2012 Kavli
Prize in
Nanoscience

Citation from the Committee

Mildred Dresselhaus, Massachusetts Institute of Technology, USA has laid the foundation for our understanding of the influence of reduced dimensionality on the fundamental thermal and electrical properties of materials. Her early work on graphite intercalation compounds and carbon fibers presaged the discoveries of C60, the fullerenes, nanotubes, and graphene. She investigated the effects of phonon confinement and electron-phonon interactions in nanostructures, and provided the key insights that underlie today’s research into nanostructured thermoelectrics. She showed that in nanostructures it is possible to decouple thermal and electrical transport, with significant implications for energy use. Thanks to Dresselhaus’s work, we have an improved understanding of how energy flows and dissipates on the nanoscale.

The story of carbon is interwoven with the story of nanoscience. The 1996 Chemistry Nobel Prize for the discovery of fullerenes, the 2008 Kavli Nanoscience Prize for the discovery of nanotubes, and the 2010 Physics Nobel Prize for graphene all recognize the remarkable phenomena that occur in highly controlled carbon-based nanostructures. As early as the 1960s, Dresselhaus led one of the very first groups that explored the carbon materials that form the basis for 2D graphene and 1D carbon nanotubes. These early experiments helped to map out the electronic band structure of these materials, critical to further understanding the unique properties they might possess. Dresselhaus studied intercalated two-dimensional graphene sheets and provided important insights into the properties of not only 2D graphene, but also of the rich interactions between graphene and the surrounding materials. Her early work on carbon fibers, beginning in the 1980s, provided Dresselhaus with the understanding and perspective to postulate the existence and unusual attributes of one-dimensional "single-wall carbon nanotubes (SWNTs)," years in advance of their actual discovery. A key prediction included the possibility that SWNTs could behave like either metals or semiconductors, depending on their chirality. Dresselhaus and coworkers pointed out that nanotubes can be viewed as arising from the folding of a single sheet of carbon, like a piece of paper that is wrapped at different spiral angles. They showed that this very simple rearrangement of their structure completely controlled their properties. This prediction was subsequently shown to be true. Through her studies of the fundamental physics of carbon-based solids, Dresselhaus laid the foundation for knowledge that has been integral to today’s nanoscience of carbon.

Dresselhaus studied the transport and optical properties of nanostructured matter with an exquisite selection of experimental techniques providing unprecedented microscopic understanding. Regarding carbon nanostructures, she pioneered Raman spectroscopy as a sensitive tool for the characterization of materials one atomic layer in wall thickness, namely carbon nanotubes and graphene. Diameter selective resonance enhancement led to the observation of Raman spectra from one single nanotube. The high sensitivity of Raman spectroscopy to diameter and chirality made the technique the prime method for the characterization of carbon nanotubes. The success story has been seamlessly adapted to the characterization of graphene and is in use in hundreds of laboratories worldwide as a fundamental diagnostic tool for carbon-based nanostructures.

Materials are held together by electrons shared between atoms. When the energy of an electron in a solid is altered, the local bonding of the solid is perturbed, resulting in a change in the position of the atoms that make up the solid. In nanoscale materials, the spatial extent of electrons and phonons can be modulated, leading to dramatically different behaviors compared with extended solids. Dresselhaus has investigated this very fundamental electron–phonon interaction in nanostructures using the powerful techniques of Raman and Resonance Raman spectroscopy.

This science also laid a foundation for practical work today aimed at controlling how energy flows. Thermoelectric materials have the ability to convert heat energy to an electrical signal or, alternatively, to utilize electrical energy to actively cool a material. Nature provides materials in which the electrical and thermal conductivity are strongly linked, resulting in a seeming limit to the achievable efficiency of a thermoelectric. Dresselhaus demonstrated that in a one-dimensional structure, it is possible to separately adjust electrical and thermal conductivity, and that this should allow the creation of new generations of thermoelectric refrigerators and new ways of scavenging waste heat for useful purposes.

Mildred S. Dresselhaus is recognized with the Kavli Prize for Nanoscience for her seminal contributions to the science of carbon-based nanostructures and for her elucidation of the electron-phonon interaction on the nanoscale.

The 2012 Nanoscience Kavli Prize explained

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.

By Nic Fleming

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 toward advancing work on carbon nanostructures.

Dresselhaus also analyzed 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 fibers 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 U.S. 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 fibers, 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 that 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 behavior 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 honor 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.”