Optoelectronics with Carbon Nanotubes

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Finally, we note the observation of EL from carbon nanotube networks. An array of many CNTs forming a thin film offers greater robustness than single-tube devices, and is more reproducible and consistent because of statistical averaging among the tubes. Thin CNT films and transistors made from such structures have garnered attention as a viable material in flexible and transparent electronics component with high mobility 90-96 . EL from a random network of SWNTs as a transistor device was first observed by Adam, et al. by ambipolar recombination of carriers 97 . The spectral analysis of the light emission revealed that it was preferentially generated in the largest diameter tubes in the sample; they found that convoluting the density of states and the Fermi-Dirac distribution of carriers reproduced the red-shift observed in the main EL peak relative to the absorption peaks. Their calculations show that the largest diameter tubes, though comprising only 4 % of their sample, dominate the EL spectrum. The quality of CNTFETs was greatly improved by Engel et al., when they combined 99% purely semiconducting tubes produced by density gradient ultracentrifugation 98 and a CNT self- assembly technique from solution 99 ; they succeeded in fabricating FETs from highly aligned CNT thin films with a high on/off ratio and a very low sheet resistance. The devices operated in ambipolar transport regime, and the EL from this material was also found to emit mainly from large-diameter tubes even at higher source-drain biases (VDS). They attribute this to the fact that the current is carried preferentially in large-diameter tubes, and that excitons created in smaller- diameter tubes can decay into large-diameter tubes before decaying radiatively. This same material was used in the film LED experiment in this work that will be discussed in much more detail later. EL has also been observed from an array of aligned SWNTs grown on a quartz substrate. It is possible to grow very straight and long carbon nanotubes that are aligned along a crystallographic plane of quartz. Zaumseil et al. achieved an efficient injection of carriers by using electrolytes instead of oxide dielectric, which enable a high capacitive coupling between the gate and the nanotube array grown on quartz 10 . They observed many ambipolar emission spots at relatively low source drain voltage (|VDS| < 3 V) thanks to the efficient gating provided by ionic liquids. However, the calculated light emission efficiency is quite low (~10 -9 ), although this may be partly due to the limits of detection. Another problem is the presence of metallic tubes in the array, resulting in high off-currents and poor on/off ratios. There is little control over the metallic/semiconducting ratio and the diameter distribution of aligned nanotubes grown 25
on quartz, which remains a significant challenge not just on quartz, but in any carbon nanotube growth method. Therefore, the purified tubes employed by Engel, et al. and in this work are the most promising in device application to date. 7. Problems with carbon-nanotube optoelectronics addressed in this work While carbon nanotubes are attractive materials for nano-scale optoelectronics, their practical use in application is still far from being realized, due to our limited understanding of EL mechanisms and unresolved challenges associated with the operation of CNT devices. PL studies have made some headway in understanding the excitonic nature of light absorption and emission, but the observations are from higher transition energies because of the ready availability of laser wavelengths in the visible range that is resonant with higher energy states, and also due to the difficulty of detection in the near infrared range below the silicon bandgap. In EL, lowest energy states are populated first, so the strongest emission is naturally from the E11 state. In addition, smaller-diameter CNTs (i.e., tubes with larger transition energies) create higher Schottky barriers, which impede electronic transport, so a certain diameter is required for electronic operation. We first tackled this problem by choosing just the right range of diameters and by using a specialized infrared camera that allowed us to detect energies as low as ~0.6 eV (see Methods). Chapter III analyzes transport and spectroscopic data thus obtained on the intensity, spectral shape and polarization of emitted photons, with the aim of understanding the characteristics of EL emission, such as carrier and phonon interactions, emission efficiency, and energy threshold for creating excitons. Other significant problems in analyzing EL emission are its very broad spectral shape and the low emission efficiency. From the analysis presented in Chapter III, it will be apparent that the very operating principle of single-tube CNTFET is the main contributor to broadening. The broad spectral shape obscures different emission peaks and greatly reduces the signal-to-noise ratio in spectra, making data analysis very difficult, if not impossible. We solved this problem by creating p-n junctions constructed with single tubes, which is discussed in Chapter IV. By doing away with high electric fields at contacts and controlling the p- and n- junctions electrostatically, we were able to obtain narrow linewidths with high signal-to-noise ratios. A 26
Page 1 and 2: Stony Brook University The official
Page 3 and 4: Stony Brook University The Graduate
Page 5 and 6: diodes nonetheless show a rectifyin
Page 7 and 8: Table of Contents Abstract ........
Page 9 and 10: Chapter I List of Figures Figure
Page 11 and 12: Figure ‎V-3 Source-drain electric
Page 13 and 14: My parents have been steadfast supp
Page 15 and 16: This work explores both fundamental
Page 17 and 18: In fact, we typically have no knowl
Page 19 and 20: (a) (b) Figure I-2 (a) Energy dispe
Page 21 and 22: (a) (b) metallic Figure I-4. One-di
Page 23 and 24: optical absorption peaks for differ
Page 25 and 26: elax rapidly to lower non-radiative
Page 27 and 28: The disorder-induced band (D-band)
Page 29 and 30: The saturation limit for semiconduc
Page 31 and 32: (a) (b) Figure I-7. (a) A schematic
Page 33 and 34: assisted tunneling through Schottky
Page 35 and 36: majority carriers that gain enough
Page 37: conventional ambipolar emission. Th
Page 41 and 42: Chapter II Methods 1. Materials One
Page 43 and 44: diameter (< 2 nm) SWNTs at IBM T. J
Page 45 and 46: devices were annealed in vacuum at
Page 47 and 48: 3. Experimental set-up The optical
Page 49 and 50: off wavelengths of 2150 nm, 2000 nm
Page 51 and 52: Chapter III Unipolar, High-Bias Emi
Page 53 and 54: Figure III-1. Semi-log plot of drai
Page 55 and 56: wetting with CNTs 57 and a relative
Page 57 and 58: Figure III-4. (Main panel) Electrol
Page 59 and 60: By plotting the current as a functi
Page 61 and 62: phonon temperature in broadening. S
Page 63 and 64: found multiple tubes bound together
Page 65 and 66: where i is the phonon mode, Tsub is
Page 67 and 68: The main panel of Figure III-10 sho
Page 69 and 70: the effect following Perebeinos’
Page 71 and 72: the optical phonon population is no
Page 73 and 74: (a) Figure III-13. (a) Spectra from
Page 75 and 76: DOP = I║ / (I┴ + I║) = 0.77.
Page 77 and 78: inding energy for perpendicular exc
Page 79 and 80: 3. Conclusions We have examined the
Page 81 and 82: In a split-gate scheme, a new level
Page 83 and 84: 3. Electroluminescence mechanism an
Page 85 and 86: After calibrating our detection sys
Page 87 and 88: (a) (b) Figure IV-3. Electrolumines
Page 89 and 90:
observed by increasing the VGS valu
Page 91 and 92:
We claimed in Chapter III that in t
Page 93 and 94:
Let us finally comment on the effic
Page 95 and 96:
Chapter V The Polarized Carbon Nano
Page 97 and 98:
(a) (b) Figure V-1. (a) SEM image o
Page 99 and 100:
oth electrons and holes can be inje
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In the reverse direction (i.e., neg
Page 103 and 104:
4. Electroluminescence characterist
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and drain pads (marked “S” and
Page 107 and 108:
(a) (b) Figure V-8. (a) EL intensit
Page 109 and 110:
We attribute the observation of the
Page 111 and 112:
(a) (b) (c) Figure V-9. Electrolumi
Page 113 and 114:
measurements (i.e. additional chemi
Page 115 and 116:
(a) (b) Figure V-10. Full-width at
Page 117 and 118:
mechanisms are the same for differe
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experiment, solid line: cosine squa
Page 121 and 122:
emission observed at higher current
Page 123 and 124:
Bibliography 1. Avouris, P.; Chen,
Page 125 and 126:
30. Miyauchi, Y.; Maruyama, S., Ide
Page 127 and 128:
56. Chen, Z.; Appenzeller, J.; Knoc
Page 129 and 130:
85. Marty, L.; Adam, E.; Albert, L.
Page 131 and 132:
112. Steiner, M.; Freitag, M.; Pere
Page 133 and 134:
140. Grüneis, A.; Saito, R.; Samso
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Optoelectronics with Carbon Nanotubes

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