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

We have known the optical properties of dispersion of Au nanoparticles into polymer matrix 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. It is known that luminescence can be enhanced near the metallic nanostructures, however in this section we wonder to know whether the optimal fluorescence enhancement can be achieved at a resonant distance between nanocomposite and emission layer.

In order to build a tunable distance between the Au particles and emissive polymer, we insert a PEDOT:PSS layer to form a spacer layer which the thickness (d) can be controlled by adjustable r.p.m. (revolution per minute) of

the spinner. Figure 4-4 shows the structure with introducing the spacer layer. The different thickness of spacer layer with 0 nm, 10 nm, 30 nm, 40 nm, 60 nm,

Figure 4-4 The inserted PEDOT:PSS layer between emission layer and nanocomposite plays an role of spacer layer, and a resonant distance may help to obtain an optimal

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and 80 nm, were respectively deposited into the location between emission layer and nanocomposite. The observed PL intensity is shown in Figure 4-5. (The 40 nm and 60 nm curves almost overlap with the other curves so that for clearing figure showing, we don’t put them on this figure.) The PL intensity of each thickness is normalized by the curve without spacer layer (d=0 nm). Figure 4-5 shows a result that the PL intensity get enhancement by adding the spacer layer, especially with the thickness of 30 nm. The result revels that luminescence can be enhanced near the metallic

nanostructures, and tuning the distances between nanocomposite and emission layer can even directly increase the radiative intensity of emissive polymer.

At a resonant distance, the optimal fluorescence enhancement can even be obtained. Dipolar mode of localized surface plasmon at gold NPs with average sizes 92 nm is more remarkably coupled with singlet polaron excitons in MEH-PPV layer, which in nature perform dipolar behavior. Furthermore, inserting a spacer layer tunes the distances of resonant coupling between dipoles and excitons. For a configuration

with a spacer layer of width d as 30 nm, the PL enhancement can be as high as 15x than the sample without spacer layer (d=0 nm). An additional result reveals that the color coordinates of different structure of each sample can be altered. It offers a color modulation way by altering layer structure (Figure 4-6).

Figure 4-6 Color modulation of different structure of samples on CIE 1931 color space

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4.4 R. Q. Y. R. (relative quantum yield ratio) V.S. Distance

R. Q. E. R. is defined as relative quantum efficiency ratio, which denotes the quantum corresponds to dipole approximation of theoretical analysis (Figure 2-6).

The R. Q. Y. R. of each various thickness from experimental results is fitted by red curve. At the region of distance d > 30𝑛𝑚 (red area), the nanoantenna approximation has been proven validly. It reveals that the enhancement of luminescence versus different d was decided by near-field attributed to the increasing non-radiative decay rate and radiative decay rate with decreasing distance. Since electric field in the sphere is a superposition of all multipole modes, all these Figure 4-7 R.Q.Y.R. as a function of d between the emitting layer and the nanoantennas. Black line shows the theoretical results by the exact electrodynamical method at the dipole approximation.

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modes contribute to the non radiative decay rate, whereas only the dipole mode is assumed to affect the radiative decay rate. The dipole approximation strongly overestimates the quantum yield at short distance and does not predict any quenching. Furthermore, the conformational defects also introduced a potential quenching channel such as trapping states at the interfaces.

4.5 Characteristic of Electroluminesecence on the Device

Current density (J)-Voltage (V), and EL spectra characteristics of various devices fabricated in this work are shown in Figure 4-8. It is noteworthy that the performance of the PLEDs is significantly improved when the PEDOT:PSS-Au nanocomposite layer is used compared to the one using PEDOT:PSS only as a hole injection layer. Especially for

Figure 4-8 (a) current density-voltage and (b) EL spectra characteristics of polymer light emitting devices made with the PEDOT:PSS and the PEDOT:PSS-Au nanocomposites with different thickness of spacer layer.

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using PEDOT:PSS-Au nanocomposite layer with a spacer layer of thickness d=30 nm, the electronic characteristic is improved strongly. At electroluminescence, highest EL intensity of proposed scheme were observed at d=30 nm as predicted by PL measurement and optical model. The results reveal that introduction of nanocomposite layer and spacer layer did not deteriorate electrical performance in PLED, whereas the luminescent quantum efficiency can be enhanced.

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Chapter 5

Conclusion and Future Work

5.1 Conclusion

Surface plasmon resonances in metallic nanoparticles are of interest for a variety of applications due to large local field enhancement that occurs in the vicinity of metal surface.

And a metallic nanoparticle sphere acts as a nanoantenna in the quasi-static dipole approximation limit of Mie theory. In this study, gold NPs-incorporated PEDOT:PSS was demonstrated in the nanocomposite form. The dipole mode of localized surface plasmon at gold NPs with diameter as average size 92 nm remarkably coupled with singlet excitons in the MEH-PPV layer, which in nature perform dipolar behavior. Theory predicted that spectral behaviors of coupled excitation in excitons are modulated by the separation due in part to the near-field effect of localized surface plasmon and in part to the Purcell effect, while the nanoantenna assumption only applied at long distances. These predictions were confirmed in

a nanostructured polymeric system by inserting a bare PEDOT:PSS layer as spacer to tune the spatial overlap between dipoles and excitons. Combination of nanocomposite and spacer layer discloses the merit of metal enhanced fluorescence (MEF) effect, as well as color tunability.

Table 5-1 Comparisons of the characteristics of various configurations

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Compared to the sample without Au particles mixing into PEDOT:PSS layer, surface plasmon resonance coupling to molecular excited state resulted in fluorescence enhancement of up to 2.3-fold in overall relative quantum efficiency. Optimal design of spacer layer (30 nm) enhances the Q.Y. as high as 30x. By inserting of nanocomposite and spacer layer, not only the optical efficiency can be effectively enhanced without sacrifice the electric properties.

Dual-functional nanocomposite plays the role of hole-injection layer in organic lighting devices, and possesses potential for photovoltaic applications.

5.1 Future work

Although the structure with optimal Q.Y. has been realized in photoluminescent results, some issues regarding electroluminescence have to be expounded. Firstly, the roughness of nanocomposite layer and interface morphology can pronouncedly affect the charged carrier injection. EL quantum efficiency _related to PL intrinsic quantum efficiency _emby

r em

     

where  is a balanced charge injection factor, which is depend on the processes of carrier injection, and _rquantifies the efficiency of the formation of a singlet exciton from a positive and a negative polaron. We expect the width of spacer layer d and the existence of

nanoparticles are relevant to . However, the mechanism depicting the interplay between injected charged carriers and nanoparticles is still ambiguous. We hope EL measurement will help us unveil the underlying relationship between _ and _em.

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