Hot exciton transport in $\mathrm{WS}{\mathrm{e}}_{2}$ monolayers

Rapid Communication

Hot exciton transport in $WS e_{2}$ monolayers

Darwin F. Cordovilla Leon, Zidong Li, Sung Woon Jang, and Parag B. Deotare

Phys. Rev. B 100, 241401(R) – Published 2 December 2019

Abstract

We experimentally demonstrate hot exciton transport in hexagonal boron nitride encapsulated $WS e_{2}$ monolayers via spatially and temporally resolved photoluminescence measurements at room temperature. We show that the nonlinear evolution of the mean-squared displacement of the nonresonantly excited hot exciton gas is primarily due to the relaxation of its excess kinetic energy and is characterized by a density-dependent fast expansion that converges to a slower, constant rate expansion. We also observe saturation of the hot exciton gas's expansion rate at high excitation densities due to the balance between Auger-assisted hot exciton generation and the phonon-assisted hot exciton relaxation processes. These measurements provide insight into a process that is ubiquitous in exciton transport measurements where nonresonant optical excitation is typically employed.

Received 31 May 2019
Revised 5 November 2019

DOI:https://doi.org/10.1103/PhysRevB.100.241401

Physics Subject Headings (PhySH)

Carrier dynamics Diffusion Excitons Luminescence Optical transient phenomena

Monolayer films Quantum wells Semiconductor compounds

Photoluminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Darwin F. Cordovilla Leon ^1,*, Zidong Li^2,*, Sung Woon Jang², and Parag B. Deotare^1,2,†

¹Applied Physics Program, University of Michigan-Ann Arbor, 450 Church Street, 1425 Randall Laboratory, Ann Arbor, Michigan 48109, USA
²Electrical and Computer Engineering Department, University of Michigan-Ann Arbor, 1301 Beal Avenue, Ann Arbor, Michigan 48109, USA

^*These authors contributed equally to this work.
^†Corresponding author: pdeotare@umich.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 100, Iss. 24 — 15 December 2019

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Evolution of the MSD of excitons in an h-BN encapsulated $WS e_{2}$ monolayer on a $Si O_{2} / Si$ substrate for increasing excitation densities. The MSD is defined as $〈 Δ σ^{2} (t) 〉 \equiv σ^{2} (t) - σ^{2} (0)$ , where $σ (t)$ represents the standard deviation of the Gaussian distribution of excitons at time $t$ . The solid lines represent the kinetic energy relaxation model fit represented by Eq. (3) and the markers are the experimental data. The temporally and spatially resolved maps which the MSD curves were extracted from are shown in Supplemental Figs. S3–S12. In addition, the results of these fits are shown in Supplemental Figs. S15 and S16.
Reuse & Permissions
Figure 2
(a) Time-resolved photoluminescence (TRPL) of an h-BN encapsulated $WS e_{2}$ monolayer transferred onto a $Si O_{2} / Si$ substrate for increasing density of excitons created per optical pulse. The fits were obtained assuming 11.5% absorption of the optical excitation at 405 nm [58] and accounting for the optical losses in our measurement setup. The inset shows the PL's initial decay emphasizing the lack of excitation density-induced decay saturation. (b) Auger constant estimated by fitting the data in (a) with Eq. (2). The $y$ axis represents the product of the initial exciton density denoted by $n (0)$ and the Auger constant $γ_{A}$ . (c) Integrated PL of the h-BN encapsulated $WS e_{2}$ monolayer for increasing density of excitons created per optical pulse. The Auger constant was estimated by fitting the PL intensity with the relation $I_{PL} \propto ln [1 + n (0) γ_{A} τ] {(γ_{A} τ)}^{- 1}$ [3]. The solid lines indicate the fits, and the markers the experimental data.
Reuse & Permissions
Figure 3
Schematic diagram illustrating (a) the timescales of exciton formation, hot and cold exciton transport regimes, as well as the evolution of the temperature of the exciton gas, and (b) exciton dispersion relation showing the exciton formation and relaxation processes. The saturation refers to the rate balance between exciton-exciton Auger scattering and exciton-phonon scattering.
Reuse & Permissions
Figure 4
Excess diffusivity of excitons in an h-BN encapsulated $WS e_{2}$ monolayer created by two lasers with different photon energies and increasing excitation densities obtained by fitting the MSD with the model $〈 Δ σ^{2} (t) 〉 = 2 t D (t) = 2 t [D_{0} + D^{*} e^{- t / τ}]$ , where the steady state diffusivity was assumed to be $D_{0} = μ_{q} k_{B} T_{0} / q$ , with $T_{0}$ set to the lattice temperature at 300 K. The saturation of the excess diffusivity and corresponding excess temperature of the exciton gas occurs at a higher excitation density for the optical excitation with lower photon energy (2.4 eV) than for the optical excitation with higher photon energy (3.1 eV). The data correspond to a sample that is different from that shown in Fig. 1. Further details of these fits can be found in Supplemental Figs. S17–S20.
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics

Hot exciton transport in $WS e_{2}$ monolayers

Darwin F. Cordovilla Leon, Zidong Li, Sung Woon Jang, and Parag B. Deotare

Phys. Rev. B 100, 241401(R) – Published 2 December 2019

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

Supplemental Material (Subscription Required)

References (Subscription Required)

Issue

Access Options

Physical Review B

covering condensed matter and materials physics

Hot exciton transport in WSe2 monolayers

Darwin F. Cordovilla Leon, Zidong Li, Sung Woon Jang, and Parag B. Deotare

Phys. Rev. B 100, 241401(R) – Published 2 December 2019

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

Supplemental Material (Subscription Required)

References (Subscription Required)

Issue

Access Options

Authorization Required

Other Options

Download & Share

Images

Figure 1

Figure 2

Figure 3

Figure 4

Log In

Search

Article Lookup

Paste a citation or DOI

Enter a citation

Hot exciton transport in $WS e_{2}$ monolayers