All forms of energy follow the law of conservation of
energy, by
which they can be neither created nor destroyed. Light-to-heat conversion
as a traditional yet constantly evolving means of converting light
into thermal energy has been of enduring appeal to researchers and
the public. With the continuous development of advanced nanotechnologies,
a variety of photothermal nanomaterials have been endowed with excellent
light harvesting and photothermal conversion capabilities for exploring
fascinating and prospective applications. Herein we review the latest
progresses on photothermal nanomaterials, with a focus on their underlying
mechanisms as powerful light-to-heat converters. We present an extensive
catalogue of nanostructured photothermal materials, including metallic/semiconductor
structures, carbon materials, organic polymers, and two-dimensional
materials. The proper material selection and rational structural design
for improving the photothermal performance are then discussed. We
also provide a representative overview of the latest techniques for
probing photothermally generated heat at the nanoscale. We finally
review the recent significant developments of photothermal applications
and give a brief outlook on the current challenges and future directions
of photothermal nanomaterials.