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

The surface effects on the fluorescence properties of the UC nanoshells were studied by comparing with that of their solid counterparts and solid NaYF4 core/UC shell nanoparticles.. To s

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

3.1 Introduction

NaYF4: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, NaYF4: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 NaYF4: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 NaYF4 core/UC shell nanoparticles Surface coatings of undoped NaYF4

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

inorganic materials, whereas the dark contrast at the periphery of the particles was caused by the NaYF4:Yb,Er shell These results confirmed the formation

of UC nanoshells The NaYF4:Yb,Er shell showed good crystallinity as indicated by the (100) lattice fringes (d100 = 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

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)

To study the surface effects on fluorescence properties of UC nanoshells, solid UC nanoparticles and solid NaYF4 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 NaYF4 core/UC shell nanoparticles The XRD spectra

confirmed the hcp structure of both solid UC nanoparticles and solid NaYF4

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 NaYF4 core/UC shell nanoparticles Since the solid NaYF4 core/UC shell nanoparticles were synthesized by coating the solid NaYF4 core (10 ± 1 nm) with UC shell, the thickness of the UC shell was calculated to be ~3 nm For solid NaYF4

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 NaYF4 (Fig 3.4c) The average hydrodynamic diameter was ~15.3 nm for the solid UC nanoparticles and

~16.5 nm for the solid NaYF4 core/UC shell nanoparticles, consistent with the TEM results (the insets of Fig 3.4b, c)

Fig 3.4 (a) XRD of the as-synthesized solid UC nanoparticles and solid

NaYF4 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 NaYF4 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 NaYF4

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 2H9/2 - 4I15/2, 2H11/2 - 4I15/2, 4S3/2 - 4I15/2, and 4F9/2 - 4I15/2

transitions of Er3+ (Fig 3.6) These transitions can be explained as follows

Sensitizer ions, Yb3+ were first excited by 980-nm NIR photons from the ground state 2F7/2 to the excited state 2F5/2 (this energy gap is ~ 1.25 – 1.27 eV

or ~976 – 992 nm) The Er3+ as activator ions, which received the energy from excited Yb3+, were then excited from the ground state 4I15/2 to the excited state 4I11/2 These excited Er3+ ions at the 4I11/2 level were further excited to higher energy level (4F7/2 level) by energy transfer of excited Yb3+ The excited Er3+ at 4F7/2 level experienced non-radiative relaxations to 2H11/2 and

4S3/2 levels, and subsequent returned to the ground state 4I15/2, leading to the

emissions of two green peaks as shown in Fig 3.5a For the blue peak (2H9/2 -

4I15/2 transition), the excited Er3+ at 4S3/2 level were further excited to the 4G11/2

level instead of returning to the ground state 4I15/2 This was followed by non-radiative relaxation to 2H9/2 and subsequent returned to ground state 4I15/2, leading to the emission peak at blue region For the red peak (4F9/2 - 4I15/2

transition), the excited Er3+ ions at the 4I11/2 level first relaxed to 4I13/2 before exciting to 4F9/2, and subsequent returned to the ground state 4I15/2

Fig 3.5  (a) Spectra and (b) photographs of fluorescence of UC nanostructures, from left to right are (b1) undoped NaYF4 core showing no

UC emission, (b2) UC nanoshells, (b3) solid NaYF4 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 Yb3+ and Er3+ 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

As expected, there was no UC emission for undoped NaYF4 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 NaYF4 core/UC shell)  (UC nanoshells) The integrated total emission intensities of UC nanoshells and solid NaYF4 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 NaYF4 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 NaYF4 with

Yb and Er dopant ions The UC active surface area is referred to as the surface of the NaYF4 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 NaYF4 core are zero In our estimations, the interface area between the doped shell (~3 nm) and the solid undoped core (~10 nm) in NaYF4

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

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 NaYF4 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 NaYF4 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 NaYF4 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

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

(~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

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)

Integrated red emission intensity

normalized to that of solid UC (size ~15

nm)

Integrated total emission intensity

normalized to that of solid UC (size ~15

nm)

r 2   r

804

2



r



1767 3



r

1620 3

1 3 2



 r

r

1587 3

1 3 2



 r

r



40 0

3

r

50 0

3

3 1 3 2

2 2



 r

r

54 0

3

3 1 3 2

2 2







r r

r r

707



r

860

2 2 1



 r

r



r 1

r 2  

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 NaYF4 core (low phonon energy material) in

the solid NaYF4 core/UC shell nanoparticles This explains the decrease of

the emission intensity of UC nanoshells compared to solid NaYF4 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

NaYF4 core/UC shell

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

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 NaYF4

with low phonon energy undoped NaYF4 shell provided an alternative to prevent such undesirable non-radiative loss and significantly enhanced the

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 NaYF4 (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 NaYF4-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 NaYF4-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