Optoelectronics with Carbon Nanotubes

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experiments 6 , and thermal heating, which strongly influences the EL of metallic as well as semiconducting SWNTs 87 , does not play a role. (a) (b) EL Intensity (arb. units) 60 40 20 0 0 50 100 150 200 Drain-Source (Current (nA)) Figure IV-2. Identification of the light emission mechanism. (a) The upper plane is an SEM image of a CNT diode. The lower plane is a surface plot of the infrared emission. A microscopy image of the device (not shown) was taken under external illumination in order to verify that the emission is localized at the position of the CNT. Infrared emission is observed at the position of the tube when the device is operated as a LED (VGS1 = -8 V, VGS2 = +8 V; left image). In contrast, no emission is observed when a unipolar current of equal magnitude is driven through the nanotube (VGS1 = -8 V, VGS2 = -8 V; right image). (b) Integrated EL intensity as a function of current. The linear fit indicates that the mechanism is threshold-less and that the emission is proportional to the carrier injection rate. (c) EL spectrum of a SWNT diode at IDS = 200 nA. The red line is a Lorentz fit. After Ref. 11. 71 (c)
After calibrating our detection system against the infrared emission from a known black- body emitter and taking into account its collection efficiency, we estimate an EL efficiency of ~0.5–1 10 -4 photons per injected electron-hole pair. Given a radiative carrier lifetime of 10– 100 ns in CNTs 33, 34, 78 , we obtain a non-radiative lifetime τL in the order of a few picoseconds. This value is smaller than what is typically observed in PL measurements 130 (~20–200 ps). However, our value for τL appears reasonable because of the interaction with the environment and a higher concentration of carriers, both of which cause an increase of the non-radiative decay rate 141 . In normal PL measurements, the tube is suspended while our tube is directly on a SiO2 substrate and is covered by a layer of ALD-deposited Al2O3, for the estimated dielectric constant of εeff = 5.7. Fig. IV-2 (c) shows the spectrally dispersed emission of a SWNT diode at IDS = 200 nA. It is composed of a single, narrow peak centered at ~0.635 eV, with a spectral width of ~50 meV. Note that this is much narrower than the widths discussed in Chapter III. Based on a correlation between the EL results with PL and Raman data 109, 142 , we assign the EL peak to emission from the lowest-energy bright exciton state E11 in the CNT. The E11 designation is also consistent with the known diameter distribution of the sample (see also the discussion of E11 peak assignment in Chapter III). Now we touch on the double-peak feature in emission spectra that was observed in many CNTFETs (Chapter III) and also evident in some of CNT diodes. We now have much narrower linewidth which allows us to analyze these two peaks quantitatively. In Figure IV-3 (a) we present the results obtained from another device. Besides the dominant emission at ~0.755 eV (labeled X), a weaker luminescence band is observed at ~65 meV lower energy (LX). We can rule out the possibility that X and LX are originating from two separate tubes in a multi-walled CNT. The small energy spacing translates into a diameter difference which is much less than twice the graphite lattice-plane distance. In order to further confirm that both emission peaks do not stem from a small bundle of CNTs, we characterized the tube by resonance Raman spectroscopy and atomic force microscopy (AFM). In the Raman measurements, we tune the excitation laser energy between 2.0 and 2.5 eV, i.e. across the E33-range that corresponds to the diameter-range of our CNT sample. Only one single radial-breathing-mode (RBM), centered at ωRBM ~ 200 cm -1 , was observed, from which the CNT diameter is determined to be 37 dt = 72
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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
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: 3. Electroluminescence mechanism an
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
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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