Andrey A. Shiryaev

Molecular-sized fluorescent nanodiamonds

Doping of carbon nanoparticles with impurity atoms is central to their application. However, doping has proven elusive for very small carbon nanoparticles because of their limited availability and a lack of fundamental understanding of impurity stability in such nanostructures. Here, we show that isolated diamond nanoparticles as small as 1.6xa0nm, comprising only ∼400 carbon atoms, are capable of housing stable photoluminescent colour centres, namely the silicon vacancy (SiV). Surprisingly, fluorescence from SiVs is stable over time, and few or only single colour centres are found per nanocrystal. We also observe size-dependent SiV emission supported by quantum-chemical simulation of SiV energy levels in small nanodiamonds. Our work opens the way to investigating the physics and chemistry of molecular-sized cubic carbon clusters and promises the application of ultrasmall non-perturbative fluorescent nanoparticles as markers in microscopy and sensing.

Nitrogen and Luminescent Nitrogen‐Vacancy Defects in Detonation Nanodiamond

An efficient method to investigate the microstructure and spatial distribution of nitrogen and nitrogen-vacancy (N-V) defects in detonation nanodiamond (DND) with primary particle sizes ranging from approximately 3 to 50 nm is presented. Detailed analysis reveals atomic nitrogen concentrations as high as 3 at% in 50% of diamond primary particles with sizes smaller than 6 nm. A non-uniform distribution of nitrogen within larger primary DND particles is also presented, indicating a preference for location within the defective central part or at twin boundaries. A photoluminescence (PL) spectrum with well-pronounced zero-phonon lines related to the N-V centers is demonstrated for the first time for electron-irradiated and annealed DND particles at continuous laser excitation. Combined Raman and PL analysis of DND crystallites dispersed on a Si substrate leads to the conclusion that the observed N-V luminescence originates from primary particles with sizes exceeding 30 nm. These findings demonstrate that by manipulation of the size/nitrogen content in DND there are prospects for mass production of nanodiamond photoemitters based on bright and stable luminescence from nitrogen-related defects.

X-ray spectroscopy study of the chemical state of “invisible” Au in synthetic minerals in the Fe-As-S system

Abstract Minerals of the Fe-As-S system are the main components of Au ores in many hydrothermal deposits, including Carlin-type Au deposits, volcanogenic massive sulfide deposits, epithermal, mesothermal, sedimentary-hosted systems, and Archean Au lodes. The “invisible” (or refractory) form of Au is present in all types of hydrothermal ores and often predominates. Knowledge of the chemical state of “invisible” Au (local atomic environment/structural position, electronic structure, and oxidation state) is crucial for understanding the conditions of ore formation and necessary for the physical-chemical modeling of hydrothermal Au mineralization. In addition, it will help to improve the technologies of ore processing and Au extraction. Here we report an investigation of the chemical state of “invisible” Au in synthetic analogs of natural minerals (As-free pyrite FeS2, arsenopyrite FeAsS, and löllingite FeAs2). The compounds were synthesized by means of hydrothermal (pyrite) and salt flux techniques (in each case) and studied by X-ray absorption fine structure (XAFS) spectroscopy in a high-energy resolution fluorescence detection (HERFD) mode in combination with first-principles quantum chemical calculations. The content of “invisible” Au in the synthesized löllingite (800 ± 300 ppm) was much higher than that in arsenopyrite (23 ± 14 ppm). The lowest Au content was observed in zonal pyrite crystals synthesized in a salt flux. High “invisible” Au contents were observed in hydrothermal pyrite (40–90 ppm), which implies that this mineral can efficiently scavenge Au even in As-free systems. The Au content of the hydrothermal pyrite is independent of sulfur fugacity and probably corresponds to the maximum Au solubility at the experimental P-T parameters (450 °C, 1 kbar). It is shown that Au replaces Fe in the structures of löllingite, arsenopyrite, and hydrothermal pyrite. The Au-ligand distance increases by 0.14 Å (pyrite), 0.16 Å (löllingite), and 0.23 Å (As), 0.13 Å (S) (arsenopyrite) relative to the Fe-ligand distance in pure compounds. Distortions of the atomic structures are localized around Au atoms and disappear at R > ∼4 Å. Chemically bound Au occurs only in hydrothermal pyrite, whereas pyrite synthesized without hydrothermal fluid contains only Au°. The heating (metamorphism) of hydrothermal pyrite results in the decomposition of chemically bound Au and formation of Au° nuggets, which coarsen with increasing temperature. Depending on the chemical composition of the host mineral, Au can play a role of either a cation or an anion: the Bader atomic partial charge of Au decreases in the order pyrite (+0.4 e) > arsenopyrite (0) > löllingite (−0.4 e). Our results suggest that other noble metals (platinum group elements, Ag) can form a chemically bound refractory admixture in base metal sulfides/chalcogenides. The content of chemically bound noble metals can vary depending on the composition of the host mineral and ore history.

Plutonium Environment in Lanthanide Borosilicate Glass

Two lanthanide borosilicate (LaBS) glasses containing 9.5 and 5.0 wt.% PuO 2 prepared at 1500 °C consisted of a vitreous phase and minor crystalline PuO 2 (or PuO 2-HfO 2 solid solution with minor HfO 2) and britholite-type phases. X-ray absorption spectra of Pu L III edge in the as-prepared and stored for various periods LaBS glasses were recorded, analyzed and compared with the spectra of crystalline PuO 2. Pu in the as-prepared glass existed in predominantly tetravalent form (Pu 4+ ions) but its storage in air results in partial oxidation as was seen from shift of peak energy values. In the structure of the asprepared glass, Pu 4+ ions had a co-ordination number (CN) close to 6 (~6.3) and were located within the axially squeezed octahedra with five equidistant oxygen ions at a distance of 2.265±0.015 A and one – at a shorter distance (2.130±0.010 A) from the Pu 4+ ion. The Pu uf8e7Pu(M) distance (second co-ordination shell) was 3.675±0.015 A. “Aging” of the LaBS glass with transformation of some fraction of Pu into penta- or/and hexavalent form was accompanied by a structural transformation.

Oxidation state and coordination surrounding of iron and uranium in sodium aluminum iron phosphate glasses

The oxidation state of Fe and U and the coordination surrounding of Fe in uranium-containing sodium aluminum iron phosphate glasses were determined by analysis of the FeK and UL3 X-ray absorption nearedge structure (XANES), X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy on 57Fe nuclei. Uranium is present in the glasses in the form of U(V) and U(VI), and iron, in the form of Fe(III) and Fe(II), mainly in the distorted octahedral surrounding. The fraction of U in various oxidation states depends on the form of untroducing uranium (UO2 or UO3) and on the oxide concentration. With an increase in the UO3 concentration in glasses, the fraction of U(VI) increases and the fraction Fe(III) relative to Fe(II) decreases.

Ion implantation in nanodiamonds: size effect and energy dependence

Nanoparticles are ubiquitous in nature and are increasingly important for technology. They are subject to bombardment by ionizing radiation in a diverse range of environments. In particular, nanodiamonds represent a variety of nanoparticles of significant fundamental and applied interest. Here we present a combined experimental and computational study of the behaviour of nanodiamonds under irradiation by xenon ions. Unexpectedly, we observed a pronounced size effect on the radiation resistance of the nanodiamonds: particles larger than 8u2009nm behave similarly to macroscopic diamond (i.e. characterized by high radiation resistance) whereas smaller particles can be completely destroyed by a single impact from an ion in a defined energy range. This latter observation is explained by extreme heating of the nanodiamonds by the penetrating ion. The obtained results are not limited to nanodiamonds, making them of interest for several fields, putting constraints on processes for the controlled modification of nanodiamonds, on the survival of dust in astrophysical environments, and on the behaviour of actinides released from nuclear waste into the environment.

Single-crystal Fe-bearing sphalerite: synthesis, lattice parameter, thermal expansion coefficient and microhardness

Sphalerite crystals (Fe,Zn)S containing up to 56xa0mol% of FeS have been synthesized by gas transport method and in molten salts in the temperature range 340–780xa0°C at various sulfur fugacities. It is shown that lattice parameter of Fe-bearing sphalerite changes with temperature and composition (x, mol% FeS in sphalerite) according to parabolic equation:

_2

a, pm ,0.0004/{text{AA}} = , left( {5.4099, pm ,0.0008} right) , + , left( {5.82, pm ,0.36} right) cdot 10^{ - 4} cdot x , + , left( { - 4.7, pm ,0.6} right) cdot 10^{ - 6} cdot x^{2} + , left( {4.2, pm ,0.4} right) cdot 10^{ - 5} cdot left( {T{-} , 298.15{text{ K}}} right)