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Photoluminescence properties of vacuum-deposited organic molecule-oxide (MePTCDI–SiO2) mixed layers
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 1384–1388 www.elsevier.com/locate/jlumin
Photoluminescence properties of vacuum-deposited organic molecule-oxide (MePTCDI–SiO2) mixed layers M. Levichkova, J. Assa1, H. Fro¨b, K. Leo Institut fu¨r Angewandte Photophysik, Technische Universita¨t Dresden, 01062 Dresden, Germany Received 31 July 2007; received in revised form 9 January 2008; accepted 10 January 2008 Available online 17 January 2008
Abstract Thin films of the perylene derivative N,N0 -dimethylperylene-3,4,9,10-bis-dicarboximide (MePTCDI) incorporated in SiO2 matrix at various concentrations are obtained by condensation of host and dye in high vacuum. Photoluminescence spectroscopy is applied to study the spectral properties of the layers. Significant alterations in luminescence spectra in dependence on dye quantity are explained as a consequence of dye aggregation and resonant energy transfer. We demonstrate that the deposition geometry and preparation conditions offer an effective way to reduce the possibilities for non-radiative transitions, thus increasing the photoluminescence quantum efficiency. r 2008 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.66.Sq Keywords: Thin films; Physical vapor deposition; Organic dye; SiO2; Photoluminescence
1. Introduction Over the last decade, the incorporation of dye molecules in matrices received renewed interest as a basic technique for application in solid-state dye lasers, luminescence conversion systems, or non-linear optics [1–3]. Here, the nature of the matrix governs nearly all characteristics of the dye: it causes a spectral shift in both absorption and emission, alters the distribution between processes which the excited state may undergo—and consequently the fluorescence lifetime—and influences the photostability [4]. In contrast to liquid systems, a solid solution of an organic dye obtained by vapor deposition of both the components, host and dye, represents a mechanical mixture. Thus, the structure of the solid system will also
Corresponding author. Tel.: +49 351 463 38770; fax: +49 351 463 37065. E-mail address: [email protected] (M. Levichkova). 1 Present address: Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 109, 1113 Sofia, Bulgaria.
0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.01.008
affect the optical response of the embedded organic molecules. For the practical application of organic solid-state systems in lighting applications, high photoluminescence (PL) quantum efficiency and photostability are required. Therefore, it is important to find systems which combine both properties. The inorganic material SiO2 exhibits high mechanical and photostability. Additionally, thin SiO2 films show only slight absorption in the visible spectral region, which is a prerequisite for an optically inactive matrix. Thus, the incorporation of proper dyes might result in beneficial photophysical and mechanical properties. The perylene derivative N,N0 -dimethylperylene-3,4,9,10-bis-dicarboximide (MePTCDI) grows usually in crystalline structures [5,6], and thus shows rather different spectral response as isolated molecule and molecular crystal. Thus, it can be used as a model system to investigate the influence of the matrix and the dye distribution on the photophysical properties. Here, we investigate the luminescent properties of mixed MePTCDI/SiO2 vacuum-deposited (VD) films as a function of their composition and structure, i.e. total dye concentration and especially dye distribution in the
ARTICLE IN PRESS M. Levichkova et al. / Journal of Luminescence 128 (2008) 1384–1388
inorganic host. For this reason, we introduce a preparation approach which we refer to as ‘‘layer-by-layer’’ (LBL). In contrast to the classical co-evaporation, this approach enables separate condensation of matrix and dye during the deposition process and variety of film structures. In order to find a correlation between the structure of the mixed layers and their optical characteristics, we compare solid solutions of MePTCDI grown by simultaneous condensation and grown by LBL. 2. Experimental The mixed layers are prepared by evaporation of dye and matrix in high vacuum, less than 5 106 mbar, and their condensation on glass substrates kept at room temperature. MePTCDI is purified by train sublimation in vacuum. The dye is thermally evaporated from a quartz crucible at a deposition rate of 0.02 A˚/s. The SiO2 matrix is prepared by e-gun deposition at rates in the range 0.5–10 A˚/s. For all samples, the total thickness is 100 nm. The films are grown following two preparation approaches. By the classical co-evaporation, guest and host materials condense simultaneously on a static substrate (static condensation). In the case of LBL growth, the substrate is mounted on a rotating holder. Additional system of plates and apertures was designed which allows for the substrate to see for one part of the rotation period only the dye crucible and for another one the e-gun for the SiO2 matrix. Thereby, it was possible to separate the condensation of dye and matrix particles. The film thus obtained is built up of thin successive sublayers of organic and inorganic material. The rotation speed of the substrate holder amounts to 12–30 rot/min. For both growth modes, the distribution of the dye molecules in depth of the film is schematically depicted in Fig. 1. An evaporation source to substrate distance of about 30 cm assures the good film homogeneity. The deposition rate of each material is monitored independently using quartz oscillators. The deposited amount of the dye is controlled by adjusting its rate with respect to that of the SiO2 host. Thus, it is possible to control the thickness of each dye or SiO2
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sublayer, and consequently the separation between the dye molecules. 3. Results and discussion The effect of intermolecular interactions on the PL efficiency of the mixed layers is followed by the evolution of the luminescence spectra in dependence on MePTCDI content. Fig. 2 depicts the PL excitation and emission spectra of MePTCDI incorporated in SiO2 by co-evaporation. Because of low absorption of the samples caused by the low dye amount, the corresponding luminescence excitation spectra were used. A comparison between the excitation spectra of the solid solutions and the absorption spectrum of dissolved MePTCDI molecules in dimethyl sulfoxid (DMSO) shows for all samples the presence of isolated molecules. The spectral features of the lowest
Fig. 2. PL excitation and emission spectra of mixed MePTCDI/SiO2 films at various concentrations of the dye. As detection wavelength for the PL excitation spectra 595 nm is used. Luminescence is recorded at excitation of 502 nm. The samples are obtained by co-evaporation. The absorption and emission spectra of dissolved in DMSO molecules are presented for comparison.
Fig. 1. Schematic representation of the dye depth distribution for layers grown by (a) co-evaporation and (b) layer-by-layer.
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S0S1 transition are clearly manifested in the PL excitation spectra: Peak 1 at 2.3, Peak 2 at 2.47, and shoulder at around 2.64 eV. Thus, as in liquid solution, the spacing between vibronic levels amounts to 0.17 eV [7]. The positions of the peaks are slightly shifted to the red with respect to the corresponding monomer peaks in DMSO (0.06 eV for the 0.2% sample). This shift to lower energies is attributed to interactions with the SiO2 matrix as a dielectric solvent environment. With increasing MePTCDI concentration, the absorption band spectrally broadens from 0.32 up to 0.39 eV. In addition, a notable change of the ratio of the heights of Peak 1 and Peak 2 occurs. Peak 2 favorably gains intensity, which denotes enhanced excitation strength of the 0–1 transition and thus evinces dimer formation. The Stokes shift between the excitation and emission spectra is comparatively small, i.e. 0.066 eV for the 0.2% sample. This is expected for a rigid matrix, since in such an environment, little rearrangement is possible [8]. In the PL emission spectra, two distinct tendencies are observed: (i) in contrast to the absorption spectra, the emission spectra significantly change in spectral shape with increasing dye content and (ii) the alteration of spectral shape is accompanied by change in luminescence intensity. The samples with low MePTCDI concentration (up to 1%) show monomer-type emission and an increase of the PL intensity at a slight shift to the red of the PL maximum. Above 1%, the shift to lower energies continues (0.02 eV for the studied concentration range). The luminescence intensity nearly linearly increases with increasing dye quantity for the samples at 0.2% and 0.4%; above 0.4% the linear dependence is violated. Additionally, the second peak becomes considerably broader, and above 2%, more intense with respect to the first peak. This spectral behavior can be explained by aggregation of molecules and resonant energy transfer (Fo¨rster transfer), two effects which are substantially influenced by changes in dye concentration. With increasing dye quantity in the matrix, the probability for aggregate formation increases. The deposition technology we use assures rather low-order arrangement of the condensed molecules. For a simple dimer complex, if the transition dipoles of the monomers are not parallel, transitions from both dimer states are optically allowed [9]. Hence, the multiplicity of possible orientations between pairs of MePTCDI molecules, sufficiently close to form a dimer, leads to the observed broadening of the emission band. Due to relaxation processes within the dimer, emission occurs primarily from the lower energy level. The induced energy shift of the exciton band splitting contributes additionally to the red shift of the emission band relative to the monomer. Also, the oscillator strength of a dimer is lower with respect to that of the corresponding monomer [9]. Therefore, the decrease in luminescence efficiency with increasing dye quantity is assigned to predominant emission of dimers or higher-order aggregates within the films.
Fig. 3. Molecular separation and probability for Fo¨rster energy transfer in dependence on dye concentration for MePTCDI diluted in SiO2 matrix.
The efficiency of the Fo¨rster transfer is strongly distance dependent. It is most effective when donor and acceptor are separated by a distance within the Fo¨rster radius [10]. Fig. 3 shows the probability for Fo¨rster energy transfer in dependence on the MePTCDI concentration in the matrix. The probability P is P¼
1 , 1 þ ðR=R0 Þ6
(1)
where R denotes the molecular separation and R0 is the Fo¨rster radius. We calculated R0 to amount to 52 A˚ by determining the spectral overlap for MePTCDI molecules dissolved in chloroform (for the calculation, the emission and absorption spectra presented in Ref. [11] were used). The value is consistent with the data for dissolved perylene derivatives reported in Ref. [12]. For the studied samples, as evident from Fig. 3, 0.4–1% dilution corresponds to a mean distance between the MePTCDI molecules in the matrix which lies in the range of the referred critical radius. If the thin film comprises both monomers and dimers, which are close enough for effective energy transfer to take place, excitation will be transferred with a probability P from the isolated MePTCDI molecules to the aggregates. As a result, the PL quantum efficiency of the MePTCDI/ SiO2 system will decrease. For dye contents X1%, the probability for Fo¨rster transfer exceeds 0.9, while for 0.1% it is only 0.09. Such an interpretation is supported by measurements of the luminescence anisotropy of thin films of diluted 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) molecules in SiO2 [13]. The transition to monomer-like emission in samples grown by co-evaporation occurs at dye concentrations usually below 1%. Fro¨b et al. [13] established a similar tendency for mixed PTCDA/SiO2 films, obtained using the same preparation approach. In contrast, for the LBLsamples, the latter transition is shifted to higher dye quantities. Fig. 4 presents emission spectra of 2% MePTCDI/SiO2 samples grown by various modes. The
ARTICLE IN PRESS M. Levichkova et al. / Journal of Luminescence 128 (2008) 1384–1388
Energy (eV) 2.3 2.2
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Wavelength (nm) Fig. 4. PL emission spectra of 2% MePTCDI/SiO2 thin films grown by various deposition conditions. As an excitation wavelength 502 nm is used. The inset shows the corresponding normalized excitation spectra.
corresponding excitation spectra are quite similar (inset in Fig. 4), but depending on the preparation approach, the samples show different emission behavior. The spectral shape and the decreased PL intensity for the sample grown using an intermediate approach (co-evaporation on rotating substrate with laterally inhomogeneous condensation of the compounds) are evidence for a smaller dimer contribution to the overall luminescence compared to the conventionally grown film, whereas the LBL-film shows intense monomer emission. We further determine the PL quantum efficiency of the mixed MePTCDI/SiO2 samples, defined relative to the internal quantum efficiency of pure MePTCDI layer (0.03 as referred in Ref. [14]) assuming equal absorption per molecule for all samples. It increases with decreasing dye concentration, reaching a value of about 0.7 at 0.2% (Fig. 5). For the LBL-samples, the integral luminescence intensity is enhanced by a factor of up to 2.5 compared to that of the co-deposited samples at equal dye quantity. This efficiency increase is explained by the different film structure obtained. Layer-by-layer growth assures condensation separated in time of both materials. The film is built up consecutively, i.e. each dye sublayer is followed by a matrix sublayer. Hence, the possibility for an encounter between two MePTCDI molecules of the same or of two neighboring dye sublayers is reduced. Also, the separating rigid and dense SiO2 sublayer restricts the dye motion and the migration of the organic molecules (compare with Ref. [15]). The LBL growth mode thus leads to a film structure with more homogeneous dye distribution. It assists the effective isolation and separation of the MePTCDI molecules in the matrix even at higher concentrations, as it is evident from the monomer-type emission spectrum in Fig. 4. We established a similar behavior for the LBLgrown solid solutions of other perylene derivatives.
Fig. 5. Relative PL quantum efficiency of mixed MePTCDI/SiO2 films, grown by simultaneous condensation and LBL (optimized samples, see Fig. 6 and text) as a function of MePTCDI concentration.
Additionally, by the LBL growth, time for relaxation between the successive layers is gained. The MePTCDI molecules thus hit the already relatively cooled surface of the matrix sublayer. Consequently, the possibility of destruction of the initial dye molecules during film growth due to thermal stress is limited. The possibility for energy transfer of the excitation to non-luminescent molecules decreases and the channels for luminescence losses are reduced. As noted, the LBL approach enables to control the thickness of the constituent sublayers by varying the evaporation rate for the organic and matrix materials. In such a way, the separation between the organic molecules in the sublayer and between the organic sublayers themselves could be varied. This leads to a different areal density of MePTCDI molecules in the final film (in-plane and in-depth film direction) and accordingly to emission from different states. Systematic study of the luminescence response depending on the film composition (i.e. dye distribution in the LBL-films) confirms this assumption. The emission of samples at the same macroscopic MePTCDI concentration, but at different sublayer coverage is compared in Fig. 6. The presented sample pairs consist of dye sublayers with MePTCDI coverage of 0.1 and 0.01 ML (1 ML corresponds to 70 molecules per 100 nm2), and respectively differ in the thickness of the separating matrix sublayers. A ten-time dilution of the dye coverage leads to decreased possibility for aggregation in the in-plane film direction. By that, even 1–1.5 A˚ thickness of the separating SiO2 sublayer is sufficient to reduce intermolecular interactions, so as to an end effect pronounced monomer emission is observed and the luminescence intensity increases further. Very low concentrations in the dye sublayers are thus optimal for high efficiency.
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can be optimized with regard to high absorption and luminescence quantum efficiency. We note that the presented correlation between the luminescent properties of the solid solutions and their composition and structure is based only on absorption and luminescence spectroscopy. Independent measurements addressing directly dye aggregation will be supportive for clarifying the observed effects. Acknowledgments The authors thank M. Hoffmann for the absorption and luminescence spectra of dissolved MePTCDI and valuable comments. The authors acknowledge funding by the EU Commission through 5th framework Research Training Network ‘‘HYTEC’’ (HPRN-CT-2002-00315) and by the BMBF (Project 13N8272). Fig. 6. PL emission spectra of mixed MePTCDI/SiO2 films at same total MePTCDI concentration of (left) 3% and (right) 2%, but different sublayer composition. Samples are excited with 495 nm.
4. Conclusions In this study, we have investigated the luminescence behavior of VD mixed MePTCDI/SiO2 films. The optical response of the solid solutions depends on dye distribution and thin film structure. In the matrix, the dilution of dye molecules results in enhanced PL quantum efficiency with respect to the pure dye film. The only slight alterations in the absorption spectra upon increasing dye quantity indicate that the films are mostly composed of isolated molecules. However, crucial for the PL efficiency is the Fo¨rster transfer since the energy transfer to dye aggregates or destructed molecules leads to a decrease of the luminescence efficiency. Further, we have demonstrated that the microscopic structure of the mixed films can be affected by the preparation technology. The introduced LBL approach allows to control the molecular distribution in two directions—in plane and in depth of the film. With respect to the classical co-deposition, LBL growth assures more homogeneous dye distribution. Hence, the film structure
References [1] A. Costela, I. Garcia-Moreno, J. Barroso, R. Sastre, J. Appl. Phys. 83 (1998) 650. [2] A. Costela, I. Garcia-Moreno, J.M. Figuera, F. Amat-Guerri, R. Mallavia, M.D. Santa- Maria, R. Sastre, J. Appl. Phys. 80 (1996) 3167. [3] P. Schlotter, R. Schmidt, J. Schneider, Appl. Phys. A 64 (1997) 417. [4] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956. [5] G. Klebe, F. Graser, E. Ha¨dicke, J. Berndt, Acta Cryst. B 45 (1989) 69. [6] A.J. Ferguson, T.S. Jones, J. Phys. Chem. B 110 (2006) 6891. [7] M. Hoffmann, K. Schmidt, T. Fritz, T. Hasche, V.M. Agranovich, K. Leo, Chem. Phys. 258 (2000) 73. [8] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, Weinheim, 1988. [9] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals, Oxford University Press, New York, 1982. [10] J.R. Lakowicz, Topics in Fluorescence Spectroscopy, vol. 1, Plenum Press, New York, 1991. [11] E. Engel, K. Schmidt, D. Beljonne, J.-L. Bre´das, J. Assa, H. Fro¨b, K. Leo, M. Hoffmann, Phys. Rev. B 73 (2006) 245216. [12] H. Langhals, J. Karolin, L.B.-A˚. Johansson, J. Chem. Soc., Faraday Trans. 94 (1998) 2919. [13] H. Fro¨b, M. Kurpiers, K. Leo, MRS Spring Meeting’99, Material Research Society, San Francisco, CA, 1999, p. 103. [14] A. Nollau, M. Hoffmann, K. Floreck, T. Fritz, K. Leo, J. Appl. Phys. 87 (2000) 7802. [15] M.M. Levichkova, J.J. Assa, H. Fro¨b, K. Leo, Appl. Phys. Lett. 88 (2006) 201912.
Journal of Luminescence 128 (2008) 1384–1388 www.elsevier.com/locate/jlumin
Photoluminescence properties of vacuum-deposited organic molecule-oxide (MePTCDI–SiO2) mixed layers M. Levichkova, J. Assa1, H. Fro¨b, K. Leo Institut fu¨r Angewandte Photophysik, Technische Universita¨t Dresden, 01062 Dresden, Germany Received 31 July 2007; received in revised form 9 January 2008; accepted 10 January 2008 Available online 17 January 2008
Abstract Thin films of the perylene derivative N,N0 -dimethylperylene-3,4,9,10-bis-dicarboximide (MePTCDI) incorporated in SiO2 matrix at various concentrations are obtained by condensation of host and dye in high vacuum. Photoluminescence spectroscopy is applied to study the spectral properties of the layers. Significant alterations in luminescence spectra in dependence on dye quantity are explained as a consequence of dye aggregation and resonant energy transfer. We demonstrate that the deposition geometry and preparation conditions offer an effective way to reduce the possibilities for non-radiative transitions, thus increasing the photoluminescence quantum efficiency. r 2008 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.66.Sq Keywords: Thin films; Physical vapor deposition; Organic dye; SiO2; Photoluminescence
1. Introduction Over the last decade, the incorporation of dye molecules in matrices received renewed interest as a basic technique for application in solid-state dye lasers, luminescence conversion systems, or non-linear optics [1–3]. Here, the nature of the matrix governs nearly all characteristics of the dye: it causes a spectral shift in both absorption and emission, alters the distribution between processes which the excited state may undergo—and consequently the fluorescence lifetime—and influences the photostability [4]. In contrast to liquid systems, a solid solution of an organic dye obtained by vapor deposition of both the components, host and dye, represents a mechanical mixture. Thus, the structure of the solid system will also
Corresponding author. Tel.: +49 351 463 38770; fax: +49 351 463 37065. E-mail address: [email protected] (M. Levichkova). 1 Present address: Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 109, 1113 Sofia, Bulgaria.
0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.01.008
affect the optical response of the embedded organic molecules. For the practical application of organic solid-state systems in lighting applications, high photoluminescence (PL) quantum efficiency and photostability are required. Therefore, it is important to find systems which combine both properties. The inorganic material SiO2 exhibits high mechanical and photostability. Additionally, thin SiO2 films show only slight absorption in the visible spectral region, which is a prerequisite for an optically inactive matrix. Thus, the incorporation of proper dyes might result in beneficial photophysical and mechanical properties. The perylene derivative N,N0 -dimethylperylene-3,4,9,10-bis-dicarboximide (MePTCDI) grows usually in crystalline structures [5,6], and thus shows rather different spectral response as isolated molecule and molecular crystal. Thus, it can be used as a model system to investigate the influence of the matrix and the dye distribution on the photophysical properties. Here, we investigate the luminescent properties of mixed MePTCDI/SiO2 vacuum-deposited (VD) films as a function of their composition and structure, i.e. total dye concentration and especially dye distribution in the
ARTICLE IN PRESS M. Levichkova et al. / Journal of Luminescence 128 (2008) 1384–1388
inorganic host. For this reason, we introduce a preparation approach which we refer to as ‘‘layer-by-layer’’ (LBL). In contrast to the classical co-evaporation, this approach enables separate condensation of matrix and dye during the deposition process and variety of film structures. In order to find a correlation between the structure of the mixed layers and their optical characteristics, we compare solid solutions of MePTCDI grown by simultaneous condensation and grown by LBL. 2. Experimental The mixed layers are prepared by evaporation of dye and matrix in high vacuum, less than 5 106 mbar, and their condensation on glass substrates kept at room temperature. MePTCDI is purified by train sublimation in vacuum. The dye is thermally evaporated from a quartz crucible at a deposition rate of 0.02 A˚/s. The SiO2 matrix is prepared by e-gun deposition at rates in the range 0.5–10 A˚/s. For all samples, the total thickness is 100 nm. The films are grown following two preparation approaches. By the classical co-evaporation, guest and host materials condense simultaneously on a static substrate (static condensation). In the case of LBL growth, the substrate is mounted on a rotating holder. Additional system of plates and apertures was designed which allows for the substrate to see for one part of the rotation period only the dye crucible and for another one the e-gun for the SiO2 matrix. Thereby, it was possible to separate the condensation of dye and matrix particles. The film thus obtained is built up of thin successive sublayers of organic and inorganic material. The rotation speed of the substrate holder amounts to 12–30 rot/min. For both growth modes, the distribution of the dye molecules in depth of the film is schematically depicted in Fig. 1. An evaporation source to substrate distance of about 30 cm assures the good film homogeneity. The deposition rate of each material is monitored independently using quartz oscillators. The deposited amount of the dye is controlled by adjusting its rate with respect to that of the SiO2 host. Thus, it is possible to control the thickness of each dye or SiO2
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sublayer, and consequently the separation between the dye molecules. 3. Results and discussion The effect of intermolecular interactions on the PL efficiency of the mixed layers is followed by the evolution of the luminescence spectra in dependence on MePTCDI content. Fig. 2 depicts the PL excitation and emission spectra of MePTCDI incorporated in SiO2 by co-evaporation. Because of low absorption of the samples caused by the low dye amount, the corresponding luminescence excitation spectra were used. A comparison between the excitation spectra of the solid solutions and the absorption spectrum of dissolved MePTCDI molecules in dimethyl sulfoxid (DMSO) shows for all samples the presence of isolated molecules. The spectral features of the lowest
Fig. 2. PL excitation and emission spectra of mixed MePTCDI/SiO2 films at various concentrations of the dye. As detection wavelength for the PL excitation spectra 595 nm is used. Luminescence is recorded at excitation of 502 nm. The samples are obtained by co-evaporation. The absorption and emission spectra of dissolved in DMSO molecules are presented for comparison.
Fig. 1. Schematic representation of the dye depth distribution for layers grown by (a) co-evaporation and (b) layer-by-layer.
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S0S1 transition are clearly manifested in the PL excitation spectra: Peak 1 at 2.3, Peak 2 at 2.47, and shoulder at around 2.64 eV. Thus, as in liquid solution, the spacing between vibronic levels amounts to 0.17 eV [7]. The positions of the peaks are slightly shifted to the red with respect to the corresponding monomer peaks in DMSO (0.06 eV for the 0.2% sample). This shift to lower energies is attributed to interactions with the SiO2 matrix as a dielectric solvent environment. With increasing MePTCDI concentration, the absorption band spectrally broadens from 0.32 up to 0.39 eV. In addition, a notable change of the ratio of the heights of Peak 1 and Peak 2 occurs. Peak 2 favorably gains intensity, which denotes enhanced excitation strength of the 0–1 transition and thus evinces dimer formation. The Stokes shift between the excitation and emission spectra is comparatively small, i.e. 0.066 eV for the 0.2% sample. This is expected for a rigid matrix, since in such an environment, little rearrangement is possible [8]. In the PL emission spectra, two distinct tendencies are observed: (i) in contrast to the absorption spectra, the emission spectra significantly change in spectral shape with increasing dye content and (ii) the alteration of spectral shape is accompanied by change in luminescence intensity. The samples with low MePTCDI concentration (up to 1%) show monomer-type emission and an increase of the PL intensity at a slight shift to the red of the PL maximum. Above 1%, the shift to lower energies continues (0.02 eV for the studied concentration range). The luminescence intensity nearly linearly increases with increasing dye quantity for the samples at 0.2% and 0.4%; above 0.4% the linear dependence is violated. Additionally, the second peak becomes considerably broader, and above 2%, more intense with respect to the first peak. This spectral behavior can be explained by aggregation of molecules and resonant energy transfer (Fo¨rster transfer), two effects which are substantially influenced by changes in dye concentration. With increasing dye quantity in the matrix, the probability for aggregate formation increases. The deposition technology we use assures rather low-order arrangement of the condensed molecules. For a simple dimer complex, if the transition dipoles of the monomers are not parallel, transitions from both dimer states are optically allowed [9]. Hence, the multiplicity of possible orientations between pairs of MePTCDI molecules, sufficiently close to form a dimer, leads to the observed broadening of the emission band. Due to relaxation processes within the dimer, emission occurs primarily from the lower energy level. The induced energy shift of the exciton band splitting contributes additionally to the red shift of the emission band relative to the monomer. Also, the oscillator strength of a dimer is lower with respect to that of the corresponding monomer [9]. Therefore, the decrease in luminescence efficiency with increasing dye quantity is assigned to predominant emission of dimers or higher-order aggregates within the films.
Fig. 3. Molecular separation and probability for Fo¨rster energy transfer in dependence on dye concentration for MePTCDI diluted in SiO2 matrix.
The efficiency of the Fo¨rster transfer is strongly distance dependent. It is most effective when donor and acceptor are separated by a distance within the Fo¨rster radius [10]. Fig. 3 shows the probability for Fo¨rster energy transfer in dependence on the MePTCDI concentration in the matrix. The probability P is P¼
1 , 1 þ ðR=R0 Þ6
(1)
where R denotes the molecular separation and R0 is the Fo¨rster radius. We calculated R0 to amount to 52 A˚ by determining the spectral overlap for MePTCDI molecules dissolved in chloroform (for the calculation, the emission and absorption spectra presented in Ref. [11] were used). The value is consistent with the data for dissolved perylene derivatives reported in Ref. [12]. For the studied samples, as evident from Fig. 3, 0.4–1% dilution corresponds to a mean distance between the MePTCDI molecules in the matrix which lies in the range of the referred critical radius. If the thin film comprises both monomers and dimers, which are close enough for effective energy transfer to take place, excitation will be transferred with a probability P from the isolated MePTCDI molecules to the aggregates. As a result, the PL quantum efficiency of the MePTCDI/ SiO2 system will decrease. For dye contents X1%, the probability for Fo¨rster transfer exceeds 0.9, while for 0.1% it is only 0.09. Such an interpretation is supported by measurements of the luminescence anisotropy of thin films of diluted 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) molecules in SiO2 [13]. The transition to monomer-like emission in samples grown by co-evaporation occurs at dye concentrations usually below 1%. Fro¨b et al. [13] established a similar tendency for mixed PTCDA/SiO2 films, obtained using the same preparation approach. In contrast, for the LBLsamples, the latter transition is shifted to higher dye quantities. Fig. 4 presents emission spectra of 2% MePTCDI/SiO2 samples grown by various modes. The
ARTICLE IN PRESS M. Levichkova et al. / Journal of Luminescence 128 (2008) 1384–1388
Energy (eV) 2.3 2.2
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800
Wavelength (nm) Fig. 4. PL emission spectra of 2% MePTCDI/SiO2 thin films grown by various deposition conditions. As an excitation wavelength 502 nm is used. The inset shows the corresponding normalized excitation spectra.
corresponding excitation spectra are quite similar (inset in Fig. 4), but depending on the preparation approach, the samples show different emission behavior. The spectral shape and the decreased PL intensity for the sample grown using an intermediate approach (co-evaporation on rotating substrate with laterally inhomogeneous condensation of the compounds) are evidence for a smaller dimer contribution to the overall luminescence compared to the conventionally grown film, whereas the LBL-film shows intense monomer emission. We further determine the PL quantum efficiency of the mixed MePTCDI/SiO2 samples, defined relative to the internal quantum efficiency of pure MePTCDI layer (0.03 as referred in Ref. [14]) assuming equal absorption per molecule for all samples. It increases with decreasing dye concentration, reaching a value of about 0.7 at 0.2% (Fig. 5). For the LBL-samples, the integral luminescence intensity is enhanced by a factor of up to 2.5 compared to that of the co-deposited samples at equal dye quantity. This efficiency increase is explained by the different film structure obtained. Layer-by-layer growth assures condensation separated in time of both materials. The film is built up consecutively, i.e. each dye sublayer is followed by a matrix sublayer. Hence, the possibility for an encounter between two MePTCDI molecules of the same or of two neighboring dye sublayers is reduced. Also, the separating rigid and dense SiO2 sublayer restricts the dye motion and the migration of the organic molecules (compare with Ref. [15]). The LBL growth mode thus leads to a film structure with more homogeneous dye distribution. It assists the effective isolation and separation of the MePTCDI molecules in the matrix even at higher concentrations, as it is evident from the monomer-type emission spectrum in Fig. 4. We established a similar behavior for the LBLgrown solid solutions of other perylene derivatives.
Fig. 5. Relative PL quantum efficiency of mixed MePTCDI/SiO2 films, grown by simultaneous condensation and LBL (optimized samples, see Fig. 6 and text) as a function of MePTCDI concentration.
Additionally, by the LBL growth, time for relaxation between the successive layers is gained. The MePTCDI molecules thus hit the already relatively cooled surface of the matrix sublayer. Consequently, the possibility of destruction of the initial dye molecules during film growth due to thermal stress is limited. The possibility for energy transfer of the excitation to non-luminescent molecules decreases and the channels for luminescence losses are reduced. As noted, the LBL approach enables to control the thickness of the constituent sublayers by varying the evaporation rate for the organic and matrix materials. In such a way, the separation between the organic molecules in the sublayer and between the organic sublayers themselves could be varied. This leads to a different areal density of MePTCDI molecules in the final film (in-plane and in-depth film direction) and accordingly to emission from different states. Systematic study of the luminescence response depending on the film composition (i.e. dye distribution in the LBL-films) confirms this assumption. The emission of samples at the same macroscopic MePTCDI concentration, but at different sublayer coverage is compared in Fig. 6. The presented sample pairs consist of dye sublayers with MePTCDI coverage of 0.1 and 0.01 ML (1 ML corresponds to 70 molecules per 100 nm2), and respectively differ in the thickness of the separating matrix sublayers. A ten-time dilution of the dye coverage leads to decreased possibility for aggregation in the in-plane film direction. By that, even 1–1.5 A˚ thickness of the separating SiO2 sublayer is sufficient to reduce intermolecular interactions, so as to an end effect pronounced monomer emission is observed and the luminescence intensity increases further. Very low concentrations in the dye sublayers are thus optimal for high efficiency.
ARTICLE IN PRESS 1388
M. Levichkova et al. / Journal of Luminescence 128 (2008) 1384–1388
can be optimized with regard to high absorption and luminescence quantum efficiency. We note that the presented correlation between the luminescent properties of the solid solutions and their composition and structure is based only on absorption and luminescence spectroscopy. Independent measurements addressing directly dye aggregation will be supportive for clarifying the observed effects. Acknowledgments The authors thank M. Hoffmann for the absorption and luminescence spectra of dissolved MePTCDI and valuable comments. The authors acknowledge funding by the EU Commission through 5th framework Research Training Network ‘‘HYTEC’’ (HPRN-CT-2002-00315) and by the BMBF (Project 13N8272). Fig. 6. PL emission spectra of mixed MePTCDI/SiO2 films at same total MePTCDI concentration of (left) 3% and (right) 2%, but different sublayer composition. Samples are excited with 495 nm.
4. Conclusions In this study, we have investigated the luminescence behavior of VD mixed MePTCDI/SiO2 films. The optical response of the solid solutions depends on dye distribution and thin film structure. In the matrix, the dilution of dye molecules results in enhanced PL quantum efficiency with respect to the pure dye film. The only slight alterations in the absorption spectra upon increasing dye quantity indicate that the films are mostly composed of isolated molecules. However, crucial for the PL efficiency is the Fo¨rster transfer since the energy transfer to dye aggregates or destructed molecules leads to a decrease of the luminescence efficiency. Further, we have demonstrated that the microscopic structure of the mixed films can be affected by the preparation technology. The introduced LBL approach allows to control the molecular distribution in two directions—in plane and in depth of the film. With respect to the classical co-deposition, LBL growth assures more homogeneous dye distribution. Hence, the film structure
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