Defects in conventional semiconductors substantially lower the photoluminescence (PL)quantum yield (QY), a key metric of optoelectronic performance that directly dictates the maximum device efficiency. Two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as monolayer MoS2, often exhibit low PL QY for as-processed samples, which has typically been attributed to a large native defect density. We show that the PL QY of as-processed MoS2 and WS2 monolayers reaches near-unity when they are made intrinsic by electrostatic doping, without any chemical passivation. Surprisingly, neutral exciton recombination is entirely radiative even in the presence of a high native defect density. This finding enables TMDC monolayers for optoelectronic device applications as the stringent requirement of low defect density is eased.Multiparticle Coulomb interactions are particularly pronounced in transition metal dichalcogenide (TMDC) monolayers, leading to a multitude of recombination pathways, each associated with the different quasiparticles produced by these interactions (1). The recombination rate of excitons formed by photogenerated carriers (2, 3), depends nonlinearly on the concentration. Because excitons interact with background charge to form trions (4-8), the Fermi level also controls the dominant recombination pathway. Thus, both the background carrier concentration and the generation rate must be tuned to investigate the complete effect of multiparticle interactions on TMDC photoluminescence (PL) quantum yield (QY).In this work, we simultaneously altered the photocarrier generation rate (G) by varying the incident pump power, and the total charge concentration (electron and hole population densities; N and P) by varying the back-gate voltage (Vg) in a capacitor structure (Fig. 1A). Surprisingly, we found that all neutral excitons recombine radiatively in as-processed monolayers of MoS2, resulting in near-unity QY at low generation rates. This high QY occurred despite a reported high

Room-temperature optoelectronic devices that operate at shortwave and midwave infrared wavelengths (1-8 μm) can be used for numerous applications 1-5 . To achieve the operating wavelength range needed for a given application, a combination of materials with different bandgaps (e.g. superlattice/heterostructure) 6,7 or the variation of semiconductor alloy composition during growth 8,9 is used; however, these approaches involve fabrication complexity and the operating range is fixed post-fabrication. Although wide-range, active, and reversible tunability of the operating wavelengths in optoelectronic devices after fabrication is a highly desirable feature, no such platform has been yet developed. Here, we demonstrate high-performance room-temperature infrared optoelectronics with actively variable spectra by presenting black phosphorus (bP) as an ideal candidate. Enabled by the * � � 𝐸𝐸 𝑔𝑔 𝑘𝑘 𝐵𝐵 𝑇𝑇 �� (2)where 𝑚𝑚 𝑒𝑒 * and 𝑚𝑚 ℎ * are the effective masses of electrons and holes, respectively, 𝑘𝑘 𝐵𝐵 is Boltzmann's constant, and 𝑇𝑇 is temperature 36,37 . Since 𝑚𝑚 𝑒𝑒 * and 𝑚𝑚 ℎ * in bP have similar values, the effective mass ratio (𝑚𝑚 𝑒𝑒 * /𝑚𝑚 ℎ * ) is much higher than that of other small bandgap semiconductors.According to equation (2), this results in suppressed Auger recombination (longer Auger lifetime), which leads to bP's theoretical QY limit being much higher than that of other small bandgap semiconductors in the high injection regime.

Most optoelectronic devices operate at high photocarrier densities, where all semiconductors suffer from enhanced nonradiative recombination. Nonradiative processes proportionately reduce photoluminescence (PL) quantum yield (QY), a performance metric that directly dictates the maximum device efficiency. Although transition metal dichalcogenide (TMDC) monolayers exhibit near-unity PL QY at low exciton densities, nonradiative exciton-exciton annihilation (EEA) enhanced by van-Hove singularity (VHS) rapidly degrades their PL QY at high exciton densities and limits their utility in practical applications. Here, by applying small mechanical strain (less than 1%), we circumvented VHS resonance and markedly suppressed EEA in monolayer TMDCs, resulting in near-unity PL QY at all exciton densities despite the presence of a high native defect density. Our findings can enable light-emitting devices that retain high efficiency at all brightness levels.

This method is expected to greatly facilitate the presently available wearable gadgets in HR computation during various physical activities.

Monolayer transition metal dichalcogenides (TMDCs) are promising materials for next generation optoelectronic devices. The exciton diffusion length is a critical parameter that reflects the quality of exciton transport in monolayer TMDCs and limits the performance of many excitonic devices. Although diffusion lengths of a few hundred nanometers have been reported in the literature for as-exfoliated monolayers, these measurements are convoluted by neutral and charged excitons (trions) that coexist at room temperature due to natural background doping. Untangling the diffusion of neutral excitons and trions is paramount to understand the fundamental limits and potential of new optoelectronic device architectures made possible using TMDCs. In this work, we measure the diffusion lengths of neutral excitons and trions in monolayer MoS2 by tuning the background carrier concentration using a gate voltage and utilizing both steady state and transient spectroscopy. We observe diffusion lengths of 1.5 μm and 300 nm for neutral excitons and trions, respectively, at an optical power density of 0.6 W cm–2 .

Be it for essential everyday applications such as bright light-emitting devices or to achieve Bose−Einstein condensation, materials in which high densities of excitons recombine radiatively are crucially important. However, in all excitonic materials, exciton−exciton annihilation (EEA) becomes the dominant loss mechanism at high densities. Typically, a macroscopic parameter named EEA coefficient (C EEA ) is used to compare EEA rates between materials at the same density; higher C EEA implies higher EEA rate. Here, we find that the reported values of C EEA for 140 different materials is inversely related to the single-exciton lifetime. Since during EEA one exciton must relax to ground state, C EEA is proportional to the single-exciton recombination rate. This leads to the counterintuitive observation that the exciton density at which EEA starts to dominate is higher in a material with larger C EEA . These results broaden our understanding of EEA across different material systems and provide a vantage point for future excitonic materials and devices.

No abstract

We propose a structure that can be used for enhanced single molecule detection using surface plasmon coupled emission (SPCE). In the proposed structure, instead of a single metal layer on the glass prism of a typical SPCE structure for fluorescence microscopy, a metal-dielectric-metal structure is used. We theoretically show that the proposed structure significantly decreases the excitation volume of the fluorescently labeled sample, and simultaneously increases the peak SPCE intensity and SPCE power. Therefore, the signal-to-noise ratio and sensitivity of an SPCE based fluorescence microscopy system can be significantly increased using the proposed structure, which will be helpful for enhanced single molecule detection, especially, in a less pure biological sample.