A. E. Aleksenskii

The structure of diamond nanoclusters

A model describing the structure of diamond nanoclusters produced by explosive shocks is proposed. The model is based on experimental data obtained from x-ray diffraction and small-angle x-ray scattering. This model considers the diamond nanocluster as a crystalline diamond core coated by a carbon shell having a fractal structure. The shell structure depends both on the cooling kinetics of the detonation products and on the method used to extract from them the diamond fraction.

Diamond-graphite phase transition in ultradisperse-diamond clusters

A systematic study of the diamond-graphite structural phase transition in ultradisperse-diamond clusters obtained by the detonation technique is reported. Samples of two types, differing in the kinetics of detonation-product cooling, were investigated. The phase transition was achieved under heating in an inert atmosphere in the temperature range 720–1400 K. The transition was identified by Raman scattering and x-ray diffraction data. Raman and x-ray characterization showed the ultradisperse diamond, irrespective of the cooling rate used, to be cluster material possessing diamond structure with a characteristic nanocrystal size of 43 Å. The diamond-graphite phase transition in ultradisperse diamond is shown to start from the cluster surface inwards at Tpt≈1200 K, i.e. at substantially lower temperatures than is the case with bulk diamond single crystals.

Optical properties of nanodiamond layers

Thin ultradisperse diamond (UDD) layers deposited from a water suspension are studied by optical and x-ray photoelectron spectroscopy (XPS). The effective band gap determined by the 104-cm−1 criterion for ozone-cleaned UDD is 3.5 eV. The broad structureless photoluminescence band (380–520 nm) is associated with radiative recombination through a system of continuously distributed energy levels in the band gap of diamond nanoclusters. The optical absorption of the material at 250–1000 nm originates from absorption on the disordered nanocluster surface containing threefold-coordinated carbon. The surface of UDD clusters subjected to acid cleaning contains nitrogen-oxygen complexes adsorbed in the form of NO3− nitrate ions. Annealing in a hydrogen atmosphere results in desorption of the nitrate ions from the cluster surface. The evolution of the oxygen (O1s) and nitrogen (N1s) lines in the XPS spectra under annealing of a UDD layer is studied comprehensively.

Detonation Nanodiamonds as Catalyst Supports

The article reports on experimental study of catalytic properties of a new system: Pt on detonation nanodiamond (Pt/DND) for the carbon monoxide oxidation reaction. The catalytic activity of Pt/DND structures as a function of platinum content in the catalyst within the 7–80 wt % intervals was studied and the structure of the Pt/DND catalyst was investigated by X-ray diffraction and HRTEM. The Pt/DND catalysts developed demonstrate a high degree of conversion of CO to CO2 at room temperature, a feature making them attractive for commercial applications as catalytic systems for purification of air from carbon monoxide in houses and industrial areas. The new catalysts were incorporated in solid-state electrochemical CO gas sensors. A statement on efficiency of the detonation nanodiamonds as a support for catalytic metals of platinum groups has been done.

Ultradisperse Diamond Cluster Aggregation Studied by Atomic Force Microscopy

The structure of ultradisperse diamond (UDD) conglomerates was studied by scanning atomic-force microscopy (AFM). The UDD layers were prepared from a detonation carbon obtained by synthesis in an aqueous medium. The finest details in the AFM images of UDD layers are of the order of 10 nm, which does not allow individual 4.5-nm diamond clusters to be distinguished. The UDD conglomerates deposited and dried on a silicon substrate surface, exhibit certain deformation and differ from the initial (apparently, spherical) shape. This may imply that cohesion between the UDD nanoparticles is comparable with their adhesion to the silicon substrate.

Ordered porous diamond films fabricated by colloidal crystal templating

We have developed a colloidal crystal templating method for preparation of diamond films with 2D and 3D ordered porous structures. The technological process involved breaks down into (a) impregnation into the pores of silica colloidal crystal (opal) films of detonation nanodiamond (DND) particles from their hydrosol; (b) microwave plasma-enhanced chemical vapor deposition (MWPECVD) regrowth with diamond of pores with high DND filling; (c) Ar(+) ion dry etching of fragments of shells of coalesced diamond crystallites which form in the course of MWPECVD on the surface of the SiO(2) beads making up the outer surface of a film and (d) wet etching of the SiO(2) template in aqueous HF solution. The final samples are either connected to the substrate or free-standing films of various thicknesses having 2D or 3D ordered porous structures. The morphology of the diamond films fabricated by this method replicates the pore network of the opal template. Raman measurements confirm the diamond structure of the synthesized ordered porous material.

Effect of hydrogen on the structure of ultradisperse diamond

The paper reports on a study of the effect of annealing in hydrogen on the structural phase transition in clusters of ultradisperse diamond (UDD) obtained by the detonation method. The samples studied were of two types, namely, prepared by the “dry” and “wet” techniques, which differ in the cooling rate of the detonation products and, accordingly, in the structure of the diamond nanocluster shell. It is shown that, irrespective of the type of synthesis, the relative content of the diamond (sp3) phase increases within the anneal temperature range of 450 to 750°C, the increase being more pronounced in the samples prepared by “dry” synthesis. A model accounting for the observed structural transformation processes is discussed. A hypothesis of the possibility of compacting UDD clusters into bulk single crystals is put forward.

Optical properties of detonation nanodiamond hydrosols

Studies of the optical properties of hydrosols of 4-nm detonation nanodiamond particles performed in the 0.2–1.1 μm range have revealed a novel effect, a strong increase of absorption at the edges of the spectral range, and provided its explanation in terms of absorption of radiation by the dimer chains (the so-called Pandey chains) fixed on the surface of a nanodiamond particle. The effect of particle size distribution in a hydrosol on the relative intensity of Rayleigh scattering and light absorption by nanodiamond particles in this range has been analyzed.

Effect of tetraethoxysilane pretreatment on synthesis of colloidal particles of amorphous silicon dioxide

The effect of the time passed after tetraethoxysilane treatment with ammonia on the diameter of particles produced by tetraethoxysilane hydrolysis in alcohol-water-ammonia media is studied. The regulation the time passed after of tetraethoxysilane treatment results in the synthesis of submicron monodisperse spherical silica particles with diameters differing by a factor of two. The difference is explained by the formation of SiO2 particles with sizes of 10–100 nm in tetraethoxysilane during 10–30 h after treatment with ammonia. These particles enhance the concentration of nucleation centers in a reaction mixture, thus decreasing the final size of monodisperse silica spheres. Opal films with a high structural perfection and pronounced photonic crystal properties are grown based on the obtained monodisperse SiO2 particles.

Single-layer graphene oxide films on a silicon surface

A method is proposed to produce large-area single-layer graphene oxide films on the surface of semiconductor silicon wafers by precipitation from aqueous suspensions. Graphene oxide is synthesized from natural crystalline graphite during chemical oxidation and represents a wide-gap insulator. Single-layer graphene with a homogeneous-fragment size up to 50 μm can be formed by the reduction of graphene oxide films, and this size is significantly larger than those achieved to date.