UV-laser-induced nanoclusters in silver ion-exchanged soda-lime silicate glass

Article abstract of DOI:10.1016/j.physb.2006.03.024

Excimer-laser irradiation (ArF: 193 nm) was performed on silver-exchanged commercial soda-lime silicate glass. Silver nanoclusters were obtained with an average size dependent on the irradiation time, without subsequent heating. Laser irradiation induces

Full text of DOI:10.1016/j.physb.2006.03.024

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Physica B 387 (2007) 32–35
www.elsevier.com/locate/physb
UV-laser-induced nanoclusters in silver ion-exchanged
soda-lime silicate glass
Ã
Jiawei Sheng , Jingwu Zheng, Jian Zhang, Chunhui Zhou, Liqiang Jiang
College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Zhaohui No.6, Hangzhou, Zhejiang 310014, China
Received 11 January 2006; received in revised form 16 March 2006; accepted 18 March 2006
Abstract
Excimer-laser irradiation (ArF: 193 nm) was performed on silver-exchanged commercial soda-lime silicate glass. Silver nanoclusters
were obtained with an average size dependent on the irradiation time, without subsequent heating. Laser irradiation induces the
reduction of silver ions and promotes the silver atoms aggregation.
r 2006 Elsevier B.V. All rights reserved.
PACS: 81.05.Kf; 61.46.+w; 61.43.Fs; 61.80.Ba
Keywords: Glass; Excimer-laser irradiation; Nanoclusters
Introducing nanosize metal clusters such as silver into
glass has been used for changing the colour of decorative
glass and recently for fabricating optical devices. During
the past decades, the physical and, in particular, the linear
and non-linear optical properties of metallic nanocluster in
glass have been investigated using numerous experimental
and theoretical approaches. Silicate glasses containing
silver nanoclusters have attracted interest during the last
years as materials for non-linear optical device fabrication.
Ion exchange process with thermal treatments is a well-
established technique for producing nanoclusters in a
silicate glass [1–5]. Recently, the high-power laser anneal-
ing is an alternative way to form nanoclusters in silver ion-
exchanged glass [5–7]. Although a substantial amount of
literature is available on properties of silver nanoclusters in
glass exhibiting high optical non-linear response [8], only
minimal work has been reported on the optical spectro-
scopic investigations of silver ion-exchanged glass. In this
work, we report on the silver nanoclusters formation
behaviour in ion-exchanged soda-lime silicate glass by UV-
laser irradiation. Once the formation mechanism of the
silver nanoclusters is well characterized, the optimum
annealing process for size-controlled silver nanoclusters
may be obtained.
Commercially available soda-lime silicate glass sub-
strates composed of (wt%): 74.2 SiO , 14.3 Na O, 1.9
Al O , 8.1 CaO, and 1.5 MgO were utilized for experi-
2
2
2
3
mentation. Samples (2 mm thick and of approximately
15 ꢀ 15 mm), which had previously been scoured and dried,
were preheated and subsequently dipped in a molten salt
bath formed by a mixture of 98 mol% NaNO and 2 mol%
3
AgNO in a crucible of Al O . The ion exchanges took
3
2
3
place at a temperature of 320 1C with a processing time of
60 min. In order to ensure that the glass structure did not
change during testing, the ion-exchange temperature was
kept well below the glass-transition temperature of the
specimen. After inter-diffusion, samples were removed
from the molten bath and washed with several portions of
distilled water and acetone to remove any silver nitrate
adhering to their surface. An ArF UV laser (193 nm) was
2
utilized at an output energy density of 30 mJ/(cm pulse),
with a repetition frequency of 10 Hz and a 20 ns pulse
duration. With these parameter settings, the laser irradia-
tion time ranged from 5 s to 30 min. Optical absorption
spectra were recorded using a spectrophotometer in the
wavelength range of 300–800 nm at ambient room tem-
perature and all recorded optical spectra were referenced to
Ã
Corresponding author. Tel.: 86 571 88320851; fax: 86 571 88320142.
E-mail address: jw-sheng@zjut.edu.cn (J. Sheng).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2006.03.024

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J. Sheng et al. / Physica B 387 (2007) 32–35
33
air. TEM images for representative samples were observed
by a transmittance electron microscope (JEOL-2010 TEM)
at electron beam energy of 300 keV.
Prior to the ion-exchange process, the glass samples were
colourless and had no measurable absorption in the visible
region. The soda-lime silicate glass was absent of any
colour change following silver-exchange experimentation
for 60 min. There was a lack of significant surface plasmon
resonance (SPR) of the silver nanoclusters in glass
6
4
2
0
Mie Theory
Experimental
(
400–430 nm) during the ion-exchange process at these
experimental conditions, indicating the silver aggregate
formation did not occur, or the silver nanoclusters, if
present, have size less than 1 nm. The formation of silver
nanoclusters is limited, primarily, to the mobility of silver
and the concentration of reducing elements present in the
glass. In our experimental procedure, the silver mobility is
minimal and the concentration of reducing agent is
relatively low.
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Experimentally recorded and theoretically calculated optical
absorption spectra of silver nanoclusters in glass.
Due to the ion-exchange process, the silver diffused up to
several micrometers in the soda-lime silicate glass. Effects
of laser irradiation were clearly visible with the glass
exemplifying a yellow colour after laser irradiation. More-
over, the colour density increased with increasing irradia-
tion time. Fig. 1 shows the absorption spectra for silver-
exchanged glasses after laser irradiations. An absorption
band at approximately 425 nm was clearly observed, which
was due to the SPR band of silver nanoclusters formed in
the glass matrix. A small blue shift of the SPR is visible in
Fig. 1 for the longest irradiation time, indicating differ-
ences in the refractive index properties of matrix or
differences in sizes of silver nanoclusters as function of
time. The laser irradiation promotes nanocluster forma-
tion, as witnessed by the appearance of the SPR band. The
intensity of the SPR band sharply increases with the
irradiation time. Fig. 2 shows the concordance of
theoretically calculated and experimentally recorded opti-
cal absorption spectra of silver nanoclusters formed after
1
0 min laser irradiation. The theoretical spectrum was
calculated using the Mie theory with the electric dipole
approximation [9–11]. For nanocluster sizes below 20 nm,
optical scattering contributions are negligible, and only the
electric dipole term in the Mie expression has to be taken
into account,
e2
a ðcmꢁ1Þ ¼ 18pQn
3
0
=2
Â
à ,
(1)
2
0
2
2
2
l ðe1 þ 2n Þ þ e
ꢁ
1
where a(cm ) is the absorption coefficient at the
wavelength of the incident light, l, Q is the volume
fraction of metallic silver per unit of the irradiated area,
and n is the effective dielectric constant of the matrix. e
and e are the frequency-dependent real and imaginary part
0
1
2
of the nanocluster, calculated from the optical constants of
their bulk metal. The absorption coefficient has its
2
0
maximum value when ðe1 þ 2n Þ ¼ 0, which is the condi-
tion for the SPR. The SPR frequency depends on the
dielectric function as well as the nanocluster size and
composition. The average radius of silver nanoclusters, r,
can be determined by means of the equation [2,12,13].
8
Blank
As ion-exchanged
vf
5
1
1
3
sec irradiation
min irradiation
0 min irradiation
0 min irradiation
r ¼
,
(2)
6
4
2
0
Do
8
where v ¼ 1:39 ꢀ 10 cm=s is the Fermi velocity for bulk
f
silver and Do is the bandwidth at half maximum (FWHM)
of the SPR band. The FWHM is determined by assuming
the absorption peak as a normal Gaussian distribution and
fitting the flank of the absorption band on the long
wavelength side. The value of Do is determined using
Do ¼ 2pcð1=l1 ꢁ 1=l2Þ, where c is the speed of light in
vacuum, l and l are the wavelength at FWHM. For the
1
2
measured FWHM of Fig. 1, the calculated radii ranged
from 0.51 to 0.98 nm, as given in Table 1. The average
nanocluster radius increased with increased irradiation
time. The sizes of nanoclusters observed from the sample
after 30 min irradiation is shown in Fig. 3, where the
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Optical absorption spectra of silver ion-exchanged glass, before
and after excimer-laser irradiation.

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J. Sheng et al. / Physica B 387 (2007) 32–35
34
Table 1
Nanocluster sizes after laser irradiation, calculated from the Mie theory
presented the effects during irradiation.
laser
Glass ꢁ! h þ e ,
þ
ꢁ
(3)
(4)
Irradiation time
Nanocluster radii (nm)
þ
ꢁ
0
5
1
1
3
s
0.51
0.76
0.84
0.98
Ag þ e ꢁ! Ag ,
min
0 min
0 min
+
ꢁ
where h is a hole and e is the electron. Substrate
temperature plays a fundamental role in the formation of
silver precipitates in silver-exchanged glass, with silver
nanoclusters usually aggregating only at a high tempera-
ture (i.e., temperatures Z 500 1C) in air. According to the
above discussion, the silver ions introduced in glass
materials by ion-exchange processes have been observed
to partly reduce to the neutral silver atom species, possibly
aggregating to form silver nanoclusters with diameter of
less than 1 nm. Although the temperature of the laser
irradiation area was far less than 500 1C, we suggest that
the neutralized silver promotes nucleation with resultant
aggregation during irradiation.
Diffusion of silver towards the surface, with consequen-
tial precipitation, are caused by thermal relaxation of the
surface tensile stress introduced by the size differential
+
+
between Ag and Na during the ion exchange process.
Tensile stress introduced by this size difference is still
present after cooling [4,14]. Following laser irradiation,
+
more Ag ions acquire sufficient energy to overcome the
static barrier potential produced by oxygen bonding and
move towards the still considerably stressed surface. The
+
activated Ag ions mobilize toward a more relaxed
surface, resulting from precipitation or nanocluster forma-
tion of silver atoms. Conversely, the dissociation of Ag–O
bonds to form Si–O and Ag–Ag bonds result in a net loss in
the system free energy [4]. Thus, the silver inclusion of the
glass network will move toward the surface and form
nanoclusters in order to maintain a minimum-energy state
within the system. Since the diffusion of silver in the glass
matrix is nominal, the above-discussed processes of silver
nanocluster formation are commonly carried out through
heat treatment.
In summary, we have been able to form silver
nanoclusters in a silver-exchanged commercial soda-lime
silicate glass by a single process of excimer-laser irradia-
tion. Moreover, it was observed that laser irradiation
promotes silver aggregation.
Fig. 3. TEM image of a glass sample after 30 min of irradiation.
spherical individual particles, with an average diameter of
2
nm, were observed.
Elemental metallic nanoclusters are generally formed in
the laser-irradiated area following heat treatment in a
reducing atmosphere. Interestingly, our results showed the
formation of silver nanoclusters only by excimer-laser
irradiation without subsequent heating, yielding a yellow
colour of glass. Currently, the mechanisms responsible for
the formation of the silver nanoclusters solely by laser
irradiation are not fully understood. In the as-exchanged
glass, silver atoms are expected to be bound to non-
bridging oxygen (NBO) atoms, after the substitution for
the sodium atoms in the glass matrix. The silver introduced
The author gratefully acknowledges the financial sup-
port for this work from Zhejiang Provincial Natural
Science Foundation of China (R404108), Zhejiang Pro-
vincial Special Hired Professor Foundation, and Scientific
Research Foundation of Zhejiang Provincial Education
Department (20040251). Dr. Satjit Brar and Bill X. Huang
of NIH/NIAAA are acknowledged for the English
improvement.
+
by ion-exchange process is comprised of Ag as the major
0
0
state, with a minor population of Ag atoms [4]. The Ag
atoms have an absorption wavelength of approximately
00 nm and have no visible absorption. The development
2
of appreciable colour occurs only after the aggregation of
0
Ag atoms to form nanoclusters larger than 1 nm. When
the silver-exchanged glass was subjected to laser irradia-
tion, a reduction of silver atoms was induced. An electron
was driven out from the 2p orbital of a NBO near the silver
ions after laser irradiation, while silver ion captured the
electron to form an atom. The following equations
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