Appl. Phys. A 80, 1109–1113 (2005)
Applied Physics A
DOI: 10.1007/s00339-003-2392-1
Materials Science & Processing
j.m. fitz-gerald1,u
g. jennings1
r. johnson2
c.l. fraser2
Matrix assisted pulsed laser deposition
of light emitting polymer thin films
1 University
2 University
of Virginia, Department of Materials Science and Engineering, Charlottesville, VA 22904, USA
of Virginia, Department of Chemistry, Charlottesville, VA 22904, USA
Received: 27 August 2003/Accepted: 21 October 2003
Published online: 19 December 2003 • © Springer-Verlag 2003
Matrix assisted laser processing allows for the
deposition of functional and fragile materials with a minimum of breakdown and decomposition. In this communication we report on light emitting thin films of ruthenium
tris(bipyridine)-centered star-shaped poly(methyl methacrylate), Ru(bpyPMMA2 )3 (PF6 )2 , grown by matrix assisted
pulsed laser deposition. A pulsed excimer laser (KrF) operating at 248 nm was used for all experiments. Due to
the absorption at 248 nm and the solubility characteristics of [Ru(bpyPMMA2 )3 ](PF6 )2 , dimethoxy-ethane (DME)
was used as a solvent [1]. Dilute solutions (2 wt. %) of
[Ru(bpyPMMA2 )3 ](PF6 )2 and DME were flash frozen in liquid
nitrogen producing a solid target. Thin films ranging from 20
to 100 nm were grown on Si in an Ar atmosphere at 200 mTorr
at a laser fluence of 0.04 J/cm2 . The deposited materials were
characterized by proton nuclear magnetic resonance (1 H NMR)
and gel permeation chromatography (GPC) equipped with refractive index (RI), and ultraviolet/visible (UV/vis) detection.
ABSTRACT
PACS 81.15.Fg;
1
79.20.Ds; 78.66.Qn; 42.70.Jk
Introduction
Next generation applications require tighter tolerances on the structural, morphological, and chemical composition of the thin films used in their fabrication. This is especially the case for the deposition of high quality thin films of
organic or polymeric materials as opposed to purely inorganic
materials where high temperatures and native oxides are used
to overcome the hurdles in their fabrication. Depending on
the particular application, it may be desirable to deposit films
containing single or multilayer structures of different organic
or polymeric materials, homogeneous composite materials, or
materials with graded compositions [2]. In many situations, it
will be necessary to deposit the films discretely, achieve conformal coverage, and provide high quality films, especially
with regard to surface coverage uniformity and thickness control. Thin films of polymeric, inorganic and organic materials
also play an important role in batteries, high performance dielectrics, optical data storage, optical communications, and
u Fax: +1-434/982-5660, E-mail: jmf8h@virginia.edu
displays based on organic electroluminescent materials [3–5].
Polymer and organic coatings are essential for the fabrication
of chemical and biochemical sensors [6, 7], and in biomedical applications ranging from passivation films for prosthetic
devices to coatings for targeted drug delivery systems [8–11].
Based on their varied structures, physical properties, and
reactivities, metal complexes can play key roles in macromolecules [12–14]. Metal complexes may serve as templates
for self-assembly, as cross-links, or as part of the polymer
backbone.
Metal ions may be labile or inert with donors or acceptors in the form of chromophores, magnetic, or conducting centers. These systems allow for the introduction of
a variety of features into polymers. One fascinating class
of inorganic polymers are site-isolated metal-centered starshaped polymers (MCSPs) as shown in Fig. 1. These materials often exhibit interesting and well defined responses
to stimuli in solution and in bulk, with significant potential for next level sensing and imaging applications [15–17].
[Ru(bpyPMMA2)3 ](PF6 )2 is in the class of polymeric metal
complexes whose electroluminescent and photoluminescent properties are being actively studied for potential application to the expanding field of light emitting polymers (LEP), among other applications. After excitation at
∼ 468 nm, a metal-to-ligand charge transfer band (MLCT),
[Ru(bpyPMMA2)3 ](PF6 )2 emits at 610 nm, producing a characteristic orange color.
Matrix assisted pulsed laser evaporation (MAPLE) was
developed at the Naval Research Laboratory in the late 90 s
for the deposition of functional organic materials for chemical sensor applications, specifically directed at the detection
of nerve and mustard gases [18]. This process has met with
moderate success in depositing specific soft materials without
structural damage (i.e. with native properties retained) [19].
This process has been described in detail elsewhere [20].
2
Experimental
In this report thin films of [Ru(bpyPMMA2)3 ](PF6 )2 were deposited on Si(001) via MAPLE using an
KrF excimer laser operating at the following conditions:
λ: 248 nm, repetition rate: 10 Hz, fluence: 0.04 J/cm2 . The
Ru polymer was prepared via metalloinitiation using atom
transfer radical polymerization (ATRP) which allows for
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Applied Physics A – Materials Science & Processing
FIGURE 1 The metal-centered star-shaped polymer, [Ru(bpyPMMA2 )3 ](PF6 )2 and the response to
optical stimuli
controlled polymer growth, control over molecular weight
distribution, and architecture [21]. A solution of 2 wt. %
[Ru(bpyPMMA2)3 ](PF6 )2 was dissolved in dimethoxyethane
(DME), vortex mixed and ultrasonicated. Dimethoxyethane is
a clear, colorless liquid with a melting temperature of −58 ◦ C
and an absorbance of 25% cm−1 (λ = 248 nm) [22]. Prior to
deposition, the solution was flash frozen in liquid nitrogen and
then transferred to a vacuum chamber with a cold stage. The
chamber was pumped down to a base pressure of 10−6 Torr
and continuously backfilled with Ar, maintaining 200 mTorr
during deposition. During deposition the beam was scanned
across the stationary target with a computer controlled translating mirror for a total of 36 k pulses.
As a first order test, following deposition, films were exposed to low intensity UV radiation from a hand held (generic
portable UV light) UV source (λ = 266 nm) and emitted in the
orange (∼ 610 nm) as shown in Fig. 2. Although the material
only needs 468 nm light for excitation, the portable UV allows
for large area exposure and also imaging with a digital camera.
The dark square areas in Fig. 2 represent the locations of Si
witness substrates. The deposited and native films were compared using conventional characterization techniques. GPC
analysis of the original star polymer in CHCl3 (flow rate =
1 mL/min), with in-line UV/vis spectroscopy, indicated the
presence of the [Ru(bpy)3]2+ chromophore co-eluting with
the polymer, clearly demonstrating that the metal complex
is associated with the native polymer. Molecular weight estimates using the RI detector with linear PMMA standards
[Ru(bpyPMMA2 )3 ](PF6 )2 thin film grown on a 4 Si wafer by
MAPLE emitting characteristic orange light (610 nm) under a hand held UV
light source
FIGURE 2
indicate a number average molecular weight, Mn , of ∼ 33 kDa
for the native polymeric metal complex. GPC analysis of the
deposited material reveals several key differences from the native material: a low molecular weight shoulder, a slight shift
of the majority peak to lower elution volume, and a high molecular weight tailing. A GPC overlay of native and deposited
polymers is provided in Fig. 3. The low molecular weight
shoulder indicates partial cleavage of the PMMA chains. This
raises the question of where polymer degradation may be occurring. Significant overlap of the shoulder with the main
peak precludes accurate molecular weight determination of
the lower molecular weight component formed in the deposition process. However, estimates using linear PMMA standards suggest an Mn of ∼ 9 – 12 kDa, approximately 1/3 of the
molecular weight of the original star polymer.
A 3-D in-line UV/vis spectrum of the MAPLE deposited
material is shown in Fig. 4. This figure clearly shows the
presence of the [Ru(bpy)3]2+ chromophore again co-eluting
with the majority peak, therefore suggesting that the star remains intact in a majority of the sample. In contrast, the
chromophore is clearly absent in the low molecular weight
shoulder, suggesting that polymer may be degraded by destruction of the metal-ligand bond (i.e. macroligand dissociation). Moreover, if scission were occurring predominantly
along the polymer backbone or at the ester functional groups
FIGURE 3 GPC overlay for deposited and native [Ru(bpyPMMA2 )3 ](PF6 )2 . The overlay clearly shows a low molecular weight fraction (higher
elution volume) in the MAPLE deposited material perhaps resulting from the
destruction of metal–ligand bonds. Also evident is the slight shift to higher
MW and overall broadening
FITZ - GERALD et al.
Matrix assisted pulsed laser deposition of light emitting polymer thin films
1111
FIGURE 4 A 3-D plot of the GPC UV-vis spectra of the MAPLE deposited
films showing the presence of the [Ru(bpy)3 ]2+ chromophore co-eluting
with the majority peak. This suggests that the star predominantly remains
intact. The chromosphere is absent in the low molecular weight shoulder
(larger elution volume) that is roughly 1/3 the molecular weight, implying
that one mechanism of polymer degradation may involve photolysis of the
metal–polymeric bpy ligand bond
that serve as the attachment point of the polymer to the metal
complex on the bipyridine ligands, the resulting metal-free
fraction would be 1/6th (weight of one arm of the original
star) or lower in molecular weight. The metal-containing
fragments would retain the tris(bipyridine) chromophore and
would be detected by in-line UV-vis and could range in molecular weight from a polymer-free [Ru(bpy)3]2+ analogue to
nearly the size of native polymer. That is, rather than two distinct fractions, a long tail from native polymer too much lower
MW fragments would be expected for either of these alternative degradation mechanisms. However, the data does not
preclude a small degree of degradation by these methods.
Careful analysis of the GPC overlay also indicates some
high molecular weight tailing in the deposited polymer.
Reasons for this may include star-star coupling as a result of laser/polymer interaction (e.g. reactive intermediates
formed at polymer chain ends or other sites could lead to
interchain coupling reactions). Additionally, a slight shift
toward lower elution volume for the major fraction after
laser deposition is also indicative of an increase in molecular weight. This increased molecular weight also has
chromophores present (and can be seen in the 3-D plot),
lending credence to this hypothesis. Comparison of 1 H
NMR spectra shown in Fig. 5 for the native and deposited
material also reveals some degradation, as evidenced by
changes in peak ratios in the methyl region (1.6 – 1.2 ppm).
However, the complexity of this region of the spectrum of
PMMA makes it difficult to determine exactly what bonds
are affected. It is conceivable for a percentage of the polymeric material to be cleaved in the polymer backbone,
and recombination or termination of any reactive species
present.
3
Conclusions
We have succeeded in depositing thin films of a visible light emitting polymer, [Ru(bpyPMMA2 )3 ](PF6 )2 by ma-
1 H NMR spectra of native (a) and laser deposited (b)
[Ru(bpyPMMA2 )3 ](PF6 )2 samples in CDCl3
FIGURE 5
trix assisted pulsed laser deposition. Under a UV light the deposited materials exhibited characteristic emission at 610 nm
(orange). Though a majority of the polymer seems to remain
intact during deposition, 1 H NMR and GPC with UV/vis analysis reveal some polymer degradation under the conditions
investigated in this preliminary study. Data are consistent with
both polymer coupling reactions and some polymeric ligand
dissociation from the ruthenium center. In future studies, we
will attempt to control polymer degradation processes and optimize deposition processes by changing the laser wavelength
and exploring other solvent matrices that ideally, for reasons
of energy and degradation effects, are highly absorbing in the
visible region.
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Applied Physics A – Materials Science & Processing
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