The discussion concerning the number of women in physics, and more generally in technical sciences, is nowadays a very vivid discussion. More and more universities are making strong efforts to encourage more women into studying technical sciences supported by numerous programs with medium success rate. Even though, the number of women studying physics has increased in time as can be seen from a graph from the American physical society presented in 2017 showing the amount of female and male students with a finished PhD in the last 55 years, the amount of female professors still lies around 20 percent in Austria.
Nevertheless, there have been, and there are, many women who have made and are making great contributions to physical science. In the context of the international women's day, we want to introduce one of them.
Mildred Dresselhaus, Queen of Carbon
Mildred Dresselhaus was an American physicist born in 1930 and died in February 2017. She attended the Hunter women’s college, where she had the idea that women could study physics just like men. Later she was awarded a Fulbright Fellowship to spend one year in Cambridge, where she realized there were only few women doing physics. "I didn't really know I wasn't supposed to do physics until I joined the mainstream. When I got my degree in 1958 it was pretty lonely - we (women) were only two percent of the physics community then", she said once.
But she was never discouraged. On the contrary, Dresselhaus was a woman who liked challenge. She started her career at Lincoln Laboratory in MIT in 1958, where she started working on superconductors. Later she changed her research focus to magneto-optics with lasers on semimetals. Ten years later she made her greatest contributions to condensed matter physics by unraveling the electronic structure of graphite. She was of the first to work with Raman spectroscopy, a technique where light interacts with a sample and produces scattered radiation with different wavelengths characteristic of the binding of the atoms in the sample.
Her research soon moved from graphite to two-dimensional (2D) ultra-thin layers of carbon, called graphene, in the 2000’s. For her longstanding work with carbon materials, she was known as the “Queen of Carbon”.
From graphite to carbon nanotubes
Carbon is an element which has many allotropes, this means it takes several forms or structures, depending on the ordering of the carbon atoms. For example, if the atoms order tetrahedrally one obtains diamond, but if they order in ultra-thin 2D layers bundled on top of each other very weakly, then one has graphite. We know graphite from pencils. The basic structure of graphite is a single 2D layer of carbon atoms arranged in a hexagonal fashion (much like a honeycomb) and this single layer graphite is known as graphene. Graphene has completely other properties than the bulk graphite material. If you take one graphene sheet and role it up then you have a single-wall carbon nanotube (SWCNT). They are known to be the hardest material ever.
Dresselhaus’ research focused on understanding the electronic properties of graphite and SWCNT with Raman spectroscopy. Their electronic properties depend on the structure of the nanotube. Rolling it at different angle, will show different electronic properties. It is a very good electrical conductor; which properties can further be manipulated by substituting carbon atoms through other one like boron or nitrogen. The results of her research helped to implement carbon nanotubes mainly as composite fibers in polymers to improve the mechanical, thermal and electrical properties of other materials.
Beyond graphite and 2D materials
Dresselhaus’ work on carbon based materials that started with the use of magneto-optical techniques on graphite in the 1960s moved on from 3D graphite to lower dimensional fullerenes, carbon nanotubes and finally to the wonder material called graphene over a period of four decades. Meanwhile, in the 1980s Dresselhaus contributed immensely to the understanding and development of methods for synthesizing lithium intercalated graphite which formed the basis of today’s successful Li-ion batteries that we all carry in our handys. She performed in-depth investigations of C60 buckministerfullerenes, where 60 carbon atoms join hands to form a closed soccer ball like structure with interesting physical properties. A major application area of the fullerenes have been in medical physics, where these tiny nanoscopic carbon structures are used as high performance magneto-resonance imaging agents, drug and gene delivery or in cancer research. The works of Dresselhaus in the emergence of the field of low dimensional thermoelectricity, which focusses mainly on the interconversion of heat and electricity using nanomaterials, paved the way for energy harvesting and environment friendly electronics.
Being a stalwart and pioneer in low dimensional carbon based material, Dresselhaus was quick to contribute to the emerging field of 2D materials starting from graphene. This 2D form of carbon carries a certain significance in the history of physics. In fact in the late 1930s, the famous Russian physicist Lev Landau showed that such 2D crystal structures are unstable and cannot be synthesized. Though it was recognized that such a 2D structure if realized would possess novel physical and chemical properties, it was pointed out by Richard Feynman in 1959 that such nanodimensional system could revolutionize our technologies. Theoretical calculations showed graphene to be the host of many new properties such as new types of charged particles in them (that are electrons dressed up in hitherto unknown attires) the massless Dirac particles with almost infinite velocities that would make electronic devices faster than ever imagined. Finally graphene was realized and the technique used for obtaining a 0.3 nanometer layer from a graphite block is mechanical exfoliation where a scotch tape is used to tear apart the graphite several times until one gets this one single atomic layer of carbon – the graphene.
In order to identify that this layer is indeed graphene, the techniques of Raman spectroscopy that Dresselhaus used extensively for studying her carbon nanotubes and fullerenes, came very handy. This technique soon became "the technique" to identify and characterize atomically thin 2D layers. Dresselhaus kept on working not only on graphene but 2D materials beyond graphene – the transition metal dichalcogenides like MoS2, single layered phosphorus or phosphorene etc. until the last day of her life.
But Dresselhaus did more than just physics. Throughout her entire life she was strongly engaged in promoting opportunities for women in science and engineering and encouraging women for studying physics. She participated in numerous projects, like a MIT seminar in engineering for undergraduates to strengthen the confidence of young female students. The seminar was very well attended, by both, male and female students. In 2017 she participated in a video together with General Electric to persuade women for a scientific career with the question “What if female scientist were celebrities?” Her efforts were successful with her granddaughter, who is also working with carbon nanotubes.
The fascinating thing about Mildred Dresselhaus was seeing her at a physics conference, where she would look like this stereotypical grandmother: a bit out of place walking around a poster session full of typical physicists. And if you would go talk to her, then all her greatness and knowledge would be revealed. It is exactly women like Dresselhaus that help others to overcome the prejudices of society that say that physics is no career for women: Because it sure is. (Andrea Navarro-Quezada, Rajdeep Adhikari, 8.3.2019)
To the German version of the article
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