Metastable atomic-ordered configurations for Al1/2Ga1/2N predicted by Monte-Carlo method based on first-principles calculations.

Metastability of Aln/12Ga1-n/12N (n= 2-10: integer) with the 1-2 monolayer (ML) in-plane configuration towards thec[0001] direction has been demonstrated recently. To theoretically explain the existence of these metastable structures, relatively large calculation cells are needed. However, previous calculations were limited to the use of small calculation cell sizes to estimate the local potential depth (Δσ) of ordered Al1/2Ga1/2N models. In this work, we were able to evaluate large calculation cells based on the interaction energies between proximate Al atoms (δEAl-Al) in AlGaN alloys. To do this,δEAl-Alvalues were estimated by first-principles calculations (FPCs) using a (5a1× 5a2× 5c) cell. Next, a survey of the possible ordered configurations using various large calculation cell models was performed using the estimatedδEAl-Alvalues and the Monte-Carlo method. Then, various Δσvalues were estimated by FPCs and compared with the configurations previously reported by other research groups. We found that the ordered configuration obtained from the (4a1× 2a2× 1c) calculation cell (C42) has the lowest Δσof -9.3 meV/cation and exhibited an in-plane configuration at thec(0001) plane having (-Al-Al-Ga-Ga-) and (-Al-Ga-) sequence arrangements observed along them11-00planes. Hence, we found consistencies between the morphology obtained from experiment and the shape of the primitive cell based on our numerical calculations.

Probing Copper and Copper-Gold Alloy Surfaces with Space-Quantized Oxygen Molecular Beam.

The orientation and motion of reactants play important roles in reactions. The small rotational excitations involved render the reactants susceptible to dynamical steering, making direct comparison between experiments and theory rather challenging. Using space-quantized molecular beams, we directly probed the (polar and azimuthal) orientation dependence of O2 chemisorption on Cu(110) and Cu3Au(110). We observed polar and azimuthal anisotropies on both surfaces. Chemisorption proceeded rather favorably with the O-O bond axis oriented parallel (vs perpendicular) to the surface and rather favorably with the O-O bond axis oriented along [001] (vs along [1̅10]). The presence of Au hindered the surface from further oxidation, introducing a higher activation barrier to chemisorption and rendering an almost negligible azimuthal anisotropy. The presence of Au also prevented the cartwheel-like rotations of O2.

Efficient Hydrogen Isotope Separation by Tunneling Effect Using Graphene-Based Heterogeneous Electrocatalysts in Electrochemical Hydrogen Isotope Pumping.

The fabrication of a hydrogen isotope enrichment system is essential for the development of industrial, medical, life science, and nuclear fusion fields, and therefore, efficient enrichment techniques with a high separation factor and economic feasibility are still being explored. Herein, we report a hydrogen/deuterium (H/D) separation ability with polymer electrolyte membrane electrochemical hydrogen pumping (PEM-ECHP) using a heterogeneous electrode consisting of palladium and graphene layers (PdGr). By mass spectroscopic analysis, we demonstrate significant bias voltage dependence of the H/D separation factor with a maximum of ∼25 at 0.15 V and room temperature, which is superior to those of conventional separation methods. Theoretical analysis demonstrated that the observed high H/D factor stems from tunneling of hydrogen isotopes through atomically thick graphene during the electrochemical reaction and that the bias dependence of H/D results from a transition from the quantum tunneling regime to the classical overbarrier regime for hydrogen isotopes transfer through the graphene. These findings will help us understand the origin of the isotope separation ability of graphene discussed so far and contribute to developing an economical hydrogen isotope enrichment system using two-dimensional materials.

Interface atom mobility and charge transfer effects on CuO and Cu2O formation on Cu3Pd(111) and Cu3Pt(111).

We bombarded [Formula: see text] and [Formula: see text] with a 2.3 eV hyperthermal oxygen molecular beam (HOMB) source, and characterized the corresponding (oxide) surfaces with synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS). At [Formula: see text], CuO forms on both [Formula: see text] and [Formula: see text]. When we increase the surface temperature to [Formula: see text], [Formula: see text] also forms on [Formula: see text], but not on [Formula: see text]. For comparison, [Formula: see text] forms even at [Formula: see text] on Cu(111). On [Formula: see text], [Formula: see text] forms only after [Formula: see text], and no oxides can be found at [Formula: see text]. We ascribe this difference in Cu oxide formation to the mobility of the interfacial species (Cu/Pd/Pt) and charge transfer between the surface Cu oxides and subsurface species (Cu/Pd/Pt).

Defluorination and adsorption of tetrafluoroethylene (TFE) on TiO2(110) and Cr2O3(0001).

Here, we show that metal oxide surfaces catalyze the formation of intermediate defluorinated tetrafluoroethylene (TFE) radicals, resulting in enhanced binding on the corresponding metal oxide surfaces. We attribute the preferential adsorption and radical formation of TFE on Cr2O3(0001) relative to TiO2(110) to the low oxygen coordination of Cr surface atoms. This hints at a possible dependence of the TFE binding strength to the surface stoichiometry of metal-oxide surfaces.

Quantitative Multilayer Cu(410) Structure and Relaxation Determined by QLEED.

Industrially relevant catalytically active surfaces exhibit defects. These defects serve as active sites; expose incoming adsorbates to both high and low coordinated surface atoms; determine morphology, reactivity, energetics, and surface relaxation. These, in turn, affect crystal growth, oxidation, catalysis, and corrosion. Systematic experimental analyses of such surface defects pose challenges, esp., when they do not exhibit order. High Miller index surfaces can provide access to these features and information, albeit indirectly. Here, we show that with quantitative low-energy electron diffraction (QLEED) intensity analyses and density functional theory (DFT) calculations, we can visualize the local atomic configuration, the corresponding electron distribution, and local reactivity. The QLEED-determined Cu(410) structure (Pendry reliability factor RP ≃ 0.0797) exhibits alternating sequences of expansion (+) and contraction (-) (of the first 16 atomic interlayers) relative to the bulk-truncated interlayer spacing of ca. 0.437 Å. The corresponding electron distribution shows smoothening relative to the bulk-determined structure. These results should aid us to further gain an atomic-scale understanding of the nature of defects in materials.

Spin-Dependent O2 Binding to Hemoglobin.

We report results of our study on the mechanism of spin-dependent O2 binding to hemoglobin, which we represent as FePIm (Fe = iron, P = porphyrin, Im = imidazole). This involves the transition between two states, viz., the oxyhemoglobin state and the deoxyhemoglobin state. The deoxyhemoglobin state pertains to FePIm and a free O2 molecule, while the oxyhemoglobin state pertains to an O2 bound to FePIm. The deoxyhemoglobin and oxyhemoglobin systems have triplet and singlet total magnetizations, respectively. We found that a spin transition from triplet to quintet to singlet mediates the O2 binding process, and this accelerates the reaction. We also found that the position of the Fe atom out of the porphyrin plane is an important indicator of O2 affinity.