Optoelectronics with Carbon Nanotubes

and drain pads (marked “S” and “D” respectively) and the top gates (marked “VTG1”
and “VTG2”). (b) EL signal from the same device as in (a). (c) Combined image of
(a) and (b), showing that the EL signal is located between the two top gates. (d)
Spatial cross sections of a reflection as in (a) against a unipolar and ambipolar
emission signals across the device channel. The two top gates (lateral spacing = 1
μm) can be recognized as double-peak feature. Detectable EL emission originates
from the device area between the two split gates (unipolar: split gate voltages off;
ambipolar: split gate voltages on).
Figure V-7. Light output characteristics (spectrally integrated EL intensity) of both a
CNT thinfilm LED (red circles) and a single-tube diode (black squares). Both
devices are biased in the forward mode. The corresponding split gate voltages
VTG1/VTG2 are -5V/+5V for the film device and -20V/+20V for the single-tube device.
After Ref. 152.
Figure V-8 (a) shows the EL intensity of a device integrated over wavelength as a
function of source-drain voltage. As a reference, we plot in the inset of Figure V-8 (a) the
electrical transport characteristics of the same device. We see from the IDS–VDS curves that this
device has the same type of top-gate dependence as the devices in Figure V-4. In the forward-
bias direction (i.e., positive VDS in Figure V-8 (a)), we observe a substantial increase of light
intensity up to a factor of 16 for a fixed VDS as we increase VTG1/VTG2 from 0V/0V to +5V/-5V.
In the reverse-bias direction, despite the high currents achieved (see the negative VDS side in
inset Figure V-8 (a)), we only observe relatively weak electroluminescence if we increase |VDS|.
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