Queen of Carbon: The Ongoing Materials Science Legacy Of Mildred Dresselhaus (Women In Science 27)
Its astounding versatility is matched only by our total and historic complacency in the face of its wonders. “Carbon? Whatever – it’s, like, all over the place. Now, protactinium, there’s an element…” Working on the logic that exotic elements must breed exotic properties, and intoxicated by the trans-Uranium revolution of the 30s and 40s, the potential of carbon to surprise us still was left unexplored for decades until Mildred Dresselhaus decided to re-open the case of this “solved” element, leading to astounding breakthroughs in materials technology we are only now learning to harness.
Dresselhaus was born in Brooklyn in 1930, on the ragged, desperate edge of the Great Depression. Her father was routinely unemployed, and the family had to make do with subsistence level governmental relief more often than not. Moving to the Bronx, she experienced directly how economic reality impacts an educational system as teachers, watching the support network of their classrooms fall apart, tumbled themselves into dispirited cynicism that made its way into their teaching. But Dresselhaus was smart, and she earned a place, not at the prestigious Bronx School of Science (it didn’t allow girls), but at the next best option, Hunter High, which came with guaranteed admission to Hunter College.
Hunter was primarily an institution for the creation of future educators, and that was Dresselhaus’s original goal, but exposure to science classes, and in particular to the lectures and stringent advice of Rosalyn Yalow, convinced her that chemistry and physics held an interest for her that couldn’t be matched by a generic liberal arts teacher education program, so she applied for, and handily won, a Fulbright Scholarship to study physics in Cambridge.
After the Second World War, superconductivity was an It topic of study, and Dresselhaus’s 1957 thesis demonstrated an interesting relation between electromagnetism and superconductivity that nobody had a ready answer for, and wouldn’t for many years. It was an intriguing enough result to earn her a spot first at Cornell, then MIT. She worked with Ben Lax, who was among the first to apply lasers to investigate the electric properties of semiconductors. Whereas most of us would be sold at “using lasers,” Dresselhaus saw potential in applying these approaches to rigorously plumb the largely ignored depths of carbon structures.
As she explained in an interview with the Kavli Foundation, “Carbon materials were attractive because they had small effective masses and widely spaced energy levels that electrons could inhabit, and it only took a small amount of energy to bump the electrons up and down between levels.” Carbon seemed an ideal substance to investigate with the new methods, and important results started flying from Dresselhaus’s lab.
The closer she looked, the more interesting properties she found in one of carbon’s most common forms, graphite. Graphite is made of numerous stacked layers of carbon sheets that are weakly attracted to each other. When you drag a pencil across a page, you are breaking those weak bonds, leaving layer after layer of carbon behind. Not content with looking at graphite’s structure, Dresselhaus investigated the individual layers of which it is composed. These one-atom-thick sheets of carbon are known as graphene, and their properties are exquisite.
To begin, unlike in, say, diamond, where each carbon is linked in a three dimensional tetrahedral structure with four other neighboring carbons via an sp3 hybridization, in graphene, each carbon is linked only to three other carbons through an sp2 hybridization, which allows the sheet to lie flat, and provides for a truly remarkable tensile strength. In the 1980s, after a series of experiments that led to the discovery of fullerenes (cylindrical or spherical molecules of carbon for which Richard Smalley, picking up on Dresselhaus’s work, would win the Nobel Prize), Dresselhaus predicted that one could make an elongated, single-atom thick tube of sp2 bonded carbon, what we know today as a carbon nanotube.
This theoretical prediction of three decades ago is now a burgeoning industry of enormous potential. Single and multiple-walled nanotubes are now a full reality, and their properties are every bit as marvelous as Dresselhaus predicted. You start with a sheet of graphene. Depending on how you roll it up (as represented by m and n, the magnitudes of the two vectors you use to denote the tube’s eventual central axis), the resulting tube will have different thermoelectric properties. Some tubes will be metallic in character, and some will be semi-conductors.
The resulting structures possess the highest tensile strength of any substance on Earth. Nanotubes have been tested at 63 Gigapascals, or 114 times the value for structural steel. Such a cable could support 14,000 lbs while having a cross section of less than one square millimeter. And not only is it strong, but its thermal conductivity is off the charts, allowing a flow of energy at ten times the rate that permitted in copper. And NOT ONLY THAT, but the electric conductivity on metal-type nanotubes (where m=n) is a thousand times that of copper.
Strong, conductive, and increasingly cheap to produce, the excitement about the micro-engineering potential of nanotubes is understandably intense, and it is all a result of one woman’s dogged determination to fully catalogue, over the course of a half century, the properties of a substance deemed too common for sustained scientific interest. At age 84, her research continues, not only deepening her investigation of the properties of nanotubes through Raman spectroscopy, but starting an inquiry into the behavior of bismuth nanowires, a new element for a new century. Awarded the Presidential Medal of Freedom in 2014, her work both as a scientist and advocate for women in science recognized the world over, Mildred Dresselhaus is a modest living legend, and a sign for us all that the universe is yet hiding some of its greatest secrets in plain sight.