sharp-tip structures with a highly focused field at the end of the tip. [7] In particular, plasmonic gap nanostructures (PGNs) are among the most promising materials for diverse applications with highly localized field and amplified optical signals in the gap including sensing, spectroscopy, optics, biomedicine, electronics, and energy applications. [8][9][10][11][12][13] The development of bottom-up or top-down synthetic methods enabled the access to numerous nanogap structures with diverse gap sizes, morphologies, and compositions. [14][15][16][17][18] The highly localized electric field inside the plasmonic nanogap and the appearance of plasmonic coupling in closely neighboring structures have been studied, leading to the discovery of interesting optical phenomena and advances in sensing such as ultrasensitive spectroscopy, singlemolecule sensing methods, and in situ detection techniques. [19][20][21] As small changes in the distance, morphology, and composition of the nanogap lead to significant variations in the optical responses, the highly precise, controllable, and high-yielding preparation of PGNs is important to achieve reproducible and reliable results. However, although numerous pioneering studies on basic synthetic methods, characteristics, and applications enabled the understanding and control over plasmonic nanogaps, key challenges still remain for the practical use and applications of PGNs. In particular, the sub-nanometer, atomic-level control of the nanogap, coupled with the scaling-up issue, and its fundamental understanding have not yet been fully accomplished. For example, in surfaceenhanced Raman spectroscopy (SERS) using plasmonic gaps, early research focused on a high enhancement factor (EF). In contrast, recent studies revealed that a deep understanding at atomic scale is required, as differences in the small regime such as picocavities or molecular orientation can have a large effect on the signal. [22][23][24] Hence, a comprehensive understanding of the nanogap is critical to the further development of the field.This progress report addresses emerging issues and challenges in the field of nanogap-based plasmonics and plasmonic nanomaterials. We first describe the challenges in the creation of intra-or intergaps by bottom-up or top-down synthesis, and discuss breakthroughs that allow for overcoming these obstacles. Next, optical properties according to gap size, morphology, and composition are summarized to provide insight into the nature of gap plasmonics. The discussion of applications in this article mainly highlights surface-enhanced Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals....
