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Carbon Nanotubes as Photoabsorbers & Defining and Refining Graphene Nanostructures from the Bottom-Up: Nanocarbon Materials for Next-Generation Electronics and Photonics

Seminar Group: 


Professor Michael S. Arnold


Materials Science and Engineering Department
University of Wisconsin, Madison


Friday, May 31, 2013 - 4:00pm


ESB 1001


Professor Ed Kramer

The crystalline extended aromatic backbone of carbon nanotubes and graphene-derived nanostructures gives rise to exceptional electronic and optoelectronic properties. In this talk, I will detail two of our recent advances in carbon-based electronic materials:

(1) First, inspired by the successes of polymer photovoltaics, we are investigating the use of bandgap-controlled and electronic-type sorted semiconducting carbon nanotubes in photovoltaic devices in which the nanotubes are the photoabsorbing “polymers”. The goal is to exploit carbon nanotubes’ strong near infrared absorptivity, ultrafast transport characteristics, and stability. We show that the nanotube exciton binding energy can be overcome in donor / acceptor heterojunction schemes, realizing an internal quantum efficiency for exciton dissociation and charge collection > 85%. We elucidate the underpinning length-scales and mechanisms for exciton migration using photocurrent and optical spectroscopy, Monte Carlo simulation, and ultrafast transient absorption. So far, we have explored these materials in simplified bilayered heterojunction devices using nanotube films ~ 5 nm in thickness (matched to the inter-nanotube diffusion length), realizing a 1% AM1.5G solar cell but with very high > 75% internal QE. Many times better power conversion efficiency will ultimately be possible if light can be harvested from more material by blending or nanostructuring and controlling the anisotropy and dynamics of exciton migration.

(2) Secondly, I will discuss our progress towards achieving high-performance graphene nanostructures via planar processing methods. We have realized new scalable routes for patterning graphene with sub-10 nm resolution by exploiting self-assembling lithography, with the potential to give rise to high performance semiconducting behavior if edge-defects can be controlled. We show that edge-defects can be decreased in the nanostructures by avoiding top-down etching and instead employing a novel bottom up growth strategy that we have developed called barrierguided chemical vapor deposition (BG-CVD). In BG-CVD, graphene growth is laterally confined within nanoscale channels using prepatterned barrier templates on Cu catalyst substrates. The approach relies on self-limiting crystal growth processes to more abruptly define edges. We demonstrate crystalline nanoribbon arrays and nanostructures with 25nm features and edgedefects reduced by 10x or more. Edges can furthermore be refined by exploiting kinetic and thermodynamic factors during CVD.