J. Phys. Chem. 1996, 100, 10707-10709
10707
Spectroscopic Characterization of Inorganic Host:Organic Dopant Films Fabricated via Laser-Assisted Molecular Beam Deposition R. L. DeLeon, W. M. K. P. Wijekoon, U. Narang, M. L. Hall, P. N. Prasad,* and J. F. Garvey* Photonics Research Laboratory and Department of Chemistry, State UniVersity of New York at Buffalo, Buffalo, New York 14260-3000 ReceiVed: February 8, 1996; In Final Form: April 16, 1996X
Thin films of hybrid inorganic:organic composites [TiO2:copper phthalocyanine, Al2O3:6-propionyl-2(dimethylamino)naphthalene, and Al2O3:rhodamine 6G] were fabricated by mixing the constituents in the gas phase via the technique of laser-assisted molecular beam deposition. These films were characterized by UV-visible, infrared, and steady-state fluorescence spectroscopic techniques. UV-visible and infrared spectra of TiO2:copper phthalocyanine films indicate that the organometallic compound, copper phthalocyanine, is trapped within the host TiO2 matrix without degradation. The local microenvironment of the organic components in these composite films were studied by steady-state fluorescence spectroscopy. Emission spectra indicate that the organic dye molecule, rhodamine 6G, is trapped within the Al2O3 matrix as the monomer without aggregation. The spectral red shift of the emission peak and the full width at the half maximum suggest the environmentally sensitive probe, 6-propionyl-2-(dimethylamino)naphthalene, molecules are embedded within the Al2O3 matrix as opposed to being adsorbed on the Al2O3 surface.
Thin films of inorganic:organic composites are of considerable interest in applications for future photonics and optoelectronic devices.1,2 Although inorganic glasses (for example, SiO2 and TiO2) are excellent photonic media, their intrinsic optical nonlinearity is several orders of magnitude smaller than that of many nonlinear optical (NLO) organic compounds. On the other hand, many excellent NLO organic compounds exhibit considerable optical losses.2 One way to prepare a highly nonlinear optically active medium with a low loss is the fabrication of inorganic:organic hybrid composites. Such composites have even been shown to have enhanced NLO response beyond that of their individual constituents.3 However, the high temperature required for processing of the inorganic oxide host matrix prevents the preparation of such composites because of the lack of thermal stability of the organic component. Approaches such as sol-gel processing can be used to prepare inorganic:organic composites. However, such composites exhibit undesirable aging effects.4,5 In some situations, these composite films crack because of the strain of shrinkage or becuase they do not fully densify to a hard glass. Therefore, it is highly desirable to develop an alternate approach for the fabircation of thin films of inorganic:organic composites. Previously, we have reported the fabrication of a thin film of an inorganic:organic composite by using the technique of laser-assisted molecular beam deposition (LAMBD).6,7 This was accomplished by mixing laser-evaporated silica clusters with organic vapors [N-4-(4-nitrophenyl)-s-prolinol] within a molecular beam expansion. During that work, it was observed that the size of the SiO2 aggregates in the film could be reduced by generating SiO2 directly within the molecular beam by ablating a silica target and introducing an oxidizing agent into the ablated plasma via a supersonic expansion.7 Following this idea, we have now fabricated thin films of several different inorganic: organic composites: Al2O3:6-propionyl-2-(dimethylamino)naphthalene (PRODAN), Al2O3:rhodamine 6G (R6G), and TiO2: copper phthalocyanine. Copper phthalocyanine was selected because of its NLO properties. R6G was chosen to provide information on the state of molecular aggregation, and PRODAN X
Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00395-4 CCC: $12.00
was chosen to provide information about the local microenvironment of the organic compound. In this paper, we report on the characterization of these films by UV-visible, infrared, and steady-state fluorescence spectroscopic techniques. The LAMBD process is described in detail in ref 6. Therefore, only a brief experimental discussion will be presented here. In the present work, gas-phase metal oxides are generated by introducing a supersonic expansion of oxygen carrier gas into a laser-evaporated metal (titanium or aluminum) plasma. Atomic emission measurements were made during the LAMBD deposition of Ti- and Al-based films. Spectral simulations that provided the best data fits indicated a temperature on the order of 15 000 K. The organic compound (copper phthalocyanine, PRODAN, or R6G) was heated in a Pyrex U-tube and expanded with the oxygen into the metal plasma. Copper phthalocyanine and R6G were heated to 0.93), and its emission spectrum provides information about its nature (monomer or aggregates, etc.) even within a film architecture.9 By contrast, the PRODAN fluorescence is highly sensitive to the physiochemical properties of its local microenvironment (emission maximum changes from 401 nm in cyclohexane to 530 nm in water).10 Thus, changes in the PRODAN emission spectrum will probe the local microenvironment surrounding the PRODAN molecule. In addition, PRODAN and R6G have separately been incorporated into an SiO2 matrix through the sol-gel process and their emission characteristics have been studied.9,11,12 Therefore, a comparison can be made between the microenvironment of the LAMBD films and that of the solgel-processed inorganic:organic composite. All fluorescence measurements were performed with a SLM 48000 MHF spectrofluorometer using a xenon arc lamp as the excitation source.9 Emission spectra were background subtracted and corrected for detector and monochromator transmission nonlinearities. Figure 4 represents the fluorescence spectra of a LAMBD film consisting of Al2O3:R6G (spectrum B) and that of a pure Al2O3 film (spectrum A), which was also fabricated by the LAMBD technique. The emission peak of
Characterization of Inorganic Films
J. Phys. Chem., Vol. 100, No. 25, 1996 10709
Figure 4. Steady-state fluorescence spectra of (A) an Al2O3 film and (B) an Al2O3:R6G film. Both films are on silica substrates.
the Al2O3:R6G film at approximately 546 nm clearly indicates that R6G is incorporated in the film in the monomeric form.9 This spectrum is essentially identical to that of low-concentration R6G (50 µM) doped SiO2 sol-gel films in which the R6G molecules exist as monomers.9 When the doping levels of R6G in the sol-gel matrix is increased from 50 µM to 250 µm and 5 mM, the peak of the emission spectrum is red-shifted from 546 nm to 556 and 568 nm, respectively, indicating the formation of aggregates. The fluorescence emission spectra of the LAMBD films do not show any apparent changes with the deposition time. This indicates that the R6G aggregation does not depend on the film thickness, indicative of a homogeneous deposition throughout the entire process. Figure 5 shows the emission spectrum of PRODAN incorporated within an Al2O3 LAMBD film and that of a film made by spreading a chloroform solution of PRODAN on a silica substrate. The changes in emission characteristics of PRODAN are a measure of the immediate microenvironment about the probe (i.e., the cybotatic region).11 As seen in Figure 5, the emission maximum of the PRODAN-doped LAMBD film (A) is slightly red-shifted and the emission spectrum is broadened compared to that of the film made by spreading a chloroform solution of PRODAN on silica (B). It should be noted that the emission profile (not shown in Figure 5) of a film made by vacuum deposition of PRODAN on an Al2O3 LAMBD film falls in between that of spectra A and B. The full widths at the half maxima (fwhm) are 5720 ( 250, 4800 ( 200, and 3670 ( 250 cm-1 for the PRODAN-doped LAMBD film, the film prepared by vacuum deposition of PRODAN on a LAMBD film of Al2O3 and the film made by spreading a chloroform solution of PRODAN on silica substrate. This indicates that PRODAN senses a more polar environment within the PRODAN-doped LAMBD film. We attribute the larger bathochromatic shift of the emission peak and larger fwhm of the PRODAN LAMBD film to the PRODAN molecules being embedded within the Al2O3 host. In conclusion, the organic component incorporates without aggregation during the LAMBD process and is sequestered within the host inorganic oxide matrix as opposed to being
Figure 5. Steady-state fluorescence spectra of (A) a thin film made by spreading a chloroform solution of pure PRODAN on a silica substrate and of (B) a LAMBD film of Al2O3:PRODAN on a silica substrate. The full width at half-maximum (fwhm) for spectra A and B are 3670 ( 250 and 5720 ( 250 cm-1, respectively. A film prepared by vacuum deposition of PRODAN on an Al2O3 LAMBD film has a fwhm of 4800 ( 200 cm-1.
adsorbed on the surface. LAMBD is an excellent lowtemperature process for incorporating organic molecules, such as nonlinearly active molecules, within an inorganic host matrix. Acknowledgment. This investigation was supported by the National Science Foundation Solid State Chemistry program. References and Notes (1) (a) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; John Wiley & Sons: New York, 1992. (b) Clusters and Cluster-Assembled Materials, Materials Research Society Symposium Proceedings, Vol. 206; Averback, R. S., Bernholc, J., David, L. N., Eds.; Materials Research Society: Pittsburgh, PA, 1991. (2) Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: Orlando, FL, 1987; Vols. I, II. (3) Fischer, G. L.; Boyd, R. W.; Gehr, R. J.; Jenekhe, S. A.; Osaheni, J. A.; Sipe, J. E.; Weller-Brophy, L. A. Phys. ReV. Lett. 1995, 74, 1871. (4) Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes; Klien, L. C., Ed.; Noyes Publications: Park Ridge, NJ, 1988. (5) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press Inc.: San Diego, CA, 1990. (6) Wijekoon, W. M. K. P.; Prasad, P. N.; Garvey, J. F. Proc. SPIE 1995, 2403, 143. (7) Wijekoon, W. M. K. P.; Lyktey, M. Y. M.; Prasad, P. N.; Garvey, J. F. Appl. Phys. Lett. 1995, 67, 1698. (8) Moser, F. H.; Thomas, A. H. The Phthalocyanines; CRC Press: Boca Raton, FL, 1983; Vols. I, II. (9) Narang, U.; Bright, F. V.; Prasad, P. N. Appl. Spectrosc. 1993, 47, 229. (10) Weber, G.; Farris, F. J. Biochemistry 1979, 18, 3705. (11) Narang, U.; Jordan, J. D.; Bright, F. V.; Prasad, P. N. J. Phys. Chem. 1994, 98, 8101. (12) Gvishi, R.; Narang, U.; Bright, F. V.; Prasad, P. N. Chem. Mater. 1995, 7, 1703.
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