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2D periodic structures patterned on 3D surfaces by interference lithography for SERS
Applied Surface Science xxx (xxxx) xxx–xxx
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
2D periodic structures patterned on 3D surfaces by interference lithography for SERS ⁎
Ivana Lettrichovaa, , Agata Laurencikovab, Dusan Pudisa, Jozef Novakb, Matej Gorausa, Jaroslav Kovac Jr.c, Peter Gasoa, Juraj Nevrelac Dept. of Physics, University of Žilina, Univerzitná 1, 010 08 Žilina, Slovakia Inst. of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia c Inst. of Electronics and Photonics, Slovak University of Technology, Ilkovičova 3, 812 19 Bratislava, Slovakia a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: 2D periodic structure surface-enhanced Raman spectroscopy Ag island Interference lithography GaP nanocones
This contribution brings novel concept of Ag islands formation in 2D periodic arrangement with square symmetry on special 3D surfaces – gallium phosphide (GaP) nanocones, for surface-enhanced Raman spectroscopy application. 2D periodic structure in square symmetry with period of app. 470 nm is patterned in photoresist layer by interference lithography using double exposure of a two-beam interference optical field. Two different techniques for photoresist deposition on 3D surface are compared: spin-coating and photoresist deposition from water level. After Ag sputtering and lift-off, GaP nanocones are decorated with Ag islands in 2D periodical arrangement. Detectable enhancement in Raman signal for Rhodamine 6G adsorbed on prepared samples is demonstrated.
1. Introduction The energy spectrum of molecular vibrations, also known as Raman scattering [1], serves as a characteristic and unique fingerprint for the chemical composition of a sample. Molecular vibrations originated from oscillations between the constituent atoms of the molecules mixed with time-harmonic optical field produce scattered radiation that is frequency-shifted from the incident radiation. Even though the Raman scattering is an extremely weak effect, it can be increased if the molecules are adsorbed onto roughened metal surfaces exploiting the enhancement associated with surface plasmons. The magnitude of Raman scattering can be considerably enhanced, if the scattering molecule is placed near a roughened metal substrate [2], usually silver, gold or copper. Thanks to surface plasmon in nanoscale roughness features of metal substrate excited by visible light, strong electromagnetic fields are generated, what leads to enhancement in Raman signal. This process with enhancement factor on the order of 106–107, or in case of resonance as high as 1012–1014 [3], is known as surface-enhanced Raman scattering (SERS). The size, shape and material of nanoscale features must be considered to determine the resonant frequency of the conduction electrons in a metallic nanostructure. A large variety of 3D-SERS active substrates has been realized to obtain SERS applications in different fields, e. g. spectroscopic characterizations, biological sensing or detection of single molecule [4–16].
⁎
It has been theoretically predicted, that Ag or Au nanocones with small apex angle are able to accumulate plasmon polaritons at the tip of the cone and thus strongly enhance the electromagnetic field, e. g. the Raman signal emitted from molecules located at the tip [4]. Coluccio et al. presents platinum/carbon nanotips decorated by Ag nanoparticles (NPs) as plasmonic 3D structures for biological sensing [5]. Liao et al. reports on SiO2 nanorods 100 nm in diameter arranged in square lattice with period of 300 nm prepared by interference lithography with Ag particles on top [6]. For periodical arrangement, nature-grown photonic structures are often used. 3D photonic architecture of butterfly wing was covered with Au NPs [7], or it was used to generate 3D metallic replicas [8]. Quasi-periodic arrays of pillars in cicada wing were covered with graphene oxide and Au NPs decorated [9], or anglecoated with Ag and Au [10]. In recent years, various 3D nanostructured arrays such as Au NPs decorated TiO2 nanorod arrays [11], Ag coated silicon nanopillars [12], ZnO nanowires decorated with Au nanorods [13] or noble-metal building blocks [14], carbon nanotube arrays with Au NPs [15], anodic aluminum oxide templates with Ag NPs [16] have been reported with improved SERS effect. In this paper, we present special 3D structures – gallium phosphide (GaP) nanocones, decorated with Ag islands in 2D periodic arrangement with square symmetry. As the photoresist deposition on 3D surfaces is challenging, two different techniques were used: standard spincoating [17] which seems in this case rather unuseful, and photoresist
Corresponding author. E-mail address: [email protected] (I. Lettrichova).
https://doi.org/10.1016/j.apsusc.2018.06.162 Received 28 February 2018; Received in revised form 12 June 2018; Accepted 18 June 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Lettrichova, I., Applied Surface Science (2018), https://doi.org/10.1016/j.apsusc.2018.06.162
Applied Surface Science xxx (xxxx) xxx–xxx
I. Lettrichova et al.
GaP nanocones was submerged in deionized water, few milimeters under the floating photoresist layer. By draining the water, the photoresist layer covered the substrate and formed conformal photoresist layer all over the sample even in the 3D surface of the GaP nanocones. After photoresist deposition, the post-baking was processed at 65 °C for 2 min and at 103 °C for 3 min to remove the solvent. In the next step, 2D structure was patterned in photoresist layer by interference lithography (Fig. 2c). The sample was exposed by 1D periodical optical field produced by interference of two laser beams arranged in Mach-Zehnder configuration. As a laser source, Toptica laser emitting at wavelength λ = 403 nm was used. The period Λ of 1D interference optical field can be adjusted by angle of interfering laser beams θ according to the equation
Λ=
λ 2sinθ
(1)
deposition from water level [18,19]. 2D periodic structure in square symmetry was prepared by interference lithography using double exposure of a two-beam interference optical field [20]. Further Ag sputtering and lift-off leads to formation of Ag islands at GaP nanocones in periodical arrangement. Application of prepared GaP nanocones with Ag islands for surface-enhanced Raman spectroscopy is demonstrated.
Square symmetry of the exposed structure was achieved by double exposure process with the in-plane sample rotation of 90° between exposures [20]. After exposure, samples were developed in 1:3 solution of AZ 400 K developer and deionized water for 8 s. After sputtering of 10 nm of Ag (Fig. 2d) followed by lift off (Fig. 2e), Ag islands in 2D periodic arrangement were formed at GaP nanocones. The sample was investigated by micro-Raman measurements using Rhodamine 6G (R6G) as the probe molecule. The R6G in aqueous solution at concentration of 10−2 M was dropped onto the prepared sample, and the solvent was evaporated under ambient condition. The micro-Raman measurements were performed in air at room temperature, with the focus of the beam of an Ar laser (488 nm) using MonoVista 750 CRS system confocal Raman Microscope (Spectroscopy &Imaging, Germany) in backscattering geometry.
2. Experimental
3. Results
As 3D structures, zinc-doped GaP nanocones grown by metal organic vapour phase epitaxy at temperatures 650 – 690 °C for 25 min were used [21]. GaP nanocones were grown on GaP〈1 1 1〉B substrates by a VLS technique using 30 nm colloidal gold particles as seeds. The nanocones have hexagonal cross-sections and their bases follow the orientation of the substrate. As the nanocone technique growth uses Au seeds, nanocones are arranged randomly within the sample. The dimension of the basis is app. 1 µm and the nanocone growth varies from 1.5 to 2 µm. In Fig. 1, there is shown scanning electron microscope (SEM) image of one GaP nanocone. The schematic process of the Ag island formation is depicted in Fig. 2. In the first step, GaP nanocones are uniformly covered by photoresist layer (Fig. 2b). Two different techniques were used to photoresist deposition, (i) standard spin-coating and (ii) deposition from water level. In the first case, a positive-tone photoresist AZ1505 was spin-coated at 4000 rpm. In the second case, the planar thin uniform AZ1505 photoresist layer was first formed in the deionized water level by dropping the photoresist on the water surface. The substrate with
Above mentioned process was used for preparation of square 2D periodic structure on 3D surface of GaP nanocones. In the first case, standard spin-coating procedure was used to photoresist deposition. Under the same conditions, we obtained the photoresist thickness of app. 600 nm on planar sample. In the case of sample with GaP nanocones, the photoresist thickness varies dramatically with the position at the nanocone. In the planar part of the sample, the photoresist thickness is app. 600 nm, it goes down at the sides of GaP nanocone, and at the tip of the nanocone, the photoresist vanishes totally. After the exposure process by interference lithography, the 2D periodic structures with period of Λ ≈ 470 nm were patterned in the photoresist layer. In SEM image in Fig. 3, there is shown GaP nanocone with patterned photoresist layer. It is obvious, that standard spin-coating process is not suitable for photoresist deposition. The SEM picture of GaP nanocone with patterned photoresist deposited from water level is shown in Fig. 4. It can be seen, that the photoresist layer is almost uniform all over the sample, even for GaP nanocone tip. The photoresist thickness for planar sample under the
Fig. 1. SEM image of GaP nanocone.
Fig. 2. Schematic process of the Ag islands formation in GaP nanocones. (a) GaP nanocone, (b) photoresist deposition, (c) interference lithography, (d) Ag sputtering, (e) lift-off. 2
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Fig. 3. SEM picture of GaP nanocone with patterned photoresist deposited by spin-coating.
Fig. 5. SEM picture of GaP nanocone with patterned Ag islands. 20000
R6G C-C-C ring in-plane vibration
15000
R6G C-H out-of-plane bend
Counts
GaP TO line 10000
GaP LO line 5000
0 300
400
500
600
700
800
-1
Raman shift [cm ]
Fig. 6. Micro-Raman spectra of R6G adsorbed on GaP nanocone with (black line) and without (red line) Ag islands. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
A shoulder at the TO band is probably related to intrinsic surface defects of the GaP nanocones. The peak position is consistent with that of GaP crystal, indicating a high degree of crystalline perfection of the nanocones. Additional Raman line belongs to GaP longitudinal optical transition (LO) line with a maximum at 402.5 cm−1 but with much lower intensity in comparison to TO line. Next intensive Raman lines at 611 and 770 cm−1 belong to the vibrational frequencies of R6G [22]. Slight enhancement in Raman signal for patterned GaP nanocone is visible. In Fig. 7, we present spectra of the R6G adsorbed in GaP nanocone (black line) and planar structure (blue line), both with Ag islands. For both peaks representing the vibrational frequencies of R6G, we can see detectable enhancement in Raman signal. Further investigation will be focused on optimization of spatial distribution of Ag islands with respect to the electromagnetic wave interaction, what should lead to surface plasmon resonance. Reflectance measurement shows weak extinction at Raman excitation wavelength 488 nm, what causes only small enhancement in Raman signal. As the extinction coefficient can be shifted by Ag nanoparticle
Fig. 4. SEM picture of GaP nanocone with patterned photoresist deposited from water level.
same conditions was app. 110 nm. Thus, the 2D periodic structure remains at the sides of GaP nanocones as well, and it is crucial for further processing to obtain Ag islands at GaP nanocones. After sputtering of 10 nm of Ag and lift off, Ag islands were formed at GaP nanocones. SEM image in Fig. 5 documents arrangement, shape and size of Ag islands. Patterned Ag islands are formed from Ag NPs rather than from compact Ag layer. The diameter of Ag NPs within the Ag islands varies from ones to tens of nanometers, which causes both absorption and scattering of the incident light. The micro-Raman measurements were performed using R6G deposited in the surface. Obtained micro-Raman spectra are shown in Figs. 6 and 7. Fig. 6 compares Raman scattering signal of the R6G in GaP nanocone samples with (black line) and without (red line) Ag islands. It follows from the figure that the dominant line was at 365.9 cm−1. It belongs to a GaP transversal optical transition (TO) line. 3
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APVV-16-0129 and APVV-14-0297. R6G C-C-C ring in-plane vibration
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R6G C-H out-of-plane bend
References [1] C.V. Raman, K.S. Krishnan, A new type of secondary radiation, Nature 121 (1928) 501–502. [2] M. Fleischmann, P.J. Hendra, A.J. McQuillan, Raman spectra of pyridine adsorbed at a silver electrode, Chem. Phys. Lett. 26 (1974) 163–166. [3] S. Nie, S.R. Emory, Probing single molecules and single nanoparticles by surface enhanced Raman scattering, Science 275 (1997) 1102–1106. [4] M.I. Stockman, Nanofocusing of optical energy in tapered plasmonic waveguides, Phys. Rev. Lett. 93 (2004) 137404-1–4. [5] M.L. Coluccio, M. Francardi, F. Gentile, P. Candeloro, L. Ferrara, G. Perozziello, E. Di Fabrizio, Plasmonic 3D-structures based on silver decorated nanotips for biological sensing, Opt. Lasers Eng. 76 (2016) 45–51. [6] P.F. Liao, J.G. Bergman, D.S. Chemla, A. Wokaun, J. Melngailis, A.M. Hawryluk, N.P. Economou, Surface-enhanced Raman scattering from microlithographic silver particle surfaces, Chem. Phys. Lett. 82 (2) (1981) 355–359. [7] Z. Mu, X. Zhao, Z. Xie, Y. Zhao, Q. Zhong, L. Bo, Z. Gu, In situ synthesis of gold nanoparticles (AuNPs) in butterfly wings for surface enhanced Raman spectroscopy (SERS), J. Mater. Chem. B 1 (2013) 1607–1613. [8] Y. Tan, J. Gu, X. Zang, W. Xu, K. Shi, L. Xu, D. Zhang, Versatile fabrication of intact three-dimensional metallic butterfly wing scales with hierarchical sub-micrometer structures, Angew. Chem. Int. Ed. 50 (2011) 8307–8311. [9] G. Shi, M. Wang, Y. Zhu, L. Shen, Y. Wang, W. Ma, Y. Chen, R. Li, A flexible and stable surface-enhanced Raman scattering (SERS) substrate based on Au nanoparticles/Graphene oxide/Cicada wing array, Opt. Commun. 412 (2018) 28–36. [10] P.R. Stoddart, P.J. Cadusch, T.M. Boyce, R.M. Erasmus, J.D. Comins, Optical properties of chitin: surface-enhanced Raman scattering substrates based on antireflection structures on cicada wings, Nanotechnology 17 (2006) 680–686. [11] H. Fang, Ch.X. Zhang, L. Liu, Y.M. Zhao, H.J. Xu, Recyclable three-dimensional Ag nanoparticle-decorated TiO2 nanorod arrays for surface-enhanced Raman scattering, Biosens. Bioelectron. 64 (2015) 434–441. [12] M.S. Schmidt, J. Hübner, A. Boisen, Large area fabrication of leaning silicon nanopillars for surface enhanced Raman spectroscopy, Adv. Mater. 24 (2012) OP11–OP18. [13] R. Kattumenu, Ch.H. Lee, L. Tian, M.E. McConney, S. Singamaneni, Nanorod decorated nanowires as highly efficient SERS-active hybrids, J. Mater. Chem. 21 (2011) 15218–15223. [14] Ch. Zhu, G. Meng, Q. Huang, X. Wang, Y. Qian, X. Hu, H. Tang, N. Wu, ZnO-nanotaper array sacrificial templated synthesis of noble-metal building-block assembled nanotube arrays as 3D SERS-substrates, Nano Res. 8 (3) (2015) 957–966. [15] S. Yick, Z.J. Hanz, K. Ostrikov, Atmospheric microplasma-functionalized 3D microfluidic strips within dense carbon nanotube arrays confine Au nanodots for SERS sensing, Chem. Commun. 49 (2013) 2861–2863. [16] N. Nuntawong, M. Horprathum, P. Eiamchai, K. Wong-ek, V. Patthanasettakul, P. Chindaudom, Surface-enhanced Raman scattering substrate of silver nanoparticles depositing on AAO template fabricated by magnetron sputtering, Vacuum 84 (2010) 1415–1418. [17] A. Ramam, S. Chua, Distribution of photoresist over GaAs mesa structures, J. Electrochem. Soc. 141 (1994) 576–578. [18] H. Zhou, B.K. Chong, P. Stopford, G. Mills, A. Midha, L. Donaldson, J.M.R. Weavera, Lithographically defined nano and micro sensors using ‘‘float coating’’ of resist and electron beam lithography, J. Vac. Sci. Technol. B 18 (6) (2000) 3594–3599. [19] P. Eliáš, D. Gregušová, J. Martaus, I. Kostič, Conformal AZ5214-E resist deposition on patterned (100) InP substrates, J. Micromech. Microeng. 16 (2006) 191–197. [20] D. Pudis, L. Suslik, J. Skriniarova, J. Kovac, J. Kovac Jr., I. Kubicova, I. Martincek, S. Hascik, P. Schaaf, Effect of 2D photonic structure patterned in the LED surface on emission properties, Appl. Surf. Sci. 269 (2013) 161–165. [21] J. Novák, A. Laurenčíková, P. Eliáš, S. Hasenöhrl, M. Sojková, E. Dobrocka, J. Kováč Jr., J. Kováč, J. Ďurišová, D. Pudiš, Nanorods and nanocones for advanced sensor applications, Appl. Surf. Sci. (2018), http://dx.doi.org/10.1016/j.apsusc.2018.04. 176 (in press). [22] C. Wu, E. Chen, J. Wei, Surface enhanced Raman spectroscopy of Rhodamine 6G on agglomerates of different-sized silver truncated nanotriangles, Colloids Surf. A: Physicochem. Eng. Aspects 506 (2016) 450–456.
Counts
GaP TO line 10000
GaP LO line 5000
0 300
400
500
600
700
800
-1
Raman shift [cm ]
Fig. 7. Micro-Raman spectra of R6G adsorbed on GaP nanocone (black line) and planar structure (blue line), both with Ag islands. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
geometry, we perform simulations to optimize the Ag nanoparticle/Ag island diameter and period. Also, to better fit 2D structure with 3D structure, it is possible to: (i) combine 2D structure in hexagonal symmetry (which can be easily obtained by interference lithography) with nanocones arranged in this symmetry, or to (ii) combine 2D structure in square or hexagonal symmetry with nanocones with circular basis. Similarly, the density of Ag islands can be simply changed with the period of the 2D patterned structure. 4. Conclusion In this paper, Ag islands in 2D periodic arrangement with square symmetry patterned on GaP nanocones for surface-enhanced Raman spectroscopy application are presented. For photoresist deposition on 3D surfaces, two different techniques are presented and compared: spin-coating and photoresist deposition from water level. The photoresist layer deposited in GaP nanocone was patterned by 2D periodic structure in square symmetry with period of app. 470 nm by interference lithography. After Ag sputtering and lift-off, GaP nanocones were decorated with Ag islands in 2D periodical arrangement. Detectable enhancement in Raman signal for R6G adsorbed on prepared samples was presented. For application in SERS, further study, as well as the interference pattern period optimization, is needed. Acknowledgement This work was supported by the Slovak National Grant Agency under the projects no. VEGA 1/0540/18 and VEGA 1/0278/15, the Slovak Research and Development Agency under the projects No.
4
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
2D periodic structures patterned on 3D surfaces by interference lithography for SERS ⁎
Ivana Lettrichovaa, , Agata Laurencikovab, Dusan Pudisa, Jozef Novakb, Matej Gorausa, Jaroslav Kovac Jr.c, Peter Gasoa, Juraj Nevrelac Dept. of Physics, University of Žilina, Univerzitná 1, 010 08 Žilina, Slovakia Inst. of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia c Inst. of Electronics and Photonics, Slovak University of Technology, Ilkovičova 3, 812 19 Bratislava, Slovakia a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: 2D periodic structure surface-enhanced Raman spectroscopy Ag island Interference lithography GaP nanocones
This contribution brings novel concept of Ag islands formation in 2D periodic arrangement with square symmetry on special 3D surfaces – gallium phosphide (GaP) nanocones, for surface-enhanced Raman spectroscopy application. 2D periodic structure in square symmetry with period of app. 470 nm is patterned in photoresist layer by interference lithography using double exposure of a two-beam interference optical field. Two different techniques for photoresist deposition on 3D surface are compared: spin-coating and photoresist deposition from water level. After Ag sputtering and lift-off, GaP nanocones are decorated with Ag islands in 2D periodical arrangement. Detectable enhancement in Raman signal for Rhodamine 6G adsorbed on prepared samples is demonstrated.
1. Introduction The energy spectrum of molecular vibrations, also known as Raman scattering [1], serves as a characteristic and unique fingerprint for the chemical composition of a sample. Molecular vibrations originated from oscillations between the constituent atoms of the molecules mixed with time-harmonic optical field produce scattered radiation that is frequency-shifted from the incident radiation. Even though the Raman scattering is an extremely weak effect, it can be increased if the molecules are adsorbed onto roughened metal surfaces exploiting the enhancement associated with surface plasmons. The magnitude of Raman scattering can be considerably enhanced, if the scattering molecule is placed near a roughened metal substrate [2], usually silver, gold or copper. Thanks to surface plasmon in nanoscale roughness features of metal substrate excited by visible light, strong electromagnetic fields are generated, what leads to enhancement in Raman signal. This process with enhancement factor on the order of 106–107, or in case of resonance as high as 1012–1014 [3], is known as surface-enhanced Raman scattering (SERS). The size, shape and material of nanoscale features must be considered to determine the resonant frequency of the conduction electrons in a metallic nanostructure. A large variety of 3D-SERS active substrates has been realized to obtain SERS applications in different fields, e. g. spectroscopic characterizations, biological sensing or detection of single molecule [4–16].
⁎
It has been theoretically predicted, that Ag or Au nanocones with small apex angle are able to accumulate plasmon polaritons at the tip of the cone and thus strongly enhance the electromagnetic field, e. g. the Raman signal emitted from molecules located at the tip [4]. Coluccio et al. presents platinum/carbon nanotips decorated by Ag nanoparticles (NPs) as plasmonic 3D structures for biological sensing [5]. Liao et al. reports on SiO2 nanorods 100 nm in diameter arranged in square lattice with period of 300 nm prepared by interference lithography with Ag particles on top [6]. For periodical arrangement, nature-grown photonic structures are often used. 3D photonic architecture of butterfly wing was covered with Au NPs [7], or it was used to generate 3D metallic replicas [8]. Quasi-periodic arrays of pillars in cicada wing were covered with graphene oxide and Au NPs decorated [9], or anglecoated with Ag and Au [10]. In recent years, various 3D nanostructured arrays such as Au NPs decorated TiO2 nanorod arrays [11], Ag coated silicon nanopillars [12], ZnO nanowires decorated with Au nanorods [13] or noble-metal building blocks [14], carbon nanotube arrays with Au NPs [15], anodic aluminum oxide templates with Ag NPs [16] have been reported with improved SERS effect. In this paper, we present special 3D structures – gallium phosphide (GaP) nanocones, decorated with Ag islands in 2D periodic arrangement with square symmetry. As the photoresist deposition on 3D surfaces is challenging, two different techniques were used: standard spincoating [17] which seems in this case rather unuseful, and photoresist
Corresponding author. E-mail address: [email protected] (I. Lettrichova).
https://doi.org/10.1016/j.apsusc.2018.06.162 Received 28 February 2018; Received in revised form 12 June 2018; Accepted 18 June 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Lettrichova, I., Applied Surface Science (2018), https://doi.org/10.1016/j.apsusc.2018.06.162
Applied Surface Science xxx (xxxx) xxx–xxx
I. Lettrichova et al.
GaP nanocones was submerged in deionized water, few milimeters under the floating photoresist layer. By draining the water, the photoresist layer covered the substrate and formed conformal photoresist layer all over the sample even in the 3D surface of the GaP nanocones. After photoresist deposition, the post-baking was processed at 65 °C for 2 min and at 103 °C for 3 min to remove the solvent. In the next step, 2D structure was patterned in photoresist layer by interference lithography (Fig. 2c). The sample was exposed by 1D periodical optical field produced by interference of two laser beams arranged in Mach-Zehnder configuration. As a laser source, Toptica laser emitting at wavelength λ = 403 nm was used. The period Λ of 1D interference optical field can be adjusted by angle of interfering laser beams θ according to the equation
Λ=
λ 2sinθ
(1)
deposition from water level [18,19]. 2D periodic structure in square symmetry was prepared by interference lithography using double exposure of a two-beam interference optical field [20]. Further Ag sputtering and lift-off leads to formation of Ag islands at GaP nanocones in periodical arrangement. Application of prepared GaP nanocones with Ag islands for surface-enhanced Raman spectroscopy is demonstrated.
Square symmetry of the exposed structure was achieved by double exposure process with the in-plane sample rotation of 90° between exposures [20]. After exposure, samples were developed in 1:3 solution of AZ 400 K developer and deionized water for 8 s. After sputtering of 10 nm of Ag (Fig. 2d) followed by lift off (Fig. 2e), Ag islands in 2D periodic arrangement were formed at GaP nanocones. The sample was investigated by micro-Raman measurements using Rhodamine 6G (R6G) as the probe molecule. The R6G in aqueous solution at concentration of 10−2 M was dropped onto the prepared sample, and the solvent was evaporated under ambient condition. The micro-Raman measurements were performed in air at room temperature, with the focus of the beam of an Ar laser (488 nm) using MonoVista 750 CRS system confocal Raman Microscope (Spectroscopy &Imaging, Germany) in backscattering geometry.
2. Experimental
3. Results
As 3D structures, zinc-doped GaP nanocones grown by metal organic vapour phase epitaxy at temperatures 650 – 690 °C for 25 min were used [21]. GaP nanocones were grown on GaP〈1 1 1〉B substrates by a VLS technique using 30 nm colloidal gold particles as seeds. The nanocones have hexagonal cross-sections and their bases follow the orientation of the substrate. As the nanocone technique growth uses Au seeds, nanocones are arranged randomly within the sample. The dimension of the basis is app. 1 µm and the nanocone growth varies from 1.5 to 2 µm. In Fig. 1, there is shown scanning electron microscope (SEM) image of one GaP nanocone. The schematic process of the Ag island formation is depicted in Fig. 2. In the first step, GaP nanocones are uniformly covered by photoresist layer (Fig. 2b). Two different techniques were used to photoresist deposition, (i) standard spin-coating and (ii) deposition from water level. In the first case, a positive-tone photoresist AZ1505 was spin-coated at 4000 rpm. In the second case, the planar thin uniform AZ1505 photoresist layer was first formed in the deionized water level by dropping the photoresist on the water surface. The substrate with
Above mentioned process was used for preparation of square 2D periodic structure on 3D surface of GaP nanocones. In the first case, standard spin-coating procedure was used to photoresist deposition. Under the same conditions, we obtained the photoresist thickness of app. 600 nm on planar sample. In the case of sample with GaP nanocones, the photoresist thickness varies dramatically with the position at the nanocone. In the planar part of the sample, the photoresist thickness is app. 600 nm, it goes down at the sides of GaP nanocone, and at the tip of the nanocone, the photoresist vanishes totally. After the exposure process by interference lithography, the 2D periodic structures with period of Λ ≈ 470 nm were patterned in the photoresist layer. In SEM image in Fig. 3, there is shown GaP nanocone with patterned photoresist layer. It is obvious, that standard spin-coating process is not suitable for photoresist deposition. The SEM picture of GaP nanocone with patterned photoresist deposited from water level is shown in Fig. 4. It can be seen, that the photoresist layer is almost uniform all over the sample, even for GaP nanocone tip. The photoresist thickness for planar sample under the
Fig. 1. SEM image of GaP nanocone.
Fig. 2. Schematic process of the Ag islands formation in GaP nanocones. (a) GaP nanocone, (b) photoresist deposition, (c) interference lithography, (d) Ag sputtering, (e) lift-off. 2
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Fig. 3. SEM picture of GaP nanocone with patterned photoresist deposited by spin-coating.
Fig. 5. SEM picture of GaP nanocone with patterned Ag islands. 20000
R6G C-C-C ring in-plane vibration
15000
R6G C-H out-of-plane bend
Counts
GaP TO line 10000
GaP LO line 5000
0 300
400
500
600
700
800
-1
Raman shift [cm ]
Fig. 6. Micro-Raman spectra of R6G adsorbed on GaP nanocone with (black line) and without (red line) Ag islands. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
A shoulder at the TO band is probably related to intrinsic surface defects of the GaP nanocones. The peak position is consistent with that of GaP crystal, indicating a high degree of crystalline perfection of the nanocones. Additional Raman line belongs to GaP longitudinal optical transition (LO) line with a maximum at 402.5 cm−1 but with much lower intensity in comparison to TO line. Next intensive Raman lines at 611 and 770 cm−1 belong to the vibrational frequencies of R6G [22]. Slight enhancement in Raman signal for patterned GaP nanocone is visible. In Fig. 7, we present spectra of the R6G adsorbed in GaP nanocone (black line) and planar structure (blue line), both with Ag islands. For both peaks representing the vibrational frequencies of R6G, we can see detectable enhancement in Raman signal. Further investigation will be focused on optimization of spatial distribution of Ag islands with respect to the electromagnetic wave interaction, what should lead to surface plasmon resonance. Reflectance measurement shows weak extinction at Raman excitation wavelength 488 nm, what causes only small enhancement in Raman signal. As the extinction coefficient can be shifted by Ag nanoparticle
Fig. 4. SEM picture of GaP nanocone with patterned photoresist deposited from water level.
same conditions was app. 110 nm. Thus, the 2D periodic structure remains at the sides of GaP nanocones as well, and it is crucial for further processing to obtain Ag islands at GaP nanocones. After sputtering of 10 nm of Ag and lift off, Ag islands were formed at GaP nanocones. SEM image in Fig. 5 documents arrangement, shape and size of Ag islands. Patterned Ag islands are formed from Ag NPs rather than from compact Ag layer. The diameter of Ag NPs within the Ag islands varies from ones to tens of nanometers, which causes both absorption and scattering of the incident light. The micro-Raman measurements were performed using R6G deposited in the surface. Obtained micro-Raman spectra are shown in Figs. 6 and 7. Fig. 6 compares Raman scattering signal of the R6G in GaP nanocone samples with (black line) and without (red line) Ag islands. It follows from the figure that the dominant line was at 365.9 cm−1. It belongs to a GaP transversal optical transition (TO) line. 3
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APVV-16-0129 and APVV-14-0297. R6G C-C-C ring in-plane vibration
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R6G C-H out-of-plane bend
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Fig. 7. Micro-Raman spectra of R6G adsorbed on GaP nanocone (black line) and planar structure (blue line), both with Ag islands. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
geometry, we perform simulations to optimize the Ag nanoparticle/Ag island diameter and period. Also, to better fit 2D structure with 3D structure, it is possible to: (i) combine 2D structure in hexagonal symmetry (which can be easily obtained by interference lithography) with nanocones arranged in this symmetry, or to (ii) combine 2D structure in square or hexagonal symmetry with nanocones with circular basis. Similarly, the density of Ag islands can be simply changed with the period of the 2D patterned structure. 4. Conclusion In this paper, Ag islands in 2D periodic arrangement with square symmetry patterned on GaP nanocones for surface-enhanced Raman spectroscopy application are presented. For photoresist deposition on 3D surfaces, two different techniques are presented and compared: spin-coating and photoresist deposition from water level. The photoresist layer deposited in GaP nanocone was patterned by 2D periodic structure in square symmetry with period of app. 470 nm by interference lithography. After Ag sputtering and lift-off, GaP nanocones were decorated with Ag islands in 2D periodical arrangement. Detectable enhancement in Raman signal for R6G adsorbed on prepared samples was presented. For application in SERS, further study, as well as the interference pattern period optimization, is needed. Acknowledgement This work was supported by the Slovak National Grant Agency under the projects no. VEGA 1/0540/18 and VEGA 1/0278/15, the Slovak Research and Development Agency under the projects No.
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