Size is one of the central issues in nanoscience. The practical meaning of the term "sub-nanometric material (SNM)" requires two aspects: (1) its size should be at the atomic level; (2) it shows unique (size-related) properties compared to its nano-counterparts with larger sizes. Here, SNMs in the form of wires (SNWs) and the unique properties arising from their special size are reviewed. First, their polymer-like behavior, including rheological behavior and self-assembly, is dicussed. Their origins may stem from the special size and the ligands around the wire. Even a slight increase in diameter would risk the polymer-like behavior. Meanwhile, the ligands on SNWs are proportional to the inorganic entity at this scale. Consequently, surface ligands should have a profound impact on the properties, like catalysis, self-assembly, optics, etc. To reveal more potential applications, their applications in energy conversion are comprehensively reviewed. To some extent, characterization can greatly influence the way things are observed. Thus, some appropriate characterization techniques are briefly introduced. Finally, another emerging part of SNWs (atomic chain material) is briefly introduced. It is hoped that this review can provide new insights to this special scale.

N ear-infrared (NIR, 780−2500 nm) light makes up to 52% of the photoenergy of the sunlight arriving at the earth's surface, 1 so many scientists have made great efforts to develop efficient NIR absorbents in the hope of making the utmost use of solar energy. 2,3 Photo-to-thermal conversion is a hotspot in the research of NIR conversion. So far, various NIR absorbents have been exploited, such as gold nanostructures, CuS, CuSe and organic dyes etc., 4−21 which could be applied in solar-driven water evaporation and the area of tumor therapy. However, there are some drawbacks restricting the development. The gold-based NIR absorbents are expensive, so they cannot fit the demand of large-scale applications. For CuS and CuSe, they are hindered by their toxicity. With regard to organic dyes, they often suffer from photobleaching and low light absorbance. Thus, it is urgent to develop an efficient NIR absorbent with high stability and photo-to-thermal conversion efficiency. Tungsten bronze (M x WO 3 , 0 < x < 1) might be a superior choice. It is a nonstoichiometric compound in which M can be alkali metal ions, ammonium ions, hydrogen ions, rare-earth metal ions or some other metal ions and tungsten ions are with mixed valence (+5 and +6). Tungsten bronze is not only a superior choice in the utilization of NIR to offer excellent photothermal performance 22−26 but also has the advantages of low cost and environmental friendliness. Recently, Guo et al. have reported several tungsten bronze nanomaterials, such as Cs x WO 3 nanorods, 22 ammonium tungsten bronze nanocubes 23 and their excellent photothermal performance was proved. And in order to improve the

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