Marlies K. Van Bael

Towards Efficient Hybrid Solar Cells Based on Fully Polymer Infiltrated ZnO Nanorod Arrays

L.B. and B.C. contributed equally to this work. The authors thank R. Rieke from Rieke Metals for useful suggestions. This work was financially supported by BOF, UHasselt, the Flemish Odysseus program, and the Interreg project Organext. A. H. is a postdoctoral research fellow of the Research Foundation-Flanders (FWO Vlaanderen).

Self-Assembled Multilayers of Vertically Aligned Semiconductor Nanorods on Device-Scale Areas

Research on colloidal nanocrystals is rapidly evolving towards applications, e.g., in the fi elds of electronics and photonics. [ 1 ] This is due to the unique combination of size and shapedependent physical properties with the suitability of nanocrystals for wet processing and self-assembly. Especially in the case of spherical nanocrystals, the formation of complex binary and ternary superstructures has been demonstrated, together with an improved understanding of the driving forces of self-assembly. [ 2 ] Interest in this direction is triggered by the possibility to create nanocomposites with physical properties different from those of the individual building blocks. On the other hand, the formation of ordered superstructures of anisotropic or branched nanocrystals proves more diffi cult, most likely due to their reduced shape symmetry. [ 3 ] Only few studies have demonstrated the controlled formation of ordered superstructures in this case. Chains of rods/tetrapods were prepared via selective attachment of molecules to the tips of rods that promoted their assembly or via welding through metal domains, [ 4 ] while micrometer-scale assemblies of vertically aligned rods were fabricated directly in solution or randomly on a substrate with several techniques. [ 5 ] An additional challenge is to achieve organization of nanocrystals on substrates on a scale compatible with practical applications. So far, the formation of a few monolayers of vertically aligned rods has been demonstrated, [ 6 ] however fi lms with such a small thickness are prone to cracking and tend to have regions with missing nanocrystals, which is a problem for devices. Here we report the formation of multilayers of vertically aligned CdSe/CdS nanorods on a cm 2 scale on a variety of substrates and we demonstrate that this assembly strategy is independent on the type of substrate, which is indicative of a pre-organization of the rods already in the solution phase. Moreover, we monitor how the degree of order varies with annealing temperature of the fi lm, and we show that the degree of order is progressively lost upon increasing the temperature, up to the melting of the nanocrystals. These results have important implications for applications in thin fi lm devices. The nanorods are synthesized following established procedures and have a narrow distribution of diameters and lengths. [ 7 ] Our self-assembly method involves a controlled solvent evaporation in a vapor-saturated atmosphere, following simple dropcasting of a highly concentrated solution of nanorods. A sketch of the setup is shown in Figure 1 a and a detailed description of the procedure can be found in the supporting information. Importantly, this technique does not require an external fi eld

Structural and optical properties of DNA layers covalently attached to diamond surfaces

Label-free detection of DNA molecules on chemically vapor-deposited diamond surfaces is achieved with spectroscopic ellipsometry in the infrared and vacuum ultraviolet range. This nondestructive method has the potential to yield information on the average orientation of single as well as double-stranded DNA molecules, without restricting the strand length to the persistence length. The orientational analysis based on electronic excitations in combination with information from layer thicknesses provides a deeper understanding of biological layers on diamond. The pi-pi* transition dipole moments, corresponding to a transition at 4.74 eV, originate from the individual bases. They are in a plane perpendicular to the DNA backbone with an associated n-pi* transition at 4.47 eV. For 8-36 bases of single- and double-stranded DNA covalently attached to ultra-nanocrystalline diamond, the ratio between in- and out-of-plane components in the best fit simulations to the ellipsometric spectra yields an average tilt angle of the DNA backbone with respect to the surface plane ranging from 45 degrees to 52 degrees . We comment on the physical meaning of the calculated tilt angles. Additional information is gathered from atomic force microscopy, fluorescence imaging, and wetting experiments. The results reported here are of value in understanding and optimizing the performance of the electronic readout of a diamond-based label-free DNA hybridization sensor.

Hydrothermal synthesis of ZnO nanorods: a statistical determination of the significant parameters in view of reducing the diameter

In this paper a 2(8-4) fractional factorial design of experiments is applied to identify the important parameters that affect the average diameter of ZnO rods, synthesized by means of a hydrothermal procedure. A water-based Zn(2+) precursor is used for the formation of one-dimensional ZnO particles, without the presence of an organic additive. Results indicate that, at the investigated levels, four of the parameters have a significant effect on the mean diameter. These are the temperature, the heating rate, stirring and an ultrasonic pre-treatment of the precursor solution. Experiments carried out with zinc acetate and zinc chloride do not show a significant difference in rod diameter. Other parameters that do not show a significant effect are the concentration of Zn(2+), the molar ratio between the hydroxyl and the zinc ions, and the reaction time. Interactions are observed between stirring and an ultrasonic pre-treatment and between the zinc concentration and the OH:Zn ratio. By fixing the significant factors at their optimal value it is possible to decrease the mean diameter. The particles are characterized by means of x-ray diffraction (XRD) and transmission electron microscopy (TEM).

Sol–gel (combustion) synthesis and characterization of different alkaline earth metal (Ca, Sr, Ba) stannates

In this study, monophasic strontium and barium stannate (SrSnO3, Sr2SnO4, BaSnO3, Ba2SnO4) powders were synthesized by means of environmentally friendly aqueous sol–gel technique under neutral conditions. However, it was established that the successful sol–gel synthesis of appropriate calcium stannates (CaSnO3 and Ca2SnO4) can be performed only at acidic sol–gel processing conditions. Moreover, the influence of nature of alkaline earth metal source on the phase purity of different metal stannates was evaluated. The thermal behaviour of Ca–Sn–O, Sr–Sn–O and Ba–Sn–O precursor gels was investigated by TG-DSC measurements. The phase purity, crystallization peculiarities and microstructural evolution of the sol–gel derived alkaline earth metal stannate powders were studied by XRD and SEM measurements.

Aqueous Solutions for Low-Temperature Photoannealing of Functional Oxide Films: Reaching the 400 °C Si-Technology Integration Barrier

Functional oxide films were obtained at low temperature by combination of aqueous precursors and a UV-assisted annealing process (aqueous photochemical solution deposition). For a PbTiO(3) model system, functional ferroelectric perovskite films were prepared at only 400 °C, a temperature compatible with the current Si-technology demands. Intrinsically photosensitive and environmentally friendly aqueous precursors can be prepared for most of the functional multimetal oxides, as additionally demonstrated here for multiferroic BiFeO(3), yielding virtually unlimited possibilities for this low-temperature fabrication technology.

Aqueous chemical solution deposition of ultrathin lanthanide oxide dielectric films

Hasselt Univ, Mat Res Inst, Lab Inorgan & Phys Chem, B-3590 Diepenbeek, Belgium. IMEC vzw, Div IMOMEC, B-3590 Diepenbeek, Belgium. IMEC vzw, B-3001 Heverlee, Belgium. Katholieke Univ Leuven, Dept Chem, B-3001 Heverlee, Belgium. XIOS Hogesch Limburg, Dept Ind Sci & Technol IWT, B-3590 Diepenbeek, Belgium.Van Bael, MK, Hasselt Univ, Mat Res Inst, Lab Inorgan & Phys Chem, B-3590 Diepenbeek, Belgium.marlies.vanbael@uhasselt.be jules.mullens@uhasselt.be

Investigation of the ferroelectric–relaxor crossover in Ce-doped BaTiO3 ceramics by impedance spectroscopy and Raman study

BaCe x Ti1− x O3 (x = 0.06, 0.10, and 0.20) solid solutions were prepared via conventional the solid-state reaction and sintered at 1500°C for 4 h, resulting in dense single phase ceramics with homogeneous microstructures. The electric field dependence of permittivity of the BaCe x Ti1− x O3 ceramics was investigated in detail, together with the ferroelectric–paraelectric phase transition features. A transformation from normal to relaxor ferroelectrics was observed by increasing the Ce concentration. For low-Ce content, a substitution of Ce on both A and B site positions was proposed from the dielectric study and confirmed by Raman analysis.

Tuning the dimensions of ZnO nanorod arrays for application in hybrid photovoltaics.

ZnO nanorod arrays are a very eligible option as electron acceptor material in hybrid solar cells, owing to their favorable electrical properties and abundance of available, easy, and low-cost synthesis methods. To become truly effective in this field, a major prerequisite is the ability to tune the nanorod dimensions towards optimal compatibility with electron-donating absorber materials. In this work, a water-based seeding and growth procedure is used to synthesize ZnO nanorods. The nanorod diameter is tuned either by modifying the zinc concentration of the seeding solution or by changing the concentration of the hydrothermal growth solution. The consequences of this morphological tailoring in the performance of hybrid solar cells are investigated, which leads to a new record efficiency of 0.82 % for hydrothermally grown ZnO nanorods of size 300 nm in combination with poly(3-hexylthiophene-2,5-diyl) (P3HT). This improvement is attributed to a combined effect of nanorod diameter and orientation, and possibly to a better alignment of the P3HT backbone resulting in improved charge transport.

Thermal behaviour of arsenic trioxide adsorbed on activated carbon

The thermal stability and desorption of arsenic trioxide (As(2)O(3)) adsorbed on activated carbon (AC) was investigated as this phenomenon is expected to influence the arsenic release during low temperature pyrolysis of chromated copper arsenate (CCA) wood waste. Firstly, a thermogravimetric (TG) experiment with arsenolite, an allotropic form of As(2)O(3), was performed. The sample starts to sublime at temperatures lower than 200 degrees C with a sublimation peak temperature of 271 degrees C. Subsequently, TG experiments with samples of As(2)O(3) adsorbed on AC revealed that only very little (max. 6+/-3 wt%) As(2)O(3) was volatilized at temperatures below 280 degrees C, while still 41.6 (+/-5)wt% of the original arsenic concentration was retained at 440 degrees C and 28.5 (+/-3)wt% at 600 degrees C. The major arsenic volatilization occurred between 300 degrees C and 500 degrees C. The kinetic parameters of desorption, activation energy of desorption (E(d)) and pre-exponential factor (A), were determined by fitting an Arrhenius model to the experimental data, resulting in E(d)=69 kJ/mol, A=1.21 x 10(4)s(-1). It can be concluded that the adsorption of As(2)O(3) on AC can contribute to the thermal stabilisation of As(2)O(3). Consequently, during low temperature pyrolysis of CCA wood arsenic release may be prevented by adsorption of As(2)O(3) on the coal-type product formed during the thermal decomposition of the wood.