Htet H. Kyaw

Highly efficient ZnO/Au Schottky barrier dye-sensitized solar cells: Role of gold nanoparticles on the charge-transfer process

Summary Zinc oxide (ZnO) nanorods decorated with gold (Au) nanoparticles have been synthesized and used to fabricate dye-sensitized solar cells (DSSC). The picosecond-resolved, time-correlated single-photon-count (TCSPC) spectroscopy technique was used to explore the charge-transfer mechanism in the ZnO/Au-nanocomposite DSSC. Due to the formation of the Schottky barrier at the ZnO/Au interface and the higher optical absorptions of the ZnO/Au photoelectrodes arising from the surface plasmon absorption of the Au nanoparticles, enhanced power-conversion efficiency (PCE) of 6.49% for small-area (0.1 cm2) ZnO/Au-nanocomposite DSSC was achieved compared to the 5.34% efficiency of the bare ZnO nanorod DSSC. The TCSPC studies revealed similar dynamics for the charge transfer from dye molecules to ZnO both in the presence and absence of Au nanoparticles. A slower fluorescence decay associated with the electron recombination process, observed in the presence of Au nanoparticles, confirmed the blocking of the electron transfer from ZnO back to the dye or electrolyte by the Schottky barrier formed at the ZnO/Au interface. For large area DSSC (1 cm2), ~130% enhancement in PCE (from 0.50% to 1.16%) was achieved after incorporation of the Au nanoparticles into the ZnO nanorods.

Structural properties, magnetic interactions, magnetocaloric effect and critical behaviour of cobalt doped La0.7Te0.3MnO3

The effect of cobalt doping on the structural, magnetic and magnetocaloric properties of electron-doped manganite La0.7Te0.3Mn1−xCoxO3 (x = 0, 0.1, 0.2, 0.25, 0.3 and 0.5) has been investigated. The parent compound La0.7Te0.3MnO3 crystallizes in a rhombohedral structure with Rc space group. With the increase in Co concentration to x = 0.2, a structural transition from rhombohedral (Rc space group) to orthorhombic (Pbnm space group) is observed. X-ray photoelectron spectroscopy (XPS) indicates that the structural transition is due to the disordered distribution of Mn2+/Mn3+ and Co2+/Co3+ ions. All the samples undergo a paramagnetic–ferromagnetic (PM–FM) phase transition. With the increase in Co content to x = 0.1, the unit cell volume increases with a decrease in both Mn–O–Mn bond angle and Tc indicating a weakening of the double exchange interaction. However, with further increase in Co concentration, Tc increases. The presence of competing ferromagnetic and antiferromagnetic interactions leads to a glassy behaviour at low temperatures for low Co doping concentrations. However, for higher Co concentrations, no such behaviour is observed. Arrott plots reveal a second order nature of magnetic transition for all the samples. The magnetic exchange interactions for x = 0.3 and 0.5 follow the mean-field model. Magnetization results show that the magnetocaloric property of the electron-doped manganite is affected by the substitution of Co at Mn sites. Relatively large values of relative cooling power and broad temperature interval of the magnetocaloric effect make the present compounds promising for sub-room temperature magnetic refrigeration applications.

Self-organization of gold nanoparticles on silanated surfaces

Summary The self-organization of monolayer gold nanoparticles (AuNPs) on 3-aminopropyltriethoxysilane (APTES)-functionalized glass substrate is reported. The orientation of APTES molecules on glass substrates plays an important role in the interaction between AuNPs and APTES molecules on the glass substrates. Different orientations of APTES affect the self-organization of AuNps on APTES-functionalized glass substrates. The as grown monolayers and films annealed in ultrahigh vacuum and air (600 °C) were studied by water contact angle measurements, atomic force microscopy, X-ray photoelectron spectroscopy, UV–visible spectroscopy and ultraviolet photoelectron spectroscopy. Results of this study are fundamentally important and also can be applied for designing and modelling of surface plasmon resonance based sensor applications.

One-Diode Model Equivalent Circuit Analysis for ZnO Nanorod-Based Dye-Sensitized Solar Cells: Effects of Annealing and Active Area

Electrical characteristics of 1-D zinc oxide (ZnO) nanorod-based dye-sensitized solar cells (DSSCs) were experimentally measured and followed by theoretical analysis using simple one-diode model. Defect sites (mostly oxygen vacancies) in ZnO are typically responsible for lower DSSC performance, which are removed by annealing the ZnO nanorods at high temperatures up to 450 °C. The DSSC performances with respect to the different annealing temperatures (250 °C, 350 °C, and 450°C) were determined by measuring their I-V characteristics at 1-sun irradiation (AM 1.5G). The variations in series and shunt resistances of DSSC were estimated by fitting the experimental I-V characteristics with the ideal I-V curve obtained from the one-diode equivalent model of the DSSC. By increasing annealing temperature, reduction in the series resistance Rs of the DSSCs with a subsequent increase in the shunt resistance Rsh was obtained. Annealing temperature of 350 °C was found to be optimum at which maximum DSSC performances with 1-cm2 cell active area showing minimum Rs (0.02 kΩ) with high Rsh (1.08 kΩ) values were observed. Reduction in the active area of the DSSCs from 1 to 0.25 cm2 and further to 0.1 cm2 demonstrated improved device performance with ~56% and ~24% enhancement in the fill factor and open-circuit voltage Voc, respectively, due to the reduced sheet resistance and lower recombination rate resulting low series resistance and high shunt resistance, respectively. At the optimum annealing temperature, maximum DSSC efficiency of 4.60% was obtained for the 0.1-cm2 cell active area.

Design of electric-field assisted surface plasmon resonance system for the detection of heavy metal ions in water

Surface Plasmon Resonance (SPR) sensors are widely used in diverse applications. For detecting heavy metal ions in water, surface functionalization of the metal surface is typically used to adsorb target molecules, where the ionic concentration is detected via a resonance shift (resonance angle, resonance wavelength or intensity). This paper studies the potential of a possible alternative approach that could eliminate the need of using surface functionalization by the application of an external electric field in the flow channel. The exerted electrical force on the ions pushes them against the surface for enhanced adsorption; hence it is referred to as “Electric-Field assisted SPR system”. High system sensitivity is achieved by monitoring the time dynamics of the signal shift. The ion deposition dynamics are discussed using a derived theoretical model based on ion mobility in water. On the application of an appropriate force, the target ions stack onto the sensor surface depending on the ionic concentration of target solution, ion mass, and flow rate. In the experimental part, a broad detection range of target cadmium ions (Cd 2+) in water from several parts per million (ppm) down to a few parts per billion (ppb) can be detected.

Plasmon Resonance Enhanced Zinc Oxide Photoelectrodes for Improvement in Performance of Dye Sensitized Solar Cells

Nanocomposites of vertically aligned zinc oxide (ZnO) nanorod arrays incorporated with gold (Au) nanoparticles have been used as photoelectrodes to fabricate dye sensitized solar cells (DSSCs). Due to the surface plasmon resonance of the Au nanoparticles, the nanocomposite photoelectrodes demonstrate enhancement in the visible light absorption resulting in ~8% higher photocurrent compared to ZnO photoelectrode based DSSCs fabricated without any Au nanoparticles. In addition to the higher optical absorption due to the gold nanoparticles, a Schottky barrier forms at the ZnO/Au interface preventing the back electron transfer from the conduction band of the semiconductor nanorods to the redox electrolyte providing improvement in the charge separation at the nanocomposite photoelectrode. Upon incorporation of Au nanoparticles, the overall efficiency of the DSSC increased from 2.41% to 3.27%. The role of Au nanoparticles on the performance of the DSSCs for varying concentration of the Au nanoparticles as well as the post-growth annealing treatment of the nanocomposite photoelectrode is reported.