Department of Physics

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We report on the fabrication and characterization of iron oxide nanoparticle thin film superlattices. The formation into different film morphologies is controlled by tuning the particle plus solvent-to-substrate interaction. It turns out that the wetting vs dewetting properties of the solvent before the self-assembly process during solvent evaporation plays a major role in determining the resulting film morphology. In addition to layerwise growth three-dimensional mesocrystalline growth is also evidenced. The understanding of the mechanisms ruling nanoparticle self-assembly represents an important step towards the fabrication of novel materials with tailored optical, magnetic or electrical transport properties. © 2014 IOP Publishing Ltd.

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Nanotechnology

Growth modes of nanoparticle superlattice thin films

A recent breakthrough in nanotechnology provides a great extent in sensor fabrication and application. The technology has emerged as a powerful technique to minimize the size of devices; amount of materials, energy and time consumption. Nanogap based sensor is one of the sensor that capable of characterizing and quantifying molecules selectively and sensitively with good electrical behavior. In this manuscript, we present a collaboration work between UniMAP, MARDI and UPM in the process development of 40 nm silicon nanogap for sensor application. The process consists of a combination of electron beam lithography (EBL) method and conventional photolithography method. Both methods were for nanogap and electrodes pattern respectively. Silicon on insulator (SOI) substrate was used to fabricate the nanogap structure and gold was used for the electrode. The ability of EBL system to fabricate a gap in nanometer scale with direct lithography technique on SOI substrate gives advantages in this development work. The developed silicon nanogap device was physically characterized with scanning electron microscope (SEM). The sensor application was accomplished by testing the device with different level of pH solutions using a dielectric analyzer. © 2014 IEEE.

IEEE International Conference on Semiconductor Electronics, Proceedings, ICSE

Process development of 40 nm silicon nanogap for sensor application

Nonlinear optical transmission of graphene oxide-(Cu, Zn, Sn, H 2) porphyrin composites was investigated using the Z-scan technique at 532 nm with picosecond (ps) and 800 nm with femtosecond laser pulses. Pure porphyrins show saturable absorption (SA) in reverse saturable absorption (RSA) behaviour and graphene oxide shows complete RSA behaviour, observed in an open aperture Z-scan curve. Interestingly, composites have shown a switch-over from reverse RSA to SA and back to RSA behaviour, observed with variation of intensity towards the focus, due to strong two-photon absorption as well as excited state absorption in the ps regime. This switching behaviour was interpreted as due to long lifetimes and saturation of the excited states. This may find application in optical switching. © 2014 IOP Publishing Ltd.

Journal of Optics (United Kingdom)

Optical transmission control in graphene oxide and its organic composites with ultrashort laser pulses

Ferromagnetic BiFeO3 nanocrystals of average size 11 nm were used to form nanocomposites (x)BiFeO3/(100 - x)NiO, x = 0, 20, 40, 50, 60, 80, and 100 by simple solvothermal process. The ferromagnetic BiFeO 3 nanocrystals embedded in antiferromagnetic NiO nanostructures were confirmed from X-ray diffraction and transmission electron microscope studies. The modification of cycloidal spin structure of bulk BiFeO3 owing to reduction in particle size compared to its spin spiral wavelength (62 nm) results in ferromagnetic ordering in pure BiFeO3 nanocrystals. High Neel temperature (TN) of NiO leads to significant exchange bias effect across the BiFeO3/NiO interface at room temperature. A maximum exchange bias field of 123.5 Oe at 300 K for x = 50 after field cooling at 7 kOe has been observed. The exchange bias coupling causes an enhancement of coercivity up to 235 Oe at 300 K. The observed exchange bias effect originates from the exchange coupling between the surface uncompensated spins of BiFeO 3 nanocrystals and NiO nanostructures. © 2014 AIP Publishing LLC.

Journal of Applied Physics

Exchange bias effect in BiFeO3-NiO nanocomposite

Investigation is focused on the effect of different parametric controls like driving frequency, amplitude of the alternating current (ac), direct current (dc) bias field and anisotropy field (Hk) on the Giant Magneto-Impedance (GMI) properties of rapidly quenched soft magnetic Co66Fe2Si13B15Cr4 amorphous wires developed by in-water quenching technique. Optimal controls of these parameters like driving frequency and amplitude of the ac were found to display a sensitive change in GMI properties at optimal frequency of 5 MHz and 5 mA driving current. The influence of these controls on the output factors like maximum GMI ratio (GMImax), transition from single peak (SP) to double peak (DP), field sensitivity of GMI (η) and anisotropy field (Hk) throw light on the application of the developed wires for sensors. In addition to high Hk, driving frequency of 5 MHz also delivered a combination of desirable sensor properties like high level of sensitivity and its enhanced span of stability. Joule heating further improved the GMI response of the material through current annealing at a current density of 5 A/mm2. © 2014 Elsevier B.V. All rights reserved.

Sensors and Actuators, A: Physical

Parametric controls on giant magnetoimpedance (GMI) behaviour of CoFeSiBCr amorphous wires for prospective sensor applications

Solid solution of nickel ferrite (NiFe2O4) and barium titanate (BaTiO3), (100-x)BaTiO3-(x) NiFe 2O4 has been prepared by solid state reaction. Compressive strain is developed in NiFe2O4 due to mutual structural interaction across the interface of NiFe2O4 and BaTiO 3 phases. Quantitative analysis of X-ray diffraction and X-ray photo electron spectrum suggest mixed spinel structure of NiFe2O 4. A systematic study of composition dependence of composite indicates BaTiO3 causes a random distribution of Fe and Ni cations among octahedral and tetrahedral sites during non-equilibrium growth of NiFe2O4. The degree of inversion decreases monotonically from 0.97 to 0.75 with increase of BaTiO3 content. Temperature dependence of magnetization has been analyzed by four sublattice model to describe complex magnetic exchange interactions in mixed spinel phase. Curie temperature and saturation magnetization decrease with increase of BaTiO 3 concentration. Enhancement of strain and larger occupancy of Ni2+ at tetrahedral site increase coercivity up to 200Oe. Magnetostructual coupling induced by BaTiO3 improves coercivity in NiFe2O 4. An increase in the demagnetization and homogeneity in magnetization process in NiFe2O4 is observed due to the interaction with diamagnetic BaTiO3. © 2014 AIP Publishing LLC.

Magnetic properties of mixed spinel BaTiO3-NiFe 2O4 composites

The failure of two superheater tubes (T1&T2) is analyzed in the present investigation. The tubes are fractured perpendicular to tube length and bowed down towards the fire side without any brittleness, as examined through naked eye as well as tensile testing. There is no material loss from inside of tubes. Outer surface wall is thinned down by erosion and corrosion mechanism through fly ash deposits. Subsequently, the non-linear heating leads the catastrophic failure after short service period. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.

Procedia Engineering

Analysis of superheater boiler tubes failed through non-linear heating

The Anderson-Evans cluster, {CrMo6O24}n-, exhibits ruby-like red emission. During our attempts to engineer new photoluminescent crystals using chromium molybdate as a molecular building block, we isolated four new rare-earth based chromium molybdates: [Gd(H 2O)7{CrMo6(OH)6O18}] ·4H2O, 1, (4-pycH)[Gd(H2O)4{CrMo 6(OH)6O18}(4-pycH)]·5H2O, 3, and (4-pycH)2[Ln(H2O)6{CrMo6(OH) 6O18}]·2H2O, (Ln = Tb, 4 and Sm, 5; H4-pyc = 4-pyridine carboxylic acid and 4-pycH = 4-pyridinium carboxylate). All are nonmolecular solids characterized by extended 1-D or 2-D interactions between rare-earth ions and the chromium molybdate cluster. All solids showed intense room-temperature Cr3+ related red emission and also exhibited weak antiferromagnetism at low temperatures. This journal is © the Partner Organisations 2014.

CrystEngComm

Photoluminescent chromium molybdate cluster coordinated with rare earth cations: Synthesis, structure, optical and magnetic properties

Many varieties of layered inorganic-organic (IO) perovskite of type (R-NH3)2MX4 (where R: organic moiety, M: divalent metal, and X: halogen) were successfully fabricated and characterized. X-ray diffraction data suggest that these inorganic and organic structures are alternatively stacked up along c-axis, where inorganic mono layers are of extended corner-shared MX6 octahedra and organic spacers are the bi-layers of organic entities. These layered perovskites show unusual room-temperature exciton absorption and photoluminescence due to the quantum and dielectric confinement-induced enhancement in the exciton binding energies. A wide spectral range of optical exciton tunability (350 to 600 nm) was observed experimentally from systematic compositional variation in (i) divalent metal ions (M = Pb2+, Sn2+, Hg2+), (ii) halides (X = I- and Br-), and (iii) organic moieties (R). Specific photoluminescence features are due to the structure of the extended MX 2- 4 network and the eventual electronic band structure. The compositionally dependent photoluminescence of these IO hybrids could be useful in various photonic and optoelectronic devices. © Society of Photo-Optical Instrumentation Engineers.

Journal of Nanophotonics

Strong room-temperature ultraviolet to red excitons from inorganic organic-layered perovskites, (R-NH3)2MX4 (M=Pb2+, Sn2+, Hg2+; X=I-, Br -)

We report static and dynamic photocurrent response from sub-stoichiometric a-SiNx:H thin films. The photocurrent spectral (PCS) response is peaked in the technologically important optical energy range of 2.2 to 4.5 eV. The transient photocurrent response with prolonged exposure is attributed to reduction in number of charge carriers due to trapping of photo-generated carriers at defect sites. The narrow PCS response is attributed to dominant photo-generation of carriers in the bandtails of stoichiometric Si3N4phase and subsequent transport through the excess Si network. © 2014 AIP Publishing LLC.