Gimenez Aurélien V, Kho Kiang W, Keyes Tia E
School of Chemical Sciences & National Centre for Sensor Research, Dublin City University Dublin 9 Ireland
Nanoscale Adv. 2020 Aug 11;2(10):4740-4756. doi: 10.1039/d0na00527d. eCollection 2020 Oct 13.
Plasmonic nanostructures are important across diverse applications from sensing to renewable energy. Periodic porous array structures are particularly attractive because such topography offers a means to encapsulate or capture solution phase species and combines both propagating and localised plasmonic modes offering versatile addressability. However, in analytical spectroscopic applications, periodic pore arrays have typically reported weaker plasmonic signal enhancement compared to particulate structures. This may be addressed by introducing additional nano-structuring into the array to promote plasmonic coupling that promotes electric field-enhancement, whilst retaining pore structure. Introducing nanoparticle structures into the pores is a useful means to promote such coupling. However, current approaches rely on either expensive top-down methods or on bottom-up methods that yield random particle placement and distribution. This report describes a low cost, top-down technique for preparation of nano-sub-structured plasmonic pore arrays in a highly reproducible manner that can be applied to build arrays extending over macroscopic areas of mm to cm. The method exploits oxygen plasma etching, under controlled conditions, of the cavity encapsulated templating polystyrene (PS) spheres used to create the periodic array. Subsequent metal deposition leads to reproducible nano-structuring within the wells of the pore array, coined in-cavity nanoparticles (icNPs). This approach was demonstrated across periodic arrays with pore/sphere diameters ranging from 500 nm to 3 μm and reliably improved the plasmonic properties of the substrate across all array dimensions compared to analogous periodic arrays without the nano-structuring. The enhancement factors achieved for metal enhanced emission and surface enhanced Raman spectroscopy depended on the substrate dimensions, with the best performance achieved for nanostructured 2 μm diameter pore arrays, where a more than 10 improvement over Surface Enhanced Raman Spectroscopy (SERS) and 200-fold improvement over Metal Enhanced Fluorescence (MEF) were observed for these substrates compared with analogous unmodified pore arrays. The experiments were supported by Finite-Difference Time-Domain (FDTD) calculations used to simulate the electric field distribution as a function of pore nano-structuring.
等离子体纳米结构在从传感到可再生能源等各种应用中都很重要。周期性多孔阵列结构特别有吸引力，因为这种形貌提供了一种封装或捕获溶液相物质的方法，并且结合了传播和局域等离子体模式，提供了通用的可寻址性。然而，在分析光谱应用中，与颗粒结构相比，周期性孔阵列通常报道的等离子体信号增强较弱。这可以通过在阵列中引入额外的纳米结构来促进等离子体耦合来解决，这种耦合可以促进电场增强，同时保留孔结构。将纳米颗粒结构引入孔中是促进这种耦合的一种有用方法。然而，目前的方法要么依赖于昂贵的自上而下的方法，要么依赖于产生随机颗粒放置和分布的自下而上的方法。本报告描述了一种低成本、自上而下的技术，用于以高度可重复的方式制备纳米亚结构等离子体孔阵列，该技术可应用于构建覆盖毫米至厘米宏观区域的阵列。该方法利用在受控条件下对用于创建周期性阵列的封装模板聚苯乙烯（PS）球体的腔体进行氧等离子体蚀刻。随后的金属沉积导致孔阵列孔内可重复的纳米结构化，即腔内纳米颗粒（icNP）。这种方法在孔径/球体直径范围从500纳米到3微米的周期性阵列中得到了验证，与没有纳米结构化的类似周期性阵列相比，在所有阵列尺寸上都可靠地改善了基底的等离子体特性。金属增强发射和表面增强拉曼光谱实现的增强因子取决于基底尺寸，对于直径为2微米的纳米结构化孔阵列实现了最佳性能，与类似的未修饰孔阵列相比，这些基底在表面增强拉曼光谱（SERS）方面提高了10倍以上，在金属增强荧光（MEF）方面提高了200倍。实验得到了有限时域差分（FDTD）计算的支持，该计算用于模拟作为孔纳米结构化函数的电场分布。
