When scientists synthesize with a dream of discovering a new material, in most cases it is a drive of curiosity that propells science. However, targeted drives often considers realistic possibilities and sometimes are based on emotions. History of exotic materials have often been accidental and sometimes unintentional but we must always remember that the follow up has been realistic and targeted towards improvement in the same functionality.
One question arises always. Is mother Earth or the Universe naturally deprived of such exotic materials or is it just that we have never known these materials because we did not find them. The answers to these questions may never be found if we do not find these materials one day by exploration. However, to create these materials is not easy.
Materials even if they have the same chemical composition may differ in phase which means structure or some physical property. This is because during synthesis the environment dominates the kinetics of the molecules and hence the formation of a phase takes place with a definite functionality. It is absolutely difficult to reproduce the exact same environment between two synthesis but most materials have a steady domain of conditions where a prefered phase is maintained. But this is not a guaranteed effect for sensitive compositions, especially in the nano regime where surface energy of the materials drive a dominant role in formation. Hence, often we find contradictory results from the same material synthesized via a slightly different route.
Let us get to the scale of the atoms and imagine that they are like social bodies trying to arrange themselves in a social pattern. Depending on the prevailing boundaries of given conditions these bodies will try to arrange themselves according to their intellect which in the case of the atom may be the instantaneous energy distribution in the atom. This distribution if made uniform by quasi-statically approaching some kinetic equilibrium state. Then only the molecules get the chance of behaving exactly in the same way.
In the social analogy it is equivalent to a social order where there is one social form being formed by gradually introducing discipline in the frame and bringing in more and more people in the same format. This is the phenomenon of crystallization of a solid when you achieve a regular structure over a large form of matter. On the other hand the glassy or amorphous phase is a frozen liquid or disturbed/perturbed state of the same composition which does not have a long range structure or has a continuously modifying arrangement of atoms comparable to societies with less law and order.
The SMART group targets to study these social behaviours in the molecular level of matter where the units are subject to various defined social pressure and extract the functionality of that certain perturbed state of matter.
RESEARCH FOCUS:
Magnetism, Multiferroics, Spintronics, Semiconductors Materials: Nanocrystals, Microcrystals, Thin films, Single Crystals, Glasses
Synthesis, Structural/electronic/magnetic characterization to investigate multiferroicity in perovskite oxides; Superconductivity in TM-intercalated Pnictogen Chalcogenide Topological Insulators and other oxides; Spintronics in dilute magnetic semiconductors Photoconduction properties of transition metal doped ZnO for device applications; Negative photoconduction in random ZnO/carbon nanotube network; Self-assembled nanofabrication and characterization for semiconductors and oxide materials (nanopartciles, nanowires) for nanodevice applications; Synthesis, structural and photoelectric / electronic properties of semiconductors
CHARACTERISATION TECHNIQUES used in analysis:
X-ray Diffraction; X-ray Absorption; X-ray photo emission spectroscopy; UV-Vis-IR spectroscopy; Raman Spectroscopy; Tunneling Electron Microscopy; Scanning Electron Microscopy; etc.
EQUIPMENTS in Lab:
X-ray Diffraction; Synthesis lab with 8 furnaces; Gas/Light Sensing; Dielectric and Electrical measurements; Hall measurement, Centrifuge, etc.
SYNTHESIS ROUTES:
Solid state synthesis; Thin film deposition techniques; Sol-gel technique; Hydrothermal technique; Co-precipitation; Electro chemical deposition
Nanoparticles of complex oxide materials are synthesized using different solution based techniques. Sol-gel based materials generally tend to be small but agglomerated. This techniques generally yields extremely high purity single phase materials. Using Hydrothermal technique, generally different morphology of the same material can be produced. However, although doping can be modified the exact amount of doping always remain a undertermined factor. Co-precipitation is a powerful method similar to solgel, which can synthesize pure paze materials but with different morphologies. However, chances of phase segregation is more than solgel process.
All nano particles are anealed in air to explore size dependent properties of the materials.
Zinc Oxide
Zinc oxide (molecular formula, ZnO) is a multifunctional material, with its unique physical and chemical properties such as high chemical stability, high electrochemical coupling coefficient, high photo stability and broad range of radiation absorption. It is recognised as a potential II VI photonic semiconductor materials due to its wide band gap (3.3 eV) and high exciton binding energy (60 meV). It possesses considerable potential for applications in optoelectronic devices such as UV lasers, LEDs, as electrode in solar cells, gas and bio sensors etc. The last few years have witnessed tremendous efforts on understanding the physical and optical properties of ZnO with particular attention on fabrication and device applications. Many synthesis routes like sol-gel, hydrothermal, co-precipitation, wet chemical method etc has been used to obtain high quality nano/microstructured ZnO material. It is also well established that ZnO optoelectronic properties strongly vary depending on its defect structure based on synthesis techniques.
Crystal Structure:
ZnO generally crystalizes in two forms: Hexagonal Wurtzite and cubic zinc blende. According to the first principle periodic Hartree-Fock linear combination of atomic orbitals theory, the hexagonal Zinc wurtzite is found to be the most thermodynamically stable form6. It belongs to the space group of P63mc which has two lattice parameters of 3.25 A and 5.20A along a and c-axes and is characterized by two interconnecting sub lattices of Zn2+ and O2- where each anion is surrounded by four cations at the corners of a tetrahedron with a typical sp3 covalent bonding. The structure of ZnO, can be described as a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions, stacked alternately along the C-axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (0001)-O polar surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis, as well as a divergence in surface energy. The zinc and oxide center in the wurtzite ZnO is tetrahedral and this tetrahedral symmetry plays an important role for polarity of ZnO. Piezoelectricity and pyroelectricity are the direct consequences of polar symmetry of ZnO along hexagonal axis. ZnO is generally found to be n-type structure. This n-type is due to the structural point defect (vacancies and interstitials) and extended defects (threading/planar dislocations). The presence of oxygen vacancies in ZnO lattice gives it n-type conductivity.
Perovskite Oxides:Ferroelectrics, Multiferroics
Pure PbTiO3 belongs to a perovskite ABO3 type family. At room-temperature, PbTiO3 is a ferroelectric material with tetragonal structure space group. Pure PbTiO3 is not commercially used as a piezoelectric material due to practical problems of making a sintered pellet or single crystal. However, it can be modified to form solid solutions with other elements for obtaining materials with excellent piezoelectric properties. PbTiO3 undergoes structural first order phase transition at 763 K into cubic paraelectric structure. The phase transition behaviour is of displacive nature as the central Ti atom and the oxygen atoms are displaced from their centro-symmetric position of the cubic form. One of the perovskite, BaTiO3 is widely studied among ferroelectric materials. Although PbTiO3 is isomorphous at room temperature with BaTiO3 ceramic, there are important differences between these ferroelectric materials. The lattice constant of PbTiO3 at room-temperature are a, b (3.902A) and c (4.156A). This gives a c/a ratio of 1.06 whereas for BaTiO3, c/a ratio is 1.01. Hence, PbTiO3 has more tetragonal distortion than BaTiO3. In tetragonal PbTiO3 phase, the position of the ions may be described by keeping the Pb ion at the origin (0, 0, 0), Ti ions (0.5, 0.5, 0.5+dz1), O1(0.5, 0.5, dz2), and O2(0.5, 0, 0.5+dz3) for Rietveld refinement. Displacement of Ti and O atoms from the ideal cubic position is denoted by dz. The shifting of Ti and O in the PbTiO3 system is in the same direction whereas BaTiO3 is in the opposite direction from the cubic system.
The covalent character of Pb and its significant feature in ferroelectric/piezoelectric properties have been widely discussed in literature of PbTiO3 and based compounds. Cohen has elaborated the difference in PbTiO3 and lead-free perovskites (BaTiO3, KNbO3) properties. It has been shown that in PbTiO3, the hybridization between Ti-O and Pb-O are important contributions towards ferroelectricity by weakening short-range repulsions. A weakening of this hybridization reduces the ferroelectricity in the PbTiO3. Pb(6s2) states hybridize with the O(2p) and Ti(3d) to O(2p) states. This hybridization makes PbTiO3-based materials important as ferroelectrics. Since it is difficult to grow single crystals of PbTiO3 of a size suitable for measurements; there have been relatively few attempts to systematically study dielectric, elastic, and piezoelectric properties of this crystal. A few complicated, experimental studies exhibit large conductivity, possibly related to a high concentrations of Pb vacancies. The most relevant difference between PbTiO3 and other perovskite ferroelectrics is a low anisotropy of permittivity and relatively low shear piezoelectric coefficients in PbTiO3. PbTiO3 has many important technological applications in electronics and microelectronics due to its high spontaneous polarization, Curie temperature, and pyroelectric coefficient. Ferroelectric PbTiO3 films deposited onto various substrates has proven advantageous in memory applications, field effect devices and pyroelectric detectors which have hence stimulated technological interest in this field of study.
Rare Earth Oxides
Fluorite structured cerium oxide (CeO2) have attracted attention from various fields in materials engineering, chemistry and physics. It has versatile applications in water-gas shift reaction, gas sensor, biomedicines, catalysis, glass polishing, fuel cells, ultraviolet absorbents, etc. Recently many researchers have worked on aliovalent (especially transition element) ion doped CeO2, with enhanced functionalities like oxygen storage, ferromagnetism, photocatalysis, differential cytotoxicity against neuroblastoma cancer cells, catalytic activity for oxidation of CO, etc. The last two functionalities are related to bandgap of these ceramic materials. CeO2 has bandgap of 6 eV which is due to O2p, valance band (VB) and Ce 5d and 6s, conduction band (CB) but in between these bands an empty Ce 4f band is present which with partially filling Ce 4f1 causes bandgap of 3 eV. With substitution defects and disorder in lattice increases which leads to formation of some new states between VB and CB and thereby reduces the bandgap. However, band structure and bandgap of CeO2 has always been seen as a controversial subject. Both direct and indirect bandgaps have been reported with a range of values of bandgaps. However, in both types only allowed transitions have been discussed in literature, whereas forbidden transitions have not been discussed much.
Titanium Oxide
Titanium dioxide occurs in three polymorph crystalline phases: anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic). While a anatase and brookite are metastable phases, rutile is thermodynamically the most stable phase. Difference in arrangement of distorted oxygen octahedron (with titanium at the center) forms these three phases of TiO2. In the rutile phase, each octahedron shares two edges with two octahedral and two corners with two other octahedral. The octahedral arrangement of rutile phase is more stable than that of anatase and brookite phases where three and four edges of each octahedral are shared respectively. The initial phase formed in a low-temperature synthesis method, e.g. solgel process, is a nano-crystalline anatase phase. Heating the samples, irreversibly converts the phase from metastable anatase to stable rutile. Anatase and rutile phases are commercially important materials. TiO2 properties are structure dependent. Bulk anatase to rutile transformation happens at 400 to 1200 C. In nano-sized powder this variation is more prominent. Anatase to rutile phase transition in TiO2 depends on many factors including particle size, annealing temperature, amount and nature of dopants, etc. The charge, size, and the bonding nature of the substituent element dictate the phase transition temperature
