Enhanced Light-Matter Interactions in Self-Assembled Plasmonic Nanoparticles on 2D Semiconductors

Single-Particle Fluorescence Spectroscopy for Elucidating Charge Transfer and Catalytic Mechanisms on Nanophotocatalysts

Photocatalysis is a cost-effective approach to producing renewable energy. A thorough comprehension of carrier separation at the micronano level is crucial for enhancing the photochemical conversion capabilities of photocatalysts. However, the heterogeneity of photocatalyst nanoparticles and complex charge migration processes limit the profound understanding of photocatalytic reaction mechanisms. By establishing the precise interrelationship between microscopic properties and photophysical behaviors of photocatalysts, single-particle fluorescence spectroscopy can elucidate the carrier separation and catalytic mechanism of the photocatalysts in situ, which provides perspectives for improving the photocatalytic efficiency. This Review primarily focuses on the basic principles and advantages of single-particle fluorescence spectroscopy and its progress in the study of plasmonic and semiconductor photocatalysis, especially emphasizing its importance in understanding the charge separation and photocatalytic reaction mechanism, which offers scientific guidance for designing efficient photocatalytic systems. Finally, we summarize and forecast the future development prospects of single-particle fluorescence spectroscopy technology, especially the insights into its technological upgrading.

Exploring Label-Free Imaging Techniques with Copper Sulfide Microspheres for Observing Breast Cancer Cells

A single breast cancer is a prevalent form of cancer, affecting over 2.3 million women worldwide, as reported by the World Health Organization. Recently, researchers have extensively explored the utilization of biomaterials in breast cancer theranostics. One notable biomaterial being investigated is various structures of copper sulfide (CuS). In this work, a microsphere (MS) structure composed of CuS was employed for label-free imaging of MCF-7 breast cancer cells and normal Vero cells, respectively. Various label-free imaging techniques, such as bright field, dark field, phase contrast (PC), and differential interference contrast (DIC), were employed to capture images of CuS MSs, cell, and intact CuS MSs within a cell. The study compared the outcomes of each imaging technique and determined that DIC imaging provided the highest resolution for cells incubated with CuS MSs. Furthermore, the combination of PC and DIC techniques proved to be effective for imaging breast cancer cells in conjunction with CuS MSs. This research underscores the potential of CuS MSs for label-free cell detection and emphasizes the significance of selecting appropriate imaging techniques to attain high-quality images in the field of cell observation.

Photoluminescence of MoS2 on Plasmonic Gold Nanoparticles Depending on the Aggregate Size

Transition metal dichalcogenides (TMDs) are promising candidates for ultrathin functional semiconductor devices. In particular, incorporating plasmonic nanoparticles into TMD-based devices enhances the light-matter interaction for increased absorption efficiency and enables control of device performance such as electronic, electrical, and optical properties. In this heterohybrid structure, manipulating the number of TMD layers and the aggregate size of plasmonic nanoparticles is a straightforward approach to tailoring device performance. In this study, we use photoluminescence (PL) spectroscopy, which is a commonly employed technique for monitoring device performance, to analyze the changes in electronic and optical properties depending on the number of MoS2 layers and the size of the gold nanoparticle (AuNP) aggregate under nonresonant and resonant excitation conditions. The PL intensity in monolayer MoS2/AuNPs increases as the size of aggregates increases irrespective of the excitation conditions. The strain induced by AuNPs causes a red shift, but as the aggregates grow larger, the effect of p-doping increases and the blue shift becomes prominent. In multilayer MoS2/AuNPs, quenched PL intensity is observed under nonresonant excitation, while enhancement is noted under resonant excitation, which is mainly contributed by p-doping and LSPR, respectively. Remarkably, the alteration in the spectral shape due to resonant excitation is evident solely in small aggregates of AuNPs across all layers.

Surface plasmon enhancement in silver nanowires and bilayer two-dimensional materials

In order to improve the low light absorption of two-dimensional (2D) transition metal dichalcogenides (TMDCs), surface plasmon (SP) nanostructures have been widely studied. However, the impact of interlayer twist on such nanostructures has rarely been studied. Here, we construct two different composite structures of silver nanowires (Ag NWs) and pristine bilayer MoS2 (pBLM) or twisted bilayer MoS2 (tBLM). The interlayer twist can further promote the light utilization of MoS2, resulting in an ∼4-fold higher spectral enhancement in Ag/tBLM than that in Ag/pBLM. In addition, the photocurrent and detectivity of the phototransistor based on the Ag/tBLM composite structure were improved by 7-fold and ∼100-fold, respectively, compared to those of the Ag/pBLM phototransistor. Theoretical simulations show that the enhancement of photocurrent can be attributed to the enhancement of the local electric field at the interface between Ag NWs and the tBLM film, which is called the 'hot spot'. These results provide a reference for understanding the modulation mechanism of SPs and interlayer twist on the optoelectronic properties of 2D materials.

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.

Interfacial engineering of halide perovskites and two-dimensional materials

Recently, halide perovskites (HPs) and layered two-dimensional (2D) materials have received significant attention from industry and academia alike. HPs are emerging materials that have exciting photoelectric properties, such as a high absorption coefficient, rapid carrier mobility and high photoluminescence quantum yields, making them excellent candidates for various optoelectronic applications. 2D materials possess confined carrier mobility in 2D planes and are widely employed in nanostructures to achieve interfacial modification. HP/2D material interfaces could potentially reveal unprecedented interfacial properties, including light absorbance with desired spectral overlap, tunable carrier dynamics and modified stability, which may lead to several practical applications. In this review, we attempt to provide a comprehensive perspective on the development of interfacial engineering of HP/2D material interfaces. Specifically, we highlight the recent progress in HP/2D material interfaces considering their architectures, electronic energetics tuning and interfacial properties, discuss the potential applications of these interfaces and analyze the challenges and future research directions of interfacial engineering of HP/2D material interfaces. This review links the fields of HPs and 2D materials through interfacial engineering to provide insights into future innovations and their great potential applications in optoelectronic devices.

Surface plasmon enhancement in the different spatial distributions of nanowire and two-dimensional material

Surface plasmon (SP) nanostructures have been widely researched to improve the low light absorption of two-dimensional transition metal dichalcogenides (TMDCs). However, the impact of the different coupling forms of them,...

Dark-field/hyperspectral microscopy for detecting nanoscale particles in environmental nanotoxicology research

Nanoscale contaminants (including engineered nanoparticles and nanoplastics) pose a significant threat to organisms and environment. Rapid and non-destructive detection and identification of nanosized materials in cells, tissues and organisms is still challenging, although a number of conventional methods exist. These approaches for nanoparticles imaging and characterisation both inside the cytoplasm and on the cell or tissue outer surfaces, such as electron or scanning probe microscopies, are unquestionably potent tools, having excellent resolution and supplemented with chemical analysis capabilities. However, imaging and detection of nanomaterials in situ, in wet unfixed and even live samples, such as living isolated cells, microorganisms, protozoans and miniature invertebrates using electron microscopy is practically impossible, because of the elaborate sample preparation requiring chemical fixation, contrast staining, matrix embedding and exposure into vacuum. Atomic force microscopy, in several cases, can be used for imaging and mechanical analysis of live cells and organisms under ambient conditions, however this technique allows for investigation of surfaces. Therefore, a different approach allowing for imaging and differentiation of nanoscale particles in wet samples is required. Dark-field microscopy as an optical microscopy technique has been popular among researchers, mostly for imaging relatively large specimens. In recent years, the so-called "enhanced dark field" microscopy based on using higher numerical aperture light condensers and variable numerical aperture objectives has emegred, which allows for imaging of nanoscale particles (starting from 5 nm nanospheres) using almost conventional optical microscopy methodology. Hyperspectral imaging can turn a dark-field optical microscope into a powerful chemical characterisation tool. As a result, this technique is becoming popular in environmental nanotoxicology studies. In this Review Article we introduce the reader into the methodology of enhanced dark-field and dark-field-based hyperspectral microscopy, covering the most important advances in this rapidly-expanding area of environmental nanotoxicology.

Rapid and Large-Scale Quality Assessment of Two-Dimensional MoS2 Using Sulfur Particles with Optical Visualization

The efficient nondestructive assessment of quality and homogeneity for two-dimensional (2D) MoS₂ is critically important to advance their practical applications. Here, we presented a rapid and large-area assessment method for visually evaluating the quality and uniformity of chemical vapor deposition (CVD)-grown MoS₂ monolayers simply with conventional optical microscopes. This was achieved through one-pot adsorbing abundant sulfur particles selectively onto as-grown poorer-quality MoS₂ monolayers in a CVD system without any additional treatment. We further revealed that this favorable adsorption of sulfur particles on MoS₂ originated from their intrinsic higher-density sulfur vacancies. Based on unadsorbed MoS₂ monolayers, superior performance field effect transistors with a mobility of ∼49 cm² V^-1 s^-1 were constructed. Importantly, the assessment approach was noninvasive due to the all-vapor-phase and moderate adsorption-desorption process. Our work offers a new route for the performance and yield optimization of devices by quality assessment of 2D semiconductors prior to device fabrication.

Growth Mechanism of Alternating Defect Domains in Hexagonal WS2 via Inhomogeneous W-Precursor Accumulation

While a hexagonal WS₂ monolayer, grown by chemical vapor deposition, exhibits distinctive patterns in photoluminescence mapping, segmented with alternating S-vacancy (SV) and W-vacancy (WV) domains in a single crystal, the formation mechanism for native alternating defect domains remains unresolved to date. Here, the formation mechanism of alternating defect domains in hexagonal WS₂ via the precursor accumulation model is experimentally elucidated. A triangular WS₂ seed is initially formed, followed by a hexagonal flake. Alternating W-rich (SV) and W-deficient (WV) domains are constructed in hexagonal WS₂ flake, which is confirmed by confocal photoluminescence mapping and secondary ion mass spectroscopy. This is explained by the accumulation or scarcity of W-precursors at the edge of the WS₂ flake. The W-precursors accumulate near the edges of the initial triangular WS₂ seed over time, while they are deficient near the corners of the triangular WS₂ , eventually forming WV domains in the truncated hexagonal domains. The heterogeneous accumulation becomes more prominent in the presence of H₂ gas through desorption of the W-precursors.

Nanoscale light-matter interactions in metal-organic frameworks cladding optical fibers

The utilization of refractive index (RI) change due to guest-host interactions between the guest volatile organic compound vapor and porous metal-organic frameworks (vapor-MOF interactions) is promising in photonic vapor sensors. Therefore, the study of light-matter interactions in nanoporous metal-organic frameworks (MOFs) is fundamental and essential for MOF-based photonic devices. In this work, the manipulation of light in MOFs to investigate the vapor-MOF interactions by using optical fiber devices is demonstrated. The vapor-MOF interactions and the light-vapor interactions (light in MOFs to sense the RI changes resulting from the vapor-MOF interactions) are investigated. The cladding mode is excited by a long-period fiber grating (LPFG) for evanescent field sensing in a ZIF-8 sensitive coating. The experimental results combining quantum chemical calculations and optical simulations reveal the relationships between the microscopic energy of vapor desorption, RI changes and evanescent field enhancement in ZIF-8 during the vapor-MOF interactions. With exceptionally large RI changes, the evanescent field of cladding mode in ZIF-8 is greatly enhanced to sense the vapor-MOF interactions. As a proof-of-concept, a LPFG sensor with ZIF-8 coating showed a high sensitivity of 1.33 pm ppm-1 in the linear range from 9.8 ppm to 540 ppm for the sensing of ethanol vapor. The investigation of light-matter interactions in ZIF-8 provides a useful guideline for the design and fabrication of MOF-based optical waveguide/fiber sensors.

A Critical Review on Enhancement of Photocatalytic Hydrogen Production by Molybdenum Disulfide: From Growth to Interfacial Activities

Ultrathin 2D molybdenum disulfide (MoS₂ ), which is the flagship of 2D transition-metal dichalcogenide nanomaterials, has drawn much attention in the last few years. 2D MoS₂ has been banked as an alternative to platinum for highly active hydrogen evolution reaction because of its low cost, high surface-to-volume ratio, and abundant active sites. However, when MoS₂ is used directly as a photocatalyst, contrary to public expectation, it still performs poorly due to lateral size, high recombination ratio of excitons, and low optical cross section. Besides, simply compositing MoS₂ as a cocatalyst with other semiconductors cannot satisfy the practical application, which stimulates the pursual of a comprehensive insight into recent advances in synthesis, properties, and enhanced hydrogen production of MoS₂ . Therefore, in this Review, emphasis is given to synthetic methods, phase transitions, tunable optical properties, and interfacial engineering of 2D MoS₂ . Abundant ways of band edge tuning, structural modification, and phase transition are addressed, which can generate the neoteric photocatalytic systems. Finally, the main challenges and opportunities with respect to MoS₂ being a cocatalyst and coherent light-matter interaction of MoS₂ in photocatalytic systems are proposed.

Large-Area and Strain-Reduced Two-Dimensional Molybdenum Disulfide Monolayer Emitters on a Three-Dimensional Substrate

Atomically thin membranes of two-dimensional (2-D) transition-metal dichalcogenides (TMDCs) have distinct emission properties, which can be utilized for realizing ultrathin optoelectronic integrated systems in the future. Growing a large-area and strain-reduced monolayer 2-D material on a three-dimensional (3-D) substrate with microstructures or nanostructures is a crucial technique because the electronic band structure of TMDC atomic layers is strongly affected by the number of stacked layers and strain. In this study, a large-area and strain-reduced MoS₂ monolayer was fabricated on a 3-D substrate through a two-step growth procedure. The material characteristics and optical properties of monolayer TMDCs fabricated on the nonplanar substrate were examined. The growth of monolayer MoS₂ on a cone-shaped sapphire substrate effectively reduced the tensile strain induced by the substrate by decreasing the thermal expansion mismatch between the 2-D material and the substrate. Monolayer MoS₂ grown on the nonplanar substrate exhibited uniform strain reduction and luminescence intensity. The fabrication of monolayer MoS₂ on a nonplanar substrate increased the light extraction efficiency. In the future, large-area and strain-reduced 2-D TMDC materials grown on a nonplanar substrate can be employed as novel light-emitting devices for applications in lighting, communication, and displays for the development of ultrathin optoelectronic integrated systems.