Abstract

Oriented assemblies of small crystals forming larger structuresare common in nature and crucial for forthcoming technologies asthey circumvent the difficulties of structural manipulation atmicroscopic scale. We have discovered two distinctive concentricassemblies of zinc oxide rods, wherein each rod has an intrinsicallypositive and a negative polar end induced by the noncentrosymmetricarrangement of Zn and O atoms. All the rods in a single assemblyemanate out of a central core maintaining a single polar direction.Due to growth along the two polar surfaces with different atomicarrangements, these assemblies are distinct in their intrinsicproperties and exhibit strong UV luminescence in the exterior ofZn-polar assemblies, unlike the O-polar assemblies. We have reportedsurface-enhanced Raman scattering (SERS) from Ag and Au nanoparticlesdecorated on thin carbon nanowalls (CNWs) grown by microwave plasmachemical vapor deposition. The Ag morphology is controlled byexposing the CNWs to oxygen plasma and through the electrodepositionprocess by varying the number of deposition cycles. The SERSsubstrates are capable of detecting low concentrations of rhodamine6G and bovine serum albumin, showing much higher Raman enhancementthan ordinary planar HOPG with Ag decoration. The major factorscontributing to this behavior include: high density of Agnanoparticles, large surface area, high surface roughness, and theunderlying presence of vertically oriented CNWs. The relativelysimple procedure of substrate preparation and nanoparticledecoration suggests that this is a promising approach forfabricating ultrasensitive SERS substrates for biological andchemical detection at the single-molecule level, while alsoenabling the study of fundamental SERS phenomena.

Abstract

Abstract

This talk concerns with various aspects of ZnO-based thin films forphotovoltaic applications. In particular, emphasis has been given tounderstand dynamics of thin film growth by studying temporal and spatialevolution of surface roughness within framework of the dynamic scalingtheory. The films deposited at room temperature by RF magnetron sputteringfrom a ceramic target follow anomalous (super-rough) scaling behaviorcharacterized by global roughness and growth exponents clearly distinctfrom the local ones. The observed anomalous scaling and high growthexponents are correlated to the shadowing effect arising due tonon-uniform flux distribution favored by the 30-tilt of thesubstrates. The scaling exponents and hence the growth dynamics are foundto be strongly dependent on the process parameter, namely the RF powerduring sputtering. While the roughness exponent α is stable at1.5±0.2, the growth exponents βlocal and β decrease with thedecrease in the RF power. A dominant anisotropic growth of crystallites athigh powers is believed to be the main reason for the power-dependentroughening behavior. Besides, thickness-dependent film properties, role ofsputtering configuration and influence of the RF power on film properties,which assume much importance from technological point of view, have beendiscussed. It has been shown that thickness-dependent stress cancounteract the typical carrier concentration dependent broadening of bandgap observed in degenerate semiconductors. The process parameters (forexample, RF power) and sputtering configuration are shown to havetremendous impact on film properties. A very brief introduction to ourother research efforts (that includes research in flexibleoptoelectronics, alternate absorber layers for inorganic solar cells,etc.) is also presented.

Abstract

Phase transitions occurring at absolute zero temperature and governedby a critical value of a variable like pressure, magnetic field,doping etc. are called quantum phase transitions and the point of thephase diagram where it happens the quantum critical point (QCP). Ahuge accumulation of entropy at the QCP leads to a competition betweendifferent ground states and hence, unusual behaviour in thethermodynamics and transport properties of systems close/at the QCP. Aone dimensional (1-d) spin ½ antiferromagnetic Heisenberg chain (AfHC)is expected to be quantum critical at a field H_s (saturation field)above which it undergoes a transition to a ferromagnetic state.Recently, it has been theoretically predicted that the divergence andsign change of Grüneisen parameter across the quantum critical pointcan be used as a novel technique to probe quantum criticality. For afield induced quantum critical point, the Grüneisen parameter is, infact, magnetocaloric effect (MCE) which is the heating or cooling of asystem adiabatically, in response to a changing magnetic field. Inthis talk, I will present measurements andtheoretical calculations of the MCE in a metal organic polymer systembuilt from Cu^2+ (S=½ ) ions, which is a very good realization of a1-d AfHC. We verify unambiguously the theoretical predictions anddemonstrate that the 1-d AfHC shows an extraordinarily large MCE and apronounced magnetic cooling effect near the QCP. Our results suggestthat quantum magnets near a H-induced QCP open up new possibilitiesfor realizing very efficient low-temperature coolants. We, therefore,suggest the MCE experiments as a new means of exploring quantumcriticality, one of the most interesting issues in modern condensedmatter physics.

Abstract