Trion Species-Resolved Quantum Beats in MoSe2

Monolayer photonic materials offer a tremendous potential for on-chip optoelectronic devices. Their realization requires knowledge of optical coherence properties of excitons and trions that have so far been limited to nonlinear optical experiments carried out with strongly inhomogenously broadened material. Here we employ h-BN encapsulated and electrically gated MoSe2 to reveal coherence properties of trion-species directly in the linear optical response. Autocorrelation measurements reveal long dephasing times up to T2=1.16+-0.05 ps for positively charged excitons. Gate dependent measurements provide evidence that the positively-charged trion forms via spatially localized hole states making this trion less prone to dephasing in the presence of elevated hole carrier concentrations. Quantum beat signatures demonstrate coherent coupling between excitons and trions that have a dephasing time up to 0.6 ps, a two-fold increase over those in previous reports. A key merit of the prolonged exciton/trion coherences is that they were achieved in a linear optical experiment, and thus are directly relevant to applications in nanolasers, coherent control, and on-chip quantum information processing requiring long photon coherence.Comment: 21 pages, 6 figures, 2 SOI figure

Linearly Polarized Excitons in Single- and Few-Layer ReS₂ Crystals

Rhenium disulfide (ReS₂), a layered group VII transition metal dichalcogenide, has been studied by optical spectroscopy. We demonstrate that the reduced crystal symmetry, as compared to the molybdenum and tungsten dichalcogenides, leads to anisotropic optical properties that persist from the bulk down to the monolayer limit. We find that the direct optical gap blueshifts from 1.47 eV in the bulk to 1.61 eV in the monolayer limit. In the ultrathin limit, we observe polarization-dependent absorption and polarized emission from the band-edge optical transitions. We thus establish ultrathin ReS₂ as a birefringent material with strongly polarized direct optical transitions that vary in energy and orientation with sample thickness

In-Plane Anisotropy in Mono- and Few-Layer ReS₂ Probed by Raman Spectroscopy and Scanning Transmission Electron Microscopy

Rhenium disulfide (ReS₂) is a semiconducting layered transition metal dichalcogenide that exhibits a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various material properties. Here, we demonstrate the strong anisotropy in the Raman scattering response for linearly polarized excitation. Polarized Raman scattering is shown to permit a determination of the crystallographic orientation of ReS₂ through comparison with direct structural analysis by scanning transmission electron microscopy (STEM). Analysis of the frequency difference of appropriate Raman modes is also shown to provide a means of precisely determining layer thickness up to four layers

Measurement of Lateral and Interfacial Thermal Conductivity of Single- and Bilayer MoS₂ and MoSe₂ Using Refined Optothermal Raman Technique

Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides (TMDCs) have attracted extensive interest in recent years, motivating investigation into multiple properties. In this work, we demonstrate a refined version of the optothermal Raman technique, to measure the thermal transport properties of two TMDC materials, MoS₂ and MoSe₂, in single-layer (1L) and bilayer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ∼40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different radial positions. For 1L MoS₂ and MoSe₂, the room-temperature thermal conductivities are 84 ± 17 and 59 ± 18 W/(m·K), respectively. For 2L MoS₂ and MoSe₂, we obtain values of 77 ± 25 W and 42 ± 13 W/(m·K). Crucially, the interfacial thermal conductance is found to be of order 0.1–1 MW/m² K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices

Analysis of bundle stability in the face of spatial differences in positive feedback (α_f), using an ODE model composed of 2 FP compartments.

<p>One FP fractional compartment (FP₂) is subject to a stimulus that increases actin polymerization by Δα_f, whereas the remaining fraction (FP₁) is subject to nominal polymerization conditions, α_f. This stimulus may either be sustained (shown in A) or transient (shown in B-G). <b>(A)</b> A 3-D plot showing steady state bundles on the vertical axis as a function of Δα_f/α_f and FP₂ (log scale). For this sustained enhancement, the bundle intensity in FP₁ (blue mesh) and FP₂ (red mesh) depend on the intensity Δα_f and the relative proportions of FP₂ to FP₁ (FP₁ + FP₂ = 1). As Δα_f increases, FPs with stronger feedback form stronger bundles. If the fraction of FPs with enhanced feedback (FP₂, red mesh) is small, the FPs with normal α_f (FP₁, blue mesh) are unperturbed, while the bundles in FP₂ are strengthened. Because there is a fixed total amount of actin, stronger bundling in FP₂ drains actin available for bundles in FP₁ until a threshold is reached at which collapse of actin bundles in FP₁ is observed. (<b>B)</b> Time course of a transient stimulus applied at t = 40. In C-E, the value of Δα_f follows this time course, with varying intensities; equal volume fractions for FP₁ and FP₂ were used, with red curves corresponding to FP₂ and blue to FP₁. (<b>C</b>) A small perturbation allows the system to return to the pre-stimulus steady state. (<b>D</b>) When the maximum Δα_f/α_f = 1 (i.e. FP₂ transiently reaches twice that of FP₁), a new stable steady state is generated where bundles have collapsed in FP₁ and increased in FP₂. (<b>E</b>) When the maximum Δα_f/α_f = 2 (i.e. FP₂ transiently reaches 3 times that of FP₁) there is a transient increase in FP₂ bundles followed by collapse and enhanced bundles in FP₁. The behavior displayed in C, D and E is the hallmark of a tristable system. See the text for an explanation. (<b>F</b>). The steady state values for concentration of bundles in fractions FP₁ (blue) and FP₂ (red) are shown as a function of stimulus intensity, consistent with C-E. (<b>G</b>) Different fractions of FP₁ and FP₂ will impact the steady state values (see also <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005433#pcbi.1005433.s008" target="_blank">S6 Fig</a>).</p

Spatial simulations of the response to perturbed bundling activity, β_b, highlighting potential compensatory mechanisms for actin instability.

<p><b>A-C</b>. Progressive loss of FPs due to a continued decrease of β_b imposed at time t = 0; snapshots at time (<b>A</b>) t = 0, <b>(B)</b> t = 500, and <b>(C)</b> t = 1500. <b>D-J.</b> Tests of combinations of transient perturbation in β_b and α_f to explore compensatory mechanisms. <b>(D)</b> Return to baseline β_b at time t₁ = 500 results in <b>(E)</b> recovery of the majority of the remaining foot process bundle concentrations at t₂ = 1500. <b>(F)</b> Decrease of α_b at time t₁ while holding β_b constant results in <b>(G)</b> similar stabilization. Finally, <b>(I)</b> increase of α_f while holding β_b constant <b>(J)</b> produces similar spatial results. All three interventions prevent progressive effacement (compare C with E, G and J). <b>(H)</b> Timecourses for spatial average of bundle concentration in the FPs identified by arrows in snapshots E, G and J (at time 1500, gray arrowhead). Linestyle follows the same pattern as arrows. The same color scale is used for all the 3-D snapshots of bundle concentrations. Parametric perturbations are listed in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005433#pcbi.1005433.s002" target="_blank">S1 Table</a>.</p

Spatial simulation showing how a transient, localized increase in positive feedback α_f in the region labeled FP₂ impacts actin bundle concentrations in neighboring foot processes.

<p><b>(A)</b> Diagram of a region of a podocyte at steady state. A transient jump in α_f, as in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005433#pcbi.1005433.s009" target="_blank">S7 Fig</a>, was locally applied to the region labeled FP₂ and the state of actin throughout the podocyte was simulated over time. The inset shows bundle concentrations over time in four FPs, two within the FP₂ segment and two outside it, identified by colored arrowheads and lines. <b>(B)</b> Zoomed snapshot of the highlighted region is at time = 200 and <b>(C)</b> time = 1000.</p

Impact of hyperactive bundling, α_b, on FP actin stability.

<p>Spatially, the cyclic behavior (triggered by sudden, but spatially uniform, increase in α_b at t = 40) gives rise to asynchronous and progressive loss of actin bundles within FPs (A-D). <b>(A)</b> Spatial steady state actin bundle concentration; simulation parameters are same as those highlighted in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005433#pcbi.1005433.g002" target="_blank">Fig 2C</a>. <b>(B)</b> Snapshots of bundle concentration in response to increased bundling, at time t = 1200 and <b>(C)</b> time t = 2400. Insets correspond to magnified and slightly rotated view of respective boxes. The colorbar represents bundle concentration in normalized arbitrary units. <b>(D)</b> Timecourse of bundle concentration at randomly picked FPs, demonstrating asynchronous collapse of bundles. <b>(E)</b> ODE solution for increase in α_b predicts cytoskeleton collapse when the actin pool is reduced to 70% of that in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005433#pcbi.1005433.g002" target="_blank">Fig 2</a>, <b>(F)</b> For systems with larger pools of actin (115%), stronger yet still unstable bundles are predicted. In spatial simulations the positive feedback, α_f, is localized to FPs only, and zero elsewhere.</p

Image segmentation, volume reconstruction, and quantitative analysis of podocyte morphology.

<p><b>(A)</b> In order to reconstruct the complete podocyte volume including all the foot processes, we manually segmented stacks of SBEM images. This was done by reviewing the entire stack and identifying all of the projections that emanate from the cell body. <b>(B)</b> At full resolution, geometric details of individual foot processes can be seen. <b>(C)</b> We then thresholded the segmented images to obtain continuous binary stacks that can be extruded in Rhinoceros and combined in Virtual Cell to form <b>(D)</b> a reconstructed 3-D volume. <b>(E)</b> We quantified the volume and surface area share of foot processes (FPs) by imposing a Gaussian surface filter in Seg3D, which removed all surface projections smaller than 440 nm. <b>(F)</b> We used a commonly used heat transfer model to identify the cell body of the cells: a cytoplasmic specie was uniformly synthesized and allowed to diffuse into the membrane until steady state. Regions with low surface area-to-volume ratios, i.e., cell body, maintain ~90% of the maximum value. <b>(G)</b> These sections were assigned as the cell body. <b>(H)</b> From the reconstructed volumes, length and angles for major processes and branches were measured using Imaris; the lime colored rendered volume represents the cell volume whereas colored internal lines are the measured paths for the branches. <b>(I)</b> For clarity, the internal lines are shown without the rendered volume. Sample branching patterns for two of the cells are shown in supplementary <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005433#pcbi.1005433.s001" target="_blank">S1 Fig</a>. <b>(J)</b> Using the volume, surface area, and branching information, a representative geometry is constructed. For computational simplicity, we assumed symmetry about the xy- and yz-planes, and hereby only half of these are shown. Rat podocytes (RP) used in this figure are RP1, RP8, RP9, RP11, and RP13, respectively, and morphological characteristics of these cells are shown in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005433#pcbi.1005433.t001" target="_blank">Table 1</a>.</p

Modulation of Quantum Tunneling <i>via</i> a Vertical Two-Dimensional Black Phosphorus and Molybdenum Disulfide p–n Junction

Diverse diode characteristics were observed in two-dimensional (2D) black phosphorus (BP) and molybdenum disulfide (MoS₂) heterojunctions. The characteristics of a backward rectifying diode, a Zener diode, and a forward rectifying diode were obtained from the heterojunction through thickness modulation of the BP flake or back gate modulation. Moreover, a tunnel diode with a precursor to negative differential resistance can be realized by applying dual gating with a solid polymer electrolyte layer as a top gate dielectric material. Interestingly, a steep subthreshold swing of 55 mV/dec was achieved in a top-gated 2D BP–MoS₂ junction. Our simple device architecture and chemical doping-free processing guaranteed the device quality. This work helps us understand the fundamentals of tunneling in 2D semiconductor heterostructures and shows great potential in future applications in integrated low-power circuits