Optoelectronics with Carbon Nanotubes

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248/ωRBM ~ 1.24 nm for this tube. From the AFM cross-section we extract a similar diameter. Those measurements, and the fact that double-peak spectra similar to the one in Figure IV-3 have also been observed in other devices, support our claim that both emission features originate from a single SWNT. The two peaks in Figure IV-3 cannot be identified as the E11 exciton transition and the E11 continuum. The exciton binding energy that we estimate for a 1.24-nm-diameter tube embedded in SiO2/Al2O3 (εeff ~ 5.7) is Eb 0.12 eV 24 , i.e. almost twice as large as the observed splitting. More importantly, the continuum transition carries only a small fraction of the spectral weight 24 (see also Figure IV-5). We can also exclude phonon-assisted emission, because of the different current-dependencies of the two peaks. Phonon assisted peaks are also not expected to be of comparable intensity to the main peak, and the energy separation does not correspond to any phonons that would couple strongly to excitons. Low-energy satellite peaks have repeatedly been observed in PL measurements and have been attributed to localized exciton states 129, 130, 143-145 . We thus assign the peak X to “free” exciton emission and LX to emission from weakly localized excitons. It is not possible to determine from our optical measurements the physical mechanism of the exciton localization. It might be due to environmental fluctuations, leading to the formation of quantum-dot-like states, or brightening of intrinsic dark CNT states at structural defect sites. We note that in one of our devices, the low-energy emission feature LX was initially not present, but developed after stressing the tube by passing a high current through the device. This observation also supports the assignment of LX to emission from a defect site. 73
(a) (b) Figure IV-3. Electroluminescence spectra. (a) EL spectrum of a CNT diode recorded at different drain-source currents IDS between 30 and 230 nA. Gate biases of VGS1 = - 9 V and VGS2 = +9 V were applied. The data can be fitted well with two Gaussians and, at low currents, we extract widths of ~45 meV (FWHM) for the individual contributions. Besides the strong exciton emission (labeled X), a weaker satellite peak at lower energy is observed (LX). It is attributed to localized exciton emission. (b) Comparison between EL spectra at two different gate biases (normalized). Solid green line: VGS1 = -7 V, VGS2 = +7 V; dashed grey line: VGS1 = -9 V, VGS2 = +9 V. The spectral width of the -/+ 7 V measurement is only ~35 meV (FWHM). (c) Red symbols: The free exciton emission (X) shows an approximately linear increase with current. Green symbols: Localized exciton emission (LX). The EL saturates as the current exceeds 100 nA. The EL versus current dependence shows no threshold behavior (see also Figure IV-2 (b)). The dashed lines are guides to the eye. (d) Same as (c), but for a different device. After Ref. 11. Figure IV-3 (c) depicts the current dependence of the X and LX emission intensity as extracted from Figure IV-3 (a). The free exciton emission X shows a linear increase with current 74 (c) (d)
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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 ........
Page 9 and 10:
Chapter I List of Figures Figure
Page 11 and 12:
Figure ‎V-3 Source-drain electric
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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
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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
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 and 38: conventional ambipolar emission. Th
Page 39 and 40: 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: After calibrating our detection sys
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
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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