The word “sub-nanometer” is increasingly used recently.  However, the size strictly confined within 1 nm is difficult to  determine, since there might be errors in most of the size conventional  characterization tools, and the featured size might  not be even in the overall structures, especially at this extremely  small scale. Moreover, whether there is or should be a critical  size level to differentiate sub-nanometric material （SNMs） from conventional nanomaterials  is not clear, in spite of the fact that it is well-accepted  that the size of nanomaterials would certainly influence their  properties. The size scale of sub-nanometer bridges the molecular  level and the conventional nanoscale, and an increasing  number of novel properties are being progressively researched.  Specially specifying this size scale might help to attract wide  interest on this special topic. We consider the materials as  the so-called SNMs when two aspects are met: 1) the size of  materials are confined at the level of a single-unit cell of crystals  or molecular level; 2) some unique (size-related) properties  comparing to nanomaterials at large sizes can be obtained. ――Ni Bing,Shi Yuang,Wang Xun,The Sub-Nanometer Scale as a New Focus in Nanoscience.[J] .Adv. Mater. Weinheim, 2018, 30: e1802031，