Control of Radiative Exciton Recombination by Charge Transfer Induced Surface Dipoles in MoS2 and WS2 Monolayers

Monolayer Semiconductor Superlattices with High Optical Absorption

Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. A uniform film much thinner than the wavelength can only absorb up to 50% of the incident light when embedded in a symmetric and homogeneous environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption by artificially stacking WS2 monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the peak absorptance of a single WS2 monolayer, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach an absorptance of 27% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer with 25% absorptance while increasing photoluminescence thanks to doping. Finally, we exploit the atomic layer deposition of alumina spacers to boost the absorptance to 31% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve strong light-matter coupling and enhance the performance of nanophotonic devices such as modulators and photodetectors.

Improving the Optical Quality of MoSe2 and WS2 Monolayers with Complete h-BN Encapsulation by High-Temperature Annealing

We improved the optical quality and stability of an exfoliated monolayer (ML) MoSe₂ and chemical vapor deposition (CVD)-grown WS₂ MLs by encapsulating and sealing them with both top and bottom few-layer h-BN, as tested by subsequent high-temperature annealing up to 873 K and photoluminescence (PL) measurements. These transition-metal dichalcogenide (TMD) MLs remained stable up to this maximum temperature, as seen visually. After the heating/cooling cycle, the integrated photoluminescence (PL) intensity at 300 K in the MoSe₂ ML was ∼4 times larger than that before heating and that from exciton and trion PL in the analogous WS₂ ML sample was ∼14 times and ∼2.5 times larger at 77 K and the exciton peak was ∼9.5 times larger at 300 K. This is attributed to the reduction of impurities, the lateral expulsion of contamination leading to clean and atomically flat surfaces, and the sealing provided by the h-BN layers that prevents the diffusion of molecules such as trace O₂ and H₂O to the TMD ML. Stability and optical performance are much improved compared to that in earlier work using top h-BN only, in which the WS₂ ML PL intensity decreased even for an optimal gas environment. This complete encapsulation is particularly promising for CVD-grown TMD MLs because they have relatively more charge and other impurities than do exfoliated MLs. These results open a new route for improving the optical properties of TMD MLs and their performance and applications both at room and higher temperatures.

N-type and p-type molecular doping on monolayer MoS2

Monolayer MoS2 has attracted much attention due to its high on/off current ratio, transparency, and suitability for optoelectronic devices. Surface doping by molecular adsorption has proven to be an effective method to facilitate the usage of MoS2. However, there are no works available to systematically clarify the effects of the adsorption of F4TCNQ, PTCDA, and tetracene on the electronic and optical properties of the material. Therefore, this work elucidated the problem by using density functional theory calculations. We found that the adsorption of F4TCNQ and PTCDA turns MoS2 into a p-type semiconductor, while the tetracene converts MoS2 into an n-type semiconductor. The occurrence of a new energy level in the conduction band for F4TCNQ and PTCDA and the valence band for tetracene reduces the bandgap of the monolayer MoS2. Besides, the MoS2/F4TCNQ and MoS2/PTCDA systems exhibit an auxiliary optical peak at the long wavelengths of 950 and 850 nm, respectively. Contrastingly, the MoS2/tetracene modifies the optical spectrum of the monolayer MoS2 only in the ultraviolet region. The findings are in good agreement with the experiments.

The Stoichiometry of TCNQ-Based Organic Charge-Transfer Cocrystals

Organic charge-transfer cocrystals (CTCs) have attracted significant research attention due to their wide range of potential applications in organic optoelectronic devices, organic magnetic devices, organic energy devices, pharmaceutical industry, etc. The physical properties of organic charge transfer cocrystals can be tuned not only by changing the donor and acceptor molecules, but also by varying the stoichiometry between the donor and the acceptor. However, the importance of the stoichiometry on tuning the properties of CTCs has still been underestimated. In this review, single-crystal growth methods of organic CTCs with different stoichiometries are first introduced, and their physical properties, including the degree of charge transfer, electrical conductivity, and field-effect mobility, are then discussed. Finally, a perspective of this research direction is provided to give the readers a general understanding of the concept.

Robust B-exciton emission at room temperature in few-layers of MoS2:Ag nanoheterojunctions embedded into a glass matrix

Tailoring the photoluminescence (PL) properties in two-dimensional (2D) molybdenum disulfide (MoS2) crystals using external factors is critical for its use in valleytronic, nanophotonic and optoelectronic applications. Although significant effort has been devoted towards enhancing or manipulating the excitonic emission in MoS2 monolayers, the excitonic emission in few-layers MoS2 has been largely unexplored. Here, we put forward a novel nano-heterojunction system, prepared with a non-lithographic process, to enhance and control such emission. It is based on the incorporation of few-layers MoS2 into a plasmonic silver metaphosphate glass (AgPO3) matrix. It is shown that, apart from the enhancement of the emission of both A- and B-excitons, the B-excitonic emission dominates the PL intensity. In particular, we observe an almost six-fold enhancement of the B-exciton emission, compared to control MoS2 samples. This enhanced PL at room temperature is attributed to an enhanced exciton-plasmon coupling and it is supported by ultrafast time-resolved spectroscopy that reveals plasmon-enhanced electron transfer that takes place in Ag nanoparticles-MoS2 nanoheterojunctions. Our results provide a great avenue to tailor the emission properties of few-layers MoS2, which could find application in emerging valleytronic devices working with B excitons.

Distinctive Field-Effect Transistors and Ternary Inverters Using Cross-Type WSe2/MoS2 Heterojunctions Treated with Polymer Acid

The electrical and optical characteristics of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be improved by surface modification. In this study, distinctive field-effect transistors (FETs) were realized by forming cross-type 2D WSe₂/MoS₂ p-n heterojunctions through surface treatment using poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-PMAA). The FETs were applied to new ternary inverters as multivalued logic circuits (MVLCs). Laser confocal microscope photoluminescence spectroscopy indicated the generation of trions in the WSe₂ and MoS₂ layers, and the intensity decreased after PMMA-co-PMAA treatment. For the cross-type WSe₂/MoS₂ p-n heterojunction FETs subjected to PMMA-co-PMAA treatment, the channel current and the region of anti-ambipolar transistor characteristics increased considerably, and ternary inverter characteristics with three stable logic states, "1", "1/2", and "0", were realized. Interestingly, the intermediate logic state 1/2, which results from the negative differential transconductance characteristics, was realized by the turn-on of all component FETs, as the current of the FETs increased after PMMA-co-PMAA treatment. The electron-rich carboxyl acid moieties in PMMA-co-PMAA can undergo coordination with the metal Mo or W atoms present in the Se or S vacancies, respectively, resulting in the modulation of charge density. These features yielded distinctive FETs and ternary inverters for MVLCs using cross-type WSe₂/MoS₂ heterojunctions.

Photoluminescence enhancement of monolayer MoS2 using plasmonic gallium nanoparticles

2D monolayer molybdenum disulphide (MoS₂) has been the focus of intense research due to its direct bandgap compared with the indirect bandgap of its bulk counterpart; however its photoluminescence (PL) intensity is limited due to its low absorption efficiency. Herein, we use gallium hemispherical nanoparticles (Ga NPs) deposited by thermal evaporation on top of chemical vapour deposited MoS₂ monolayers in order to enhance its luminescence. The influence of the NP radius and the laser wavelength is reported in PL and Raman experiments. In addition, the physics behind the PL enhancement factor is investigated. The results indicate that the prominent enhancement is caused by the localized surface plasmon resonance of the Ga NPs induced by a charge transfer phenomenon. This work sheds light on the use of alternative metals, besides silver and gold, for the improvement of MoS₂ luminescence.

Composition-Tunable Synthesis of Large-Scale Mo1- xW xS2 Alloys with Enhanced Photoluminescence

Alloying two-dimensional transition metal dichalcogenides (2D TMDs) is a promising avenue for band gap engineering. In addition, developing a scalable synthesis process is essential for the practical application of these alloys with tunable band gaps in optoelectronic devices. Here, we report the synthesis of optically uniform and scalable single-layer Mo_{1- x}W _xS₂ alloys by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The amount of W content ( x) in the Mo_{1- x}W _xS₂ alloy is systemically controlled by the co-sputtering technique. The post-laser process allows layer-by-layer thinning of the Mo_{1- x}W _xS₂ alloys down to a single-layer; such a layer exhibits tunable properties with the optical band gap ranging from 1.871 to 1.971 eV with variation in the W content, x = 0 to 1. Moreover, the predominant exciton complexes, trions, are transitioned to neutral excitons with increasing W concentration; this is attributed to the decrease in excessive charge carriers with an increase in the W content of the alloy. Photoluminescence (PL) and Raman mapping analyses suggest that the laser-thinning of the Mo_{1- x}W _xS₂ alloys is a self-limiting process caused by heat dissipation to the substrate, resulting in spatially uniform single-layer Mo_{1- x}W _xS₂ alloy films. Our findings present a promising path for the fabrication of large-scale single-layer 2D TMD alloys and the design of versatile optoelectronic devices.

Photo-induced excitonic structure renormalization and broadband absorption in monolayer tungsten disulphide

Atomically thin transition metal dichalcogenides (TMDCs) have emerged as a new class of two-dimensional (2D) material for novel optoelectronic applications. In particular, 2D TMDCs are viewed as intriguing and appealing materials to construct Q-switching and mode-locked modulators, due to their broadband saturable absorption even of photon energy below their excitonic energies. However, the dynamics and mechanism of saturable absorption inside TMDCs has yet to be investigated. In this paper, the relaxation dynamics of monolayer tungsten disulphide (WS₂) was investigated considering different excitonic transitions. WS₂ illustrates dramatic changes in optical responses when excited by intense laser pulses, which are characterized by the broadband photo-induced nonresonance absorption and the giant excitonic bands renormalization process. The experimental results show that strong photo-induced restructuring of excitonic bands has picosecond lifetime and full recovery of optical responses takes hundreds of picosecond. Additionally, our observations reveal that heavy renormalization and overlap of excitonic bands are induced by strong many-body Coulomb interactions. Moreover, the broadband absorption feature of WS₂ opens up new applications in broadband saturable absorbers and ultrafast photonic devices.

Synergetic photoluminescence enhancement of monolayer MoS2via surface plasmon resonance and defect repair

The weak light-absorption and low quantum yield (QY) in monolayer MoS₂ are great challenges for the applications of this material in practical optoelectronic devices. Here, we report on a synergistic strategy to obtain highly enhanced photoluminescence (PL) of monolayer MoS₂ by simultaneously improving the intensity of the electromagnetic field around MoS₂ and the QY of MoS₂. Self-assembled sub-monolayer Au nanoparticles underneath the monolayer MoS₂ and bis(trifluoromethane)sulfonimide (TFSI) treatment to the MoS₂ surface are used to boost the excitation field and the QY, respectively. An enhancement factor of the PL intensity as high as 280 is achieved. The enhancement mechanisms are analyzed by inspecting the contribution of the PL spectra from A excitons and A⁻ trions under different conditions. Our study takes a further step to developing high-performance optoelectronic devices based on monolayer MoS₂.

A synergistic strategy is reported to obtain a highly enhanced photoluminescence (PL) of monolayer MoS₂ by simultaneously improving the intensity of the electromagnetic field around MoS₂ and the QY of MoS₂.

Synthesis of uniform single layer WS2 for tunable photoluminescence

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained great interest due to their unique tunable bandgap as a function of the number of layers. Especially, single-layer tungsten disulfides (WS₂) is a direct band gap semiconductor with a gap of 2.1 eV featuring strong photoluminescence and large exciton binding energy. Although synthesis of MoS₂ and their layer dependent properties have been studied rigorously, little attention has been paid to the formation of single-layer WS₂ and its layer dependent properties. Here we report the scalable synthesis of uniform single-layer WS₂ film by a two-step chemical vapor deposition (CVD) method followed by a laser thinning process. The PL intensity increases six-fold, while the PL peak shifts from 1.92 eV to 1.97 eV during the laser thinning from few-layers to single-layer. We find from the analysis of exciton complexes that both a neutral exciton and a trion increases with decreasing WS₂ film thickness; however, the neutral exciton is predominant in single-layer WS₂. The binding energies of trion and biexciton for single-layer WS₂ are experimentally characterized at 35 meV and 60 meV, respectively. The tunable optical properties by precise control of WS₂ layers could empower a great deal of flexibility in designing atomically thin optoelectronic devices.

Strain-Modulated Electronic Structure and Infrared Light Adsorption in Palladium Diselenide Monolayer

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) exhibit intriguing properties for both fundamental research and potential application in fields ranging from electronic devices to catalysis. Based on first-principles calculations, we proposed a stable form of palladium diselenide (PdSe₂) monolayer that can be synthesized by selenizing Pd(111) surface. It has a moderate band gap of about 1.10 eV, a small in-plane stiffness, and electron mobility larger than that of monolayer black phosphorus by more than one order. Additionally, tensile strain can modulate the band gap of PdSe₂ monolayer and consequently enhance the infrared light adsorption ability. These interesting properties are quite promising for application in electronic and optoelectronic devices.