RESUMO
It is critical to understand the laws of quantum mechanics in transformative technologies for computation and quantum information science applications to enable the ongoing second quantum revolution calls. Recently, spin qubits based on point defects have gained great attention, since these qubits can be initiated, selectively controlled, and read out with high precision at ambient temperature. The major challenge in these systems is controllably generating multiqubit systems while properly coupling the defects. To address this issue, we began by tackling the engineering challenges these systems present and understanding the fundamentals of defects. In this regard, we controllably generate defects in MoS2 and WS2 monolayers and tune their physicochemical properties via proton irradiation. We quantitatively discovered that the proton energy could modulate the defects' density and nature; higher defect densities were seen with lower proton irradiation energies. Three distinct defect types were observed: vacancies, antisites, and adatoms. In particular, the creation and manipulation of antisite defects provides an alternative way to create and pattern spin qubits based on point defects. Our results demonstrate that altering the particle irradiation energy can regulate the formation of defects, which can be utilized to modify the properties of 2D materials and create reliable electronic devices.
RESUMO
The titanomagnetites (Fe2-xTixO4,x⩽ 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe2TiO4were deposited by molecular beam epitaxy (MBE) on MgO(001) at 250-375 °C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe3O4and rutile TiO2. Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe3+, Fe2+, Fe0). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe2+. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe2+was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe2+was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe2TiO4by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates.
RESUMO
Recent investigations on spinel CoMn2O4have shown its potential for applications in water splitting and fuel cell technologies as it exhibits strong catalytic behavior through oxygen reduction reactivity. To further understand this material, we report for the first time the synthesis of single-crystalline Co1+xMn2-xO4thin films using molecular beam epitaxy. By varying sample composition, we establish links between cation stoichiometry and material properties using in-situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, x-ray absorption spectroscopy, and spectroscopic ellipsometry. Our results indicate that excess Co ions occupy tetrahedral interstitial sites at lower excess Co stoichiometries, and become substitutional for octahedrally-coordinated Mn at higher Co levels. We compare these results with density functional theory models of stoichiometric CoMn2O4to understand how the Jahn-Teller distortion and hybridization in Mn-O bonds impact the ability to hole dope the material with excess Co. The findings provide important insights into CoMn2O4and related spinel oxides that are promising candidates for inexpensive oxygen reduction reaction catalysts.
