Preface
The 15 elements from lanthanum (atomic number 58, 4f 0) to lutetium (atomic
number 71, 4f
14
) are termed as lanthanides. They are simply characterized by an
incompletely filled 4f shell. All the elements in the group have a close chemical
resemblance, which rendered their separation and characterization quite difficult.
The recent upsurge of interest in optical devices, such as lasers and optical
amplifiers, based on electronic transitions of rare-earth ions (lanthanides) has
generated an impressive momentum to work in different materials doped with these
ions. The optical properties of lanthanide ions became important when techniques
were developed to separate the different lanthanide ions to high purity. An
attractive feature of these ions is their luminescent properties. Lanthanide
compounds have the characteristic property of line-like emission, which results in a
high color purity of the emitted light. The emission color depends on the lanthanide
ion but is largely independent of the environment of a given lanthanide ion. Several
lanthanide ions show luminescence in the visible or near-infrared spectral regions
upon irradiation with ultra violet radiation. The color of the emitted light depends
on the lanthanide ion. For instance, Eu3+ emits red light, Tb3+ green light, Sm3+
orange light, and Tm3+ blue light. Yb3+, Nd3+ and Er3+ are well-known for their
near-infrared luminescence, but other lanthanide ions (Pr3+, Sm3+, Dy3+, Ho3+ and
Tm3+) also show transitions in the near-infrared region. Gd3+ emits in the
ultraviolet region, but its luminescence can only be observed in the absence of
organic ligands with low-lying singlet and triplet levels. The lanthanide emission is
due to transitions inside the 4f shell, thus intraconfigurational f-f transitions.
Because the partially filled 4f shell is well shielded from the environment by the
closed 5s2 and 5p6 shells, the ligands in the first and second co-ordination sphere
perturb the electronic configurations of the trivalent lanthanide ions only to a
limited extent. This shielding is responsible for the specific properties of lanthanide
luminescence, more particularly for the narrow-band emission and for the long
lifetime of the excited states.
Materials containing lanthanide ions play a critical role in a wide variety of
technological applications. Their optical applications span the spectral range from
the ultraviolet to the infrared region. Advances in such diverse fields as phosphors
for fluorescent lighting, display monitors, and X-ray imaging; scintillators; lasers;
and amplifiers for fiber-optic communication rely on the development of improved
lanthanide-doped materials that provide enhanced optical performance. The
luminescence efficiency of these materials is often limited by the dynamics of the
lanthanide ion, which depends on its interactions with the insulating host. Through
innovative design and synthesis of the host matrix, the optical response of the
dopant
population
can
be
influenced.
Particularly,
in
lanthanide-doped
nanomaterials successful manipulation of the host structure, while maintaining the
material properties required for device construction and operation, is essential to
refine the material behavior in existing applications and extend the use of the
material into new areas. Moreover, reduction of the particles size in a crystalline
system can result in significant modification of their properties compared to the
bulk due to high surface-to-volume ratio and quantum confinement effect. It is
especially this luminescence property of lanthanides in nano size which fascinated
to carry out the present research work.
The present study reports the synthesis, structural and photoluminescence studies
of rare-earth doped vanadates, phosphates and borates synthesized by coprecipitation technique. Much emphasis has been laid on the study of
photoluminescence properties of various rare-earth ions embedded in different host
matrix. The accompanying change in their properties upon the addition of various
metal ions and change of solvents have been also studied.
Chapter 1 deals with the general introduction of lanthanides and nanoparticles and
the theme of the thesis. This chapter highlights the fact that present day advanced
technology heavily relies on the exciting spectroscopic properties of lanthanide
ii
(rare-earth) ions and more importantly, on the lanthanide doped nanoparticles.
They have the uncommon features of both sharp, weak 4f-4f transitions and broad,
intense 4f-5d transitions. Various methods employed for synthesizing the
nanoparticles and their advantages and disadvantages have also been summarized.
In short, the first chapter briefly highlights the underlying concept for carrying out
the present work.
Chapter 2 gives a brief review on the significance of luminescent trivalent
lanthanide ions and their application in numerous fields; more importantly as
phosphors for fluorescent lighting, display monitors, and X-ray imaging,
scintillators, lasers and amplifiers for fiber-optic communication. In this review, we
focus on the synthesis, characterization and important developments in the field of
lanthanide doped luminescent nanomaterials during the past few decades and wish
to demonstrate that the study of luminescent nanomaterials is still challenging in
spite of its long history.
Chapter 3 serves the plan of the present work. This include the synthesis of rareearth doped nanoparticles using suitable solvents such as ethylene glycol, dimethyl
formamide, water, etc. which can act as capping agent as well as reaction medium,
with special emphasis on their physico-chemical properties, study the effect of
addition of some metal ions with a view to tuning the physico-chemical properties
of the synthesized nanoparticles and the effect of heat treatment on the lanthanide
doped nanoparticles for increased luminescence and targeted applications.
Method of preparation and techniques used for characterizing the samples are
briefly outlined in Chapter 4. Lanthanum vanadate (LaVO4), Cerium phosphate
(CePO4) and Yttrium borate (YBO3) are the three different host used for the present
study with various lanthanide ions viz. Eu3+, Dy3+, Sm3+ and Tb3+ as dopants.
Ethylene glycol, water, N,N'-dimethyl formamide (DMF) and their mixed media
have been used as solvents. The experimental procedure for the preparation of
these nanoparticles has been described in detail. This chapter also provides an
iii
insight into the various instruments or techniques employed for characterizing the
nanoparticles.
Chapter 5 describes the synthesis and characterization of Ln3+ (= Dy, Sm and Eu)
doped LaVO4 nanophosphors. Here, anomalous enhancement in the luminescence
behavior of the samples prepared in water is witnessed which is accountable to the
phase change induced by the strong interaction between water and the surface of
LaVO4 nanoparticles. This study suggests that water plays an important role in the
synthesis of tetragonal rare earth doped LaVO4 nanoparticles. It shows that higher
crystal symmetry produces stronger fluorescence. The luminescence intensity of
Ln3+ (= Dy, Sm, Eu) nanophosphors further enhanced when various metal ions is
introduced as co-activator. The sensitizing effect of various metal ions (Mn+) on
Ln3+ emission has also been studied. Lifetime study of the co-activated samples
and those prepared in water suggests better luminescence properties and might
promote both academic interest in lanthanide chemistry and other novel
applications in nanotechnology. The synthesized nanoparticles can be re-dispersed
in methanol and incorporated in polymer film (PVA). Greenish blue light for Dy3+,
reddish-orange light for Sm3+ and intense red light for Eu3+ doped LaVO4
nanoparticles is observed under UV irradiation. Moreover, these nanomaterials
may find industrial applications due to their properties, simplicity of process, low
cost and availability of raw materials.
Chapter 6 is confined to the study of synthesis and characterization of Dy3+ and
Tb3+ doped CePO4 nanophosphors. Monoclinic and hexagonal phases of Dy3+ and
Tb3+ doped CePO4 nanophosphors have been obtained. Change of solvent in CePO4
system induces different shapes of the nanophosphors. The luminescence intensity
of the samples prepared in EG with monoclinic phase are found to be most
prominent than those prepared in other solvents. While hexagonal phase, obtained
for the samples prepared in water and water mixed solvents, transformed to
monoclinic phase after heating the sample at 900 oC. The luminescence emission
intensity of the as-prepared samples in CePO4:Dy3+ greatly increases after heat
iv
treatment and upon the introduction of metal ions (Li+, Ba2+ and Bi3+). In the case
of CePO4:Tb3+, the luminescence emission intensity of the as-prepared samples
increases significantly after introduction of Li+ ions only while the addition of Ba2+
and Bi3+ reduces the emission intensity. The as-prepared samples are dispersible in
polar solvents and can be incorporated in PVA film. These nanomaterials may find
industrial applications due to their properties, simplicity of process, low cost and
availability of raw materials.
The synthesis and characterization of Eu3+ and Tb3+ doped YBO3 nanophosphors
have been described in Chapter 7. The luminescence intensity of 5D0→7F2
transition at 612 nm (red) is more prominent than that of 5D0→7F1 transition at 588
nm (orange) for all the samples. The strongest emission was obtained at 544 nm
corresponding to 5D4→7F5 (green) transition of Tb3+. Concentration dependent
luminescence study shows that the luminescence intensity is optimum at 7 at.%
and 20 at.% for Eu3+ and Tb3+ respectively and decreases above this due to
concentration quenching. The introduction of M+ ions significantly enhances the
emission intensity of Eu3+ and Tb3+ in YBO3. The samples show strong deep red
emission with very high R/O values suggesting the improved chromaticity upon the
introduction of M+ ions into YBO3 matrix. This is especially vital for a red
phosphor such as YBO3:Eu3+, as their application has always been hampered owing
to its relatively poor chromaticity. The CIE coordinates fits well in the deep red and
green region of the chromaticity diagram, thereby strengthening the result that the
chromaticity is improved upon the introduction of metal ions. The prepared
nanophosphor could therefore serve as a good source of red and green light. The
nanophosphors are well re-dispersed in polar solvents and can be incorporated into
polymer films. The re-dispersible capability of the samples could extend its
application in biological assays, biological fluorescence labelling, etc.
v
In Chapter 8, the important findings of our present study are summarized as
follows:
(i)
Highly crystalline re-dispersible nanophosphors of Eu3+, Dy3+, Sm3+ and Tb3+
doped in different host matrices viz. LaVO4, CePO4 and YBO3 have been
successfully prepared in different solvents within a short duration at relatively
low temperature via co-precipitation method.
(ii)
In the case of vanadates, anomalous enhancement in the luminescence
behaviour of the samples prepared in water is witnessed which is accountable
to the phase change induced by the strong interaction between water and the
surface of LaVO4 nanoparticles.
(iii) The luminescence intensity of Ln3+ (= Dy, Sm, Eu) nanophosphors further
enhanced when various metal ions is introduced as co-activator. The
sensitizing effect of various metal ions (Mn+) on Ln3+ emission varies with
Mn+ concentration.
(iv) Lifetime study of the co-activated samples and those prepared in water
suggests better luminescence properties and might promote both academic
interest
in
lanthanide
chemistry and
other
novel
applications
in
nanotechnology.
(v)
Change of solvents in both Dy3+ and Tb3+ doped CePO4 system induces
different shapes of the nanophosphors. The luminescence intensity of the
samples prepared in EG with monoclinic phase are found to be most
prominent than those prepared in other solvents. While hexagonal phase,
obtained for the samples prepared in water and water mixed solvents,
transformed to monoclinic phase after heating the sample at 900 oC.
(vi) The luminescence emission intensity of the as-prepared samples in
CePO4:Dy3+ greatly increases after heat treatment and upon the introduction
of metal ions (Li+, Ba2+ and Bi3+). In the case of CePO4:Tb3+, the
luminescence emission intensity of the as-prepared samples increases
significantly after introduction of Li+ ions only while the addition of Ba2+ and
Bi3+ reduces the emission intensity.
vi
(vii) In Eu3+ and Tb3+ doped YBO3 host, the intensity of 5D0→7F2 transition of
Eu3+ strongly dominates the emission spectra even prior to the addition of
metal ions which may be attributed to the higher population of Eu3+ ions in
asymmetric environment in the YBO3 matrix. The strongest emission was
obtained at 544 nm corresponding to 5D4→7F5 (green) transition of Tb3+. High
R/O values of YBO3:Eu3+ and co-doped with M+ ions strongly indicates the
improvement in chromaticity of the nanophosphors.
(viii) The CIE coordinates fits well in the deep red and green region of the
chromaticity diagram for YBO3:Ln3+ nanophosphors, thereby strengthening
the result that the chromaticity is improved upon the introduction of metal
ions. The prepared nanophosphor could therefore serve as a good source of
red and green light. It could find application in PDPs.
(ix) The synthesized nanoparticles can be re-dispersed in polar solvents such as
ethanol, methanol and water and incorporated in polymer film (PVA).
Greenish blue light for Dy3+, reddish-orange light for Sm3+, intense red light
for Eu3+ and bright green light for Tb3+ doped LaVO4 nanoparticles is
observed under UV irradiation. The re-dispersible capability of the samples
could extend its application in biological assays, biological fluorescence
labelling, etc. These nanomaterials could find industrial applications due to
their properties, simplicity of process, low cost and availability of raw
materials.
vii