Anjana Asthana

In situ observation of size-scale effects on the mechanical properties of ZnO nanowires

In this investigation, the size-scale in mechanical properties of individual [0001] ZnO nanowires and the correlation with atomic-scale arrangements were explored via in situ high-resolution transmission electron microscopy (TEM) equipped with atomic force microscopy (AFM) and nanoindentation (NI) systems. The Youngs modulus was determined to be size-scale-dependent for nanowires with diameter, d, in the range of 40 nm ≤ d ≤ 110 nm, and reached the maximum of ∼ 249 GPa for d = 40 nm. However, this phenomenon was not observed for nanowires in the range of 200 nm ≤ d ≤ 400 nm, where an average constant Youngs modulus of ∼ 147.3 GPa was detected, close to the modulus value of bulk ZnO. A size-scale dependence in the failure of nanowires was also observed. The thick ZnO nanowires (d ≥ 200 nm) were brittle, while the thin nanowires (d ≤ 110 nm) were highly flexible. The diameter effect and enhanced Youngs modulus observed in thin ZnO nanowires are due to the combined effects of surface relaxation and long-range interactions present in ionic crystals, which leads to much stiffer surfaces than bulk wires. The brittle failure in thicker ZnO wires was initiated from the outermost layer, where the maximum tensile stress operates and propagates along the (0001) planes. After a number of loading and unloading cycles, the highly compressed region of the thinner nanowires was transformed from a crystalline to an amorphous phase, and the region near the neutral zone was converted into a mixture of disordered atomic planes and bent lattice fringes as revealed by high-resolution images.

In situ probing of electromechanical properties of an individual ZnO nanobelt

We report here, an investigation on electrical and structural-microstructural properties of an individual ZnO nanobelt via in situ transmission electron microscopy using an atomic force microscopy (AFM) system. The I-V characteristics of the ZnO nanobelt, just in contact with the AFM tip indicates the insulating behavior, however, it behaves like a semiconductor under applied stress. Analysis of the high resolution lattice images and the corresponding electron diffraction patterns shows that each ZnO nanobelt is a single crystalline, having wurtzite hexagonal structure (a=0.324 nm, c=0.520 66 nm) with a general growth direction of [101¯0].

Investigations on the structural disordering of neutron-irradiated highly oriented pyrolytic graphite by X-ray diffraction and electron microscopy

Light and heavy neutron-irradiation damage of highly oriented pyrolytic graphite (HOPG) crystals was examined by means of X-ray diffraction and high-resolution high-voltage transmission electron microscopy (TEM). From the X-ray data analysis, it was found that there is an average increase of about 3% in the c-axis lattice parameter of the unit cell of graphite for lightly neutron-irradiated HOPG. However, the c-axis lattice parameter could not be estimated from the HOPG sample having the highest dose of neutron irradiation under the present investigation, because the X-ray profile was highly asymmetrical. This increase in the c-axis lattice parameter is attributed to lattice expansion due to the static displacement of atoms after neutron irradiation. Local structure analysis by TEM shows that the 0002 lattice spacing for the above-mentioned HOPG samples has been increased by up to 10% as a result of the neutron irradiation. This increase in c-axis lattice spacing can be ascribed to the fragmentation of the crystal lattice into nanocrystallites, breaking and bending of the 0002 straight lattice fringes, appearance of dislocation loops, and extra interstitial planes within the fragmented nanocrystallites. All these changes are a result of the static displacement of atoms after neutron irradiation.

Multicolored and white-light phosphors based on doped GdF3 nanoparticles and their potential bio-applications

Rare-earth-doped gadolinium fluoride nanocrystals were synthesized by a single step synthesis employing ethylene glycol as solvent. Based on X-ray diffraction studies, stabilization of hexagonal modification of GdF(3) has been inferred. The microscopic studies show formation of uniformly distributed nanocrystals (~15 nm). The nanoparticles are readily dispersible in water and show bright luminescence in colloidal solution. The luminescence properties have been investigated as a function of activator concentrations, and enhanced optical properties have been attributed to efficient energy transfer from the Gd(3+) to the activator RE(3+) ions, which has further been confirmed by steady-state and time-resolved optical studies. It has been demonstrated that on doping appropriate amount of activators in host GdF(3), a novel white-light-emitting phosphor is obtained with CIE co-ordinates and correlated color temperature (CCT) very close to broad daylight. This can have promising applications as phosphor for white-light ultraviolet-light-emitting diodes (UV-LEDs). Our experiments showed efficient labeling of human breast carcinoma cells (MCF-7) by Tb(3+)-doped GdF(3) nanoparticles. The fluorescence intensity was found to be dependent on the surface modifying/coating agent, and the results were validated using confocal microscopy in terms of localization of these functionalized nanoparticles.

Real time observation of mechanically triggered piezoelectric current in individual ZnO nanobelts

The detection of piezoelectric current in one-dimensional semiconductor materials has been a controversial issue due to the possibility of charge annihilation at nanoscale dimensions. We report here, the mechanically triggered electrical current in uniaxially compressed individual ZnO nanobelts under no applied bias. The measurements were carried out in situ by using a transmission electron microscope. In contrast to the bending mode, the magnitude of the electrical current increased with the increase of uniaxial compression, which indicates the load-mode dependency of the detected current. The flow of electrical current through the ZnO nanobelts under applied stress was explained based on the separation of ionic charges along the two ends of the nanobelts due to the applied compressive force. The charge separation is expected to induce an internal electric field inside the nanobelt and facilitate the movement of free charge carriers through the nanobelt. Due to the separation and accumulation of charges, the metal–semiconductor system becomes forward biased when contact is established, leading to the flow of electrons through the Schottky barrier.

Structural-microstructural characteristics and its correlations with the superconducting properties of in situ PIT-processed MgB2 tapes with ethyltoluene and SiC powder added

We performed structural?microstructural investigations of pure MgB2, ethyltoluene and both ethyltoluene-?and SiC-added MgB2/Fe tapes. The analysis of the microstructure shows that the grain size for the pure and ethyltoluene-added MgB2 tape sample is in the range of 10?100?nm. However, with the addition of both ethyltoluene and SiC, the grain size decreases to about 5?80?nm. The a-axis length of the ethyltoluene-added tape samples is slightly decreased, whereas for both ethyltoluene-?and SiC-added samples, the a-axis length is decreased by 0.4% as compared to the pure MgB2 tape sample, showing the amount of carbon substitution is less in ethyltoluene-added tape samples. The reason for the higher Jc values of the ethyltoluene-added MgB2 tape sample as compared to the pure MgB2 tapes is the presence of a lesser amount of the impurity phase, MgO. The large improvement in Jc?B properties for the ethyltoluene-?and SiC-added MgB2 tape sample is attributed to (1)?the enhancement of upper critical field, Bc2, by the substitution of carbon for boron, (2)?pinning by nanosized (5?20?nm) particles of Mg2Si and other silicides, (3)?enhanced grain boundary pinning due to the smaller grain size and (4)?the presence of a lesser amount of impurity phase, MgO, as compared to the pure sample.

A study on the modulation of the electrical transport by mechanical straining of individual titanium dioxide nanotube

We report here, the deformation driven modulation of the electrical transport properties of an individual TiO2 nanotube via in situ transmission electron microscopy (TEM) using a scanning tunneling microscopy system. The current-voltage characteristics of each individual TiO2 nanotube revealed that under bending deformation within the elastic limit, the electrical conductivity of a TiO2 nanotube can be enhanced. High resolution TEM and electron diffraction pattern reveal that TiO2 nanotubes have tetragonal structure (a=0.378 nm, c=0.9513 nm, I41/amd). Analysis based on a metal-semiconductor-metal model suggests that in-shell, surface defect-driven conduction modes and electron–phonon coupling effect are responsible for the modulated semiconducting behaviors.

New Flexible Channels for Room Temperature Tunneling Field Effect Transistors

Tunneling field effect transistors (TFETs) have been proposed to overcome the fundamental issues of Si based transistors, such as short channel effect, finite leakage current, and high contact resistance. Unfortunately, most if not all TFETs are operational only at cryogenic temperatures. Here we report that iron (Fe) quantum dots functionalized boron nitride nanotubes (QDs-BNNTs) can be used as the flexible tunneling channels of TFETs at room temperatures. The electrical insulating BNNTs are used as the one-dimensional (1D) substrates to confine the uniform formation of Fe QDs on their surface as the flexible tunneling channel. Consistent semiconductor-like transport behaviors under various bending conditions are detected by scanning tunneling spectroscopy in a transmission electron microscopy system (in-situ STM-TEM). As suggested by computer simulation, the uniform distribution of Fe QDs enable an averaging effect on the possible electron tunneling pathways, which is responsible for the consistent transport properties that are not sensitive to bending.

Unusual magnetic properties of Mn-doped ThO2 nanoparticles

We report the synthesis of Th 1– x Mn x O 2 ( x = 0, 0.001, 0.002, 0.004, and 0.01 wt%) nanoparticles by the urea combustion method using thorium nitrate gel followed by heat treatment at a higher temperature ( T ). The obtained Th 1– x Mn x O 2 nanocrystals were characterized by x-ray diffraction (XRD), direct-current magnetization ( M ) measurements and electron paramagnetic resonance (EPR). XRD analysis revealed that Th 1– x Mn x O 2 crystallizes in the cubic structure ( Fm 3 m ). M measurements showed ferromagnetic ordering at room temperature for Th 0.99 Mn 0.01 O 2 samples annealed at 775 K. An intense and broad ferromagnetic resonance (FMR) having linewidth of ∼1200 G, was observed at relatively lower fields in the EPR spectra of Th 0.99 Mn 0.01 O 2 samples annealed at 775 K, indicating the presence of a ferromagnetic phase at room temperature. EPR measurements were used to estimate the number of spins involved in the ferromagnetic ordering. Out of the total Mn present in Th 0.99 Mn 0.01 O 2 samples, about 25% of the Mn 2+ ions were found to be responsible for the ferromagnetic ordering. In addition to the FMR signal, a weak hyperfine sextet was observed at g = 2.0048 ( 55 Mn, I = 5/2), which corresponds to the −1/2 ↔ +1/2 transition of Mn 2+ ions, suggesting its presence at thorium sites (uncoupled spins). X-ray photoelectron spectra indicated that the manganese ions exist mainly as Mn 2+ , Mn 3+ , and Mn 4+ . The room-temperature ferromagnetism may be attributed to the coupling between these Mn 2+ ions at thorium sites in ThO 2 rather than due to the formation of any metastable secondary phases.

Improved superconducting properties of MgB2 bulk materials prepared by sintering

We report here the improvement of superconducting properties of MgB2 bulk samples by sintering at ambient pressure, employing RF induction heating. It has been found that Tc (R = 0) transition temperature of the sintered sample gets enhanced to ~40.00 K as compared to Tc (R = 0) ~25 K for the unsintered MgB2 sample. A high critical current density of ~5.45×104 A cm−2 (20 K) has been achieved for a sintered MgB2 sample whereas for an unsintered sample, the Jc is found to be ~1.20×103 A cm2 (20 K). Detailed microstrucural investigations by transmission electron microscope (TEM) reveal that most of the intergrain interfaces correspond to low angle grain boundaries. Based on TEM explorations, these boundaries have been recognized to be dominantly low angle tilt boundaries. Surface microstructural features of the MgB2 sample as investigated by scanning electron microscopy reveal a well-connected uniform grain structure for the sintered samples.