practical applications as a reliable power source. To address this issue, the integration of PSCs with supercapacitors (SCs) in the form of integrated photo-supercapacitors (IPSs) has gathered significant attention. This integration can balance energy availability and demand, reduce energy wastage, and stabilize power output for portable and wearable electronics. Meanwhile, the excellent optoelectronic properties with mixed electronic and ionic conductivity of metal halide perovskites (MHPs) have expanded their application as electrode and electrolyte materials for SCs and photo-supercapacitors (PSs) applications. This review provides an all-inclusive summary of the current state-of-the-art research progress of PSCs-IPSs and MHPs-based SCs and PSs by highlighting their basics and integration approaches. It also discusses the challenges and prospects of these materials and technologies.

Rewritable Photoluminescence and Structural Color Display for Dual-Responsive Optical Encryption.

Optical encryption using coloration and photoluminescent (PL) materials can provide highly secure data protection with direct and intuitive identification of encrypted information. Encryption capable of independently controlling wavelength-tunable coloration as well as variable light intensity PL is not adequately demonstrated yet. Herein, a rewritable PL and structural color (SC) display suitable for dual-responsive optical encryption developed with a stimuli-responsive SC of a block copolymer (BCP) photonic crystal (PC) with alternating in-plane lamellae, of which a variety of 3D and 2D perovskite nanocrystals is preferentially self-assembled with characteristic PL, is presented. The SC of a BCP PC is controlled in the visible range with different perovskite precursor doping times. The perovskite nanocrystals developed in the BCP PC are highly luminescent, with a PL quantum yield of ≈33.7%, yielding environmentally stable SC and PL dual-mode displays. The independently programmed SC and PL information is erasable and rewritable. Dual-responsive optical encryption is demonstrated, in which true Morse code information is deciphered only when the information encoded by SCs is properly combined with PL information. Numerous combinations of SC and PL realize high security level of data anticounterfeiting. This dual-mode encryption display offers novel optical encryption with high information security and anti-counterfeiting.

Achieving Multicolor Emitting of Antimony-Doped Indium-Based Halide Perovskite via Monovalent Metal Induced Phase Engineering.

Lead-free metal halide perovskites have attracted attention because of their excellent optical properties and nontoxicity. Here, we report the synthesis of Sb3+-doped indium halide perovskite Cs2InCl5·H2O:Sb3+ by an improved solution coprecipitation method. The treatment of the Sb3+-doped indium halide perovskite with selected monovalent cation halides led to Cs2MInCl6 (Ag+, K+, Na+) in different crystal structures or phases. Sb3+ has an isolated ns2 electron, and Sb3+-doped metal halide acts as the luminescence center and exhibits bright broadband emission that originated from self-trapped excitons. Under UV light excitation, these phosphors with different crystal structures emitted multicolored luminescence ranging from blue, green, yellow, and red depending on whether or not or which monovalent metal ion was used. The phosphor samples were used to print high-resolution 2D color barcodes for security and anticounterfeiting applications. The study presented here provides a new approach for the design and synthesis of lead-free metal halide perovskites with different crystal structures and unique optical properties.