**3. Conclusions**

The geometry of plasmonic structures plays a key role and is an essential property of a plasmonic system. For SERS systems composed of metal nanoparticles without a supporting substrate or on a non-metal substrate, a single particle may not create significant enhancement, while their agglomeration demonstrates a much more enhanced signal both theoretically and experimentally. This is due to the existence of the hotspots that are created in small gap between the single elements. For a gap-mode SERS system, a good overall enhancement factor can be expected as the hotspots are created between the particle and the substrate, and/or between the plasmonic particles themselves. Finally, metal nanostructure arrays provide a versatile tool in the SERS catalog. Their optical resonance behavior can be tuned *via* changing their shapes using different approaches. For TERS systems, the lightening rod effect, which is induced by any geometric anisotropy, and the LSPR, which is influenced by the tip geometry and material, are the two most critical experimental and theoretical parameters, while the real experiment is usually performed in the so-called gap-mode TERS using metal substrates with metal tips. Despite the flexibility of the simulation tools addressing TERS experiments, the modeling of such experiments is still challenging due to the difficulties providing different and precise dimensions of active tips and

substrates. Nevertheless, the support of experimental results by the simulations or the planning of any SERS or TERS experiment using simulation tools is of major importance toward the understanding of the physics of plasmonic systems and in providing a better control over the measurement itself.
