The fundamentals of nanomaterials

Nanomaterials are single-phase or multi-phase polycrystals which have at least one dimension of their basic building blocks (i.e. crystallites or grains) in the range of a few nanometers. A recent report states that ‘Nanoscience is the study of phenomenon and manipulation of materials at atomic, molecular and macromolecular scales where properties differ significantly from those at larger scale’.  

Nanomaterials have been classified into different categories according to the number of directions in which their dimensionality approaches the nanometeric range or equivalently based on the number of degrees of freedom (d_f) of their carriers. The three-dimensional nanomaterials (d_f=3) are for example those materials which are made up of nanometer-sized grains, while two-dimensional nanomaterials (d_f=2) have one dimension in the nanoscale and include thin films and surface coatings. On the other hand nanowires and nanotubes constitute one-dimensional nanomaterials (d_f=1) with two dimensions in nanoscale and zero-dimensional nanomaterials (d_f=0) have all the three dimensions in nanoscale. The category of zero-dimensional nanomaterials includes nanostructures such as precipitates, colloids and quantum dots.

There are two fundamental approaches to produce nanomaterials; the top down approach and the bottom up approach. The top down approach creates nanomaterials from bulk by refining their size, while the bottom up approach builds them from building blocks such as atoms and molecules.  The starting material to generate a nanomaterial can be in solid, liquid or gas form. It must be mentioned here though that the method of synthesis of a nanomaterial affects the microstructure of the grain interiors and the grain boundaries, a fact, which needs due attention in any discussion on the structural aspects of nanomaterials.  For example the nanomaterials formed by the condensation techniques are usually defect free with low residual strains  while the milling generated nanomaterials have a high density of defects and are highly strained. On the other hand nanomaterials generated by crystallization of amorphous phases are usually free of pores and voids.

Two dimensional model of nanomaterials. The blue filled and open circles indicate atoms in crystalline regions and boundary regions, respectively.

The nanomaterials may contain crystalline, quasicrystalline or amorphous phases and can be metals, semiconductors, ceramics or composites. As a consequence of the small dimensions of their basic building blocks, nanomaterials are characterized by two distinct features, namely; (i) a large volume fraction of grain or interface boundaries and consequently a substantial fraction of atoms residing in regions other than the crystallite or the grain interiors and (ii) ‘quantum confinement’ effects; which is a result of crystallite size being reduced to a scale where it becomes of the order of the critical length scales of the physical phenomenon such as the mean free path of the electrons, coherency length, screening length etc.. These two features cause nanomaterials to possess physical, mechanical and chemical properties which are different from both molecular clusters as well as bulk materials and therein lies the origin and importance of the field of nanoscience. The nanomaterials for example are known to exhibit depressed melting points, improved chemical reactivity, enhanced band gap, higher specific heats and thermal expansion coefficients and superior soft magnetic properties as compared to their bulk counterparts.

Solids are known to crystallize in different stable or metastable forms depending on conditions of temperature and pressure, in addition to various other factors. In the case of the nanomaterials, size plays the role of an additional variable which also controls its crystal structure and has an effect on the different cooperative phenomenon such as magnetism, superconductivity and ferroelectricity. The particle size dependent modification in phase diagram of materials has been reported.