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Abstract
We present the enhancement of the hot-electron emissions by the enhanced electric field with deep UV surface plasmon resonance (DUV-SPR). An aluminum disk–hole array was designed using the finite-difference time-domain (FDTD) method for enhancing the electric field by the disk–hole cavity coupling. We found that the photoelectron emission efficiencies were experimentally improved by four times and that the disk–hole distance was a key factor to induce the strong disk–hole coupling. The aluminum disk–hole array with DUV-SPR would be expected for many applications, such as the highly sensitive photodetectors, the photoelectron guns, and the efficient photocatalysts.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface plasmon resonance (SPR) has attracted many attentions due to the characteristics of electric field enhancement and field confinement effects on a metal surface. It has been applied to many applications, such as high-sensitivity biosensors [1,2,3], cancer therapy [4], photovoltaics [5], nanolasers [6], photodetectors [7–9], refractive index sensors [10], optical waveguides [11], fluorescence measurements [12], color filters [13,14], and surface-enhanced Raman scattering [15]. In recent years, the excitation range of surface plasmons has shifted to the deep ultraviolet (DUV) region, where fundamental and applied research has been demonstrated [16–19]. Because the photon energy of DUV light is higher than that of visible light or near-infrared light, it is expected to be applied to simultaneous excitation of multi-stained fluorescent dyes [20], tip-enhanced Raman scattering [21], and photoelectron emission from metal surfaces [22–24].
Some studies have been reported on the application of the enhancement of hot-electron emission from metal surfaces by using deep ultraviolet surface plasmon resonance (DUV-SPR) [25–35]. Yan Shen et al. applied SPR to multiphoton absorption process and improved the quantum efficiency to 40.16% by incorporating gold particles into a graphene emitter [34]. Péter Dombi et al. elucidated the electron generation from metallic nanoparticles through SPR using ultrashort laser pulses [35]. It has also been reported that the plasmon-enhanced photoelectrons improve the reaction efficiency of photocatalysts [36–38]. Macek et al. reported in 1972 the enhancement of the photoelectron yield by DUV-SPR [22]. Our group had also achieved the photocurrent enhancement by DUV-SPR in the Kretschmann configuration [23,24]. The incident intensity, anode–cathode bias voltage, and time dependence of the photoelectron emission efficiency were measured to further elucidate the relationship between surface plasmons and photoelectron emission [24].
In this study, we applied an aluminum (Al) periodic nanostructure for the excitation of DUV-SPR and improved the photoelectron emission efficiency. A periodic array structure would be useful for the device applications because it allows SPR excitation at a normal incidence of light. We propose the periodic nanostructure composed of Al nano-disk array and nano-hole array (Fig. 1).
The Al hole array is used not only for SPR excitation but also for the cathode electrode. By combining with the disk array, the strong electric filed enhancement at the hole array would be expected owing to their SPR coupling between the disk array and hole array. The disk array and hole array are electrically isolated. When DUV light with a wavelength of 266 nm is irradiated into the Al disk–hole array from the disk array side, the surface plasmons are excited and the electric field intensity is greatly enhanced at the edge of the disk and the hole. Hot-electrons are emitted from the hole array side by applying a bias voltage. Enhanced electric field intensity increases the hot-electron emission efficiency according to Fermi's golden rule. The disk–hole distance is one of the important structural parameters because it determines the coupling effect of SPR between the disk array and hole array and it brings further improvement of the emission efficiency. We optimized the structural parameters of period, diameter, Al thickness, and disk–hole distance to excite DUV-SPR at a wavelength of 266 nm. The advantages of the disk–hole array structure over the only hole array structure were also discussed based on the simulations. We have experimentally demonstrated the hot-electron emission enhancement with DUV-SPR in the optimized structure.
2. Hot-electron emission by DUV light and the structural model
Figure 2(a) shows the energy band diagram of the hot-electron emission process. Electrons in the Fermi energy level in metal are emitted into the vacuum when the photon energy is higher than the work function ϕ of the metal. In other words, when the kinetic energy Ekin of the excited electron is greater than zero, the electrons are emitted into the vacuum. The kinetic energy Ekin of the hot-electron is expressed by Eq. (1):
where h is the Planck’s constant, ν is a frequency of the incident light, and Eb is a binding energy of the electrons in a metal. The work function of Al used as photocathode is 4.28 eV, so we need a shorter wavelength less than 290 nm for photoelectron emission excitation. The photon energy used in this study is 4.66 eV (wavelength: 266 nm). Under these conditions, the kinetic energy of the electron excited at the Fermi level is 0.38 eV, which satisfies the conditions for photoelectron emission.
Fig. 2. (a) Energy band diagram of the hot-electron emission. (b) Simulation model of the Al disk–hole array.
Figure 2(b) shows a simulation model of the unit cell for the Al disk–hole array. The structure has four structural parameters: period, diameter, Al thickness, disk–hole distance. Quartz which is transparent for DUV light was used as the substrate. The excitation light is normally incident from the quartz side. The disk and hole arrays are periodically arranged in a hexagonal lattice. We optimized the structural parameters using the finite-difference time-domain (FDTD) method (FullWAVE, Synopsys, Inc.).
3. Simulation results for optimization of structural parameters
The optimized structural parameters with a resonant wavelength of 266 nm were a period of 160 nm, a diameter of 86 nm, an Al thickness of 30 nm, and a disk–hole distance of 86 nm. We also simulated the only hole array for comparison. The optimized parameters for the only hole array were a period of 165 nm, a diameter of 120 nm, and an Al thickness of 30 nm. Figure 3(a) shows the reflectance spectra of their optimized Al disk–hole array and Al only hole array. Reflectance dip was observed at a wavelength of 266 nm in both structure as their optimization. Since the dip of the disk–hole array was sharper than that of the only hole array, a higher efficient resonance was obtained at the disk–hole array. This is due to the coupling effect caused by the holes and disks, the structure confines an incident light energy because of cavity-like behavior between the disk array and hole array. The clear difference in the structural parameters between the disk–hole array and only hole array was the hole diameter. In terms of the fabrication, it is desirable to decrease the aspect ratio of diameter to Al thickness. In other words, the disk–hole array has a smaller aspect ratio leading to easier fabrication.
Fig. 3. Comparison of the spectra and electric field enhancement effect between the disk–hole array and only hole array by using the FDTD method. (a) Simulated reflectance spectra of the disk–hole array (red) and only hole array (blue). (b) Simulated absorption spectra of the disk–hole array (red), only hole array (blue), and only disk array (green). (c) Electric field intensity distribution in the x–z cross-section of the disk–hole array. (d) Electric field intensity distribution in the x–z cross-section of the only hole array.
Figure 3(b) shows the simulated absorption spectra of the disk–hole array (red), only hole array (blue), and only disk array (green) with the structural parameters optimized for disk–hole array. The resonance wavelengths of the disk–hole array, only hole array, and only disk array were 266 nm, 290 nm, and 320 nm, respectively. The resonance wavelength of the disk–hole array shifted to the shorter wavelength due to the coupling of the plasmon oscillations of the disk array and hole array, indicating that the antibonding mode was excited. Clausen et al. reported that, in the antibonding mode, the resonance in the disk–hole array was higher than that in the only hole array or the only disk array structure [39].
Figures 3(c) and 3(d) show the electric field intensity distribution of the disk–hole array and only hole array, respectively. The electric field was significantly enhanced at the edges in both structures. The electric field intensity was greatly enhanced at the wider region in the disk–hole array than that in the only hole array.
Figure 4(a) shows the disk–hole distance dependence of the averaged electric filed intensity on the hole array surface. The averaged intensity was sinusoidally oscillated on the distance with a period of approximately 100 nm, which corresponds to a half of propagation wavelength λneff. Here, effective refractive index neff was calculated by averaging each refractive index of air and SiO2 according to their area ratio. The maximum intensity at a distance of 86 nm was enhanced by more than twice compared to at the other distances.
Fig. 4. The electric field intensity distribution in the disk–hole array depending on the disk–hole distance. (a) Disk–hole distance dependence of the averaged electric field intensity on the hole array surface. (b-e) Electric field intensity distributions when the disk–hole distances with the resonance point are selected. Disk–hole distances were (b) 86 nm, (c) 170 nm, (d) 270 nm, and (e) 370 nm.
Figures 4(b)–4(e) show the electric field intensity distributions when the disk–hole distances were 86 nm, 170 nm, 270 nm, and 370 nm, respectively. The electric field intensity was enhanced at the edge of the structure, and the enhancement was significantly increased at a disk–hole distance of 86 nm. The strengthened coupling effect was induced by the proximity between the disk array and hole array.
4. Experimental results of photoelectron emission
Al disk–hole array was fabricated based on the structural parameters optimized by the simulation for the resonance at a DUV wavelength of 266 nm. The following is the fabrication procedure. A quartz substrate was cleaned with piranha solution for 10 min. After hexamethyldisilazane treatment, a positive-type electron beam resist (ZEP520A, Zeon Corporation) was spin-coated on the substrate and pre-baked at 180 °C for 2 min. Third, hole patterns were exposed to the resist using an electron beam lithography system (ELS-7700K, ELIONIX). The resist was developed by o-xylene for 2 min. Hole patterns of the resist were transferred by etching the quartz substrate using the reactive ion etching (RIE) system (RIE-10NR, Samco). This etching time was controlled for the disk–hole distance. After RIE, the resist was removed by oxygen ashing for 10 min. Finally, Al was evaporated onto the etched quartz substrate using a resistance heating evaporation system.
Figure 5(a) shows the scanning electron microscope (SEM) image of the fabricated disk–hole array. The hole pattern was periodically arranged in a hexagonal lattice. The period and the diameter were 161 ± 1 nm, and 98 ± 3 nm, respectively. The Al thickness and the disk–hole distance were measured using the surface profiler (Alpha-Step IQ, KLA-Tencor), and their values were 24 ± 3 nm, and 60 ± 1 nm, respectively.
Fig. 5. (a) SEM image of the fabricated disk–hole array surface. The scale bar is 100 nm. (b) Measured (solid line) and simulated (broken line) reflection spectra of the disk–hole array. (c) Resonance wavelength as a function of the disk–hole distance.
Figure 5(b) shows the reflection spectra of the disk–hole array samples obtained using the reflection spectrometer (FE-3000, Otsuka Electronics Co., Ltd). The solid and the broken lines express the measured reflection spectrum and the simulated reflection spectrum, respectively. The reflectance dip at 266 nm indicates that the fabricated Al nano disk–hole array excites DUV-SPR at this wavelength. The measured spectrum exhibits lower reflectance than the simulated spectrum in over the DUV to visible wavelength range. We consider that the reason why it exhibits low reflectance is because incident light was scattered by the fabrication imperfection such as surface roughness, edges of holes and disks. Although the measured reflectance dip drops to almost zero, some incident light was used for the scattering. It will result in reduced photoelectron emission efficiency. Figure 5(c) shows the measured resonance wavelength as a function of the disk–hole distance. The resonance wavelength was red-shifted in proportion to the disk-hole distance. The disk–hole distance was measured at the alignment mark near position of the sample by using surface profiler. Although the absolute values of these measured distance may include the etching rate fluctuation due to the difference of opening size between holes and alignment mark, the resonance wavelength has a linear dependence to the disk–hole distance.
To evaluate the photoelectron emission characteristics, we measured the incident light intensity dependence of the photoelectron emission efficiency and the incident light polarization angle dependence of the enhancement factor. The photoelectron emission efficiency was calculated by dividing the photocurrent by the incident light power. The enhancement factor is defined as the efficiency of the disk–hole array divided by that of the aluminum thin film without the nanostructure.
Figure 6 shows a schematic of the optical setup used to measure the photoelectron emission efficiency. The fourth harmonic of the Nd: YAG laser (FQCW 266-10, CryLaS) was used as the DUV excitation light. The laser power can be varied up to 0.1 mW with a neutral-density filter. The fabricated samples were placed in a vacuum chamber. A vacuum level was 10−5 Pa. The laser beam was focused at a spot area of 0.083 mm2 on the sample by a lens with a focal length of 300 mm.
Figure 7(a) shows the incident laser power dependence of the photocurrent. The red circular dots show the photocurrent of the disk–hole array with a disk–hole distance of 60 nm optimized for excitation DUV-SPR at a wavelength of 266 nm, and the photoelectron emission efficiency was achieved to 0.87 nA/mW. The blue triangular dots were the results of the Al thin film without nanostructure, and the efficiency was 0.22 nA/mW. These results show that the photocurrent linearly increases with incident laser power. We found that the disk–hole array structure could enhance the photoelectron emission efficiency by approximately 4 times compared to the Al thin film. In our previous research with Kretschmann configuration, we performed 2.9 nA/mW emission efficiency. We considered that all energy of the incident light was consumed for SPR excitation in the Kretschmann configuration, and electric field was uniformly enhanced on the metal surface. On the other hand, in the disk–hole array structure, the incident energy was lost by the electrically isolated disk array, and the electric field was locally enhanced at the edge of the holes. We expect the local enhancement would be higher than Kretschmann configuration owing to the cavity resonance, but averaging the emissions in excitation area decrease the efficiency. However, this array structure has an advantage in the sense for the practical applications because SPR was excited by normal incidence.
Fig. 7. Enhancement of the hot-electron emission efficiency by the disk–hole array. (a) Incident light power dependence of photoelectron emission efficiency. Red: disk–hole array. Blue: Al thin film. (b) Polarization dependence of the enhancement factor. (c) Enhancement factor as a function of disk–hole distance.
Figure 7(b) shows the incident light polarization angle dependence of the enhancement factor. The incident light polarization angle was changed from 0° to 360° using a half-wave plate, and the enhancement of the photoelectron emission efficiency was obtained for each polarization angle. As shown in Fig. 7(b), the enhancement factor was independent on the polarization angle owing to the hexagonal array. As a result of them, the proposed nanodisk–hole array can be applied to randomly polarized light, and it has a potential application for the photoelectric conversion of ambient light.
Figure 7(c) shows the experimental results of the enhancement factor as a function of disk–hole distance. The maximum enhancement factor of four times was obtained at a disk–hole distance of 60 nm. This result indicates that the disk–hole distance control is crucial for the efficient SPR induced enhancement of hot electron emission.
5. Conclusion
In this study, we demonstrated that the hot-electron emission efficiency was enhanced by DUV-SPR with the Al disk–hole array. The structural parameters of period, diameter, Al thickness, and disk–hole distance were optimized for the resonance at a wavelength of 266 nm. By combining hole array with disk array in the short distance of 86 nm, larger enhancement of DUV-SPR was obtained owing to the cavity resonance. A hot-electron emission efficiency of 0.87 nA/mW, which is four times higher than that of the thin film, was obtained. The Al disk–hole array structure would be applied to improve the efficiencies of the photomultiplier tubes, the silicon photodetectors, the photocatalysts, and the highly efficient UV sterilizations.
Funding
Japan Science and Technology Agency (JPMJCR2003).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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