Charge Separation in Monolayer WSe₂ by Strain Engineering: Implications for Strain-Induced Diode Action

Abstract

Strain-engineering band structure in transition-metal dichalcogenides (TMDC) is a promising avenue toward capabilities in optoelectronics. For example, controlling the flow of optically generated quasiparticles can be achieved by a localized strain field which reduces the bandgap and generates an energy-band gradient that funnels neutral excitons to the strain apex. It would be even more advantageous to mimic a diode's internal field, where both conduction and valence bands bend in the same direction, to separate electrons and holes. This can be achieved if the strain in the TMDC layer lowers both the conduction band minimum as well as the valence band maximum during strain-induced band narrowing. Here, we have used density functional theory (DFT) calculations of monolayer WSe 2 electronic structure under biaxial strain to show that WSe 2 has this property. To test the band bending experimentally, we combined localized strain with electrostatic doping to follow photoluminescence from excitons and positive or negative trions. In unstrained WSe 2 , both positive and negative trion emissions dominate over excitons away from charge neutrality. In contrast, for strained areas, negative trions accumulate, while positive trion emission is near zero away from charge neutrality, indicating a lack of holes. Hence, strain bends both conduction and valence bands down, similarly to the band bending in a PN-diode depletion region, providing an opportunity to separate electrons and holes via localized strain.

Analysis of Strain and Defects in Tellurium-WSe2 Moiré Heterostructures Using Scanning Nanodiffraction

“…In the case of WSe 2 , biaxial strain bends down both the conduction band minimum and valence band maximum at different rates, leading to an overall bandgap narrowing. 36 In trigonal Te, shear (hydrostatic or uniaxial) strain causes the material to change from a trivial insulator to a strong topological insulator. 37 Ultimately, both the observed defects and strain will influence the behavior of this moireh eterostructure.…”

“…In addition, the observed strains in WSe 2 and Te are known to alter the optoelectronic properties. In the case of WSe 2 , biaxial strain bends down both the conduction band minimum and valence band maximum at different rates, leading to an overall bandgap narrowing . In trigonal Te, shear (hydrostatic or uniaxial) strain causes the material to change from a trivial insulator to a strong topological insulator .…”

Improving Strain-localized GaSe Single Photon Emitters with Electrical Doping

“…Such a design allows both functions of parallel capacitor and field-effect transistor (FET) for 2D materials under strain. 16 To apply biased gate voltages to the GaSe flake, only the electrodes S and G are connected to form a parallel capacitor-like device. A detailed description of the substrate design, patterning, and fabrication is in Figure S1 and the Methods section.…”

“…The GaSe flake forms a tented structure as reported in our previous work. , Both ends of the GaSe flake are connected with the electrodes S and D, respectively. Such a design allows both functions of parallel capacitor and field-effect transistor (FET) for 2D materials under strain . To apply biased gate voltages to the GaSe flake, only the electrodes S and G are connected to form a parallel capacitor-like device.…”

“…While trions (charged excitons) are commonly observed in TMDC monolayers, ,, trions in GaSe are not generally observed due to nonradiative decay at low photogeneration rates , and the formation of biexcitons at high photogeneration rates. , The XX 1 biexciton is observed on the higher energy side of the exciton X with a negative binding energy of −7 meV, due to exciton–exciton repulsion. − The XX 2 biexciton is observed on the lower energy side of the exciton X with a positive binding energy of +5 meV due to attractive exciton–exciton interactions. The formation of biexcitons was attributed to the high exciton densities in this Type-I energy funnel in our previous work …”