Enhanced Pair-Density-Wave Vertices in a Bilayer Hubbard Model at Half Filling.

Motivated by the pair-density-wave (PDW) state found in the one-dimensional Kondo-Heisenberg chain, we report on a determinant quantum Monte Carlo study of pair fields for a two-dimensional half-filled Hubbard layer coupled to an itinerant, noninteracting layer with one electron per site. In a specific range of interlayer hopping, the pairing vertex associated with PDW order becomes more attractive than that for uniform d-wave pairing, although both remain subdominant to the leading antiferromagnetic correlations at half filling. Our result sheds light on where one potentially may find a PDW state in such a model.

Self-supervised generative models for crystal structures.

Inspired by advancements in natural language processing, we utilize self-supervised learning and an equivariant graph neural network to develop a unified platform for training generative models capable of generating inorganic crystal structures, as well as efficiently adapting to downstream tasks in material property prediction. To mitigate the challenge of evaluating the reliability of generated structures during training, we employ a generative adversarial network (GAN) with its discriminator being a cost-effective reliability evaluator, significantly enhancing model performance. We demonstrate the utility of our model in optimizing crystal structures under predefined conditions. Without external properties acquired experimentally or numerically, our model further displays its capability to help understand inorganic crystal formation by grouping chemically similar elements. This paper extends an invitation to further explore the scientific understanding of material structures through generative models, offering a fresh perspective on the scope and efficacy of machine learning in material science.

Application of Metal Halide Perovskite in Internet of Things.

The Internet of Things (IoT) technology connects the real and network worlds by integrating sensors and internet technology, which has greatly changed people's lifestyles, showing its broad application prospects. However, traditional materials for the sensors and power components used in the IoT limit its development for high-precision detection, long-term endurance, and multi-scenario applications. Metal halide perovskite, with unique advantages such as excellent photoelectric properties, an adjustable bandgap, flexibility, and a mild process, exhibits enormous potential to meet the requirements for IoT development. This paper provides a comprehensive review of metal halide perovskite's application in sensors and energy supply modules within IoT systems. Advances in perovskite-based sensors, such as for gas, humidity, photoelectric, and optical sensors, are discussed. The application of indoor photovoltaics based on perovskite in IoT systems is also discussed. Lastly, the application prospects and challenges of perovskite-based devices in the IoT are summarized.

Bridging-Solvent Strategy for Quasi-Single-Crystal Perovskite Films and Stable Solar Cells.

Controllable fabrication of formamidinium (FA)-based perovskite solar cells (PSCs) with both high efficiency and long-term stability is the key to their further commercialization. However, the diversity of PbI2 complexes and perovskite compositions usually leads to light sensitive PbI2 residues and phase impurities in the film, which can accelerate the device degradation. Here, the crystallization kinetics of FA-based perovskite films are studied and a bridging-solvent strategy is proposed to modulate the reaction kinetics between PbI2 and ammonium salts by prohibiting the formation of undesired intermediates. N-methylpyrrolidone (NMP) solvent is introduced into the PbI2 precursor solution to obtain stable and homogeneous PbI2-NMP complex films. The strong interaction between NMP and formamidinium iodide (FAI) molecules promotes the conversion from PbI2-NMP into (001)-oriented quasi-single-crystal perovskite films with negligible impurities, long carrier lifetime of 1.5 µs and a large grain size of 3 µm. The optimized PSCs exhibit a high power conversion efficiency of 24.1%, as well as superior shelf stability which maintains 95% initial efficiency after storage in air for 1200 h (T95 = 1200 h), and operating stability with T96 = 300 h under continuous working at the maximum power point. This work offers a simple and reproducible method for fabricating phase-pure and uniaxially oriented perovskite films.

Synthesis and Optical Properties of CdSeTe/CdZnS/ZnS Core/Shell Nanorods.

Semiconductor nanorods (NRs) have great potential in optoelectronic devices for their unique linearly polarized luminescence which can break the external quantum efficiency limit of light-emitting diodes (LEDs) based on spherical quantum dots. Significant progress has been made for developing red, green, and blue light-emitting NRs. However, the synthesis of NRs emitting in the deep red region, which can be used for accurate red LED displays and promoting plant growth, is currently less explored. Here, we report the synthesis of deep red CdSeTe/CdZnS/ZnS dot-in-rod core/shell NRs via a seeded growth method, where the doping of Te in the CdSe core can extend the NR emission to the deep red region. The rod-shaped CdZnS shell is grown over CdSeTe seeds. By growing a ZnS passivation shell, the CdSeTe/CdZnS/ZnS NRs exhibit a photoluminescence emission peak at 670 nm, a full width at a half maximum of 61 nm and a photoluminescence quantum yield of 45%. The development of deep red NRs can greatly extend the applications of anisotropic nanocrystals.

High-Efficiency and Stable Colloidal One-Dimensional Core/Shell Nanorod Light-Emitting Diodes.

Anisotropic nanocrystals such as nanorods (NRs) display unique linearly polarized emission, which is expected to break the external quantum efficiency (EQE) limit of quantum dot-based light-emitting diodes (LEDs). However, the progress in achieving a higher EQE using NRs encounters several challenges, primarily involving a low photoluminescence quantum yield (PLQY) of NRs and imbalanced charge injection in NR-LEDs. In this work, we investigated NR-LEDs based on CdSe/CdZnS/ZnS rod-in-rod NRs with a high PLQY and higher linear polarization compared to those of dot-in-rod NRs. The balanced charge injection is achieved using ZnMgO nanoparticles as the electron transport layer and poly-TPD {poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine]} as the hole transport layer. Therefore, the NR-LEDs exhibit a maximum EQE of 21.5% and a maximum luminance of >120 000 cd/m2 owing to the high level of in-plane transitions with a dipole moment of 90%. The NR-LEDs also have greatly inhibited droop in EQE under a high current density as well as outstanding operation lifetime and cycle stability.

Uniaxial-Oriented Perovskite Films with Controllable Orientation.

Perovskite films with large crystal size, preferred orientation, and facile fabrication process, combining advantages of single-crystal and polycrystalline films, have gained considerable attention recently. However, there is little research on the facet properties of perovskite films. Here, (111)- and (001)-oriented perovskite films with bandgaps ranging from 1.53 to 1.77 eV, and systematically investigated their orientation-dependent properties are achieved. The (111)-oriented films show electron-dominated traps and the (001)-oriented films show hole-dominated traps, which are related to their atomic arrangement at the surface. Compared with the (001)-oriented films, the (111)-oriented films exhibit lower work function and superior water/oxygen robustness. For the wide-bandgap films, the lattice of the (001)-oriented film provides an unobstructed passage for ion migration. Comparably, the (111)-oriented films exhibit suppressed ion migration and excellent phase stability. The optimized unencapsulated solar cells based on both (001) and (111) orientations show a similar high efficiency of ≈23%. The (111)-oriented solar cell exhibits excellent stability, maintaining 95% of its initial efficiency after 1500 h maximum power point (MPP) tracking test, and 97% initial efficiency after 3000 h aging in ambient conditions. This work paves the way for the rational design, controllable synthesis, and targeted optimization of uniaxial-oriented perovskite films for various electronic applications.

22% Record Efficiency in Nanorod Light-Emitting Diodes Achieved by Gradient Shells.

The external quantum efficiency (EQE) in light-emitting diodes (LEDs) based on isotropic quantum dots has approached the theoretical limit of close to 20%. Anisotropic nanorods can break this limit by taking advantage of their directional emission. However, the progress towards higher EQE by using CdSe/CdS nanorods (NRs) faces several challenges, primarily involving the low quantum yield and unbalanced charge injection in devices. Herein, the seeded growth method is modified and anisotropic nanorods are obtained with photoluminescence quantum yield up to 98% by coating a gradient alloyed CdZnSe shell around conventional spherical CdSe seeds. This intermediate alloyed CdZnSe shell combined with a subsequent rod-shaped CdZnS/ZnS shell can effectively suppress the electron delocalization in the typical CdSe/CdS nanorods due to their small conduction bandgap offset. Additionally, this alloyed shell can reduce the hole-injection barrier and create a larger barrier for electron injection, both effects promoting a balanced injection of electrons and holes in LEDs. Hence, LEDs are reached with high brightness (160341 cd m-2) and high efficiency (EQE = 22%, current efficiency = 23.19 cd A-1), which are the highest values to date for nanorod LEDs.

Ultrastable and High-Efficiency Deep Red QLEDs through Giant Continuously Graded Colloidal Quantum Dots with Shell Engineering.

Quantum dot (QD) based light-emitting diodes (QLEDs) hold great promise for next-generation lighting and displays. In order to reach a wide color gamut, deep red QLEDs emitting at wavelengths beyond 630 nm are highly desirable but have rarely been reported. Here, we synthesized deep red emitting ZnCdSe/ZnSeS QDs (diameter ∼16 nm) with a continuous gradient bialloyed core-shell structure. These QDs exhibit high quantum yield, excellent stability, and a reduced hole injection barrier. The QLEDs based on ZnCdSe/ZnSeS QDs have an external quantum efficiency above 20% in the luminance range of 200-90000 cd m-2 and a record T95 operation lifetime (time for the luminance to decrease to 95% of its initial value) of more than 20000 h at a luminance of 1000 cd m-2. Furthermore, the ZnCdSe/ZnSeS QLEDs have outstanding shelf stability (>100 days) and cycle stability (>10 cycles). The reported QLEDs with excellent stability and durability can accelerate the pace of QLED applications.

(111)-Dominated Perovskite Films by Antisolvent Engineering.

Fabricating perovskite films with a dominant crystal orientation is an effective path to realizing quasi-single-crystal perovskite film, which can eliminate the fluctuation of the electrical properties in films arising from grain-to-grain variations, and improve the performance of perovskite solar cells (PSCs). Perovskite (FAPbI3 ) films based on one-step antisolvent methods usually suffer from chaotic orientations due to the inevitable intermediate phase conversion from intermediates of PbI2 •DMSO, FA2 Pb3 I8 •4DMSO, and δ-FAPbI3 to α-FAPbI3 . Here, a high-quality perovskite film with (111) preferred orientation ((111)-α-FAPbI3 ) using a short-chain isomeric alcohol antisolvent, isopropanol (IPA) or isobutanol (IBA), is reported. The interaction between IPA and PbI2 leads to a corner-sharing structure instead of an edge-sharing PbI2 octahedron, sidestepping the formation of these intermediates. With the volatilization of IPA, FA+ can replace IPA in situ to form α-FAPbI3 along the (111) direction. Compared to randomly orientated perovskites, the dominantly (111) orientated perovskite ((111)-perovskite) exhibits improved carrier mobility, uniform surface potential, suppressed film defects and enhanced photostability. PSCs based on the (111)-perovskite films show 22% power conversion efficiency and excellent stability, which remains unchanged after 600 h continuous working at maximum power point, and 95% after 2000 h of storage in atmosphere environment.

Rugged bialkali photocathodes encapsulated with graphene and thin metal film.

Protection of free-electron sources has been technically challenging due to lack of materials that transmit electrons while preventing corrosive gas molecules. Two-dimensional materials uniquely possess both of required properties. Here, we report three orders of magnitude increase in active pressure and factor of two enhancement in the lifetime of high quantum efficiency (QE) bialkali photocathodes (cesium potassium antimonide (CsK2Sb)) by encapsulating them in graphene and thin nickel (Ni) film. The photoelectrons were extracted through the graphene protection layer in a reflection mode, and we achieved QE of ~ 0.17% at ~ 3.4 eV, 1/e lifetime of 188 h with average current of 8.6 nA under continuous illumination, and no decrease of QE at the pressure of as high as ~ 1 × 10-3 Pa. In comparison, the QE decreased drastically at 10-6 Pa for bare, non-protected CsK2Sb photocathodes and their 1/e lifetime under continuous illumination was ~ 48 h. We attributed the improvements to the gas impermeability and photoelectron transparency of graphene.

Spectral and polarization based imaging in deep-ultraviolet excited photoelectron microscopy.

Using photoelectron emission microscopy, nanoscale spectral imaging of atomically thin MoS2 buried between Al2O3 and SiO2 is achieved by monitoring the wavelength and polarization dependence of the photoelectron signal excited by deep-ultraviolet light. Although photons induce the photoemission, images can exhibit resolutions below the photon wavelength as electrons sense the response. To validate this concept, the dependence of photoemission yield on the wavelength and polarization of the exciting light was first measured and then compared to simulations of the optical response quantified with classical optical theory. A close correlation between experiment and theory indicates that photoemission probes the optical interaction of UV-light with the material stack directly. The utility of this probe is then demonstrated when both the spectral and polarization dependence of photoemission observe spatial variation consistent with grains and defects in buried MoS2. Taken together, these new modalities of photoelectron microscopy allow mapping of optical property variation at length scales unobtainable with conventional light-based microscopy.

Surface-capping engineering for electrically neutral surface of perovskite films and stable solar cells.

Metal halide perovskite solar cells (PSCs) have developed rapidly in recent years, due to their high performance and low-cost solution-based fabrication process. These excellent properties are mainly attributed to the high defect tolerance of polycrystalline perovskite films. Meanwhile, these defects can also facilitate ion migration and carrier recombination, which cause the device performance and the long-term stability of PSCs to deteriorate heavily. Therefore, it is critical to passivate the defects, especially at the surfaces of perovskite grains where the defects are most concentrated due to the dangling bonds. Here we propose a surface-capping engineering (SCE) method to construct 'dangling-bond-free' surfaces for perovskite grains. Diamine iodide (methylenediammonium diiodide, MDAI2) was used to construct an electroneutral PbX6-MDA-PbX6(X = Cl, Br or I) layer at the perovskite surfaces. Compared to the monovalent FA+which can only coordinate one [PbX6]4-slab, the bivalent MDA2+can coordinate two [PbX6]4-slabs on both sides, thus realizing a dangling-bond-free surface. Solar cells based on SCE-perovskite films exhibited a higher power conversion efficiency (PCE) of 21.6%, compared with 19.9% of the control group; and maintained over 96% of its initial PCE after 13 h during the maximum power point tracking test under continuous AM1.5G illumination, whereas the control group only lasted 1.5 h. Constructing a dangling-bond-free capping layer on the grain boundary opens new avenues for the fabrication of ultralow-defect polycrystalline semiconductors, paving the way to further improve the PCE and lifetime of PSCs.

Halide perovskites and perovskite related materials for particle radiation detection.

Radiation detectors are widely used in physics, materials science, chemistry, and biology. Halide perovskites are known for their superior properties including tunable bandgaps and chemical compositions, high defect tolerance, solution-processable synthesis of films and crystals, and high carrier diffusion length. Recently, halide perovskites have attracted enormous interest as particle radiation detectors for both charged (α and ß) and uncharged (neutrons) particles. Solid-state detectors based on single crystal perovskites can detect α particles and thermal neutrons with energy-resolved spectra. Halide perovskite scintillators are also able to detect ß particles and fast neutrons. In this review, we briefly introduce the fundamentals of radiation detection and summarize the recent progress on halide perovskite detectors for particle radiation.

Photoemission from Bialkali Photocathodes through an Atomically Thin Protection Layer.

Photocathodes are essential components for various applications requiring photon-to-free-electron conversion, for example, high-sensitivity photodetectors and electron injectors for free-electron lasers. Alkali antimonide thin films are widely used as photocathode materials owing to their high quantum efficiency (QE) in the visible spectral range; however, their lifetime can be limited even in ultrahigh vacuum due to their high reactivity to residual gases and sensitivity to ion back-bombardment in these applications. An ambitious technical challenge is to extend the lifetime of bialkali photocathodes by coating them with suitable materials that can isolate the photocathode films from residual gases while still maintaining their highly emissive properties. We propose the use of graphene, an atomically thin two-dimensional material with gas impermeability, as a promising candidate for this purpose. Here, we report that high-quality bialkali antimonide can be grown on a two-layer (2L) suspended graphene substrate with a peak QE of 15%. More importantly, by comparing the photoemission through varying layers of graphene, we demonstrate that photoelectrons can transmit through few-layer graphene with a maximum QE of over 0.7% at 4.5 eV for 2L graphene, corresponding to a transmission efficiency of 5%. These results demonstrate important progress toward fully encapsulated bialkali photocathodes having both high QEs and long lifetimes using atomically thin protection layers.

Cesium-Coated Halide Perovskites as a Photocathode Material: Modeling Insights.

Photocathodes emit electrons when illuminated, a process utilized across many technologies. Cutting-edge applications require a set of operating conditions that are not met with current photocathode materials. Meanwhile, halide perovskites have been studied extensively and have shown a lot of promise for a wide variety of optoelectronic applications. Well-documented halide perovskite properties such as inexpensive growth techniques, improved carrier mobility, low trap density, and tunable direct band gaps make them promising candidates for next-generation photocathode materials. Here, we use density functional theory to explore the possible application of pure inorganic perovskites (CsPbBr3 and CsPbI3) as photocathodes. It is determined that the addition of a Cs coating improved the performance by lowering the work function anywhere between 1.5 and 3 eV depending on the material, crystal surface, and surface coverage. A phenomenological model, modified from that developed by Gyftopoulos and Levine, is used to predict the reduction in work function with Cs coverage. The results of this work aim to guide the further experimental development of Cs-coated halide perovskites for photocathode materials.

Highly efficient photoelectric effect in halide perovskites for regenerative electron sources.

Electron sources are a critical component in a wide range of applications such as electron-beam accelerator facilities, photomultipliers, and image intensifiers for night vision. We report efficient, regenerative and low-cost electron sources based on solution-processed halide perovskites thin films when they are excited with light with energy equal to or above their bandgap. We measure a quantum efficiency up to 2.2% and a lifetime of more than 25 h. Importantly, even after degradation, the electron emission can be completely regenerated to its maximum efficiency by deposition of a monolayer of Cs. The electron emission from halide perovskites can be tuned over the visible and ultraviolet spectrum, and operates at vacuum levels with pressures at least two-orders higher than in state-of-the-art semiconductor electron sources.

Role of the Metal-Semiconductor Interface in Halide Perovskite Devices for Radiation Photon Counting.

Halide perovskites are promising optoelectronic semiconductors. For applications in solid-state detectors that operate in low photon flux counting mode, blocking interfaces are essential to minimize the dark current noise. Here, we investigate the interface between methylammonium lead tri-iodide (MAPbI3) single crystals and commonly used high and low work function metals to achieve photon counting capabilities in a solid-state detector. Using scanning photocurrent microscopy, we observe a large Schottky barrier at the MAPbI3/Pb interface, which efficiently blocks dark current. Moreover, the shape of the photocurrent profile indicates that the MAPbI3 single-crystal surface has a deep fermi level close to that of Au. Rationalized by first-principle calculations, we attribute this observation to the defects due to excess iodine on the surface underpinning emergence of deep band-edge states. The photocurrent decay profile yields a charge carrier diffusion length of 10-25 µm. Using this knowledge, we demonstrate a single-crystal MAPbI3 detector that can count single γ-ray photons by producing sharp electrical pulses with a fast rise time of <2 µs. Our study indicates that the interface plays a crucial role in solid-state detectors operating in photon counting mode.

A sensitive and robust thin-film x-ray detector using 2D layered perovskite diodes.

Solid-state radiation detectors, using crystalline semiconductors to convert radiation photons to electrical charges, outperform other technologies with high detectivity and sensitivity. Here, we demonstrate a thin-film x-ray detector comprised with highly crystalline two-dimensional Ruddlesden-Popper phase layered perovskites fabricated in a fully depleted p-i-n architecture. It shows high diode resistivity of 1012 ohm·cm in reverse-bias regime leading to a high x-ray detecting sensitivity up to 0.276 C Gyair -1 cm-3. Such high signal is collected by the built-in potential underpinning operation of primary photocurrent device with robust operation. The detectors generate substantial x-ray photon-induced open-circuit voltages that offer an alternative detecting mechanism. Our findings suggest a new generation of x-ray detectors based on low-cost layered perovskite thin films for future x-ray imaging technologies.