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Non-stabilized europium-doped lanthanum oxyfluoride and fluoridenanoparticles well dispersed in thin silica films†

Emille M. Rodrigues, Ernesto R. Souza, Jorge H. S. K. Monteiro, Rafael D. L. Gaspar, Italo O. Mazaliand Fernando A. Sigoli*

Received 24th July 2012, Accepted 17th September 2012

DOI: 10.1039/c2jm34901a

Well dispersed non-stabilized lanthanum oxyfluoride and fluoride nanoparticles were prepared in situ in

thin silica films from rapid thermal decomposition of lanthanum tris-trifluoroacetate under nitrogen

atmosphere. The thin silica films were obtained from sol–gel method and spin-coating. The

spectroscopic properties of the non-stabilized nanoparticles as well the nanoparticles dispersed into

thin silica films were studied in order to apply the system in future photonic applications such as

erbium(III)-doped waveguide amplifiers. The non-stabilized nanoparticles were characterized by XRD,

FT-IR, Transmission Electron Microscopy, Confocal Raman Spectroscopy and steady-state and time-

resolved Luminescence Spectroscopy and these characterizations were used as a starting point to

characterize the nanoparticles dispersed into the films. According to the temperature of the thermal

treatments, the non-stabilized nanoparticles may present Eu(III)-doped LaOF in tetragonal and

rhombohedral phases as well as a mixed phase of Eu(III)-doped LaF3 and LaOF. The tetragonal LaOF

phase has C4v La(III) point symmetry and is more symmetric than the rhombohedral LaOF phase,

where the La(III) ion has C3v symmetry, consequently tetragonal LaOF presented lower U2 values than

rhombohedral LaOF. Theoretical calculations of Judd–Ofelt intensity parameters were also performed

and were in good agreement with the experimental values. The samples containing the mixed phase of

LaF3 and LaOF presented lower values of intensity parameters than pure LaOF phases. The samples

containing the mixed phase presented higher values of emission lifetimes and quantum efficiencies.

Confocal Raman spectroscopy of these samples complements the luminescence studies and indicates

which LaOF phase is present in the mixed phase of LaF3 and LaOF. The rapid thermal decomposition

of the precursor tris-trifluoroacetate on thin silica films results in well-dispersed 10 nm nanoparticles.

The mixed phase of LaF3 and LaOF phases is also present in thin films. The luminescence of the Eu(III)

and Er(III)/Yb(III)-doped LaF3/LaOF nanoparticles containing thin silica films presented broad

emission bands suggesting that in the future the systems may be applied as erbium(III)-doped waveguide

amplifiers.

Introduction

The luminescence of a lanthanide ion-doped inorganic material

arises from the electronic transitions of the partially filled 4f

orbitals, consequently they have low molar absorption coeffi-

cients. However, Laporte’s rule may be relaxed due to a mixing

between the wave function of the f and d orbitals. The spin rules

are also relaxed because of spin–orbit coupling, particularly

when lanthanide ions are present. These relaxations of the

selection rules result in observable electronic transition bands,

sometimes with high intensities. As the 4f orbitals are strongly

Laboratory of Functional Materials – Institute of Chemistry – Universityof Campinas – UNICAMP, P. O. Box 6154, Campinas, Sao Paulo,Brazil, 13083-970. E-mail: fsigoli@iqm.unicamp.br

† Electronic supplementary information (ESI) available. See DOI:10.1039/c2jm34901a

This journal is ª The Royal Society of Chemistry 2012

shielded by the filled 5s and 5p orbitals, their emission bands are

narrow, enabling their use in technological applications such as

organic light emitting diodes (OLEDs),1 solar cells,2 optical

amplifiers3 and biological imaging agents.4

Eu(III) is the most common lanthanide ion for luminescence

studies, because its energy level diagram has non-degenerate

emitting and ground states (5D0 and7F0, respectively). The main

electronic transitions arise from the excited state 5D0 to the 7FJ

(J ¼ 0, 1, 2, 3, 4, 5, 6) levels. The Eu(III) transitions provide

important information about the chemical environment around

the Eu(III) ion and make it a probe of the chemical environment

in which it is located.5 The presence of just one narrow (FWHM

¼ 7 cm�1) non-degenerate 5D0 / 7F0 transition indicates that

the Eu(III) ions may lie in one non-centrosymmetric site. The

hypersensitive 5D0 / 7F2 transition is allowed by both forced

electric dipole and dynamic coupling mechanisms and its

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intensity depends on the charge density distribution around the

Eu(III) in the host matrix. If this distribution is nearly symmetric,

this transition is strictly forbidden and its intensity is lower than

the 5D0 /7F1 magnetic dipole transition. The latter transition is

not affected by the charge distribution surrounding the ion, and

it is used as the reference transition for determination of Judd–

Ofelt parameters. Analyzing the 5D0 / 7F4 transition, it is

possible to evaluate the interaction between Eu(III) ion sites in the

crystal structure of the host matrix.6 The so-called Judd–Ofelt

intensity parameters7,8 (Ul) are another way to obtain informa-

tion about the chemical properties of the lanthanide(III) envi-

ronment. The U2 parameter is related to the local symmetry and

the U4 parameter is related to the long range effects.

The luminescent materials have a basic structure, composed of

a host matrix and an activator ion, that is the luminescent center.

In some cases, a sensitizer is used to improve the activator

luminescence, and also to produce the up-conversion phenom-

enon.9 These materials are largely used due to the shielding of the

luminescent ion from the outside environment.

The most common host matrixes are fluorides and oxy-

fluorides, mainly LaF3 and LaOF, due to their low phonon

energy, high chemical stability, high refractive index and high

transparence in the UV-Vis-NIR region. Bischoff et al.10

prepared and studied the optical properties of LaF3 thin films in

CaF2 substrates and demonstrated the high refractive index and

low extinction coefficient of the samples, being suitable for

optical applications. Buchal et al.11 studied Er(III)-doped thin

films of LaF3, LuF3 and YF3 for photoluminescence and found

LaF3 as the most appropriate host matrix for the Er(III) ion,

being largely used in up-conversion applications. Armelao et al.12

prepared LaOF thin films and concluded that they have potential

for visible and infrared emitting devices. Lanthanide-doped LaF3

core–shell nanoparticles for waveguide amplifiers (Ln(III)-doped

LaF3 core/LaF3 shell)13,3 and water dispersible nanoparticles14–17

are well studied and their luminescent properties are well estab-

lished. Thin silica films containing citrate stabilized trivalent

Rare-Earth – RE(III) doped LaF3 nanoparticles were studied by

Sudarsan et al.17 and they found that the incorporation of RE(III)

in these nanoparticles is better than the direct incorporation of

RE(III) ions on the thin silica films, because the LaF3 host matrix

shields these ions from the high phonon and O–H oscillators of

the sol–gel SiO2 matrix. Besides that, this method prevents the

prejudicial clustering of the RE(III) ions. However, the thin films

are not composed only of LaF3 nanoparticles, as a lanthanum

silicate phase was detected by XRD on film samples treated at

900 �C in an air atmosphere. Sivakumar et al.9,18,19 reported

white, red, green and blue light emission from Ln(III) doped LaF3

nanoparticles co-doped with Yb(III) ions in thin silica films

thermally treated for 12 h in an air atmosphere. They concluded

that this material has potential for application in waveguide

amplifiers, light emitting appliances, etc. Fujihara et al.20

prepared LaF3 crystals from lanthanum tris-trifluoroacetate in a

sol–gel silica matrix and noted that particle growth was pre-

vented when they were inside the SiO2 matrix, and they obtained

10–30 nm nanoparticles. Their TGA/DTA analysis showed the

possibility of the crystallization of La2O3 with LaF3 around

380 �C, but the XRD technique did not detect the oxide phase.

This work proposes a simple route to prepare well dispersed

Eu(III)-doped LaF3/LaOF nanoparticles, submitting the Eu(III)-

24110 | J. Mater. Chem., 2012, 22, 24109–24123

doped-lanthanum tris-trifluoroacetate to different thermal

treatment conditions and characterizing the resulting material

before and after dispersion in thin sol–gel silica films. The sol–gel

method was used by Sigoli et al. for the preparation of rare earth-

doped SiO2:GeO221 and SiO2:HfO2

22 films for application as

planar waveguides, and this method combined with rapid

thermal treatment under a nitrogen atmosphere was used in the

present work to prepare Eu(III)-doped LaF3/LaOF-containing

thin silica films. It was possible to study their luminescent

properties and identify a mixed phase of LaF3 and LaOF from

luminescence and Raman spectroscopies.

Experimental section

Eu(III)-doped LaF3 and LaOF particles were prepared by

thermal treatment of the Eu(III)-doped lanthanum tris-tri-

fluoroacetate (15 mol%), prepared as described by Ribeiro et al.23

To 0.8146 g (2.5 mmol) of La2O3 (Aldrich, 99.9%) and 0.3663 g

(1.0 mmol) of Eu2O3 (Aldrich, 99.9%) was added a stoichio-

metric amount of HCl (Synth, 36.5–38%) in order to obtain

0.2 mol l�1 and 0.1 mol l�1 LaCl3 and EuCl3 solutions, respec-

tively, which were used to obtain Eu(III)-doped LaOHCO3

through a homogeneous precipitation with urea in aqueous

medium. To the Eu(III)-doped LaOHCO3 were added a stoi-

chiometric amount of CF3COOH (Aldrich, 98%) and water. This

solution was stirred and heated to 80 �C until the resulting

powder was completely dry. The Eu(III)-doped lanthanum tris-

trifluoroacetate was treated in different atmospheres, at different

temperatures and for different time periods as follows: (1)

synthetic air (conventional furnace, EDG10P-S) – 650 �C/10 min;

900 �C/10 min; 1100 �C/10 min and (2) nitrogen (tubular furnace

EDG10P-S) – 650 �C/1 min followed by 900 �C/2 min; 650 �C/1min followed by 1100 �C/2 min. The thermal treatments under

nitrogen atmosphere were carried out in order to simulate and

compare the thermal treatments that the fluoride particles would

be exposed to during the densification process of the thin silica

films when they are inserted on thin films and when they are as

isolated particles.

Silica nanoparticles were obtained by the sol–gel method,22

using the basic hydrolysis of tetraethyl orthosilicate (TEOS) in

ethanol. The thin silica films were obtained over a silicon

substrate by spin coating. The densification of the films was

carried out using a rapid thermal treatment with a conventional

tubular furnace (EDG10P-S) and a rapid treatment process

furnace (RTP-Jipelec First 150) at 650 �C/5 min, 900 �C/2 min,

1100 �C/2 min, and 1150 �C/2 min.

The powder and thin film samples were characterized by FT-

IR (Bomen FTLA 2000) in the range of 4000 to 400 cm�1 with 4

cm�1 resolution using KBr pellets for the nanoparticles with

0.5% weight concentration, X-ray diffraction (XRD-7000

CuKa) in the 2q range of 4–70� at the rate of 2� min-1. The

photoluminescent data were obtained at room temperature and

at 77 K in a Fluorolog-3 spectrofluorometer (Horiba FL3-

22iHR320), with double gratings (1200 gr mm�1, 330 nm blaze)

in the excitation monochromator and double gratings (1200 gr

mm�1, 500 nm blaze) in the emission monochromator with a

Xenon lamp ozone free 450 W (Ushio) as radiation source. The

excitation spectra were corrected in real time according to the

lamp intensity and optical system of the excitation

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monochromator using a silicon diode as a reference. The

emission spectra were corrected according to the optical system

of the emission monochromators and the photomultipliers

(Hamamatsu UV-Vis-R928P and NIR-H10330A) responses.

The emission spectrum in the NIR region was obtained by

exciting the Er/Yb co-doped thin silica films at 980 nm using a

laser (CrystaLaser – DL series) at 100 mW. The 5D0 emission

decay curves were measured using a multichannel system (Flu-

oroHub-B) linked with a pulsed 150 W Xenon lamp using the

multichannel system and 1024 channels. The absolute quantum

yields were measured using a Quanta-4 (Horiba F-309) inte-

grating sphere equipped with an optical fibers bundle (NA ¼0.22-Horiba-FL-3000/FM4-3000). The refractive index and

thickness of the thin films were obtained by M-lines prism

coupling (Metricon 2010) using a 1532 nm laser. Confocal

Raman spectroscopy was performed using a Horiba Jobin-

Yvon T64000 with a He–Ne laser at 632.8 nm and 1.5 mW.

Transmission electron microscopy (TEM) images were obtained

in a TEM-MSC (JEOL 2010) with 0.25 nm resolution operating

at 200 kV.

Results and discussion

Eu(III)-doped LaF3 and LaOF samples

Fig. 1 shows the FT-IR spectra of the samples under thermal

treatment in synthetic air at: 400 �C/3h; 500 �C/3h and500 �C/15h.The band at 418 cm�1 in the infrared spectra (Fig. 1) is

attributed to n(La–O) and this band is displaced to higher

wavenumbers as the temperature is raised or the period of the

thermal treatment is longer. This change suggests that the LaOF

phase is present in the samples treated at higher temperatures

and for longer times, compared with the ones with lower

temperatures and short periods of thermal treatment. The

infrared spectra analysis brings no information regarding the

presence of the LaF3 phase, because the n(La–F) is observed

below 400 cm�1.

Fig. 1 FT-IR spectra of Eu(III)-doped nanoparticles obtained by

different thermal treatments.

This journal is ª The Royal Society of Chemistry 2012

Fig. 2a shows the X-ray diffraction of the samples thermally

treated in synthetic air for 10 min and at different temperatures:

900 �C and 1000 �C. Under these conditions only LaOF is

formed. However, different phases are formed depending on the

thermal treatment conditions. At 900 �C the LaOF is formed as a

tetragonal phase (PDF 44-121, space group P4/nmmS)24 and at

1000 �C the phase is rhombohedral (PDF 6-281, space group

R�3mR).25

Fig. 2b shows the diffractograms of the samples thermally

treated under nitrogen atmosphere at 650 �C/1 min followed by

900 �C/2 min and 650 �C/1 min followed by 1100 �C/2 min. In

both cases only the hexagonal LaF3 phase is observed (PDF 32-

483, space group: P�3c1).26 Zalkin et al.27 studied the X-ray

diffraction data for LaF3 monocrystals and found that the LaF3

crystals are trigonal instead of hexagonal, with space group P�3c1

(D3d4), which was confirmed by powder neutron diffraction28 and

Raman29 studies. Carnall et al.30 assumed that LaF3 belongs to

the P�3c1 space group with a point symmetry of La(III) ion as C2.

Fig. 2 X-ray patterns of samples thermally treated in synthetic air at

900 �C/10 min (tetragonal LaOF:Eu(III)) and 1000 �C/10 min (rhombo-

hedral LaOF:Eu(III)) (a) and thermally treated under nitrogen atmo-

sphere at 650 �C/1 min, followed by 900 �C/2 min and 650 �C/1 min,

followed by 1100 �C/2 min (b).

J. Mater. Chem., 2012, 22, 24109–24123 | 24111

Fig. 4 Luminescence spectra ((a) excitation (b) emission) at 77 K of

Eu(III)-doped LaOF nanoparticles obtained by thermal treatment under a

synthetic air atmosphere for 10 min and 900 �C (tetragonal phase) and

1000 �C (rhombohedral phase).

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All the LaF3 and LaOF samples prepared were doped with

Eu(III) ion, because its luminescence properties are well known

as a probe of the chemical environment of a host matrix.31 In

this work, the Eu(III) luminescence is used to study the differ-

ences in the chemical environment in tetragonal and rhombo-

hedral LaOF obtained by thermal treatment under a synthetic

air atmosphere. In this case, only the local symmetry around

the Eu(III) ion is changed while the surrounding ions (O2� and

F�) remain the same. The influence of the surrounding ions

and the charge distribution on the Eu(III) luminescence when

the host matrix is changed from LaF3 to LaOF is used to

monitor the presence of LaOF phase in the samples obtained

by thermal treatment under a nitrogen atmosphere that,

according to the X-ray diffractograms (Fig. 2a), are composed

only of LaF3.

With the crystallographic data, it was possible to obtain the

point group of the Eu(III) ion in the LaOF host matrix in both

phases: C4v for the tetragonal LaOF phase and C3v for the

rhombohedral LaOF phase. The symmetry of the Eu(III) ion in

both phases is shown in Fig. 3.

It is known that lanthanum oxyfluoride may be obtained with

two crystalline phases and some studies have been made

regarding the differences in the luminescence spectra of Eu(III)-

doped LaOF.32 The differences in the Judd–Ofelt parameters

regarding a more symmetric environment of the tetragonal

phase than the rhombohedral one were also analyzed in this

work.

Luminescence and Raman Spectroscopy of the samples treated

under synthetic air atmosphere. Fig. 4 shows the luminescence

spectra of the samples thermally treated in a synthetic air

atmosphere, obtained at 77 K: In Fig. 4a a broad band is

observed around 270 nm that is attributed to a charge transfer

from O2� / Eu3+. This transition is very intense, because it is

allowed by the spin and parity selection rules. The Eu(III) tran-

sitions 7F0 /5D4,

7F0 /5L6 and

7F0 /5D2 are also observed,

but with low intensity compared with the charge transfer one,

since they are forbidden by all selection rules. From the emission

spectra obtained under 270 nm excitation (Fig. 4b) it is possible

to notice different spectral profiles for each sample, with the

hypersensitive 5D0 /7F2 transition being the most affected by

changes in the local symmetry. But the 5D0 /7F1 transition also

has different splitting patterns for each LaOF phase. The 5D0 /7F0 transition has its intensity near to the 5D0/

7F1 transition in

the tetragonal phase, while in the rhombohedral phase, its

Fig. 3 Symmetry of the Eu(III) ions. (a) Tetragonal LaOF – C4v. (b)

Rhombohedral LaOF – C3v.

24112 | J. Mater. Chem., 2012, 22, 24109–24123

intensity is very low, evidence that the J-mixing effect is not very

pronounced in this phase.

In this work it can be observed that the Eu(III) site symmetry

in tetragonal LaOF is C4v, which is more symmetric than the

C3v in rhombohedral LaOF. As powder samples were analyzed,

light scattering from their surface depends on the sample

position in the sample holder and also on particle size, which

makes it difficult to compare the intensity between different

samples. The intensity parameters Ul and the integrated

intensity of the bands in the emission spectra are more faithful

and reproducible parameters than the total intensity, because

they are not dependent on the light scattering from the sample

surface.

The intensity parameters U2 and U4 were calculated using eqn

(1) and (2):33–35

A0J ¼ A01 ��I0J

I01

���s01

s0J

�(1)

where A0J and A01 are the Einstein coefficients of the sponta-

neous emission; I0J and I01 are integral intensities; s0J and s01 are

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the barycenter energies of 5D0 /7FJ and

5D0 /7F1 transitions,

respectively, with l ¼ 2, 4. The A01 value is given as 50 s�1.36

Using the A0J values, it is possible to calculate the intensity

parameters according to eqn (2).33–35 The total sum of the A0J

values (l ¼ 1, 2, 4) give the radiative emission rate (Arad), as

shown in eqn (3).

Ul ¼ 3h-c3A0J

4e2u3ch7FJkU ðJÞk5D0i2

(2)

Arad ¼ A01 + A02 + A04 (3)

where u is the transition centroid and c is the Lorentz local field

correction with the refractive index n ¼ 1.5, given by:

c ¼ n� ðn2 þ 2Þ29

(4)

The values for h7FJkU ðJÞk5D0i2 are 0.0032 and 0.0023 for J ¼2 and J ¼ 4, respectively.35

The parameters R02 and R12 are calculated by the ratio

between the integrated areas (I0J) of5D0 /

7F0/5D0 /

7F2 and5D0 /

7F1/5D0 /

7F2 transitions and this ratio is given by eqn

(5) and (6) as follows:

R02 ¼ I00

I02(5)

R12 ¼ I01

I02(6)

The measurements of the emission lifetime of the metastable5D0 emitting level (s), combined with the values of the radiative

emission coefficient (Arad), allow calculation of the non-radiative

emission rate (Anrad) according to eqn (7):

Atot ¼ 1

s¼ Arad þ Anrad (7)

Using the values of the radiative and non-radiative emission rates

it is also possible to calculate the quantum efficiency (h), using

eqn (8):

h ¼ Arad

Arad þ Anrad

(8)

All these parameters were experimentally obtained for each of

the Eu(III)-doped LaOF crystalline phases, and the values are

shown in Table 1.

In tetragonal LaOF the Eu(III) ions that substitute La(III) ions

are placed in sites that have C4v symmetry, while in rhombohe-

dral LaOF these ions are placed on the less symmetric C3v sites

(Fig. 3). Several papers in the literature show that the U2

parameter is related to the symmetry and theU4 parameter to the

Table 1 Intensity parameters, radiative, non-radiative and total emission ratecrystal phases of LaOF

Sample phaseU2/10

�20

cm2U4/10

�20

cm2 R02 R

Tetragonal LaOF 4.4 5.5 0.224 0.Rhombohedral LaOF 6.6 3.4 0.025 0.

This journal is ª The Royal Society of Chemistry 2012

long range effects and electronic density surrounding the rare

earth ion.32 Some recent evidence has physically demonstrated

that U4 is also affected by the symmetry.37

As the tetragonal LaOF phase has a higher symmetric envi-

ronment (C4v local symmetry) than the rhombohedral LaOF

phase (C3v local symmetry), the 5D0 / 7F2 transition is more

forbidden, and the U2 parameter is expected to be lower than the

rhombohedral one. In fact, it can be observed that the tetragonal

U2 value is lower than the rhombohedral one (Table 1).

The U4 parameter has the opposite tendency compared to U2,

because in higher symmetries, the U4 values are higher than in

lower symmetries. The tetragonal LaOF is more symmetric than

the rhombohedral LaOF phase, and that is reflected in a higher

value of U4, as expected, according to what has already been

observed in other Eu(III)-doped inorganic samples by Ferreira

et al.37 The experimental data in Table 1 seem to be in agreement

with these tendencies.

The 4f–4f intensity theory38 is a very good approach to

determine the polarizability and charge factors around lantha-

nide ions and also to explain the values of experimentally

observed Judd–Ofelt intensity parameters. The 4f–4f intensity

theory is widely used for lanthanide complexes.39–42

The calculation of the intensity parameters was performed for

LaOF:Eu3+ in rhombohedral and tetragonal phases in order to

compare the effects of point symmetry on Judd–Ofelt intensity

parameters and the correlation with polarizability. The coordi-

nation polyhedron and spherical polar coordinates were

obtained from crystallographic information24,25 and the polar

spherical coordinates for LaOF:Eu3+ in rhombohedral and

tetragonal phase are shown in Tables S1 and S2, respectively, in

the ESI†.

According to the theory of 4f–4f intensities,38 the Judd–Ofelt

intensity parameters are given by:

Ul ¼ ð2lþ 1ÞXt; p

��Bltp

��2ð2tþ 1Þ (9)

where Bltp contains the contributions of the electric dipole (ed)

and dynamic coupling (dc) mechanisms: Bltp ¼ Bedltp + Bdc

ltp. The

electric dipole is generated by the presence of the charges around

the lanthanide ion and the dynamic coupling is generated by the

interaction of the charges and electromagnetic radiation due to

the polarizability of ligands atoms. Therefore, the function Bltp is

calculated using eqn (10):38

Bltp ¼ 2

DE

�rtþ1

�qðt; pÞgt

p

��ðl� 1Þð2lþ 3Þ

ð2lþ 1Þ�hrtirð2bÞlþ1

�f kCðlÞkf �Gt

pdt;lþ1 (10)

s, lifetime of emitting state 5D0 and quantum efficiency values for different

12 Arad/s�1 Anr/s

�1 Atot/s�1 s/ms h/%

422 252.9 820.1 1073.0 0.93 23.6281 286.7 2100.0 2387.0 0.42 12.0

J. Mater. Chem., 2012, 22, 24109–24123 | 24113

Fig. 5 Emission decay curves of the 5D0 metastable state of europium-

doped LaOF in tetragonal and rhombohedral phases.

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The factor r(2b)l+1comes from the simple overlap model.38

From that model it is stated that the charges are points distrib-

uted to half-length of the metal–ligand atom bond distance and

are proportional to the overlap between ligand and 4f wave-

functions. In eqn (10) two other functions appear: gtp (odd-rank

ligand field parameters) and Gtp (ligand polarizability dependent

term). These equations are:38

gtp ¼

Xj

�4p

2tþ 1

�1=2

e2gjYt;p

Rtþ1j

(11)

Gtp ¼

�4p

2tþ 1

�1=2 Xj

aj

Yt;p

Rtþ1j

(12)

where gj represents the charge factor and aj represents the

polarizability of the jth ligand atom. The spherical coordinates

are obtained from Tables S1 and S2 from ESI.† The parameters a

(polarizability) and g (charge factor) can be determined by

adjusting the intensity parameters obtained theoretically with the

experimental ones. The results obtained for the chemical bond

polarizabilities (a), charge factors (g) and the calculated Judd–

Ofelt intensity parameters (U), are shown in Table 2.

The U2 values obtained from the 4f–4f intensity theory are in

very good agreement with the experimental ones (Table 1) in

both cases, while the U4 values are not in good numerical

agreement but have the same tendency when compared with

experimental ones. These results show that the changes of the

point symmetry around Ln3+ change the intensity parameters

(U2 andU4) but the polarizability of the system remains the same.

This observation illustrates the intensity parameter dependency

on the point symmetry around Ln3+ and the difficulty of corre-

lating these parameters with the covalence of the chemical bonds.

Regarding the 5D0 metastable state emission lifetime (Fig. 5),

as rhombohedral LaOF has lower symmetry than tetragonal

LaOF, the mixing between the f and d wavefunctions may be

higher, relaxing the Laporte selection rule. And this could be one

reason that the emission lifetime of the emitting state becomes

shorter than the one for the tetragonal LaOF phase. Grzyb and

Lis32 also observed the same tendency and by studying the rise

time of tetragonal and rhombohedral phases, they concluded

that the two phases have different rise time mechanisms, justi-

fying the differences in the lifetime decay values that are found

for each of the LaOF phases.

The Raman spectra of tetragonal and rhombohedral phases

are presented in Fig. 6. The spectra profile is very different for

each phase. Using factor group analysis, for tetragonal LaOF

with C4v symmetry, six Raman active modes are expected: A1g +

2B1g + 3Eg.43 From these, five modes are clearly observed from

100 to 450 cm�1. H€olsa et al.43 observed almost the same profile

Table 2 Theoretical values of intensity parameters

Sample phase

4f–4f intensity theory

a/�A3 g/�A

Tetragonal LaOF 3.8 (F�) 1.15 (O2�) 1.0Rhombohedral LaOF

24114 | J. Mater. Chem., 2012, 22, 24109–24123

for non-doped LaOF tetragonal phases, but in their spectrum the

mode near 270 cm�1 has a higher intensity than the modes near

125 and 225 cm�1, while in our Raman spectra, the opposite

intensity ratio was observed. This difference is probably due to

the distortion on the crystal structure of LaOF when Eu(III) ion is

inserted into the host matrix. For rhombohedral LaOF, the six

Raman active modes that are expected are 3A1g and 3Eg44 and

from these, five modes are observed from 100 to 600 cm�1. No

Raman spectra of rhombohedral LaOF were found in the liter-

ature, so it was not possible to compare the relative intensity of

the modes between our samples and a non-doped LaOF sample.

Luminescence, Raman Spectroscopy and TEM of the samples

treated under a nitrogen atmosphere. The excitation spectra

monitoring the 5D0 /7F1 Eu(III) transition at 590 nm (Fig. 7a)

present the typical Eu(III) transition lines and no charge transfer

band is detected. In the excitation spectra monitored for the 5D0

/ 7F2 Eu(III) transition at 610 nm (Fig. 7c), besides the Eu(III)

lines, it is possible to observe the beginning of a charge transfer

band around 260 nm. In the emission spectra, excited at 396 nm

(Fig. 7b) the 5D0 /7F1 transition has a higher intensity than the

5D0 /7F2 transition. On the other hand, the emission spectra

under 270 nm excitation (Fig. 7d) show exactly the opposite, and

the Eu(III) emission profile is similar to that observed for the

LaOF samples discussed above. These results suggest that LaOF

is present in LaF3 samples. The decomposition of tris-tri-

fluoroacetate under the conditions described leads to a mixed

phase of LaF3 and LaOF that can be detected by luminescence

3 U2/10�20 cm2 U4/10

�20 cm2

(F�) 2.0 (O2�) 4.4 0.566.6 0.41

This journal is ª The Royal Society of Chemistry 2012

Fig. 6 Raman scattering spectra of tetragonal and rhombohedral Eu(III)-

doped LaOF obtained with a laser excitation at 632.8 nm, P ¼ 1.5 mW.Fig. 8 Symmetry surrounding the Eu(III) ions in LaF3 host matrix.

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spectroscopy, but not by X-ray diffractometry (Fig. 2b), indi-

cating the formation of a small amount of LaOF phase. The

reason for the presence of LaOF is that even under a nitrogen

atmosphere some La–O chemical bonds may stay intact after the

thermal decomposition of the precursor.

As previously discussed, the point symmetry of La(III) in LaF3

is C2, but this point symmetry could also be approximated by a

C2v or D3h when LaF3 is doped with Eu(III). Using crystallo-

graphic information, it was possible to obtain the space group for

the Eu(III) ion on LaF3 host matrix (Fig. 8). As can be observed

in the emission spectra of Fig. 7b, the forced electric dipole

Fig. 7 Luminescence spectra at 77 K of Eu(III)-doped samples obtained by the

and (c) excitation; (b) and (d) emission.

This journal is ª The Royal Society of Chemistry 2012

transition 5D0 /7F2 is strictly forbidden, as expected for a site

symmetry with an inversion centre, but none of these point

symmetries has an inversion centre. The explanation relies on the

fact that the electric dipole and the dynamic coupling mecha-

nisms that influence the intensity of the 5D0 /7F2 transition are

very low in the ionic environment of LaF3 host matrix. This fact

makes LaF3 a suitable host matrix for the rare earth emission

levels calculation, as already discussed by Carnall et al.30

The intensity parameters U2 and U4 were also calculated for

each sample from the emission spectra obtained at room

temperature (data not shown), and the values obtained are given

rmal treatment under a nitrogen atmosphere at different temperatures. (a)

J. Mater. Chem., 2012, 22, 24109–24123 | 24115

Table 3 Intensity parameters, radiative, non-radiative and total emission rates, lifetime of emitting state 5D0 and quantum efficiency values for differentsamples of Eu(III)-doped LaF3

Sample lexc

U2/10�20

cm2U4/10

�20

cm2 R02 R12 Arad/s�1 Anr/s

�1 Atot/s�1 s/ms sa

a/ms h/%

900 �C 396 0.9 0.1 — 1.997 79.2 76.2 155.0 6.43 — 51.0900 �C 270 2.1 0.3 0.099 0.861 116.0 550.7 666.7 0.85/2.83 1.50 17.41100 �C 396 1.1 0.2 — 1.578 87.6 30.6 118.2 8.46 — 74.11100 �C 270 2.1 3.0 0.098 0.857 174.7 474.7 649.4 0.78/5.43 1.54 26.9

a sa ¼ average lifetime.

Table 4 Theoretical values of intensity parameters and experimentalones

Sample

4f–4f intensity theory

a/�A3 g/�A3 U2/10�20 cm2

U4/10�20

cm2

1100 �C 1.5 (F�) 1.0 (F�) 1.1 0.11

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in Table 3. It is known that Eu(III)-doped LaF3 has a higher ionic

character than Eu(III)-doped LaOF, but in both compounds, the

Eu(III) site symmetry does not have an inversion center. As

already discussed, the 5D0 /7F2 transition is weak in LaF3 due

to the high ionic character, and this is reflected in the intensity

parameters U2 and U4 that are expected to be lower for LaF3

than for LaOF. In LaF3 samples, the selective excitation suggests

that two phases (LaF3 and LaOF) are present, so the intensity

parameters are expected to have the contribution of both. Two

lifetime values are also expected. To calculate the average lifetime

eqn (13) was used:

sa ¼ ðC1 � s1Þ þ ðC2 � s2ÞC1 þ C2

(13)

where C1 and C2 are the coefficients of the two lifetime values (s1and s2 respectively) obtained from second order exponential

decay.

Using this approximation of the lifetime value and eqn (7) and

(8), the total emission rate, the non-radiative emission rate and

the quantum efficiency were calculated for the samples with two

lifetime values.

Analyzing the samples treated at 900 �C with different exci-

tation wavelengths it can be observed that for excitation at 396

nm, U2 and U4 have lower values than for excitation at 270 nm

(Table 3). As already discussed, higher values of these parameters

are associated with the presence of a LaOF phase. The intensity

parameters have a greater contribution of the Eu(III) ions in the

LaF3 phase when these samples are excited at 396 nm, while for

270 nm excitation, the intensity parameters also have the

contribution of Eu(III) ions in the LaOF phase. More evidence

for this proposition is that with 270 nm excitation the 5D0 /7F0

transition can be observed because of the J-mixing effect, which

is more pronounced in LaOF than in LaF3.

For samples treated at 1100 �C, the same tendency is observed.

Again, the lifetime measurements show that with 396 nm exci-

tation only one phase is observed (LaF3:Eu(III)) while under 270

nm excitation, there are at least two different Eu(III) ions

(LaF3:Eu(III) and LaOF:Eu(III)).

The theoretical calculation of the intensity parameters was

also performed for LaF3, similar to the LaOF samples, in order

to compare the experimental values with the theoretical ones.

The coordination polyhedron and spherical polar coordinates

were obtained from crystallographic information,24 and the polar

spherical coordinates for LaF3 hexagonal phase are shown in

Table S3 of ESI.†

The U2 and U4 values obtained from 4f–4f intensity theory are

in very good agreement with the experimental ones (Table 3),

24116 | J. Mater. Chem., 2012, 22, 24109–24123

showing that the 4f–4f intensity theory can also be applied to

ionic solids and not only to complexes with some degree of

covalence (Table 4).

The lifetimes of the Eu(III) 5D0 metastable state in the LaF3

phase (measured with 396 nm excitation) were very high (6.43

and 8.46 ms for the 900 and 1100 �C samples, respectively), due

to the low phonon energy of the LaF3 host matrix. The lifetime

decay curves are given in Fig. 9. The lifetime and quantum effi-

ciency values are high (Table 3). Sudarsan et al.31 found lifetime

values of the 5D0 metastable state as 6.01 ms and 4.5 ms for di-n-

octadecyldithiophosphate ligand stabilized 5% Eu(III) and 10%

Eu(III) doped LaF3 nanoparticles, respectively. The values

determined in this work are higher than those, even for samples

doped with 15 mol% of Eu(III). However this method produces

particles with a mixed phase of LaF3 and LaOF, as previously

discussed, and not pure LaF3 nanoparticles, as indicated in

ref. 31.

The comparison between the intensity parameters, emission

lifetime values, absolute quantum yield and quantum efficiencies

for the pure LaOF tetragonal phase and for the sample with the

mixed phase of LaF3 and LaOF is shown in Table 5. The abso-

lute quantum yields of the samples were obtained by direct

excitation on the Eu(III) ion transition 7F0 /5L6 at 396 nm. The

LaF3 present in the mixed phase has very low phonon energy,

contributing to a high lifetime, compared with pure LaOF.

Besides, non-radiative processes are minimal in matrixes with

low phonon energy such as LaF3, resulting in a high value of the

quantum efficiency (See eqn (7) and (8)), compared with LaOF.

However, the absolute quantum yield of Eu(III)-doped LaF3 is

low compared with the one for LaOF. The absolute quantum

yield is a measure of the efficiency of the whole emission process,

beginning with the sample excitation followed by the energy

decay to the emitting state, from where the emission occurs. On

the other side, the quantum efficiency is a measure of the emis-

sion efficiency from the emitting state, in this case, the 5D0 level

of Eu(III) ion. For the Eu(III)-doped LaF3, the absolute quantum

yield is low (1.8%), but the quantum efficiency is high (74.1%),

This journal is ª The Royal Society of Chemistry 2012

Fig. 9 Emission decay curves of the 5D0 metastable state of europium-

doped samples obtained under different excitation wavelengths.

Table 5 Intensity parameters (U2 and U4), emission lifetime values,absolute quantum yield and quantum efficiencies of tetragonal LaOFunder 270 nm excitation and the mixed phase of the LaF3 and LaOF(sample treated at 1000 �C in a nitrogen atmosphere) under 397 nmexcitation

SampleU2/10

�20

cm2U4/10

�20

cm2 s/ms 4abs/% h/%

Tetragonal LaOF 4.4 5.5 0.93 16.7 23.6LaF3 1100

�C 1.1 0.2 8.46 1.8 74.1

Fig. 10 Raman scattering spectra of the samples obtained by thermal

treatment under nitrogen atmosphere at different temperatures. Bulk

LaF3 and bulk LaF3:Eu(III) spectra are also presented. P ¼ 1.5 mW.

Fig. 11 Raman scattering spectra of the sample obtained at 900 �C and

of bulk LaF3:Eu(III) as well as tetragonal and rhombohedral LaOF,

showing phase mixing between LaF3, tetragonal LaOF and rhombohe-

dral LaOF. P ¼ 1.5 mW.

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suggesting that the energy transfer from the 5L6 level to the 5D0

level is not efficient, but the emission from the 5D0 is very effi-

cient. Probably these results indicate that the Eu(III)-doped LaF3

possesses more defects in the crystal structure than Eu(III)-doped

LaOF sample providing an alternative route for non-radiative

decay from 5L6 during the ion–ion energy transfer. It should be

noted that the absolute quantum yield for both samples were

obtained by direct excitation at the 7F0 /5L6 transition of the

Eu(III) ion, while the quantum efficiency for LaOF was obtained

by excitation on the O2� / Eu3+ charge transfer band. As

already discussed, the U2 and U4 values are lower for LaF3 than

for LaOF phase because of the high ionic character of LaF3, and

the presence of LaF3 in the mixed phase justifies the lower values

of these parameters, compared with the ones of pure LaOF.

Raman spectroscopy brings complementary information

about the mixed phase of the samples. The Raman spectra of the

LaF3 samples thermally treated at 900 and 1100 �C, as well asbulk LaF3 and bulk LaF3:Eu(III), are given in Fig. 10.

Comparing the bulk LaF3 and bulk LaF3:Eu(III) spectra, it is

interesting to observe that there is a significant change in the

relative intensities of the bands around 360 and 400 cm�1. In the

bulk LaF3 spectra, the band around 360 nm is more intense than

the one at 400 cm�1, and this intensity ratio is inverted when the

samples are doped with Eu(III). The introduction of Eu(III) ion

results in small distortions of the crystal structure that are

reflected in the Raman spectra. Another interesting aspect of the

LaF3 samples treated at 900 �C and 1100 �C is that the band

This journal is ª The Royal Society of Chemistry 2012

around 400 nm for both of them is very large. This observation

suggests that LaOF phase is also present, corroborating with the

luminescence spectroscopy data previously discussed.

In Fig. 11 and 12 the Raman scattering spectra of LaF3

samples treated at 900 and 1100 �C, respectively, are given

separately to better analyze the mixed phase.

Observing the spectra in Fig. 11 and 12 it is clear that the

900 �C sample (Fig. 11) is composed of a mixed phase of LaF3,

tetragonal LaOF and rhombohedral LaOF, while the 1100 �Csample is composed mainly of LaF3 and rhombohedral LaOF.

The tetragonal LaOF phase is formed before the rhombohedral

one. So at the thermal treatment of the precursor at 650 �C/1 min

followed by 900 �C/2 min, some La(III) tris-trifluoroacetate was

converted to LaF3 and an amount of that was converted to

LaOF, part of which remained as tetragonal LaOF while the

other part was converted into rhombohedral LaOF. The

J. Mater. Chem., 2012, 22, 24109–24123 | 24117

Fig. 12 Raman scattering spectra of the sample obtained at 1100 �C and

bulk LaF3:Eu(III) as well as tetragonal LaOF, showing phase mixing

between LaF3 and rhombohedral LaOF. P ¼ 1.5 mW.

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presence of 3 crystalline phases in the sample treated at 900 �Ccauses a non-homogeneous enlargement of the emission bands of

this sample, mainly when the excitation is made at 270 nm

(Fig. 7d). Regarding the sample treated at 650 �C/1 min followed

by 1100 �C/2 min, the Raman spectra suggests the presence of a

Fig. 13 Transmission electron microscopy of the sample treated at 650�C/1 min followed by 1100 �C/2 min. (a) Low magnification image. (b)

High magnification image.

24118 | J. Mater. Chem., 2012, 22, 24109–24123

mixed phase between LaF3 and rhombohedral LaOF (see the

band at 500 cm�1) that is formed at high temperatures.

Transmission electron microscopy (Fig. 13) of the Eu(III)-

doped LaF3 samples treated at 650 �C/1 min followed by

1100 �C/2 min shows that after the thermal treatment the parti-

cles are very crystalline. There are large particles, as it can be seen

in Fig. 13a, but there are some regions composed of small

nanoparticles that are crystalline. Depending on the region, the

indexed planes may correspond to (0 1 2) planes of rhombohe-

dral LaOF (PDF 06-281) or (�1 2 1) planes of LaF3 (PDF 32-

483), corroborating with the luminescence and Raman spec-

troscopy data, suggesting that these samples are composed by a

mixed phase of LaF3 and rhombohedral LaOF.

Eu(III)-doped LaF3 and LaOF nanoparticle-containing thin silica

films

The samples were prepared as described in the experimental

section, and the densification process was carried out at 900 �C,1100 �C and 1150 �C. Fig. 14 shows the FT-IR spectra of these

samples and the Si–O–Si45 stretching at 1090 cm�1 and defor-

mation at 807 and 465 cm�1 can be observed. No information

regarding LaOF or LaF3 phases is observed, because La–O and

La–F stretching appears below 400 cm�1. Moreover, no band at

620 cm�1, attributed to a-cristobalite, is observed, showing that

the rapid thermal treatment does not crystallize the thin silica

films.46 Based on the detection of the band at 960 cm�1, this

treatment is also effective for elimination of the silanol groups, as

the band disappeared when the samples were thermally treated

above 900 �C.The X-ray diffractograms also show no information about

LaF3 and LaOF phases in the thin films due to the very low

concentration, it being difficult to detect them by this technique.

Nevertheless, their X-ray diffractograms are shown in Fig. 15

and the absence of a peak at 2q ¼ 22� due to the a-cristobalite

phase is in agreement with the infrared spectral data that the thin

silica films are not crystalline.

Fig. 16 presents the luminescence spectra of thin silica film

thermally treated at 650 �C for 5 minutes. The excitation spectra

Fig. 14 Infrared spectra of the LaF3:Eu(III)-containing thin silica films

under different thermal treatment conditions.

This journal is ª The Royal Society of Chemistry 2012

Fig. 15 Diffractograms of Eu(III)-doped LaF3 nanoparticle-containing

thin silica films at different thermal treatments.

Fig. 16 Luminescence spectra of the nanoparticle-containing thin silica

films thermally treated at 650 �C for 5 minutes. (a) Excitation spectrum

(lem ¼ 613 nm). (b) Emission spectrum (lexc ¼ 250 nm).

This journal is ª The Royal Society of Chemistry 2012

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of the thin films were obtained by monitoring the emission band

at 613 nm. The emission spectra were recorded with 250 nm

excitation. Fig. 17 presents the luminescence spectra of silica thin

film thermally treated at 1150 �C for 2 minutes.

The excitation spectra for the sample before (Fig. 16a) and

after the densification process (Fig. 17a) present part of a charge

transfer transition band. The Eu(III) transitions 7F0 /5L6 (392

nm) and 7F0 / 5D2 (462 nm) can be seen only with an ampli-

fication of the region between 380 and 500 nm, which is shown as

an inset in Fig. 17a. The emission spectra of these samples have

almost the same profile characteristics of the Eu(III) ion in a

chemical environment where the forced electric dipole 5D0/7F2

transition is more intense than the allowed magnetic dipole 5D0

/ 7F1 transition. The data suggest the presence of a LaOF phase

in the thin silica films, before the densification process. The

emission spectra show broad emission bands because the sample

is composed by nanoparticles where most of the Eu(III)

Fig. 17 Luminescence spectra of the nanoparticle-containing thin silica

films thermally treated at 1150 �C for 2 minutes. The inset shows a

magnification of the same spectrum with some Eu(III) transitions. (a)

Excitation spectrum (lem ¼ 613 nm) and (b) emission spectrum (lexc ¼250 nm).

J. Mater. Chem., 2012, 22, 24109–24123 | 24119

Fig. 19 Thicknesses and refractive indexes as functions of the temper-

ature of thermal treatment.

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crystallographic sites are located at the surface and probably are

not well defined, leading to a non-homogeneous broadening of

the emission bands. The same effect was expected to occur with

the Er(III)/Yb(III) co-doped thin silica films. However, as shown

with the Er(III) emission at 1550 nm (Fig. 18) by excitation at

980 nm, the full width at half maximum (FWHM) is around 30

nm. Regarding the potential application as erbium-doped

waveguide amplifiers, Stouwdam and van Veggel47 obtained a

FWHM of 70 nm for 5% Er(III)-doped LaF3 nanoparticles;

Sudarsan et al.17 obtained FWHM equal to 69 and 56 nm for

citrate stabilized Er(III)-doped LaF3 nanoparticles pure and

dispersed in thin silica films, respectively. Sigoli et al.22 obtained

FWHM equal to 34 nm for Er(III)/Yb(III) co-doped thin

SiO2:HfO2 and more recently Cunha et al.48 have found a

considerably low FWHM for Er(III)/Yb(III) co-doped thin

SiO2:ZrO2 films. All these systems were described as candidates

for application in devices aiming at the amplification of the

1.5 mm telecommunication window. Therefore, the Er(III)/Yb(III)

doped thin silica films presented in this work have comparable

values of FWHM to those found in the literature indicating that

these films have some potential for the development of erbium-

doped waveguide amplifiers.

Prism coupling measurements were done in order to obtain the

thicknesses and the refractive indexes of the thin silica films

thermally treated under different conditions, in order to check

which of these conditions is more suitable to obtain a dense thin

film. The thickness and the refractive index values and their

tendency with temperature variation are given in Fig. 19.

As the thermal treatment temperature is raised, a decrease of

the film thickness is observed, because as-prepared films are

porous (thermal treatment at 650 �C for 5 min) and, as they

undergo the thermal treatment, their porosities as well as their

thicknesses decrease. From Fig. 19, one may observe a small

increase of the refractive index and a small decrease of the

thickness from 650 �C to 1100 �C followed by an abrupt decrease

of the thickness and increase of the refractive index from 1100 �C

Fig. 18 Photoemission spectrum (lexc ¼ 980 nm laser at 100 mW) of the4I13/2 / 4I15/2 transition of the 5% Er(III)/10% Yb(III)-doped LaF3

nanoparticles-containing thin silica film thermally treated at 1100 �C for

30 seconds.

24120 | J. Mater. Chem., 2012, 22, 24109–24123

to 1150 �C, showing that the complete densification is reached

only at 1150 �C. The final film thickness is 2.05 mm and the

refractive index is 1.4573. Considering the refractive index of

pure silica at 1536 nm (h ¼ 1.4444) and comparing it with the

value obtained with the densified thin film (h ¼ 1.4573), it is

possible to observe that the thin film has a higher refractive index

than pure silica (Dh ¼ 0.0130), which means that light would be

guided in these films, and they could be used as waveguides for

photonic applications.49 These light guiding properties will be

explored in the future.

The refractive index of the thin film depends not only on the

thermal treatment conditions or the porosity degree of the film,

but also on its composition. The films were prepared choosing a

composition with a molar fraction of 0.55 mol of LaF3, that was

supposed to result in a refractive index equal to 1.4521, as

calculated from the Lorentz–Lorenz equation (eqn (14)):

h2 � 1

h2 þ 2¼ fa

ðh2a � 1Þ

ðh2a þ 2Þ þ fb

ðh2b � 1Þ

ðh2b þ 2Þ (14)

where: h represents the final refractive index of the film; ha and hbrepresent the refractive indexes of the a and b components (SiO2

and LaF3); fa and fb represent the molar fractions of the a and b

components.

The expected values of refractive index as a function of the film

composition, calculated from the Lorentz–Lorenz equation, are

given in Fig. 20. These values were calculated if the thin films

were composed only of SiO2 and LaF3 or SiO2 and La2O3. The

refractive index of the thin film sample treated at 1150 �C is

intermediate between the curves, which is further evidence that

this sample is composed of at least some LaOF or, as already

observed in the powder samples, of a mixed phase of LaF3 and

LaOF. As already discussed, the luminescence spectra suggest

only the presence of LaOF phase. Due to the low thickness of the

film samples, it was not possible to verify if the mixed phase is

really present using Raman spectroscopy as it was done for the

powder samples. However, the TEM images obtained for the

thin films (Fig. 21 and 22) bring some information regarding

phase formation.

This thin film sample was not densified, so the crystalline

phases dispersed are not well formed, and the crystalline planes

This journal is ª The Royal Society of Chemistry 2012

Fig. 20 Refractive index values of the LaF3 and La2O3 phases based on

the Lorentz–Lorenz equation and the experimental values.

Fig. 21 Transmission electron microscopy of the nanoparticle contain-

ing a non-densified thin silica film (650 �C 5 min). (a) Low magnification

image. (b) High magnification image.

Fig. 22 Transmission electron microscopy of the nanoparticle contain-

ing densified thin silica film (1150 �C 2 min). (a) Low magnification

image. (b) High magnification image.

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cannot be seen clearly, but they could still be indexed to the

correspondent patterns of hexagonal LaF3. As can be observed

in Fig. 21a, the nanoparticles are well dispersed and most of them

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are very small. As shown in Fig. 21b, they are near 10 nm in size.

Analyzing the crystallographic planes of the particles, it was

found that some of them correspond to the (0 3 0) plane of the

LaF3 phase (PDF 32-483), with an interplanar distance of 2.12�A,

while others correspond to the (�1 2 0) planes of rhombohedral

LaOF (PDF 06-281), as shown in Fig. S1 of ESI,† with an

interplanar distance of 2.03 �A, suggesting that the thin films are

also composed of a mixed phase between LaF3 and rhombohe-

dral LaOF. However, the TEM image in Fig. 21b shows that a

part of the sample is composed only of the LaF3 phase.

The densified thin silica film has more crystalline nanoparticles

and, comparing Fig. 21 and 22, even after the densification

process the nanoparticles remain well dispersed and small, which

is advantageous because no stabilizing agent was used. The

nanoparticles remained near 10 nm in size on the thin silica film.

Again, the attribution of crystalline planes in the TEM images

indicates that there is a mixed phase of LaF3, tetragonal and

rhombohedral LaOF (see Fig. S2a and b of ESI†). Comparing

with the powder sample treated at 650 �C/1 min and 1100 �C/2min, the densified thin film (1150 �C/2 min) has smaller nano-

particles, but it contains two LaOF phases that are probably

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present in higher amounts because the luminescence spectros-

copy is typical of Eu(III) in the LaOF phase. Once again, the high

magnification image (Fig. 22b) was taken from a sample area

containing only the LaF3 phase.

Conclusions

Eu(III) doped nanoparticles of pure LaOF in the tetragonal and

rhombohedral phases as well as a mixed phase of LaF3 and

LaOF were prepared, and their luminescence properties were

studied. The analysis of the luminescence properties of the

Eu(III)-doped LaOF gives information regarding the symmetry

of each of the phases. Tetragonal LaOF with La(III) C4v site

symmetry is more symmetric than rhombohedral LaOF with

La(III) C3v site symmetry and this is reflected in a lower value of

U2 and a higher value of U4 for the Eu(III)-doped tetragonal

LaOF. The comparison of the experimental and theoretical

values of U2 and U4 parameters shows that these parameters

depend on the symmetry and are independent of ligand polar-

izability. The mixed phase of LaF3 and LaOF was not detected

by X-ray diffraction measurements, probably because the

quantity of LaOF is low. The latter phase was detected by

luminescence and Raman spectroscopy and in TEM images. The

mixed phase particles have U2 and U4 values lower than the pure

LaOF ones. Raman spectroscopy and TEM images complement

the luminescence spectroscopy data, giving information about

which LaOF phase is present in the samples that present the

mixed phase of LaF3 and LaOF.

These particles were dispersed in thin silica films and a phase

mixing between LaF3 and LaOF phases was present even before

the densification process. The film thicknesses were 2.05 mm and

the refractive index 1.4573. The thin film preparation method

presented here is interesting, because the nanoparticles are well

dispersed and stabilized on the thin films without stabilizing

agents. By luminescence spectroscopy it was not possible to infer

which LaOF phase was present or even identify the LaF3 emis-

sion by selective excitation, indicating that this phase is present in

small amounts in the sample. The emission of Er(III)/Yb(III) co-

doped thin SiO2:LaF3 films in the 1.5 mm region suggests that this

system may be used in future work as an erbium(III)-doped

waveguide amplifier.

Acknowledgements

EMR thanks CAPES for a fellowship and for Crystal Structures

Database availability. IOM and FAS are grateful to CNPq and

FAPESP for financial support. The authors would like to thank

Prof. C. H. Collins (IQ-UNICAMP, Campinas, Brazil) for

English revision and the Multiuser Laboratory of Advanced

Optical Spectroscopy (LMEOA/IQ-UNICAMP/FAPESP 2009/

54066-7) for use of its equipment. This work is a contribution of

the National Institute of Science and Technology in Complex

Functional Materials (CNPq-MCT/Fapesp).

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