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Optoelectronics with Carbon Nanotubes

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measurements (i.e. additional chemical processing), so the dominant peak (around 0.60 eV) is<br />

assumed to be from the defect-bound states. For current film LED spectra, we took the spectral<br />

data in forward bias first, followed by the reverse bias involving higher currents, which would<br />

explain that the free-energy states are still observable in the spectra for the former during which<br />

up to 10.4 μA was passed through the device. By the time the second spectral measurements<br />

were conducted, there was hardly any emission from free-exciton states, although there was still<br />

a hint of it around 0.65 eV at a higher intensity (Figure V-9 (b)).<br />

Figure V-10 shows the widths (FWHMs) of the same spectral data as a function of<br />

applied power and of emission intensity. The devices were operated in three different modes; as<br />

a split-gate diode LED, as a resistor (i.e., no applied gate voltage), and as a global back-gated<br />

FET. We found that the Gaussian functional form fits our data well (Figure V-10, inset), from<br />

which to extract the widths. There are several features in the plots that distinguish the emission<br />

behaviors of film devices that we can compare to spectral characteristics of single-tube devices.<br />

First, the overall widths are much larger than in single-tube LEDs in which we achieved<br />

the smallest width observed in carbon-nanotube electroluminescence. Film LEDs are<br />

constructed of tubes <strong>with</strong> a range of diameters and are operated at much higher overall power, so<br />

the larger width is not particularly surprising. The single-tube LED is constructed from a clean<br />

CVD-grown nanotube and is operated at the level 0.01 W/m. The film device is fabricated <strong>with</strong><br />

many chemically-processed nanotubes deposited onto a substrate and requires at least 100 times<br />

larger power input to observe luminescence <strong>with</strong> the same sensitivity. This means that we do not<br />

have sufficient data at the lower end of the power range to extrapolate the behavior of<br />

luminescence intensity in film devices to that of single-tube devices. Since one of the objectives<br />

of constructing the film LED is to scale up the light output of nanotube-based devices rather than<br />

to elucidate fundamental principles of the emission process, the comparison between the single-<br />

tube and film LEDs is not particularly useful.<br />

Consequently, we discuss here some of the mechanisms that are relevant to the spectral<br />

width in film devices, <strong>with</strong> an emphasis on the effect of the electric field, which was found to be<br />

the dominant source of bias-dependent broadening in single-tube FET emission. Figure V-10 (a)<br />

shows a width of ~135 meV for the FWHM in the film device operating as a single-gate FET at<br />

the input power of 250 W/m. Let us recall that the FWHM was about 300 meV at the same input<br />

100

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