This reduces not only the clustering effect and energy transfer between ions but also allows energy transfers from crystals to RE ions[4,5], which enhances the luminescence quan-tum yiel
Trang 1Tin-dioxide nanocrystals as Er 3 þ luminescence sensitizers: Formation
Lidia Zura,b,*, Lam Thi Ngoc Tranc,b,d, Marcello Meneghettie,b, Van Thi Thanh Tranf,
Anna Lukowiakg, Alessandro Chiaserab, Daniele Zontac,b,h, Maurizio Ferrarib,a,
Giancarlo C Righinia,i
a Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Piazza del Viminale 1, 00184, Roma, Italy
b IFN-CNR CSMFO Lab., and FBK Photonics Unit, via alla Cascata 56/C Povo, 38123, Trento, Italy
c Department of Civil, Environmental and Mechanical Engineering, Trento University Via Mesiano, 77, 38123, Trento, Italy
d Ho Chi Minh City University of Technical Education, 1 Vo Van Ngan Street, Linh Chieu Ward, Thu Duc District, Ho Chi Minh City, Viet Nam
e Dipartimento di Fisica, Universita di Trento, via Sommarive 14, Povo, 38123, Trento, Italy
f University of Science, Vietnam National University, Ho Chi Minh City, 227 Nguyen Van Cu Street, Ward 4, District 5, Ho Chi Minh City, Viet Nam
g Institute of Low Temperature and Structure Research, PAS, Okolna St 2, 50-422, Wroclaw, Poland
h Department of Civil and Environmental Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, G11XJ, UK
i IFAC e CNR, MiPLab., 50019, Sesto Fiorentino, Italy
a r t i c l e i n f o
Article history:
Received 15 June 2016
Received in revised form
19 July 2016
Accepted 29 August 2016
Available online xxx
Keywords:
Glass-ceramics
Thin films
SnO 2 -SiO 2
Rare-earths
Luminescence
Energy transfer
a b s t r a c t
Silica-tin dioxide thinfilms doped with Er3þions were fabricated and investigated Different parameters such as heat-treatment temperatures, molar concentrations of SnO2as well as Er3þions concentration were changed in order to obtain the best properties of presented thinfilms Using several techniques, thinfilms were characterized and proved to be crack-free, water-free and smooth after a heat-treatment
at 1200 C Aiming to application in optics, the transparency of thin films was also evidenced by transmission spectra Based on the photoluminescence measurements, the mechanism of energy transfer from SnO2nanocrystals to Er3þions was examined and discussed
© 2016 Elsevier B.V All rights reserved
1 Introduction
Silica-based materials doped with rare earths (RE) are widely
used in photonics They are promising materials that have found
application in high brightness displays and laser emitters [1]
Among others, Er3þion is one of the most studied RE dopants It can
be used in optical amplifiers in the main optical telecommunication
window at the 1.5mm[2] Nowadays, fabrication of miniaturized,
efficient integrated optics devices requires a high concentrations of
rare earths in a small volume; one of the main problem arising is
the concentration quenching effect due to the high content of active
dopants This effect is observed for all RE after reaching a certain
concentration, characteristic for each of these ions, due to clusters formation The ion-ion interactions in the clusters lead to reduction
of luminescence efficiency[3] In order to overcome this problem, glass-ceramic materials containing RE embedded in nanocrystals can be prepared This reduces not only the clustering effect and energy transfer between ions but also allows energy transfers from crystals to RE ions[4,5], which enhances the luminescence quan-tum yield and compensates the small absorption cross section of the majority of rare earth ions Additionally, the glass-ceramic materials play a significant role because they merge the mechani-cal and optimechani-cal properties of the glasses with a crystal-like envi-ronment for the RE ions[6]
One of the promising materials that could be used to solve this problem is tin dioxide SnO2 is a wide-band gap semiconductor (Eg¼ 3.6 eV at 300 K) with a maximum phonon energy below
630 cm1 [7] Some papers about the RE doped to SnO2
* Corresponding author Centro Fermi/CNR-IFN, Via alla Cascata 56/C, 38123,
Povo, Trento, Italy.
E-mail address: zur@fbk.eu (L Zur).
Contents lists available atScienceDirect Optical Materials
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m/ l o ca t e / o p t m a t
http://dx.doi.org/10.1016/j.optmat.2016.08.041
0925-3467/© 2016 Elsevier B.V All rights reserved.
Trang 2nanocrystals have already been published[8,9], and investigations
on increasing SnO2concentration in silicate matrix were carried out
[10,11] Particularly, strong enhancement of the Eu3þemission was
observed, thanks to energy transfer from the SnO2nanocrystals to
Eu3þions[12]
In this paper, xSnO2-(100-x)SiO2glass-ceramic thinfilms doped
with Er3þions are presented Influence of the composition and
heat-treatment process on the structural, optical, and spectroscopic
properties of tin dioxide based glass-ceramics has been
investi-gated by several characterization techniques Especially, we focused
on the energy transfer between SnO2and Er3þions as well as on the
influence of rare earths on crystallization of SnO2
2 Experimental
xSnO2e(100-x)SiO2 (x ¼ 10, 20, and 30 mol %) glass-ceramic
thinfilms doped with different concentrations of Er3þ(0, 1, 2, and
3 mol %) were fabricated by sol-gel route using dip-coating
tech-nique Precursors, SnCl2$2H2O and Er(NO3)3$5H2O, were dissolved
in ethanol and added to the starting solution that had been
pre-pared by mixing tetraethyl orthosilicate (TEOS), ethanol, de-ionized
water, and hydrochloric acid with the molar ratio of 1:21:2:0.01
Thefinal mixture was stirred for 16 h Then, thin films activated by
Er3þwere deposited on pure SiO2 and silicon substrates by
dip-coating with the deeping rate 8 cm/min After thefirst layer was
dip-coated, there was a temporary annealing at 150C during 3 min
before next dip-coating step Final thinfilms having 3 layers were
heat-treated in air at various temperatures ranging from 500C to
1200C for 1 h
Structure, surface, transparency, and photoluminescence
prop-erties of the resulting thin films were characterized Using the
EQUINOX 55 spectrometer, Fourier-transform infrared (FTIR)
transmission measurements were performed to identify the
structure, phases and especially existence of water and solvents in
obtained thinfilms The crystallization of tin dioxide component
was studied with X-ray diffraction patterns performed by
D8eADVANCE The Atomic Force Microscopic images were taken
by AFM-D3100 to supply the surface characterization of the thin
films Then, the transparency of thin films was provided by the
ultravioletevisible spectra Finally, using Horiba spectrometer, the
photoluminescence of the Er3þ ions in the surrounding
glass-ceramics environment was recorded upon an excitation at
300 nm, evidencing energy transfer from SnO2crystals to Er3þ
3 Results and discussion
3.1 Structural and morphological characterization
FT-IR transmission spectra of the thinfilms xSnO2e(100-x)SiO2
doped with Er3þ dip-coated on the silicon substrate have been
analyzed FT-IR analysis is used to study rare earth doped materials,
giving important information about the structure, and allows
tracking the process of removing organic groups as well as hydroxyl
groups Hydroxyl groups are well known as luminescence
quenching agents [13e15], so the heat treatment process is
essential to remove these groups in order to achieve efficient
luminescence of Er3þ Fig 1 presents the infrared transmission
spectra of 30SnO2e70SiO2thinfilms doped with 1% of Er3 þ
heat-treated at different temperatures After annealing at 600 C,
peaks at about 1630 and 3404 cm1were observed that are typical
for bending and stretching vibration of OH group coming from
residual water in thin films When films were heat-treated at
higher temperatures (800÷1200 C), disappearance of these two
bands implies that hydroxyls groups were eliminated by the
annealing These results are in accordance with previously
published data[16,17] The majority of amorphous SiO2 is responsible for the broad absorption of asymmetric stretching vibration of ^SieOeSi^ linking bonds at 1062 cm1 There are also other absorption bands concerning SieOeSi bindings: 811 cm1 assigned to symmetric
stretching and 457 cm1corresponding to the bending vibrational mode between adjoining SiO4rings[18e20] The next weak band taken into account is the vibration absorption around 667 cm1of
d(OeSneO) bonding [17] Finally, the weakest band appears at
551 cm1after heat treatment at 1200C and is assigned to the vibration of SneOeSn bonding
FT-IR spectra of thinfilms containing different SnO2contents are presented in Fig 2 Increasing concentration of SnO2 affects in broadening of the peak observed around 653 cm1, connected with
d(OeSneO) bonding The band at 561 cm1assigned to the
vibra-tion of SneOeSn bonding is broadening with increasing concen-tration of SnO2
One of the requirements of the xSnO2e(100-x)SiO2 glass-ceramic thinfilms is formation of SnO2 nanocrystals containing
Er3þions, uniformly distributed in SiO2matrix Thereby, the energy transfer from SnO2 nanocrystals to erbium is more efficient
Fig 1 Infrared spectra of 30SnO 2 e70SiO 2 thin film doped with 1% Er 3þ as a function of annealing temperature.
Fig 2 Infrared spectra of xSnO 2 e(100-x)SiO 2 thin films doped with 1% Er 3þ as a function of SnO 2 content.
Trang 3Primarily, the influence of heat-treatment process on formation of
SnO2 nanocrystals was carefully examined Fig 3 shows the
diffraction patterns of the 1%Er3þ:70SiO2e30SnO2 thin films
annealed at 800C, 1000C and 1200C The diffraction peaks at
2q¼ 26.9, 34.1, 38.2, and 52.0, which can be assigned to the
(110), (101), (200), (211), and (112) planes of SnO2 rutile crystal
phase (JCPDS 41-1445), are present only after the heat-treatment at
1000 C or higher temperature In addition, several diffraction
peaks with low intensity appear (marked with asterisk) According
to previous results[21], it is possible to conclude that a new
crys-talline phase is formed; however, no known cryscrys-talline structure
associated with Er/O, Er/Si, or Er/O/Si compounds matches with the
data of XRD on Fig 3 SnO2 nanocrystal size calculated from
Scherrer formula forfilms annealed at 1000C and 1200C shows
that the heat-treatment temperature affects slightly the crystals
size Previously published results shown that the sintering process
influences the agglomeration and the particle size growth The
sintering temperature promotes enlargement of grain boundaries
and consequently particle size increases as a function of annealing
temperature[22,23]
The structure of the xSnO2e(100-x)SiO2 thin films was also
investigated as a function of the SnO2 content, and presented in
Fig 4(a) Based on diffraction patterns, using Scherrer formula, the
size of the tin dioxide nanocrystals was calculated and displayed in
Table 1 We observed that the size of nanocrystals increases with
SnO2content Moreover, one can see that increasing concentration
of SnO2do not cause formation of new phase in investigatedfilms
Obtained results are in accordance to the FT-IR results discussed
above
In order to provide complete examination of xSnO2e(100-x)
SiO2system, the structure of thinfilms as a function of Er3 þ
con-centration has been investigated (Fig 4(b)) Due to formation of
SnO2nanocrystals, part of the dopant is in amorphous SiO2and part
is embedded in SnO2nanocrystals Incorporation of Er3þinto SnO2
will cause defects in the crystal lattice, causing the loss of symmetry
in the crystal Thus, as a result, slight change in the size of SnO2
nanocrystals is observed with increasing concentration of Er3þions
Similar behavior has been observed before in SnO2doped with Er3þ
[24] Additionally, no other phase was formed with increasing
concentration of erbium
The microstructure of thinfilms is also a relevant factor for
application of planar waveguides AFM images of thinfilms with 1%
of Er3þand various concentrations of SnO2annealed at 1200C are presented in Fig 5(a) Annealing at this temperature promoted formation of the SnO2nanocrystals, with a dense and homogenous distribution on the thinfilm surface It should be noted that when the content of SnO2is low, the size of formed nanocrystals is small (what was confirmed by XRD analysis), which consequently leads
to lower average value of surface roughness Increasing concen-tration of tin dioxide caused increase of the surface roughness The values of root mean square roughness (Rrms) of the thinfilms are 6.30 nm, 8.53 nm and 10.11 nm for SnO2content of 10%, 20% and 20%, respectively
Fig 3 X-Ray diffraction patterns of 30SnO 2 e70SiO 2 thin film doped with 1% Er 3þ as a
function of heat-treatment temperature.
Fig 4 X-Ray diffraction patterns of xSnO 2 e(100-x)SiO 2 thin films as a function of (a) SnO 2 content and (b) Er3þconcentration.
Table 1 The average SnO 2 crystallite sizes in xSnO 2 e(100-x)SiO 2 thin films doped with Er 3þ
in function of composition.
Er3þ[mol%] SnO 2 [mol%]
Trang 4The influence of rare earths concentration on the surface
roughness was also examined, as shown in Fig 5(b) Thin films
containing 30% of SnO2with the concentration of Er3þincreased up
to 3% have been chosen for doing this analysis Obtained results
show the influence of erbium on formation of SnO2nanocrystals It
can be seen that the average roughness of thefilms decreases due
to increasing concentration of Er3þ The root mean square
rough-ness (Rrms) of the thinfilms decreases with increasing content of
Er3þand is equal to 10.11 nm (1% of Er3þ), 9.97 nm (2% of Er3þ) and
5.40 nm (3% of Er3þ) We can conclude that the incorporation of
Er3þions inhibits the growth of SnO2crystals
Taking into account the results obtained from AFM
micro-structure analysis as a function of SnO2and Er3þconcentration, the
proper composition can be chosen as a compromise between the
SnO2and Er3þcontent in the thinfilms
3.2 Optical characterization
SnO2eSiO2thinfilms doped with Er3þions dip-coated on pure
silica substrate were heat-treated at 500 C for 1 h in order to
collect UV-VIS transmittance spectra The spectra were recorded for
samples with different SnO2and Er3þconcentration, as shown in
Fig 6 It is clearly seen that, irrespective of the thinfilms
compo-sition, the transmittance always remains close to 90% Moreover,
the transmittance spectra recorded for 30SnO2e70SiO2thinfilms
with different Er3þ ions concentration (Fig 6(b)) show that the
increasing concentration of Er3þhas no effect on the transparency
of investigated thinfilms
3.3 Spectroscopic characterization Spectroscopic properties of Er3þions doped into xSnO2e(100-x) SiO2thinfilms dip-coated on the silicon substrates and heat treated
at 1200C for 1 h have been analyzed as a function of thefilm composition Deep and entire analysis of SiO2eSnO2 system was possible by changing the concentration of SnO2in the range from
10 to 30%, while the Er3þcontent was set at 1% Infrared photo-luminescence spectra of discussed glass-ceramic thinfilms were recorded upon an excitation at 300 nm and are presented in
Fig 7(a) This excitation wavelength corresponds to the interband electronic transition of the SnO2nanocrystals The narrowing of the emission peaks, associated to the Stark multiplets, evidences that the rare earth ions were embedded in the SnO2nanocrystals The emission intensity increases with the SnO2content This behavior
reflects an increase in the number of the Er3þions embedded in the SnO2nanocrystals with the SnO2content When the concentration
of SnO2increased from 10% up to 30%, the luminescence intensity of
1550 nm region was enhanced about 10 times With increasing concentration of SnO2, the density as well as the size of nano-crystals increases, which makes easier the process of incorporation
of the rare earth ions in the nanocrystals Therefore, more Er3þions are excited due to exciton mediated energy transfer from SnO2
nanocrystals After analysis of obtained results, 30SnO2e70SiO2
composition has been selected to examine the spectroscopic properties as a function of Er3þconcentration.Fig 7(b) shows the luminescence spectra for 30SnO2e70SiO2thinfilms doped with 1, 2 and 3% of Er3þ It is well known that with increasing concentration
of rare earth ions the quenching effect is observed, due to clustering
Fig 5 3D AFM images of the surface of xSnO 2 e(100-x)SiO 2 thin films as a function of (a) SnO 2 and (b) Er3þconcentration.
Trang 5and ions-ions interaction[25,26] Different investigation are carried
out in order to allow higher doping of rare earth ions in different
materials which is important from application point of view One of
the solutions of this problem is the existence of nanocrystals in
amorphous matrix, making the RE distribution easier In
xSnO2e(100-x)SiO2 glass-ceramic thin films, SnO2 nanocrystals
would appear to ease the dispersion of Er3þions in silica matrix
Nevertheless, the quenching effect remains The emission intensity
decreases very slightly when the concentration of Er3þreaches 2%,
then some reduction of intensity is observed for thin film
con-taining 3% of Er3þ
Fig 8 shows the PL emission spectra for the 1Er3þ:
30SnO2e70SiO2 thin film as a function of the excitation
wave-lengths The spectra exhibit similar shapes, but different intensities
It appears that the most suitable pumping region is around
290e300 nm This is not surprising, as this region corresponds to
the SnO2band gap
4 Conclusions
The effects of SnO2and Er3þconcentration on the structure and
distribution of erbium ions in glass-ceramic thin films were
investigated by FT-IR spectroscopy, X-ray diffraction and Atomic
Force Microscopy Obtained films are crack-free and exhibit a
transmittance of around 90% over the 400 nme1.1mm region Size
of the nanocrystals increases with the SnO2content and reaches about 61 nm for 2% Er3þ:30SnO2e70SiO2thinfilm The properties
of erbium as well as the role of SnO2 nanocrystals as Er3þ lumi-nescence sensitizers were experimentally confirmed based on emission spectra Thin films with composition 30SnO2e70SiO2
Fig 6 Ultravioletevisible spectra of xSnO 2 -(100-x)SiO 2 thin films as a function of (a)
SnO 2 and (b) Er3þcontent. Fig 7 Photoluminescence spectra of Er
3þ doped to xSnO 2 e(100-x)SiO 2 as a function of (a) SnO 2 and (b) Er3þconcentration in thin films with 1%Er 3þ (a) and 30%SnO 2 (b).
Fig 8 Photoluminescence spectra of 30SnO 2 e70SiO 2 thin films doped with 1% of Er 3þ
as a function of different excitation wavelengths.
Trang 6doped with 2% of Er3þappears to be the most attractive in terms of
luminescence efficiency
Acknowledgements
The research was performed in the framework of the CNR-PAS
joint Project (2014e2016), Cost Action MP1401, Centro Fermi
PLANS project and a bilateral project“Plasmonics for a better
effi-ciency of solar cells” between South Africa and Italy (contributo del
Ministero degli Affari Esteri e della Cooperazione Internazionale,
Direzione Generale per la Promozione del Sistema Paese) This
research is funded by Vietnam National Foundation for Science and
Technology Development (NAFOSTED) under grant number
103.03-2015.34 Thi Ngoc Lam Tran acknowledges the scholarship
of the Ministry of Education and Training, Vietnam International
Education Development
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