Measurement of Quantum Yields of Monolayer TMDs Using Dye-Dispersed PMMA Thin Films

Nonlinear Nano-Imaging of Interlayer Coupling in 2D Graphene-Semiconductor Heterostructures

The emergent electronic, spin, and other quantum properties of 2D heterostructures of graphene and transition metal dichalcogenides are controlled by the underlying interlayer coupling and associated charge and energy transfer dynamics. However, these processes are sensitive to interlayer distance and crystallographic orientation, which are in turn affected by defects, grain boundaries, or other nanoscale heterogeneities. This obfuscates the distinction between interlayer charge and energy transfer. Here, nanoscale imaging in coherent four-wave mixing (FWM) and incoherent two-photon photoluminescence (2PPL) is combined with a tip distance-dependent coupled rate equation model to resolve the underlying intra- and inter-layer dynamics while avoiding the influence of structural heterogeneities in mono- to multi-layer graphene/WSe2 heterostructures. With selective insertion of hBN spacer layers, it is shown that energy, as opposed to charge transfer, dominates the interlayer-coupled optical response. From the distinct nano-FWM and -2PPL tip-sample distance-dependent modification of interlayer and intralayer relaxation by tip-induced enhancement and quenching, an interlayer energy transfer time ofτ ET ≈ ( 0 . 35 - 0.15 + 0.65 ) $\tau _{\rm ET} \approx (0.35^{+0.65}_{-0.15})$  ps consistent with recent reports is derived. As a local probe technique, this approach highlights the ability to determine intrinsic sample properties even in the presence of large sample heterogeneity.

Exciton engineering of 2D Ruddlesden-Popper perovskites by synergistically tuning the intra and interlayer structures

Designing two-dimensional halide perovskites for high-performance optoelectronic applications requires deep understanding of the structure-property relationship that governs their excitonic behaviors. However, a design framework that considers both intra and interlayer structures modified by the A-site and spacer cations, respectively, has not been developed. Here, we use pressure to synergistically tune the intra and interlayer structures and uncover the structural modulations that result in improved optoelectronic performance. Under applied pressure, (BA)2(GA)Pb2I7 exhibits a 72-fold boost of photoluminescence and 10-fold increase of photoconductivity. Based on the observed structural change, we introduce a structural descriptor χ that describes both the intra and interlayer characteristics and establish a general quantitative relationship between χ and photoluminescence quantum yield: smaller χ correlates with minimized trapped excitons and more efficient emission from free excitons. Building on this principle, we design a perovskite (CMA)2(FA)Pb2I7 that exhibits a small χ and an impressive photoluminescence quantum yield of 59.3%.

Optical grade transformation of monolayer transition metal dichalcogenides via encapsulation annealing

Monolayer transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for optoelectronic applications due to their direct band gap and strong light-matter interactions. However, exfoliated TMDs have demonstrated optical characteristics that fall short of expectations, primarily because of significant defects and associated doping in the synthesized TMD crystals. Here, we report the improvement of optical properties in monolayer TMDs of MoS2, MoSe2, WS2, and WSe2, by hBN-encapsulation annealing. Monolayer WSe2 showed 2000% enhanced photoluminescence quantum yield (PLQY) and 1000% increased lifetime after encapsulation annealing at 1000 °C, which are attributed to dominant radiative recombination of excitons through dedoping of monolayer TMDs. Furthermore, after encapsulation annealing, the transport characteristics of monolayer WS2 changed from n-type to ambipolar, along with an enhanced hole transport, which also support dedoping of annealed TMDs. This work provides an innovative approach to elevate the optical grade of monolayer TMDs, enabling the fabrication of high-performance optoelectronic devices.

Biaxial strain tuned upconversion photoluminescence of monolayer WS2

Monolayer tungsten disulfide (1L-WS2) is a direct bandgap atomic-layered semiconductor material with strain tunable optical and optoelectronic properties among the monolayer transition metal dichalcogenides (1L-TMDs). Here, we demonstrate biaxial strain tuned upconversion photoluminescence (UPL) from exfoliated 1L-WS2 flakes transferred on a flexible polycarbonate cruciform substrate. When the biaxial strain applied to 1L-WS2 increases from 0 to 0.51%, it is observed that the UPL peak position is redshifted by up to 60 nm/% strain, while the UPL intensity exhibits exponential growth with the upconversion energy difference varying from - 303 to - 120 meV. The measured power dependence of UPL from 1L-WS2 under biaxial strain reveals the one photon involved multiphonon-mediated upconversion mechanism. The demonstrated results provide new opportunities in advancing TMD-based optical upconversion devices for future flexible photonics and optoelectronics.

Synergetic Enhancement of Quantum Yield and Exciton Lifetime of Monolayer WS2 by Proximal Metal Plate and Negative Electric Bias

The efficiency of light emission is a critical performance factor for monolayer transition metal dichalcogenides (1L-TMDs) for photonic applications. While various methods have been studied to compensate for lattice defects to improve the quantum yield (QY) of 1L-TMDs, exciton-exciton annihilation (EEA) is still a major nonradiative decay channel for excitons at high exciton densities. Here, we demonstrate that the combined use of a proximal Au plate and a negative electric gate bias (NEGB) for 1L-WS2 provides a dramatic enhancement of the exciton lifetime at high exciton densities with the corresponding QY enhanced by 30 times and the EEA rate constant decreased by 80 times. The suppression of EEA by NEGB is attributed to the reduction of the defect-assisted EEA process, which we also explain with our theoretical model. Our results provide a synergetic solution to cope with EEA to realize high-intensity 2D light emitters using TMDs.

Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structures

Hybrid structures with an interface between two different materials with properly aligned energy levels facilitate photo-induced charge separation to be exploited in optoelectronic applications. Particularly, the combination of 2D transition metal dichalcogenides (TMDCs) and dye molecules offers strong light-matter interaction, tailorable band level alignments, and high fluorescence quantum yields. In this work, we aim at the charge or energy transfer-related quenching of the fluorescence of the dye perylene orange (PO) when isolated molecules are brought onto monolayer TMDCs via thermal vapor deposition. Here, micro-photoluminescence spectroscopy revealed a strong intensity drop of the PO fluorescence. For the TMDC emission, in contrast, we observed a relative growth of the trion versus exciton contribution. In addition, fluorescence imaging lifetime microscopy quantified the intensity quenching to a factor of about 10³ and demonstrated a drastic lifetime reduction from 3 ns to values much shorter than the 100 ps width of the instrument response function. From the ratio of the intensity quenching that is attributed to hole or energy transfer from dye to semiconductor, we deduce a time constant of several picoseconds at most, pointing to an efficient charge separation suitable for optoelectronic devices.

Exciton-dominant photoluminescence of MoS2 by a functionalized substrate

Transition metal dichalcogenides (TMDs) have been considered as promising candidates for transparent and flexible optoelectronic devices owing to their large exciton binding energy and strong light-matter interaction. However, monolayer (1L) TMDs exhibited different intensities and spectra of photoluminescence (PL), and the characteristics of their electronic devices also differed in each study. This has been explained in terms of various defects in TMDs, such as vacancies and grain boundaries, and their surroundings, such as dielectric screening and charged impurities, which lead to non-radiative recombination of trions, low quantum yield (QY), and unexpected doping. However, it should be noted that the surface conditions of the substrate are also a critical factor in determining the properties of TMDs located on the substrate. Here, we demonstrate that the optical and electrical properties of 1L MoS₂ are strongly influenced by the functionalized substrate. The PL of 1L MoS₂ placed on the oxygen plasma-treated SiO₂ substrate was highly p-doped owing to the functional groups of -OH on SiO₂, resulting in a strong enhancement of PL by approximately 20 times. The PL QY of 1L MoS₂ on plasma-treated SiO₂ substrate increased by one order of magnitude. Surprisingly, the observed PL spectra show the suppression of non-radiative recombination by trions, thus the exciton-dominant PL led to a prolonged lifetime of MoS₂ on the plasma-treated substrate. The MoS₂ field-effect transistors fabricated on plasma-treated SiO₂ also exhibited a large hysteresis in the transfer curve owing to charge trapping of the functional groups. Our study demonstrates that the functional groups on the substrate strongly affect the characteristics of 1L MoS₂, which provides clues as to why MoS₂ exfoliated on various substrates always exhibited different properties in previous studies.

Defect-suppressed submillimeter-scale WS2 single crystals with high photoluminescence quantum yields by alternate-growth-etching CVD

Defects, such as uncontrollable vacancies, will intensively degrade the material properties and device performance of CVD-grown transition metal dichalcogenides (TMDs). Although vacancies can be repaired by some post-processing measures, these treatments are usually time-consuming, complicated and may introduce uncontrollable chemical contaminants into TMDs. How to efficiently suppress the uncontrollable defects during CVD growth and acquire intrinsic high-quality CVD-grown TMDs without any after-treatment remains a critical challenge, and has not yet been well resolved. Here, an alternate-growth-etching (AGE) CVD method was demonstrated to fabricate defect-suppressed submillimeter-scale monolayer WS₂ single crystals. Compared with normal CVD, the grain size of the as-grown WS₂ can be enlarged by 4-5 times (∼520 μm) and the growth rate of ∼14.4 μm min^-1 is also at a high level compared to reported results. Moreover, AGE-CVD can efficiently suppress atomic vacancies in WS₂. In every growth-etching cycle, the etching of WS₂ occurs preferentially at the defective sites, which will be healed at the following growth stage. As a result, WS₂ monolayers obtained by AGE-CVD possess higher crystal quality, carrier mobility (8.3 cm² V^-1 s^-1) and PL quantum yield (QY, 52.6%) than those by normal CVD. In particular, such a PL QY is the highest value ever reported for in situ CVD-grown TMDs without any after-treatment, and is even comparable to the values of mechanically exfoliated samples. This AGE-CVD method is also appropriate for the synthesis of other high-quality TMD single crystals on a large-scale.

Optoelectronic system and device integration for quantum-dot light-emitting diode white lighting with computational design framework

We propose a computational design framework to design the architecture of a white lighting system having multiple pixelated patterns of electric-field-driven quantum dot light-emitting diodes. The quantum dot of the white lighting system has been optimised by a system-level combinatorial colour optimisation process with the Nelder-Mead algorithm used for machine learning. The layout of quantum dot patterns is designed precisely using rigorous device-level charge transport simulation with an electric-field dependent charge injection model. A theoretical maximum of 97% colour rendering index has been achieved with red, green, cyan, and blue quantum dot light-emitting diodes as primary colours. The white lighting system has been fabricated using the transfer printing technique to validate the computational design framework. It exhibits excellent lighting performance of 92% colour rendering index and wide colour temperature variation from 1612 K to 8903 K with only the four pixelated quantum dots as primary.

Transition Metal Dichalcogenide Dimer Nanoantennas for Tailored Light-Matter Interactions

Transition metal dichalcogenides have emerged as promising materials for nanophotonic resonators because of their large refractive index, low absorption within a large portion of the visible spectrum, and compatibility with a wide range of substrates. Herein, we use these properties to fabricate WS₂ double-pillar nanoantennas in a variety of geometries enabled by the anisotropy in the crystal structure. Using dark-field spectroscopy, we reveal multiple Mie resonances, to which we couple WSe₂ monolayer photoluminescence and achieve Purcell enhancement and an increased fluorescence by factors up to 240 for dimer gaps of 150 nm. We introduce postfabrication atomic force microscope repositioning and rotation of dimer nanoantennas, achieving gaps as small as 10 ± 5 nm, which enables a host of potential applications, including strong Purcell enhancement of single-photon emitters and optical trapping, which we study in simulations. Our findings highlight the advantages of using transition metal dichalcogenides for nanophotonics by exploring applications enabled by their properties.

Low Temperature Step Annealing Synthesis of the Ti2AlN MAX Phase to Fabricate MXene Quantum Dots

We present the synthesis of the Ti2AlN MAX phase using two-step annealing at temperatures of 600 °C and 1100 °C, the lowest synthesis temperatures reported so far. After the successful synthesis of the Ti2AlN MAX phase, two-dimensional Ti2N MXene was prepared through wet chemical etching and further fragmented into light emitting MXene quantum dots (MQDs) with a size of 3.2 nm by hydrothermal method. Our MQDs displayed a 6.9% quantum yield at a 310 nm wavelength of excitation, suggesting promising nanophotonic applications.

Flexible Silicon Dimer Nanocavity with Electric and Magnetic Enhancement

High-index dielectrics have recently been regarded as promising building blocks in nanophotonics owing to optical electric and magnetic Mie resonances. In particular, silicon is gaining great interest as the backbone of modern technology. Here, silicon dimer nanocavities with different sizes of silicon nanospheres were constructed using a probe nanomanipulation method and interacted with a few-layered R6G membrane to investigate the enhancement of electric and magnetic mode coupling. The evidence of the enhancement of fluorescence and slightly prolonged lifetime of R6G indicated the existence of nanocavities. In addition, the simulated electric and magnetic field distributions and decomposed mode of nanocavity were used to analyze the contribution of electric and magnetic modes to the R6G enhanced fluorescence. Such silicon dimer is a flexible nanocavity with electric and magnetic mode enhancement and has promising applications in sensing and all-dielectric metamaterials or nanophotonic devices.

Optimization Strategies for High Photoluminescence Quantum Yield of Monolayer Chemical Vapor Deposition Transition Metal Dichalcogenides

Chemical vapor deposition (CVD) is a promising method to obtain monolayer transition metal dichalcogenides (TMDCs) with high quality and enough size to meet the requirements of practical photoelectric devices. However, the as-grown monolayers often exhibit a lower PL performance due to the stress between the as-grown TMDCs flakes and the substrate. Therefore, finding a facile method to effectively promote the photoluminescence quantum yield (PL QY) of CVD monolayer TMDCs with a clean surface is highly desirable for practical applications. In this work, based on the CVD monolayers MoS₂ and MoSe₂, the effect of various stress relaxation methods on the TMDCs PL enhancement is systemically studied. By comparing the different kinds of volatile solution treatment processes, as well as the traditional transfer process, it can be found that the volatile solution with a moderate volatilization rate such as ethanol or IPA is a preferred option to improve the PL performance of the CVD monolayer TMDCs, which also surpasses the traditional transfer method by avoiding wrinkles, defects, and contamination to the samples. PL QY of ethanol-treated CVD samples could increase by 6 times on average. Significantly, PL QY of CVD MoSe₂ treated by ethanol can reach ∼16%, which is at the forefront of the previous reports of 2D MoSe₂. Our study demonstrated an optimized method to enhance the PL QY of CVD monolayer TMDCs, which would facilitate TMDCs optoelectronics.

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

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

Experimental and Computational Investigation of Layer-Dependent Thermal Conductivities and Interfacial Thermal Conductance of One- to Three-Layer WSe2

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs) have received extensive research interest and investigations in the past decade. In this research, we used a refined opto-thermal Raman technique to explore the thermal transport properties of one popular TMDC material WSe₂, in the single-layer (1L), bilayer (2L), and trilayer (3L) forms. This measurement technique is direct without additional processing to the material, and the absorption coefficient of WSe₂ is discovered during the measurement process to further increase this technique's precision. By comparing the sample's Raman spectroscopy spectra through two different laser spot sizes, we are able to obtain two parameters-lateral thermal conductivities of 1L-3L WSe₂ and the interfacial thermal conductance between 1L-3L WSe₂ and the substrate. We also implemented full-atom nonequilibrium molecular dynamics simulations (NEMD) to computationally investigate the thermal conductivities of 1L-3L WSe₂ to provide comprehensive evidence and confirm the experimental results. The trend of the layer-dependent lateral thermal conductivities and interfacial thermal conductance of 1L-3L WSe₂ is discovered. The room-temperature thermal conductivities for 1L-3L WSe₂ are 37 ± 12, 24 ± 12, and 20 ± 6 W/(m·K), respectively. The suspended 1L WSe₂ possesses a thermal conductivity of 49 ± 14 W/(m·K). Crucially, the interfacial thermal conductance values between 1L-3L WSe₂ and the substrate are found to be 2.95 ± 0.46, 3.45 ± 0.50, and 3.46 ± 0.45 MW/(m²·K), respectively, with a flattened trend starting the 2L, a finding that provides the key information for thermal management and thermoelectric designs.

Deterministic fabrication of arbitrary vertical heterostructures of two-dimensional Ruddlesden-Popper halide perovskites

Ruddlesden-Popper lead halide perovskites have emerged as a new class of two-dimensional semiconductors with tunable optoelectronic properties, potentially offering unlimited heterostructure configurations for exploration. However, the practical realization of such heterostructures is challenging because of the difficulty in achieving controllable direct synthesis or van der Waals integration of halide perovskites due to their mobile and fragile crystal lattices. Here we report direct growth of large-area nanosheets of diverse phase-pure Ruddlesden-Popper perovskites with thicknesses down to one monolayer at the solution-air interface and a reliable approach for gently transferring and stacking these nanosheets. These advances enable the deterministic fabrication of arbitrary vertical heterostructures and multi-heterostructures of Ruddlesden-Popper perovskites with greater structural degrees of freedom that define the electronic structures of the heterojunctions. Such rationally designed heterostructures exhibit interesting interlayer properties, such as interlayer carrier transfer and reduction of the photoluminescence linewidth, and could enable the exploration of exciton physics and optoelectronic applications.

Growth and Interlayer Engineering of 2D Layered Semiconductors for Future Electronics

Layered materials that do not form a covalent bond in a vertical direction can be prepared in a few atoms to one atom thickness without dangling bonds. This distinctive characteristic of limiting thickness around the sub-nanometer level allowed scientists to explore various physical phenomena in the quantum realm. In addition to the contribution to fundamental science, various applications were proposed. Representatively, they were suggested as a promising material for future electronics. This is because (i) the dangling-bond-free nature inhibits surface scattering, thus carrier mobility can be maintained at sub-nanometer range; (ii) the ultrathin nature allows the short-channel effect to be overcome. In order to establish fundamental discoveries and utilize them in practical applications, appropriate preparation methods are required. On the other hand, adjusting properties to fit the desired application properly is another critical issue. Hence, in this review, we first describe the preparation method of layered materials. Proper growth techniques for target applications and the growth of emerging materials at the beginning stage will be extensively discussed. In addition, we suggest interlayer engineering via intercalation as a method for the development of artificial crystal. Since infinite combinations of the host-intercalant combination are possible, it is expected to expand the material system from the current compound system. Finally, inevitable factors that layered materials must face to be used as electronic applications will be introduced with possible solutions. Emerging electronic devices realized by layered materials are also discussed.