arXiv subjects

Alina Vlad

Publications and source records attributed to Alina Vlad.

5 recordsLinked to original sources

arXiv2025

Transient Surface Oxides Form on Pt(111) - But Vanish During Ammonia Oxidation

Ammonia oxidation on platinum catalysts is pivotal for industrial nitric acid production and environmental abatement, yet the role of surface oxides in this process remains debated. Using operando surface X-ray diffraction (SXRD), crystal truncation rod (CTR) analysis, and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we reveal that Pt(111) does not form stable surface oxides under ammonia oxidation conditions. Instead, transient hexagonal monolayers and a Pt(111)-(8x8) superstructure emerge under oxygen-rich atmospheres and above the catalyst light-off temperature, but vanish upon ammonia exposure. Real-time mass spectrometry and NAP-XPS demonstrate that the reaction proceeds via a Langmuir-Hinshelwood mechanism, where adsorbed NHx and O species availability dictate selectivity toward NO or N2. Reducing the oxygen pressure by an order of magnitude slows the kinetics of oxide growth, only detected after 24 hr, and facilitated by transient and precursor structures.

cond-mat.mtrl-sci↗

arXiv2024

X-ray diffraction reveals the consequences of strong deformation in thin smectic films: dilation and chevron formation

Smectic liquid crystals can be viewed as model systems for lamellar structures for which there has been extensive theoretical development. We demonstrate that a nonlinear energy description is required with respect to the usual Landau-de Gennes elasticity in order to explain the observed layer spacing of highly curved smectic layers. Using X-ray diffraction we have quantitatively determined the dilation of bent layers distorted by antagonistic anchoring (as high as 1.8% of dilation for the most bent smectic layers) and accurately described it by the minimal nonlinear expression for energy. We observe a 1° tilt of planar layers that are connected to the curved layers. This value is consistent with simple energetic calculations, demonstrating how the bending energy impacts the overall structure of a thin distorted smectic film. Finally, we show that combined X-ray measurements and theoretical modeling allow for the quantitative determination of the number of curved smectic layers and of the resulting thickness of the dilated region with unprecedented precision.

cond-mat.soft↗

arXiv2020

Gas-induced selective re-orientation of Au-Cu nanoparticles on TiO 2 (110)

Au-Cu bimetallic nanoparticles (NPs) grown on TiO 2 (110) have been followed in-situ using grazing incidence x-ray diffraction and x-ray photoemission spectroscopy from their synthesis to their exposure to a CO/O 2 mixture at low pressure (P < 10-5 mbar) and at different temperatures (300 K-470 K). As-prepared samples are composed of two types of alloyed NPs: randomly oriented and

physics.chem-ph↗

arXiv2016

Wide bandgap semiconductor from a hidden 2D incommensurate graphene phase

Producing a usable semiconducting form of graphene has plagued the development of graphene electronics for nearly two decades. Now that new preparation methods have become available, graphene's intrinsic properties can be measured and the search for semiconducting graphene has begun to produce results. This is the case of the first graphene "buffer" layer grown on SiC(0001) presented in this work. We show, contrary to assumptions of the last forty years, that the buffer graphene layer is not commensurate with SiC. The new modulated structure we've found resolves a long standing contradiction where ab initio calculations expect a metallic buffer, while experimentally it is found to be a semiconductor. Model calculations using the new incommensurate structure show that the semiconducting $π$-band character of the buffer comes from partially hybridized graphene incommensurate boundaries surrounding unperturbed graphene islands.

cond-mat.mes-hall↗

arXiv2015

Symmetry of the Fermi surface and evolution of the electronic structure across the paramagnetic-helimagnetic transition in MnSi/Si(111)

MnSi has been extensively studied for five decades, nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevented from a comprehensive understanding the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at T$_C$ $\sim$ 40 K, along with the appearance of sharp quasiparticle emission below T$_C$. The shape of the resulting FS is found to fulfill robust nesting effects. These effects can be at the origin of strong magnetic fluctuations not accounted for by state-of-art quasiparticle self-consistent GW approximation. From this perspective, the unforeseen quasiparticle damping detected in the paramagnetic phase and relaxing only below T$_C$, along with the persistence of the d-bands splitting well above T$_C$, at odds with a simple Stoner model for itinerant magnetism, open the search for exotic magnetic interactions favored by FS nesting and affecting the quasiparticles lifetime.

cond-mat.str-el↗