Schematic presentation of a domain of finite size (4 × 4) of the brickwall (BW) phase of PTCDA with indicated unit cell. The S0/S1 transition dipole is indicated on one molecule. In addition interaction energies of this molecule with next neighbor and next-next neighbor molecules are given.
Fluorescence (FL) and fluorescence excitation (FLE) spectra of PTCDA in the BW (top) and the diluted (d) phase (bottom), both measured at a cryostat temperature of 20 K. The excitation wavelengths were 471 nm and 465 nm, respectively, whereas the detection wavelength (λdet) for the FLE spectra was 555 nm and 534 nm, respectively.
Exciton band structure of the BW phase, calculated for a very large (400 × 400 molecules) domain and representation of the Brillouin zone.
Comparison of the 0-0 line shape in the fluorescence excitation (FLE) spectra of the BW and the d phase at 20 K (λdet = 625 nm and 534 nm, respectively). The spectrum of the d phase was shifted by −405 cm−1 for alignment of the peak maxima. The absence of the vibronic modes in the spectrum of the BW phase, that are seen for the d phase, e.g., at 19 660, and 19 700 cm−1, and 19 840 cm−1 is due to the superradiance of the BW phase. In addition, a spectrum of a very diluted phase, where the molecules are solely adsorbed at steps sites, is displayed for comparison to illustrate the site broadening of the spectrum of the d phase. The peak position of this spectrum was shifted by about −535 cm−1. For further details see text.
(a) Comparison of the experimental FLE spectrum (black) of the BW phase at 20 K and λdet = 658 m (black) and calculated spectra for three domains of different sizes ( ) (red) with 4, 7, and 15. The experimental and calculated spectra were aligned at the positions of the peak maxima. For further details see text. (b) Comparison of the experimental FLE spectrum (black) of the BW phase as above with calculated spectra. The red stick spectrum is the spectrum for a single domain of 7 × 7 molecules. The green spectrum includes additional site disorder. The blue spectrum is the spectrum for a Poissonian distribution of domain sizes with the mean of 7 in the x and y directions. The purple spectrum is obtained from the blue by convolution with a Lorentzian with a FWHM of 5 cm−1.
Fluorescence spectra of the BW phase as a function of temperature demonstrating the superradiance. The excitation wavelength was 476 nm. The inset shows the number of coherently emitting molecules N c determined from the enhancement of the 0-0 transition with decreasing temperature. For further details see text.
Experimental (black) and calculated (red) fluorescence excitation spectra of the 0-0 line of the BW phase for a domain size ( ) (red) with 7 at three different temperatures (λdet = 625 nm (100 K, 160 K) and 548 nm (260 K)). The inset shows the temperature dependence of the parameter Δɛ. Note the compressed scale of the wavenumber axis with respect to Fig. 5(a) .
Article metrics loading...
Full text loading...