Optoelectronics with Carbon Nanotubes

diameter (< 2 nm) SWNTs at IBM T. J. Watson Research Center where all the devices were
fabricated.
CNT film devices were made from laser-ablation grown bulk nanotubes dispersed in a
surfactant (sodium dodecyl sulphate or SDS) solution. Via density gradient ultracentrifugation,
the sample was purified to ~99 % semiconducting CNTs with a narrow diameter range 98 . The
average length of individual tubes after the processing was about 1 μm. Stripes of individual
tubes and bundles of ~10 μm width were self-assembled by a mechanical slip-stick process 99 .
The SDS was subsequently removed by annealing at 600 °C for 60 seconds in argon. The
sample used for this work had been assembled and processed previously for a CNT film FET
study 99 .
2. Device fabrication
i. CNTFETs
For single-tube devices, CNTs produced by laser ablation or arc-discharge method were
suspended in 1, 2-dichloroethane by sonication at a medium power setting for about 30 minutes,
which was found sufficient to separate CNT bundles into individual tubes. The resulting tubes
range in length from sub-micron to ~10 μm, with the majority of them in the 1 to 5 μm range.
The solution was drop-cast onto a highly p-doped Si wafer with a 100 to 300 nm thermally-
grown SiO2 layer. After waiting about 30 seconds for tubes to adhere to the surface by van der
Waal forces, the extra solution was blown off by dry nitrogen gas. The solution concentration
was adjusted until the surface density of the nanotubes imaged with an SEM was determined to
be spatially separated, in the order of 10 CNT per 1000 μm 2 . While the spatial resolution of the
SEM does not approach nanometers, imaging at the accelerating voltage of 1 kV distinctly
scatters electrons off of CNT surface and enables us to determine the placement of individual
tubes.
Contact electrodes were written via standard electron-beam lithography. The distance
between electrodes is 1 μm. Ti = 0.5 nm/Pd = 20~30 nm/Au = 20~30 nm were deposited via e-
beam evaporation. The very thin Ti layer is used for adhesion and does not uniformly cover the
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