Giant Enhancement of Photoluminescence Emission in WS2-Two-Dimensional Perovskite Heterostructures

Hot carrier dynamics in the BA2PbBr4/MoS2 heterostructure

Herein, we investigated the carrier-phonon relaxation process in a two-dimensional (2D) BA2PbBr4 perovskite and its heterostructure with MoS2. Energy transfer was observed in the van der Waals heterostructure of 2D perovskite and monolayer MoS2, leading to enhancement in the photoluminescence intensity of MoS2. Femtosecond pump-probe spectroscopy was used to study the carrier and lattice dynamics of pristine 2D materials and their heterostructure. A generalized two-temperature model was introduced to include competing effects of electron cooling in the rate equation of electron and lattice relaxation dynamics. The hot phonon bottleneck effect is more enhanced in the BA2PbBr4/MoS2 heterostructure than in pristine BA2PbBr4, resulting in a longer electron relaxation time. By developing a heterostructure platform with 2D BA2PbBr4 and MoS2 hybrid materials, this work provides a unique opportunity to understand and tailor carrier dynamics, interfacial coupling, and long-lived hot electrons, ultimately enhancing the efficiency of optoelectronic devices.

Moiré superlattices in twisted two-dimensional halide perovskites

Moiré superlattices have emerged as a new platform for studying strongly correlated quantum phenomena, but these systems have been largely limited to van der Waals layer two-dimensional materials. Here we introduce moiré superlattices leveraging ultrathin, ligand-free halide perovskites, facilitated by ionic interactions. Square moiré superlattices with varying periodic lengths are clearly visualized through high-resolution transmission electron microscopy. Twist-angle-dependent transient photoluminescence microscopy and electrical characterizations indicate the emergence of localized bright excitons and trapped charge carriers near a twist angle of ~10°. The localized excitons are accompanied by enhanced exciton emission, attributed to an increased oscillator strength by a theoretically predicted flat band. This research showcases the promise of two-dimensional perovskites as unique room-temperature moiré materials.

Enhancing photoluminescence of WSe2 in vapor grown WSe2/VOCl bilayer heterojunctions via surface passivation

Monolayer tungsten selenide (WSe2) has attracted attention due to its direct bandgap-generated strong light emission and light-matter interaction. Herein, vertical WSe2/VOCl bilayer heterojunctions with enhanced PL of WSe2 were synthesized by the vapor growth method. The morphology, crystal structure, and chemical composition of the WSe2/VOCl heterojunctions were systematically investigated, which confirmed the successful formation of the heterojunctions. The PL emission intensity of WSe2 obtained from the WSe2/VOCl heterojunction was about 2.4 times higher than that of the WSe2 monolayer, demonstrating the high optical quality of the WSe2/VOCl heterojunction, which was further confirmed by time-resolved PL measurements. The insulator top VOCl, which was deposited on the surface of the semiconductor bottom WSe2 as a surface passivation material, reducing the impurities and resulting in an atomically clean surface, successfully enhanced the PL emission of the bottom WSe2. This vertical WSe2/VOCl bilayer heterojunction with PL enhancement could provide a promising platform for optical devices.

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.

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.

Topological quantum devices: a review

The introduction of the concept of topology into condensed matter physics has greatly deepened our fundamental understanding of transport properties of electrons as well as all other forms of quasi particles in solid materials. It has also fostered a paradigm shift from conventional electronic/optoelectronic devices to novel quantum devices based on topology-enabled quantum device functionalities that transfer energy and information with unprecedented precision, robustness, and efficiency. In this article, the recent research progress in topological quantum devices is reviewed. We first outline the topological spintronic devices underlined by the spin-momentum locking property of topology. We then highlight the topological electronic devices based on quantized electron and dissipationless spin conductivity protected by topology. Finally, we discuss quantum optoelectronic devices with topology-redefined photoexcitation and emission. The field of topological quantum devices is only in its infancy, we envision many significant advances in the near future.

Valley-Polarized Interlayer Excitons in 2D Chalcogenide-Halide Perovskite-van der Waals Heterostructures

Interlayer excitons (IXs) in two-dimensional (2D) heterostructures provide an exciting avenue for exploring optoelectronic and valleytronic phenomena. Presently, valleytronic research is limited to transition metal dichalcogenide (TMD) based 2D heterostructure samples, which require strict lattice (mis) match and interlayer twist angle requirements. Here, we explore a 2D heterostructure system with experimental observation of spin-valley layer coupling to realize helicity-resolved IXs, without the requirement of a specific geometric arrangement, i.e., twist angle or specific thermal annealing treatment of the samples in 2D Ruddlesden-Popper (2DRP) halide perovskite/2D TMD heterostructures. Using first-principle calculations, time-resolved and circularly polarized luminescence measurements, we demonstrate that Rashba spin-splitting in 2D perovskites and strongly coupled spin-valley physics in monolayer TMDs render spin-valley-dependent optical selection rules to the IXs. Consequently, a robust valley polarization of ∼14% with a long exciton lifetime of ∼22 ns is obtained in type-II band aligned 2DRP/TMD heterostructure at ∼1.54 eV measured at 80 K. Our work expands the scope for studying spin-valley physics in heterostructures of disparate classes of 2D semiconductors.

Trends in energy and charge transfer in 2D and integrated perovskite heterostructures

Two-dimensional (2D) van der Waals (vdW) heterostructured transition metal dichalcogenides (TMDs) open up new possibilities for a wide range of optoelectronic applications. Interlayer couplings are responsible for several fascinating physics phenomena, which are in addition to the multifunctionalities that have been discovered in the field of optoelectronics. These couplings can influence the overall charge, or the energy transfer processes via stacking, separation, and dielectric angles. This focused review article summarizes the most recent and promising strategies for interlayer exciton emission in 2D or integrated perovskites and TMD heterostructures. These types of devices require a thorough comprehension and effective control of interlayer couplings in order to realize their functionalities and improve performance, which is demonstrated in this article with the energy or charge transfer mechanisms in the individual devices. An ideal platform for examining the interlayer coupling and the related physical processes is provided by a summary of the recent research findings in 2D perovskites and TMDs. Furthermore, it would encourage more investigation into the comprehension and regulation of excitonic effects and the related optoelectronic applications in vdW heterostructures over a broad spectral response range. Finally, the current challenges and prospects are summarized in this paper.

Revealing Giant Exciton Fine-Structure Splitting in Two-Dimensional Perovskites Using van der Waals Passivation

Organic-inorganic layered perovskites are currently some of the most promising 2D van der Waals materials. Low crystal quality usually broadens the exciton line width, obscuring the fine structure of the exciton in conventional photoluminescence experiments. Here, we propose a mechanical approach to reducing the effect of spectral diffusion by means of hBN capping on layered perovskites, revealing the exciton fine structure. We used a stochastic model to link the reduction of the spectral line width with the population of charge fluctuation centers present in the organic spacer. van der Waals forces between both lattices cause the partial clamping of the perovskite organic spacer molecules, and hence the amplitude of the overall spectral diffusion effect is reduced. Our work provides a low-cost solution to the problem of accessing important fine-structure excitonic state information, along with an explanation of the important carrier dynamics present in the organic spacer that affect the quality of the optical emission.

Atomic and structural modifications of two-dimensional transition metal dichalcogenides for various advanced applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and their heterostructures have attracted significant interest in both academia and industry because of their unusual physical and chemical properties. They offer numerous applications, such as electronic, optoelectronic, and spintronic devices, in addition to energy storage and conversion. Atomic and structural modifications of van der Waals layered materials are required to achieve unique and versatile properties for advanced applications. This review presents a discussion on the atomic-scale and structural modifications of 2D TMDs and their heterostructures via post-treatment. Atomic-scale modifications such as vacancy generation, substitutional doping, functionalization and repair of 2D TMDs and structural modifications including phase transitions and construction of heterostructures are discussed. Such modifications on the physical and chemical properties of 2D TMDs enable the development of various advanced applications including electronic and optoelectronic devices, sensing, catalysis, nanogenerators, and memory and neuromorphic devices. Finally, the challenges and prospects of various post-treatment techniques and related future advanced applications are addressed.

Effects of Tellurium Doping on Environmental Stability and Luminous Performance of CsPbBr3 Quantum Dots

The effects of elemental tellurium doping and decorating on the photoluminescence quantum yield (PL QY) and the environmental stability of the CsPbBr₃ quantum dots (QDs) have been systematically studied. The PL spectra blue-shifts from 520 to 464 nm gradually with the increase in the amount of Te, and the full width at half-maximum (FWHM) increases from 20 to 62 nm and decreases to 27 nm accordingly. The morphology of the untreated samples has a rectangular shape with distinct boundaries, whereas the Te-doped samples have a semi-core-shell structure with partially coated CsPb₂Br₅ after tellurium doping. Furthermore, the apparent size of the nanocomposites increases to 20 nm, but the crystal size of the core decreases slightly according to the broadened peaks of X-ray diffraction (XRD). Further investigation by X-ray photoelectron spectroscopy shows that the binding energy of Pb-Br increases and Pb-Te bonds are formed in Te-doped samples, which can enhance the stability of QDs from the view of strengthening the chemical bonds and inhibiting the detaching behavior of bromine under moisture. At the nominal content of Pb/Te = 1:0.4, the thermal decomposition temperature of the QDs increases from 300 to 500 °C; the maximum of PL QY increases to 70% for the 1:0.4 sample and the relative PL peak intensity maintains 50% of the initial value after a 60 h aging simulation. Finally, the nanocomposite materials are fabricated into a white light-emitting device (WLED). Under the illumination of a commercial GaN chip, the device shows a good Commission Internationale de lEclairage (CIE) color coordination of (0.3291,0.3318).

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.

Dominating Interlayer Resonant Energy Transfer in Type-II 2D Heterostructure

Type-II heterostructures (HSs) are essential components of modern electronic and optoelectronic devices. Earlier studies have found that in type-II transition metal dichalcogenide (TMD) HSs, the dominating carrier relaxation pathway is the interlayer charge transfer (CT) mechanism. Here, this report shows that, in a type-II HS formed between monolayers of MoSe₂ and ReS₂, nonradiative energy transfer (ET) from higher to lower work function material (ReS₂ to MoSe₂) dominates over the traditional CT process with and without a charge-blocking interlayer. Without a charge-blocking interlayer, the HS area shows 3.6 times MoSe₂ photoluminescence (PL) enhancement as compared to the MoSe₂ area alone. In a completely encapsulated sample, the HS PL emission further increases by a factor of 6.4. After completely blocking the CT process, more than 1 order of magnitude higher MoSe₂ PL emission was achieved from the HS area. This work reveals that the nature of this ET is truly a resonant effect by showing that in a similar type-II HS formed by ReS₂ and WSe₂, CT dominates over ET, resulting in a severely quenched WSe₂ PL. This study not only provides significant insight into the competing interlayer processes but also shows an innovative way to increase the PL emission intensity of the desired TMD material using the ET process by carefully choosing the right material combination for HS.

Van der Waals MoS2/Two-Dimensional Perovskite Heterostructure for Sensitive and Ultrafast Sub-Band-Gap Photodetection

Two-dimensional (2D) hybrid perovskites have been extensively studied as the promising light-sensitive materials in the photodetectors owing to their improved structural stability over that of their three-dimensional counterparts. However, the application of the 2D perovskite-based photodetector in the near-infrared (NIR) region is obstructed by the large intrinsic optical band gap. Herein, we develop a novel van der Waals heterostructure composed of few-layer 2D perovskite/MoS₂ nanoflakes, which exhibits high-sensitivity detection performance over a broad spectral region, from the visible region to the telecommunication wavelength (i.e., 1550 nm). In particular, the photoresponsivity and specific detectivity under an 860 nm laser reach 121 A W^-1 and 4.3 × 10¹⁴ Jones, respectively, whereas the individual nanoflakes show no response under the same wavelength. Meanwhile, the response time at the microsecond (μs) level is obtained, shortened by around 3 orders of magnitude compared to that of the constituting layers. The sensitive and ultrafast photoresponse at the NIR wavelength stems from the strong interlayer transition of sub-band-gap photons and the rapid separation of the photogenerated carriers by the built-in field within the heterojunction area. Our results not only provide an effective approach to achieve sub-band-gap photodetection in 2D perovskite-based structures but also suggest a universal strategy to fabricate high-performance optoelectronic devices.

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.

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.

Metal Halide Perovskite/2D Material Heterostructures: Syntheses and Applications

The past decade has witnessed the great success achieved by metal halide perovskites (MHPs) in photovoltaic and related fields. However, challenges still remain in further improving their performance, as well as, settling the stability issue for future commercialization. Recently, MHP/2D material heterostructures that combining MHPs with the low-cost and solution-processable 2D materials have demonstrated unprecedented improvement in both performance and stability due to the distinctive features at hetero-interface. The diverse fabrication techniques of MHPs and 2D materials allow them to be assembled as heterostructures with different configurations in a variety of ways. Moreover, the large families of MHPs and 2D materials provide the opportunity for the rational design and modification on compositions and functionalities of MHP/2D materials heterostructures. Herein, a comprehensive review of MHP/2D material heterostructures from syntheses to applications is presented. First, various fabrication techniques for MHP/2D material heterostructures are introduced by classifying them into solid-state methods and solution-processed methods. Then the applications of MHP/2D heterostructures in various fields including photodetectors, solar cells, and photocatalysis are summarized in detail. Finally, current challenges for the development of MHP/2D material heterostructures are highlighted, and future opportunities for the advancements in this research field are also provided.

Giant Photoluminescence Enhancement and Resonant Charge Transfer in Atomically Thin Two-Dimensional Cr2Ge2Te6/WS2 Heterostructures

Hybridization of two-dimensional (2D) magnetic semiconductors with transition-metal dichalcogenides (TMDC) monolayers can significantly engineer the light-matter interactions and provide a promising platform for enhanced excitonic systems with artificially tailored band alignments. Here, we report the fabrication of heterostructures with monolayer WS₂ on 2D Cr₂Ge₂Te₆ (CGT), which displayed giant photoluminescence enhancement at specific CGT layer numbers. The highly enhanced quantum yield obtained can be explained by novel photoexcited carrier dynamics, facilitated by alternate relaxation channels, resulting in resonance charge transfer at the heterointerface. 2D CGT revealed a strongly layer-dependent work function (up to ∼750 meV), which greatly modulates the band positioning in the heterostructure. These heterostructures conceived both type I and type II band alignments, which are verified by Kelvin probe force microscopy and PL measurements. In addition to layer modulation, we uncover temperature and power dependence of the resonance charge transfer in the multilayer heterostructure. Our findings provide further insights into the ultrafast charge dynamics occurring at the atomic interfaces. The results may pave the way for novel optoelectronics based on van der Waals heterostructures.

Semiconductor physics of organic-inorganic 2D halide perovskites

Achieving technologically relevant performance and stability for optoelectronics, energy conversion, photonics, spintronics and quantum devices requires creating atomically precise materials with tailored homo- and hetero-interfaces, which can form functional hierarchical assemblies. Nature employs tunable sequence chemistry to create complex architectures, which efficiently transform matter and energy, however, in contrast, the design of synthetic materials and their integration remains a long-standing challenge. Organic-inorganic two-dimensional halide perovskites (2DPKs) are organic and inorganic two-dimensional layers, which self-assemble in solution to form highly ordered periodic stacks. They exhibit a large compositional and structural phase space, which has led to novel and exciting physical properties. In this Review, we discuss the current understanding in the structure and physical properties of 2DPKs from the monolayers to assemblies, and present a comprehensive comparison with conventional semiconductors, thereby providing a broad understanding of low-dimensional semiconductors that feature complex organic-inorganic hetero-interfaces.

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

Two-dimensional (2D) Ruddlesden-Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage. Here, we address the photovoltaic effect and the photodetection performance in tungsten disulfide (WS₂) monolayer/2D perovskite (C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) vertical heterostructures. A vertical device geometry with separate graphene contacts to both heterointerface constituents acted as a photovoltaic device and self-driven photodetector. The photovoltaic device exhibited an open circuit voltage of -0.57 V and a short circuit current of 41.6 nA. A photoresponsivity of 0.13 mA/W at the WS₂/PEPI heterointerface was achieved, which was signified by a factor of 5 compared to that from the individual WS₂ region. The current on/off ratio of the self-driven photodetector was approximately 1500. The photoresponsivity and external quantum efficiency of the self-driven photodetector were estimated to be 24.2 μA/W and 5.7 × 10^-5, respectively. This work corroborates that 2D perovskites are promising light absorbing layers in optoelectronic devices with a TMDC-based heterointerface.

Excitonic Energy Transfer in Heterostructures of Quasi-2D Perovskite and Monolayer WS2

Quasi-two-dimensional (2D) organic-inorganic hybrid perovskite is a re-emerging material with strongly excitonic absorption and emission properties that are attractive for photonics and optoelectronics. Here we report the experimental observation of excitonic energy transfer (ET) in van der Waals heterostructures consisting of quasi-2D hybrid perovskite (C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) and monolayer WS₂. Photoluminescence excitation spectroscopy reveals a distinct ground exciton resonance feature of perovskite, evidencing ET from perovskite to WS₂. We find unexpectedly high photoluminescence enhancement factors of up to ∼8, which cannot be explained by single-interface ET. Our analysis reveals that interlayer ET across the bulk of the layered perovskite also contributes to the large enhancement factor. Further, from the weak temperature dependence of the lower-limit ET rate, which we found to be ∼3 ns^-1, we conclude that the Förster-type mechanism is responsible.

CuCrO2 Nanoparticles Incorporated into PTAA as a Hole Transport Layer for 85 °C and Light Stabilities in Perovskite Solar Cells

High-mobility inorganic CuCrO2 nanoparticles are co-utilized with conventional poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) (PTAA) as a hole transport layer (HTL) for perovskite solar cells to improve device performance and long-term stability. Even though CuCrO2 nanoparticles can be readily synthesized by hydrothermal reaction, it is difficult to form a uniform HTL with CuCrO2 alone due to the severe agglomeration of nanoparticles. Herein, both CuCrO2 nanoparticles and PTAA are sequentially deposited on perovskite by a simple spin-coating process, forming uniform HTL with excellent coverage. Due to the presence of high-mobility CuCrO2 nanoparticles, CuCrO2/PTAA HTL demonstrates better carrier extraction and transport. A reduction in trap density is also observed by trap-filled limited voltages and capacitance analyses. Incorporation of stable CuCrO2 also contributes to the improved device stability under heat and light. Encapsulated perovskite solar cells with CuCrO2/PTAA HTL retain their efficiency over 90% after ~900-h storage in 85 °C/85% relative humidity and under continuous 1-sun illumination at maximum-power point.

One-pot ultrasonic synthesis of multifunctional Au nanoparticle-ferrocene-WS2 nanosheet composite for the construction of an electrochemical biosensing platform

Structuring of noble metal nanoparticles on transition metal dichalcogenide nanosheets induces significantly enhanced electronic, optical, and catalytic functions. However, the synthesis of multifunctional hybrids is always time-consuming and involves multiple steps. Herein, a ternary Au nanoparticle-ferrocene-WS₂ nanosheet (AFW) composite has been prepared by a facile one-step sonochemical approach. Stripped WS₂ nanosheets were functionalized with ferrocene monocarboxylic acid (FMC) and gold nanoparticles (AuNPs) by making use of the strong coordinative interaction of carboxyl groups with tungsten atoms. The AuNPs decorating the WS₂ nanosheet not only increase the water solubility of WS₂ nanosheet and surface area of the modified electrode, but also act as electron transport bridges to aid the tunneling of electrons from the small redox molecule, FMC, through the space to the electrode on which they are mounted. Furthermore, the ternary AFW nanocomposite could effectively avoid the leaching of FMC from the nanocomposite matrix and provided a suitable environment for the immobilized biomolecules. Combining the immune magnetic beads technology and the AFW nanocomposite with aforementioned advantages, a high-performance electrochemical immunosensor was successfully developed using carbohydrate antigen 72-4 (CA72-4) as a model analyte. A linear relationship in the range of 2-50 U/L for the detection of CA72-4 was found with a low detection limit of 0.6 U/L. In addition, the biosensor showed excellent performance in selectivity, stability, and reproducibility. Thus, this work not only proposes a facile avenue for preparing a 2D WS₂ nanocomposite with multifunctional properties but also opens up a new method to extend the application of WS₂-based materials in biological sensing.