A team of researchers from Denmark has solved one of the biggest challenges in making effective nanoelectronics based on graphene. The new results have just been published in Nature Nanotechnology.
For 15 years, scientists have tried to exploit the “miracle material”
graphene to produce nanoscale electronics. On paper, graphene should be
great for just that: it is ultra-thin – only one atom thick and
therefore two-dimensional, it is excellent for conducting electrical
current, and holds great promise for future forms of electronics that
are faster and more energy efficient. In addition, graphene consists of
carbon atoms – of which we have an unlimited supply.
In theory, graphene can be altered to perform many different tasks within e.g. electronics, photonics or sensors simply
by cutting tiny patterns in it, as this fundamentally alters its
quantum properties. One “simple” task, which has turned out to be
surprisingly difficult, is to induce a band gap – which is crucial for
making transistors and optoelectronic devices. However, since graphene
is only an atom thick all of the atoms are important and even tiny
irregularities in the pattern can destroy its properties.
“Graphene is a fantastic material, which I think will play a crucial
role in making new nanoscale electronics. The problem is that it is
extremely difficult to engineer the electrical properties,” says Peter
Bøggild, professor at DTU Physics.
The Center for Nanostructured Graphene at
DTU and Aalborg University was established in 2012 specifically to
study how the electrical properties of graphene can be tailored by
changing its shape on an extremely small scale. When actually patterning
graphene, the team of researchers from DTU and Aalborg experienced the
same as other researchers worldwide: it didn’t work.
“When you make patterns in a material like graphene, you do so in
order to change its properties in a controlled way – to match your
design. However, what we have seen throughout the years is that we can
make the holes, but not without introducing so much disorder and
contamination that it no longer behaves like graphene. It is a bit
similar to making a water pipe that is partly blocked because of poor
manufacturing. On the outside, it might look fine, but water cannot flow
freely. For electronics, that is obviously disastrous,” says Peter
Bøggild.
Now, the team of scientists have solved the problem. The results are published in Nature Nanotechnology. Two
postdocs from DTU Physics, Bjarke Jessen and Lene Gammelgaard, first
encapsulated graphene inside another two-dimensional material –
hexagonal boron nitride, a non-conductive material that is often used
for protecting graphene’s properties.
Next, they used a technique called electron beam lithography to
carefully pattern the protective layer of boron nitride and graphene
below with a dense array of ultra small holes. The holes have a diameter
of approx. 20 nanometers, with just 12 nanometers between them –
however, the roughness at the edge of the holes is less than 1
nanometer, or a billionth of a meter. This allows 1000 times more
electrical current to flow than had been reported in such small graphene
structures. And not just that.
“We have shown that we can control graphene’s band structure and
design how it should behave. When we control the band structure, we have
access to all of graphene’s properties – and we found to our surprise
that some of the most subtle quantum electronic effects survive the
dense patterning – that is extremely encouraging. Our work suggests that
we can sit in front of the computer and design components and devices –
or dream up something entirely new – and then go to the laboratory and
realize them in practice,” says Peter Bøggild. He continues:
“Many scientists had long since abandoned attempting nanolithography
in graphene on this scale, and it is quite a pity, since nanostructuring
is a crucial tool for exploiting the most exciting features of graphene
electronics and photonics. Now we have figured out how it can be done;
one could say that the curse is lifted. There are other challenges, but
the fact that we can tailor electronic properties of graphene is a big
step towards creating new electronics with extremely small dimensions,”
says Peter Bøggild.
About the Center for Nanostructured Graphene
• Funded by the Danish National Research Foundation with a total
budget of 100 million DKK for the ten-year period 2012 – 2022. It
focuses on basic research, but all its research projects have long-term
perspectives for applications.
• The team is also part of the Graphene Flagship, which with a budget
of €1 billion represents a new form of joint, coordinated research on
an unprecedented scale, and is Europe’s biggest ever research
initiative. It is tasked with bringing together academic and industrial
partners to take graphene from the realm of academic laboratories into
society in the space of 10 years, thus generating economic growth, new
jobs and new opportunities for Europe.
