Optoelectronics with Carbon Nanotubes

numerical aperture of the objective lens, we are collecting in the solid angle that covers about 4%
at 90-degree angle to the tube direction.
Figure V - 12 clearly shows that the electroluminescence is linearly polarized in such a
way that intensity maxima are obtained if the transmission direction of the polarizer is aligned
with the CNT direction. The cosine-squared fits reveal intensity ratios Imax/Imin (i.e. ratios with
polarizer at 0°/180° over 90°/270° in Figure V-12) close to 3.5. The degree of polarization is of
the same order as those obtained in electroluminescence 73, 87 , photoluminescence and Raman 137
as well as photoconductivity measurements 135, 136 performed on single nanotubes, demonstrating
the extraordinary level of alignment in the CNT film.
We did not observe any statistically significant difference in the degree of light
polarization when the CNT film diode is operated in the reverse mode (see Figure V-12 (b)).
This is not unexpected, since the polarization dependence of the electroluminescence
measurement reflects the nature of exciton recombination rather than exciton creation. Since we
do not observe light emission from the CNT-metal-contacts that are covered by the metallic gates
(see Figure V-6 (d)), the polarization dependence in both forward and reverse mode reflects
solely the alignment of the CNTs in the “active area” between the split gates (see Figure 1).
(a) (b)
Figure V-12. Polarization dependence of the electroluminescence intensity from a
CNT thin film diode in the (a) forward mode and (b) reverse mode (VDS=10V for
both (a) and (b)) for two different split gate settings VTG1/VTG2 (solid symbols:
105