Abstract
We report on femtosecond (130 femtoseconds) laser pulse removal and modification in Carbon thin films. Micro-size constructions on the surface of exposed thin films were produced and were observed using Scanning Electron Microscope (SEM). The results show that femtosecond laser irradiation considerably modifies the optical properties and surface morphology of Carbon thin films. Self-prepared shapes like wrinkles, cellular erections, groups of spots, and cracks are observed by SEM studies.
1.
Introduction
Laser-induced modifications are a subject of interest for many applications. The laser-induced changes in different kinds of surfaces have been extensively studied. For
example, the laser produces periodic structures at the surface of diamond-like
carbon thin film, etc. Nanosecond and femtosecond lasers are being widely used
for the fabrication of the different structures on thin film surfaces as well
as changes in different properties are also observed. So, in this work, the
femtosecond laser-induced modifications and modifications in the optical properties
of diamond-like carbon thin films have been studied.
2.
Experimental Details
The experimental part is comprised of two
steps. In the first step, the diamond-like carbon thin film was deposited on a glass substrate (size: 4cm × 1cm) using the rf magnetron sputtering process. Later
the thin films were irradiated with a different number of femtosecond laser
pulses. In this experiment, Ti: Sapphire
laser of wavelength 400 nm (second harmonic), energy 2 mW, repletion rate 1kHz, and pulse duration is 130fs is used to interact with the diamond-like carbon thin
films. A laser beam is focused on the sample by the visible lens. The argon gas flow
near the target to avoid oxidation on carbon thin film. The number of pulses
used is 2500, 5000, 7500, and 10000 in four places in square form. The squares
are separated by a 1mm distance. So, the size of each square is 0.5 mm. The
number of circular spots in each square is given by 11×11=121 circular spots.
Square 1 is at 2500, second at 5000 third and fourth at 7500 and 10000
respectively. Square 1 represents the number of laser pulses at 2500. The results
have been analyzed by using a UV-Visible spectrophotometer and Scanning Electron
Microscopy (SEM).
3. Results and Discussion
The results have been
studied by using a UV-Visible spectrophotometer and Scanning Electron Microscopy
(SEM). The details are given in the following:
3.1 Optical Properties
The optical properties of
pristine and femtosecond laser irradiated spots with a different number of laser
pulses are studied by using a UV-Visible spectrophotometer.
Figure 1 shows the optical spectra of
unirradiated and femtosecond laser irradiated spots with 2500, 5000, 7500, and 10000 laser pulses. The black line near the zero percent transmission
is the baseline. It can be seen that the percentage transmission decreases
constantly when the wavelength changes from 800nm to 350 nm, and a fast
decrease in percentage transmission is observed from 350 nm to 200 nm. So,
the absorption edge is observed near 350 nm. The percentage transmission
becomes zero below 300 nm for all the samples. The kink that appeared at 340 nm is
not true data but it appears due to source changes during measurement. The
increase in percentage transmission is observed as the number of pulses
increases.
The UV-visible transmission spectra are
shown in the figure which shows the high transmission in the UV region. The
fundamental formulas used to calculate the absorption coefficient (α) are given
below
I= Io eαt , A =
log (Io/I) …………. (3.1)
α = 2.303 (Abs/t) …………. (3.2)
where (t) shows the thickness and Abs
represents the absorbance. Tauc’s relation has been used to determine the
optical band gaps for all the films.
(αhν) n = A (hν – Eg) …………. (3.3)
Where A is the constant, hν is the
photon energy and n is an index and it defines the optical absorption process
and is theoretically equal to n=2 for direct and n = 1/2 for indirect band gap.
The energy band gap values have been deduced by plotting hν vs. (αhν)2.
The linear extrapolation at the energy axis gives the value of the band gap energy.
The linear shape of the graph confirms the direct band gap nature of the
diamond-like carbon thin film as shown in figure 2. The values of the
optical band gap of the diamond-like carbon thin film irradiated by the different
number of pulses are shown in figure 3. It can be seen that the
optical band gap of as-deposited diamond-like carbon thin film is 3.54 eV which
shows random variation in optical band gap when the number of laser pulses is
increased to 10000. The curve in the figure is the linear fitting of the data
points, which shows the decreasing trend in optical bandgap as a function of a number of laser pulses. As the optical band gap of a diamond is ~5.5 eV and of
graphite is zero, therefore the decreasing trend of the diamond-like carbon
thin films is an indication that the transformation of diamond-like carbon
thin films into more graphitic films.
3.2 Scanning
Electron Microscopy
Scanning electron microscopy is done
to analyze the femtosecond laser irradiation of thin films. Figure 4 shows
the scanning electron micrographs at 300 K magnification of laser irradiated
square-shaped spots in which there is an array of 11×11 circular spots. Figure 4
(a) shows the circular spots formed due to irradiation of 2500 laser
pulses. Similarly, figure 4(b), (c), and (d) shows the laser irradiated spots at
5000, 7500, and 10000 laser pulses. It can be seen that the laser
decay occurs more when the number of laser pulses is increased from zero to
10000. The figure shows the dark and bright portions on the surface of the thin film.
The dark portion shows the laser decay zero when the laser produces some
thermal and non-thermal effects. The thermal effects arise due to the rise of
laser irradiation. The non-thermal effects may also be present which occur due
to direct bond breaking. Figure 4 shows the scanning electron microscopy images
of laser irradiated diamond-like carbon thin films at a magnification of 7.74 K
and 11.25 K for 2500 and 5000 pulses. The micrographs show the
function of elongated ripples-type microstructures. These kinds of
microstructures appear due to the well-known interference phenomena of laser
light on the surface of thin films. The non-uniform ablation and effect on thin
film samples are changed to the non-uniform energy distribution of laser spots.
4. Conclusions
The results obtained of diamond-like carbon thin film irradiated by femtosecond laser are reported in this work. The optical transmission is found to increase and the optical band gap calculated by Tauc’s plots decreases as the number of laser pulses is increased. The decreasing trend of diamond-like carbon thin films shows the transformation of diamond-like carbon thin film into a more graphitic thin film. The scanning electron micrographs did analyze the femtosecond laser irradiated diamond-like carbon thin films. The heat-affected zone is reduced by increasing the number of laser pulses. The inhomogeneous ablation of the surface, ripple formation, and other effects on the thin film show the non-uniform energy distribution.
