A scanning tunneling microscopy and potentiometry study of epitaxial thin films of La0.7Ca0.3MnO3

To investigate the role of grain boundaries and other growth related microstructure in manganite films, a scanning tunneling microscope is used to simultaneously probe surface topography and local potential distribution under current flow at nanometer level in films of epitaxial thin films of La0.7Ca0.3MnO3 deposited on single crystal SrTiO3 and NdGaO3 substrate by laser ablation. We have studied two types of films strained and strain relaxed. Thin (50nm) films (strained due to lattice mismatch between substrate and the film) show step growth (unit cell steps) and have very smooth surfaces. Relatively thicker films (strain relaxed, thickness 200nm) do not have these step growths and show rather smooth well connected grains. Charge transport in these films is not uniform on the nanometer level and is accompanied by potential jumps at the internal surfaces. In particular scattering from grain boundaries results in large variations in the local potential resulting in fields as high as 104-105V/cm located near the grain boundaries. We discuss the role of local strain and strain inhomogeneties in determining the current transport in these films and their resistance and magnetoresistivity. In this paper we attempt to correlate between bulk electronic properties with microscopic electronic conduction using scanning tunneling microscopy and scanning tunneling potentiometry.

The effect of host glass on optical absorption and fluorescence of Nd3+ in xNa(2)O-(30-x)K2O-70B(2)O(3) glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Nd3+ has been studied in mixed alkali borate glasses of the type xNa(2)O-(30-x)K2O-69.5B(2)O(3)-0.5Nd(2)O(3) (X = 5,10,15,20 and 25). Various spectroscopic parameters such as Racah (E-1, E-2 and E-3), spin-orbit (xi(4f)) and configuration interaction (alpha, beta) parameters have been calculated. The Judd-Ofelt intensity parameters (Omega(lambda)) have been calculated and the radiative transition probabilities (A(rad)), radiative lifetimes (tau(r)), branching ratios (beta) and integrated absorption cross sections (Sigma) have been obtained for certain excited states of the Nd3+, ion and are discussed with respect to x. From the fluorescence spectra, the effective fluorescence line widths (Deltalambda(eff)) and stimulated emission cross sections (sigma(p)) have been obtained for the three transitions F-4(3/2) --> I-4(9/2), F-4(3/2) --> I-4(11/2) and F-4(3/2) --> I-4(13/2) of Nd3+. The stimulated emission cross section (sigma(p)) values are found to be in the range (2.0-4.8) x 10(-2)0 cm(2) and they are large enough to indicate that the mixed alkali borate glasses could be potential laser host materials.

Fluorescence quenching of biologically active carboxamide by aniline and carbon tetrachloride in different solvents using Stern-Volmer plots

Fluorescence quenching of biologically active carboxamide namely (E)-2-(4-chlorobenzylideneamino)-N-(2-chlorophenyl)-4,5,6,7-tetrahydrobe nzo[b]thiophene-3-carboxamide [ECNCTTC] by aniline and carbon tetrachloride (CCl(4)) quenchers in different solvents using steady state method and time resolved method using only one solvent has been carried out at room temperature to understand the role of quenching mechanisms. The Stern-Volmer plot has been found to be linear for all the solvents studied. The probability of quenching per encounter p (p') was determined in all the solvents and was found to be less than unity. Further, from the studies of rate parameters and life time measurements in n-heptane and cyclohexane with aniline and carbon tetrachloride as quenchers have been shown that, the phenomenon of quenching is generally governed by the well-known Stern-Volmer (S-V) plot. The activation energy E(a) (or E(a)') of quenching was determined using the literature values of activation energy of diffusion E(d) and the experimentally determined values of p (or p'). It has been found that, the activation energy E(a) (E(a)') is greater than the activation energy for diffusion E(d) in all solvents. Hence, from the magnitudes of E(a) (or E(a)') as well as p (or p') infer that, the quenching mechanism is not solely due to the material diffusion, but there is also contribution from the activation energy. (C) 2011 Elsevier B.V. All rights reserved.

Effect of oxygen pressure on the grain and domain structure of polycrystalline 0.85PbMg(1/3)Nb(2/3)O(3)-0.15PbTiO(3) thin films studied by scanning probe microscopy

0.85PbMg(1/3)Nb(2/3)O(3)-0.15PbTiO(3) ferroelectric-relaxor thin films have been deposited on La(0.5)nSr(0.5)CoO(3)/(1 1 1) Pt/TiO(2)/SiO(2)/Si by pulsed laser ablation at various oxygen partial pressures in the range 0.05 to 0.4 Torr. All the films have a rhombohedral perovskite structure. The grain morphology and orientation are drastically affected by the oxygen pressure, studied by x-ray diffraction and scanning electron microscopy. The domain structure investigations by dynamic contact electrostatic force microscopy have revealed that the distribution of polar nanoregions and their dynamics is influenced by the grain morphology, orientation and more importantly, oxygen vacancies. The correlation length extracted from autocorrelation function images has shown that the polarization disorder decreases with oxygen pressure up to 0.3 Torr. The presence of polarized domains and their electric field induced switching is discussed in terms of internal bias field and domain wall pinning. Film deposited at 0.4 Torr presents a curious case with unique triangular grain morphology and large polarization disorder.

Enhanced endocytosis of nano-curcumin in nasopharyngeal cancer cells: An atomic force microscopy study

Recent studies in drug development have shown that curcumin can be a good competent due to its improved anticancer, antioxidant, anti-proliferative, and anti-inflammatory activities. A detailed real time characterization of drug (curcumin)-cell interaction is carried out in human nasopharyngeal cancer cells using atomic force microscopy. Nanocurcumin shows an enhanced uptake over micron sized drugs attributed to the receptor mediated route. Cell membrane stiffness plays a critical role in the drug endocytosis in nasopharyngeal cancer cells. (C) 2011 American Institute of Physics. [doi:10.1063/1.3653388]

Functionalized and Postfunctionalizable Porous Polymeric Films through Evaporation-Induced Phase Separation Using Mixed Solvents

Condensation of water droplets during rapid evaporation of a polymer solution, under humid conditions, has been known to generate uniformly porous polymer films. Similar porous films are also formed when a solution of the polymer in THF containing small amounts of water, is allowed to evaporate rapidly under air flow; this suggests that water droplets may be formed during the final stages of film formation. In the presence of added surfactants, the interface of water droplets could become lined with the surfactants and consequently the internal walls of the pores generated, upon removal of the water, could become decorated with the hydrophilic head groups of the surfactant molecules. In a series of carefully designed experiments, we have examined the effect of added surfactants, both anionic and cationic, on the formation of porous PMMA films; the films were prepared by evaporating a solution of the polymer in THF containing controlled amounts of aqueous surfactant solutions. We observed that the average size of the pores decreases with increasing surfactant concentration, while it increases with increasing amounts of added water. The size of the pores and their distribution were examined using AFM and IR imaging methods. Although IR imaging possessed inadequate resolution to confirm the presence of surfactants at the pore surface, exchange of the inorganic counterion, such as the sodium-ion of SDS, with suitable ionic organic dyes permitted the unequivocal demonstration of the presence of the surfactants at the interface by the use of confocal fluorescence microscopy.

Nanostructure of giant magnetoresistive oxide film Nd2/3Sr1/3MnO3 by scanning tunneling microscopy

Scanning tunneling microscopy was used to study the surface nanostructure of the epitaxial film Nd2/3Sr1/3MnO3 that shows giant magnetoresistance. The surface morphology of the film consists of a number of overlapping platelets of about 30–35 Å diameter that grow at an angle of 35°–45° to the surface normal. The peak to peak height of the platelets are multiples of the c‐axis lattice parameter of 7.85 Å showing that the growth of the platelets takes place by the layer by layer addition of one formula unit. The mean surface roughness is about 10 Å. In the range of a few microns the film exhibits no defects or dislocations. The film is unstable in ambient atmosphere and tends to get covered by an adsorbate layer. Tip‐surface interactions cause the adsorbate to be dislodged exposing the surface nanostructure. The degradation of the film in real time when imaged in air was recorded. The adsorbates increase the surface roughness of the film.

The fluorescence of Nd3+ in lead borate and bismuth borate glasses with large stimulated emission cross section

The stimulated emission cross section σp for the 1060 nm transition of Nd3+ in lead borate and bismuth borate glasses has been determined from fluorescence measurements. The compositional dependence of σp, which has been evaluated using radiative transition probability, refractive index of the host glass, effective fluorescence linewidth, and position of the band, with PbO/Bi2O3 content is investigated. The σp values of the 1060 nm band of Nd3+ for lead borate and bismuth borate glasses are found to be in the range 2.6–5.7×10−20 cm2 at 298 K and 3.0–6.3×10−20 cm2 at 4.2 K. The σp values are comparatively large suggesting the possible utilization of these materials in laser applications.

Analysis of integrated optofluidic lab-on-a-chip fluorescence biosensor based on transmittance of light through a fluidic gap

Sensitivity analysis is an important aspect to be looked into while designing lab-on-a-chip systems. In this paper we will be showing with appropriate design that the best sensitivity of the fluorescence biosensor is achieved for an optimal width of fluidic gap, corresponding to a particular mode spot size. We will be also showing that the sensitivity of the biosensor is affected by efficiency of light coupling, which is influenced by changes in the width of fluidic gap, refractive index of the fluid and higher order modes.

Spatial filter based 3D resolution improvement and polarization properties of multiphoton multiple-excitation-spot-optical microscopy

Three-dimensional (3D) resolution improvement in multi-photon multiple-excitation-spot-optical microscopy is proposed. Specially designed spatial filter is employed for improving the overall 3D resolution of the imaging system. An improvement up to a factor of 14.5 and sub-femto liter volume excitation is achieved. The system shows substantial sidelobe reduction (<4%) due to the non-linear intensity dependence of multiphoton process. Polarization effect on x-oriented and freely rotating dipoles shows dramatic change in the field distribution at the focal-plane. The resulting point-spread function has the ability to produce several strongly localized polarization dependent field patterns which may find applications in optical engineering and bioimaging.