Many-Body Complexes in 2D Semiconductors

Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Progress in Advanced Infrared Optoelectronic Sensors

Infrared optoelectronic sensors have attracted considerable research interest over the past few decades due to their wide-ranging applications in military, healthcare, environmental monitoring, industrial inspection, and human-computer interaction systems. A comprehensive understanding of infrared optoelectronic sensors is of great importance for achieving their future optimization. This paper comprehensively reviews the recent advancements in infrared optoelectronic sensors. Firstly, their working mechanisms are elucidated. Then, the key metrics for evaluating an infrared optoelectronic sensor are introduced. Subsequently, an overview of promising materials and nanostructures for high-performance infrared optoelectronic sensors, along with the performances of state-of-the-art devices, is presented. Finally, the challenges facing infrared optoelectronic sensors are posed, and some perspectives for the optimization of infrared optoelectronic sensors are discussed, thereby paving the way for the development of future infrared optoelectronic sensors.

Can 2D Semiconductors Be Game-Changers for Nanoelectronics and Photonics?

2D semiconductors have interesting physical and chemical attributes that have led them to become one of the most intensely investigated semiconductor families in recent history. They may play a crucial role in the next technological revolution in electronics as well as optoelectronics or photonics. In this Perspective, we explore the fundamental principles and significant advancements in electronic and photonic devices comprising 2D semiconductors. We focus on strategies aimed at enhancing the performance of conventional devices and exploiting important properties of 2D semiconductors that allow fundamentally interesting device functionalities for future applications. Approaches for the realization of emerging logic transistors and memory devices as well as photovoltaics, photodetectors, electro-optical modulators, and nonlinear optics based on 2D semiconductors are discussed. We also provide a forward-looking perspective on critical remaining challenges and opportunities for basic science and technology level applications of 2D semiconductors.

Atomic-scale visualization of the interlayer Rydberg exciton complex in moiré heterostructures

Excitonic systems, facilitated by optical pumping, electrostatic gating or magnetic field, sustain composite particles with fascinating physics. Although various intriguing excitonic phases have been revealed via global measurements, the atomic-scale accessibility towards excitons has yet to be established. Here, we realize the ground-state interlayer exciton complexes through the intrinsic charge transfer in monolayer YbCl3/graphite heterostructure. Combining scanning tunneling microscope and theoretical calculations, the excitonic in-gap states are directly profiled. The out-of-plane excitonic charge clouds exhibit oscillating Rydberg nodal structure, while their in-plane arrangements are determined by moiré periodicity. Exploiting the tunneling probe to reflect the shape of charge clouds, we reveal the principal quantum number hierarchy of Rydberg series, which points to an excitonic energy-level configuration with unusually large binding energy. Our results demonstrate the feasibility of mapping out the charge clouds of excitons microscopically and pave a brand-new way to directly investigate the nanoscale order of exotic correlated phases.

A photoelectrochemical aptasensor based on double Z-scheme α-Fe2O3/MoS2/Bi2S3 ternary heterojunction for sensitive detection of circulating tumor cells

A novel photoelectrochemical (PEC) aptasensor based on a dual Z-scheme α-Fe2O3/MoS2/Bi2S3 ternary heterojunction for the ultrasensitive detection of circulating tumor cells (CTCs) was developed. The α-Fe2O3/MoS2/Bi2S3 nanocomposite was prepared via a step-by-step route, and the photoproduced electron/hole transfer path was speculated by conducting trapping experiments of reactive species. α-Fe2O3/MoS2/Bi2S3-modified electrodes exhibited greatly enhanced photocurrent under visible light due to the double Z-scheme charge transfer process, which met the requirement of the PEC sensor for detecting larger targets. After the aptamer was conjugated on the photoelectrode through chitosan (CS) and glutaraldehyde (GA), when MCF-7 cells were presented and captured, the photocurrent of the PEC biosensing system decreased due to steric hindrance. The current intensity had a linear relationship with the logarithm of MCF-7 cell concentration ranging from 10 to 1×105 cells mL-1, with a low detection limit of 3 cell mL-1 (S/N = 3). The dual Z-scheme α-Fe2O3/MoS2/Bi2S3 ternary heterojunction-modified PEC aptasensor exhibited high sensitivity and excellent specificity and stability. Additionally, MCF-7 cells in human serum were determined by this PEC aptasensor, exhibiting great potential as a promising tool for clinical detection.

Disentangling the multiorbital contributions of excitons by photoemission exciton tomography

Excitons are realizations of a correlated many-particle wave function, specifically consisting of electrons and holes in an entangled state. Excitons occur widely in semiconductors and are dominant excitations in semiconducting organic and low-dimensional quantum materials. To efficiently harness the strong optical response and high tuneability of excitons in optoelectronics and in energy-transformation processes, access to the full wavefunction of the entangled state is critical, but has so far not been feasible. Here, we show how time-resolved photoemission momentum microscopy can be used to gain access to the entangled wavefunction and to unravel the exciton's multiorbital electron and hole contributions. For the prototypical organic semiconductor buckminsterfullerene (C60), we exemplify the capabilities of exciton tomography and achieve unprecedented access to key properties of the entangled exciton state including localization, charge-transfer character, and ultrafast exciton formation and relaxation dynamics.

Demonstration of Two-Dimensional Exciton Complex Palette

Exciton complexes in two-dimensional semiconductors, encompassing bright and dark excitons, biexcitons, and defect-bound excitons, have shown significant potential across a wide range of research areas. These applications range from exploring quantum many-body phenomena to developing nonclassical light sources and quantum transport devices. To fully leverage their dynamic and interactive properties and extend the capabilities of excitonic devices, realizing systematic engineering and mixing of the exciton complexes are crucial. Unlike conventional material methods, which often lead to undesired changes in the electronic band structure and binding energy, optical methods provide a means to manipulate the radiative decay dynamics of individual exciton complexes in a purely environmental manner. Here, we employ a specialized photonic platform, analogous to an artist's palette, to arrange and mix exciton complexes on an identical two-dimensional transition metal dichalcogenide medium. Essentially, a gradient thickness mirror (GTM) continuously tunes the local distribution of optical vacuum field interference. The GTM platform enables us to create and examine five distinct compositions of the exciton complexes of the WSe2 monolayer and their contributions to the photoluminescence spectrum. Moreover, the exciton complex palette facilitates the observation of dark and defect-bound excitons, even at high temperatures of 70 K, and its performance can be further managed by simple postprocessing manipulations.

Comparison of electron scattering by acoustic-phonons in two types of quantum wells with GaAs and GaN materials

In this work, we report a detailed comparison of electron-acoustic-phonon (EAP) interaction strength in symmetric (parabolic) and asymmetric (semi-parabolic) quantum-wells (QWs) for both GaAs and GaN materials. The operator projection method will be utilized to calculate the acoustic-phonon-assisted cyclotron resonance (CR) absorption power. The EAP interaction strength is determined by measuring the full width at half maximum (FWHM) of the acoustic-phonon-assisted CR absorption peak based on the profile of the curve describing the dependence of the acoustic-phonon-assisted CR absorption power on the photon energy. The studied result reveals that the EAP interaction strengths in the symmetric and asymmetric QWs are functions of the electron temperature (ET), external magnetic field (EMF), and confined potential frequency (CPF). Namely, the larger the ET, the EMF, and the CPF, the stronger the EAP interaction strengths in the symmetric and asymmetric QWs are for both GaN and GaAs materials. More importantly, the obtained result demonstrates that under the influence of the structural (CPF) and external (ET and EMF) parameters, the EAP interaction strength in the symmetric QW is always much stronger than that in the asymmetric QW for both GaN and GaAs materials. Simultaneously, the EAP interaction strength in the GaN material is much stronger than that in the GaAs material for both the symmetric and asymmetric QWs.

Enhanced interactions of excitonic complexes in free-standing WS2

Excitonic complexes, bound states of electrons and holes, provide a promising platform in monolayer transition-metal dichalcogenide (TMDC) semiconductors for investigating diverse many-body interaction phenomena. The surrounding dielectric environment has been found to strongly influence the excitonic properties of the TMDC monolayers. While the impact of different dielectric surroundings on two-dimensional semiconductor materials and their strong correlations have been well studied, the effects on exciton formation and its properties resulting from a further reduction in dielectric screening remain elusive. In this study, we examined free-standing tungsten disulfide (WS2) monolayers, where the efficient generation of higher-order correlated excitonic complexes is readily observed. This phenomenon arises from the effective mutual interactions among excitons and internal carriers, attributed to the modulated exciton dynamics generated by the further reduced dielectric screening effect in the freestanding structure. The formation efficiency of excitonic complexes is enhanced and the multiple biexciton species (five particles such as charged biexcitons and acceptor/donor-bound biexcitons) are successfully induced under low excitation intensity and moderate temperature conditions. Our findings offer valuable insights into the influence of the dielectric environment on exciton interactions and enable a productive avenue for exploring fundamental many-body interactions, providing new possibilities for dielectric engineering of atomic thin semiconductors.

Negative Capacitance Field Effect Transistors based on Van der Waals 2D Materials

Steep subthreshold swing (SS) is a decisive index for low energy consumption devices. However, the SS of conventional field effect transistors (FETs) has suffered from Boltzmann Tyranny, which limits the scaling of SS to sub-60 mV dec-1 at room temperature. Ferroelectric gate stack with negative capacitance (NC) is proved to reduce the SS effectively by the amplification of the gate voltage. With the application of 2D ferroelectric materials, the NC FETs can be further improved in performance and downscaled to a smaller dimension as well. This review introduces some related concepts for in-depth understanding of NC FETs, including the NC, internal gate voltage, SS, negative drain-induced barrier lowering, negative differential resistance, single-domain state, and multi-domain state. Meanwhile, this work summarizes the recent advances of the 2D NC FETs. Moreover, the electrical characteristics of some high-performance NC FETs are expressed as well. The factors which affect the performance of the 2D NC FETs are also presented in this paper. Finally, this work gives a brief summary and outlook for the 2D NC FETs.

Increased Mobility and Reduced Hysteresis of MoS2 Field-Effect Transistors via Direct Surface Precipitation of CsPbBr3-Nanoclusters for Charge Transfer Doping

Transition-metal dichalcogenides (TMDs) and metal halide perovskites (MHPs) have been investigated for various applications, owing to their unique physical properties and excellent optoelectronic functionalities. TMD monolayers synthesized via chemical vapor deposition (CVD), which are advantageous for large-area synthesis, exhibit low mobility and prominent hysteresis in the electrical signals of field-effect transistors (FETs) because of their native defects. In this study, we demonstrate an increase in electrical mobility by ∼170 times and reduced hysteresis in the current-bias curves of MoS2 FETs hybridized with CsPbBr3 for charge transfer doping, which is implemented via solution-based CsPbBr3-nanocluster precipitation on CVD-grown MoS2 monolayer FETs. Electrons injected from CsPbBr3 into MoS2 induce heavy n-doping and heal point defects in the MoS2 channel layer, thus significantly increasing mobility and reducing hysteresis in the hybrid FETs. Our results provide a foundation for improving the reliability and performance of TMD-based FETs by hybridizing them with solution-based perovskites.

Negative Valley Polarization of the Intralayer Exciton via One-Step Growth of H-Type Heterobilayer WS2/MoS2

Vertical type II van der Waals heterobilayers of transition metal dichalcogenides (TMDs) have attracted wide attention due to their distinctive features mostly arising from the emergence of intriguing electronic structures that include moiré-related phenomena. Owing to strong spin-orbit coupling under a noncentrosymmetric environment, TMD heterobilayers host nonequivalent +K and -K valleys of contrasting Berry curvatures, which can be optically controlled by the helicity of optical excitation. The corresponding valley selection rules are well established by not only intralayer excitons but also interlayer excitons. Quite intriguingly, here, we experimentally demonstrate that unusual valley switching can be achieved using the lowest-lying intralayer excitons in H-type heterobilayer WS₂/MoS₂ prepared by one-step growth. This TMD combination provides an ideal case for interlayer coupling with an almost perfect lattice match, thereby also in the momentum space between +K and -K valleys in the H-type heterostructure. The underlying valley-switching mechanism can be understood by bright-to-dark conversion of initially created electrons in the valley of WS₂, followed by interlayer charge transfer to the opposite valley in MoS₂. Our suggested model is also confirmed by the absence of valley switching when the lowest-lying excitons in MoS₂ are directly generated in the heterobilayer. In contrast to the H-type case, we show that no valley switching is observed from R-type heterobilayers prepared by the same method, where interlayer charge transfer does not occur between the opposite valleys. We compare the case with the series of valley polarization data from other heterobilayer combinations obtained under different excitation energies and temperatures. Our valley switching mechanism can be utilized for valley manipulation by controlling the excitation photon energy together with the photon helicity in valleytronic devices derived from H-type TMD heterobilayers.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Surface acoustic wave induced phenomena in two-dimensional materials

Surface acoustic wave (SAW)-matter interaction provides a fascinating key for inducing and manipulating novel phenomena and functionalities in two-dimensional (2D) materials. The dynamic strain field and piezo-electric field associated with propagating SAWs determine the coherent manipulation and transduction between 2D excitons and phonons. Over the past decade, many intriguing acoustic-induced effects, including the acousto-electric effect, acousto-galvanic effect, acoustic Stark effect, acoustic Hall effect and acoustic exciton transport, have been reported experimentally. However, many more phenomena, such as the valley acousto-electric effect, valley acousto-electric Hall effect and acoustic spin Hall effect, were only theoretically proposed, the experimental verification of which are yet to be achieved. In this minireview, we attempt to overview the recent breakthrough of SAW-induced phenomena covering acoustic charge transport, acoustic exciton transport and modulation, and coherent acoustic phonons. Perspectives on the opportunities of the proposed SAW-induced phenomena, as well as open experimental challenges, are also discussed, attempting to offer some guidelines for experimentalists and theorists to explore the desired exotic properties and boost practical applications of 2D materials.

Observation of carrier transfer in a vertical 0D-CsPbBr3/2D-MoS2 mixed-dimensional van der Waals heterojunction

Two-dimensional transition metal dichalcogenides with outstanding properties open up a new way to develop optoelectronic devices such as phototransistors and light-emitting diodes. Heterostructure with light-harvesting materials can produce many photogenerated carriers via charge and/or energy transfer. In this paper, the ultrafast dynamics of charge transfer in zero-dimensional CsPbBr₃ quantum dot/two-dimensional MoS₂ van der Waals heterostructures are investigated through femtosecond time-resolved transient absorption spectroscopy. Hole and electron transfers in the ps and fs magnitude at the interfaces between MoS₂ and CsPbBr₃ are observed by modulating pump wavelengths of the pump-probe configurations. Our study highlights the opportunities for realizing the exciton devices based on quantum dot/two-dimensional semiconductor heterostructures.

Lead-Free Perovskite Cs3Bi2I9-Derived Electroactive PVDF Composite-Based Piezoelectric Nanogenerators for Physiological Signal Monitoring and Piezo-Phototronic-Aided Strain Modulated Photodetectors

In recent years, lead-free perovskite materials are exponentially emerging in photovoltaic and optoelectronic applications due to their low toxicity and superior optical properties. On the other hand, the demand for flexible, wearable, and lightweight optoelectronic devices is significantly growing in sensor and actuator technologies. In this scenario, lead-free perovskite-based flexible piezoelectric polymer composites have sparked considerable attention in this field due to their excellent piezo-, pyro-, ferroelectric, and photovoltaic properties. Thus, in this work, a long-term stable lead-free Cs₃Bi₂I₉-PVDF composite is introduced. The in situ growth of the Cs₃Bi₂I₉ perovskite induces 92% yield of the electroactive phase in the PVDF matrix. The possible mechanism behind the electroactive β-phase transformation is presented via interfacial interactions of PVDF moieties with the Cs₃Bi₂I₉ (CBI) perovskite, which also give rise to long-term environmental stability. Next, a piezoelectric nanogenerator (PNG) has been fabricated with the Cs₃Bi₂I₉-PVDF composite for mechanical energy harvesting, biophysiological motion monitoring, and voice recognitions that have potential utility in the health-care sector. Furthermore, a photodetector is developed to realize the piezo-phototronic effect. It exhibits a fast photoswitching behavior with rise and decay times of 141 and 278 ms, respectively. Thus, it is confirmed that the flexible Cs₃Bi₂I₉-PVDF composite has shown tremendous potential to be used as an optical signal-modulated piezo-responsive wearable sensor.

Towards ballistic transport CVD graphene by controlled removal of polymer residues

Polymer-assisted wet transfer of chemical vapor deposited (CVD) graphene has achieved great success towards the true potential for large-scale electronic applications, while the lack of an efficient polymer removal method has been regarded as a crucial factor for realizing high carrier mobility in graphene devices. Hereby, we report an efficient and facile method to clean polymer residues on graphene surface by merely employing solvent mixture of isopropanol (IPA) and water (H₂O). Raman spectroscopy shows an intact crystal structure of graphene after treatment, and the x-ray photoelectron spectroscopy indicates a significant decrease in the C-O and C=O bond signals, which is mainly attributed to the removal of polymer residues and further confirmed by subsequent atomic force microscopy analysis. More importantly, our gated measurements demonstrate that the proposed approach has resulted in a 3-fold increase of the carrier mobility in CVD graphene with the electron mobility close to 10 000 cm²V^-1S^-1, corresponding to an electron mean free path beyond 100 nm. This intrigues the promising application for this novel method in achieving ballistic transport for CVD graphene devices.

Nano-engineering and nano-manufacturing in 2D materials: marvels of nanotechnology

Two-dimensional materials have attracted significant interest and investigation since the marvellous discovery of graphene. Due to their unique physical, mechanical and optical properties, van der Waals (vdW) materials possess extraordinary potential for application in future optoelectronics devices. Nano-engineering and nano-manufacturing in the atomically thin regime has further opened multifarious avenues to explore novel physical properties. Among them, moiré heterostructures, strain engineering and substrate manipulation have created numerous exotic and topological phenomena such as unconventional superconductivity, orbital magnetism, flexible nanoelectronics and highly efficient photovoltaics. This review comprehensively summarizes the three most influential techniques of nano-engineering in 2D materials. The latest development in the marvels of moiré structures in vdW materials is discussed; in addition, topological structures in layered materials and substrate engineering on the nanoscale are thoroughly scrutinized to highlight their significance in micro- and nano-devices. Finally, we conclude with remarks on challenges and possible future directions in the rapidly expanding field of nanotechnology and nanomaterial.

Directly Visualizing Photoinduced Renormalized Momentum-Forbidden Electronic Quantum States in an Atomically Thin Semiconductor

Resolving the momentum degree of freedom of photoexcited charge carriers and exploring the excited-state physics in the hexagonal Brillouin zone of atomically thin semiconductors have recently attracted great interest for optoelectronic technologies. We demonstrate a combination of light-modulated scanning tunneling microscopy and the quasiparticle interference (QPI) technique to offer a directly accessible approach to reveal and quantify the unexplored momentum-forbidden electronic quantum states in transition metal dichalcogenide (TMD) monolayers. Our QPI results affirm the large spin-splitting energy at the spin-valley-coupled Q valleys in the conduction band (CB) of a tungsten disulfide monolayer. Furthermore, we also quantify the photoexcited carrier density-dependent band renormalization at the Q valleys. Our findings directly highlight the importance of the excited-state distribution at the Q valley in the band renormalization in TMDs and support the critical role of the CB Q valley in engineering the quantum electronic valley degree of freedom in TMD devices.

An Assessment of MXenes through Scanning Probe Microscopy

Recently, exploring the unique properties of 2D materials has constituted a new wave of research, which lead these materials to enormous applications ranging from optoelectronics to healthcare systems. Due to the profusion of surface terminated functionalities, MXenes have become an emerging class of 2D materials that can be easily integrated with other materials. The versatility of MXenes allows to tune their finest material properties for further device applications. This review initiates with the classification of preparation methods of MXenes, where the authors elaborate on the significance of top-down approaches including the exfoliation of solid layers. Next, the focus is diverted toward the materials analysis of MXenes including their terminations analysis as well as their intriguing electrical and mechanical behaviors through scanning probe microscopy. Finally, critical challenges and perspectives for MXenes analysis and applications are explored and discussed. Therefore, this comprehensive review can encourage researchers, and offer a precise direction to employ MXenes in various applications.

Optical absorption of interlayer excitons in transition-metal dichalcogenide heterostructures

Interlayer excitons, electron-hole pairs bound across two monolayer van der Waals semiconductors, offer promising electrical tunability and localizability. Because such excitons display weak electron-hole overlap, most studies have examined only the lowest-energy excitons through photoluminescence. We directly measured the dielectric response of interlayer excitons, which we accessed using their static electric dipole moment. We thereby determined an intrinsic radiative lifetime of 0.40 nanoseconds for the lowest direct-gap interlayer exciton in a tungsten diselenide/molybdenum diselenide heterostructure. We found that differences in electric field and twist angle induced trends in exciton transition strengths and energies, which could be related to wave function overlap, moiré confinement, and atomic reconstruction. Through comparison with photoluminescence spectra, this study identifies a momentum-indirect emission mechanism. Characterization of the absorption is key for applications relying on light-matter interactions.

Nanocomposite hydrogels for biomedical applications

Nanomaterials' unique structures at the nanometer level determine their incredible functions, and based on this, they can be widely used in the field of nanomedicine. However, nanomaterials do possess disadvantages that cannot be ignored, such as burst release, rapid elimination, and poor bioadhesion. Hydrogels are scaffolds with three‐dimensional structures, and they exhibit good biocompatibility and drug release capacity. Hydrogels are also associated with disadvantages for biomedical applications such as poor anti‐tumor capability, weak bioimaging capability, limited responsiveness, and so on. Incorporating nanomaterials into the 3D hydrogel network through physical or chemical covalent action may be an effective method to avoid their disadvantages. In nanocomposite hydrogel systems, multifunctional nanomaterials often work as the function core, giving the hydrogels a variety of properties (such as photo‐thermal conversion, magnetothermal conversion, conductivity, targeting tumor, etc.). While, hydrogels can effectively improve the retention effect of nanomaterials and make the nanoparticles have good plasticity to adapt to various biomedical applications (such as various biosensors). Nanocomposite hydrogel systems have broad application prospects in biomedicine. In this review, we comprehensively summarize and discuss the most recent advances of nanomaterials composite hydrogels in biomedicine, including drug and cell delivery, cancer treatment, tissue regeneration, biosensing, and bioimaging, and we also briefly discussed the current situation of their commoditization in biomedicine.

2D Functional Minerals as Sustainable Materials for Magneto-Optics

Liquid crystal devices using organic molecules are nowadays widely used to modulate transmitted light, but this technology still suffers from relatively weak response, high cost, toxicity and environmental concerns, and cannot fully meet the demand of future sustainable society. Here, an alternative approach to color-tunable optical devices, which is based on sustainable inorganic liquid crystals derived from 2D mineral materials abundant in nature, is described. The prototypical 2D mineral of vermiculite is massively produced by a green method, possessing size-to-thickness aspect ratios of >10³ , in-plane magnetization of >10 emu g^-1 , and an optical bandgap of >3 eV. These characteristics endow 2D vermiculite with sensitive magneto-birefringence response, been several orders of magnitude larger than organic counterparts, as well as capability of broad-spectrum modulation. The finding consequently permits the fabrication of various magnetochromic or mechanochromic devices with low or even zero-energy consumption during operation. This work creates opportunities for the application of sustainable materials in advanced optics.

Highly Enhanced Photoluminescence of Monolayer MoS2 in Plasmonic Hybrids with Double-Layer Stacked Ag Nanoparticles

In this work, a feasible method was proposed to prepare MoS₂-based plasmonic hybrid systems with high photoluminescence (PL) emission enhancement. The enhancement effect of plasmonic hybrids on the PL emission of MoS₂ has been systematically studied on MoS₂/Ag spherical nanoparticle (SP) hybrid systems with different architectures by changing the stacking position of Ag SPs. It is demonstrated that the sandwich-like hybrid composed of monolayer MoS₂ and dielectric Al₂O₃ layer between two layers of Ag SPs has the highest PL enhancement. Remarkably, after adding an Al₂O₃ layer under MoS₂, the PL intensity enhancement up to 209 times was achieved in the sandwich-like hybrid system. Compared with the hybrid with single-layer SPs, the sandwich-like hybrid system with double-layer Ag SPs exhibited an obvious blue shift as a result of the selective enhancement of the A⁰ exciton in MoS₂. These results demonstrate that MoS₂/Ag SP hybrid nanosystems have significant implications for sensing and photoelectronic devices.

Halide Perovskite Nanocrystal-Enabled Stabilization of Transition Metal Dichalcogenide Nanosheets

Transition metal dichalcogenide (TMD) nanosheets exfoliated in the liquid phase are of significant interest owing to their potential for scalable and flexible photoelectronic applications. Although various dispersants such as surfactants, oligomers, and polymers are used to obtain highly exfoliated TMD nanosheets, most of them are electrically insulating and need to be removed; otherwise, the photoelectric properties of the TMD nanosheets degrade. Here, inorganic halide perovskite nanocrystals (NCs) of CsPbX₃  (X = Cl, Br, or I) are presented as non-destructive dispersants capable of dispersing TMD nanosheets in the liquid phase and enhancing the photodetection properties of the nanosheets, thus eliminating the need to remove the dispersant. MoSe₂ nanosheets dispersed in the liquid phase are adsorbed with CsPbCl₃  NCs. The CsPbCl₃ nanocrystals on MoSe₂ efficiently withdraw electrons from the nanosheets, and suppress the dark current of the MoSe₂ nanosheets, leading to flexible near-infrared MoSe₂  photodetectors with a high ON/OFF photocurrent ratio and detectivity. Moreover, lanthanide ion-doped CsPbCl₃  NCs enhance the ON/OFF current ratio to >10⁶ . Meanwhile, the dispersion stability of the MoSe₂  nanosheets exfoliated with the perovskite NCs is sufficiently high.

One order enhancement of charge carrier relaxation rate by tuning structural and optical properties in annealed cobalt doped MoS2 nanosheets

The effective manipulation of excitons is crucial for the realization of exciton-based devices and circuits, and doping is considered a good strategy to achieve this.

Interfacing transition metal dichalcogenides with chromium germanium telluride quantum dots for controllable light-matter interactions

In this work, we unravel a facile solution-based method to prepare chromium germanium telluride, Cr₂Ge₂Te₆ (CGT) quantum dots (QDs), which present strong light-matter interactions with monolayer transition metal dichalcogenides (TMDs) in their CGT/TMD vertical heterostructures. The heterostructures' optoelectronic properties were controlled by simply varying the QDs thickness. We observed contrasting emissions from monolayer TMDs in the various CGT QDs-TMDs (of WS₂, WSe₂ and MoS₂) heterostructures depending on the density of QDs in the heterostructures. Low-density CGT QDs-based heterostructures demonstrated a reduced light emission intensity compared to the isolated monolayers, but with an increased trion ratio due to the electron doping effect of CGT QDs. In contrast, high-density CGT QDs-based heterostructures showed an increased light emission intensity and a broadened, red-shifted emission peak in comparison to the bare TMDs, attributed to the enhanced optical absorption in the heterostructures arising from the assembled CGT QDs. Finally, proof-of-concept field-effect transistor (FET) and photodetector devices based on the created CGT QDs-WS₂ heterostructures were designed, which showed an enhanced optoelectronic performance.

Two-dimensional multiferroics

Due to unprecedented application prospects such as high-density and low-power multistate storage, spintronics and nanoelectronics, two-dimensional (2D) multiferroics, coupled with at least two ferroic orders, have gotten a lot of interest in recent years. Multiple functions can be achieved in 2D multiferroics via coupling phenomena such as magnetoelectricity, piezoelectricity, and magnetoelasticity, which offers technical support for the creation of multifunctional devices. The research progress of 2D ferromagnetic-ferroelectric multiferroic materials, ferroelectric-ferroelastic multiferroic materials, and ferromagnetic-ferroelastic materials in recent years is reviewed in this paper. The categorization of 2D multiferroics is explored in terms of the multiple sources of ferroelectricity. The top-down approaches and the bottom-up methods used to fabricate 2D multiferroics materials are introduced. Finally, the authors outline potential research prospects and application scenarios for 2D multiferroic materials.

Optical anticounterfeiting photonic bilayer film based on handedness of solid-state helicoidal structure

Anticounterfeiting photonic bilayer films were fabricated by sandwiching two solid-state cholesteric liquid crystal films having different handedness. The fabricated photonic bilayer films were successfully applied to patterning by selective photopolymerization. This photonic bilayer film as a new cryptographic technology is of interest for its anticounterfeiting application.

Photoluminescence of monolayer MoS2 modulated by water/O2/laser irradiation

The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate their PL efficiencies. Herein, we use PL/Raman microspectroscopy and the corresponding in situ mapping, atomic force microscopy, and field-effect transistor (FET) characterization to investigate the changes in the structural and optical properties of monolayer MoS₂. Relatively low power density (<4.08 × 10⁵ W cm^-2) of laser irradiation in ambient air can cause a slight PL suppression effect on monolayer MoS₂, whereas relatively high power density (∼1.02 × 10⁶ W cm^-2) of laser irradiation brings significant PL enhancement. Experiments under different atmospheres reveal that the laser-irradiation-induced enhancement only occurs in the atmosphere containing O₂ and is more remarkable in pure O₂. In addition, physically adsorbed water can also induce PL enhancement of monolayer MoS₂. FET devices suggest that the adsorbed water produces a p-doping effect on MoS₂, and the laser irradiation in ambient air generates an n-doping effect, and both types of doping can enhance the PL intensity. The island-shaped defects caused by laser irradiation can be stabilized by oxygen atoms and act as trapping centers for excited trions or electrons, thus reducing the non-radiative recombination ratio and enhancing the PL intensity. The physically adsorbed water works in a similar way. A low power density of laser irradiation can sweep away the originally adsorbed H₂O on the surface, thus reducing the PL.

2D Graphdiyne: A Rising Star on the Horizon of Energy Conversion

Two-dimensional (2D) graphdiyne (GDY), a rapidly rising star on the horizon of carbon materials, is a new carbon allotrope featuring sp- and sp² -cohybridized carbon atoms and 2D one-atom-thick network. Since the first successful synthesis of GDY by Professor Li's group in 2010, GDY has attached great interests from both scientific and industrial viewpoints based on its unique structure and physicochemical properties, which provides a fertile ground for applications in various fields including electrocatalysis, energy conversion, energy storage and optoelectronic devices. In this work, various potential properties of the GDY-based electrocatalysts and their recent advances in energy conversion are reviewed, including atomic catalysts, heterogeneous catalysts, and metal-free catalysts. The critical role of GDY in improving catalytic activity and stability is analyzed. The perspectives of the challenges and opportunities faced by GDY-based materials for energy conversion are also outlined.

Atomic-Layer-Deposition-Based 2D Transition Metal Chalcogenides: Synthesis, Modulation, and Applications

Transition metal chalcogenides (TMCs) are a large family of 2D materials with different properties, and are promising candidates for a wide range of applications such as nanoelectronics, sensors, energy conversion, and energy storage. In the research of new materials, the development and investigation of industry-compatible synthesis techniques is of key importance. In this respect, it is important to study 2D TMC materials synthesized by the atomic layer deposition (ALD) technique, which is widely applied in industries. In addition to the synthesis of 2D TMCs, ALD is used to modulate the characteristic of 2D TMCs such as their carrier density and morphology. So far, the improvement of thin film uniformity without oxidation and the synthesis of low-dimensional nanomaterials on 2D TMCs have been the research focus. Herein, the synthesis and modulation of 2D TMCs by ALD is described, and the characteristics of ALD-based TMCs used in nanoelectronics, sensors, and energy applications are discussed.

Ambipolar Photoresponsivity in an Ultrasensitive Photodetector Based on a WSe2/InSe Heterostructure by a Photogating Effect

Ambipolar photoresponsivity mainly originates from intrinsic or interfacial defects. However, these defects are difficult to control and will prolong the response speed of the photodetector. Here, we demonstrate tunable ambipolar photoresponsivity in a photodetector built from vertical p-WSe₂/n-InSe heterostructures with photogating effect, exhibiting ultrahigh photoresponsivity from -1.76 × 10⁴ to 5.48 × 10⁴ A/W. Moreover, the photodetector possesses broadband photodetection (365-965 nm), an ultrahigh specific detectivity (D*) of 5.8 × 10¹³ Jones, an external quantum efficiency of 1.86 × 10⁷%, and a rapid response time of 20.8 ms. The WSe₂/InSe vertical architecture has promising potential in developing high-performance nano-optoelectronics.

Substrate Effects on the Bandwidth of CdSe Quantum Dot Photodetectors

We investigate the time-resolved photocurrent response of CdSe quantum dot (QD) thin films sensitized with zinc β-tetraaminophthalocyanine (Zn4APc) (Kumar , ACS Appl. Mater. Interfaces, 2019, 11, 48271-48280) on three different substrates, namely, silicon with 230 nm SiO₂ dielectric, glass, and polyimide. While Si/SiO₂ (230 nm) is not suitable for any transient photocurrent characterization due to an interfering photocurrent response of the buried silicon, we find that polyimide substrates invoke the larger optical bandwidth with 85 kHz vs 67 kHz for the same quantum dot thin film on glass. Upon evaluation of the transient photocurrent, we find that the photoresponse of the CdSe quantum dot films can be described as a combination of carrier recombination and fast trapping within 2.7 ns followed by slower multiple trapping events. The latter are less pronounced on polyimide, which leads to the higher bandwidth. We show that all devices are resistance-capacitance (RC)-time limited and that improvements of photoresistance are the key to further increasing the bandwidth.

Novel electrical properties and applications in kaleidoscopic graphene nanoribbons

As one of the representatives of nano-graphene materials, graphene nanoribbons (GNRs) have more novel electrical properties, highly adjustable electronic properties, and optoelectronic properties than graphene due to their diverse geometric structures and atomic precision configurations. The electrical properties and band gaps of GNRs depend on their width, length, boundary configuration and other elemental doping, etc. With the improvement of the preparation technology and level of GNRs with atomic precision, increasing number of GNRs with different configurations are being prepared. They all show novel electrical properties and high tunability, which provides a broad prospect for the application of GNRs in the field of microelectronics. Here, we summarize the latest GNR-based achievements in recent years and summarize the latest electrical properties and potential applications of GNRs.

Covalent modification of black phosphorus with alkoxy groups to improve the solubility and ambient stability

Black phosphorus (BP), a new 2D material as a layered allotrope of phosphorus, has regained attention due to its outstanding semiconductor characteristics. However, the major hurdles of using few-layer BP for applications are its poor solution processability and low ambient stability. Here, we report a covalent modification of BP nanosheets by a chemical reaction with sodium alkoxide. Fourier transform infrared spectra, Raman spectra, X-ray photoemission spectra and thermogravimetric analyses all confirmed the successful introduction of alkoxy groups on the BP surface with P-O-C bonds, which increased the solubility and ambient stability of BP. The introduced alkoxy groups as soluble side chains on the BP surface not only increase the solubility of BP nanosheets by almost 3 times, but also decrease the degradation ratio of the modified BP by about 2 times because of the encapsulation. In this work we developed a facile synthetic strategy to covalently modify BP by introducing soluble side chains, suggesting an effective way to realize its full potential application in electronics.

Generation of 2D nonlayered ferromagnetic VO2(M) nanosheets induced by strain engineering of CO2

Two-dimensional (2D) nonlayered ferromagnets displaying high Curie temperatures, sizable magnetic anisotropy levels, and large spin polarizations are emerging as promising 2D ferromagnetics. However, the difficulties in synthesizing 2D nonlayered intrinsic ferromagnets have largely limited their development. Herein, defect-rich 2D nonlayered VO₂(M) nanosheets have been fabricated by deploying straining engineering of CO₂ on the metal-insulator transition (MIT) of VO₂. Above T_MIT, the strong strain engineering of CO₂ in the R phase of VO₂ generated a very large number of atomic defects in its 3D crystal structure, and as a result facilitated conversion of the defective 3D network to 2D nanosheets along the c-axis. The as-prepared 2D defective VO₂(M) nanosheets displayed unique room-temperature ferromagnetism, attributed to the symmetry breaking triggered by the disordered atomic structure combined with the 3D-to-2D transformation.