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Investigating the remarkable effects in two-dimensional (2-D) semiconductors and controlling their can unlock the full potential of 2-D materials for future applications in 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 (ReSe₂) on a back-gated graphene device. They used (STS) and 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 (shifts in physical qualities) and extraordinary excitonic effects due to and reduced . Light-matter interactions in these systems are governed by enhanced excitonic effects, which physicists have studied to develop . A unique feature of 2-D semiconductors is their unprecedented tunability relative to both electric and optical properties due to .

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 , and . 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 and the . 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 (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 .

Qui et al. first imaged the monolayer ReSe₂ to show a distorted 1T structure with 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 .

To probe carrier-dependent excitonic effects, the scientists first transferred a monolayer ReSe₂ flake on to a clean back-gated graphene . The device constituted of several components according to to include a SiO₂ substrate, which contrasted with the constituent ) 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 to monolayer ReSe₂.

After STM imaging the atomically resolved image revealed a diamond chain-like structure with a distorted 1T atomic structure. The scientists observed the stacking alignment of the material along two crystallographic orientations as , where monolayer ReSe₂ containing a triclinic lattice symmetry lay on graphene with a honeycomb lattice.

When they probed the local electronic properties of ReSe₂ using STS (scanning tunneling spectroscopy) the scientists observed differential conductance (dI/dV) spectra in several moiré regions to exhibit similar features. As a unique feature of the study, Qiu et al. probed the quasiparticle (QP) band structures as a function of gate voltage.

The optical bandgap (Eopt) remained nearly constant at all gate voltages in contrast to the monotonic reduction of Eg, in agreement with . To verify this, they performed photoluminescence measurements of the monolayer ReSe₂/graphene/h-BN sample at different gate voltages at room temperature (RT). The gate-dependent photoluminescence spectra revealed a nearly constant Eopt of monolayer ReSe₂.

The scientists then determined the exciton binding energy and derived a large, gate-tunable bandgap renormalization for ReSe₂ in the hybrid device. They sought the physical origins of the gate-tunable QP bandgap renormalization and exciton binding energy in the monolayer ReSe₂ by excluding contributions from the out-of-plane field-induced polarization wave functions and substantiated their origin from gate-induced free carriers in graphene. Theoretical results of the study also showed that moderate doping in graphene could substantially reduce exciton binding energy (Eb) by hundreds of milli-electron volts as the free-carrier concentration in graphene increased. In addition, Qiu et al. directly compared the theory with their experimental results.

In this way, Zhizhan Qiu and co-workers successfully tailored the QP bandgap and exciton binding energy in a 2-D semiconductor by controlling doping of the underlying graphene with electrostatic gating. The results showed that screening from a graphene substrate had profound impact on Coulomb interactions that lead to broad tunability of the electronic band gap and exciton binding energy. The findings revealed in hybrid 2-D semiconductors or graphene systems. The work will pave the way to control excitonic effects and precisely tune the exciton binding energies in 2-D semiconductors for a variety of technical applications.

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