金属Bowtie结构耦合电场强度的调控及表面增强拉曼散射研究

It is now widely recognized that the extreme sensitivity of surface-enhanced Raman scattering (SERS) is dominated by the electromagnetic enhancement. A particularly intriguing feature of the electromagnetic enhancement is associated with the presence of the so-called “nanogap” effect where local SERS enhancement factors (EF) sufficient for the detection of single molecules level have been observed. In these nanostructures with the characteristics of nanogap coupling, metal bowties nanostructures have been shown to higher localized field enhancement and better spatial confinement of the applied electric field than other coupled plasmon resonant nanoparticle pair geometries. At present, the nanogap of bowties based on the fabrication of electron beam lithography (EBL) can be down to ~5 nm. Further reducing the gap distance is very difficult due to electron wave properties. However, the smaller gap corresponds to stronger coupling and thus it is necessary to reduce the gap down to 5 nm..We have demonstrated that the strongest plasmonic electric field of ultrathin tetrahedral amorphous carbon (ta-C) coated Ag nanoparticle is not trapped within the ta-C layer but is released to its outside surface, while leaving the weaker electric field inside ta-C layer. In this project, based on this finding, we firstly offer a method to further reduce the gap of bowtie combining the conventional EBL method with coating an ultrathin ta-C film. Firstly, ultrathin ta-C film can further reduce the gap distance of bowtie. Secondly, the strong released electric field of single ta-C coated prism can induce a stronger coupling each other within a relative smaller gap distance so that higher electric field can be produced which is better to the single molecules SERS. Thirdly, we will study the substrate effect on the electric field. Normally, part of electric field will penetrate the substrate due to its high dielectric which is not useful to SERS. By coating a ta-C with a higher dielectric constant than substrate, the electric field can partly decouple from the substrate. Finally, we will use nanoimprint lithography to fabricate the bowtie/ta-C nanostructures with the low cost and high throughput which can open the doors of wide application in SERS.

金属Bowtie由于其良好的空间局域性及两尖端间隙中强耦合所形成的局域电场可实现单分子SERS成为研究热点，如何调控间隙是实现强耦合的关键。由于电子波动性导致的束斑大小限制，基于微加工制备Bowtie间隙最小为5nm。我们以前发现当超薄高介电ta-C薄膜包覆金属纳米粒子后，导致最强电场从介电层内表面转移到外表面。本项目将基于此效应第一次提供可控的调节Bowtie间隙的工具。主要通过在Bowtie上沉积超薄ta-C对其间隙的进一步调控，使间隙可以小于5nm，进而研究两个在ta-C外的释放电场在相对小的间隙内的强耦合模式，以期获得更强的局域电场，从而研究单分子SERS效应；其次探讨Bowtie/ta-C体系的电子运动导致的损耗对增强电场的影响；最后，研究高介电薄膜对降低基底耦合效应的机制，并结合纳米压印研究Bowtie/ta-C结构的批量化制备，评估其作为SERS衬底的稳定性。

金属微纳结构表面等离激元效应可在金属表面产生增强的局域电磁场，实现对其附近光学过程效率的增强。而金属耦合结构由于相互之间存在耦合效应可实现最优电磁场增强，尤其是可满足单分子拉曼检测。本项目主要针对金属有序耦合结构精确制备中所存在的由于电子波动性引起的耦合结构中间隙尺寸较大，无法实现最优耦合效应等缺陷，提出采用超薄高介电ta-C薄膜包覆金属耦合结构，从而可调控间隙尺寸到达最优耦合效应，获得更强的耦合电磁场，并研究其在表面增强拉曼中的应用。.本项目主要进行了以下相关研究。第一，研究了超薄ta-C及其他介质薄膜包覆金属微纳结构的表面等离激元特性，首先设计并制备金属/介质/金属耦合结构，由于耦合结构的耦合效应在超小间隙下受经典电磁理论和量子隧穿效应影响，不同的介质材料会引起不同的势垒高度，所对应的最优介质间隙是不同的，从而耦合电磁场的增强是不同的。我们发现从SERS应用来看，必须选择电子亲合势较小的介电材料，才能得到经典电磁模型所主导的强耦合电场。第二，研究了超薄ta-C包覆Ag耦合结构在单分子拉曼检测中的应用，我们通过纳米压印及超薄ta-C沉积，借助微毛细管力的作用，得到了间隙大小为2nm的Ag纳米手指闭合阵列结构，发现耦合电场不再局域在间隙介电层中，而是溢出到ta-C介电层的外表面，其增强电磁场可实现单分子SERS检测。第三，研究了悬空的超薄ta-C包覆Au耦合结构电磁场增强，发现可在金结构上下表面形成双电磁场增强，并实现了对儿童尿液中塑化剂的检测。.我们通过超薄ta-C薄膜包覆金属耦合结构形成金属/ta-C/金属耦合结构体系，实现了对耦合结构中间隙尺寸的精确调控，并突破了由传统微加工中间隙尺寸太大的问题，从而实现了由经典电磁理论和量子力学效应决定下最优耦合电磁场效应。这种结构体系除了在表面增强拉曼生物检测应用外，在非线性光学增强、光催化、光学器件中都有广泛的科学价值和应用前景。