Self-Template-Directed Synthesis of Porous Perovskite Nanowires at Room Temperature for High-Performance Visible-Light Photodetectors

Recent Advances in Low-Dimensional Nanomaterials for Photodetectors

New emerging low-dimensional such as 0D, 1D, and 2D nanomaterials have attracted tremendous research interests in various fields of state-of-the-art electronics, optoelectronics, and photonic applications due to their unique structural features and associated electronic, mechanical, and optical properties as well as high-throughput fabrication for large-area and low-cost production and integration. Particularly, photodetectors which transform light to electrical signals are one of the key components in modern optical communication and developed imaging technologies for whole application spectrum in the daily lives, including X-rays and ultraviolet biomedical imaging, visible light camera, and infrared night vision and spectroscopy. Today, diverse photodetector technologies are growing in terms of functionality and performance beyond the conventional silicon semiconductor, and low-dimensional nanomaterials have been demonstrated as promising potential platforms. In this review, the current states of progress on the development of these nanomaterials and their applications in the field of photodetectors are summarized. From the elemental combination for material design and lattice structure to the essential investigations of hybrid device architectures, various devices and recent developments including wearable photodetectors and neuromorphic applications are fully introduced. Finally, the future perspectives and challenges of the low-dimensional nanomaterials based photodetectors are also discussed.

Towards high photoresponse of perovskite nanowire/copper phthalocyanine heterostructured photodetector

One-dimensional nanowire structures composed of perovskite are widely recognized for their exceptional optoelectronic performance and mechanical properties, making them a popular area of investigation in photodetection research. In this work, a perovskite nanowire/copper phthalocyanine heterojunction-based photodetector was fabricated, which exhibits high photoresponse in the visible-near-infrared region. The incorporation of a heterojunction significantly enhanced the photoelectric performance. Specifically, the photoresponsivity and external quantum efficiency of the nanowire-based device were elevated from 58.5 A W-1and 1.35 × 104% to 84.5 A W-1and 1.97 × 104% at 532 nm, respectively. The enhanced photoresponse of the heterojunction device can be attributed to the unique microstructure of nanowire arrays. The wrapping of the nanowires by copper phthalocyanine forms heterojunctions with a larger dissociation area, which facilitated exciton dissociation and enhanced device performance. This work provides a promising example for optimizing the performance of nanowire devices.

CH3NH3PbI3/Au/Mg0.2Zn0.8O Heterojunction Self-Powered Photodetectors with Suppressed Dark Current and Enhanced Detectivity

Interface engineering of the hole transport layer in CH₃NH₃PbI₃ photodetectors has resulted in significantly increased carrier accumulation and dark current as well as energy band mismatch, thus achieving the goal of high-power conversion efficiency. However, the reported heterojunction perovskite photodetectors exhibit high dark currents and low responsivities. Herein, heterojunction self-powered photodetectors, composed of p-type CH₃NH₃PbI₃ and n-type Mg_0.2Zn_0.8O, are prepared through the spin coating and magnetron sputtering. The obtained heterojunctions exhibit a high responsivity of 0.58 A/W, and the EQE of the CH₃NH₃PbI₃/Au/Mg_0.2Zn_0.8O heterojunction self-powered photodetectors is 10.23 times that of the CH₃NH₃PbI₃/Au photodetectors and 84.51 times that of the Mg_0.2ZnO_0.8/Au photodetectors. The built-in electric field of the p-n heterojunction significantly suppresses the dark current and improves the responsivity. Remarkably, in the self-supply voltage detection mode, the heterojunction achieves a high responsivity of up to 1.1 mA/W. The dark current of the CH₃NH₃PbI₃/Au/Mg_0.2Zn_0.8O heterojunction self-powered photodetectors is less than 1.4 × 10^-1 pA at 0 V, which is more than 10 times lower than that of the CH₃NH₃PbI₃ photodetectors. The best value of the detectivity is as high as 4.7 × 10¹² Jones. Furthermore, the heterojunction self-powered photodetectors exhibit a uniform photodetection response over a wide spectral range from 200 to 850 nm. This work provides guidance for achieving a low dark current and high detectivity for perovskite photodetectors.

Nanostructured perovskites for nonvolatile memory devices

Perovskite materials have driven tremendous advances in constructing electronic devices owing to their low cost, facile synthesis, outstanding electric and optoelectronic properties, flexible dimensionality engineering, and so on. Particularly, emerging nonvolatile memory devices (eNVMs) based on perovskites give birth to numerous traditional paradigm terminators in the fields of storage and computation. Despite significant exploration efforts being devoted to perovskite-based high-density storage and neuromorphic electronic devices, research studies on materials' dimensionality that has dominant effects on perovskite electronics' performances are paid little attention; therefore, a review from the point of view of structural morphologies of perovskites is essential for constructing perovskite-based devices. Here, recent advances of perovskite-based eNVMs (memristors and field-effect-transistors) are reviewed in terms of the dimensionality of perovskite materials and their potentialities in storage or neuromorphic computing. The corresponding material preparation methods, device structures, working mechanisms, and unique features are showcased and evaluated in detail. Furthermore, a broad spectrum of advanced technologies (e.g., hardware-based neural networks, in-sensor computing, logic operation, physical unclonable functions, and true random number generator), which are successfully achieved for perovskite-based electronics, are investigated. It is obvious that this review will provide benchmarks for designing high-quality perovskite-based electronics for application in storage, neuromorphic computing, artificial intelligence, information security, etc.

Epitaxial Stabilization of Tetragonal Cesium Tin Iodide

A full range of optoelectronic devices has been demonstrated incorporating hybrid organic-inorganic halide perovskites including high-performance photovoltaics, light emitting diodes, and lasers. Tin-based inorganic halide perovskites, such as CsSnX₃ (X = Cl, Br, I), have been studied as promising candidates that avoid toxic lead halide compositions. One of the main obstacles for improving the properties of all-inorganic perovskites and transitioning their use to high-end electronic applications is obtaining crystalline thin films with minimal crystal defects, despite their reputation for defect tolerance in photovoltaic applications. In this study, the single-domain epitaxial growth of cesium tin iodide (CsSnI₃) on closely lattice matched single-crystal potassium chloride (KCl) substrates is demonstrated. Using in situ real-time diffraction techniques, we find a new epitaxially-stabilized tetragonal phase at room temperature that expands the possibility for controlling electronic properties. We also exploit controllable epitaxy to grow multilayer two-dimensional quantum wells and demonstrate epitaxial films in a lateral photodetector architecture. This work provides insight into the phase control during halide perovskite epitaxy and expands the selection of epitaxially accessible materials from this exciting class of compounds.

Probing reversible photoluminescence alteration in CH3NH3PbBr3 colloidal quantum dots for luminescence-based gas sensing application

Methylammonium lead bromide (CH₃NH₃PbBr₃) colloidal quantum dots (QDs) exhibit strong green photoluminescence (PL) with high photoluminescence quantum yield (PLQY) making it valuable for various optoelectronic applications. Under the influence of polar gaseous molecules, hybrid halide perovskites show changes in its structural and electrical properties. We, for the first time, have investigated the influence of NH₃ gas molecules on the optical properties of CH₃NH₃PbBr₃ colloidal QDs. The investigations carried out under a controlled environment reveal that even the presence of 37 ppm of ammonia (NH₃) gas molecules causes a significant reduction in the PL intensity of CH₃NH₃PbBr₃ colloidal QDs. The reduction rate of PL intensity can be tuned with the concentration of NH₃ gas molecules. We propose that the decrease in PL intensity is because of the formation of a non-luminescent NH₄PbBr₃ phase under the presence of NH₃ gas molecules. Further, the non-luminescent NH₄PbBr₃ retransformed into luminescent CH₃NH₃PbBr₃ on the introduction of methylamine (CH₃NH₂) gas molecules. This reversible alternation in PL properties enables us to demonstrate its application for (NH₃) gas sensing. The advantage of using CH₃NH₃PbBr₃ colloidal QDs for luminescence-based sensing is that its green emission is visible with the naked eye even under daylight, which is easy to detect.

Alloying n-Butylamine into CsPbBr3 To Give a Two-Dimensional Bilayered Perovskite Ferroelectric Material

Cesium-lead halide perovskites (e.g. CsPbBr₃ ) have gained attention because of their rich physical properties, but their bulk ferroelectricity remains unexplored. Herein, by alloying flexible organic cations into the cubic CsPbBr₃ , we design the first cesium-based two-dimensional (2D) perovskite ferroelectric material with both inorganic alkali metal and organic cations, (C₄ H₉ NH₃ )₂ CsPb₂ Br₇ (1). Strikingly, 1 shows a high Curie temperature (T_c =412 K) above that of BaTiO₃ (ca. 393 K) and notable spontaneous polarization (ca. 4.2 μC cm^-2 ), triggered by not only the ordering of organic cations but also atomic displacement of inorganic Cs⁺ ions. To our knowledge, such a 2D bilayered Cs⁺ -based metal-halide perovskite ferroelectric material with inorganic and organic cations is unprecedented. 1 also shows photoelectric semiconducting behavior with large "on/off" ratios of photoconductivity (>10³ ).

Highly Luminescent CsPbX3 (X=Cl, Br, I) Nanocrystals Achieved by a Rapid Anion Exchange at Room Temperature

Cesium lead halide perovskite (CsPbX₃ ) nanocrystals (NCs) exhibit an excellent photoelectric performance, which is directly governed by fine-tuning of the composition and preparation of materials with a special phase structure and morphology. However, it is still facing challenges to achieve highly stable and luminescent CsPbX₃ NCs at room temperature. Herein, we report on a novel exchange reaction, in which metal halides MX₂ (M=Zn, Mg, Cu, or Ca; X=Cl, Br, or I) solids act as anion source to directly prepare CsPbX₃ NCs at room temperature without any pretreatment. Introducing small amount of oleic acid or oleylamine speed up the exchange reaction through different promotion mechanisms. Oleic acid coordinates to the surface of the NCs, which increases the reaction activity, and oleylamine greatly enhances the dissolution of ZnCl₂ . XRD and TEM tests demonstrate that the cubic phase structure and the morphology of the parent CsPbX₃ were well preserved. Moreover, the band-gap energies and photoluminescence (PL) spectra were readily tunable over the entire visible spectral region of λ=406-685 nm. Our findings could open up the possibilities of using metal halide solids as new anion sources to prepare high-quality CsPbX₃ NCs at room temperature.

Two-Dimensional Materials for Halide Perovskite-Based Optoelectronic Devices

Halide perovskites have high light absorption coefficients, long charge carrier diffusion lengths, intense photoluminescence, and slow rates of non-radiative charge recombination. Thus, they are attractive photoactive materials for developing high-performance optoelectronic devices. These devices are also cheap and easy to be fabricated. To realize the optimal performances of halide perovskite-based optoelectronic devices (HPODs), perovskite photoactive layers should work effectively with other functional materials such as electrodes, interfacial layers and encapsulating films. Conventional two-dimensional (2D) materials are promising candidates for this purpose because of their unique structures and/or interesting optoelectronic properties. Here, we comprehensively summarize the recent advancements in the applications of conventional 2D materials for halide perovskite-based photodetectors, solar cells and light-emitting diodes. The examples of these 2D materials are graphene and its derivatives, mono- and few-layer transition metal dichalcogenides (TMDs), graphdiyne and metal nanosheets, etc. The research related to 2D nanostructured perovskites and 2D Ruddlesden-Popper perovskites as efficient and stable photoactive layers is also outlined. The syntheses, functions and working mechanisms of relevant 2D materials are introduced, and the challenges to achieving practical applications of HPODs using 2D materials are also discussed.

LowDimensional Halide Perovskites and Their Advanced Optoelectronic Applications

Metal halide perovskites are crystalline materials originally developed out of scientific curiosity. They have shown great potential as active materials in optoelectronic applications. In the last 6 years, their certified photovoltaic efficiencies have reached 22.1%. Compared to bulk halide perovskites, low-dimensional ones exhibited novel physical properties. The photoluminescence quantum yields of perovskite quantum dots are close to 100%. The external quantum efficiencies and current efficiencies of perovskite quantum dot light-emitting diodes have reached 8% and 43 cd A-1, respectively, and their nanowire lasers show ultralow-threshold room-temperature lasing with emission tunability and ease of synthesis. Perovskite nanowire photodetectors reached a responsivity of 10 A W-1 and a specific normalized detectivity of the order of 1012 Jones. Different from most reported reviews focusing on photovoltaic applications, we summarize the rapid progress in the study of low-dimensional perovskite materials, as well as their promising applications in optoelectronic devices. In particular, we review the wide tunability of fabrication methods and the state-of-the-art research outputs of low-dimensional perovskite optoelectronic devices. Finally, the anticipated challenges and potential for this exciting research are proposed.

Passivated Single-Crystalline CH3NH3PbI3 Nanowire Photodetector with High Detectivity and Polarization Sensitivity

Photodetectors convert light signals into current or voltage outputs and are widely used for imaging, sensing, and spectroscopy. Perovskite-based photodetectors have shown high sensitivity and fast response due to the unprecedented low recombination loss in this solution processed semiconductor. Among various types of CH₃NH₃PbI₃ morphology (film, single crystal, nanowire), single-crystalline CH₃NH₃PbI₃ nanowires are particularly interesting for photodetection because of their reduced grain boundary, morphological anisotropy, and excellent mechanical flexibility. The concomitant disadvantage associated with the CH₃NH₃PbI₃ nanowire photodetectors is their large surface area, which catalyzes carrier recombination and material decomposition, thus significantly degrading device performance and stability. Here we solved this key problem by introducing oleic acid soaking to passivate surface defects of CH₃NH₃PbI₃ nanowires, which leads to a device with much improved stability and unprecedented sensitivity (measured detectivity of 2 × 10¹³ Jones). By taking advantage of their one-dimensional geometry, we also showcased, for the first time, the linear dichroic photodetection of our CH₃NH₃PbI₃ nanowire photodetector.

Organolead Halide Perovskite Nanocrystals: Branched Capping Ligands Control Crystal Size and Stability

CH3 NH3 PbBr3 perovskite nanocrystals (PNCs) of different sizes (ca. 2.5-100 nm) with high photoluminescence (PL) quantum yield (QY; ca. 15-55 %) and product yield have been synthesized using the branched molecules, APTES and NH2 -POSS, as capping ligands. These ligands are sterically hindered, resulting in a uniform size of PNCs. The different capping effects resulting from branched versus straight-chain capping ligands were compared and a possible mechanism proposed to explain the dissolution-precipitation process, which affects the growth and aggregation of PNCs, and thereby their overall stability. Unlike conventional PNCs capped with straight-chain ligands, APTES-capped PNCs show high stability in protic solvents as a result of the strong steric hindrance and propensity for hydrolysis of APTES, which prevent such molecules from reaching and reacting with the core of PNCs.

Electron-beam irradiation induced transformation of Cu2(OH)3NO3 nanoflakes into nanocrystalline CuO

The transmission electron microscope electron-beam (TEM e-beam) as a material modification tool has been demonstrated. The material modification is realised in the high-resolution TEM mode (largest condenser aperture, 150 μm, and 200 nm spot size) at a 200 keV beam energy. The Cu2(OH)3NO3 (CHN) nanoflakes used in this study were microwave solution processed that were layered single crystals and radiation sensitive. The single domain CHN flakes disintegrate into a large number of individual CuO crystallites within a 90 s span of time. The sequential bright-field, dark-field, and selected area electron diffraction modes were employed to record the evolved morphology, microstructural changes, and structural transformation that validate CHN modification. High-resolution transmission electron microscopy imaging of e-beam irradiated regions unambiguously supports the growth of CuO nanoparticles (11.8(3.2) nm in diameter). This study demonstrates e-beam irradiation induced CHN depletion, subsequent nucleation and growth of nanocrystalline CuO regions well embedded in the parent burnt porous matrix which can be useful for miniaturized sensing applications. NaBH4 induced room temperature reduction of CHN to elemental Cu and its printability on paper was also demonstrated.

Synthesis, properties, and optical applications of low-dimensional perovskites

This feature article provides an overview of synthesis, properties and applications of low-dimensional perovskites.

High-Performance Planar-Type Photodetector on (100) Facet of MAPbI3 Single Crystal

Recently, the discovery of organometallic halide perovskites provides promising routes for fabricating optoelectronic devices with low cost and high performance. Previous experimental studies of MAPbI₃ optoelectronic devices, such as photodetectors and solar cells, are normally based on polycrystalline films. In this work, a high-performance planar-type photodetector fabricated on the (100) facet of a MAPbI₃ single crystal is proposed. We demonstrate that MAPbI₃ photodetector based on single crystal can perform much better than that on polycrystalline-film counterpart. The low trap density of MAPbI₃ single crystal accounts for the higher carrier mobility and longer carrier diffusion length, resulted in a significant performance increasement of MAPbI₃ photodetector. Compared with similar planar-type photodetectors based on MAPbI₃ polycrystalline film, our MAPbI₃ single crystal photodetector showed excellent performance with good stability and durability, broader response spectrum to near-infrared region, about 10² times higher responsivity and EQE, and approximately 10³ times faster response speed. These results may pave the way for exploiting high-performance perovskites photodetectors based on single crystal.

Ultrasensitive solution-processed broad-band photodetectors using CH₃NH₃PbI₃ perovskite hybrids and PbS quantum dots as light harvesters

Sensing from ultraviolet-visible to infrared is critical for both scientific and industrial applications. In this work, we demonstrate solution-processed ultrasensitive broad-band photodetectors (PDs) utilizing organolead halide perovskite materials (CH3NH3PbI3) and PbS quantum dots (QDs) as light harvesters. Through passivating the structural defects on the surface of PbS QDs with diminutive molecular-scaled CH3NH3PbI3, both trap states in the bandgap of PbS QDs for charge carrier recombination and the leakage currents occurring at the defect sites are significantly reduced. In addition, CH3NH3PbI3 itself is an excellent light harvester in photovoltaics, which contributes a great photoresponse in the ultraviolet-visible region. Consequently, operated at room temperature, the resultant PDs show a spectral response from 375 nm to 1100 nm, with high responsivities over 300 mA W(-1) and 130 mA W(-1), high detectivities exceeding 10(13) Jones (1 Jones = 1 cm Hz(1/2) W(-1)) and 5 × 10(12) Jones in the visible and near infrared regions, respectively. These device performance parameters are comparable to those from pristine inorganic counterparts. Thus, our results offer a facile and promising route for advancing the performance of broad-band PDs.