**2. Classification of nanostructure materials**

The nanomaterials can broadly be classified in four categories. They are (a) zero dimensional (0D), (b) one dimensional (1D), (c) two dimensional (2D) and (d) three dimensional (3D). (a) Zero dimensional nanostructures (e.g., quantum dots and nanoparticles) have all the dimensions within the nanoscale region. They are also addressed as artificial atoms as their size restriction, particularly quantum dots, can be found within a few nanometer ranges like atoms/molecules. They have grown themselves up in recent years as suitable applicant in the field of optoelectronics, biomedical, energy, textile industries, etc. (b) One dimensional nanostructure (e.g., nanowires, nanotubes, nanorods, etc.) have diameter/thickness in nanometer range, however its length to width ratio may be more than 1000 nm, i.e., several micrometers. It means that in this kind of material, one dimension of the nanostructure is not in the nanometer range. This specific class of nanomaterial has immense applications in electronics, in addition to other domains of applications like energy and environment [19]. (c) Two dimensional nano structures (e.g., nanofilms, nanosheets, nanowires, graphene, etc.) have two dimensions outside the nanometer range. It indicates that although the area (length and breadth) of these kinds of materials is more than nanometer scale, but the thickness is always within the nanoscale region. They are very useful in the applications of sensors, templates, photocatalysis, nanoreactors and nanocontainers. (d) Three dimensional nanostructures (e.g., bulk powders, nanocomposites, bundle of nanotubes/nanowires, multilayer films, nanotextured surfaces, etc.) are the nanomaterials in which, all the three dimensions of a material are not within nanoscale region. This class of materials has applications in all such fields where introduction of other dimensional nanomaterials is required in high scale, including the fields where electronics, optical, mechanical and magnetic properties are necessary.
