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Abstract
We demonstrate that the plasmonic properties of Au nanodisk structures fabricated by an electron beam lithography can be improved by very simple heat treatments, and that the resonance wavelength can be tuned by temperature. With a Ni adhesion layer, the resonance peak increased and blue-shifted due to improvement of metal quality without changing the sizes of nanostructures, while without the adhesion layer, the resonance wavelength can be tuned over a wider wavelength range by intentionally reducing the size of nanostructures through annealing. For Ag nanodisks with the adhesive layer, the plasmon resonance wavelength was blue-shifted due to the size reduction of nanodisks through thermal annealing. Full-color tuning of plasmonic resonance should be possible by controlling the diameter and height of Ag nanodisks under appropriate temperature conditions of heat treatment.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, surface plasmon resonance has attracted attention in various fields, such as improving the emission efficiency of quantum dots [1–5] and quantum wells [6–11], biosensing [12–17], gas sensing [18–20], and high-resolution cell imaging [21–23]. Since the resonance wavelength of localized surface plasmon resonance induced by nanostructures can be easily tuned by changing the size of the nanostructures a wide variety of structures have been fabricated by annealing after evaporation [24–29], chemical synthesis and transfer to a substrate [30–32], and top-down lithographic techniques [33–38]. Each method has its advantages and disadvantages. Although the way of annealing an evaporated metal thin film can easily fabricate nanoparticles, the shape, size, and particle-to-particle distance are random and difficult to control [24]. Nanostructures fabricated by chemical synthesis are single-crystal, atomically smooth, and have a high quality factor, but it is difficult to precisely arrange them to fabricate complex structures [39]. Electron beam lithography (EBL) is widely used to fabricate asymmetric and periodic nanostructures for localized surface plasmon resonance because of its ability to precisely control shape, size, and interparticle distance. Using a lithographic techniques, the shape, size, and inter-particle distance can be precisely controlled to fabricate asymmetric and periodic structures that are difficult to obtain naturally [33–38]. However, EBL-fabricated structures have plasmon damping due to surface roughness, grain boundaries and adhesion layers [40,41]. In this paper, we have shown that the simple heat treatments of nanostructures fixed by an adhesion layer improves the plasmonic properties without changing size of nanostructures, allowing tuning of the resonance wavelength. Although the structure cannot be fixed without an adhesion layer, the resonance wavelength can be tuned over a wider range by improving the plasmonic properties and intentionally reducing the size of the nanostructure.
2. Methods
Figure 1 shows the experimental procedure for fabricating nanodisk structures. The cover glass (No. 3, Matsunami Glass Industry) was ultrasonically cleaned with acetone for 5 and 10 min and ultrapure water for 10 min at 25°C. ZEP 520A (Zeon Corporation) resist was spin-coated for 120 sec at 5000 rpm on its glass and baked for 3 min on a hot plate at 180°C. Espacer 300Z (Showa Denko K.K.), a charge dissipating layer, was spin-coated for 60 sec at 1500 rpm and baked for 1 min at 100°C. Electron beam exposure was performed using an EBL system (ELS-7500EX, Elionix) according to the digital pattern of the periodic nanodisk structure created by AutoCAD (Autodesk). The exposure was performed at 50 kV accelerating voltage, 300 µm × 300 µm write field and 60000 dot count. Dose time was optimized by actually fabricating the structure. After exposure, the sample was soaked in ultrapure water for 1 min to remove the Espacer 300Z, developed with xylene for 3 min, and rinsed with IPA for 20 sec. After development, Ni was deposited as an adhesion layer followed by Au by resistance thermal evaporation (SVC-700TM, Sanyu Electron), and lift-off was performed in a 2-butanone, followed by rinse with IPA for 20 sec. Every time the samples were cleaned and rinsed, they were dried with a nitrogen gun. Annealing of the samples was performed in an electric furnace under air atmosphere for 10 min.
Extinction spectra and transmission colors were analyzed by the optical microscope (BX51, Olympus) and the xenon lamp (BPS-X150B, Bunkoukeiki), and the surface structure was evaluated by the scanning electron microscope (SEM) (FlexSEM 1000 II, Hitachi High-Tech). The absorption colors were obtained by inverting the transmission colors. The SEM images were obtained around the center of the lithographic area, where there is little or no charging effect. The similar disk structures were observed throughout the lithographic area.
3. Results and discussion
3.1 Flexible tuning of the peak wavelength with the Ni adhesion layer
Nanodisk structures with diameter of 100 nm, Ni adhesion layer height of 2 nm and Au height of 30 nm were fabricated and annealed from 150°C to 400°C. The height of the Au nanodisks was set to 30 nm to obtain the peak of plasmon resonance within the visible band. Figure 2 shows (a) the SEM images, (b) extinction spectra and (c) transmission and absorption color before and after annealing. The dotted line in Fig. 2(b) indicates the extinction spectrum of the Au nanodisk structure with a diameter of 100 nm and a height of 30 nm calculated by the finite difference time domain (FDTD) method [26–28]. The SEM image shows that nanodisk structures are obtained as designed. Before annealing, the resonance wavelength is located on the longer wavelength side and broader than the calculated spectrum, due to the plasmon dumping of adhesion layer and grain boundaries effects. However, after annealing, the resonance wavelength shifts to the short wavelength side and the peak becomes sharper and stronger, even at a low annealing temperature of 150°C, while the size of the structure remains unchanged. It is considered that the electron oscillation becomes stronger because the quality of Au nanodisks is improved by thermal annealing. This is attributed to the reduction of grain boundaries and subsequently increase of the mean free path of the electrons [42,43]. As the annealing temperature was further increased, the peak wavelength was blue-shifted with temperature, and approached the calculated resonance wavelength at annealing temperature of 200°C to 250°C. Furthermore, annealing above 350°C resulted in a smaller wavelength shift and a saturation tendency of the blue-shift. SEM images after annealing at high temperatures also confirmed that the diameter of the structure remained almost unchanged. As shown in Fig. 2(c), transmission color observed by optical microscopy was dull blue before annealing, and then changed to bright light blue, blue, and red-purple after thermal treatment. The absorption color, which is the complementary color of transmission, changed from brown to orange, ochre, and dull yellow. The resonance wavelength of the Au nanodisk structure can be shifted by more than 100 nm, and accordingly the absorption color of the plasmon resonance changes significantly. Therefore, a simple annealing process can improve the plasmonic properties of nanostructures fabricated by EBL due to the reduction of grain boundaries, and a 2 nm Ni adhesion layer enables resonance wavelength (color) tuning with almost no change in the diameter of the structure.
Fig. 2. Annealing temperature dependence of nanodisk with diameter of 100 nm, Ni adhesion layer height of 2 nm and Au height of 30 nm. (a) SEM images before and after annealing at 150°C to 400°C. (b) Extinction spectra before and after annealing at 150°C to 400°C. The dotted line is calculated extinction spectrum of the nanodisk structure with a diameter of 100 nm and a height of 30 nm by FDTD method. (c) Transmission and absorption colors before and after annealing at 150°C to 400°C.
In order to investigate the improvement effects of annealing nanodisk with larger diameter, nanodisk structure with diameter of 150 nm, Ni height of 2 nm and Au height of 20 nm were fabricated and annealed at 150°C to 500°C. The height of Au nanodisks was set to 20 nm to expand the tuning range of surface plasmon resonance obtained by the size reduction of the nanodisk by thermal annealing. The lower height, i.e. smaller volume, of the nanodisk is considered to be more effective to induce the size reduction in case of large nanodisk diameter or without Ni adhesive layer. Figure 3 shows (a) the SEM images and (b) extinction spectra before and after annealing. Due to the enlarged diameter of nanodisk structures, the change in the extinction spectrum is small for annealing at 150°C, and the spectrum begins to change significantly for annealing above 200°C. A remarkable increase in the resonance peak was observed for annealing at 350°C. However, the resonance wavelength shift became smaller above 350°C. Annealing up to 500°C hardly changed the resonance wavelength, and a saturation tendency of the blue-shift was observed. Therefore, as the diameter of the structure increases, the annealing temperature at which the plasmonic properties are greatly improved becomes higher, and the change in resonance wavelength due to the reduction of grain boundaries begins to saturate at about 350°C, similar to the case of smaller diameter.
Fig. 3. Annealing temperature dependence of nanodisk with diameter of 150 nm, Ni adhesion layer height of 2 nm and Au height of 20 nm. SEM images (a) and extinction spectra (b) before and after annealing at 150°C to 500°C.
3.2 Effect of the Ni adhesion layer
It is reported that the optimal thickness of Ti is 0.5 nm as an adhesion layer for Au thin film, and that if the Ti is thick enough, it will diffuse through the grain boundaries of Au to the surface when annealed, reducing the thermal stability of the Au thin film [44]. Thus, we investigated enhancement of plasmonic properties and tunability of resonance wavelength by annealing of the nanodisk structure without or with thin Ni layer. Figure 4 shows (a) the SEM images, (b) extinction spectra, (c) transmission and absorption color of nanodisk structures fabricated with Au height of 20 nm and without Ni layer before and after annealing at 100°C to 400°C. The diameters of the fabricated Au disks were approximately 100 nm to 120 nm. Although the SEM image before annealing shows that the structure could be fabricated without the Ni adhesion layer, there were wide distribution in size of nanostructures and some lacking areas. Since there was no plasmon damping due to the adhesion layer, the peak intensity of extinction spectrum was somewhat larger even before annealing. As shown in Fig. 4 (b), when the sample was annealed at low temperature below 200°C, while the size of the structure was maintained, blue-shift and sharpening of the resonance peak were observed, similar to the case with the adhesion layer. As the annealing temperature was further increased, the blue-shift of resonance wavelength and decrease of peak intensity were observed, reflecting the clear size reduction of nanodisk seen in SEM images. The diameter of the nanodisk was estimated to decrease from 120 ± 5 nm before heating to 106 ± 6 nm after heating at 400 °C by particle analysis of the SEM images. Figure 5 shows calculated extinction spectra of the Au nanodisk with a height of 20 nm and a diameter from 60 to 120 nm. The calculation result indicates that the resonance peak is blue-shifted and its intensity decreases with reducing diameter, showing similar dependence to the experimental result. The peak shifts of extinction spectra experimentally obtained by thermal annealing of 200 °C and 400 °C, where the size reduction was observed, were in good agreement with the peak shifts between the 120 nm and 100 nm diameter nondisks. From a different perspective, it is possible to intentionally control the size by annealing, and thus tuning of the resonance wavelength over a wider range. The absorption color changes from brown to orange, yellowish green, and green, indicating that the resonance wavelength without Ni adhesion layer is tuned over a wider range than that with 2 nm Ni layer.
Fig. 4. Annealing temperature dependence of nanodisk with diameter of 100 to 120 nm, no Ni adhesion layer and Au height of 20 nm. SEM images (a), extinction spectra (b) and transmission and absorption colors (c) before and after annealing at 100°C to 400°C.
Figure 6 shows (a) SEM images, (b) extinction spectra and (c) transmission and absorption color of the nanodisk with a height of 20 nm of Au and 0.5 nm of Ni before and after annealing at 100°C to 400°C. Even though the adhesion layer is thin, and a nanodisk structure with a uniform size of 120 to 130 nm in diameter and no lacking areas can be fabricated. The peak of the extinction spectrum before annealing was slightly broadened due to the plasmon damping. When this sample was also annealed at low temperature below 200°C, there was almost no change in size of the structure and the plasmonic properties were enhanced. As the temperature increased, a clear size reduction was observed at annealing temperature above 300°C. However, this size reduction was smaller than that when there was no adhesion layer. Absorption color changes from brown to orange to dull yellow, and this shift of resonance wavelength is comparable to 2 nm Ni thickness and not as great as without Ni layer. Therefore, Ni adhesion layer thickness of 2 nm is more suitable for improving the plasmonic properties while keeping the structure fixed, and no Ni layer is desirable for tuning of the resonance wavelength over a wider range.
Fig. 5. Calculated extinction spectra of the Au nanodisk with a height of 20 nm and a diameter from 60 to 120 nm.
Fig. 6. Annealing temperature dependence of nanodisk with diameter of 120 to 130 nm, Ni adhesion layer height of 0.5 nm and Au height of 20 nm. SEM images (a), extinction spectra (b) and Transmission and absorption colors (c) before and after annealing at 100°C to 400°C.
3.3 Flexible tuning of the peak wavelength of the Ag nanodisk
Nanodisk structures with diameter of 100 nm, Ni adhesion layer height of 2 nm, and Ag height of 40 nm were fabricated. The height of the Ag nanodisks was set to 40 nm, taking into account that Ag nanodisks easily change the shape and evaporate at lower temperature, reflecting lower melting point than Au. The peak wavelength of plasmon resonance for Ag nanodisk with height of 40 nm appears in the visible band around green, allowing the size reduction to be easily recognized by spectral measurements. Figure 7 shows (a) SEM images and (b) extinction spectra of the structures when annealed at 150°C to 300°C. Unlike Au nanodisk structure, some parts of the structure were lacking, even though with 2 nm Ni adhesion layer. The nanodisk could not be fixed and the size of the nanodisk shrank as the annealing temperature was increased, and some even broke apart. The extinction spectra show a significant decrease in peak intensity and a blue-shift due to size reduction for the annealing temperature above 250°C. Annealing at 150°C did not reduce the size as in the case of Au nanodisk, and there was no improvement of the plasmonic properties unlike for the Au nanodisk structure. Therefore, the plasmon damping due to grain boundaries in Ag nanostructures fabricated by EBL is quite small, and annealing treatment is not necessary for the purpose of improving plasmonic properties. It is conceivable that the adhesion layer is not needed because the nanostructure cannot be fixed during annealing. However, without an adhesion layer, the silver came off too much during lift-off process and the size of resulting nanodisk became smaller than the designed size, making it difficult to fabricate a uniform structure. Therefore, the adhesion layer is necessary for the stable fabrication, not for the fix of the nanostructure during the annealing process. Since the resonance wavelength of Ag exists from the blue wavelength region, it might be possible to tune the wavelength in the visible region from blue to red (full-color) through size control by annealing, if the initial size of the Ag disk structure is enlarged.
Fig. 7. Annealing temperature dependence of nanodisk of Ni adhesion layer height of 2 nm and Ag height of 40 nm with diameters of 100 nm. SEM images (a), extinction spectra (b) before and after annealing at 150°C to 300°C.
4. Conclusion
In this paper, we have shown a method to improve the plasmonic properties of EBL-fabricated nanodisks by very simple heat treatments. By fixing the structure with a 2 nm Ni adhesion layer, annealing improved the plasmonic properties without changing the size of the structure, allowing tuning of wavelengths over 100 nm. As the disk diameter was increased, the higher annealing temperature was required to improve the plasmonic properties. Without the adhesion layer, in addition to improved plasmonic properties, the blue-shift due to size reduction allows wider range tuning of resonance wavelengths. Furthermore, the results of fabricating and annealing Ag nanodisk structures indicate that Ag nanostructures prepared by EBL were almost unaffected by plasmon damping due to grain boundaries. If the diameter of the initial Ag disk structure is increased, full-color tuning might be possible by intentionally reducing the size through annealing. This very simple and effective method proposed here enable the flexible tuning of peak wavelength and intensity of localized surface plasmon resonance, and expected to open up a wide range of applications in the fields of plasmonics and nanophotonics.
Funding
Japan Society for the Promotion of Science (JP19H05627, JP20H05622, JP20K04521, JP21K19218).
Acknowledgments
The author wish to thank Prof. K. Tamada of Kyushu University, Prof. Y. Kawakami, Prof. M. Funato, and Prof. Terazima of Kyoto University for valuable discussions and support.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data supporting the findings of this study are available from the corresponding author upon reasonable request.
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