Dissertations

High entropy alloy films of AlCoCrFeNi B2-ordered structure are formed during an ultrafast heating process by reactive Ni/Al multilayers. The self-propagating high-temperature reaction occurring in reactive Ni/Al multilayers after ignition represents an ultrafast heat source which is used for the transformation of a thin films Al/CoFe/CrNi multilayer structure into a single-phase high entropy alloy film. The materials design of the combined multilayers thus determines the phase formation. Conventional rapid thermal annealing transforms the multilayer into a film with multiple equilibrium phases. Ultrafast combustion synthesis produces films with ultrafine-grained single-phase B2-ordered compound alloy. The heating rates during the combustion synthesis are in the order of one million K/s, much higher than those of the rapid thermal annealing, which is about 7 K/s. The results are compared with differential scanning calorimetry experiments with heating rates ranging from about 100 K/s up to 25000 K/s. It is shown that the heating rate clearly determines the phase formation in the multilayers. The rapid kinetics of the combustion prevents long-range diffusion and promotes the run-away transformation. Thus, multilayer combustion synthesis using reactive Ni/Al multilayers as heat source represents a new pathway for the fabrication of single phase high-entropy alloy films.

We demonstrate that the modulated surface photovoltage spectroscopy (modulated SPS) technique can be applied to investigate interface states in the bandgap, i.e. interface passivation, of crystalline silicon coated with a downshift layer such as hydrogenated aluminum nitride with embedded terbium ions by suppressing straylight with a cut-off filter. Different hydrogen contents influence the surface photovoltage spectra at photon energies below the bandgap of crystalline silicon. Modulated SPS reveals that at higher hydrogen content there is a lower signal and, thus, a lower density of surface defect states. Our experiments show that modulated SPS can become a powerful tool for characterizing defect states at interfaces which cannot be easily studied by other methods.

Development of broadband absorption materials for solar energy harvesting is an important strategy to address global energy issues. Herein, it is demonstrated that an ultrablack silicon structure with abundant surface texturing can absorb about 98.7% solar light within the wavelength range of 300 to 2500 nm, i.e., a very large range and amount. Under 1 sun irradiation, the ultrablack silicon sample's surface temperature can increase from 21.2 to 51.2 ˚C in 15 min. During the photothermal water evaporation process, the ultrablack silicon sample's surface temperature can still reach a highest temperature of 43.2 ˚C. The average photothermal conversion efficiency (PTCE) can be as high as 72.96%. The excellent photothermal performance to the excellent light-trapping ability of the pyramidal surface nanostructures during solar illumination, which leads to extremely efficient absorption of light, is attributed. In addition, the large water contact area also enables fast vapor transport. The stability of the photothermal converter is also examined, presenting excellent structure and performance stabilities over 10 cycles. This indicates that the ultrablack Si absorber can be a promising photothermal conversion material for seawater desalination, water purification, photothermal therapy, and more.

We present structural, optical and electrical investigations of layered WS2 films prepared on tungsten. A two-step technique has been used to synthesize layered WS2 films using sulfurization of W films sputtered with thinner (1 and 2 nm) and thicker (14 and 28 nm) thicknesses at 800 ˚C. XRD analysis revealed that the examined films are polycrystalline with texture and have a 2H-WS2 hexagonal microstructure. Using Raman spectroscopy with the 532 nm laser excitation, the presence of E12g and A1g vibration modes was observed and the layered nature of WS2 was confirmed. FE SEM observations showed two different surface morphologies. The samples grown on thinner W films were not compact over the surface and agglomeration of nanosize grains in combination of triangles and flakes was visible. In another group the surface was lamellar and contained plenty of nanorods embedded vertically and/or inclined at different angles to the surface. Layered WS2 films exhibited a direct band gap in the range of 2.1-2.5 eV and they were n-type semiconductors with the sheet resistance in the order of several MΩ at room temperature.