Photon Upconversion Spectroscopy
397
1.5 Combination of Host Lattice and Lanthanides for Photon
Upconversion Process
Lanthanide based UC materials consist of an inorganic host doped with a small
concentration of lanthanide ions, usually in 3+ oxidation state [25]. The dopant
lanthanide ions act as luminescent centre and the host lattice provides a matrix for
spatial arrangement of dopant ions. An optimum combination of host lattice as well
as dopant ions renders highly efficient UC emission. In designing lanthanide-based
UC materials, host matrix is of great importance. The host matrix should be of
low phonon frequency and there should be a close lattice match to the dopant ions.
Phonon frequency plays a vital role in reducing multi-phonon relaxation among
close lying energy levels. Host matrix containing heavy halogenides present low nonradiative losses (phonon energies <300 cm
−1 ) but they are hygroscopic and suffer
from low chemical stability. Metal oxides offer high chemical stability, however, their
phonon frequency is very high (above 500 cm
−1 ). In comparison, fluorides are more
promising host lattice for UC emission due to their low phonon energies (∼500 cm
−1 )
and of high chemical stability [26]. Among all the fluoride UC materials, hexagonal
NaYF 4 has been recognised as the most efficient host matrix for lanthanide UC so far.
A detail of different synthesis processes for such hosts can be found in our previous
work [14].
Trivalent lanthanide ions (such as Tm
3+ , Er
3+ , Ho
3+ , and Tb
3+ ) possess ladder like
energy states and so they have been frequently used as activator ions [12, 13, 19]. The
absorption cross section of Yb
3+ ion (at λ = 980 nm) is of the order of 11.7 × 10
−21
cm
2 which is quite high than any of the lanthanide ions. Therefore, Yb
3+ ion has been
typically used as universal sensitizer for NIR (980 nm) to visible UC processes [19].
The
2 F 7/2 →
2 F 5/2 transition of Yb
3+ is well matched with different transitions present
in Er
3+ , Tm
3+ , and Ho
3+ , therefore, Yb
3+ provides efficient energy transfer to these
ions. Thus, Yb
3+ ion has been typically co-doped with Er
3+ , Tm
3+ , and Ho
3+ as a
sensitizer to get efficient UC emission. Er
3+ and Yb
3+ co-doped system gives intense
green emission at around 520/540 nm due to
2 H 11/2 /
4 S 3/2 →
4 I 15/2 transitions while
a red emission band (at around 650 nm) appears via
4 F 9/2 →
4 I 15/2 transition [12].
Ho
3+ and Yb
3+ co-doped system also gives green and red emissions at 540 nm and
647 nm due to
5 S 2 /
5 F 4 →
5 I 8 and
5 F 5 →
5 I 8 transitions, respectively [13]. Whereas,
Tm
3+ -Yb
3+ co-doped system gives intense IR transition at 800 nm due to
3 H 4 →
3 H 6
transition. Other relatively weak bands are also observed at 479, 450, and 350 nm
due to
1 G 4 →
3 H 6 ,
1 D 2 →
3 F 4 , and
1 D 2 →
3 H 6 transitions, respectively [27].
Furthermore, concentration of dopant ions is another important parameter to
enhance the UC emission intensity. As we keep on increasing the concentration of the
activator ion, optical emission will increase. However, after a certain concentration
a sharp reduction in the optical emission occurs, which is known as concentration
quenching. Dexter and Schulman were the first to propose a generalized theory of
concentration quenching [28]. This theory states that, if the activator concentration
increases (above a critical concentration) it brings down the mutual distance between
the activator ions. This initiates transfer of energy from excited ions to unexcited ions
397
1.5 Combination of Host Lattice and Lanthanides for Photon
Upconversion Process
Lanthanide based UC materials consist of an inorganic host doped with a small
concentration of lanthanide ions, usually in 3+ oxidation state [25]. The dopant
lanthanide ions act as luminescent centre and the host lattice provides a matrix for
spatial arrangement of dopant ions. An optimum combination of host lattice as well
as dopant ions renders highly efficient UC emission. In designing lanthanide-based
UC materials, host matrix is of great importance. The host matrix should be of
low phonon frequency and there should be a close lattice match to the dopant ions.
Phonon frequency plays a vital role in reducing multi-phonon relaxation among
close lying energy levels. Host matrix containing heavy halogenides present low nonradiative losses (phonon energies <300 cm
−1 ) but they are hygroscopic and suffer
from low chemical stability. Metal oxides offer high chemical stability, however, their
phonon frequency is very high (above 500 cm
−1 ). In comparison, fluorides are more
promising host lattice for UC emission due to their low phonon energies (∼500 cm
−1 )
and of high chemical stability [26]. Among all the fluoride UC materials, hexagonal
NaYF 4 has been recognised as the most efficient host matrix for lanthanide UC so far.
A detail of different synthesis processes for such hosts can be found in our previous
work [14].
Trivalent lanthanide ions (such as Tm
3+ , Er
3+ , Ho
3+ , and Tb
3+ ) possess ladder like
energy states and so they have been frequently used as activator ions [12, 13, 19]. The
absorption cross section of Yb
3+ ion (at λ = 980 nm) is of the order of 11.7 × 10
−21
cm
2 which is quite high than any of the lanthanide ions. Therefore, Yb
3+ ion has been
typically used as universal sensitizer for NIR (980 nm) to visible UC processes [19].
The
2 F 7/2 →
2 F 5/2 transition of Yb
3+ is well matched with different transitions present
in Er
3+ , Tm
3+ , and Ho
3+ , therefore, Yb
3+ provides efficient energy transfer to these
ions. Thus, Yb
3+ ion has been typically co-doped with Er
3+ , Tm
3+ , and Ho
3+ as a
sensitizer to get efficient UC emission. Er
3+ and Yb
3+ co-doped system gives intense
green emission at around 520/540 nm due to
2 H 11/2 /
4 S 3/2 →
4 I 15/2 transitions while
a red emission band (at around 650 nm) appears via
4 F 9/2 →
4 I 15/2 transition [12].
Ho
3+ and Yb
3+ co-doped system also gives green and red emissions at 540 nm and
647 nm due to
5 S 2 /
5 F 4 →
5 I 8 and
5 F 5 →
5 I 8 transitions, respectively [13]. Whereas,
Tm
3+ -Yb
3+ co-doped system gives intense IR transition at 800 nm due to
3 H 4 →
3 H 6
transition. Other relatively weak bands are also observed at 479, 450, and 350 nm
due to
1 G 4 →
3 H 6 ,
1 D 2 →
3 F 4 , and
1 D 2 →
3 H 6 transitions, respectively [27].
Furthermore, concentration of dopant ions is another important parameter to
enhance the UC emission intensity. As we keep on increasing the concentration of the
activator ion, optical emission will increase. However, after a certain concentration
a sharp reduction in the optical emission occurs, which is known as concentration
quenching. Dexter and Schulman were the first to propose a generalized theory of
concentration quenching [28]. This theory states that, if the activator concentration
increases (above a critical concentration) it brings down the mutual distance between
the activator ions. This initiates transfer of energy from excited ions to unexcited ions