Tech Briefs Magazine - February 2023 - 25
them to accurately extract its thermal
expansion coefficient.
Because 2D materials are so small - perhaps
just a few microns in size - standard
tools aren't sensitive enough to directly
measure their expansion. Plus, the materials
are so thin they must be bonded to a
substrate such as silicon or copper. If the
2D material and its substrate have different
TECs, they will expand differently when
temperatures increase, which causes thermal
stress.
For instance, if a 2D material is bonded
to a substrate with a higher TEC, when the
device is heated the substrate will expand
more than the 2D material, which stretches
it. This makes it difficult to measure the actual
TEC of a 2D material since the substrate
affects its expansion.
The researchers overcame these problems
by focusing on the atoms that make
up the 2D material. When a material is
heated, its atoms vibrate at a lower frequency
and move farther apart, which causes
the material to expand. They measure
these vibrations using a technique called
micro-Raman spectroscopy, which involves
hitting the material with a laser. The vibrating
atoms scatter the laser's light, and this
interaction can be used to detect their vibrational
frequency.
But as the substrate expands or compresses,
it impacts how the 2D material's
atoms vibrate. The researchers needed to
decouple this substrate effect to zero in on
the material's intrinsic properties. They did
this by measuring the vibrational frequency
of the same 2D material on three different
substrates: copper, which has a high TEC;
fused silica, which has a low TEC; and a silicon
substrate dotted with tiny holes. Because
the 2D material hovers above the
holes on the latter substrate, they can
A green laser shines onto an atomically thin material. By probing how atoms vibrate when deposited
on substrates and upon heating, researchers turn Raman spectroscopy into a scientific " ruler "
to understand how 2D materials expand. (Image: Yang Zhong and Lenan Zhang)
Tech Briefs, February 2023
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Tech Briefs Magazine - February 2023
Table of Contents for the Digital Edition of Tech Briefs Magazine - February 2023
Tech Briefs Magazine - February 2023 - Intro
Tech Briefs Magazine - February 2023 - Sponsor
Tech Briefs Magazine - February 2023 - Cov1
Tech Briefs Magazine - February 2023 - Cov2
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Tech Briefs Magazine - February 2023 - Cov3
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Tech Briefs Magazine - February 2023 - STCov1
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Tech Briefs Magazine - February 2023 - STCov4
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