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Two-dimensional materials are
crystalline substances with a thickness
of a few atoms or less.
The most famous of the 2D materials is graphene.
A single layer of carbon atoms called a monolayer,
which is hundreds of times stronger than steel,
conducts electricity better than copper,
and is completely flexible.
Beyond graphene there exists a host of 2D materials
with a wide range of different properties.
One group called transition metal dichalcogenides,
and known as TMDs,
combine the incredible thinness of graphene
with exceptional semiconductor properties.
Unlike graphene, a TMD monolayer is three atoms thick.
Each sheet consists of a layer of transition metal atoms,
such as molybdenum
or tungsten,
between two planes of chalcogen atoms,
such as sulfur
or selenium.
TMD monolayers are direct bandgap semiconductors,
meaning they strongly emit light
when excited electrically or optically.
But for thicker TMD sheets such as bilayers
and trilayers
the bandgap is indirect and
the excellent luminescence properties are lost.
So, to utilise light emission from TMDs
individual monolayers must be isolated from a bulk crystal.
To do this researchers at the University of Sheffield
use a method known as mechanical exfoliation.
Although atoms within an individual layer
are strongly bonded to one another,
adjacent sheets are bound together very weakly
through the van der Waals interaction.
By simply applying sticky tape to the crystal
this weak interlayer bonding can be broken
and thin films of material
can be removed from the crystal surface.
By then applying this tape onto a substrate,
such as glass, and slowly peeling it back off
interlayer bonding between layers
within the thin films can be broken further
and single monolayers can be
transferred onto the surface.
Assembling different 2D materials into vertical stacks,
one layer at a time,
creates new artificial materials
called van der Waals heterostructures.
By careful control of the sheet order
within the heterostructure
the properties of each individual layer can be combined
to produce optoelectronic devices
with tailor-made properties.
This allows the construction of
electroluminescent devices
for light-emitting applications,
as well as photodetectors
for optical imaging sensors,
and 2D material transistors
for the basis of flexible computational elements.
Combining these different
electrical and optical elements
based upon van der Waals heterostructures
has incredible potential for
the development of new flexible
electronic devices that could
revolutionize current technology.
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