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Plasmonic properties and optimization of ultraviolet surface-enhanced Raman spectroscopy
Optics Communications 456 (2020) 124631
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Plasmonic properties and optimization of ultraviolet surface-enhanced Raman spectroscopy Huan Pei a , Yong Wei b,c ,∗, Qiyuan Dai b , Fengmin Wang c a
College of Information Science and Engineering, Yanshan University, Qinhuangdao, 066004, China Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao, 066004, China c College of Iiren, Yanshan University, Qinhuangdao, 066004, China b
ARTICLE
INFO
Keywords: Surface enhanced Raman spectroscopy Localized surface plasmon Ultraviolet Aluminum nanoparticles
ABSTRACT The investigation of plasmonic properties and optimization of ultraviolet surface-enhanced Raman spectroscopy (UV-SERS) are essential for improving the detective capability in terms of high sensitivity and application range. In this work, the finite element method (FEM) are performed for the UV-SERS using Al nanoparticles (NPs) dimer. We theoretically explore the influence of incident angle, radius and separation of NPs dimer on the electric enhancement. The simulation results show that the optimized radius and separation of NPs dimer are 30 nm and 1 nm for UV-SERS measurements under the normal incidence condition, where the strongest Raman enhancement factor reach as high as 9 orders with excitation wavelength of 215 nm. Our results may help to find the optimal geometric factor for further improving the performance of the UV-SERS system.
1. Introduction After four decades of development, surface-enhanced Raman spectroscopy (SERS) with ultrahigh detection sensitivity and rich structural information of target molecules has been a powerful characterization technique in the research of surface science, food safety inspection, environmental detecting and material science [1–6]. The SERS phenomenon is primarily attributed to the enhanced electromagnetic field resulting from the localized surface plasmon resonance (LSPR) in the vicinity of noble metallic nanostructures [7,8], especially, the tremendous electromagnetic field enhancement at tiny nanogaps between neighboring nanostructures can greatly promote the absorption and emission cross sections of the molecule, making even single molecule measure possible [9–11]. In traditional SERS experiments, the excitation laser is usually chosen in the visible and near-infrared range by using Au or Ag nanoparticles as substrate [12–14]. Because most of fluorescence of target molecules occurs in these region, the detected Raman signals are often disturbed by the strong fluorescence background. The solution is to remove the excitation light from the visible range. For example, one way is to move the excited wavelength to the infrared band, however, the photon energy of infrared wavelength is low, which leading to lower sensitivity of Raman measurement, especially for some biological molecules with smaller scattering cross section. Previous studies demonstrate that many biological molecules have strong resonance absorption in the ultraviolet (UV) range [15,16].
Therefore, selecting UV as excitation light can greatly improve the measurement sensitivity. In the search for UV-SERS plasmonic substrates, it is found that aluminum (Al) is an ideal candidate material because of the reasonably small imaginary part of dielectric function and free-electron-like property in the UV range [17–19]. Meanwhile, compared with Au and Ag, Al is the most abundant and cost-effective non-noble plasmonic material, which further expand the application of SERS technology to various fields [20]. Experimentally, Yang et al. have prepared the Al nanoparticles (NPs) with long-term stability and good reproducibility by using a simple and efficient approach, and it exhibit the stronger Raman enhancement from UV to visible range [21]. In order to find optimal geometric parameters of Al NPs dimer for the experiment, the underlying LSPR properties of gap-mode UVSERS is studied theoretically in this paper. The finite element method (FEM) is used to calculate the enhancement factors of electromagnetic field with different incident angle, radius and separation of NPs dimer. Subsequently, we compare the Raman enhancement factors in detail for achieving higher sensitivity in UV-SERS measurements. 2. Simulation model and method For introducing the plasmonic coupling in UV-SERS, a metallic dimer consisting of two spherical Al NPs is chosen as the investigated substrate in Fig. 1. The radius of each Al NPs and the separation
∗ Corresponding author at: Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao, 066004, China. E-mail address: [email protected] (Y. Wei).
https://doi.org/10.1016/j.optcom.2019.124631 Received 25 May 2019; Received in revised form 31 August 2019; Accepted 24 September 2019 Available online 27 September 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
H. Pei, Y. Wei, Q. Dai et al.
Optics Communications 456 (2020) 124631
shift. It is noticed that the high energy peaks at shorter wavelength are in the deep UV region (the wavelength 𝜆 < 300 nm). Meanwhile, the low energy peaks at longer wavelength shift the UV region to the visible region with the increasing of the radius. The above phenomenon fully indicates the varying radius of NPs dimer can effectively tune the position of LSPR peak. Then we calculate the dependence of Raman enhancement on the radius, as shown in Fig. 3. Here the LSPR peaks around 215 nm and 240 nm at the radius of 30 nm and 40 nm are chosen as excitation wavelength, respectively. Fig. 3(a) indicates that Raman enhancement decreases gradually with the increase of the radius in the UV region, except for the larger enhancement with radius of 40 nm in the visible region (𝜆 > 410 nm). Fig. 3(b) shows that Raman enhancement with radius of 40 nm is larger except for the wavelength from 300 nm to 360 nm. By comparing Fig. 3(a) and (b), it is found that the optimal radius should be 30 nm under excitation wavelength of 215 nm, which could be mainly attributed to the stronger excitation enhancement than other radius in Fig. 2(b). To further find out the optimum configuration for UV-SERS measurements, we also study the dimer separation effect of electric field and Raman enhancements in Fig. 4. Here, we fixed the Al NPs size of 30 nm and the excitation wavelength of 215 nm. It is noticed that the gap of the Al NP dimer is larger than 1.0 nm in which the electron potential between the NP dimer is characterized by a large potential barrier that can effectively prevent electron tunneling effect. As a result, the electron tunneling effect is so weak that is not considered in all calculations. Fig. 4(a) presents that the electric field enhancement will decrease sharply and blue shift with the increases of dimer separation from 1 nm to 3 nm. In the UV-SERS system, the huge electric coupling effect at shorter dimer separation has dominating contribution to the Raman enhancement in this spectral region. As a result, we observe the best Raman enhancement can be obtained with a separation of 1 nm, corresponding to the maximum enhancement factor of 109 in the UV region from Fig. 4(b), which is approaching that of the experimental results using noble metallic substrate in the visible region [27,28].
Fig. 1. The model of UV-SERS using Al NPs dimer in the simulations.
between two NPs are denoted as 𝑟 and 𝑑, respectively. The whole model is irradiated by a p-polarized plane wave with electric magnitude 𝐸 along the 𝑍 axis, and its incident angle is 𝜃. The local electric field enhancement (𝑀) and corresponding Raman enhancement (𝐸) can be given as follows [22–25]. |𝑀(𝜔𝐼 )|2 = |𝐸(𝜔𝐼 )|2 ∕|𝐸0 (𝜔𝐼 )|2 2
2
2
(1)
|𝑀(𝜔𝑆 )| = |𝐸(𝜔𝑆 )| ∕|𝐸0 (𝜔𝑆 )|
(2)
2
(3)
2
𝑅𝐸 = |𝑀(𝜔𝐼 )| |𝑀(𝜔𝑆 )|
where 𝑀(𝜔𝐼 ) and 𝑀(𝜔𝑆 ) are the excitation and emission enhancement through Eqs. (1) and (2), respectively, in which 𝐸(𝜔𝐼 ) and 𝐸(𝜔𝑆 ) refer to the enhanced amplitudes of local excitation and emission fields, and 𝐸0 is the amplitude of incident field with the magnitude of 1 V/m in the simulation. In this work, the numerical simulations of local electric field in the middle of Al NPs dimer are performed with FEM. To prevent unwanted reflections, the boundary condition with perfectly matched layer is used in the simulation. Meanwhile, the calculated freedom degrees are around 200 million, which is large enough to ensure the accuracy of the results in the near-field. 3. Results We have firstly considered the influence of the incident angle of the incoming light to local electric enhancement in the middle of two Al NPs, as shown in Fig. 2(a). In the simulations the radius and the separation of the NPs dimer are set as 40 nm and 1 nm, respectively. Our calculation demonstrates that the electric enhancements are highly sensitive to the incident angle. The electric enhancement at the nanogap will decrease significantly when it decreases from 𝜃 = 0◦ to 𝜃 = 80◦ . The reason for this optimal angle is due to the vertical field component of electric magnitude along the Z axis plays a major contribution to the LSPR effect. As a result, it is found that the best electric enhancement can be obtained for UV-SERS under the normal incidence condition because of the maximum of the vertical field component, which is agreeing with the recent experiments [21]. In addition, the LSPR peaks in Fig. 2(a) are due to the coupling effect between the excited wavelength and plasmon modes at the nanogap. This is the case when the LSPR is close to the Rayleigh anomaly wavelength of the Al NP dimer Therefore, we take only this angle in the following calculations [26]. To quantitatively clarify the dependence of LSPR activity on the radius of Al NPs, Fig. 2(b) shows the enhancement factors of electric field at the center of the dimer gap with the radius from 30 nm to 60 nm, where the separation between two NPs is fixed at 𝑑 = 1 nm. For all the radius size considered, it can be seen that two distinguished LSPR peaks are observed clearly in the UV and visible region from 200 nm to 500 nm, and both two peaks decrease rapidly with the increasing of the radius size. This is mainly because that NPs with smaller radius have higher surface charge density, which leads to larger resonance peaks under the stronger plasmon coupling effect in the nanogap. By decreasing the radius, the peaks appear a gradual blue
4. Conclusion In conclusion, a systematic FEM investigations of the plasmonic properties of an Al NPs dimer has been developed. Based on the simulation results, the local enhancement of electric field could be controlled by modifying the incident angle and the geometry of Al NPs dimer. The optimized NPs radius of 30 nm and separation of 1 nm suggest an efficiency acquisition of UV-SERS under the normal incidence condition. The calculated Raman enhancement can reach 109 in the UV region with excitation wavelength of 215 nm. Our researches provide a prospective avenue for highly sensitive UV-SERS experiment. Furthermore, the application of plasmonic coupling effect for strain sensors has achieved rapid development in recent years, the current research shows that reducing the gap and designing the geometry of noble metallic dimers is a key factor for designing new strain sensors [29–31]. Therefore, the plasmonic Al NP dimer with low cost can also be useful for strain sensor technology by adjusting appropriate size and gap of NP dimer. Acknowledgments This work was supported by the Natural Science Foundation of Hebei province, China (Grant Nos. B2018203112), the 100 Talents Project of Hebei Province, China (Grant No. E2016100003), the National Science Foundation for Young Scientists of China (Grant No. 61802334), and the Science and Technology Research and Development Plan of Qinhuangdao City, China (201602A006). 2
H. Pei, Y. Wei, Q. Dai et al.
Optics Communications 456 (2020) 124631
Fig. 2. The dependence of local electric field enhancement on the (a) incident angle and (b) Al NPs radius, where the separation of the dimer is set as 1 nm.
Fig. 3. The dependence of Raman enhancement on the radius with excitation wavelength of (a) 215 nm and (b) 240 nm, respectively. The separation of the dimer is set as 1 nm.
Fig. 4. The dependence of (a) electric field enhancement and (b) Raman enhancement on the dimer separation. Here, the radius and the excitation wavelength are set as 30 nm and 215 nm, respectively.
References
[9] S.Y. Ding, E.M. You, Z.Q. Tian, M. Moskovits, Electromagnetic theories of surface-enhanced Raman spectroscopy, Chem. Soc. Rev. 46 (13) (2017) 4042. [10] X. Tian, Y. Fang, M. Sun, Formation of enhanced uniform chiral fields in symmetric dimer nanostructures, Sci. Rep. 5 (1) (2015) 17534. [11] P.A. Mosierboss, Review of SERS substrates for chemical sensing, Nanomaterials 7 (6) (2017) 142. [12] A.X. Wang, X. Kong, Review of recent progress of plasmonic materials and nano-structures for surface-enhanced Raman scattering, Materials 8 (6) (2015) 3024. [13] H. Guo, Z. Zhang, B. Xing, A. Mukherjee, C. Musante, J.C. White, L. He, Analysis of silver nanoparticles in antimicrobial products using surface-enhanced Raman spectroscopy (SERS), Environ. Sci. Technol. 49 (7) (2015) 4317–4324. [14] S.M. Ansar, M. Gadogbe, K. Siriwardana, J.Y. Howe, S. Dogel, H. Hosseinkhannazer, W.E. Collier, J. Rodriguez, S. Zou, D. Zhang, Dispersion stability, ligand structure and conformation, and SERS activities of 1-alkanethiol functionalized gold and silver nanoparticles, J. Phys. Chem. C 118 (43) (2014) 24925–24934. [15] X.-F. Lin, B. Ren, Z.-L. Yang, G.-K. Liu, Z.-Q. Tian, Surface-enhanced Raman spectroscopy with ultraviolet excitation, J. Raman Spectrosc. 36 (6?7) (2005) 606–612. [16] S. Tian, O. Neumann, M.J. Mcclain, X. Yang, L. Zhou, C. Zhang, P. Nordlander, N.J. Halas, Aluminum Nanocrystals: A sustainable substrate for quantitative SERS-based DNA detection, Nano Lett. 17 (8) (2017) 5071–5077. [17] P.M. Schwab, M. Carola, D. Katja, E. Hans-Jürgen, Oxide mediated spectral shifting in aluminum resonant optical antennas, Opt. Express 23 (20) (2015) 26533. [18] P.R. West, S. Ishii, G.V. Naik, N.K. Emani, V.M. Shalaev, A. Boltasseva, Searching for better plasmonic materials, Laser Photonics Rev. 4 (6) (2010) 795–808.
[1] M. Moskovits, Surface enhanced spectroscopy, Rev. Modern Phys. 57 (3) (1985) 783–826. [2] L. Anran, L. Shuzhou, Large-volume hot spots in gold spiky nanoparticle dimers for high-performance surface-enhanced spectroscopy, Nanoscale 6 (21) (2014) 12921–12928. [3] W. Zhou, Q. Li, H. Liu, J. Yang, D. Liu, Building electromagnetic hot spots in living cells via target-triggered nanoparticle dimerization, ACS Nano 11 (4) (2017) 3532. [4] K.N. Kanipe, P.P.F. Chidester, G.D. Stucky, C.D. Meinhart, M. Moskovits, Properly structured, any metal can produce intense surface enhanced Raman spectra, J. Phys. Chem. C 121 (26) (2017) 14269–14273. [5] R. Zhang, Y. Zhang, Z.C. Dong, S. Jiang, C. Zhang, L.G. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. Luo, Chemical mapping of a single molecule by plasmon-enhanced Raman scattering, Nature 498 (7452) (2013) 82–86. [6] Y.J. Ai, P. Liang, Y.X. Wu, Q.M. Dong, J.B. Li, Y. Bai, B.J. Xu, Z. Yu, D. Ni, Rapid qualitative and quantitative determination of food colorants by both Raman spectra and Surface-enhanced Raman Scattering (SERS), Food Chem. 241 (2018) 427–433. [7] G. Luca, G. Duncan, Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications, Chem. Soc. Rev. 41 (21) (2012) 7085–7107. [8] S. Chen, Z. Yang, L. Meng, J.F. Li, C.T. Williams, Z.Q. Tian, Electromagnetic enhancement in shell-isolated nanoparticle-enhanced Raman scattering from gold flat surfaces, J. Phys. Chem. C 119 (9) (2015) 5246–5251. 3
H. Pei, Y. Wei, Q. Dai et al.
Optics Communications 456 (2020) 124631 [26] J. Marae-Djouda, R. Caputo, N. Mahi, G. Lvque, A. Akjouj, P.M. Adam, T. Maurer, Angular plasmon response of gold nanoparticles arrays: Approaching the Rayleigh limit, Nanophotonics 6 (1) (2016) 279–288. [27] Y. Wei, L. Li, D.X. Sun, M.L. Wang, Y.Y. Zhu, Density-matrix evaluation of the enhancement to resonant Raman scattering and fluorescence of molecules confined in metallic nanoparticle dimers, Sci. Rep. 8 (1) (2018) 1832. [28] Q. Fu, Z. Zhan, J. Dou, X. Zheng, R. Xu, M. Wu, Y. Lei, Highly reproducible and sensitive SERS substrates with Ag inter-nanoparticle gaps of 5 nm fabricated by ultrathin aluminum mask technique, ACS Appl. Mater. Interfaces 7 (24) (2015) 13322–13328. [29] T. Maurer, J. Marae-Djouda, U. Cataldi, A. Gontier, G. Montay, Y. Madi, B. Panicaud, D. Macias, P.M. Adam, G. Lvque, The beginnings of plasmomechanics: Towards plasmonic strain sensors, Front. Mater. Sci. 9 (2) (2015) 170–177. [30] S. Aurlien, G. Arthur, P. Alexandre, B. Jrmie, Y.L. Hlne, B. Jean-Louis, P. JroMe, A. Pierre-Michel, M. Thomas, Single step synthesis and organization of gold colloids assisted by copolymer templates, Nanotechnology 25 (22) (2014) 225603. [31] J. Marae-Djouda, A. Gontier, R. Caputo, G. Leveque, B. Bercu, Y. Madi, G. Montay, P.M. Adam, M. Molinari, S. Stagon, Dense brushes of tilted metallic nanorods grown onto stretchable substrates for optical strain sensing, ACS Appl. Nano Mater. 1 (5) (2018) acsanm.8b00441.
[19] M.W. Knight, L. Liu, Y. Wang, L. Brown, S. Mukherjee, N.S. King, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum plasmonic nanoantennas, Nano Lett. 12 (11) (2012) 6000–6004. [20] L. Zhang, X. Li, Y. Wang, K. Sun, X. Chen, H. Chen, J. Zhou, Reproducible plasmonic nanopyramid array of various metals for highly sensitive refractometric and surface-enhanced Raman biosensing, ACS Omega 3 (10) (2018) 14181–14187. [21] X.M. Li, M.H. Bi, L. Cui, Y.Z. Zhou, J. Yang, 3D aluminum hybrid plasmonic nanostructures with large areas of dense hot spots and long-term stability, Adv. Funct. Mater. 27 (10) (2017) 1605703. [22] X. Hongxing, W. Xue-Hua, M.P. Persson, H.Q. Xu, K. Mikael, J. Peter, Unified treatment of fluorescence and raman scattering processes near metal surfaces, Phys. Rev. Lett. 93 (24) (2004) 11945. [23] Y. Zhilin, L. Qianhong, F. Yurui, S. Mengtao, Deep ultraviolet tip-enhanced Raman scattering, Chem. Commun. 47 (32) (2011) 9131. [24] W. Yong, L. Li, S. Dexin, Z. Yanying, T. Guangjun, The effect of silica shell on the surface enhanced Raman scattering and fluorescence with Ag nanoparticles: A three-dimensional finite element method investigation, Opt. Commun. 47 (2018) 426–432. [25] L. Meng, M. Sun, J. Chen, Z. Yang, A nanoplasmonic strategy for precision in-situ measurements of tip-enhanced Raman and fluorescence spectroscopy, Sci. Rep. 6 (2016) 19558.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Plasmonic properties and optimization of ultraviolet surface-enhanced Raman spectroscopy Huan Pei a , Yong Wei b,c ,∗, Qiyuan Dai b , Fengmin Wang c a
College of Information Science and Engineering, Yanshan University, Qinhuangdao, 066004, China Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao, 066004, China c College of Iiren, Yanshan University, Qinhuangdao, 066004, China b
ARTICLE
INFO
Keywords: Surface enhanced Raman spectroscopy Localized surface plasmon Ultraviolet Aluminum nanoparticles
ABSTRACT The investigation of plasmonic properties and optimization of ultraviolet surface-enhanced Raman spectroscopy (UV-SERS) are essential for improving the detective capability in terms of high sensitivity and application range. In this work, the finite element method (FEM) are performed for the UV-SERS using Al nanoparticles (NPs) dimer. We theoretically explore the influence of incident angle, radius and separation of NPs dimer on the electric enhancement. The simulation results show that the optimized radius and separation of NPs dimer are 30 nm and 1 nm for UV-SERS measurements under the normal incidence condition, where the strongest Raman enhancement factor reach as high as 9 orders with excitation wavelength of 215 nm. Our results may help to find the optimal geometric factor for further improving the performance of the UV-SERS system.
1. Introduction After four decades of development, surface-enhanced Raman spectroscopy (SERS) with ultrahigh detection sensitivity and rich structural information of target molecules has been a powerful characterization technique in the research of surface science, food safety inspection, environmental detecting and material science [1–6]. The SERS phenomenon is primarily attributed to the enhanced electromagnetic field resulting from the localized surface plasmon resonance (LSPR) in the vicinity of noble metallic nanostructures [7,8], especially, the tremendous electromagnetic field enhancement at tiny nanogaps between neighboring nanostructures can greatly promote the absorption and emission cross sections of the molecule, making even single molecule measure possible [9–11]. In traditional SERS experiments, the excitation laser is usually chosen in the visible and near-infrared range by using Au or Ag nanoparticles as substrate [12–14]. Because most of fluorescence of target molecules occurs in these region, the detected Raman signals are often disturbed by the strong fluorescence background. The solution is to remove the excitation light from the visible range. For example, one way is to move the excited wavelength to the infrared band, however, the photon energy of infrared wavelength is low, which leading to lower sensitivity of Raman measurement, especially for some biological molecules with smaller scattering cross section. Previous studies demonstrate that many biological molecules have strong resonance absorption in the ultraviolet (UV) range [15,16].
Therefore, selecting UV as excitation light can greatly improve the measurement sensitivity. In the search for UV-SERS plasmonic substrates, it is found that aluminum (Al) is an ideal candidate material because of the reasonably small imaginary part of dielectric function and free-electron-like property in the UV range [17–19]. Meanwhile, compared with Au and Ag, Al is the most abundant and cost-effective non-noble plasmonic material, which further expand the application of SERS technology to various fields [20]. Experimentally, Yang et al. have prepared the Al nanoparticles (NPs) with long-term stability and good reproducibility by using a simple and efficient approach, and it exhibit the stronger Raman enhancement from UV to visible range [21]. In order to find optimal geometric parameters of Al NPs dimer for the experiment, the underlying LSPR properties of gap-mode UVSERS is studied theoretically in this paper. The finite element method (FEM) is used to calculate the enhancement factors of electromagnetic field with different incident angle, radius and separation of NPs dimer. Subsequently, we compare the Raman enhancement factors in detail for achieving higher sensitivity in UV-SERS measurements. 2. Simulation model and method For introducing the plasmonic coupling in UV-SERS, a metallic dimer consisting of two spherical Al NPs is chosen as the investigated substrate in Fig. 1. The radius of each Al NPs and the separation
∗ Corresponding author at: Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao, 066004, China. E-mail address: [email protected] (Y. Wei).
https://doi.org/10.1016/j.optcom.2019.124631 Received 25 May 2019; Received in revised form 31 August 2019; Accepted 24 September 2019 Available online 27 September 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
H. Pei, Y. Wei, Q. Dai et al.
Optics Communications 456 (2020) 124631
shift. It is noticed that the high energy peaks at shorter wavelength are in the deep UV region (the wavelength 𝜆 < 300 nm). Meanwhile, the low energy peaks at longer wavelength shift the UV region to the visible region with the increasing of the radius. The above phenomenon fully indicates the varying radius of NPs dimer can effectively tune the position of LSPR peak. Then we calculate the dependence of Raman enhancement on the radius, as shown in Fig. 3. Here the LSPR peaks around 215 nm and 240 nm at the radius of 30 nm and 40 nm are chosen as excitation wavelength, respectively. Fig. 3(a) indicates that Raman enhancement decreases gradually with the increase of the radius in the UV region, except for the larger enhancement with radius of 40 nm in the visible region (𝜆 > 410 nm). Fig. 3(b) shows that Raman enhancement with radius of 40 nm is larger except for the wavelength from 300 nm to 360 nm. By comparing Fig. 3(a) and (b), it is found that the optimal radius should be 30 nm under excitation wavelength of 215 nm, which could be mainly attributed to the stronger excitation enhancement than other radius in Fig. 2(b). To further find out the optimum configuration for UV-SERS measurements, we also study the dimer separation effect of electric field and Raman enhancements in Fig. 4. Here, we fixed the Al NPs size of 30 nm and the excitation wavelength of 215 nm. It is noticed that the gap of the Al NP dimer is larger than 1.0 nm in which the electron potential between the NP dimer is characterized by a large potential barrier that can effectively prevent electron tunneling effect. As a result, the electron tunneling effect is so weak that is not considered in all calculations. Fig. 4(a) presents that the electric field enhancement will decrease sharply and blue shift with the increases of dimer separation from 1 nm to 3 nm. In the UV-SERS system, the huge electric coupling effect at shorter dimer separation has dominating contribution to the Raman enhancement in this spectral region. As a result, we observe the best Raman enhancement can be obtained with a separation of 1 nm, corresponding to the maximum enhancement factor of 109 in the UV region from Fig. 4(b), which is approaching that of the experimental results using noble metallic substrate in the visible region [27,28].
Fig. 1. The model of UV-SERS using Al NPs dimer in the simulations.
between two NPs are denoted as 𝑟 and 𝑑, respectively. The whole model is irradiated by a p-polarized plane wave with electric magnitude 𝐸 along the 𝑍 axis, and its incident angle is 𝜃. The local electric field enhancement (𝑀) and corresponding Raman enhancement (𝐸) can be given as follows [22–25]. |𝑀(𝜔𝐼 )|2 = |𝐸(𝜔𝐼 )|2 ∕|𝐸0 (𝜔𝐼 )|2 2
2
2
(1)
|𝑀(𝜔𝑆 )| = |𝐸(𝜔𝑆 )| ∕|𝐸0 (𝜔𝑆 )|
(2)
2
(3)
2
𝑅𝐸 = |𝑀(𝜔𝐼 )| |𝑀(𝜔𝑆 )|
where 𝑀(𝜔𝐼 ) and 𝑀(𝜔𝑆 ) are the excitation and emission enhancement through Eqs. (1) and (2), respectively, in which 𝐸(𝜔𝐼 ) and 𝐸(𝜔𝑆 ) refer to the enhanced amplitudes of local excitation and emission fields, and 𝐸0 is the amplitude of incident field with the magnitude of 1 V/m in the simulation. In this work, the numerical simulations of local electric field in the middle of Al NPs dimer are performed with FEM. To prevent unwanted reflections, the boundary condition with perfectly matched layer is used in the simulation. Meanwhile, the calculated freedom degrees are around 200 million, which is large enough to ensure the accuracy of the results in the near-field. 3. Results We have firstly considered the influence of the incident angle of the incoming light to local electric enhancement in the middle of two Al NPs, as shown in Fig. 2(a). In the simulations the radius and the separation of the NPs dimer are set as 40 nm and 1 nm, respectively. Our calculation demonstrates that the electric enhancements are highly sensitive to the incident angle. The electric enhancement at the nanogap will decrease significantly when it decreases from 𝜃 = 0◦ to 𝜃 = 80◦ . The reason for this optimal angle is due to the vertical field component of electric magnitude along the Z axis plays a major contribution to the LSPR effect. As a result, it is found that the best electric enhancement can be obtained for UV-SERS under the normal incidence condition because of the maximum of the vertical field component, which is agreeing with the recent experiments [21]. In addition, the LSPR peaks in Fig. 2(a) are due to the coupling effect between the excited wavelength and plasmon modes at the nanogap. This is the case when the LSPR is close to the Rayleigh anomaly wavelength of the Al NP dimer Therefore, we take only this angle in the following calculations [26]. To quantitatively clarify the dependence of LSPR activity on the radius of Al NPs, Fig. 2(b) shows the enhancement factors of electric field at the center of the dimer gap with the radius from 30 nm to 60 nm, where the separation between two NPs is fixed at 𝑑 = 1 nm. For all the radius size considered, it can be seen that two distinguished LSPR peaks are observed clearly in the UV and visible region from 200 nm to 500 nm, and both two peaks decrease rapidly with the increasing of the radius size. This is mainly because that NPs with smaller radius have higher surface charge density, which leads to larger resonance peaks under the stronger plasmon coupling effect in the nanogap. By decreasing the radius, the peaks appear a gradual blue
4. Conclusion In conclusion, a systematic FEM investigations of the plasmonic properties of an Al NPs dimer has been developed. Based on the simulation results, the local enhancement of electric field could be controlled by modifying the incident angle and the geometry of Al NPs dimer. The optimized NPs radius of 30 nm and separation of 1 nm suggest an efficiency acquisition of UV-SERS under the normal incidence condition. The calculated Raman enhancement can reach 109 in the UV region with excitation wavelength of 215 nm. Our researches provide a prospective avenue for highly sensitive UV-SERS experiment. Furthermore, the application of plasmonic coupling effect for strain sensors has achieved rapid development in recent years, the current research shows that reducing the gap and designing the geometry of noble metallic dimers is a key factor for designing new strain sensors [29–31]. Therefore, the plasmonic Al NP dimer with low cost can also be useful for strain sensor technology by adjusting appropriate size and gap of NP dimer. Acknowledgments This work was supported by the Natural Science Foundation of Hebei province, China (Grant Nos. B2018203112), the 100 Talents Project of Hebei Province, China (Grant No. E2016100003), the National Science Foundation for Young Scientists of China (Grant No. 61802334), and the Science and Technology Research and Development Plan of Qinhuangdao City, China (201602A006). 2
H. Pei, Y. Wei, Q. Dai et al.
Optics Communications 456 (2020) 124631
Fig. 2. The dependence of local electric field enhancement on the (a) incident angle and (b) Al NPs radius, where the separation of the dimer is set as 1 nm.
Fig. 3. The dependence of Raman enhancement on the radius with excitation wavelength of (a) 215 nm and (b) 240 nm, respectively. The separation of the dimer is set as 1 nm.
Fig. 4. The dependence of (a) electric field enhancement and (b) Raman enhancement on the dimer separation. Here, the radius and the excitation wavelength are set as 30 nm and 215 nm, respectively.
References
[9] S.Y. Ding, E.M. You, Z.Q. Tian, M. Moskovits, Electromagnetic theories of surface-enhanced Raman spectroscopy, Chem. Soc. Rev. 46 (13) (2017) 4042. [10] X. Tian, Y. Fang, M. Sun, Formation of enhanced uniform chiral fields in symmetric dimer nanostructures, Sci. Rep. 5 (1) (2015) 17534. [11] P.A. Mosierboss, Review of SERS substrates for chemical sensing, Nanomaterials 7 (6) (2017) 142. [12] A.X. Wang, X. Kong, Review of recent progress of plasmonic materials and nano-structures for surface-enhanced Raman scattering, Materials 8 (6) (2015) 3024. [13] H. Guo, Z. Zhang, B. Xing, A. Mukherjee, C. Musante, J.C. White, L. He, Analysis of silver nanoparticles in antimicrobial products using surface-enhanced Raman spectroscopy (SERS), Environ. Sci. Technol. 49 (7) (2015) 4317–4324. [14] S.M. Ansar, M. Gadogbe, K. Siriwardana, J.Y. Howe, S. Dogel, H. Hosseinkhannazer, W.E. Collier, J. Rodriguez, S. Zou, D. Zhang, Dispersion stability, ligand structure and conformation, and SERS activities of 1-alkanethiol functionalized gold and silver nanoparticles, J. Phys. Chem. C 118 (43) (2014) 24925–24934. [15] X.-F. Lin, B. Ren, Z.-L. Yang, G.-K. Liu, Z.-Q. Tian, Surface-enhanced Raman spectroscopy with ultraviolet excitation, J. Raman Spectrosc. 36 (6?7) (2005) 606–612. [16] S. Tian, O. Neumann, M.J. Mcclain, X. Yang, L. Zhou, C. Zhang, P. Nordlander, N.J. Halas, Aluminum Nanocrystals: A sustainable substrate for quantitative SERS-based DNA detection, Nano Lett. 17 (8) (2017) 5071–5077. [17] P.M. Schwab, M. Carola, D. Katja, E. Hans-Jürgen, Oxide mediated spectral shifting in aluminum resonant optical antennas, Opt. Express 23 (20) (2015) 26533. [18] P.R. West, S. Ishii, G.V. Naik, N.K. Emani, V.M. Shalaev, A. Boltasseva, Searching for better plasmonic materials, Laser Photonics Rev. 4 (6) (2010) 795–808.
[1] M. Moskovits, Surface enhanced spectroscopy, Rev. Modern Phys. 57 (3) (1985) 783–826. [2] L. Anran, L. Shuzhou, Large-volume hot spots in gold spiky nanoparticle dimers for high-performance surface-enhanced spectroscopy, Nanoscale 6 (21) (2014) 12921–12928. [3] W. Zhou, Q. Li, H. Liu, J. Yang, D. Liu, Building electromagnetic hot spots in living cells via target-triggered nanoparticle dimerization, ACS Nano 11 (4) (2017) 3532. [4] K.N. Kanipe, P.P.F. Chidester, G.D. Stucky, C.D. Meinhart, M. Moskovits, Properly structured, any metal can produce intense surface enhanced Raman spectra, J. Phys. Chem. C 121 (26) (2017) 14269–14273. [5] R. Zhang, Y. Zhang, Z.C. Dong, S. Jiang, C. Zhang, L.G. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. Luo, Chemical mapping of a single molecule by plasmon-enhanced Raman scattering, Nature 498 (7452) (2013) 82–86. [6] Y.J. Ai, P. Liang, Y.X. Wu, Q.M. Dong, J.B. Li, Y. Bai, B.J. Xu, Z. Yu, D. Ni, Rapid qualitative and quantitative determination of food colorants by both Raman spectra and Surface-enhanced Raman Scattering (SERS), Food Chem. 241 (2018) 427–433. [7] G. Luca, G. Duncan, Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications, Chem. Soc. Rev. 41 (21) (2012) 7085–7107. [8] S. Chen, Z. Yang, L. Meng, J.F. Li, C.T. Williams, Z.Q. Tian, Electromagnetic enhancement in shell-isolated nanoparticle-enhanced Raman scattering from gold flat surfaces, J. Phys. Chem. C 119 (9) (2015) 5246–5251. 3
H. Pei, Y. Wei, Q. Dai et al.
Optics Communications 456 (2020) 124631 [26] J. Marae-Djouda, R. Caputo, N. Mahi, G. Lvque, A. Akjouj, P.M. Adam, T. Maurer, Angular plasmon response of gold nanoparticles arrays: Approaching the Rayleigh limit, Nanophotonics 6 (1) (2016) 279–288. [27] Y. Wei, L. Li, D.X. Sun, M.L. Wang, Y.Y. Zhu, Density-matrix evaluation of the enhancement to resonant Raman scattering and fluorescence of molecules confined in metallic nanoparticle dimers, Sci. Rep. 8 (1) (2018) 1832. [28] Q. Fu, Z. Zhan, J. Dou, X. Zheng, R. Xu, M. Wu, Y. Lei, Highly reproducible and sensitive SERS substrates with Ag inter-nanoparticle gaps of 5 nm fabricated by ultrathin aluminum mask technique, ACS Appl. Mater. Interfaces 7 (24) (2015) 13322–13328. [29] T. Maurer, J. Marae-Djouda, U. Cataldi, A. Gontier, G. Montay, Y. Madi, B. Panicaud, D. Macias, P.M. Adam, G. Lvque, The beginnings of plasmomechanics: Towards plasmonic strain sensors, Front. Mater. Sci. 9 (2) (2015) 170–177. [30] S. Aurlien, G. Arthur, P. Alexandre, B. Jrmie, Y.L. Hlne, B. Jean-Louis, P. JroMe, A. Pierre-Michel, M. Thomas, Single step synthesis and organization of gold colloids assisted by copolymer templates, Nanotechnology 25 (22) (2014) 225603. [31] J. Marae-Djouda, A. Gontier, R. Caputo, G. Leveque, B. Bercu, Y. Madi, G. Montay, P.M. Adam, M. Molinari, S. Stagon, Dense brushes of tilted metallic nanorods grown onto stretchable substrates for optical strain sensing, ACS Appl. Nano Mater. 1 (5) (2018) acsanm.8b00441.
[19] M.W. Knight, L. Liu, Y. Wang, L. Brown, S. Mukherjee, N.S. King, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum plasmonic nanoantennas, Nano Lett. 12 (11) (2012) 6000–6004. [20] L. Zhang, X. Li, Y. Wang, K. Sun, X. Chen, H. Chen, J. Zhou, Reproducible plasmonic nanopyramid array of various metals for highly sensitive refractometric and surface-enhanced Raman biosensing, ACS Omega 3 (10) (2018) 14181–14187. [21] X.M. Li, M.H. Bi, L. Cui, Y.Z. Zhou, J. Yang, 3D aluminum hybrid plasmonic nanostructures with large areas of dense hot spots and long-term stability, Adv. Funct. Mater. 27 (10) (2017) 1605703. [22] X. Hongxing, W. Xue-Hua, M.P. Persson, H.Q. Xu, K. Mikael, J. Peter, Unified treatment of fluorescence and raman scattering processes near metal surfaces, Phys. Rev. Lett. 93 (24) (2004) 11945. [23] Y. Zhilin, L. Qianhong, F. Yurui, S. Mengtao, Deep ultraviolet tip-enhanced Raman scattering, Chem. Commun. 47 (32) (2011) 9131. [24] W. Yong, L. Li, S. Dexin, Z. Yanying, T. Guangjun, The effect of silica shell on the surface enhanced Raman scattering and fluorescence with Ag nanoparticles: A three-dimensional finite element method investigation, Opt. Commun. 47 (2018) 426–432. [25] L. Meng, M. Sun, J. Chen, Z. Yang, A nanoplasmonic strategy for precision in-situ measurements of tip-enhanced Raman and fluorescence spectroscopy, Sci. Rep. 6 (2016) 19558.
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