Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures

Abstract

Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.

Van Der Waals Heterostructures between Small Organic Molecules and Layered Substrates

“…MoS2 is composed by one layer of Mo atoms stacked between two sulfide layers and used as channel materials in FETs because of its high on/off ratio, low sub-threshold swing value, and good carrier mobility [21][22][23][24]. G, MoS2, and h-BN are all chemically inert, avoiding bonding strongly when integrating with different dimensional crystals at the interface and leading to 2D-2D and 0D-2D (organic molecules-2D) van der Waals (vdW) heterostructures [25][26][27]. For example, MoS2-G vdW heterostructure shows that close to the Fermi level, no significant charge transfer doping is detected from MoS2 to G [28,29].…”

Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures

“…IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electronhole wave functions resulting in dense IX ensembles that are thermalized to the lattice temperature. Besides IX ensembles in III-V HS [5][6][7][8][9][10][11][12][13][14][15], hetero-bilayers prepared from semiconducting transition metal dichalcogenides (TMDs) exhibit superior potential for studying interacting IX ensembles [3,[16][17][18][19][20][21][22][23][24][25] with intriguing spin-and valley-properties [26][27][28]. At the same time, TMDs are truly two-dimensional (2D) crystals coupled to each other or to substrates by van der Waals forces.…”

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe2

“…12, 13 Electrons and holes in these materials can form excitons with unusually large binding energies of many hundreds of millielectronvolts, 14−19 making these quasiparticles stable even at elevated temperatures and high carrier densities. 20,21 The properties of excitons in 2D TMDCs are a topic of intense research, investigating, for example, rapid exciton−exciton scattering, 22 interlayer excitons, 23 charged excitons and excitonic molecules, 24,25 ultrafast recombination dynamics, 19,26−28 or efficient coupling to light and lattice vibrations. 4,19,29,30 In many experiments, excitons are created indirectly through nonresonant optical excitation or electronic injection, which may prepare unbound charge carriers with energies far above the exciton resonance.…”