Surface-enhanced Raman spectroscopy (SERS) is a vibrational
spectroscopic
technique with molecular fingerprinting capability and high sensitivity,
even down to the single-molecule level. As it is 50 years since the
observation of the phenomenon, it has now become an important task
to discuss the challenges in this field and determine the areas of
development. Electromagnetic enhancement has a mature theoretical
explanation, while a chemical mechanism which involves more complex
interactions has been difficult to elucidate until recently. This
article focuses on the 2D material-based platforms where chemical
enhancement (CE) is a significant contributor to SERS. In the context
of a diverse range (transition metal dichalcogenides, MXenes, etc.)
and categories (insulating, semiconducting, semimetallic, and metallic)
of 2D materials, the review aims to realize the influence of various
factors on SERS response such as substrates (layer thickness, structural
phase, etc.), analytes (energy levels, molecular orientation, etc.),
excitation wavelengths, molecular resonances, charge-transfer transitions,
dipole interactions, etc. Some examples of special treatments or approaches
have been outlined for overcoming well-known limitations of SERS and
include how CE benefits from the defect-induced physicochemical changes
to 2D materials mostly via the charge-transport ability or surface
interaction efficiency. The review may help readers understand different
phenomena involved in CE and broaden the substrate-designing approaches
based on a diverse set of 2D materials.