Phonon Confinement Effects in the Raman Spectrum of Nanodiamond

Nanodiamonds (ND) exhibit unique properties due to their small size and high surface-to-volume ratio compared to bulk diamonds. A reduction in crystal size also affects ND Raman spectra. The confinement of optical phonons in nanocrystals (\u3c10 nm) results in asymmetrically broadened Raman lines, which are shifted toward lower wavenumbers. The phonon confinement model (PCM) relates the observed changes in the Raman spectra to the crystal size and can be used for size characterization at the nanoscale. While the PCM was successfully applied to a variety of materials including Si and BN, results remained unsatisfactory in the case of ND. In order to improve the agreement between the predictions of the model and experimental Raman spectra of ND, effects such as crystal size distribution, lattice defects, and the energy dispersion of the phonon modes were taken into consideration and incorporated into the PCM. This work has shown that phonon wave vectors from small vibrational domains lead to a broad shoulder peak at ~1250 cm-1, that is often observed in the Raman spectrum of ND

Defining the most suitable cycling routes for certain scenarios using the multicriteria analysis : Master's Thesis

Prikazana je metodologija odabira biciklističkih ruta za pojedine scenarije koja se temelji na metodama sustavnog inženjerstva i to na višekriterijalnoj analizi uz korištenje GIS-a (Geografskog informacijskog sustava). Predložena metodologija omogućava cjelovito i sustavno rješavanje problema, pri čemu su u obzir uzeti mnogobrojni elementi. Rezultat je prijedlog biciklističkih ruta koje predstavljaju najbolje kompromisno rješenje u skladu s kriterijima i preferencijama sudionika.The methodology for cycling routes for certain scenarios selection, based on the systems engineering methods, including multicriteria analysis and GIS, is presented. The proposed methodology enables an integral and systemic resolution of problems, with many considered elements. The result is the cycling routes proposal that constitutes the best possible compromise solution

Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon

Electrochemical capacitors, also called supercapacitors, store energy in two closely spaced layers with opposing charges, and are used to power hybrid electric vehicles, portable electronic equipment and other devices¹. By offering fast charging and discharging rates, and the ability to sustain millions of ²⁻⁵, electrochemical capacitors bridge the gap between batteries, which offer high energy densities but are slow, and conventional electrolytic capacitors, which are fast but have low energy densities. Here, we demonstrate microsupercapacitors with powers per volume that are comparable to electrolytic capacitors, capacitances that are four orders of magnitude higher, and energies per volume that are an order of magnitude higher. We also measured discharge rates of up to 200 V s⁻¹, which is three orders of magnitude higher than conventional supercapacitors. The microsupercapacitors are produced by the electrophoretic deposition of a several micrometre-thick layer of nanostructured carbon onions⁶‚⁷ with diameters of 6-7 nm. Integration of these nanoparticles in a microdevice with a high surface-to-volume ratio, without the use of organic binders and polymer separators, improves performance because of the ease with which ions can access the active material. Increasing the energy density and discharge rates of supercapacitors will enable them to compete with batteries and conventional electrolytic capacitors in a number of applications

Bending Rigidity of Two-Dimensional Titanium Carbide (MXene) Nanoribbons: A Molecular Dynamics Study

The interest in nanodiamond applications in biology and medicine is on the rise over recent years. This is due to the unique combination of properties that nanodiamond provides. Small size (~5 nm), low cost, scalable production, negligible toxicity, chemical inertness of diamond core and rich chemistry of nanodiamond surface, as well as bright and robust fluorescence resistant to photobleaching are the distinct parameters that render nanodiamond superior to any other nanomaterial when it comes to biomedical applications. The most exciting recent results have been related to the use of nanodiamonds for drug delivery and diagnostics—two components of a quickly growing area of biomedical research dubbed theranostics. However, nanodiamond offers much more in addition: it can be used to produce biodegradable bone surgery devices, tissue engineering scaffolds, kill drug resistant microbes, help us to fight viruses, and deliver genetic material into cell nucleus. All these exciting opportunities require an in-depth understanding of nanodiamond. This review covers the recent progress as well as general trends in biomedical applications of nanodiamond, and underlines the importance of purification, characterization, and rational modification of this nanomaterial when designing nanodiamond based theranostic platforms

Understanding Chemistry of Two-Dimensional Transition Metal Carbides and Carbonitrides (MXenes) with Gas Analysis

MXenes, a large family of two-dimensional materials that are intensely investigated for a broad range of applications, are unstable in water, spontaneously forming TiO2. Several hypotheses have been proposed recently to explain the transformations of MXenes in aqueous environments based on characterization of solid products and measurements of solution pH. However, no studies of the gaseous products of these reactions have been reported. In this work, we demonstrate the use of Raman spectroscopy and gas chromatography techniques to study the gaseous reaction products of Ti2C, Ti3C2, Ti3CN, and Nb2C MXenes in aqueous environments. Based on the analysis of gases, the reactivities of MXenes with different monolayer thickness and chemical composition have been analyzed. We demonstrate the analysis of gases produced during MXene transformations as a powerful technique that can be used for better understanding of their nontrivial chemistry

Molecular Dynamic Study of the Mechanical Properties of Two-Dimensional Titanium Carbides Tiₙ₊₁Cₙ (MXenes)

Two-dimensional materials beyond graphene are attracting much attention. Recently discovered 2D carbides and nitrides (MXenes) have shown very attractive electrical and electrochemical properties, but their mechanical properties have not been characterized yet. There are neither experimental measurements reported in the literature nor predictions of strength or fracture modes for single-layer MXenes. The mechanical properties of two-dimensional titanium carbides were investigated in this study using classical molecular dynamics. Young\u27s modulus was calculated from the linear part of strain-stress curves obtained under tensile deformation of the samples. Strain-rate effects were observed for all Tin+1Cn samples. From the radial distribution function, it is found that the structure of the simulated samples is preserved during the deformation process. Calculated values of the elastic constants are in good agreement with published DFT data

Environment-Sensitive Photoresponse of Spontaneously Partially Oxidized Ti₃C₂ MXene Thin Films

A large family of two-dimensional transition metal carbides and nitrides (MXenes) has increasingly raised interest for electronic and optoelectronic applications due to their high electrical conductivity, potentially tunable electronic structure, nonlinear optical properties, and ability to be manufactured in the thin film state. During delamination and storage in ambient air environment, spontaneous oxidation of MXene flakes leads to formation of titanium oxide, a process that, as we demonstrate here, can be harnessed for manufacturing MXene–titania composites for optoelectronics, sensing, and other applications. We show that partially oxidized MXene thin films containing the <i>in situ</i> formed phase of titanium oxide have a significant photoresponse in the UV region of the spectrum. The relaxation process of photoexcited charge carriers takes a long time (∼24 h) but can be accelerated in the presence of oxygen and water vapor in the atmosphere. These properties of spontaneously formed MXene–titania thin films make them attractive materials for photoresistors with memory effect and sensitivity to the environment, as well as many other photo- and environment-sensing applications

Dispersions of Two-Dimensional Titanium Carbide MXene in Organic Solvents

Two-dimensional titanium carbide (Ti₃C₂T_<i>x</i>) MXene has attracted a great deal of attention in the research community and has already showed promise in numerous applications, but only its dispersions in aqueous solutions have previously been available. Here we show that Ti₃C₂T_<i>x</i> can be dispersed in many polar organic solvents, but the best dispersions were achieved in <i>N</i>,<i>N</i>-dimethylformamide, <i>N</i>-methyl-2-pyrrolidone, dimethyl sulfoxide, propylene carbonate, and ethanol. The dispersions were examined by measuring the concentration and absorbance spectra of MXene in organic solvents as well as correlating the concentration to solvent physical properties, such as surface tension, boiling point, and polarity index. Hildebrand and Hansen solubility parameters were additionally used to provide an initial understanding of how Ti₃C₂T_<i>x</i> MXene behaves in organic media and potentially develop quantitative correlations to select solvents and their combinations that can disperse Ti₃C₂T_<i>x</i> and other MXenes. Using this analysis, we have outlined a range of organic solvents, which can disperse Ti₃C₂T_<i>x</i>, expanding the opportunities for processing techniques, such as mixing MXenes with other nanomaterials or polymers to form composites, preparing inks for printing, and deposition requiring solution processable materials, allowing the use of Ti₃C₂T_<i>x</i> in a multitude of applications

Effect of Defects on Graphitization of SiC

Epitaxial graphene and carbon nanotubes (CNTs) grown on SiC have shown big potential in electronics. The motivation to produce faster and smaller electronic devices using less power opened the way to a study of how to produce controlled epitaxial graphene and CNTs on SiC. Since defects are among the important tools to control the properties of materials, the effects of defects on the carbon formation on SiC have been analyzed. In this study, the effects of defects on the carbon formation on SiC have been analyzed. We produced carbon films on the surface of four different SiC materials (polycrystalline sintered SiC disks, single crystalline SiC wafers, SiC whiskers, and nanowhiskers) by chlorination and vacuum annealing with the goal to understand the effects of surface defects on the carbon structure and the SiC decomposition rate. We have shown that grain boundaries, dislocations, scratches, surface steps, and external surfaces may greatly enhance the reaction rate and affect the final structure of carbon derived from SiC