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The word "nano", just a fraction that indicates one billionth of a unit Quantity until recently; however, the same is redefining the understanding of matter at an
extraordinary pace every day. Noble prize winning inventions of bucky balls and carbon
fullerene structures, first electron microscope image of the carbon microtubules, later
called as carbon nanotubes (CNTs), the invention of inorganic fullerenes and
anisotropic nanostructures can be termed as major breakthroughs in the field of
nanoscience and technology. Synthesis of size and shape controlled nanostructures
(triangles, cubes, tubes, wires, rods, fibers, tetrapods, etc.), their self-assembly, properties and
possible applications are under rigorous research. Realizing the importance of
nanotechnology, state of the art technology centers with excellent processing, characterization and device fabrication facilities are being developed.
The significantly different physical properties of these nanostructured
materials have been ascribed to their characteristic structural features in between the
isolated atoms and the bulk macroscopic materials. ''Quantum confinement'', the most
popular term in the nano-world, is essentially due to the changes in the atomic structure
as a result of the direct influence of the ultra-small length scale on the energy band structure. The exceptional electronic, mechanical, optical and magnetic properties of the
nanoscale materials can all be attributed to the changes in the total energy and structure
of the system. In the free electron model the energies of the electronic states and the
spacing between energy levels, both vary as a function of 1/L2, with L being the
dimension in that direction. At nanoscale dimensions the normally collective electronic
properties of the solid become severely distorted and the electrons at this length scale
tend to follow the ''particle in a box'' model, might often require higher order
calculations to account for band structure. The electronic states are more like those found
in the localized molecular bonds than the macroscopic solids. The main implication of
such confinement is the change in the system total energy; and hence the overall
thermodynamic stability. The chemical reactivity, being a function of the system
structure and the occupation of the outermost energy levels, will be significantly affected
at such a length scale, causing a corresponding change in the physical properties .
Literature shows that in the past few years, attention has been alerted on the research field of one dimensional (1D) nanostructure materials, such as nanowires and nanorods, because of both their basic significance and the broad range of their possible applications in nanodevices[2, 3].
Recently, one-dimensional (1D) or quasi-1D ZnO nanostructured materials have received special attention due to their unique properties and numerous potential applications.
High chemical stability, low threshold intensity, wide band gap (3.37 eV), and large exciton binding energy (60 meV) make ZnO an excellent candidate for the fabrication of electronic and optoelectronic nanodevices. In order to obtain different ZnO nanostructured materials, varieties of methods have been developed. By far, a large number of ZnO nanostructured materials with different structures and morphologies have been synthesized. Considering the quantum confinement effect, peculiar properties of ZnO nanostructure may be expected to be obtained through controlling the shape and dimensionality of the materials. The Nano tetrapods structured materials with special shape and structure which are significantly different from other ZnO nanostructures, have remarkable optical, electric, magnetic, and mechanical properties, and they possess promising applications in nanoelectronics and photonics. Thus, much effort has been devoted to the synthesis and investigation of their characteristics . Several experiments confirmed that ZnO is very resistive to high energy radiation making it a very suitable candidate for space applications. ZnO is easily etched in all acids and alkalis, and this provides an opportunity for fabrication of devices with considerable ease.
The present regeneration is based on the possibility to grow epitaxial
layers, quantum wells, nanorods and related objects or quantum dots and on the hope
a material for blue/UV optoelectronics, including light emitting or even laser diodes in addition to (or instead of) the GaN-based structures,
a radiation-hard material for electronic devices in a corresponding environment,
a material for electronic circuits that is transparent in the visible and/or usable at elevated temperature
a diluted- or ferro- magnetic material, when doped with Co, Mn, Fe, V, etc., for semiconductor spintronics,
a transparent highly conducting oxide when doped with Al, Ga, In, etc., as a cheaper alternative to ITO.
For several of the above-mentioned applications a stable, high, and reproducible p-doping is obligatory. Though progress has been made in this crucial field this aspect still forms a major problem. The emphasis of the present very active period of ZnO research is essentially on the same topics as before, but including nanostructures, new growth and doping techniques and focusing more on application- related aspects.
1.1 CRYSTAL STRUCTURE
Most of the group-II-VI binary compound semiconductors crystallize in
either cubic zinc-blende or hexagonal wurtzite structure where each anion is surrounded
by four cations at the corners of a tetrahedron, and vice versa. This tetrahedral
coordination is typical of sp3 covalent bonding, but these materials also have a substantial
ionic character. ZnO is a II-VI compound semiconductor whose ionicity resides at the
borderline between covalent and ionic semiconductor. The crystal structures shared by
ZnO are wurtzite B4, zinc blende B3, and rock salt B1, as schematically shown in Fig.
1.1. At ambient conditions, the thermodynamically stable phase is wurtzite. The zincblende
ZnO structure can be stabilized only by growth on cubic substrates, and the rock
salt NaCl structure may be obtained at relatively high pressures.
Stick and ball representation of ZnO crystal structures: (a) cubic rock salt
(B1), (b) cubic zinc blende (B3), and (c) hexagonal wurtzite (B4). Shaded gray and
black spheres denote Zn and O atoms, respectively.
Zinc oxide hexagonal wurtzite-type structure has a polar hexagonal axis, the c-axis, chosen to be parallel to z. The primitive translation vectors a and b lay in the x-y plane, are of equal length, and include an angle of 120Â°, while c is parallel to the zaxis. The point group is in the various notations 6mm or C6v, the space group P63mc or C46v. One zinc ion is surrounded tetrahedral by four oxygen ions and vice versa. The primitive unit cell contains two formula units of ZnO. The values of the primitive translation vectors are at room temperature a = b â‰ˆ 0.3249 nm and c â‰ˆ 0.5206 nm. The ratio c/a of the elementary translation vectors deviates with values around 1.602 slightly from the ideal value c/a = 8/3 = 1.633. In contrast to other II-VI semiconductors, which exist both in the cubic zinc blende and the hexagonal wurtzite-type structures (like ZnS, which gave the name to both structures) ZnO crystallizes with great preference in the wurtzite-type structure. The two latter structures both form a face-centered cubic lattice (FCC), however with different arrangements of the atoms within the unit cell, i.e. different bases.
1.2 LATTICE PARAMETERS:
The lattice parameters of a semiconductor usually depend on the following factors:
Free electron concentration acting via deformation potential of a conduction band minimum occupied by these electrons
Concentration of foreign atoms and defects and their difference of ionic radii with respect to the substituted matrix ion
External strains ( for example, those induced by substrate)
The lattice parameters of any crystalline material are commonly and most
accurately measured by high resolution x-ray diffraction (HRXRD) by using the Bond
method  for a set of symmetrical and asymmetrical reflections.
For the wurtzite ZnO, lattice constants at room temperature determined by
various experimental measurements and theoretical calculations are in good agreement.
The lattice constants mostly range from 3.2475 to 3.2501 Å for the a parameter and from
5.2042 to 5.2075 Å for the c parameter. The c/a ratio vary in a slightly wider range, from
1.593 to 1.6035 .The deviation from that of the ideal wurtzite crystal is probably due to
lattice stability and ionicity. It has been reported that free charge is the dominant factor
responsible for expanding the lattice proportional to the deformation potential of the
conduction-band minimum and inversely proportional to the carrier density and bulk
modulus. The point defects such as zinc antisites, oxygen vacancies, and extended
defects, such as threading dislocations, also increase the lattice constant.
For the zinc-blende polytype of ZnO, the calculated lattice constants based
on a modern ab initio technique are predicted to be 4.60 and 4.619 Å. Ashrafi et al.
characterized the zinc-blende phase of ZnO films grown by plasma assisted metalorganic
molecular beam epitaxy using reflection high-energy electron-diffraction
(RHEED), x-ray diffraction (XRD), transmission electron microscope (TEM), and
atomic-force microscope (AFM) measurements.
A high-pressure phase transition from the wurtzite to the rocksalt structure
decreases the lattice constant down to the range of 4.271-4.294 Å. The experimental
values obtained by x-ray diffraction are in close agreement. The predicted lattice
parameters of 4.058-4.316 Å using various calculation techniques, such as the HF, are
about 5% smaller or larger than the experimental values.
1.3 ELECTRONIC BAND STRUCTURE
The band structure of a given semiconductor is pivotal in determining its
potential utility. Consequently, an accurate knowledge of the band structure is critical if
the semiconductor in question is to be incorporated in the family of materials considered
for device applications. Several theoretical approaches of varying degrees of complexity
have been employed to calculate the band structure of ZnO for its wurzite, zinc-blende,
and rock salt polytypes. Besides, a number of experimental data have been published
regarding the band structure of the electronic states of wurtzite ZnO. X-ray- or UV
reflection/absorption or emission techniques have conventionally been used to measure
the electronic core levels in solids. These methods basically measure the energy
difference by inducing transitions between electronic levels (for example, transitions
from the upper valence-band states to the upper conduction-band states, and from the
lower valence-band states) or by exciting collective modes (for example, the upper core
states to the lower edge of the conduction band and to excitations of plasmons). Another
important method for the investigation of the energy region is based on the photoelectric
effect extended to the x-ray region, namely, photoelectron spectroscopy (PES). The peaks
in emission spectrum correspond to electron emission from a core level without inelastic
scattering, which is usually accompanied by a far-less-intense tail region in the spectrum.
More recently, angle-resolved photoelectron spectroscopy (ARPES) technique has started
to be used. This technique together with synchrotron radiation excitation has been
recognized as a powerful tool that enables experimental bulk and surface electronic bandstructure determinations under the assumptions of k conservation and single nearly-freeelectron-like final band.
Since ZnO is a direct gap semiconductor with the global extrema of the uppermost valence and the lowest conduction bands (VB and CB, respectively) at the same point in the Brillouin zone, namely at k = 0, i.e. at the Ð“-point, we are mainly interested in this region. The lowest CB is formed from the empty 4s states of Zn2+ or the anti-bonding sp3 hybrid states. A typical representation of the band structure looks as follows :
Nanostructured ZnO has following main motivation factors for my research:-
First, it is an important oxide, exhibiting near-ultraviolet emission and transparent conductivity because it has a direct wide band gap of 3.37 eV and a large excitation binding energy (60 meV).
Second, because ZnO has non-central symmetry so it is piezoelectric. This property is used in sensors and transducers, nano-generator etc.
Also it is non toxic, bio-safe and biocompatible.
Among all the known material it has richest family of nano-structures both in structures and properties.
Finally to work on fabrication and characterization of ZnO nanostructures, the basics fabrication and Characterization tools are available in the NANO-TECH and HI-TECH Labs of department of Physics UAJK Muzaffarabad. So I inspired to avail this chance.