Advances in Nanoparticles, 2013, 2, 223-228 Published Online August 2013 (
Nonlinear Coefficient Determination of Au/Pd Bimetallic
Nanoparticles Using Z-Scan
José Luis Jiménez Pérez1*, Rubén Gutiérrez-Fuentes2, José Francisco Sánchez Ramírez1,
Omar Uriel García Vidal1, Daniel Erick Téllez-Sánchez1, Zormy Nacary Correa Pacheco1,
Alfredo Cruz Orea3, Jesús Antonio Fuentes García1
1Unidad Profesional Interdisciplinaria en Ingeniería y Tecnologías Avanzadas del IPN, México, D.F., México
2Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, México D.F., México
3Departamento de Física, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, México, D.F., México
Email: *
Received March 22, 2013; revised April 22, 2013; accepted April 30, 2013
Copyright © 2013 José Luis Jiménez Pérez et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this paper we present the nonlinear optical characterization of Au/Pd nanoparticles in order to obtain the nonlinear
refractive indices using the Z-scan technique. The experiments were performed using a 514 nm laser beam Ar+, with 14
Hz of modulation frequency, as excitation source. By using a lens the excitation beam was focused to a small spot and
the sample was moved across the focal region along the z-axis by a motorized translation stage. Seven samples with
different concentration ratio of Au/Pd nanoparticles were prepared by simultaneous reduction of gold and palladium
ions in presence of poly (N-vinyl-2-pirrolidone) (PVP) using ethanol as a reducing agent. In this work, we report the
application of the Z-scan technique, to generate optical transmission of laser light as a function of the z position for so-
lutions containing bimetallic nanoparticles of Au (core)/Pd (shell) with average sizes ranging from 3 to 5 nm. The mag-
nitude of the obtained nonlinear refractive index was in the order of 108 cm2/W. Our results show that the nonlinear
refractive index has a nonlinear behavior when the (Au/Pd) ratio was increased.
Keywords: Nanofluids; Nanoparticles; Nonlinear Refractive Indices; Z-Scan
1. Introduction
Bimetallic nanoparticles (NPs) have attracted great in-
terest among the scientific and technological community
since they lead to many interesting size dependent elec-
trical, chemical, and optical properties [1-3]. Specifically
they often exhibit enhanced catalytic properties which
differ from that of their monometallic counterparts, in
thin films [4]. On the other hand, the studies for new
nonlinear optical materials have increasing interest in the
past years, due to the numerous applications in various
fields such as optical communication devices and elec-
tronics [5]. Nanoparticles colloids in solution have been
studied extensively because of their large third-order
nonlinear susceptibilities and nonlinear optical response
[6,7], chemical and biological sensors [8,9], optical en-
ergy transport [10,11], nonlinear thermal material [12,13],
thermal therapy [14,15] and medicine [16].
Z-scan technique is one of the simplest and effective
tools for measuring third order of nonlinear optics such
as nonlinear refraction coefficient and absorption. It has
been widely used in material characterization [17,18].
This method utilizes a focused laser beam that is intense
enough to access nonlinearities in a sample. As the sam-
ple passes through the focal point of the beam, changes in
its transmittance, due to nonlinear absorption and nonlinear
refraction, can be measured by using an open aperture
and close aperture experimental setup, respectively. In
the open aperture technique the beam is focused into a
detector, after passed through the sample. As the sample
travels through the focus of the initial beam, the trans-
mittance either increase or decrease (depending on the
nonlinearity of the sample) and the detector receives more
or less light than the linear transmittance, yielding a hump
or dip in the curve of transmittance as a function of the
sample position. For nonlinear refraction, after the beam
passes through the sample, it is attenuated by a semi-
closed aperture that usually allows less than 50% of the
initial beam to be detected by the detector. The converg-
ing and diverging of the beam (allowing more and less of
the beam to pass through the aperture, respectively) due
*Corresponding author.
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to the changes in the refractive index, a pre-focal valley
and post-focal peak are observed for a positive change in
refractive index. While a pre-focal peak and a post-focal
valley is observed for a negative change in refraction
In this paper we studied the effect of the concentration
of Au/Pd ratio on their nonlinear refraction coefficient.
Therefore, we report this effect using a single beam Z-
scan technique. The results of this method are found to
be in good agreement with the values of other works.
2. Methods
2.1. Sample Preparation
Colloidal dispersion of Au/Pd bimetallic nanoparticles
was prepared by simultaneous reduction of gold and pal-
ladium ions in presence of poly (N-vinyl-2-pirrolidone)
(PVP) using ethanol as a reducing agent [19-22]. In a
typical synthesis process, ethanol solutions of palladium
chloride (0.033 mmol in 25 ml of ethanol) were prepared
in advance by stirring dispersions of PdCl2 powder in
ethanol for 48 hrs. Solutions of tetrachloroauric acid
(0.033 mmol in 25 ml of water) were prepared by dis-
solving the HAuCl4·3H2O crystals in water. For prepar-
ing bimetallic nanoparticles, solutions containing two
metal ions were mixed in 50 ml of pure ethanol/water
(1/1 v/v). 151 mg of PVP (Aldrich, average molecular
weight 10,000) was added to the total metal ion content
of 6.66 × 105 mol. The mixture solution was stirred and
refluxed at about 100˚C for 1 hr. For the preparation of
bimetallic nanoparticles with different Au/Pd ratios (10/1,
5/1, 2/1, 1/1,1/2, 1/5 and 1/10), metal ion solutions of
corresponding molar ratios were mixed and refluxed at
100˚C for 1 hr under agitation. The ethanol/water volu-
metric ratio of 1:1 and total ion content of 6.6 × 105 mol
was maintained in the final 50 ml solution. The same
procedure was followed to prepare the monometallic
palladium (Pd) particles. For the preparation of mono-
metallic gold (Au) particles, 23.5 ml of PVP solution
(75.5 g in 23.5 ml of water) was added to the gold ion
solution, and then an aqueous solution of NaBH4 (0.066
mmol in 1.5 ml of water) was added to the resulting solu-
tion at 25˚C. The colloidal dispersions thus prepared are
stable, with 3 - 5 nm in average diameter and narrow size
distributions. In order to obtain the aqueous nanofluids
containing Au/Pd bimetallic nanoparticles, the colloidal
dispersions were subsequently dried in vacuum and the
metallic particles were dissolved in water maintaining the
same initial concentrations of metal content and were
placed in a quartz cuvette of 1 mm thick for the optical
and thermal optical measurements. All the experiments
were performed at room temperature.
2.2. Z-Scan Technique
Among the techniques used to characterize the complex
susceptibility, the most popular is the Z-scan method.
Since its introduction in 1989 [23], this technique has
gained importance due to its simplicity compared to other
techniques used to measure the nonlinear refraction and
absorption of optical materials. In addition to the sim-
plicity of the experimental setup, a Z-scan measurement
provides a sensitive method for the determination of the
signal on values of the real and imaginary parts of x. The
technique relies on the basic idea of relating the beam
center intensity variation to the refractive index variation.
This can be done by monitoring the normalized transmit-
tance as a function of the sample path along the incident
beam, for a positive nonlinearity. In the case of a nega-
tive nonlinearity, the curve is inverted.
The transmittance variation between peak and valley
positions is proportional to the induced phase shift, ΔΦo
and therefore to the nonlinear refractive index (n2) is
calculated using the standard relations by means of the
following equations [24,25]:
00 00
,14 1T zzzzzzz9
nI Leffk
 (2)
where z is the position, z0 is the Rayleigh length, ΔΦ0 is
the phase change due to the nonlinear refraction, n2 is the
nonlinear refractive index, k = 2π/λ is the wave vector, I0
= 1.8 × 103 W/cm2 is the on-axis irradiance at focus (i.e.,
z = 0), and
Leff1 expL
 
is the effective
length of nonlinear medium, α0 is the linear absorption
coefficient of the samples (L denotes the sample thick-
ness). The nonlinear refractive index, n2 was calculated
being this parameter the value of peak to
valley of data transmittance from the closed aperture Z-
scan measurement which can be described as [24]:
 (3)
Here S is the linear transmittance of the aperture. Fig-
ure 1 shows the schematic diagram of a single beam Z-
scan experimental setup used in the present measurement.
The experiments were performed using a CW Ar+ laser
beam, 514 nm wavelength (model Cyonics, Uniphase).
The beam was focused to a small spot using a lens and
Figure 1. Experimental setup for the nonlinear laser spec-
troscopy. The closed and open aperture signals are propor-
tional to the real and imaginary parts of n2, respectively.
Copyright © 2013 SciRes. ANP
the sample was scanned along a Z-axis by a motorized
translation stage (Zaber). The transmitted light, in the far
field, passed through the closed and open apertures and
was recorded by the detectors Dc, Do and Dr (reference
detector). The laser beam waist ω0 at the focus length
was measured to be 10.9 µm and the Rayleigh length was
found to be satisfied the basic criteria of the Z-scan ex-
A quartz optical cell, 1 mm thick, containing the speci-
men solution was translated across the focal region along
the z-axial direction. All the measurements were carried
out at room temperature for both closed aperture and
open aperture configurations.
Optical absorption spectra of the fluid samples were
measured using a UV-Vis-NIR double beam spectropho-
tometer (Shimadzu UV3101PC).
For transmission electron microscopic (TEM) obser-
vations, a drop of fluids was spread on a carbon coated
copper microgrid. A JEOL JEM200 microscope was used
for the low magnification of the samples.
3. Results and Discussion
The absorption spectra of the Au/Pd nanofluids at dif-
ferent ratios (10/1, 5/1, 2/1, 1/1, 1/2, 1/5, 1/10) were
measured by spectrophotometer. The measurements of
absorption spectra were carried out at room temperature
for visible wavelength, 350 - 800 nm. The spectra are
shown in Figure 2.
Figure 3 displays a typical TEM images showing the
particles with a uniform distribution and uniform shape.
The average particle size obtained from TEM images was
3.9 nm.
For closed aperture setup normalized transmittance is
attributed to the nonlinearity of the refractive index which
was considered here [26]. In Figures 4-10 the closed
aperture Z-scan curves obtained for Au/Pd nanofluid, at
different concentration, at beam intensity of I0 = 1.8 ×
Figure 2. Optical absorption spectra of bimetallic nanopar-
ticles with different Au/Pd mole ratios.
Figure 3. TEM image of Au/Pd particles with an average
size of 3.9 nm.
Figure 4. Closed aperture Z-scan curve for Au/Pd nanopar-
ticles at a concentration 10/1. The solid line is the best fit-
ting of the standard closed aperture equations to the ex-
perimental data.
Figure 5. Closed aperture Z-scan curve for Au/Pd nanopar-
ticles at a concentration 5/1. The solid line is the best fitting
of the standard closed aperture equations to the experi-
mental data.
103 W/cm2 are shown. The circle symbols represent the
experimental data while the solid lines are theoretical fits
Copyright © 2013 SciRes. ANP
Figure 6. Closed aperture Z-scan curve for Au/Pd nanopar-
ticles at a concentration 2/1. The solid line is the best fitting
of the standard closed aperture equations to the experi-
mental data.
Figure 7. Closed aperture Z-scan curve for Au/Pd nanopar-
ticles at a concentration 1/1. The solid line is the best fitting
of the standard closed aperture equations to the experi-
mental data.
Figure 8. Closed aperture Z-scan curve for Au/Pd nanopar-
ticles at a concentration 1/2. The solid line is the best fitting
of the standard closed aperture equations to the experi-
mental data.
Figure 9. Closed aperture Z-scan curve for Au/Pd nanopar-
ticles at a concentration 1/5. The solid line is the best fitting
of the standard closed aperture equations to the experi-
mental data.
Figure 10. Closed aperture Z-scan curve for Au/Pd na nopa r-
ticles at a concentration 1/10. The solid line is the best fit-
ting of the standard closed aperture equations to the ex-
perimental data.
to the closed aperture.
The theoretical transmittance curves presented in Fig-
ures 4-10 are fitted very well with the experimental data
and they show symmetry curves. The peak follow by
valley illustrate a self-focusing effect for a negative
change in refraction. The solid line shows the theoretical
fitting using a well-known normalized transmittance.
The nonlinear refraction coefficients n2 (cm2/W) to-
gether with the values of linear absorption of all samples
obtained in the present work are listed on Table 1.
Figure 11 shows the variation of the nonlinear refrac-
tion index coefficient as a function of concentrations for
different ratios. The nonlinear refraction coefficient de-
crease with increasing of concentration (Au/Pd (10/1) to
Au/Pd (2/1)). By other hand, if the concentration is in-
verse (Au/Pd (1/1) to Au/Pd (1/10)), the nonlinear refrac-
tion coefficient increases with the increasing of concen-
tration. However the increase of nonlinear of refractive
index coefficient with the concentration did not show a
Copyright © 2013 SciRes. ANP
Table 1. Nonlinear optical properties of Au/Pd nanofluid at
different concentrations. Molar ratios of 10/1, 5/1, 2/1, 1/1,
1/2, 1/5, 1/10 were measured at 514 nm laser beam.
Au/Pd ΔTpv Δφ0 α (cm1) n2 (cm2/W)
10/1 1.48 3.99 13.5 13.0 ×108
5/1 1.78 5.21 10 8.59 ×108
2/1 1.03 3.02 7.9 3.93 ×108
1/1 1.27 3.72 6.9 4.23 ×108
1/2 1.45 4.25 8.9 6.23 ×108
1/5 1.24 3.63 6.28 3.76 ×108
1/10 1.84 5.39 6.84 6.08 ×108
Figure 11. Variation of the nonlinear refraction index coef-
ficient as a function of Au/Pd ratio.
linear relationship as displayed in Figure 11. Shahriari et
al. [26] studied the effect of concentration and particle
size on nonlinearity of Au nanofluid prepared by γ (60Co)
radiation. From the experimental results [26], the authors
obtained values of refractive index coefficient nonlinear
of the same order found in this work.
4. Conclusion
The nonlinearity properties of Au/Pd nanofluid prepared
at different concentrations have been successfully inves-
tigated using a single beam Z-scan method. A CW, Ar+
laser beam, at wavelength 514 nm, was used as excitation
source. The Au/Pd nanofluid shows a good nonlinear
response. The sign of the nonlinear refractive index is
found to be negative and the magnitude is in the order of
108 cm2/W. A nonlinear relationship was obtained for
nonlinear refractive index as a function of Au/Pd ratio.
Until our knowledge these results are reported for the
first time in the literature. These results showed that
Au/Pd nanofluid has significant values of nonlinear re-
fractive index, thus it could be good candidate for nonlin-
ear thermal material.
5. Acknowledgements
The authors are thankful to the Mexican Agencies,
CONACYT, CGPI-IPN and COFAA-IPN for financial
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