Schematic
illustration of the electron-hole pairs (electron: pink, hole: blue), which are
formed by absorption of light in the two-layer molybdenum disulfide layer.
Credit: Nadine Leisgang and Lorenzo Ceccarelli, Department of Physics,
University of Basel
By
layering different two-dimensional materials, physicists at the University of
Basel have created a novel structure with the ability to absorb almost all
light of a selected wavelength. The achievement relies on a double layer of
molybdenum disulfide. The new structure's particular properties make it a
candidate for applications in optical components or as a source of individual
photons, which play a key role in quantum research. The results were published
in the scientific journal Nature Nanotechnology.
Novel
two-dimensional materials are currently a hot research topic around the world.
Of special interest are van der Waals heterostructures, which are made up of
individual layers of different materials held together by van der Waals forces.
The interactions between the different layers can give the resulting material
entirely new properties.
Double
layer unlocks crucial properties
There are
already van der Waals heterostructures that absorb up to 100 percent of light.
Single-layers of molybdenum disulfide offer absorption capacities in this
range. When light is absorbed, an electron vacates its original position in the
valence band, leaving behind a positively charged hole. The electron moves to a
higher energy level, known as the conduction band, where it can move freely.
The
resulting hole and the electron are attracted to each other in accordance with
Coulomb's law, giving rise to bound electron-hole pairs that remain stable at
room temperature. However, with single-layer molybdenum disulfide there is no
way to control which light wavelengths are absorbed. "It is only when a
second layer of molybdenum disulfide is added that we get tunability, an
essential property for application purposes," explains Professor Richard
Warburton of the University of Basel's Department of Physics and Swiss
Nanoscience Institute.
Absorption
and tunability
Working in
close collaboration with researchers in France, Warburton and his team have
succeeded in creating such a structure. The physicists used a double layer of
molybdenum disulfide sandwiched between an insulator and the electrical
conductor graphene on each side.
"If
we apply a voltage to the outer graphene layers, this generates an electric
field that affects the absorption properties of the two molybdenum disulfide
layers," explains Nadine Leisgang, a doctoral student in Warburton's team
and lead author of the study. "By adjusting the voltage applied, we can
select the wavelengths at which the electron-hole pairs are formed in these
layers."
Richard
Warburton adds, "This research could pave the way for a new approach to
developing optoelectronic devices such as modulators." Modulators are used
to selectively change a signal's amplitude. Another potential application is
generating individual photons, with important implications for quantum
technology.