Optoelectronics with Carbon Nanotubes
Optoelectronics with Carbon Nanotubes Optoelectronics with Carbon Nanotubes
(a) Figure III-15. EL spectra with a polarizer aligned along (a) and normal to (b) the tube direction. Symbols are experimental data and solid lines are linear combinations of Lorentz function and the blackbody spectrum, as in Figure III-10. (Inset) Integrated intensity of Lorentz peaks (without background) as a function of power. The fits are exponential functions. As we have already seen in Figure III-13, there is evidence for the E12 (or combination of E12 and E21) transition in some of our EL spectra when both E11 and E22 are observed. Since the E12 state couples more strongly to the perpendicular field and the E11 and E22 states to the parallel field, one should be able to observe the difference in relative intensities of these three peaks if the polarization is changed. We examined under polarizer the device used for Figure III-13, where there was a very weak emission peak about half way between E11 and E22 in the unpolarized measurement. The results are shown in Figure III-16 (a) and (b), with just the E12 peak extracted and normalized in panels (c) and (d). The E12 transition is clearly more prominent in the perpendicular polarization, though it is also seen very weakly in the parallel direction. Also, the overall intensity is weaker than that of E11 or E22, as the depolarization effect of the perpendicularly polarized light causes the weak overall coupling between the E12 excitons and the light emission. A large blue shift in the position of E12 in optical absorption is expected theoretically 19, 133 , which has been observed in PLE experiments. Miyauchi et al. found a smaller exciton 63 (b)
inding energy for perpendicular excitation than for parallel excitation, which is responsible for the blue shift 139 . On the contrary, in our data, no blue shift is observed in the E12 transition (Figure III-16 (b)). A theoretical work by Uryu et al. predicts a large blue shift of the E12 peak that depends on the strength of the Coulomb interaction in SWNTs 133 ; their results show that in the absence of a strong Coulomb interaction, E12 is close to (E11 + E22)/2. In the PL measurements in which large blue shifts were observed in transverse polarization, the tubes were suspended structurally or kept in a surfactant suspension in order to reduce interaction with the environment. In contrast, our devices are directly on the substrate, which significantly increases the dielectric constant of the environment that screens the Coulomb interaction. In our analysis, ε = 3.3 is used which includes ε = 3.9 of the silicon oxide substrate. This significantly reduces the Coulomb interaction in the CNTs on substrate, which could account for the absence of a blue shift of the E12 transition. Lastly, E12 and E21 transitions are degenerate in the single-particle framework, but this no longer applies if the asymmetry between valence and conduction bands are taken into account 140 . Miyauchi et al. has found about a 100 meV difference between the two peaks in perpendicular excitations 30 . However, as has been discussed, all excitonic peaks are significantly broadened and we are not able to determine whether what we consider the E12 transition is actually a double peak consisting of E12 and E21 signatures. 64
- 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: DOP = I║ / (I┴ + I║) = 0.77.
- 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
- 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
(a)<br />
Figure III-15. EL spectra <strong>with</strong> a polarizer aligned along (a) and normal to (b) the tube<br />
direction. Symbols are experimental data and solid lines are linear combinations of<br />
Lorentz function and the blackbody spectrum, as in Figure III-10. (Inset) Integrated<br />
intensity of Lorentz peaks (<strong>with</strong>out background) as a function of power. The fits are<br />
exponential functions.<br />
As we have already seen in Figure III-13, there is evidence for the E12 (or combination of<br />
E12 and E21) transition in some of our EL spectra when both E11 and E22 are observed. Since the<br />
E12 state couples more strongly to the perpendicular field and the E11 and E22 states to the parallel<br />
field, one should be able to observe the difference in relative intensities of these three peaks if<br />
the polarization is changed. We examined under polarizer the device used for Figure III-13,<br />
where there was a very weak emission peak about half way between E11 and E22 in the<br />
unpolarized measurement. The results are shown in Figure III-16 (a) and (b), <strong>with</strong> just the E12<br />
peak extracted and normalized in panels (c) and (d). The E12 transition is clearly more prominent<br />
in the perpendicular polarization, though it is also seen very weakly in the parallel direction.<br />
Also, the overall intensity is weaker than that of E11 or E22, as the depolarization effect of the<br />
perpendicularly polarized light causes the weak overall coupling between the E12 excitons and<br />
the light emission.<br />
A large blue shift in the position of E12 in optical absorption is expected theoretically 19,<br />
133 , which has been observed in PLE experiments. Miyauchi et al. found a smaller exciton<br />
63<br />
(b)
Change language