Huge exchange energy and fine structure of excitons in an organic-inorganic quantum well material

Photoluminescence and spin-flip Raman scattering of excitons in an organic-inorganic quantum well material ${\left({\mathrm{C}}_4{\mathrm{H}}_9\mathrm{N}{\mathrm{H}}_3\right)}_2\mathrm{Pb}{\mathrm{Br}}_4$ have been investigated at low temperature. The estimated exchange energy of the exciton is greatly enhanced by strong confinement and by image charge effects, and ranges from $28\text{to}32\mathrm{meV}$. These values are one or two orders of magnitude larger than those for quantum wells made from the more usual inorganic materials. The ratio of the oscillator strength of the dipole-allowed exciton to that of the dark (dipole-forbidden) exciton is estimated to be of order 800.

Figure 1

Photoluminescence spectra of ${\left({\mathrm{C}}_4{\mathrm{H}}_9\mathrm{N}{\mathrm{H}}_3\right)}_2\mathrm{Pb}{\mathrm{Br}}_4$ excited above the band-gap energy (a) excited by ultrashort pulses at $10\mathrm{K}$ and (b) excited by a continuous-wave laser at $1.5\mathrm{K}$. The spectral resolution in (b) is higher than in (a).

Figure 2

Schematic representations of (a) exciton energy levels and relaxation processes, and (b) spin-flip Raman and ${\Gamma }_2^{-}$ emission processes with inhomogeneous broadening.

Figure 3

Photoluminescence and spin-flip Raman spectra of ${\left({\mathrm{C}}_4{\mathrm{H}}_9\mathrm{N}{\mathrm{H}}_3\right)}_2\mathrm{Pb}{\mathrm{Br}}_4$ at $1.5\mathrm{K}$ for various excitation energies within the inhomogeneously broadened ${\Gamma }_5^{-}$ exciton resonance.

Figure 4

Photoluminescence and spin-flip Raman spectra of ${\left({\mathrm{C}}_4{\mathrm{H}}_9\mathrm{N}{\mathrm{H}}_3\right)}_2\mathrm{Pb}{\mathrm{Br}}_4$ at $1.5\mathrm{K}$ for various magnetic fields applied in the $\pi$ configuration in Voigt geometry. The excitation energy is $3.0195\mathrm{eV}$. Each spectrum is horizontally shifted by $0.01\mathrm{eV}$ to show the changes clearly. The inset shows the ${\Gamma }_2^{-}$ emission intensities relative to the intensity at zero magnetic field. The dotted line shows the calculation using Eq. (4) for $f_5∕f_2=800$.

Figure 5

The observed Raman shift and the corresponding exchange energy as a function of the excitation energy. The dashed straight line is drawn as a guide for the eye.