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利用磁控溅射技术在二氧化硅衬底上依次沉积单层金薄膜以及双层金银薄膜,每层金属薄膜厚度均为20 nm,利用飞秒激光在金属薄膜表面直接进行加工。将钛-蓝宝石飞秒激光器通过衰减器进行调整,首先设定功率为0.5 W,聚焦物镜的数值孔径选择20×(NA=0.4),X、Y轴的扫描间距设定为0.01 mm,扫描速率为0.005 mm/s。打开照明光源,样品表面反射的光将会沿着原路返回,其中一部分光通过分束镜进入CCD,利用计算机软件观测样品,调整z轴,使得计算机可以呈现清晰的图像。利用电子扫描显微镜(SEM)表征金属等离子体纳米结构的直径,原子力显微镜(AFM)表征高度,观测结果如图4所示。可以看出,此时单层金膜结构直径为8 μm,高度为400 nm。金银双层金属薄膜结构直径为19 μm,高度为820 nm。为了得到更加准确的实验数据,调整激光功率为0.3、0.4、0.6 W,其他参数保持不变。观察金属等离子体纳米结构的尺寸变化,表1所示为等离子体纳米结构直径的变化,表2所示为高度的变化。
图 4 (a)单层金薄膜制备所得等离子体纳米结构的SEM图;(b)单层金薄膜制备所得等离子体纳米结构的AFM图;(c)单层金膜等离子体纳米结构的高度曲线图;(d)金银双层薄膜制备所得等离子体纳米结构的SEM图;(e)金银双层薄膜制备所得等离子体纳米结构的AFM图;(f)金银双层薄膜制备所得等离子体纳米结构的高度曲线图
Figure 4. (a) SEM plot of plasma nanostructure obtained from single-layer Au film preparation; (b) AFM plot of plasma nanostructure obtained from single-layer Au film preparation; (c) Height profile of plasma nanostructure obtained from single-layer Au film; (d) SEM plot of plasma nanostructure obtained from Au-Ag bilayer film preparation; (e) AFM plot of plasma nanostructure obtained from Au-Ag bilayer film preparation; (f) Height profile of plasma nanostructure obtained from Au-Ag bilayer film preparation
表 1 飞秒激光辐照后纳米结构的SEM图
Table 1. SEM images of nanostructures after femtosecond laser irradiation
Structures Power/W 0.3 0.4 0.5 0.6 Formation of plasma nanostructures
on monolayer Au filmsFormation of plasma nanostructures
on Au-Ag bilayer films表 2 飞秒激光辐照后纳米结构的AFM图
Table 2. AFM images of nanostructures after femtosecond laser irradiation
Structures Power/W 0.3 0.4 0.5 0.6 Formation of plasma nanostructures
on monolayer Au filmsFormation of plasma nanostructures
on Au-Ag bilayer films通过表格对比发现,随着激光功率的不断增加,金属等离子体纳米结构的直径和高度逐渐增加。相同功率下,金银双层金属等离子体纳米结构相较于单层金属直径显著变大,高度也明显增加。当功率为0.6 W时,金银双层金属等离子体纳米结构的直径和高度接近单层金属等离子体纳米结构的两倍。局域表面等离子体共振效应很大程度上取决于纳米颗粒的尺寸、形状、颗粒间的相互作用以及局部环境等。理论上,调节任一参数即可以改变共振强度。因此,随着等离子体纳米结构的逐渐增加,引起局域表面等离子体共振效应的变化。
利用罗丹明(R6G)溶液作为测试分子对金属薄膜以及等离子体纳米结构进行评价。文中测试使用的是Nanobasse公司的共聚焦拉曼光谱分析成像仪器。R6G是一种表征表面增强拉曼谱的常用染色剂,具有很强的荧光性,在SERS信号探测中具有良好的应用。在测量之前,分别配制10−2、10−4、10−6 M不同浓度的R6G溶液,所有的样品均被滴上10 μL的R6G。经过测试发现,金属薄膜在10−2 M浓度下SERS信号强度没有明显的变化。随后,在单层金膜、单层金膜制备得到的等离子体纳米结构以及金银双层金属膜制备所得的等离子体纳米结构均匀滴上10 μL的R6G,浓度为10−4 M和10−6 M,激光波长为532 nm,物镜为40X,激光功率为4.3 W,积分时间10 s,每次累积次数1,每次测量都是在表面选择任意20个点的位置,然后将所得的信号峰值取平均值。拉曼信号测试结果如图5、图6所示,图均为对原始数据进行减基线并平滑处理之后的结果。红色曲线为单层平面金膜的拉曼光谱图,可以看到基本没有峰值出现;蓝色曲线和黑色曲线分别代表单层金膜制备的等离子体纳米结构的拉曼光谱曲线和金银双层金属薄膜制备所得等离子体纳米结构的拉曼光谱曲线,通过对比发现,相较于单层平面金膜,此时出现了明显的峰值信号,而黑色曲线的峰值显著高于蓝色曲线。所有的拉曼峰(760, 1200, 1520)均表现为金属等离子体纳米结构的特征SERS信号。数值分析表明,在1520 nm处,金银双层薄膜制备的等离子体纳米结构的SERS信号峰值最高位置是单层金膜制备的等离子体纳米结构的8倍。
图 6 平面金膜、单层金膜等离子体纳米结构、金银双层等离子体纳米结构的拉曼信号图谱(R6G浓度为10−6 M)
Figure 6. Raman signal patterns of planar Au film, single-layer Au film plasma nanostructure, and Au-Ag bilayer plasma nanostructure (R6G concentration is 10−6 M)
从图5和图6可以看出,随着R6G浓度的降低,金银双层金属等离子体纳米结构显示出更强的拉曼信号,单层平面金膜仍然没有峰值出现。测试结构表明金银双层金属等离子体纳米结构能够大大提高局部电场的强度,从而使SERS信号获得极大的增强。
Femtosecond laser processing and Raman detection applications of multi-plasmon resonance nanostructures
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摘要: 以多元金属纳米薄膜(金、银)为基底,利用飞秒激光加工技术制备得到多元等离子体纳米结构,并研究了其局域表面等离子体共振效应( Local Surface Plasmon Resonance,LSPR)和表面增强拉曼散射(Surface Enhanced Raman Scattering,SERS)性能。利用时域有限差分(Finite Difference Time Domain,FDTD)软件模拟了不同情况下(单层金膜、金银双层金属薄膜的平面以及阵列结构)的电场分布情况。根据仿真结果,相较于平面金属膜来说,飞秒激光制备的微纳结构阵列附近区域产生电磁场增强,集中在结构边缘处,且其强度变化与预期结果基本保持一致。此外,使用浓度为10−4 M和10−6 M的罗丹明(R6G)溶液进行SERS性能测试。测试的结果表明,单层平面金膜基本没有SERS峰值信号出现,而单层金膜上制备的等离子体纳米结构附近出现峰值信号,双层金属薄膜上制备的等离子体纳米结构展现出更高的SERS峰值信号。多元金属等离子体纳米结构展示出更强的局域表面等离子体共振效应,从而在表面增强拉曼散射、光催化、生物传感等领域具有广泛的应用。
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关键词:
- 飞秒激光加工 /
- 多元等离子体纳米结构 /
- 局域表面等离子体共振 /
- 表面增强拉曼散射
Abstract:Objective Plasma nanostructures composed of multiple metals have been widely applied in various fields such as photocatalysis, medical imaging, solar cells, surface-enhanced Raman scattering (SERS), biosensors, and information technology, due to their localized optical near-field properties and surface plasmon resonance effects. Compared with single-metal nanostructures, multi-metal plasma nanostructures exhibit significant enhanced resonance effects in the UV-VIS wavelength range. At present, there are few studies on multi-metal plasmonic nanostructures, and the fabrication methods are complicated, such as tedious processing, poor controllability, and long preparation period. Therefore, in this study, a scheme based on multi-metal thin film plasma nanostructures was designed, and simulation methods were used to demonstrate that the designed multi-metal plasma nanostructures have the characteristic of enhanced electric field. Furthermore, multi-metal plasma nanostructures were fabricated and evaluated using Rhodamine 6G (R6G) with a femtosecond laser direct writing system, demonstrating the enhanced SERS signal of the structure. Methods This article describes the construction of a femtosecond laser direct writing system. A titanium-sapphire oscillator laser (with an output power of 3.5 W, a central wavelength of 800 nm, and a repetition frequency of 85 MHz) is used as the femtosecond laser source (Fig.1). Magnetron sputtering technology was used to deposit a dual-layered gold-silver metal film on a silicon dioxide substrate. Rhodamine (R6G) solution was used as the test molecule for evaluating the SERS performance of multi-metal plasmonic nanocavity structures. Confocal Raman spectroscopy imaging was used to analyze the SERS performance of the multi-metal plasmonic nanocavity structures. Results and Discussions A multi-metal plasmonic nano-cavity structure was fabricated using a femtosecond laser direct writing system. Different sizes of nanoparticles were produced by adjusting the laser power and pulse irradiation time. The three-dimensional morphology of the experimental results was characterized using AFM and SEM, verifying the size variation law of multi-metal plasmonic nanostructures fabricated by femtosecond laser processing (Tab.1, Tab.2). The FDTD simulation software was used to simulate and analyze the changes in the electric field intensity. The electric field distribution of the planar metal was clearly reorganized, mainly concentrated at the edge of the metal plasmonic nanostructure, and the electric field intensity of the multi-metal plasmonic nanostructure was significantly enhanced compared to that of the single metal, usually manifested as an increase in the localized surface plasmon resonance effect (Fig.2, Fig.3). Evaluation using Rhodamine (R6G) solution showed that the gold-silver bilayer metal plasmonic nanostructure exhibited a stronger Raman signal, while the single-layer planar metal film still did not show any peak (Fig.5, Fig.6). Conclusions Based on the high-precision, high-flexibility, simple and convenient femtosecond laser processing technology, the metal plasmonic nanostructures were directly fabricated on the surface of metal thin films in this study. Through continuous optimization of processing parameters, uniform and regular nanostructures were obtained, and the structure was characterized to demonstrate the significant enhancement of localized surface plasmon resonance in multi-metal plasmonic nanostructures. Surface-enhanced Raman scattering (SERS) signal enhancement was verified using Rhodamine (R6G). The Raman test results showed that the structure had excellent SERS signal enhancement performance. Experimental simulations were performed using FDTD software, and the results showed that the electric field intensity between multi-metal plasmonic nanostructures was significantly enhanced. Femtosecond lasers can be used to process any material, such as semiconductors, polymers, alloys, and others, with various processing methods. In the future, spatiotemporally shaped femtosecond laser direct writing technology will be used to expand the size processing range of femtosecond lasers and control more material properties. -
Key words:
- femtosecond laser processing /
- multiple plasma nanostructures /
- LSPR /
- SERS
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图 4 (a)单层金薄膜制备所得等离子体纳米结构的SEM图;(b)单层金薄膜制备所得等离子体纳米结构的AFM图;(c)单层金膜等离子体纳米结构的高度曲线图;(d)金银双层薄膜制备所得等离子体纳米结构的SEM图;(e)金银双层薄膜制备所得等离子体纳米结构的AFM图;(f)金银双层薄膜制备所得等离子体纳米结构的高度曲线图
Figure 4. (a) SEM plot of plasma nanostructure obtained from single-layer Au film preparation; (b) AFM plot of plasma nanostructure obtained from single-layer Au film preparation; (c) Height profile of plasma nanostructure obtained from single-layer Au film; (d) SEM plot of plasma nanostructure obtained from Au-Ag bilayer film preparation; (e) AFM plot of plasma nanostructure obtained from Au-Ag bilayer film preparation; (f) Height profile of plasma nanostructure obtained from Au-Ag bilayer film preparation
表 1 飞秒激光辐照后纳米结构的SEM图
Table 1. SEM images of nanostructures after femtosecond laser irradiation
Structures Power/W 0.3 0.4 0.5 0.6 Formation of plasma nanostructures
on monolayer Au filmsFormation of plasma nanostructures
on Au-Ag bilayer films表 2 飞秒激光辐照后纳米结构的AFM图
Table 2. AFM images of nanostructures after femtosecond laser irradiation
Structures Power/W 0.3 0.4 0.5 0.6 Formation of plasma nanostructures
on monolayer Au filmsFormation of plasma nanostructures
on Au-Ag bilayer films -
[1] Liu J, He H, Xiao D, et al. Recent advances of plasmonic nanoparticles and their applications [J]. Materials, 2018, 11(10): 1833. doi: 10.3390/ma11101833 [2] Huang Z, Lei X, Liu Y, et al. Tapered optical fiber probe assembled with plasmonic nanostructures for surface-enhanced Raman scattering application [J]. ACS Applied Materials & Interfaces, 2015, 7(31): 17247-17254. doi: 10.1021/acsami.5b04202 [3] Zhang W, Li C, Gao K, et al. Surface-enhanced Raman spectroscopy with Au-nanoparticle substrate fabricated by using femtosecond pulse [J]. Nanotechnology, 2018, 29(20): 205301. doi: 10.1088/1361-6528/aab294 [4] Paul K K, Sreekanth N, Biroju R K, et al. Solar light driven photoelectrocatalytic hydrogen evolution and dye degradation by metal-free few-layer MoS2 nanoflower/TiO2 (B) nanobelts heterostructure [J]. Solar Energy Materials and Solar Cells, 2018, 185: 364-374. doi: 10.1016/j.solmat.2018.05.056 [5] Linic S, Christopher P, Ingram D B. Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy [J]. Nature Materials, 2011, 10(12): 911-921. doi: 10.1038/nmat3151 [6] Pang B, Yang X, Xia Y. Putting gold nanocages to work for optical imaging, controlled release and cancer theranostics [J]. Nanomedicine, 2016, 11(13): 1715-1728. doi: 10.2217/nnm-2016-0109 [7] Hou B, Xie M, He R, et al. Microsphere assisted super-resolution optical imaging of plasmonic interaction between gold nanoparticles [J]. Scientific Reports, 2017, 7(1): 13789. doi: 10.1038/s41598-017-14193-3 [8] Rifat A A, Mahdiraji G A, Sua Y M, et al. Highly sensitive multi-core flat fiber surface plasmon resonance refractive index sensor [J]. Optics Express, 2016, 24(3): 2485-2495. doi: 10.1364/OE.24.002485 [9] Akjouj A, Mir A. Design of silver nanoparticles with graphene coatings layers used for LSPR biosensor applications [J]. Vacuum, 2020, 180: 109497. doi: 10.1016/j.vacuum.2020.109497 [10] Chang T H P, Mankos M, Lee K Y, et al. Multiple electron-beam lithography [J]. Microelectronic Engineering, 2001, 57: 117-135. doi: https://doi.org/10.1016/S0167-9317(01)00528-7 [11] Garg V, Mote R G, Fu J. Focused ion beam direct fabrication of subwavelength nanostructures on silicon for multicolor generation [J]. Advanced Materials Technologies, 2018, 3(8): 1800100. doi: 10.1002/admt.201800100 [12] Chou S Y, Krauss P R, Renstrom P J. Imprint of sub-25 nm vias and trenches in polymers [J]. Applied Physics Letters, 1995, 67(21): 3114-3116. doi: 10.1063/1.114851 [13] Chen W, Tymchenko M, Gopalan P, et al. Large-area nanoimprinted colloidal Au nanocrystal-based nanoantennas for ultrathin polarizing plasmonic metasurfaces [J]. Nano Letters, 2015, 15(8): 5254-5260. doi: 10.1021/acs.nanolett.5b02647 [14] Liu D, Li C, Zhou F, et al. Capillary gradient-induced self-assembly of periodic Au spherical nanoparticle arrays on an ultralarge scale via a bisolvent system at air/water interface [J]. Advanced Materials Interfaces, 2017, 4(10): 1600976. doi: 10.1002/admi.201600976 [15] Haring A P, Khan A U, Liu G, et al. 3D printed functionally graded plasmonic constructs [J]. Advanced Optical Materials, 2017, 5(18): 1700367. doi: 10.1002/adom.201700367 [16] Fritzler K B, Prinz V Y. 3D printing methods for micro-and nanostructures [J]. Physics-Uspekhi, 2019, 62(1): 54. doi: 10.3367/UFNe.2017.11.038239 [17] Yang L, Wei J, Ma Z, et al. The fabrication of micro/nano structures by laser machining [J]. Nanomaterials, 2019, 9(12): 1789. doi: 10.3390/nano9121789 [18] Ahmmed K M T, Grambow C, Kietzig A M. Fabrication of micro/nano structures on metals by femtosecond laser micro-machining [J]. Micromachines, 2014, 5(4): 1219-1253. doi: 10.3390/mi5041219 [19] Liu X, Du D, Mourou G. Laser ablation and micromachining with ultrashort laser pulses [J]. IEEE Journal of Quantum Electronics, 1997, 33(10): 1706-1716. doi: 10.1109/3.631270 [20] Xia F, Zhang X, Wang M, et al. Analysis of the laser oxidation kinetics process of In-In2O3 MTMO photomasks by laser direct writing [J]. Optics Express, 2015, 23(22): 29193-29201. doi: 10.1364/OE.23.029193 [21] Xia F, Zhang X, Wang M, et al. Numerical analysis of the sub-wavelength fabrication of MTMO grayscale photomasks by direct laser writing [J]. Optics Express, 2014, 22(14): 16889-16896. doi: 10.1364/OE.22.016889 [22] Han W, Jiang L, Li X, et al. Controllable plasmonic nanostructures induced by dual-wavelength femtosecond laser irradiation [J]. Scientific Reports, 2017, 7(1): 17333. doi: 10.1038/s41598-017-16374-6 [23] Jradi S, Zaarour L, Chehadi Z, et al. Femtosecond direct laser-induced assembly of monolayer of gold nanostructures with tunable surface plasmon resonance and high performance localized surface plasmon resonance and surface enhanced Raman scattering sensing [J]. Langmuir, 2018, 34(51): 15763-15772. doi: 10.1021/acs.langmuir.8b00413