Toshiaki Enoki
Department of Chemistry, Tokyo Institute of Technology

Photo credit: Paul McGrath
Photo credit: Paul McGrath
Millie, Gene, Miho Sugihara, my family, 1984.
Millie, Gene, Miho Sugihara, my family, 1984.

My interaction with Millie started in 1984, when I worked in her labs as a visiting scientist. Our research subject was the electronic structure of hydrogenated potassium-graphite intercalation compounds KHxC4. At that time, her labs were not so well organized for chemists, since her activities were devoted mostly to experiments on condensed matter physics. So I built a handmade hydrogenation reaction system out of glass, inside one of the rooms in her lab, which allowed me to prepare KHxC4.

2D Megallic hydrogen, 1990.
2D Megallic hydrogen, 1990.

After I finished my preparations, I began experiments on Shubnikov-de Haas oscillations using Bitter magnets at the Francis Bitter Magnet Lab. The samples were air sensitive and our machine-time was limited, so we were very nervous about handling the samples at low temperatures under high magnetic fields. Our experimental results, together with H-NMR investigations, lead us to discover the presence of two-dimensional metallic hydrogen structures in the graphitic galleries.

Preparation & STM observation of nanographene (10 nm), 2001.
Preparation & STM observation of nanographene (10 nm), 2001.

My collaboration with Millie continued even after I left her lab. In our ongoing collaborations, we were often joined by Peter C. Eklund and Robert Haddon, who have (unfortunately) already passed away. We also collaborated with several students from Millie’s lab, as well. Millie and I were interested in researching many materials together, during our collaborations, but activated carbon fibers (ACFs) proved particularly interesting to us, since they showed novel electronic and magnetic properties.  This was fascinating and differentiated them from other carbon-based materials. ACFs consist of 3D disordered network of nanographites.

STM obs. & DFT cal. of edge states in a hydrogenated zigzag edge, 2013.
STM obs. & DFT cal. of edge states in a hydrogenated zigzag edge, 2013.

High temperature annealing induced fusions, resulting in a transition from insulating to a metallic state around 1600 K, accompanied by a change in magnetism from Curie-type one to Pauli paramagnetic one. In the vicinity of the insulator-to-metal transition, a spin glass state appeared. Around the same time, Mitsutaka Fujita, who passed away later at the age of 38, theoretically predicted the presence of a nonbonding π-electron state in the vicinity of graphene zigzag edges and named it “edge state”.

Millie and T. Enoki, 1997
Millie and T. Enoki, 1997

We had a number of intense discussions with Fujita, and in 2000, we assigned the origin of the Curie spins to be of the edge state. In 2001, we felt challenged to find nano-fragments of graphene experimentally, and we found them in STM observations. We named this “nanographene”. This is the first time that the term “nanographene” was ever utilized. We confirmed the detailed electronic structures of edge state experimentally, using STM/STS observations, in 2005.

Overall, I am very thankful to have had the opportunity to collaborate with Millie over so many years.  I will always remember her whenever I read a paper about nanographene.

K. Wakabayashi, Millie, T. Enoki, 2011.
K. Wakabayashi, Millie, T. Enoki, 2011.