Coupling effects of NaYF4Yb,Er upconversion nanoshells and au ag metallic nanoshells 5

Chapter 5 The plasmonic effects of Au-Ag nanoshells on fluorescence properties of UC nanoshells 5.1 Introduction The interactions of noble metals with fluorophores such as organic dyes

Chapter 5 The plasmonic effects of Au-Ag nanoshells on fluorescence properties of UC nanoshells

5.1 Introduction

The interactions of noble metals with fluorophores such as organic dyes and quantum dots have been extensively investigated.122,123 A previous study reported the fluorescence of organic dyes was quenched when they were located near Au nanoparticles.68 However, other studies demonstrated the fluorescence of organic dyes could be enhanced by nearby Au particles due to the enhanced local field induced by the LSPR of Au particles.124,125 The fluorescence enhancement depended on the distance between the fluorophores and metallic particles.60,126 For a fluorophore sandwiched between two individual Au nanoparticles, the fluorescence was dependent on the separation distance between the two Au nanoparticles and polarization angle.127 The local field enhancement of metallic particles strongly depended on the separation distance between the adjacent metallic particles.128,129 Further study on Au nanoparticles and quantum dots layer-by-layer assembled on a substrate demonstrated that the Au nanoparticle concentration significantly affected the fluorescence quenching efficiency.65

Recently, the plasmonic effects of metallic particles have been utilized

to enhance the fluorescence of UC nanoparticles.5,130 However, the effects of concentration and distance of the metallic particles on fluorescence of UC nanoparticles are not yet well-understood It was reported that metallic nanoparticles generated heat in the presence of electromagnetic radiation and subsequent transferred the heat to the surrounding matrix.131,132 In many of

these studies, the photothermal effects of metallic particles on the fluorescence

of nearby UC nanoparticles were not considered into account Therefore, the interactions of the metallic particles with the nearby UC nanoparticles warranted further investigations

Local field enhancement of metallic nanoshells may be larger than that

of their solid counterparts as discussed in Chapter 1 The LSPR peak of metallic nanoshells can be tuned close to the excitation wavelength of a fluorophore, creating the even greater near-field enhancement under the excitation wavelength.49 Therefore, the metallic nanoshells may be a good candidate to enhance the fluorescence of UC nanostructures

In this chapter, the plasmonic effects of Au-Ag nanoshells on the fluorescence of UC nanoshells were investigated Silica film was used as a spacer to control the distance between the Au-Ag nanoshell layer and UC nanoshell layer assembled on a substrate The distance-dependent fluorescence of the UC nanoshells was studied The effects of Au-Ag nanoshell concentration (coverage %) on the substrates were also investigated The results showed the fluorescence of the UC nanoshells was either enhanced

or quenched by Au-Ag nanoshells, depending on the silica film thickness and the surface coverage % of the Au-Ag nanoshells The local field enhancement and photothermal effects of Au-Ag nanoshells on the surface coverage- and distance-dependent fluorescence of the UC nanoshells are discussed

5.2 Au-Ag nanoshell layer

Au-Ag nanoshells layer on a glass substrate was prepared using spin-coating The spin speed was 1500 rpm for 30 seconds for each cycle The larger Au-Ag nanoshells (which were synthesized from 43-nm Ag templates) were used for UC fluorescence enhancement since they had a larger field intensity enhancement around the nanoshell surface than their corresponding smaller nanoshells (which were obtained from 20-nm Ag nanoparticles) (Appendix Fig E.1) Figure 5.1 shows the SEM images of Au-Ag nanoshells deposited on the substrates using spin-coating for different number of coating cycles The Au-Ag nanoshells were randomly distributed to cover the substrate surface and most of the nanoshells appeared to form a single layer on the substrate The nanoshells were closer with each other with increasing number of coating cycles

Fig 5.1 SEM images of as-synthesized Au-Ag nanoshells spin-coated with

different number of coating cycles on the glass substrates: (a) 1 cycle, (b) 5 cycles, (c) 10 cycles, (d) 20 cycles, (e) 30 cycles, and (f) 40 cycles The scale

The SEM images showed the surface area of the substrates covered by Au-Ag nanoshells increased with increasing number of spin-coating cycles In this thesis, the surface coverage was defined as the surface area occupied by Au-Ag nanoshells divided by the total analyzed surface area The average surface coverage % of the Au-Ag nanoshell layer was calculated from the SEM images using Java image processing and analysis program (ImageJ) Figure 5.2a showed the calculated surface coverage % for different number of spin-coating cycles The surface coverage increased from 0 to 46% when the spin-coatings increased from 0 to 40 cycles Zero surface coverage % was referred to the absence of Au-Ag nanoshells on the substrates It was reported that the ~20% coverage of Au particles on a glass slide showed surface plasmon effects on the most part of fluorophore layer deposited on the Au particle layer.124

Figure 5.2b shows the LSPR extinction spectra of the Au-Ag nanoshell layer with different surface coverage % They had a similar extinction peak wavelength at ~613 nm However, the intensity of the extinction peak increased with the surface coverage %, as more Au-Ag nanoshells deposited

on the substrate surface with increasing number of spin-coating cycles

Fig 5.2 (a) Surface coverage of Au-Ag nanoshell layer on the glass substrate

calculated from the SEM images using Java image processing and analysis program (ImageJ) (b) LSPR extinction spectra of the Au-Ag nanoshell layer with different surface coverage % normalized to that of the glass substrate

5.3 Effects of surface coverage

The Au-Ag nanoshell layer with different surface coverage % on the substrates was further coated with silica film using sputtering Their extinction peaks red shifted from ~613 nm to ~640 nm after coating with the

silica film since the refractive index of silica (n = 1.45) is higher than that of air (n = 1.00), consistent with previous studies.57,133 Further, as-synthesized

UC nanoshells (7-nm interior cavity/4-nm shell) were deposited on the silica film-coated Au-Ag nanoshell layer to form an assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer using the deposition method as discussed

in Chapter 2 (Fig 2.2) Figure 5.3a shows an SEM image of the UC nanoshell layer The UC nanoshells were well-distributed and covered most of the surface The schematic for the cross-section of the assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer on the substrate is shown in Fig

5.3b Here, the decahedral Au-Ag nanoshells were used since the Au-Ag nanoshells mainly consisted of decahedral shape as discussed earlier (Chapter 4) In the schematic, the decahedral Au-Ag nanoshells lying down on the substrate wereviewed from side of the nanoshells This assembly was used to study the plasmonic effects of Au-Ag nanoshells on the fluorescence of UC nanoshells under 980-nm NIR laser

Fig 5.3 (a) SEM image of the UC nanoshell layer (b) Schematic for the

cross-section of the assembly decahedral Au-Ag nanoshell layer/silica film/UC nanoshell layer on the glass substrate, which under 980-nm NIR laser In the schematic, the decahedral Au-Ag nanoshells lying down on the substrate wereviewed from side of the nanoshells

Figure 5.4 shows the UC fluorescence spectra of the assembly of

Au-Ag nanoshell layer/silica film/UC nanoshell layer for different surface coverage % of the Au-Ag nanoshell layer Zero surface coverage % was referred to as the assembly without Au-Ag nanoshell layer (assembly of silica film/UC nanoshell layer) The UC fluorescence intensity was enhanced for the assembly in the presence of Au-Ag nanoshell layer at low surface coverage

% as compared with that in the absence of Au-Ag nanoshell layer The UC

fluorescence intensity initially increased with increasing Au-Ag surface coverage % When the surface coverage of Au-Ag nanoshell layer was higher than ~22%, the UC fluorescence intensity decreased with further increasing the surface coverage % Further, the intensity ratio of two green emission peaks changed with increasing the surface coverage of Au-Ag nanoshell layer This may be attributed to thermal effects from Au-Ag nanoshells, which is discussed in Section 5.3.2

Fig 5.4 UC fluorescence spectra of Au-Ag nanoshell layer/30-nm silica

film/UC nanoshell layer for different surface coverage % of Au-Ag nanoshell layer under 980-nm NIR excitation Zero surface coverage % was referred to

as the assembly without Au-Ag nanoshell layer (assembly of silica film/UC nanoshell layer)

Relative fluorescence factor (RFF) is defined as the total integrated UC

emission intensity of the assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer normalized by that of the assembly of silica film/UC nanoshell

(zero Au-Ag surface coverage %) When the RFF is higher than unity, it

indicates a fluorescence enhancement A fluorescence quenching is indicated

when the RFF is lower than unity Figure 5.5 shows the RFF calculated from the UC fluorescence spectra shown in Fig 5.4 The RFF was higher than

unity for the assembly in the presence of Au-Ag nanoshell layer at low surface

coverage % (region I), indicating the UC fluorescence enhancement The RFF

increased with increasing the Au-Ag surface coverage % and reached a

maximum value of ~2.5 at surface coverage of ~22% The RFF decreased

with further increasing the surface coverage from ~22% to ~46% (region II)

The RFF was observed to be ~1.0 at 46% surface coverage of Au-Ag

nanoshell layer

Fig 5.5 (a) Relative fluorescence factor (RFF) at different surface coverage

% of Au-Ag nanoshell layer RFF is defined as the total integrated UC

emission intensity of the assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer normalized by that of the assembly of silica film/UC nanoshells

5.3.1 Local field enhancement

The increase of surface coverage of Au-Ag nanoshell layer would lead

to an increase in the density of Au-Ag nanoshells on the substrates and decrease of separation distance between two adjacent Au-Ag nanoshells (Appendix Fig E.2) This would allow for a greater interaction between the adjacent Au-Ag nanoshells, leading to a larger local field enhancement.127 Figure 5.6a, b shows the calculated field intensity enhancement (| | | |⁄ )

of two adjacent Au-Ag nanoshells coated by silica film at the plasmon resonant and 980-nm NIR wavelengths, respectively, for different separation distances of the two Au-Ag nanoshells The field intensity enhancement was calculated at the surface of silica film (the insets of Fig 5.6a, b)

The calculation of field intensity enhancement was performed using the CST studio suite 2012 simulation software, frequency domain solver based

on the finite element method (FEM) The setting condition is described as follows The boundary condition for X and Y axis (parallel to the surface of glass substrate) was periodic and the Z axis (perpendicular to the surface of glass substrate) was open The plane wave propagation (k) was in direction of

45o to the substrate surface Drude model was used for the refractive index of

Au and Ag In this simulation, we assumed the Au-Ag nanoshell formed an Au-Ag alloy and the interior cavity was filled with toluene which used as a solvent in the synthesis of Au-Ag nanoshells, as discussed in Chapter 4 The composition of Au and Ag was measured to be 65% and 35%, respectively The dielectric function of the Au-Ag alloy was calculated from the dielectric function of Au and Ag based on their respective mole fraction.112,134

Fig 5.6 The calculated field intensity enhancement (|E|2/|E o|2) for different separation distance of two Au-Ag nanoshells at (a) the plasmon resonant and (b) 980-nm NIR wavelengths The field intensity enhancement was calculated

at the surface of the silica film (30 nm in thickness) as shown by the insets of

(a) and (b) The plane wave propagation (k) was in the direction of 45o to the glass surface The decahedral Au-Ag nanoshells (~39-nm interior

cavity/~6-nm shell) were used in the calculation The decahedral Au-Ag nanoshells lying down on the substrate were viewed from side of the nanoshells It assumed the interior cavity of the nanoshells was filled with toluene, which was used as a solvent in synthesis of the nanoshells as discussed in Chapter 4

The calculations showed the field intensity enhancement at the plasmon resonant and UC excitation (980 nm) wavelengths increased when the separation distance of the two Au-Ag nanoshells decreased from 90 nm to

2 nm The increase of the field intensity may be attributed to the increased coupling between the two adjacent Au-Ag nanoshells with decreasing their separation distance.135 This suggested higher surface coverage % of Au-Ag nanoshell layer would result in a higher local field enhancement since the separation distance between two adjacent Au-Ag nanoshells decreased with

increasing the Au-Ag surface coverage Therefore, the increase of RFF in the

region I (Fig 5.5) could be associated to the increase of local field intensity

When the surface coverage was higher than 22% (the region II), the RFF

decreased with increasing the Au-Ag surface coverage %, despite the increase

of local field intensity enhancement as predicted by Fig 5.6 The decrease of

RFF in the region II could be attributed to other factors that quench the UC

fluorescence, which are discussed in the following section

5.3.2 Thermal effects

Previous studies showed metallic particles could efficiently generate heat under electromagnetic radiation.136,137 The amount of generated heat increased with the number of metallic particles The heat would transfer from the metallic particles to surrounding matrix, increasing the temperature of the surrounding matrix, such as silica film and UC nanoshells in our present study

It was also reported the fluorescence emission of UC nanoparticles was quenched with increasing temperature.138

Figure 5.7a shows two green emission peaks of UC spectra for the assemblies of Au-Ag nanoshell layer/silica film/UC nanoshell layer and silica film/UC nanoshell under 980-nm NIR excitation These two emission peaks,

~525 nm and ~545 nm were attributed to 2H11/2 - 4I15/2 and 4S3/2 - 4I15/2

transitions of Er3+, respectively (Appendix Fig F.1) Previous studies reported the intensity ratio of 2H11/2 - 4I15/2 to 4S3/2 - 4I15/2 transitions (R HS) increased with increasing temperature.139,140 In this thesis, the R HS of UC nanoshells increased for the assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer as compared with the silica film/UC nanoshell layer under the 980-nm NIR irradiation This suggested that the temperature of UC nanoshells in the assembly with Au-Ag nanoshell layer increased compared with the assembly