402
P. Singh et al.
a perovskite solar cells and obtained 7.9% enhancement in the power conversion
efficiency [35]. Recently many other such applications were also suggested in Er,
Yb based UC systems.
In 2012, Ostrowski et al. dispersed the UCNPs onto SiN TEM grids and scanned
with 980 nm excitation using confocal microscopy (Fig. 7c(i, ii)) [36]. They reported
the single-particle imaging of 9 nm core/shell β-phase NaYF 4 : 20% Yb
3+ , 2%
Er
3+ /NaYF 4 nanoparticles and showed that they exhibit no measurable photobleaching or blinking. Similarly, Wu et al. also demonstrated a non-blinking and
photostable UC luminescence in a single NaYF 4 : Er, Yb nanoparticle [37]. Boyer
and co-workers designed NaYF 4 : Er(Tm), Yb-PMMA nanocomposite which can
emit white light along with blue and green lights and suggested its application in
display devices [38]. Wang et al. demonstrated the application of NaYF 4 : Yb, Er
UCNPs for fluorescent label to develop latent fingerprints on various surface as
shown in Fig. 7d [39]. Thus, integration of Yb
3+ , Er
3+ doped UCNPs with other
type of nanomaterials opens a wide range of opportunity for its versatile applications
[40]. In another work, Lin and his group synthesized uniform hollow nanospheres of
Y 2 O 3 : Yb/Er-CuxS and demonstrated their application as bio-imaging agent. They
used it for photothermal therapy of tumors also [41]. Wang et al. also reported a
biosensor based on fluorescence resonance energy transfer from UCNPs to carbonnanoparticles for direct determination of metalloproteinase-2 (MMP-2) [42]. Liang
et al. prepared core/double-shell-structured β-NaYF 4 :Er
3+ ,Yb
3+ @SiO 2 @TiO 2 and
showed their application for dye-sensitized solar cells [43].
3.2 Upconversion in Tm 3+ /Yb 3+ Doped System and Their
Applications
Tm/Yb co-doped UCNPs are another class of UCNPs after Er/Yb doped systems
which are used most widely for different applications. Mechanism of the UC emission
in Tm
3+ /Yb
3+ system is shown in Fig. 8 [44]. The Yb
3+ ions absorb 980 nm photons
and are excited to
2 F 5/2 state. The phonon assisted energy transfer from excited
Yb
3+ ions to Tm
3+ ions populates the
3 H 5 state of Tm
3+ ions. Subsequently, excited
Tm
3+ ions non-radiatively populate
3 F 4 state from where
3 F 2 state is populated either
through excited state absorption (of 980 nm photons) or through co-operative energy
transfer from Yb
3+ ions to Tm
3+ ions. A weak emission at 700 nm arises from
3 F 2
state to ground state.
3 F 2 state may further populate
3 H 4 non-radiatively from where
an intense NIR emission peaking at 802 nm is observed. Further, Yb
3+ ions pump
Tm
3+ ions (in
3 H 4 state) through ET process and
1 G 4 state of Tm
3+ is populated. UC
emission from
1 G 4 state (
1 G 4 →
3 H 6 and
1 G 4 →
3 F 4 ) results blue and red emission
peaks at 476 and 650 nm [44, 45]. The NIR-to-NIR UC emission of Tm
3+ /Yb
3+
UCNPs are of particular interest because it offers both the excitation as well as
emission wavelengths in the NIR range.
P. Singh et al.
a perovskite solar cells and obtained 7.9% enhancement in the power conversion
efficiency [35]. Recently many other such applications were also suggested in Er,
Yb based UC systems.
In 2012, Ostrowski et al. dispersed the UCNPs onto SiN TEM grids and scanned
with 980 nm excitation using confocal microscopy (Fig. 7c(i, ii)) [36]. They reported
the single-particle imaging of 9 nm core/shell β-phase NaYF 4 : 20% Yb
3+ , 2%
Er
3+ /NaYF 4 nanoparticles and showed that they exhibit no measurable photobleaching or blinking. Similarly, Wu et al. also demonstrated a non-blinking and
photostable UC luminescence in a single NaYF 4 : Er, Yb nanoparticle [37]. Boyer
and co-workers designed NaYF 4 : Er(Tm), Yb-PMMA nanocomposite which can
emit white light along with blue and green lights and suggested its application in
display devices [38]. Wang et al. demonstrated the application of NaYF 4 : Yb, Er
UCNPs for fluorescent label to develop latent fingerprints on various surface as
shown in Fig. 7d [39]. Thus, integration of Yb
3+ , Er
3+ doped UCNPs with other
type of nanomaterials opens a wide range of opportunity for its versatile applications
[40]. In another work, Lin and his group synthesized uniform hollow nanospheres of
Y 2 O 3 : Yb/Er-CuxS and demonstrated their application as bio-imaging agent. They
used it for photothermal therapy of tumors also [41]. Wang et al. also reported a
biosensor based on fluorescence resonance energy transfer from UCNPs to carbonnanoparticles for direct determination of metalloproteinase-2 (MMP-2) [42]. Liang
et al. prepared core/double-shell-structured β-NaYF 4 :Er
3+ ,Yb
3+ @SiO 2 @TiO 2 and
showed their application for dye-sensitized solar cells [43].
3.2 Upconversion in Tm 3+ /Yb 3+ Doped System and Their
Applications
Tm/Yb co-doped UCNPs are another class of UCNPs after Er/Yb doped systems
which are used most widely for different applications. Mechanism of the UC emission
in Tm
3+ /Yb
3+ system is shown in Fig. 8 [44]. The Yb
3+ ions absorb 980 nm photons
and are excited to
2 F 5/2 state. The phonon assisted energy transfer from excited
Yb
3+ ions to Tm
3+ ions populates the
3 H 5 state of Tm
3+ ions. Subsequently, excited
Tm
3+ ions non-radiatively populate
3 F 4 state from where
3 F 2 state is populated either
through excited state absorption (of 980 nm photons) or through co-operative energy
transfer from Yb
3+ ions to Tm
3+ ions. A weak emission at 700 nm arises from
3 F 2
state to ground state.
3 F 2 state may further populate
3 H 4 non-radiatively from where
an intense NIR emission peaking at 802 nm is observed. Further, Yb
3+ ions pump
Tm
3+ ions (in
3 H 4 state) through ET process and
1 G 4 state of Tm
3+ is populated. UC
emission from
1 G 4 state (
1 G 4 →
3 H 6 and
1 G 4 →
3 F 4 ) results blue and red emission
peaks at 476 and 650 nm [44, 45]. The NIR-to-NIR UC emission of Tm
3+ /Yb
3+
UCNPs are of particular interest because it offers both the excitation as well as
emission wavelengths in the NIR range.