Elimination of charge carrier trapping in diluted semiconductors.

In 1962, Mark and Helfrich demonstrated that the current in a semiconductor containing traps is reduced by N/Nt(r), with N the amount of transport sites, Nt the amount of traps and r a number that depends on the trap energy distribution. For r > 1, the possibility opens that trapping effects can be nearly eliminated when N and Nt are simultaneously reduced. Solution-processed conjugated polymers are an excellent model system to test this hypothesis, because they can be easily diluted by blending them with a high-bandgap semiconductor. We demonstrate that in conjugated polymer blends with 10% active semiconductor and 90% high-bandgap host, the typical strong electron trapping can be effectively eliminated. As a result we were able to fabricate polymer light-emitting diodes with balanced electron and hole transport and reduced non-radiative trap-assisted recombination, leading to a doubling of their efficiency at nearly ten times lower material costs.

Development of highly stable and luminescent aqueous CdS quantum dots with the poly(acrylic acid)/mercaptoacetic acid binary coating system.

Highly stable and luminescent CdS quantum dots (QD) were prepared in aqueous solutions via in situ capping of the crystals with the poly(acrylic acid) (PAA) and mercaptoacetic acid (MAA) binary mixtures. The effect of reaction temperature and coating composition on the particle size, colloidal stability and luminescence were investigated and discussed in detail. CdS QDs coated with either PAA or MAA were also prepared and compared in terms of properties. CdS-MAA QDs were highly luminescent but increasing reaction temperature caused an increase in the crystal size and a significant decrease in the quantum yield (QY). Although less luminescent and bigger than CdS-MAA, CdS-PAA QDs maintained the room temperature size and QY at higher reaction temperatures. CdS-MAA QDs lacked long-term colloidal stability whereas CdS-PAA QDs showed excellent stability over a year. Use of PAA/MAA mixture as a coating for CdS nanoparticles during the synthesis provided excellent stability, high QY and ability to tune the size and the color of the emission. Combination of all of these properties can be achieved only with the mixed coating. CdS coated with PAA/MAA at 40/60 ratio displayed the highest QY (50% of Rhodamine B) among the other compositions.

CeO2-x nanorods with intrinsic urease-like activity.

The large-scale production and ecotoxicity of urea make its removal from wastewater a health and environmental challenge. Whereas the industrial removal of urea relies on hydrolysis at elevated temperatures and high pressure, nature solves the urea disposal problem with the enzyme urease under ambient conditions. We show that CeO2-x nanorods (NRs) act as the first and efficient green urease mimic that catalyzes the hydrolysis of urea under ambient conditions with an activity (kcat = 9.58 × 101 s-1) about one order of magnitude lower than that of the native jack bean urease. The surface properties of CeO2-x NRs were probed by varying the Ce4+/Ce3+ ratio through La doping. Although La substitution increased the number of surface defects, the reduced number of Ce4+ sites with higher Lewis acidity led to a slight decrease of their catalytic activity. CeO2-x NRs are stable against pH changes and even to the presence of transition metal ions like Cu2+, one of the strongest urease inhibitors. The low costs and environmental compatibility make CeO2-x NRs a green urease substitute that may be applied in polymer membranes for water processing or filters for the waste water reclamation. The biomimicry approach allows the application of CeO2-x NRs as functional enzyme mimics where the use of native or recombinant enzyme is hampered because of its costs or operational stability.

Zinc phosphate as versatile material for potential biomedical applications Part II.

Surface chemical reactivity of two modifications of synthetic zinc phosphate tetrahydrate (alpha - and beta -form of Hopeite, alpha -,beta -ZPT) has been studied by selective chemical and e-beam etching in presence of diluted phosphoric acid and ammonia by Scanning Electron Microscopy (SEM) and microelectrophoresis (zeta potential measurements) in correlation with the corresponding bulk properties and crystal size distributions. The subtitle crystallographic differences between alpha -and beta -ZPT originating from a unique hydrogen bonding pattern, induce drastic variations of both surface potential and surface charge. Biogenic Hydroxyapatite (HAP) and one of its metastable precursors, a calcium dihydrogen phosphate dihydrate (DCPD) or Brushite were used to underline this resulting variation of chemical reactivity in zinc phosphates. In-situ monitoring of the transformation of Brushite in Hydroxyapatite is also reported.

Zinc phosphate as versatile material for potential biomedical applications Part 1.

Synthetic alpha - and beta -Hopeite, two polymorphs of zinc phosphate tetrahydrates (ZPT) have been synthesized by hydrothermal crystallization from aqueous solution at 20 degrees C and 90 degrees C respectively. Aside from their sub-title crystallographic differences originating from a unique hydrogen bonding pattern, their thermodynamic interrelation has been thoroughfully investigated by means of X-Ray diffraction (XRD) and differential scanning calorimetry (DSC), combined with thermogravimetry (TGA-MS). Using a new heterogeneous step-reaction approach, the kinetics of dehydration of the two forms of ZPT was studied and their corresponding transition temperature determined. Low temperature DRIFT, FT-Raman and (1)H, (31)P MAS-NMR reveal an oriented distortion of the zinc phosphate tetrahedra, due to a characteristic hydrogen bonding pattern and in accordance with the molecular tetrahedral linkage scheme of the phosphate groups. Biogenic Hydroxyapatite (HAP) and one of its metastable precursors, a calcium dihydrogen phosphate dihydrate (DCPD) or Brushite were also obtained and used to underline the resulting variations of chemical reactivity in zinc phosphates.

Thermodynamic confinement and alpha-helix persistence length in poly(gamma-benzyl-L-glutamate)-b-poly(dimethyl siloxane)-b-poly(gamma-benzyl-L-glutamate) triblock copolymers.

The structure and the associated dynamics of a series of poly(gamma-benzyl-L-glutamate)-b-poly(dimethyl siloxane)-b-poly(gamma-benzyl-L-glutamate) (PBLG-b-PDMS-b-PBLG) triblock copolymers were investigated using small- and wide-angle X-ray scattering, NMR, transmission electron microscopy, and dielectric spectroscopy, respectively. The structural analysis revealed phase separation in the case of the longer blocks with defected alpha-helical segments embedded within the block copolymer nanodomains. The alpha-helical persistence length was found to depend on the degree of segregation; thermodynamic confinement and chain stretching results in the partial annihilation of helical defects.