  Topotactic chemical reaction (TCR) is a chemical process that transforms one
crystalline phase to another while maintaining one or more of the original
structural frameworks, typically induced by the local insertion, removal, or
replacement of atoms in a crystal. The utilization of TCR in atomic-layer
materials and surfaces of bulk crystals leads to exotic quantum phases, as
highlighted by the control of topological phases, the emergence of
two-dimensional (2D) superconductivity, and the realization of 2D
ferromagnetism. Advanced surface-sensitive spectroscopies such as
angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling
microscopy (STM) are leading techniques to visualize the electronic structure
of such exotic states and provide us a guide to further functionalize material
properties. In this review article, we summarize the recent progress in this
field, with particular emphasis on intriguing results obtained by combining
spectroscopies and TCR in thin films.
