Coupling effects of NaYF4Yb,Er upconversion nanoshells and au ag metallic nanoshells 3

38

Chapter 3 Synthesis and characterization of NaYF
4
:Yb,Er UC nanoshells

3.1 Introduction
NaYF
4
:Yb,Er nanoparticles with hcp phase, one of the most efficient
NIR-to-visible UC materials,
26
has received growing interests due to its
potential applications in bioimaging. Recently, NaYF
4
:Yb,Er UC nanoshells
with interior cavity have been of interest as they may find combined use in
bioimaging and drug-delivery applications.
79,80
However, the UC nanoshells
have even larger numbers of surface ions due to both their inner and outer
surfaces, accentuating the surface-dependent properties. The surface effects of
UC nanoshells on the fluorescence properties are not well understood.
In this thesis, hcp NaYF
4
:Yb,Er UC nanoshells were synthesized via
thermal decomposition.
81
The optical properties of the UC nanoshells were
studied. The surface effects on the fluorescence properties of the UC
nanoshells were studied by comparing with that of their solid counterparts and

solid NaYF
4
core/UC shell nanoparticles. Surface coatings of undoped NaYF
4

on the inner and outer surfaces of the UC nanoshells showed a significant UC
emission enhancement.

3.2 Microstructure and crystal structure
Figure 3.1 shows the TEM images of the as-synthesized UC nanoshells
collected at different magnifications. The high-resolution TEM (HRTEM)
image (Fig. 3.1c) shows notably different contrasts between the center (light)
and the periphery (dark) of equiaxed nanoshells. The light contrast in the
center was caused by the lack of diffraction contrasts due to the absence of the
39

inorganic materials, whereas the dark contrast at the periphery of the particles
was caused by the NaYF
4
:Yb,Er shell. These results confirmed the formation
of UC nanoshells. The NaYF
4
:Yb,Er shell showed good crystallinity as
indicated by the (100) lattice fringes (d
100
= 0.52 nm) (Fig. 3.1c). The arrows
in Fig. 3.1e show the discontinuity in contrast at the shells, suggesting that
some of these shells were partially fractured due to the stress buildup during
the growth, similar to the previous report.
78

The average interior cavity and
shell thickness, estimated by random measurements of ~100 particles from
TEM images, was found to be 7 ± 2 nm and 4 ± 1 nm, respectively. The
average hydrodynamic diameter measured by DLS was ~15.1 nm (Fig. 3.2),
consistent with the TEM data. This result also confirmed that the UC
nanoshells were well dispersed.


Fig. 3.1 (a) to (f) are TEM images of UC nanoshells showing a low mass
density at the center (light contrast) and UC shell (dark contrast) of the
nanoshells. These TEM images were collected at different magnifications
from 50,000 to 400,000X.

40



Fig. 3.2 The particle size distribution of oleylamine-stabilized UC nanoshells
in hexane was measured using DLS. The average hydrodynamic diameter was
~15.1 nm.

The crystal structure of the UC nanoshells was confirmed to be hcp as
shown in the XRD spectra, compared to the reference of hcp sodium yttrium
fluoride (JCPDS file number PDF 16-334) (Fig. 3.3).


Fig. 3.3 XRD of the as-synthesized UC nanoshells matched well with the
reference of hcp sodium yttrium fluoride (JCPDS file number PDF 16-334).

41



To study the surface effects on fluorescence properties of UC
nanoshells, solid UC nanoparticles and solid NaYF
4
core/UC shell
nanoparticles with same crystal structure and similar size were synthesized.
Figure 3.4a-c shows the XRD and TEM images of the as-synthesized solid UC
nanoparticles and solid NaYF
4
core/UC shell nanoparticles. The XRD spectra
confirmed the hcp structure of both solid UC nanoparticles and solid NaYF
4

core/UC shell nanoparticles. The average size, estimated by random
measurements of ~100 particles from TEM images, was 15 ± 3 nm for the
solid UC nanoparticles and ~16 ± 3 nm for the solid NaYF
4
core/UC shell
nanoparticles. Since the solid NaYF
4
core/UC shell nanoparticles were
synthesized by coating the solid NaYF
4
core (10 ± 1 nm) with UC shell, the
thickness of the UC shell was calculated to be ~3 nm. For solid NaYF
4

core/UC shell nanoparticles, the diffraction contrast in the TEM image due to
dopants of Yb and Er would not be expected to be observed since their

undoped core and UC shell consisted of NaYF
4
(Fig. 3.4c). The average
hydrodynamic diameter was ~15.3 nm for the solid UC nanoparticles and
~16.5 nm for the solid NaYF
4
core/UC shell nanoparticles, consistent with the
TEM results (the insets of Fig. 3.4b, c).

42


Fig. 3.4 (a) XRD of the as-synthesized solid UC nanoparticles and solid
NaYF
4
core/UC shell nanoparticles matched well with the reference of hcp
sodium yttrium fluoride (JCPDS file number PDF 16-334). TEM images and
particle size distribution by DLS (inset) of (b) solid UC nanoparticles (~15 nm
in size) and (c) solid NaYF
4
core (~10 nm)/UC shell (~3 nm) nanoparticles.


3.3 UC emission
The fluorescence properties of the as-synthesized UC nanoshells were
studied and compared with that of the solid UC nanoparticles and solid NaYF
4

core/UC shell nanoparticles. Figure 3.5a, b shows the spectra and photograph
of fluorescence of as-synthesized UC nanostructures normalized by UC active

mass, respectively. The fluorescence spectra showed four emission peaks
attributed to the
2
H
9/2
-
4
I
15/2
,
2
H
11/2
-
4
I
15/2
,
4
S
3/2
-
4
I
15/2
, and
4
F
9/2
-

4
I
15/2

transitions of Er
3+
(Fig. 3.6). These transitions can be explained as follows.
43

Sensitizer ions, Yb
3+
were first excited by 980-nm NIR photons from the
ground state
2
F
7/2
to the excited state
2
F
5/2
(this energy gap is ~ 1.25 – 1.27 eV
or ~976 – 992 nm). The Er
3+
as activator ions, which received the energy
from excited Yb
3+
, were then excited from the ground state
4
I
15/2

to the excited
state
4
I
11/2
. These excited Er
3+
ions at the
4
I
11/2
level were further excited to
higher energy level (
4
F
7/2
level) by energy transfer of excited Yb
3+
. The
excited Er
3+
at
4
F
7/2
level experienced non-radiative relaxations to
2
H
11/2
and

4
S
3/2
levels, and subsequent returned to the ground state
4
I
15/2
, leading to the
emissions of two green peaks as shown in Fig. 3.5a. For the blue peak (
2
H
9/2
-
4
I
15/2
transition), the excited Er
3+
at
4
S
3/2
level were further excited to the
4
G
11/2

level instead of returning to the ground state
4
I

15/2
. This was followed by non-
radiative relaxation to
2
H
9/2
and subsequent returned to ground state
4
I
15/2
,
leading to the emission peak at blue region. For the red peak (
4
F
9/2
-
4
I
15/2

transition), the excited Er
3+
ions at the
4
I
11/2
level first relaxed to
4
I
13/2

before
exciting to
4
F
9/2
, and subsequent returned to the ground state
4
I
15/2
.





44


Fig. 3.5 (a) Spectra and (b) photographs of fluorescence of UC
nanostructures, from left to right are (b1) undoped NaYF
4
core showing no
UC emission, (b2) UC nanoshells, (b3) solid NaYF
4
core/UC shell, and (b4)
solid UC nanoparticles. The nanoparticles were dispersed in hexane and
excited using 980-nm NIR irradiation at room temperature. The emission
intensities of as-synthesized UC nanostuctures were normalized by UC active
mass.

Fig. 3.6 A schematic energy-level diagram of Yb
3+
and Er
3+
ions and the
upconversion mechanisms under 980-nm NIR excitation. The dashed-dotted,
dashed, and dotted lines represent photon excitation, energy transfer, and
multiphonon relaxation, respectively. The blue, green, and red solid lines
indicate the blue, green, and red emissions, respectively.
45

As expected, there was no UC emission for undoped NaYF
4
cores (~10
nm in size) as they did not contain UC active ions (Fig. 3.5a, b). The UC
emission intensity decreased in the order of (solid UC nanoparticles)  (solid
NaYF
4
core/UC shell)  (UC nanoshells). The integrated total emission
intensities of UC nanoshells and solid NaYF
4
core (~10 nm)/UC shell (~3 nm)
nanoparticles decreased by 73% and 53% compared with that of solid UC
nanoparticles (~15 nm), respectively. The decrease in UC emission could be
explained by surface effects, which discussed in the following sections.

3.4 Effects of surface
Assuming a spherical microstructure, the UC active surface area and

the UC active volume-normalized surface area of the UC nanoshells, solid
NaYF
4
core/UC shell, and solid UC nanoparticles (with their respective UC
emission intensities shown in Fig. 3.5a) were calculated as shown in Table 3.1.
The UC active volume is defined as the effective volume of the NaYF
4
with
Yb and Er dopant ions. The UC active surface area is referred to as the
surface of the NaYF
4
with Yb and Er dopant ions that directly interacts with
high phonon energy environment (e.g. oleylamine) (Appendix Fig. C.1). By
this definition, the UC active volume and UC active surface area of the
undoped NaYF
4
core are zero. In our estimations, the interface area between
the doped shell (~3 nm) and the solid undoped core (~10 nm) in NaYF
4

core/UC shell nanoparticles was ignored. This was because the undoped core
provided a protective interior environment against undesirable non-radiative
losses, as compared to the interior cavity of UC nanoshells that might contain
trapped organic materials from the reactions. UC active volume-normalized
46

surface area is a ratio of UC active surface area to UC active volume. The UC
active volume-normalized surface area is an indication of the ratio of UC
active surface-to-bulk ions.
The integrated green, red, and total emission intensities decreased in

the order of (solid UC)  (solid NaYF
4
core/UC shell)  (UC nanoshells).
For the UC nanoshells, the green, red, and total emission intensities were 26%,
30%, and 27% of those of the solid UC (~15 nm) nanoparticles, respectively.
The integrated emission intensities of solid NaYF
4
core/UC shell nanoparticles
normalized to that of solid UC (~15 nm) nanoparticles were 45%, 55%, and
47% for green, red, and total emission intensities, respectively. In this study,
these UC nanostructures were stabilized using the same surfactant
(oleylamine) and dispersed in hexane. Therefore, the significant decrease of
emission intensity from the solid nanoparticles to nanoshells could not be
associated to the surfactant and solvent. The composition ratios of Y, Yb, and
Er ions in the solid UC nanoparticles (~15 nm), solid NaYF
4
core/UC shell
nanoparticles, and UC nanoshells were similar (Table 3.2). Therefore, the
significant differences of their emission intensities could not be attributed to
the concentration of Y, Yb, and Er ions.
Among the microstructures (Table 3.1), the UC nanoshells with
interior cavity had the highest UC active volume-normalized surface area,
indicating they had the largest number of UC active surface ions due to their
inner and outer surfaces. These UC active surface ions would interact with
high phonon energy environments such as oleylamine and hexane, leading to
undesirable non-radiative losses and decreasing the UC emission. Thus, the
47

emission intensity of UC nanoshells decreased compared with the solid UC
nanoparticles.

Table 3.1

The calculation of UC active volume-normalized surface area for
the UC nanostuctures and their normalized emission intensities.

Nanoparticles NaYF
4
core
Solid UC
(~15 nm)
Solid
NaYF
4
core /
UC Shell
UC
nanoshells
Schematic microstructures



r = 5 nm
r = 7.5 nm

r
1
= 5.0 nm
r
2

= 8.0 nm
r
1
= 3.5 nm
r
2
= 7.5 nm
Calculated UC active surface area
(nm
2
)
0



Calculated UC active volume (nm
3
)
0

UC active volume-normalized surface
area (nm
-1
)
Not
applicable
Integrated green emission intensity
normalized to that of solid UC (size ~15
nm
)

0 1 0.45 0.26
Integrated red emission intensity
normalized to that of solid UC (size ~15
nm
)

0 1 0.55 0.30
Integrated total emission intensity
normalized to that of solid UC (size ~15
nm
)

0 1 0.47 0.27



r
r
1

r
2

r
804
4
2
2


r

1767
3
4
3

r



1620
3
4
3
1
3
2

 rr


1587
3
4
3
1
3
2


 rr

40.0
3

r
50.0
3
3
1
3
2
2
2

 rr
r


54.0
3
3
1
3
2
2
2
2
1




rr
rr
707
4
2

r



860
4
2
2
2
1

 rr

r
1
r
2

48

The contents trapped inside the interior cavity of the UC nanoshells,
possibly oleylamine, and other organics from reaction,

81
would provide an
environment with high phonon energy compared to that of the UC shell
surrounding the undoped solid NaYF
4
core (low phonon energy material) in
the solid NaYF
4
core/UC shell nanoparticles. This explains the decrease of
the emission intensity of UC nanoshells compared to solid NaYF
4
core/UC
shell. The solid UC nanoparticles showed the highest emission intensity since
these particles contained fewer UC active surface ions compared with others.

Table 3.2 The mol ratios of Y, Yb, and Er ions measured by ICP-OES.
 
Nanoparticles Y (mol%) Yb (mol%) Er (mol%)
UC nanoshells 78.6 19.5 1.9
Solid UC nanoparticles 78.3 19.7 1.9
NaYF
4
core/UC shell
nanoparticles
80.6 17.7 1.7


The relation between the integrated emission intensity and the UC
active volume-normalized surface area for different microstructures of UC
nanoparticles (Appendix Table C.1) is demonstrated in Fig. 3.7. This result

showed the UC emission intensities decreased with increasing UC active
volume-normalized surface area (ratio of surface ions to bulk ions). The UC
active surface ions may interact with high-phonon energy environment,
leading to an of increase non-radiative losses that compete with radiative
transfer process.
85,86
Further, the larger local disorder, OH impurities at the
49

particle surface, and surface compositional segregation of the UC active ions
may enhance the non-radiative loss mechanisms.
31,34



Fig. 3.7 Relation between the integrated emission intensities and the UC
active volume-normalized surface area for various microstructures of UC
nanoparticles. The legends of green (G), red (R), and (G+R) indicate the
integrated green, red, and total emissions, respectively.


3.5 Effects of surface coatings
To minimize the non-radiative loss, the UC active surface ions of the
particle core were passivated by surface coating of low phonon energy
materials that are commonly undoped host materials.
35,36
The surface coating
of undoped host on the UC core, referred to as undoped shell, provide a barrier
to prevent undesired interactions between the UC active surface ions of the
UC nanostructures and high phonon energy environment.

It has recently been shown that capping the UC active doped NaYF
4

with low phonon energy undoped NaYF
4
shell provided an alternative to
prevent such undesirable non-radiative loss and significantly enhanced the
50

emission intensity.
32,73
There however appeared a critical thickness of the
undoped shell beyond which no further emission enhancement was observed
when the thickness exceeded 3 nm.
32
In this thesis, the surface coatings of
undoped NaYF
4
(thickness of ~3 nm) on both the inner and outer surfaces of
the UC nanoshells increased the total emission intensity by ~19 times of that
of the UC nanoshells, and ~5 times of that of solid UC nanoparticles (~15 nm)
(Fig. 3.8). The UC nanoshells after the inner and outer surface coatings are
referred to as undoped NaYF
4
-coated UC nanoshells.


Fig. 3.8 UC spectra and photographs (bottom inset) of the fluorescence of the
UC nanostructures. For the bottom inset: (a) UC nanoshells, (b) solid UC
(~15 nm), and (c) undoped NaYF

4
-coated UC nanoshells. The measurements
were normalized by UC active mass for the UC samples dispersed in hexane
and excited using 980-nm NIR irradiation at room temperature.


3.6 Formation mechanism
The crystal structures and microstructures of the samples were
investigated at different temperatures and reaction time to understand the
formation mechanism of the UC nanoshells. During the synthesis process of
51

UC nanoshells, the samples of were collected from 300 to 340
o
C and 5 to 30
min at 340
o
C for XRD analysis (Fig. 3.9). The XRD spectra showed the
crystal structure of all the samples collected was hcp structure. These results
indicated the hcp NaYF
4
:Yb,Er was formed at the initial stage of reaction.
The microstructures during the different reaction stages were
investigated using TEM images (Fig. 3.10). The results show small solid core
particles were observed and simultaneously small voids were already formed
in some particles at the initial stage of reaction where the temperature reached
300
o
C (Fig. 3.10a). After 5 min at 340
o

C, more voids in each particle were
observed and the solid cores disappeared (Fig. 3.10b). These voids appeared
to start coalescing with each other. The small voids are thermodynamically
less stable than the big voids since the small ones have a larger surface energy
than the big ones. Eventually, a single large void in each particle was found,
whereas the small voids disappeared after 10 min at 340
o
C (Fig. 3.10c),
leading to the formation of a nanoshell structure. The gradual coalescence of
small voids into big single ones was previously reported in formation of
hollow cobalt sulfide nanocrystals through the Kirkendall effect.
78
The UC
nanoshells showed little differences in microstructure after 30 min at 340
o
C
(Fig. 3.10d) compared to that of 10 min at 340
o
C.
52


Fig. 3.9 XRD of (a) reference of hcp sodium yttrium fluoride (JCPDS file
number PDF 16-334) and the UC samples collected during the reaction at (b)
300
o
C, (c) 320
o
C, (d) 5 min at 340
o

C, (e) 10 min at 340
o
C, and (f) 30 min at
340
o
C.


Fig. 3.10 TEM images of samples obtained at different temperatures and
times of reaction: (a) at 300
o
C, (b) 5 min at 340
o
C, (c) 10 min at 340
o
C, and
(d) 30 min at 340
o
C.

53

Ostwald ripening mechanism was observed for the sample collected at
300
o
C (Fig 3.11a, b), suggesting this mechanism also contributed to the
growth of the nanoshells. In this mechanism, larger particles grow at the
expense of small particles, thus lowering the surface energy of the system.
According to our results, the formation of UC nanoshells involved the vacancy
diffusion, likely due to the Kirkendall effect and the Ostwald ripening

mechanism.


Fig. 3.11 TEM images of samples obtained at 300
o
C. (a) & (b) show the
Ostwald ripening mechanism (arrow signs) was observed in the sample
collected at 300
o
C.


3.7 Summary
UC nanoshells with hcp crystal structure were successfully synthesized
using thermal decomposition of trifluoroacetate. The interior cavity and UC
shell thickness were ~7 nm and ~4 nm, respectively. The total emission
intensity of the doped nanoshells significantly decreased when compared to
that of the solid ones (~15 nm) and solid NaYF
4
core/UC shell nanoparticles
due to the nanoshells had a higher UC active volume-normalized surface area.
Investigations of the different microstructures of the UC nanoshells and solid
54

UC nanostructures confirmed that the green, red, and total emission intensities
decreased with increasing the UC active volume-normalized surface area. The
surface coatings of undoped NaYF
4
(thickness of ~3 nm) on both the inner and
outer surfaces of the UC nanoshells led to emission enhancement of ~19 and

~5 times compared to that of the UC nanoshells and the solid UC
nanoparticles (~15 nm), respectively.