Self-healing in B12P2 through Mediated Defect Recombination

Citation: Self-healing in B12P2 through Mediated Defect Recombination. S. P. Huber, E. Gullikson, C. D. Frye, J. H. Edgar, R. W. E. van de Kruijs, F. Bijkerk, and D. Prendergast. Chemistry of Materials 28 8415--8428 (2016) 10.1021/acs.chemmater.6b04075The icosahedral boride B12P2 has been reported to exhibit “self-healing” properties, after transmission electron microscopy recordings of sample surfaces, which were exposed to highly energetic particle beams, revealed little to no damage. In this work, employing calculations from first-principles within the density functional theory (DFT) framework, the structural characteristics of boron interstitial and vacancy defects in B12P2 are investigated. Using nudged elastic band simulations, the diffusion properties of interstitial and vacancy defects and their combination, in the form of Frenkel defect pairs, are studied. We find that boron icosahedra maintain their structural integrity even when in a degraded state in the presence of a vacancy or interstitial defect and that the diffusion activation energy for the recombination of an interstitial vacany pair can be as low as 3 meV, in line with the previously reported observation of “self-healing”

Detection of defect populations in superhard semiconductor boron subphosphide B12P2 through X-ray absorption spectroscopy

Citation: Detection of defect populations in superhard semiconductor boron subphosphide B12P2 through X-ray absorption spectroscopy. S. P. Huber, E. Gullikson, J. Meyer-Ilse, C. D. Frye, J. H. Edgar, R. W. E. van de Kruijs, F. Bijkerk, and D. Prendergast J. Mater. Chem. A 5 5737--5749 (2017) 10.1039/c6ta10935gRecent theoretical work has shown for the first time how the experimentally observed property of “self-healing” of the superhard semiconductor boron subphosphide (B12P2) arises through a process of mediated defect recombination. Experimental verification of the proposed mechanism would require a method that can detect and distinguish between the various defect populations that can exist in B12P2. X-ray absorption near-edge spectroscopy (XANES) is such a method and in this work we present experimentally collected spectra of B12P2samples with varying crystalline qualities. By simulating the X-ray spectroscopic signatures of potential crystallographic point defects from first-principles within the density functional theory framework, the presence of defect populations can be determined through spectroscopic fingerprinting. Our results find an increasing propensity for the presence of phosphorus vacancy defects in samples deposited at lower temperatures but no evidence for comparable populations of boron vacancies in all the samples that have been studied. The absence of large amounts of boron vacancies is in line with the “self-healing” property of B12P2

Relation between composition and fracture strength in off-stoichiometric metal silicide free-standing membranes

In this work, we investigated the influence of composition on the polycrystalline structure, elastic properties and fracture strength, of ZrxSi1-x, NbxSi1-x, and MoxSi1-x free-standing thin films that were deposited by magnetron sputtering and subsequently annealed at 500 °C. Despite deviations from the stoichiometric composition, the crystalline structure of all films, except for the most Zr-rich ZrxSi1-x, corresponded to their respective stoichiometric disilicide structures, without the formation of a second-phase. Off-stoichiometry was found to be accompanied by the presence of lattice defects and a decrease of the grain size, which bring about a lower tensile stress in the films. The dependence of the fracture strength on the composition was remarkably similar for the three silicides, with the lowest and highest strength values occurring for samples with 30% and 37–40% of metal content, respectively. The observed dependence of strength on composition was attributed to the combination of the Hall-Petch effect, changes in the morphology and strength of grain boundaries, and the enhancement of crystal plasticity due to lattice defects induced by off-stoichiometry

A semi-analytical approach for the characterization of ordered 3D nano structures using grazing-incidence X-ray fluorescence

Following the recent demonstration of grazing-incidence X-ray fluorescence (GIXRF) based characterization of the 3D atomic distribution of different elements and dimensional parameters of periodic nanoscale structures, this work presents a new computational scheme for the simulation of the angular dependent fluorescence intensities from such periodic 2D and 3D nanoscale structures. The computational scheme is based on the dynamical diffraction theory in many-beam approximation, which allows to derive a semi-analytical solution to the Sherman equation in a linear-algebraic form. The computational scheme has been used to analyze recently published GIXRF data measured on 2D Si3N4 lamellar gratings, as well as on periodically structured 3D Cr nano pillars. Both the dimensional and structural parameters of these nanostructures have been reconstructed by fitting numeric simulations to the experimental GIXRF data. Obtained results show good agreement with nominal parameters used in the manufacturing of the structures, as well as with reconstructed parameters based on the previously published finite element method simulations, in case of the Si3N4 grating

Chemically mediated diffusion of d-metals and B through Si and agglomeration at Si-on-Mo interfaces

Chemical diffusion and interlayer formation in thin layers and at interfaces is of increasing influence in nanoscopic devices, such as nanoelectronics and reflective multilayer optics. Chemical diffusion and agglomeration at interfaces of thin Ru, Mo, Si, and B4C layers have been studied with x-ray photoelectron spectroscopy, cross section electron energy loss spectroscopy, high-angle annular dark field scanning transmission electron microscopy, and energy dispersive x-ray in relation to observations in Ru-on-B4C capped Mo/Si multilayers. Rather than in the midst of the Si layer, silicides and borides are formed at the Si-on-Mo interface front, notably RuSix and MoBx. The interface apparently acts as a precursor for further chemical diffusion and agglomeration of B, Ru, and also other investigated d-metals. Reversed “substrate-on-adlayer” interfaces can yield entirely suppressed reactivity and diffusion, stressing the influence of surface free energy and the supply of atoms to the interface via segregation during thin layer growth

Nitridation and contrast of B4C/La interfaces and X-ray multilayer optics

Chemical diffusion and interlayer formation in thin layers and at interfaces is of increasing influence in nanoscopic devices such as nano-electronics, magneto-optical storage and multilayer X-ray optics. We show that with the nitridation of reactive B4C/La interfaces, both the chemical and optical contrast can be greatly enhanced. Although interaction and diffusion of N2 from the substrate towards the adlayer does occur, this surfactant mediated growth contributes to chemical and optical interface properties that enable major reflectivity improvements of multilayer optics for 6.7 < λ < 7.0 nm.\ud \u