Y. F. Lu

Photocatalytic synthesis and photovoltaic application of Ag-TiO2 nanorod composites.

A photocatalytic strategy has been developed to synthesize colloidal Ag-TiO2 nanorod composites in which each TiO2 nanorod contains a single Ag nanoparticle on its surface. In this rational synthesis, photoexcitation of TiO2 nanorods under UV illumination produces electrons that reduce Ag(I) precursor and deposit multiple small Ag nanoparticles on the surface of TiO2 nanorods. Prolonged UV irradiation induces an interesting ripening process, which dissolves the smaller nanoparticles by photogenerated oxidative species and then redeposits Ag onto one larger and more stable particle attached to each TiO2 nanorod through the reduction of photoexcited electrons. The size of the Ag nanoparticles can be precisely controlled by varying the irradiation time and the amount of alcohol additive. The Ag-TiO2 nanorod composites were used as electron transport layers in the fabrication of organic solar cells and showed notable enhancement in power conversion efficiency (6.92%) than pure TiO2 nanorods (5.81%), as well as higher external quantum efficiency due to improved charge separation and transfer by the presence of Ag nanoparticles.

Solution‐Processed Fullerene‐Based Organic Schottky Junction Devices for Large‐Open‐Circuit‐Voltage Organic Solar Cells

A solution-processed fullerene-based organic Schottky junction photovoltaic device is fabricated to produce a large open circuit voltage, 0.85-0.95 V, which is higher than that of most organic bulk-heterojunction devices. A power conversion efficiency of 5% is achieved in fullerene-derivative [6,6]-phenyl-C71-butyric acid methylester-based Schottky junction devices.

Fluorine substituted thiophene–quinoxaline copolymer to reduce the HOMO level and increase the dielectric constant for high open-circuit voltage organic solar cells

This study reported a novel fluorinated copolymer (FTQ) and shown it to exhibit a significantly higher open circuit voltage (VOC) in bulk heterojunction solar cells with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) compared to the low band-gap polymer Thiophene–Quinoxaline (TQ). Fluorination lowers the polymer HOMO level effectively which pushes down the highest occupied molecular orbital (HOMO) level of the TQ from −5.36 eV to −5.51 eV and increases the relative dielectric constant from 4.2 to 5.5, resulting in a high VOC. The highest VOC of 950 mV was achieved in the FTQ/PCBM solar cell device. For these optimized blends, the device made of FTQ:PC71BM with a 1 : 1 weight ratio yielded a high power conversion efficiency of 5.3% after a very short time thermal annealing process. These findings will be of importance for achieving high-performance of polymer solar cells by functional group substitution in low band gap polymers.

Accuracy improvement of quantitative analysis by spatial confinement in laser-induced breakdown spectroscopy

To improve the accuracy of quantitative analysis in laser-induced breakdown spectroscopy, the plasma produced by a Nd:YAG laser from steel targets was confined by a cavity. A number of elements with low concentrations, such as vanadium (V), chromium (Cr), and manganese (Mn), in the steel samples were investigated. After the optimization of the cavity dimension and laser fluence, significant enhancement factors of 4.2, 3.1, and 2.87 in the emission intensity of V, Cr, and Mn lines, respectively, were achieved at a laser fluence of 42.9 J/cm(2) using a hemispherical cavity (diameter: 5 mm). More importantly, the correlation coefficient of the V I 440.85/Fe I 438.35 nm was increased from 0.946 (without the cavity) to 0.981 (with the cavity); and similar results for Cr I 425.43/Fe I 425.08 nm and Mn I 476.64/Fe I 492.05 nm were also obtained. Therefore, it was demonstrated that the accuracy of quantitative analysis with low concentration elements in steel samples was improved, because the plasma became uniform with spatial confinement. The results of this study provide a new pathway for improving the accuracy of quantitative analysis of LIBS.

Resonant excitation of precursor molecules in improving the particle crystallinity, growth rate and optical limiting performance of carbon nano-onions

A catalyst-free and highly efficient synthetic method for growing carbon nano-onions (CNOs) in open air has been developed through the laser resonant excitation of a precursor molecule, ethylene, in a combustion process. Highly concentric CNO particles with improved crystallinity were obtained at a laser wavelength of 10.532 µm through the resonant excitation of the CH(2) wagging mode of the ethylene molecules. A higher growth rate up to 2.1 g h( - 1) was obtained, compared with that without a laser (1.3 g h( - 1)). Formation of the CNOs with ordered graphitic shells is ascribed to the decomposition of polycyclic aromatic hydrocarbons (PAHs) into C(2) species. The optical limiting performances of the CNOs grown by the combustion processes were investigated. CNOs grown at 10.532 µm laser excitation demonstrated improved optical limiting properties due to the improved crystallinity.

Detection of trace phosphorus in steel using laser-induced breakdown spectroscopy combined with laser-induced fluorescence

Monitoring of light-element concentration in steel is very important for quality assurance in the steel industry. In this work, detection in open air of trace phosphorus (P) in steel using laser-induced breakdown spectroscopy (LIBS) combined with laser-induced fluorescence (LIF) has been investigated. An optical parametric oscillator wavelength-tunable laser was used to resonantly excite the P atoms within plasma plumes generated by a Q-switched Nd:YAG laser. A set of steel samples with P concentrations from 3.9 to 720 parts in 10(6) (ppm) were analyzed using LIBS-LIF at wavelengths of 253.40 and 253.56 nm for resonant excitation of P atoms and fluorescence lines at wavelengths of 213.55 and 213.62 nm. The calibration curves were measured to determine the limit of detection for P in steel, which is estimated to be around 0.7 ppm. The results demonstrate the potential of LIBS-LIF to meet the requirements for on-line analyses in open air in the steel industry.

Laser induced selective removal of metallic carbon nanotubes

Distinguishing between carbon nanotubes (CNTs) according to their individual electronic properties is of significant importance for developing CNT-based electronics and devices. In this study, selective removal of metallic CNTs from CNT mixtures on silicon substrates was investigated using controlled laser irradiation. Free electron movement and eddy currents are induced within the metallic CNTs by the strong electric field and optical near-field effects caused by the laser irradiation. Selective heating of metallic CNTs in air results in selective removal of metallic CNTs when the laser fluence and wavelength are properly selected. Through this process, metallic nanotubes are successfully removed from the CNT mixtures. This technique provides an efficient single-step approach for selective removal of metallic CNTs from CNT mixtures.

Controlled synthesis and defect dependent upconversion luminescence of Y2O3: Yb, Er nanoparticles

Er3+ and Yb3+ co-doped Y2O3 nanoparticles have been prepared by using a coprecipitation method followed by a post-thermal-treatment, in which a surfactant (cetyltrimethylammonium bromide, CTAB) plays an important role in the size-controlling and upconversion luminescence tuning. The green (2H11/2, 4S3/2 → 4I15/2) and red emission (4F9/2 → 4I15/2) intensity can be effectively tuned by varying the surfactant concentration, which can induce the defects in the as-obtained products. The probability of quenching and nonradiative relaxation from 4F7/2, 2H11/2, and 4S3/2 to 4F9/2 could be increased as the number of defects introduced by the surfactant increases, and thus the ratio of red to green emission is also changed. The upconversion mechanism has been analyzed and discussed, which may be a new complement for upconversion luminescence.

Utilizing insulating nanoparticles as the spacer in laminated flexible polymer solar cells for improved mechanical stability

Roll-to-roll lamination is one promising technique to produce large-area organic electronic devices such as solar cells with a large through output. One challenge in this process is the frequent electric point shorting of the cathode and anode by the excess or concentrated applied stress from many possible sources. In this paper, we report a method to avoid electric point shorting by incorporating insulating and hard barium titanate (BaTiO(3)) nanoparticles (NPs) into the active layer to work as a spacer. It has been demonstrated that the incorporated BaTiO(3) NPs in poly(3-hexylthiophene):[6,6]-phenyl-c-61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction solar cells cause no deleterious effect to the power conversion process of this type of solar cell. The resulting laminated devices with NPs in the active layer display the same efficiency as the devices without NPs, while the laminated devices with NPs can sustain a ten times higher lamination stress of over 6 MPa. The flexible polymer solar cell device with incorporated NPs shows a much smaller survivable curvature radius of 4 mm, while a regular flexible device can only sustain a bending curvature radius of 8 mm before fracture.

Laser-induced resonant excitation of ethylene molecules in C2H4/C2H2/O2 reactions to enhance diamond deposition

Vibrational resonant excitation of ethylene (C2H4) molecules using a carbon dioxide laser was employed to promote reactions in precursors of ethylene, acetylene (C2H2), and oxygen to enhance diamond deposition. One of the vibrational modes (CH2 wag mode, v7) of the C2H4 molecules was selected to achieve the resonant excitation in the reactions. Optical emission spectroscopy was used to study the effects of laser resonant excitation on the reactions for diamond deposition. The optical emissions of CH and C2 species were enhanced with the laser excitation, indicating that there are more active species generated in the reactions. Thicknesses and grain sizes of the deposited films were increased correspondingly. Temperature calculations from the line set in the R-branch of CH emission spectra indicated that a nonthermal process is involved in the enhanced diamond deposition.