How do you define the cutting edge of nanotechnology these days? Or should I say how do you define the edge states – topological materials cropped up more than once in the talks at Trends in Nanotechnology 2019 , as did 2D materials and a number of other topics. Researchers from around the world convened in San Sebastian in Spain for the 20th anniversary of the conference and I tagged along for a couple of days to get the low down.
Top-trending topological materials
The existence of materials where certain regions have a separate portfolio of properties to the bulk has captivated a lot of research groups, and for some the theories to describe these systems are infiltrating the way other systems are considered. “Symmetry and topology are the corner stones of physics nowadays,” Ikerbasque research fellow Maia Vergniory told attendees – a bold claim, but as she pointed out herself, she is not the first to say so. What her group are among the first to do is to put the weight of numbers behind the claim. Treating her audience to a whistle stop tour of Euler’s theorem for the vortices, faces and edges of polyhedra, Bloch’s theorem for periodic structures, the impact of spin-orbit coupling on densities of states, and comparisons of triangular, hexagonal and kagome lattices, she then outlined a predictive theory for topological materials that makes the link between real space orbitals and momentum space topology. With this theory she highlighted work analysing thousands of atomic limits found in nature that led to the conclusion that 27% of materials are in fact topologically “nontrivial” (for more details see Hamish Johnston’s article on Physics World from earlier this year). She then invited those interested to try the online tool she and her collaborators have developed that allows you to combine elements of your choice and identify whether or not the resulting material will have topologically nontrivial properties.
Ubiquity of topological materials revealed in catalogues containing thousands of substances
With topological behaviour so unexpectedly common it is little wonder that this line of research is having such an impact in nanotechnology and materials science. While a number of fundamental questions about the nature of these topological states still remain, Eugene Chulkov and Mikhail Otrokov were happy to relieve attendees of one of them – the existence of an antiferromagnetic topological insulator. Taking us through a series of options for achieving magnetism in a non-magnetic topological insulator Chulkov – UPV/EHU professor and Materials Physics Center (CFM) and Donostia International Physics Center (DIPC) researcher – highlighted a paper currently on arXiv but accepted for publication in a scholarly journal soon, where they report predictions and the first observations of an antiferromagnetic topological insulator. Those keen to know more of the details had not long to wait before further updates on the work from CFM’s Otrokov later in the afternoon. You can find out more too in a Physics World news story coming soon.
2D stays in trend
Of course, the topic that inspired and cross-fertilized the most talks, posters and discussions was graphene and 2D materials. Although many have been chafing at the bit for years to see the commercial impact of graphene Amaia Zurutuza, scientific director of Graphenea advised her audience to be patient, adding “things take time – there are many examples of materials taking more than 20 years to get to market”. In the meantime research in the field remains prolific, which has been good news for Graphenea who plan to release two new single crystal graphene products later this year and have even moved the company’s output up the product line to supply field effect transistors. You can hear her describe developments herself in the audio clip.
In terms of the practical aspects of fabricating graphene devices much has been learned. “There is some hope on the horizon that we can manufacture devices from 2D materials at around an-8 inch wafer,” AMO GmbH’s Daniel Neumaier told attendees following a round-up of progress in graphene device fabrication, adding, “But we need to evaluate a complete value chain.” Stephan Hofmann from Cambridge University highlighted some of the complexities in the chemical vapour deposition growth process favoured for graphene crystal growth that are not always appreciated in aims to yield reproducible high-quality samples. He described it as a “heterogenous growth” process because of all the different facets on the growing graphene islands, each with a potentially different growth-limited step be that dissociation-limited, interface-limited or something otherwise. How much imperfections such as holes in graphene pose a problem he also called into question, highlighting experiments suggesting that the size of the hole seemed to be less of a factor than the Debye length. As he described efforts to connect fundamental science with the road to market he stressed the need to consider the “interplay between software and hardware” and to think beyond CMOS and its prescriptive perfect crystal requirements. Talking with him over the coffee break he laughed at my suggestion that the holy grail of graphene research might be holey after all.
One fabrication technique that raised eyebrows a few years ago is laser-induced graphene, where the intensity of the laser converts the polymer into graphene. The approach is attractive for flexible electronics, such as the in-plane microcapacitors Joan Marti Gonzalez, a PhD student at the University of Barcelona described in his poster, and has since been demonstrated on an unlikely range of materials including toast. (“Next must be skin right,” said Marti Gonzalez only half jokingly it seemed.) Flexible devices is an increasingly active field of research asevident in the talk by Elvira Fortunato at the University of Lisbon. In addition, the 2019 Chemistry Nobel Prize for lithium ion batteries has highlighted the importance of this research for some (many were already persuaded of the field’s significance) and these and other energy storage devices will likely stay on trend in nanotechnology research for some time to come.
At the other end of the spectrum Diego Peña Gil from the Universidade de Santiago de Compostela in Spain described some of the Cordon bleu of bottom-up graphene synthesis from monomers carefully tailored for bespoke nanoribbon structures. He listed “beauty” among the motivations for the research, alongside the search for new materials, synthesis methods and structures and properties, adding “Why not? We produce very expensive buildings because they are beautiful and we are the same, we are organic architects.”
The poster session featured a lot of work on 2D materials from β12 borophene doped with transition metals to aluminium-doping for better blue phosphorene sensors. The emphasis on graphene and 2D materials may be no great surprise but it perhaps gives pause for thought to recognize that neither graphene nor topological insulators really existed as research topics twenty years ago when the first Trends in Nanotechnology conference took place. “I came 15 years ago and it was totally different – there was no graphene it was all magnetic nanoparticles,” Neumaier told me over coffee.
The enduring attraction of magnetism and nanotechnology
While some things change others stay the same. Magnetism certainly still cropped up a lot although with a radical makeover in the form of magnetic 2D materials, novel magnetic proximity effects, and – another phenomena that had not yet been experimentally observed 20 years ago – skyrmions. Javier Junquera from Universidad de Cantabria described his group’s ultimately successful hunt for an electronic analogue to these magnetic vortex-like quasiparticles, despite several explanations why their apparent absence so far was unsurprising. The interaction primarily responsible for the emergence of magnetic skyrmions – the Dzyaloshinskii–Moriya interaction – has no electronic analogue. Another factor is that electronic dipoles – like those in a crystal lattice when a positive cation is slightly displaced with respect to the counter charges around it – can ultimately disappear, which is not the case for spin. The trick was to look at an interface layer between regions of opposing polarity. You can hear him describe the hunt himself too in the audio clip.
Organic nanostructures also featured, such as the impact of “unconventional” substrates on polymer growth. (The Ulmer reaction for polymerization typically only takes place on gold and silver surfaces.) Michele Pisarra at IMDEA Nanocienca described catalytic effects of the nanostructured Moiré patterns from graphene grown on ruthenium. His talk described reactions between the organic molecule TCNQ and CH2CN but catching up with him in the break it seems the possibility of catalytic effects on other reactions are already under investigation.
I attended my first Trends in Nanotechnology conference in Tenerife in 2011. The calibre of the speakers is another aspect of the conference that doesn’t seem to have changed, so it was with genuine regret that I could only attend the first couple of days this year and hence missed a lot of photonics and bio-oriented talks among others scheduled for later in the week. The programme and attendance may be lean compared to some other conferences but the organizers seem to have mastered the kind of boutique conference that neatly blends prestigious presentations with a healthy student attendance and plenty of conferring time in breaks. Trends in Nanotechnology is organized by the Phantoms Foundation headed by Antonio Correia.