• Volume 31, Issue 3

June 2008,   pages  199-584

• Foreword

• Polystyrene as a zwitter resist in electron beam lithography based electroless patterning of gold

The resist action of polystyrene (𝑀w, 2,600,000) towards electroless deposition of gold on Si(100) surface following cross-linking by exposing to a 10 kV electron beam, has been investigated employing a scanning electron microscope equipped with electron beam lithography tool. With a low dose of electrons (21 𝜇C/cm2), the exposed regions inhibited the metal deposition from the plating solution due to cross-linking—typical of the negative resist behaviour of polystyrene, with metal depositing only on the developed Si surface. Upon increased electron dosage (160 𝜇C/cm2), however, Au deposition took place even in the exposed regions of the resist, thus turning it into a positive resist. Raman measurement revealed amorphous carbon present in the exposed region that promotes metal deposition. Further increase in dosage led successively to negative (220 𝜇C/cm2) and positive (13,500 𝜇C/cm2) resist states. The zwitter action of polystyrene resist has been exploited to create line gratings with pitch as low as 200 nm and gap electrodes down to 80 nm.

• Synthesis of size-controlled Bi particles by electrochemical deposition

Small sized bismuth particles are prepared by an electrochemical method using a triple voltage pulse technique. The bath composition and electrochemical parameters are optimized to yield monodisperse particles. The particles have been characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, UV-visible spectroscopy and X-ray diffraction technique. The particles, as deposited, are highly crystalline in nature and the particle size and shape get tuned depending on the conditions of deposition.

• Optical properties of polymer nanocomposites

Nanomaterials have emerged as an area of interest motivated by potential applications of these materials in light emitting diodes, solar cells, polarizers, light – stable colour filters, optical sensors, optical data communication and optical data storage. Nanomaterials are of particular interest as they combine the properties of two or more different materials with the possibility of possessing novel mechanical, electronic or chemical behaviour. Understanding and tuning such effects could lead to hybrid devices based on these nanocomposites with improved optical properties. We have prepared polymer nanocomposites of well-defined compositions and studied the optical properties of powders and their thin films. UV-vis absorption spectroscopy on nanocomposite powders and spectroscopic ellipsometry measurements on thin films was used to study the effect of interfacial morphology, interparticle spacing and finite size effects on optical properties of nanocomposites. Systematic shift in the imaginary part of the dielectric function can be seen with variation in size and fraction of the gold nanoparticle. The thickness of the film also plays a significant role in the tunability of the optical spectra.

• Reactivity and resizing of gold nanorods in presence of Cu2+

Due to the inherent anisotropy of the system, gold nanorods behave differently in comparison to their spherical counterparts. Reactivity of gold nanorods, in presence of cupric ions, was probed in an attempt to understand the chemistry of anisotropic particles. The reaction progresses through a series of intermediates. It can be arrested at any stage to get nanorods of desired dimension and therefore, can be used for their reshaping. The presence or absence of cetyltrimethylammonium bromide (CTAB) on the nanorod surface was found to be determining the site of initiation of the reaction. When a large concentration of CTAB is present in the system, selective etching of the tips of the nanorod occurs and when the nanorods are purified to reduce the amount of CTAB in the solution, the side faces of the nanorod also get reacted. Gold nanorods are converted to particles by further surface reconstructions in a systematic surface specific chemistry.

• Hydrogen-induced electrical and optical switching in Pd capped Pr nanoparticle layers

In this study, modification in the properties of hydrogen-induced switchable mirror based on Pr nanoparticle layers is reported. The reversible changes in hydrogen-induced electrical and optical properties of Pd capped Pr nanoparticle layers have been studied as a function of hydrogenation time and compared with the conventional device based on Pd capped Pr thin films. Faster electrical and optical response, higher optical contrast and presence of single absorption edge corresponding to Pr trihydride state in hydrogen loaded state have been observed in the case of nanoparticle layers. The improvement in the electrical and optical properties have been explained in terms of blue shift in the absorption edge due to quantum confinement effect, larger number of interparticle boundaries, presence of defects, loose adhesion to the substrate and enhanced surface to volume atom ratio at nanodimension.

• Formation of InN nanoparticle and nanorod structures by nitrogen plasma annealing method

In the present study, a novel method involving nitrogen plasma annealing has been reported for preparing InN nanoparticle/nanorod structures and for improving the properties of InN nanoparticle layers. Plasma annealed structures have been characterized by X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy techniques. InN nanoparticle layers have been prepared using activated reactive evaporation set up. It has been observed that there is a remarkable improvement in the conductivity and crystallinity of InN nanoparticle layers on annealing in nitrogen plasma. This has been attributed to the increase in the nitrogen content of the samples. Experiments involving plasma annealing of In nanorods deposited oxide template has also been carried out. It was found that on plasma treatment In nanorods get converted to mixed phase InN nanorods with hexagonal and cubic fractions.

• Nanomagnetics with lasers

Both liquid and vapour phase pulsed laser deposition (PLD) techniques have been used to synthesize nanophase magnetic alloys of CoPt. While the liquid route results in soft phase (disordered 𝑓𝑐𝑐) nanoparticles of CoPt near equiatomic composition dispersed in a surfactant–polymer matrix, the conventional vapour phase PLD allows growth of high coercivity nanoscale structures of CoPt on (001) SrTiO3. The magnetization, 𝑀(𝑇), dynamics of the colloidal particles is examined. Two distinct particle distributions are established from analysis of 𝑀(𝑇) data, in conformity with results of electron microscopy. In vapour deposited films at low growth rate (∼ 0.4 Å/s), morphology changes from a self-similar fractal to nanodots as the deposition temperature is raised from 700–800°C. The large lattice mismatch between (001) SrTiO3 and the 𝑎𝑐/𝑏𝑐 plane of 𝐿10 ordered phase imparts tensile strain to the films whose morphological manifestations can be suppressed at high growth rates.

• Soft lithography meets self-organization: Some new developments in meso-patterning

This is a brief review of our recent and ongoing work on simple, rapid, room temperature, pressure- less and large area (∼ cm2) imprinting techniques for high fidelity meso-patterning of different types of polymer films. Examples include soft solid polymer films and surfaces like cross-linked polydimethylsiloxane (PDMS) and polyacrylamide (PAA) based hydrogels, thermoplastics like polystyrene (PS), polymethylmethacrylate (PMMA) etc both on planar and curved surfaces. These techniques address two key issues in imprinting:

1. attainment of large area conformal contact with the stamp, especially on curved surfaces, and

2. ease of stamp detachment without damage to the imprinted structures.

The key element of the method is the use of thin and flexible patterned foils that readily and rapidly come into complete conformal contact with soft polymer surfaces because of adhesive interfacial interactions. The conformal contact is established at all length scales by bending of the foil at scales larger than the feature size, in conjunction with the spontaneous deformations of the film surface on the scale of the features. Complex two-dimensional patterns could also be formed even by using a simple one-dimensional master by multiple imprinting. The technique can be particularly useful for the bulk nano applications requiring routine fabrication of templates, for example, in the study of confined chemistry phenomena, nanofluidics, bio-MEMS, micro-imprinting, optical coatings and controlled dewetting.

• Synthesis of nanocomposites using glasses and mica as templates

Various nanocomposites were synthesized using either a silica-based glass or mica crystallites as the medium. In some cases by an oxidation or a sulfidation treatment a core-shell nanostructure could be generated. Iron–iron oxide core-shell structured nanocomposites exhibited excellent humidity sensing behaviour. Gold–gold sulfide core-shell nanorods exhibited a number of optical absorption peaks which arose because of their structural characteristics. Nanoparticles of silver and silver oxide could be aligned in a polymethylmethacrylate film by an a.c. electric field of 1 MHz frequency. The composites showed large sensitivity to relative humidity. Lead sulfide nanowires of diameter, 1.2 nm, were grown within the nanochannels of Na-4 mica. These exhibited a semiconductor to metal transition at around 300 K. This arose because of high pressure generated on the nanowires. Copper sulfide nanowires grown within the Na-4 mica channels showed metallic behaviour. Silver core–silver orthosilicate shell nanostructures developed within a silicate glass medium showed discontinuous changes in resistivity at some specific temperatures. This was explained as arising due to excitation of Lamb modes at certain pressures generated because of thermal expansion mismatch of the core and the shell phases. Optical properties of iron core–iron oxide shell nanocomposites when analysed by effective medium theory led to the result of a metal non-metal transition for particle diameters below a critical value. Similar results were obtained from optical absorption data of silver nanoparticles grown in a tetrapeptide solution.

• Growth of two-dimensional arrays of uncapped gold nanoparticles on silicon substrates

A method of preparing large area patterned 2D arrays of uncapped gold (Au) nanoparticles has been developed. The pattern has been formed using self-assembly of uncapped Au nanoparticles. The Au nanoparticles were synthesized via toluene/water two phase systems using a reducing agent and colloidal solution of Au nanoparticles was produced. These nanoparticles have been prepared without using any kind of capping agent. Analysis by TEM showed discrete Au nanoparticles of 4 nm average diameter. AFM analysis also showed similar result. The TEM studies showed that these nanoparticles formed self-assembled coherent patterns with dimensions exceeding 500 nm. Spin coating on silicon substrate by suitably adjusting the speed can self-assemble these nanoparticles to lengths exceeding 1 𝜇m.

• Growth of compact arrays of optical quality single crystalline ZnO nanorods by low temperature method

We report the synthesis and optical properties of compact and aligned ZnO nanorod arrays (dia, ∼ 50–200 nm) grown on a glass substrate with varying seed particle density. The suspension of ZnO nanoparticles (size, ∼ 15 nm) of various concentrations are used as seed layer for the growth of nanorod arrays via selfassembly of ZnO from solution. We studied the effect of various growth parameters (such as seeding density, microstructure of the seed layer) as well as the growth time on the growth and alignment of the nanorods. We find that the growth, areal density and alignment of the nanorods depend on the density of seed particles which can be controlled. It is observed that there is a critical density of the seed particles at which nanorod arrays show maximum preferred orientation along [002] direction. The minimum and maximum radius of the aligned nanorods synthesized by this method lie in the range 50–220 nm which depend on the seeding density and time of growth. These nanorods have a bandgap of 3.3 eV as in the case of bulk crystals and show emission in the UV region of the spectrum (∼ 400 nm) due to excitonic recombination and defect related emission in the visible region.

• New route for preparation of luminescent mercaptoethanoate capped cadmium selenide quantum dots

We report a synthesis of cadmium selenide quantum dots (Q-CdSe) by refluxing a mixture of cadmium acetate, selenium powder, sodium sulfite and 2-mercaptoethanol in N,N′-dimethyl formamide (DMF)/water solution. X-ray and electron diffractions suggest the formation of hexagonal phase of size quantized CdSe. Based on TEM analysis, the formation of nanoparticles with an average diameter of 3.5 ± 0.5 nm is inferred. Their sols in DMF and dimethyl sulphoxide (DMSO) gave characteristic absorption peaks at 300 nm and 327 nm, which is attributed to the formation of high quality, size quantized CdSe particles. Extracted particles from the sol were readily redispersed in DMF and DMSO, which were diluted further with water without losing their optical and colloidal properties. FTIR spectroscopy suggested the formation of 2-mercaptoethanol thiolate on the particle surface, with free –OH groups available for linkage. Sols in DMSO and their solutions in water displayed an intense photoluminescence (PL).

• Empirical pseudo-potential studies on electronic structure of semiconducting quantum dots

Theoretical investigations of electronic structure of quantum dots is of current interest in nanophase materials. Empirical theories such as effective mass approximation, tight binding methods and empirical pseudo-potential method are capable of explaining the experimentally observed optical properties. We employ the empirical pseudo-potential to calculate the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as a function of shape and size of the quantum dots. Our studies explain the building up of the bulk band structure when the size of the dot is much larger than the bulk Bohr exciton radius. We present our investigations of HOMO–LUMO gap variation with size, for CdSe, ZnSe and GaAs quantum dots. The calculated excitonic energies are sensitive to the shape and size of quantum dots and are in good agreement with experimental HOMO–LUMO gaps for CdSe quantum dots. The agreement improves as experimentally observed lattice contraction is incorporated in pseudo-potential calculations for ZnSe quantum dots. Electronic structure evolution, as the size of quantum dot increases, is presented for CdSe, ZnSe and GaAs quantum dots.

• New approach towards imaging 𝜆-DNA using scanning tunneling microscopy/spectroscopy (STM/STS)

A new methodology to anchor 𝜆-DNA to silanized 𝑛-Si(111) surface using Langmuir Blodget trough was developed. The 𝑛-Si (111) was silanized by treating it with low molecular weight octyltrichlorosilane in toluene. Scanning tunneling microscopy (STM) image of 𝜆-DNA on octyltrichlorosilane deposited Si substrate shows areas exhibiting arrayed structures of 700 nm length and 40 nm spacing. Scanning tunneling spectroscopy (STS) at different stages depict a broad distribution of defect states in the bandgap region of 𝑛-Si(111) which presumably facilitates tunneling through otherwise insulating DNA layer.

• Investigation on carbon nanomaterials: Coaxial CNT-cylinders and CNT-polymer composite

The macroscopic coaxial carbon cylinders (dia. ∼ 0.5 cm with varying lengths, ∼ 7–10 cm) consisting of aligned carbon nanotube (CNT) stacks have been prepared by controlled spray pyrolysis method. The coaxial carbon cylinders of CNT stacks have been formed directly inside the quartz tube. Another study is done on multi-walled CNTs (MWNTs)–polymer (e.g. polyethylene oxide (PEO), polyacrylamide (PAM)) composite films. We have investigated the structural, electrical and mechanical properties of MWNTs–PEO composites. Composites with different wt% (between 0 and 50 wt% of MWNTs) have been prepared and characterized by the scanning electron microscopic technique. Enhanced electrical conductivity and mechanical strength were observed for the MWNTs–PEO composites. We have also studied the electrical property of MWNTs–PAM composite films.

• Synthesis, characterization and application of semiconducting oxide (Cu2O and ZnO) nanostructures

In the present study, we report the synthesis, characterization and application of nanostructured oxide materials. The oxide materials (Cu2O and ZnO) have been synthesized by electrolysis based oxidation and thermal oxidation methods. Cuprous oxide (Cu2O) nanostructures have been synthesized by anodic oxidation of copper through a simple electrolysis process employing plain water (with ionic conductivity, ∼ 6 𝜇S/m) as electrolyte. In this method no special electrolytes, chemicals and surfactants are needed. The method is based on anodization pursuant to the simple electrolysis of water at different voltages. Two different types of Cu2O nanostructures have been found. One type got delaminated from copper anode and was collected from the bottom of the electrochemical cell and the other was located on the copper anode itself. The nanostructures collected from the bottom of the cell are either nanothreads embodying beads of different diameters, ∼ 10–40 nm or nanowires (length, ∼ 600–1000 nm and diameter, ∼ 10–25 nm). Those present on the copper anode were nanoblocks with preponderance of nanocubes (nanocube edge, ∼ 400 nm). The copper electrode served as a sacrificial anode for the synthesis of different nanostructures. Aligned ZnO nanorod array has been successfully synthesized by simple thermal evaporation catalyst free method. Detailed structural characterizations revealed that the as synthesized aligned ZnO nanorods are single crystalline, with a hexagonal phase, and with growth along the [0001] direction. The room-temperature photoluminescence spectra showed a weak ultraviolet emission at 380 nm, a broad blue band at 435 nm and a strong orange–red emission at 630 nm. Structural/microstructural characterization of these nanomaterials have been carried out employing scanning (XL-20) and transmission electron microscopic (Philips EM, CM-12 and Technai 20G2) techniques and X-ray diffraction techniques having graphite monochromater with CuK𝛼 radiation (𝜆 = 1.54439 Å) (X’Pert PRO PAN analytical). The UV-visible absorption spectra were recorded on Model–VARIAN, Cary 100, and Bio UV-visible spectrophotometer. The photoluminescence (PL) measurement was carried out at room temperature with a He–Cd, a laser excited at 325 nm.

• Ab initio studies on [bmim][PF6]–CO2 mixture and CO2 clusters

Ab initio molecular dynamics studies have been carried out on the room temperature ionic liquid, 1,n-butyl,3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and supercritical carbon dioxide mixture at room temperature and experimental density. Partial radial distribution functions (RDF) for different sites have been computed to see the organization of CO2 molecules around the ionic liquid. Several partial RDFs around the carbon atom of CO2 molecule are compared to find out that the CO2 has specific interaction with a carbon atom present in the imidazolium ring. The CO2 is also found to be very well organized around the terminal carbon atom of the butyl chain. The partial RDFs for the oxygen atoms around oxygen and carbon atoms of the CO2 suggests that there is very good organization of CO2 molecules around themselves even in the [bmim][PF6] – CO2 mixture. The instantaneous quadrupole moment tensor has been calculated for the anion and the cation. The ensemble average of diagonal components of quadrupole moment tensor of the cation have finite values, whereas the off-diagonal components of the cation and both the diagonal and off-diagonal components of the anion have the value of zero with a large standard deviation. The CPMD studies performed on CO2 clusters reveals the greater tendency of the clusters with more CO2 units, to deviate from the linear geometry.

• Effect of curvature on structures and vibrations of zigzag carbon nanotubes: A first-principles study

First-principles pseudopotential-based density functional theory calculations of atomic and electronic structures, full phonon dispersions and thermal properties of zigzag single wall carbon nanotubes (SWCNTs) are presented. By determining the correlation between vibrational modes of a graphene sheet and of the nanotube, we understand how rolling of the sheet results in mixing between modes and changes in vibrational spectrum of graphene. We find that the radial breathing mode softens with decreasing curvature. We estimate thermal expansion coefficient of nanotubes within a quasiharmonic approximation and identify the modes that dominate thermal expansion of some of these SWCNTs both at low and high temperatures.

• Control of nanostructures in PVA, PVA/chitosan blends and PCL through electrospinning

Aqueous solutions of polyvinyl alcohol (PVA) were electrospun and its characteristics were studied as a function of applied potential, tip-target distance and solution flow rate. Solutions of PVA and chitosan were homogeneously mixed and electrospun to result in blend nanofibres and their properties were investigated. Conditions were revealed under which multiscale bi-modal fibres could be electrospun in a single step, producing structures that have potential applications in tissue engineering. Electrospun fibres having a bimodal size distribution of poly(caprolactone) (PCL) were also fabricated using a modified electrospinning setup. Nanofibrous microporous PVA scaffolds were fabricated using a cryogenic grinding method with subsequent compaction. Such multiscale porous structures would offer ideal matrices for tissue engineering applications.

• Comparative study of electron conduction in azulene and naphthalene

We have studied the feasibility of electron conduction in azulene molecule and compared with that in its isomer naphthalene. We have used non-equilibrium Green’s function formalism to measure the current in our systems as a response of the external electric field. Parallely we have performed the Gaussian calculations with electric field in the same bias window to observe the impact of external bias on the wave functions of the systems. We have found that the conduction of azulene is higher than that of naphthalene inspite of its intrinsic donor–acceptor property, which leads a system to more insulating state. Due to stabilization through charge transfer the azulene system can be fabricated as a very effective molecular wire. Our calculations show the possibility of huge device application of azulene in nano-scale instruments.

• Nanorheology of regenerated silk fibroin solution

We have investigated the rheological properties of regenerated silk fibroin (RSF), a viscoelastic material at micro and nano length scales, by video microscopy. We describe here the principles and technique of video microscopy as a tool in such investigations. In this work, polystyrene beads were dispersed in the matrix of RSF polymer and the positions of the embedded beads diffusing were tracked using video microscopy. An optical tweezer was used to transport and locate the bead at any desired site within the micro-volume of the sample, to facilitate the subsequent free-bead video analysis. The position information of the beads was used to obtain the time dependant mean squared displacement (MSD) of the beads in the medium and hence to calculate the dynamic moduli of the medium. We present here the results of rheological measurements of the silk polymer network in solution over a frequency range, whose upper limit is the frame capture rate of our camera at full resolution. The technique is complementary to other microrheological techniques to characterize the material, but additionally enables one to characterize local inhomogeneities in the medium, features that get averaged out in bulk characterization procedures.

• Synthesis and characterization of silver molybdate nanowires, nanorods and multipods

Silver molybdate nanowires, nanorods and multipods like structures have been prepared by an organic free hydrothermal process using ammonium molybdate and silver nitrate solutions. The powder X-ray diffraction (PXRD) patterns reveal that the silver molybdate belongs to anorthic structure. The thickness, 200–500 nm, for silver molybdate nanorods/wires and 2–5 𝜇m for microrods are identified from SEM images. UV-visible spectrum of silver molybdate nanorods/nanowires shows maximum absorbance at 408 nm. Photoluminescence (PL) spectrum exhibits UV emission at 335 nm, violet emission at 408 nm and a weak green emission at 540 nm. The influence of hydrothermal synthesis conditions on silver molybdate nanowires, nanorods and multipods compositions were established.

• Influence of mechanical milling and thermal annealing on electrical and magnetic properties of nanostructured Ni–Zn and cobalt ferrites

The present article reports some of the interesting and important electrical and magnetic properties of nanostructured spinel ferrites such as Ni0.5Zn0.5Fe2O4 and CoFe2O4. In the case of Ni0.5Zn0.5Fe2O4, d.c. electrical conductivity increases upon milling, and it is attributed to oxygen vacancies created by high energy mechanical milling. The real part of dielectric constant (𝜀') for the milled sample is found to be about an order of magnitude smaller than that of the bulk nickel zinc ferrite. The increase in Néel temperature from 538 K in the bulk state to 611 K on the reduction of grain size upon milling has been explained based on the change in the cation distribution. The dielectric constant is smaller by an order of magnitude and the dielectric loss is three orders of magnitude smaller for the milled sample compared to that of the bulk. In the case of cobalt ferrite, the observed decrease in conductivity, when the grain size is increased from 8–92 nm upon thermal annealing is clearly due to the predominant effect of migration of some of the Fe3+ ions from octahedral to tetrahedral sites, as is evident from in-field Mössbauer and EXAFS measurements. The dielectric loss (tan 𝛿) is an order of magnitude smaller for the nano sized particles compared to that of the bulk counterpart.

• Microrheology of concentrated DNA solutions using optical tweezers

Semiflexible biopolymers play a vital role in shaping cellular structure and rigidity. In this work, we report the determination of microrheological properties of concentrated, double-stranded calf thymus DNA (CT-DNA) solutions using passive, laser-scattering based particle-tracking methodology. From power spectral analysis, we obtain dynamic shear moduli of the polymer solutions stretching over three decades of frequency (100–103 Hz) and over concentration ranges spanning from very dilute to concentrated regime. We also study the effects of altered ionic strength and denaturation on the shear modulus. Our results indicate that (CT-DNA) exhibits predominantly elastic behaviour in the concentration range we probed. From the measurements of the plateau shear modulus, 𝐺p, we conclude that DNA generally behaves like a semiflexible polymer in a good solvent even at low ionic strength. We have thus demonstrated application of passive microrheological method using optical tweezers to DNA solutions. Further extensions of the technique and its applications are discussed.

• Model for thermal conductivity of CNT-nanofluids

This work presents a simple model for predicting the thermal conductivity of carbon nanotube (CNT) nanofluids. Effects due to the high thermal conductivity of CNTs and the percolation of heat through it are considered to be the most important reasons for their anomalously high thermal conductivity enhancement. A new approach is taken for the modeling, the novelty of which lies in the prediction of the thermal behaviour of oil based as well as water based CNT nanofluids, which are quite different from each other in thermal characteristics. The model is found to correctly predict the trends observed in experimental data for different combinations of CNT nanofluids with varying concentrations.

• Kinetics of self-organization of polyampholyte nanoparticles in solutions

An appropriate composition of a binary solvent, water-ethanol, provides the necessary thermodynamic environment for the polyampholyte (gelatin) molecules to form self-assembled nano-clusters having fractal dimension, ≈ 2.6, in the bulk (3-D). The aggregation in the bulk of the solution appears to be an anomalous process and could be explained through Smoluchowski aggregation model. It gives a diffusion limited aggregation (DLA) type fractal dimension to the cluster in bulk, but shows extremely low polydispersity, which in fact is a signature of a slowly growing reaction limited cluster aggregation (RLA) process. Experimental results obtained from light scattering and electrophoresis experiments enable us to probe the kinetics of such growth processes.

• Enhanced catalytic activity of nanoscale platinum islands loaded onto SnO2 thin film for sensitive LPG gas sensors

In the present study, different catalysts (∼ 10 nm thick) including metals, noble metals and metal oxides, were loaded in dotted island form over SnO2 thin film for LPG gas detection. A comparison of various catalysts indicated that the presence of platinum dotted islands over SnO2 thin film deposited by r.f. sputtering exhibited enhanced response characteristics with a high sensitivity, ∼ 742, at an operating temperature of ∼ 280°C. Different characterization techniques have been employed such as atomic force microscopy, X-ray diffraction and UV–vis spectroscopy, to study the surface morphology, grain size and optical properties of the deposited thin films. The results suggest the possibility of utilizing the sensor element with the present novel method of catalyst dispersal for the efficient detection of LPG.

• Time resolved spectroscopic studies on some nanophosphors

Time resolved spectroscopy is an important tool for studying photophysical processes in phosphors. Present work investigates the steady state and time resolved photoluminescence (PL) spectroscopic characteristics of ZnS, ZnO and (Zn, Mg)O nanophosphors both in powder as well as thin film form. Photoluminescence (PL) of ZnS nanophosphors typically exhibit a purple/blue emission peak termed as self activated (SA) luminescence and emission at different wavelengths arising due to dopant impurities e.g. green emission for ZnS : Cu, orange emission for ZnS : Mn and red emission for ZnS : Eu. The lifetimes obtained from decay curves range from ns to ms level and suggest the radiative recombination path involving donor–acceptor pair recombination or internal electronic transitions of the impurity atom. A series of ZnMgO nanophosphor thin films with varied Zn : Mg ratios were prepared by chemical bath deposition. Photoluminescence (PL) excitation and emission spectra exhibit variations with changing Mg ratio. Luminescence lifetime as short as 10-10 s was observed for ZnO and ZnMgO (100 : 10) nanophosphors. With increasing Mg ratio, PL decay shifts into microsecond range. ZnO and ZnMgO alloys up to 50% Mg were prepared as powder by solid state mixing and sintering at high temperature in reducing atmosphere. Time resolved decay of PL indicated lifetime in the microsecond time scale. The novelty of the work lies in clear experimental evidence of dopants (Cu, Mn, Eu and Mg) in the decay process and luminescence life times in II–VI semiconductor nanocrystals of ZnS and ZnO. For ZnS, blue self activated luminescence decays faster than Cu and Mn related emission. For undoped ZnO nanocrystals, PL decay is in the nanosecond range whereas with Mg doping the decay becomes much slower in the microsecond range.

• Metal oxide/polyaniline nanocomposites: Cluster size and composition dependent structural and magnetic properties

Nanocomposites of iron oxide with conducting polymer in the form of powders of varying compositions have been studied to understand the effects of particle size, cluster size and magnetic inter-particle interactions. The sizes of the nanoparticles were estimated to be ∼ 10–20 nm from the X-ray diffraction (XRD) and the transmission electron micrographs (TEM). XRD shows a single crystalline phase for the 𝛾-Fe2O3. The presence of conducting polymer was confirmed through Fourier transform infrared (FTIR) spectroscopy. The amount of polymer present in the composite, the transition temperature of iron oxide and the thermal stability of polymer was determined through thermogravimetric and differential thermal analysis (TGA–DTA). The room temperature magnetic hysteresis measurements show reduction in saturation magnetization with increasing polymer concentrations. A low value of coercivity was observed for low polymer composites. On increasing the polymer concentration, the coercivity and remanence become negligible indicating a superparamagnetic phase at room temperature. Beyond a certain composition, the system shows paramagnetic behaviour which is also confirmed through zero field cooled–field cooled (ZFC–FC) measurements. We also report preliminary results on the magnetic properties of self standing sheets prepared using 𝛾-Fe2O3 and NiFe2O4 nanoparticles and conducting polymers.

• Synthesis of nanocrystalline materials through reverse micelles: A versatile methodology for synthesis of complex metal oxides

We have been successful in obtaining monophasic nanosized oxides with varying chemical compositions using the reverse micellar method. Here we describe our methodology to obtain important metal oxides like ceria, zirconia and zinc oxide. The oxalate of cerium, zirconium and zinc were synthesized using the reverse micellar route. While nanorods of zinc oxalate with dimension, 120 nm in diameter and 600 nm in length, could be obtained, whereas spherical particles of size, 4–6 nm, were obtained for cerium oxalate. These precursors were heated to form their respective oxides. Mixture of nanorods and nanoparticles of cerium oxide was obtained. ZrO2 nanoparticles of 3–4 nm size were obtained by the thermal decomposition of zirconium oxalate precursor. ZnO nanoparticles (55 nm) were obtained by the decomposition of zinc oxalate nanorods. Photoluminescence (PL) studies at 20 K shows the presence of three peaks corresponding to free excitonic emission, free to bound and donor–acceptor transitions. We also synthesized nanoparticles corresponding to Ba1–𝑥Pb𝑥ZrO3 using the reverse micellar route. The dielectric constant and loss were stable with frequency and temperature for the solid solution.

• Optical materials based on molecular nano/microcrystals and ultrathin films

Methodologies that we developed recently for the fabrication of molecular crystals with size variation in the nano to micro regime and polyelectrolyte templated mono and multilayer Langmuir–Blodgett films, are reviewed. The electronic absorption and strong fluorescence in the molecular nano/microcrystals are found to be size-dependent. Crystal structure and computational investigations provide a unified model to explain these observations. Role of polyelectrolyte templating in achieving stable and enhanced optical second harmonic generation response from LB films based on a hemicyanine amphiphile is highlighted.

• Biomimetic synthesis of hybrid nanocomposite scaffolds by freeze-thawing and freeze-drying

The aim of this study is to biomimetically synthesize hydroxyapatite–hydrophilic polymer scaffolds for biomedical applications. This organic–inorganic hybrid has been structurally characterized and reveals a good microstructural control as seen by the SEM analysis and the nanosize of the particulates is confirmed by AFM microscopy. The characterization of such nano-structured composites would allow researchers to design new systems, tailoring properties for different applications.

• Studies on crystallization behaviour and mechanical properties of Al–Ni–La metallic glasses

Alloy ingots with nominal composition, Al92–𝑥Ni8La𝑥 (𝑥 = 4 to 6) and Al94–𝑥Ni6La𝑥 (𝑥 = 6, 7), were prepared by induction melting in a purified Ar atmosphere. Each ingot was inductively re-melted and rapidly solidified ribbons were obtained by ejecting the melt onto a rotating copper wheel in an argon atmosphere. The crystallization behaviour of melt-spun amorphous ribbon was investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry and transmission electron microscopy. DSC showed that Al86Ni8La6 alloy undergoes a three-stage and rest of the alloys undergo a two-stage crystallization process upon heating. The phases responsible for each stage of crystallization were identified. During the first crystallization stage fcc-Al precipitates for low La-containing alloys and for higher La-containing alloys a bcc metastable phase precipitates. The second crystallization stage is due to formation of intermetallic compounds along with fcc-Al. Microhardness of all the ribbons was examined at different temperatures and correlated with structural evolutions. Precipitation strengthening of nano-size fcc-Al is responsible for maximum hardness in these annealed alloys.

• Synthesis, characterization and electrochemical studies of LiNi0.8M0.2O2 cathode material for rechargeable lithium batteries

LiNiO2 and substituted nickel oxides, LiNi0.8M0.2O2 and LiCo0.8M0.2O2 (M = Mg2+, Ca2+, Ba2+), have been synthesized using simple solid state technique and used as cathode active materials for lithium rechargeable cells. Physical properties of the synthesized products are discussed in the structural (XRD, TEM, SEM with EDAX) and spectroscopic (FTIR) measurements. XRD results show that the compounds are similar to LiNiO2 in structure. TEM and SEM analyses were used to examine the particle size, nature and morphological aspects of the synthesized oxides. The composition of the materials was explored by EDAX analysis. Electrochemical studies were carried out in the range 3–4.5 V (vs Li metal) using 1 M LiBF4 in ethylene carbonate/dimethyl carbonate as the electrolyte. The doping involving 20% Mg resulted in a discharge capacity of 185 mAhg-1 at 0.1 mA/cm2 and remained stable even after 25 cycles. Discharge capacity retention for Mg doped lithium nickelate at 25th cycle was noted to be nearly 7% higher than for the undoped material.

• Structure of nanocomposites of Al–Fe alloys prepared by mechanical alloying and rapid solidification processing

Structures of Al-based nanocomposites of Al–Fe alloys prepared by mechanical alloying (MA) and subsequent annealing are compared with those obtained by rapid solidification processing (RSP). MA produced only supersaturated solid solution of Fe in Al up to 10 at.% Fe, while for higher Fe content up to 20 at.% the nonequilibrium intermetallic Al5Fe2 appeared. Subsequent annealing at 673 K resulted in more Al5Fe2 formation with very little coarsening. The equilibrium intermetallics, Al3Fe (Al13Fe4), was not observed even at this temperature. In contrast, ribbons of similar composition produced by RSP formed fine cellular or dendritic structure with nanosized dispersoids of possibly a nano-quasicrystalline phase and amorphous phase along with 𝛼-Al depending on the Fe content in the alloys. This difference in the product structure can be attributed to the difference in alloying mechanisms in MA and RSP.

• Structure–property relationship of specialty elastomer–clay nanocomposites

The present work deals with the synthesis of specialty elastomer [fluoroelastomer and poly (styrene-𝑏-ethylene-co-butylene-𝑏-styrene (SEBS)]–clay nanocomposites and their structure–property relationship as elucidated from morphology studies by atomic force microscopy, transmission electron microscopy and X-ray diffraction and physico-mechanical properties. Due to polarity match, hydrophilic unmodified montmorillonite clay showed enhanced properties in resulting fluoroelastomer nanocomposites, while hydrophobic organo-clay showed best results in SEBS nanocomposites.

• Upconversion emission of BaTiO3:Er nanocrystals

Here, we report the dopant concentration and pump-power dependence upconversion emission properties of erbium doped BaTiO3 nanocrystals derived from sol–emulsion–gel method. Green (550 nm) and red (670 nm) upconversion emissions were observed at room temperature from the ${}^{4}S_{3/2}$ and ${}^{4}F_{9/2}$ levels of Er3+ : BaTiO3 nanocrystals. It is found that at 850 mW of cw excitation power (at 980 nm) the total luminescence was 17130 Cd/m2 for 1000°C heated 0.25 mol% Er-doped BaTiO3 nanocrystals. It is worthwhile to mention that the unusual power-dependent upconversion luminescence (saturation) is observed at higher dopant concentration (2.5 mol%) and high pump power. Our analysis confirms that the depletion of the excited state is responsible for the relevant fluorescence upconversion. We have again confirmed that a twophoton excited state absorption process occurs for all samples.

• Nanocrystalline silicon prepared at high growth rate using helium dilution

Growth and optimization of the nanocrystalline silicon (nc-Si : H) films have been studied by varying the electrical power applied to the helium diluted silane plasma in RF glow discharge. Wide optical gap and conducting intrinsic nanocrystalline silicon network of controlled crystalline volume fraction and oriented crystallographic lattice planes have been obtained at a reasonably high growth rate from helium diluted silane plasma, without using hydrogen. Improving crystallinity in the network comprising ∼ 10 nm Si-nanocrystallites and contributing optical gap widening, conductivity ascending and that obtained during simultaneous escalation of the deposition rate, promises significant technological impact.

• Nanodielectrics with giant permittivity

Nanodielectrics is an emerging area of research because of its potential application in energy storage and transducers. One-dimensional metallic nanostructures with localized electronic wave functions show giant dielectric constant. Following the prediction, during the last couple of years we have investigated the effect of giant permittivity in one-dimensional systems of conventional metals and conjugated polymer chains. In this article, we have tried to summarize the works on giant permittivity and finally the fabrication of nanocapacitor using metal nanowires, which shows giant permittivity is also discussed.

• Metal nanoparticle doped coloured coatings on glasses and plastics through tuning of surface plasmon band position

Several noble metal nanoparticles doped sol–gel derived thin coloured films have been synthesized and characterized. These are pure (Ag, Au, Cu and Pt), mixed/alloy (Ag–Cu, Au–Cu, Au–Ag and Au–Pt) nanoparticles in SiO2, Au in mixed SiO2–TiO2 and SiO2–ZrO2, Au and Ag nanoparticles in inorganic–organic hybrid film matrices etc. This investigation leads to the development of tailor-made coloured coatings by tuning the surface plasmon resonance (SPR) band positions originating from the embedded nanometals by controlling mainly

1. refractive index of the film matrices and

2. nanoalloy composition. In the later case a new layerby-layer (two-layer) synthetic protocol has been developed to prepare binary nanoalloy particles with controlled atomic ratios.

• Electrochemical and in situ spectroelectrochemical studies of gold nanoparticles immobilized Nafion matrix modified electrode

Electrochemical and in situ spectroelectrochemical behaviours of phenosafranine (PS+) were studied at the gold nanoparticles (AuNps) immobilized Nafion (Nf) film coated glassy carbon (GC) and indium tin oxide (ITO) electrodes. Cyclic voltammetric studies showed that the PS+ molecules strongly interact with the AuNps immobilized in the Nf matrix through the electrostatic interaction. The presence of AuNps in the Nf film improved the electrochemical characteristics of the incorporated dye molecules. The emission spectra of Nf–AuNps–PS+ films showed that the incorporated PS+ was quenched by AuNps and it could be explained based on the electronic interaction between the AuNps and PS+ molecules. The in situ spectroelectrochemical study showed an improved electrochemical characteristic of the incorporated PS+ molecules at the ITO/Nf–AuNps electrode when compared to the ITO/Nf electrode.

• CdS/CdSSe quantum dots in glass matrix

The compositions containing 55 and 60% of silica have been formulated for preparation of glass filters having sharp cut-off at 475 and 575 nm. To achieve cut-off at these wavelengths, the glasses have been doped with CdS/CdSSe and melted at 1200–1300°C. The glass samples were transparent and pale yellow in colour due to presence of CdS/CdSSe tiny nano crystal (Q-dots). in situ growth of CdS/CdSSe nano crystals imparts the yellow/orange/red colour to these glasses. Optical study shows that as prepared glasses have optical cut-off in the range 350–370 nm. The linear crystal growth of CdS/CdSSe in glasses exhibits red shift in optical cut-off. The optical filter having cut-off at 475 nm can be prepared by doping CdS and cut-off filter of wavelength 575 nm by CdSSe. The TEM results show that the CdS/CdSSe nano crystals (Q-dots) ranging from 2–5 nm are uniformly distributed into the glass matrix.

• Doped barium titanate nanoparticles

We have synthesized nickel (Ni) and iron (Fe) ion doped BaTiO3 nanoparticles through a chemical route using polyvinyl alcohol (PVA). The concentration of dopant varies from 0 to 2 mole% in the specimens. The results from X-ray diffractograms and transmission electron micrographs show that the particle diameters in the specimen lie in the range 24–40 nm. It is seen that the dielectric permittivity in doped specimens is enhanced by an order of magnitude compared to undoped barium titanate ceramics. The dielectric permittivity shows maxima at 0.3 mole% doping of Fe ion and 0.6 mole% of Ni ion. The unusual dielectric behaviour of the specimens is explained in terms of the change in crystalline structure of the specimens.

• Nanocrystalline spinel ferrites by solid state reaction route

Nanostructured NiFe2O4, MnFe2O4 and (NiZn)Fe2O4 were synthesized by aliovalent ion doping using conventional solid-state reaction route. With the doping of Nb2O5, the size of NiFe2O4 is reduced down to 33 nm. Similarly, nanostructured manganese ferrites (MnFe2O4) with diameters in the range of 45–30 nm were synthesized by Ti4+ ion doping. Particle diameters in all the specimens are found to decrease with increasing dopant content. The substitution of Nb5+ or Ti3+ ions essentially breaks up the ferrimagnetically active oxygen polyhedra. This created nanoscale regions of ferrites. Saturation magnetization and coercive field show a strong dependence on the size of the ferrite grains. Superparamagnetic behaviour is observed from the Mössbauer spectra of nanostructured NiFe2O4, if the particle size is reduced to 30 nm. Zero field cooled and field cooled curves from 30 nm sized MnFe2O4 particles showed a peak at 𝑇B (∼ 125 K), typical of superparamagnetic blocking temperature. These results are explained in terms of core/shell structure of the materials. The d.c. resistivity of the doped specimens decreases by atleast five orders of magnitude compared to pure sample. This is ascribed to the presence of an interfacial amorphous phase between the sites.

• Study of optical phonon modes of CdS nanoparticles using Raman spectroscopy

The reduction in the grain size to nanometer range can bring about radical changes in almost all of the properties of semiconductors. CdS nanoparticles have attracted considerable scientific interest because they exhibit strongly size-dependent optical and electrical properties. In the case of nanostructured materials, confinement of optical phonons can produce noticeable changes in their vibrational spectra compared to those of bulk crystals. In this paper we report the study of optical phonon modes of nanoparticles of CdS using Raman spectroscopy. Nanoparticle sample for the present study was synthesized through chemical precipitation technique. The CdS nanoparticles were then subjected to heat treatment at low temperature (150°C) for extended time intervals. The crystal structure and grain size of the samples were determined using X-ray diffraction and HRTEM. The Raman spectra of the as-prepared and heat treated samples were recorded using conventional Raman and micro-Raman techniques. The spectrum of as prepared sample exhibited an intense, broad peak at 301 cm-1 corresponding to the LO phonon mode. Higher order phonon modes were also observed in the spectra. A noticeable asymmetry in the Raman line shape indicated the effect of phonon confinement. Other features in the spectra are discussed in detail.

• Synthesis of a new generation of amphiphiles with multi-cryptand headgroups: A comparative study at air–water interface

A laterally non-symmetric aza cryptand has been derivatized with two hydrophobic chains to afford amphiphiles with one cryptand headgroup and two hydrophobic tails. Three such units readily attach to 1,3,5-benzenetricarbonyl trichloride, to form a new generation of amphiphilic molecules with three cryptand headgroups and six hydrophobic chains. These molecules are studied at the air–water interface in a Langmuir trough. They readily form LB-films on a number of substrates that are characterized.

• Water structure near single and multi-layer nanoscopic hydrophobic plates of varying separation and interaction potentials

We have performed a series of molecular dynamics simulations of water containing two nanoscopic hydrophobic plates to investigate the modifications of the density and hydrogen bond distributions of water in the vicinity of the surfaces. Our primary goal is to look at the effects of plate thickness, solute–solvent interaction and also interplate separation on the solvent structure in the confined region between two graphite-like plates and also near the outer surfaces of the plates. The thickness of the plates is varied by considering single and triple-layer graphite plates and the interaction potential is varied by tuning the attractive strength of the 12–6 pair interaction potential between a carbon atom of the graphite plates and a water molecule. The calculations are done for four different values of the tuning parameter ranging from fully Lennard–Jones to pure repulsive pair interactions. It is found that both the solvation characteristics and hydrogen bond distributions can depend rather strongly on the strength of the attractive part of the solute–water interaction potential. The thickness of the plates, however, is found to have only minor effects on the density profiles and hydrogen bond network. This indicates that the long range electrostatic interactions between water molecules on the two opposite sides of the same plate do not make any significant contribution to the overall solvation structure of these hydrophobic plates. The solvation characteristics are primarily determined by the balance between the loss of energy due to hydrogen bond network disruption, cavity repulsion potential and offset of the same by attractive component of the solute–water interactions. Our studies with different system sizes show that the essential features of solvation properties, e.g. wetting and dewetting characteristics for different interplate separations and interaction potentials, are also present in relatively smaller systems consisting of a few hundred atoms.

• Tuning photoluminescence of ZnS nanoparticles by silver

We report the results of investigation of the interaction of silver with presynthesized ZnS nanoparticles (NPs) that was stabilized by cetyl trimethyl ammonium bromide (CTAB). The photoluminescence properties of ZnS NPs were followed in the presence of Ag+ ions, Ag NPs and by the synthesis of Ag@ZnS core-shell nanoparticles. We observed that CTAB stabilized ZnS NPs emitted broadly in the region from 350–450 nm, when excited by 309 nm light. In the presence of Ag+ ions the emission peak intensity up to 400 nm was reduced, while two new and stronger peaks at 430 nm and 550 nm appeared. Similar results were obtained when Ag NPs solution was added to ZnS solution. However, when Ag@ZnS NPs were synthesized, the emission in the 350–450 nm region was much weaker in comparison to that at 540 nm, which itself appeared at a wavelength shorter than that of Ag+ ion added ZnS NPs. The observations have been explained by the presence of interstitial sulfur and Zn2+, especially near the surface of the nanocrystals and their interaction with various forms of silver. In addition, our observations suggest that Ag+ ions diffuse into the lattice of the preformed ZnS NPs just like the formation of Ag+ doped ZnS NPs and thus changes the emission characteristics. We also have pursued similar experiments with addition of Mn2+ ions to ZnS and observed similar results of emission characteristics of Mn2+ doped ZnS NPs. We expect that results would stimulate further research interests in the development of fluoremetric metal ion sensors based on interaction with quantum dots.

• Tuning luminescence intensity of RHO6G dye using silver nanoparticles

The photoluminescence (PL) from rhodamine (RHO6G) dye dispersed in ethanol has been studied in the presence of different amounts of citrate stabilized silver nanoparticles of size, ∼10 nm. Enhancement as well as quenching of luminescence intensity has been observed and it was found that luminescence intensity can be tuned by adding various amounts of silver nanoparticles to the RHO6G dye dispersion. The luminescence spectra of dye consist of two peaks at 440 nm and 550 nm. Peak at 440 nm shows an enhancement in intensity at all the concentrations of added silver nanoparticles with the maximum intensity for dye with 0.25 ml silver nanoparticles (82% enhancement in the luminescence intensity). PL intensity of intense peak at 550 nm of dye molecules was found to be quenched in presence of silver nanoparticles and maximum quenching was found to be 41% for the dye with 1 ml silver nanoparticles. However, for lowest concentration of silver nanoparticles viz. (0.01 ml), enhancement in intensity was observed (13% enhancement than the dye molecules). The quenching as well as enhancement in the intensity can be understood by considering the possibility of three different phenomena. It has been reported earlier that when metal nanoparticles are in close proximity to the fluorophores, quenching of luminescence occurs, whereas when metal nanoparticles are located at certain distance, enhancement in luminescence is observed. This effect has been explained by coupling of surface plasmon resonance from metal nanoparticles with fluorophore, resulting in the increase of excitation and emission rate of the fluorophore in the localized electromagnetic field. The quenching and enhancement of luminescence intensity of the dye molecules can also be explained as the transfer of electrons from dye to the silver nanoparticles and to an extent it can be attributed to the aggregation of dye molecules upon addition of silver nanoparticles.

• On the synthesis, characterization and photocatalytic applications of nanostructured TiO2

Nanocrystalline semiconducting materials are attracting much attention due to their potential applications in solar energy conversion, nonlinear optics, and heterogeneous photocatalysis. In the present investigation, we have synthesized nanostructured TiO2 photocatalysts, which have been used in the photocatalytic degradation of phenol (one of the most common water pollutants). These catalysts have been prepared through sol–gel technique using titanium tetra-isopropoxide as a raw material for synthesis. Characterization techniques such as XRD, SEM and TEM have been employed for structural/microstructural investigations. XRD results show that the as synthesized TiO2 nanopowder exhibit anatase phase, TiO2. The average sizes of the TiO2 nanopowders are ∼ 5–10 nm. The optical properties of the samples were investigated through UVvisible and fluorescence techniques. It has been observed that absorption edge corresponds to ∼ 410 nm (bandgap, ∼ 3.02 eV). The emission peak in the fluorescence spectrum at ∼ 418 nm corresponds to the bandgap energy of ∼ 2.97 eV. Concentration of phenol (initial concentration, ∼ 100 ppm) with illumination time was monitored by measuring the absorbance of pure and illuminated phenol through UV-visible spectrophotometer. Salient feature of this study relates to the fact that the present sol–gel synthesized TiO2 nanopowders have been found to be better photocatalysts for phenol degradation than the presently employed commercial TiO2 (P-25, Degussa) photocatalyst. Thus, whereas phenol concentration, with the presently synthesized TiO2 nanopowders, the concentration of phenol decreases up to ∼ 32% but for commercial TiO2 nanopowder (P-25, Degussa), it decreased only up to ∼ 25%. The improved surface area is considered as an important factor for the aforesaid decrease in phenol concentration.

• ZnO 1-D nanostructures: Low temperature synthesis and characterizations

ZnO is one of the most important semiconductors having a wide variety of applications in photonic, field emission and sensing devices. In addition, it exhibits a wide variety of morphologies in the nano regime that can be grown by tuning the growth habit of the ZnO crystal. Among various nanostructures, oriented 1-D nanoforms are particularly important for applications such as UV laser, sensors, UV LED, field emission displays, piezoelectric nanogenerator etc. We have developed a soft chemical approach to fabricate well-aligned arrays of various 1-D nanoforms like nanonails, nanowires and nanorods. The microstructural and photoluminescence properties of all the structures were investigated and tuned by varying the synthesis parameters. Field emission study from the aligned nanorod arrays exhibited high current density and a low turn-on field. These arrays also exhibited very strong UV emission and week defect emission. These structures can be utilized to fabricate efficient UV LEDs.

• Luminescence in Mn-doped CdS nanocrystals

We have synthesized Mn-doped CdS nanocrystals (NCs) with size ranging from 1.8–3 nm. Photoluminescence (PL) spectra of the doped NCs differ from that of the undoped NCs with an additional peak due to Mn 𝑑–𝑑 transitions. Electron paramagnetic resonance spectra along with X-ray absorption spectroscopy and PL spectra confirm the incorporation of Mn in the CdS lattice. The fact that emissions from surface states and the Mn 𝑑 levels occur at two different energies, allowed us to study the PL lifetime decay behaviour of both kinds of emissions.

• Effect of coordination on bond properties: A first principles study

We have used density functional theory to obtain the binding curves for a variety of hypothetical periodic structures of Al, Si, Pb, Sn and Au. Upon examining the resulting database of results for equilibrium bond lengths and radial force constants (within a nearest-neighbour model), we find that both decrease smoothly as coordination is reduced. The effect of dimensionality appears to be small. We find that the force constants at equilibrium vary as the inverse eighth power of the equilibrium bond length. We also find evidence that the force constants are sensitive only to the bond length, and not to the coordination number. We believe these results will be useful in formulating interatomic potentials, e.g., for nanosystems.

• In situ high temperature XRD studies of ZnO nanopowder prepared via cost effective ultrasonic mist chemical vapour deposition

Ultrasonic mist chemical vapour deposition (UM–CVD) system has been developed to prepare ZnO nanopowder. This is a promising method for large area deposition at low temperature inspite of being simple, inexpensive and safe. The particle size, lattice parameters and crystal structure of ZnO nanopowder are characterized by in situ high temperature X-ray diffraction (XRD). Surface morphology of powder was studied using transmission electron microscopy (TEM) and field emission electron microscope (FESEM). The optical properties are observed using UV-visible spectrophotometer. The influence of high temperature vacuum annealing on XRD pattern is systematically studied. Results of high temperature XRD showed prominent 100, 002 and 101 reflections among which 101 is of highest intensity. With increase in temperature, a systematic shift in peak positions towards lower 2𝜃 values has been observed, which may be due to change in lattice parameters. Temperature dependence of lattice constants under vacuum shows linear increase in their values. Diffraction patterns obtained from TEM are also in agreement with the XRD data. The synthesized powder exhibited the estimated direct bandgap (𝐸g) of 3.43 eV. The optical bandgap calculated from Tauc’s relation and the bandgap calculated from the particle size inferred from XRD were in agreement with each other.

• Raman spectroscopy of graphene on different substrates and influence of defects

We show the evolution of Raman spectra with a number of graphene layers on different substrates, SiO2/Si and conducting indium tin oxide (ITO) plate. The 𝐺 mode peak position and the intensity ratio of 𝐺 and 2𝐷 bands depend on the preparation of sample for the same number of graphene layers. The 2𝐷 Raman band has characteristic line shapes in single and bilayer graphene, capturing the differences in their electronic structure. The defects have a significant influence on the 𝐺 band peak position for the single layer graphene: the frequency shows a blue shift up to 12 cm-1 depending on the intensity of the 𝐷 Raman band, which is a marker of the defect density. Most surprisingly, Raman spectra of graphene on the conducting ITO plates show a lowering of the 𝐺 mode frequency by ∼ 6 cm-1 and the 2𝐷 band frequency by ∼ 20 cm-1. This red-shift of the 𝐺 and 2𝐷 bands is observed for the first time in single layer graphene.

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 6
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019