Nina I. Kovtyukhova

Non-oxidative intercalation and exfoliation of graphite by Brønsted acids

Graphite intercalation compounds are formed by inserting guest molecules or ions between sp(2)-bonded carbon layers. These compounds are interesting as synthetic metals and as precursors to graphene. For many decades it has been thought that graphite intercalation must involve host-guest charge transfer, resulting in partial oxidation, reduction or covalent modification of the graphene sheets. Here, we revisit this concept and show that graphite can be reversibly intercalated by non-oxidizing Brønsted acids (phosphoric, sulfuric, dichloroacetic and alkylsulfonic acids). The products are mixtures of graphite and first-stage intercalation compounds. X-ray photoelectron and vibrational spectra indicate that the graphene layers are not oxidized or reduced in the intercalation process. These observations are supported by density functional theory calculations, which indicate a dipolar interaction between the guest molecules and the polarizable graphene sheets. The intercalated graphites readily exfoliate in dimethylformamide to give suspensions of crystalline single- and few-layer graphene sheets.

Layer-by-layer self-assembly strategy for template synthesis of nanoscale devices

Abstract A combined membrane replication/layer-by-layer synthetic approach to preparing nanoscale rod-shaped rectifiers is described. Alumina and polycarbonate (PC) membranes (pore diameters 200, 100 and 70 nm) were used as templates for the electrochemical preparation of free-standing Au nanowires several microns in length. Wet layer-by-layer self-assembly of nanoparticle (TiO2 or ZnO)/polymer multilayer films was performed inside the membrane pores in two ways. (1) Growing the film between metal electrodeposition steps to give in-wire junctions; (2) first coating the membrane walls with multilayer films, and then growing nanowires inside the resulting tubules to give concentric structures. TiO2/PSS, ZnO/PSS (PSS=polystyrenesulfonate) and ZnO/PAN (PAN=polyaniline) assembly was driven by electrostatic and covalent-coordination interactions, respectively. The current–voltage (I–V) characteristics of nanowires containing semiconductor nanoparticles show current rectifying behavior. Current rectification appears to arise at the oxide semiconductor–metal interface. Switching behavior and hysteresis, which was found in all devices, was particularly evident in junctions containing anionic PSS and cationic TiO2 particles, and less evident in ZnO-containing devices.

Reversible intercalation of hexagonal boron nitride with Brønsted acids.

Hexagonal boron nitride (h-BN) is an insulating compound that is structurally similar to graphite. Like graphene, single sheets of BN are atomically flat, and they are of current interest in few-layer hybrid devices, such as transistors and capacitors, that contain insulating components. While graphite and other layered compounds can be intercalated by redox reactions and then converted chemically to suspensions of single sheets, insulating BN is not susceptible to oxidative intercalation except by extremely strong oxidizing agents. We report that stage-1 intercalation compounds can be formed by simple thermal drying of h-BN in Brønsted acids H2SO4, H3PO4, and HClO4. X-ray photoelectron and vibrational spectra, as well as electronic structure and molecular dynamics calculations, demonstrate that noncovalent interactions of these oxyacids with the basic N atoms of the sheets drive the intercalation process.

Ultrathin nanoparticle ZnS and ZnS: Mn films: surface sol-gel synthesis, morphology, photophysical properties

Abstract Ultrathin films of ZnS and Mn-doped ZnS were grown on silicon substrates using surface sol–gel reactions, and the film growth process was characterized by ellipsometry, atomic force microscopy, X-ray photoelectron spectroscopy, UV-visible absorbance and photoluminescence (PL) spectroscopy. The Si substrates were pre-treated by chemical oxidation. On the oxidized Si/SiO x surface, nanoparticulate films of ZnS and Mn-doped ZnS were grown by sequential immersion in aqueous metal acetate and sodium sulfide solutions. During the first four adsorption cycles, there was little film growth, but thereafter the amount of material deposited was linear with the number of adsorption cycles. This behavior is consistent with the formation of ZnS nuclei at low coverage, followed by particle growth in subsequent cycles. PL spectra are consistent with incorporation of Mn 2+ into the ZnS nanoparticles.

Self-assembly of nanostructured composite ZnO/polyaniline films

Abstract Ultrathin composite ZnO/polyaniline (PAN) films were deposited from organic solutions on silicon and ITO substrates using wet layer-by-layer self-assembly technique. The film growth process was characterized by transmission electron microscopy, ellipsometry, atomic force microscopy, IR and UV-visible absorbance and photoluminescence spectroscopy, and electrical measurements. The Si substrates were pre-treated by chemical oxidation. Multilayer films were grown by sequential immersion of the substrate in an ethanolic ZnO sol and PAN solution in dimethyl formamide. The first adsorption cycle resulted in well-packed monoparticulate ZnO layer almost completely covering the substrate, which predetermined the regular growth of densely packed and quite smooth ten-layer ZnO/PAN film. Photoluminescence and IR data assumed chemical interaction between the components in neighbouring layers. The multilayer (ZnO/PAN) 9 ZnO film sandwiched between ITO and Pt electrodes exhibited strong photoelectrical response while both the components were photoelectrically inactive in our experimental conditions. The reversible conversion from insulating to conducting state was observed under irradiation by light with a wavelength below 350 nm.

Self-assembled thin films from lamellar metal disulfides and organic polymers

The intercalation/exfoliation reactions of MoS2 and SnS2 yield lamellar colloids, which may be grown layer-by-layer on cationic surfaces by alternate adsorption of cationic clusters or polymers.

Atomically Thin Layers of Graphene and Hexagonal Boron Nitride Made by Solvent Exfoliation of Their Phosphoric Acid Intercalation Compounds

The development of scalable and reliable techniques for the production of the atomically thin layers of graphene and hexagonal boron nitride (h-BN) in bulk quantities could make these materials a powerful platform for devices and composites that impact a wide variety of technologies (Nature 2012, 490, 192-200). To date a number of practical exfoliation methods have been reported that are based on sonicating or stirring powdered graphite or h-BN in common solvents. However, the products of these experiments consist mainly of few-layer sheets and contain only a small fraction of monolayers. A possible reason for this is that splitting the crystals into monolayers starts from solvent intercalation, which must overcome the substantial interlayer cohesive energy (120-720 mJ/m2) of the van der Waals solids. Here we show that the yield of the atomically thin layers can be increased to near unity when stage-1 intercalation compounds of phosphoric acid are used as starting materials. The exfoliation to predominantly monolayers was achieved by stirring them in medium polarity organic solvents that can form hydrogen bonds. The exfoliation process does not disrupt the sp2 π-system of graphene and is gentle enough to allow the preparation of graphene and h-BN monolayers that are tens of microns in their lateral dimensions.

Integration of Si biocomplex building blocks into porous Si for self-organization of multifunctional structures

We developed the nanotechnology of the self-organization of the multifunctional structures from Si biocomplex and nano-Si crystals in por-Si using the integration of Si biocomplex building blocks into por-Si under UV illumination and optical control of self-organization process. Physico-chemical electronic and optical of the nanostructures were detected by AFM, IR spectroscopy. Intense photoluminescence with the maxima at 455 HM (2.73 eV) and 750 HM (1.65 eV), the maximal reflectance at 370 nm, diode properties with/without UV-illumination are characteristic for these structures. Some experimental evidences of bioactivity of these structures have been obtained.

Nanostructured multilayer metal/por-Si structures based on CoSi2 film: optical and electronic properties

Abstract To create nanostructured multilayer Co/por-Si structures based on CoSi2 film with determined size and distribution of the nanocrystals, the interaction between the 6.5-nm Co layer and the por-Si layer surface in vacuum was used. The formation of the self-ordered system based on top layer of CoSi2 nanocrystals, intermediated layer (130–150 nm) contained 3–11 nm Si nanocrystals, and por-Si layer (1.1; 1.2; 1.4 μm) grown on the single crystal Si was experimentally confirmed by TEM, AFM, scanning tunnelling, IR, and UV–VIS spectroscopies. The formed por-CoSi2/por-Si structures have novel optical and electronic properties in comparison with por-Si: the IR bands of maximal absorption (648–1275 cm−1) and maximal reflectance (2000–3200 cm−1); the maximal reflectance (up to 80%) at 800–900 nm, the optical bandgap of Si nanocrystals is Eg=1.2–2.6 eV, and the height of the barrier of CoSi2/nano-Si structures is 0.7–0.95 eV.

Self-Assembly of Nanoblocks and Molecules in Optical Thin-Film Nanostructures

The growing interest in developing techniques to prepare ultrathin semiconductor nanoparticle films is motivated by the size-dependent electronic and optical properties of semiconductors, which lead to a range of potential applications in electronic and optoelectronic devices, solar cells, photoelectrodes, photocatalysts, and sensors. The wet chemical synthesis of ultrathin semiconductor films represents, in principle, a simple and inexpensive alternative to more technologically demanding chemical vapor deposition (CVD) and physical techniques [1]. However, the realization of practical devices from wet chemical synthesis requires the development of film growth techniques that give similar or better quality films than vapor-phase methods. In particular, precise control of film thickness, crystallinity, and morphology are significant problems to be overcome in wet chemical synthesis.