Organic ferromagnets, which exhibit exchange interactions between unpaired
electrons in π-orbitals, are rare, and the
origin of ferromagnetism in these compounds has so far remained unexplained.
...Tetrakis(dimethylamino)ethylene-fullerene60 (TDAE-C60)
shows a transition to a ferromagnetic state with fully saturated s
= 1/2 molecular spins at the relatively high Curie temperature (for organic
materials) of 16 K (ref. 4). It has been
suggested that the orientations of the C60
molecules may be important for ferromagnetism in this material, but in the
absence of structural data at low temperatures there has been little progress
towards understanding these microscopic interactions. Here we report the results
of a comparative structural study of two different magnetic forms of TDAE-C
60 crystals at low temperatures, correlating the structural properties-in
particular, the intermolecular orientations-with the magnetic properties.
We find that both ferromagnetism and spin-glass-like ordering are possible
in this material, and depend on the orientational state of C60
molecules. This resolves the apparent contradictions posed by different
macroscopic measurements, and opens the
way to a microscopic understanding of π-electron ferromagnetic exchange
interactions in organic materials.
•TiO2 films with different average grain sizes were grown by atomic layer deposition.•Anatase phase of TiO2 shows a high photodegradation rate of methylene blue solutions.•Photocatalytic activity of ...TiO2 can be controlled by the average grain size.•The maximum photocatalytic activity was found for films with 200 nm grains.•The existence of maximum implies two competing mechanisms.
We have studied the effect of grain size on the photocatalytic activity of TiO2 thin films grown by atomic layer deposition technique. A number of thin films of different crystallinity (anatase, rutile, amorphous) and grain sizes (55 nm to 715 nm) were grown on Si substrates under different processing conditions, by varying the reactor temperature, deposition time or buffer layers. The films are characterized by scanning electron microscopy, grazing incidence x-ray diffraction, x-ray photoelectron spectroscopy, secondary ion mass spectrometry and atomic force microscopy. The photocatalytic activity was examined by measuring the degradation rates of methylene blue in aqueous solutions under the ultraviolet light irradiation. The photocatalytic activity exhibits a maximum for grains of average size of 200 nm, indicating existence of two competing effects, which occur when the average grain size decreases: increase of active surface area (which promotes a high photocatalytic activity) and increase of grain boundaries (which degrades the photocatalytic activity).
High abundance and low toxicity make zinc oxide very attractive for broad application in the field of photocatalysis for remediation of toxic compounds from water. Its performance could be elegantly ...improved by inducing native defects into ZnO semiconductor nanocrystals. However, this typically requires post-processing at high temperatures, during which many of the benefits of the original batch of monodisperse small grains are lost due to agglomeration and/or crystal growth. Within this research we show how variations in defect population are inherent to recrystallization process and that they can be tuned to some extent by choosing optimal pathways. The defects were studied by Raman, photoluminescence and EPR spectroscopy, while the surface chemistry of the ZnO nanorods, solvothermally recrystallized from ZnO nanodots, was evaluated by X-ray photoelectron spectroscopy. The produced nanorods with different dimensions were also utilized to show how effective the samples are for degradation of a model pollutant, namely caffeine. Unexpectedly, we found that the surface chemistry of our samples is not the main factor in photocatalytic efficiency. Instead, we show that the differences in photocatalytic activity are mainly determined by the defect population within the bulk formed during the recrystallization process under altered solvothermal conditions.
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•We can effectively dope DNA by complexing it with divalent transition metal cations – Zn2+, Co2+ and Ni2.•Electrical conductivity of the complex M-DNA is orders of magnitude higher ...comparing to the bare DNA conductivity.•M-DNA molecules can convey an electrical current at distances longer than several hundreds of nanometer.•Additional free charge carriers in M-DNA form a correlated 1D system.
We present results of the first measurements of DC conductivity in single M-DNA molecules and its temperature dependence between 25K and 300K. The conductance at room temperature in the M-DNA samples is up to five orders of magnitude higher comparing to the bare DNA. The results for the temperature dependence of the M-DNA DC conductivity show a broad temperature range above 100K where the electric conductivity is practically temperature-independent. Those findings corroborate conclusions based on previous ESR and microwave conductivity measurements that additional free carriers in M-DNA form a correlated 1D system.
The photocatalytic activity of ZnO films, grown by atomic layer deposition on sapphire, was investigated for different amounts of residual hydrogen incorporated unintentionally into the matrix during ...the crystal growth. A close correlation was found between the level of incorporated hydrogen and the rate of photocatalytic degradation of methylene blue on ZnO films. The rate of degradation is consistent with predominantly zero-order reaction kinetics. An enhanced photocatalytic activity, observed for films of predominantly (001)-oriented grains and low concentration of residual hydrogen, is explained by the reduced number of hydrogen-related defects responsible for recombination of charge carriers in combination with the preferential adsorption of water on polar (001) surfaces of ZnO grains.
In the present study, we investigate the photocatalytic activity of ZnO thin films doped by copper atoms in successive steps during the film growth by the atomic layer deposition method. Scanning ...electron microscopy and x-ray diffraction measurements reveal a polycrystalline structure of samples with degrading crystallinity for films with higher Cu content. X-ray photoelectron spectroscopy analysis is consistent with the presence of copper in the Cu+ state for samples with a relative Cu content lower than 1 at.%. Those samples exhibit p-type conductivity indicating that copper ions occupy substitutional, CuZn, acceptor sites in ZnO. As established from UV–vis measurements, Cu-doped ZnO films also show enhanced optical absorption in the visible region. A reduced electron-hole recombination rate, due to much lower intrinsic free charge carrier concentrations in the doped samples, and the increased light absorption in the visible region, lead to a large enhancement of photocatalytic efficiency observed in doped films under simulated sunlight irradiation.
We present spectroscopic measurements on the first example of a chemically electron-doped metal-DNA complex. We show that a doping level as high as the stoichiometric limit of one electron per base ...pair can be achieved. The doped unpaired electrons occupy lowest unoccupied molecular orbital levels of the nucleobases as detected with electron spin resonance and x-ray absorption near edge structure measurements. Delocalization and strong correlations between the unpaired electrons are evident from a temperature-independent spin susceptibility and a microwave conductivity above 100 K.
We have studied properties of thin protective coating layers of Al2O3, grown by atomic layer deposition (ALD) technique on surfaces of cellulose fibrous materials, using scanning electron microscopy ...(SEM), scanning transmission microscopy (STEM), energy dispersive x-ray spectroscopy (EDS), high-resolution transmission microscopy (HR-TEM), x-ray diffraction (XRD) and thermal gravimetry (TGA). A minimum thickness of about 50nm was required to completely inhibit oxygen diffusion into the material and prevent self-ignition at elevated temperatures. No substantial differences were found in morphology or protection performance of Al2O3 coatings produced by thermal or plasma-enhanced ALD (PEALD) methods. However, PEALD technique has shown almost a double growth rate, while, at the same time, it was successfully applied at temperatures as low as 40°C without degrading quality of the coating.
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•(PE)ALD produces thin, conformal protective coatings of Al2O3 on cellulose fibres.•Al2O3 coating protects cellulose fibres from self-ignition at elevated temperatures.•The minimum thickness of an effective Al2O3 coating is 50nm.•Pyrolysis of a coated cellulose fibre results in a stable hollow structure of Al2O3.
We have studied properties of thin protective coating layers of Al2O3, grown by atomic layer deposition (ALD) technique on surfaces of cellulose fibrous materials, using scanning electron microscopy ...(SEM), scanning transmission microscopy (STEM), energy dispersive x-ray spectroscopy (EDS), high-resolution transmission microscopy (HR-TEM), x-ray diffraction (XRD) and thermal gravimetry (TGA). A minimum thickness of about 50 nm was required to completely inhibit oxygen diffusion into the material and prevent self-ignition at elevated temperatures. No substantial differences were found in morphology or protection performance of Al2O3 coatings produced by thermal or plasma-enhanced ALD (PEALD) methods. However, PEALD technique has shown almost a double growth rate, while, at the same time, it was successfully applied at temperatures as low as 40 °C without degrading quality of the coating.
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