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Figure 7. (a) Schematic: a single exfoliated crystal of monolayer WSe2 is transferred and positioned over the 3.5 μm diameter silica core of a single-mode optical fiber (not drawn to scale). The WSe2 flake is encapsulated with either hexagonal boron nitride (as depicted) or other material to tune the dielectric environment. The resulting assembly is physically robust and ensures that light passes only through the monolayer flake and does not move with respect to the flake, even in the cryogenic bore of a 65 T pulsed magnet. (b) Plots show the quadratic diamagnetic shift of the exciton transition, from which the exciton’s radius rX (and also its binding energy) can be inferred. The diamagnetic shift is measured for three different dielectric environments depicted by the diagrams.
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Los Alamos researchers and their collaborators have used the high magnetic fields generated at the National High Magnetic Field Laboratory’s Pulsed Field Facility at Los Alamos to directly measure how the properties of excitons (bound electron-hole pairs) in atomically thin semiconductors are influenced by the surrounding environment. Their results demonstrate how both the size and binding energy of excitons might be tuned in future 2D devices that are based on assembly of atomically thin semiconductors and other monolayer materials for optoelectronic and energy applications. Nano Letters published their findings.
Dielectric screening plays an essential role in semiconductor physics. It modifies the interactions between electronic carriers and therefore strongly impacts the transport phenomena and optoelectronic properties via its influence on both the size and binding energy of excitons. The excitons in the new family of atomically thin 2D semiconductors, such as molybdenum sulfide (MoS2) and tungsten selenide (WSe2), lie close to a surface. 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 unaffected by the surrounding medium.
To overcome this barrier, the team exploited the fact that an exciton’s physical size is directly proportional to the small quadratic energy shift of the exciton’s optical transition in high magnetic fields – the “diamagnetic shift.” The researchers used exfoliated WSe2 monolayer flakes affixed to single-mode optical fibers. The investigators tuned the surrounding dielectric environment by encapsulating the flakes with different materials, and then performed polarized low temperature magneto-absorption studies to 65 T. Their results quantify for the first time the systematic increase of the exciton’s size (and concurrent reduction of exciton’s binding energy) as dielectric screening from the surrounding materials is increased. This insight demonstrates how exciton properties could be tuned in future 2D optoelectronic devices. Moreover, the experimental technique they developed could be broadly applicable to a wide variety of new and interesting 2D materials for studies of fundamental exciton and optical properties.
Reference: “Probing the Influence of Dielectric Environment on Excitons in Monolayer WSe2: Insight from High Magnetic Fields,” Nano Letters, (2016); doi: 10.1021/acs.nanolett.6b03276.
Authors: Andreas V. Stier and Scott A. Crooker (Condensed Matter and Magnet Science, MPA-CMMS), Nathan P. Wilson, Genevieve Clark, and Xiaodong Xu (University of Washington). Billy Vigil (MPA-CMMS) supported the project by building one of the high-field probes.
The research benefitted from the use of the 65 T capacitor-driven pulsed magnets at the National High Magnetic Field Laboratory, which the National Science Foundation (NSF) and the state of Florida fund. The work supports the Lab’s Energy Security mission area and Materials for the Future and Science of Signatures science pillars through the development of methods to investigate the properties of 2D materials for semiconductors and other energy applications. Technical contact: Scott Crooker
Period | May 31 2017 |
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Media coverage
Media coverage
Title Influence of dielectric environment on excitons in atomically thin semiconductors Date 05/31/17 Persons Andreas V Stier, Scott A Crooker, Billy Norman Vigil, Andreas V Stier, Scott A Crooker
Media Type
- STE Highlight
Keywords
- LALP 17-001
STE Mission
- Energy Security
STE Pillar
- Materials for the Future
- Science of Signatures
STE Publication Year
- 2017