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SERS characteristics analysis of composite Ag/SiO2 sinusoidal grating

Cheng XIAO Zhi-bin CHEN Meng-ze QIN Dong-xiao ZHANG

肖程, 陈志斌, 秦梦泽, 张冬晓. 复合Ag/SiO2正弦光栅基底SERS特性分析[J]. 中国光学, 2019, 12(1): 59-74. doi: 10.3788/CO.20191201.0059
引用本文: 肖程, 陈志斌, 秦梦泽, 张冬晓. 复合Ag/SiO2正弦光栅基底SERS特性分析[J]. 中国光学, 2019, 12(1): 59-74. doi: 10.3788/CO.20191201.0059
XIAO Cheng, CHEN Zhi-bin, QIN Meng-ze, ZHANG Dong-xiao. SERS characteristics analysis of composite Ag/SiO2 sinusoidal grating[J]. Chinese Optics, 2019, 12(1): 59-74. doi: 10.3788/CO.20191201.0059
Citation: XIAO Cheng, CHEN Zhi-bin, QIN Meng-ze, ZHANG Dong-xiao. SERS characteristics analysis of composite Ag/SiO2 sinusoidal grating[J]. Chinese Optics, 2019, 12(1): 59-74. doi: 10.3788/CO.20191201.0059

复合Ag/SiO2正弦光栅基底SERS特性分析

doi: 10.3788/CO.20191201.0059
基金项目: 

国防科技项目基金 2004053

详细信息
    作者简介:

    肖程(1989—),男,江苏张家港人,博士在读,2012年于南京邮电大学获得学士学位,现为陆军工程大学在读博士,主要从事微纳光学领域SERS(表面增强拉曼散射)应用方面的研究,特别是SERS用于痕量爆炸物检测方面的应用研究。E-mail:

    陈志斌(1965—),男,湖南益阳人,博士,研究员,博士生导师,主要从事光电信息探测与加密传输方面的研究。E-mail:

  • 中图分类号: TN305.7

SERS characteristics analysis of composite Ag/SiO2 sinusoidal grating

Funds: 

National Defense Science Technology Project Fund 2004053

More Information
    Author Bio:

    XIAO Cheng(1989—), male, from Zhangjiagang, Jiangsu, is a current Ph.D. student who received a bachelor′s degree from Nanjing University of Posts and Telecommunications in 2012. He is currently studying at Army Engineering University and is mainly engaged in SERS(Surface Enhanced Raman Scattering) applications research with particular attention to SERS′s applications in explosives detection.E-mail:xc_nanking@163.com

    CHEN Zhibin(1965—), male, from Yiyang, Hunan, has a Ph.D. and is a researcher and doctoral tutor who is mainly engaged in research on photoelectric information detection and transmission encryption. E-mail:shangxingboy@163.com

    Corresponding author: XIAO Cheng.E-mail:shangxingboy@163.com
  • 摘要: 当前微流控表面增强拉曼散射(SERS)检测领域常用的贵金属纳米颗粒溶胶单位体积内热点区域数量有限且热点区域范围较小,而贵金属纳米三维阵列结构加工时间长,成本高昂并存在"记忆效应"。本文提出了集成到微流道的复合Ag/SiO2正弦光栅SERS基底结构,可以利用激光干涉光刻技术进行制备,无需预制掩膜版,可实现大面积、低成本SERS基底简易快速制备。利用严格耦合波分析方法(RCWA)建立了复合正弦光栅表面电场增强数学评估模型,推导了表面等离子体共振(SPP)耦合吸收率数学模型,分析了入射光、复合正弦光栅结构与外界环境介电常数的优化匹配关系,得到了入射光785 nm条件下的最佳复合正弦光栅结构。通过制备加工并实验验证了复合正弦光栅的SERS性能,SERS增强因子(EF)能够达到104
  • 图  1  (a)激光干涉图样光强度分布;(b)复合正弦光栅截面示意图

    Figure  1.  (a) Light intensity distribution of laser interference; (b)Section diagram of the composite sinusoidal grating

    图  2  (a)吸收率随衍射级次变化曲线;(b)吸收率随入射光波长变化曲线;(c)吸收率随入射角变化曲线

    Figure  2.  (a) Absorbance as a function of diffraction grades; (b)Absorbance as a function of incident light wavelength; (c)Absorbance as a function of incident angle

    图  3  (a)吸收率随光栅周期变化曲线;(b)吸收率随光栅振幅变化曲线;(c)吸收率随光栅银层厚度变化曲线

    Figure  3.  (a) Absorbance as a function of the grating period; (b)Absorbance as a function of grating amplitude; (c)Absorbance as a function of grating silver layer thickness

    图  4  吸收率随光栅表面环境介电常数变化曲线

    Figure  4.  Absorbance as a function of the dielectric constant above the grating surface

    图  5  (a)复合正弦银光栅表面增强电场分布;(b)复合正弦金光栅表面增强电场分布;(c)复合矩形银光栅表面增强电场分布

    Figure  5.  (a) Electric field distribution of composite sinusoidal silver grating; (b)Electric field distribution of composite sinusoidal gold grating; (c)Electric field distribution of composite rectangular silver grating

    图  6  (a)复合正弦光栅结构示意图;(b)干涉光刻系统

    Figure  6.  (a) Structural diagram of the composite sinusoidal grating; (b)Interference photolithography system

    图  7  (a)复合正弦光栅SEM表征图;(b)R6G溶液的SERS光谱图;(c)R6G拉曼特征峰处信号强度的均值误差棒

    Figure  7.  (a) SEM image of the composite sinusoidal grating; (b)SERS spectrum of R6G solutions; (c)Error bars of the mean intensity of R6G Raman peaks from five average spectra

    表  1  Theoretical EF values of the three gratings

    Table  1.   Theoretical EF values of the three gratings

     Grating category EF
    Sine Water/Ag/SiO2 grating 7.4×104
    Sine Water/Au/SiO2 grating 2.0×104
    Rectangular Water/Ag/SiO2 grating 4.4×104
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  • 收稿日期:  2018-03-14
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  • 刊出日期:  2019-02-01

SERS characteristics analysis of composite Ag/SiO2 sinusoidal grating

doi: 10.3788/CO.20191201.0059
    基金项目:

    国防科技项目基金 2004053

    作者简介:

    肖程(1989—),男,江苏张家港人,博士在读,2012年于南京邮电大学获得学士学位,现为陆军工程大学在读博士,主要从事微纳光学领域SERS(表面增强拉曼散射)应用方面的研究,特别是SERS用于痕量爆炸物检测方面的应用研究。E-mail:

    陈志斌(1965—),男,湖南益阳人,博士,研究员,博士生导师,主要从事光电信息探测与加密传输方面的研究。E-mail:

    通讯作者: XIAO Cheng.E-mail:shangxingboy@163.com
  • 中图分类号: TN305.7

摘要: 当前微流控表面增强拉曼散射(SERS)检测领域常用的贵金属纳米颗粒溶胶单位体积内热点区域数量有限且热点区域范围较小,而贵金属纳米三维阵列结构加工时间长,成本高昂并存在"记忆效应"。本文提出了集成到微流道的复合Ag/SiO2正弦光栅SERS基底结构,可以利用激光干涉光刻技术进行制备,无需预制掩膜版,可实现大面积、低成本SERS基底简易快速制备。利用严格耦合波分析方法(RCWA)建立了复合正弦光栅表面电场增强数学评估模型,推导了表面等离子体共振(SPP)耦合吸收率数学模型,分析了入射光、复合正弦光栅结构与外界环境介电常数的优化匹配关系,得到了入射光785 nm条件下的最佳复合正弦光栅结构。通过制备加工并实验验证了复合正弦光栅的SERS性能,SERS增强因子(EF)能够达到104

English Abstract

肖程, 陈志斌, 秦梦泽, 张冬晓. 复合Ag/SiO2正弦光栅基底SERS特性分析[J]. 中国光学, 2019, 12(1): 59-74. doi: 10.3788/CO.20191201.0059
引用本文: 肖程, 陈志斌, 秦梦泽, 张冬晓. 复合Ag/SiO2正弦光栅基底SERS特性分析[J]. 中国光学, 2019, 12(1): 59-74. doi: 10.3788/CO.20191201.0059
XIAO Cheng, CHEN Zhi-bin, QIN Meng-ze, ZHANG Dong-xiao. SERS characteristics analysis of composite Ag/SiO2 sinusoidal grating[J]. Chinese Optics, 2019, 12(1): 59-74. doi: 10.3788/CO.20191201.0059
Citation: XIAO Cheng, CHEN Zhi-bin, QIN Meng-ze, ZHANG Dong-xiao. SERS characteristics analysis of composite Ag/SiO2 sinusoidal grating[J]. Chinese Optics, 2019, 12(1): 59-74. doi: 10.3788/CO.20191201.0059
    • In recent years, with the rapid development of laser technology, nanotechnology and computer technology, SERS microfluidic technology has been widely used, especially in the fields of interface and surface science, material analysis, biology, medicine, food safety, environmental monitoring and national security[1-5]. At present, there are two widely used Raman-enhanced substrates in SERS microfluidic detection technology: single metal nanoparticle sols, including gold nanoparticle sols and silver nanoparticle sols[6]; nanoscopic three-dimensional array structures, including spherical nanoarray structures, columnar nano-array structures, cone-shaped nano-array structures, etc.[7-9]. Noble metal nanoparticle sols are advantageous because of their low cost and simple preparation procedure. In order to improve the SERS detection sensitivity of the noble metal nanoparticle sol, the influence of factors such as the shape, size and metal type of the noble metal nanoparticle were studied early in research[10-11], thereby forming stronger particle gap electric field hotspots. However, the noble metal nanoparticle sol still has the issue of sparse hot spot regions within a unit of volume and the range of these hotspot regions being small[12]. The nano-three-dimensional array structure prepared by plasma etching and traditional lithography processing technology has good structural periodicity and SERS signal reproducibility but the process is costly and time-consuming[13]. Additionally, since the surface of the composite sinusoidal grating substrate does not have a nano-rough structure, the molecules of the substance to be detected are not easily adsorbed on the grating surface. There is also a "memory effect"[14], creating the need to clean the substrate before proceeding to another test and therefore causing the process to fail to meet the requirements of continuous SERS detection.

      For this reason, a composite sinusoidal grating structure substrate is proposed. The intensity distribution function of the pattern formed by the interference of two coherent beams has a sinusoidal function. After "linear rinsing" of the substrate on which two-beam interference fringe were recorded, the surface of the substrate will appear as a sinusoidal grating surface. Therefore, the composite sinusoidal grating structure can be processed by laser interference lithography to prepare a large-area, low-cost, simple and fast SERS substrate. In addition, since there is no nano-rough structure on the surface of the composite sinusoidal grating substrate, the molecules of the substance to be detected are not easily adsorbed on the surface of the grating, which will inhibit the "memory effect" of the SERS substrate. In this paper, the enhanced electric field distribution near the sinusoidal grating surface is analyzed in detail by using the rigorous coupled wave analysis method(RCWA). The optimal relationship between of the influencing factors on the electric field enhancement near the surface of the composite sinusoidal grating is studied, allowing the optimal substrate structure of composite sinusoidal grating to be found under certain incident light conditions. The theoretical optimized substrate structure was prepared and processed and the SERS performance experiment was conducted.

    • The intensity distribution of the pattern formed by the interference of two coherent beams is shown in Fig. 1(a). A schematic diagram of the cross-section of the one-dimensional composite sinusoidal grating proposed in this paper is shown in Fig. 1(b). In the figure, P is the period of the grating, and d is the thickness of the noble metal layer, the sinusoidal amplitude of the grating surface is A and the incident angle of light is θ0.

      图  1  (a)激光干涉图样光强度分布;(b)复合正弦光栅截面示意图

      Figure 1.  (a) Light intensity distribution of laser interference; (b)Section diagram of the composite sinusoidal grating

      The grating structure is divided into three regions. Since the composite grating structure will be integrated into the inner surface of the microchannel, the flow medium in the microchannel is mainly water in SERS microfluidic detection, so the medium of region 1 is water(refractive index of n1), region 3 is the SiO2 basal layer(refractive index of n3), and region 2 is the periodic distribution of the noble metal layer(refractive index of complex n2), water and SiO2, and the dielectric constant can be expanded by the Fourier series:

      (1)

      where the expansion factor ej is:

      (2)

      The difference of the specific grating structure is mainly reflected in the expansion coefficient of the dielectric constant. The specific value of the dielectric constant is substituted into equation (2) to obtain the expansion coefficient of the arbitrary periodic dielectric constant.

      When the incident light is in TM polarization mode, its magnetic field vector is perpendicular to the xoy plane. At this time, the electric field vector of the incident light has two components, x and y, and the magnetic field vector only has a z component, wherein the magnetic field is:

      (3)

      where k0 is the wave vector value of the incident light in vacuum. The magnetic field in Zone 1 and Zone 3 only has the z component and can be expressed as:

      (4)
      (5)

      where Rm and Tm respectively represent normalized complex amplitudes of Rayleigh reflected wave and transmitted wave with corresponding order m; the wave vector x component kxm of the two regions can be expressed as:

      (6)

      The y component can be expressed as:

      (7)

      In the grating region 2, the z component of the magnetic field and the x component of the electric field can be expanded into a superposition of spatial harmonics using a Fourier series:

      (8)
      (9)

      where ε0 is the dielectric constant in a vacuum, μ0 is the magnetic permeability in vacuum, Sxm(y), Uzm(y) are respectively the electric field complex amplitude and magnetic field complex amplitude of the mth harmonic, satisfying Maxwell′s equations:

      (10)

      Substituting equations (8), (9), and (1) into equation (10) and simplifying them into a matrix form it can be obtained that:

      (11)

      Where matrix ,

      (12)
      (13)

      After elimination of the Sx component, it can be simplified to:

      (14)

      For convenience, the spatial harmonic component of the electric field vector′s x component and the magnetic field vector′s z component in the tth layer is represented by the eigenvector and eigenvalues of the layer:

      (15)
      (16)

      where , dt is the height of the tth layer, λt, n is the eigenvalue of the matrix Bt in the tth layer, and wt, n is the corresponding feature vector. There is a boundary condition for each layer of the grating region, where the interface of the first layer has:

      (17)

      where Wt is the eigenvector matrix of the matrix EBt, and the diagonal element of the diagonal matrix Qt is the square root of the eigenvalue of the matrix EBt. The boundary conditions at the interface between the t-1th layer and the tth layer are:

      (18)

      The boundary conditions at the interface between the last layer and region 3 are:

      (19)

      Xt is a diagonal array and the diagonal element is . The important factors in equations (17), (18), and (19) are the reflection coefficient R and the transmission coefficient T, so the final boundary condition needs to be reduced to a system of equations with only R and T and then make use of the Gauss elimination method to solve for R and T.

      The diffraction efficiency DE of a given order of diffracted light is the ratio of the energy of the diffracted beam to the energy of the incident beam. In other words, it is the ratio of the energy density of the diffracted beam to the incident beam:

      (20)

      where Em and Hm are the electric field vector and magnetic field vector of the mth order diffracted light, Einc and Hinc are the electric field vector and the magnetic field vector of the incident light. These corresponding values are substituted into the equation (20) to obtain the diffraction efficiency of diffracted beams with different orders.

      For incident light in the TM polarization mode, the incident light energy is:

      (21)

      The energy of the mth reflected beam is:

      (22)

      The energy of the mth transmitted beam is:

      (23)

      By substituting equations (21), (22), and (23) into equation (20), the reflectance and transmittance of the mth order diffracted light can be obtained:

      (24)
      (25)

      When the composite sinusoidal grating is applied to SERS, it is necessary to produce surface plasmon resonance excitation on the surface. At this time, part of the incident light energy is coupled into the surface plasmon resonance and an enhanced electric field distribution is formed on the grating surface. Ab(Absorbance) can be expressed as:

      (26)

      With a higher coupling absorbance, there is a stronger plasmon resonance effect in the grating surface and a stronger enhanced electric field formed on the surface of the composite sinusoidal grating.

    • In the previous section, the surface-enhanced electric field and surface plasmon resonance coupling absorbance of the composite sinusoidal grating under TM incident light were mathematically modelled. According to the above model, the relationship between the incident light, composite sinusoidal grating structure and the dielectric constant with regards to the external environment is studied under the condition of the strongest surface plasmon resonance effect.

    • Strictly speaking, the higher the diffraction order, the better the convergence of the coupled wave algorithm. However, in numerical calculations, the diffraction order cannot be infinitely large but can be appropriately truncated. For example, using sinusoidal silver grating, it can be obtained through simulation calculation that the convergence effect of the algorithm is better when N=301, as shown in Fig. 2(a).

      图  2  (a)吸收率随衍射级次变化曲线;(b)吸收率随入射光波长变化曲线;(c)吸收率随入射角变化曲线

      Figure 2.  (a) Absorbance as a function of diffraction grades; (b)Absorbance as a function of incident light wavelength; (c)Absorbance as a function of incident angle

      Using the RCWA algorithm, it can be found that there is an optimal wavelength for a fixed-period sinusoidal silver grating, which is the response wavelength of the grating surface plasmon resonance, causing the SPP coupling absorbance of the incident light to have a maximum value. As shown in Fig. 2(b), when the sinusoidal silver grating has a period P=570 nm, an amplitude of A=20 nm, a silver layer thickness of d=100 nm, a grating surface dielectric constant of ε=1.77, an incident angle of θ0=0 °, and an incident light wavelength of λ=785 nm, the SPP coupling absorbance has a maximum value.

      In addition, for fixed-period grating and a given wavelength of incident light, there is an optimal incident angle that makes the grating surface plasmon resonance its strongest. As shown in Fig. 2(c), when the sinusoidal silver grating has a period of P=570 nm, an amplitude of A=20 nm, a silver layer thickness of d=100 nm, a grating surface dielectric constant of ε=1.77 and an incident light wavelength of λ=785 nm, the SPP coupling absorbance has a maximum value at an incident angel of θ0=0.1°.

    • For a certain wavelength of incident light, there is an optimal grating period that causes the grating surface plasmon resonance to be its strongest. As shown in Fig. 3(a), when incident light wavelength is λ=785 nm, the incident angle is θ0= 0 °, the grating surface dielectric constant is ε=1.77, the sinusoidal silver grating amplitude is A=20 nm, and the silver layer thickness is d=100 nm, there is a maximum value for the SPP coupling absorbance at a grating period of P=570 nm.

      图  3  (a)吸收率随光栅周期变化曲线;(b)吸收率随光栅振幅变化曲线;(c)吸收率随光栅银层厚度变化曲线

      Figure 3.  (a) Absorbance as a function of the grating period; (b)Absorbance as a function of grating amplitude; (c)Absorbance as a function of grating silver layer thickness

      For a given wavelength of incident light and a fixed period grating, there is an optimal grating amplitude that makes the grating surface plasmon resonance its strongest. As shown in Fig. 3(b), when the incident light wavelength is λ=785 nm, the incident angle is θ0=0 °, the grating surface dielectric constant is ε=1.77, the sinusoidal silver grating period is P=570 nm, and the silver layer thickness is d=100 nm, there is a maximum value for the SPP coupling absorbance at the grating amplitude of A=15 nm. Furthermore, when the grating amplitude is within a certain range, the surface plasmon resonance has a larger SPP coupling absorbance.

      In addition, for a given wavelength of incident light and a fixed period grating, there is a range of thickness of the silver layer of a sinusoidal silver grating to make the surface plasmon resonance of the grating in a strong state. As shown in Fig. 3(c), when the incident light wavelength is λ=785 nm, the incident angle is θ0=0°, the grating surface dielectric constant is ε=1.77, the sinusoidal silver grating period is P=570 nm, and the grating amplitude A=15 nm, the SPP coupling absorbance can hold a larger value when the silver layer thickness is d > 80 nm.

    • For conditions in which sinusoidal grating structure and incident light are fixed, there is an optimal value for the dielectric constant of the grating surface that causes the grating surface plasmon resonance to be its strongest. As shown in Fig.(4), when the incident light wavelength is λ=785 nm, the incident angle is θ0=0 °, the grating period is P=570 nm, the amplitude is A=15 nm, and the thickness of the silver layer is d=100 nm, the SPP coupling absorbance has a maximum value at an surface dielectric constant of ε=1.77(water). It should be noted that if the dielectric constant of the grating surface changes, namely the liquid medium flowing in the microchannel changes, then the grating period must be adjusted so as to cause surface plasmon resonance when the wavelength of the incident light is constant.

      图  4  吸收率随光栅表面环境介电常数变化曲线

      Figure 4.  Absorbance as a function of the dielectric constant above the grating surface

    • Based on the above analysis, when the wave- length of incident light is λ=785 nm and the medium on the grating surface is water, the corresponding optimal sinusoidal silver grating structure has a grating period of P=570 nm, an amplitude of A=15 nm and a silver layer thickness of d=100 nm. At this time, the surface enhanced electric field distribution of the sinusoidal silver grating can be obtained using a Matlab simulation, as shown in Fig. 5(a). Thus also calculates the SERS enhancement factor(EF), , where E0(ω) is the electric field intensity of the incident light, and E(ω) is the electric field strength of the surface of the SERS substrate excited by incident light. The specific EF calculated values are shown in Tab. 1.

      图  5  (a)复合正弦银光栅表面增强电场分布;(b)复合正弦金光栅表面增强电场分布;(c)复合矩形银光栅表面增强电场分布

      Figure 5.  (a) Electric field distribution of composite sinusoidal silver grating; (b)Electric field distribution of composite sinusoidal gold grating; (c)Electric field distribution of composite rectangular silver grating

      表 1  3种光栅的理论EF

      Table 1.  Theoretical EF values of the three gratings

       Grating category EF
      Sine Water/Ag/SiO2 grating 7.4×104
      Sine Water/Au/SiO2 grating 2.0×104
      Rectangular Water/Ag/SiO2 grating 4.4×104

      Using the same method as above, the structure of the sinusoidal gold grating with incident light wavelength λ=785 nm is optimized. Finally, the surface electric field distribution is simulated. As shown in Fig. 5(b), the EF value is shown in Tab. 1. It can be seen that the EF values of the optimized sinusoidal gold grating and the sinusoidal silver grating EF are of the same order of magnitude.

      In order to further illustrate the advantages in the SERS performance of the composite sinusoidal silver grating, we compared it to the optimized rectangular silver grating. As shown in Fig. 5(c), the electric field distribution of the surface of the rectangular silver grating is calculated with the same EF magnitude, as demonstrated in Tab. 1. The sinusoidal grating can be processed by laser interference lithography, which does not require a mask and easily achieves single-instance large field-of-view exposures. Also, the processing of rectangular grating requires a pre-fabricated mask so the processing cost and time are both increased.

    • Fig. 6(a) is a schematic diagram of a composite sinusoidal grating structure. The light gray area is an SiO2 substrate. A photoresist(NOA-63) is spin-coated on a clean SiO2 substrate and a laser with a wavelength of 266 nm and a power of 30 mW is used(Coherent MBD-266). The 266 nm beam emitted from the laser is collimated and expanded and then passes through a beam splitter(reflecting 30%, transmission 70%), wherein the transmitted light is reflected by a mirror and interfere with the first beam at the sample. In order to ensure the surface uniformity of the sinusoidal grating structure, an attenuator is installed on both interference optical paths to obtain same intensity of the final two beams of interference light. The interference experiment distance with the best interference effect can be found by adjusting the angle of the incident light and meeting position of the sample. This interference lithography system is shown in Fig. 6(b). The period of the interference pattern formed is P=sin(θ/2)λ0/2, where λ0 is the wavelength of the coherent light and θ is the angle between the two coherent lights. When the required sinusoidal grating period is 570 nm, the angle between the two coherent beams should be controlled to be 27°. In Fig. 6(a), the light blue portion is a cured photoresist and the dark gray portion is a vapor-deposited silver layer. The exposure time is adjusted by the photoelectric shutter to change the amplitude of the grating. The thickness of the silver layer can be controlled by adjusting the evaporation time.The exposure time and evaporation time in the preparation process are 800ms and 1000s, respectively. Fig. 7(a) is an SEM representation of a prepared sinusoidal grating.

      图  6  (a)复合正弦光栅结构示意图;(b)干涉光刻系统

      Figure 6.  (a) Structural diagram of the composite sinusoidal grating; (b)Interference photolithography system

      图  7  (a)复合正弦光栅SEM表征图;(b)R6G溶液的SERS光谱图;(c)R6G拉曼特征峰处信号强度的均值误差棒

      Figure 7.  (a) SEM image of the composite sinusoidal grating; (b)SERS spectrum of R6G solutions; (c)Error bars of the mean intensity of R6G Raman peaks from five average spectra

      Due to the ideal vibration characteristics of an R6G molecule, such a molecule was used as a probe molecule for detecting the SERS performance of the composite sinusoidal grating substrate. Under experimental conditions, the analytical enhancement factor(AEF) is used to calculate the base SERS enhancement ability. The expression is:

      (27)

      among them, ISERS is the SERS base Raman signal intensity, CSERS is the concentration of the substance to be detected on the SERS substrate, IRS is the non-SERS base Raman signal intensity and CRS is the concentration of the substance to be detected on the non-SERS substrate.

      In the experiment, a silver-plated flat glass substrate was selected as the contrast substrate. The thickness of the silver layer was 100 nm. The concentration of R6G measured by the silver-plated glass substrate was 10-3 M, as shown in Fig. 7(b), and the relative intensity of the Raman signal was 185, while the composite sinusoidal silver grating can measure the concentration of 10-6 M and the relative intensity of the Raman signal at the peak of 1 650 cm-1 is 4 316, where the Raman signal intensity is on the sinusoidal silver grating substrate, and its the average Raman signal strength measured at five different points. After calculation, the SERS analysis enhancement factor(AEF) of the composite sinusoidal silver grating substrate is 2.3×104, which is consistent with the theoretical SERS enhancement factor EF. The experimental results show that the composite sinusoidal silver grating has strong SERS enhancement performance. To further evaluate the SERS performance stability of the substrate, the relative standard deviation of the signal intensities at the four Raman peaks of the R6G Raman spectrum was calculated. Fig. 7(c) shows the mean and standard deviation of the Raman signal strength at the four characteristic peaks. The standard deviations of Raman signal intensities are 5.8%, 13.4%, 9.4%, 11.8%, respectively, at the corresponding peaks of 1 311, 1 363, 1 510, and 1 650 cm-1. In this way, they met the higher performance SERS substrate requirements[13].

    • The hots spots areas in a noble metal nanoparticle sols are small and scarce while the noble metal nano-three-dimensional array structure has high processing cost, a large time requirement and "memory effect". The composite sinusoidal grating SERS base structure using a laser was proposed. Interferometric lithography enables large-area, low-cost, simple and fast composite sinusoidal grating substrate preparation. Based on the RCWA method to analyze the surface electric field distribution and SPP coupling absorption of the composite sinusoidal grating, the theoretical analysis shows that the SERS enhancement factor EF of the best composite sinusoidal silver grating structure is 7.4×104. The experimental results show that the SERS analysis enhancement factor AEF of the composite sinusoidal silver grating is 2.3×104, which is basically consistent with theoretical predictions. It is therefore proven that composite sinusoidal silver grating is an effective SERS substrate. In future studies, a composite sinusoidal grating will be integrated into a microchannel to research the microfluidic SERS detection performance.

      ——中文对照版——

    • 近些年, 随着激光技术、纳米科技和计算机技术的迅猛发展, 表面增强拉曼散射(SERS)微流控技术已经在界面和表面科学、材料分析、生物、医学、食品安全、环境监测和国家安全等领域得到了广泛应用[1-5]。目前SERS微流控检测技术中, 广泛使用的拉曼增强基底主要有两种:单一金属纳米颗粒溶胶, 包括金纳米颗粒溶胶和银纳米颗粒溶胶[6]; 纳米三维阵列结构, 包括球状纳米阵列结构、柱状纳米阵列结构、锥状纳米阵列结构等[7-9]。贵金属纳米颗粒溶胶具有成本低、制备简单等优点, 为了提高贵金属纳米颗粒溶胶的SERS检测灵敏度, 前期对贵金属纳米颗粒的形状、尺寸以及金属种类等影响因素进行了研究[10-11], 从而形成更强的颗粒间隙电场热点。但是贵金属纳米颗粒溶胶仍存在热点区域的数量在单位体积内有限且热点区域范围较小的缺点[12]。利用等离子体刻蚀和传统光刻加工技术制备的纳米三维阵列结构具有很好的结构周期性和SERS信号重现性, 但是加工成本高昂, 加工时间长[13]。另外, 待检测分子容易吸附在三维阵列纳米结构的表面, 存在"记忆效应"[14], 导致下次检测之前需要对基底进行清洗, 因此无法满足可连续SERS检测的要求。

      为此本文提出了复合正弦光栅结构SERS基底, 两束相干光干涉形成的图样强度分布函数具有正弦函数的形式, 记录了两光束干涉条纹的底片经"线性冲洗"后, 就会呈现出表面为正弦曲线的光栅表面。因此, 该复合正弦光栅结构可以采用激光干涉光刻技术进行制备加工, 可实现大面积、低成本、简易快速的SERS基底制备。另外, 由于复合正弦光栅基底表面不存在纳米粗糙结构, 待检测物质分子不易吸附在光栅表面, 会起到抑制SERS基底"记忆效应"的效果。本文利用严格耦合波分析方法(RCWA)详细分析了复合正弦光栅表面增强电场分布情况, 研究了复合正弦光栅表面电场增强的影响因素及优化匹配关系, 给出了一定入射光条件下的最佳复合正弦光栅SERS基底结构, 并对理论优化基底结构进行了制备加工, 开展了SERS性能实验验证。

    • 两束相干光干涉形成的图样强度分布如图 1(a)所示, 本文提出的一维复合正弦光栅截面简要示意图, 如图 1(b)所示, 图中P为光栅的周期, d为贵金属层的厚度, 光栅表面正弦曲线振幅为A, 光入射角为θ0

      将光栅结构分为3个区域。由于该复合光栅结构将被集成到微流道内表面, SERS微流控检测中微流道内流动介质主要为水, 因此设区域1为介质水(折射率为n1), 区域3为SiO2基底层(折射率为n3), 区域2为贵金属层(折射率为复数n2)、水和SiO2这2种介质为周期性分布, 其介电常数可以用傅立叶级数展开:

      (1)

      其中展开系数ej为:

      (2)

      具体的光栅结构的差别主要体现在介电常数的展开系数上, 将介电常数的具体值代入式(2)可以求得任意周期性介电常数的展开系数。

      当入射光为TM偏振模式时, 其磁场矢量垂直于xoy平面, 此时入射光的电场矢量有xy两个分量, 而磁场矢量仅有z分量, 其中磁场为:

      (3)

      其中, k0为入射光在真空中的波矢值。在区域1和区域3中的磁场只有z分量, 可以表示为:

      (4)
      (5)

      其中, RmTm分别表示归一化对应级次为m的瑞利反射波和透射波复振幅, 两个区域的波矢x分量kxm可表示为:

      (6)

      y分量则可以表示为:

      (7)

      在光栅区域2中, 磁场的z分量和电场的x分量可以用傅立叶级数展开为空间谐波的叠加:

      (8)
      (9)

      其中, ε0为真空中的介电常数, μ0为真空中的磁导率, Sxm(y), Uzm(y)分别为第m级谐波的电场复振幅和磁场复振幅, 满足麦克斯韦方程组:

      (10)

      将式(8)、(9)以及(1)代入式(10)中化简并写成矩阵形式可得:

      (11)

      其中矩阵,

      (12)
      (13)

      最终消去Sx分量可以化简得到:

      (14)

      为方便处理, 第t层的电场矢量x分量和磁场矢量z分量的空间谐波分量用该层的矩阵特征向量和特征值表示:

      (15)
      (16)

      其中, , dt为第t层的高度, λt, n为第t层中矩阵Bt的特征值, wt, n为对应的特征向量。光栅区域的每一分层都存在一个边界条件, 其中第一层的交界面上有:

      (17)

      其中, Wt是矩阵EBt的特征向量矩阵, 对角阵Qt的对角元为矩阵EBt的特征值的平方根。第t-1层与第t层的交界面上的边界条件为:

      (18)

      在最后一层与区域3的交界面上的边界条件为:

      (19)

      其中, Xt是一个对角阵, 对角元为。式(17)、(18)和(19)中有意义的元素是反射系数R和透射系数T, 所以需要先将最后的边界条件化简为只具有RT的方程组, 然后再用Gauss消元法解出RT的值。

      定义某级衍射光的衍射效率DE(diffraction efficiency)为衍射光束的能量与入射光束能量之比, 也就是衍射光束与入射光束各自能流密度的比值:

      (20)

      其中, EmHm为第m级衍射光的电场矢量和磁场矢量, EincHinc为入射光的电场矢量和磁场矢量, 将相应各值代入式(20)中即可求得光栅各级衍射光束的衍射效率。

      对于TM偏振模式的入射光, 入射光能量为:

      (21)

      m级反射光束的能量为:

      (22)

      m级透射光束的能量为:

      (23)

      将式(21)、(22)、(23)代入式(20)可以得到第m级衍射光的反射率和透射率:

      (24)
      (25)

      当复合正弦光栅应用于SERS时, 需要使其表面产生表面等离子体共振激励过程, 此时部分入射光能量会耦合到表面等离子共振中, 在光栅表面形成增强电场分布, 此时入射光耦合吸收率Ab(Absorbance)可以表示成:

      (26)

      耦合吸收率越高, 说明光栅表面等离子体共振效应越强, 在复合正弦光栅表面形成的增强电场越强。

    • 上节对TM入射光情况下的复合正弦光栅表面增强电场和表面等离子体共振耦合吸收率进行了数学建模, 下面根据上述模型对表面等离子体共振效应最强情况下的入射光、复合正弦光栅结构与外界环境介电常数的优化匹配关系进行研究。

    • 严格来说, 衍射级次取得越多越好, 而且耦合波算法的收敛性与衍射级次有关, 但在实际数值计算中, 衍射级次不可能取到无穷大, 可以进行适当的截断。这里以正弦银光栅为例, 通过仿真计算可得, 在N=301时算法收敛效果比较好, 如图 2(a)所示。

      利用RCWA算法分析可得, 对于固定周期的正弦银光栅, 存在一个最佳波长, 即导致光栅表面等离子体共振的响应波长, 使得入射光的SPP耦合吸收率有一个最大值。如图 2(b)所示, 当正弦银光栅周期P=570 nm, 振幅A=20 nm, 银层厚度d=100 nm, 光栅表面介质常数ε=1.77, 入射角θ0=0°时, 当入射光波长λ=785 nm时, SPP耦合吸收率存在一个极大值。

      另外, 对于固定周期光栅和一定波长的入射光, 存在一个最佳入射角使光栅表面等离子体共振最强。如图 2(c)所示, 当正弦银光栅周期P=570 nm, 振幅A=20 nm, 银层厚度d=100 nm, 光栅表面介电常数ε=1.77, 入射光波长λ=785 nm时, 有入射角θ0=0.1°, SPP耦合吸收率存在一个极大值。

    • 对于一定波长的入射光, 存在一个最佳光栅周期使光栅表面等离子体共振最强。如图 3(a)所示, 当入射光波长λ=785 nm, 入射角θ0=0°, 光栅表面介质常数ε=1.77, 正弦银光栅振幅A=20 nm, 银层厚度d=100 nm时, 有光栅周期P=570 nm, 此时SPP耦合吸收率存在一个极大值。

      对于一定波长的入射光和固定周期光栅, 存在一个最佳光栅振幅使光栅表面等离子体共振最强。如图 3(b)所示, 当入射光波长λ=785 nm, 入射角θ0=0°, 光栅表面介质常数ε=1.77, 正弦银光栅周期P=570 nm, 银层厚度d=100 nm时, 有光栅振幅A=15 nm, 此时SPP耦合吸收率存在一个极大值。此外, 光栅振幅在一定范围内时, 表面等离子体共振有较大的SPP耦合吸收率。

      另外, 对于一定波长的入射光和固定周期光栅, 存在一个正弦银光栅的银层厚度范围使光栅表面等离子体共振处于较强状态。如图 3(c)所示, 当入射光波长λ=785 nm, 入射角θ0=0°, 光栅表面介电常数ε=1.77, 正弦银光栅周期P=570 nm, 光栅振幅A=15 nm时, 有银层厚度d > 80 nm, 此时SPP耦合吸收率能够保持一个较大值。

    • 针对固定的正弦光栅结构和入射光条件, 光栅表面环境介电常数存在一个最佳值使得光栅表面等离子体共振最强。如图(4)所示, 当入射光波长λ=785 nm, 入射角θ0=0°, 光栅周期P=570 nm, 振幅A=15 nm, 银层厚度d=100 nm时, 有表面环境介电常数ε=1.77(水), 此时SPP耦合吸收率有一个极大值。需要说明的是, 如果光栅表面环境介电常数发生变化, 即微流道内流动的液体介质发生变化, 在入射光波长不变的情况下, 光栅周期必须做出调整以满足表面等离子体共振的条件。

    • 综合以上分析, 可得入射光波长λ=785 nm, 光栅表面环境为水时, 对应的最优正弦银光栅结构为:光栅周期P=570 nm, 振幅A=15 nm, 银层厚度d=100 nm。此时通过Matlab仿真计算可得正弦银光栅表面增强电场分布, 如图 5(a)所示, 由此可计算SERS的增强因子(EF), 是入射光的电场强度, E(ω)是入射光激励下SERS基底表面的电场强度, 具体EF计算值见表 1

      利用上述同样的方法, 针对入射光波长λ=785 nm时的正弦金光栅进行了结构优化, 最后对其表面电场分布进行了仿真计算, 如图 5(b)所示, 其EF值见表 1, 可以看出优化后的正弦金光栅和正弦银光栅EF量级相同。

      为了进一步说明复合正弦银光栅的SERS优势, 与优化过的矩形银光栅做了对比, 如图 5(c)所示为矩形银光栅表面电场分布, 通过计算可得两者的EF量级相同, 见表 1。但是正弦光栅可以利用激光干涉光刻技术加工制备得到, 无需掩膜版, 容易实现一次性大视场曝光, 而加工矩形光栅需要预制掩膜版, 加工成本和时间都会提高。

    • 图 6(a)为复合正弦光栅结构示意图, 浅灰色部分为SiO2基底, 在清洁的SiO2基底上旋涂光刻胶(NOA-63), 采用波长为266 nm, 功率为30 mW的激光器(Coherent MBD-266)。从激光器发出的266 nm激光经过准直扩束后, 通过分束器(反射30%, 透射70%), 其中透射光经过全反镜的再次反射与第一束光在样品的位置产生干涉现象。为了保证正弦光栅结构表面的均匀性, 在两束干涉光路上安装了衰减器, 用于控制使两束干涉光最终的光强相同。通过调整入射光的角度以及样品的接收位置可以找到干涉效果最好的干涉实验距离, 干涉光刻系统如图 6(b)所示。形成的干涉图案周期为P=sin(θ/2)λ0/2, 其中λ0是相干光波长, θ是两束相干光的夹角。当所需正弦光栅周期为570 nm时, 应控制两束相干光夹角为27°。图 6(a)中浅蓝色部分为固化后的光刻胶, 深灰色部分为蒸镀银层。通过光电快门调整曝光时间来改变光栅的振幅, 而银层的厚度可以通过调整蒸镀时间来控制, 制备过程中的曝光时间和蒸镀时间分别为800 ms和1 000 s。图 7(a)为制备的正弦光栅SEM表征图。

      由于R6G分子良好的振动特性, 实验中将其作为检测复合正弦光栅基底SERS性能的探针分子。实验条件下, 定义分析增强因子(AEF)来计算基底SERS增强能力, 表达式为:

      (27)

      其中, ISERS是SERS基底拉曼信号强度, CSERS是SERS基底上待检测物质的浓度, IRS是非SERS基底拉曼信号强度, CRS是非SERS基底上待检测物质的浓度。

      实验中选取了镀银平面玻璃基底作为对比基底, 银层厚度为100 nm, 镀银玻璃基底能够测到的R6G浓度为10-3 M, 如图 7(b)所示, 在特征峰1 650 cm-1处拉曼信号的相对强度为185, 而复合正弦银光栅能够测到的10-6 M, 在特征峰1 650 cm-1处拉曼信号的相对强度为4 316, 这里的拉曼信号强度是在正弦银光栅基底上5个不同点测得的拉曼信号强度平均值。经过计算, 复合正弦银光栅基底的SERS分析增强因子AEF为2.3×104, 与之前推导的理论SERS增强因子EF数量级一致, 实验证明了复合正弦银光栅具有较强的SERS增强性能。为了进一步评估该基底的SERS性能稳定性, 计算了R6G拉曼光谱的4个拉曼特征峰处信号强度相对标准差。如图 7(c)所示为4个特征峰处拉曼信号强度的平均值和标准差, 可得特征峰1 311、1 363、1 510和1 650 cm-1处拉曼信号强度的相对标准差分别为5.8%、13.4%、9.4%和11.8%, 满足了较高性能的SERS基底要求[13]

    • 针对贵金属纳米颗粒溶胶单位体积内热点区域数量有限且热点区域范围较小, 而贵金属纳米三维阵列结构加工成本高、时间长并存在"记忆效应"的缺陷, 提出复合正弦光栅SERS基底结构。利用激光干涉光刻技术可实现大面积、低成本、简易快速的复合正弦光栅基底制备。在采用RCWA方法分析复合正弦光栅表面增强电场分布和SPP耦合吸收率的基础上, 理论分析表明最佳复合正弦银光栅结构的SERS增强因子EF为7.4×104。实验验证复合正弦银光栅SERS分析增强因子AEF为2.3×104, 与理论分析结果基本一致, 证明复合正弦银光栅是一种有效的SERS基底。下一步, 将把复合正弦光栅集成到微流道内, 开展微流控SERS检测性能研究。

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