Monday, February 4, 2013 - 16:00 to 17:00
Weniger 153
Event Speaker: 
Dr. Matt W. Graham, Kavli Institute at Cornell for Nanoscale Science and Laboratory for Atomic and Solid State Physics, Cornell University
Local Contact: 
Michael Zwolak

When bulk carbon is reduced to nanoscale dimensions many-body interactions and correlations are remarkably enhanced, drastically changing the optical and electronic properties in highly unpredictable ways. For instance in 1D, electronic correlations and screening in semiconducting carbon nanotubes result in strongly-bound “molecular-like” exciton states. By time-resolving exciton-state energy transfer with 2D Fourier-transform electronic spectroscopy, we establish a mechanism for “off-resonant” absorption via phonon side bands. In two dimensions, carbon becomes graphene. We develop a tool that provides ultrafast movies of this 1-atom thick membrane on most any substrate. These resulting time-space graphene images show tuneable exciton-like absorption resonances arise at precise graphene layer staking orientations. Within ~0.2 eV of the Fermi energy, stringent energy-momentum restrictions in graphene predict very inefficient energy dissipation. We study this low-energy regime by fabricating graphene p-n junctions and time-resolving the photocurrent generated. The photocurrent generated functions as an ultrafast hot electron thermometer. This measure of electron cooling rates near the Fermi energy demonstrates a new “supercollision” mechanism for graphene electron cooling, where the stringent energy-momentum constraints are relaxed.