Optoelectronic Properties of a van der Waals WS2 Monolayer/2D Perovskite Vertical Heterostructure

Critical Role of Carrier Cooling Mechanism in WS2/CsPbBr3 Hybrid Nanocomposites for Enhanced Photodetector Performances

Heterostructures and nanocomposites comprising transition metal dichalcogenides (TMDCs) and halide perovskite nanocrystals (NCs) are prominently used in several optoelectronic devices. Hot carriers (HCs) are the charge carriers possessing higher kinetic energy than surrounded thermal distributions. Properly utilizing these HCs by slowing down their cooling mechanism reduces the energy losses in optoelectronic devices. Herein, employing the femtosecond transient absorption spectroscopy (fs-TAS) technique, the slowdown processes of HC relaxations are reported in WS2/CsPbBr3 hybrid-nanocomposites due to the hot-phonon bottleneck. HCs relaxation time increases from ≈6 ps in CsPbBr3 NCs to ≈10 ps in WS2/CsPbBr3 nanocomposites at an excitation fluence of 17.7 µJ cm-2. The maximum HCs temperature TC increased to 1181 K in WS2/CsPbBr3 nanocomposites with an observed TC of 856 K in pristine NCs. The electron transfer process from NCs to WS2 nanosheets has been observed in these nanocomposites with time component t2 ≈38.0-102.4 ps in pristine NCs and 20.9-66.9 ps in nanocomposites, became faster at excitation-fluence of 17.7-99.8 µJ cm-2. Furthermore, a significant enhancement in nanocomposite-based photodetector confirmed the efficient charge transfer at the heterojunction, resulting ≈400%, ≈420%, and ≈200% increase in the photocurrent, responsivity, and detectivity, respectively, compared to the pristine devices.

Structure and stability of copper nanoclusters on monolayer tungsten dichalcogenides

Layered materials, such as tungsten dichalcogenides (TMDs), are being studied for a wide range of applications, due to their unique and varied properties. Specifically, their use as either a support for low dimensional catalysts or as an ultrathin diffusion barrier in semiconductor devices interconnect structures are particularly relevant. In order to fully realise these possible applications for TMDs, understanding the interaction between metals and the monolayer they are deposited on is of utmost importance. The morphology that arises due to given metal-substrate combinations determines their possible applications and thus is a central characteristic. Previous theoretical studies typically focus on the effects which single metal adatoms, or dopants, have on a TMDs' electronic and optical properties, thereby leaving a knowledge gap in terms of thin film nucleation on TMD monolayers. To address this, we present a density functional theory (DFT) study of the adsorption of small Cu clusters on a range of TMD monolayers, namely WS2, WSe2, and WTe2. We explore how metal-substrate and metal-metal interactions contribute to both the stability of these Cu clusters and their morphology, and investigate the role of the chalcogen in these interactions. We find that single Cu atoms adsorb most strongly to the adsorption site above the W atom, however as nanocluster size increases, Cu tends to be adsorbed atop the chalcogen atoms in the monolayer to facilitate Cu-Cu bond formation. We show that Cu-Cu interactions drive the stability of the adsorbed Cu nanoclusters, with a clear preference for 3D structures on all 3 monolayers studied. Furthermore, significant Cu migration occurs during 0 K relaxation. This, combined with the small activation barriers found for Cu migration suggest facile and dynamic cluster behaviour at finite temperature on all three monolayers. Finally, we find that Cu clusters are generally most stable on WTe2 and least stable on WSe2. This difference however is typically only in the range of 0.1 eV.

Robust Growth of 2D Transition Metal Dichalcogenide Vertical Heterostructures via Ammonium-Assisted CVD Strategy

Two dimension (2D) transition metal dichalcogenides (TMD) heterostructures have opened unparalleled prospects for next-generation electronic and optoelectronic applications due to their atomic-scale thickness and distinct physical properties. The chemical vapor deposition (CVD) method is the most feasible approach to prepare 2D TMD heterostructures. However, the synthesis of 2D vertical heterostructures faces competition between in-plane and out-of-plane growth, which makes it difficult to precisely control the growth of vertical heterostructures. Here, a universal and controllable strategy is reported to grow various 2D TMD vertical heterostructures through an ammonium-assisted CVD process. The ammonium-assisted strategy shows excellent controllability and operational simplicity to prevent interlayer diffusion/alloying and thermal decomposition of the existed TMD templates. Ab initio simulations demonstrate that the reaction between NH4Cl and MoS2 leads to the formation of MoS3 clusters, promoting the nucleation and growth of 2D MoS2 on existed 2D WS2 layer, thereby leading to the growth of vertical heterostructure. The resulting 2D WSe2/WS2 vertical heterostructure photodetectors demonstrate an outstanding optoelectronic performance, which are comparable to the performances of photodetectors fabricated from mechanically exfoliated and stacked vertical heterostructures. The ammonium-assisted strategy for robust growth of high-quality vertical van der Waals heterostructures will facilitate fundamental physics investigations and device applications in electronics and optoelectronics.

High-Throughput Computational Study and Machine Learning Prediction of Electronic Properties in Transition Metal Dichalcogenide/Two-Dimensional Layered Halide Perovskite Heterostructures

Heterostructures formed by transition metal dichalcogenides (TMDs) and two-dimensional layered halide perovskites (2D-LHPs) have attracted significant attention due to their unique optoelectronic properties. However, theoretical studies face challenges due to the large number of atoms and the need for lattice matching. With the discovery of more 2D-LHPs, there is an urgent need for methods to rapidly predict and screen TMDs/2D-LHPs heterostructures. This study employs first-principles calculations to perform high-throughput computations on 602 TMDs/2D-LHPs heterostructures. Results show that different combinations exhibit diverse band alignments, with MoS2 and WS2 more likely to form type-II heterostructures with 2D-LHPs. The highest photoelectric conversion efficiency of type-II structures reaches 23.26%, demonstrating potential applications in solar cells. Notably, some MoS2/2D-LHPs form type-S structures, showing promise in photocatalysis. Furthermore, we found that TMDs can significantly affect the conformation of organic molecules in 2D-LHPs, thus modulating the electronic properties of the heterostructures. To overcome computational cost limitations, we constructed a crystal graph convolutional neural network model based on the calculated data to predict the electronic properties of TMDs/2D-LHPs heterostructures. Using this model, we predicted the bandgaps and band alignment types of 9,360 TMDs/2D-LHPs heterostructures, providing a comprehensive theoretical reference for research in this field.

Long-Lived Interlayer Excitons in WS2/Ruddlesden-Popper Perovskite van der Waals Heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and perovskites hold substantial promise for various optoelectronic applications such as light emission, photodetection, and energy harvesting. However, each of these materials possesses certain limitations that can be overcome by synergistically combining them to form heterostructures, thereby unveiling intriguing optical properties. In this study, we present an uncomplicated technique for crafting a van der Waals (vdW) heterojunction comprising monolayer WS2 and a Ruddlesden-Popper (RP) perovskite, namely (TEA)2PbI4. By utilizing ultrafast transient absorption (TA) spectroscopy, we explored the charge carrier dynamics within the WS2/(TEA)2PbI4 heterostructure. Our findings uncover a type-II band alignment in the heterostructure, facilitating rapid (within 260 fs) hole transfer from WS2 to the perovskite and leading to the formation of interlayer excitons (IXs) with a much longer lifetime (728 ps). This strategic approach has the potential to contribute to the development of hybrid systems aimed at achieving high-performance optoelectronic devices.

Recent advances in 2D transition metal dichalcogenide-based photodetectors: a review

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as a highly promising platform for the development of photodetectors (PDs) owing to their remarkable electronic and optoelectronic properties. Highly effective PDs can be obtained by making use of the exceptional properties of 2D materials, such as their high transparency, large charge carrier mobility, and tunable electronic structure. The photodetection mechanism in 2D TMD-based PDs is thoroughly discussed in this article, with special attention paid to the key characteristics that set them apart from PDs based on other integrated materials. This review examines how single TMDs, TMD-TMD heterostructures, TMD-graphene (Gr) hybrids, TMD-MXene composites, TMD-perovskite heterostructures, and TMD-quantum dot (QD) configurations show advanced photodetection. Additionally, a thorough analysis of the recent developments in 2D TMD-based PDs, highlighting their exceptional performance capabilities, including ultrafast photo response, ultrabroad detectivity, and ultrahigh photoresponsivity, attained through cutting-edge methods is provided. The article conclusion highlights the potential for ground-breaking discoveries in this fast developing field of research by outlining the challenges faced in the field of PDs today and providing an outlook on the prospects of 2D TMD-based PDs in the future.

Energy Funneling in a Noninteger Two-Dimensional Perovskite

Energy funneling is a phenomenon that has been exploited in optoelectronic devices based on low-dimensional materials to improve their performance. Here, we introduce a new class of two-dimensional semiconductor, characterized by multiple regions of varying thickness in a single confined nanostructure with homogeneous composition. This "noninteger 2D semiconductor" was prepared via the structural transformation of two-octahedron-layer-thick (n = 2) 2D cesium lead bromide perovskite nanosheets; it consisted of a central n = 2 region surrounded by edge-lying n = 3 regions, as imaged by electron microscopy. Thicker noninteger 2D CsPbBr3 nanostructures were obtained as well. These noninteger 2D perovskites formed a laterally coupled quantum well band alignment with virtually no strain at the interface and no dielectric barrier, across which unprecedented intramaterial funneling of the photoexcitation energy was observed from the thin to the thick regions using time-resolved absorption and photoluminescence spectroscopy.

Trends and prospects of 2-D tungsten disulphide (WS2) hybrid nanosystems for environmental and biomedical applications

Recently, 2D layered transition metal dichalcogenides (TMDCs) with their ultrathin sheet nanostructure and diversified electronic structure have drawn attention for various advanced applications to achieve high-performance parameters. Unique 2D TMDCs mainly comprise transition metal and chalcogen element where chalcogen element layers sandwich the transition metal element layer. In such a case, various properties can be enhanced and controlled depending on the targeted application. Among manipulative 2D TMDCs, tungsten disulphide (WS2) is one of the emerging nano-system due to its fascinating properties in terms of direct band gap, higher mobility, strong photoluminescence, good thermal stability, and strong magnetic field interaction. The advancement in characterization techniques, especially scattering techniques, can help in study of opto-electronic properties of 2D TMDCs along with determination of layer variations and investigation of defect. In this review, the fabrication and applications are well summarized to optimize an appropriate WS2-TMDCs assembly according to focused field of research. Here, the scientific investigations on 2D WS2 are studied in terms of its structure, role of scattering techniques to study its properties, and synthesis routes followed by its potential applications for environmental remediation (e.g., photocatalytic degradation of pollutants, gas sensing, and wastewater treatment) and biomedical domain (e.g., drug delivery, photothermal therapy, biomedical imaging, and biosensing). Further, a special emphasis is given to the significance of 2D WS2 as a substrate for surface-enhanced Raman scattering (SERS). The discussion is further extended to commercial and industrial aspects, keeping in view major research gaps in existing research studies.

Carrier transfer in quasi-2D perovskite/MoS2 monolayer heterostructure

Two-dimensional layered semiconductors have attracted intense interest in recent years. The van der Waals coupling between the layers tolerates stacking various materials and establishing heterostructures with new characteristics for a wide range of optoelectronic applications. The interlayer exciton dynamics at the interface within the heterostructure are vitally important for the performance of the photodetector and photovoltaic device. Here, a heterostructure comprising two-dimensional organic-inorganic Ruddlesden-Popper perovskites and transition metal dichalcogenide monolayer was fabricated and its ultrafast charge separation processes were systematically studied by using femtosecond time-resolved transient absorption spectroscopy. Significant hole and electron transfer processes in the ps and fs magnitude at the interface of the heterostructure were observed by tuning pump wavelengths of the pump-probe geometries. The results emphasize the realization of the exciton devices based on semiconductor heterostructures of two-dimensional perovskite and transition metal dichalcogenide.

Thermal stability and decomposition kinetics of mixed-cation halide perovskites

Organic-inorganic halide perovskites (OIHPs) have emerged as one of the most efficient photovoltaic materials due to their superior properties. However, improving their stability remains a key challenge. Herein, we investigate the thermal decomposition properties of OIHP FAxMA1-xPbI3 with mixed cations of formamidinium (FA) and methylammonium (MA). Using thermogravimetric analysis together with Fourier transform infrared spectroscopy, we identify and monitor the gaseous decomposition products as a function of temperature and cation composition. Thermal decomposition products of both MA and FA cations were observed at all stages of the thermal decomposition process, contrary to previous expectations. The yield, release sequence and kinetics of the organic gaseous products were found to depend strongly on the ratio between FA and MA cations. Furthermore, cesium ion doping was investigated as a potential strategy to improve the thermal stability of mixed cation perovskites. These results provide new insights into the effect of cation mixing on perovskite stability, suggesting that optimizing the cation ratios and decomposition pathways can guide approaches to boost the stability and performance of mixed cation perovskites.

Improved Photo-Excited Carriers Transportation of WS2 -O-Doped-Graphene Heterostructures for Solar Steam Generation

Two-dimensional (2D) transition metal dichalcogenides and graphene have revealed promising applications in optoelectronic and energy storage and conversion. However, there are rare reports of modifying the light-to-heat transformation via preparing their heterostructures for solar steam generation. In this work, commercial WS₂ and sucrose are utilized as precursors to produce 2D WS₂ -O-doped-graphene heterostructures (WS₂ -O-graphene) for solar water evaporation. The WS₂ -O-graphene evaporators demonstrate excellent average water evaporation rate (2.11 kg m^-2  h^-1 ) and energy efficiency (82.2%), which are 1.3- and 1.2-fold higher than WS₂ and O-doped graphene-based evaporators, respectively. Furthermore, for the real seawater with different pH values (pH 1 and 12) and rhodamine B pollutants, the WS₂ -O-graphene evaporators show great average evaporation rates (≈2.08 and 2.09 kg m^-2  h^-1 , respectively) for producing freshwater with an extremely low-grade of dye residual and nearly neutral pH values. More interestingly, due to the self-storage water ability of WS₂ -O-graphene evaporators, water evaporation can be implemented without the presence of bulk water. As a result, the evaporation rate reaches 3.23 kg m^-2  h^-1 , which is ≈1.5 times higher than the regular solar water evaporation system. This work provides a new approach for preparing 2D transition metal dichalcogenides and graphene heterostructures for efficient solar water evaporation.

The role of Pb oxidation state of the precursor in the formation of 2D perovskite microplates

Two-dimensional (2D) lead halide perovskites are an exciting class of materials currently being extensively explored for photovoltaics and other optoelectronic applications. Their ionic nature makes them ideal candidates for solution processing into both thin films and nanostructured crystals. Understanding how 2D lead halide perovskite crystals form is key towards full control over their physical properties, which may enable new physical phenomena and devices. Here, we investigate the effects of the Pb oxidation state of the initial inorganic precursor on the growth of pure-phase (n = 1) - Popper 2D perovskite BA₂PbI₄ in single-step synthesis. We examine the different crystallisation routes in exposing PbO₂ and PbI₂ powders to a BAI : IPA organo-halide solution, by combining in situ optical microscopy, UV-VIS spectroscopy and time-resolved high performance liquid chromatography. So far, works using PbO₂ to synthesise 3D LHPs introduce a preceding step to reduce PbO₂ into either PbO or PbI₂. In this work, we find that BA₂PbI₄ is directly formed when exposing PbO₂ to BAI : IPA without the need for an external reducing agent. We explain this phenomenon by the spontaneous reduction/oxidation of PbO₂/BAI that occurs under iodine-rich conditions. We observe differences in the final morphology (rectangles vs. octagons) and nanocrystal growth rate, which we explain through the different chemistry and iodoplumbate complexes involved in each case. As such, this work spans the horizon of usable lead precursors and offers a new turning knob to control crystal growth in single-step LHP synthesis.

Interlayer excitons in MoSe2/2D perovskite hybrid heterostructures - the interplay between charge and energy transfer

van der Waals crystals have opened a new and exciting chapter in heterostructure research, removing the lattice matching constraint characteristics of epitaxial semiconductors. They provide unprecedented flexibility for heterostructure design. Combining two-dimensional (2D) perovskites with other 2D materials, in particular transition metal dichalcogenides (TMDs), has recently emerged as an intriguing way to design hybrid opto-electronic devices. However, the excitation transfer mechanism between the layers (charge or energy transfer) remains to be elucidated. Here, we investigate PEA₂PbI₄/MoSe₂ and (BA)₂PbI₄/MoSe₂ heterostructures by combining optical spectroscopy and density functional theory (DFT) calculations. We show that band alignment facilitates charge transfer. Namely, holes are transferred from TMDs to 2D perovskites, while the electron transfer is blocked, resulting in the formation of interlayer excitons. Moreover, we show that the energy transfer mechanism can be turned on by an appropriate alignment of the excitonic states, providing a rule of thumb for the deterministic control of the excitation transfer mechanism in TMD/2D-perovskite heterostructures.

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.

Perovskite-Type 2D Materials for High-Performance Photodetectors

Photodetectors are light sensors in widespread use in image sensing, optical communication, and consumer electronics. In current smart optoelectronic technology, conventional semiconductors have encountered a bottleneck caused by inflexibility and opacity. With the ever-increasing demands for versatile optoelectronic applications, perovskite-type 2D materials demonstrate great potential for advanced photodetectors inspired by molecularly thin 2D materials. Through the reduction of thickness to thin or molecularly thin levels, single-crystalline 2D perovskites can exhibit superior optoelectronic performance characteristics, such as tunable absorption property by chemical design, enhanced carrier separation by remarkable photosensing capability, and improved carrier extraction by versatile band engineering. More importantly, perovskite-type 2D materials exhibit great potential for large-scale monolithic integration to achieve all-in-one sensing-memory-computing optoelectronic devices. In this Perspective, recent progress in 2D perovskite-based photodetectors is presented in detail. The focus is on growth strategies for reducing thickness, thickness-dependent optical and electrical properties, device engineering, heterojunction fabrication, and device performance. Finally, the current challenges and future prospects in this field are presented.

Science of 2.5 dimensional materials: paradigm shift of materials science toward future social innovation

The past decades of materials science discoveries are the basis of our present society - from the foundation of semiconductor devices to the recent development of internet of things (IoT) technologies. These materials science developments have depended mainly on control of rigid chemical bonds, such as covalent and ionic bonds, in organic molecules and polymers, inorganic crystals and thin films. The recent discovery of graphene and other two-dimensional (2D) materials offers a novel approach to synthesizing materials by controlling their weak out-of-plane van der Waals (vdW) interactions. Artificial stacks of different types of 2D materials are a novel concept in materials synthesis, with the stacks not limited by rigid chemical bonds nor by lattice constants. This offers plenty of opportunities to explore new physics, chemistry, and engineering. An often-overlooked characteristic of vdW stacks is the well-defined 2D nanospace between the layers, which provides unique physical phenomena and a rich field for synthesis of novel materials. Applying the science of intercalation compounds to 2D materials provides new insights and expectations about the use of the vdW nanospace. We call this nascent field of science '2.5 dimensional (2.5D) materials,' to acknowledge the important extra degree of freedom beyond 2D materials. 2.5D materials not only offer a new field of scientific research, but also contribute to the development of practical applications, and will lead to future social innovation. In this paper, we introduce the new scientific concept of this science of '2.5D materials' and review recent research developments based on this new scientific concept.

Fabrication of 2D PdSe2/3D CdTe Mixed-Dimensional van der Waals Heterojunction for Broadband Infrared Detection

Room-temperature infrared photodetectors are in great demand because of their vitally important applications in the military and civilian fields, which has inspired intensive studies in recent decades. Here, we present the fabrication of a large-size PdSe₂/CdTe mixed-dimensional van der Waals (vdW) heterojunction for room-temperature infrared detection. Taking advantage of the wide light absorption of the multilayer PdSe₂, high-quality vdW interface, and unique mixed-dimensional device geometry, the present device is capable of detecting an ultrawide light up to long-wave infrared (LWIR) of 10.6 μm at room temperature. In addition, our photodetector exhibits a good capability to follow short pulse infrared signals with a quick response rate of 70 ns, a large responsivity of 324.7 mA/W, and a reasonable specific detectivity of 3.3 × 10¹² Jones. Significantly, the assembled photodetector is highly sensitive to a polarized infrared light signal with a decent polarization sensitivity of 4.4. More importantly, an outstanding LWIR imaging capability based on the PdSe₂/CdTe vdW heterojunction at nonrefrigeration condition is demonstrated. Our work paves a novel route for the design of highly polarization-sensitive, room-temperature infrared photodetectors.

Two-Dimensional Hybrid Perovskite-Based van der Waals Heterostructures

Two-dimensional (2D) hybrid perovskites, as newly emerging materials, have become the center of attention in optoelectronic fields within a few years because of their unique optoelectronic properties, including tunable bandgap, long carrier diffusion length, and high absorption coefficient. 2D perovskite-based van der Waals heterostructures via integration of 2D perovskites with other layered materials provide new platforms for many optoelectronic devices with prominent performance, such as photodetectors, light-emitting diodes (LEDs), and phototransistors. In this Perspective, the unique properties of 2D perovskites will be first introduced to explore why this material is attractive and popular. Subsequently, various types of heterostructures based on 2D perovskites will be illustrated, including the heterostructure construction approaches as well as their optical and optoelectronic applications. Finally, potential research directions based on 2D perovskite heterostructures are also proposed.

Exciton-Photonics: From Fundamental Science to Applications

Semiconductors in all dimensionalities ranging from 0D quantum dots and molecules to 3D bulk crystals support bound electron-hole pair quasiparticles termed excitons. Over the past two decades, the emergence of a variety of low-dimensional semiconductors that support excitons combined with advances in nano-optics and photonics has burgeoned an advanced area of research that focuses on engineering, imaging, and modulating the coupling between excitons and photons, resulting in the formation of hybrid quasiparticles termed exciton-polaritons. This advanced area has the potential to bring about a paradigm shift in quantum optics, as well as classical optoelectronic devices. Here, we present a review on the coupling of light in excitonic semiconductors and previous investigations of the optical properties of these hybrid quasiparticles via both far-field and near-field imaging and spectroscopy techniques. Special emphasis is given to recent advances with critical evaluation of the bottlenecks that plague various materials toward practical device implementations including quantum light sources. Our review highlights a growing need for excitonic material development together with optical engineering and imaging techniques to harness the utility of excitons and their host materials for a variety of applications.

Nonradiative Energy Transfer and Selective Charge Transfer in a WS2/(PEA)2PbI4 Heterostructure

van der Waals heterostructures are currently the focus of intense investigation; this is essentially due to the unprecedented flexibility offered by the total relaxation of lattice matching requirements and their new and exotic properties compared to the individual layers. Here, we investigate the hybrid transition-metal dichalcogenide/2D perovskite heterostructure WS₂/(PEA)₂PbI₄ (where PEA stands for phenylethylammonium). We present the first density functional theory (DFT) calculations of a heterostructure ensemble, which reveal a novel band alignment, where direct electron transfer is blocked by the organic spacer of the 2D perovskite. In contrast, the valence band forms a cascade from WS₂ through the PEA to the PbI₄ layer allowing hole transfer. These predictions are supported by optical spectroscopy studies, which provide compelling evidence for both charge transfer and nonradiative transfer of the excitation (energy transfer) between the layers. Our results show that TMD/2D perovskite (where TMD stands for transition-metal dichalcogenides) heterostructures provide a flexible and convenient way to engineer the band alignment.

Upconversion Photovoltaic Effect of WS2/2D Perovskite Heterostructures by Two-Photon Absorption

Photovoltaic devices work by converting sunlight energy into electric energy. The efficiency of current photovoltaic devices, however, is significantly limited by the transmission loss of photons with energies below the bandgap of channel semiconductors, which can be circumvented by photon energy upconversion. Energy upconversion has been widely employed to improve the efficiency of traditional solar cells. However, the employment of energy upconversion in two-dimensional (2D) heterostructure photovoltaic devices has not been investigated yet. Here, we report the upconversion photovoltaic effect of WS₂ monolayer/(C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) 2D perovskite heterostructures by below-bandgap two-photon absorption via a virtual intermediate state. An open circuit voltage of 0.37 V and short circuit current of 7.4 pA are obtained with a photoresponsivity of 771 pA/W and current on/off ratio of 130:1. This work demonstrates that upconversion by two-photon absorption may potentially be a strategy for boosting the efficiency of 2D material-based photovoltaic devices by virtue of the absorption of photons below the bandgap energy of channel semiconductors.

Photoluminescence upconversion of 2D materials and applications

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interactions, where the energy of emitted photons is higher than that of the incident photons. PL upconversion is an intriguing process in two-dimensional materials and specifically designed 2D heterostructures, which have potential upconversion applications in optoelectronic devices, bioimaging, and semiconductor cooling. In this review, we focus on the recent advances in photoluminescence upconversion in two-dimensional materials and their heterostructures. We discuss the upconversion mechanisms, applications, and future outlook of upconversion in two-dimensional materials.

Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities

Atomic defects, being the most prevalent zero-dimensional topological defects, are ubiquitous in a wide range of 2D transition-metal dichalcogenides (TMDs). They could be intrinsic, formed during the initial sample growth, or created by postprocessing. Despite the majority of TMDs being largely unaffected after losing chalcogen atoms in the outermost layer, a spectrum of properties, including optical, electrical, and chemical properties, can be significantly modulated, and potentially invoke applicable functionalities utilized in many applications. Hence, controlling chalcogen atomic defects provides an alternative avenue for engineering a wide range of physical and chemical properties of 2D TMDs. In this article, we review recent progress on the role of chalcogen atomic defects in engineering 2D TMDs, with a particular focus on device performance improvements. Various approaches for creating chalcogen atomic defects including nonstoichiometric synthesis and postgrowth treatment, together with their characterization and interpretation are systematically overviewed. The tailoring of optical, electrical, and magnetic properties, along with the device performance enhancement in electronic, optoelectronic, chemical sensing, biomedical, and catalytic activity are discussed in detail. Postformation dynamic evolution and repair of chalcogen atomic defects are also introduced. Finally, we offer our perspective on the challenges and opportunities in this field.