Thermally Driven Crossover from Indirect toward Direct Bandgap in 2D Semiconductors: MoSe₂ versus MoS₂

TLDR

Transition‑metal chalcogenide layers transition from indirect to direct bandgaps when reduced from bulk to monolayer. The study aims to reversibly tune the indirect‑to‑direct bandgap crossover in multilayer samples for device applications. The crossover is achieved by mechanically exfoliating multilayers down to a single layer. In few‑layer MoSe₂, heating thermally decouples layers, driving the nearly degenerate indirect and direct gaps toward a direct‑gap 2D state, markedly boosting photoluminescence and revealing a 1.55‑eV bandgap suitable for solar‑cell applications.

Abstract

Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe2, which shows stark contrast from the well-explored MoS2 where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe2 is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe2, identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices.