Chapter 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 85    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. fluorescence of the UC nanoshells was studied. The distance-dependent 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. 86    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) cycle, (b) cycles, (c) 10 cycles, (d) 20 cycles, (e) 30 cycles, and (f) 40 cycles. The scale bars for the images (a – f) are m. 87    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 to 46% when the spin-coatings increased from 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. 88    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 (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. 89    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 were viewed 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 were viewed from side of the nanoshells. Figure 5.4 shows the UC fluorescence spectra of the assembly of AuAg 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 90    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 91    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. 92    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 93    Fig. 5.6 The calculated field intensity enhancement (|E|2/|Eo|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/~6nm 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 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. 94    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 (RHS) increased with increasing temperature.139,140 In this thesis, the RHS 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 95    with no Au-Ag nanoshell layer. For the UC assembly with Au-Ag nanoshell layer, the LSPR of the Au-Ag nanoshells that coupled with emitted lights from the UC nanoshells or 980-nm NIR lights from the excitation laser would generate heat. Since the 980-nm excitation light from the laser was much stronger than the emitted light from UC nanoshells, therefore, the heat may be mainly generated from the absorption of 980-nm NIR light by Au-Ag nanoshells. These generated heat could transfer to the silica thin film and subsequently the UC nanoshells, increasing the temperature of UC nanoshells. The RHS decreased when Au-Ag nanoshell layer were removed from the assembly (silica film/UC nanoshell layer) as shown in Fig. 5.7a, indicating the decrease in temperature of the UC nanoshells. The temperature of the UC nanoshells in the assembly was calculated from the RHS using by Boltzmann law, which can be expressed as I I ln / / ln / ∆ 5.1 / ∆ 5.2 where I (2H11/2 – 4I15/2) and I (2S3/2 – 4I15/2) are emission intensity due to transitions of 2H11/2 – 4I15/2 and 2S3/2 – 4I15/2, respectively, ∆ between 2H11/2 and 2S3/2 energy levels, is energy gap is Boltzmann constant, C is a constant, and T is absolute temperature. The detailed calculation is shown in Appendix F. 96    Fig. 5.7 (a) UC emission spectra of the assemblies of Au-Ag nanoshell layer/30-nm silica film/UC nanoshell layer (Au-Ag surface coverage of 46%) and silica film/UC nanoshell layer at green region. (b) The calculated temperature of UC nanoshells in the assembly of Au-Ag nanoshell layer/30nm silica film/UC nanoshell layer for different surface coverage % of Au-Ag nanoshell layer. Figure 5.7b shows the calculated temperature of UC nanoshells in the assembly increased as the surface coverage % of Au-Ag nanoshell layer increased. This suggested more heat was generated with increasing number of Au-Ag nanoshells. This heat would transfer from Au-Ag nanoshells to the UC nanoshells, increasing the temperature of UC nanoshells. It was reported that UC fluorescence intensity of Er3+ doped chalcogenide glasses (Ga2S3: La2O3) decreased with increasing temperature from 23 oC to 200 oC due to nonradiative losses.141 This temperature-dependent UC fluorescence was further confirmed using hcp NaYF4:Yb,Er UC nanoparticles.138,142 Figure 5.7b demonstrates that a sharp temperature increase was observed when the surface coverage of Au-Ag nanoshells was higher than 22%. This sharp increase in temperature could contribute to a significant fluorescence quenching of the 97    UC nanoshells, decreasing the UC emission intensity. This explains the decrease of RFF when the Au-Ag surface coverage was higher than 22% (the region II) as shown in Fig 5.5. At these regions, the RFF decreased with increasing the Au-Ag surface coverage %. When the surface coverage increased to 46%, the RFF decreased to ~1.0. This result suggested the fluorescence quenching by photothermal effects and the fluorescence enhancement by local field enhancement could occur, depending on the coverage of metallic nanoshells. 5.4 Distance-dependent UC fluorescence To study the effects of distance between Au-Ag nanoshell layer and UC nanoshell layer on the UC fluorescence, Au-Ag nanoshell layer on the substrate was coated with different thicknesses of silica films (5 – 180 nm) to form silica film-coated Au-Ag nanoshell layer (Appendix Fig. G.1), followed by the deposition of UC nanoshells. The silica film was used as a spacer between the UC nanoshell layer and Au-Ag nanoshell layer. Therefore, the distance between the UC nanoshell layer and Au-Ag nanoshell layer was controlled by thickness of the silica film. In this study, Au-Ag nanoshell layer with surface coverage ~22% was used since it produced a maximum UC fluorescence enhancement as discussed earlier. The distance-dependent UC fluorescence of the UC nanoshells in the assembly was then investigated under 980-nm NIR excitation. Figure 5.8a shows the RFF of the assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer for different thicknesses of silica film. The result showed the RFF was lower than unity for the assembly with silica 98    film smaller than nm. This indicated the UC fluorescence was quenched when the silica thickness was less than nm, even though the calculated field intensity enhancement of Au-Ag nanoshells was high at this region (Fig. 5.8b, c). A possible explanation for this result is given as follows. Previous reports showed the fluorescence of fluorophores (e.g. organic dyes and quantum dots) was quenched when they were too close or in direct contact with metallic nanoparticles due to non-radiative energy transfer from the fluorophores to the metallic nanoparticles.143,144  The non-radiative energy transfer from the fluorophores to the metallic particles at short separation distance is commonly associated to fluorescence resonance energy transfer (FRET). The quenching efficiency of the fluorophores to metallic particles sharply increased as the distance from the fluorophores to the metallic particles decreased.65,69 However, there was almost no fluorescence quenching to the metallic nanoparticles when the distance increased to ~30 nm.70 By using the similar explanation, in our present study, when silica film thickness < nm, the UC nanoshells may be too close with Au-Ag nanoshells to allow non-radiative losses from the excited UC nanoshells to Au-Ag nanoshells, leading to the UC fluorescence quenching. The result suggested, at these distances, the magnitude of fluorescence quenching due to non-radiative losses to Au-Ag nanoshells were probably greater than the magnitude of fluorescence enhancement caused by the local field intensity enhancement of Au-Ag nanoshells, leading to a RFF lower than unity. When the silica thickness increased to nm, the RFF increased to 1.0. This could be the distance whereby the fluorescence enhancement by enhanced local field intensity and the quenching due to the non-radiative 99    losses equally contributed, resulting in no change in the total fluorescence intensity. When the silica thickness increased from to 30 nm, the RFF increased and reached a maximum value of ~2.5 at 30-nm silica thickness. At these silica thicknesses, the magnitude of UC fluorescence enhancement due to the enhanced local field intensity by Au-Ag nanoshell layer was probably larger than the magnitude of fluorescence quenching due to the non-radiative losses, leading to fluorescence enhancement. Further increasing the silica thickness from 30 – 180 nm, the RFF decreased toward unity (unenhanced value) as the local field intensity enhancement decreased with increasing distance from the surface of Au-Ag nanoshells and toward unenhanced field intensity (Fig. 5.8b, c). When the UC nanoshells were far from the Au-Ag nanoshells separated by thick silica film (> 120 nm), the UC fluorescence intensity of the assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer was similar to that of the assembly of silica film/UC nanoshell layer. A 3D contour plot of the field intensity enhancement for Au-Ag nanoshells on the glass substrates is shown in Fig. 5.8d. The above results indicated that there existed an optimized coverage of metallic nanoshells and the separation distance between metallic nanoshells and UC nanoshells, where the effects of local field enhancement predominated the non-radiative effects due to the metallic shell’s coupling with UC nanoshells. The fluorescence enhancement or quenching would depend on the complicated competitions among these factors. 100    Fig. 5.8 (a) Relative fluorescence factor (RFF) of the assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer with different thicknesses of silica film. Calculated field intensity enhancement (|E|2 ̸ |Eo|2) for Au-Ag nanoshells coated with different thicknesses of silica films at (b) the plasmon resonant and (c) 980-nm NIR wavelengths. The field intensity enhancement was calculated using the CST studio suite 2012 simulation software at the silica surface (the insets of (b) and (c)), where the UC nanoshells were deposited on. The setting condition was similar with that discussed earlier. (d) The 3D  contour plot of the field intensity enhancement at the plasmon resonant wavelength for Au-Ag nanoshell on the glass substrate.   The color scale is logarithmic. 101    5.5 Summary This study demonstrated that the UC emission of the UC nanoshells can be either enhanced or quenched by Au-Ag nanoshells, depending on the silica film thickness and the surface coverage % of the Au-Ag nanoshells. When the silica thickness was fixed, for low Au-Ag surface coverage (< 22%), the UC fluorescence intensity increased with increasing surface coverage % due to the increase of local field intensity enhancement of Au-Ag nanoshells. When the surface coverage was higher than 22%, the UC fluorescence intensity decreased with increasing surface coverage % due to the thermal effects of Au-Ag nanoshells. When the surface coverage % of Au-Ag nanoshell layer was fixed, the UC fluorescence quenching, enhancement, and subsequent unenhanced UC fluorescence intensity were found to be related to increasing thickness of silica film. 102    [...]... nanoshell layer (Au- Ag surface coverage of 46%) and silica film/UC nanoshell layer at green region (b) The calculated temperature of UC nanoshells in the assembly of Au- Ag nanoshell layer/30nm silica film/UC nanoshell layer for different surface coverage % of Au- Ag nanoshell layer Figure 5. 7b shows the calculated temperature of UC nanoshells in the assembly increased as the surface coverage % of Au- Ag nanoshell... intensity enhancement for Au- Ag nanoshells on the glass substrates is shown in Fig 5. 8d The above results indicated that there existed an optimized coverage of metallic nanoshells and the separation distance between metallic nanoshells and UC nanoshells, where the effects of local field enhancement predominated the non-radiative effects due to the metallic shell’s coupling with UC nanoshells The fluorescence... emission of the UC nanoshells can be either enhanced or quenched by Au- Ag nanoshells, depending on the silica film thickness and the surface coverage % of the Au- Ag nanoshells When the silica thickness was fixed, for low Au- Ag surface coverage (< 22%), the UC fluorescence intensity increased with increasing surface coverage % due to the increase of local field intensity enhancement of Au- Ag nanoshells When... 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, ~52 5 nm and ~54 5 nm were attributed to 2H11/2 - 4I 15/ 2 and 4S3/2 - 4I 15/ 2 transitions of Er3+, respectively (Appendix Fig F.1) Previous studies reported the intensity ratio of 2H11/2 - 4I 15/ 2... distance from the surface of Au- Ag nanoshells and toward unenhanced field intensity (Fig 5. 8b, c) When the UC nanoshells were far from the Au- Ag nanoshells separated by thick silica film (> 120 nm), the UC fluorescence intensity of the assembly of Au- Ag nanoshell layer/silica film/UC nanoshell layer was similar to that of the assembly of silica film/UC nanoshell layer A 3D contour plot of the field intensity... 5. 1 / ∆ 1 5. 2 where I (2H11/2 – 4I 15/ 2) and I (2S3/2 – 4I 15/ 2) are emission intensity due to transitions of 2H11/2 – 4I 15/ 2 and 2S3/2 – 4I 15/ 2, respectively, ∆ between 2H11/2 and 2S3/2 energy levels, is energy gap is Boltzmann constant, C is a constant, and T is absolute temperature The detailed calculation is shown in Appendix F 96    Fig 5. 7 (a) UC emission spectra of the assemblies of Au- Ag. .. study the effects of distance between Au- Ag nanoshell layer and UC nanoshell layer on the UC fluorescence, Au- Ag nanoshell layer on the substrate was coated with different thicknesses of silica films (5 – 180 nm) to form silica film-coated Au- Ag nanoshell layer (Appendix Fig G.1), followed by the deposition of UC nanoshells The silica film was used as a spacer between the UC nanoshell layer and Au- Ag nanoshell... non-radiative losses from the excited UC nanoshells to Au- Ag nanoshells, leading to the UC fluorescence quenching The result suggested, at these distances, the magnitude of fluorescence quenching due to non-radiative losses to Au- Ag nanoshells were probably greater than the magnitude of fluorescence enhancement caused by the local field intensity enhancement of Au- Ag nanoshells, leading to a RFF lower than... absorption of 980-nm NIR light by Au- Ag nanoshells These generated heat could transfer to the silica thin film and subsequently the UC nanoshells, increasing the temperature of UC nanoshells The RHS decreased when Au- Ag nanoshell layer were removed from the assembly (silica film/UC nanoshell layer) as shown in Fig 5. 7a, indicating the decrease in temperature of the UC nanoshells The temperature of the UC nanoshells. .. layer and Au- Ag nanoshell layer was controlled by thickness of the silica film In this study, Au- Ag nanoshell layer with surface coverage ~22% was used since it produced a maximum UC fluorescence enhancement as discussed earlier The distance-dependent UC fluorescence of the UC nanoshells in the assembly was then investigated under 980-nm NIR excitation Figure 5. 8a shows the RFF of the assembly of Au- Ag . 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 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. 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,