Advancing Basic Science for Humanity
2012 Nanoscience Prize Explanatory Notes
Nanoscientists, like other scientists, naturally make progress by building on the previous work of colleagues. If asked to acknowledge the foundations upon which they have based their studies, many would highlight the contributions of one researcher in particular who has done more than most to add to fundamental knowledge in the field.In making their award, the Kavli Nanoscience Prize committee has selected a scientist whose work, over more than five decades, has improved understanding of how and why the thermal, electrical and other characteristics of materials structured at the nanoscale can be dramatically different from those of the same materials at larger dimensions.
Computer artwork of a carbon nanotube showing its hexagonal carbon structure. Often measuring only a few nanometres wide and anything up to several millimetres long, their unusual and potentially useful properties include very high tensile strength, high electrical and heat conductivity, and relative chemical stability. Credit: Science Photo Library
Mildred Dresselhaus, of the Massachusetts Institute of Technology, in Cambridge, Massachusetts, in the United States, began researching graphene and its properties in the 1960s when few others were doing so. The “electronic structure”, or distribution of electrons in energy bands around the nuclei of atoms, is fundamental to the material’s properties, such as whether it is a metal, semiconductor or an insulator. Professor Dresselhaus was one of the first researchers to map out the electronic structure of graphite, using various magneto-optical techniques. These involve using light and magnetic fields to probe the electronic structure of materials.
From the early 1970s, Professor Dresselhaus applied her expertise in the use of these techniques to graphite intercalation compounds, materials in which different chemical species are sandwiched between graphite layers to alter properties such as electrical and thermal conductivity. This work provided an important stepping stone towards advancing work on carbon nanostructures.
Dresselhaus also analysed graphite using techniques including laser ablation in which energy from a laser beam is used to strip material from a surface. Using this technique she demonstrated that liquid carbon is metallic rather than semiconducting. During these experiments she observed that large carbon clusters were being ejected. At this time researchers at Exxon were studying the properties of carbon clusters up to about 15 atoms. After speaking to Professor Dresselhaus about her laser ablation work they began to look at how carbon atoms are bound together. These discussions directly contributed to work which led to the publication two years later of the first observation of the buckminsterfullerene, the famous football-shaped “buckyball” molecule made up of 60 carbon atoms (C60) with a range of interesting and useful properties such as superconductivity and light absorption. Three of the paper’s authors, Robert Curl, Richard Smalley and Sir Harry Kroto, were jointly awarded the Nobel Prize in Chemistry for the discovery in 1996.
Carbon fibres are made up of strands of carbon atoms and have greater strength than steel despite being significantly lighter in weight. After writing a book on the topic, Professor Dresselhaus was invited to give a talk at a US Department of Defense workshop in 1990. At this event she was joined in a podium discussion by Professor Smalley. When asked about the connections between C60 and carbon fibres, they discussed the possibility of adding rings of ten carbon atoms to elongate the football to become C70. The conversation proceeded to the idea of C80, C100 and eventually a tubular structure that later became famous as the single-walled (one atom thick) carbon nanotube (SWNT).
Professor Dresselhaus went on to explore the concept further. SWNTs can be seen as sheets of graphene cut in such a way that they can be rolled into a cylinder. Depending on the spiral angle along which the cylinder is rolled relative to the hexagons into which the carbon atoms are arranged, a SWNT can either form a symmetrical pattern or a non-symmetrical, “chiral”, one. Professor Dresselhaus’ group predicted this aspect of their geometry would control their properties, determining whether they were either semiconducting or metallic, for example. This was later confirmed when the first papers on the observation of SWNTs were published from 1993 on.
As with all materials, SWNTs and graphene are held together by electrons shared between atoms. When the energies of electrons are altered, there are changes to the bonds within the material, and therefore to the elastic and thermal behaviour of the material. The way in which the electrons and the elastic response interact provides a unique fingerprint that can be detected using Raman spectroscopy, which involves observing how light from a laser is scattered by a material.
From 1996 Professor Dresselhaus pioneered the use of Raman spectroscopy to characterise SWNTs and graphene. The technique has proved to be a powerful way to study the quantum properties of electrons and uniform oscillations of elastic arrangements of atoms or molecules called phonons, which are fundamental to a material’s properties. Professor Dresselhaus made the first observation of how carbon atoms in nanotubes vibrate in a coordinated fashion along the direction of the nanotube’s radius. She investigated and helped explain the fundamental interactions between electrons and phonons in nanostructures, and by the year 2000 was able to isolate the Raman spectra from individual nanotubes. Raman spectroscopy has become the prime method for the characterisation of carbon nanotubes and has also been adopted for investigating the properties of graphene.
This fundamental science has laid the foundations for work on technologies capable of controlling how energy flows. Thermoelectric materials can convert heat energy into an electrical signal or use electrical energy to cool a material. Those that occur in nature are limited in their efficiency and utility by the strong links between their electrical and thermal conductivity. Professor Dresselhaus showed that these links can be changed for nanoscale structures, since it is possible to separately adjust the electrical and thermal conductivity of carbon nanotubes.
Professor Arne Skjeltorp, of the University of Oslo, and chairman of the Kavli Nanoscience Prize Committee, said that, while an award could have been made for Professor Dresselhaus’ work in the field as a whole, members of the committee wanted to honour her for her specific advances in the study of phonons, electron-phonon interactions and thermal transport in nanostructures.
“Mildred Dresselhaus laid the foundation for our understanding of the influence of reduced dimensionality on the fundamental thermal and electrical properties of materials,” said Professor Skjeltorp. “Her work has provided a series of seminal contributions to the science of carbon-based nanostructures.”