大面积图案化电场增强薄膜的设计及制备研究

采用有限时域差分（FDTD）法仿真了不同闪耀光栅结构上的银（Ag）薄膜模型。在633 nm的激发光下，闪耀光栅上周期为1/1200 mm、厚度为15 nm的Ag薄膜模型产生了较强的局域表面等离子体共振（LSPR）效应。利用机械刻划工艺和电子束蒸发镀膜工艺成功制备了这种Ag光栅薄膜，从而大幅降低了图案化电场增强薄膜的制备成本和难度。利用该电场增强Ag薄膜，基于表面增强拉曼散射（SERS），对亚甲基蓝染料进行检测，SERS信号强度增强，与FDTD仿真结果吻合。同时，基底不同位置处的主要特征峰强度的相对标准偏差（RSD）值都小于17%，薄膜表现出良好的均匀性和再现性。

Abstract

Objective

Currently, there are various technologies for controlling the electric field using the micro-nano structure of noble metal films, including electron beam lithography (EBL) and focused ion beam (FIB) etching. Most of these processing methods are complicated, with limited processing areas and high costs. These limitations significantly restrict the development of photoelectric functional films with electric field enhancement characteristics. According to current theories, the enhancement mechanism of surface-enhanced Raman scattering (SERS) technology can be divided into electromagnetic enhancement mechanism (EM) and chemical enhancement mechanism (CM). Compared to CM, EM generally exhibits stronger increases in the Raman scattering signals. Therefore, it is important to develop EM-based SERS substrates. The enhancement of the electromagnetic field mainly originates from the local surface plasmon resonance (LSPR) of the metal nanoparticles, which is independent of the adsorbed molecules and is an inherent property of metal nanoparticles. Therefore, classical electronic dynamics methods such as the finite difference time domain (FDTD) method can be used to describe the electromagnetic field. The purpose of this study is to develop a large-area, low-cost, and reproducible SERS substrate using FDTD simulations combined with mechanical grating ruling and electron beam evaporation deposition.

Methods

In this study, Silver (Ag) films with different blazed grating structures are simulated using FDTD method. The LSPR of the Ag films with blazed grating structures is significantly enhanced by adjusting the period. Under excitation at 633 nm, we find that Ag films with a grating period of 1/1200 mm generate a significant LSPR effect. Large-area electric-field-enhanced Ag grating films that can be produced in batches are realized using a mechanical grating ruling process and electron beam evaporation deposition. We successfully apply this electric field-enhanced Ag film to SERS to detect methylene blue dye. The Ag grating films significantly enhance the SERS intensity, and the test results show good uniformity and reproducibility of the substrate, consistent with the FDTD simulation results.

Results and Discussions

As shown in Fig. 1, FDTD software is used to design and simulate two types of blazed grating film structures, and the electric field intensity and absorption spectra of the Ag films are compared. The results of the FDTD simulation (Fig. 2) show that compared to the Ag film with a grating structure, the pure Ag film exhibits lower electric field intensity and light absorption. Compared with other two types of structures, the Ag films on Grating 1 exhibit the strongest electric field intensity and absorbance in the local area, indicating that the Ag films with a grating structure exhibit a stronger LSPR effect at 633 nm. Blazed Grating 1 and Grating 2 structures are prepared using a mechanical scribing process and the required diffraction gratings are produced in batches through grating replication. Finally, the processed gratings are coated with Ag films through electron beam evaporation deposition to obtain Ag grating films with the required electric field enhancement effect. The prepared Ag films with grating structures are characterized using X-ray diffraction (XRD) (Fig. 4), atomic force microscopy, and optical microscopy (Fig. 5). The microscopic images and XRD results illustrate that the large-area Ag films with a grating structure are successfully prepared. The surface uniformity of the prepared gratings is good with equal spacing between the lines. A Raman spectrometer is used to verify the electric-field enhancement ability of films on Grating 1. As shown in Fig. 6(a), the Ag thin films with a grating structure produce more 'hot spots,' which significantly enhances the local electromagnetic field on the surfaces of the grating samples, thus substantially improving the Raman scattering signal of methylene blue. Figures. 6(b)‒(d) show that the corresponding relative standard deviation (RSD) values of the characteristic peaks at 1623 cm^-1 and 1394 cm^-1 for methylene blue in different sites of the Grating 1 sample are less than 17%, confirming the good reproducibility of the SERS substrate.

Conclusions

In this study, a simple and low-cost technology for fabricating large-area patterned electric-field-enhanced Ag films is proposed. First, Ag films with a blazed grating structure is simulated using the FDTD method, and a film structure with strong electric field enhancement at 633 nm wavelength is obtained by adjusting the period, which effectively tunes the LSPR effect on the film surface. Ag thin films with corresponding structures are prepared using mechanical grating ruling and electron-beam evaporation deposition. Through verification by a Raman spectrometer, the Ag films with a blazed grating structure improve the SERS intensity, consistent with the simulation results. This electric-field-enhanced film provides an efficient substrate for the enhancement of Raman scattering signals and an idea for the actual detection of trace molecules.

1　引言

当外部入射光作用于贵金属薄膜表面时，如果其频率和波矢量与金属薄膜表面自由电子振荡频率和波矢量相匹配，就会产生电子的集体共振现象，从而出现局域表面等离子体共振（LSPR）效应^［1-2］。这种集体振荡效应可使薄膜表面电场显著放大^［3］。LSPR效应在生物传感^［4］、荧光增强^［5］、非线性光学^［6］、表面增强拉曼散射（SERS）^［7］等众多光电子学领域中得到广泛应用。金、银、铂等贵金属材料因介电常数为负且绝对值较大，常被设计为特定的微纳结构以形成电场增强^［8］。目前，制备微纳结构金属薄膜的工艺多种多样，其中以电子束曝光（EBL）^［9］和聚焦离子束刻蚀（FIB）^［10］等方法为代表。这些加工方法大多较为复杂，加工面积有限，成本相对高昂，这大大限制了具备电场增强特性的光电功能薄膜的发展。因此，开发制备工艺简单、成本低廉、能够大面积生产的电场增强金属薄膜具有重要的意义。

与上述过程繁琐、价格昂贵的加工方法相比，基于机械刻划和复制技术制备衍射光栅^［11］的工艺流程相对简单，生产成本也大幅降低。通过设计合适的光栅结构，可以获得具有电场增强效果的贵金属薄膜。文献［12］报道，贵金属薄膜的电场增强效果不仅取决于材料的选择，还取决于薄膜表面微结构设计。通过改变每毫米光栅划线数、光栅平面与槽面夹角、金属薄膜厚度等参数，可以高效、便捷地对不同波段电场增强金属薄膜进行研究。

本文首先通过时域有限差分（FDTD）法进行仿真，建立了银光栅薄膜模型，研究了在633 nm入射光下不同结构模型的电场增强效果。根据仿真结果，使用机械刻划工艺制备所需的大面积高精度衍射光栅母板，再通过光栅复制的方式批量生产所需的衍射光栅，最后在加工好的光栅微结构上，通过电子束蒸发镀膜法，镀上相应厚度的金属薄膜，即可得到具有电场增强效果的金属光栅薄膜。

2　FDTD仿真

表面等离子体激元是一种沿着金属表面传播的波。当入射光的偏振方向平行于入射面时，在合适的边界条件下求解麦克斯韦方程组^［13］，可以得到等离子体激元的传输波矢量大小（ $k_{s p}$ ）：

k_{s p} = \frac{ω}{c} \sqrt[]{\frac{ε_{d} ε_{m}}{ε_{d} + ε_{m}}}

，（1）

式中： $ε_{m}$ 是金属的介电常数； $ε_{d}$ 是介质的介电常数； $c$ 是光速； $ω$ 是光的角频率。由于动量不匹配，只有在 $ε_{m}$ 与 $ε_{d}$ 实部符号相反的界面上才能激发倏逝波。

根据Drude模型^［14］，当满足共振条件时，有

ω_{m a x} = \frac{ω_{p}}{\sqrt[]{2 ε_{m} + 1}}

，（2）

式中： $ω_{m a x}$ 表示表面等离子体共振峰的频率； $ω_{p}$ 表示表面等离子体频率。由公式 $ε_{m} = n^{2}$ 可知， $ε_{m}$ 与介质环境的折射率（ $n$ ）有关。共振波长（ $λ_{0}$ ）与 $n$ 的关系为

λ_{0} = \frac{2 π c \sqrt[]{1 + \frac{1 - L}{L} ε_{d}}}{ω_{p}} = λ_{p} \sqrt[]{1 + n^{2} (\frac{1}{L} - 1)}

，（3）

式中： $L$ 是金属颗粒的形状因子； $λ_{p}$ 是金属颗粒的等离子体波长。

由式（3）可以看出，消光光谱的波长 $λ_{0}$ 主要由 $L$ 、 $λ_{p}$ 、 $n$ 、 $ε_{d}$ 决定。

为了实现633 nm波段大面积电场增强的图案化银薄膜，使用FDTD法设计和模拟了图1所示的两种闪耀光栅薄膜结构，并与单独Ag薄膜的电场强度和吸收光谱进行了对比。

图 1. 仿真结构示意图

Fig. 1. Schematic of simulation structure

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在仿真中设置闪耀角均为8°，周期分别为1/1200 mm与1/2400 mm。将周期为1/1200 mm与1/2400 mm的闪耀光栅分别标记为Grating 1和Grating 2。xy方向采用周期边界条件。同时，633 nm的平面光垂直入射在仿真样品的xy面上。图2（a）所示样品为没有光栅结构的15 nm厚的Ag薄膜，由于其表面相对光滑致密，电场强度也十分微弱。图2（d）所示的15 nm厚的Ag薄膜对600~700 nm波段的光束几乎没有吸收，因此薄膜表面也没有产生LSPR效应。图2（b）、（c）是仿真的两种闪耀光栅结构，并分别在表面涂覆了15 nm厚的银膜层。如图2（b）所示，633 nm入射光照射在Ag膜层表面，在斜面上形成了强烈的LSPR效应。图2（e）也揭示了Grating 1结构上的Ag薄膜在620 nm左右产生了共振吸收峰，与电场仿真结果吻合。图2（c）是Grating 2结构上的Ag薄膜在633 nm入射光下的电场仿真结果，可以看出，相比于图2（a），其在光栅斜面上表现出比较弱的LSPR效应。如图2（f）所示，在600~700 nm波段，这种周期光栅结构上的Ag薄膜的吸光率较低，没有产生明显的共振吸收峰。

图 2. 采用FDTD法仿真获得的在633 nm入射光下的电场强度和吸收图谱。（a）（d）15 nm厚的Ag薄膜；（b）（e）Grating 1上的Ag薄膜；（c）（f）Grating 2上的Ag薄膜

Fig. 2. Electric field intensity and absorption spectra obtained by FDTD simulation at 633 nm incident light. (a)(d) 15 nm thick Ag film; (b)(e) Ag film on Grating 1; (c)(f) Ag film on Grating 2

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图3为三种Ag薄膜结构在633 nm平面光照射下的局域电场强度与吸光率对比。局域电场强度的变化趋势与吸光率一致。相比于光栅结构上的Ag薄膜，单纯的Ag薄膜表现出较低的电场强度和吸光率。而Grating 1上的Ag薄膜则在局域表现出最强的电场强度和吸光率，这表明这种光栅结构上的Ag薄膜在633 nm处相比于其他两种薄膜表现出更强的LSPR效应。

图 3. 不同Ag薄膜在633 nm入射光下的电场强度与吸光率

Fig. 3. Electric field intensity and absorptivity of different Ag films at 633 nm incidence light

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3　实验

3.1　样品制备

实验首先制备母版光栅，采用K9玻璃作为光栅的毛胚，利用真空镀膜机在表面镀上铬作为底层，然后镀铝以增加K9玻璃与铝层的黏结牢固度，最后利用光栅刻划机进行刻划，刻划完毕后利用光栅复制工艺得到闪耀角为8°、周期分别为1/1200 mm和1/2400 mm的透射式闪耀光栅。在所得光栅表面利用电子束蒸发技术分别沉积15 nm 厚的Ag薄膜。薄膜沉积时本底真空度为5×10^-4 Pa，电子束电压为9.3 kV。最终周期为1/1200 mm与1/2400 mm的闪耀光栅上的Ag薄膜样品分别标记为Grating 1样品和Grating 2样品。为了方便对比，没有光栅结构的15 nm Ag薄膜也被用于对照。最后在不同样品上分别滴加1×10^-4 mol/L浓度的亚甲基蓝（MB）染料以测试其SERS性能。

3.2　样品表征

采用X射线衍射仪分析薄膜样品的晶相结构，扫描角度为10°~70°。采用原子力显微镜表征样品的表面形貌。采用共聚焦拉曼显微镜表征薄膜的染料探针分子的拉曼散射光谱，其激发波长为633 nm。所有样品测试均在室温下进行。

4　结果分析

4.1　X射线衍射图谱

图4为三种15 nm厚Ag薄膜的X射线衍射（XRD）图谱。可以看出，Ag薄膜和Grating 2光栅结构上的Ag薄膜均未出现明显的衍射峰。这是因为沉积的薄膜厚度太薄，薄膜的生长杂乱无序，主要呈现出非晶结构。而Grating 1结构上的Ag薄膜光栅则在38.3°处出现了一个小的衍射峰，这个衍射峰对应Ag晶体结构的（111）晶面^［15］。Grating 1样品出现了对应于（111）晶面的衍射峰，可能是由于在相同的闪耀角下，较大的周期使其具有更高的垂直高度。在膜料沉积过程中，距离蒸发源更近的位置首先吸附凝结，形成晶核生长，具有相对更大的厚度，从而提高了结晶性^［16］，因此产生了对应于具有最低表面结合能的（111）晶面的衍射峰^［17］。

图 4. 不同Ag薄膜结构的XRD图谱

Fig. 4. XRD patterns of Ag films with different structures

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4.2　原子力显微镜图谱和光学显微镜图谱

图5为不同周期15 nm厚度的Grating 1样品和Grating 2样品的原子力显微镜（AFM）图像和光学显微镜图像。图5（a）、（c）表明，大面积的电场增强的Ag薄膜结构得到成功制备，其中插图为尺寸为1.5 μm×1.5 μm的AFM图像。制备的Ag薄膜表面均匀度较好，刻线间距相等。从AFM图像可以看出，所制备的两种周期的光栅Ag薄膜都表现出连续的锯齿状条纹。图5（b）、（d）显示了Grating 1样品和Grating 2样品的表面均方根（RMS）粗糙度，分别为4.997 nm和4.728 nm，而纯Ag薄膜的RMS粗糙度为1.198 nm。

图 5. 不同样品的光学图像

Fig. 5. Optical images of different samples

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4.3　SERS图谱

当染料探针分子与Ag薄膜结构的距离足够小时，LSPR效应导致的电场增强会放大拉曼散射信号，从而出现表面增强拉曼散射，即SERS效应^［18］。为了进一步验证这种光栅结构上的电场增强Ag薄膜的潜在应用，我们采用拉曼光谱仪研究了纯Ag薄膜和光栅结构上的Ag薄膜的SERS效应。采用亚甲基蓝作为SERS探针分子，其浓度为1×10^-4 mol/L，激发波长为633 nm。测试结果如图6（a）所示，可以看出，单独的Ag薄膜对MB分子的检测效果有限，Grating 1结构上的Ag薄膜的检测能力最强。明显观察到以1393 cm^-1和1623 cm^-1为中心的特征峰，分别对应于C—N键的对称伸缩振动模式和C—C键的环伸缩振动模式^［19］。在633 nm激发光的激发下，Ag薄膜相对于其他一些金属材料呈现出更好的SERS效果^［20］。此外，光栅结构上的Ag薄膜样品相比于纯Ag薄膜产生了更多尖端和“热点”，在633 nm激发光的激发下，光栅结构上的Ag薄膜表面的局域电磁场得到显著增强，从而大幅增强了拉曼散射信号^［21］。为了对样品的SERS均匀性进行表征，我们将Grating 1样品浸泡在1×10^-4 mol/L的MB染料中，等待干燥，然后在样品表面随机选取10个点进行测试。如图6（b）~（d）所示，我们计算了MB在1394 cm^-1和1623 cm^-1处的振动峰强度的相对标准偏差（RSD）值，发现RSD值都小于17%，这验证了我们所制备的大面积SERS基底具有良好的再现性。

图 6. 实验结果。（a）MB在不同样品上的SERS图谱；（b）Grating 1样品上10个随机位点收集的MB分子的SERS图谱；（c）1623 cm^-1和（d）1394 cm^-1处特征峰的RSD值

Fig. 6. Experimental results. (a) SERS spectra of MB on different samples; (b) SERS spectra of MB molecules collected at 10 random sites on Grating 1 sample; RSD values of characteristic peaks at (c) 1623 cm^-1 and (d) 1394 cm^-1

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5　结论

提出了一种工艺简单、成本低廉的制备大面积图案化电场增强薄膜的技术。首先通过FDTD法仿真了闪耀光栅结构上的Ag薄膜，通过调整周期在633 nm波长下获得了强烈电场增强的薄膜结构，该结构有效调谐了薄膜表面的LSPR效应。然后采用机械刻化和电子束蒸发工艺大面积制备了相应结构的Ag薄膜。在633 nm激发光下通过拉曼光谱仪验证发现，闪耀光栅结构上的Ag薄膜有助于提升SERS强度，与仿真结果一致。这种电场增强薄膜可增强拉曼散射信号，是一种高效的基底，为实际微量分子检测提供了一种思路。

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