This paper is devoted  to the study of the relationship between structure and optical properties of silica glasses doped with optically active Er3+ ions and reported the influence of the suitably selected heat treatment on the optical properties of silica glasses. The physico-chemical properties of Er3+doped silica are presented, which has been related with the effects of hydroxyl groups on the luminescence property  of Er3+ in the host matrix of silica

Binh, Pham Thanh

Chien, Nguyen Duc

Long, Nguyen Viet

Xuan Thao, Du Thi

English

Vietnam Academy of Science and Technology: Journals Online

114
Journal of Chemistry, Vol. 45 (1), P. 114 - 119, 2006 
Structure and Physico-Chemical Properties of Silica 
Gels Doped with Optically Activated Er3+ ions by  
Sol-Gel Process 
Received 7 July 2005 
Nguyen Viet Long1, Du Thi Xuan Thao2, Nguyen Duc Chien3, Pham thanh binh4
1International Faculty of Higher Education Teaching, Posts and Telecommunications Ins. Tech.  
2University of Mining and Geology 
3Institute of Engineering Physics, HUT 
4International  Training Institute of Materials Science ITIMS, HUT 
summary 
This paper is devoted  to the study of the relationship between structure and optical 
properties of silica glasses doped with optically active Er3+ ions and reported the influence of the 
suitably selected heat treatment on the optical properties of silica glasses. The physico-chemical 
properties of Er3+doped silica are presented, which has been related with the effects of hydroxyl 
groups on the luminescence property  of Er3+ in the host matrix of silica. 
 
I - Introduction 
The development of new photonic materials 
has been studied for the purpose to generate, 
guide, switch, multiplex, and amplify light, 
which has become very necessary in many areas 
in photonic technology such as high power 
fiber, solid state lasers. To date, Er3+-doped 
silica is one of the most attractive and potential 
photonic materials for Er3+-doped fiber 
amplifiers (EDFA). Moreover, sol-gel 
technology has been used to incorporate erbium 
ions into silica gel. Specially, trivalent erbium is 
characterized by an optical transition at 1.54 
µm, an important standard wavelength in optical 
telecommunication, which is utilized for optical 
amplification to compensate the loss of 
information signal occurring in the long-
distance transmission networks [1]. The main 
problems that influence the optical performance 
of sol-gel method-based devices are the none-
radiative relaxation channels (due to rare earth 
concentration quenching) and vibrations of the 
OH groups [2 - 3]. However, the relationships 
between the structure and physico-chemical 
properties of Er3+ doped silica glasses have not 
been expressed clearly in the most of works, 
especially the effects of hydroxyl groups on the 
optical quality of Er3+ doped glasses [1-4]. The 
main object of this paper is to focus on the two 
issues concerning the above silica materials. 
II - Experimental procedure 
1. Sol-gel Process 
The starting materials in the experiments are 
tetraethylorthosilicate (TEOS), ethanol or 
isopropanol, deionized water and acid catalyst. 
TEOS, ethanol and deionized water were used in 
the molar ratios 1: 4: 4, respectively. Erbium 
was introduced by ErCl3 or Er(NO3)3.9H2O. The 
basic chemical reactions of sol-gel process on 
undergoing hydrolysis and condensation under 
HCl catalyst are:  
115
(a) 
(b) 
ErO6
4 2 4Si(OR) 4 H O Si(OH) 4 ROHn n n n+  + (1) 
 4 2 2Si(OH) SiO 2 H On n n + (2) 
 
Fig. 1: Disordered covalent polymeric networks 
(shaded areas represent pores) with Si-O-Si 
bondes [5] and a ErO6 configuration with 
covalent bonds ([6]) 
 
The solutions were maintained at 40oC until 
gelled. After gelation, the samples were dried 
slowly at 90oC to form monolithic silica gel 
doped with erbium. Gelation occurs and an 
extensive amorphous three-dimensional network 
of Si-O-Si bonds is formed with a diversity of 
pores, where erbium ions can be incorporated 
and the covalent bonds between Er and Si or O 
are formed. The schematic representation of 
structures on the silica surface, as discovered 
from infrared spectra by Wood et al. [5], is 
shown in figure 1 and 2. The figures show the 
formation of clusters of Si(OH)m.(H2O)n.
However, there are still many difficulties in sol-
gel process, mainly concerning with the 
presence of a large number of hydroxyl groups 
in gel bulk and on surface of silica gel, which 
decreases the emission of the rare earth ions at 
1.5 µm.  
So far, a great number of researches dealing 
with the hydroxyl problems has been carried 
out. It is evidently seen that a lot of papers 
concerning with different methods removing 
hydroxyl groups. 
2. Heat treatment process for obtaining Er3+ 
doped silica glass with high optical  quality 
 
Fig. 2a: Schematic representations of structures 
on the silica surface present in gel as revealed 
from IR spectra. Hydrogen bonds are represented 
by dotted lines. Fig. 2b. Diagram of changes 
during the gelation of particulate sols. Dots 
represent hydrogen bonds; curved lines represent 
micelle surfaces [5] 
116
The main objective of this paper is to 
present a suitable heat treatment aim to remove 
hydroxyl groups which are formed in silica gel 
doped with certain concentration of erbium for 
purpose to increase the emission performance of 
Er3+ ions. For the purpose of reducing hydroxyl 
groups, isothermal treatment was carried out for 
interval periods of time. Samples were 
isothermally annealed at 200oC for 24 h, and then 
temperature was increased slowly with a heating 
rate of 0.5oC/min, to 400oC and isothermally 
annealed at 400oC for 24 h and so on to the next 
heating stage at 600oC, 800oC, 950oC, 1100oC….
This selected heat treatment process allows 
getting the final silica glasses doped with erbium 
with a minimised content of hydroxyl groups. 
However, an amount of -OH groups is existent, 
which of course exhibits the natural property of 
sol-gel derived glasses. The samples of silica gel 
contained 2 - 7 wt.%Er2O3. After heat treatment, 
the color of  the samples looks slightly pink (this 
color is typical for Er3+ doped silica glasses). Er3+-
doped monolithic silica glasses have been 
prepared with high optical quality by stepwise 
heat treatment process. The detail of experimental 
measurement of silica glasses could be seen in 
Long, et. al [7, 8].  
III - Results and discussion 
1. Effects of hydroxyl groups on optical 
property of Er3+ doped silica glass 
It has been known that one impurity 
strongly quenching emission of Er is OH 
groups, as the second overtone of the OH stretch 
vibration is resonant with the first excited state 
of Er. In highly Er3+ doped glass, in which 
energy migration can take place over a 
significant distance, a small concentration of 
OH will cause significant quenching. In fact, the 
quenchers come from water in the solvents used 
to prepare these glasses and these unwanted 
channels decrease the fluorescence intensity of 
Er3+ ion. It is composed of a superposition of 
SiO-H stretching vibrations: (i) vicinal free SiO-
H stretching, (ii) isolated free SiO-H stretching 
vibrations, (iii) vicinal H-bonded-SiOH---OH2
stretching vibrations, and (iv) isolated H-
bonded-SiOH---OH2 stretching vibrations; (v) 
cluster H-bonded H2O [5]. In addition to the 
envelope of vibrations corresponding to O-H 
stretching, and also the band at 1643 cm-1 was 
formed due to a deformation mode of adsorbed 
molecular water. It has been known that there 
are three possible quenching mechanisms: (i) 
temperature quenching for the measurements 
depending on temperature (small and not 
dominant), (ii) quenching by the nearest C-H 
neighbours (very small and not dominant), (iii) 
quenching by different hydroxyl groups 
(dominant). OH groups are present in the liquids 
(mainly alcohols) used in the preparation of the 
Er3+-doped glasses, and they can be present also 
in the solvent used in the experimental 
measurements. It is therefore likely that OH 
groups are coordinated to the Er3+. It has been 
found that one OH molecule can penetrate the 
first coordination shell of the ion Er3+. In 
particular, water may preferentially solvate the 
Er3+; therefore, OH groups decrease the 
fluorescence of Er3+ ions. It is the most potent 
quencher of the rare-earth luminescence, and has 
a stronger interaction to the rare earth. Because 
its second vibrational overtone (E0 = 3400 cm
-1)
is strongly resonant with the 4I13/2  4I15/2 
transition of Er3+ ion [6] (E = 6500 cm-1 
corresponding to  = 1540 nm), an OH group 
only near Er3+ ions will result in efficient 
quenching of the luminescence. This is an 
undesirable effect leading to limit optical gains.   
2. Investigation of Fourier Transform 
Infrared (FTIR) spectra  
The experimental FTIR spectra were 
recorded in the frequency range 400 - 4000    
cm-1. All of infrared investigations of the 
fundamental network vibrations often make 
dilution in infrared transparent media with KBr. 
In FTIR spectra, the most intense peaks around 
1080 cm-1 and 1220 cm-1 were found. Variations 
in the peak positions and the number of peaks 
can be seen in FTIR spectra with different 
structural morphologies of silica from 
amorphous structure to crystal structure and 
then structure of quartz, which pointed out that 
possibly crystallization occurs for samples 
annealed at 1000, 1200oC. Most of the peaks in 
the FTIR spectra are consistent with the peaks in 
117
the Raman spectra except the bands at 1220   
cm-1 and 460 cm-1. It has been obvious that the 
major features of FTIR spectra associated with 
vibrational network of Er3+-doped silica glasses 
are located at 460, 800, 1080, and 1220 cm-1 [7 - 
9]. The most intense 1080 and 1220  cm-1 bands 
in the FTIR spectra are assigned to the TO and 
LO modes of the asymmetric Si-O- stretching 
vibration of microstructure of silica glasses, 
respectively. An O-H stretching broadband 
between about 3000 and 3800 cm-1 centered at 
near 3440 cm-1 corresponding to the 
fundamental stretching combined vibrations of 
different hydroxyl groups comes from the 
vibration of surface isolated silanol groups or 
silanol groups on the surface, which is hydrogen 
bonded to molecular water. The vibrational 
intensity of this band decreased significantly 
(not shown here) during the heating treatment 
and the fluorescence of Er3+ ions increased (the 
OH groups are the most potent quenchers of the 
Er3+ luminescence).  
Transeverse optical rocking vibration (TO1), 
symmetric stretching vibration (TO2) and 
asymmetric stretching vibration (TO3) are 
vibrations of 460 cm-1, 800 cm-1 and 1080 cm-1 
are assigned to the Si-O-Si vibrational modes, 
respectively. A strong shoulder at about 1220 
cm-1 is assigned to longitudinal optical 
component (LO3) of the high-frequency 
vibration of SiO2. All of spectra exhibit a band 
centered at near 940 - 960 cm-1, which is due to 
stretching vibrations of Si-OH. This band did 
not appear in the samples annealed at 800, 950, 
1100, and 1200oC. Moreover, there is an 
evidence in the FTIR spectra with the samples 
annealed at 1100, 1200oC for symmetric ring 
breathing modes due to the D2 vibrations in the 
Raman spectra [8].  Finally, we suggested that 
optical transparent glasses and specific 
vibrational rings best describe the structure and 
optical properties of silica glasses. In other 
words, FTIR spectra are useful in distinguishing 
between physically adsorbed water and various 
hydrogen-boned silanol species. The main 
effects of hydroxyl impurities (in figure not 
shown here) is seen that the spectral region 
centered at 3440 cm-1 consists of combination 
bands associated with various silanol species, 
adsorbed water, and residual organics and the 
band at 1643 cm-1 was formed due to a 
deformation mode of adsorbed molecular water. 
It has been also shown that hydroxyl groups can 
be present even at high annealing temperature 
and functionalize the sol-gel-derived glasses. In 
fact, to understand the behavior of the structural 
and dynamic changes during gel-to-glass 
transitions is to improve the optical quality of 
Er3+ doped silica glass. It have been suggested 
that the SiO4 units (in which a silicon (Si
4+) atom 
(ion) in the tetrahedron is covalently ionically 
bonded to four oxygen atoms (ions) which may 
be a fundamental tetrahedral microstructure unit 
of silica glasses and exhibit the nature of Si-O-
Si vibrational modes. Further studies have 
shown that Er3+ ions incorporated into glassy 
matrix in form of ErO6 bonded cluster 
configuration. See more in the works of A. 
Polman in [6]. Moreover, controlling a gel-to-
glass process can be accomplished with a great 
ease by using suitably selected heat treatment 
for obtaining the optimal amorphous structure, 
which is necessary to get the highest 
fluorescence of Er3+. However, there was an 
important thing to determine the temperature 
range, which pores were collapsed for more Er3+ 
ions incorporating into matrix network of SiO2.
But it is also seen that at the beginning of 
gelation most of Er3+ ion has been incorporated 
into polymer matrix. It means that selected heat 
treatment process has only reduced the content 
of OH hydroxyl population. In my point of 
view, it has been seen that this temperature 
range for reducing most of remaining OH 
groups of samples to being in the range of 800 - 
1100oC.  
3. X ray diffraction 
Sharp diffraction peaks show the presence of 
two phase crystalline quartz of SiO2, hexagonal 
and tetragonal respectively, which leads to 
decrease the fluorescence of Er3+ ion in the 
matrix of silica host. The above effects due to 
the structural changes are very important in 
order to prepare silica materials with the 
desirable structure and properties as well as 
optically improved quality. According to the 
above results about X-ray and FTIR, some 
remarks on the heat treatment procedure to the 
118
1400 1450 1500 1550 1600 1650 1700
4 wt. % Er
2
O
3
(a) 800 oC
(b) 950 oCP
L
I n
t e
n s
i t y
( a
. u
)
Wavelength (nm)
evolution of structure silica glass are described 
as below: (i) It is obviously shown that the 
vibrations are present mainly for OH vibrational 
groups and matrix vibrations of with specific 
bonds existing in silica gel in the temperature 
range of 25 - 150oC;  (ii) It is clearly shown that 
in the heat treatment range of 100 - 400oC, the 
intensity of OH vibrational groups was reduced 
significantly. In the intermediate heat treatment 
range of 400 - 800oC, a continuous decrease of 
OH groups as shown in FTIR spectra, which 
leads to being nearly a minimum of OH groups. 
Many authors have determined that the 
densification of silica gel is achieved at about 
800oC, see more in the works in [7, 8]; (iii) It is 
clearly shown that most of remaining OH 
groups are reduced in the heat treatment range 
of 900 - 1100oC to be completely minimised; 
however, it is also emphasized that the 
crystallization process of SiO2 occurs 
simultaneously. At 1200oC, the crystallization 
process had occurred; (iv) It is shown that the 
amorphous structural phase of silica gel by 
synthesizing sol-gel method was gradually 
conversed into the different crystal phases at 
higher temperature heat treatment and the 
structure of SiO2 exists both amorphous. Crystal 
phases of SiO2 are formed in the higher 
temperature range of 1000 - 1200oC.  
4. Photoluminescence spectra (PL) 
The 4I13/2  4I15/2 fluorescence of Er3+ at 1.5 
µm from silica samples was shown in figure 3. 
The structure of this transition is of particular 
interest for application in wavelength division 
multiplexed signal amplifiers (WDM) in 
telecommunication. We used the samples with  
~4 wt.% Er2O3 because of the optically active Er 
concentration to get the highest luminescence as 
presented in our previous report. This is a 
suitable concentration in application for Er3+-
doped amplifiers (EDFA). 
Erbium-doped silica glasses have been 
successfully fabricated by sol-gel process. The 
structure of glassy network has been 
investigated by FTIR and X-ray diffraction 
spectroscopy. It has been shown that 
characterization of sol-gel derived glasses is the 
fact that a OH content is left in the final silica 
glass samples. It has seen that the O-H 
stretching vibration affects the fluorescence 
decay at 1.5 µm because two O-H vibrations are 
enough to bridge the gap of about 6500 cm-1
between the ground state 4I15/2 and the first 
excited state 4I13/2 of the Er
3+ ion. This problem 
was dealt with by suitably isothermal annealed 
processes at higher temperature to minimise the 
residual OH content and make good optical 
silica glasses. It have been also found that the 
intensity of Er3+ fluorescence in sol-gel silica 
glasses increases with increasing heat treatment 
temperature due to the removal of adsorbed 
water or residual hydroxyl groups. An important 
advantage of isothermally selected heat 
treatment at 950oC for 24 h lies on the fact that 
the structure of SiO2 was still amorphous, and 
that an amount of residual OH groups was 
significantly minimised leading to the highest 
increase of fluorescence.  
 
Fig. 3: Room temperature 4I13/2  4I15/2 
transition of Er3+ ions of 4 wt.% Er2O3 silica 
samples 
Acknowledgements: This research was 
partially supported by the Research and 
Development program of Posts and Telecoms 
Institute of Technology and the State Program 
of Fundamental Research, grand number 
KHCB-441204. 
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Structure and physico-chemical properties of silica gels doped with optically activated Er3+ ions by sol-gel process

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Structure and physico-chemical properties of silica gels doped with optically activated Er3+ ions by sol-gel process

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