Optoelectronics with Carbon Nanotubes

Chapter V The Polarized Carbon Nanotube Thin Film LED
1. Introduction
In the previous chapter, a light-emitting CNT p-i-n junction was demonstrated with a
single CNT. Such a device is a basic photonic building block and paves the way for application
of CNTs in nano-optics and photonics 111 . The possibility of using a single CNT device as a
quantum light source has also been explored 151 . The high quantum efficiency of our devices
made it possible to produce narrow emission lines that in turn enabled us to analyze individual
emission peaks in detail. While the single-tube LED has such advantages when studying the
underlying physics of electrical emission, the likelihood of a device failure is understandably
high. Moreover, light output is limited by the maximum current a single CNT can carry. Small-
diameter CNTs (d ~1 nm), when aligned at mid-gap with the Fermi level of the metal, have a
typical device resistance in the order of 1 to 10 MΩ, which sets the current level at a few
hundred nanoamperes at normal operating voltages. The operating drain-source and gate
voltages are mostly limited by the breakdown of the gate dielectric and cannot be increased
beyond a certain level, typically a few volts in the geometry used. One solution is to increase the
number of channels so that the total current-carrying capacity is increased while the load per tube
is decreased. This reduces the probability of a tube failure, and even in the event of a single tube
breakdown, the device can continue to function.
Consequently, the light emission properties of CNT films and networks 10, 97, 99 have been
attracting recent interest beyond single CNT applications. CNT films can be readily assembled
from solution and allow for the possibility of scaling up both the current and the amount of light.
In addition, CNT films achieve more consistent and stable output among devices by “averaging-
out” the heterogeneities of individual CNTs. Furthermore, self-assembled and highly aligned
arrays with many CNTs in parallel are expected to preserve the polarization provided by the one-
dimensional character of individual CNTs. Since emission and absorption of light that is
polarized parallel to the long axis of a CNT occurs with higher efficiency, an aligned CNT film
could be a promising building block for future laser applications.
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