-
[show abstract]
[hide abstract]
ABSTRACT: Careful tuning of formation (calcination) temperature of Sr(2+) doped BiFeO(3) multiferroic ceramics results in tailorable particle morphologies ranging from spherical to pillar-like. Based on the minimization of Gibb's free energy approach, the dominant homogeneous mechanism for particle growth is suggested. The chemical substitution of a trivalent ion (Bi(3+)) by a divalent ion (Sr(2+)) causes the transformation of certain fraction of Fe(3+) to Fe(4+) and/or the appearance of oxygen vacancies. This has been respectively proved by the analysis of XPS and refinement of neutron diffraction data. Although significant modification in the particle morphology is observed, the crystal unit cell remains rhombohedral with a R3c space group but interesting variations in physical properties are achieved. O-vacancies induced strong and sharp photoluminescence activity in the IR region, similar to ZnO, is reported for the first time. This observation opens up a new application for multiferroic ceramics. SQUID M-H data confirms the straightening of the canted spin structure of BiFeO(3), which in turn results in magnetism similar to ferromagnetic materials. Findings of the magneto-dielectric effect are also discussed to claim the multiferroic nature of the sample.
Journal of Physics Condensed Matter 01/2013; 25(5):055303. · 2.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: SnO2 nanowires and nanobelts have been grown by the thermal evaporation of Sn powders. The growth of nanowires and nanobelts has
been investigated at different temperatures (750–1000°C). The field emission scanning electron microscopic and transmission
electron microscopic studies revealed the growth of nanowires and nano-belts at different growth temperatures. The growth
mechanisms of the formation of the nanostructures have also been discussed. X-ray diffraction patterns showed that the nanowires
and nanobelts are highly crystalline with tetragonal rutile phase. UV-visible absorption spectrum showed the bulk bandgap
value (∼ 3–6 eV) of SnO2. Photoluminescence spectra demonstrated a Stokes-shifted emission in the wavelength range 558–588 nm. The Raman and Fourier
transform infrared spectra revealed the formation of stoichiometric SnO2 at different growth temperatures.
KeywordsSnO2
-nanostructures-growth mechanism-optical properties
Bulletin of Materials Science 04/2012; 33(4):357-364. · 0.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: ZnO nanostructures were fabricated on copper substrates by hydrothermal method at an optimized growth temperature of -95 degrees C. Structural properties were investigated by field emission scanning electron and transmission electron microscopy. Distinct morphologies were found to be formed at different growth times. The formation of nanotubes mainly involved the initial nucleation followed by the growth of nanorods at 95 degrees C, and then with the increase of dissolution time at room temperature, the preferential chemical dissolution of the metastable Zn-rich [0001] polar surfaces resulted in removing the atoms from the surfaces, thus leading to the thinning of the wall of the nanostructures. Completely hollow ZnO nanotubes could be obtained at a high dissolution time. The room temperature photoluminescence and optical absorption properties of ZnO nanotubes have been studied as a function of dissolution time. The efficacy of ZnO nanotubes for glucose sensing applications has been studied.
Journal of Nanoscience and Nanotechnology 01/2012; 12(1):308-15. · 1.56 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Composite thin films of (SrBi2Ta2O9)(1−x)–(La0.67Sr0.33MnO3)x are prepared for the first time using the pulsed-laser deposition technique with ablation occurring from two individual targets. X-ray diffraction and field-emission scanning electron microscopy reveal the formation of ferromagnetic La0.67Sr0.33MnO3 embedded in the ferroelectric SrBi2Ta2O9 phase. Complex impedance spectroscopy is carried out on the composite at different temperatures. Bulk resistance calculated from the complex impedance plot decreases with the increase in temperature up to 80 °C, an observation contradicting earlier results. However, the increase in resistivity of the La0.67Sr0.33MnO3 phase with temperature as the metal-to-insulator transition temperature (80 °C) is approached explains the observation. Also, the resistivity decreases with the increase in La0.67Sr0.33MnO3 content at a particular temperature. Electric modulus, dielectric spectroscopy and ac conductivity are used to study the transport property of the films. Activation energy, from the Arrhenius plot, is studied to discuss the conduction mechanism in the composite thin films.
Journal of Physics D Applied Physics 02/2011; 44(9):095403. · 2.54 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Ge nanocrystals embedded in HfO <sub>2</sub> matrices were prepared by rf magnetron sputtering technique. Transmission electron micrographs reveal the formation of spherical shape Ge nanocrystals of 4–6 nm diameters for 800 ° C and 6–9 nm for 900 ° C annealed samples. X-ray photoelectron spectroscopy confirms the formation of surface oxidized Ge nanocrystals. Embedded Ge nanocrystals show strong photoluminescence peaks in visible and ultraviolet region even at room temperature. Spectral analysis suggests that emission in 1.58 and 3.18 eV bands originate from T <sub>Σ</sub>( T <sub>Π</sub>)→ S <sub>0</sub> , and T <sub>Π</sub><sup>′</sup>→ S <sub>0</sub> optical transitions in GeO color centers, respectively, and those in the range 2.0–3.0 eV are related to Ge/O defects at the interface of the oxidized nanocrystals. Temperature dependent photoluminescence study has revealed additional fine structures with lowering of temperature, the origin of which is attributed to the strong coupling of electronic excitations with local vibration of germanium oxides at the surface.
Journal of Applied Physics 10/2010; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Defect characterization in 1.2 MeV Ar <sup>8+</sup> irradiated polycrystalline ZnO has been carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM) along with electrical resistivity, and photoluminescence (PL) measurements at room temperature (RT). Interestingly, irradiation with the initial fluence (1×10<sup>15</sup> ions / cm <sup>2</sup>) changes the color of the sample from white to orange while the highest irradiation fluence (5×10<sup>16</sup> ions / cm <sup>2</sup>) makes it dark reddish brown that appears as black. XRD study reveals no significant change in the average grain size of the samples with irradiation fluence. Increase in surface roughness due to sputtering is clearly visible in SEM with highest fluence of irradiation. RT PL spectrum of the unirradiated sample shows intense ultraviolet (UV) emission (∼3.27 eV ) and less prominent defect level emissions (2–3 eV). The overall emission is largely quenched due to initial irradiation fluence. Increasing the fluence of Ar beam further, UV emission is enhanced along with prominent defect level emissions. Remarkably, the resistivity of the irradiated sample with highest fluence is reduced by four orders of magnitude compared to that of the unirradiated sample. This is due to an increase in donor concentration as well as their mobility induced by high fluence of irradiation. Change in color in the irradiated samples indicates dominant presence of oxygen vacancies. It is now well known that oxygen vacancies are deep donors in ZnO. So oxygen vacancies, in principle, are not the source of conductivity in ZnO at RT. Simultaneous evolution of coloration and conductivity in ZnO, as is seen in this study, indicate that oxygen vacancies strongly influence the stability of shallow donors, pr-
-
esumably zinc interstitial related (highly mobile Zn interstitials also need to form defect pair/complex to be stable), which act as major source of carriers. Such a contention is in conformity with most recent theoretical calculations.
Journal of Applied Physics 07/2010; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This paper proposes a mobile station (MS)-controlled fast MAC-layer handover (HO) scheme to reduce the HO latency in Mobile WiMAX. Based on the received signal strength (RSS) from any Base Station (BS), the MS can estimate its present distance from that BS. This capability allows the MS, through utilization of the broadcast messages by its Serving BS (SBS) and through few scannings of the Neighbouring BSs (NBS), to self-track its own direction of motion relative to the SBS and the NBSs and thus look ahead to perform an intelligent choice of the Target BS (TBS) for the impending HO. Simulation studies have shown a considerable reduction in the HO latency in the proposed method.
Wireless Communications and Networking Conference (WCNC), 2010 IEEE; 05/2010
-
[show abstract]
[hide abstract]
ABSTRACT: Hexagonal shaped radial and quasialigned arrays of ZnO nanorods with diameter of about 40–60 nm have been deposited on p -Si (100) substrates by vapor-solid method using Zn as the source material. x-ray diffraction, field emission scanning electron microscopy, temperature dependent-photoluminescence, and impedance spectroscopy have been used to characterize the structural, optical and electrical transport properties of the grown nanostructures. At room temperature, a strong free excitonic emission peak at 3.311 eV with very weak defect emissions is observed. At low temperatures, near-band-edge steady-state photoluminescence spectra of ZnO nanorods are dominated by neutral-donor-bound-exciton (D<sup>0</sup>X) transitions with corresponding transverse and longitudinal optical phonon replicas. The impedance spectra as a function of bias voltage and temperature have been studied in detail. The differences in characteristics of p -Si / ZnO / Al and Al/ZnO/Al devices are discussed by using one and two RC equivalent circuits. A comparative study of the impedance spectra for bulk ZnO and ZnO nanostructures is presented. The activation energy of ZnO nanorods is found to be 0.08 eV, which is slightly greater than the reported bulk value.
Journal of Applied Physics 08/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Multiwalled carbon nanotube-CdS/polyvinyl alcohol (MWCNT-CdS/PVA) composites have been grown by a simple chemical process on one-dimensional templates. The plane-view transmission electron micrographs clearly indicate the formation of nanocrystalline CdS on the nanotube surfaces. The superior dielectric behavior of the MWCNT-CdS nanostructures over MWCNT and PVA host matrices has been demonstrated. The dc and ac transport properties of CdS carbon nanotube-insulating polymer nanocomposites have been studied using impedance spectroscopy. An enhancement in optical band gap of nanocomposites over the bulk CdS has been observed due to the quantum confinement effect in CdS nanocrystals.
Journal of Applied Physics 07/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: CdS/Ge nanowire heterojunction has been grown by chemical deposition of CdS on Ge nanowire templates. Transmission electron micrographs show the growth of core-shell Ge/CdS nanowire radial heterostructures. Raman spectra reveal the confinement of phonons in nanocrystalline CdS shell grown on Ge nanowires. A diodelike behavior in I-V characteristics of Ge/CdS heterojunction is observed due to the formation of rectifying junction between CdS shells and Ge nanowire cores. An improved photocurrent spectrum of Ge/CdS heterojunction nanowires with broadband response from visible to near-IR region is demonstrated.
Applied Physics Letters 07/2009; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: CdS nanocomposites have been grown in polyvinyl alcohol matrix by a chemical synthesis process. The transmission electron micrographs of nanocomposites synthesized at 70–90 ° C temperature showed the growth of needlelike and junctionlike nanostructures. X-ray photoelectron spectroscopy analysis revealed the growth of stoichiometric CdS without the formation of any intermediate phases at the CdS-polyvinyl alcohol interface. Raman spectra of first order longitudinal optical phonon peak has been analyzed using phonon dispersion model to detect the surface phonon modes in CdS nanoneedles and wires. The origin of negative differential resistance behavior in current-voltage characteristics for junctionlike CdS nanocomposites has been discussed.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have synthesized Ge-based magnetic diode composed of a Mn-doped Ge film grown on lightly As-doped Ge on silicon substrate. p-Ge1−xMnx/n-Ge heterostructure behaves like a conventional diode under forward and reverse biases and works like a spin valve below Curie temperature ( ∼ 50 K) under zero (B = 0) and nonzero (B = 300 mT) magnetic fields at forward bias (+2 V). A hysteretic behavior of p-n junction current with small coercive magnetic field implies the nonvolatility of the diode. Thus, a single element of p-Ge1−xMnx/n-Ge on silicon substrate deserves nonvolatility, rectification, and spin-valve-like functionality.
Applied Physics Letters 03/2009; 94(12):122505-122505-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Polycrystalline SrBi <sub>2</sub> Ta <sub>2</sub> O <sub>9</sub> (SBT) thin films were grown on Pt / Ti / SiO <sub>2</sub>/ Si substrates by pulsed laser ablation technique. Phase analyses of the deposited films were studied by grazing incidence x-ray diffraction. Microstructural and interfaces of the SBT film were investigated using a field emission scanning electron microscope. The dc leakage current behavior was studied at different temperatures, and the current transport mechanism was investigated. The calculated activation energies from the Arrhenius plot were attributed to the shallow traps existing near the conduction band of the SBT thin films. The current-voltage plot could be clearly separated into three regions, i.e., Ohm’s law, trap-filled limited, and Child’s law. At a low electric field, the current density–voltage characteristics showed the Ohmic behavior. Lampert’s theory of space charge limited conduction was found to be suitable to explain the current conduction through SBT films. The trap-filled limited voltage increases with increasing temperature up to 100 ° C and then decreases with temperature.
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Hexagonal and preferentially oriented three-dimensional ZnO tripods have been grown on p -Si [100] substrates by a simple vapor-solid technique without using any catalysts. The tripodal-core growth route involves the formation of a hexagonal disk with [0002] larger surfaces. The tripods show irreversible shape transformation to tetrapods at higher temperature and prolonged growth time. Temperature-dependent photoluminescence characteristics of ZnO tripods have been investigated in the range from 10 to 300 K. Multiple LO phonon-assisted emissions associated with both bound and free excitons are observed for tripods, the origin of which have been explained by Permogorov’s theory for II–VI semiconductors.
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Building large high-speed multi-field packet classifiers is an acknowledged challenge to researchers. PC rules specify ranges, IP prefixes or wildcards. Current PCs inefficiently search a range as multiple prefixes. The present paper proposes generalized hardware-specific wire-speed direct search of ranges and wildcards. Associative search tables, created through preprocessing of the rule database, are searched using RAM-based pipelined CAMs (PCAM) which are likely to provide a better alternative to TCAMs. The proposed range-based search engine has constant search time, irrespective of rule database sizes and field value widths.
Communication Systems and Networks and Workshops, 2009. COMSNETS 2009. First International; 02/2009
-
[show abstract]
[hide abstract]
ABSTRACT: Ge nanowires were grown on Au-coated Si (100) substrates using vapor-liquid-solid technique. Temperature dependent photoluminescence spectra of Mn doped nanowires showed dominant impurity emissions due to transition from an intermediate <sup>4</sup>T<sub>1</sub> to <sup>6</sup>A<sub>1</sub> ground state of Mn <sup>2+</sup> in Ge nanowire. Magnetic hysteresis loop showed paramagnetic to ferromagnetic transition with decreasing temperature. Temperature dependent electrical resistivity indicated the formation of Mn related acceptor level in Ge at 0.159 eV from the valence band.
Journal of Applied Physics 02/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have investigated the interfacial and frequency dependent electrical properties of metal-ferroelectric-insulator-semiconductor capacitors with SrBi <sub>2</sub> Ta <sub>2</sub> O <sub>9</sub> (SBT) ferroelectric films grown on ZrO <sub>2</sub> buffer layer coated Si. Heterostructure SBT and ZrO <sub>2</sub> thin films were deposited using rf magnetron sputtering. Interfacial and surface roughness parameters of heterostructures were extracted from the simulation of specular x-ray reflectivity data. The structure exhibited clockwise capacitance-voltage hysteresis with a maximum memory window of 2.0 V at a bias voltage of ±7 V . Frequency dependent (5 kHz–1 MHz) measurements at room temperature indicated that the clockwise hysteresis originates from the ferroelectric domain reversal. A minimum leakage current density of 4×10<sup>-8</sup> A / cm <sup>2</sup> of fabricated capacitors at an applied voltage of ±5 V revealed that the ZrO <sub>2</sub> buffer layer prevents the interfacial diffusion between SBT thin film and the substrate, resulting in an improvement of interface quality. The charge retention time of the ferroelectric capacitor was studied as a function of buffer layer thickness.
Journal of Applied Physics 10/2008; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Self-assembled Ge islands have been grown using a Stranski–Krastanov growth mechanism on Si (001) substrates by solid source molecular beam epitaxy. We performed time varying annealing experiments at a representative temperature of 650 ° C to study the shape and size evolution of islands for a relatively high Ge coverage. Islands are found to coarsen due to heat treatment via structural and compositional changes through continuous strain relaxation. Different island morphologies, namely, “pyramids,” “transitional domes,” and “domes” are found during the annealing sequence. The dominant coarsening mechanisms for the temporal evolution of islands of as-grown and annealed samples are explained by the comprehensive analysis of Rutherford back scattering, Raman spectroscopy, high-resolution x-ray diffraction, and atomic force microscopy. A correlation of the morphological evolution with the composition and strain relaxation of grown islands is presented.
Journal of Applied Physics 07/2008; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Ge nanocrystals (NCs) embedded in silicon dioxide ( SiO <sub>2</sub>) matrix are grown by radio-frequency magnetron sputtering and studied in order to understand the origin of ultraviolet (UV) and blue photoluminescence (PL) from the NC-SiO <sub>2</sub> system. Ge NCs of diameter 7–8 nm are formed after postdeposition annealing, as confirmed by transmission electron microscopy and Raman scattering studies. Optical Raman studies indicate the presence of strain in the embedded Ge NCs. Polarization dependent low frequency Raman studies reveal surface symmetrical and surface quadrupolar acoustic phonon modes of Ge NCs. PL studies with 488 nm excitation shows a broad emission band peaked at ∼545 nm , which is attributed to oxygen deficient defects in the SiO <sub>2</sub> matrix. PL studies with 325 nm excitation show additional strong peaks in the 377–400 nm region. Time resolved PL studies in the UV-blue range show double exponential decay dynamics in the nanosecond time scale, irrespective of the NC size. Comparative studies of PL emission from SiO <sub>2</sub> layers with no Ge content and with Ge content show that the ∼400 nm PL emission is originated from a defective NC / SiO <sub>2</sub> interface and the band is not unique to the presence of Ge. PL excitation spectroscopy measurements show large Stokes shift for the UV emission bands. We propose that the intense UV peaks at ∼377 nm is originated from the twofold coordinated silicon defect at the interface between NC and SiO <sub>2</sub> matrix and it is not necessarily specific to the presence of Ge in the oxide matrix. It is believed that due to-
the influence of strain on the NCs and interface states, PL from quantum confined carriers may be partially quenched for the embedded Ge NCs.
Journal of Applied Physics 06/2008; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Ferroelectric SrBi2Ta2O9 (SBT) thin films have been deposited by the radio-frequency magnetron sputtering technique on bare p-Si as well as on HfO2 insulating buffer p-Si. XRD patterns revealed the formation of a well-crystallized SBT perovskite thin film on the HfO2 buffer layer. The electrical properties of the metal–ferroelectric–insulator–semiconductor (MFIS) structure were characterized by varying thicknesses of the HfO2 layer. The MFIS structure exhibits a maximum clockwise C–V memory window of 1.60 V when the thickness of the HfO2 layer was 12 nm with a lower leakage current density of 6.20 × 10−7 A cm−2 at a positive applied voltage of 7 V. However, the memory window reaches a maximum value of 0.7 V at a bias voltage of ±5 and then decreases due to charge injection in the case of the insulating buffer layer thickness of 3 nm. The density of oxide trapped charges at/near the buffer layer–ferroelectric interface is studied by the voltage stress method. Capacitance–voltage (C–V) and leakage current density (J–V) characteristics of the Al/SBT/HfO2/Si(1 0 0) capacitor indicate that the introduction of the HfO2 buffer layer prevents interfacial diffusion between the SBT thin film and the Si substrate effectively and improves the interface quality. Furthermore, the Al/SBT/HfO2/Si structures exhibit excellent retention characteristics, the high and low capacitance values clearly distinguishable for over 1 h and 30 min. This shows that the proposed Al/SrBi2Ta2O9/HfO2/Si structure is ideally suitable for high performance ferroelectric memories.
Journal of Physics D Applied Physics 04/2008; 41(9):095408. · 2.54 Impact Factor
