Large lattice distortions and size-dependent bandgap modulation in epitaxial halide perovskite nanowires

On-Wire Design of Axial Periodic Halide Perovskite Superlattices for High-Performance Photodetection

Precise synthesis of all-inorganic lead halide perovskite nanowire heterostructures and superlattices with designable modulation of chemical compositions is essential for tailoring their optoelectronic properties. Nevertheless, controllable synthesis of perovskite nanostructure heterostructures remains challenging and underexplored to date. Here, we report a rational strategy for wafer-scale synthesis of one-dimensional periodic CsPbCl3/CsPbI3 superlattices. We show that the highly parallel array of halide perovskite nanowires can be prepared roughly as horizontally guided growth on an M-plane sapphire. A periodic patterning of the sapphire substrate enables position-selective ion exchange to obtain highly periodic CsPbCl3/CsPbI3 nanowire superlattices. This patterning is further confirmed by micro-photoluminescence investigations, which show that two separate band-edge emission peaks appear at the interface of a CsPbCl3/CsPbI3 heterojunction. Additionally, compared with the pure CsPbCl3 nanowires, photodetectors fabricated using these periodic heterostructure nanowires exhibit superior photoelectric performance, namely, high ION/IOFF ratio (104), higher responsivity (49 A/W), and higher detectivity (1.51 × 1013 Jones). Moreover, a spatially resolved visible image sensor based on periodic nanowire superlattices is demonstrated with good imaging capability, suggesting promising application prospects in future photoelectronic imaging systems. All these results based on the periodic CsPbCl3/CsPbI3 nanowire superlattices provides an attractive material platform for integrated perovskite devices and circuits.

Replicating CD Nanogrooves onto PDMS to Guide Nanowire Growth for Monolithic Flexible Photodetectors with High Bending-Stable UV-vis-NIR Photoresponse

Guided nanowires grown on polymer surfaces facilitate their seamless integration as flexible devices without post-growth processing steps. However, this is challenging due to the inability of polymer films to provide the required lattice-matching effect. In this work, this challenge is addressed by replicating highly aligned nanogrooves from a compact disc (CD) onto a casted flexible polydimethylsiloxane (PDMS) surface. Leveraging the replicated nanogrooves, copper hexadecafluorophthalocyanine (F16CuPc) and various metal phthalocyanines are guided into large-area, self-aligned nanowires. Subsequently, by employing specifically designed shadow masks during electrode deposition, these nanowires are seamlessly integrated as either a monolithic flexible photodetector with a large sensing area or on-chip flexible photodetector arrays. The resulting flexible photodetectors exhibit millisecond and long-term stable response to UV-vis-NIR light. Notably, they demonstrate exceptional bending stability, retaining stable and sensitive photoresponse even at a curvature radius as low as 0.5 cm and after enduring 1000 bending cycles. Furthermore, the photodetector array showcases consistent sensitivity and response speed across the entire array. This work not only proves the viability of guided nanowire growth on flexible polymer surfaces by replicating CD nanogrooves but also underscores the potential for large-scale monolithic integration of guided nanowires as flexible devices.

Trivalent Europium-Doped CsCl Quantum Dots for MA-Free Perovskite Solar Cells with Inherent Bandgap through Lattice Strain Compensation

Cesium-formamidinium (Cs-FA) perovskites have garnered widespread interest owing to their excellent thermal- and photostability in achieving stable perovskite solar cells (PSCs). However, Cs-FA perovskite typically suffers from Cs+ and FA+ mismatches, affecting the Cs-FA morphology and lattice distortion, resulting in an enlarged bandgap (Eg ). In this work, "upgraded" CsCl, Eu3+ -doped CsCl quantum dots, are developed to solve the key issues in Cs-FA PSCs and also exploit the advantage of Cs-FA PSCs on stability. The introduction of Eu3+ promotes the formation of high-quality Cs-FA films by adjusting the Pb-I cluster. CsCl:Eu3+ also offsets the local strain and lattice contraction induced by Cs+ , which maintains the inherent Eg of FAPbI3 and decreases the trap density. Finally, a power conversion efficiency (PCE) of 24.13% is obtained with an excellent short-circuit current density of 26.10 mA cm-2 . The unencapsulated devices show excellent humidity stability and storage stability, and an initial PCE of 92.2% within 500 h under continuous light illumination, and bias voltage conditions is achieved. This study provides a universal strategy to address the inherent issues of Cs-FA devices and maintain the stability of MA-free PSCs to satisfy future commercial criteria.

On-site growth of perovskite nanocrystal arrays for integrated nanodevices

Despite remarkable progress in the development of halide perovskite materials and devices, their integration into nanoscale optoelectronics has been hindered by a lack of control over nanoscale patterning. Owing to their tendency to degrade rapidly, perovskites suffer from chemical incompatibility with conventional lithographic processes. Here, we present an alternative, bottom-up approach for precise and scalable formation of perovskite nanocrystal arrays with deterministic control over size, number, and position. In our approach, localized growth and positioning is guided using topographical templates of controlled surface wettability through which nanoscale forces are engineered to achieve sub-lithographic resolutions. With this technique, we demonstrate deterministic arrays of CsPbBr₃ nanocrystals with tunable dimensions down to <50 nm and positional accuracy <50 nm. Versatile, scalable, and compatible with device integration processes, we then use our technique to demonstrate arrays of nanoscale light-emitting diodes, highlighting the new opportunities that this platform offers for perovskites' integration into on-chip nanodevices.

Flexible Microspectrometers Based on Printed Perovskite Pixels with Graded Bandgaps

Miniaturized spectrometers have attracted much attention due to their capability to detect spectral information within a small size. However, such technology still faces challenges including large-scale preparation and performance repeatability. In this work, we overcome these challenges by demonstrating a microspectrometer constructed with a series of pixelized graded-bandgap perovskite photodetectors fabricated with inkjet printing. High-quality perovskite films with minimal pinholes and large grains are deposited by optimizing printing conditions including substrate temperature and surface modification. The resulting perovskite photodetectors show decent photosensing performance, and the different photodetectors based on perovskite films with different bandgaps exhibit various spectral responsivities with different cutoff wavelength edges. Microspectrometers are then constructed with the array of the pixelized graded-bandgap perovskite photodetectors, and incident spectra are algorithmically reconstructed by combining their output currents. The reconstruction performance of the miniaturized spectrometer is evaluated by comparing the results to the spectral curve measured with a commercial bulky spectrometer, indicating a reliable spectral reconstruction with a resolution of around 10 nm. More significantly, the miniaturized spectrometers are successfully fabricated on polymer substrates, and they demonstrate excellent mechanical flexibility. Therefore, this work provides a flexible miniaturized spectrometer with large-scale fabricability, which is promising for emerging applications including wearable devices, hyperspectral imaging, and internet of things.

Light-Induced Structural Dynamics and Charge Transport in Layered Halide Perovskite Thin Films

The dynamic nature of the metal halide perovskite lattice upon photoexcitation plays a vital role in their properties. Here we report an observation of light-induced structure dynamics in quasi-2D Ruddlesden-Popper phase perovskite thin films and its impact on the carrier transport properties. By a time-resolved X-ray scattering technique, we observe a rapid lattice expansion upon photoexcitation, followed by a slow relaxation over the course of 100 ns in the dark. Theoretical modeling suggests that the expansion originates from the lattice's thermal fluctuations caused by photon energy deposition. Power dependent optical spectroscopy and photoconductivity indicate that high laser powers triggered a strong local structural disorder, which increased the charge dissociation activation energy that results in localized transport. Our study investigates the impact of laser energy deposition on the lattices and the subsequent carrier transport properties, that are relevant to device operations.

Quasi-One-Dimensional Metallicity in Compressed CsSnI3

Low-dimensional metal halides exhibit strong structural and electronic anisotropies, making them candidates for accessing unusual electronic properties. Here, we demonstrate pressure-induced quasi-one-dimensional (quasi-1D) metallicity in δ-CsSnI₃. With the application of pressure up to 40 GPa, the initially insulating δ-CsSnI₃ transforms to a metallic state. Synchrotron X-ray diffraction and Raman spectroscopy indicate that the starting 1D chain structure of edge-sharing Sn-I octahedra in δ-CsSnI₃ is maintained in the high-pressure metallic phase while the SnI₆ octahedral chains are distorted. Our experiments combined with first-principles density functional theory calculations reveal that pressure induces Sn-Sn hybridization and enhances Sn-I coupling within the chain, leading to band gap closure and formation of conductive SnI₆ distorted octahedral chains. In contrast, the interchain I...I interactions remain minimal, resulting in a highly anisotropic electronic structure and quasi-1D metallicity. Our study offers a high-pressure approach for achieving diverse electronic platforms in the broad family of low-dimensional metal halides.

Self-Healing Ability of Perovskites Observed via Photoluminescence Response on Nanoscale Local Forces and Mechanical Damage

The photoluminescence (PL) of metal halide perovskites can recover after light or current-induced degradation. This self-healing ability is tested by acting mechanically on MAPbI3 polycrystalline microcrystals by an atomic force microscope tip (applying force, scratching, and cutting) while monitoring the PL. Although strain and crystal damage induce strong PL quenching, the initial balance between radiative and nonradiative processes in the microcrystals is restored within a few minutes. The stepwise quenching-recovery cycles induced by the mechanical action is interpreted as a modulation of the PL blinking behavior. This study proposes that the dynamic equilibrium between active and inactive states of the metastable nonradiative recombination centers causing blinking is perturbed by strain. Reversible stochastic transformation of several nonradiative centers per microcrystal under application/release of the local stress can lead to the observed PL quenching and recovery. Fitting the experimental PL trajectories by a phenomenological model based on viscoelasticity provides a characteristic time of strain relaxation in MAPbI3 on the order of 10-100 s. The key role of metastable defect states in nonradiative losses and in the self-healing properties of perovskites is suggested.

Improved extraction efficiency of CsPbBr3 perovskite light-emitting diodes due to anodic aluminum oxide nanopore structure

In this work, we investigate the improvement in the performance of a CsPbBr₃ perovskite light-emitting diode (PeLED) due to an anodic aluminum oxide (AAO) nanopore structure. The AAO structure in the CsPbBr₃ PeLED structure can improve the light extraction efficiency of CsPbBr₃ PeLEDs in two ways: the emission light in the side direction being redirected to the normal direction due to the light scattering effect caused by aluminum oxide nanopores and the effective emission area as a result of the rough surface of the AAO structure. The peak luminance, current efficiency, and external quantum efficiency (EQE) were 11,460 cd/m², 2.03 cd/A, and 0.69% at a bias of 6.0 V, respectively. For comparison, the luminance, current efficiency, and EQE values of CsPbBr₃ PeLEDs with the AAO structure using 50 V of pore-expanding voltage demonstrated improvements of 282%, 190%, and 1280%, respectively, over CsPbBr₃ PeLEDs without the AAO structure.

Insight on noncovalent interactions and orbital constructs in low-dimensional antimony halide perovskites

Reported is a series of eight antimony halide perovskite derivatives synthesized from acidic aqueous solutions of antimony oxide and halogen substituted pyridines. These materials feature anionic one-dimensional antimony halide (SbX; X = Cl, Br, I) chains or ribbons charge-balanced by organic para-halopyridinium cations (XPy; X = H, Cl, Br) which assemble into three-dimensional networks via halogen and hydrogen noncovalent interactions (NCIs) between ion pairs. Computational density functional theory (DFT) based natural bonding orbital (NBO) and density of state (DOS) methods were utilized to map the band structure and quantify and categorize noncovalent interaction strength and type. Moreover, we determined the presence of hybridized intermediate bands which are responsible for the small bandgap energies within this family and arise from mixing of the halide p-states and the Sb s-states. We note that the degree of hybridization, and thus optical properties, is influenced primarily by changes about inner sphere bonding and independent of second sphere interactions. This report is the first to specifically monitor the evolution of haloantimonate(III) hybrid perovskite atomic and molecular orbitals involved in optical behavior as a function of inner and outer sphere effects.

Holistic Determination of Optoelectronic Properties using High-Throughput Spectroscopy of Surface-Guided CsPbBr3 Nanowires

Optoelectronic micro- and nanostructures have a vast parameter space to explore for modification and optimization of their functional performance. This paper reports on a data-led approach using high-throughput single nanostructure spectroscopy to probe >8000 structures, allowing for holistic analysis of multiple material and optoelectronic parameters with statistical confidence. The methodology is applied to surface-guided CsPbBr3 nanowires, which have complex and interrelated geometric, structural, and electronic properties. Photoluminescence-based measurements, studying both the surface and embedded interfaces, exploits the natural inter nanowire geometric variation to show that increasing the nanowire width reduces the optical bandgap, increases the recombination rate in the nanowire bulk, and reduces the rate at the surface interface. A model of carrier recombination and diffusion ascribes these trends to carrier density and strain effects at the interfaces and self-consistently retrieves values for carrier mobility, trap densities, bandgap, diffusion length, and internal quantum efficiency. The model predicts parameter trends, such as the variation of internal quantum efficiency with width, which is confirmed by experimental verification. As this approach requires minimal a priori information, it is widely applicable to nano- and microscale materials.

Potential Application of Perovskite Structure for Water Treatment: Effects of Band Gap, Band Edges, and Lifetime of Charge Carrier for Photocatalysis

Perovskite structures have attracted scientific interest as a promising alternative for water treatment due to their unique structural, high oxidation activity, electronic stability, and optical properties. In addition, the photocatalytic activity of perovskite structures is higher than that of many transition metal compounds. A critical property that determines the high-performance photocatalytic and optical properties is the band gap, lifetime of carrier charge, and band edges relative to the redox potential. Thus, the synthesis/processing and study of the effect on the band gap, lifetime of carrier charge, and band edges relative to the redox potential in the development of high-performance photocatalysts for water treatment are critical. This review presents the basic physical principles of optical band gaps, their band gap tunability, potentials, and limitations in the applications for the water treatment. Furthermore, it reports recent advances in the synthesis process and comparatively examines the band gap effect in the photocatalytic response. In addition to the synthesis, the physical mechanisms associated with the change in the band gap have been discussed. Finally, the conclusions of this review, along with the current challenges of perovskites for photocatalysis, are presented.

Controlling the Phase Transition in CsPbI3 Nanowires

Cesium lead iodide (CsPbI₃) is a promising semiconductor with a suitable band gap for optoelectronic devices. CsPbI₃ has a metastable perovskite phase that undergoes a phase transition into an unfavorable nonperovskite phase in an ambient environment. This phase transition changes the optoelectronic properties of CsPbI₃ and hinders its potential for device applications. Therefore, it is of central importance to understand the kinetics of such instability and develop strategies to control and stabilize the perovskite phase. Here, we use ultralong CsPbI₃ nanowires as a model platform to investigate the phase transition kinetics. Our results depict the role of environmental stressors (moisture and temperature) in controlling the phase transition dynamics of CsPbI₃, which can serve as guiding principles for future phase transition studies and the design of related photovoltaics. Furthermore, we demonstrate the controllability of phase propagation on individual nanowires by varying the moisture level and temperature.

Single-Crystalline Perovskite Nanowire Arrays for Stable X-ray Scintillators with Micrometer Spatial Resolution

X-ray scintillation detectors based on metal halide perovskites have shown excellent light yield, but they mostly target applications with spatial resolution at the tens of micrometers level. Here, we use a one-step solution method to grow arrays of 15-μm-long single-crystalline CsPbBr₃ nanowires (NWs) in an AAO (anodized aluminum oxide) membrane template, with nanowire diameters ranging from 30 to 360 nm. The CsPbBr₃ nanowires in AAO (CsPbBr₃ NW/AAO) show increasing X-ray scintillation efficiency with decreasing nanowire diameter, with a maximum photon yield of ∼5 300 ph/MeV at 30 nm diameter. The CsPbBr₃ NW/AAO composites also display high radiation resistance, with a scintillation-intensity decrease of only ∼20-30% after 24 h of X-ray exposure (integrated dose 162 Gy_air) and almost no change after ambient storage for 2 months. X-ray images can distinguish line pairs with a spacing of 2 μm for all nanowire diameters, while slanted edge measurements show a spatial resolution of ∼160 lp/mm at modulation transfer function (MTF) = 0.1. The combination of high spatial resolution, radiation stability, and easy fabrication makes these CsPbBr₃ NW/AAO scintillators a promising candidate for high-resolution X-ray imaging applications.

Peculiar Steric Hindrance Assists Monoclinic Phase Formation toward High-Quality All-Inorganic Perovskites

Bromine-containing metal halide all-inorganic perovskite CsPbI₂Br exhibits excellent photoelectric performance and supreme thermal and structural stabilities; it is thus attractive for use as photoabsorbing layers in perovskite solar cells (PSCs). However, when steric hindrance molecules are introduced, the complicated phase transition mechanism and the difficult-to-control crystallization process in CsPbI₂Br are not well understood. Here, we introduce a class of sterically hindered cesium naphthenate small molecules to control the crystallization process of CsPbI₂Br films. Of interest, a new intermediate monoclinic phase has been discovered which leads to formation of dense and nonporous polycrystalline perovskite films. This phenomenon was also explained by density functional theory. The residues of steric hindrance molecules inside the CsPbI₂Br film also improve its stability. We further show that as the ring number of cycloalkanes increases, the hindrance for the crystallization becomes more significant. Thus, by choosing the suitable steric hindrance, the optimal photovoltaic efficiency is 15.45%.

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.

Upconversion Perovskite Nanocrystal Heterostructures with Enhanced Luminescence and Stability by Lattice Matching

Lead halide perovskite quantum dots (PQDs) exhibit excellent photoelectric and optical properties, but their poor stability and low multiphoton absorption efficiency greatly limit their biological applications. Efforts have been made to combine upconversion nanoparticles (UCNPs) with PQDs to produce a composite material that is NIR-excitable, upconverting, and emission-tunable due to the unique optical properties of UCNPs, which converts tissue-penetrating near-infrared light into visible light based on an upconversion multiphoton excitation process. However, it is challenging to make such a nanocrystal heterostructure and maintain good optical properties and stability of both UCNPs and PQDs because they have different crystal structures. Here, we report the synthesis of heterostructured UCNP-PQD nanocrystals to bring hexagonal-phase NaYF₄ UCNPs and cubic-phase CsPbBr₁X₂ PQDs in close proximity in a single nanocrystal, leading to efficient Förster resonance energy transfer (FRET) from the UCNP to the PQD under NIR excitation, as compared to their counterparts in solution. Moreover, by further improving the lattice matching between the UCNP and PQD using Gd to replace Y, heterostructured CsPbBr₃-NaGdF₄:Yb,Tm nanocrystals are successfully synthesized, with much enhanced luminescence and stability at high temperatures or in polar solvents or under continuous ultraviolet light excitation as compared to those of the CsPbBr₃-NaYF₄:Yb,Tm nanocrystals and pure PQDs.

Synthesis and Structural Analysis of Ternary Ca–Al–Fe Layered Double Hydroxides with Different Iron Contents

Hydrocalumite structured layered double hydroxides (LDHs) with various Fe3+ ratios were prepared through a coprecipitation method. In order to control the Fe3+ content in LDH, binary Ca–Fe LDHs were first synthesized with various Ca/Fe ratios. The X-ray diffraction pattern showed that only a limited Ca/Fe ratio resulted in LDH formation. The Fe3+ content in LDH was controlled by applying Al3+ while the divalent and trivalent metal ratio was set to 2. Through X-ray diffraction patterns, ternary LDHs with Ca–Al–Fe composition were successfully synthesized without significant impurities, with the Al increasing crystallinity. Quantification showed that Al moiety participated in the formation of the LDH framework more than Ca and Fe, implying a structural stabilization in the presence of Al. In order to investigate the global and local structure of Fe moiety in the LDH, both solid state UV-vis and X-ray absorption spectroscopies were carried out. Both spectroscopies revealed that the existence of Al induced slight local distortion in coordination but global crystal stabilization.

Remanent Polarization and Strong Photoluminescence Modulation by an External Electric Field in Epitaxial CsPbBr3 Nanowires

Metal halide perovskites (MHPs) have unique characteristics and hold great potential for next-generation optoelectronic technologies. Recently, the importance of lattice strain in MHPs has been gaining recognition as a significant optimization parameter for device performance. While the effect of strain on the fundamental properties of MHPs has been at the center of interest, its combined effect with an external electric field has been largely overlooked. Here we perform an electric-field-dependent photoluminescence study on heteroepitaxially strained surface-guided CsPbBr₃ nanowires. We reveal an unexpected strong linear dependence of the photoluminescence intensity on the alternating field amplitude, stemming from an induced internal dipole. Using low-frequency polarized-Raman spectroscopy, we reveal structural modifications in the nanowires under an external field, associated with the observed polarity. These results reflect the important interplay between strain and an external field in MHPs and offer opportunities for the design of MHP-based optoelectronic nanodevices.

Extraordinary phase coherence length in epitaxial halide perovskites

Inorganic halide perovskites have emerged as a promising platform in a wide range of applications from solar energy harvesting to computing and light emission. The recent advent of epitaxial thin film growth of halide perovskites has made it possible to investigate low-dimensional quantum electronic devices based on this class of materials. This study leverages advances in vapor-phase epitaxy of halide perovskites to perform low-temperature magnetotransport measurements on single-domain cesium tin iodide (CsSnI₃) epitaxial thin films. The low-field magnetoresistance carries signatures of coherent quantum interference effects and spin-orbit coupling. These weak anti-localization measurements reveal a micron-scale low-temperature phase coherence length for charge carriers in this system. The results indicate that epitaxial halide perovskite heterostructures are a promising platform for investigating long coherent quantum electronic effects and potential applications in spintronics and spin-orbitronics.

•

Epitaxial halide perovskites with extraordinary quantum phase coherence

•

Quantum transport properties with weak antilocalization observed in tetragonal CsSnI₃

•

Demonstration of quasi-2d charge carrier behavior with of spin-orbit coupling

•

Epitaxial halide perovskites emerging materials for quantum electronic applications

Solution Epitaxy of Halide Perovskite Thin Single Crystals for Stable Transistors

Halide perovskites hold promise for energy and optoelectronic applications due to their fascinating photophysical properties and facile processing. Among various forms, epitaxial thin single crystals (TSCs) are highly desirable due to their high crystallinity, reduced defects, and easy epitaxial integration with other materials. However, a cost-effective method for obtaining TSCs with perfect epitaxial features remains elusive. Here, we demonstrate a direct epitaxial growth of high-quality all-inorganic perovskite CsPbBr₃ TSCs on various substrates through a facile solution process under near-ambient conditions. Structural characterizations reveal a high-quality epitaxy between the obtained perovskite TSCs and substrates, thus leading to efficiently reduced defects. The resultant TSCs display a low trap density (∼10¹¹ cm^-3) and a long carrier lifetime (∼10.16 ns). Top-gate/top-contact transistors based on these TSCs exhibit high on/off ratios of over 10⁵, an optimal hole mobility of 3.9 cm² V^-1 s^-1, almost hysteresis-free operation, and high stability at room temperature. Such a facile approach for the high-yield production of perovskite epitaxial TSCs will enable a broad range of high-performance electronic applications.

Thin Layer Buckling in Perovskite CsPbBr3 Nanobelts

Flexible semiconductor materials, where structural fluctuations and transformation are tolerable and have low impact on electronic properties, focus interest for future applications. Two-dimensional thin layer lead halide perovskites are hailed for their unconventional optoelectronic features. We report structural deformations via thin layer buckling in colloidal CsPbBr₃ nanobelts adsorbed on carbon substrates. The microstructure of buckled nanobelts is determined using transmission electron microscopy and atomic force microscopy. We measured significant decrease in emission from the buckled nanobelt using cathodoluminescence, marking the influence of such mechanical deformations on electronic properties. By employing plate buckling theory, we approximate adhesion forces between the buckled nanobelt and the substrate to be F_adhesion ∼ 0.12 μN, marking a limit to sustain such deformation. This work highlights detrimental effects of mechanical buckling on electronic properties in halide perovskite nanostructures and points toward the capillary action that should be minimized in fabrication of future devices and heterostructures based on nanoperovskites.

Oriented Halide Perovskite Nanostructures and Thin Films for Optoelectronics

Oriented semiconductor nanostructures and thin films exhibit many advantageous properties, such as directional exciton transport, efficient charge transfer and separation, and optical anisotropy, and hence these nanostructures are highly promising for use in optoelectronics and photonics. The controlled growth of these structures can facilitate device integration to improve optoelectronic performance and benefit in-depth fundamental studies of the physical properties of these materials. Halide perovskites have emerged as a new family of promising and cost-effective semiconductor materials for next-generation high-power conversion efficiency photovoltaics and for versatile high-performance optoelectronics, such as light-emitting diodes, lasers, photodetectors, and high-energy radiation imaging and detectors. In this Review, we summarize the advances in the fabrication of halide perovskite nanostructures and thin films with controlled dimensionality and crystallographic orientation, along with their applications and performance characteristics in optoelectronics. We examine the growth methods, mechanisms, and fabrication strategies for several technologically relevant structures, including nanowires, nanoplates, nanostructure arrays, single-crystal thin films, and highly oriented thin films. We highlight and discuss the advantageous photophysical properties and remarkable performance characteristics of oriented nanostructures and thin films for optoelectronics. Finally, we survey the remaining challenges and provide a perspective regarding the opportunities for further progress in this field.

Two-dimensional overdamped fluctuations of the soft perovskite lattice in CsPbBr3

Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr₃ crystals using momentum-resolved neutron and X-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional sheets of correlated rotations in real space, and could represent precursors to proposed two-dimensional polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimization of their optical and thermal properties.

Engineering the Bandgap and Surface Structure of CsPbCl3 Nanocrystals to Achieve Efficient Ultraviolet Luminescence

Herein, we report the design of novel ultraviolet luminescent CsPbCl₃ nanocrystals (NCs) with the emission peak at 381 nm through doping of cadmium ions. Subsequently, a surface passivation strategy with CdCl₂ is adopted to improve their photoluminescence quantum yield (PLQY) with the maximum value of 60.5 %, which is 67 times higher than that of the pristine counterparts. The PLQY of the surface passivated NCs remains over 50 % after one week while the pristine NCs show negligible emission. By virtue of density functional theory calculations, we reveal that the higher PLQY and better stability after surface passivation may result from the significant elimination of surface chloride vacancy (V_Cl ) defects. These findings provide fundamental insights into the optical manipulation of metal ion-doped CsPbCl₃ NCs.

Using In Situ Optical Spectroscopy to Elucidate Film Formation of Metal Halide Perovskites

The research interest in halide perovskites has gained momentum enormously over the last recent years, also due to the demonstration of high-efficient perovskite-based optoelectronic devices. A prerequisite for such highly efficient devices is to realize high-quality perovskite layers, which requires a deep understanding about the perovskite formation and good process control. In that context, in situ optical spectroscopy during the processing of halide perovskites has become increasingly popular. Even though it is a relatively easily accessible yet powerful tool for studying perovskite formation, there exist some technical and analytical aspects that need to be considered to unfold its full potential. In this Perspective, we give an overview of the latest developments in the field of in situ optical spectroscopy to control and better understand the film processing of halide perovskites. We highlight possibilities and pitfalls regarding the analysis of measured optical data, discuss the development of technical concepts, and address future prospects of optical in situ spectroscopy.

Vertically Aligned CsPbBr3 Nanowire Arrays with Template-Induced Crystal Phase Transition and Stability

Metal halide perovskites show great promise for a wide range of optoelectronic applications but are plagued by instability when exposed to air and light. This work presents low-temperature solution growth of vertically aligned CsPbBr₃ nanowire arrays in AAO (anodized aluminum oxide) templates with excellent stability, with samples exposed to air for 4 months still exhibiting comparable photoluminescence and UV stability to fresh samples. The single-crystal nanowire length is adjusted from ∼100 nm to 5 μm by adjusting the precursor solution amount and concentration, and we observe length-to-diameter ratios as high as 100. Structural characterization results indicate that large-diameter CsPbBr₃ nanowires have an orthorhombic structure, while the 10 nm- and 20 nm-diameter nanowires adopt a cubic structure. Photoluminescence shows a gradual blue-shift in emission with decreasing nanowire diameter and marginal changes under varying illumination power intensity. The CsPbBr₃-nanowires/AAO composite exhibits excellent resistance to X-ray radiation and long-term air storage, which makes it promising for future optoelectronic applications such as X-ray scintillators. These results show how physical confinement in AAO can be used to realize CsPbBr₃ nanowire arrays and control their morphology and crystal structure.

Understanding homoepitaxial growth of horizontal kinked GaN nanowires

Epitaxial horizontal nanowires (NWs) have attracted much attention due to their easily large-scale integration. From the reported literature, epitaxial growth is usually driven by minimization of strain between NW and substrate, which governs the growth along with specific crystallographic orientation. Here, we report the first homoepitaxial growth of horizontal GaN NWs from a surface-directed vapor-liquid-solid growth method. The NWs grow along with six symmetry-equivalent 〈1-100〉 (m-axis) directions, exhibiting a random 60°/120° kinked configuration. Owing to homoepitaxial growth, strain could be eliminated. From the obtained results, we suggest that the formation the horizontal NWs, and their growth direction /orientation is not directly related to the strain minimization. A general rule based on the epitaxial relationship and potential low-index growth orientation is proposed for understanding the arrangement of epitaxial horizontal NWs. It is deduced that kinking of the horizontal NWs was attributed to unintentional guided growth determined by the roughness of the substrates' surface. This study provides an insight for a better understanding of the evolution of epitaxial horizontal NWs, especially for the growth direction/orientation.

Binary ligand-mediated morphological evolution of methylammonium lead bromide nanocrystals

Binary ligands of carboxylic acids and alkylamines induce the morphological evolution of MAPbBr3 nanocubes to 1D nanowires and 2D nanosheets during the aging process.

In Situ Imaging of Ferroelastic Domain Dynamics in CsPbBr3 Perovskite Nanowires by Nanofocused Scanning X-ray Diffraction

The interest in metal halide perovskites has grown as impressive results have been shown in solar cells, light emitting devices, and scintillators, but this class of materials have a complex crystal structure that is only partially understood. In particular, the dynamics of the nanoscale ferroelastic domains in metal halide perovskites remains difficult to study. An ideal in situ imaging method for ferroelastic domains requires a challenging combination of high spatial resolution and long penetration depth. Here, we demonstrate in situ temperature-dependent imaging of ferroelastic domains in a single nanowire of metal halide perovskite, CsPbBr3. Scanning X-ray diffraction with a 60 nm beam was used to retrieve local structural properties for temperatures up to 140 °C. We observed a single Bragg peak at room temperature, but at 80 °C, four new Bragg peaks appeared, originating in different real-space domains. The domains were arranged in periodic stripes in the center and with a hatched pattern close to the edges. Reciprocal space mapping at 80 °C was used to quantify the local strain and lattice tilts, revealing the ferroelastic nature of the domains. The domains display a partial stability to further temperature changes. Our results show the dynamics of nanoscale ferroelastic domain formation within a single-crystal perovskite nanostructure, which is important both for the fundamental understanding of these materials and for the development of perovskite-based devices.

Halide perovskite nanocrystal arrays: Multiplexed synthesis and size-dependent emission

Halide perovskites have exceptional optoelectronic properties, but a poor understanding of the relationship between crystal dimensions, composition, and properties limits their use in integrated devices. We report a new multiplexed cantilever-free scanning probe method for synthesizing compositionally diverse and size-controlled halide perovskite nanocrystals spanning square centimeter areas. Single-particle photoluminescence studies reveal multiple independent emission modes due to defect-defined band edges with relative intensities that depend on crystal size at a fixed composition. Smaller particles, but ones with dimensions that exceed the quantum confinement regime, exhibit blue-shifted emission due to reabsorption of higher-energy modes. Six different halide perovskites have been synthesized, including a layered Ruddlesden-Popper phase, and the method has been used to prepare functional solar cells based on single nanocrystals. The ability to pattern arrays of multicolor light-emitting nanocrystals opens avenues toward the development of optoelectronic devices, including optical displays.

Self-Assembly of Perovskite CsPbBr3 Quantum Dots Driven by a Photo-Induced Alkynyl Homocoupling Reaction

Herein, we report the facile growth of three-dimensional CsPbBr₃ perovskite supercrystals (PSCs) self-assembled from individual CsPbBr₃ perovskite quantum dots (PQDs). By varying the carbon chain length of a surface-bound ligand molecule, 1-alkynyl acid, different morphologies of PSCs were obtained accompanied by an over 1000-fold photoluminescence improvement compared with that of PQDs. Systematic analyses have shown, for the first time, that under UV irradiation, CsBr, the byproduct formed during PQDs synthesis, could effectively catalyze the homocoupling reaction between two alkynyl groups, which further worked as a driving force to push forward the self-assembly of PQDs.

Impact of excitation energy on the excitonic luminescence of cesium lead bromide perovskite nanosheets

An excitation-energy-dependent luminescence phenomenon is reported in cesium lead bromide (CsPbBr₃) perovskite nanosheets. At 10 K, the relative integrated luminescence intensity between the trapped exciton (TX) emission and the free exciton (FX) emission shows an interesting tendency with increasing the optical excitation energy from 2.431 eV (510 nm) to 3.758 eV (330 nm). To interpret such phenomenon, we develop a quantitative model on the basis of the biological population growth theory. A good agreement between experiment and theory is obtained. It is thus revealed that the lower capture coefficient of the TX level to the excited excitons via multiphonon emission relative to the FX level shall be the major cause of the observed phenomenon. These findings may help to deepen the current understanding of the complex luminescence mechanisms of these emerging light-emitting materials.