Mental Enhancement Fluorescence

Fluorescence detection is the basis of many measurements in biological research, and it provides numerous measurement opportunities including studies of biological macromolecules, cell imaging, and DNA sequencing. In our research, we investigate the fluorescence quantum yield of MEH-PPV of its separation from a light irradiated, spherical gold nanoparticle. Our approach based on the use of gold nanoantennas is promising for various applications such as enhancement of weakly fluorescing systems.

From section 2.2.1, 2.2.2, the spherical nanoparticle acts as an electric dipole when the particle size is much smaller than the wavelength of light in the surrounding medium (d <<λ ), and as a light is irradiated on the particle, the electric field 𝐄 is described by equation (2-10b). To consider a single molecule located at 𝐫_m and represented by a three-level system with transition dipole moment p and transition frequency ω. The extrinsic quantum yield (Q.Y) can be represented as a three-level process which involves the excitation probability η_exc and the intrinsic quantum yield (emission decay rate ratio) η_em = γ^r , with γγ _r and γ being the radiative decay rate and the total decay rate, respectively (Figure 2-5). The extrinsic quantum yield can then be written as

Q. Y = number of radiative photons

number of incident photons = η_exc ∙ η_em= η_exc ∙_γ ^γr

r+γnr (2-14) where γ_nr = γ − γ_r is the nonradiative decay rate and η_exc ∝ 𝐄(𝐫_m, ω) ² is the excitation rate depending on the total local excitation field 𝐄(𝐫_m, ω) (incident plus scattered) [21][22].

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From equation (2-14), the fluorescence is the product of two processes：(1) Excitation by the incident field which was influenced by the local environment, and (2) emission of radiation influenced by the balance of radiative and

nonradiative decay. The source of process (1) is the external radiation field, and in process (2) it is the molecule itself which makes up the source.

Based on the pioneering work by Purcell, the emission rate of excited molecule depends not only its life time but also of the environment. It was realized that the modification of the life time is influenced by the radiative decay rate due to photon emission and by the irradiative decay rate due to energy dissipation in the

Figure 2-5 Sketch of a molecule represented as a three-level system

Figure 2-6 Nanoantenna enhanced quantum efficiency by exact electrodynamical method

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environment. For atoms or molecules close to metal surfaces both rates can be enhancement.

Excited-state lifetimes of single molecules have been measured as a function of their separation from light-irradiated metal boundaries and satisfactory agreement with theory has been achieved. However, previous experiments showed either fluorescence enhancement or fluorescence quenching. The problem originates that the competing effects between local field enhancement leads to an increased excitation probability and nonradiative energy transfer to the particle leads to a decrease of the quantum yield (quenching). The competing factors can be taken into accounts by the equation (2-14), η_exc , γ_r , and γ_nr .By the exact electrodynamical method at the dipole approximation, the relative factors (η_exc, η_em) corresponded to quantum yield (Equation 2-14) was shown in Figure 2-6, which is plotted by exact electrodynamical.

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Chapter 3

Experiment

3.1 Introduction

In order to see how the LSP modifies the spectroscopic properties of fluorescent polymer, we deposited an layer structure containing Au NPs (~92 nm diameter) dispersed in PEDOT:PSS as the solvent, and a layer (100 nm thickness) of MEH-PPV play a role of active layer. The separation (deposited with PEDOT:PSS) between gold nanoparticle and emission layer can be altered to obtain an optimal quantum yield. For discussing photoluminescence, the unpolarized light with specific wavelength (depends on the peak of absorption spectrum of individual sample) is incident into samples on the side of emission layer. And the detector is on the same side of the incident light with the same incident angle to the normal of the sample.

An important advantage of this polymeric system is not only the photoluminescence can be discussed but also electrolumiscence does, when the structure (Figure 1-3) is sandwiched by the ITO (Indium Tin Oxide) layer (anode) and aluminum layer (cathode). Once the voltage is applied to electrodes, the electrolumiscence can be discussed. At this time, nanocomposite layer functions as not only a hole-injection layer but also a metallic nanostructure layer.

We propose not only a simple way but also a stable technology to fit the ideal experimental structure we designed (layer-by-layer structure). That is the spin-coating fabrication (Figure 3-1). It provides a convenient step-by-step method for precise and uniform deposition of thin films and coating, and the materials including polymer or nanocomposite can be easily deposited on quartz or ITO substrates. At the following parts, the detail experimental-process will be presented step by step.

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Figure 3-2 Fabrication flow for optical and electrical measurement 3.2 Fabrication Flow

In our research, we investigate the fluorescence quantum yield of MEH-PPV of its

separation from a light irradiated, spherical gold nanoparticles. However, in our system, once the fluorescence is discussed, the measurement of photoluminescent (PL) and electroluminescent (EL) are good ways to examined. We presented a feasible experimental process of fabricating the samples for optical measurement (PL) and devices for electrical measurement (EL). For the electroluminescent measurement, the ITO substrate has to be Figure 3-1 Spin coating is an economical and fast method to produce homogeneous layers.

An excess amount of the solvent is placed on the substrate, which is then rotated at high speed by spinner in order to spread the fluid by centrifugal force. The film thickness can be adjusted by varying the rotation speed, the rotation time, and the concentration of the used solution.

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Figure 3-3 Size distributions. Insets show SEM images.

patterned, cleaned, and layer deposited sequentially, and in the final the device is finished to be measured. For the photoluminescent measurement, the electrodes of metal deposition can be ignored. After cleaning the quartz substrate and depositing polymer layers sequentially, the samples for photoluminescence measurement is completed. The Figure 3-2 shows the fabrication flow.

3.2.1 Cleaned ITO & Quartz Substrate and Surface Treatment ITO or Quartz substrate (GE214/124) was oxygen residues and showing a good wetting property before PEDOT:PSS film coating. absorption and scattering properties of gold NPs, a conclusive gold NP parameter over sizes and

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concentration would be decided. 3 different average size of Au nanoparticles distribution 63±29, 72±33 and 92±36 nm were synthesized. Figure 3-3 shows the particles size distributions of synthesized Au NPs as a function of reaction time. Inset shows SEM images.

3.2.3 Preparation of PEDOT:PSS-Au Nanocomposite & MEH-PPV

PEDOT:PSS (Baytron® . P， Al 4083) has been widely adopted as a hole injection material in achieving highly efficient PLED. As it forms layer structure by spin-coating, the layer is a transparent and conductive polymer. It both flattens the surface and matches the work function of the anode of the device. Before making use of PEDOT:PSS solution, it must be filtered through a 0.22 um membrance filter (PVDF). 10 w.t. % hybrid PEDOT:PSS-Au nanocomposite solutions were prepared and then to be put into an ultrasonic bath for 30 min.

MEH-PPV is conjugated, hole-transporting polymer. In this research, it is prepared for 0.8 w.t.

% using toluene as solvent. The molecule structure of PEDOT:PSS and MEH-PPV are shown in Figure 3-4(a) and (b), respectively.

Figure 3-4 Molecular structure of (a) PEDOT:PSS (b) MEH-PPV.

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Figure 3-5 The cross-section of sample

Figure 3-6 Schematic description of pattering ITO/quartz substrate.

3.2.4 Preparation of Samples

After cleaning process on quartz substrate, the PEDOT:PSS-Au nanocomposite solution was spin-coated at 2000 rpm onto the cleaned ITO/quartz substrates, and then baked at 140℃

for 1 hr. Spacer layer (PEDOT:PSS) with various thickness was subsequently spin-coated on the nanocomposite layer. In the final, the emitting layer (MEHPPV) with the thickness of 100 nm was deposited on top of the spacer layer. Figure 3-5 shows the structure.

3.2.5 Patterning ITO/quartz substrate ITO (Indium Tin Oxide) is a transparent conducting material. It is always coated on the quartz substrate.

Before fabricating devices, ITO must be patterned for defining efficient emitting area. A general patterning flow is shown in Figure 3-6.

At first, the tapes are applied to ITO substrate where we want to retain. This substrate is then put into shallow container filled with HCl

MEH-PPV

Quartz PEDOT:PSS + Au NPs

PEDOT:PSS

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Figure 3-7 The patterning ITO/quartz substrate which we design.

liquid for 2min at 70℃, and sequentially to put the substrate into the container filled with D.I.

water as fast as possibile for rinsing out the residual HCl. The parts which are mantled by tapes will be left, the others will be etched off by HCl liquid. In the final, using the tweezer to pick up the residual tapes, and a patterened ITO/quartz is finished. Figure 3-7 shows the ITO substrate after patterening.

3.2.6 Device Fabrication

The various thickness of device structures, ITO/PEDOT:PSS-Au/ PEDOT:PSS (spacer) /MEH-PPV/Al, were prepared in the following manner for EL measurement.For all structures, the patterned indium-tin-oxide quartz substrates were cleaned ultrasonically with acetone, detergent ,and DI water each for 40min. After cleaning, 10% hybrid PEDOT:PSS-Au nanocomposite solutions was spin-coated at 2000 rpm onto the cleaned ITO substrates and baked at 110 °C for 4 h. Spacer layer (PEDOT: PSS) were subsequently spin-coated on the PEDOT-Au nanocomposite layer; a 100-nm layer of emission layer (MEH-PPV) were sequentially deposited. For applying the voltage to active areas, the rest parts of ITO have to be rinsed. After spin coating each layer on quartz, the sample must be cleaned with individual solvent to remove the polymer layer almost everywhere from the sample, except for the middle region that the active areas located at. And then the samples are attached to a designed

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Figure 3-8 The cross-section of the device and the functions of each layer.

mask to deposit a layer of Al. Like Figure 3-9 shows. The battle used as a insulator for isolating each emission area. The emitting area of the device was defined as 3 mm _ 3 mm.

3.2.7 Package

After depositing the cathode, we package the sample with a smaller piece of glass in the glove box under nitrogen environment. The package processes includes applying the transparent UV-curably glue around the small glass, covering it on our sample and finally expose it to UV-light for 40 seconds. After exposure, the UV-curable adhesive holds together the glass and quartz, so that the active layers between them are protected well.

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3.3 Measurements

PL and optical absorption were carried out by Acton Research Spectra Pro-150 and PerkinElmer Lambda 650, respectively. The thickness of film was measured by alpha-step and AFM. The voltage-current-luminance characteristics were measured using an optical power meter ( PR-650 ) and digital source meter ( Keithley 2400 ). In our research, Ultraviolet-Visible spectroscopy and photoluminescence spectroscopy offer rigorus measurements of plasmon resonance band of Au NPs and the system between light-irradiated particle and active layer, respectively. In the following section (3-9), (3-10), a detail introduction of them will be given.

Figure 3-9 The perspective drawing of the finished device.

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3.3.1 Ultraviolet-Visible (UV-Vis) Spectroscopy

A scheme of the components of a simplified spectrometer (Figure 3-10) is shown in the following diagram. An incident light (colored red) comes from two sources, visible or UV

light. Each single wavelength is extracted by a prism or diffraction grating, and the beam of light is split into two equal intensity beams by a half-mirrored component. The sample beam (Green magenta) passes through the examined sample. The reference beam (colored blue) passes through an identical substrate of sample. The substrate depends on what the materials you want to make it as a reference. For gold colloid, it is cuvette containing only the D.I.

water, and for quartz-substrate samples, it is only the quartz substrate. The intensities of these light beams are then measured by electronic detectors and compared. The intensity of the reference beam, which should have suffered little or no light absorption, is defined as I₀. The

Figure 3-10 Scheme of the components of UV-Vis Spectroscopy

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intensity of the sample beam is defined as I. Over a short period of time, the spectrometer automatically scans all the component wavelengths in the manner described. The ultraviolet (UV) region scanned is from 200 to 400 nm, and the visible portion is from 400 to 800 nm.

Absorption is defined as A= log I₀/I. If no absorption has occurred, A= 0. The wavelength of maximum absorbance is a characteristic value, designated as λmax. Different examined samples may have very different absorption maxima and distribution of absorption. For absorbing intensely sample, it must be examined in dilute solution, so that the passed light received by the detector is significant, and to use transparent solvents to prevent extra absorption.

3.3.2 Photoluminescence Spectroscopy

Figure 3-11 (a) A simple scheme of the Photoluminescene Spectroscopy (b) An energy-transfer diagram of photoluminescence

(a) (b)

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A scheme of the photoluminescence apparatus is shown in Figure 3-11(a).

Photoluminescence is excited by an unpolarized light (wavelength of the light is adjustable).

The luminescence radiation is filtered by a monochromator and detected by a detector. The PL signal is amplified by a Photomultiplier tube (PMT). And the detector is on the same side of the incident light with the same incident angle to the normal of the sample.

Photoluminescence (abbreviated as PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons (Figure 3-11(b)). Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. This is one of many forms of luminescence (light emission) and is distinguished by photoexcitation (excitation by photons). The period between absorption and emission is typically extremely short, in the order of 10 nanoseconds. Under special circumstances, however, this period can be extended into minutes or hours. More interesting processes occur when the chemical substrate undergoes internal energy transitions before re-emitting the energy from the absorption event.

The most familiar such effect is fluorescence, which is also typically a fast process, but in which some of the original energy is dissipated so that the emitted light photons are of lower energy than those absorbed. The generated photon in this case is said to be red shifted.

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Chapter 4

Results and Disscussions

4.1 Surface Plasmon Characterization of Gold Nanoparticles

When gold nanoparticles interact with an electromagnetic wave, a unique resonance behavior occurs at visible spectrum. The phenomenon leads to localized surface plasmon which depicts the non-propagating excitations of conduction electrons of metallic nano-structures coupled to photons. At the beginning, we explored the physical insight into the optical characteristics of gold nanoparticles.

Figure 4-1 Surface plasmon extinction spectra of spherical Au nanoparticles, with (a) different Au nanoparticles sizes and (b) different Au nanoparticles concentration (C1, C2, C3,and C4)

Extinction spectra for average Au nanoparticles size 63, 72, and 92 nm in diameter suspended in aqueous solution are shown in Figure 4-1(a).The extinction spectra of gold NPs

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exhibit a dependence of particle sizes due to resonantly enhanced absorption and scattering.

From Figure 4-1, it can be observed that the surface plasmon absorption resulted in red shift with increasing the particle size.

For nanoparticles radius r much small smaller than the wavelength of the incident light ( roughly 2r <^λmax₁₀ ), dipole oscillation significantly contributes to the extinction cross-section and explained by dipole approximation of the Mie theory [20]. However, for larger nanoparticles where the dipole approximation is no longer valid, the plasmon resonance depends explicitly on the particle as a function of particle radius. The larger the particle become, the more important are the higer-order modes. At this time, the light can no longer polarize the nanoparticles homogenously. The higher-order modes peak at lower energies and therefore the plasmon band redshifts with increasing particle size.

On the other hand, the concentration increase results in a widening bandwidth, while the plasmon bandwidth is associated with the dephasing of the coherent electron oscillation Figure 4-1(b). Large bandwidth corresponds to increasing loss of the coherent electron motion. In the following section, NPs with the most dilute concentration (C₄= 0.1 w.t.%) and with the largest radius (r=92±36 nm) were adopted since their absorption peak overlaps that of the fluorescence emitter.

4.2 Fluorencense Enhancement by nanocomposite

In this section, we blend gold nanoparticles into PEDOT:PSS to form a nanocomposite material which is capable to manipulate the photon coupling from emitting photons with the non-propagating surface plasmon. Au-incorporated nanocomposite offers the ability to study local electromagnetic interactions in the plasmon-fluorenscense system. The example of fluorescent emitter, MEH-PPV, located in proximity to the surface plasmon polariton field of the metal is a case in point here.

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Three samples were prepared as shown in Figure 4-2. Sample R is a bare MEH-PPV layer spin-coated on the quartz substrate as a reference, sample A is the PEDOT:PSS layer inserted into the structure of sample R, and sample B replaces the PEDOT:PSS layer with Au

Figure 4-2 Cross-section of the samples (R, A, and B)

Figure 4-3 Absorption and photoluminescence spectra of samples with (sample B) and without (sample A) gold-incorporated nanocomposite layer.

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nanoparticles-incorporated PEDOT:PSS nanocompsite layer. The sample design includes practical features which aim at the utilization of the exciton-plasmon interaction in an optoelectronic device.

The observed PL as well as absorption spectra is plotted in Figure 4-3. While fluorescence emitted from MEH-PPV passed through the PEDOT:PSS layer as a reference, the FWHM of the PL spectrum is about one-half of the absorption spectrum and has a vibronic feature in the shoulder at the long-wavelength region [24]. The respective emission peaks ( normalized ) represent vibrational transitions between energy levels ：0-0 and 0-1. However, as the fluorescence coupled with nanocomposites, the presence of noble metal surfaces can significantly impact the manner in which incident photoexcitation was converted into photons.

The relative contribution of absorption and radiative scattering to the overall extinction spectrum of a metal nanostructure is a crucial factor that dictates the conversion efficiency of incident photons to fluorescence emission from the nanoparticles/emitter system.

This interesting phenomenon can be explained by the radiating plasmon (RP) model [1], where plasmons induced by resonant near-field interactions of fluorescence emitter with

Table 4-1 Optical properties of test samples and their corresponding photoluminescence (PL) characteristics. R.Q.E.R. is defined as a relative quantum efficiency ratio, which denotes the quantum efficiency in contrast to the no nanoparticles incorporated sample (sample A).

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metallic nanostructures (having dominant scattering properties) will radiate the spectral properties of the excited state intensely, thereby leading to observations of emission enhancements even from the fluorescence emitter with high free space quantum yields. The lower shoulder peak accounting for the 0-1 state energy transition (at 614 nm) is relatively suppressed for no surface plasmon mediated interplay with the nanoparticles, and FWHM of overall emission spectrum was thus minimized. According to Table 4-1, the emission of sample B peaked at 577 nm with a narrow FWHM, that incurs a red saturated color corresponding to 1931 Commission Internationale de l’Éclairage (CIE) color coordinates of (0.55, 0.45). To sum up, the narrowed FWHM of bandwidth implied the rising amount of radiative emission into visible region and the furthering of the radiative efficiency, which is a promising characteristic for electroluminescence of potential applications as PLEDs.

To make a brief summary, gold NPs-incorporated PEDOT:PSS was demonstrated in nanocomposite forms. We have also investigated the optical properties of insertion of nanocomposite layer which led to surface plasmon mediated photoluminescence enhancement from the emissive layer, MEH-PPV. Surface plasmon polariton resonance coupling to molecular excited state resulted in enhancement of up to 2-fold in overall relative quantum efficiency. Meanwhile, the photoluminescence spectra peaked a more saturated color corresponding to 1931 CIE coordinates of (0.55, 0.45) stemmed from the more narrow bandwidth feature. These samples designed with practical features target at the eventually performance optimization of the polymeric optoelectronic device by exciton-plasmon interaction.

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4.3 Optimal Fluorencense Enhancement by nanocomposite

4.3 Optimal Fluorencense Enhancement by nanocomposite