High-Quality Organic Single Crystalline Thin Films for Nonlinear Optical Applications by Vapor Growth Ashutosh Choubey, O-Pil Kwon,* Mojca Jazbinsek, and Peter Gu¨nter Nonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 402-405
ReceiVed September 14, 2006; ReVised Manuscript ReceiVed NoVember 2, 2006
ABSTRACT: We report on the production of organic single crystalline thin films for integrated nonlinear optical applications by a vapor growth technique. The configurationally locked polyene chromophore 2-{3-[2-(4-dimethylaminophenyl)vinyl]-5,5dimethylcyclohex-2-enylidene} malononitrile (DAT2) that was used exhibits a noncentrosymmetric crystalline arrangement with monoclinic point group symmetry 2. It shows a second harmonic generation signal of about 2 orders of magnitude greater than that of urea. The grown DAT2 films are single crystalline and exhibit good optical quality and suitable size (area of ∼5 × 3 mm2 and the thickness in the range of 0.2-5 µm) for fabrication of photonic devices. Growth rates of a few hours have been achieved. Furthermore, the edges of the films are sharp and flat, which greatly reduces the additional processing requirements for applications. Introduction Nonlinear optical materials are key materials for numerous photonic applications.1 Organic nonlinear optical materials exhibit large and extremely fast nonlinearities compared to their inorganic counterparts. A great challenge for the development of high-speed and broadband all-optical photonic devices is to achieve simultaneously the ease of fabrication typical for polymers and high nonlinearities and stability typical for crystals. Therefore, a majority of studies on the crystals have focused on large macroscopic nonlinearities and an easy, practical, and reliable crystal growth method. The former is achieved by high supramolecular acentricity of chromophores with large microscopic molecular hyperpolarizability β. Furthermore, single crystalline thin films with high optical quality are essential to the production of integrated photonic devices.2 In addition, the challenge is also to obtain sharp edges of the films to reduce the light coupling losses into such devices. Nonlinear optical chromophores with large molecular hyperpolarizability β have been identified during the past decade.3-7 However, there are only a few reports on the successful development of reliable and stable crystalline thin films with large macroscopic nonlinearity.8-11 For integrated optics, one is interested in fabrication techniques for organic nonlinear optical crystalline films with a thickness in the range of 0.210 µm and a large area. This is because recently developed long π-conjugated chromophores with large dipole moments usually exhibit a high tendency for the antiparallel dipole-dipole aggregation, leading to centrosymmetric arrangements, and most of the crystals grown exhibit three-dimensional (3D) bulk morphology. Obviously, for bulk crystals complicated and timeconsuming cutting and polishing procedures are required to fabricate waveguiding devices. Furthermore, even for the bulk crystals, solution and melt growth techniques are most common. The former is limited to low growth rates and solvent or solution inclusion problems, while the latter does not possess these restrictions, but it is only applicable to organic molecules with a sufficient thermal stability.2 The vapor growth method is well-known to overcome these problems and is one of the simplest techniques to obtain crystals of high purity and lattice perfection. However, as a * To whom correspondence should be addressed. Fax: +41 1 633 1056. Tel: +41 1 633 3258. E-mail:
[email protected].
consequence of the physical vapor deposition process at a low vapor pressure, vapor growth often exhibits low growth rates in the range 0.1-0.01 mm/h for high crystalline quality,12 which is mostly not sufficient for obtaining crystals of a reasonable size.2 Recently, we have developed a series of nonlinear optical crystals based on configurationally locked polyene chromophores.13 The π-conjugated triene segments of the chromophores were incorporated into a ring system to improve the thermal- and photochemical stability of the molecules.14,15 One of the chromophores, 2-{3-[2-(4-dimethylaminophenyl)vinyl]5,5-dimethylcyclohex-2-enylidene}malononitrile (DAT2, see Figure 1a), exhibits a noncentrosymmetric arrangement in the solid crystalline state and therefore shows second harmonic generation (SHG) activity. On the basis of the large difference between the melting and recrystallization temperatures and high thermal stability of DAT2, we demonstrated the growth of quasitwo-dimensional (2D) platelet single crystals by a Bridgmantype melt growth technique.13a In the present work, we report on the realization of DAT2 thin electro-optic films by a vapor growth technique. The grown DAT2 films are single crystalline and exhibit good optical quality and appropriate size for the fabrication of the active integrated photonic devices (i.e., single crystalline area of ∼5 × 3 mm2 and the thickness in the range of 0.2-5 µm) with fast growth rates of few hours. Additionally, the edges of the films are very flat and sharp, which is promising for easy device fabrication without the need for additional polishing of the edges. Experimental Section Materials. The configurationally locked polyene chromophore 2-{3[2-(4-dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene}malononitrile (DAT2) was synthesized by two consecutive Knoevenagel condensations according to the literature.13a,15 The DAT2 materials for all experiments were purified by recrystallization in methylenechloride/ methanol solution for several times. Powder SHG Measurements. The relative second harmonic generation (SHG) activity of the DAT2 crystals was measured by using the Kurtz and Perry powder test.16 We used a tunable output of an optical parametric amplifier (TOPAS, from Light Conversion Ltd.). The idler wave at a wavelength of 1907 nm was pumped by an amplified Ti:sapphire laser (Clark MXR Inc, CPA 2001). It was used for determining the nonresonant second harmonic generation (SHG) efficiency. Powder X-ray Diffraction. Powder X-ray diffraction studies were performed using a STOE powder X-ray diffractometer (Cu KR
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Figure 1. Chemical structure (a) and TGA and DSC curves (b) of the investigated configurationally locked polyene DAT2 chromophore. The sublimation temperature Tsub and the melting temperature Tm were defined here as the starting point of the weight loss and endothermic transition at the heating scan rate of 10 °C/min, respectively. radiation, λ ) 1.54056 Å) equipped with a mini position sensitive detector (PSD) detector and a Ge monochromator on the primary beam. The measurements were performed in the transmission and reflection modes. X-ray Crystal Structure. X-ray crystal structure for the DAT2 crystal grown in methanol solution:13a C21H23N3, Mr ) 317.42, monoclinic, space group P21, a ) 6.1303(7) Å, b ) 7.4239(9) Å, c ) 20.258(4) Å, R ) 90°, β ) 96.750(8)°, γ ) 90°, V ) 915.6(2) Å3, T ) 295 K. Detail crystallographic data (excluding structure factors) for the DAT2 crystal structure have been deposited with the Cambridge Crystallographic Data Center as supplementary publication no. CCDC278087.
Results and Discussion Characterization. The chemical structure of the configurationally locked polyene DAT2 chromophore used in this study is shown in Figure 1a. The DAT2 chromophores consist of π-conjugated phenylhexatriene bridge between dimethylamino electron donor and dicyanomethylidene electron acceptor. As determined by X-ray crystallography, the DAT2 crystals have a noncentrosymmetric monoclinic structure with space group symmetry P21 (point group 2).13a The long axis of the DAT2 molecule, which is the main direction of charge delocalization, is aligned at an angle of θp ) 71° relative to the polar b-axis (cos θp ) 0.33). In the Kurtz and Perry powder test16 at a fundamental wavelength of 1907 nm, the DAT2 crystal shows a strong powder SHG signal of about 2 orders of magnitude greater than that of urea, due to the large microscopic hyperpolarizability βz and noncentrosymmetric packing. Thermal Properties. The relative thermal stability of DAT2 chromophores has been investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) under nitrogen atmosphere at a scan rate of 10 °C/min as shown in Figure 1b. The DAT2 chromophore does not show any evidence of decomposition up to at least 300 °C in the DSC measurement. The starting temperature of melting transition is 216 °C. The sublimation temperature Tsub, defined here as the starting point of the weight loss in the heating TGA scan, appear at about 245 °C at a heating rate of 10 °C/min. However, the actual sublimation temperature at a slow heating rate of 0.5 °C/min, like at the actual vapor growth conditions, can also be near the starting point of melting (216 °C). Resulting from the thermal stability and the sublimation temperatures, the DAT2 crystals could be possibly grown also by a vapor growth technique based on thermal evaporation (or sublimation).
Figure 2. Schematic illustration for the vapor growth technique in a closed ampule configuration used for the growth of DAT2 crystals.
Vapor Crystal Growth. Because of the high thermal stability of the DAT2 molecules, the vapor crystal growth process was performed at normal atmospheric pressure using the closed ampule configuration as shown schematically in Figure 2. The DAT2 material was placed into the Pyrex glass ampule with a length of 40 mm from a bottom to a conical top and a diameter of 20 mm. The ampule was filled with Argon gas with normal atmospheric pressure. The process of the complete evacuation and filling of Argon gas was repeated three times to prevent the eventual decomposition of the DAT2 molecules caused by thermal oxidation. The ampule was finally sealed at the conical end and placed into a homemade closed furnace. The furnace is 10 cm long with a diameter of 7 cm. Figure 2 also shows the schematic horizontal temperature profile of the furnace from one edge of the ampule to the other. As shown in Figure 2, the temperatures at the edge and the center of the furnace were 223 and 150 °C during the crystal growth. The temperature near the wall of the furnace (223 °C) is near the actual sublimation temperature (∼216 °C). The furnace was heated at 30 °C/h from room temperature up to 223 °C and kept at this temperature during 5 h to grow DAT2 crystals. Finally, the ampule was cooled down to room temperature at a cooling rate of a 30 °C/h. The crystals grew from the wall at the conical end of the ampule as illustrated in Figure 2. The inset of Figure 3 shows a photograph of the typical DAT2 crystal grown by the above method. The crystals are transparent and red in color. Typically, the largest crystal shown in this figure has the shape of a thin plate with an area of ∼5 × 3 mm2 and a thickness of about ∼3 µm. To examine the optical quality of the grown crystals, we investigated them between the crossed polarizers in a polarizing microscope (see Figure 3). The crystals are homogeneously transparent and appear completely opaque when rotating them between the crossed polarizers (by 45° from the position of Figure 4). Therefore, the grown crystals are believed to be single crystalline. These crystal edges are very straight and, as seen through the crossed polarizers, parallel to the optical indicatrix axes, which is also advantageous for easy device fabrication. The quality of the crystals and the edges was also investigated by scanning electron microscopy (SEM). Figure 4 shows the as-grown crystals. They exhibit thicknesses in the range of
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Figure 3. The DAT2 single crystalline thin film grown from the vapor as observed between the crossed polarizers in a microscope. The inset is a photograph of a typical DAT2 thin film.
Figure 5. X-ray diffraction patterns for DAT2 crystals: from top to bottom, crystalline powder grown from the methanol solution and crystalline thin film grown by the vapor in the transmission and reflection mode.
Figure 4. Images of the scanning electron microscopy (SEM) for a thicker (a) and thinner (b) DAT2 crystalline films.
0.2-5 µm, which is very suitable for waveguiding applications. Another remarkable observation from the SEM micrograph is that the edges of the films are very sharp and flat, and therefore additional sharp-edge polishing to reduce the coupling losses may not be needed, which is a big benefit from the point of fabrication of optical waveguide devices. Morphology of the Grown Crystals. To understand the morphology of the grown DAT2 single crystalline thin films, we measured flat-plate X-ray diffraction patterns in the transmission mode (incidence normal to the surface) as well as in the reflection mode as shown in Figure 5. We also measured the X-ray powder diffraction pattern of the crystals grown from the methanol solution for a comparison. The diffracted peaks
Figure 6. Crystal packing diagram of two unit cells of DAT2 crystal projected along the principal crystallographic a-axis. The charge-transfer axis (defined here between the nitrogen atom of donor group and the central carbon atom of acceptor group) is indicated by arrows. The occupancy of hydrogen of the dimethylamino (CH3)2N- donor groups is 0.5, which is due to rotational CH3 disorder modeled by the two most probable situations.
of the powder crystals were assigned according to the previous X-ray structural analysis.13a As shown in Figure 5, for a single crystalline DAT2 thin film, the transmission mode exhibits one sharp peak assigned to the (020) reflection, therefore corresponding to the principal crystallographic b-axis. The reflection mode exhibits sharp peaks assigned to (002h), (003h), (004h), (005h), (006h), (007h) reflections, which correspond only to the principal crystallographic c-axis. This indicates that, in the grown thin film, the polar b-axis and the c-axis are respectively parallel and perpendicular to the large surface area of the film as shown in Figure 6, with high crystal perfection without any major defects. Note that the digital electro-optic and nonlinear optical coefficients according to the structure are r222 and d222, which
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can be utilized for electric fields parallel to the b-axis, i.e., in the plane of the film. Conclusion We have grown organic nonlinear optical single crystalline thin films of the DAT2 molecules with a thickness between 0.2 and 5 µm by the vapor deposition technique in closed ampules. The single crystalline thin films with sharp edges and good optical quality were grown with fast growth rates of a few hours, and also they show favorable crystal geometry with a large area of ∼5 × 3 mm2 and a few micrometers thickness like 2D morphology. The efficient growth of single crystalline thin films is promising for the development of highly integrated and stable nonlinear optical waveguide devices for photonic applications. Acknowledgment. This work has been supported by the Swiss National Science Foundation. References (1) Bosshard, Ch.; Bo¨sch, M.; Liakatas, I.; Ja¨ger, M.; Gu¨nter, P. In Nonlinear Optical Effects and Materials; Gu¨nter, P. Ed.; SpringerVerlag: Berlin, 2000; Chapter 3. (2) Bosshard, Ch.; Sutter, K.; Preˆtre, Ph.; Hulliger, J.; Flo¨rsheimer, M.; Kaatz, P.; Gu¨nter, P. In Organic Nonlinear Optical Materials, Volume 1 of AdVances in Nonlinear Optics; Gordon and Breach Science Publishers: New York, 1995.
Crystal Growth & Design, Vol. 7, No. 2, 2007 405 (3) Marder, S. R.; Beratan, D. N.; Cheng, L. T. Science 1991, 252, 103. (4) Marder, S. R.; Perry, J. W.; Yakymyshyn, C. P. Chem. Mater. 1994, 6, 1137. (5) Nalwa, H. S.; Watanabe, T.; Miyata, S. In Nonlinear Optics of Organic Molecules and Polymers; Nalwa, H. S.; Miyata, S., Eds.; CRC Press: Boca Raton, FL, 1997; Chapter 4. (6) Kuzyk, M. G.; Dirk, C. W. In Characterization Techniques and Tabulations for Organic Nonlinear Optical Materials, Maercel Dekker, Inc.: New York, 1998. (7) Dalton, L. R. J. Phys. Condens. Matter. 2003, 15, 897. (8) Manetta, S.; Ehrensperger, M.; Bosshard, Ch.; Gu¨nter, P. C. R. Physique 2002, 3, 449. (9) Swamy, R. K.; Kutty, S. P.; Titus, J.; Khatavkar, S.; Thakur, M. Appl. Phys. Lett. 2004, 85, 4025. (10) Khan, R. U. A.; Kwon, O. P.; Tapponnier, A.; Rashid, A. N.; Gu¨nter, P. AdV. Funct. Mater. 2006, 16, 180. (11) Facchetti, A.; Annoni, E.; Beverina, L.; Morone, M.; Zhu, P.; Marks, T. J.; Pagani, G. A. Nat. Mater. 2004, 3, 910. (12) Sherwood, J. N. Pure Appl. Opt. 1998, 7, 229. (13) (a) Kwon, O. P.; Ruiz, B.; Choubey, A.; Mutter, L.; Schneider, A.; Jazbinsek, M.; Gramlich, V.; Gu¨nter, P. Chem. Mater. 2006, 18, 4049. (b) Kwon, O. P.; Kwon, S. J.; Jazbinsek, M.; Choubey, A.; Losio, P. A.; Gramlich, V.; Gu¨nter, P. Cryst. Growth Des. 2006, 6, 2327. (c) Kwon, S. J.; Kwon, O. P.; Jazbinsek, M.; Gramlich, V.; Gu¨nter, P. Chem. Commun. 2006, 3729. (14) Shu, C. F.; Tsai, W. J.; Jen, A. K-Y. Tetrahedron Lett. 1996, 37, 7055. (15) Ermer, S.; Lovejoy, S. M.; Leung, D. S.; Warren, H.; Moylan, C. R.; Twieg, R. J. Chem. Mater. 1997, 9, 1437. (16) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798.
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