# Enhanced Mechanical Properties in Organofluorosilica    Thin Films

Authors: Mario Pagliaro; Giovanni Palmisano; Eric Le Bourhis; Rosaria Ciriminna; Laura M. Ilharco; Alexandra Fidalgo
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Fluorinated hybrid organic-inorganic silicates (ORMOSIL) thin films display exceptional mechanical properties in terms of both hardness and elastic modulus that can be finely tuned by varying the angular velocity of the spin coating process. Hence, as traditional alkyl-modified silica xerogels generally show poor mechanical behavior, these materials offer a solution to a major limitation to applicability of ORMOSIL-based films.


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## Body

## 1. Introduction

We recently
reported that the mechanical properties of thin films of hybrid
organic-inorganic silicates (ORMOSIL) can be largely tuned in a wide range by
changing the degree of organic modification [1]. These findings are important since in thin
functional coatings, small amount of ORMOSIL impart improved new properties to
materials that are already commercialized in microelectronics as dielectric
materials [2],
and are about to find ubiquitous diffusion as versatile OLEDs [3].
Another structural characteristic of practical
importance is that in such coatings, strong chemical adhesion between the glass
substrate hydroxyls and the film occurs limiting delamination (see Figure 1) [4].Figure 1
Strong chemical bonding to glass substrates in sol-gel ORMOSIL thin films ensures prevention of delamination.Finally, the external
surface of ORMOSIL is depleted of silanol groups leading to low optical loss in
the near infrared, which is an essential requirement for optical applications
such as waveguides.Returning to alkyl-modified silica
thin films, we explained the drastic variation in mechanical response on the basis
of the structural changes in the organosilica molecular structure [1]. In general,
trifunctional alkoxysilanes and silicon alkoxides are excellent cross-linkers
and upon the sol-gel polycondensation, they lead to hybrid materials with high
Young’s modulus (E> 10 GPa) and
high (or even no) glass transition temperature (Tg).  A major limitation to applicability, however, often
resides in their poor mechanical behavior [4]
and this is partly the case also for alkyl-modified silica xerogels [1]. It is therefore
of great importance that fluorinated ORMOSIL thin films display, as shown in
this report, exceptionally good mechanical properties in terms of both hardness
and elastic modulus. Moreover, delicate fine tuning of the mechanical
properties of sol-gel coatings can easily be achieved by varying the angular
velocity of the spin coating process.


## 2. Experimental Section

A series of thin
films was obtained by spin-coating 800μL of precursor solution containing
3,3,3-trifluoropropyl-trimethoxysilane (TFPTMS), methanol, water, and fluoride in molar ratios Si:H2O:NaF:MeOH = 1:4:10-4:8) or 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecaoctafluoro-triethoxysilane
(TDOFTES), using tridecafluoroethanol as cosolvent, over microscope slides,
using a chemat technology KW-4A spin coater changing the spin-off speed following
spin-up at equal speed (1000 rpm for 18 seconds). The preparation for the first
series of samples was repeated under acid catalyzed conditions. The sol
compositions and preparation conditions of the film are summarized in Table 1. A
reference microscope slide (substrate) was added to the series showing that all
films are actually as stiff as (and even stiffer for some of these) the slide.Table 1
Sol compositions and identification of the thin films prepared. The spin-up conditions were 18 seconds at 1000 rpm for all the films.t2—spin-off period in seconds; r2—spin-off velocity in rotations per minute TFPTMS: trifluoropropyl-trimethoxysilane, TDOFTES: 3,3,4,4,5, 5,6,6,7,7,8,8,8-tridecaoctafluoro-triethoxysilane, TDF: tridecafluoroethanol.
Film designationSol compositionTotal volume/μLHydrolysis timet2/s;  r2/rpmvolume/μLTFPTTDOFTESCH3OHNaFH2OTDFHClMS(10 mM)(10 mM)TFP3(a)700—115135.6220——2106.615 min60; 3000TFP5700—115135.6220——2106.615 min60; 5000PF03(b)—1384115135.62203176—5966.615 min60; 3000PF04—1384115135.62203176—5966.615 min60; 4000PF05—1384115135.62203176—5966.615 min60; 5000PF06—1384115135.62203176—5966.615 min60; 6000PF07—1384115135.62203176—5966.615 min60; 7000PF08—1384115135.62203176—5966.615 min60; 8000TFA3(a)700—1151—220—35.62106.67 h60; 3000TFA5700—1151—220—35.62106.67 h60; 5000Si0Me——60; 3000Si100Me——60; 3000(a)Molar ratios: Si:H2O:NaF:MeOH = 1:4:10-4:8
(b)Molar ratios: Si:H2O:NaF:MeOH:TDF = 1:4:10-4:8:4.All the resulting
films were thus tested using a nanoindenter from CSM Instruments (Peseux, Switzerland).
The instrument was equipped with a Berkovich indenter, with equivalent
semiopening angle of 70.3 degrees. The tests were performed in air with the
force-control mode of the machine. The loading-unloading procedure was as
follows: loading to maximum load in 30 seconds, plateau at maximum load for 30
seconds, unloading in 30 seconds. The tests were repeated 15 times under a
maximum load of 2 mN. The calibration procedure suggested by Oliver and Pharr [5] was used to correct the data for
the load-frame compliance of the apparatus and for the imperfect shape of the
indenter tip, that is, the
relation between contact depth and contact area (The area function, i.e., the relation between contact depth and contact area, was calibrated performing several indentations on fused silica with penetration depth in the range 10-300 nm, with good reproducibility. The reduced elastic modulus of fused silica (Er = 69.6 GPa) was used as input to evaluate the contact area at each contact depth, and the plot was fitted with the Oliver and Pharr dependence obtaining the area function).  The infrared spectra of the films,  in attenuated
total reflection mode (ATR),  were recorded using a Perkin-Elmer (Wiesbaden, Germany)  1760
X FTIR plus micro-FTIR   spectrometer in the range 400–4000 cm−1,
at 4 cm−1 resolution, and are the ratio of 1000 scans for each film
against the same number of scans for the clean ATR crystal.


## 3. Results and Discussion

The thickness,
hardness, modulus, and contact penetration values for all the films are given
in Table2 and plotted in Figure 2.Table 2
Hardness (H) and modulus (Er) for the
fluorinated thin films.
FilmH/GPaEr/GPaTFP37.980.0TFP56.877.6PF037.679.3PF046.978.4PF056.878.3PF066.777.0PF077.078.2PF087.179.1TFA36.877.4TFA56.877.1Si0Me4.233.4Si100Me1.48.6Substrate7.178.9(a) Hardness and (b) elastic modulus for fluorinated organosilica thin films coated at different spin-off speed.
Data for TFP- and PFO-modified samples are shown. Spin-off speed in labelled specimens is ordered as
in Table1.
(a)(b)Hardness is the
mean contact pressure, that is,
the ratio of applied load and the resulting contact depth. The elastic modulus
is evaluated by the Oliver and Pharr
procedure, which allows one to evaluate the so-called reduced elastic modulus,Er, by fitting the unloading
curve with a power-law relation. In particular, following the equations derived
by Sneddon describing the elastic contact between a rigid indenter and an
elastic half-space, this is accomplished after evaluating the derivative of the
latter curve at the point of maximum penetration depth through (1) [6]: (1)S=dFdh|h=hmax=2β⋅Er⋅Acπ, where S is named
contact stiffness, F is applied load, h penetration depth, Ac is
the contact area.  β  is a pure
geometric correction factor, which takes also into account that the indenter is
not a perfect cone (King determined, by FEM simulations, for the Berkovich
indenter a β  value of
1.034) [7].
Following this approach, and taking into account also the limitations pointed
out by Chaudhri on the use of the concept of reduced elastic modulus [6],
the indenter was assumed to be rigid with respect to our films, so that the
original Sneddon equation can be applied (see (2), in which the Poisson ratio, ν, is assumed to be equal to 0.3): (2)Efilm=(1−ν2)Er. As expected, contact
penetration increases with the spin-coating speed, from about 76.5±4.4 nm for the
trifluoropropyl-modified silica sample prepared at 3000 rpm, to 83.9±2.1 nm for the
analogous film obtained at 5000 rpm. All films studied were mechanically stable
and no time dependence of the response was observed. Similarly, the nanoindentation measurements are reproducible as shown by
consecutive force curves (not displayed) giving penetration standard deviation
of less than 6%.Figure3 shows
the representative ATR of the film PFO6 in a micro-FTIR. The photograph of the
sample, which has the lowest H and Er, shows that the film is very
homogeneous, both the front and the back faces. The spectrum of sample PFO6 (front
in red in Figure 3) shows a band at ∼1077 cm−1, similar to that of TFA
films.ATR spectra of films TFP3,
PF03, PF06, and TFA3 in the silica fingerprint region.
(a)(b)(c)As previously reported, the elastic
moduli vary with nanoindentation depth, pointing to a clear substrate effect [8].
Taking therefore such effects into consideration, and thus limiting
measurements at lower loads at which penetration depth is compatible with
respect to the thickness of the films, intrinsic hardness, and elasticity
values for both of the ORMOSIL films are given in Table 2.Trends in Table2 and Figure 2
reveal valuable  information. The modulus values are about 80 GPa and hardness
values are about and   higher than 7 GPa,  which compared with, respectively, 8.6 and 1.4 GPa for a 100%
methyl-modified film obtained under the same spin coating conditions. Interestingly,  for lower spin-off velocity (3000 rpm), the perfluorinated film has slightly
worse mechanical properties than the trifluoropropyl-modified counterpart (PF3
versus TFP3). For both series, increasing the spin coating speed lowers both
hardness and modulus, which is in accordance with the formation of less thick films. When the
speed exceeds 6000 rpm, the substrate effect becomes predominant and eventually
at 8000 rpm both values correspond to those of the glassy substrate.Such exceptionally
good mechanical properties can be explained in terms of the secondary polymeric
structure of the organofluorosilica gels. Indeed, hardness and modulus of the
TFP3 sample are about two and three times higher compared to the corresponding values
of unmodified SiO2 (4.2 and 33.4 GPa, resp.). We propose that the increased
stiffness—the resistance of
the resulting elastic structure to deflection by an applied force—is due to the
presence of fluorine in the CF3−(CH2)2−[Si−O−]n chains, which is well known to promote orientation by lateral interactions [9].
Too high a level of fluorination, such as in the case of the perfluorinated
samples, produces an adverse effect, due to the dominance of interactions within
the polymer chains [10].  On the other hand,
the influence of the degree of the sol-gel reactions at the moment of spinning
is clear comparing films prepared at low spin velocity from sols with the same
precursor but with different hydrolysis conditions (TFP3 and TFA3). The latter
has lower modulus and hardness due to incomplete hydrolysis of the organosilane
precursor, which under acidic conditions takes places at slower rate. On the
other hand, for sufficiently high spin-off speeds, the nature of precursors and
the stage of the sol-gel process become irrelevant, yielding small differences
in ORMOSILs’ hardness values: indeed, all the samples prepared at 5000 rpm
(TFP5, PF05, and TFA5) have
almost identical mechanical properties.The mechanical properties of the
films correlate well with the structural information retrieved from infrared
analysis. Figure3 shows the ATR spectra of films TFP3, PF03, PF06, and TFA3 in
the silica fingerprint region.The films with
better mechanical properties (TFP3 and PF03), obtained with lower spin-off velocity,
have similar spectra, with two maxima at 896 and 760 cm−1, assigned
to the νasSi−O−Si and νsSi−O−Si modes, respectively. Those with poorer
mechanical behavior (PF06 and TFA3) present the νsSi−O−Si mode much shifted (at 750 cm−1),
and the νasSi−O−Si band splits in two components at 876 and 1072 cm−1, which can be assigned to the transverse and longitudinal
optical components, respectively. This LO-TO splitting is usually observed in
infrared spectra of thinner silica layers, with a higher degree of long
distance order in the silica network, which points to a separation between
silica rich and fluoroalkyl rich nanodomains.Selected
force-penetration curves are shown for three different films in Figure4 with
loading and unloading separated portions since the films deform plastically.
The curves were obtained under the same maximum force. Under a given load
however, penetration changes from one sample to the other. These changes could
be analyzed in terms of hardness and modulus as extracted from unloading curve.Figure 4
Force-penetration curve of
selected fluorinated organosilica thin films coated at different spin-off
speed.As
previously reported, the elastic moduli vary with nanoindentation depth,
pointing to a clear substrate effect [1]. Taking therefore such effects into
consideration, and thus limiting measurements at lower loads at which penetration
depth is compatible with respect to the thickness of the films, intrinsic
hardness and elasticity values for both the ORMOSIL film series are given in
Table 2.In conclusion, we
report the first case of hybrid ORMOSIL thin films
displaying superior mechanical properties. The materials are organofluorosilica
xerogels obtained by sol-gel polycondensation catalyzed by fluoride. A limited amount
of work has been devoted to the study of fluorinated organosilica sol-gels,
despite the fact that such fluorinated materials [11]  show exceptional
properties in terms, for instance, of enhanced CO2-philicity in
catalysis in supercritical carbon dioxide [12]
or  O2 permeability such as in the case of optical oxygen sensors [13].
As previously emphasized [1, 14], intelligent
design of organosilica thin films for a number of new applications should be
based on understanding the structural features governing the behavior of these
materials, and thus on mastering the sol-gel process at their origin. Weak
interactions amongst the fluorinated siloxane chains in the organosilica matrix
likely ensure excellent stiffness and similarly good hardness and elasticity. For comparison,
excellent periodic mesoporous organosilica (PMO) films obtained by surfactant template
polycondensation display 0.6 GPa hardness and 10 GPa elastic modulus values [2].  The
versatile chemistry of sol-gel-assembled ORMOSIL offers abundant opportunity
for tailoring the mechanical properties of organosilica thin films. Hence, as
organosilica sol-gel coatings are about to find a multiplicity of practical
applications, these findings open the route to the preparation of silica-based
films with enhanced mechanical properties.



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*Source: 964046-2008-11-10.xml*