Exciton States in a New Monolayer Semiconductor

Published February 27, 2018

Circularly-polarized optical spectra from 0-65 teslas of monolayers of WSe2.

Analogous to the unique spectral fingerprint of any atom or molecule, researchers have measured the spectrum of optical excitations in monolayer tungsten diselenide (WSe₂), which is a member of a new family of ultrathin semiconductors that are just one atomic layer thick.

What did scientists discover?

The optical spectrum of an atomically-thin semiconductor — a single atomic layer of tungsten diselenide, or WSe₂ — was measured in extremely strong pulsed magnetic fields up to 65 teslas.

Why is this important?

The high magnetic fields allowed us to directly measure, for the first time, the mass of the fundamental particles in this new family of two-dimensional (2D) materials. In all semiconductors, the fundamental optical excitation is the exciton (a bound electron-hole pair), which plays an essential role in technological applications such as solar cells or light-emitting diodes (LEDs). These excitons have a "weight" (i.e., mass), which influences nearly all aspects of device performance. In the recently-discovered class of 2D semiconductors such as WSe₂ or MoS₂, the exciton’s mass had until now only been inferred from theory. In this work, we provide the first experimental measurement of this important and fundamental material parameter. These 2D semiconductors are expected to form the basis of a new generation of ultrathin, lightweight, and efficient optoelectronic devices.

Who did the research?

Andreas V. Stier¹, Nathan P. Wilson², Kirill A. Velizhanin³, Junichiro Kono⁴, Xiaodong Xu², Scott A. Crooker¹

¹National MagLab-Los Alamos; ²University of Washington; ³Los Alamos National Lab; ⁴Rice University

Why did they need the MagLab?

The most reliable way to measure the mass of excitons in a semiconductor is to study their behavior in high magnetic fields. These new materials require much higher magnetic fields than traditional semiconductors like silicon and GaAs.