Tip-enhanced Raman spectroscopy (TERS) has reached nanometer spatial resolution for measurements performed at ambient conditions and subnanometer resolution at ultrahigh vacuum. Super-resolution (beyond the tip apex diameter) TERS has been obtained mostly in the gap mode configuration, where a conductive substrate localizes the electric fields. Here we present experimental and theoretical TERS to explore the field distribution responsible for spectral enhancement. We use gold tips of $40\pm 10\text{nm}$ apex diameter to measure TERS on graphene, a spatially delocalized two-dimensional sample, sitting on different substrates: (i) glass, (ii) a thin layer of gold and (iii) a surface covered with $12\text{nm}$ diameter gold spheres, for which $6\text{nm}$ resolution is achieved at ambient conditions. The super-resolution is due to the field configuration resulting from the coupled tip-sample-substrate system, exhibiting a nontrivial spatial surface distribution. The field distribution and the symmetry selection rules are different for nongap versus gap mode configurations. This influences the overall enhancement which depends on the Raman mode symmetry and substrate structure.

中文翻译：

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基材对尖端增强拉曼光谱的影响：场分布模拟与石墨烯测量之间的比较

尖端增强拉曼光谱（TERS）已达到纳米空间分辨率，可用于在环境条件下进行的测量以及在超高真空下的亚纳米分辨率。已经在间隙模式配置中获得了超分辨率（超过尖端顶点直径）的TERS，在该模式下，导电基板可以使电场局部化。在这里，我们提出实验和理论上的TERS，以探索负责频谱增强的场分布。我们使用的金提示$40\pm 10\text{纳米}$ 顶点直径，用于测量石墨烯（一种空间离域的二维样本）的石墨烯，该石墨烯位于不同的基材上：（i）玻璃，（ii）薄金层和（iii）覆盖有石墨烯的表面 $12\text{纳米}$ 直径金球，为此 $6\text{纳米}$分辨率是在环境条件下实现的。超分辨率是由于耦合的尖端-样品-基底系统产生的场配置所致，表现出非平凡的空间表面分布。对于非间隙和间隙模式配置，场分布和对称选择规则是不同的。这影响了取决于拉曼模式对称性和衬底结构的整体增强。