Chapter Synthesis and characterization of NaYF4: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 38    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 ± nm and ± 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. 39    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). 40    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 ± nm for the solid UC nanoparticles and ~16 ± 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 ± 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). 41    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. 42    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 S3/2 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 I15/2 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 nonradiative 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. 43    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. 44    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 45    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 46    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 NaYF4 core r = nm (nm2) Calculated UC active volume (nm3) UC active volume-normalized surface area (nm-1) (~15 nm) Not applicable Solid NaYF4 core / UC Shell r r  Schematic microstructures Calculated UC active surface area Solid UC UC nanoshells r1 r1  r2  r = 7.5 nm r2  r1 = 5.0 nm r1 = 3.5 nm r2 = 8.0 nm r2 = 7.5 nm  4r 4r22 4 r12  r22  707  804  860 r  1767 r  0.40   r23  r13  1620 3r22 r23  r13  0.50     r23  r13  1587  r12  r22 r23    r13  0.54 Integrated green emission intensity normalized to that of solid UC (size ~15 0.45 0.26 0.55 0.30 0.47 0.27 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) 47    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.       Nanoparticles Y (mol%) Yb (mol%) Er (mol%) UC nanoshells 78.6 19.5 1.9 Solid UC nanoparticles 78.3 19.7 1.9 NaYF4 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 48    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 49    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 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 50    UC nanoshells, the samples of were collected from 300 to 340 oC and to 30 at 340 oC 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 NaYF4: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 oC (Fig. 3.10a). After at 340 oC, 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 at 340 oC (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 at 340 oC (Fig. 3.10d) compared to that of 10 at 340 oC. 51    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 oC, (c) 320 oC, (d) at 340 oC, (e) 10 at 340 oC, and (f) 30 at 340 oC. Fig. 3.10 TEM images of samples obtained at different temperatures and times of reaction: (a) at 300 oC, (b) at 340 oC, (c) 10 at 340 oC, and (d) 30 at 340 oC. 52    Ostwald ripening mechanism was observed for the sample collected at 300oC (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 oC. (a) & (b) show the Ostwald ripening mechanism (arrow signs) was observed in the sample collected at 300 oC. 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 NaYF4 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 53    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 NaYF4 (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. 54    [...]... of (a) reference of hcp sodium yttrium fluoride (JCPDS file number PDF 16 -33 4) and the UC samples collected during the reaction at (b) 30 0 oC, (c) 32 0 oC, (d) 5 min at 34 0 oC, (e) 10 min at 34 0 oC, and (f) 30 min at 34 0 oC Fig 3. 10 TEM images of samples obtained at different temperatures and times of reaction: (a) at 30 0 oC, (b) 5 min at 34 0 oC, (c) 10 min at 34 0 oC, and (d) 30 min at 34 0 oC 52    Ostwald... 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...   UC nanoshells, the samples of were collected from 30 0 to 34 0 oC and 5 to 30 min at 34 0 oC 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 NaYF4: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)... min at 34 0 oC (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 34 0 oC (Fig 3. 10d) compared to that of 10 min at 34 0 oC 51    Fig 3. 9 XRD of (a)... 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. .. undesirable non-radiative loss and significantly enhanced the 49    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... 30 0oC (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. .. emission intensities decreased with increasing the UC active volume-normalized surface area The surface coatings of undoped NaYF4 (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 54    ... 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 50 ... doped nanoshells significantly decreased when compared to that of the solid ones (~15 nm) and solid NaYF4 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 53   UC nanostructures confirmed that the green, red, and total emission intensities decreased with increasing the . 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  . 16 -33 4) and the UC samples collected during the reaction at (b) 30 0 o C, (c) 32 0 o C, (d) 5 min at 34 0 o C, (e) 10 min at 34 0 o C, and (f) 30 min at 34 0 o C. Fig. 3. 10 TEM images of. 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