Colloidal Single-Layer Quantum Dots with Lateral Confinement Effects on 2D Exciton

Synthesis and Modulation of Low-Dimensional Transition Metal Chalcogenide Materials via Atomic Substitution

In recent years, low-dimensional transition metal chalcogenide (TMC) materials have garnered growing research attention due to their superior electronic, optical, and catalytic properties compared to their bulk counterparts. The controllable synthesis and manipulation of these materials are crucial for tailoring their properties and unlocking their full potential in various applications. In this context, the atomic substitution method has emerged as a favorable approach. It involves the replacement of specific atoms within TMC structures with other elements and possesses the capability to regulate the compositions finely, crystal structures, and inherent properties of the resulting materials. In this review, we present a comprehensive overview on various strategies of atomic substitution employed in the synthesis of zero-dimensional, one-dimensional and two-dimensional TMC materials. The effects of substituting elements, substitution ratios, and substitution positions on the structures and morphologies of resulting material are discussed. The enhanced electrocatalytic performance and photovoltaic properties of the obtained materials are also provided, emphasizing the role of atomic substitution in achieving these advancements. Finally, challenges and future prospects in the field of atomic substitution for fabricating low-dimensional TMC materials are summarized.

Competitive Effects of Oxidation and Quantum Confinement on Modulation of the Photophysical Properties of Metallic-Phase Tungsten Dichalcogenide Quantum Dots

Metallic-phase transition metal dichalcogenide quantum dots (TMDs-mQDs) have been reported in recent years. However, a dominant mechanism for modulating their intrinsic exciton behaviors has not been determined yet as their size is close to the Bohr radius. Herein, we demonstrate that the oxidation effect prevails over quantum confinement on metallic-phase tungsten dichalcogenide QDs (WX₂-mQDs; X = S, Se) when the QD size becomes larger than the exciton Bohr radius. WX₂-mQDs with a diameter of ~12 nm show an obvious change in their photophysical properties when the pH of the solution changes from 2 to 11 compared to changing the size from ~3 nm. Meanwhile, we found that quantum confinement is the dominant function for the optical spectroscopic results in the WX₂-mQDs with a size of ~3 nm. This is because the oxidation of the larger WX₂-mQDs induces sub-energy states, thus enabling excitons to migrate into the lower defect energy states, whereas in WX₂-mQDs with a size comparable to the exciton Bohr radius, protonation enhances the quantum confinement.

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.

A colloidal route to semiconducting tungsten disulfide nanosheets with monolayer thickness

Transition metal dichalcogenides (TMDs) are a class of materials that have been extensively studied in the last decade, with molybdenum disulfide (MoS₂) being the main protagonist. Typically, the interesting TMD properties, e.g. a direct band gap transition, or broken inversion symmetry, are only present in monolayer thick TMDs, and in the absence of strong lateral confinement, we require different materials or alloys thereof when we want to obtain TMDs with varying (direct) band gap energies. With this in mind, tungsten disulfide (WS₂) is emerging as a direct competitor of MoS₂ due to its similar properties but larger band gap energy. While several colloidal strategies have been reported for the synthesis of WS₂, the synthesis of monolayer WS₂ and detailed studies on the effect of synthesis parameters on the synthesis outcome have remained elusive. In this work we therefore focused on a colloidal synthesis method for monolayer WS₂ using a design of experiment (DOE) approach. After optimization, we obtained nanosheets with a band gap transition consistent with the expected value for a monolayer. The thickness was further confirmed by Raman spectroscopy. While we could identify two temperature ranges where we could obtain a monolayer, sample characterization by XPS spectroscopy revealed the presence of different ratios of the metallic phase, with the sample synthesized at lower temperature displaying a lower concentration of the metallic phase.

pH-Dependent Photophysical Properties of Metallic Phase MoSe2 Quantum Dots

Fluorescence properties of quantum dots (QDs) are critically affected by their redox states, which is important for practical applications. In this study, we investigated the optical properties of MoSe₂-metallic phase quantum-dots (MoSe₂-mQDs) depending on the pH variation, in which the MoSe₂-mQDs were dispersed in water with two sizes (Φ~3 nm and 12 nm). The larger MoSe₂-mQDs exhibited a large red-shift and broadening of photoluminescence (PL) peak with a constant UV absorption spectra as varying the pH, while the smaller ones showed a small red-shift and peak broadening, but discrete absorption bands in the acidic solution. The excitation wavelength-dependent photoluminescence shows that the PL properties of smaller MoSe₂-mQDs are more sensitive to the pH change compared to those of larger ones. From the time-resolved PL spectroscopy, the excitons dominantly decaying with an energy of ~3 eV in pH 2 clearly show the shift of PL peak to the lower energy (~2.6 eV) as the pH increases to 7 and 11 in the smaller MoSe₂-mQDs. On the other hand, in the larger MoSe₂-mQDs, the exciton decay is less sensitive to the redox states compared to those of the smaller ones. This result shows that the pH variation is more critical to the change of photophysical properties than the size effect in MoSe₂-mQDs.

Localization of Laterally Confined Modes in a 2D Semiconductor Microcavity

Monolayer semiconductor embedded planar microcavities are becoming a promising light-matter interacting system to uncover a wealth of photonic, excitonic, and polaritonic physics at the two-dimensional (2D) limit. In these 2D semiconductor microcavities employing the longitudinal Fabry-Perot resonance, major attention has been paid to the coupling of excitons with vertically confined cavity photons; by contrast, the lateral confinement effect on exciton-photon interactions is still elusive. Here we observe the localized distribution of laterally confined modes with discrete energies in a 2D semiconductor embedded microcavity. Monolayer tungsten disulfides with equilateral triangular geometries but varied edge lengths are selected as the active media incorporated into a dielectric planar microcavity. With the shortening of the edge length, photoluminescence mappings of active regions present spatially localized emission patterns, which are attributed to the presence of in-plane triangular waveguiding resonance caused by total internal reflection at the one-dimensional closed boundary between the monolayer semiconductor and its surrounding cavity material. Unlike the conventional quantum confinement effect of native excitons appearing at the nanometer scale, the mode emission at the active-medium center exhibits apparent size-dependent features at the micrometer scale due to the optical confinement effect correlated with its photonic nature. By reducing the area of active media, single-mode dominant emission is achieved together with its nondispersive energy and improved directionality. Our work highlights the crucial role of lateral mode control in monolayer semiconductor embedded planar microcavities and encourages the investigation of the quantum billiard problem in 2D semiconductors.

Observation of quantum-confined exciton states in monolayer WS2 quantum dots by ultrafast spectroscopy

Monolayer transition metal dichalcogenide quantum dots (TMDC QDs) could exhibit unique photophysical properties, because of both lateral quantum confinement effect and edge effect. However, there is little fundamental study on the quantum-confined exciton dynamics in monolayer TMDC QDs, to date. Here, by selective excitations of monolayer WS₂ QDs in broadband transient absorption (TA) spectroscopy experiments, the excitation-wavelength-dependent ground state bleaching signals corresponding to the quantum-confined exciton states are directly observed. Compared to the time-resolved photophysical properties of WS₂ nanosheets, the selected monolayer WS₂ QDs only show one ground state bleaching peak with larger initial values for the linear polarization anisotropy of band-edge excitons, probably due to the expired spin-orbit coupling. This suggests a complete change of the band structure for monolayer WS₂ QDs. In the femtosecond time-resolved circular polarization anisotropy experiments, a valley depolarization time of ∼100 fs is observed for WS₂ nanosheets at room temperature, which is not observed for monolayer WS₂ QDs. Our findings suggest a strong state-mixing of band-edge valley excitons responsible for the large linear polarization in monolayer WS₂ QDs, which could be helpful for understanding the exciton relaxation mechanisms in colloidal monolayer TMDC QDs.

Incorporation of Two-Dimensional WSe2 into MAPbI3 Perovskite for Efficient and Stable Photovoltaics

Achieving reduced defect density and efficient charge carrier extraction plays a vital role for efficient and stable perovskite solar cells (PSCs). Over the course of technical development, it is desired to use one single material or approach to synergistically passivate defects and enhance the charge extraction. In this work, we developed an effective strategy for obtaining efficient and stable PSCs via incorporating quasi-monolayer two-dimensional WSe₂ into the MAPbI₃ perovskite layer. The addition of WSe₂ helps with the formation of perovskite film with higher quality and also passivates the Pb-related defects through Pb-Se coordination bonding. MAPbI₃/WSe₂ shows a more matched energy-level alignment between the perovskite layer and hole transport layer for accelerated hole extraction. Consequently, the performances of PSCs significantly improved with power conversion efficiency increase from 19.2% to 21.2% after the incorporation of WSe₂. Accordingly, the MAPbI₃/WSe₂-based PSCs exhibit well-improved photostability with suppression of Pb⁰ defect formation.

Recent Advances in Two-Dimensional Quantum Dots and Their Applications

Two-dimensional quantum dots have received a lot of attention in recent years due to their fascinating properties and widespread applications in sensors, batteries, white light-emitting diodes, photodetectors, phototransistors, etc. Atomically thin two-dimensional quantum dots derived from graphene, layered transition metal dichalcogenide, and phosphorene have sparked researchers' interest with their unique optical and electronic properties, such as a tunable energy bandgap, efficient electronic transport, and semiconducting characteristics. In this review, we provide in-depth analysis of the characteristics of two-dimensional quantum dots materials, their synthesis methods, and opportunities and challenges for novel device applications. This analysis will serve as a tipping point for learning about the recent breakthroughs in two-dimensional quantum dots and motivate more scientists and engineers to grasp two-dimensional quantum dots materials by incorporating them into a variety of electrical and optical fields.

Colloidal Nanostructures of Transition-Metal Dichalcogenides

ConspectusLayered transition-metal dichalcogenides (TMDs) are intriguing two-dimensional (2D) compounds where metal and chalcogen atoms are covalently bonded in each monolayer, and the monolayers are held together by weak van der Waals forces. Distinct from graphene, which is chemically inert, layered TMDs exhibit a wide range of electronic, optical, catalytic, and magnetic properties dependent upon their compositions, crystal structures, and thicknesses, which make them fundamentally and technologically important. TMD nanostructures are traditionally synthesized using gas-phase chemical deposition methods, which are typically limited to small-scale samples of substrate-bound planar materials. Colloidal synthesis has emerged as an alternative synthesis approach to enable the scalable synthesis of free-standing TMDs. The judicious selection of precursors, solvents, and capping ligands together with the optimization of synthesis parameters such as concentrations and temperatures leads to the fabrication of colloidal TMD nanostructures exhibiting tunable properties. In addition, understanding the formation and transformation of TMD nanostructures in solution contributes to the discovery of important structure-function relationships, which may be extendable to other anisotropic systems.In this Account, we summarize recent progress in the colloidal synthesis, characterization, and applications of TMD nanostructures with tunable compositions, structures, and thicknesses. On the basis of the preparation of Mo- and W-based disulfide, diselenide, and ditelluride nanostructures, we discuss examples of phase engineering where various metastable TMD compounds can be directly accessed at low temperatures in solution. We also analyze the chemistry involved in broadly tuning the composition across the MoSe₂-WSe₂, WS₂-WSe₂, and MoTe₂-WTe₂ solid solutions as well as atomic-level microscopic characterization and the resulting composition-tunable properties. We then highlight how the high densities of defects in the colloidally synthesized TMD nanostructures enable unique catalytic properties, including their ability to facilitate the selective hydrogenation of substituted nitroarenes using molecular hydrogen. Finally, using this library of colloidal TMD nanostructures as substrates, we discuss the pathways by which noble metals deposit onto them in solution. We highlight the importance of the relative strengths of the interfacial metal-chalcogen bonds in determining the sizes and morphologies of the deposited noble metal components. These synthesis capabilities for colloidal TMD nanostructures, which have been generalized to a library of composition-tunable phases, enable new systematic studies of structure-property relationships and chemical reactivity in this important class of 2D materials.

Measurement of Exciton and Trion Energies in Multistacked hBN/WS2 Coupled Quantum Wells for Resonant Tunneling Diodes

Quantum confinements, especially quantum in narrow wells, have been investigated because of their controllability over electrical parameters. For example, quantum dots can emit a variety of photon wavelengths even for the same material depending on their particle size. More recently, the research into two-dimensional (2D) materials has shown the availability of several quantum mechanical phenomenon confined within a sheet of materials. Starting with the gapless semimetal properties of graphene, current research has begun into the excitons and their properties within 2D materials. Even for simple 2D systems, experimental results often offer surprising results, unexpected from traditional studies. We investigated a coupled quantum well system using 2D hexagonal boron nitride (hBN) barrier as well as 2D tungsten disulfide (WS₂) semiconductor arranged in stacked structures to study the various 2D to 2D interactions. We determined that for hexagonal boron nitride-tungsten disulfide (hBN/WS₂) quantum well stacks, the interaction between successive wells resulted in decreasing bandgap, and the effect was pronounced even over a large distance of up to four stacks. Additionally, we observed that a single layer of isolating hBN barriers significantly reduces interlayer interaction between WS₂ layers, while still preserving the interwell interactions in the alternative hBN/WS₂ structure. The methods we used for the study of coupled quantum wells here show a method for determining the respective exciton energy levels and trion energy levels within 2D materials and 2D materials-based structures. Renormalization energy levels are the key in understanding conductive and photonic properties of stacked 2D materials.

Enhanced Light-Driven Charge Separation and H2 Generation Efficiency in WSe2 Nanosheet-Semiconductor Nanocrystal Heterostructures

Semiconductor-catalyst heterostructures have shown promising performances for light-driven H₂ generation, although further development of these materials is hindered by the lack of cost-effective and efficient catalysts. In this paper, we adopt a colloidal method to prepare few-layer WSe₂ nanosheets without exfoliation and apply them as catalysts for forming heterostructures with a wide range of semiconductor absorbers (CdS nanorods, CdSe/CdS dot-in-rods, TiO₂ nanoparticles, g-C₃N₄ nanosheets). These WSe₂-semiconductor heterostructures show enhanced solar-to-hydrogen conversion efficiencies compared to semiconductors without WSe₂. The detailed mechanism of this enhancement has been investigated using WSe₂ nanosheet-decorated CdSe/CdS dot-in-rods as a model system, which display ∼5.5-fold higher hydrogen generation apparent quantum efficiency compared to free CdSe/CdS dot-in-rods. Transient absorption spectroscopic studies reveal efficient charge separation in WSe₂-decorated CdSe/CdS dot-in-rods, suggesting its key role in enhancing the H₂ generation efficiency of WSe₂-semiconductor heterostructures. This work demonstrates the great potentials of WSe₂ nanosheets as catalysts for light-driven hydrogen production and the important effect of forming WSe₂-semiconductor heterostructures in facilitating charge separation and photocatalysis.

Size- and temperature-dependent photoluminescence spectra of strongly confined CsPbBr3 quantum dots

Lead-halide perovskite nanocrystals (NCs) are receiving much attention as a potential high-quality source of photons due to their superior luminescence properties in comparison to other semiconductor NCs. To date, research has focused mostly on NCs with little or no quantum confinement. Here, we measured the size- and temperature-dependent photoluminescence (PL) from strongly confined CsPbBr₃ quantum dots (QDs) with highly uniform size distributions, and examined the factors determining the evolution of the energy and linewidth of the PL with varying temperature and QD size. Compared to the extensively studied II-VI QDs, the spectral position of PL from CsPbBr₃ QDs shows an opposite dependence on temperature, with weaker dependence overall. On the other hand, the PL linewidth is much more sensitive to the temperature and size of the QDs compared to II-VI QDs, indicating much stronger coupling of excitons to the vibrational degrees of freedom both in the lattice and at the surface of the QDs.

Energy Storage Materials as Emerging Nano-contaminants

New and emerging nanotechnologies are increasingly using nanomaterials that undergo significant chemical reactions upon exposure to environmental conditions. The rapid advent of lithium ion batteries for energy storage in mobile electronics and electric vehicles is leading to rapid increases in the manufacture of complex transition metal oxides that incorporate elements such as Co and Ni that have the potential for significant adverse biological impact. This Perspective summarizes some of the important technological drivers behind complex oxide materials and highlights some of the chemical transformations that need to be understood in order to assess the overall environmental impact associated with energy storage technologies.

Phototherapy with layered materials derived quantum dots

Quantum dots (QDs) originating from two-dimensional (2D) sheets of graphitic carbon nitride (g-C₃N₄), graphene, hexagonal boron nitride (h-BN), monoatomic buckled crystals (phosphorene), germanene, silicene and transition metal dichalcogenides (TMDCs) are emerging zero-dimensional materials. These QDs possess diverse optical properties, are chemically stable, have surprisingly excellent biocompatibility and are relatively amenable to surface modifications. It is therefore not difficult to see that these QDs have potential in a variety of bioapplications, including biosensing, bioimaging and anticancer and antimicrobial therapy. In this review, we briefly summarize the recent progress of these exciting QD based nanoagents and strategies for phototherapy. In addition, we will discuss about the current limitations, challenges and future prospects of QDs in biomedical applications.

Linearly Polarized Luminescence of Atomically Thin MoS2 Semiconductor Nanocrystals

Atomically thin layers of transition-metal dichalcogenides semiconductors, such as MoS₂, exhibit strong and circularly polarized light emission due to inherent crystal symmetries, pronounced spin-orbit coupling, and out-of-plane dielectric and spatial confinement. While the layer-by-layer confinement is well-understood, the understanding of the impact of in-plane quantization in their optical spectrum is far behind. Here, we report the optical properties of atomically thin MoS₂ colloidal semiconductor nanocrystals. In addition to the spatial-confinement effect leading to their blue wavelength emission, the high quality of our MoS₂ nanocrystals is revealed by narrow photoluminescence, which allows us to resolve multiple optically active transitions, originating from quantum-confined excitons (coupled electron-hole pairs). Surprisingly, in stark contrast to monolayer MoS₂, the luminescence of the lowest-energy levels is linearly polarized and persists up to room temperature, meaning that it could be exploited in a variety of light-emitting applications.

Electrically Pumped White-Light-Emitting Diodes Based on Histidine-Doped MoS2 Quantum Dots

MoS₂ quantum dots (QDs)-based white-light-emitting diodes (QD-WLEDs) are designed, fabricated, and demonstrated. The highly luminescent, histidine-doped MoS₂ QDs synthesized by microwave induced fragmentation of 2D MoS₂ nanoflakes possess a wide distribution of available electronic states as inferred from the pronounced excitation-wavelength-dependent emission properties. Notably, the histidine-doped MoS₂ QDs show a very strong emission intensity, which exceeds seven times of magnitude larger than that of pristine MoS₂ QDs. The strongly enhanced emission is mainly attributed to nitrogen acceptor bound excitons and passivation of defects by histidine-doping, which can enhance the radiative recombination drastically. The enabled electroluminescence (EL) spectra of the QD-WLEDs with the main peak around 500 nm are found to be consistent with the photoluminescence spectra of the histidine-doped MoS₂ QDs. The enhanced intensity of EL spectra with the current increase shows the stability of histidine-doped MoS₂ based QD-WLEDs. The typical EL spectrum of the novel QD-WLEDs has a Commission Internationale de l'Eclairage chromaticity coordinate of (0.30, 0.36) exhibiting an intrinsic broadband white-light emission. The unprecedented and low-toxicity QD-WLEDs based on a single light-emitting material can serve as an excellent alternative for using transition metal dichalcogenides QDs as next generation optoelectronic devices.

In-plane Aligned Colloidal 2D WS2 Nanoflakes for Solution-Processable Thin Films with High Planar Conductivity

Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS₂ colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS₂ nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS₂ thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 μS, comparable to the values reported for epitaxially grown WS₂ monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.

Using ligands to control reactivity, size and phase in the colloidal synthesis of WSe2 nanocrystals

Colloidal synthesis enables size- and phase-tuning of WSe₂ nanocrystals.

2D Crystals in Three Dimensions: Electronic Decoupling of Single-Layered Platelets in Colloidal Nanoparticles

2D crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley- and spin-orbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, the formation of stacks of 2D crystals intercalated with small surfactant molecules is proposed. It is shown, using first principles calculations, that the very short surfactant methyl amine electronically decouples the layers. The indirect-direct band gap transition characteristic for Group 6 transition metal dichalcogenides is demonstrated experimentally by observing the emergence of a strong photoluminescence signal for ethoxide-intercalated WSe₂ and MoSe₂ multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the 2D crystals with electronic properties typical of monolayers. Variation of the surfactant's chemical potential allows fine-tuning of electronic properties and potentially elimination of trap states caused by defects.

Synthesis of ultrathin WSe2 nanosheets and their high-performance catalysis for conversion of amines to imines

Tungsten diselenide (WSe2) is the material with the lowest thermal conductivity in the world. Most physical methods are used for the synthesis of tungsten diselenide. Here, a simple colloidal method is reported for the synthesis of WSe2 nanosheets. The composition, valence, size, morphology and properties of the samples were characterized and measured. Results showed that the obtained WSe2 nanosheets with a thickness of 0.7 nm had strong blue fluorescence. Significantly, the synthesized WSe2 nanosheets exhibited excellent catalytic activity for the aerobic coupling of amines to imines, with 100% yield under visible light irradiation and air atmosphere. As a photocatalyst, it exhibited excellent recyclability, and maintained a high yield after 5 cycles. It was found that this reaction could also happen in the presence of natural light by slightly extending the reaction time. Moreover, H2O was used as a solvent in the catalytic process, avoiding expensive and toxic organic solvents. This work provides an efficient, economical and sustainable process for the synthesis of imines and shows the great potential of WSe2 nanosheets as photocatalysts for organic synthesis.

Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures

The precise control in size/thickness, composition, crystal phases, doping, defects, and surface properties of two-dimensional (2D) layered transition metal chalcogenide (TMC) is important for the investigation of interwoven relationship between structures, functions, and practical applications. Of the multiple synthetic routes, solution-based top-down and bottom-up chemical methods have been uniquely important for their potential to control the size and composition at the molecular level in addition to their scalability, competitive production cost, and solution processability. Here, we introduce an overview of the recent advances in the solution-based preparation routes of 2D layered TMC nanostructures along with important scientific developments.

Precise Control of Quantum Confinement in Cesium Lead Halide Perovskite Quantum Dots via Thermodynamic Equilibrium

Cesium lead halide (CsPbX₃) nanocrystals have emerged as a new family of materials that can outperform the existing semiconductor nanocrystals due to their superb optical and charge-transport properties. However, the lack of a robust method for producing quantum dots with controlled size and high ensemble uniformity has been one of the major obstacles in exploring the useful properties of excitons in zero-dimensional nanostructures of CsPbX₃. Here, we report a new synthesis approach that enables the precise control of the size based on the equilibrium rather than kinetics, producing CsPbX₃ quantum dots nearly free of heterogeneous broadening in their exciton luminescence. The high level of size control and ensemble uniformity achieved here will open the door to harnessing the benefits of excitons in CsPbX₃ quantum dots for photonic and energy-harvesting applications.

Preferential horizontal growth of tungsten sulfide on carbon and insight into active sulfur sites for the hydrogen evolution reaction

Transition metal dichalcogenides (TMDs) have attracted considerable attention as active electrocatalysts for the hydrogen evolution reaction (HER). Since TMD catalysts are commonly supported on carbon to endow electrical conductivity, understanding the growth behaviour of TMDs on carbon surfaces is crucial, and yet remains to be explored. In this work, we investigated the growth behaviour of tungsten sulfide (WS_x) on carbon surfaces inside the confined nanopores. Experimental and computational studies revealed the preferential bonding between the basal planes of WS_x and carbon surfaces, as well as the subsequent horizontal growth of WS_x. As a result, subnanometer WS_x clusters were formed at a low WS_x loading, and grew into monolayer WS₂ nanoplates with increased WS_x loadings. In contrast, a TMD analogue, MoS₂, favors edge plane bonding with carbon surfaces and subsequent stacking of nanoplate layers, leading to multilayer MoS₂ nanoplates with increased MoS₂ loadings. A time-dependent growth of WS_x further corroborated the formation of WS₂ nanoplates at the expense of ultrasmall WS_x nanoclusters. Interestingly, the sample prepared with a short sulfidation time, which was mainly comprised of WS_x nanoclusters, showed higher HER activity compared to the sample prepared with a prolonged sulfidation time, which mostly contained WS₂ nanoplates. The higher HER activity of WS_x nanoclusters is attributed to the larger density of active bridging S₂^2- sites, compared to the WS₂ nanoplates. These findings may provide important insights into the growth behaviour of layered TMD materials at the nanoscale, as well as potential active species in WS_x for the HER.

Transition metal dichalcogenide quantum dots: synthesis, photoluminescence and biological applications

We introduce the synthesis strategy, photoluminescence features and biological applications of TMD QDs.

Effects of Direct Solvent-Quantum Dot Interaction on the Optical Properties of Colloidal Monolayer WS2 Quantum Dots

Because of the absence of native dangling bonds on the surface of the layered transition metal dichalcogenides (TMDCs), the surface of colloidal quantum dots (QDs) of TMDCs is exposed directly to the solvent environment. Therefore, the optical and electronic properties of TMDCS QDs are expected to have stronger influence from the solvent than usual surface-passivated QDs due to more direct solvent-QD interaction. Study of such solvent effect has been difficult in colloidal QDs of TMDC due to the large spectroscopic heterogeneity resulting from the heterogeneity of the lateral size or (and) thickness in ensemble. Here, we developed a new synthesis procedure producing the highly uniform colloidal monolayer WS₂ QDs exhibiting well-defined photoluminescence (PL) spectrum free from ensemble heterogeneity. Using these newly synthesized monolayer WS₂ QDs, we observed the strong influence of the aromatic solvents on the PL energy and intensity of monolayer WS₂ QD beyond the simple dielectric screening effect, which is considered to result from the direct electronic interaction between the valence band of the QDs and molecular orbital of the solvent. We also observed the large effect of stacking/separation equilibrium on the PL spectrum dictated by the balance between inter QD and QD-solvent interactions. The new capability to probe the effect of the solvent molecules on the optical properties of colloidal TMDC QDs will be valuable for their applications in various liquid surrounding environments.

Recent Progress in the Preparation, Assembly, Transformation, and Applications of Layer-Structured Nanodisks beyond Graphene

Layered nanodisks with confined thickness and lateral size have been emerging as a unique type of two-dimensional (2D) nanomaterials in recent years. Inheriting some properties of 2D nanosheets and meanwhile possessing the size-confinement effect, these layered nanodisks exhibit unique optical, electronic, and chemical properties, which endow them with great promise in a wide range of applications. Here, the recent progress of layered nanodisks is introduced. The synthetic strategies, assembly, structural/compositional transformation, and applications of layered nanodisks are systematically described and discussed, with emphasis on their new appealing structures and functions. Finally, some perspectives and future research directions of this promising field are given.

Size-tunable Lateral Confinement in Monolayer Semiconductors

Three-dimensional confinement allows semiconductor quantum dots to exhibit size-tunable electronic and optical properties that enable a wide range of opto-electronic applications from displays, solar cells and bio-medical imaging to single-electron devices. Additional modalities such as spin and valley properties in monolayer transition metal dichalcogenides provide further degrees of freedom requisite for information processing and spintronics. In nanostructures, however, spatial confinement can cause hybridization that inhibits the robustness of these emergent properties. Here, we show that laterally-confined excitons in monolayer MoS₂ nanodots can be created through top-down nanopatterning with controlled size tunability. Unlike chemically-exfoliated monolayer nanoparticles, the lithographically patterned monolayer semiconductor nanodots down to a radius of 15 nm exhibit the same valley polarization as in a continuous monolayer sheet. The inherited bulk spin and valley properties, the size dependence of excitonic energies, and the ability to fabricate MoS₂ nanostructures using semiconductor-compatible processing suggest that monolayer semiconductor nanodots have potential to be multimodal building blocks of integrated optoelectronics and spintronics systems.