Optoelectronics with Carbon Nanotubes

This work explores both fundamental physical properties and application possibilities of
electroluminescence (EL) from semiconducting CNT devices. While there has been much work
on photoluminescence (PL) from CNTs in solution and suspended in air (see, for example, the
chapter on PL by Lefebvre, et al. in Ref. 6 and references therein), electrically induced
luminescence has been less explored because of complexity of device fabrication, its typically
low luminescence efficiency, e.g., as low as 10 -10 in some cases 10 , and limitations in the
detection technology of infrared radiation. Additionally, until 2009, when EL from CNT p-n
junction was observed for the first time 11 , very high fields and accompanying high
current/heating was a hindrance to elucidating the details of luminescence characteristics. While
laser-induced PL studies have done a great deal to help us understand the fundamental physics of
CNTs, it is vital that we understand their EL mechanisms and apply the knowledge to the
development of carbon-based, nano-scale optoelectronics in order to capitalize on the unique
properties of CNTs in application.
In the effort to contribute to these endeavors, this thesis reports on the findings from three
distinctly different CNT device structures: (1) conventional bottom-gated CNT field-effect
transistors (CNTFETs) operated in the unipolar transport and emission regime; (2) single-tube,
top split-gate CNT p-n junction devices as light-emitting diodes (LEDs); and (3) top split-gate p-
n junction LEDs with self-aligned, purified semiconducting SWNTs. Each device probes
different aspects of EL mechanisms, the results of which, we hope, will come together to play an
important part in advancing the science and technology of this fascinating material. In the rest of
this chapter, fundamentals of carbon nanotube physics are discussed with a particular emphasis
on their transport and luminescent properties to provide background information for subsequent
chapters.
1. Structure of single-wall carbon nanotubes (SWNTs)
With the diameter in the order of only a nanometer and the length ranging up to microns
and even millimeters, SWNTs are a prime example of almost perfect one-dimensional structures.
This reduced dimensionality has several significant consequences for their electronic and optical
properties, which are discussed in this and the following sections.
2