Auxiliary Information
A. Camposeo1*, M. Polo1,2, S. Tavazzi 3, L. Silvestri3, P. Spearman3, R. Cingolani1 and D.
Pisignano1,2
1
National Nanotechnology Laboratory of CNR-INFM, c/o Distretto Tecnologico, Università del
Salento, via Arnesano I-73100, Lecce, Italy
2 Scuola Superiore ISUFI, Università del Salento, via Arnesano I-73100, Lecce, Italy
3
Dipartimento Scienza dei Materiali, Università di Milano Bicocca,Via Cozzi 53, I-20125 Milano
Italy
PACS Numbers: 42.50.Nn, 78.47.jf, 78.55.kz
* Corresponding author. Tel: +390832298147. Fax: +390832298146. E-mail address: andrea.camposeo@unile.it
1. Role of self-absorption, polarization mixing, and waveguiding
The analysis of the intrinsic excitonic emission in tetracene (TCN) single crystal needs a careful
evaluation of the self-absorption. Self-absorption can strongly affect both the photoluminescence
(PL) spectral shape and temporal evolution in the high energy tail of the PL spectrum, where
overlap between absorbance and emission spectra occurs. In order to minimize the contribution of
such effect in our study, we analyze a single crystal with a thickness of about 70 nm (see Fig. 1).
For this crystal, we estimate an optical density of 0.29 at the emission peak wavelength (530 nm),
thus making self-absorption negligible. The optical density is deduced from the absorption spectra
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reported in Ref. [1]. Furthermore, for detecting the PL emission we use an experimental
configuration similar to Ref. [2], namely the PL emission is collected from the irradiated surface of
the crystal. This set-up, combined with an excitation wavelength close to maximum absorbance, can
limit the path length of emitted photons in the crystal, thus reducing self absorption. A number of
experimental observations support our choice, namely:
i) At room temperature, the x-polarized PL vibronic replicas of TCN crystals are centered at about
565 nm and 615 nm. The centre of the corresponding 0-0 peak is therefore expected at about 530
nm (2.34 eV) (the maximum of the 0-0 absorption band for the same polarization is found at 2.39
eV [1]). In the measured PL spectrum, the maximum of the 0-0 is detected at about 530 nm, as
expected, only for the sample having a thickness of 70 nm (Fig. 2a). The PL spectrum of a 280 nm
thick crystal (optical density higher than 1 at the energy of the emission), reported in Fig. 2a for
reference, is red-shifted and peaked at 536 nm, evidencing the role of self-absorption. For
comparison, in Fig. 2a we also display the PL spectrum of a TCN single crystal reported in Ref. [2].
Also this spectrum is red-shifted by 3.6 nm with respect to the 70 nm thick crystal.
ii) The analysis of the spectral shape evidences a higher ratio between the intensity of the (0-0) and
(0-1) bands in the PL spectrum of the 70 nm thick crystal compared to the 280 nm thick sample and
to data from Ref. [2] (Fig. 2a). This observation also supports the minor role played by selfabsorption in the thin crystal.
iii) The comparison of the PL spectra at low temperature (Fig. 2b) of the 70 nm and 280 nm thick
TCN crystal shows that only in the thinner sample the (0-0) transition can be clearly identified. On
the contrary, in the 280 nm thick crystal such transition is detectable as a small shoulder in the high
energy tail of the spectrum. We point out that in the low temperature spectra of the TCN crystal
reported in Ref. [2], the (0-0) transition was not spectrally resolved, also being identified as a
shoulder of the high energy region of the emission spectrum.
The choice of a 70 nm thick crystal allows us to rule out polarization mixing effects. In fact, neither
the x axis nor the axis orthogonal to x in the ab plane are principal axes, hence the spectra collected
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with these polarizations can contain contributions from different transitions and polarization mixing
effects are also expected. However, we have shown (Ref. [1]) that such polarization mixing can be
ruled out for crystals with thickness < 300 nm, as those used in this work.
We have also pointed out that the role of waveguiding for TCN crystals with thickness below 70
nm is marginal [1]. In fact, by measuring the angular distribution of the PL intensity in a plane
perpendicular to the ab crystal face (Fig. 3), the PL intensity is maximum along the direction
perpendicular to the ab crystal face and minimum at ±90°, corresponding to emission from crystal
edge.
2. F and F’ states: time-resolved and temperature data analysis
The emission spectra of our samples are measured at different temperatures in the range 11-290 K
both upon cooling and heating of the sample. In Figure 4, we report the integrated PL intensity as a
function of temperature for a cooling-heating cycle. The inset shows the spectra acquired at 130 K
upon cooling (empty blue squares) and heating (continuous red line). We do not observe any
significant hysteresis. On the contrary, a strong hysteresis is reported in Ref. [3] during the phase
transition of TCN samples, which is considered the origin of the appearance of the F’ state.
In Figure 5, we display a typical time-resolved x-polarized emission integrated in the spectral region
520-550 nm. The temporal decay cannot be described by a mono-exponential function, that is
indicative of different emitting species. Moreover, the low temperature PL spectra (Figure 2 of the
manuscript) show the existence of at least two emitting species in the considered spectral range, i.e.
the (0-0) excitonic transition peaked at 526 nm (F state) and a state peaked at 539 nm (F’). The
comparison with recent works on anthracene [4] suggests that the two state are coupled. The authors
propose a model based on a three level system, composed by a ground state and two excited states
(an excitonic state and a defect state) with PL lifetimes F and D, respectively. The two states are
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connected by non-radiative decay with rate, kF-D’=1/F-D . The overall emission I(t) is given by the
relation:
I (t )  A exp( t /  F )  A' exp( t /  D )
(1)
where A and A’ are related to the intensities of the emission from the two excited states [5,6]. Both
the temperature- and time-resolved PL spectra measured on our TCN single crystals are in
agreement with the model described above. Therefore, the time-resolved data at the different
temperatures are analyzed by fitting the experimental data with Eq. (1), after convolution with a
Gaussian instrument response function with a FWHM of 20 ps. An example of the fitting results is
reported in Fig. 5.
References and notes
[1] S. Tavazzi, L. Raimondo, L. Silvestri, P. Spearman, A. Camposeo, M. Polo and D. Pisignano, J.
Chem. Phys. 128, 154709 (2008).
[2] H. Nishimura, T. Yamaoka, A. Matsui, K. Mizuno, and G. J. Sloan, J. Phys. Soc. Jap. 54, 1627
(1985).
[3] U. Sondermann, A. Kutoglu and H. Bässler, J. Chem. Phys. 89, 1735 (1985).
[4] T.-S-Ahn, A. M. Müller, O. Al-Kaysi, F. C. Spano, J. E. Norton, D. Beljonne, J.-L. Bredas and
C. J. Bardeen, J. Chem. Phys. 128, 054505 (2008).
[5] S.-H. Lim, T. G. Bjorklund and C. J. Bardeen, J. Phys. Chem. B 108, 4289 (2004).
[6] Joseph R. Lakowicz , Principles of fluorescence spectroscopy, (Springer, 2006), p. 263
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Figure Captions
Figure 1. (a) Atomic force microscopy topographic image of a 70 nm thick TCN single crystal and
(b) height profile along the blue line displayed in (a). Inset: image of a TCN crystal taken under
optical microscope. Scale bar: 1.5 mm.
Figure 2. (a) x-polarized PL of a 70 nm thick (dotted red line) and of a 280 nm thick TCN single
crystal (dashed blue line) at room temperature. For comparison, the emission spectrum of a TCN
single crystal as reported in Ref. [2] (continuous green line) is displayed. (b) x-polarized PL of a 70
nm thick (dotted red line) and of a 280 nm thick TCN single crystal (dashed blue line) at 11 K.
Figure 3. Integrated PL intensity vs collection angle. Data are obtained by collecting the emission
with an optical fiber (collection angle of 8×10-3 sr) in a plane perpendicular to the ab crystal face.
The solid line is a guide for the eyes.
Figure 4. Integrated PL intensity vs temperature measured during a cooling-heating (full blue
circles and empty red circles, respectively) cycle. In the inset the spectra acquired at 130 K upon
cooling (empty blue squares) and heating (continuous red line) are displayed.
Figure 5. x-polarized time-resolved emission integrated in the spectral range 520-550 nm, measured
at 110 K. The continuous line is a fit to the data with Equation 1.
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A. Camposeo et al., Figure 1
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A. Camposeo et al. Figure 2
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A. Camposeo et al., Figure 3
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A. Camposeo et al., Figure 4
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A. Camposeo et al., Figure 5
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