© Copyright 2006 American Chemical Society
AUGUST 1, 2006 VOLUME 22, NUMBER 16
Letters Fluorescent Plasma Nanocomposite Thin Films Containing Nonaggregated Rhodamine 6G Laser Dye Molecules A. Barranco*,†,‡ and P. Groening† Nanotech@surfaces Laboratory, EMPA Materials Science and Technology, Feuerwerkerstrasse 39 CH-3602 Thun, Switzerland, and Instituto de Ciencia de Materiales de SeVilla (CSIC-UniVersidad de SeVilla) c/Ame´ rico Vespucio s/n 41092 SeVilla, Spain ReceiVed December 6, 2005. In Final Form: May 24, 2006 This letter reports a novel methodology for the synthesis of dye-containing nanocomposite thin films containing fluorescent rhodamine 6G (Rh6G) laser dye molecules. The nanocomposites are deposited in one step at room temperature in a downstream microwave plasma operating at low pressure and power. By controlling the plasma chemistry, it is possible to reduce the formation of dye dimers and higher aggregates that quench the fluorescence of the dye molecules. The films are intensely absorbent and fluorescent, insoluble in water, mechanically stable, and present good adhesion to the substrate. Besides, the method is compatible with the present silicon technology and therefore particularly interesting for the fabrication of integrated optoelectronic devices.
Nanocomposite thin films containing fluorescent rhodamine 6G (Rh6G) laser dye molecules are deposited in one step at room temperature by the plasma polymerization of rhodamine 6G in an electron cyclotron resonance microwave plasma operating at low pressure and power. Typically, the interaction of organic precursor molecules (i.e., a dye) with a plasma leads to a high fragmentation of the molecules producing cross-linked and optically inactive polymeric materials. Nonetheless, the sublimation in the downstream region of the plasma permits us to control the interaction of the precursor molecules and the active species of the plasma. For the first time, in this work we have obtained by this methodlogy polymeric thin films containing intact molecules of the rhodamine 6G laser dye. Films of only tens of nanometers are intensely absorbent, indicating a high concentration of unreacted laser dye molecules in the layers. Moreover, by controlling the plasma chemistry it has been possible to reduce the formation of dye dimers and higher aggregates that quench the fluorescence of the dye molecules. The films are insoluble * Corresponding author. E-mail:
[email protected]. Fax: +34 95 446 06 65. † EMPA Materials Science and Technology. ‡ Instituto de Ciencia de Materiales de Sevilla.
in water, mechanically stable, and present good adhesion to the substrate. The deposition method is compatible with the present silicon technology and therefore suitable for the fabrication of integrated optoelectronic devices. Rh6G is a xanthene derivative used as a gain medium in dye lasers.1-3 Rh6G exhibits strong absorption in the visible and a very high fluorescence quantum yield.3 In recent years, an increasing number of works have tried the incorporation of Rh6G in inorganic and organic matrixes for application in fields such as solid-state lasing, optoelectronics, optical filters, and so forth.4-12 A serious problem for these applications is the aggregation of the dye molecules when trying to incorporate (1) Scha¨fer F. P., Ed. Dye Lasers; Topics in Applied Physics; SpringerVerlag: New York, 1973; Vol. 1. (2) Brackmann, U. Laser Dyes, 3rd ed. Landa Physics AG: Go¨ttingen, Germany, 2000. (3) Valeur B. Molecular Fluorescence: Principles and Applications; Wiley: New York, 2001. (4) Yang P.; Wirnsberger G.; Huanng H. C.; Cordero S. R.; McGehee M. D.; Scott B.; Deng T.; Whitesides G. M.; Cmelka G. F.; Buratto S. K.; Stucky G. D. Science 2000, 287, 465. (5) Vogel R.; Meredith P.; Kartini I.; Harvey M.; Riches J. D.; Bishop A.; Heckenberg N.; Trau M.; Ruinsztein-Dunlop H. ChemPhysChem 2003, 4, 595. (6) Vogel, R.; Meredith, P.; Harvey, M. D.; Rubinsztein-Dunlop, H. Spectrochim. Acta, Part A 2004, 60, 245.
10.1021/la053304d CCC: $33.50 © 2006 American Chemical Society Published on Web 07/07/2006
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them in a high concentration in solid host materials prepared by sol-gel methods.4,5,11 This shortcoming occurs because Rh6G aggregates quench the fluorescence of the rhodamine molecules, decreasing the optical activity of the layers.5,6,11,13 This phenomenon is especially important in compact films and can also be observed in aqueous and alcoholic solutions at relatively high dye concentrations. Different chemical synthetic strategies, as the incorporation of the dye in mesostructured layers or porous gels, have been developed to decrease the percentage of aggregates and increase the fluorescence emission.4-6,8,11 For example, Rh6G-doped sol-gel mesoestructured silica waveguides have recently shown amplified laser emission at pumping thresholds as low as 10 kW/cm2.4 Plasma-enhanced chemical vapor deposition (PECVD) and plasma polymerization are well-known techniques developed during the past decades for the deposition of thin films of oxides, polymers, metals, and so forth. The technique is scaleable and easily integrable within the procedures to fabricate electronic and optoelectronic components.14-17 When an organic monomer is introduced into a plasma, the monomer is fragmented, yielding neutral and charged molecular fragments and atomic species that usually produce highly cross-linked layers without the retention of the monomer functionalities.14-17 During the last few years, interest in plasma processes has shifted to the deposition of less-cross-linked materials or highly functionalized films that retain the chemical functionalities of the monomer in the deposited films.18 In this regard, only a few papers in the references have reported the plasma polymerization of dye molecules to obtain colored films for different applications,19,20 the encapsulation of dye aggregates,21 or the use of plasma to sublimate dye molecules.22,23 In this letter, we report a new methodology to prepare fluorescent nanocomposite thin films by PECVD. The developed procedure yields insoluble, highly absorbent, fluorescent thin films that are deposited in one step at room temperature. The films present very good adhesion to the substrate and high mechanical stability. To our knowledge, this is the first time that direct synthesis by plasma polymerization of light-absorbing and fluorescent thin films of rhodamine has been achieved. As compared to the well-known sol-gel procedures, this physical methodology permits the synthesis of the films without any solvent, drying steps, or thermal processes. The films are obtained in one step in only a few minutes. The procedure is fully (7) Kalogeras, I. M.; Neagu, E. R.; Vassilikou-Dova, A. Macromolecules 2004, 37, 1042. (8) Loerke J.; Marlow F. AdV. Mater. 2004, 14, 1745. (9) Ohishi T. J. Non-Cryst. Solids 2003, 332, 80. (10) Wirnsberger G.; Yang P.; Huang H. C.; Scott B.; Deng T.; Whitesides G. M.; Chmelka B. F.; Stucky G. D. J. Phys. Chem. B 2001, 105, 6307. (11) Del Monte, F.; Mackenzie, J. D.; Levy, D. Langmuir 2000, 16, 7377. (12) Nasr, C.; Liu, D.; Hotchandani, S.; Kamat, P. C. J. Phys. Chem. 1996, 100, 11054. (13) Bojarski P. Chem. Phys. Lett. 1997, 278, 225. (14) Grill, A. Cold Plasma in Materials Fabrication; IEE Press: New York, 1994. (15) Yasuda, H. Plasma Polymerization; Academic Press: Orlando, FL, 1985. (16) d’Agostino, R., Favia, P., Fracassi, F., Eds. Plasma Processing of Polymers; Kluwer Academic: Dordrecht, The Netherlands, 1996. (17) Gro¨ning, P. Cold Plasma Processes in Surface Science and Technology. Nalwa, H. S., Ed. In Handbook of Thin Film Materials; Academic Press: San Diego, CA, 2001; Vol. 1, p 219. (18) Denes, F. S.; Manolache, S. Prog. Polym. Sci. 2004, 29, 815. (19) Inoue, M.; Morita, H.; Takai, Y.; Mizutani, T.; Ieda, M. Jpn. J. Appl. Phys. 1988, 73, 1059. (20) Osada, Y.; Mizumoto, A. J. Appl. Phys. 1986, 5, 1776. (21) Homilius, F.; Heilmann, A.; von Borczyskowski, C. Surf. Coat. Technol. 1994, 74-75, 594. (22) Maggioni, G.; Carturan, S.; Quaranta, A.; Patelli, A.; Della Mea, G. Chem. Mater. 2002, 14, 4790. (23) Maggioni, G.; Carturan, S.; Quaranta, A.; Patelli, A.; G. Della Mea, V. Regato, Surf. Coat. Technol. 2003, 174-175, 1151.
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compatible with the standard microelectronic wafer-scale fabrication including in situ deposition and etching procedures (i.e., with an oxygen plasma), the use of masks, or the integration of the films in a multilayer structure. Besides, the optical properties of the films are not the result of the specific interaction of the dye molecule as a guest in an inorganic host matrix. Thus, solid nanocomposites with a very high concentration of dye laser molecules (i.e., highly absorbent films) can be obtained. In addition, we are able to effectively control the aggregation of the dye molecule in the compact thin films by reducing the formation of nonfluorescent dimers and higher aggregates (i.e., strongly fluorescent films). The plasma deposition process is carried out in a remote configuration by sublimating Rh6G (Aldrich) in the downstream region of an inert (Ar) or reactive (trimethylsilane (TMS)/Ar mixture) microwave ECR plasma, with R being the ratio (mass flow of TMS)/(mass flow of Ar). The characteristics of the plasma source have been described elsewhere.24 The sample holder is located ∼5 cm from the plasma source. It is interesting that the sublimation of the dye inside the excitation zone of the plasma chamber produces cross-linked and transparent polymeric films, which will not be studied here. The downstream plasma polymerization of the TMS precursor, in the absence of dye sublimation, yields transparent SiOxCyHz thin films at a growth rate of ∼8 nm/min for R ) 1.5. The plasma copolymerization of Rh6G and TMS produces Si-containing nanocomposites with a Si percentage in the films of as low as ∼10 at. % for R ) 1.5 as determined by X-ray photoelectron spectroscopy (XPS). Figure 1 a and b shows two atomic force microscopy (AFM) images of a TMS plasma-polymerized film (Figure 1a) and a copolymerized Rh6G/TMS film (Figure 1b) with similar thickness (∼90 nm). Both layers are homogeneous and crack-free. The AFM images show that the film surfaces are very smooth, especially the plasma-polymerized Rh6G thin films. The corresponding rms values are ∼0.4 and ∼0.6 nm, respectively. The Rh6G/TMS copolymerized film (Figure 1b) shows the typical granular structure of plasma-polymerized TMS materials.25 In both examples, the layers are very flat without aggregates or dye crystallites. This type of microstructure is crucial to the integration of the films in multilayer structures. The nanocomposite thin films can be easily patterned after deposition by using a shadow mask and oxygen plasma treatments. An example of this procedure is shown in Figure 1c. The absorption spectra of selected nanocomposite thin films have been plotted in Figure 2. The spectra of vacuum-evaporated Rh6G films is also shown for comparison. It is worth noting that the evaporated films are formed by aggregates of dye molecules. These films are not stable mechanically and can be removed easily by paper brushing or by immersion in water. The absorption spectrum of Rh6G in aqueous and alcoholic solutions,6 colloids suspensions,12 or dye-doped films5,6,11,13 is typically characterized by an absorption maximum between 525 and 540 nm and a high-energy shoulder. It is commonly accepted that the main absorption peak appearing at low energy corresponds to the absorption by free Rh6G monomer molecules.5,6,11,13 The position of this maximum changes with the environment of the dye molecule in each system.3,7,13 The high-energy shoulder corresponds to light absorption by Rh6G dimers and higher aggregates.5,6,11,13 These two absorption features can be observed clearly in the spectra of the evaporated Rh6G film (Figure 2) at ∼550 and ∼515 nm. However, the spectra of the plasma(24) Nowak, S.; Gro¨ning, P.; Kuttel, O. M.; Collaud, M.; Dietler, G. J. Vac. Sci. Technol., A 1992, 10, 3419. (25) Barranco, A.; Cotrino, J.; Yubero, F.; Espino´s, J. P.; Benı´tez, J.; Clerc, C.; Gonza´lez-Elipe, A. R. Thin Solid Films. 2001, 401, 150.
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Figure 3. (a) Fluorescence spectra (λexc ) 500 nm) and (b) excitation spectra ((λem ) 600 nm) corresponding to the nanocomposite thin film of Figure 2.
Figure 1. (a) AFM taping mode images (1 × 1 µm2) of a 100nm-thick plasma-polymerized Rh6G film and (b) AFM taping mode images (1 × 1 µm2) of a 100-nm-thick plasma-copolymerized Rh6G/ TMS film deposited with R ) 1. (c) SEM micrograph of a 100nm-thick oxygen plasma Rh6G/TMS film deposited with R ) 1 patterned after deposition by an oxygen plasma using a shadow mask.
Figure 2. Absorption spectra of a vacuum-sublimated Rh6G and three plasma nanocomposite thin films deposited with increasing R ratios. The inset shows several plasma-polymerized Rh6G films on quartz substrates.
polymerized Rh6G film (R ) 0) and the copolymerized Rh6G/ TMS films are quite different from that of the evaporated one. The intensity of the high-energy shoulder and the width of the absorption peaks decrease dramatically as R increases. In fact, the absorption curve of the films synthesized with R ) 1.5 shows
a nearly symmetrical profile centered at ∼524 nm. Besides, the absorption analysis of the films shows that the interaction of the downstream plasma with the sublimated dye molecules produces an effective modification of the structure of the deposited films that leads to a large decrease in the degree of aggregation of the dye molecules. We have found that this process is particularly efficient when a flow of TMS is added to the Ar discharge at an optimum value of R ) 1.5. The fluorescence emission spectra of selected films (λexc ) 500 nm) have been plotted in Figure 3a. The Figure shows that the fluorescence emission is blue-shifted as the R ratio increases from 581 nm for R ) 0 to 554 nm for R ) 1.5. In addition, the width of the peaks decreases as R increases. The increment in the width of the fluorescence peaks can be attributed to the fluorescence of dye aggregates by analogy to the effect measured for Rh6G solutions at elevated concentrations and in dye-doped sol-gel oxides.5-7,12,13 The fluorescence excitation spectra of the films have been plotted in Figure 3b. The spectra are mirror images of the absorption spectra of Figure 2, depicting a blue shift and a decrease in width as R increases. The decrease in the intensity of the high-energy shoulder of the excitation spectra can also be interpreted as a reduction in the aggregation of the dye in the nanocomposite.5,6,11,13 The shapes of the fluorescence and excitation spectra of the plasma nanocomposites, especially for R ) 1.5, are similar to those reported for diluted alcohol solutions of Rh6G-TiO2 nanocomposites5,6 and nonaggregated Rh6G adsorbed in mesostructured SiO2 and TiO2 thin films and in porous silica gels.4-6,11 However, they are narrower than for dye-intercalated clay minerals characterized by Rh6G dimers.26 A low-intensity feature at ∼670 nm can be observed in the film obtained at R ) 0. This emission may be possibly because of a partial modification of the structure of the dye in these films. A preliminary XPS and Fourier transform infrared (FTIR) characterization of the deposited films shows clearly that the plasma produces partial fragmentation and cross linking of the (26) Martı´nez Martı´nez, V.; Lo´pez Arbeloa, F.; Ban˜uelos Prieto, J.; Arbeloa Lo´pez, T.; Lo´pez Arbeloa, I. Langmuir 2004, 20, 5709.
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Rh6G molecules in the films. Both fragmented and isolated dye molecules contribute to the XPS and FTIR spectra. Thus, the FTIR spectra of the nanocomposite films are formed by broad bands that can be interpreted as the result of the cross linking of Rh6G molecules during plasma deposition (i.e., extensive fragmentation of the dye molecules in the layer) and the complete or partial hindering of vibrational modes of the portion of nonfragmented dye molecules confined in the cross-linked polymeric matrix. This fragmentation process is congruent with the decrease in the light absorption and increase in the fluorescence emission intensity observed for the plasma polymer thin films, in comparison with vacuum-sublimated films of similar thicknesses. It is interesting that this fragmentation and the partial destruction of the molecular structure of the dye in the films are necessary to optimize the optical, chemical, and mechanical properties of the nanocomposite. Moreover, the molecular fragmentation of the dye is required to produce a cross-linked matrix that is mechanically stable and insoluble. Our results have also shown that the control of the partial fragmentation of the dye can be used to reduce the formation of dimers and/or higher aggregates of the dye, a feature that determines the final luminescence properties of the nanocomposites. In this context, the copolymerization of Rh6G with TMS leads to a structure containing mainly isolated optically active molecules. In summary, for the first time, a one-step process has been developed for the synthesis of insoluble, highly absorbing, fluorescent thin films by the plasma-assisted deposition of rhodamine molecules. The absorption and fluorescence emission and excitation spectra of the films prove the presence of intact dye molecules in a polymeric matrix formed by cross-linked dye or dye-TMS fragments. Because of their strong absorption in
Letters
a relatively narrow wavelength, these films could be directly applied to the fabrication of wavelength-selective filters for optical applications and optical filter coatings of LCD displays.9 In addition, the deposition process is compatible with the sequential deposition of compact and microstructured oxide thin films (i.e., TiO2, SiO2, ZnO, etc.27-29) and the use of patterning processes that have potential applications in the development of complex optical devices. All of these procedures are fully compatible with standard microelectronic wafer-scale fabrication. Besides, the method provides a highly effective reduction of the degree of dye aggregation in solid films at high dye concentrations, avoiding fluorescence quenching processes. These novel materials are very promising for the fabrication of wavelength-selective filters, lasing media, and other applications that require highly colored, fluorescent ultrathin films. Acknowledgment. We thank MECD (Nanolambda ref NAN2004-09317-C04-01 and Ramon y Cajal ref 2004/00001198) and EMPA for financial support and R. Widmer (EMPA) for the AFM characterization. Supporting Information Available: FTIR transmission spectra of evaporated and plasma-polymerized Rh6G layers. This material is available free of charge via the Internet at http://pubs.acs.org. LA053304D (27) Barranco, A.; Cotrino, J.; Yubero, F.; Gonza´lez-Elipe, A. R. Chem. Mater. 2003, 15, 3041. (28) Barranco, A.; Cotrino, J.; Yubero, F.; Girardeau, T.; Camelio, S.; Gonza´lezElipe, A. R. Surf. Coat. Technol. 2004, 180-181, 244. (29) Martin, A.; Espinos, J. P.; Justo, A.; Holgado, J. P.; Yubero, F.; GonzalezElipe, A. R. Surf. Coat. Technol. 2002, 151-152, 189.
