Advanced Materials Research Vol. 501

Abstract: Solid-state microwave as a method was used to synthesize quaternary compounds Pb1-xYbxSe0.2Te0.8 from initial components to obtain polycrystalline ingots with large grain size. Thin films of Pb1-xYbxSe0.2Te0.8 were then deposited onto glass substrates using thermal evaporation in vacuum, which have a polycrystalline rock salt (NaCl) structure for powders, and thin films. The field emission scanning electron microscopy (FESEM) images reveal that the Pb1-xYbxSe0.2Te0.8 thin films with Yb-doped have uniform crystal grain size and dense nanostructure. The thermoelectric properties of thin films were measured in the temperature range 298–523 K. The Seebeck coefficient of films increased with x when x=0.015, 0.03 and 0.045, while decreased for x=0.06−0.105.

Abstract: High efficiency is one of the characteristics of a good solar cell. The objective of the experiment is to investigate the effect of electric field treatment on the performance of organic salt tetrabutylammonium hexafluorophosphate (TBAPF₆) doped solar cell with ITO/MEHPPV:PCBM/Al structure where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. The devices were build by depositing 20 % of TBAPF₆ (weight percentage with respect to MEHPPV) doped MEHPPV:PCBM thin film onto the ITO by spin coating technique, and followed by deposition of Al by using electron gun evaporation technique. The performance of the devices was analyzed through the current-voltage (I-V) curve under illumination with a solar simulator at 100 mW/cm². Electric field treatment was given by applying different constant voltages of 6 V, 8 V and 10 V for 60 s to the devices before the I-V characterization. A device without electric field treatment was also investigated for comparison. The efficiency of the devices increased with constant voltage given and reached an optimum value at treated voltage of 8 V. The optimum device exhibited the highest efficiency of 0.16% with short circuit current density of 0.88 mA/cm², open circuit voltage of 0.60 V and fill factor of 30%.

Abstract: SnO2 has been successfully grown on Si substrate using a thermal evaporation method, under different percentages of argon and oxygen gases. High purity Sn metal was used as reactants with deposition temperature were set at 900°C. Different oxygen concentrations range between 1.6 and 10% were used during thermal evaporation process. The as-prepared metal oxides were analyzed using SEM-EDS, XRD and UV-VIS. SnO2 nanowhiskers were obtained at lower oxygen concentrations (1.6 to 6% oxygen), while nanowires structures were formed at higher oxygen percentages. XRD results revealed SnO2 produced were highly crystalline and no other impurity phase diffraction peaks were detected. EDS analysis revealed that only Sn and O elements were present in the sample which are consistent with XRD results. UV-VIS result revealed that the optical band gap energy of the metal oxides produced have low significant effect with increasing oxygen concentration. Optical band gap energy for was within 3.3 eV, which was lower than the optical band gap energy of bulk SnO2.

Abstract: Nanocrystalline diamond films were grown by hot filament chemical vapour deposition (HFCVD) in a mixture of methane and hydrogen gases. Three straight parallel wires filament configuration were used in the HFCVD system for the deposition of the films studied in this work. The deposition pressure for the growth of diamond films in this hot filament chemical vapour deposition (HFCVD) reactor have been optimized to be at 20 torr with the methane and hydrogen flow-rates fixed at 2 and 200 sccm respectively. The films studied in this work were grown at low deposition pressures of 2 and 5 torr using the same gas flow-rates used for the optimized diamond film growth including an additional film grown at pressure of 5 mbar with the methane flow-rate reduced to 1 sccm. The morphology showed the formation of closed packed diamond grains for the film grown at 5 torr with methane and hydrogen flow-rates fixed at 2 and 200 sccm. Decrease in pressure and methane flow-rate produced significant changes to the morphology of the diamond grains formed. X-ray diffraction showed that diamond phase phases were dominant in the films deposited at higher pressure. Raman and photoluminescence (PL) spectral analysis were performed using spectra acquired at 325 and 514 nm excitation energies. Raman analysis revealed that increase in deposition pressure from 2 to 5 Torr resulted in the transformation of the film structure from diamond-like-carbon to nanocrystalline diamond structure. UV excitation produced high PL emission intensity at 2.1 eV and the PL intensity was highest for the films deposited at the lowest pressure. Visible excitation on the other hand produced low intensity broad PL emission for all the films between 1.2 and 2.5 eV and the PL intensity was high for the films deposited at the highest deposition pressure.

Abstract: In this research we introduce an inexpensive method to produce highly crystalline GaN Nanowires (NWs) grown on porous zinc oxide (PZnO) using commercial GaN powder, either in argon gas or combination of nitrogen and Ar gas atmosphere, by thermal evaporation. Morphological structural studies using transmission electron microscope (TEM) and scanning electron microscopy (SEM) measurements showed the role of porosity and different gas flowing, in the alignment and nucleation of these NWs. The NWs grown under flow of mix gases have very different diameters of between 50 and 200 nm, but those which were grown in Ar gas atmosphere, have rather uniform diameter of around 50 nm. The length of the GaN NWs was uniform, (around 10 µm). Optical and structural characterizations were performed by energy-dispersive X-ray spectroscopy (EDX) and high resolution X-ray diffraction (HR-XRD). Results revealed that these NWs are of single-crystal hexagonal GaN with [oooı] and [ıoīı] growth directions for the NWs grown under Ar and mixed gas flow.

Abstract: Fourier transform infrared (FTIR) spectroscopy has been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary AlxlnyGa1-x-yN thin films at room temperature. The AlxlnyGa1-x-yN films were grown on c-plane (0001) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with indium (In) mole fraction y = 0.0 to 0.10 and constant aluminium (Al) mole fraction x = 0.06. The experimental results indicated that the AlxlnyGa1-x-yN alloys had two-mode behavior, for the A1 (LO) and E1 (TO) modes.

Abstract: The effect of Ca substitution by Na, Mg and Yb on the structural and transport properties of Bi1.6Pb0.4Sr2Ca2-xMxCu3Oy (M = Na, Mg and Yb) (x = 0.0 and 0.2) superconducting samples have been investigated. The samples were prepared by the coprecipitation (COP) method. The samples were characterized by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), electrical resistivity measurement and critical current density. The critical current density (Jc) and transition temperature (Tc zero) of Na, Mg and Yb substituted with x = 0.2 were found to be lower than the pure sample. Tc zero varies between 100 K and 63 K. Mg concentration was found to give the highest Tc zero of 93 K. Tc zero gradually decreased from Mg, Yb to Na corresponding to a small change in the carrier concentration. Jc decreased with Mg, Yb and Na substitution, and it was measured to be 7.4611 A/cm2, 0.0667 A/cm2, 1.4579 A/cm2 and 1.2479 A/cm2 for pure, Na, Mg and Yb substitution, respectively at 60 K. XRD analysis showed that the decrease of the volume fraction for the 2223 phase and increase of the volume fraction for the 2212 phase with substitution of Na, Mg and Yb. The proportion of Bi-2223/Bi-2212 (%) was estimated from 78.13/21.87 for pure to 51.71/48.29 for Na substitution.

Abstract: The influence of calcium substitution at the barium site of porous Y(Ba1-xCax)2Cu3Oδ (x= 0.00, 0.10, 0.20 and 0.30) samples prepared via solid-state reaction method have been investigated. The structure, morphology, critical temperature and critical current were determined by x-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and four-point probe method, respectively. Generally, the curves of normalized resistance for all samples displayed normal metallic behavior above Tc onset. The Tc zero was between 84 K and 71 K with increase in Ca concentration corresponding to a small change of carrier concentration. The critical current density, Jc decreases with increase in Ca concentration. The highest Jc of 2.657 A/cm2 at 50 K was obtained in Ca-free porous YBCO which is higher than that of Ca-free non porous YBCO. Further substitution of Ca at Ba site decreased Jc monotonously. The increase of Ca concentration decreased the volume of unit cell but the crystallographic structure remains in the orthorhombic form where a≠b≠c. The grains are highly compacted and randomly distributed while the grain size decreased as the Ca concentration increased.

Abstract: The effect of Ca substitution in Y1-xCaxBa2Cu3O7 superconductor prepared via co-precipitation method has been investigated. The concentration of Ca substitution was varied from x = 0.05 to x = 0.20. The samples were characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (SEM), resistivity measurement and current density measurement (without magnetic field). The critical temperature of the samples decreased as the concentration of Ca be increased whereby the highest value of critical temperature (Tc zero) was 83 K at x = 0.05. The crystallographic structure of pure sample was evidenced to be orthorhombic form where a ≠ b ≠ c.. At x = 0.10, the microstructure showed an improvement in grain alignment compared to other Ca concentration. The critical current density (Jc) increased with Ca concentration. The value of Jc for x = 0.05 at 50 K and 60 K was 1.51 A/cm2 and 1.00 A/cm2, respectively.

Abstract: We investigated the elastic properties of MgB2 and 5 wt.% SiC-doped MgB2 superconductors utilizing the pulse-echo overlap technique. Longitudinal and shear ultrasound velocities were measured for each sample at 80 K and 300 K. The measured velocities at 80 K were used to calculate the various elastic moduli, i.e. longitudinal (CL), shear (G), bulk (B) and Young’s (Y) modulus, and the Debye temperature D, for both samples. The high D at 694 K and 706 K obtained for MgB2 and SiC-doped MgB2, respectively, provide strong evidences via direct acoustic measurements to support numerous theoretical and non-acoustic data-based calculations. At 300 K, the higher longitudinal velocity of the pure MgB2 compared to the doped MgB2 but converging towards the same value of longitudinal modulus at 80 K, seems to suggest that the SiC-doped MgB2 undergoes greater elastic stiffening through temperature range of 300 K down to 80 K. Using the D values, the electron-phonon coupling constant  was calculated within the BCS framework and the two dimensional van Hove scenario. The results led us to conclude that MgB2 is a moderately-strong coupled superconductor.