Confinement of long-lived interlayer excitons in WS2/WSe2 heterostructures (original) (raw)
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Nanomaterials
Two-dimensional transition metal dichalcogenides (2D-TMDs) hold a great potential to platform future flexible optoelectronics. The beating hearts of these materials are their excitons known as XA and XB, which arise from transitions between spin-orbit split (SOS) levels in the conduction and valence bands at the K-point. The functionality of 2D-TMD-based devices is determined by the dynamics of these excitons. One of the most consequential channels of exciton decay on the device functionality is the defect-assisted recombination (DAR). Here, we employ steady-state absorption and emission spectroscopies, and pump density-dependent femtosecond transient absorption spectroscopy to report on the effect of DAR on the lifetime of excitons in monolayers of tungsten disulfide (2D-WS2) and diselenide (2D-WSe2). These pump-probe measurements suggested that while exciton decay dynamics in both monolayers are driven by DAR, in 2D-WS2, defect states near the XB exciton fill up before those near ...
ACS Applied Electronic Materials, 2021
Two dimensional (2D) van der Waals heterostructures from transition metal dichalcogenide (TMDC) semiconductors show a new class of spatially separate excitons with extraordinary properties. The interlayer excitons (XI) have been studied extensively, yet the mechanisms that modulate XI are still not well understood. Here, we introduce several organic-layer-embedded hybrid heterostructures, MoS2/organic/WSe2, to study the binding energy of XI. We discover that the dielectric screening of the quasi-particle is reduced with organic molecules due to decreased dielectric constant and greater separation distance between the TMDC layers. As a result, a distinct blueshift is observed in interlayer emission. We also find that the band alignment at the heterointerface is critical. When the organic layer provides a staggered energy state, interlayer charge transfer can transition from tunneling to band-assisted transfer, further increasing XI emission energies due to a stronger dipolar interaction. The formation of XI may also be significantly suppressed with electron or hole trapping molecules. These findings should be useful in realizing XI-based optoelectronics.
Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures
Nature communications, 2016
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 transfe...
Infrared Interlayer Exciton Emission in MoS2/WSe2 Heterostructures
Physical Review Letters, 2019
We report light emission around 1 eV (1240 nm) from heterostructures of MoS 2 and WSe 2 transition metal dichalcogenide monolayers. We identify its origin in an interlayer exciton (ILX) by its wide spectral tunability under an out-of-plane electric field. From the static dipole moment of the state, its temperature and twist-angle dependence, and comparison with electronic structure calculations, we assign this ILX to the fundamental interlayer transition between the K valleys in this system. Our findings gain access to the interlayer physics of the intrinsically incommensurate MoS 2 =WSe 2 heterostructure, including moiré and valley pseudospin effects, and its integration with silicon photonics and optical fiber communication systems operating at wavelengths longer than 1150 nm.
ACS Photonics, 2020
In 1H monolayer transition metal dichalcogenide, the inversion symmetry is broken, while the reflection symmetry is maintained. On the contrary, in the bilayer, the inversion symmetry is restored, but the reflection symmetry is broken. As a consequence of these contrasting symmetries, here we show that bilayer WS 2 exhibits a quantum confined Stark effect (QCSE) that is linear with the applied out-of-plane electric field, in contrary to a quadratic one for a monolayer. The interplay between the unique layer degree of freedom in the bilayer and the field driven partial 1 arXiv:2011.06790v1 [cond-mat.mes-hall] 13 Nov 2020 inter-conversion between intra-layer and inter-layer excitons generates a giant tunability of the exciton oscillator strength. This makes bilayer WS 2 a promising candidate for an atomically thin, tunable electro-absorption modulator at the exciton resonance, particularly when stacked on top of a graphene layer that provides an ultra-fast non-radiative relaxation channel. By tweaking the biasing configuration, we further show that the excitonic response can be largely tuned through electrostatic doping, by efficiently transferring the oscillator strength from neutral to charged exciton. The findings are prospective towards highly tunable, atomically thin, compact and light, on chip, reconfigurable components for next generation optoelectronics.
The trilayer exciton emission in WSe2/WS2/MoS2 van der Waals heterostructures
Applied Physics Letters
In this Letter, we present the experimental observation of trilayer exciton emission in the WSe2/WS2/MoS2 heterostructures by photoluminescence spectroscopy at a low temperature of ∼10 K. It is found that the trilayer exciton has a lower exciton binding energy than that in the WSe2/MoS2 heterobilayer due to the insertion of monolayer WS2, which is manifested by the blueshift of this trilayer exciton peak relative to the bilayer exciton. Through the measurements of the exciton peak position as a function of excitation power, the trilayer excitons exhibited the density-dependent repulsive interaction among them, and the reduction of binding energy ∼8 meV is deduced. Moreover, the trilayer exciton has a longer lifetime than the bilayer exciton, resulting from the reduction of electron–hole wavefunction overlap.
Excitons and trions in WSSe monolayers
arXiv (Cornell University), 2022
The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited on SiO2/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the "parent" WS2 and WSe2 MLs, we assign the WSSe MLs to the ML family with the dark ground exciton state. We find that, in addition to the neutral bright A exciton line, three observed emission lines are associated with negatively charged excitons. The application of in-plane and out-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin-and momentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbidden complex. Furthermore, the existence of the single photon emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to enlarge the wavelength range for potential future quantum applications of S-TMDs.
2021
Qianhui Shi, En-Min Shih, Daniel Rhodes, Bumho Kim, Katayun Barmak, Kenji Watanabe, Takashi Taniguchi, Zlatko Papić, Dmitry A. Abanin, James Hone, and Cory R. Dean Department of Physics, Columbia University, New York, NY, USA Department of Mechanical Engineering, Columbia University, New York, NY, USA Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK Department of Theoretical Physics, University of Geneva, 24 quai Ernest-Ansermet, 1211 Geneva, Switzerland (Dated: August 25, 2021)
Ultrafast Exciton and Trion Dynamics in High‐Quality Encapsulated MoS 2 Monolayers
physica status solidi (b)
Monolayers of transition metal dichalcogenides (1L-TMDs) are promising semiconductors with unique electrical and optical properties. The quantum confinement experienced by electrons and holes in the 2D structure and the reduced Coulomb screening lead to the appearance of direct bandgap excitonic transitions showing high binding energies (up to 0.5 eV) and very large oscillator strengths up to room temperature. [1-3] Moreover, the breaking of spatial inversion symmetry in the 2D lattice and the large spin-orbit coupling generate spin-valley locked excitons at the K and K 0 valleys, optically addressable by circularly polarized light. [4] The strong Coulomb interactions in atomically thin TMDs also enhance the stability of manybody complexes resulting from the correlations of excitons with charge carriers. In this case, TMD excitons are dressed by a Fermi sea of free charges generated in the material by natural or artificial doping, forming three-particle bound states (trions) in the presence of low doping levels, or manifesting emerging many-body phenomena at elevated doping regimes. [5-7] 1Ls TMDs also offer the tremendous advantage of being stackable to form van der Waals heterostructures (HSs), with atomically perfect interfaces without any lattice mismatch limitation. [8] This allows to sandwich the 1Ls between few-layers of transparent high bandgap hexagonal boron nitride (hBN), ensuring a good protection from external contaminants and dielectric insulation. hBN encapsulation has proven to be a key requirement for obtaining good optical quality from small flakes of mechanically exfoliated TMD 1Ls. [9,10] Cryogenic temperatures and encapsulation in hBN narrow the exciton lines, [11] reaching the homogeneous linewidth regime, in which the dephasing rate is dominated by radiative recombination. [12] The high quality of encapsulated TMDs reveals a Rydberg series of excitonic states below the free particle bandgap, which has been previously shown to be slightly nonhydrogenic. [13] In addition, the encapsulation of TMD bilayers recently allowed to unveil hybridized intra-inter layer excitonic species, [14,15] showing new types of many-body interactions. [16] Femtosecond pump-probe spectroscopy [17-19] is a powerful technique to measure the temporal dynamics of excitons in 1L