Nanotechnology and Nanoscience
The term „nanotechnology‟ was initially coined by Prof. Norio Taniguchi in 1974, describing it as the “processing of separation, consolidation and deformation of metals by one atom or one molecule”. Although, nanotechnology was coined lately, existence of real nanotechnology dates back 4th -18th centuries with the usage of nanotechnology in Lycurgus cup, stained glass windows in churches, lustrous ceramic pottery and glasses and carbon nanotubes and cementite nanowires reinforced sabre blades providing high strength to the steel. A report by Michael Faraday in 1857 suggested the size dependent optical properties of gold particles, although he was not aware that the prepared colloidal particles were in nano range. The real development came into picture when Prof. R. Feynman delivered an intriguing talk on designing the things at nanoscale and forecasted the adaptability to manipulate, control, assemble, produce and manufacture things at atomic precision. With the invention of scanning tunnelling microscope by Binning and Roher in 1981, real time morphology of the atomic/molecular based architecture could be envisaged. This crucial discovery was later recognized with Nobel Prize in 1986. An artifact to literature “Engines of Creation”, authored by K. Eric Drexler in 1986 came up as the first book on nanotechnology, with the simultaneous discovery of atomic force microscope. Afterwards, enormous growth taken place which resulted in the development of Bucky ball, carbon nanotubes (CNTs) and advancements of diverse nanomaterials, finally, leading to products in the consumer market by early 2000 (Cao et al., 2004).
Nanoscience is the study of phenomenon and manipulation of matter at smaller (molecular/atomic) scale, where properties (e.g. optical, electrical, mechanical, etc.) of materials become considerably different from the properties of larger scale materials. Nanotechnologies are the applications of nanoscience especially to commercial and industrial purposes (Filipponi et al., 2010).
The nanoscale is typically defined as 1 to 100nm. So, a nanomaterial should have at least 1 dimension in the scale of nanometre. Nanomaterials exhibit distinctive performance owing to its quantum confinement effect and large surface area. The energy quantization in nanomaterials dramatically alters its electronic band structure and hence modifies its properties like magnetic, optical, and electronic properties. So, the significances of quantum confinement in nanoscale are technologically important, besides being fundamental. 1 nanometre (nm) measurement is equal to 1-billionth of a meter, or we may say that it is about 10 water molecules or about the 6 carbon atoms width. The width of a red blood cell is about 7000 nm and the width of a human hair is about 80,000 nm. The size of an atom is also smaller than 1 nm whereas in case of molecules e.g. proteins, the size extends between 1 nm and higher. Nanoparticles possess wide-range applications in the fields of biotechnology, biomedicines, material sciences, bionanosensors, DNA analytics etc. A potential application of nanoparticles is its photocatalytic activity for the purpose of elimination or reduction of contaminants present in water and nanotechnology provides us methods for the purification of air and water by using semiconductor nanoparticles as sensing systems or as catalytic agents. On the basis of dimensions, nanomaterials can be classified as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three dimensional (3D) as shown in figure 1.1 (Mandal et al., 2011).
Figure 1.1: Classification of nanomaterials as (a) zero dimensional (Quantum dots), (b) one dimensional (Nanorods/ tubes, fibers), (c) 2 dimensional (Nano sheets/ thin paper), (d) 3 dimensional (Polycrystal). Ref: (Gusev, 2007).
1.3. Zinc Oxide
Zinc oxide (ZnO) is one of the versatile functional materials which have very appealing and broad-ranging history in diverse applications. ZnO has extensive use in various industrial applications such as a pigment in enamel coatings and paints, as an important ingredient in glass, cements, glue, tires, white ink, matches, cosmetics and dental cements. The average annual production of ZnO to fulfil industrial demand is reported as ~ 100,000 tonnes. The vast applications rely on various physical and chemical properties of ZnO such as the chemical activity, thermal and electrical conductivity, bioactivity and its UV absorption capability. It can also be categorized in the list of ‘old’ semiconductors. Reports on synthesis and characterization of ZnO have been available from as early as 1935. In 1960’s improvements in the growth of single crystalline ZnO in epitaxial and bulk forms have transformed attentions in ZnO. Since then, synthesis of ZnO thin films has been an attractive field because of the strong potential for applications such as light emitters, transducers, sensors and catalyst. ZnO Nanostructures have attracted massive attention in recent period due to potential technological advantages and applications (Wang, 2004).
Zinc Oxide is a promising material and it has unique desirable properties such as wide band gap (3.37eV) and large exciton energy (60meV). ZnO is thermally more stable in hexagonal wurtzite structure at room temperature than in cubic zinc blend structure. In ZnO crystal, Zn2+ and O2- planes are formed alternatively along the c-axis and it has P63mc space group. The basic hexagonal wurtzite crystal structure of ZnO is shown in Figure 1.2 (Look D. C, 2001).