Observation of the triplet metastable dtate of shallow donor pairs in AlN crystals with a negative-U behavior: A high-frequency EPR and ENDOR study

Theoretical predictions about the n-type conductivity in nitride semiconductors are discussed in the light of results of a high-frequency EPR an ENDOR study. It is shown that two types of effective-mass-like, shallow donors with a delocalized wave function exist in unintentionally doped AlN. The experiments demonstrate how the transformation from a shallow donor to a deep (DX) center takes place and how the deep DX center can be reconverted into a shallow donor forming a spin triplet and singlet states. © 2008 The American Physical Society

Determination of the optical bandgap and disorder energies of thin amorphous SiC and AlN films produced by radio frequency magnetron sputtering

Amorphous aluminum nitrite and silicon carbide (a-AlN and a-SiC) thin films were prepared by radio frequency magnetron sputtering. Due to the deposition method and production conditions the deposited films grown in amorphous state. We systematically measure the optical bandgap through optical transmission spectroscopy and its change with a cumulative thermal annealing. The results show a linear relation between the Tauc-gap and the Tauc-slope for both AlN and SiC films, which can be explained analytically from the existence of an Urbach focus, and therefore can be used to determine the Urbach focus or to ensure the correct usage of the bandgap determination methods

Emission pattern of an aluminium nitride target for radio frequency magnetron sputtering

Thin amorphous aluminium nitride films, (a-AlN) have been produced by radio frequency magnetron sputtering at rf power 120W from a highly pure AlN target. The target is mounted below the substrate holder such that its position can be adjusted inside the vacuum chamber. The emission pattern is determined by means of thickness distribution of the deposited material obtained from optical transmission measurements. Holding a set of the process parameters constant and only varying the target-sample distance a three dimensional emission pattern of the AlN target was determined. The deposition rate and emission pattern for 120W and 180W (studied before) were compared. This comparison allows us to consider the target and shielding dimensions of our magnetron to predict the thickness and the sputtering rate distribution for any process parameter and sample target geometry

A facile and green route to terpene derived acrylate and methacrylate monomers and simple free radical polymerisation to yield new renewable polymers and coatings

We present new acrylic monomers derived directly from abundant naturally available terpenes via a facile, green and catalytic approach. These monomers can be polymerised to create new polymers with a wide range of mechanical properties that positions them ideally for application across the commodity and specialty plastics landscape; from packaging, cosmetic and medical, through to composites and coatings. We demonstrate their utility through formation of novel renewable polymer coatings

Reprogramming the assembly of unmodified DNA with a small molecule

The ability of DNA to store and encode information arises from base pairing of the four-letter nucleobase code to form a double helix. Expanding this DNA ‘alphabet’ by synthetic incorporation of new bases can introduce new functionalities and enable the formation of novel nucleic acid structures. However, reprogramming the self-assembly of existing nucleobases presents an alternative route to expand the structural space and functionality of nucleic acids. Here we report the discovery that a small molecule, cyanuric acid, with three thymine-like faces reprogrammes the assembly of unmodified poly(adenine) (poly(A)) into stable, long and abundant fibres with a unique internal structure. Poly(A) DNA, RNA and peptide nucleic acid all form these assemblies. Our studies are consistent with the association of adenine and cyanuric acid units into a hexameric rosette, which brings together poly(A) triplexes with a subsequent cooperative polymerization. Fundamentally, this study shows that small hydrogen-bonding molecules can be used to induce the assembly of nucleic acids in water, which leads to new structures from inexpensive and readily available materials

THE ROLE OF SPIN-ORBIT COUPLING AND HYPERFINE COUPLING IN OPTICAL PUMPING OF F-CENTRES

The ground state polarization of the electronic spins achieved by optical pumping of isolated F-centres is strongly dependent on the magnetic field and the wavelength of the pumping light. Experimentally it does not show the simple relationship to the magnetic circular dichroism (MCD) that has generally been assumed. A closer theoretical analysis shows indeed that the phenomena depend critically on the kind of spin mixing that prevails in the pumping cycle. If spin orbit coupling in the absorption band is the dominant spin mixing mechanism the sign of the pumping effect will be the same everywhere in the absorption band, if some other mechanism like spin mixing by radiation less transitions or hyperfine coupling to the surrounding nuclear spins prevails, the sign will follow the MCD. In strong magnetic fields experiments argue in favour of the first alternative, in low fields spin mixing by hyperhe coupling becomes important. On this basis the mechanism of optical pumping of F-centres and applications to optical detection of spin resonances are discussed