Md. Ahamad Mohiddon

Cost effective nanostructured copper substrates prepared with ultrafast laser pulses for explosives detection using surface enhanced Raman scattering

Ultrafast laser pulses induced surface nanostructures were fabricated on a copper (Cu) target through ablation in acetone, dichloromethane, acetonitrile, and chloroform. Surface morphological information accomplished from the field emission scanning electron microscopic data demonstrated the diversities of ablation mechanism in each case. Fabricated Cu substrates were utilized exultantly to investigate the surface plasmon (localized and propagating) mediated enhancements of different analytes using surface enhance Raman scattering (SERS) studies. Multiple utility of these substrates were efficiently demonstrated by collecting the SERS data of Rhodamine 6G molecule and two different secondary explosive molecules such as 5-amino-3-nitro-l,2,4-triazole and trinitrotoluene on different days which were weeks apart. We achieved significant enhancement factors of >105 through an easily adoptable cleaning procedure.

Detection of phase transitions from the study of whispering gallery mode resonance in liquid crystal droplets

We report studies on the whispering gallery mode (WGM) resonance of liquid crystal (LC) droplets across the smectic-A (SmA) to nematic phase transition. The quality factor (Q) in the SmA phase decreases rapidly with a characteristic slope change at the SmA-N transition. In the SmA phase, Q-factor is linearly proportional to the birefringence (Δn). We discuss the effect of topological defect transformation on the WGM resonance. The study of WGM resonance is expected to be very useful for detecting subtle phase transitions among LC mesophases.

Structural and dielectric properties of (Li0.5Na0.5) doped strontium barium niobate

The alkali (Li0.5Na0.5) modified strontium barium niobate (SBN) was prepared by a conventional mixed oxide method. X-ray diffraction was carried out to study the phase changes developed in the sample. Quantitative x-ray analysis was also carried out to study the effect of (Li0.5Na0.5) on the “extent of reaction” in SBN reaction kinematics. From the surface morphology study it was found that grains preferred to grow in unidirection forming tetragonal shaped grains creating porosity, which in turn affects the dielectric and grain boundary resistance properties. Detailed dielectric analysis revealed that, all the SBN samples show diffuse phase transitions and alkali doping results in increasing the transition temperature along with decreasing maximum dielectric constant. Complex impedance analysis shows that (Li0.5Na0.5) doping in SBN increases its grain boundary resistance.

Effect of Nb doping on structural and electric properties of PZT (65/35) ceramic

The aim of this study is to determine the effect of Nb5+ doping on PZT (65/35) based bulk materials in their structure, micro structure and electrical properties. The Nb content was chosen 0-9 mole%. These materials were prepared by conventional mixed oxide method. X ray diffraction studies suggest the compound to be of rhombohedral perovskite phase. Excess addition of Nb result in pyrochlore and fluorite phase develops in PZT (65/35) sample. Detailed studies of dielectric constant as a function of temperature (40degC to 500degC) and frequency (100 Hz to 1 MHz) suggest that the compounds undergo diffuse type of phase transition. Maximum dielectric constant and resistivity were found to be strongly dependent on doping and measuring frequencies. Using complex impedance analysis micro structural parameters such as bulk and grain boundary resistance, bulk charge carrier concentration and relaxation time were determined

Morphologically manipulated Ag/ZnO nanostructures as surface enhanced Raman scattering probes for explosives detection

The detection of secondary explosive molecules (e.g., ANTA, FOX-7, and CL-20) using Ag decorated ZnO nanostructures as surface enhanced Raman scattering (SERS) probes is demonstrated. ZnO nanostructures were grown on borosilicate glass substrates by rapid thermal oxidation of metallic Zn films at 500 °C. The oxide nanostructures, including nanosheets and nanowires, emerged over the surface of the Zn film leaving behind the metal residue. We demonstrate that SERS measurements with concentrations as low as 10 μM, of the three explosive molecules ANTA, FOX-7, and CL-20 over ZnO/Ag nanostructures, resulted in enhancement factors of ∼107, ∼107, and ∼104, respectively. These measurements validate the high sensitivity of detection of explosive molecules using Ag decorated ZnO nanostructures as SERS substrates. The Zn metal residue and conditions of annealing play an important role in determining the detection sensitivity.

Low temperature Au induced crystallization of titanium dioxide thin films for resistive switching applications

Metal induced decrease of crystallization temperature of sol–gel derived titanium dioxide (TiO2) thin films is reported. It is shown that the Au induced onset of crystallization occurs at a temperature of 250 °C as compared to 400 °C when it is deposited directly on the same substrate. The crystallization process is probed using X-ray diffraction and confirmed by Raman spectroscopy. The onset of crystallization is evidenced by the appearance of the diffraction peak from the (101) plane of anatase TiO2 and the peak due to A1g + B1g Raman mode at 515 cm−1. Polarized optical microscopy and Raman imaging indicated that the spatial spread of crystallization across the surface of the film increases with the increase in temperature. Unipolar resistive switching is demonstrated by fabricating an Au/TiO2/Au stack, which shows formation at 9 V, set voltage of 0.5 V and reset voltage of 3.3 V. The maximum set–reset resistance ratio achieved was 105. The mechanism of resistive switching is established by a correlation with photoluminescence spectra which indicates the presence of defects that aid in the switching process.

Effect of heating rate on dielectric and pyroelectric properties of double doped PZT

Abstract Double doped lead zirconium titanate (PZT) was synthesised by solid state reaction method. Calcination and sintering was carried out at various heating rates in order to study the effect of heating rate on the extent of phase formation of double doped PZT. Furthermore, the effect of heating rate on dielectric and pyroelectric properties was also investigated. Rhombohedral perovskite phase was confirmed in the double doped PZT samples. Quantitative X-ray diffraction (XRD) analysis suggests that the extent of PZT formation decreases beyond 8°C min−1 heating rate. The crystallite and grain sizes calculated from Scherrers equation and field emission scanning electron microscope (FE-SEM) photographs respectively show the decreasing behaviour with increasing heating rate. Dielectric and pyroelectric properties show the increasing behaviour up to 8°C min−1 and decreasing behaviour beyond this heating rate.

Chromium oxide as a metal diffusion barrier layer: An x-ray absorption fine structure spectroscopy study

The interaction at the interface between chromium and amorphous Silicon (a-Si) films in the presence of a sandwich layer of chromium oxide is investigated using X-ray absorption fine structure (XAFS) spectroscopy. The oxidized interface was created, in situ, prior to the deposition of a 400 nm tick a-Si layer over a 50 nm tick Cr layer. The entire stack of substrate/metallic Cr/Cr2O3/a-Si was then annealed at temperatures from 300 up to 700 °C. Analysis of the near edge and extended regions of each XAFS spectrum shows that only a small fraction of Cr is able to diffuse through the oxide layer up to 500 °C, while the remaining fraction is buried under the oxide layer in the form of metallic Cr. At higher temperatures, diffusion through the oxide layer is enhanced and the diffused metallic Cr reacts with a-Si to form CrSi2. At 700 °C, the film contains Cr2O3 and CrSi2 without evidence of unreacted metallic Cr. The activation energy and diffusion coefficient of Cr are quantitatively determined in the two tem...

Effect of calcium doping on dielectric and pyroelectric properties of PLZT

Effect of calcium doping on structural, dielectric and pyroelectric properties of Pb_0.92-xLa_0.08Cax(Zr_0.65Ti_0.35)_0.98O₃ (PLCZT) composition derived by the solid state reaction method was studied. Rhombohedral perovskite phase was confirmed in all the PLCZT samples. Calcium doping did not show any structural change in PLCZT (8/65/35) up to 2.5 % doping, however grain size and crystallite size was found to increase up to 1%, but no systematic variation of transition temperature was found with Ca doping. P-E hysteresis loop parameters (E_c and P_r) and pyroelectric properties were found to decrease with Ca doping up to 1% and there after increased.

Electrical and thermal tuning of quality factor and free spectral range of optical resonance of nematic liquid crystal microdroplets

We experimentally study the effect of temperature and electric field on the quality (Q) factor and free spectral range (FSR) of whispering-gallery-mode optical resonance of dye-doped nematic liquid crystal microdroplets. Both the Q factor and the FSR are highly sensitive to the temperature and electric field and are tunable. The Q factor decreases, whereas the FSR increases substantially, with increasing temperature and electric field. The variation of the Q factor and FSR is understood based on the change in the effective refractive index and the dynamic size of the microdroplets.