JOURNAL OF APPLIED PHYSICS 108, 064906 共2010兲
Synthesis of nanoparticles in laser ablation of aluminum in liquid
Bhupesh Kumar and Raj K. Tharejaa兲
Department of Physics, Indian Institute of Technology Kanpur, Kanpur 208016, UP, India
共Received 10 June 2010; accepted 6 August 2010; published online 21 September 2010兲
We report the synthesis of aluminum nanoparticles using pulsed laser ablation in water confined
plasma. Nanoparticles have spherical shape and size distribution depends on laser fluence. Strong
blue photoluminescence peaks at 405 nm 共3.06 eV兲 and 430 nm 共2.89 eV兲 due to oxygen deficient
defects 共F, F+, and F++ centers兲 is reported with different UV excitations. A comparative study of
plasma in deionized water and air ambient reveals enhanced line broadening and higher electron
density in water confined plasma compared to that in air, in agreement with radiative recombination
model. The temporal dependence of spectral radiant energy density of plasma is also discussed.
© 2010 American Institute of Physics. 关doi:10.1063/1.3486517兴
I. INTRODUCTION
Investigation of laser ablation process has been an active
area of research owing to its wide range of applications such
as analysis of solid materials and liquids using laser induced
breakdown spectroscopy, synthesis of nanoparticles 共NPs兲,
microstructure fabrication, etc.1–7 High energy nanosecond
laser beam focused onto material surface results in rapid
heating of the surface to high temperature and eventually to
plasma formation.8 The plasma parameters such as electron
temperature and electron density of the expanding plasma
vary both spatially and temporally. The species of the expanding plasma interact among themselves and with the ambient atmosphere resulting in excitations and ionization.8 The
high pressure is generated at the solid-liquid interface region
compared to that generated at the solid–gas interface.9,10 The
condensation of plasma species in the ambient results in the
formation of NPs.11,12 At the termination of the laser pulse,
plasma cools adiabatically and thermodynamic state of the
plasma governs the nucleation and growth kinetics of the
NPs. Further, the magnitude of background pressure experienced by plasma plume in liquid being higher than that in the
gas, the plume dynamics is restricted and confined to smaller
volume. To understand formation of NPs from plasma as
precursor in liquid ambient, it is essential to investigate the
plasma parameters and plume dynamics of plasma confined
in liquid ambient. The liquid and gas ambient offers a unique
opportunity to blend the materials with desired functional
properties due to ultra fast reaction dynamics between
plasma species and ambient. Using pulsed laser ablation
共PLA兲 in liquid environment nanocrystals of cubic-C3N4
nanocrystals, diamond nanocrystals with cubic and hexagonal structures and boron nitride nanocrystals have been
synthesized.13–15 The optical properties of zinc oxide nanocomposites synthesized by laser ablation of zinc in various
organic liquids and deionized water has been studied.16,17
Gold NPs by ablating gold in water using femtosecond laser
has also been reported.18 The pulsed laser irradiation of solid
particles suspended in liquids has shown reduction in the size
of spherical metal particles and transformation of rods to
a兲
Electronic mail: [email protected].
0021-8979/2010/108共6兲/064906/6/$30.00
spheres. Although formation and collapse of laser generated
cavity near solid-liquid boundary have been reported, the
laser processing of material in ambient liquid is poorly understood. The rate of bubble generation for metal etching in
water, EtOH, and Perfluorocarbon 共PFC兲 has been reported.19
The motivation for this work is to comprehend the physical
conditions initiating the NP formation during PLA in water,
identify the defect aggregates which are responsible for luminescence in nanostructures of aluminum oxide. Further, it
is interesting to investigate the effect of ambient atmosphere
on plume dynamics and spectral radiant energy of plasma.
The ignition probability of fuel mixtures of aluminum and
aluminum oxide NPs is significantly higher than that of pure
diesel.20 Aluminum oxide sols are also used to produce protective films on the surfaces of various materials and
composites.21 However, it is not easy to synthesize aluminum
oxide NPs using existing techniques such as reduction in
metal salts followed by oxidation or decomposition of organometallic precursors.22 Thus, it is imperative to develop
techniques for synthesis and conservation of colloidal systems with a high concentration of aluminum oxide NPs. In
the present paper, we report the synthesis of the aluminum
NPs using PLA of aluminum in deionized water ambient.
The NPs were characterized by transmission electron microscopy 共TEM兲, x-ray diffraction 共XRD兲, and photoluminescence 共PL兲 measurement. An attempt is made to correlate the
properties of plasma, source of NPs, created in liquid environment with that of evolution of NPs. The characteristic
difference of plasma created in air and liquid ambient are
discussed based on radiative recombination 共RR兲, spectral
radiant energy density, and collision excitation parameters.
II. EXPERIMENT
In order to investigate the laser ablation of aluminum in
air and water ambient, an aluminum disk of diameter 25 mm
was used as a target and the laser beam was focused to a spot
of diameter ⬃100 ␮m on it. The target was continuously
rotated by a stepper motor to avoid pit formation during
ablation. Neodymium-doped yttrium aluminum garnet;
Nd:Y3Al5O12 共Nd:YAG兲 pulsed laser 共Model Lab190-10,
Spectra Physics, 1064 nm, pulse width ⬃8 ns, rep rate 10
108, 064906-1
© 2010 American Institute of Physics
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B. Kumar and R. K. Thareja
J. Appl. Phys. 108, 064906 共2010兲
FIG. 2. XRD pattern of the NPs synthesized by laser ablation of aluminum
in water. The inset shows variation in NP size with laser fluence.
FIG. 1. 关共a兲 and 共b兲兴 TEM images of NPs at fluence 20 J cm−2 and
55 J cm−2, respectively. NPs were synthesized by laser ablation of Al in
water. 共c兲 shows the particle size distribution.
Hz兲 was used for PLA. To synthesize the NPs, laser fluence
in the range 10– 55 J cm−2 was used for laser ablation in
water ambient for 60 min. The laser fluence of 55 J cm−2
was used for emission spectroscopy measurements and
plasma plume imaging in air and water ambient, respectively.
A spectrograph 共Shamrock 303 i, Andor Technology兲 and
intensified charge couple device 共ICCD兲 共Andor i Star, Andor Technology兲 with a 1200 lines/mm grating was used to
record the temporal evolution of the plasma emission spectra. A camera was attached to ICCD to record plasma images
at various time delays with respect to ablating pulse. The
gate width and gate step were set to 50 ns for imaging and
spectroscopy. The plasma emission was collected by a lens
and then imaged onto the detector slit. The water containing
ablated aluminum became colloidal solution which was dried
to get powder at temperature ⬃50 ° C. The powder obtained
was characterized using PL and XRD measurements. The PL
measurements were done using Fluorolog-3 共Jobin Yvon兲
with the UV excitation wavelengths of 266 and 355 nm. The
XRD measurement was done using x-ray wavelength of 1.54
Å from Cu K␣ 共X’pert Pro, Philips兲.The samples for TEM
共FEI Technai 20 U兲 measurements were prepared on carbon
coated copper grid by putting a drop of colloidal solution and
dried in ambient conditions.
tively. Figure 1共c兲 shows the size distribution of NPs obtained by statistical analysis of the particles in images from
the different regions of the copper grid. Gaussian fit to the
size distributions show that the peak shifts from 25 to 28 nm
and full width at half maxima 共FWHM兲 from 22 to 25 nm on
changing fluence from 20 to 55 J cm−2. The particle size
estimated from XRD pattern using Debye–Scherrer formula
at various fluences is shown in the inset of Fig. 2; it shows
similar trend in size variation.23,24 Figure 2 shows the XRD
measurement of synthesized NPs, two prominent peaks at
45.7° and 66.8° correspond to Al 共200兲 and Al 共220兲, respectively, and a broad peak at 38.7° corresponds to Al2O3.25 It
follows from Figs. 1 and 2 that the final synthesized products
strongly depend on the constituents of the surfactant and the
laser fluence used.18,24,26 The broad nature of particle size
distribution may be due to variation in absorption of laser
energy by the particles in colloid for long ablation time. The
varying collision probability of the ablated species with ambient may also contribute to broad range of size
distribution.27 Although no dissociation of water was observed in absence of aluminum target, however, energetic
plasma species of aluminum in presence of aluminum target
may result in dissociation of water molecules. Figure 3
shows the emission spectrum of Al plasma in water at laser
fluence of 55 J cm−2. It shows the Al I transitions
共 2S1/2 – 2P1/2兲 and 共 2S1/2 – 2P3/2兲 at 394 nm and 396 nm, respectively, O I transition 共 5P3 – 5S2兲 at 777 nm, H␣ transition
共 2P3/2 – 2S1/2兲 at 656 nm, and H␤ transition 共 2D5/2 – 2S3/2兲 at
III. RESULTS AND DISCUSSION
The colloidal solution containing ablated aluminum was
dried on carbon coated copper grid for TEM measurements
to determine the size and shape of the particles present in the
colloid. Figures 1共a兲 and 1共b兲 show the spherical shape of
NPs at laser fluence of 20 J cm−2 and 55 J cm−2, respec-
FIG. 3. Spectrum of water dissociation observed in the presence of Al target
with line emission of Al I 共394.4 and 396.1 nm兲, H␣ 共656 nm兲, H␤ 共486 nm兲,
and O I 共777 nm兲.
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064906-3
J. Appl. Phys. 108, 064906 共2010兲
B. Kumar and R. K. Thareja
FIG. 4. PL emission spectra of NPs synthesized by laser ablation of aluminum in water at the excitation wavelengths of 266 nm and 355 nm, respectively. The arrows show the position of PL peaks at 405, 422, and 430 nm.
486 nm. An OH band 共306–308 nm兲 is also observed at
longer delays and could not be resolved due to poor resolution of monochromator.28 The presence of oxygen and hydrogen emission lines confirms the dissociation of water
molecules in the ambient. The interaction of ablated plasma
species and dissociation products results in the formation of
NPs as confirmed in XRD, Fig. 2. Figure 4 shows PL profile
of the NPs synthesized at laser fluence of 55 J cm−2. PL
measurement was done at room temperature; NPs exhibit a
broad band PL with peaks at 405 nm and 430 nm for 266 nm
and 355 nm excitation wavelengths, respectively. The broad
nature of PL peaks may be due to the presence three different
kinds of oxygen vacancies 共F, F+, and F++兲 or their combination arising due to inhomogeneous distribution in NPs. The
peaks at 405 nm 共3.06 eV兲 and at 430 nm 共2.89 eV兲 correspond to aluminum oxide, F+ centers and transition between
the energy levels of the F centers, respectively.29,30 Further, a
small hump observed at 422 nm 共2.94 eV兲 is similar to that
observed in nanostructured aluminum oxide NPs due to the
defects. The PL peak position shifts toward the longer wavelength as the excitation wavelength is increased from 266 to
355 nm. The redshift in PL peaks may be due to the inhomogeneous distribution of defect centers participating in PL
emission under different excitations.
To investigate the physical conditions initiating the NPs
formation in water ambient; temperature and electron density
of plasma are estimated. Further, we have compared the
plasma parameters in water and air ambient to envisage the
characteristic difference in plasma. Figure 5共a兲 shows the
plume images recorded at various time delays with respect to
ablating pulse at fluence of 55 J cm−2 in both the ambient. It
is apparent from the images that plume expansion is larger in
air and confined to smaller size in water due to strong background pressure. Figure 5共b兲 shows the axial position of
plume front at various time delays in air and water ambient.
The plume expansion rate is comparable at the initial times
but in water it saturates much earlier 共⬃300 ns兲 than that in
air 共⬃800 ns兲. The shock wave model R = ␣tn is fitted to the
plume front position 共R兲 at various time delays 共t兲 with ␣
and n as the fitting parameters. The ratio of the values of ␣
for water and air ambient is 1.53, which shows the magnitude of pressure in water due to high density of ambient is
much larger compared to that in air.9,10
FIG. 5. 共Color online兲 共a兲 Images of plasma plume at various time delays in
air and water ambient. 共b兲 Plasma plume front position at various time
delays in air and water ambient.
The electron density and electron temperature were estimated using Stark broadened profile of Al I and ratio of
intensity of H␣ and H␤ lines, respectively. The electron temperature was estimated to ⬃4760 K at the delay of 100 ns
using the ratio of intensity of H␣ and H␤ lines.31 The electron
density of the plasma is estimated using Stark broadened
profile of Al I 共 2S1/2 – 2P1/2兲.32 The Stark broadening of a
emission line arises due to the Coulomb interaction of electrons and ions and is given by33
ne =
冉 冊
⌬␭
1016 cm−3 ,
2w
共1兲
where ne is the electron density, ⌬␭ 共angstrom兲 is the
FWHM, w is the electron-impact half width parameter which
is weakly temperature dependent. The minimum value of
electron density necessary for the validity of local thermal
equilibrium 共LTE兲 of plasma can be obtained using, ne
3
−3
ⱖ 1.4⫻ 1014T1/2
where Te is the electron teme 共⌬E兲 cm
perature and ⌬E the energy separation between the upper and
lower states of the Stark broadened profile of Al I.34 For our
experimental conditions the electron density estimated at delay of 100 ns in water confined plasma is larger by three
orders than minimum electron density necessary for the validity of LTE. This is evident that during the initial stage of
the plume expansion, plasma is characterized by high pres-
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064906-4
J. Appl. Phys. 108, 064906 共2010兲
B. Kumar and R. K. Thareja
sure, high density and temperature at the solid–liquid interface. The nucleation for NPs formation occurs within a very
short time when the pressure is much larger at the initial
stages. Enhanced collisions due to reduced inter particle
separation in plasma confined to smaller volume in water
environment also contribute to nucleation initiation for the
NPs. Further, as the pressure and temperature of the plasma
plume decreases the condensation of plasma results in the
formation of NPs.11 However, at longer time delays, NPs
follow a steady growth rate through collisions and aggregations of atoms and clusters.11,35 In addition, the possibility of
NP formation in liquid due to direct ablation and etching of
target surface due to the high pressure is not ruled out.35
In order to compare the temporal evolution of plasma,
time resolved emission spectra and imaging of the plasma in
air and liquid ambient were recorded. Figure 6共a兲 shows the
temporal evolution of the plasma emission spectra in water
and air ambient. Emission spectra are dominated by continuum emission close to surface of target and at earlier time
of the plume expansion, however, the extent of continuum is
much larger for ablation in liquid due to possible water molecules dissociation. Figure 6共b兲 shows the emission spectra
at 200 ns with respect to ablating pulse in water and air
ambient. The prominent emission lines observed in air ambient in the spectral range 370–415 nm are Al I transitions
共 2S1/2 – 2P1/2兲 at 394.4 and 共 2S1/2 – 2P3/2兲 at 396.1 nm and Al
II transition 共 3F4 – 3D3兲 at 399.5 nm. However, Al II transition is absent in liquid ambient due to possible suppression
of higher ionic species in the continuum. Further, the lines at
394.4 and 396.1 nm merge due to enhanced broadening of
lines in water confined plasma. The plasma species both
from Al, oxygen, and hydrogen ions from dissociated water
molecules may also contribute to broadening.
Figure 7 shows temporal variation in electron density of
plasma in water and air confined plasma. The solid lines
show the electron density calculated using RR model
whereas solid circles and squares are the estimated values of
density using Stark broadening. The inset in Fig. 7 shows the
Lorentzian profile fitted to experimental data of Stark broadened profile. The increased and distortion in FWHM in water
may be due to enhanced collisions acting as additional perturbation. The electron density estimated for plasma confined
in water is higher compared to that in air is consistent with
the available observations under similar conditions.36 The
higher electron density in water is attributed to the combined
effect of strong confinement of plasma and the contribution
from the background. The faster temporal decay profile of
electron density in water ambient compared to that in air
may be due to the strong compression of the plume into a
smaller region, hence favoring higher electron–ion recombination and rapid decay of species in the initial stages of
expansion of plume.37 An attempt is made to understand the
electron density decay profile in terms of RR model. Since
the observed spectra are dominantly contributed of neutral
species, the simplest probable recombination process may be
represented by
FIG. 6. 共Color online兲 共a兲 Temporal evolution of emission spectra from
plasma in water and air ambient. 共b兲 Emission spectrum of plasma in water
and air at time delay of 200 ns.
A+ + e → Aⴱ + h␯ ,
共2兲
where A+, e, and Aⴱ denote ion, electron, and electronically
excited atom, respectively. The rate equation for the recombination process can be written as33
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064906-5
J. Appl. Phys. 108, 064906 共2010兲
B. Kumar and R. K. Thareja
FIG. 7. Temporal variation in electron density in aluminum plasma confined
in water and air ambient. The solid line represents the density estimated
using RR model. Inset shows the line profiles of Al I transitions at 394.4 and
396.1 nm for water and air fitted using Lorentzian function. Solid lines
represent theoretical fitting to experimental data.
⳵ ne
= − ␣ n en + ,
⳵t
共3兲
where ␣共⬃10−12 cm3 s−1兲 is the recombination constant and
ne and n+ represent the electron and ion density, respectively.
Assuming the local charge neutrality condition ne = n+, the
solution of the above equation is given by
1
1
=
+ ␣t,
n共r,t兲 n0共r兲
共4兲
where n0 is electron density at t = 0. Assuming the continuum
emission is mainly due to RR process the value of n0 can be
extrapolated. The density estimated using RR fits well to
experimental data in water compared to that in air as shown
in Fig. 7. However, the possibility of three body recombination cannot be ruled out at longer time delays in both the
ambient as can be inferred from the slow decrease in electron
density.
The optical thickness of the plasma being different in the
two environments, the emissive power and hence spectral
radiant energy density, 关␳共t兲兴 for the plasma may be different.
To estimate ␳共t兲, we assumed that the region of maximum
emission intensity is in spherical shape of the expanding
plume as shown in Fig. 5共a兲. The spectral radiant flux density
关I共t兲兴 can be determined from the difference between absorption and emission and its spatial gradient along the line of
observation using the following relation:32,38
I共t兲 = 4␲
␧共t兲
共1 − e−␬共t兲R共t兲兲,
␬共t兲
共5兲
where ␧共t兲 and ␬共t兲 are the emission and absorption coefficients, respectively, and R共t兲 is the radius of the spherical
plasma plume. The coefficients ␧共t兲 and ␬共t兲 are proportional
to the densities of electrons and ions32 and their temporal
behavior can be assumed to follow exponentially decaying
function given as
冉 冊
␧共t兲 = ␧0 exp −
t
,
␶P
FIG. 8. Temporal variation in spectral radiant energy density, ␳共t兲 of aluminum plasma confined in water and air ambient.
冉 冊
␬共t兲 = ␬0 exp −
t
,
␶P
共6兲
where ␧0 and ␬0 are the emission and absorption coefficients
at t = 0, and ␶ P is the time constant estimated from the time
resolved spectra of the plasma emission. The values of ␶ P
were estimated to ⬃200 ns and ⬃115 ns from temporal
evolution of electron density of plasma confined in water and
air ambient, respectively. Therefore, the spectral radiant energy density is given by
␳共t兲 =
4␲ ␧共t兲
共1 − e−␬共t兲R共t兲兲,
c ␬共t兲
共7兲
where c is the speed of light. Apart from other factors, the
physical dimension of the plasma plume R also plays significant role in determining spectral radiant energy density ␳共t兲.
Figure 8 shows the temporal variation in spectral radiant
energy density ␳共t兲 estimated for plasma in air and liquid
confined plasma up to 1200 ns. The energy density is lower
for liquid confined plasma because of lower value of spectral
intensity due to partial losses in the ambient medium. It is
observed that ␳共t兲 in liquid changes slowly compared to that
in air. This could be attributed to the collision induced excitation and de-excitation processes dominant in liquid confined plasma. Also, the lower rate of adiabatic cooling of
plasma facilitates to exist it for longer time. Hence, strong
confinement effect is playing an important role in governing
the thermodynamic state of the plasma. Assuming collisions
are the dominant process for excitation and de-excitation, we
compared the collision excitation parameter ␩共t兲 in water
and air confined plasma 共Fig. 9兲. It is assumed that ␩共t兲
decays exponentially with a characteristic time 共␶c兲 which is
different from the plasma life time ␶ P 共Ref. 38兲
冉 冊
␩共t兲 = ␩0 exp −
t
,
␶c
共8兲
where ␩0 is the rate parameter for the collision excitation at
t = 0 and ␶c is a time constant. It is assumed that plasma
species have equal probability of collision at initial time in
irrespective of the environments. However, at later time as
the plume expands in different ambient, the probability of
collision also differs. The values of ␶c were estimated to
⬃564 ns and 176 ns from temporal variation in intensity of
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064906-6
J. Appl. Phys. 108, 064906 共2010兲
B. Kumar and R. K. Thareja
4
FIG. 9. Temporal variation in collision excitation parameter, ␩共t兲 in aluminum plasma confined in water and air ambient.
spectral emission in water and air confined plasma, respectively. The larger values of ␶c than ␶ P can be related to the
recombination of free electrons and ions resulting energetic
species and their participation in the collision process. The
excitation parameter ␩共t兲 is proportional to the density of
energetic collision partners.38 The temporal variation in ␩共t兲
is shown in Fig. 9. The decay of the energy of plasma and the
decrease in the number density of the collision partners can
be directly associated with the estimated temporal behavior
of the electron density.
IV. CONCLUSION
We have synthesized the aluminum NPs using PLA of
aluminum in water ambient at various laser fluences. The
NPs are spherical in shape and have broad size distribution
centered on ⬃25 nm. The NPs show strong blue PL in the
visible range with peaks at 405, 422, and 430 nm under
different UV excitations. PL emission is attributed to oxygen
deficiency defect centers in aluminum oxides. A comparative
investigation of plasma parameters in liquid and gas ambient
has been done. Electron density of aluminum plasma is
higher in liquid ambient and its temporal evolution nicely fits
to RR model. This is supported by slow varying spectral
radiant energy density and enhanced collision induced excitations in liquid as strong confinement is assumed to enhance
the plasma life time.
ACKNOWLEDGMENTS
Work is partly supported by DRDO 共New Delhi兲.
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