World Journal of Nano Science and Engineering, 2012, 2, 181-188
http://dx.doi.org/10.4236/wjnse.2012.24024 Published Online December 2012 (http://www.SciRP.org/journal/wjnse)
Observed Enhancement in LIBS Signals from Nano vs.
Bulk ZnO Targets: Comparative Study of
Plasma Parameters
Ashraf M. El Sherbini1,2, Abdel-Nasser M. Aboulfotouh1, Farid F. Rashid3, Sami H. Allam1,
Ashraf El Dakrouri4,5, Tharwtt M. El Sherbini1
1Laboratory of Lasers and New Materials, Department of Physics, Faculty of Science, Cairo University, Giza, Egypt
2Department of Physics, Collage of Science, Al-Imam Muhammad Ibn Saud Islamic University (AIMSIU), Riyadh, KSA
3Department of Laser and Opto-Electronic Engineering, University of Technology, Baghdad, Iraq
4Collage of Applied Medical Science, King Saud University, Riyadh, KSA
5The National Institute of Laser Enhanced Sciences, Cairo University, Giza, Egypt
Received October 2, 2012; revised October 30, 2012; accepted November 8, 2012
ABSTRACT
In this article, we will report an experimental evidence of enhanced LIBS emission upon replacing a Bulk-Based ZnO
target by the corresponding Nano-Based target. The plasma was initiated via interaction of a Nd:YAG laser at the fun-
damental wavelength with both targets in open air under the same experimental conditions. The measurements show an
enhanced emission from the Zn I-lines at the wavelengths of 328.26, 330.29, 334.55, 468.06, 472.2, 481.01, 636.38 nm.
The measurements were repeated at different delay times in the range from 1 to 5 μs at constant irradiation level and
fixed gate time of 1 μs. The average enhancement over the different Zn I-lines was found increases exponentially up to
8-fold with delay time. The electron density to each plasma was measured utilizing the Hα-line appeared in the emitted
spectra from each plasma and was found to give similar values. The electron temperatures were measured via Boltz-
mann plot method utilizing the relative intensities of the Zn I-lines and were found to give very close values. Moreover,
the relative population density of the ground state of the zinc atoms (relative concentration) was measured spectro-
scopically utilizing the Boltzmann plot method and was found to increase in a very similar trend to that of enhancement.
The results of the spectroscopic analysis conclude that these signal enhancements can be attributed to the higher con-
centration of neutral atoms in the Nano-Based material plasma with respect to the corresponding Bulk-based ZnO mate-
rial.
Keywords: LIBS; Enhancement; ZnO Nonmaterial; Hα-Line; Zn I-Lines; Spectroscopy
1. Introduction
LIBS (acronym standing for laser induced breakdown
spectroscopy) is one of the potential growing fields of
analytical applications. It was successfully used for ele-
mental analysis [1], quality of metal industry [2], clean-
ing [3], studying of old archeology [4], characterization
of soils [5] and in jewelry industry [6]. This passive
spectroscopy technique is based on utilizing the light
emitted from plasma, assuming that the emitted radiation
is sufficiently influenced by the plasma properties [7,8].
However, the relatively small signal to background ra-
tio presents one major problem which imposes a limita-
tion on the use of this technique, hence the poor the abil-
ity of the LIBS technique to detect the very small con-
centration of the different matrix elements [9,10]. Diffe-
rent techniques were devised to overcome this diffi-
culty, such as the double pulse technique [11,12] in
which the target material is irradiated by consequent
double laser pulses separated by a certain delay time.
Moreover, the introduction of the femtosecond laser
source provides one basic advantage, which is the very
small continuum component appeared under the emitted
lines from plasma, hence the better the signal to back-
ground ratio [13,14].
On the other hand, the nanomaterials are categorized
as those which have structured components with at least
one dimension less than 100 nm [15]. Two principal fac-
tors cause the properties of nanomaterials to differ sig-
nificantly from bulk materials namely; the tremendous
increase in the relative surface area and the quantum ef-
fects. These factors can substantially change and/or en-
hance the well known bulk properties, such as chemical
reactivity [16], mechanical strength [17], electrical and
C
opyright © 2012 SciRes. WJNSE

A. M. EL SHERBINI ET AL.
182
magnetic [18] and optical characteristics [19]. As a parti-
cle size decreases, a greater proportion of atoms are
found at the surface compared to those inside [15]. The
quantum effects can begin to dominate the properties of
matter as its size is reduced to the nano-scale. Therefore,
nanoparticles are of interest because of its inherent new
properties when compared with larger particles of the
same materials. For example, titanium dioxide and zinc
oxide become transparent at the nano-scale however they
are able to absorb and reflect UV light [20].
The accelerating progress in the field of nanomaterial
science may provide a new facility to reach signal en-
hancement. To the best of our knowledge no work has
been done in a systematic manner to examine the optical
signal emitted from the laser produced plasmas utilizing
nanomaterials base targets instead of bulky ones.
In this work we will demonstrate the ability of com-
bining the LIBS technique with Nano-Based materials
(used as a target) to get strong enhanced optical signals.
2. Experimental Setup
The experimental setup is shown in Figure 1(a). The
irradiation system comprises a Nd:YAG laser able to
deliver an energy of 650 mJ/pulse at the fundamental
wavelength 1064 nm, at constant duration of 5 ns. The
detection system consists of an SE-200 echelle type
spectrograph equipped with time control iStar®-ICCD
camera. The emitted light from plasma is spatially inte-
grated and collected at the entrance hole of spectrograph
via a 25 μm quartz fiber, positioned with the help of pre-
cise xyz-translational stage holder at 15 mm from the
laser-plasma axis. The relative spectral response of the
camera-MCP (micro channel plate) and spectrograph
including optical fiber over the entire wavelength win-
dow from 200 to 1000 nm was measured using a Deute-
rium-Halogen lamp (type, DH-2000-CAL) with the re-
sult as shown in Figure 2(a). The processing of the ex-
perimental data was carried out using home-made routine
built under the MATLAB7®package [21]. Each data
(a) (b)
Figure 1. (a) Experimental setup; (b) TEM image of the ZnO-20 nm size (as supplied by manufacturerer).
(a) (b)
Figure 2. In part (a), shown is the spectral response curve of the whole experimental setup over the entire wavelength region
from 200 to 1000 nm; in part (b), an example on the multi-element Boltzmann plot between Nano-Based target plasma (upper
dashed line) and the Bulk-Based (lower dashed line) plasma shows points of intersection with vertical axis and the method to
calculate the relative concentration η.

A. M. EL SHERBINI ET AL. 183
point was taken as the average over three different single
laser shots on fresh target condition which enable us to
account for any fluctuations in measurements. A monitor
to the incident laser power on the target was made via a
beam splitter which reflects 4%, absorb 4% and with the
help of absolutely calibrated power-meter (type Ophier).
Unless specified, the ZnO (MKNANO-ZnO-020) material
(as shown in Figure 1(b)) with almost spherical shaped
powder samples was purchased from “MKNANO” and was
used without further purification.
The nano and bulk powder targets were compressed to
a disk shape (Pressure ~ 5 ton/cm2), under the same am-
bient conditions for a time period of 5 min. Both tables
were positioned side by side perpendicular to laser axis
on a xy-translational stage. It is worth noting that, no
further chemical or heat treatment was carried out and to
the best of our knowledge, both of the nano and bulk
materials were synthesized using the same base material.
3. Plasma Parameters
The plasma state can be described by two measurable
parameters namely; the electron density and temperature
[7,8]. At relatively large electron density ~ 1019 cm–3, the
plasma state is said to be in complete thermodynamical
equilibrium (CTE). In this situation the emitted light
from the plasma displays a continuous spectrum and the
plasma is characterized by single temperature. This con-
dition is rarely verified in the laser induced plasma ex-
periments [7].
In LIBS experiments, it was verified that the electron
density is in the range from 1016 to 1018 cm–3 and the ra-
diation field is dominated by a line rather than continu-
ous spectrum. This state is called local thermodynamical
equilibrium (LTE), at which the particle species are
characterized by unique temperature which is different
from the temperature of the radiation field.
At a rather lower electron density regimes (1016 > ne >
109 cm–3), the electron gas in plasma tends to divide the
energy levels of the atoms into two main categories. We
call this state as partial local thermodynamic equilibrium
(PLTE) [7,8].
3.1. Measurement of Plasma Electron Density
Spectroscopically, the electron number density can be
measured by different methods namely; measurement of
the optical refractivity of the plasma [7], calculation of
the principal quantum number at the series limit [7,8],
measurement of to stark profile of certain optically thin
emitted spectral lines [22], the measurement of the abso-
lute emission coefficient (spectral intensity) of spectral
line [7] and finally from the measurement of the absolute
emissivity of the continuum emission [8].
Among the different proposed methods, the Stark
broadening of emitted lines has been the most widely
used method [8]. This method is based on the assumption
that the Stark effect is the dominant broadening over the
Doppler broadening and the other pressure broadening
mechanisms resulted from collisions with neutral atoms
(i.e., resonance and Van der Waals broadenings) [22].
The theoretical calculation of Stark broadening parame-
ters of hydrogenic lines is described in detail by Griem
[8].
For the Hα-line, the following expression can be used
to calculate the electron density [22]:
 
32
12 3
1/2
8.02 10cm
s
e
H
nH





 
 



(1)
In this expression 12

is the half width of the reduced
Stark profiles in Å, it is a weak function of electron den-
sity and temperature through the ion-ion correlation and
Debye-shielding correction and the velocity dependence
of the impact broadening.
3.2. Measurement of Plasma Temperature
The second important plasma parameter is the electron
temperature, which determines the strength of the differ-
ent distribution functions [7]. In laser produced plasmas,
a combination of the line and continuum spectra is ap-
peared in the emission spectra. In this condition, the LTE
is almost being fulfilled and one should employ the opti-
cal emission spectroscopy technique to measure the tem-
perature [7,8].
The most direct method to estimate the temperature is
via measurement of the relative spectral intensity of two
or more lines emerging from the same element and ioni-
zation stage with small wavelength separation and large
separation in upper state excitation energy. Moreover,
these lines should be optically thin [7,8]. The following
expression is recommended:
1112 22
112 2
lnln 4π
o
rr
oe
hcN ICI CE
Ag AgUkT

 


 
 



(2)
In this expression, I, λ, A, g are the spectral intensity,
wavelength, transition probability and statistical weight
of the upper state respectively. The subscript numbers
indicate different lines. N an d Uo are the population den-
sity and the parathion function of the atom at temperature
Te. The constants h and c are the Planck constant and
speed of light, respectively.
The lines intensities should be corrected by the relative
response factors Cr1,2, at the different emitted wave-
lengths; these factors are saved from the absolute calibra-
tion curve, shown in Figure 2(a). Mathematically, the
plot of the LHS of Equation (2) with excitation energy
(RHS) yields a straight line of negative slope which de-
termines the temperature.

A. M. EL SHERBINI ET AL.
184
3.3. Measurement of Relative Concentration
In order to measure the relative concentration of the zinc
atoms in both plasmas created from the nanomaterial and
the corresponding bulk material targets, we have devel-
oped the following simple method. From Equation (2),
the term

4π
oo
hcN U

is a constant defined by the in-
tersection of the backward extrapolation of the Boltz-
mann line with the vertical axis. This method is not new,
since it was suggested before [23,24] in what is called
multi-element Saha-Boltzmann plot in which the relative
concentration of the different matrix elements in the tar-
get material can be estimated.
After the construction of the two Boltzmann plots, one
for the plasma emerging from the Nano-Based ZnO ma-
terial target and the other for the plasma created from the
Bulk-Based ZnO target, we should get two straight lines
as shown in Figure 2(b).
The upper line proves a higher concentration than the
lower one. The two points of intersection with the verti-
cal axis should give two similar quantities


N
ano
4π
oo
hcN Ufor the Nano-Based target and


Bulk
4π
oo
hcN U for the Bulk-Based target.
This means that, if the lower bulk line is multiplied by
factor η we can make the two straight lines coincide (of
course, with little orientation defined by the different
temperatures of the two plasmas). This factor represents
the relative concentration of the atoms (stoichiometry) in
the plasmas created from the Nano vs. Bulk targets i.e.




Nano Bulkoo N
NN CC  
B
.
4. Results and Discussion
Figure 3(a) shows an example of one emitted optical
signal at the Zn I-lines from the plasmas created from the
Nano-Based target material (red colored) in comparison
(a)
(b)
Figure 3. (a) Demonstration to emission enhancement at the Zn I-lines from the Nano-Based target (red color) in comparison
to that from the Bulk-Based material (black color), showing the emission from the Zn I-lines as depicted in figure; (b) Dem-
onstration to enhanced spectral radiance from the Zn I-lines at different wavelength regions from the Nano-Based target (red
colored) in comparison to that from the Bulk-Based target at different three arbitrary delay times of 1, 3, 5 s, are shown in
subsequent rows.

A. M. EL SHERBINI ET AL. 185
o signals at the stame wavelengths arise from the corre-
in contrast to the behavior of the spectral inten-
si
nce in the en-
hancement factor at the different Zn I-lines (This will be
hich causes this be-
ha
sponding Bulk-Based material target (black colored) at
an arbitrary delay time of 4 μs. Qualitatively, one can
notice the clear enhancement in the spectral intensity
from the nano to bulk plasmas. Moreover, Figure 3(b)
displays the same comparison between the two signals at
the same wavelengths at regular delay times of 1, 3 and 5
μs, respectively, which demonstrates the temporal in-
crease in the signal enhancement as the delay time in-
creases.
This is
ty of the Hα-line. The reason for the decrease in the
spectral intensity is not yet been resolved which grasps a
need for more extensive investigations.
Moreover, there is an obvious differe
treated in a separate publication).
An attempt to know the reason w
vior can be achieved via careful examination of the
different plasma parameters (electron density and tem-
perature) in both plasmas. The electron density was
measured utilizing the optically thin Hα-line appeared in
both spectra under the same condition. Utilizing Equa-
tion (1), in conjunction with special software routine
written in MATLAB7®. This program was constructed to
compare the spectral line shape of the measured Hα-line
to the theoretically constructed Voigt profile [21] and
hence to extract the Lorentzian component of the line
FWHM used to calculate the electron density as shown in
Figure 4 (first two columns).
Figure 4. Demonstration of the H

-line fitting and the measured electron densities at three different delay times of 1, 3, 5 s
from the Nano-Based plasma target (black color) and the Bulk-Based plasma (blue color). The third column demonstrates
the measurement of the plasma temperature at three delay times of 1, 3, 5 s from the Nano-Based plasma (upper red line) in
comparison to Bulk-Based plasma (lower black line) in addition to method of calculation f the relative concentration
 

A. M. EL SHERBINI ET AL.
186
An example of the fitting of the Hα-line at regular de-
ay times of 1, 3, 5 μs to both plasmas arises from Nano- l
Based target (first column with red curves) in compari-
son to the second column in Figure 4, which shows the
fitting of the Hα-line appeared in the spectra arises from
the Bulk-Based plasma (blue curves) together with the
estimated electron densities.
On the other hand, in order to calculate the plasma
electron temperature, we have constructed the Boltzmann
pl
ted
fr
both plasmas (fac-
to
olid squares)
an
ral intensity from the Zn
I-
Wavelength
(n
Transition Statistical weight Energy of upper
)
ots utilizing the spectral lines intensities emitted by the
Zn I-lines at 330.29, 334.55, 472.2, 481.01, 636.38 nm
with the results as shown in Figure 4 (third column).
The atomic parameters of the Zn I-lines used to con-
struct the Boltzmann plot are presented in Table 1.
Also, Figure 4 (third column) reveals that; first, the
concentration of the zinc atoms in the plasma crea
om the Nano-Based target is higher than the corre-
sponding plasma arising from the Bulk-Based material
and second, both plasmas show nearly similar tempera-
tures at the different delay times, since both of the
Boltzmann lines are almost parallel.
Finally, in order to calculate the relative concentration
(stoichiometry) of the neutral zinc in
r η); we have utilized the predictions of set of Boltz-
mann plots in Figure 4 in which the factor η was calcu-
lated at each delay time. The overall result of the meas-
urement of the average enhancement factor over the dif-
ferent emission wavelengths from the neutral Zn I-lines
at the wavelengths of 328.26, 330.29, 334.55, 468.06,
472.2, 481.01, 636.38 nm with delay time is shown in
Figure 5, (red squares) whereas an obvious increase in
an exponential manner to the signal enhancement from
the nano vs. bulk based materials is shown.
For a comparison reason, we have plotted both of the
measured relative electron density (blue s
d the relative electron temperature (black disks) as
well as the measured relative concentration (inverted
green triangles) with delays times in the range from 1 to
5 μs, respectively, in Figure 5.
Figure 5 reveals the following conclusions, first, it
ensures that the enhanced spect
lines can’t be attributed to the plasma temperature dif-
ference or the difference in the electron densities. Second,
Table 1. The atomic parameters of the Zn I-lines.
m) λ probability A ( s–1) g state E (eV
481.01 7.00 × 107 3 6.6674
472.2 4.58 × 107 3 6.6673
Figure 5. This figure represents the temporal variation of
the relative parameters; spectral intensity (enhancement;
red squares) and the relative concentration (
 inverted
green triangles) and the relative electron density (


eN eB
nn
temperature
;
black disks) and finally, the relative electron
(


eN eB
TT blue squares) with de lay time .
from this figure it is clear that the signal enhancement
depends only on the relative atomic concentre
plasma created from the Nano-Based material with re-
spulk-Based plasma. Neverthe
ation th
ect to Bless, the relative
oncentration (inverted green triangles) increases in a
tting
st
ity is in the order of 10
cm
c
very combatable manner with signal enhancement.
This fact can be qualitatively explained in terms of the
collisional radiative modeling: The intense emission from
the nanomaterial plasma with respect to the bulk one
under the same experimental conditions can be related to
the higher number of excited atoms to the upper emi
ate (labeled j). This state can be populated via different
atomic processes e.g. collisional excitation from all lower
laying states, especially from the ground state and/or
collisional de-excitation from the upper states, especially
from the ground state of the next ionization stage and/or
radiative decay from the upper excited states, especially
recombination of both types (radiative or non-radiative
three particle recombination).
All the relevant processes can contribute by a certain
amount depending on the predominant rate coefficients
and the electron density present in the plasma, as well as
the electron temperature [7]. In the laser produced plas-
mas (LIBS), the electron dens17
–3, which means that the collisional processes are the
dominant ones. For close approximation, the number of
collisional processes from the ground state leading to
population of the upper emitting state j per unit volume
per unit time is in direct proportion to the spectral inten-
sity of the emitted radiation. Hence the relative intensity
from either plasma (Enhancement) should amount to;
330.29 1.07 × 108 5 7.7975
334.55 1.50 × 108 7 7.7980
636.38 4.65 × 107 5 7.7585
jNjN oN eNN
jBjBoBeBB
INNnR
I
NNnR
 
 

 


 

 
(3)
whereas, Ij, Nj, No are the spectral intensity of lines and

A. M. EL SHERBINI ET AL. 187
the population density of this upper state and the popula-
tion density of the ground state, respectively. R, ne are the
electron-atom collision excitation rate coeffi
electron density, respectively. The subscripts N, B ind
ca
cient and
i-
te nano and the corresponding bulk quantity. Equation
(3) indicates that the relative spectral intensity would
increase linearly with both of relative concentration

oN oB
NN   and the relative electron density

eN eB
nnand the relative electron temperature through
temperature dependant electron-atom collisional excita-
tion rate coefficient [7].
Experimentally, the observed invariance of the relative
electature, density and together with the
relative enhancement compatible relative con-
centration variation under different conditions leads to
conclusion that the higher concentration of atoms in the
N
ron temper
measured
nO. This signal enhancem
igation of the different plasma pa-
uted to the larger concentration of the
n, Washington DC, 2005). As well as the Col-
ive
a
Spectroscopy,” Critical Reviews in Analytical
Chemistry, Vol. 27, No. 4, 1997, pp. 257-290.
doi:10.1080/10
ano-Based plasma is decisive in the observed en-
hancement phenomenon.
5. Conclusion
An enhanced emission optical signal from the Nano-
Based material target was observed in comparison to
Bulk-Based plasma from Zent,
after a careful invest
rameters, was attrib
zinc atoms in the plasma created from the Nano-Based
material with respect to that created from the Bulk-based
one. Further investigation would be appreciated to ex-
plore more the effects of the different laser wavelengths
and laser fluence as well as the type of the material tar-
get.
6. Acknowledgements and Dedication
This work is financially supported in part by the Iraq
Scholar Rescue Fund Project (Institute of International
Educatio
lage of Applied Medical Science, King Saud Un
KSA at which the calculations and the written m
rsity,
terial
were prepared. The authors address their gratitude to
Prof’s. N. Omenetto, H.-J. Kunze, and A. El Dawyyan
for their valuable discussions and guidance.
This work is completely dedicated to the memory of
souls of the Egyptian martyrs during our revolution on
Jan. 25, 2011.
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