Modern technology is defined by a relatively small number of basic materials – and the key to the future success of graphene and other 2d materials is finding the right combination to unleash new potential and new capabilities.
That was the message from Nobel Prize-winning physicist Kostya Novoselov in his keynote address to the 2015 Graphene Week conference in Manchester.
Novoselov noted that electronics is dominated by silicon, civil engineering by steel, and aerospace by aluminium. In the case of 2d materials, researchers are rapidly building up a library of crystals that may be mixed and matched to engineer new materials – heterostructures – with functionalities tailored for specific practical applications.
In his presentation, Novoselov discussed a number of desirable characteristics of such engineered heterostructures, and gave examples of technologies that would benefit from this approach to materials science and engineering.
The centrepiece of the Graphene Flagship calendar, the 10th Graphene Week conference is taking place at the University of Manchester, attended by more than 600 delegates. Home to the UK’s National Graphene Institute, Manchester is also the research base of Novoselov and his colleague Andre Geim, who shared 2010 Nobel Prize in Physics for their pioneering work on the properties of graphene.
Other guest speakers include Harvard University’s Philip Kim, who talked about physics near the charge neutrality of graphene. The Dirac points in graphene – zero energy crossing points of two energy dispersion curves, which define the material as a semi-metal, and therefore devoid of band gap – are an important consideration. The physics here can be exotic, given that, near the Dirac point, physical behaviour is governed by disorder.
Connecting graphene to a superconductor and then reducing the Fermi energy in graphene to a level smaller than that of the superconductor gap, we see evidence of specular Andreev reflections, in which superconducting Cooper pairs split into pairs of electrons and holes across the charge neutrality point. In simple physics terms, this is conservation of momentum at work. In a second example, there is a dramatic enhancement of electronic thermal conductivity in boron nitride-encapsulated graphene at the neutral point, as the Fermi energy there becomes smaller than the thermal energy.
IBM researcher Shu-Jen Han discussed nanoelectronics based on low-dimensional carbon materials. Many of us understand the outline at least of Moore's Law, which describes the reducing length scale and increasing density over time of microelectronic components. We may also be aware that disruptive technologies are required to extend Moore's Law to ever smaller physical scales.
Down-scaling transistor size is more than an engineering challenge, as there is fundamental physics to consider. For example, frequency and power limits are saturating at the length scales of tens of nanometres that prevail today. To continue with such scale reduction, power is a critically important factor. But how to reduce power flowing through electronic components?
Delegates also heard from Rod Ruoff, a veteran nanoscientist from the US who is now at the Centre of Multidimensional Carbon Materials in South Korea. Ruoff spoke of graphene and new carbon materials, including the material known as bilayer diamond. He also discussed the stacking of graphene layers, the application of ‘negative curvature’ carbon in electrical energy storage, pattern graphite, and graphene coatings that could massively reduce friction.
An important point made by Ruoff is that graphene will not have a major impact until we can produce high-quality bulk material in ways other than chemical vapour deposition. Growth of graphene on copper or other metal substrates is too slow, he said, so we must come up with new methods.
Wolfgang Templ of Alcatel-Lucent in Stuttgart presented his vision for graphene in telecommunications applications. There is much popular talk of graphene and related materials replacing silicon in electronics applications , but, according to Templ, it is the wafer-scale integration of graphene with silicon components that offers considerable promise at an affordable price.
As a proof-of-concept, Aachen-based research firm AMO has, together with Temple and his Alcatel-Lucent colleagues, demonstrated a graphene-based photodetector on a silicon waveguide, with transmission rates of up to 50 gigabits per second. Graphene-based optoelectronics is a rapidly developing field, in which basic physics is tightly integrated with the search for practical engineering solutions.