Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals

Room Temperature, Twist Angle Independent, Momentum Direct Interlayer Excitons in van der Waals Heterostructures with Wide Spectral Tunability

Interlayer excitons (IXs) in van der Waals heterostructures with static out of plane dipole moment and long lifetime show promise in the development of exciton based optoelectronic devices and the exploration of many body physics. However, these IXs are not always observed, as the emission is very sensitive to lattice mismatch and twist angle between the constituent materials. Moreover, their emission intensity is very weak compared to that of corresponding intralayer excitons at room temperature. Here we report the room-temperature realization of twist angle independent momentum direct IX in the heterostructures of bulk PbI2 and bilayer WS2. Momentum conserving transitions combined with the large band offsets between the constituent materials enable intense IX emission at room temperature. A long lifetime (∼100 ns), noticeable Stark shift, and tunability of IX emission from 1.70 to 1.45 eV by varying the number of WS2 layers make these heterostructures promising to develop room temperature exciton based optoelectronic devices.

Toward High-Performance Self-Powered Near-Ultraviolet Photodetection by Constructing a Unipolar Heterojunction

Constructing a unipolar heterojunction is an effective energy band engineering strategy to improve the performance of photoelectric devices, which could suppress dark current and enhance detectivity by modulating the transfer of carriers. In this work, unipolar heterojunctions of Si/PbI2 and GaSb/PbI2 are constructed successfully for high-performance self-powered near-ultraviolet photodetection. Owing to the unique band offset of unipolar heterojunctions, the transport of holes is blocked, and only photogenerated electrons in PbI2 can flow unimpeded under the driving force of the built-in electric field. Thus, the recombination of photogenerated electron-hole pairs is suppressed, contributing to high-performance near-ultraviolet photodetection. The as-fabricated Si/PbI2 self-powered near-ultraviolet photodetector exhibits a low dark current of 10-13 A, a high Ilight/Idark ratio of 104, and fast response times of 26/24 ms, which are much better than those of the PbI2 metal-semiconductor-metal photodetector. Furthermore, the as-fabricated GaSb/PbI2 unipolar heterojunction photodetector also exhibits impressive self-powered near-ultraviolet photodetection behaviors. Evidently, this work shows the potential of unipolar heterojunctions for next-generation Si-based and GaSb-based high-performance photodetection.

Contact Geometry-Dependent Excitonic Emission in Mixed-Dimensional van der Waals Heterostructures

Manipulation of excitonic emission in two-dimensional (2D) materials via the assembly of van der Waals (vdW) heterostructures unlocks numerous opportunities for engineering their photonic and optoelectronic properties. In this work, we introduce a category of mixed-dimensional vdW heterostructures, integrating 2D materials with one-dimensional (1D) semiconductor nanowires composed of vdW layers. This configuration induces spatially distinct localized excitonic emissions through a tailored interfacial heterolayer atomic arrangement. By precisely adjusting both the axial and sidewall facet orientations of bottom-up grown PbI2 vdW nanowires and by transferring them onto 1L WSe2 flakes, we establish vdW heterointerfaces with either perpendicular or parallel interatomic arrangements. The edge-standing heterojunction, featuring perpendicular PbI2 layers atop WSe2, promotes efficient charge transfer through the edges and coupled localized states, leading to an enhanced redshifted excitonic emission. Conversely, the layer-by-layer heterointerface, where PbI2 layers are in parallel contact with WSe2, exhibits substantial quenching due to deep midgap states in a type-II alignment, as evidenced by power-dependent measurements and first-principle calculations. Our results introduce a method for actively manipulating excitonic emissions in 2D transition metal dichalcogenides (TMDs) through edge engineering, highlighting their potential in the development of various quantum devices.

Growth of 1D Carbon Nanotube@Perovskite Core-Shell van der Waals Heterostructures through Chemical Vapor Deposition

Perovskite is an emerging material with immense potential in the field of optoelectronics. 1D perovskite nanowires are crucial building blocks for the development of optoelectronic devices. However, producing perovskite nanowires with high quality and controlled alignment is challenging. In this study, the direct epitaxial growth of perovskite on oriented carbon nanotube (CNT) templates is presented through a chemical vapor deposition method. The deposition process of lead iodide and methylammonium iodide is systematically investigated, and a layer plus island growth mechanism is proposed to interpret the experimental observations. The aligned long CNTs serve as 1D templates and allow the growth of CNT@perovskite core-shell heterostructure with a high aspect ratio to withstand large deformation. The obtained 1D perovskite materials can be easily manipulated and transferred, enabling the facile preparation of microscale flexible devices. For proof of concept, a photodetector based on an individual CNT@methylammonium lead iodide heterostructure is fabricated. This work provides a new approach to prepare 1D hetero-nanostructure and may inspire the design of novel flexible nanophotodetectors.

Electric-field tunable Type-I to Type-II band alignment transition in MoSe2/WS2 heterobilayers

Semiconductor heterojunctions are ubiquitous components of modern electronics. Their properties depend crucially on the band alignment at the interface, which may exhibit straddling gap (type-I), staggered gap (type-II) or broken gap (type-III). The distinct characteristics and applications associated with each alignment make it highly desirable to switch between them within a single material. Here we demonstrate an electrically tunable transition between type-I and type-II band alignments in MoSe2/WS2 heterobilayers by investigating their luminescence and photocurrent characteristics. In their intrinsic state, these heterobilayers exhibit a type-I band alignment, resulting in the dominant intralayer exciton luminescence from MoSe2. However, the application of a strong interlayer electric field induces a transition to a type-II band alignment, leading to pronounced interlayer exciton luminescence. Furthermore, the formation of the interlayer exciton state traps free carriers at the interface, leading to the suppression of interlayer photocurrent and highly nonlinear photocurrent-voltage characteristics. This breakthrough in electrical band alignment control, interlayer exciton manipulation, and carrier trapping heralds a new era of versatile optical and (opto)electronic devices composed of van der Waals heterostructures.

Control of positive and negative photo- and thermal-responses in a single PbI2@CH3NH3PbI3 micro/nanowire-based device for real-time sensing, nonvolatile memory, and logic operation

CH3NH3PbI3 has shown great potential for photodetectors and photovoltaic devices due to its excellent positive response to visible light. However, its real-time response characteristics hinder its application in optical memory and logic operation; moreover, the presence of excessive PbI2 is a double-edged sword. Herein, we constructed a dual-terminal device using a single CH3NH3PbI3 micro/nanowire with two Ag electrodes, and then in situ introduced PbI2 quantum dots (QDs) as hole trap centres by thermal decomposition at 160 °C. An anomalous negative photoconductivity (NPC) effect for sub-bandgap light below the PbI2 bandgap is obtained. Importantly, an electrically erasable nonvolatile photomemory can be realized. Furthermore, the device also exhibits an abnormal positive thermal resistance (PTR)-related thermomemory effect, and the thermal-induced high-resistance state (HRS) can be erased by a large bias or an illumination of 365 nm super-bandgap UV light. Additionally, logical "OR" gate operations are achieved through a combination of 650 nm sub-bandgap light and a 70 °C temperature-induced HRS, as well as a large bias and 365 nm super-bandgap light-triggered low-resistance state. These effects are attributed to the excitation and injection of holes in QDs and structural defect traps. This multifunctional device, integrating real-time sensing, nonvolatile memory, and logical operation, holds significant potential for novel electronic and optoelectronic applications.

Growth of millimeter-sized 2D metal iodide crystals induced by ion-specific preference at water-air interfaces

Conventional liquid-phase methods lack precise control in synthesizing and processing materials with macroscopic sizes and atomic thicknesses. Water interfaces are ubiquitous and unique in catalyzing many chemical reactions. However, investigations on two-dimensional (2D) materials related to water interfaces remain limited. Here we report the growth of millimeter-sized 2D PbI2 single crystals at the water-air interface. The growth mechanism is based on an inherent ion-specific preference, i.e. iodine and lead ions tend to remain at the water-air interface and in bulk water, respectively. The spontaneous accumulation and in-plane arrangement within the 2D crystal of iodide ions at the water-air interface leads to the unique crystallization of PbI2 as well as other metal iodides. In particular, PbI2 crystals can be customized to specific thicknesses and further transformed into millimeter-sized mono- to few-layer perovskites. Additionally, we have developed water-based techniques, including water-soaking, spin-coating, water-etching, and water-flow-assisted transfer to recycle, thin, pattern, and position PbI2, and subsequently, perovskites. Our water-interface mediated synthesis and processing methods represents a significant advancement in achieving simple, cost-effective, and energy-efficient production of functional materials and their integrated devices.

Highly Reliable Van Der Waals Memory Boosted by a Single 2D Charge Trap Medium

Charge trap materials that can store carriers efficiently and controllably are desired for memory applications. 2D materials are promising for highly compacted and reliable memory mainly due to their ease of constructing atomically uniform interfaces, however, remain unexplored as being charge trap media. Here it is discovered that 2D semiconducting PbI2 is an excellent charge trap material for nonvolatile memory and artificial synapses. It is simple to construct PbI2 -based charge trap devices since no complicated synthesis or additional defect manufacturing are required. As a demonstration, MoS2 /PbI2 device exhibits a large memory window of 120 V, fast write speed of 5 µs, high on-off ratio around 106 , multilevel memory of over 8 distinct states, high reliability with endurance up to 104 cycles and retention over 1.2 × 104 s. It is envisioned that PbI2 with ionic activity caused by the natively formed iodine vacancies is unique to combine with unlimited 2D materials for versatile van der Waals devices with high-integration and multifunctionality.

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

Aqueous-Phase Formation of Two-Dimensional PbI2 Nanoplates for High-Performance Self-Powered Photodetectors

The process of the aqueous synthesis of nanomaterials has gained considerable interest due to its ability to eliminate the need for complex organic solvents, which aligns with the principles of green chemistry. Fabricating nanostructures in aqueous solutions has gained recognition for its potential to develop ultrasensitive, low-energy, and ultrafast optoelectronic devices. This study focuses on synthesizing lead iodide (PbI2) nanoplates (NPs) using a water-based solution technique and fabricating a planar photodetector. The planar photodetectors (ITO/PbI2 NPs/Au) demonstrated a remarkable photosensitivity of 3.9 × 103 and photoresponsivity of 0.51 mA/W at a wavelength of 405 nm. Further, we have carried-out analytical calculations for key performance parameters including open-circuit voltage (Voc), short-circuit current (Isc), on-off ratio, responsivity (R), and specific detectivity (D*) at zero applied bias, while photodetector operating in self-powered mode. These values are as follows: Voc = 0.103 V, Isc = 1.93 × 10-8, on-off ratio = 103, R = 4.0 mA/W, and D* = 3.3 × 1011 Jones. Particularly, the asymmetrical output properties of ITO/PbI2 NPs/Au detector provided additional evidence of the effective creation of a Schottky contact. Therefore, the photodetector exhibited a photo-response even at 0 V bias (rise/decay time ~1 s), leading to the realization of self-powered photodetectors. Additionally, the device exhibited a rapid photo-response of 0.23/0.38 s (-5 V) in the visible range. This study expands the scope of aqueous-phase synthesis of PbI2 nanostructures, enabling the large-area fabrication of high-performance photodetectors.

A Droplet Method for Synthesis of Multiclass Ultrathin Metal Halides

2D metal halides have attracted increasing research attention in recent years; however, it is still challenging to synthesize them via liquid-phase methods. Here it is demonstrated that a droplet method is simple and efficient for the synthesis of multiclass 2D metal halides, including trivalent (BiI3 , SbI3 ), divalent (SnI2 , GeI2 ), and monovalent (CuI) ones. In particular, 2D SbI3 is first experimentally achieved, of which the thinnest thickness is ≈6 nm. The nucleation and growth of these metal halide nanosheets are mainly determined by the supersaturation of precursor solutions that are dynamically varying during the solution evaporation. After solution drying, the nanosheets can fall on the surface of many different substrates, which further enables the feasible fabrication of related heterostructures and devices. With SbI3 /WSe2 being a good demonstration, the photoluminescence intensity and photo responsivity of WSe2 is obviously enhanced after interfacing with SbI3 . The work opens a new pathway for 2D metal halides toward widespread investigation and applications.

Bioinspired "cage traps" for closed-loop lead management of perovskite solar cells under real-world contamination assessment

Despite the remarkable progress made in perovskite solar cells, great concerns regarding potential Pb contamination risk and environmental vulnerability risks associated with perovskite solar cells pose a significant obstacle to their real-world commercialization. In this study, we took inspiration from the ensnaring prey behavior of spiders and chemical components in spider web to strategically implant a multifunctional mesoporous amino-grafted-carbon net into perovskite solar cells, creating a biomimetic cage traps that could effectively mitigate Pb leakage and shield the external invasion under extreme weather conditions. The synergistic Pb capturing mechanism in terms of chemical chelation and physical adsorption is in-depth explored. Additionally, the Pb contamination assessment of end-of-life perovskite solar cells in the real-world ecosystem, including Yellow River water and soil, is proposed. The sustainable closed-loop Pb management process is also successfully established involving four critical steps: Pb precipitation, Pb adsorption, Pb desorption, and Pb recycling. Our findings provide inspiring insights for promoting green and sustainable industrialization of perovskite solar cells.

TiO2@COF Nanowire Arrays: A "Filter Amplifier" Heterojunction Strategy to Reverse the Redox Nature

Surface modification is a promising method to change the surface properties of nanomaterials, but it is limited in enhancing their intrinsic redox nature. In this work, a "filter amplifier" strategy is proposed for the first time to reverse the intrinsic redox nature of materials. This is demonstrated by coating a COF-316 layer with controlled thickness on TiO₂ to form core-sheath nanowire arrays. This unique structure forms a Z-scheme heterojunction to function as "a filter amplifier" which can conceal the intrinsic oxidative sites and increase the extrinsic reductive sites. Consequently, the selective response of TiO₂ is dramatically reversed from reductive ethanol and methanol to oxidative NO₂. Moreover, TiO₂@COF-316 provides remarkably improved sensitivity, response, and recovery speed, as well as unusual anti-humidity properties as compared with TiO₂. This work not only provides a new strategy to rationally modulate the surface chemistry properties of nanomaterials but also opens an avenue to design high-performance electronic devices with a Z-scheme heterojunction.

Order-disorder phase transition driven by interlayer sliding in lead iodides

A variety of phase transitions have been found in two-dimensional layered materials, but some of their atomic-scale mechanisms are hard to clearly understand. Here, we report the discovery of a phase transition whose mechanism is identified as interlayer sliding in lead iodides, a layered material widely used to synthesize lead halide perovskites. The low-temperature crystal structure of lead iodides is found not 2H polytype as known before, but non-centrosymmetric 4H polytype. This undergoes the order-disorder phase transition characterized by the abrupt spectral broadening of valence bands, taken by angle-resolved photoemission, at the critical temperature of 120 K. It is accompanied by drastic changes in simultaneously taken photocurrent and photoluminescence. The transmission electron microscopy is used to reveal that lead iodide layers stacked in the form of 4H polytype at low temperatures irregularly slide over each other above 120 K, which can be explained by the low energy barrier of only 10.6 meV/atom estimated by first principles calculations. Our findings suggest that interlayer sliding is a key mechanism of the phase transitions in layered materials, which can significantly affect optoelectronic and optical characteristics.

Recent Development of Halide Perovskite Materials and Devices for Ionizing Radiation Detection

Ionizing radiation such as X-rays and γ-rays has been extensively studied and used in various fields such as medical imaging, radiographic nondestructive testing, nuclear defense, homeland security, and scientific research. Therefore, the detection of such high-energy radiation with high-sensitivity and low-cost-based materials and devices is highly important and desirable. Halide perovskites have emerged as promising candidates for radiation detection due to the large light absorption coefficient, large resistivity, low leakage current, high mobility, and simplicity in synthesis and processing as compared with commercial silicon (Si) and amorphous selenium (a-Se). In this review, we provide an extensive overview of current progress in terms of materials development and corresponding device architectures for radiation detection. We discuss the properties of a plethora of reported compounds involving organic-inorganic hybrid, all-inorganic, all-organic perovskite and antiperovskite structures, as well as the continuous breakthroughs in device architectures, performance, and environmental stability. We focus on the critical advancements of the field in the past few years and we provide valuable insight for the development of next-generation materials and devices for radiation detection and imaging applications.

Enhancing Photodetection Ability of MoS2 Nanoscrolls via Interface Engineering

Van der Waals semiconductors have been really confirmed in two-dimensional (2D) layered systems beyond the traditional limits of lattice-matching requirements. The extension of this concept to the 1D atomic level may generate intriguing physical functionalities due to its non-covalent bonding surface. However, whether the curvature of the lattice in such rolled-up structures affects their optoelectronic features or the performance of devices established on them remains an open question. Here, MoS₂-based nanoscrolls were obtained by virtue of an alkaline solution-assisted method and the 0D/1D (BaTiO₃/MoS₂) strategy to tune their optoelectronic properties and improve the light sensing performance was explored. The capillary force generated by a drop of NaHCO₃ solution could drive the delamination of nanosheets from the underlying substrate and a spontaneous rolling-up process. The package of BaTiO₃ particles in MoS₂ nanoscrolls has been evident by TEM image, and the optical characterizations were mirrored via micro-Raman spectroscopy and photoluminescence. These bare MoS₂ nanoscrolls reveal a reduced photoresponse compared to the plane structures due to the curvature of the lattice. However, such BaTiO₃/MoS₂ nanoscrolls exhibit a significantly improved photodetection (R_hybrid = 73.9 A/W vs R_only = 1.1 A/W and R_2D = 1.5 A/W at 470 nm, 0.58 mW·cm^-2), potentially due to the carrier extraction/injection occurring between BaTiO₃ and MoS₂. This study thereby provides an insight into 1D van der Waals material community and demonstrates a general approach to fabricate high-performance 1D van der Waals optoelectronic devices.

Lithographic Multicolor Patterning on Hybrid Perovskites for Nano-Optoelectronic Applications

Ultrathin hybrid perovskites, with exotic properties and two-dimensional geometry, exhibit great potential in nanoscale optical and optoelectronic devices. However, it is still challenging for them to be compatible with high-resolution patterning technology toward miniaturization and integration applications, as they can be readily damaged by the organic solvents used in standard lithography processes. Here, a flexible three-step method is developed to make high-resolution multicolor patterning on hybrid perovskite, particularly achieved on a single nanosheet. The process includes first synthesis of precursor PbI₂ , then e-beam lithography and final conversion to target perovskite. The patterns with linewidth around 150 nm can be achieved, which can be applied in miniature optoelectronic devices and high-resolution displays. As an example, the channel length of perovskite photodetectors can be down to 126 nm. Through deterministic vapor-phase anion exchange, a perovskite nanosheet can not only gradually alter the color of the same pattern in a wide wavelength range, but also display different colors simultaneously. The authors are optimistic that the method can be applied for unlimited perovskite types and device configurations for their high-integrated miniature applications.

Flexible Omnidirectional Self-Powered Photodetectors Enabled by Solution-Processed Two-Dimensional Layered PbI2 Nanoplates

Realizing omnidirectional self-powered photodetectors is central to advancing next-generation portable and smart photodetector systems. However, the traditional omnidirectional photodetector is typically achieved by integrating complex hemispherical microlens on multiple photodetectors, which makes the detection system cumbersome and restricts its application in the portable field. Here, facile and high-performance flexible omnidirectional self-powered photodetectors are achieved by solution-processed two-dimensional (2D) layered PbI₂ nanoplates on transparent conducting substrates. Characterization of PbI₂ nanoplates microstructural/compositional and their photodetection properties have been systematically characterized. Under the irradiation of a 405 nm laser, the photodetectors exhibit an impressively low dark current of 10^-13 A, a high light on/off ratio up to 10⁶, and a fast rise/decay response time of 2/3 ms. Importantly, when light irradiates the photodetector at 5°, it can still maintain high photodetection properties, realizing almost 360° omnidirectional self-powered photodetection. What is more, these self-powered photodetectors exhibit robust omnidirectional photoresponse stability of flexibility even after bending for 1200 cycles. Thus, this work broadens the applicability of 2D layered nanoplates for further extending its applications in advanced optoelectronic devices.

Interfacial Coupling and Modulation of van der Waals Heterostructures for Nanodevices

In recent years, van der Waals heterostructures (vdWHs) of two-dimensional (2D) materials have attracted extensive research interest. By stacking various 2D materials together to form vdWHs, it is interesting to see that new and fascinating properties are formed beyond single 2D materials; thus, 2D heterostructures-based nanodevices, especially for potential optoelectronic applications, were successfully constructed in the past few decades. With the dramatically increased demand for well-controlled heterostructures for nanodevices with desired performance in recent years, various interfacial modulation methods have been carried out to regulate the interfacial coupling of such heterostructures. Here, the research progress in the study of interfacial coupling of vdWHs (investigated by Photoluminescence, Raman, and Pump-probe spectroscopies as well as other techniques), the modulation of interfacial coupling by applying various external fields (including electrical, optical, mechanical fields), as well as the related applications for future electrics and optoelectronics, have been briefly reviewed. By summarizing the recent progress, discussing the recent advances, and looking forward to future trends and existing challenges, this review is aimed at providing an overall picture of the importance of interfacial modulation in vdWHs for possible strategies to optimize the device's performance.

Two-Dimensional Cs2AgBiBr6/WS2 Heterostructure-Based Photodetector with Boosted Detectivity via Interfacial Engineering

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers have been widely used for optoelectronic devices because of their ultrasensitivity to light detection acquired from their direct gap properties. However, the small cross-section of photon absorption in the atomically thin layer thickness significantly limits the generation of photocarriers, restricting their performance. Here, we integrate monolayer WS₂ with 2D perovskites Cs₂AgBiBr₆, which serve as the light absorption layer, to greatly enhance the photosensitivity of WS₂. The efficient charge transfer at the Cs₂AgBiBr₆/WS₂ heterojunction is evidenced by the shortened photoluminescence (PL) decay time of Cs₂AgBiBr₆. Scanning photocurrent microscopy of Cs₂AgBiBr₆/WS₂/graphene reveals that improved charge extraction from graphene leads to an enhanced photoresponse. The 2D Cs₂AgBiBr₆/WS₂/graphene vertical heterostructure photodetector exhibits a high detectivity (D*) of 1.5 × 10¹³ Jones with a fast response time of 52.3 μs/53.6 μs and an on/off ratio of 1.02 × 10⁴. It is worth noting that this 2D heterostructure photodetector can realize self-powered light detection behavior with an open-circuit voltage of ∼0.75 V. The results suggest that the 2D perovskites can effectively improve the TMDC layer-based photodetectors for low-power consumption photoelectrical applications.

Deeper Insight into the Role of Organic Ammonium Cations in Reducing Surface Defects of the Perovskite Film

Organic ammonium salts (OASs) have been widely used to passivate perovskite defects. The passivation mechanism is usually attributed to coordination of OASs with unpaired lead or halide ions, yet ignoring their interaction with excess PbI₂ on the perovskite film. Herein, we demonstrate that OASs not only passivate defects by themselves, but also redistribute excess aggregated PbI₂ into a discontinuous layer, augmenting its passivation effect. Moreover, alkyl OAS is more powerful to disperse PbI₂ than a F-containing one, leading to better passivation and device efficiency because F atoms restrict the intercalation of OAS into PbI₂ layers. Inspired by this mechanism, exfoliated PbI₂ nanosheets are adopted to provide better dispersity of PbI₂ , further boosting the efficiency to 23.14 %. Our finding offers a distinctive understanding of the role of OASs in reducing perovskite defects, and a route to choosing an OAS passivator by considering substitution effects rather than by trial and error.

Nonlinear microscopy of lead iodide nanosheets

Lead iodide (PbI₂) is a van der Waals layered semiconductor with a direct bandgap in its bulk form and a hexagonal layered crystalline structure. The recently developed PbI₂ nanosheets have shown great promise for high-performance optoelectronic devices, including nanolasers and photodetectors. However, despite being widely used as a precursor for perovskite materials, the optical properties of PbI₂ nanomaterials remain largely unexplored. Here, we determine the nonlinear optical properties of PbI₂ nanosheets by utilising nonlinear microscopy as a non-invasive optical technique. We demonstrate the nonlinearity enhancement dependent on excitonic resonances, crystalline orientation, thickness, and influence of the substrate. Our results allow for estimating the second- and third-order nonlinear susceptibilities of the nanosheets, opening new opportunities for the use of PbI₂ nanosheets as nonlinear and quantum light sources.

Visualizing Van der Waals Epitaxial Growth of 2D Heterostructures

Understanding the growth mechanisms of 2D van der Waals (vdW) heterostructures is of great importance in exploring their functionalities and device applications. A custom-built system integrating physical vapor deposition and optical microscopy/Raman spectroscopy is employed to study the dynamic growth processes of 2D vdW heterostructures in situ. This allows the identification of a new growth mode with a distinctly different growth rate and morphology from those of the conventional linear growth mode. A model that explains the difference in morphologies and quantifies the growth rates of the two modes by taking the role of surface diffusion into account is proposed. A range of material combinations including CdI₂ /WS₂ , CdI₂ /MoS₂ , CdI₂ /WSe₂ , PbI₂ /WS₂ , PbI₂ /MoS₂ , PbI₂ /WSe₂ , and Bi₂ Se₃ /WS₂ is systematically investigated. These findings may be generalized to the synthesis of many other 2D heterostructures with controlled morphologies and physical properties, benefiting future device applications.

Orientation-Controlled Large-Area Epitaxial PbI2 Thin Films with Tunable Optical Properties

Lead iodide (PbI₂) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI₂ with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI₂ remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI₂ thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI₂ thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI₂ on other substrates and heterostructures of PbI₂ on single-crystalline graphene. The ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI₂ and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.

Transition of the Type of Band Alignments for All-Inorganic Perovskite van der Waals Heterostructures CsSnBr3/WS2(1-x)Se2x

In general, two-dimensional semiconductor-based van der Waals heterostructures (vdWHs) can be modulated to achieve the transition of band alignments (type-I, type-II, and type-III), which can be applied in different applications. However, it is rare in three-dimensional perovskite-based vdWHs, and it is challenging to achieve the tunable band alignments for a single perovskite-based heterostructure. Here, we systematically investigate the electronic and optical properties of all-inorganic perovskite vdWHs CsSnBr₃/WS_2(1-x)Se_2x based on density functional theory (DFT) calculation. The calculated results show that the transitions of band alignment from type-II to type-I and type-III to type-II are achieved by modulating the doping ratio of the Se atom in the WS_2(1-x)Se_2x monolayer for SnBr₂/WS_2(1-x)Se_2x and CsBr/WS_2(1-x)Se_2x heterostructures, respectively, in which the CsBr and SnBr₂ represent two different terminated surfaces of CsSnBr₃. The change of band alignments can be attributed to the conduction band minimum (CBM) transforming from the W 5d to Sn 5p orbital in SnBr₂/WS_2(1-x)Se_2x vdWHs, and the valence band maximum (VBM) and CBM change from an overlapped state to a separated one in CsBr/WS_2(1-x)Se_2x vdWHs. This work can provide a theoretical basis for the dynamic modulation of band alignments in perovskite-based vdWHs.

Multiple-Dimensionally Controllable Nucleation Sites of Two-Dimensional WS2/Bi2Se3 Heterojunctions Based on Vapor Growth

Two-dimensional (2D) heterojunctions have attracted great attention due to their excellent optoelectronic properties. Until now, precisely controlling the nucleation density and stacking area of 2D heterojunctions has been of critical importance but still a huge challenge. It hampers the progress of controlled growth of 2D heterojunctions for optoelectronic devices because the potential relation between numerous growth parameters and nucleation density is always poorly understood. Herein, by cooperatively controlling three parameters (substrate temperature, gas flow rate, and precursor concentration) in modified vapor deposition growth, the nucleation density and stacking area of WS₂/Bi₂Se₃ vertical heterojunctions were successfully modulated. High-quality WS₂/Bi₂Se₃ vertical heterojunctions with various stacking areas were effectively grown from single and multiple nucleation sites. Moreover, the potential nucleation mechanism and efficient charge transfer of WS₂/Bi₂Se₃ vertical heterojunctions were systematically studied by utilizing the density functional theory and photoluminescence spectra. This modified vapor deposition strategy and the proposed mechanism are helpful in controlling the nucleation density and stacking area of other heterojunctions, which plays a key role in the preparation of electronic and optoelectronic nanodevices.

Direct Synthesis and Enhanced Rectification of Alloy-to-Alloy 2D Type-II MoS2(1- x ) Se2 x /SnS2(1- y ) Se2 y Heterostructures

The interfacial tunable band alignment of heterostructures is coveted in device design and optimization of device performance. As an intentional approach, alloying allows band engineering and continuous band-edge tunability for low-dimensional semiconductors. Thus, combining the tunability of alloying with the band structure of a heterostructure is highly desirable for the improvement of device characteristics. In this work, the single-step growth of alloy-to-alloy (MoS_{2(1- x )} Se_{2 x} /SnS_{2(1- y )} Se_{2 y} ) 2D vertical heterostructures is demonstrated. Electron diffraction reveals the well-aligned heteroepitaxial relationship for the heterostructure, and a near-atomically sharp and defect-free boundary along the interface is observed. The nearly intrinsic van der Waals (vdW) interface enables measurement of the intrinsic behaviors of the heterostructures. The optimized type-II band alignment for the MoS_{2(1- x )} Se_{2 x} /SnS_{2(1- y )} Se_{2 y} heterostructure, along with the large band offset and effective charge transfer, is confirmed through quenched PL spectroscopy combined with density functional theory calculations. Devices based on completely stacked heterostructures show one or two orders enhanced electron mobility and rectification ratio than those of the constituent materials. The realization of device-quality alloy-to-alloy heterostructures provides a new material platform for precisely tuning band alignment and optimizing device applications.

Room temperature near unity spin polarization in 2D Van der Waals heterostructures

The generation and manipulation of spin polarization at room temperature are essential for 2D van der Waals (vdW) materials-based spin-photonic and spintronic applications. However, most of the high degree polarization is achieved at cryogenic temperatures, where the spin-valley polarization lifetime is increased. Here, we report on room temperature high-spin polarization in 2D layers by reducing its carrier lifetime via the construction of vdW heterostructures. A near unity degree of polarization is observed in PbI2 layers with the formation of type-I and type-II band aligned vdW heterostructures with monolayer WS2 and WSe2. We demonstrate that the spin polarization is related to the carrier lifetime and can be manipulated by the layer thickness, temperature, and excitation wavelength. We further elucidate the carrier dynamics and measure the polarization lifetime in these heterostructures. Our work provides a promising approach to achieve room temperature high-spin polarizations, which contribute to spin-photonics applications.

Engineering the Phases and Heterostructures of Ultrathin Hybrid Perovskite Nanosheets

Low-dimensional perovskites have gained increasing attention recently, and engineering their material phases, structural patterning and interfacial properties is crucial for future perovskite-based applications. Here a phase and heterostructure engineering on ultrathin perovskites, through the reversible cation exchange of hybrid perovskites and efficient surface functionalization of low-dimensional materials, is demonstrated. Using PbI₂ as precursor and template, perovskite nanosheets of varying thickness and hexagonal shape on diverse substrates is obtained. Multiple phases, such as PbI₂ , MAPbI₃ and FAPbI₃ , can be flexibly designed and transformed as a single nanosheet. A perovskite nanosheet can be patterned using masks made of 2D materials, fabricating lateral heterostructures of perovskite and PbI₂ . Perovskite-based vertical heterostructures show strong interfacial coupling with 2D materials. As a demonstration, monolayer MoS₂ /MAPbI₃ stacks give a type-II heterojunction. The ability to combine the optically efficient perovskites with versatile 2D materials creates possibilities for new designs and functionalities.

Type-II Interface Band Alignment in the vdW PbI2-MoSe2 Heterostructure

Energy band alignments at heterostructure interfaces play key roles in device performance, especially between two-dimensional atomically thin materials. Herein, van der Waals PbI₂-MoSe₂ heterostructures fabricated by in situ PbI₂ deposition on monolayer MoSe₂ are comprehensively studied using scanning tunneling microscopy/spectroscopy, atomic force microscopy, photoemission spectroscopy, and Raman and photoluminescence (PL) spectroscopy. PbI₂ grows on MoSe₂ in a quasi layer-by-layer epitaxial mode. A type-II interface band alignment is proposed between PbI₂ and MoSe₂ with the conduction band minimum (valence band maximum) located at PbI₂ (MoSe₂), which is confirmed by first-principles calculations and the existence of interfacial excitons revealed using temperature-dependent PL. Our findings provide a scalable method to fabricate PbI₂-MoSe₂ heterostructures and new insights into the electronic structures for future device design.

Interfacial Engineering of W2 N/WC Heterostructures Derived from Solid-State Synthesis: A Highly Efficient Trifunctional Electrocatalyst for ORR, OER, and HER

To meet the practical demand of overall water splitting and regenerative metal-air batteries, highly efficient, low-cost, and durable electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are required to displace noble metal catalysts. In this work, a facile solid-state synthesis strategy is developed to construct the interfacial engineering of W₂ N/WC heterostructures, in which abundant interfaces are formed. Under high temperature (800 °C), volatile CN_x species from dicyanodiamide are trapped by WO₃ nanorods, followed by simultaneous nitridation and carbonization, to form W₂ N/WC heterostructure catalysts. The resultant W₂ N/WC heterostructure catalysts exhibit an efficient and stable electrocatalytic performance toward the ORR, OER, and HER, including a half-wave potential of 0.81 V (ORR) and a low overpotential at 10 mA cm^-2 for the OER (320 mV) and HER (148.5 mV). Furthermore, a W₂ N/WC-based Zn-air battery shows outstanding high power density (172 mW cm^-2 ). Density functional theory and X-ray absorption fine structure analysis computations reveal that W₂ N/WC interfaces synergistically facilitate transport and separation of charge, thus accelerating the electrochemical ORR, OER, and HER. This work paves a novel avenue for constructing efficient and low-cost electrocatalysts for electrochemical energy devices.

Black phosphorus-based van der Waals heterostructures for mid-infrared light-emission applications

Mid-infrared (MIR) light-emitting devices play a key role in optical communications, thermal imaging, and material analysis applications. Two-dimensional (2D) materials offer a promising direction for next-generation MIR devices owing to their exotic optical properties, as well as the ultimate thickness limit. More importantly, van der Waals heterostructures-combining the best of various 2D materials at an artificial atomic level-provide many new possibilities for constructing MIR light-emitting devices of large tuneability and high integration. Here, we introduce a simple but novel van der Waals heterostructure for MIR light-emission applications built from thin-film BP and transition metal dichalcogenides (TMDCs), in which BP acts as an MIR light-emission layer. For BP-WSe₂ heterostructures, an enhancement of ~200% in the photoluminescence intensities in the MIR region is observed, demonstrating highly efficient energy transfer in this heterostructure with type-I band alignment. For BP-MoS₂ heterostructures, a room temperature MIR light-emitting diode (LED) is enabled through the formation of a vertical PN heterojunction at the interface. Our work reveals that the BP-TMDC heterostructure with efficient light emission in the MIR range, either optically or electrically activated, provides a promising platform for infrared light property studies and applications.

Few-Layer PbI2 Nanoparticle: A 2D Semiconductor with Lateral Quantum Confinement

Inspired by the superior optoelectronic performances of various 2D semiconductors, their new compositions and structures are being actively pursued in order to foster novel fundamental physics and device applications. As a layered semiconductor with a direct bandgap, few-layer PbI₂ should have drawn much research attention due to their capability of emitting photons at short wavelengths of the visible spectrum. Here we chemically synthesize few-layer PbI₂ flakes and nanoparticles, which demonstrate unique exciton properties that have rare counterparts in other 2D semiconductors. For three layers and more, the single PbI₂ flakes can be utilized to show how the bandgap energy of a 2D semiconductor evolves with the changing layer thickness. The single PbI₂ nanoparticles are associated with an ultranarrow photoluminescence line width of ∼1 meV, thus reflecting the influence of lateral quantum confinement on the energy-level structures of a 2D semiconductor. The above findings mark the emergence of a potent 2D platform that is more than complementary to well-studied transition-metal dichalcogenide monolayers.

Surface group-modified MXene nano-flake doping of monolayer tungsten disulfides

Exciton/trion-involved optoelectronic properties have attracted exponential amount of attention for various applications ranging from optoelectronics, valleytronics to electronics. Herein, we report a new chemical (MXene) doping strategy to modulate the negative trion and neutral exciton for achieving high photoluminescence yield of atomically thin transition metal dichalcogenides, enabled by the regulation of carrier densities to promote electron-bound trion-to-exciton transition via charge transfer from TMDCs to MXene. As a proof of concept, the MXene nano-flake-doped tungsten disulfide is demonstrated to obtain an enhanced PL efficiency of up to ∼five folds, which obviously exceeds the reported efficiency upon electrical and/or plasma doping strategies. The PL enhancement degree can also be modulated by tuning the corresponding surface functional groups of MXene nano-flakes, reflecting that the electron-withdrawing functional groups play a vital role in this charge transfer process. These findings offer promising clues to control the optoelectronic properties of TMDCs and expand the scope of the application of MXene nano-flakes, suggesting a possibility to construct a new heterostructure junction based on MXenes and TMDCs.

PbI2-MoS2 Heterojunction: van der Waals Epitaxial Growth and Energy Band Alignment

van der Waals (vdW) epitaxy offers a promising strategy without lattice and processing constraints to prepare atomically clean and electronically sharp interfaces for fundamental studies and electronic device demonstrations. Herein, PbI₂ was thermally deposited at high-vacuum conditions onto CVD-grown monolayer MoS₂ flakes in a vdW epitaxial manner to form 3D-2D heterojunctions, which are promising for vdW epitaxial growth of perovskite films. X-ray diffraction, X-ray photoemission spectroscopy, Raman, and atomic force microscopy measurements reveal the structural properties of the high-quality heterojunctions. Photoluminescence (PL) measurements reveal that the PL emissions from the bottom MoS₂ flakes are greatly quenched compared to their as-grown counterparts, which can be ascribed to the band alignment-induced distinct interfacial charge-transfer behaviors. Strong interlayer excitons can be detected at the PbI₂/MoS₂ interface, indicating an effective type II band alignment, which can be further confirmed by ultraviolet photoemission spectroscopy measurements. The results provide a new material platform for the application of the vdW heterojunctions in electronic and optoelectronic devices.

Unique and Tunable Photodetecting Performance for Two-Dimensional Layered MoSe2/WSe2 p-n Junction on the 4H-SiC Substrate

MoSe₂/WSe₂ two-dimensional transition-metal dichalcogenide (TMDC) heterojunction photodetectors based on epitaxial n-doped 4H-silicon carbide (SiC) substrate are investigated and exhibited low leakage, high stability, and fast photoresponse. The efficient separation of photogenerated carriers occurs between TMDCs and 4H-SiC, as indicated by the photoluminescence spectrum and the band alignment analysis under 532 nm. The MoSe₂/WSe₂/4H-SiC photodetector shows an obvious rectification behavior and unique current-gate voltage ( I- V_g) characteristics. The gate tunable photocurrent scanning maps display the highest photocurrent in the MoSe₂/WSe₂ region including a certain intensive current region in individual TMDCs/4H-SiC junctions under a 532 nm laser. Besides, the maximum responsivity of the heterojunction photodetectors is 7.17 A·W^-1 with the V_g of 10 V at positive bias. The corresponding maximum external quantum efficiency and detectivity also significantly increase to 1.67 × 10³% and 5.51 × 10¹¹ jones with the largest I_light/ I_dark ratio of ∼10³. Moreover, the MoSe₂/4H-SiC photodetector delivers an enhanced photoresponse behavior with gate modulation, which is different from the previous paper. These results of our study demonstrate that MoSe₂/WSe₂ heterojunction photodetectors based on the n-doped 4H-SiC substrate will be a promising candidate for future optoelectronics applications in spectral responsivity.