12 research outputs found
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 ReS<sub>2</sub> Crystals
Rhenium disulfide (ReS<sub>2</sub>), 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 ReS<sub>2</sub> 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 ReS<sub>2</sub> Probed by Raman Spectroscopy and Scanning Transmission Electron Microscopy
Rhenium
disulfide (ReS<sub>2</sub>) 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 ReS<sub>2</sub> 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 MoS<sub>2</sub> and MoSe<sub>2</sub> 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, MoS<sub>2</sub> and MoSe<sub>2</sub>, 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 MoS<sub>2</sub> and MoSe<sub>2</sub>, the
room-temperature thermal conductivities are 84 ± 17 and 59 ±
18 W/(m·K), respectively. For 2L MoS<sub>2</sub> and MoSe<sub>2</sub>, 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/m<sup>2</sup> 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<sub>f</sub>), using an ODE model composed of 2 FP compartments.
<p>One FP fractional compartment (FP<sub>2</sub>) is subject to a stimulus that increases actin polymerization by Δα<sub>f</sub>, whereas the remaining fraction (FP<sub>1</sub>) is subject to nominal polymerization conditions, α<sub>f</sub>. 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<sub>f</sub>/α<sub>f</sub> and FP<sub>2</sub> (log scale). For this sustained enhancement, the bundle intensity in FP<sub>1</sub> (blue mesh) and FP<sub>2</sub> (red mesh) depend on the intensity Δα<sub>f</sub> and the relative proportions of FP<sub>2</sub> to FP<sub>1</sub> (FP<sub>1</sub> + FP<sub>2</sub> = 1). As Δα<sub>f</sub> increases, FPs with stronger feedback form stronger bundles. If the fraction of FPs with enhanced feedback (FP<sub>2</sub>, red mesh) is small, the FPs with normal α<sub>f</sub> (FP<sub>1</sub>, blue mesh) are unperturbed, while the bundles in FP<sub>2</sub> are strengthened. Because there is a fixed total amount of actin, stronger bundling in FP<sub>2</sub> drains actin available for bundles in FP<sub>1</sub> until a threshold is reached at which collapse of actin bundles in FP<sub>1</sub> is observed. (<b>B)</b> Time course of a transient stimulus applied at t = 40. In C-E, the value of Δα<sub>f</sub> follows this time course, with varying intensities; equal volume fractions for FP<sub>1</sub> and FP<sub>2</sub> were used, with red curves corresponding to FP<sub>2</sub> and blue to FP<sub>1</sub>. (<b>C</b>) A small perturbation allows the system to return to the pre-stimulus steady state. (<b>D</b>) When the maximum Δα<sub>f</sub>/α<sub>f</sub> = 1 (i.e. FP<sub>2</sub> transiently reaches twice that of FP<sub>1</sub>), a new stable steady state is generated where bundles have collapsed in FP<sub>1</sub> and increased in FP<sub>2</sub>. (<b>E</b>) When the maximum Δα<sub>f</sub>/α<sub>f</sub> = 2 (i.e. FP<sub>2</sub> transiently reaches 3 times that of FP<sub>1</sub>) there is a transient increase in FP<sub>2</sub> bundles followed by collapse and enhanced bundles in FP<sub>1</sub>. 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 FP<sub>1</sub> (blue) and FP<sub>2</sub> (red) are shown as a function of stimulus intensity, consistent with C-E. (<b>G</b>) Different fractions of FP<sub>1</sub> and FP<sub>2</sub> 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<sub>b</sub>, highlighting potential compensatory mechanisms for actin instability.
<p><b>A-C</b>. Progressive loss of FPs due to a continued decrease of β<sub>b</sub> 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<sub>b</sub> and α<sub>f</sub> to explore compensatory mechanisms. <b>(D)</b> Return to baseline β<sub>b</sub> at time t<sub>1</sub> = 500 results in <b>(E)</b> recovery of the majority of the remaining foot process bundle concentrations at t<sub>2</sub> = 1500. <b>(F)</b> Decrease of α<sub>b</sub> at time t<sub>1</sub> while holding β<sub>b</sub> constant results in <b>(G)</b> similar stabilization. Finally, <b>(I)</b> increase of α<sub>f</sub> while holding β<sub>b</sub> 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<sub>f</sub> in the region labeled FP<sub>2</sub> 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<sub>f</sub>, 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 FP<sub>2</sub> 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 FP<sub>2</sub> 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<sub>b</sub>, on FP actin stability.
<p>Spatially, the cyclic behavior (triggered by sudden, but spatially uniform, increase in α<sub>b</sub> 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<sub>b</sub> 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<sub>f</sub>, 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 (MoS<sub>2</sub>) 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–MoS<sub>2</sub> 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