Graphene is a two-dimensional (2D) hexagonal array of carbon atoms in sp2-hybridized states. Graphene presents unique and exceptional electronic, thermal and mechanical properties. However, in its pristine state graphene is a gapless semiconductor, which poses some limitations to its use in some transistor electronics. Because of this there is a renewed interest in other possible two-dimensional carbon-based structures similar to graphene. Examples of this are graphynes and graphdiynes, which are two-dimensional structures, composed of carbon atoms in sp2 and sp-hybridized states. Graphdiynes (benzenoid rings connecting two acetylenic groups) were recently synthesized and they can be intrinsically nonzero gap systems. These systems can be easily hydrogenated and the amount of hydrogenation can be used to tune the band gap value. In this work we have investigated, through fully atomistic molecular dynamics simulations with reactive force field (ReaxFF), the structural and dynamics aspects of the hydrogenation mechanisms of graphdiyne membranes. Our results showed that depending on whether the atoms are in the benzenoid rings or as part of the acetylenic groups, the rates of hydrogenation are quite distinct and change in time in a very complex pattern. Initially, the most probable sites to be hydrogenated are the carbon atoms forming the triple bonds, as expected. But as the amount of hydrogenation increases in time this changes and then the carbon atoms forming single bonds become the preferential sites. The formation of correlated domains observed in hydrogenated graphene is no longer observed in the case of graphdiynes. We have also carried out ab initio DFT calculations for model structures in order to test the reliability of ReaxFF calculations.

Magnetic properties at nano-scale provide a whole spectrum of new phenomena that can be beneficial for spintronic devices characterized with ultra-short response time, high sensitivity to magnetic field and miniature size. The properties and stability of a magnetic system can be enhanced by creating ordered arrays of ferromagnetic nano-particles. Here we report a considerable reduction of coercitivity for a magnetic array using triangular, square and hexagonal particle arrangement. The reduction of coercitivity can be explained by fine-tuning of dipole-dipole interaction between magnetic particles, which is to large degree influenced by the number of nearest neighbors and distance between the particles.

In this paper, we report new phase of crystalline silicon, quasicrystalline silicon thin-film on glass substrate. The surface topography of these films reveal simultaneous existence of sixfold and fivefold symmetry. We found an array of quasi-unit cell in 2-D that formed quasicrystalline solid. This is first time demonstration of quasicrystalline for single element, silicon (Si). Raman spectra suggests that we found crystalline silicon structure on glass substrate that is not single-crystal silicon (c-Si) but very close to c-Si.

The surface structure of oxide materials may be the limiting factor in controlling switching properties at interfaces. Here we investigate and correlate the surface structure and electronic properties of BaTiO3 substrates. By using low energy electron diffraction and scanning tunneling microscopy we are able to identify surface reconstructions based on annealing treatments. We then investigate the effect of contact size on the transport properties on oxide surfaces utilizing atomic force microscopy. Our results show the critical importance of controlling surface structure to optimize electronic properties at oxide interfaces.

Photocatalytic properties of titanium oxide depend on the material size and shape, which can favour a higher interaction between reactants and catalyst. Most of the studies reported until now, show that reducing size down to the nanoscale increases the photocatalytic efficiency. We demonstrate that a multiscale shape design, integrating surface roughness, particle shape, and material 1D processing and orientation, can favour photocatalytic properties in the solid-gas regime, especially mineralization (conversion into CO2), when the material hierarchical 1D orientation is combined with unidirectional gas flow. Several materials with hierarchical structure were prepared and characterized. They have been tested for the photocatalytic mineralization of gaseous acetone, and compared with commercial catalysts. Our study reveals that a suitable combination of multiscale design can favour high mineralization.

Various metallic structures of complex shape, resembling natural objects such as plants, mushrooms, and seashells, were produced when growing nanowires by means of pulsed current electroplating in porous membranes. These structures occur as the result of nanowires self-assembling (biomimetics) if the electroplating is continued after the nanowires reach the membrane surface. By varying the membrane geometry and the pulsed current parameters, and alternating electroplating from two baths with different electrolytes, various models were fabricated, including a hollow container with wall thickness of 10-30 nm. The possibility of shape regulation for models was demonstrated: in certain conditions, mushroom- and shell-like convex-concave models of the same kind were obtained. The hierarchical structure of models at the nano-, micro- and mesoscopic levels is shown through fragmentation and chemical etching. This biomimetic method suggests an analogy between the shape-forming processes of natural plants and their metallic models. Nanostructured mesoscopic objects of metals (Ag, Pd, Rh, Ni, Bi), alloys (PdNi, PdCo, PbIn) as well as their combinations (PdNi/ Pb, PdNi/ PbIn) were obtained. The technological simplicity of the present method makes it suitable for fabricating nanostructured materials that may be efficient in catalysis, superhydrophobic applications, medical filters, and nanoplasmonics.

In this work, a novel low dielectric constant (low-k) pore sealing approach was engineered by depositing firstly a sub-2 nm SAMs and then a 3 nm TiN barrier film. The low-k film was pretreated by plasma to introduce hydroxyl groups onto the surface, followed by SAMs deposition. Then a TiN film was deposited from tetrakis(dimethylamino)titanium (TDMAT) via ALD as a dielectric barrier. Penetration of Ti atoms into low-k was measured and used to evaluate the sealing ability of SAMs. For the samples covered with SAMs, around 90% reduction of Ti atoms penetration was achieved. The pore radius was reduced to below 0.5 nm after the barrier deposition. The ∆k after pretreatment and after SAMs are 0.1 and 0.16, respectively.

Density functional theory and statistical calculations are combined to address the chemical stability and structure of epoxy functionalizations of single-layer graphene. Our computations show that at oxidation levels of O:C<0.5, the Gibbs free energy of formation per epoxide amounts to about 0.6 eV, and the structure of the epoxy functionalizations presents local order and long-range disorder. The positive energy value indicates that in air at p=1 bar and room temperature, epoxy functionalizations of graphene are unstable and prone to spontaneous reduction. Our calculations show also that formation and release of O2 is a slow process whose kinetics is controlled by large energy barriers, the formation of very stable intermediate species, and unlikely electronic transitions.

We report on the maskless integration of micron-sized GaAs crystals on patterned Si substrates by metal organic vapor phase epitaxy. In order to adapt the mismatch between the lattice parameter and thermal expansion coefficient of GaAs and Si, 2 μm tall Ge crystals were first grown as virtual substrate by low energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs structures grown on top of the Ge crystals at the transition towards full pyramids with energetically stable {111} facets. A substantial release of strain is shown in GaAs crystals with a height of 2 μm and lateral sizes up to 15×15 μm2 by both X-ray diffraction and photoluminescence.

The percolation threshold in a ceramic composite depends on the processing conditions used to fabricate them along with the size and shape of the filler. In this study, borosilicate glass microspheres were used as the matrix material and nanosized antimony tin oxide (ATO) particles were used as the filler. The microsphere/ATO composites were fabricated by hot pressing around the glass transition temperature in order to control the viscosity. The pressure and temperature applied allowed the ATO to be confined to the spaces between certain glass particles, forming percolating networks at low volume fractions of the ATO. The electrical properties were examined using ac impedance spectroscopy. The impedance, electric modulus, and tan δ were studied which allowed for valuable insights in structure-property-processing relationships in these materials, along with determination of the percolation behavior in these composites. This analysis on samples right before percolation indicated that there was a highly resistive component affecting long range conductivity which is likely due to porosity at the triple points while the dielectric response is affected by the clusters of ATO nanoparticles. Based on this, the percolation of ATO should reduce down to lower concentrations if the processing conditions are improved to reduce this porosity and further segregate the ATO.

A modified critical point model dielectric function for graphene is derived here and used to analyze spectroscopic ellipsometry data obtained over a wide spectral range from 3 to 9 eV. Critical point and exciton resonance energies are extracted and discussed. Our findings indicate that epitaxial graphene on SiC to exhibits equivalent exciton behavior to that of suspended graphene. We further apply our model dielectric function to evaluate dielectric function data for highly oriented pyrolytic graphite reported in the literature. Excellent agreement is found between the critical point model developed here and the literature data even for the low energy spectral range up to 1 eV.

A methodology is described for atomistic simulations of shock-compressed materials that incorporates quantum nuclear effects on the fly. We introduce a modification of the multi-scale shock technique (MSST) that couples to a quantum thermal bath described by a colored noise Langevin thermostat. The new approach, which we call QB-MSST, is of comparable computational cost to MSST and self-consistently incorporates quantum heat capacities and Bose-Einstein harmonic vibrational distributions. As a first test, we study shock-compressed methane using the ReaxFF potential. The Hugoniot curves predicted from the new approach are found comparable with existing experimental data. We find that the self-consistent nature of the method results in the onset of chemistry at 40% lower pressure on the shock Hugoniot than observed with classical molecular dynamics. The temperature change associated with quantum heat capacity is determined to be the primary factor in this shift.

The effectiveness of lightweight aggregate (LWA) as an internal curing agent (ICA) to reduce concrete shrinkage is evaluated for repair concrete used in cultural heritage works (RCCHW) using curing periods of 30 days. Normal weight aggregate is replaced by LWA at volume replacement levels ranging from 10 to 14%. The mixtures contain Portland cement maintaining the paste content at approximately 24.1% of concrete volume. Comparisons are made with mixtures containing low-absorption granite and high-absorption limestone normal weight coarse aggregates. At the replacement levels used in this study, LWA results in a small reduction in concrete density, no appreciable effect on concrete compressive strength, and a decrease in concrete shrinkage for drying periods up to 30 days. With a curing period of 14 days, all mixtures with LWA exhibited less shrinkage than the mixtures with either low- or high-absorption normal weight aggregates.

In this paper, we demonstrate deposition methods and conditions that allow the control of the electrical properties of doped ZnTe grown by RF magnetron sputtering using both nitrogen and copper as dopants. The carrier density of the films was characterized using a van der Pauw Hall effect measurement method. We demonstrate how the concentration of nitrogen in the plasma during the growth of the film impacts the conductivity of the ZnTe films. Films with hole concentrations in excess of 1018 cm-3 and a high degree of crystallinity were successfully grown. Similarly, we demonstrate that the hole concentration in the Cu-doped ZnTe can be varied by varying the amount of copper introduced in the films. We also observe that annealing the copper doped ZnTe films increases the carrier density, whereas annealing the nitrogen doped ZnTe films causes a decrease in carrier concentration and conductivity.

Optical absorption efficiency, an important metric for sensing, radiometric and energy harvesting applications, has been studied theoretically and experimentally in porous, ordered nanostructures, including multi-walled- (MW) carbon nanotubes (CNTs) and single-walled- (SW) CNTs. We have characterized the absorption efficiencies in the 350 nm -7000 nm wavelength range of vertically aligned MWCNT arrays with high site densities synthesized directly on metallic substrates using a plasma-enhanced (PE)- chemical vapor deposition (CVD) process. Our ultra-thin absorbers exhibit a reflectance as low as ∼ 0.02 % (100 X lower than the benchmark). Such high efficiency absorbers are particularly attractive for radiometry, as well as energy harnessing applications. This work increases the portfolio of materials that can be integrated with such absorbers due to the potential for reduced synthesis temperatures arising from a plasma process. Optical modeling calculations were conducted that enabled a determination of the extinction coefficient in the films.

Papers in the Appendix were published in electronic format as Volume 1534