214 research outputs found
CdV2O4: A rare example of a collinear multiferroic spinel
By studying the dielectric properties of the geometrically frustrated spinel
CdV2O4, we observe ferroelectricity developing at the transition into the
collinear antiferromagnetic ground state. In this multiferroic spinel,
ferroelectricity is driven by local magnetostriction and not by the more common
scenario of spiral magnetism. The experimental findings are corroborated by
ab-initio calculations of the electric polarization and the underlying spin and
orbital order. The results point towards a charge rearrangement due to
dimerization, where electronic correlations and the proximity to the
insulator-metal transition play an important role.Comment: 4+ pages, 3 figure
Changes in the soluble nitrogen fraction of milk throughout PDO Grana Padano cheese-making
The behaviour of soluble nitrogen compounds during Grana Padano cheese-making was studied at eight dairies. Raw milk, skimmed milk, sweet whey and the derived natural whey culture, collected from 24 processes, were analysed for soluble whey proteins (\u3b1-lactalbumin and \u3b2-lactoglobulin), proteose-peptones (PP), small peptides (SP), caseinomacropeptides (CMPs), and free amino acids (FAAs). The PP fraction increased during milk natural creaming, then part of it was selectively retained in the curd and the rest degraded in the first few hours of whey fermentation, together with \u3b1-lactalbumin, CMPs and part of SP. Features outlined for the whey culture were confirmed on 30 samples collected at six different dairies. A time course study of the whey fermentation showed that degradation of \u3b1-lactalbumin began when the pH dropped below 4, whereas \u3b2-lactoglobulin content did not change. Uptake of specific FAAs was shown to support the initial growth of lactic acid bacteria in whey
Ab-initio study of the relation between electric polarization and electric field gradients in ferroelectrics
The hyperfine interaction between the quadrupole moment of atomic nuclei and
the electric field gradient (EFG) provides information on the electronic charge
distribution close to a given atomic site. In ferroelectric materials, the loss
of inversion symmetry of the electronic charge distribution is necessary for
the appearance of the electric polarization. We present first-principles
density functional theory calculations of ferroelectrics such as BaTiO3, KNbO3,
PbTiO3 and other oxides with perovskite structures, by focusing on both EFG
tensors and polarization. We analyze the EFG tensor properties such as
orientation and correlation between components and their link with electric
polarization. This work supports previous studies of ferroelectric materials
where a relation between EFG tensors and polarization was observed, which may
be exploited to study ferroelectric order when standard techniques to measure
polarization are not easily applied.Comment: 9 pages, 6 figures, 5 tables, corrected typos, as published in Phys.
Rev.
Polar distortions in hydrogen bonded organic ferroelectrics
Although ferroelectric compounds containing hydrogen bonds were among the
first to be discovered, organic ferroelectrics are relatively rare. The
discovery of high polarization at room temperature in croconic acid [Nature
\textbf{463}, 789 (2010)] has led to a renewed interest in organic
ferroelectrics. We present an ab-initio study of two ferroelectric organic
molecular crystals, 1-cyclobutene-1,2-dicarboxylic acid (CBDC) and
2-phenylmalondialdehyde (PhMDA). By using a distortion-mode analysis we shed
light on the microscopic mechanisms contributing to the polarization, which we
find to be as large as 14.3 and 7.0\,C/cm for CBDC and PhMDA
respectively. These results suggest that it may be fruitful to search among
known but poorly characterized organic compounds for organic ferroelectrics
with enhanced polar properties suitable for device applications.Comment: Submitte
Theory of band gap bowing of disordered substitutional II-VI and III-V semiconductor alloys
For a wide class of technologically relevant compound III-V and II-VI
semiconductor materials AC and BC mixed crystals (alloys) of the type
A(x)B(1-x)C can be realized. As the electronic properties like the bulk band
gap vary continuously with x, any band gap in between that of the pure AC and
BC systems can be obtained by choosing the appropriate concentration x, granted
that the respective ratio is miscible and thermodynamically stable. In most
cases the band gap does not vary linearly with x, but a pronounced bowing
behavior as a function of the concentration is observed. In this paper we show
that the electronic properties of such A(x)B(1-x)C semiconductors and, in
particular, the band gap bowing can well be described and understood starting
from empirical tight binding models for the pure AC and BC systems. The
electronic properties of the A(x)B(1-x)C system can be described by choosing
the tight-binding parameters of the AC or BC system with probabilities x and
1-x, respectively. We demonstrate this by exact diagonalization of finite but
large supercells and by means of calculations within the established coherent
potential approximation (CPA). We apply this treatment to the II-VI system
Cd(x)Zn(1-x)Se, to the III-V system In(x)Ga(1-x)As and to the III-nitride
system Ga(x)Al(1-x)N.Comment: 14 pages, 10 figure
Influence of sulfur oxidation state and substituents on sulfur-bridged luminescent copper(I) complexes showing thermally activated delayed fluorescence
C.M.B. thanks Dr. Maria B. Ezhova for helpful discussions regarding NMR spectra, and Dr. Saeid Kamal for assistance with the TCSPC data. C.M.B. and M.O.W. acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Peter Wall Institute for Advanced Studies for financial support and the Laboratory for Advanced Spectroscopy for Imaging Research (LASIR) for facilities access. Z.X. thanks Compute Canada for computing resources for DFT calculations. C.L. thanks the Prof. & Mrs. Purdie Bequests Scholarship and AstraZeneca PhD Studentship. E.Z.-C. and I.D.W.S thank EPSRC (grants EP/R035164/1 and EP/L017008/1) for financial support.Copper(I) complexes are seen as more sustainable alternatives to those containing metal ions such as iridium and platinum for emitting devices. Copper(I) complexes have the ability to radiatively decay via a thermally activated delayed fluorescence (TADF) pathway, leading to higher photoluminescent quantum yields. In this work we discuss six new heteroleptic Cu(I) complexes of the diphosphineâdiimine motif. The diphosphine ligands employed are (oxydi- 2,1-phenylene)bis(diphenylphosphine) (DPEPhos) and the diimine fragments are sulfur- bridged dipyridyl ligands (DPS) which are functionalized at the 6,6âČ-positions of the pyridyl rings (R = H, Me, Ph), and have varying oxidation states at the bridging sulfur atom (S, SO2). The proton ( Cu-DPS, Cu-DPSO2 ) and phenyl ( Cu-Ph-DPS, Cu-Ph-DPSO2 ) substituted species are found to form monometallic complexes, while those with methyl substitution ( Cu-Me-DPS, Cu-Me-DPSO2 ) are found to have a âGoldilocksâ degree of steric bulk leading to bimetallic species. All six Cu(I) complexes show emission in the solid state, with the photophysical properties characterized by low temperature steady-state and time-resolved spectroscopies and variable temperature time-correlated single photon counting (TCSPC). Cu- DPS , Cu-DPSO2 , Cu-Me-DPS , Cu-Me-DPSO2 and Cu-Ph-DPSO2 were shown to emit via a TADF mechanism, while Cu-Ph-DPS showed photoluminescence properties consistent with triplet ligand-centered (3LC) emission.PostprintPeer reviewe
CO adsorption on neutral iridium clusters
The adsorption of carbon monoxide on neutral iridium clusters in the size
range of n = 3 to 21 atoms is investigated with infrared multiple photon
dissociation spectroscopy. For each cluster size only a single v(CO) band is
present with frequencies in the range between 1962 cm-1 (n = 8) and 1985 cm-1
(n = 18) which can be attributed to an atop binding geometry. This behaviour is
compared to the CO binding geometries on clusters of other group 9 and 10
transition metals as well as to that on extended surfaces. The preference of Ir
for atop binding is rationalized by relativistic effects on the electronic
structure of the later 5d metals
CdS and Zn1âxSnxOy buffer layers for CIGS solar cells
Thin film solar cells based on Cu(In,Ga)Se2 (CIGS), where just the buffer layer is changed, were fabricated and studied. The effects of two different buffer layers, CdS and ZnxSn1-xOy (ZnSnO), are compared using several characterization techniques. We compared both devices and observe that the ZnSnO-based solar cells have similar values of power conversion efficiency as compared to the cells with CdS buffer layers. The ZnSnO-based devices have higher values in the short-circuit current (Jsc) that compensate for lower values in fill factor (FF) and open circuit voltage (Voc) than CdS based devices. Kelvin probe force microscopy (KPFM) results indicate that CdS provides junctions with slightly higher surface photovoltage (SPV) than ZnSnO, thus explaining the lower Voc potential for the ZnSnO sample. The TEM analysis shows a poly-crystalline ZnSnO layer and we have not detected any strong evidence of diffusion of Zn or Sn into the CIGS. From the photoluminescence measurements, we concluded that both samples are being affected by fluctuating potentials, although this effect is higher for the CdS sample.publishe
Formation and Stability of small well-defined Cu- and Ni oxide particles
Well-defined and -structured Cu/Cu2O and Ni/NiO composite nanoparticles have been prepared by physical-vapor deposition on vacuum-cleaved NaCl(001) single crystal facets. Epitaxial growth has been observed due to the close crystallographic matching of the respective cubic crystal lattices. Distinct particle morphologies have only been obtained for the Ni/NiO particles, comprising truncated half-octahedral, rhombohedral- and pentagonal-shaped outlines. Oxidation of the particles in the temperature range 473-673 K in both cases led to the formation of well-defined CuO and NiO particles with distinct morphologies. Whereas CuO possibly adopts its thermodynamical equilibrium shape, NiO formation is accompanied by entering a Kirkendall-like state, that is, a hollow core-shell structure is obtained. The difference in the formation of the oxides is also reflected by their stability under reducing conditions. CuO transforms back to a polycrystalline mixture of Cu metal, Cu2O and CuO after reduction in hydrogen at 673 K. In contrast, as expected from theoretical stability considerations, the formation of the hollow NiO structure is reversed upon annealing in hydrogen at 673 K and moreover results in the formation of a Ni-rich silicide structure Ni3Si2. The discussed systems present a convenient way to tackle and investigate various problems in nanotechnology or catalysis, including phase transformations, establishing structure/activity relationships or monitoring intermetallic particles, starting from well-defined and simple models.(VLID)2852386Accepted versio
Atomically precise semiconductor-graphene and hBN interfaces by Ge intercalation
The full exploration of the potential, which graphene offers to nanoelectronics requires its integration into semiconductor technology. So far the real-world applications are limited by the ability to concomitantly achieve large single-crystalline domains on dielectrics and semiconductors and to tailor the interfaces between them. Here we show a new direct bottom-up method for the fabrication of high-quality atomically precise interfaces between 2D materials, like graphene and hexagonal boron nitride (hBN), and classical semiconductor via Ge intercalation. Using angle-resolved photoemission spectroscopy and complementary DFT modelling we observed for the first time that epitaxially grown graphene with the Ge monolayer underneath demonstrates Dirac Fermions unaffected by the substrate as well as an unperturbed electronic band structure of hBN. This approach provides the intrinsic relativistic 2D electron gas towards integration in semiconductor technology. Hence, these new interfaces are a promising path for the integration of graphene and hBN into state-of-the-art semiconductor technology
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