Zsolt Szekrényes

Nanowires of Methylammonium Lead Iodide (CH3NH3PbI3) Prepared by Low Temperature Solution-Mediated Crystallization

We report the synthesis of Methylammonium Lead Iodide (CH(3)NH(3)PbI(3)) nanowires by a low temperature solution processed crystallization using a simple slip-coating method. The anisotropic particle shape exhibits advantages over nanoparticles in terms of charge transport under illumination. These results provide a basis for solvent-mediated tailoring of structural properties like the crystallite size and orientation in trihalide perovskite thin films, which, once implemented into a device, may ultimately result in an enhanced charge carrier extraction.

Characterization of luminescent silicon carbide nanocrystals prepared by reactive bonding and subsequent wet chemical etching

Fabrication of nanosized silicon carbide crystals is a crucial aspect for many biomedical applications. Here, we report an effective fabrication method of silicon carbide nanocrystals based on the reactive bonding method followed by electroless wet chemical etching. Our samples show strong violet-blue emission in the 410–450 nm region depending on the used solvents. Raman and infrared measurements unraveled the surface bonding structure of the fabricated nanoparticles being different from silicon carbide microcrystals. This might give an opportunity to use standard chemistry methods for biological functionalization of such nanoparticles.

Interactions and chemical transformations of coronene inside and outside carbon nanotubes

By exposing flat and curved carbon surfaces to coronene, a variety of van der Waals hybrid heterostructures are prepared, including coronene encapsulated in carbon nanotubes, and coronene and dicoronylene adsorbed on nanotubes or graphite via π-π interactions. The structure of the final product is determined by the temperature of the experiment and the curvature of the carbon surface. While at temperatures below and close to the sublimation point of coronene, nanotubes with suitable diameters are filled with single coronene molecules, at higher temperatures additional dimerization and oligomerization of coronene occurs on the surface of carbon nanotubes. The fact that dicoronylene and possible higher oligomers are formed at lower temperatures than expected for vapor-phase polymerization indicates the active role of the carbon surface used primarily as template. Removal of adsorbed species from the nanotube surface is of utmost importance for reliable characterization of encapsulated molecules: it is demonstrated that the green fluorescence attributed previously to encapsulated coronene is instead caused by dicoronylene adsorbed on the surface which can be solubilized and removed using surfactants. After removing most of the adsorbed layer, a combination of Raman spectroscopy and transmission electron microscopy was employed to follow the transformation dynamics of coronene molecules inside nanotubes.

Bundle versus network conductivity of carbon nanotubes separated by type

We report wide-range optical investigations on transparent conducting networks made from separated (semiconducting, metallic) and reference (mixed) single-walled carbon nanotubes, complemented by transport measurements. Comparing the intrinsic frequency-dependent conductivity of the nanotubes with that of the networks, we conclude that higher intrinsic conductivity results in better transport properties, indicating that the properties of the nanotubes are at least as much important as the contacts. We find that HNO3 doping offers a larger improvement in transparent conductive quality than separation. Spontaneous dedoping occurs in all samples but is most effective in films made of doped metallic tubes, where the sheet conductance returns close to its original value within 24 h.

Melting of Hydrogen Bonds in Uracil Derivatives Probed by Infrared Spectroscopy and ab Initio Molecular Dynamics

The thermal response of hydrogen bonds is a crucial aspect in the self-assembly of molecular nanostructures. To gain a detailed understanding of their response, we investigated infrared spectra of monomers and hydrogen-bonded dimers of two uracil-derivative molecules, supported by density functional theory calculations. Matrix isolation spectra of monomers, temperature dependence in the solid state, and ab initio molecular dynamics calculations give a comprehensive picture about the dimer structure and dynamics of such systems as well as a proper assignment of hydrogen-bond affected bands. The evolution of the hydrogen bond melting is followed by calculating the C═O···H-N distance distributions at different temperatures. The result of this calculation yields excellent agreement with the H-bond melting temperature observed by experiment.

On the formation of blisters in annealed hydrogenated a-Si layers

Differently hydrogenated radio frequency-sputtered a-Si layers have been studied by infrared (IR) spectroscopy as a function of the annealing time at 350°C with the aim to get a deeper understanding of the origin of blisters previously observed by us in a-Si/a-Ge multilayers prepared under the same conditions as the ones applied to the present a-Si layers. The H content varied between 10.8 and 17.6 at.% as measured by elastic recoil detection analysis. IR spectroscopy showed that the concentration of the clustered (Si-H)n groups and of the (Si-H2)n (n ≥ 1) polymers increased at the expense of the Si-H mono-hydrides with increasing annealing time, suggesting that there is a corresponding increase of the volume of micro-voids whose walls are assumed from literature to be decorated by the clustered mono-hydride groups and polymers. At the same time, an increase in the size of surface blisters was observed. Also, with increasing annealing time, the total concentration of bonded H of any type decreases, indicating that H is partially released from its bonds to Si. It is argued that the H released from the (Si-H)n complexes and polymers at the microvoid surfaces form molecular H2 inside the voids, whose size increases upon annealing because of the thermal expansion of the H2 gas, eventually producing plastic surface deformation in the shape of blisters.

Fabrication and characterization of ultrathin dextran layers: Time dependent nanostructure in aqueous environments revealed by OWLS

Surface coatings of the polysaccharide dextran and its derivatives are key ingredients especially in label-free biosensors for the suppression of non-specific binding and for receptor immobilization. Nevertheless, the nanostructure of these ultrathin coatings and its tailoring by the variation of the preparation conditions have not been profoundly characterized and understood. In this work carboxymethylated dextran (CMD) was prepared and used for fabricating ultrathin surface coatings. A grafting method based on covalent coupling to aminosilane- and epoxysilane-functionalized surfaces was applied to obtain thin CMD layers. The carboxyl moiety of the CMD was coupled to the aminated surface by EDC-NHS reagents, while CMD coupling through epoxysilane molecules was performed without any additional reagents. The surface analysis following the grafting procedures consisted of X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared spectroscopy (ATR-IR), spectroscopic ellipsometry, atomic force microscopy (AFM) and optical waveguide lightmode spectroscopy (OWLS). The XPS and AFM measurements showed that the grafting resulted in a very thin dextran layer of a few nanometers. The OWLS method allowed devising the structure of the interfacial dextran layers by the evaluation of the optogeometrical parameters. The alteration in the nanostructure of the CMD layer with the chemical composition of the silane coverage and the pH of the grafting solution was revealed by in situ OWLS, specifically, lain down chains were found to be prevalent on the surface under neutral and basic conditions on epoxysilylated surfaces. The developed methodologies allowed to design and fabricate nanometer scale CMD layers with well-controlled surface structure, which are very difficult to characterize in aqueous environments using present instrumentations and highly hydrated surface layers.

Si surface preparation and passivation by vapor phase of heavy water

In our previously published paper [1, 2] we demonstrated that deuterium adsorbs on Si surface at room temperature much stronger than hydrogen [3, 4]. Moreover, in case of deuterium passivated wafers the vacuum storage can be omitted without risking the non-controlled native oxidation of silicon for up to 5 hours or more. It could be a suitable and more robust surface cleaning and passivation process for the industry, but heavy water is expensive. As a cheaper procedure, we present in this paper the results of our studies in which the Si surface is treated in vapor phase of heavy-water (D2O) + 50% HF (e.g. 20:1) mixture at 25, 40, 50 and 65 °C, for 1, 10 and 60 minutes. The surface evolution of the D-passivated surface was followed by contact angle measurements, by spectroscopic ellipsometry (SE), by atomic force microscopy (AFM), by X-ray photoelectron spectroscopy (XPS), by transmission electron microscopy (TEM) and by infrared absorption spectroscopy (IR) qualification and the results were compared to the H-passivated Si surface. It turned out that 1 min vapor phase treatment at 65 °C was enough to remove the native oxide and to passivate the Si surface without any degradation of the atomic surface flatness. Combination of D (or H) passivation with rapid thermal process (RTP) based on the thermal desorption kinetics of the adsorbed D and/or H layers on Si is a promising method for improved interface engineering and for better initial reactions in case of ultra thin dielectric layer formations.

Direction-dependent secondary bonds and their stepwise melting in a uracil-based molecular crystal studied by infrared spectroscopy and theoretical modeling

Abstract Three types of supramolecular interactions are identified in the three crystallographic directions in crystals of 1,4-bis[(1-hexylurac-6-yl) ethynyl]benzene, a uracil-based molecule with a linear backbone. These three interactions, characterized by their strongest component, are: intermolecular double H-bonds along the molecular axis, London dispersion interaction of hexyl chains connecting these linear assemblies, and π – π stacking of the aromatic rings perpendicular to the molecular planes. On heating, two transitions happen, disordering of hexyl chains at 473 K, followed by H-bond melting at 534 K. The nature of the bonds and transitions was established by matrix-isolation and temperature-dependent infrared spectroscopy and supported by theoretical computations.

From nano voids to blisters in hydrogenated amorphous silicon

AFM and FTIR spectroscopy were applied to study the relationship between surface blisters and nanovoids in annealed hydrogenated a-Si. The influence of the H bonding configuration on the way the nanovoids give rise to the blisters is discussed. Annealing causes an increase of the polymers density. As they reside on the voids walls their density increase causes an increase of the voids volume. The polymers may release H inside the voids with creation of H2 gas, whose expansion, upon annealing, further contributes to the volume increase of the voids till the formation of surface blisters.