Solubility of Hydrogen in a WMoTaNbV High-Entropy Alloy.

The WMoTaNbV alloy has shown promise for applications as a solid state hydrogen storage material. It absorbs significant quantities of H directly from the atmosphere, trapping it with high energy. In this work, the dynamics of the absorption of hydrogen isotopes are studied by determining the activation energy for the solubility and the solution enthalpy of H in the WMoTaNbV alloy. The activation energy was studied by heating samples in a H atmosphere at temperatures ranging from 20 °C to 400 °C and comparing the amounts of absorbed H. The solution activation energy EA of H was determined to be EA=0.22±0.02 eV (21.2 ± 1.9 kJ/mol). The performed density functional theory calculations revealed that the neighbouring host atoms strongly influenced the solution enthalpy, leading to a range of theoretical values from -0.40 eV to 0.29 eV (-38.6 kJ/mol to 28.0 kJ/mol).

Irradiation Damage Independent Deuterium Retention in WMoTaNbV.

High entropy alloys are a promising new class of metal alloys with outstanding radiation resistance and thermal stability. The interaction with hydrogen might, however, have desired (H storage) or undesired effects, such as hydrogen-induced embrittlement or tritium retention in the fusion reactor wall. High entropy alloy WMoTaNbV and bulk W samples were used to study the quantity of irradiation-induced trapping sites and properties of D retention by employing thermal desorption spectrometry, secondary ion mass spectrometry, and elastic recoil detection analysis. The D implantation was not found to create additional hydrogen traps in WMoTaNbV as it does in W, while 90 at% of implanted D is retained in WMoTaNbV, in contrast to 35 at% in W. Implantation created damage predicted by SRIM is 0.24 dpa in WMoTaNbV, calculated with a density of 6.044×1022 atoms/cm3. The depth of the maximum damage was 90 nm. An effective trapping energy for D in WMoTaNbV was found to be about 1.7 eV, and the D emission temperature was close to 700 °C.

Amphoteric Be in GaN: Experimental Evidence for Switching between Substitutional and Interstitial Lattice Sites.

We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of Be_{Ga} in the positron annihilation signals in Be-doped GaN, while the emergence of V_{Ga} at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.

Understanding and Control of Bipolar Self-Doping in Copper Nitride.

Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (Cu3N) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind this behavior in Cu3N is currently lacking in the literature. In this work, we use combinatorial growth with a temperature gradient to demonstrate both conduction types of phase-pure, sputter-deposited Cu3N thin films. Room temperature Hall effect and Seebeck effect measurements show n-type Cu3N with an electron density of 1017 cm-3 for low growth temperature (≈ 35 °C) and p-type with a hole density between 1015 cm-3 and 1016 cm-3 for elevated growth temperatures (50 °C to 120 °C). Mobility for both types of Cu3N was ≈ 0.1 cm2/Vs to 1 cm2/V. Additionally, temperature-dependent Hall effect measurements indicate that ionized defects are an important scattering mechanism in p-type films. By combining X-ray absorption spectroscopy and first-principles defect theory, we determined that VCu defects form preferentially in p-type Cu3N while Cui defects form preferentially in n-type Cu3N; suggesting that Cu3N is a compensated semiconductor with conductivity type resulting from a balance between donor and acceptor defects. Based on these theoretical and experimental results, we propose a kinetic defect formation mechanism for bipolar doping in Cu3N, that is also supported by positron annihilation experiments. Overall, the results of this work highlight the importance of kinetic processes in the defect physics of metastable materials, and provide a framework that can be applied when considering the properties of such materials in general.

Identification of vacancy defect complexes in transparent semiconducting oxides ZnO, In2O3 and SnO2.

Positron annihilation spectroscopy, when combined with supporting high-quality modeling of positron states and annihilation in matter, is a powerful tool for detailed defect identification of vacancy-type defects in semiconductors and oxides. Here we demonstrate that the Doppler broadening of the positron annihilation radiation is a very sensitive means for observing the oxygen environment around cation vacancies, the main open-volume defects trapping positrons in measurements made for transparent semiconducting oxides. Changes in the positron annihilation signal due to external manipulation such as irradiation and annealing can be correlated with the associated changes in the sizes of the detected vacancy clusters. Our examples for ZnO, In2O3 and SnO2 demonstrate that oxygen vacancies in oxides can be detected directly using positron annihilation spectroscopy when they are complexed with cation vacancies.

Tailoring the chain packing in ultrathin polyelectrolyte films formed by sequential adsorption: nanoscale probing by positron annihilation spectroscopy.

Depth profiling experiments by positron annihilation spectroscopy have been used to investigate the free volume element size and concentration in films assembled using the layer-by-layer (LbL) adsorption method. Films prepared from strong polyelectrolytes, weak polyelectrolytes, hydrogen-bonding polymers, and blended polyelectrolyte multilayers have different chain packing that is reflected in the free volume characteristics. The influence of various parameters on free volume, such as number of bilayers, salt concentration, solution pH, and molecular weight, has been systematically studied. The free volume cavity diameters vary from 4 to 6 Å, and the free volume concentrations vary from (1.1-4.3) × 10(20) cm(-3), depending on the choice of assembly polymers and conditions. Films assembled from strong polyelectrolytes have fewer free volume cavities with a larger average size than films prepared from weak polyelectrolytes. Blending the weak polyanion poly(acrylic acid), PAA, with the strong polyanion poly(styrene sulfonate), PSS, to layer alternately with the polycation poly(allyamine hydrochloride), PAH, is shown to be a viable method to achieve intermediate free volume characteristics in these LbL films. An increase in salt concentration of the adsorption solutions for films prepared from strong polyelectrolytes makes these films tend toward weaker polyelectrolyte free volume characteristics. Hydrogen-bonded layered films show larger free volume element size and concentration than do their electrostatically bonded counterparts, while reducing the molecular weight of these hydrogen-bonded polymers results in slightly reduced free volume size and concentration. A study of the effect of solution pH on films prepared from weak polyelectrolytes shows that when both polyelectrolytes are substantially charged in solution (assembly pH = 7.5), the chains pack similarly to strong polyelectrolytes (i.e., lower free volume concentration), but with smaller average cavity sizes. These results give, for the first time, a clear indication of how the free volume profile develops in LbL thin films, offering numerous methods to tailor the Ångström-scale free volume properties by judicious selection of the assembly polymers and conditions. These findings can be potentially exploited to tailor the properties of thin polymer films for applications spanning membranes, sensing, and drug delivery.

Temperature-induced structural transition in-situ in porcine lens--changes observed in void size distribution.

The function of mammalian ocular lens is to provide a sharp image to the retina. Accordingly, the lens needs to be transparent and minimize light scattering. To do so the lens fiber cells first loose intracellular organelles, organize the cytoplasm and arrange the fiber cell membranes. Because the fiber cells are metabolically inactive, the plasma membrane becomes the only cellular organelle and consequently, the phase behavior of these membranes determines the physiological state of the lens. Previous studies have shown that lipids extracted from the nuclear and cortical region of human lens show a temperature-induced phase transition close to the body temperature. Yet, the physiological function of this phase transition is not known, and even the presence of the phase transition in intact lenses is unknown. Positron annihilation lifetime spectroscopy (PALS) was used to characterize the sub-nanometer-sized local structure of intact porcine lens and these studies were complemented with differential scanning calorimeter and mass spectrometric analysis in extracted porcine lens lipids. Using PALS, we present evidence for the presence of a temperature-dependent structural transition centered at 35.5 degrees C in-situ in clear extracted porcine lenses. Further studies employing extracted lens lipids and purified egg-yolk sphingomyelin and cholesterol mixtures suggest that the nano-scale transition emerges from the phase behavior of lens lipids. Based on our results, PALS seems to be a viable method for gaining additional information on biological tissues, especially since it enables non-destructive studies on intact tissues.

Probing biomembranes with positrons.

Free volume pockets inside a cell membrane play a prominent role in a variety of dynamic processes such as the permeability of small molecules across membranes and the diffusion of, e.g., lipids, drugs, and electron carriers in the plane of the membrane. Nonetheless, by now the chances for characterizing free volume voids in a nonperturbative manner through experiments have been very limited. Here we use lipid membranes as an example to show how positron annihilation spectroscopy (PALS) together with atomistic simulations can be employed to gauge changes in free volume pockets in biological macromolecular complexes. The measurements show that PALS is a viable technique to probe free volume in biomolecular systems. As examples, we consider the gel-to-fluid transition and the role of increasing cholesterol concentration in a lipid membrane. Further applications proposed in this work for PALS are likely to provide a great deal of insight into the understanding of the role of free volume in the dynamics of biomolecular complexes.

Evidence of PPII-like helical conformation and glass transition in a self-assembled solid-state polypeptide-surfactant complex: poly(L-histidine)/docylbenzenesulfonic acid.

We present lamellar self-assembly of cationic poly(L-histidine) (PLH) stoichiometrically complexed with an anionic surfactant, dodecyl benzenesulfonic acid (DBSA), which allows a stabilized conformation reminiscent of polyproline type II (PPII) left-handed helices. Such a conformation has no intrapeptide hydrogen bonds, and it has previously been found to be one source of flexibility, e.g., in collagen and elastin, as well as an intermediate in silk processing. PLH(DBSA)1.0 complexes were characterized by Fourier transform infrared spectroscopy (FTIR), circular dichroism (CD), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). The PPII-like conformation in PLH(DBSA)1.0 is revealed by characteristic CD and FTIR spectra, where the latter indicates absence of intrachain peptide hydrogen bonds. In addition, a glass transition was directly verified by DSC at ca. 135 degrees C for PLH(DBSA)1.0 and indirectly by SAXS and TEM in comparison to pure PLH at 165 degrees C, thus indicating plasticization. Glass transitions have not been observed before in polypeptide-surfactant complexes. The present results show that surfactant binding can be a simple scheme to provide steric crowding to stabilize PPII conformation to tune the polypeptide properties, plasticization and flexibility.