Raman and Photoluminescence Properties of Red and Yellow

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Raman and photoluminescence properties of red and yellow rubrene crystals Miri Sinwani, and Yaakov R. Tischler J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5032338 • Publication Date (Web): 30 May 2014 Downloaded from http://pubs.acs.org on June 2, 2014

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The Journal of Physical Chemistry

Raman and Photoluminescence Properties of Red and Yellow Rubrene Crystals Miri Sinwani and Yaakov R. Tischler Department of Chemistry, and Bar Ilan Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 5290002, Israel.

ABSTRACT In this paper, we use a combination of micro-Raman and micro-photoluminescence measurements to demonstrate that rubrene single crystals which appear as yellow platelets and red needles are two distinct orientations of the same crystallographic unit cell. As confirmed by X-ray diffraction and polarization microscopy, we show that red and yellow crystals represent the crystallographic orientations (020) and (002) respectively, with the crystallographic c-axis being the optically activated orientation in red crystals, and the b-axis in yellow crystals. Raman measurements of red crystals are characterized by a pronounced Raman shift at 217 cm-1, which to the best of our knowledge is the first report of this peak at its predicted spectral position. The Raman and photoluminescence spectra obtained from yellow crystals changes abruptly at their borders into the spectra of red crystals, confirming the distinct spectroscopic properties of each crystal orientation. The combination of Raman and photoluminescence measurements applied at different focal planes within red crystals reveals that the 217 cm-1 peak can be classified as a surface active mode. 1 ACS Paragon Plus Environment


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KEY WORDS: Rubrene, crystal, semiconductor, Raman, photoluminescence.

1. INTRODUCTION In the past two decades, many researchers have studied organic semiconductor materials in the field of optoelectronic applications due to their remarkable advantages in the large scale fabrication of low cost, low weight, and high efficiency optoelectronic devices such as OLEDs1–4, OPVs5–7, and OFETs8–13. Investigating the intrinsic optical properties and charge transport properties of these materials is critical for improving the efficiency and performance of organic based optoelectronic devices. Organic single crystals, particularly single crystals of small molecules, are the appropriate system from which intrinsic characterization can be extracted due to their wide-scale, uniform, and repeatable molecular arrangement, and high chemical purity, which leads to high electrical mobility and conductivity, and long range exciton diffusion. OFET structures of organic single crystals (e.g. anthracene, tetracene, etc.) have shown high charge motilities typically in the range of 0.1 – 10 cm2V-1s-1,9,14–16 reaching the charge mobility values of amorphous silicon.17 The highest measured hole mobility of 20 – 40 cm2V-1s-1 at room temperature18,19 for organic small molecules has been observed in rubrene single crystals. This result has motivated us, among many other research groups, to investigate the optical properties of rubrene crystals in order to understand better the properties of this organic semiconductor, and as a consequence to improve the efficiency of optoelectronic applications in the future.

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The rubrene molecule (5,6,11,12-tetraphenylnaphthacene) is a small molecule with a backbone of four fused benzene rings substituted with four phenyl groups on both sides of the two internal benzene rings (Figure 1a). Rubrene single crystals grown by the physical vapor transport (PVT) method are characterized with orthorhombic structure corresponding to the calculated crystallographic axes: a=7.18 Å, b=14.43 Å, and c=26.90 Å.20 The optically active molecular transition dipole moments in orthorhombic rubrene crystals are the M axis – parallel to the c-axis, and the L axis – partially parallel to the baxis (See figures 1b, 1c). Recently, El Helou et al.21 revealed based on transmission measurements that the ac and the ab facets of rubrene single crystals appear with different colors due to different crystallographic directions. The ac facet possesses a red color, and commonly corresponds to a needle-like shape, and the ab facet possesses a yellow color, and is also known for its platelet-like shape. These distinctions in color and appearance allowed us to assign crystals that have ac and ab facets at their surface as red and yellow crystals, respectively. Raman spectroscopy is a non-destructive tool for probing intramolecular and intermolecular (phonons) vibrations of materials. In the field of organic semiconductors, many studies have investigated the charge transport properties of these materials, though their mechanism is not fully understood. Detecting the intermolecular vibrations of organic single crystals can assist in better comprehension of the charge transport mechanism in organic semiconductors. Moreover, probing intermolecular vibrations along a certain crystallographic axis can assist in explaining the charge transport properties along a particular direction, as Podzorov et al. demonstrated for rubrene single crystals.18 Previous studies on crystalline rubrene have identified the Raman shifts at low 3 ACS Paragon Plus Environment


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energies and attributed them to various intermolecular vibrational modes, as demonstrated in the work by Ren et al.22 and in accordance with simulations performed by Venuti et al23 who also performed polarized Raman along different crystallographic axes of crystalline rubrene.

Nevertheless, it is essential to investigate further the

characteristic intermolecular vibrations of rubrene single crystals along different crystallographic axes. In this paper, we combined micro-Raman and co-located micro-photoluminescence (PL) measurements for studying the characteristic vibrations of red and yellow rubrene crystals. The combination of an integrated micro-Raman/PL measurement technique allowed us to interpret the Raman spectra with great accuracy based on the co-located PL measurements. First, we measured Raman spectra in the low frequency region (50 – 250 cm-1) to detect lattice vibrations associated with the intermolecular interactions. Then, we measured the crystals in the high frequency region (800 – 1800 cm-1) to characterize the intramolecular interactions. In addition, the integrated micro-Raman/PL measurement method was used for examining the boundaries of the crystals both in the lateral and vertical directions, and led us to new insights. We should mention that each crystal was also characterized by optical polarization and X-ray Diffraction measurements to confirm its crystallographic structure and orientation.


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Figure 1. (a) Molecular structure of rubrene with the representative dipole moments M and L, (b) the active dipole moment in orthorhombic rubrene when the incoming light is parallel to the b-axis or to the a-axis, (c) and the active dipole moment in orthorhombic rubrene when incoming light is parallel to the c-axis or to the a-axis.

2. EXPERIMENTAL 2.1. Crystal growth Rubrene crystals were grown by using the PVT method.24 In our process, rubrene crystals were grown in a three zone furnace (Carbolite Model HZS 12/600) from sublimed grade rubrene powder purchased from Sigma-Aldrich (purity 99.99%). The powder was placed in an alumina crucible in the first zone under a nitrogen flow in the range of 100 – 300 sccm. The first zone, the second zone, and the third zone were heated separately to 300°C, 250°C and 200°C, respectively. Once the desired temperature gradient was achieved, the sublimed rubrene molecules were allowed to crystallize in either the second or the third zone of the furnace (depending on the flow rate of nitrogen). The growth duration was varied between 5 to 20 hours to obtain different size crystals. The crystals were grown on the walls of the reactor, and then transferred to a glass 5 ACS Paragon Plus Environment


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substrate for further characterizations. The optical characterizations concluded polarized transmission microscopy, photoluminescence, and micro-Raman spectroscopy, followed by X-ray diffraction (XRD) for structural characterization.

2.2. Optical and structural characterization Optical polarization measurements were performed using a transmission microscope (NJF-120A) with a rotatable sheet polarizer (Edmund Optics 200MM SQ TS) situated between the light source and the sample plane to control the polarization of the illumination. Images of light transmission show uniform optical polarization for each crystal. XRD was carried out to characterize the structure of the crystals, and to investigate which crystallographic planes are present in the crystal surface of each crystal. Photoluminescence and co-located Raman spectra were measured in a dual laser confocal micro-Raman/PL system (LabRam HR) with the aid of automated laser and filter switching that enabled stable and accurate measurements. For the Raman characterization, we used an excitation wavelength of 784 nm – to avoid fluorescence – and the power of the laser was adjusted to 1mW in order to detect the weak Raman signals. For the PL characterization we used an excitation wavelength of 532 nm and adjusted the laser power to 5µW due to strong fluorescence from the crystals. In each measurement, an excitation laser beam was first transmitted through an objective of either 50X (N.A. = 0.75) or 100X (N.A. = 0.9) to a certain spot on the crystal surface, then the scattered light was collected by the same objective and directed to the spectrometer portion of the instrument. 6 ACS Paragon Plus Environment

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3. RESULTS AND DISCUSSION 3.1. Optical polarization The two facets of rubrene crystals showed completely uniform optical polarization anisotropy behavior in which yellow crystals changed their color from yellow to orange by a 90° rotation of the polarizer sheet, and red crystals changed their color from red to orange under the same conditions, as illustrated in Figure 2. This optical anisotropy phenomenon occurs due to different structural orientations which allow different components of a molecular dipole moment to overlap and interact with the incoming polarized light (E). As mentioned above (Figure 1), each molecular dipole moment is oriented parallel or partially parallel to one of the crystal's crystallographic axes. Therefore, in agreement to previous work21, when light is polarized to the a-axis (E//a), to the b-axis (E//b), or to the c-axis (E//c), three different colors are observed: orange, yellow, and red, respectively. Based on the color distinction, we were able to simply determine which crystallographic plane is lying in each crystal surface: in yellow crystals, it is the ab plane and in red crystals, it is the ac plane. The final observations were further confirmed by XRD.



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Figure 2. The color of a platelet-like rubrene crystal changes from (a) yellow to (b) orange, as the light polarization is change by 900, and the color of a needle-like rubrene crystal changes from (c) red to (d) orange under the same conditions.

3.2. X-Ray diffraction (XRD) XRD measurements of the red and yellow crystals showed that they both have the same orthorhombic unit cell (corresponding to the lattice parameters mentioned in the introduction) with different sets of planes being scattered, in agreement with previous works.21,25 The scattered plane (002) from yellow crystals is very intense (Figure 3a), revealing a single crystal structure, and implies that the c-axis is vertical to the crystal surface as was determined from the optical polarization measurements. In contrast, the scattering intensity from the red crystals’ planes (Figure 3b) is lower by an order of magnitude than the yellow crystals, and the existence of two scattered planes at (002) and (020) indicates these are not single crystals. Nevertheless, the relatively strong diffraction intensity of the (020) plane in comparison to the (002) scattered plane strengthen our findings that for the red crystals, the b-axis is perpendicular to the ac plane which lies in 8 ACS Paragon Plus Environment

the crystal surface as argued above. After the structural orientation was established for the red and yellow crystals, the Raman behavior was characterized.

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Figure 3. XRD of (a) a yellow crystal, and of (b) a red crystal

3.3. Photoluminescence and Raman characterization PL and Raman spectra obtained from red crystals were consistently different from the PL and Raman spectra obtained from yellow crystals. Red crystals were characterized with PL spectrum containing one single peak at 565 nm (2.19 eV), and yellow crystals were characterized with PL spectrum containing two peaks at 565 nm



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(2.19 eV) and at 605 nm (2.05 eV) as illustrated in Figure 3a. The single emission peak at 565 nm in the red crystals is driven by the dipole moment M along the c-axis, and according to literature26 the detection of this peak in the PL spectra of the yellow crystals is attributed to PL “leakage” from the c-axis polarization. However, the second emission peak at 605 nm in the yellow crystals is driven by the dipole moment L along the b-axis, and therefore it does not appear in the PL spectra of red crystals, as was recently published by Irkhin et al.26 Another distinction we consistently notice is in the PL intensities of the two crystals. The PL intensity from red crystals was higher by an order of magnitude than from yellow crystals. We attribute the differences in PL intensity that we observed to the much larger absorption length of yellow oriented crystals compared to red ones, with respect to the microscope objective that we used for PL excitation and collection. In order to enable depth profiling of the spectrum, in all PL measurements a 50X objective with NA = 0.75 was used, which produced a depth of field of 625 nm for 532 nm excitation. This choice of NA gave a balance between obtaining high localization of the laser focus and minimizing the possibility of photo-bleaching the samples. According to the literature26, at the wavelength of 532 nm, yellow crystals have an absorption length of 6 µm (along the b-axis polarization), while red crystals have an absorption length of 0.2 µm (along the c-axis polarization). Thus, within the depth of field of the objective used, less than 10% of the excitation is absorbed in the yellow crystals, compared to effectively 100% in the red crystals, and therefore for the same laser power, an order of magnitude more PL photons are generated and collected from within the depth of field of the red versus yellow crystals.


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Raman spectra of red and yellow crystals were first measured in the low frequencies region (50 – 250 cm-1) where lattice vibrations are expected to be detected. In agreement to previous studies22,23,27, yellow crystals exhibit three clear Raman shifts at 104.18 cm-1, 107.61 cm-1, and 138.70 cm-1, with an additional two moderate Raman shifts at 79 cm-1, and 86 cm-1, as illustrated in Figure 4b. In contrast, red crystals show four clear and intense Raman shifts in the range of 75 – 140 cm-1. In the red crystals, we also found an intense Raman shift at 217.89 cm-1 (Figure 4b) that perfectly matches density functional theory (DFT) calculations which until now were not completely confirmed by experimental observations.23 After measuring more than ten yellow crystals and ten red crystals, we observed that the 217 cm-1 Raman shift appears only in red crystals, indicating that this peak originates from light-matter interaction with the c-axis which lies in the surface of the red crystal. To confirm this observation, we decided to measure PL and Raman spectra from the borders of a yellow crystal, with the hypothesis that at the border, we would be able to detect properties of the vertically aligned c-axis.

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Figure 4. (a) Typical PL spectra of both red and yellow crystals followed with (b) their corresponding Raman shift measurements.



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In order to probe the c-axis, we performed integrated Raman/PL mapping of yellow crystals from the surface through its borders. First, we focused the PL excitation beam on the crystal surface as shown in Figure 5b. We measured the characteristic PL of yellow crystals (Figure 5a), characterized by two main peaks at 565 nm and 605 nm. Additional peaks at 650 nm and at 700 nm were also observed which were attributed to the vibronic progression.28 Then, by using micro-PL automatic mapping, PL spectra were measured along the crystal surface and across its borders until no PL signal was obtained. This method was originated by Beams et al for Raman measurements.29 As expected, the crystal surface was consistently characterized by two peaks at 565 nm and 605 nm. At the border of the crystal, a single photoluminescence peak at 565 nm was obtained, as illustrated in Figure 5a, indicating that the excitation laser beam (Figure 5c) successfully detected the c-axis of the crystal. Similarly, Raman spectra obtained from the crystal surface demonstrated characteristic Raman shifts of yellow crystals, whereas Raman spectrum obtained by the crystal border revealed near perfect correspondence with the Raman shift spectrum of the red crystal surface, with a clear Raman shift at 217.46 cm-1 and only four Raman shifts below 150 cm-1 as demonstrated in Figure 6a. Figures 6b and 6c illustrate the location of the Raman excitation laser beam on the measured points at the crystal surface and borders. From these measurements, we conclude that the newly observed 217 cm-1 Raman shift is attributed to the crystallographic c-axis of the orthorhombic rubrene crystals.


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Figure 6. (a) Raman shift spectra in the low frequency region of a yellow crystal measured at the crystal surface and at its border, with the corresponding images that illustrate the measuring laser beam on the measured points: (b) at the crystal surface, and (c) its border.

Figure 5. (a) PL spectrum of a yellow crystal measured at the crystal surface and at its border, followed with images of the laser beam used for excitation (b) incident on the surface of the crystal, and (c) incident on its border.

As previously reported,23 the spectral shifts in both the high frequency region and low frequency region are driven by

structural orientation of the crystal. Therefore, we

compared the Raman spectra of red versus yellow crystals in the high frequency region (800 – 1800 cm-1), where we would expect that the intramolecular vibrational modes of yellow crystal borders would be similar to those of red crystals surface, and different from the Raman shifts of yellow crystals surface. As illustrated in Figure 7, the Raman spectra obtained from the surface of the red crystal and the border of the yellow crystal are in fact highly correlated while the Raman from the surface of the yellow crystal is different in two levels: intensity and spectral peaks. Regarding intensity, very weak Raman signals are generated by the yellow crystal surface in comparison to the intense 13 ACS Paragon Plus Environment


Raman signals from the red crystal surface and the yellow crystal borders. For example, vibrational peaks at 999.57 cm-1, 1298.31 cm-1, and 1538.69 cm-1 (marked with ‘○’ in the spectrum) belong to molecular vibrations30 lying in cyclic aromatic planes, and therefore are strongly expressed in red crystals by the tetracene backbone and the phenyl side groups of the rubrene molecule. However, these peaks are hardly detectable from the yellow crystals’ surface, in agreement to literature.23 Regarding the second difference, the Raman shifts that were measured in red crystals and yellow crystal borders at 1163.53 cm-1, 1264.96 cm-1, and 1596.66 cm-1 (marked with ‘□’ in the spectrum) were not detected in yellow crystals. These observations reconfirm that each crystal orientation possesses a particular molecular orientation, and therefore reveals a specific set of active molecular modes in the high frequencies in Raman spectroscopy.

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Figure 7. Raman shift spectra of red crystal, yellow crystal surface and yellow crystal border. The vibronic signals were collected in the high energy region of 800 - 1800 cm-1.


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3.4. Characterization of the Raman shift at 217 cm-1 The Raman shift at 217 cm-1 was further investigated by integrated Raman/PL measurements along different heights in the red crystals in order to understand the relationship between this Raman shift and the other four Raman modes below 150 cm-1. It should be mentioned that in a regular procedure of measuring Raman spectrum, the 784 nm excitation laser beam is focused on the crystal surface with a penetration depth of 5 µm, meaning the obtained spectra comes from both bulk and surface located molecules. In order to collect localized Raman scattering from the crystal surface, we decided to raise the focus of the excitation laser beam, by using automatic focusing system, 5 µm up from the crystal surface. This ‘out of crystal’ measurement was compared to a regular Raman measurement, and to a ‘bulk of crystal’ Raman measurement where the focus of the laser beam was lowered 5 µm into the bulk of the crystal. In addition, we measured PL spectra for the same three depths mentioned above in order to obtain a baseline to clarify the unusual enhancement obtained at the 217 cm-1 Raman peak.

All

measurements of relative intensity that are discussed in the following paragraphs were performed on a spectrometer that was both wavelength and intensity calibrated, using the same diffraction grating and confocal pin-hole diameter. The stability of the lasers’ power was greater than 99% and constant integration times of 120 seconds and 1 second were used for all Raman and PL spectra, respectively. As illustrated in Figure 8a, the PL spectra measured at the three different heights share the same order of magnitude for intensity, with slight differences. The regular ‘surface of crystal’ point showed approximately 18% higher relative PL intensity compared to the PL measured at the ‘bulk of crystal’ point due to fewer emitted photons being lost to 15 ACS Paragon Plus Environment


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scattering throughout the crystal. Similarly, this reason explains the 33% higher relative PL intensity obtained from the ‘out of crystal’ measurement compared to the PL intensity of the regular ‘surface of crystal’ point. However, when we compared the intensities of the corresponding Raman shifts, we noticed a clear distinction between the Raman shift at 217 cm-1 versus the other four shifts below 150 cm-1. The low energy shifts showed moderate Raman signals in the ‘bulk’ measurement, three times more intense signals at the regular ‘surface of crystal’ point, and an additional 30% increase in relative intensity at the ‘out of crystal’ measurement, as shown in Figure 8b. In contrast, the Raman shift at 217 cm-1 exhibits a very different trend in intensity. As before, the Raman intensity obtained from the regular surface was three times higher than the Raman intensity measured at the ‘bulk’ height, in complete correlation to the intensity trend shown by the four shifts from the bulk to the regular surface measurements. However, when the Raman spectrum was measured at the ‘out of crystal’ height, the detected Raman intensity was approximately 300% higher than the Raman intensity obtained from the ‘surface of crystal’ measurement versus the nearly 30% intensity increase for the four low energy Raman shifts. This unexpected increase shows that the 217 cm-1 shift is more active at the top surface with an intensity that is higher by one order of magnitude than the signal obtained at the bulk crystal spot. Therefore, we suggest that this Raman shift at 217 cm-1 is a surface active mode of the caxis in rubrene crystals. This observation should be confirmed by further investigation.


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Figure 8. (a) PL spectra of rubrene red crystal at different heights in the crystal, and (b) the corresponding Raman spectra.

4. CONCLUSIONS In this article, we determined characteristic Raman and photoluminescence properties of orthorhombic needle-like red and platelet-like yellow rubrene crystals. We showed based on optical polarization and XRD measurements that red and yellow crystals reveal different PL and Raman shift characterization due to different active molecular dipole moments lying in the b and c crystallographic axes of the crystal orientations, respectively. Specifically, we showed that red crystals are characterized by a Raman shift at 217 cm-1 in the low frequencies region, and with intense peaks in the high frequencies 17 ACS Paragon Plus Environment


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region, as opposed to the moderate Raman peaks obtained by yellow crystals. Using integrated co-located Raman/PL measurements, we confirmed that the 217 cm-1 Raman peak originates from the c crystallographic axis by carefully detecting this peak at the borders of yellow crystals. Finally, integrated Raman/PL spectra along different heights of red crystals suggested that the 217 cm-1 peak may be classified as a surface mode and should be further studied.

ACKNOWLEDGMENTS This work was supported by a grant from the Israel Science Foundation (ISF). The authors gratefully acknowledge Dr. Gili Taguri for performing XRD, Dr. Hagit Aviv for help with setting up the Raman measurements, and Dr. Daniel Nessim for lending us the 3-zone tube furnace.


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10μm

10μm

c

b

a

a


Raman and Photoluminescence Properties of Red and Yellow

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