It is demonstrated that photoluminescence of DCJTB can be enhanced by surface plasmons occurred in silver nanoparticle arrays on glass substrates fabricated by using nanosphere lithography (NSL) combined with reactive ion etching (RIE). By changing the size of the seed polystyrene nanosphere with fixed thickness of SiO2 film as a buffer layer between silver nanoparticles and fluorescent dye, we systematically studied the interaction between surface plasmons in Ag nanostructures and fluorescent dye by measuring the photoluminescence and time-resolved photoluminescence (TRPL) of the samples. As compared with pure DCJTB, it is observed that PL enhancement as high as 9.4 times and life time shortening from 0.966 ns shortened to 0.63 ns can be achieved with polystyrene nanosphere 430nm in diameter. The physical origin due to plasmonic excitation has been clarified from 3D finite element simulations, as well as the assistance of UV-visible reflectance spectrum.
© 2013 OSA
Metal enhanced fluorescence (MEF) recently attracted great interest for enhancing the fluorescence intensity and shortening the fluorescence lifetime of fluorescent molecules adsorbed on metallic nanostructures. With proper choice of the wavelength of incident light and geometry of metallic nanostructures, surface plasmon resonance (SPR) around the metallic nanostructures will be excited and lead to giant electromagnetic field strength for enhancing the absorption efficiency of the fluorescent substance, thus enhancing itsfluorescence intensity [1, 2]. Many studies on the MEF substrate have been reported [3,4] and the MEF can be applied to diverse research fields, such as enhancing OLED luminescent intensity , biosensors [6, 7], biomedical applications [8, 9], and single-molecule fluorescence .
Surface plasmon resonance (SPR) is a phenomenon occurred at the interface between metal and dielectric material with free electrons of the metal surface interfered by external electromagnetic field . Under certain conditions, the charge density of free electrons, driven by horizontal component of the electric field of external electromagnetic wave, will induce longitudinal dipole oscillation, leading to transport phenomena of collective longitudinal resonance effect in the metal surface. Since SPR is excited at the metal-dielectric surface, the resonance condition for SPR excitation is sensitive to the local refractive index change of environment and giant electromagnetic field will be generated around the metallic nanostructures. With these unique optical properties, SPR is also widely applied to the research fields of biosensors [12, 13] and surface-enhanced Raman scattering (SERS) [14–19]. Different shapes and distributions of metallic nanoparticles have been fabricated and applied to the research fields of MEF [20–23]. With excitation of SPR, fluorescence of molecules adsorbed on the nanoparticles could be enhanced. It is pointed out that when the absorption peak of the surface plasmon of the nanoparticles overlapped with the excitation and emission spectrum peaks of the fluorescent molecules, the fluorescence emission of adsorbed molecules can be strongly enhanced [22, 23]. With the absorption spectra of fluorescent molecules matched the SPR spectrum of nanostructure; we can thus achieve the purpose of optimal fluorescence enhancement by adjusting the geometry of metallic nanostructure. Recently, periodic Ag nanoparticle arrays have been fabricated by electron beam lithography (e-beam lithography) to enhance the emission intensity of LED. By tuning the coupling between InGaN multiple quantum wells and surface plasmon excited at the Ag nanoparticle arrays, 2.8-fold enhancement in peak photoluminescence intensity is demonstrated . However, the cost and fabrication process of e-beam lithography are quite expensive and complicated, alternative low-cost and east-to-operate fabrication method will make MEF more promising in practical applications.
Nanosphere lithography (NSL) is able to produce a large area of periodic-array nanostructure. Several advantages of NSL include the applicability to different substrates, fast fabrication speed, and low-cost. The period and size of nanoparticle array could be controlled by changing the size of nanosphere, leading to adjustable SPR absorption band of the resulting metallic nanostructure. This method can also generate two particular kinds of structures: arrays of triangular nanoparticles with lift-off; and nanoparticles without lift-off. We refer to this structure as “metal film over nanosphere” (MFON) , for example, it is called AgFON when the metal is silver . In this paper, we demonstrated that photoluminescence of DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) can be enhanced by surface plasmons occurred in silver nanoparticle arrays on glass substrates fabricated by using nanosphere lithography (NSL) combined with reactive ion etching (RIE).
DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) is a red-luminescent dye, which is commonly employed in organic light-emitting diode (OLED) as a guest luminescent material [27–30]. By changing the size of seed polystyrene nanosphere with fixed thickness of SiO2 film as a buffer layer between silver nanoparticles and fluorescent dye; we studied the interaction between surface plasmons in Ag nanostructures and DCJTB by measuring the photoluminescence (PL) and time-resolved photoluminescence (TRPL) of the samples. With the introduction of Ag nanoparticle arrays fabricated by NSL method, it is thus possible to enhance the photoluminescence efficiency of DCJTB in practical OLED applications by the excitation of surface plasmon.
Figure 1 shows side view of plasmon-enhanced DCJTB with the Ag nanoparticle arrays fabricated by using NSL method. Ag nanoparticle array was fabricated by drop-coating polystyrene nanospheres with different diameters onto glass substrates. The substrates (Matsunsmi cover glass, 22 × 22 mm, 0.12–0.17 mm thickness) were first cleaned using acetone and methanol in an ultrasonic bath for 30 min. The diameters of polystyrene nanospheres were 430 nm, 500 nm, and 600 nm, respectively. The nanospheres were self-assembled into a single layer of hexagonally close-packed 2D colloidal crystals onto the substrate. Nanowell structure with 5 nm in depth was fabricated by using reactive ion etching (RIE) with the work gas O2:SF6 (5 sccm: 20 sccm), the RF power 20 W and the etching time 2 minutes . The presence of nanowell structure could increase the adhesion between the Ag nanoparticles and glass substrate. Ag film with 50 nm in thickness was deposited with a deposition rate of 4 Ǻ/s by thermal evaporation under the pressure of 5 × 10−6 Torr. The samples were then immersed in an ultrasonic bath filled with methanol for lift-off process to remove the nanospheres, resulting in triangular Ag nanoparticle arrays left on the glass. SiO2 with 20 nm in thickness as a buffer layer was then deposited by using E-gun evaporator. DCJTB (LT-E704, Luminescence Technology Corp.) with 75 nm in thickness was finally deposited onto the SiO2 buffer layer with thermal evaporation. The purpose of introducing SiO2 buffer layer between Ag nanoparticle arrays and DCJTB is to prevent quench process of fluorescent molecules. If the distance between metallic nanostructures and fluorescent molecules is too close, the energy of excited molecules will possibly be absorbed by the metallic nanoparticles. On the contrary, if the distance is too large, the field strength of plasmons will become too low. Thus there persists an optimal thickness of buffer layer for maximum plasmonic coupling efficiency . We have tried to make the thickness of SiO2 buffer layer as small as 5 nm. However, precise thickness-control in the manufacture of less than 5nm SiO2 is not an easy task and the intensity enhancement is comparable no matter the thickness of SiO2 is 5nm or 20 nm. We therefore choose the thickness of SiO2 buffer layer to be 20 nm to demonstrate the idea in our experiment. Atomic force microscope (AFM, Nanosurf Mobile S) was used to analyze the morphology of the samples. Silicon nanoprobe tips with a diameter of 10 nm were used.
All of the AFM images were collected in the contact mode with an applied force of 18 nN, and the scanning range was 5 μm × 5 μm. PL and TRPL measurement systems are composed of a cw and pulsed mode switchable 375 nm diode laser (PicoQuant, PDL800-D), an inverted microscope and spectrometer (Jobin Yvon, MicroHR) with two exits. One exit of the spectrometer is equipped with photomultiplier tube (PMT) and another is equipped with photon counter (TimeHarp 200). In PL measurement, the laser light (CW mode, 11mW) is passing through a mirror into an inverted microscope and focused onto the sample by using an objective lens with 100X magnification and 0.9 NA. The spot size focused on the sample is about 20 μm × 10 μm. The fluorescence light scattered from the sample is then passing through a high-pass filter with cutoff wavelength at 385 nm to reject the incident laser light and then analyzed by the spectrometer with PMT detector. In TRPL measurement, the diode laser is operated in pulse mode with pulse-width 55 ps at FWHM and the laser light is passing through the same apparatus as PL measurement, except that the fluorescence light is collected and analyzed by spectrometer equipped with photon counter. The spectrometer is at first tuned to a measured wavelength and the time delay between laser pulse and photon of measured wavelength is detected. By accumulation the time-delay signals from photon counter (TimeHarp 200), probability distribution of the fluorescence light versus time can be obtained. By fitting the distribution with exponential function, lifetime of the sample can then be found.
3. Results and discussions
Triangular Ag nanoparticle arrays were fabricated by using NSL method by drop-coating polystyrene nanospheres with diameters 430 nm, 500 nm, and 600 nm onto glass substrates. With lift-off process to remove the nanospheres, triangular Ag nanoparticle arrays will be left on the glass. Figure 2 shows the SEM images of triangular Ag nanoparticle array fabricated with nanosphere 430 nm in diameter and Ag 50 nm in thickness. This result shows that large area and periodic nanoparticle array can be fabricated by NSL method. The size of individual nanoparticle αSL and the distance between triangular nanoparticles αip,SL can be tuned by changing the size of nanosphere D according to αSL = 0.223D and αip,SL = 0.577D . As the size of nanoparticle changes, the corresponding plasmonic response of the metallic nanostructure will also changes. UV-visible reflectance was employed to measure the absorption spectra of the fluorescent dye and substrates. Figure 3(a) shows the measuredresults of DCJTB and the substrates with nanospheres 430nm, 500nm and 600nm in diameter, respectively, together with the absorption spectrum of the molecule. The two peaks observed in each of the spectra of the substrate can be verified to be originated from different plasmonic modes of the system, with the higher frequency one from the peripheral mode and the low frequency one from the apex mode. We have confirmed these using numerical simulations (COMSOL) with dielectric constant of silver adopted from modified Drude model . Among these three substrates, the silver nanoparticle array with nanosphere 430 nm in diameter is most suitable to act as plasmonic enhanced substrate for DCJTB due to the fact that the absorption peak of DCJTB is very close to the 490 nm and 590 nm absorption peaks of silver nanoparticle array with nanosphere 430 nm in diameter.
After deposition of 20 nm SiO2 buffer layer and 75 nm DCJTB, PL measurement was employed to examine the plasmonic enhancement in luminescent intensity of DCJTB with different size of silver nanoparticle arrays as substrate. Figure 3(b) shows the results of PL measurement from DCJTB on glass substrate and DCJTB on Ag nanoparticle arrays with nanosphere 430nm, 500nm and 600nm in diameter. In PL analysis, the laser power density is 5.5 kW/cm2 with acquisition time 1s. Irradiating the same place several times, damage really occurs. If we measure different places with the same power density and acquisition time, then the PL signal remains a constant. One may avoid sample damage by shortening the acquisition time. From Fig. 3(a) with Fig. 3(b), it is observed that fluorescent molecules will emit more photons due to strong electric field coupling when the plasmon resonance of substrate nanostructure is highly overlapped with the absorption band of molecules. By integrating all the area of luminescent spectrum and comparing the intensity from DCJTB on Ag nanoparticle arrays and glass substrate, the enhancement factor as high as 9.41 can be achieved by using the substrate with 430nm nanosphere. The rest of enhancement factors are 6.28 and 4.05 for substrate with 500nm and 600 nanospheres, respectively. By the way, DCJTB is often used as a doping material and the distance between discrete molecules should be large enough to maintain its unique spectral properties. Since the powder of DCJTB is directly thermal-evaporated onto the substrates, aggregation of the molecules of DCJTB will lead to quenching of photo-excited molecules and red-shift and broadening of its fluorescence spectral.
TRPL measurement was then employed to examine the fluorescent lifetime of DCJTB with different size of silver nanoparticle arrays as substrate. Figure 4 shows the TRPL results of the four samples as described in Fig. 3. The lifetimes of DCJTB on glass substrate, Ag nanoparticle arrays with nanosphere 430nm, 500nm and 600nm in diameter were fitted to be 0.966ns, 0.63ns, 0.702ns and 0.669ns, respectively. It is obvious that lifetime of DCJTB shrank 35% as it deposited on Ag nanoparticle arrays with nanosphere 430nm in diameter due to strong field coupling occurred when the plasmon resonance of substrate nanostructure is highly overlapped with the absorption band of molecules.
In order to understand the observed enhanced fluorescence intensity (Fig. 3b), we have performed the following theoretical analysis. Starting from the following well-established result for enhanced-fluorescence yield :Fig. 5 the field enhancement factor using COMSOL for the computation of the electric fields at 20 nm from the apex of neighbor nanoparticles with incident light at the wavelength of 500 nm. It is obvious that the computed field enhancement results are completely in consistency with the increasing of the enhanced fluorescence intensity with the decrease of nanoparticle size. As the computation for the lifetimes and quantum yields for such a complicated geometry will be extremely involved even using commercial software such as COMSOL, here we shall consider a simplified model of a radiating molecular dipole interacting with a sphere to mimic the real situation. The analytical results are well available from the literature . Figure 6(a) shows the variation of the total decay rate of a molecule (with radial dipole moment) as a function of the sphere radius while Fig. 6(b) shows the same variation for the quantum yield. The location of the molecule is at 20 nm from the sphere and the emission wavelength is set at 500 nm. It is clear from these results that one does expect an overall decrease in both the total decay (hence an increase in lifetime) and the quantum yield as the particle size increases. This accounts for part of the observation in Fig. 4, except that the results show reversed behavior for the cases with nanoparticle sizes of 500 nm and 600 nm. This probably shows the limit of our oversimplified model. Moreover, the overall decrease of quantum yield (Fig. 6(b)) does correlate very well with what was observed in Fig. 3(b). Note that the oscillatory (interference) behavior in Fig. 6 will likely disappear when one considers a real geometry with deviation from a highly symmetric one like the sphere.
We have sucessfully demonstrated that photoluminescence of DCJTB can be enhanced by surface plasmons occurred in silver nanoparticle arrays on glass substrates fabricated by using nanosphere lithography (NSL) combined with reactive ion etching (RIE). By changing the size of seed polystyrene nanosphere with fixed thickness of SiO2 film as a buffer layer between silver nanoparticles and fluorescent dye; we systematically studied the interaction between surface plasmons in Ag nanostructures and fluorescent dye by measuring the photoluminescence and time-resolved photoluminescence (TRPL) of the samples. As compared with pure DCJTB, it is observed that PL enhancement as high as 9.4 times and life time shortening from 0.966 ns shortened to 0.63 ns can be achieved with polystyrene nanosphere 430nm in diameter. Since nanosphere lithography (NSL) is able to produce a large area of periodic-array nanostructure with several advantages including the applicability to different substrates, fast fabrication speed, and low-cost, we believe that MEF based on the substrates fabricated by using NSL will become a promising technique with wide branch of applications.
Authors acknowledge financial support from the National Science Council of ROC under grant number NSC 100-2112-M-019-003-MY3. We also acknowledge Dr. Hung-Yi Chung of the Academia Sinica for his help in numerical computations.
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