Mechanically controllable strain junctions: targeting quantum effects and strong plasmonic coupling in ultra-narrow gaps

在过去的十年里，带隙天线得到了广泛的研究和广泛的应用，因为带隙内的耦合电场可以在很大的范围内进行频谱调谐。许多新兴的纳米光子技术依赖于这种等离子体耦合的仔细控制，包括用于化学和生物应用的高灵敏度传感器的光学纳米天线和改进的光伏器件。通常，金属纳米结构之间的距离从几十纳米到几纳米不等。直到最近，几个小组才成功地产生并测量了亚纳米能隙，这显示了相干量子隧道等新现象。这些效应可能成为纳米光电子学的关键，并可能为单分子光电子学铺平道路。然而，实现具有亚纳米级间隙的可重复和稳定的实验条件仍然是一项具有挑战性的任务，要求很高。此外，到目前为止所展示的方法在许多情况下不适合于制备具有预先选择的光学频率的共振等离子体耦合，因为它们的等离子体性质由于对随机制备过程的强烈依赖而不是很好地控制。在本工作中，将通过发展机械可控应变结(MCSJ)来修正使用机械可控断口结(MCBJ)的方法。即使在环境条件下，MCBJ在测量隧道效应和单分子电导方面也表现出了很好的性能，但不幸的是，MCBJ对形成的电极间隙几何形状的控制很差。MCSJ缝隙区域的等离子体尖端的形状是在预拉伸的衬底上预先定义的，随后将被可控地释放以接近尖端。这项工作的初始部分致力于测试MCBJ的测量。这些实验用于验证拉伸控制设置，并建立与文献结果的链接。该项目的目标是开发一种定义明确、可调的实验装置，用于研究金属天线之间纳米间隙在较大参数变化下的光学和电学性质之间的相互作用，包括纯间隙和分子连接的间隙，并在环境条件下进行亚纳米控制。在这种情况下，将解决强耦合和量子等离子激元的问题。从发光，电子输运和拉曼研究的结果，都是在完全相同的系统上变化的天线几何形状，间隙大小，偏置电压和分子桥接将被收集。因此，该项目旨在获得对强耦合区域中等离子体激元模式分布、消失近场的作用、电偏置和分子电导率的新见解。

Within the last decade gap antennas have been widely studied and are commonly used due to the strongly enhanced coupled electrical fields within their gap, which can be spectrally tuned over a wide range. Many emerging nano-photonic technologies depend on the careful control of this plasmonic coupling, including optical nanoantennas for high-sensitivity sensors in chemical and biological applications and improved photovoltaic devices. Typically the distances between the metallic nanostructures range from several tens of nm down to a few nm. Only recently several groups managed to produce and measure sub-nanometer gaps, which show new phenomena such as coherent quantum tunnelling. These effects could become crucial in nanoscale optoelectronics and may pave the way to single molecule opto-electronics.Nevertheless, achieving reproducible and stable experimental conditions with sub-nanometer sized gaps remains a challenging task in high demand. In addition, the methods demonstrated so far are in many cases not suitable for preparing resonant plasmonic coupling with a preselected optical frequency, since their plasmonic properties are not very well-controllable due to a strong dependence on the random preparation processes. In the present work, the approach of using mechanically controllable break junctions (MCBJs) will be revised by developing mechanically controllable strain junctions (MCSJs). MCBJs have shown great performance in measuring tunneling effects and single molecule conductance even at ambient conditions, but unfortunately exhibit poor control of the formed electrode gap geometry itself. The shape of the plasmonic tips in the gap region of a MCSJ is lithographically pre-defined on a pre-stretched substrate, which afterwards will be controllably released to approach the tips.The initial part of this work is dedicated to test measurements on MCBJs. These experiments are used to validate the stretching control setup and establish a link to literature results. The objectives of this project are to develop a well-defined, tunable experimental setup for investigating the interplay between the optical and electronic properties of a nano-gap between metal antennas under wide parameter variation, both for pure gaps and for gaps bridged by molecules, with sub-nanometer control under ambient conditions. With this setup the regime of strong coupling and quantum plasmonics will be addressed. The results from luminescence, electronic transport and Raman studies, all on exactly the same system under variation of antenna geometry, gap size, bias voltage, and molecular bridging will be collected. The project thus aims at gaining new insight into the plasmonic mode distribution, role of the evanescent near-field, electrical biasing, and molecular conductivity in the strong coupling regime.