9056
J. Phys. Chem. C 2008, 112, 9056–9060
Metal-Induced Photoluminescence Quenching of Organic Molecular Crystals Thomas Dienel, Holger Proehl, Roman Forker, Karl Leo, and Torsten Fritz* Institut für Angewandte Photophysik, Technische UniVersität Dresden, D-01062 Dresden, Germany ReceiVed: October 4, 2007; ReVised Manuscript ReceiVed: March 12, 2008
We present a detailed study of the thickness dependence of the photoluminescence of ultrathin organic films on Au(111). For the evaluation of the observed intensities a new differential analysis method is introduced. The obtained quantum yield indicates a complete quenching in the thickness range below 4 monolayers (ML) for the two crystal polymorphs and saturates above 12 ML. Several models of energy transfer were applied to our data. We conclude that only consideration of both exciton diffusion and dissociation at the organic-metal interface in combination with classical energy transfer results in a complete and satisfactory description. I. Introduction Thin films consisting of small organic molecules became a widely used basis of miscellaneous electronic applications. Besides the already well established organic light emitting devices (OLEDs),1 a growing number of applications, including photovoltaic cells,2 thin film transistors,3 and structures for gas sensing4 exists. Phthalocyanines are a large class of molecules that can be utilized in such devices and are already well-known as pigments in the dye industry. They consist of four isoindole groups linked by nitrogen atoms, whereby a completely π-conjugated ring system is formed, which binds either hydrogen or metals. The molecule investigated here is the titanyl phthalocyanine (TiOPc), whose phthalocyanine ring centers a titaniumoxide group (TiO2+). One of the distinctive characteristics is the permanent electric dipole moment,5 due to the oxygen standing out of the phthalocyanine plane (cf. Figure 1), which contributes to the formation of various crystal polymorphs.6–9 Many attempts were made to force distinct TiOPc polymorphs to grow, e.g., by changing the type of the substrate or by some postdeposition modifications. For the latter, solvent vapor treatment is one possibility, where the molecular film is recrystallized by the vapor of a selected solvent, like tetrahydrofurane, chlorobenzene-water mixtures, or xylene,10,11 ethanol,12 and water.13 The influence of the substrate on the arrangement of the molecules were shown exemplarily for metals.14,15 An additional heating of the substrate during the deposition of the TiOPc molecules, as shown by Tsushima et al.,16,17 also leads to the growth of distinct crystal polymorphs. This temperature-induced phase selection was then expanded to dielectric substrates with crystalline surfaces, such as sapphire18,19 or mica,20 and to amorphous glass as well.21 The most exciting finding from these studies of different crystal polymorphs of TiOPc is their clear assignment by their absorption spectra only, as proven by comparative studies with X-ray diffraction measurements.19,22,23 TiOPc is thus an ideal model substance for an optical in situ study of the initial growth modes of the film. We reported already an absorption spectroscopy study on the TiOPc-gold system,24 using differential reflectance spectroscopy (DRS),25,26 which provides an excellent nondestructive technique with a high sensitivity to study the * To whom correspondence should be addressed. E-mail: torsten.fritz@ iapp.de.
Figure 1. Structure of the molecule TiOPc investigated in this study.
growth even in the submonolayer range (in terms of coverage equivalents). The results for the dielectric functionε of TiOPc on Au(111) were related to scanning tunneling microscopy (STM)27 and low-energy electron diffraction (LEED) investigations. This substrate was chosen since it is especially important to have metals in direct contact with the organic molecules for applications where electronic contacts are indispensable. In such cases, the quenching of electronically excited states by the metal becomes important. One way to learn more about the processes contributing to the quenching and their distance dependence is to study the luminescence from molecular layers on a metal surface. For that purpose, one can use self-assembled monolayers composed of a luminescent group on top of an alkyl chain, covalently linked to a gold surface. With variation of the chain length one determines the separation of the chromophore from the substrate.28,29 A different way to influence the distance of the luminescent molecules is to insert a separate optically inert spacer layer, as done by Barnes and co-workers,30,31 where the influence of the spacer thickness on the lifetime of the excited states of metal-organic complexes containing phosphorescent Eu3+ ions and the excited state of rhodamine molecules were investigated. This traces back to the early studies by Drexhage et al.32 about monomolecular dye layers. The third approach consists of the deposition of metal layers on top of a previously evaporated molecular film. Studies by Choong et al.33–35 and other groups36,37 reported photoluminescence (PL) changes of tris-(8-hydroxyquinoline) aluminum, which is widely used in OLEDs, or of polymer materials,38–41 being in contact with different metals. The mechanisms included to describe the luminescence quenching are the dissociation of excitons at the metal-organic interface, the exciton diffusion toward this interface followed by subsequent dissociation, and, according to earlier classical studies,42 nonradiative energy transfer to the metal. As we will show later, consideration of all three of these nonradiative decay channels is necessary to
10.1021/jp709718t CCC: $40.75 2008 American Chemical Society Published on Web 05/23/2008
PL Quenching of Organic Molecular Crystals show the best agreement with our experimental results. Unfortunately, the deposition of metals on top of molecular layers leads typically in a very rough interface, due to the surface of the molecular film, and the diffusion of metal atoms into the film. Both effects have in common that the thickness dependence is somewhat smeared out and therefore no longer well defined. Consequently, a well-defined interface is achieved only when the metal is used as substrate. Further, gold is well-known to bind aromatic hydrocarbons by van der Waals interaction only so that chemical modifications can be excluded. Here we use a modified DRS setup43 with HeNe laser as excitation light source in combination with organic molecular beam epitaxy to study the thickness-dependent PL of TiOPc on Au(111) during the growth. This paper is organized as follows: After the Experimental section (section II), we present the development of the PL spectra of TiOPc films, based on a new evaluation procedure going beyond the standard assumption of homogeneous excitation. II. Experimental Section Although we use a completely nonluminescent substrate, the PL spectra have to be corrected, where possible luminescence of the windows in the light path and stray light are taken into account. To evaluate the luminescence quenching by the gold substrate, we applied Gaussian peak fits to the emission spectra and used the respective peak areas for further calculation. As we observe the PL under increasing film thickness, one has to take into account the increasing absorption as well. Further, we have to consider that our films are characterized by the coexistence of several phases, i.e., polymorphs. The TiOPc was purchased from Syntec Wolfen and was purified by two cycles of gradient sublimation. The organic thin films were deposited under ultrahigh vacuum conditions (base pressure below 1 × 10-9 mbar) from low flux Knudsen cells at 400 °C. After introduction of the material into the vacuum chamber, the source was degassed for more than two hours at a temperature close to the sublimation point. The Au(111) single crystalline substrate was prepared by several cycles of sputtering with Ar + ions (600 eV, 30 min) and subsequent annealing at 550 °C while the surface quality was controlled in vacuum by LEED, Auger electron spectroscopy, and STM. After this procedure, the surface exhibits the well-known (22 × 3) reconstruction. Subsequently, we deposited the TiOPc molecules with a molecular flux of around 0.7 ML/min, while the substrate was held at temperatures of 200 °C to promote crystalline growth of the films. A HeNe laser (632 nm ( 1.96 eV, 20 mW) was focused onto the sample as excitation light source for PL experiments (exposure area about 1 mm2). The emitted light was collimated by a lens in close proximity to the sample. The evaporation beam, and the light beams have all an angle of approximately 20° to the surface normal of the sample. As detection system we used an optical multichannel analyzer (OMA) consisting of a grating-mirror-spectrograph (Acton Research SpectraPro-150, 300 gr/mm blazed grating), equipped with a back-illuminated single-stage Peltier-cooled CCD (Roper Scientific, SpectruMM 250B with UV-enhancement coating), which allows a fast spectra collection while providing a sufficient signal-to-noise ratio. An edge filter (Eedge ) 1.73 eV) was used to suppress the excitation light from the detection system. The spectral sensitivity of the CCD was calibrated using a halogen lamp (Micropack Halogen Light Source HL-2000 CAL, color temperature 2960 K).
J. Phys. Chem. C, Vol. 112, No. 24, 2008 9057
Figure 2. In situ PL spectra of TiOPc molecules on gold held at 200 °C during film deposition (scale on the left). Above a thickness of 4 ML the emission from the molecules was detectable. The peak at 1.52 eV, which shifts to smaller energies with further film growth, is assigned to be the emission of phase I crystallites, while the second peak, remaining at an energetically constant position of 1.31 eV, belongs to phase II crystallites. Modulations in the low energy range are due to interference effects in the CCD.
Here, we introduce a new procedure for the analysis of the thickness-dependent quenching: The partial PL intensity IPL of a certain phase j (manifesting itself as phase-assignable peak in the PL spectra) at the total film thickness d can be written as
IPL, j ∝
∫0d qj(z)e(z)fj(z)dz
(1)
where qj(z) is the thickness-dependent quantum yield of emission, being influenced by all kinds of quenching processes as discussed above. The product e(z)fj(z) is the thickness-dependent excitation of the volume fraction fj(z) of the corresponding crystal polymorph. As the absorption of a distinct slab in a film depends on the whole thickness of the film, e.g., on the number of layers the light has passed, the excitation e(z) of such a slab is represented by the differential absorption. The latter can be calculated by using the function “absorption by layer” implemented in the commercially available software FilmWizard.44 As we work in the thickness range up to 10 nm, the functions e(z) show a linear z-dependence (see Supporting Information for calculation details and corresponding Supporting Figure 5). Note that even the linear increase of e(z) differs significantly from the common assumption of a thickness-independent excitation in thin films, which would directly lead to a horizontal line. The volume fractions are obtained from the respective peak areas Aj(z) in the ε′′ spectra24 from fj(z) ) Aj(z)/∑nAn(z) (see Supporting Figure 4). Since IPL ) ∫dIPL by definition of dIPL, the differential PL yield ηj can be written as
ηj(z) )
dIPL, j(z) ∝ qj(z)e(z)fj(z) dz
(2)
Commonly in literature the PL yield is defined as difference quotient of the entire PL intensity at a certain total layer thickness and the corresponding thickness. Since qj(z) is independent of the thickness-dependent excitation e(z), we will discuss the thickness-dependence of the excited-state quenching by using this quantity, rather than the differential PL yield ηj
9058 J. Phys. Chem. C, Vol. 112, No. 24, 2008
qj(z) ∝
ηj(z) dIPL, j(z)/dz ) e(z)fj(z) e(z)fj(z)
Dienel et al.
(3)
Here, the derivative is calculated numerically. Finally, we would like to stress that our so defined quantum efficiency is a relative measure, since the proportionality constant is not known. III. Results In Figure 2 the thickness-dependent PL spectra, where the background was subtracted and the sensitivity of the chargecoupled device was regarded by an energy-dependent correction factor, are presented. Detectable emission intensities were only observed for films thicker than 4 ML, which will be related to a substrate induced quenching and discussed in detail later. At a thickness of 7.1 ML, the measured emission spectrum consists of two features, at 1.52 eV and a smaller one at 1.31 eV. The energetic position of the first one is close to the corresponding absorption band, indicating phase I emission in accordance with the literature data.16,17,22 The corresponding Stokes shift between absorption and emission can be determined to vary from 0.12 to 0.09 eV in the thickness range from 4.7 to 18.4 ML (for absorption spectra, i.e., ε′′, compare with ref 24). The phase I luminescence feature consists not only of the peak discussed so far but also of a shoulder toward higher energies, whose belonging to the phase I emission is not quite clear from literature. In comparison with the spectra of the dielectric function ε′′,24 one can assume that the shoulder in the PL spectra might be artificial, due to possible reabsorption by the phase II absorption. Focusing our examination on the peak at 1.31 eV, it is likely to assume that the peak originates from the phase II crystal polymorph, due to the energetically lower position compared to the phase I emission (Stokes shift varies from 0.21 eV for 4.7 ML to 0.16 eV for 18.4 ML). This is in compliance with the literature data for phase II TiOPc films, produced by vapor phase treatment.12 An emission feature located further to the red (1.20 eV), as reported by Tsushima et al.,16,17 was not observed here. By comparison of our PL spectra to the absorption spectra, it is again apparent that phase I and phase II grow simultaneously.24
Figure 4. Normalized quantum yield qj of the phase I peak (top) and the phase II peak (bottom) vs film thickness d of a growing TiOPc film on Au(111) held at 200 °C. Dashed lines are based on the classical theories (dash-dot-dotted according to Chance, Prock, and Silbey (CPS), and dashed according to Kuhn). The dotted lines describe the exciton decay at the Au(111)/organic interface and the solid lines account for both the exciton diffusion and dissociation. All curves are normalized to 1 at d ) 18 ML. Uncertainties stemming from the Gaussian curve fitting, the determination of the absorption correction, and the volume fraction as well as from the film thickness d are indicated as error bars.
Interestingly, the phase II emission peak is almost constant in position with increasing film thickness (cf. Figure 2). Consequently, the phase II peak position is also not altered by the observed rearrangement of the molecular structure, being still at 1.31 eV 30 min after ending the film deposition red (cf. spectrum of the realigned film in Supporting Figure 1). During this realignment, the phase I luminescence peak is reduced in intensity relative to the phase II, which confirms the observation from the absorption spectra that molecules originally arranged in phase I gradually realign toward phase II. Additionally, the phase II emission is intensified by the roughening of the entire TiOPc film, which increases the separation of parts of some crystallites from the substrate. IV. Discussion Figure 3. Thickness-dependent quantum yield qj(d) vs film thickness d of the two crystal polymorphs j ) I, II of TiOPc grown on Au(111) held at 200 °C. The derivative of the PL intensities, the latter obtained by Gaussian curve fitting of the spectra in Figure 2, were corrected for the thickness-dependence of the volume fraction and the absorption of the film (solid symbols). The traditional evaluation of the PL yield by dividing the intensities by the entire film thickness d is shown for comparison (open symbols).
To extract the intensities of the corresponding peaks of the two crystal polymorphs grown in the TiOPc film, Gaussian curve fits were applied to the PL spectra in Figure 2 (for thicknessdependence of PL intensities see Supporting Figure 2). In Figure 3, we compare the traditional evaluation of dividing the PL intensity by the total film thickness d (based on the assumption of a thickness-independent excitation) and the volume fraction
PL Quenching of Organic Molecular Crystals
J. Phys. Chem. C, Vol. 112, No. 24, 2008 9059
of the corresponding crystal phase, to our new approach outlined in the Experimental section, where the derivative of the PL intensity is corrected by the differential absorption and the volume fraction to yield the thickness-dependent quantum efficiency (compare eq 3 and Supporting Figure 3 for derivatives of PL intensities). One can clearly see from the data points a lack of emission for the thinnest films and a sudden onset of the emission at more than 4 ML. Both evaluation methods reveal an increase of the respective yield of around 2 orders of magnitude with increasing thickness. The fact that no luminescence from either phase was observed for the thinner films underlines the layerby-layer growth of these layers, since otherwise emission from thicker parts in an island film would have caused detectable emission in films with a lower nominal thickness. The results nicely demonstrate the necessity to study optical properties of thin films during growth of the film, since stepwise experiments can not exclude luminescence from thicker island, caused by realignment of the molecules after film deposition (increased time between deposition and measurement). However, the main obvious difference between the two evaluation methods is the clear saturation of the excitationcorrected quantum yield curves around 12 ML, while the PL yield (PL divided by d) does not show this behavior. The reason for this is the rigid behavior of the conventional PL yield evaluation method: while the quantum yield qj(d) obtained from the derivative of the PL intensity takes into account the different quantum efficiencies of molecules at any specific z coordinate, the PL intensity divided by the film thickness is an averaged value over the complete film thickness, assuming a molecular yield independent from the position of the corresponding molecule in the film.45 In the following, we will discuss the quantum yields qj(d), calculated according to eq 3, separately for the two TiOPc polymorphs. In a first step we are going to test whether the classical energy transfer models, the CPS-theory45,46 and the donor-acceptor approach by Kuhn et al.,47 are valid to describe our results. Assuming a constant radiative decay rate krad, j ) const, one gets
qj(d) )
krad, j krad, j + knonrad, j
( ( ))
(5)
( ( ))
(6)
qj(d) ∝ 1 + for the CPS theory46 and
(4)
qj(d) ∝ 1 +
d0, j d d0, j d
3 -1
4 -1
for the model proposed by Kuhn and co-workers.47,48 In both cases d is the thickness of the entire film, and the parameter d0, j defines the range of interaction. The fits to our experimental data reveal quenching distances d0 of 8.2 ML for phase I and 9.2 ML for phase II for the CPS theory and for Kuhn’s model 7.8 ML for phase I and 8.5 ML for phase II, respectively (dash-dot-dotted and dashed curves in Figure 4). It is obvious from the figure that the slopes of both the CPS and Kuhn’s theory are too small to explain our observed steep decrease for film thickness d below 5 ML. To overcome this discrepancies, Gebauer et al.45 proposed a charge transfer mechanism at the interface between substrate and the PTCDA film in their study and demonstrated a simple model. Thereby, they assumed a constant PL yield qj for all molecules within the film thickness d, except of a certain number
of layers (represented by thickness d0, j) next to the substrate, where they set qj ) 0.
q qj(d) ∝ (d - d0, j) d
(7)
The curves fitted to our data (dotted lines in Figure 4) describe our data almost correctly, with a dead layer (from where no luminescence intensity is emitted) of thickness d0 ) 4.5 ML for phase I and 4.9 ML for phase II. The luminescence starts when the fifth layer of TiOPc molecules is deposited. The quantitative agreement, especially in the thickness range with the steep decrease, is quite well but slightly overestimates the signal in the thickness range between 7 and 10 ML. To further improve the model, Gebauer et al. included exciton diffusion toward the substrate, in addition to the interface dissociation
[
(
qj(d) ∝ q 1 - exp -
d - d0, j Lj
)]
(8)
The full lines in Figure show the improved results obtained with the following parameters: for phase I d0 ) 4.4 ML and L ) 4.9 ML; for phase II d0 ) 4.8 ML and L ) 5.5 ML. Please note that the observed diffusion lengths of around 5 ML applies only to the direction of growth of the film, i.e., perpendicularly to the interface. In conclusion, we have shown that the combination of exciton diffusion and dissociation at the organic-metal interface is required to model the luminescence quenching adequately. V. Conclusion and Summary The thickness-dependent PL of TiOPc thin films on Au(111) has been investigated by in situ spectroscopy during the actual growth of the molecular films. The emission spectra showed measurable intensities only above 4 ML. In accordance with the absorption study, where a simultaneous growth of phase I and phase II was reported,24 the PL spectra also indicate the simultaneous growth of the two polymorphs with their distinct emission peaks. For the evaluation of the PL intensities a new differential method is introduced. We show that it is crucial to take the thickness-dependent absorption due to the field enhancement near the metal and the interference effects into account, although the total film thicknesses are much smaller than the wavelength of light. The obtained quantum yield exhibits a complete quenching in the thickness range below 4 ML for the two crystal polymorphs and saturates above 12 ML. Several models of energy transfer were applied to our data. We conclude that only consideration of exciton diffusion and dissociation at the organic-metal interface results in a complete and satisfactory description. Acknowledgment. Financial support by the “Deutsche Forschungsgemeinschaft” grants FR875/6 and FR875/9 is gratefully acknowledged. We thank M. Koch for several cycles of sublimation of the TiOPc raw material and K. Walzer for fruitful discussions. Supporting Information Available: PL spectra for the film rearranged from phase I to phase II; thickness dependence of the PL intensities, the corresponding derivatives, and the volume fractions; differential absorption for TiOPc on Au(111) exhibiting linear behavior with increasing distance. This material is free of charge via the Internet at http://pubs.acs.org.
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JP709718T
