Probing the influence of dielectric environment on excitons in monolayer WSe2: Insight from high magnetic fields (1608.05093v1)

Abstract: Excitons in atomically-thin semiconductors necessarily lie close to a surface, and therefore their properties are expected to be strongly influenced by the surrounding dielectric environment. However, systematic studies exploring this role are challenging, in part because the most readily accessible exciton parameter -- the exciton's optical transition energy -- is largely \textit{un}affected by the surrounding medium. Here we show that the role of the dielectric environment is revealed through its systematic influence on the \textit{size} of the exciton, which can be directly measured via the diamagnetic shift of the exciton transition in high magnetic fields. Using exfoliated WSe$_2$ monolayers affixed to single-mode optical fibers, we tune the surrounding dielectric environment by encapsulating the flakes with different materials, and perform polarized low-temperature magneto-absorption studies to 65~T. The systematic increase of the exciton's size with dielectric screening, and concurrent reduction in binding energy (also inferred from these measurements), is quantitatively compared with leading theoretical models. These results demonstrate how exciton properties can be tuned in future 2D optoelectronic devices.

Influence of Dielectric Environment on Excitons in Monolayer WSe$_2$ Under High Magnetic Fields

The paper "Probing the influence of dielectric environment on excitons in monolayer WSe$_2$: Insight from high magnetic fields" details a refined investigation into the manipulation of exciton properties by dielectric environments in two-dimensional (2D) semiconductors, specifically monolayer WSe$_2$. This work leverages high magnetic fields to achieve unprecedented insight into how the dielectric surroundings impact exciton behavior, focusing on the diamagnetic shift as a principal metric for exciton size and binding energy adjustment.

A comprehensive understanding of dielectric screening is crucial in both the theoretical and practical realms of semiconductor physics, particularly concerning excitons, which are electron-hole pairs bound together by electrostatic forces. The research challenges the simplistic view that changes in the surrounding material minimally affect exciton optical transition energies. Instead, it posits that these changes significantly influence the exciton's size and binding energy, affecting future applications in optoelectronic devices.

For measurement, the authors employed exfoliated monolayer WSe$_2$, strategically modified with various dielectrics. By doing so, they conducted magneto-absorption spectroscopy at cryogenic temperatures, reaching magnetic fields of up to 65 T. These experiments elucidated the influence of dielectric environments on the exciton radius and binding energy. The findings demonstrated a measurable increase in exciton size, from approximately 1.2 nm to 1.6 nm, and a concurrent drop in binding energy, from around 480 meV to 220 meV, as the dielectric constant increased from 1.55ε₀ to 3.30ε₀.

These observations are juxtaposed against theoretical predictions derived from the widely used Keldysh model, which characterizes non-local dielectric screening in 2D materials. Despite capturing general trends, the model underestimated the sensitivity of exciton properties to environmental screening. This indicates a potential limitation in the model or a need for refinement to accommodate 2D material dielectric responses more accurately.

Key experimental advances include a fiber-coupled setup that enables precise light absorption measurements in high magnetic fields. By encapsulating the WSe$_2$ within standardized dielectric materials like hexagonal boron nitride (hBN) or transparent polymers, the experiments maintained high precision and reproducibility despite challenging environmental conditions.

These results have implications for the theoretical modeling of 2D materials, suggesting enhancements to current models to include more pronounced dielectric effects. The findings also offer practical pathways for tailoring exciton properties in semiconductor applications by carefully selecting or engineering surrounding materials.

Looking forward, the techniques and findings from this paper pave the way for further explorations into multiferroic and heterostructured 2D systems, with implicational bridges to future quantum computing and photonics, where fine-tuned material responses are indispensable. Such studies emphasize the nuanced roles dielectrics play, beyond simple insulating layers, into active modulators of quantum properties at atomic scales.