quantum dots, metallic nanoparticles, and lanthanide ions doped upconversion

Chapter 1 Introduction 1.1 Background Nanostructured materials have significantly different characteristics from their bulk counterparts. 1 Inorganic nanoparticles such as semiconductor quantum dots, metallic nanoparticles, and lanthanide ions doped upconversion nanoparticles have attracted interests due to their size- and shape-dependent optical properties. 2,3 Recently, the combination of metallic nanostructures and lanthanide ions doped upconversion nanostructures have gained a growing interest due to their potential applications in bioimaging and photothermal therapy of cancer cells. 4,5 The fluorescence of fluorophores, such as organic dyes or quantum, dots was enhanced when they were located near metallic nanoparticles due to the plasmonic effects. 6,7 The interactions of these fluorophores with metallic nanoparticles have been extensively investigated. 8,9 Recently, the plasmonic effects from the metallic nanoparticles have been proposed to enhance the fluorescence of upconversion nanoparticles. 10 In this chapter, lanthanide ions doped upconversion nanoparticles, metallic nanostructures, and their unique optical properties are discussed in detail. The fluorescence coupled with the plasmonic effects is also discussed. 1.2 Upconversion Upconversion (UC) commonly refers to a nonlinear optical processes in which the sequential absorption of two or more incident photons leads to 1

the emission of a photon at a shorter wavelength than the excitation wavelength. 11 For example, near infrared (NIR) lights can be converted into visible lights via the UC process. This NIR-to-visible UC technique has potential applications in three-dimensional (3D) displays, 12 white lightemitting diodes (LED), 13 solar cells, 14 and bioimaging. 15 Successful synthesis of UC nanoparticles led to exploration of NIR-tovisible bioimaging. 16,17,18 NIR lights as an excitation source can reduce autofluorescence from biological specimens, improving signal-to-noise ratios compared with ultraviolet (UV) lights commonly used in quantum dots, conventional organic dye, or fluorescent proteins. 19 The NIR excitation source has high penetration depth in biological specimens. For example, NIR lights can penetrate as deep as a few to 10 cm into biological tissues, whereas UV light can penetrate only 1-2 mm. 20 NIR excitation source can also minimize photodamage to biological tissues as its energy is lower than the UV source. 1.2.1 UC mechanism The UC mechanism commonly consists of excited state absorption (ESA) and energy transfer upconversion (ETU). 21,22 Both mechanisms involve the sequential absorption of two or more photons (Fig. 1.1). In ESA mechanism, a single dopant ion is excited from the ground state G to the first exited state E1 by an incident photon (Fig. 1.1a). A second incident photon promotes the excited ion from E1 to the higher excited state E2. UC emission is produced when the excited ion returns to the ground state G from the exited state E2. 2

Fig. 1.1 UC mechanisms: (a) Excited state absorption (ESA) and (b) energy transfer upconversion (ETU). The dashed-dotted, dashed, and solid red lines represent photon excitation, energy transfer, and emission processes, respectively. In contrast to ESA, ETU process involves non-radiative energy transfers between two neighboring ions. In ETU process, the two neighboring ions individually absorb a photon with same energy; thereby this ion is excited from its ground state to the higher energy state E1 (Fig. 1.1b). Non-radiative energy transfer process promotes one of the ions to the upper state E2 while the other relaxes back to the ground state G. UC emission is produced when the ion at energy state E2 returns to its ground state. The UC efficiency of an ETU process is strongly influenced by the dopant ion concentration which determines the average distance between the neighboring dopant ions. It is important to note that photon avalance (PA) is the other UC mechanism based on the sequential absorption of two or more photons. This mechanism is less observed in UC process than the ESA and ETU mechanisms. In the UC mechanism, at least two lower energy photons are required to generate one higher energy photon. However, not all of the energy absorbed is emitted as radiation. The excited ions can also undergo non- 3

radiative relaxation by transferring part of its energy to the host lattices as heat when returning to the ground states. This undesirable non-radiative relaxation mechanism always competes with the radiative transition in the UC process. 1.2.2 UC materials UC materials commonly consist of a crystalline host material and dopants. The dopant ions in the host provide characteristic UC luminescence properties. Selection of host materials, dopants, and dopant concentration are essential to obtain a highly efficient UC process. A. Selection of host materials Efficient hosts should have low phonon energy. Low phonon energy host materials result in higher UC emission intensity since it can minimize the non-radiative loss of electron transition from the excited states to the ground states of lanthanide ions. This is because a larger number of phonons are required for the non-radiative relaxation of excited electrons in the low phonon energy hosts, leading to a lower probability of non-radiative transitions. Heavy halide based materials such as chlorides, bromides, and iodides have low phonon energy (less than 300 cm -1 ). 23 However, these materials are undesirably hygroscopic. The fluorides (e.g. NaYF 4 and NaGdF 4 ) and oxides (e.g. Y 2 O 3 ) exhibit low phonon energies, ~400 and ~600 cm -1, respectively. They have high chemical and thermal stability, thus they are often used as a host of UC materials. Host materials also require that its cations have ionic radii close to the dopant ions in order to reduce lattice strain in the doped host. Hosts based on 4

Na +, Ca 2+ and Y 3+ cations are commonly used for UC materials as their cations have ionic radii close to lanthanide dopant ions. The crystal structure of the host material also significantly influences the optical properties. 24 For example, Yb and Er ions doped hexagonal close-packed (hcp) NaYF 4 bulk materials showed an emission about an order of magnitude higher than their cubic phase counterparts. 25 This phase-dependent optical property is attributed to the different crystal-fields around lanthanide ions in the hosts. To date, NaYF 4 with hcp crystal structure is one of the most efficient hosts for UC materials. 26 A. Dopants Lanthanide (rare earth) ions are commonly used as a dopant for UC materials. They exist in their most stable oxidation state as trivalent ions (Ln 3+ ). The 4f electrons in the lanthanide ions are shielded from the surroundings by filled outer 5s 2 and 5p 6 orbitals. Therefore, the 4f energy structures of lanthanide ions are not strongly affected by the host environments. The electron transitions within the 4f energy states are Laporteforbidden, resulting in a low transition probability. Therefore, the lanthanide elements themselves are not UC active. However, the 4f-4f transition would occur when the trivalent lanthanide ions (Ln 3+ ) are doped into a crystalline host. The surrounding ligand ions generate a crystal field around the dopant ions, increasing the 4f 4f transition probabilities of the lanthanide ions. 23 The ladder-like energy levels of the 4f states allow the lanthanide ions for sequentially absorbing multiple photons with suitable energy to reach a higher excited state. When the energy gaps between three or more subsequent 5

energy levels are very similar, the sequential excitation by a single monochromatic light source to a higher excited state is possible since each absorption step requires the same photon energy. Useful UC emission would be produced when the excited ions return to its ground state. In the UC materials, the lanthanide dopants may be categorized into sensitizer and activator ions. A sensitizer is a donor of the energy, whereas an activator is an acceptor of energy from the sensitizer and also an emitter of radiation. The sensitizers can be excited by a photon, for example NIR, and capable of transferring its energy to the neighboring activator ions. 27 Activator ions, after receiving the energy from the sensitizer ions, subsequently emit photons with shorter wavelength than that of the excitation wavelength in its relaxation. Lanthanide sensitizers commonly have a large absorption cross-section at the excitation wavelengths to obtain high UC efficiency. For example, Yb 3+ ion is widely used as the sensitizer in UC materials due to its large absorption cross-section at 980-nm NIR excitation wavelength. The absorption band of Yb 3+ ion located around 980 nm is attributed to the 2 F 7/2-2 F 5/2 transition (Fig. 1.2). The 2 F 7/2-2 F 5/2 transition energy gap of Yb 3+ ion is matched well with many 4f 4f transitions energy gap of other lanthanide ions (e.g. Er 3+, Tm 3+, and Ho 3+ ) which are commonly used as the activator ions. This promotes efficient energy transfer from Yb 3+ ions to the other neighboring lanthanide activator ions in the UC materials. 6

Fig. 1.2 A schematic 4f energy-level diagram of Yb 3+ (sensitizer ion) and Er 3+ (activator ion). Lanthanide activators have the energy levels to absorb the transfer of energy from the excited sensitizer ions and then efficiently generate emission. The energy difference between each excited level and its ground state in 4f orbital of the activator ions should be close enough to photon absorption by the sensitizer to facilitate the energy transfer steps. Doping concentration of lanthanide ions is also essential since it affects the distance between the dopant ions in the hosts, assuming a homogeneous distribution. In principle, the absorption can be improved by increasing the concentration of the lanthanide dopants in UC materials. However, there appears an optimum doping concentration of the lanthanide ions to obtain high UC efficiency. At a low doping concentration, UC emission intensity increases with increasing the concentration of activator ions and would reach a maximum at a certain concentration. Further increasing the concentration 7

would lead to a decrease of UC emission due to concentration quenching. For example, the doping concentration of Er 3+ did not exceed 3 % in most Er 3+ doped UC materials. 24 However, the absorption by the dopant at such low concentration is not sufficient. To increase the absorption, a higher concentration of Yb 3+ sensitizer is codoped into the UC materials. The concentration of Yb 3+ doped in UC materials is commonly 18 20 %. To date, hcp phase NaYF 4 codoped with Yb 3+ and Er 3+ is one of the most efficient NIR-to-visible UC materials. 26 The hcp NaYF 4 :20%Yb,2%Er is selected for detailed study in this thesis. 1.2.3 Surface-dependent optical properties In UC, the emission is produced through radiative transitions of electrons from the excited states to the ground states in 4f orbitals of the lanthanide ions. For example, under 980-nm NIR excitation, NaYF 4 :Yb,Er nanoparticles produce the UC emission through the 4f-4f transitions of Er 3+. Optically active 4f electrons of lanthanide ions are shielded by filled outer 5s 2 and 5p 6 orbitals, hence quantum confinement effects on electronic states of these localized electrons are not expected for UC nanoparticles. 28 Therefore, the wavelength of UC emission peak is independent from the particle size. As the size decreases, the ratio of surface-to-bulk atoms or ions however increases, thus the surface effects on the optical properties of the materials become more apparent compared to that of the bulk counterparts. The local atomic environment of the surface atoms may be significantly different from that of the interior atoms, accentuating the surface-dependent optical properties. 29 For example, these surface atoms with fewer adjacent 8

coordination atoms and more unsaturated dangling bonds interact with the surrounding environment. The UC nanoparticles are commonly rendered dispersible using long chain organic surfactants (e.g. oleylamine and oleic acid) to prevent aggregation. These surfactants however possess undesirably high vibrational energy functional groups (typically~1500 cm -1 and ~ 3000 cm -1 ) 30, and may interact with the UC active surface ions of UC nanoparticles, leading to undesirable non-radiative losses and decrease of the UC emission. 25,31,32 The ratio of surface-to-bulk ions increases with decreasing particle size, thus the emission of the UC nanoparticles is less than that of bulk counterparts. For example, the emission intensity of UC nanoparticles with 8 30 nm in size was only 0.2 3 % of that of their bulk counterparts. 33 Further, the compositional segregation of dopant ions and OH impurities at the particle surface may enhance the non-radiative mechanisms, decreasing the UC emission intensity. 31,34 1.2.4 Surface passivation To minimize the non-radiative losses, the UC active surface of UC nanoparticles are commonly passivated by surface coating of low phonon energy inorganic materials. 35,36 The surface coating would provide a barrier to prevent undesired interactions between the UC active surface ions of UC nanoparticles and high phonon energy environment such as surfactants and solvents. The undoped host materials are usually used as the coating materials due to low phonon energy and the similar lattice parameter as the doped UC core materials. This would allow the shell deposition and epitaxial growth of the shell on the core surface that may result in a better coverage and protection 9

of the doped nanoparticle core against the surrounding environment. 37 The undoped hosts coated on the UC cores are commonly referred to as undoped shells. The undoped shells would protect the surface of UC cores from high phonon energy environments, preventing the undesirable non-radiative losses and enhancing the UC emission intensity. It was shown that the UC emission intensity of UC core/undoped shell nanoparticles increased with increasing thickness of the undoped shell, with no further enhancement deserved when the thickness exceeded 3 nm. 32 The 3-nm undoped shell was sufficiently thick to prevent undesirable interactions with phonons of surfactant or other molecules in the environment. The total UC emission enhancement of UC cores/undoped shell increased by 15 times compared with that without the intermediate undoped shell. Thus, the surface passivation by the undoped shells is a powerful method to enhance the fluorescence of UC nanoparticles. Recently, the plasmonic effects of metallic nanostructures have been proposed for the fluorescence enhancement of UC nanoparticles, 10 which is discussed in the following sections. 1.3 Metallic nanostructures Metallic nanoparticles are of interests because of potential applications in biomedical imaging, 38 photothermal therapy, 39,40 and fluorescence enhancement. 41 Different from UC nanoparticles, the optical properties of metallic nanoparticles arise from the interaction between an electromagnetic wave (e.g. light) and the conduction electrons in the metal, leading to the absorption and/or scattering at resonant wavelengths due to the excitation of plasmon oscillations. For examples, the plasmon resonance at ~520 nm is 10

responsible for the ruby red colour displayed by the Au colloids. This optical phenomenon has been used for centuries. The ruby red of stained glass windows arises from Au nanoparticles, formed by the reduction of its metallic ions in the glass-forming process. The optical properties of metallic nanostructures may be tailored by controlling their composition, size, shape, and structure. Au nanostructures are one of the most studied due to its good biocompatibility, thermal, and chemical stability. Recently, Au nanostructures have found interests due to their tunable localized surface plasmon resonance, local field enhancement around the particle surface, and localized heating. 42 1.3.1 Localized surface plasmon resonance (LSPR) Plasmon resonance is an optical phenomenon arising from collective oscillations of free electrons against the fixed (lattice of) positive ions in a metal induced by an electromagnetic wave (light). 43 The presence of an external electric field, for example from incident light, causes displacements of the free electrons in the metal. A restoring force from the positive ions in the opposite direction to this displacement lead to the free electrons oscillate backwards and forwards with respect to the fixed positive ions. The plasmon frequency is determined by the restoring force and effective mass of the electron. 42 The plasmon resonance caused by surface electrons are commonly referred to as surface plasmon resonance. 44 For metallic nanoparticles with dimensions smaller than the wavelength of incident light, a strong interaction with the incident light through plasmon resonances that confined within the particle surface is widely known as localized surface plasmon resonance (LSPR) as shown in Fig. 1.3. 45 The LSPR causes enhanced optical extinction 11

(absorption + scattering) with a maximum at the plasmon resonant frequency. The contribution of scattering relative to absorption increases as the particle size increases. The theoretical frequency of LSPR is / 3 for a metallic nanosphere placed in vacuum, where / is the plasmon frequency of a bulk metal, is the number density of conduction electrons, is the dielectric constant of vacuum, is the charge of an electron, and is the effective mass of an electron. Fig. 1.3 Schematic diagram of localized surface plasmon resonance (LSPR) of metallic nanospheres. 45 The LSPR extinction peak of the metallic nanoparticles is dependent on the size, shape, chemical composition, and surrounding medium. The extinction peak red shifts with increasing particle size mainly due to retardation effects. 46 This can be understood as the distance between regions of oscillation-induced charges at opposite interfaces (surfaces) of the nanoparticle increases with increasing size, leading to a smaller restoring force and subsequent lower resonant frequency. Therefore, the LSPR extinction peak shifts to longer wavelength with increasing particle size. For a sphere of 12

volume V and dielectric function in the quasi-static limit, the explicit expression for the extinction cross section is 9 / 2 1.1 where is dielectric constant of surrounding medium, and are real and imaginary parts, respectively. is a measure of the total effective area that the EM fields perceive when interacting with the particle. It would reach a maximum when the denominator in the above equation is minimum, a condition where = 2. This shows the dependence of the LSPR extinction peak on the surrounding dielectric medium. The details of absorption and scattering cross sections for the metallic particles are discussed in Appendix A. 1.3.2 Local field enhancement Metallic particles (e.g. Au, Ag) are known to significantly enhance electromagnetic field around them under incident light due to plasmon resonance. The enhancement of the electromagnetic field intensities around the metallic particles is produced due to the coupling between incident light and collective oscillation of free electrons at the particle surface. The displacements of the free electrons with respect to the fixed positive ions in the metallic nanoparticles caused by an external electric field from the incident light create charges at opposite surfaces, enhancing the local electric field around the particles. The plasmon-induced electric field enhancement depends on various parameters, such as wavelength, distance from the 13

particles, metallic element, surrounding medium, size, and shape of the metallic particles. 47,48 Field intensity enhancement is commonly defined as the intensity ratio between the electric fields around a metallic object under incident fields and the incident fields in absence of the metallic object. 49 Figure 1.4 shows a schematic configuration of a metallic sphere under an uniform incident electric field ( ). The electric field intensity is maximized at the direction 0, for most cases and the field intensity enhancement ( ) can be expressed as 1 2 2 1.2 where is electric field at a point of interest near the metallic particles in an environment in which there exists a incident field, and are the permittivity of the metal particle and the surrounding medium, respectively, is a function of the frequency of incident light, a is the radius of the metal sphere which << of incident light, and r is the radius vector from the particle center to a point of interest where the electric field is calculated. Equation 1.2 shows the field intensity enhancement decreases with increasing distance from the particle surface. For most cases, the field intensity enhancement reaches a maximum at the plasmon resonant frequency. 49 The local field enhancement around the metallic particles induced by their LSPR has been utilized for a number of applications such as surface-enhanced Raman spectroscopy (SERS) and fluorescence enhancement of the nearby fluorophores. 50,51 14

Fig. 1.4 Schematic configuration of a metallic nanoparticle under uniform incident electric field (E o ). 1.3.3 Galvanic replacement reaction Galvanic replacement reaction has been exploited as a powerful method to synthesize hollow metallic nanostructures. 52 Galvanic replacement reaction is driven by electrochemical potential difference between two metals, with the higher potential metal serving as a cathode and the lower one as an anode. The anode is defined as the electrode where oxidation occurs and the cathode is the electrode where the reduction takes place. A conventional example is Zn strip in a solution containing Cu 2+ ions. Since the Zn 2+ /Zn standard reduction potential (-0.76 V) is more negative than that of Cu 2+ /Cu (0.34 V), Zn is oxidized to Zn 2+ and Cu 2+ is reduced to Cu. This principle has been extended to synthesis of hollow metallic nanostructure, which the metal strip is replaced by metallic nanoparticles. For example, Ag nanoparticles widely used as a sacrificial template in the galvanic replacement reaction, reacted with AuCl 4 - solution to form hollow Au or Au-Ag alloy nanostructures. 53 The standard reduction potential of the AuCl 4 - /Au pair (0.99 15

V) is more positive than that of the AgCl/Ag pair (0.22 V), thus the oxidation of Ag nanoparticles by AuCl - 4 would take place to form Au, AgCl, and HCl. 54 The galvanic replacement reaction between Ag and HAuCl 4 is expressed as: 3Ag (s) + HAuCl 4 Au (s) + 3AgCl (s) + HCl (aq). In this reaction, Au formed from the reduction of AuCl 4 - would deposit on Ag nanoparticles. Interior cavity is formed when most of Ag solids have been oxidized, followed by the formation of hollow Au or Au-Ag metallic nanoshells. The shape of the metallic nanoshells depends on the shape of the sacrificial Ag templates. Since the reaction takes place in the solution, the surrounding mediums (e.g. solvents or surfactants) may be trapped in the interior cavity. 1.3.4 Metallic nanoshells Hollow metallic nanoshells may have tunable optical properties and a larger local field enhancement compared with that of their solid counterparts. 55 The LSPR extinction peak of hollow metallic nanoshells shifts to longer wavelength than that of their solid counterparts. The extinction peak of the nanoshells red shifts with decreasing shell thickness or increasing interior cavity. 56 For example, the extinction peak of Au solid nanopsheres with 50 nm in diameter is calculated to be ~530 nm in water medium and their corresponding Au nanoshells (diameter of 50 nm and shell thickness of 6 nm) have the peak at ~624 nm (Fig. 1.5a). The field intensity enhancement around the particle surface is larger for Au nanoshells compared with that of their solid counterparts (Fig. 1.5b) since the nanoshells showed a stronger coupling with the light due to the plasmon hybridization of both sphere and cavity. 49 The calculated field 16

intensity enhancement at the (outer) particle surface is as high as ~250 times for the Au nanoshells and ~34 times for the corresponding solid nanoparticles at their respective extinction peaks. Therefore, the metallic nanoshells may be a good candidate for the fluorescence enhancement of UC nanoparticles compared with their solid counterparts. The field intensity enhancement decreased with increasing the distance from the particle surface, consistent with Eq. 1.2. Fig. 1.5 (a) Calculated LSPR extinction spectra and (b) Calculated field intensity enhancement of Au solid nanospheres (50 nm in diameter) and spherical Au nanoshells (50 nm in diameter and 6 nm in shell thickness) in water medium. The LSPR extinction peak of metallic nanoshells is also more sensitive to the change of surrounding medium than that of their corresponding solid nanoparticles. LSPR sensitivity is commonly determined by the shift in wavelength of the extinction peak for a corresponding change in medium refractive index ( /Δ ) as measured in nm/refractive index unit (RIU). For example, the LSPR sensitivity was 60 and 408 nm/riu for solid Au 17

nanoparticles (diameter of 50 nm) and hollow Au nanoshells (diameter of 50 nm, shell thickness of 4.5 nm), respectively. 57 The optical sensitivity of hollow metallic nanoshells can be explained by plasmon hybridization, as discussed in Appendix B. 58,59 1.4 Plasmon-coupled fluorescence Previous studies showed interactions between metallic particles and fluorophores (e.g. organic dye, quantum dots) led to an increase or decrease in the fluorescence of the fluorophores, depending on the relative magnitudes of the fluorescence enhancement and quenching. 60,61 The interactions between the metallic particles and the nearby fluorophores may be described as follows: (1) enhanced light absorption of the nearby fluorophores due to field enhancement induced by the LSPR of the metallic particles (2) enhanced radiative emission of the fluorophores due to coupling to LSPR of the metallic particles, and (3) metal-dipole interaction leads to non-radiative energy transfer to the metal particles. 62 The first two terms lead to the fluorescence enhancement, whereas the third term causes the fluorescence quenching. All the three mechanisms are dependent on the distance of the fluorophores to the metallic particles. 63 The enhanced field around the metallic particles can concentrate the local excitation density, increasing light absorption of the nearby fluorophores and enhancing the fluorescence. The radiative emission is influenced by the balance of radiative and non-radiative decay rates. When the fluorophores are too close or in direct contact with the metallic particles, non-radiative energy transfer to the metallic particles would increase. 64,65 This non-radiative energy 18

transfer to the metallic particles leads to a decrease of the quantum yield (quenching) of the nearby fluorophores. For a fluorophore located close to a metallic particle, the enhancement of excitation rate can be expressed as 66 1.3 where and is the excitation rate in the present and absence of metallic particles, respectively, is a unit vector pointing in direction of transition dipole moment, is incident electric field, and is electric field at position of the fluorophore near the metallic particle. For electric field intensity at the direction 0 (parallel direction to incident field ) (Fig. 1.4), the excitation rate enhancement can be simplified to 1.4 Fluorescence intensity is proportional to the excitation rate and the quantum yield. 67 Fluorescence enhancement of a fluorophore located near a metallic particle is determined by the ratio between the fluorescence rate of the fluorophore close to the metallic particle and that in absence of the metallic particle. Therefore, the fluorescence enhancement (F) can be written as 1.5 19

where is the quantum yield of the fluorophore located near the metallic particle and is the intrinsic quantum yield (in absence the metallic particle). Intrinsic quantum yield is defined as the ratio between the intrinsic radiative decay rate ( ) and the total intrinsic decay rate of the fluorophore ( ). In the absence of the metallic particle, the total intrinsic decay rate is given by the sum of the intrinsic radiative and intrinsic non-radiative decay rates. The intrinsic quantum yield can be expressed as:, where is intrinsic non-radiative decay rate. For the fluorophores close to metallic particles, the relative quantum yield may be expressed as 60 1 1.6 where is the additional non-radiative decay due to the absorption by the metallic particles. Since the radiative and non-radiative decay rates are influenced by the distance from the fluorophores to the metallic particles, 68 the quantum yield in the presence of metallic particles is also distance-dependent. Theoretical calculations of the distance dependence of the non-radiative energy transfer rate from a dye molecule to a metal nanoparticle followed a distance dependence at large distances, whereas small deviations were observed at shorter distances. 69 However, recent studies showed that the resonance energy transfer rate is a distance dependence. 64,70 20

1.5 Motivation and Objectives The NIR-to-visible UC nanostructures have gained interests for a number of potential applications as discussed earlier. Today, it remains a big challenge to synthesize UC nanoparticles that have similar emission intensity of their bulk counterparts since the nanoparticles have large surface area (large number of surface atoms). For example, the emission intensity of UC nanoparticles with 8 30 nm in size was only 0.2 3 % of that of their bulk counterparts. 33 Therefore, it is necessary to find methods to enhance fluorescence of UC nanostructures. UC nanoshells with interior cavity have gained scientific interests due to their potential applications in bioimaging and drug delivery. These nanoshells have even a larger number of surface atoms than their solid counterparts due to their inner and outer surfaces. Hence, UC nanoshells may be a good candidate for studying the effects of surface and the UC fluorescence enhancement by surface coatings and plasmonic effects from metallic particles. NaYF 4 :Yb,Er with hcp phase is one of the most efficient UC materials. 26 Therefore, this material was selected to fabricate the UC nanoshells in this thesis. The fluorescence enhancement by plasmonic effects from metallic particles has mainly involved the molecule dyes or quantum dots. Recently, the plasmon-enhanced fluorescence has been applied to UC nanostructures. To date, plasmonic effects on the surface coverage- and distance-dependent fluorescence of UC nanostructures are not well-understood. Metallic nanoparticles can efficiently generate heat in the presence of electromagnetic radiation and subsequently transfer the heat to surrounding matrix. Most of 21

the previous studies have not considered the photothermal effects from the metallic particles on the fluorescence properties of nearby UC nanostructures. The local field enhancement near the particle surface may be larger for the metallic nanoshells than their corresponding solid nanoparticles. 49 Thus, the metallic nanoshells may be a good candidate to enhance the fluorescence of UC nanostructures. In this thesis, Au-Ag metallic nanoshells were selected as they have a strong plasmonic interaction with incident lights, leading to local field enhancement. To address the UC fluorescence issues, the effects of surface, surface coatings, and plasmonic on the fluorescence of the UC nanoshells were systematically studied. This thesis includes the followings: 1. Synthesis of NaYF 4 :Yb,Er UC nanoshells via thermal decomposition was carried out. The microstructure and optical properties were investigated. The effects of surface and surface coatings of undoped NaYF 4 on the fluorescence of NaYF 4 :Yb,Er nanoshells were studied 2. Synthesis of Au-Ag metallic nanoshells was conducted via galvanic replacement reaction of Ag templates with HAuCl 4. The microstructure and plasmonic properties of Au-Ag nanoshells were studied. The transformation from the Ag templates to Au-Ag nanoshells was systematically investigated. 3. The plasmonic effects of Au-Ag nanoshells on the fluorescence properties of NaYF 4 :Yb,Er nanoshells were studied. The layer-by-layer assembly of Au-Ag nanoshell layer/silica film/nayf 4 :Yb,Er nanoshell layer was prepared for different surface coverage % of Au-Ag nanoshell layer and different thicknesses of silica film (to control the distance from the 22

NaYF 4 :Yb,Er nanoshell layer to the Au-Ag nanoshell layer). Thus, the surface coverage- and distance-dependent fluorescence of the NaYF 4 :Yb,Er nanoshells were investigated. 1.6 Outline of the thesis The outline of this thesis is as follows: 1. Synthesis and characterization of NaYF 4 :Yb,Er nanoshells and the effects of surface and surface coatings on the UC fluorescence. 2. Synthesis and characterization of Au-Ag metallic nanoshells. The transformation from Ag templates to Au-Ag nanoshells via galvanic replacement reaction. 3. The plasmonic effects of Au-Ag metallic nanoshells on the fluorescence properties of NaYF 4 :Yb,Er nanoshells. In the following chapters, the use of term UC often refers to NaYF 4 :Yb,Er. 23