Advances in Materials Physics and Chemistry, 2011, 1, 64-69
doi:10.4236/ampc.2011.13011 Published Online December 2011 (http://www.SciRP.org/journal/ampc)
Two Photon Absorption in Nanostructure Wide Band Gap
Semiconductor CdS Using Femtosecond Laser
Abdulla M. Suhail, Raied K. Jamal, Hani J. Kbashi
Department of Physics, College of Sciences, Baghdad University, Baghdad, Iraq
Received September 11, 2011; revised October 18, 2011; accepted October 29, 2011
Abstract
Strong nonlinear absorption in CdS nanoparticles of dimensions in the range 4 - 50 nm when irradiated with
a femtosecond pulsed laser at 800 nm is observed. The CdS nanoparticles sizes were controlled by changing
the reaction time of aqueous solutions of thiourea and cadmium chloride. The structure of the prepared CdS
nanoparticles were tested and their optical properties were investigated and the nonlinear absorption coeffi-
cients were measured using a fully computerized z-scan unit. It is found that the formation of the CdS nanos-
tructuresfor are responsible for the observation of the strong tow photon absorption.
Keywords: Cadmium Sulfide, Nanostructure, Multiphoton Absorption
1. Introduction
Nanoparticle semiconductor materials with large nonlin-
ear coefficients have attracted much attention in recent
years [1-5]. This is interest due to their unique optical
properties when their size decreased from bulk to few
nanometers. Such unusual properties may have techno-
logical applications such as optical switching devices,
Q-dot lasers, and there is great potential in biophotonics
[6,7]. The linear optical properties of the semiconductor
nanoparticles depends strongly upon the particle size [8].
The nonlinear optical properties, the absorption and re-
fractive index, were observed in nanoparticles semicon-
ductor CdS material [9-11]. The dynamic of the CdS
excited state have been studied by femtosecond laser and
by the photoluminescence (PL) analysis [12-14]. In this
study, a CdS nanofilm was deposited using a simple and
low cost spray pyrolysis technique. A fully computerized
Z-Scan setup with femtosecond laser was used to investi-
gate the effect of surface state formation in CdS nanoparti-
cles on the nonlinear dynamics parameters.
2. Experimental
The CdS nanofilm was prepared by mixing an 0.1 M
aqueous solution of thiourea (NH2CSNH2) and an 0.2 M
aqueous solution of Cadmium Chloride (CdCl2) as start-
ing solution. They were mixed and dissolved in distilled
water in the ratio of 3:2. The solution was mixed thor-
oughly and the final solution was sprayed onto a heated
substrate kept at a temperature of 300˚C. Spray pyrolysis
is a useful alterative to the traditional methods for ob-
taining cadmium sulfide nanofilms, because of its sim-
plicity, low cost and minimal waste production. The
spray pyrolysis process allows the coating of a large sur-
face and it is easy to include in an industrial production
line. With spray pyrolysis, the solution is sprayed di-
rectly onto the substrate. A stream of nitrogen gas can be
used to help the spraying of solution through the nozzle.
In the study, a CdS nanofilm was deposited by the
spray pyrolysis technique on a glass substrates. The flow
rate of the solution experimentally was 5 ml/min and the
substrate temperature was held constant at 300˚C. The
nozzle to substrate distance was 28 cm and the diameter
of the nozzle was 0.8 mm. The number of sprays was 15.
The spraying time was controlled by solenoid valve. The
heated substrate was left for 10 s after each spraying run
to give time for the deposited CdS layer to be dry and
also to prevent excessive cooling of the substrate. This
yielded a uniformly grown CdS film on the substrate.
The schematic representation of the spray system is
given in Figure 1. When the solution is sprayed the fol-
lowing reaction takes place at the surface of the heated
substrate.


22 2
2
42
CdClNHCS2H O
CdS2NH ClCO


 
During the chemical reaction gas and water vapor is

A. M. SUHAIL ET AL.65
Heater
sample
compressor
Chamber
solution
Figure 1. Schematic representation of the spray system.
obtained from this reaction due to the high temperature
of the substrate. At the end of the reaction a yellow pre-
cipitates remains as a nanofilm of CdS material.
After deposition the film, the material was cooled to
room temperature gradually. There are several experi-
mental parameters which are control the homogeneity
and the thickness of the nanofilm. These parameters are
the spraying time, the height of the atomizer and the
pressure of the nitrogen gas carrier. The topography of
the prepared nanofilm was studied using Scanning Elec-
tron Microscopy (SEM) type ULTRA 55 with different
magnifications; as shown in Figure 2. The figures show
nanocrystals of size ~4 - 50 nm. The sample was scanned
in all zones before the picture was taken. The micro-
graphs reveal that the particles were hexagonal in shape.
The X-ray diffraction (XRD) pattern of the CdS nano-
film was recorded using an XRD 2000 system. The
X-ray diffractometer used a copper tube radiation line
with a wavelength of 1.54 Å and a 2θ range from 20˚ to
60˚. Scan rate was 1 deg/min. The UV-VIS absorption
and transmission spectra of the sample were recorded by
a Hitachi U-4100 spectrometer covering 200 - 1100 nm.
The photoluminescence (PL) spectrum was studied using
an SL1174 spectrophotometer in the range 300 - 900 nm.
The nonlinear absorption study at the near resonant re-
gime was carried out using the single-beam femtosecond
open aperture z-scan technique (OA). The z-scan setup is
illustrated by the schematic diagram shown in Figure 3.
A femtosecond laser with a pulse duration of 51 fs and
average power of 351 mW was used as a laser source.
The pulse duration was measured using an autocorrela-
tior and the energy was measured using a pyroelectric
energy probe of model type (PDA36A), covering the
wavelength range 350 - 1100 nm from THORLABS. The
beam profile was adjusted using a spatial filter, leading
(a)
(b)
(d)
(c)
50nm
500n
m
Air
Substrate
Thinfilm 900 nm
(c)
Figure 2. Top-view SEM images of CdS nanofilm at differ-
ent magnification powers (a)-(c) and cross-section (d).

A. M. SUHAIL ET AL. 66
Z
800nm
51fs
250kHz
R PSF
Power Controller BS1
D2 S BS
L
-Z Z
Com
p
uter
D2
Au
Figure 3. Schematic of the z-scan setup recording the non-
linear absorption, R—Rotator, P—Polarizer, SF—Spatial
filter, BS1, BS2—Beam Splitter, Au—Autocorrelation, D1,
D2—Detectors, L—Lens, S—Sample.
to spatial intensity profile that was near-Gaussian with
beam quality of M2 ≈ 1.36. The laser beam was focused
by a lens with 15 cm focal length to produce a waist of
32.7 µm. The sample was translated along the beam axis
(z-axis) through the Rayleigh distance 4200 µm.
3. Results and Discussion
The topography study of the prepared film shows the
formations of the CdS nano structure and the film thick-
ness was in the range of 0.5 - 2 µm. The XRD pattern
was recorded for the nanofilm CdS spray-deposited film
as shown in Figure 4. The spectrum through 2θ = 20˚ to
2θ = 60˚ indicates that the CdS nanofilm has a polycrys-
talline structure. The observed values of the XRD peaks
are compared with American Society for Testing and
Materials (ASTM) data for hexagonal CdS. The figure
shows broad peaks which give evidence of the formation
of the nanostructure. Using the width of the (002) peak
which appears at an angle of 26.8˚ on the 2θ scale in
Scherrer’s formula [15]:
0.94 cosd

  (1)
where d is the average crystalline grain size, λ is the
wavelength, β represents the full width at half maximum
(FWHM) in redian that equal 0.0087 and θ is the Bragg
diffraction angle in degree, the size of the formed nano-
particles was found to be about 50 nm. The absorption
spectrum of the CdS nanoparticles film is shown in Fig-
ure 5. The film is highly absorbing at wavelength below
500 nm.
The energy band gap of CdS film was estimated using
Tauc relation which can be written as [16]:
 
hvA hvEgn

 (2)
0
100
200
300
400
500
600
700
800
900
20 30 40 50 60
2θ
(100)
(101)
(002)
(102)
Intensity (arb. Units)
2θ (degree)
Figure 4. XRD pattern of CdS nanofilm deposited on a glass
substrate at 300˚C.
0
10
20
30
40
50
60
70
80
90
100
300 400 500 600 700 800
Absorption
Wl h(
)
Wavelength (nm)
Absorption (%)
Figure 5. Optical absorption spectra of CdS nanofilm.
where A is a constant, α absorption coefficient, hν the
photon energy (Eg) the band gap, n = 1/2 for the direct
transitions.
Referring to the data extracted from the absorption
spectrum, the absorption coefficient was calculated as
function of wavelength. Assuming an allowed transition,
direct band gap transition, the dependence of (αhν)2 on
hν is plotted in Figure 6.
The extrapolation of the linear part of the plot to
(αhν)2 = 0 gives rise to an estimate the energy gap value
of the CdS nanoparticles which was found to be 2.7 eV.
This value is comparable to the values found by the other
workers [17]. The optical transmittance spectrum of the
CdS nanofilm is shown in Figure 7.
The transmittance is high in the visible region with a
sharp increasing beyond the 520 nm, this indicates that
the CdS nanofilm has high absorption below this value
because of the occurrence of the linear and nonlinear
absorption. The (PL) emission spectrum of CdS nanopar-
ticles excited by a 350 nm line is shown in Figure 8.
The spectrum shows a peak at 485 nm which can be re-

A. M. SUHAIL ET AL.67
0
200
400
600
800
1000
1200
1400
012345
Energy Gap (eV)
(ahv)
2
(cm
–1
eV)
2
Eg = 2.7 eV
Figure 6. (αhν)2 versus enrgy gap.
0
10
20
30
40
50
60
70
80
90
100
300 400 500 600 700 800
Transmittance
Transmittance %
Wavelength (nm)
Figure 7. Transmission spectrum of CdS nanofilm.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0
.
9
380 430 480 530 580 630 680
Fluorescence (arb .Uint)
Wavelength (nm)
Figure 8. Photoluminescence emission spectrum of CdS
nanofilm.
ferred to the direct band gap transition and two peaks
around 580 and 630 nm which are attributed to the for-
mation of surface states. The observation of surface
states in the photoluminescence spectrum of the CdS
nanoparticles has been reported by many authors [18,19].
The nanocrystals synthesized with elemental sulfur have
more surface defects which act as traps on the nanocrys-
tals surface [20].
The z-scan transition curves for different laser intensi-
ties incident on the CdS nanofilm are presented in Fig-
ure 9. The normalized transmittance of the open aperture
z-scan is given by [21]:


2
12
1ln 1expd
πo
o
Tzqx x
q







 (3)
where, qo = βIo Leff,

22
1
oo
I
Izz

 is the excitation
intensity at the position z, 2 π
oo
zz

  where zo is the
Rayleigh range, ωo is the minimum beam waist at focal
point (z = 0), λ is the laser free-space wavelength, Leff =
[1-exp(-αoL)]/αo is the effective sample length for 2PA
processes; L is the sample length and αo is the linear ab-
sorption coefficient. The open aperture z-scan graphs are
always normalized to linear transmittance i.e., transmit-
tance at large values of │z│. The 2PA coefficient can be
extracted from the best fit between Equation (3) and the
experiment (OA) z-scan curve.
If qo < 1 Equation (3) can be expanded in a Taylor se-
ries as [21]:


32
0
11
m
mo
m
q
T
m


 
 (4)
Furthermore, if the higher order terms are ignored, the
transmission as a function of the incident intensity is
given by [21,22]:
32
12
oeff
I
L
T

 (5)
0.9
0.95
1
1.05
-2-1.5-1-0.50 0.5 11.52
β=0.977 cm/GW from fitting
Z-position (cm)
Normalized Transmittance
● I
o
= 671 GW/cm
2
▲ I
o
=207 GW/cm
2
■ I
o
= 68 GW/cm
2
Figure 9. OA z-scan curves measured with different excita-
tion irradiance at a wavelength of 800 nm and a pulse dura-
tion 51 fs and repetition rate of 250 kHz. The solid lines are
the fitted curves by employing the z-scan theory, described
in the text, on 2PA.

A. M. SUHAIL ET AL. 68
The sold curve in Figure 9 is the best fit for Equation
(5). The Equation (5) shows clearly that the depth of the
absorption dip is linearly proportional to the 2PA coeffi-
cient β, but the shape of the trace is primarily determined
by the Rayleigh range of the focused Gaussian beam.
The fitted value of ß is on the order of 50 cm/GW. This
value is ten times of magnitudes higher than the value
observed with bulk CdS sample. This results is in a good
agreement with values mentioned in [23]. The natural
logarithm of the (1-T) values are plotted as a function of
the natural logarithm of the incident intensity Io in Fig-
ure 10. The curve can be reasonably fitted with a straight
line with a slope of 0.97. This indicates that the 2PA was
occur in CdS pump by 800 nm laser source of 51 fs pulse
duration as shown in Figure 10.
The formation of surface defects may contribute to the
absorption mechanism of the prepared film due to small
increase in the linear absorption cross section [24]. The
formation of the surface defects in CdS and in the other
sulfur compound increase the nonlinear scattering lead-
ing to decreasing in the nonlinear absorption coefficient.
This was observed by viewing the transmittance light
through IR camera.
4. Conclusions
A CdS nanocrystalline film was prepared by the chemi-
cal spray pyrolysis technique. The nonlinear absorption
coefficient was measured by fully computerized the z-
scan technique. The measurements show that the nonlin-
ear absorption coefficient for the nanocrystallites is one
order of magnitude higher than that of the bulk CdS ma-
terial. This increase in the nonlinear responsivity when
the crystalline size approaches nano-scale dimensions
may be attributed to collimating of the incident intensity
of the pumped laser which led to improve the nonlinear
-6
-5
-4
-3
-2
-1
0
012345
Ln(1-T)
Ln(I
0
)
Figure 10. Plot of Ln(1-T) vs. Ln(Io) at 800 nm wavelength,
the solid line is the example of the linear fit at 800 nm with
slope s = 0.97.
dynamic of the CdS nanocrystallite.
5. Acknowledgements
This work has been carried out in the physics Department,
School of Engineering and Applied Sciences, Harvard
University. The authors would like to thanks Mazur Re-
search Group in Harvard University for their help
through this work. Thank also to Christopher C. Evans,
Jonthan D. B. Bradley, and Eric Mazur for their interest,
guide and useful discussion. We thanks also the Ministry
of Higher Education in the Republic of Iraq for support
this work.
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