Optoelectronics with Carbon Nanotubes

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Figure IV-5. Comparison between ambipolar and unipolar light emission. Green line: EL of a CNT diode. Red line: EL emission from a FET device. The upper curve is offset for clarity. Both devices undergo the same processing steps with the only difference being the absence of top-gate electrodes in the FET. Inset: In the FET emission measurements, biases of VDS = +5 V and VGS = -20 V were applied to the drain- and (silicon) back-gate electrodes, respectively. An electric field of >25 V/μm occurs near the drain electrode. Electron-hole pairs are generated by impact excitation. Green line: Calculated absorption spectrum of a 1.4 nm diameter tube (εeff = 6.0) at zero field. X is the exciton transition, FC denotes the band-to-band (free carrier) transitions. Red line: Calculated absorption spectrum for a field of 25 V/µm. After Ref. 11. 79
Let us finally comment on the efficiency of the CNT LEDs. Measurements of the PL efficiency of SWNTs yielded values up to 77, 79 ~1 × 10 -2 , whereas in our devices we obtain at most ~10 -4 photons per injected electron-hole pair. This difference of about two orders of magnitude can be understood by taking the following two factors into account. First, only a fraction of electrically induced electron-hole pairs possesses the right spin to populate radiative singlet exciton states; [1 3 exp( /kT)] times as many populate non-radiative triplet states (k is the Boltzmann constant, T = 300 K, and Δ is the singlet-triplet splitting). Using literature values 33, 34 for Δ, we estimate that this effect reduces the efficiency by about an order of magnitude. Second, the short non-radiative lifetime leads to an efficiency reduction by another order of magnitude. Possible routes for improving the efficiency hence would be brightening of the triplet states – for example, by adding magnetic nanoparticles 143 – and/or suspending the CNT to increase the non-radiative lifetime. 4. Conclusions P-n junction devices were created with single carbon nanotubes using top split gates that electrostatically control spatially-separated p- and n- regions. The transport shows a clear rectifying behavior when the gates are biased in opposite voltages. Compared to conventional ambipolar devices as seen in devices with a global back gate 73, 74 , the p-n junction thus created and controlled emits light much more efficiently and at low input power, leading to a narrow linewidth that approaches that of photoluminescence. The narrow spectra allow us to analyze emission characteristics in greater detail than was previously possible. In some devices, two peaks associated with the E11 transition were observed, separated by ~65 meV. We assign the higher-energy peak to free excitons and the lower-energy peak to weakly localized excitons. The latter may be bound to local potential fluctuations, such as defects. As we increase current, we observe a saturation of emission intensity from such localized excitons, suggesting a higher exciton density and a local annihilation mechanism by exciton-exciton interaction. We also observe no shift in the free exciton peak, which indicates a constant local charge density as expected in the recombination region of the p-i-n structure. Conversely, the lower-energy peak red-shifts with increased 80
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Stony Brook University The official
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Stony Brook University The Graduate
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diodes nonetheless show a rectifyin
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Table of Contents Abstract ........
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Chapter I List of Figures Figure
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Figure ‎V-3 Source-drain electric
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My parents have been steadfast supp
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This work explores both fundamental
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In fact, we typically have no knowl
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(a) (b) Figure I-2 (a) Energy dispe
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(a) (b) metallic Figure I-4. One-di
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optical absorption peaks for differ
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elax rapidly to lower non-radiative
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The disorder-induced band (D-band)
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The saturation limit for semiconduc
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(a) (b) Figure I-7. (a) A schematic
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assisted tunneling through Schottky
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majority carriers that gain enough
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conventional ambipolar emission. Th
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on quartz, which remains a signific
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: We claimed in Chapter III that in t
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
Page 101 and 102: In the reverse direction (i.e., neg
Page 103 and 104: 4. Electroluminescence characterist
Page 105 and 106: 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
Page 119 and 120: 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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