Photoacoustic Studies of Colloidal Silica Particles after MeV Ion-Induced Shape Deformation

doi:10.4236/njgc.2011.12011

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New Journal of Glass and Ceramics, 2011, 1, 63-68

doi:10.4236/njgc.2011.12011 Published Online July 2011 (http://www.SciRP.org/journal/njgc)

Photoacoustic Studies of Colloidal Silica Particles

after MeV Ion-Induced Shape Deformation*

Ulises Morales1, Rosalba Castañeda-Guzmán2, Santiago Jesús Pérez-Ruiz2, Juan-Carlos Cheang-Wong1

1Instituto de Física, Universidad Nacional Autónoma de México, México, D.F., Mexico; 2Centro de Ciencias Aplicadas y Desarrollo

Tecnológico, Universidad Nacional Autónoma de México, México, D.F., Mexico.

Email: [email protected]

Received May 17th, 2011; revised June 14th, 2011; accepted June 21st, 2011.

ABSTRACT

Ordered arrays of colloidal submicrometer-sized silica particles deposited onto silicon wafers were irradiated with

MeV Si ions. The spherical silica particles tu rned into oblate particles as a result o f the increase of the particle dimen-

sion perpendicular to the ion beam direction and the decrease in the parallel direction. Pulsed laser photoacoustic

spectroscopy was used to study the structural changes of the silica particles after the ion-induced shape deformation.

Our purpose is to correlate the mechanical vibrations generated by the pulsed laser as a function of the Si irradiation

parameters: ion energy and fluence. Fast Fourier transform analysis of the photoacoustic signal was carried out in

order to obtain the normal vibration modes of the system. The size, size distribution and shape of the silica particles

were determined by scanning electron microscopy. Our results revealed significant structural differences between the

spherical and the deformed silica particles.

Keywords: P u l sed L a s e r Phot oacoustic Spectroscopy, Silica Particles, Ion Irradiation

1. Introduction

Periodic arrays of colloidal silica particles are very at-

tractive because of their potential applications in coating

technology, optoelectronic/plasmonics devices, and na-

nolithography [1,2]. The properties of these arrays of

SiO2 particles depend on their shape, size and spatial

distribution, which in turn determine the different roles

they can play as electronic substrates, thin film substrates,

electrical and thermal insulators, photonic band gap

crystals, masks for lithographic patterning, etc. Changes

in the mechanical properties of these colloidal silica par-

ticles, as an integral part of the particle- substrate system,

are important for the fundamental understanding of fric-

tion, wear, and contact mechanics at the nanoscale, as

well as for applications. Elastic parameters are sensi-

tively related to the microstructure in these arrays.

Ion irradiation is a widely used technique to modify

the structure and composition of materials. It is well

known that MeV ion-irradiated silica particles can un-

dergo extreme deformations [3,4]. The ion-induced

shape deformation has been described in terms of the

ion hammering theory, in which an amorphous material

undergoes a shape modification when bombarded with

fast heavy ions [5]. In the case of spherical colloidal

silica particles, ion irradiation turns them into oblate

particles, as a result of the increase of the particle di-

mension perpendicular to the ion beam and the de-

crease in the parallel direction. Our previous works on

this kind of samples showed that this anisotropic shape

deformation increases with the ion fluence and with the

silica particle size [4], and it depends on the impinging

ion, the ion energy and the irradiation temperature in a

way that merits further detailed studies [6,7]. Moreover,

the mechanical or elastic properties of these structures

are poorly studied. It seems that the mechanical char-

acteristics of these deformed silica particles are not

only size-dependent, but also shape dependent. Then,

one of the aims of the present work is to study the

structural changes induced by the ion irradiation on

these silica particles as a function of the irradiation

parameters (ion energy and fluence).

We characterize the elastic properties of our arrays

of silica particles by means of the Pulsed Laser Photo-

acoustic (PLPA) spectroscopy. PLPA relies on the ab-

sorption of a short laser pulse (nanosecond range) by a

sample and the subsequent measurement of a nonradia-

*This work was supported by DGAPA-UNAM projects IN-101210,

IN-117208 and CONACYT grants No. 128274, 123143 and 82919.

Photoacoustic Studies of Colloidal Silica Particles after MeV Ion-Induced Shape Deformation

tive relaxation by the detection of ultrasonic pressure

waves. Fast Fourier transform (FFT) analysis of the

photoacoustic signals allows us to correlate the vibra-

tion modes of the system in the region around the 30

MHz with the structural changes of the silica particles

as a function of the irradiation parameters. The com-

bination of pulsed laser and piezoelectric detection

lead to the simplicity of the experimental setup. In

general, in coated materials the surface wave propaga-

tion velocity depends on the frequency and, as an es-

sential property of surface waves, the depth of wave

motion is proportional to the wavelength, hence it is

smaller at higher frequencies [8]. In our case we con-

sider all the waves generated by the laser, since the

useful dimensions of the sample are very small to con-

sider just the surface waves. It is also important to take

into account the existing weak bonding of the silica

particles with one another and as well with the sub-

strate. The aim of the present work is to study by

means of pulsed laser photoacoustic spectroscopy the

vibrations at high frequency (30 MHz) of colloidal

silica particles with different degrees of deformation

and correlate them with micro- and nanostructural

changes. Such a shape deformation was achieved after

room temperature irradiations with MeV Si ions at dif-

ferent energies and fluences.

2. Experimental Details

2.1. Sample Preparation

Spherical submicrometer-sized colloidal silica particles

were prepared according to a modified Stöber process

[4], using a reaction mixture containing tetraethoxisi-

lane (Si{OC2H5}4), deionized water, ammonia and

ethanol. A drop of the colloidal dispersion containing

the silica particles was deposited onto previously clea-

ned Si(100) substrates by means of a micropipette. The

particle mean size was about 520 nm in diameter. The

samples were then irradiated at room temperature with

Si2+ ions at 6 MeV, following an angle of 45° with re-

spect to the sample surface, using the 3 MV Tandem

accelerator (NEC 9SDH-2 Pelletron) facility at the

Instituto de Física of the National University of Mex-

ico. Homogeneous implantation over an area of 1 × 1

cm2 was achieved by means of an electrostatic raster

scanner. In order to keep a small virgin area in the ir-

radiated samples, an adequate mask was used to limit

the beam exposed area. The irradiation fluence was in

the range 1.5 × 1015 - 5 × 1015 Si/cm2. Similar samples

were also prepared by using a spin coater system. The

particles cover an area to 3 × 5 mm2 onto the Si(100)

substrate, forming a homogeneous monolayer. These

spin coated samples were irradiated with 4 MeV Si

ions at 45˚, and a fluence of 4 × 1015 Si/cm2. The size

distribution and shape of the silica particles were de-

termined by SEM measurements (JEOL JSM 5600-LV).

2.2. Photoacoustic Experiments

Figure 1 shows the photoacoustic experimental setup

developed for previous investigations [9]. The beam

source was provided by a p-polarized Nd: YAG laser

(Continuum’s Surelite, USA), with a 5 ns pulse width,

and a repetition rate of 10 Hz at a wavelength of 532

nm. The laser light is focused onto the surface of the

sample, with a spot diameter of 0.5 mm. A fast photo-

diode detector (Thorlabs Inc., model 201/579/7227)

with a rise time < 1 ns is implemented to receive part

of the laser beam, triggering a digital oscilloscope in

order to monitor the acoustic signals. The studied sam-

ples are mounted on a 240 kHz PZT or a 5 MHz ultra-

sonic transducer (GE Inspection Technologies). The

sensor transforms the generated PLPA wave signals

into electronic pulses, which are visualized and ana-

lyzed on a digital oscilloscope (Tektronix TDS 540),

and then these signals are processed in a PC encoded in

MatLab.

Photoacoustic signal generation is based on the

model of Tam [10]. Time signals coming from the os-

cilloscope are analyzed, specially the amplitude varia-

tion for each sample. A data analysis by means of RMS

(Root Mean Square) gives the vibration energy for

each case. In order to deal with a domain in frequency

a Fourier transform is applied to time signals. The re-

sulting spectrum is studied at high frequency, where

the vibration modes of the particle arrays can be fol-

lowed by looking for the changes in the peaks either

for the spherical or the deformed particles, for different

ion fluences and energies. We studied a monolayer of

silica particles, which time signal, after a data treat-

ment, allows us to determine the maximum rate of

change for spherical and deformed particles. Compared

with the spherical particles, in the case of the deformed

ones there is a variation in the speed of sound as a

sensor

sample

Oscilloscope



Nd:YAG@532nm

Figure 1. Pulsed laser photoacoustic experimental setup.

Photoacoustic Studies of Colloidal Silica Particles after MeV Ion-Induced Shape Deformation 65

consequence of ion-induced shape deformation. FFT

analysis of the photoacoustic signals includes the lon

gitudinal, transverse and surface waves and it was

necessary to discriminate the substrate vibrations (at

low frequency) from those due to the particles.

3. Results and Discussion

The as-prepared samples consisted of ~520 nm diame-

ter colloidal silica particles deposited onto a Si(100)

substrate, arranged in a continuous and homogeneous

monolayer, forming a 2-D hexagonally-ordered struc-

ture. In most cases quite narrow particle size distribu-

tions were obtained, indicating that essentially mono-

disperse SiO2 particles were synthesized [7]. Figure 2

shows a series of SEM micrographs corresponding to

as-prepared silica particles (viewed under normal inci-

dence) and those irradiated with a 6 MeV Si ions at

various fluences up to 4.5 × 1015 Si/cm2. One can ob-

serve that the spherical silica particles experimented

extreme deformations under exposure to MeV Si ions

at room temperature, and turned into oblate ellipsoids

as a result of the increase of the particle dimension

perpendicular to the ion beam and the decrease in the

direction parallel to the ion beam [6,7]. This anisot-

ropic plastic deformation increases with the ion fluence

and depends on the ion beam energy, as described in

our previous papers [6,7].

Concerning the PLPA characterization of the sam-

ples, we have performed comparative experiments for

different samples throughout the study of their time

signals as a function of the irradiation energy and flu-

ence. By using a 240 kHz PZT transducer, the photo-

acoustic signals were obtained from the bare Si(100)

substrate and from the samples consisting of silica par-

ticles deformed by 6 MeV Si ions with the following

fluences: (1.5, 3, 3.5, 4, and 4.5) × 1015 Si/cm2.

Figure 3 shows the corresponding set of PLPA sig-

nals exhibiting amplitude oscillations which decay as

the acoustic wave loses energy. Compared with the

other samples, the bare Si substrate shows a similar

PLPA signal up to 0.05 ms (region not shown), and

then its amplitude rises to a higher value as can be

clearly seen in Figure 3. Indeed, the important feature

that we would like to highlight from this figure is the

noticeable difference in the PLPA signals from the bare

substrate and the spherical and oblate particles.

Spherical and deformed particles show similar signals,

but the amplitude for the spherical ones is consistently

greater over the entire time spectrum. In the case of the

deformed particles, the higher the deformation, the

lower the signal amplitude and the signal shifts slightly

to shorter times (to the left) as a function of the ion

fluence.

Figure 2. SEM micrographs corresponding to: (a) As-prepared

spherical silica particles (top view); (b) 1.5 × 1015 Si/cm2, (c)

3 × 1015 Si/cm2, (d) 4.5 × 1015 Si/cm2 irradiated samples

(viewed in a direction perpendicular to the irradiation

beam, under an angle of 45° with respect to the sample

surface). The scale bar is 1 μm, except for (b), where it cor-

responds to 2 μm.

Figure 4 shows in detail the beginning of the previ-

ous signals once smoothed. Appearing after the initial

noise signal, the first local maxima were analyzed for

the different samples. Time signal exhibits a maximum

in amplitude for the spherical particles. For the irradi-

ated samples, their amplitude decreases systematically

as a function of the fluence up to 4.5 × 1015 Si/cm2. As

the shape deformation increases, the local maximum

corresponds to shorter arrival time. Therefore, the

speed of sound has different values for different de-

formation fluences. In order to compare the behaviour

of the samples studied in Figure 4, a Root Mean

Square (RMS) analysis is carried out as a function of

the fluence (see Figure 5). For the spherical particles

the vibration energy starts at around 0.325 mV, and

then it decreases systematically as the particle defor-

mation increases with fluence.

Studies close related with the structural characteristics

of the samples can be accurately performed by PLPA and

the adequate data analysis [11]. The frequency spectra

show the normal vibrational modes produced by the sub

strate-particles system, and the high-frequency signals

give us information about the structural differences of

the particles. For the following experiments, the PLPA

signal was obtained by using a 5 MHz high-frequency

sensor. In this case three samples were studied: the bare

Si substrate, the sample with spherical particles, and the

one irradiated with 6 MeV Si ions at the highest fluence

(5 × 1015 Si/cm2). Fourier transform was applied with the

MatlabTM software. Figure 6 shows a plot of the rela-

tive amplitude signals as a function of the frequency.

Photoacoustic Studies of Colloidal Silica Particles after MeV Ion-Induced Shape Deformation

Figure 3. PLPA time signals corresponding to: bare Si sub-

strate, spherical silica particles and the series of oblate par-

ticles irradiated at different fluences (smoothed curves).

0 1 2 34 5 6 78

-0.2

0.2

0.4

0.6

0.8

Time [ s]

Amplitude [mV]



Figure 4. Beginning of the PLPA time signals correspond-

ing to Figure 3 (smoothed curves): substrate (▬▬▬),

spherical particles (●●●●) and oblate particles irradiated at

different fluences: 1.5 × 1015 (●●●●), 3 × 1015 (××××), 3.5 ×

1015 (○○○○), 4 × 1015 (▬ ▬ ▬) and 4.5 × 1015 (∆∆∆∆)

Si/cm2.

This kind of plot allows us to observe the high-frequency

part of the spectrum. For the substrate the signal in this

frequency range has negligible amplitude and the higher

amplitude peaks are produced by the spherical particles.

The spherical particles exhibit two peaks, at 18 MHz and

28 MHz. The deformed particles do not show the peak at

18 MHz and the second peak is shifted to 31 MHz, with

a quite short amplitude. Based on the previous data, a

second analysis was carried out using samples irradiated

at different fluences: (1.5, 3, and 5) ×1015 Si/cm2. The

results are shown in Figure 7, and it is important to no

tice that again the irradiated samples do not exhibit the

01 2 3 4 5

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Fluence [10

Si/cm

]

RMS [mV]

Figure 5. RMS analysis obtained from the PLPA signals in

Figure 4 as a function of the irradiation fluence.

peak at 18 MHz, and the peak at 28 MHz seems to move

toward higher frequencies as the deformation fluence

increases. This shift may be due to a change in the struc-

ture of the particles.

In the case of the samples prepared by spin coating the

silica particles cover a monolayer area of 2 × 5 mm2. The

studied sample was deformed by a 4 MeV Si irradiation

at an incident angle of 45˚ and a fluence of 4 × 1015

Si/cm2. It is important to mention that the deformation

rate of the silica particles is different at 4 MeV and 6

MeV [7]. The results of the PLPA measurements for

these samples are shown in Figure 8. The local maxi-

mum of the relative amplitude at 18 MHz can be ob-

served again, but this time it appears in both the spherical

and the deformed particles. The peaks presented by the

deformed particles are more intense than the ones corre-

sponding to the spherical particles, even if the signal

amplitudes around 30 MHz are similar for both kinds of

particles. It is also to be noticed that the signal due to the

deformed particles is slightly smaller with respect to the

one obtained in the previous study at 6 MeV, and this

behavior may be due to a variation of the deformation

rate at 4 MeV [7].

We performed a sophisticated data analysis of the ar-

rival times of photoacoustic signals in the case of a

monolayer of silica particles, in order to determine a

value for the parameters related to the speed of sound in

the samples. A signal analysis method based on Hilbert

and wavelet transforms was used in this study. After the

ion irradiation, the result is not only a shape transforma-

tion into oblate silica particles, but also a change in the

arrival times of the photoacoustic signals. This fact

clearly means that the speed of sound changed in the

case of the oblate particles. By means of the Hilbert

transform, the envelope of the time signal for both the

spherical and irradiated particles (4 × 1015 Si/cm2 flu-

ence) was calculated. Then a wavelet db5 reconstruction

Photoacoustic Studies of Colloidal Silica Particles after MeV Ion-Induced Shape Deformation 67

Figure 6. PLPA frequency signals (smoothed): bare sub-

strate, spherical silica particles and oblate particles irradi-

ated with 6 MeV Si ions at 45º, fluence 5 × 1015 Si/cm2.

20 25 3035

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Frequency (MHz)

Relative Amplitude

Fluence 5 x 1015

Fluence 3 x 1015

Fluence 1.5 x 1015

Spherical

Figure 7. PLPA frequency signals (smoothed): spherical

silica particles and oblate particles irradiated with 6 MeV

Si ions at 45˚ at different fluences: 1.5 × 1015, 3 × 1015 and 5

× 1015 Si/cm2.

analysis was used to obtain the maximum of the deriva-

tive, revealing significant differences between the arrival

times for the spherical and the oblate particles. Our re-

sults showed that the arrival times were 4.4304 × 10–5 s

for the spherical particles and 4.4089 × 10–5 s for de-

formed ones, leading to a difference in the arrival time of

0.215 μs. These experiments confirmed that the speed of

sound in the silica particles changed due to the shape

deformation of the particles after the ion irradiation.

4. Concluding Remarks

By means of the PLPA technique it is possible to follow

the structural changes due to MeV ion-induced deforma-

tion of colloidal silica particles irradiated with Si ions at

different energies and fluences. The signal analysis as a

Figure 8. PLPA frequency signals (smoothed): bare sub-

strate, monolayer of spherical silica particles and oblate

particles irradiated with 4 MeV Si ions at 45º, fluence 4 ×

1015 Si/cm2.

function of time for both spherical and 6 MeV irradiated

particles at different fluences showed that the arrival

time of the photoacoustic signal decreases as well as its

amplitude when the deformation of the particle increases.

The RMS test proves this behavior by showing a lower

vibrational energy for the deformed particles. Moreover,

our high-frequency signal study also showed a shift to-

ward higher frequencies as the deformation fluence in

creases, revealing structural differences of the particles

after the irradiation. Finally, the data analysis of the ar-

rival time of the photoacoustic signals allowed us to de-

termine a significant difference between the arrival times

for samples with spherical and oblate particles. This dif-

ference in the speed of sound for both kinds of particles

can be related to changes in their structure and mechani-

cal properties.

5. Acknowledgements

The authors acknowledge K. López and F.J. Jaimes for

accelerator operation and C. Magaña for SEM operation.

U. Morales thanks the ICyTDF for the financial support.

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