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Optical Properties of Perylene Thin Films on Cu(110) Qiao Chen*,† and N. V. Richardson‡ Department of Chemistry and Biochemistry, UniVersity of Sussex, Brighton, BN1 9QJ, United Kingdom, School of Chemistry, UniVersity of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, United Kingdom ReceiVed: August 29, 2009; ReVised Manuscript ReceiVed: February 10, 2010
We present a detailed, in situ study of photoluminescence of ultrathin perylene films on a Cu(110) surface. Temperature-dependent measurement has been correlated to the surface phase transition process. Different components of the fluorescence emission have been assigned to excimer, defect states, and monomer excitons. Most importantly, a coverage-dependent measurement has allowed us to identify that only emission from the first layer of the perylene is quenched by the metal substrate. 1. Introduction The optical and electronic properties of thin crystalline organic films depend crucially on their orientation and the longrange ordering,1-6 which, in turn, determine their suitability for molecular electronic devices such as FETs, LEDs, etc.7,8 It has been suggested9,10 that improved internal ordering of the organic thin film could enhance field-effect carrier mobilities, together with increased electrical conductivity and reduced activation energy for electrical conduction. The optical properties of thin films of π-conjugated molecules, such as 3,4,9,10-perylenetetracarboxylic acid-dianhydride (PTCDA)11,12 and perylene diimide (PTCDI),13 deposited by vacuum sublimation on welldefined single crystalline surfaces have become a topic of growing interest over the past few years. The optical activity of the molecules relies on the chromophore of the conjugated π system, such as perylene in PTCDA. However, to our knowledge, so far there is very little study of the optical properties of the perylene thin films. Naturally formed perylene crystals have two types of molecular arrangement, R and β forms.14-17 The R-type crystal contains four molecules per unit cell. The molecules form two pairs of dimers with the longer molecular axis perpendicular to the crystal ab plane. Within the dimers, the molecular planes are aligned almost face to face leading to a herringbone structure along the b-axis. However, the β-type crystal only contains two molecules but also forms a herringbone structure. No π-π interacting dimers are formed in the β crystal. Only in the R-modification of perylene crystals are excimers formed due to its dimeric, nearly face-to-face molecular packing. Although the process and the dynamics of excimer formation in solids are not yet fully understood, both experimental and theoretical studies have shown that the excimer Stoke shift, line shape, and temperature dependence are determined by the dimeric geometry. Our previous study18 has shown that a novel, well-ordered perylene multilayer structure can be formed on clean Cu(110) surfaces. Figure 1A shows a typical STM image of a highquality epitaxial growth of a perylene multilayer structure with a film thickness of 3.6 nm. The multilayer has a centered orthorhombic structure with dimensions a ) 20.7 Å, b ) 19.3 * To whom correspondence should be addressed. Phone: (+44)1273678492. E-mail:
[email protected]. † University of Sussex. ‡ University of St. Andrews.
Figure 1. Ordered perylene multilayer on Cu(110). (A) A typical STM image (11 nm × 11 nm, sample bias V ) 0.01 V, tunneling current I ) 0.47 nA) of the perylene multilayer deposited at room temperature. The arrow indicates the 〈110〉 azimuth of the substrate. The unit cell is marked as the rectangular with a dimension of 19.3 Å × 20.7 Å. (B) The model of the centered orthorhombic perylene structure on Cu(110). The blue molecules form the first layer on the substrate and the red molecules are the top layer. The green molecules are in the centered position. The wire frame indicates the 3D unit cell.
Å, c ) 3.4 Å, and β ) 90°, commensurate along the substrate 〈110〉 direction (indicated with arrow), but not along the 〈001〉 azimuth. The model is shown in Figure 1B. Completely flatlying molecules form one-dimensional chains in the 〈110〉 azimuth with a 90° azimuthal rotation between neighboring molecules along the chain. In this multilayer crystallized structure, molecular planes are aligned and form a perfect sandwiched π-stack with a gap distance of 3.4 Å (c-axis, perpendicular to the copper substrate). Therefore, the 3.6 nm thickness corresponds to 20 layers of perylene. The structure has no resemblance to either R- or β-type crystal. In general, a commensurate overlayer often has a structure different from that of the bulk crystal, while an incommensurate overlayer would ignore the substrate structural influences and its symmetric elements and maintain its own bulk crystal structures. The influence of the substrate on the overlayer structures offers opportunities of controlling the thin film structures in order to optimize their physical properties. In this study, we will focus on the film’s intrinsic optical characteristics correlated with film structures and preparation parameters. We report photoluminescence measurement of perylene thin films on Cu(110) as a function of surface coverage. Temperature-dependent measurements indicate an irreversible phase transition at a temperature around 320 K. Emission intensities are monitored as a function of thickness.
10.1021/jp908354e 2010 American Chemical Society Published on Web 03/15/2010
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Figure 2. Photoluminescence spectrum (blue) of multilayer disordered perylene on Cu(110) at low temperature (130 K). The spectrum is fitted with two Gaussian components (red).
2. Experimental Section The experiments were carried out in a UHV instrument equipped with low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS) (VSW HIB 1000 double pass spectrometer). A Hiden quadrupole mass spectrometer was used to monitor the molecular beam. The Cu(110) crystals were cut and polished mechanically to a mirror finish before insertion into UHV where they were cleaned by standard Ar+ bombardment (typically, 500 eV, 30 µA cm-2) and annealing (773 K) procedures until a clean surface was obtained, characterized by sharp (1 × 1) LEED patterns. Perylene was degassed for 3 h at 333 K before dosing. The doser consists of a glass tube with heating wire and thermocouple sensor, so the dosing temperature is well controlled and the reproducibility is ensured. The chemical was dosed at 433 K, with the substrate at a controlled temperature with a dosing -9 pressure about 1 × 10 mbar. The sample can be cooled by liquid nitrogen allowing a temperature range from 130 to 800 K. In the in situ photoluminescence measurement, perylene thin films were excited with a 408 nm (24510 cm-1) diode laser. The emission signal was collected by an optical fiber at an angle of 45° relative to the surface normal, coupled into the UHV system. A grating spectrometer (USB2000, Ocean Optics) was used to analyze the fluorescence signals. 3. Results and Discussion 3.1. Photoluminescence of Perylene Thin Films As a Function of Annealing Temperature between 130 and 240 K. With the sample cooled to 130 K, the deposition of multilayer perylene films forms a disordered overlayer structure with an estimated film thickness of 3.6 nm. Figure 2 shows the typical photoluminescence of such films which give rise to an asymmetric peak shape centered at 17 146 cm-1. The spectrum can be satisfactorily fitted with two Gaussian peaks centered at 16 431 and 17 340 cm-1 with an intensity ratio of 1.9. The lower energy component has a broader line shape with a width of 1631 cm-1 while the higher energy component is narrower with a width of 939 cm-1. The significant Stoke’s shift from the lowest 0-0 transition of monomer emission19 at 20 833 cm-1 implies the formation of excimers during the excitation process. In general, the peak shape is in good agreement with excimer emission from a crystal, in the region of 16 000 to 17 500 cm-1
Figure 3. Temperature-dependent photoluminescence of perylene multilayer on Cu(110) following the annealing process. The change of peak shape is an indication of a phase transition.
depending on the sample temperature.19 At this stage, although the overlayer is disordered, we can reasonably assume that dimeric structures with molecular planes nearly face-to-face stacked are formed with maximum π-π interactions. The temperature-dependent emission following annealing is shown in Figure 3. By increasing the sample temperature, the excimer emission shifts toward the lower energy accompanied by a reduction of maximum peak intensity to 240 K while the peak shape is more or less maintained. At this stage, it is necessary to note that this peak shift as a function of temperature is contradictory to that of the R crystal,14 which could reflect their structural differences suggesting the disordered film is not formed of R-type microcrystals. It is likely, although disordered, that the structure of the film is affected by the metal substrate. Further increasing the sample temperature, the peak intensity recovers without significant shift of peak position up to 280 K, although there is increasing intensity on the higher energy side of the band. Further annealing the sample, a clear shoulder at high energy is developed and the main peak is further shifted down. At temperatures above 310 K, the overall intensity decreases continuously to zero at 330 K. The quantitative analysis of the emission peak position, full width of half-maximum (fwhm) of the peak, and integrated emission intensities are summarized in Figure 4. It is clear that the annealing process can be divided into three stages: from 130 to 240 K, 240 to 290 K, and 290 to 330 K. The increase of fwhm (Figure 4A) and decrease of peak intensity (Figure 4B) between 130 and 240 K suggests that it is dominated by a dynamic process competing with a nonradiation decay of excimers, such as a transition into the self-trapped excitons. As the temperature increases, the lifetime of the excimer decreases which results in the increase of peak width and the decrease of peak intensity. For this type of thermal quenching of total luminescence intensity, a biexponential expression of fluorescence intensity as a function of temperature was recently discussed for crystalline PTCDA20 and Cl4MePTCDI13 films, similar to the inorganic semiconductors. The biexponential function is the result of two activation processes. The first rate limited process involves the relaxation barrier to the self-trapped states. The second process involves the coupling of the excimer to the phonons of the lattice. Our biexponential fitting of the
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Figure 4. (A) fwhm, (B) integrated intensity, (C) the position of the excimer emission as a function of annealing temperature, and (D) the biexponential fitting of the decay of emission intensity between 128 and 220 K. The fitted result (solid line) is overlapped on the measured data (cross markers).
Figure 5. LEED pattern (recorded at 28 eV) of ordered perylene multilayer film after annealing at 290 K. The unit cell and high symmetry substrate vector are indicated on the pattern.
integrated fluorescence intensity as a function of sample temperature is shown in Figure 4D, which gives activation barriers of 15.6 and 100.0 meV, respectively. This is similar to those found in PTCDA (21.2 and 131.5 meV) films.20 3.2. Fluorescence Variation Induced by Phase Transition and Thermal Desorption from 240 to 320 K. At surface temperatures between 240 and 290 K, the fluorescence peak position is almost constant, as shown in Figure 4C. However, the peak intensity increases and peak width decreases, which indicates that they are more sensitive to the temperature changes (Figure 4, parts B and A). This process is irreversible and a clear multilayer LEED pattern appears at the end, shown in Figure 5. The LEED pattern is recorded at 28 eV with offcentered sample position, which shows a unit cell of roughly 20 Å × 10 Å ((1.5 Å). No substrate spots can be observed at this stage. The same LEED pattern can be observed across the sample, therefore the formation of molecular clusters and local ordering can be excluded. This structure is identical with that of the multilayer film grown at room temperature.18 Therefore, it is reasonable to assume that the change of photoluminescence signal at this temperature range is due to a structural transition leading to long-range ordering. The highest intensity is slightly higher than that of the original disordered film, which is related to their structural difference, as well as temperature difference. There is no significant film thickness change during the phase transition, since the transition temperature is lower than the
multilayer desorption temperature (see the next section). It is therefore that quantum efficiency of the ordered film is higher than the disordered film. The ordering temperature threshold of 240 K suggests an energy barrier (diffusion barrier) of 600 meV. At a temperature above 290 K, a further improvement of surface ordering is evidenced by the appearance of a high-energy shoulder at 300 K, as shown in Figure 3. However, slightly above this temperature, a sudden, irreversible drop of fluorescence intensity without any change of peak shape or peak position is observed. We believe that this is due to the thermal desorption of multilayer structure with an energy barrier of 725 meV, slightly higher than the diffusion energy barrier. We are aware that the leading edge of the desorption temperature is lower than our doser degassing and final dosing temperature. This can be explained as related to the error in measuring the absolute temperature of the doser. 3.3. Temperature Dependence on Fluorescence from Ordered Perylene Film. To understand the temperaturedependent optical properties of the ordered perylene thin film, the photoluminescence of a freshly prepared film at room temperature with the centered orthorhombic structure was measured with the temperature controlled between 280 and 130 K. Figure 6a shows the sequence of emission spectra in this temperature range. The well-defined fluorescence band from the ordered perylene film can be decomposed into three Gaussian components with the strongest peak (A) at 16 549 cm-1 and two weaker peaks (B and C) at 19 060 and 20 833 cm-1 respectively, at room temperature (280 K), as shown in Figure 6b. Cooling to low temperature (130 K), the intensity of peak A increases by a factor of 2.6 while that of peaks B and C decreases. This temperature dependence is reversible on heating and cooling, which suggests that the system is under thermodynamic equilibrium. So the equilibrium between three excited states corresponding to three emission peaks is controlled by the temperature. The existence of an isosbestic point at 18 442 cm-1 suggests that the excited states corresponding to peaks A and B have the identical quantum efficiency for fluorescence
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Figure 6. (a) Temperature dependence on photoluminescence spectra of ordered multilayer perylene on Cu(110). (b) A three-component Gaussian fitting of a typical ordered layer emission spectrum.
and the total density of excited states at A and B is constant. Thus, the low-lying state B is the dominate channel for the decay of state A. This also suggests that the probability of nonradiative decay, such as interaction with defects, is very low, which implies a high-quality, well-ordered multilayer thin film. The emission of peak C at 20 833 cm-1 is in agreement with the single molecule’s lowest transition, which can be assigned to free exciton luminescence.19 Peak A has a significant Stoke shift of 4284 cm-1, which can be assigned as the excimer luminescence. Similar excimer emission has been observed from disordered perylene film and R-type perylene crystal.19 The blueshifted band B corresponds to a less relaxed excimer state. Following several cycles of annealing and cooling between 130 and 290 K, the intensity of this band gradually reduced. Therefore, it is very likely that peak B is related to defects states in the ordered film, similar to the Y band observed in PTCDA on Ag(111).11 For the centered orthorhombic ordered structure, all the molecules are aligned with their molecular plane exclusively parallel to the substrate. The STM images also reveal that within the unit cell, molecules are paired with their π rings face-to-face aligned at a separation of 3.4 Å, as shown in Figure 1. This geometry maximizes the excimer excitations. Thermal energy is required to overcome the energy barrier for the transition from the relaxed excimer states to the high-energy defect states or free exciton state. Therefore, at low temperature, those transitions becomes less likely and their intensities decrease, accompanied by the increase of the excimer band. This analysis is consistent with the temperature behavior of the fwhm of the excimer emission, which is reduced at lower temperature, indicating an increased lifetime. When the temperature changes from 130 to 290 K, the fwhm of peak A changes from 1825 to 2750 cm-1 while for the excimer emission of R-phase perylene crystal,17 the fwhm changes from 2500 to 3500 cm-1. Such a narrower line width implies the multilayer thin film has lower density of defects as excimer decay channel. A similar assignment can be applied to the two excimer components of the disordered film deposited at low temperature (130 K). The peak at 16431 cm-1 is contributed from the fully relaxed excimer emission, while the 17 340 cm-1 peak could be contributed from the defects. This assignment is consistent with the fact that the intensity of this defect emission is much stronger than that in the ordered film. 3.4. Film Thickness Dependence of Fluorescence Intensity and Metal Surface Quench Effects. With in situ photoluminescence measurements, we are able to further establish the correlation between film thickness and fluorescence intensity
Figure 7. HREELS spectra of multilayer ordered perylene on Cu(110) at different surface coverages.
by monitoring the adsorption or desorption process. In each case, the surface coverage is calibrated by the high-resolution electron energy loss spectroscopy (HREELS) using the intensity measured at monolayer of perylene on Cu(110). The desorption temperature (>600 K) for the monolayer is much higher than the desorption temperature of the multilayer (300 K). Therefore it is a relatively reliable method to create a monolayer by annealing the sample at about 500 K. Here, we assume that the intensity in HREELS is proportional to the film thickness for relatively thin layers. It is also critical that the molecules of the monolayer and multilayer adopt the same flat-lying geometry, so the metal dipole selection rule, molecular geometry, and polarization have no influence on the vibration intensities. It is also important to mention that the measured HREELS intensity has a perfect linear relationship with the dosing time at fixed doser temperature and doser-sample geometry. Therefore, the thickness can be controlled by the dosing time and calibrated by HREELS. Typical HREELS spectra of perylene on Cu(110) with a variety of film thicknesses are shown in Figure 7. The spectra are dominated by a feature at 740 cm-1, which corresponds to the out-of-plane bending mode for the C-H bonds. This confirms that the molecular plane is parallel to the substrate at any thickness, when it is deposited at room temperature. All the in-plane stretching and deformation modes are screened by the free electrons of the metal substrate. The emission intensity as a function of film thickness is shown in Figure 8, following deposition (A) or desorption (B). The dosing rate is about 0.1 ML/min, as calibrated from HREELS intensities with sample at room temperature. The in situ PL intensity measurement following the desorption process was recorded at constant heating power, which increases the sample temperature at 0.2 deg/s. The slight discrepancy between the PL signal intensities during the deposition and desorption process is due to the multilayer structure of the deposited film being less ordered than the film in the thermal desorption process. No measurable emission is observed before the first layer is formed. However, beyond monolayer, the emission increases sharply. At higher coverage, the emission increases slowly. This trend of coverage dependence is in good agreement with the behavior of other organic thin films.11,21 During deposition and desorption processes, the emission intensities
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Figure 8. Photoluminescence intensities as a function of surface coverage during (a) deposition and (b) desorption. The surface coverage is calibrated with HREELS loss intensity. The dashed curve on the deposited film is fitted with a classic CPS model with a quenching distance of 2.9 ML.
have very similar thickness dependence. This suggests a layerby-layer mechanism in both growth and desorption processes while perylene is in the same adsorption geometry. Most importantly, both deposition and desorption confirm that the first layer does not have any measurable emission. This implies that the strong π-d interaction between the molecule and the substrate has quenched the emission by reducing its excited states lifetime. This result is in clear contrast to that of PTCDA and quaterthiophene on Ag(111),11 which shows that the first two-layer-thick film displays no photoluminescence, while some earlier work on disordered films observed emission even at the submonolayer thickness regime.22 The discrepancy between perylene on Cu(110) (in this work) and PTCDA/quaterthiophene on Ag(111) is possibly due to a relatively strong interaction between the Ag(111) substrate and the molecules containing oxygen and sulfur atoms. However, it was originally suggested that the quenching of the second layer is due to tunneling effects or charge transfer, mediated through the first layer molecules.11 Although the quenching of the emission for the first layer molecule is well expected, it is the first time that it is supported by clear experimental evidence for perylene on the Cu(110) system. Meanwhile, the PL intensity as a function of film thickness during the deposition can be fitted very well with the classic Chance, Prock, and Silbey (CPS) theory,11 with a quenching distance of 2.9 ML. The fitted curve is shown in Figure 8a.
Chen and Richardson 4. Conclusion Using in situ photoluminescence measurement, we have analyzed the different emission signals from disordered and ordered perylene thin films. An energy barrier for surface ordering is identified at 600 meV, while a desorption energy barrier is found to be 725 meV. For disordered films, a biexponential analysis has revealed two exciton decay channels with associated energy barriers at 15.6 and 100.0 meV, respectively. For the ordered perylene film, three components have been identified at 16 549, 19 060, and 20 833 cm-1 which have been assigned to excimers, defect states, and monomer excitons. The thickness dependent emission intensity measurement, following both adsorption and desorption processes, has confirmed that only the first layer of perylene excitation is quenched on the metal substrate. References and Notes (1) Toda, Y.; Yanagi, H. Appl. Phys. Lett. 1996, 69, 2315–2317. (2) Bo¨hler, A.; Urbach, P.; Scho¨bel, J.; Dirr, S.; Johannes, H. H.; Wiese, S.; Ammermann, D.; Kowalsky, W. Phys. E 1998, 2, 562–572. (3) Cho, K. J.; Shim, H. K.; Kim, Y. I. Synth. Met. 2001, 117, 153– 155. (4) Colle, M.; Tsutsui, T. Synth. Met. 2000, 111, 95–97. (5) Feng, W.; Fujii, A.; Lee, S.; Wu, H.; Yoshino, K. J. Appl. Phys. 2000, 88, 7120–7123. (6) Gerasimova, N. B.; Komolov, A. S.; Aliaev, Y. G.; Sidorenko, A. G. Phys. Low-Dimens. Struct. 2001, 1-2, 119–125. (7) Ozaki, H. J. Chem. Phys. 2000, 113, 6361–6375. (8) Yamaguchi, T. J. Phys. Soc. Jpn. 1999, 68, 1321–1330. (9) Salih, A. J.; Lau, S. P.; Marshall, J. M.; Maud, J. M.; Bowen, W. R.; Hilal, N.; Lovitt, R. W.; Williams, P. M. Appl. Phys. Lett. 1996, 69, 2231– 2233. (10) Karl, N.; Marktanner, J. Mol. Cryst. Liq. Cryst. 2001, 355, 149– 173. (11) Gebauer, W.; Langner, A.; Schneider, M.; Sokolowski, M.; Umbach, E. Phys. ReV. B 2004, 69, 155431. (12) Schneider, M.; Umbach, E.; Sokolowski, M. Chem. Phys. 2006, 325, 185–192. (13) Graaf, H.; Unold, T.; Mattheus, C.; Schlettwein, D. J. Phys. D: Appl. Phys. 2008, 41, 105112. (14) Tanaka, J. Bull. Chem. Soc. Jpn. 1963, 36, 1237–1249. (15) Tanaka, J.; Kishi, T.; Tanaka, M. Bull. Chem. Soc. Jpn. 1974, 47, 2376–2381. (16) Port, H.; Walker, B.; Wolf, H. C. J. Luminesc. 1984, 31-32, 780– 782. (17) Walker, B.; Port, H.; Wolf, H. C. Chem. Phys. 1985, 92, 177–185. (18) Chen, Q.; Rada, T.; McDowall, A.; Richardson, N. V. Chem. Mater. 2002, 14, 743–749. (19) Nishimura, H.; Yamaoka, T.; Mizuno, K.; Iemura, M.; Matsui, A. J. Phys. Soc. Jpn. 1984, 53, 3999–4008. (20) Bala, W.; Dalasinski, P.; Rebarz, M.; Bratkowski, A. Opt. Mater. 2006, 28, 94. (21) Dienel, T.; Proehl, H.; Forker, R.; Leo, K.; Fritz, T. J. Phys. Chem. C, 2008, 112, 9056–9060. (22) Daffertshofer, M.; Port, H.; Wolf, H. C. Chem. Phys. 1995, 200, 225–233.
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