Optoelectronics with Carbon Nanotubes

mechanisms are the same for different operating modes, other than the small field that exists
between the top gates in the diode mode.
Figure V-11. Total electroluminescence intensity as a function of current, i.e., injected
carriers. The intensity is extracted as the area of the Gaussian fit (see Figure V-10,
inset). The graph shows that the emission originates from carrier recombination that
requires a higher total current in the unipolar mode (black triangles) in which the
number of minority carriers per unit current is small. Emission per current is
comparable for split-gate and zero-gate modes because the ratio of majority to minority
carriers is similar. See Figure V-10 captions for procedural details of fitting Gaussian
function.
Finally, we investigate the polarization dependence of the electroluminescence emission
of the CNT film diode. Based on SEM (see Figure V-1 (a)) and Raman imaging, individual
CNTs (mostly in bundles) forming the film have been measured to lie almost parallel, within a
range of 5° with respect to one another 99 . Since single CNTs are known to emit light that is
strongly polarized in the direction of their long axes, we expect that the high degree of CNT
alignment in the film translates into a pronounced light polarization effect. We inserted a linear
polarizer in the optical path (Figure II-3) and measured the transmitted intensity as a function of
the angle between the tube direction and the polarizer. We should point out that this measures
the polarization property of the light itself, not the intensity as a function of the collection angle
with respect to the device position and orientation, as in the dipole radiation. Based on the
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