Coupling effects of NaYF4Yb,Er upconversion nanoshells and au ag metallic nanoshells 1

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.. Low phonon e

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

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 light-emitting diodes (LED),13 solar cells,14 and bioimaging.15

Successful synthesis of UC nanoparticles led to exploration of visible 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

NIR-to-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

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-

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 NaYF4 and NaGdF4) and oxides (e.g Y2O3) 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

Na+, Ca2+ and Y3+ 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) NaYF4 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, NaYF4 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 (Ln3+) The 4f electrons in the lanthanide ions are shielded from the surroundings by filled outer 5s2 and 5p6 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 Laporte-forbidden, 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 (Ln3+) 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

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, Yb3+ 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 Yb3+ ion located around 980 nm is attributed to the 2F7/2 - 2F5/2 transition (Fig 1.2) The 2F7/2 - 2F5/2 transition energy gap of Yb3+ ion is matched well with many 4f–4f transitions energy gap of other lanthanide ions (e.g Er3+, Tm3+, and Ho3+) which are commonly used as the activator ions This promotes efficient energy transfer from Yb3+ions to the other neighboring lanthanide activator ions in the UC materials

Fig 1.2 A schematic 4f energy-level diagram of Yb3+ (sensitizer ion) and Er3+

(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

would lead to a decrease of UC emission due to concentration quenching For example, the doping concentration of Er3+ did not exceed 3 % in most Er3+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 Yb3+ sensitizer is codoped into the UC materials The concentration of Yb3+ doped in UC materials is commonly 18 – 20 % To

date, hcp phase NaYF4 codoped with Yb3+ and Er3+ is one of the most efficient NIR-to-visible UC materials.26 The hcp NaYF4:20%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, NaYF4:Yb,Er nanoparticlesproduce the UC emission through the 4f-4f transitions of Er3+

Optically active 4f electrons of lanthanide ions are shielded by filled outer 5s2 and 5p6 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

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.33Further, 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

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

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