Optoelectronics with Carbon Nanotubes

Chapter II Methods
1. Materials
One of the most important characteristics of the CNTs for optoelectronic experiments is
their diameter distribution since the E11 transition energy depends inversely on the diameter.
Practically speaking, an appropriate distribution of diameters is required for the emission signal
to fit within the measurement energy window of about 0.55 eV to 1.2 eV. This energy range
approximately corresponds to the diameter range of 0.7 nm to 1.5 nm. The empirical data show
that the transition energy actually varies a great deal depending on the chirality, not just the
diameter 15 , so it is likely that the practical diameter range extends further.
Of the several standard ways to determine the CNT diameter, transmission electron
microscopy (TEM) is the most direct and accurate, but it cannot be conducted on a wafer surface.
Atomic force microscopy (AFM) has a measurement error of at least 0.2 nm (about 0.5 nm for
the AFM system that was available to us), too large a fraction of a CNT diameter to measure it to
any useful precision. A more promising approach is to use Raman spectroscopy to determine the
phonon frequency of the tube’s radial breathing mode (RBM), which is inversely proportional to
the diameter 37 . However, Raman signals, especially the RBM signal, are very weak from a CNT
on a wafer surface (in contrast to suspended tubes), unless the laser excitation energy happens to
be in resonance with the CNT’s E33 absorption energy. This means that the excitation energy
needs to be tuned while searching for Raman signals, which is a very slow and labor-intensive
process. Given such limitations, it is not practical to determine individual CNT diameters in
advance, so the samples were chosen on the basis of their statistically-known diameter
distribution. Attempts to measure individual diameters by Raman spectroscopy afterwards were
not always successful.
Several reliable methods have been developed to produce different types of carbon
nanotubes (CNTs). For this work, the tubes used were grown using arc-discharge, laser-ablation
and chemical-vapor deposition (CVD) methods. Of the three methods, laser ablation and arc-
discharge produce bulk bundles of nanotubes in quantities of milligrams and even grams, while
the CVD method grows individual nanotubes directly on a SiO2 surface from catalyst particles.
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