Fluorescent plasma nanocomposite thin films containing nonaggregated rhodamine 6G laser dye molecules.

Abstract

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.

Full text

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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
fluorescentrhodamine6G(Rh6G)laserdyemolecules.Thenanocompositesaredepositedinonestepatroomtemperature
in a downstream microwave plasma operating at low pressure and power. By controlling the plasma chemistry, it is
possibletoreducetheformationofdyedimersandhigheraggregatesthatquenchthefluorescenceofthedyemolecules.
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)laserdyemoleculesaredepositedinonestepatroom
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 sublima-
tioninthedownstreamregionoftheplasmapermitsustocontrol
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 concentra-
tion of unreacted laser dye molecules in the layers. Moreover,
bycontrollingtheplasmachemistryithasbeenpossibletoreduce
the formation of dye dimers and higher aggregates that quench
the fluorescence of the dye molecules. The films are insoluble
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.1-3Rh6G exhibits strong absorption in the visible and a
very high fluorescence quantum yield.3In recent years, an
increasingnumberofworkshavetriedtheincorporationofRh6G
in inorganic and organic matrixes for application in fields such
as solid-state lasing, optoelectronics, optical filters, and so
forth.4-12A serious problem for these applications is the
aggregation of the dye molecules when trying to incorporate
* Corresponding author. E-mail: angelbar@icmse.csic.es. Fax: +34 95
446 06 65.
†EMPA Materials Science and Technology.
‡Instituto de Ciencia de Materiales de Sevilla.
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© Copyright 2006
American Chemical Society
AUGUST 1, 2006
VOLUME 22, NUMBER 16
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.4,5,11This shortcoming occurs because Rh6G
aggregatesquenchthefluorescenceoftherhodaminemolecules,
decreasing the optical activity of the layers.5,6,11,13This phe-
nomenon is especially important in compact films and can also
beobservedinaqueousandalcoholicsolutionsatrelativelyhigh
dye concentrations. Different chemical synthetic strategies, as
the incorporation of the dye in mesostructured layers or porous
gels,havebeendevelopedtodecreasethepercentageofaggregates
and increase the fluorescence emission.4-6,8,11For 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
duringthepastdecadesforthedepositionofthinfilmsofoxides,
polymers, metals, and so forth. The technique is scaleable and
easily integrable within the procedures to fabricate electronic
andoptoelectroniccomponents.14-17Whenanorganicmonomer
isintroducedintoaplasma,themonomerisfragmented,yielding
neutral and charged molecular fragments and atomic species
that usually produce highly cross-linked layers without the
retention of the monomer functionalities.14-17During the last
fewyears,interestinplasmaprocesseshasshiftedtothedeposition
ofless-cross-linkedmaterialsorhighlyfunctionalizedfilmsthat
retainthechemicalfunctionalitiesofthemonomerinthedeposited
films.18In this regard, only a few papers in the references have
reported the plasma polymerization of dye molecules to obtain
colored films for different applications,19,20the encapsulation
of dye aggregates,21or the use of plasma to sublimate dye
molecules.22,23
In this letter, we report a new methodology to prepare
fluorescentnanocompositethinfilmsbyPECVD.Thedeveloped
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
mechanicalstability.Toourknowledge,thisisthefirsttimethat
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,dryingsteps,orthermalprocesses.Thefilmsareobtained
in one step in only a few minutes. The procedure is fully
compatible with the standard microelectronic wafer-scale fab-
ricationincludinginsitudepositionandetchingprocedures(i.e.,
with an oxygen plasma), the use of masks, or the integration of
thefilmsinamultilayerstructure.Besides,theopticalproperties
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,weareabletoeffectivelycontroltheaggregationofthe
dyemoleculeinthecompactthinfilmsbyreducingtheformation
of nonfluorescent dimers and higher aggregates (i.e., strongly
fluorescent films).
The plasma deposition process is carried out in a remote
configurationbysublimatingRh6G(Aldrich)inthedownstream
region of an inert (Ar) or reactive (trimethylsilane (TMS)/Ar
mixture) microwave ECR plasma, with R being the ratio (mass
flowofTMS)/(massflowofAr).Thecharacteristicsoftheplasma
source have been described elsewhere.24The 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
chamberproducescross-linkedandtransparentpolymericfilms,
which will not be studied here. The downstream plasma
polymerization of the TMS precursor, in the absence of dye
sublimation, yields transparent SiOxCyHzthin films at a growth
rate of ∼8 nm/min for R ) 1.5. The plasma copolymerization
ofRh6GandTMSproducesSi-containingnanocompositeswith
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
copolymerizedRh6G/TMSfilm(Figure1b)withsimilarthickness
(∼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 cor-
responding 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.25In
bothexamples,thelayersareveryflatwithoutaggregatesordye
crystallites.Thistypeofmicrostructureiscrucialtotheintegration
of the films in multilayer structures. The nanocomposite thin
films can be easily patterned after deposition by using a shadow
maskandoxygenplasmatreatments.Anexampleofthisprocedure
is shown in Figure 1c.
The absorption spectra of selected nanocomposite thin films
havebeenplottedinFigure2.Thespectraofvacuum-evaporated
Rh6Gfilmsisalsoshownforcomparison.Itisworthnotingthat
theevaporatedfilmsareformedbyaggregatesofdyemolecules.
These films are not stable mechanically and can be removed
easilybypaperbrushingorbyimmersioninwater.Theabsorption
spectrum of Rh6G in aqueous and alcoholic solutions,6colloids
suspensions,12ordye-dopedfilms5,6,11,13istypicallycharacterized
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
absorptionbyfreeRh6Gmonomermolecules.5,6,11,13Theposition
of this maximum changes with the environment of the dye
molecule in each system.3,7,13The high-energy shoulder corre-
sponds to light absorption by Rh6G dimers and higher ag-
gregates.5,6,11,13These 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.
(7) Kalogeras,I.M.;Neagu,E.R.;Vassilikou-Dova,A.Macromolecules2004,
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.PlasmaPolymerization;AcademicPress: Orlando,FL,1985.
(16) d’Agostino,R.,Favia,P.,Fracassi,F.,Eds.PlasmaProcessingofPolymers;
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.;DellaMea,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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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,
theabsorptioncurveofthefilmssynthesizedwithR)1.5shows
a nearly symmetrical profile centered at ∼524 nm. Besides, the
absorption analysis of the films shows that the interaction of the
downstreamplasmawiththesublimateddyemoleculesproduces
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
fluorescenceofdyeaggregatesbyanalogytotheeffectmeasured
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
absorptionspectraofFigure2,depictingablueshiftandadecrease
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.5,6,11,13The shapes of the fluorescence and
excitation spectra of the plasma nanocomposites, especially for
R)1.5,aresimilartothosereportedfordilutedalcoholsolutions
of Rh6G-TiO2 nanocomposites5,6and nonaggregated Rh6G
adsorbed in mesostructured SiO2 and TiO2 thin films and in
porous silica gels.4-6,11However, 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.
Figure 1. (a) AFM taping mode images (1 × 1 µm2) of a 100-
nm-thickplasma-polymerizedRh6Gfilmand(b)AFMtapingmode
images(1×1µm2)ofa100-nm-thickplasma-copolymerizedRh6G/
TMS film deposited with R ) 1. (c) SEM micrograph of a 100-
nm-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.
Figure3. (a)Fluorescencespectra(λexc)500nm)and(b)excitation
spectra ((λem) 600 nm) corresponding to the nanocomposite thin
film of Figure 2.
LettersLangmuir, Vol. 22, No. 16, 2006 6721

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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
fragmentationofthedyemoleculesinthelayer)andthecomplete
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
thedecreaseinthelightabsorptionandincreaseinthefluorescence
emission intensity observed for the plasma polymer thin films,
in comparison with vacuum-sublimated films of similar thick-
nesses. 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
rhodaminemolecules.Theabsorptionandfluorescenceemission
and excitation spectra of the films prove the presence of intact
dyemoleculesinapolymericmatrixformedbycross-linkeddye
or dye-TMS fragments. Because of their strong absorption in
a relatively narrow wavelength, these films could be directly
appliedtothefabricationofwavelength-selectivefiltersforoptical
applications and optical filter coatings of LCD displays.9In
addition,thedepositionprocessiscompatiblewiththesequential
depositionofcompactandmicrostructuredoxidethinfilms(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,
avoidingfluorescencequenchingprocesses.Thesenovelmaterials
are very promising for the fabrication of wavelength-selective
filters, lasing media, and other applications that require highly
colored, fluorescent ultrathin films.
Acknowledgment.
NAN2004-09317-C04-01andRamonyCajalref2004/00001198)
andEMPAforfinancialsupportandR.Widmer(EMPA)forthe
AFM characterization.
We thank MECD (Nanolambda ref
SupportingInformationAvailable: FTIRtransmissionspectra
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 ´lez-
Elipe, A. R. Surf. Coat. Technol. 2004, 180-181, 244.
(29) Martin,A.;Espinos,J.P.;Justo,A.;Holgado,J.P.;Yubero,F.;Gonzalez-
Elipe, A. R. Surf. Coat. Technol. 2002, 151-152, 189.
6722 Langmuir, Vol. 22, No. 16, 2006 Letters