Investigating the remarkable excitonic effects in two-dimensional (2-D) semiconductors and controlling their exciton binding energies can unlock the full potential of 2-D materials for future applications in photonic and optoelectronic devices. In a recent study, Zhizhan Qiu and colleagues at the interdisciplinary departments of chemistry, engineering, advanced 2-D materials, physics and materials science in Singapore, Japan and the U.S. demonstrated large excitonic effects and gate-tunable exciton binding energies in single-layer rhenium diselenide (ReSe2) on a back-gated graphene device. They used scanning tunneling spectroscopy (STS) and differential reflectance spectroscopy to measure the quasiparticle (QP) electronic and optical bandgap (Eopt) of single-layer ReSe2 to yield a large exciton binding energy of 520 meV.
The scientists achieved continuous tuning of the electronic bandgap and exciton binding energy of monolayer ReSe2 by hundreds of milli-electron volts via electrostatic gating. Qiu et al. credited the phenomenon to tunable Coulomb interactions arising from the gate-controlled free carriers in graphene. The new findings are now published on Science Advances and will open a new avenue to control bandgap renormalization and exciton binding energies in 2-D semiconductors for a variety of technical applications.
Atomically thin two-dimensional (2-D) semiconductors usually display large bandgap renormalization (shifts in physical qualities) and extraordinary excitonic effects due to quantum confinement and reduced dielectric screening. Light-matter interactions in these systems are governed by enhanced excitonic effects, which physicists have studied to develop exciton-based devices at room temperature. A unique feature of 2-D semiconductors is their unprecedented tunability relative to both electric and optical properties due to doping and environmental screening.
Researchers can engineer theoretically predicted and experimentally demonstrated Coulomb interactions in 2-D semiconductors to tune the quasiparticle bandgap (Eg) and exciton binding energies (Eb) of samples, with methods such as chemical doping, electrostatic gating and engineering environmental screening. Among the reported techniques, electrostatic gating offers additional advantages such as continuous tunability and excellent compatibility for integration in modern devices. However, an overlap of the band-edge absorption step with strong excitonic resonances makes it challenging to accurately determine the Eg of 2-D semiconductors from their optical absorption spectrum alone.
Scientists had therefore used scanning tunneling spectroscopy and optical spectroscopy to directly probe the Eb of 2-D semiconductors and measure Eg and the optical bandgap (Eopt). In the present work, Qiu et al. similarly used this approach to demonstrate gate-tunable Eg and excitonic effects in monolayer ReSe2 on a back-gated graphene field-effect transistor (FET) device. They observed a large Eb of 520 meV for monolayer ReSe2 at zero gate voltage, followed by continuously tuning from 460 to 680 meV via electrostatic gating due to gate-controlled free carriers in graphene. The ability to precisely tune the bandgap and excitonic effects of 2-D graphene semiconductors will provide a new route to optimize interfacial charge transport or light-harvesting efficiency. Qui et al. expect the present findings to profoundly impact new electronic and optoelectronic devices based on artificially engineered van der Waals heterostructures.
Qui et al. first imaged the monolayer ReSe2 to show a distorted 1T structure with triclinic symmetry. The four Re atoms slipped from their regular octahedral sites due to charge decoupling to form a 1D chain-like structure with interconnected diamond-shaped units. Due to the topological features, the monolayer ReSe2 exhibited unique in-plane anisotropic electronic and optical properties useful for near-infrared polarization-sensitive optoelectronic applications.
To probe carrier-dependent excitonic effects, the scientists first transferred a monolayer ReSe2 flake on to a clean back-gated graphene FET (field effect transistor) device. The device constituted of several components according to a previously established recipe to include a SiO2 substrate, which contrasted with the constituent atomic flatness of hexagonal boron nitride (hBN) that markedly reduced surface roughness and charge inhomogeneity in graphene. The use of graphene allowed direct scanning tunneling microscopy (STM) measurements of the gated single-layer ReSe2 while improving the electrical contact to monolayer ReSe2.
After STM imaging the atomically resolved image revealed a diamond chain-like structure as expected for monolayer ReSe2 with a distorted 1T atomic structure. The scientists observed the stacking alignment of the material along two crystallographic orientations as moiré patterns, where monolayer ReSe2 containing a triclinic lattice symmetry lay on graphene with a honeycomb lattice.
To probe carrier-dependent excitonic effects, the scientists first transferred a monolayer ReSe2 flake on to a clean back-gated graphene FET (field effect transistor) device. The device constituted of several components according to a previously established recipe to include a SiO2 substrate, which contrasted with the constituent atomic flatness of hexagonal boron nitride (hBN) that markedly reduced surface roughness and charge inhomogeneity in graphene. The use of graphene allowed direct scanning tunneling microscopy (STM) measurements of the gated single-layer ReSe2 while improving the electrical contact to monolayer ReSe2.
After STM imaging the atomically resolved image revealed a diamond chain-like structure as expected for monolayer ReSe2 with a distorted 1T atomic structure. The scientists observed the stacking alignment of the material along two crystallographic orientations as moiré patterns, where monolayer ReSe2 containing a triclinic lattice symmetry lay on graphene with a honeycomb lattice.
