Electro‐Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling

Abstract

Ultrafast interlayer charge transfer is one of the most distinct features of van der Waals (vdW) heterostructures. Its dynamics competes with carrier thermalization such that the energy of nonthermalized photocarriers may be harnessed by band engineering. In this study, nonthermalized photocarrier energy is harnessed to achieve near‐infrared (NIR) to visible light upconversion in a metal–insulator–semiconductor (MIS) vdW heterostructure tunnel diode consisting of few‐layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS2). Photoexcitation of the electrically biased heterostructure with 1.58 eV NIR laser in the linear absorption regime generates emission from the ground exciton state of WS2, which corresponds to upconversion by ≈370 meV. The upconversion is realized by electrically assisted interlayer transfer of nonthermalized photoexcited holes from FLG to WS2, followed by formation and radiative recombination of excitons in WS2. The photocarrier transfer rate can be described by Fowler–Nordheim tunneling mechanism and is electrically tunable by two orders of magnitude by tuning voltage bias applied to the device. This study highlights the prospects for realizing novel electro‐optic upconversion devices by exploiting electrically tunable nonthermalized photocarrier relaxation dynamics in vdW heterostructures.

Thermalization of photoexcited carriers in two-dimensional transition metal dichalcogenides and internal quantum efficiency of van der Waals heterostructures

“…Equation 14is somewhat similar to the transport time formula for a triangular barrier, employed previously [7,64] to describe electron tunneling in the Fowler-Nordheim regime. In our case, we assume a low bias and a thin barrier such that the voltage does not appear in τ tun explicitly, but it may influence 0 .…”

Excitonic Emission in Atomically Thin Electroluminescent Devices

“…a) Reproduced with permission. [ 47 ] Copyright 2020, Wiley‐VCH. b,c) Reproduced with permission.…”

“…[17,31] Furthermore, since interlayer interactions in layered materials are van der Waals force, different 2D layered materials, such as insulating hexagonal boron nitride (h-BN), semimetallic graphene and semiconducting TMDCs (H-phase) and BP can be arbitrarily stacked into heterostructures to form p-n junctions, [32] tunneling junctions [33] or type II heterobilayers [34] that are of fundamental importance for exploration of exciton related physics and electrically driven excitonic emission. [35] So far, multiple types of 2DLEDs, including vertical and lateral p-n junctions, [11,14,32,[36][37][38][39][40][41][42][43] quantum well (QW) structures, [15,33,44,45] metal-semiconductor-insulator (MIS) structures, [46][47][48][49][50][51] and transient-2DLEDs, [52][53][54] have been fabricated. Among these devices, emission wavelength, intensity, quantum efficiency, and luminescent area that are considered as key parameters for LEDs vary according to different structures, emission mechanisms, and active materials (luminescent materials).…”

“…e) EL spectra of the device in (d) under the configuration of electron doping (black) and hole doping (red).The inset shows the spectra in semilog scale. a) Reproduced with permission [47]. Copyright 2020, Wiley-VCH.…”

Polarization‐Resolved Broadband MoS2/Black Phosphorus/MoS2 Optoelectronic Memory with Ultralong Retention Time and Ultrahigh Switching Ratio

“…Of these, NOMs based on 2D semiconductors and their van der Waals (vdWs) heterostructures have elicited tremendous attention due to their high absorption coefficient, tunable bandgap, structural flexibility, as well as significant photo‐generated charge trapping caused by their large surface‐to‐volume ratio. [ 13–19 ]…”