Magneto-optical studies of single-wall carbon nanotubes

We report a detailed study of the magnetophotoluminescence of single-wall carbon nanotubes at various temperatures in fields up to $58\mathrm{T}$. We give direct experimental evidence of the diameter dependence of the Aharanov-Bohm phase-induced band gap shifts. Large increases in intensity are produced by the magnetic field at low temperatures which are also significantly chiral index $\left[(n,m)\right]$ dependent. These increases are attributed to the magnetic field induced mixing of the wave functions of the exciton states. A study of the emission from nanotubes aligned perpendicular to the applied magnetic field shows even larger field-induced photoluminescence intensity enhancements and unexpectedly large redshifts in band gap energies, not predicted theoretically.

Figure 1

(Color online) Schematic view of the optical configurations used in the continuous field experiments. Both excitation and collection are made using a seven fiber bundle, with excitation via the central fiber.

Figure 2

(Color online) Absorbance change at $1.25\mathrm{eV}$ as a function of time compared with the variation of applied $B$ field during a single magnetic field pulse, for a CoMoCat-NLS solution at room temperature. A tungsten halogen light source was used with excitation and collection polarized parallel to the magnetic field.

Figure 3

(Color online) (a) Magnetic field dependent PL from CoMoCat-NLS solution at room temperature, measured using unpolarized $670\text{−}\mathrm{nm}$ excitation in the Voigt $(k\perp B)$ geometry. The peak labeled laser is from scattered laser light in the second order of the diffraction grating. (b) Detail for (8, 3) peak. Zero intensity levels are offset for clarity.

Figure 4

(Color online) (a) Relative energy shifts $(\delta E_g∕E_g)$ in PL from $0\text{to}58\mathrm{T}$ for SWCNT solutions at room temperature and $4.2\mathrm{K}$, measured using unpolarized light in the Voigt $(k\perp B)$ configuration. (b) Relative energy shift in PL from 0 to $19.5\mathrm{T}$ for frozen HiPCO-PVP solution at $4.2\mathrm{K}$, measured using polarized light in the Voigt configuration. Theoretical AA shifts (red solid lines) are given by Eq. (1) and averaged AA shifts, assuming random nanotube orientation, are given by $0.5\times$ Eq. (1) (green broken lines).

Figure 5

(Color online) (a) Magnetic field dependent PL from frozen CoMoCat-NLS solution at $4.2\mathrm{K}$, measured using unpolarized $670\text{−}\mathrm{nm}$ excitation in the Voigt $(k\perp B)$ geometry and (b) detail for (7,5) and (8,3) peaks. Zero intensity levels offset for clarity. All lines a guide for the eye only.

Figure 6

(Color online) PL peak positions as a function of applied magnetic field for the (7,6) and (10,5) tubes up to $58\mathrm{T}$. Data taken from pulsed-field measurements of frozen HiPCO-SDBS solution at $4.2\mathrm{K}$ using 670- and $800\text{−}\mathrm{nm}$ excitation, respectively. Solid (black) lines are linear fits to the data. Predicted AA splittings are given by dashed (red) lines, and 50% averaged AA splittings are given by dashed-dot (green) lines.

Figure 7

(Color online) PLE maps for frozen $\left(4.2\mathrm{K}\right)$ HiPCO-PVP solution at (a) $0\mathrm{T}$ and (b) $19.5\mathrm{T}$ taken in the Voigt $(k\perp B)$ geometry with polarization parallel to the applied field. The false color scale is the same in both plots.

Figure 8

(Color online) (a) PLE maps for frozen HiPCO-SDBS solution at $1.6\mathrm{K}$ in steady magnetic fields of 0, 5, 10, 15, and $19.5\mathrm{T}$ taken in the Voigt $(k\perp B)$ geometry with excitation polarized parallel to the applied field. (b) Magnetic field induced intensity enhancements $I\left(B\right)∕I\left(0\right)$.

Figure 9

(Color online) PLE maps for frozen $\left(4.2\mathrm{K}\right)$ HiPCO-PVP solution at (a) $0\mathrm{T}$ and (b) $19.5\mathrm{T}$ taken in the Faraday geometry $(k\parallel B)$. False color scale equivalent in both plots.

Figure 10

(Color online) PL shifts for frozen $\left(4.2\mathrm{K}\right)$ HiPCO-PVP solution at $19.5\mathrm{T}$ taken in the Faraday $(k\parallel B)$ geometry. Theoretical AA shifts (red solid lines) for the Voigt geometry are shown for comparison, as given by Eq. (1) which was shown to agree well with experiment in Fig. 4.