[MOS] March 3, 2009

[Zina Queen]

Seminar on

Modern Optics and Spectroscopy

Optical spectroscopy of individual carbon nanotube p-n diodes

Ji Ung Lee,
  University of Albany, SUNY

Tuesday, March 3, 2009

12:00 - 1:00 p.m.

The p-n junction diode is the basis for nearly all-modern 
semiconductor electronics. It is the basis for most transistors and 
optical devices. The p-n structure is also useful for studying 
fundamental materials properties. Here, we show that a carbon 
nanotube p-n diode can provide a comprehensive probe of the optical 
and electronic transitions in SWNTs.  The p-n doping is achieved 
using a split gate structure that electrostatically dopes the two 
ends of a single nanotube.  The resulting diode can exhibit an ideal 
diode behavior, the theoretical limit of performance for any diode. 
In the photocurrent spectra, an alternating sequence of resonant 
peaks from the dissociation of excitons and exciton-phonon bound 
states is observed, for the lowest and higher electronic subbands. 
At an intermediate energy, the onset of continuum is observed that 
allows measurements of exciton binding energies. The measured binding 
energies are large (>0.25eV), and both the binding energy and the 
onset of continuum follow the inverse diameter relation as expected 
from general theory of optical transitions in nanotubes.  In addition 
to the energy levels revealed in the photocurrent spectra, detailed 
transport measurements provide a complete set of energy levels of the 
p-n structure.  Specifically, we demonstrate that bandgap 
renormalization, the shrinkage of the fundamental bandgap due to 
many-body exchange-correction properties of interaction electrons, 
dramatically alters the electronic structure, resulting in the 
formation of heterointerfaces along a homogeneous material. 


Grier Room, MIT Bldg 34-401
Refreshments served after the lecture
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