Biennial Report 2016/2017

Reports
precursor thin films was determined
gravimetrically.
Precursor UV irradiation was carried out in a
homemade chamber [1.2 m 3 h -1 nitrogen gas flow,
*
0.25 % oxygen], equipped with a Xe 2 excimer
lamp having a continuous emission in the VUV
wavelength range at 160 nm < λ < 195 nm
[photon energy 7.75 > E Ph > 6.36 eV, 614 kJ mol -1
< ȂE < 748 kJ mol -1 ], enough for photoinitiated
radical bond cleavage reactions of C-C
(348 kJ mol -1 ), C-O (358 kJ mol -1 ) and C-H
(413 kJ mol -1 ) bonds in the organic ligands of
titanium(IV) ethoxide [Ti(OEt) 4 ] as precursor.
During irradiation the temperature at sample
surface did not exceeded 34 °C. ATR-FTIR
spectra on pristine and irradiated Ti(OEt) 4 thin
films were recorded under flowing nitrogen
atmosphere in absorbance mode. Gaseous
reaction products were enriched on a solid phase
microextraction fiber assembly and analyzed
using a gas chromatography system coupled with
a mass selective detector (GC-MS).
Film morphology was obtained by scanning
electron microscopy (SEM) at an acceleration
voltage of 1 kV. Thin film depth profiles were
produced by multiple X-ray photoelectron
spectroscopy (XPS) analysis cycles together with
material removal via sputtering using argon ions
(0.5 kV). The thin films structure was investigated
using X-ray diffraction (XRD) in Bragg-Brentano
geometry with Cu Kα 1 radiation (λ = 0.15406 nm),
in the 2θ range of 20°-70° and a step width of
0.05°. X-ray reflectivity (XRR) measurements
were performed using a combined high-resolution
reflectometer/diffractometer setup with parallelbeam
geometry. X-ray radiation was collimated by
a multilayer-gradient X-ray mirror. Cu Kα 1
radiation was exclusively obtained by passing a 2-
bounce Ge(220) crystal monochromator.
Measurements were carried out in the range
between 0°-1.25° (step widths of 0.002°). The
intensity of the radiation which was reflected from
the sample was determined by a scintillation
detector. UV-Vis transmittance of thin films onto
quartz was measured in the range of 300-1000
nm. Film thicknesses of the spin-coated and VUV
irradiated samples was measured by
spectroscopic ellipsometry (SE) in the spectral
range from 400-1700 nm at three angles of
incidence (65°, 70° and 75°). Water vapour
transmission rates of TiO x covered PET films
were determined at 38 °C and 90 % relative
humidity.
Quantum chemical calculations employing
Density Functional Theory (DFT, B3LYP, Jaguar
program package) helped us to understand the
underlying molecular mechanisms. Structures and
energies of possible transformations of the
molecules were calculated at B3LYP/LACVP*
level of theory. The frequency analysis was done
at the same level of theory to characterize the
stationary points on the potential surface and to
obtain total enthalpy (H) and Gibbs free energy
(G) at a standard temperature of 298.15 K using
unscaled vibrations. ∆H and ∆G were calculated
as the difference of the calculated H and G
between reactants and products. For better
understanding of observed experimental IRspectra,
frequency analysis was done on possible
structures, which could be built during
transformations of studied geometries after UV
excitation (E Photon = 7.2 eV). The electronic
excitation energies were calculated using the
Time Dependent (TD) DFT method at the
B3LYP/LACVP* level of theory.
Results and Discussion
Ti(OEt) 4 conversion and decomposition kinetics
(Fig. 1) for different precursor layer thicknesses
were obtained under the assumption that ATR-
FTIR peak areas of C-O-based vibrations
between 1200-980 cm -1 represent the remaining
precursor contents. Measured data of the initial C-
O bond scission initiated by VUV photons can be
fit by a first-order law (correlation r 2 > 0.97) with
apparent rate constants of 2.4x10 -2 s -1 ,
1.6x10 -2 s -1 , 0.6x10 -2 s -1 for precursor film
thicknesses of 270, 550 and 1060 nm,
Fig. 1: Area of normalized ATR-FTIR peaks for C-O based
vibrations in the range of 1200-980 cm -1 (A IR1200-980 cm-1) after
VUV irradiation (t irradiation) and different radiant exposure (H e)
and a logarithmic plot of the initial reaction rates (k app) for
different precursor thin film thicknesses (d Ti(OEt)4) (inset)
[average radiant exposure per minute (H e/t) =
0.31 J cm -2 min -1 ].
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