Publikationen

A two-dimensional numerical model for self-propagating reactions in Al/Ni multilayer foils was developed. It was used to study thermal properties, convective heat loss, and the effect of initial temperature on the self-propagating reaction in Al/Ni multilayer foils. For model adjustments by experimental results, these Al/Ni multilayer foils were fabricated by the magnetron sputtering technique with a 1:1 atomic ratio. Heat of reaction of the fabricated foils was determined employing Differential Scanning Calorimetry (DSC). Self-propagating reaction was initiated by an electrical spark on the surface of the foils. The movement of the reaction front was recorded with a high-speed camera. Activation energy is fitted with these velocity data from the high-speed camera to adjust the numerical model. Calculated reaction front temperature of the self-propagating reaction was compared with the temperature obtained by time-resolved pyrometer measurements. X-ray diffraction results confirmed that all reactants reacted and formed a B2 NiAl phase. Finally, it is predicted that (1) increasing thermal conductivity of the final product increases the reaction front velocity; (2) effect of heat convection losses on reaction characteristics is insignificant, e.g., the foils can maintain their characteristics in water; and (3) with increasing initial temperature of the foils, the reaction front velocity and the reaction temperature increased.

The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in nanostructures. Time-resolved microphotoluminescence in MoS2 monolayers (MLs) and bilayers (BLs), both unstrained and compressively strained, in a wide temperature range (10-300 K), is studied to distinguish between exciton states optically allowed and forbidden, both in spin and in momentum, as well as to estimate their characteristic decay times and contributions to the total radiation intensity. The decay times are found to either increase or decrease with increasing temperature, indicating the lowest bright or lowest dark state, respectively. The results unambiguously show that, in an unstrained ML, the spin-allowed state is the lowest for a series of A excitons (1.9 eV), with the dark state being <2 meV higher, and that the splitting energy can increase several times at compression. In contrast, in the indirect exciton series in BLs (1.5 eV), the spin-forbidden state is the lowest, being about 3 meV below the bright one. The strong effect of strain on the exciton spectrum can explain the large scatter among the published data and must be taken into account to realize the desired optical properties of 2D MoS2.

Efficient and green exfoliation of bulk MoS2 into few-layered nanosheets in the semiconducting hexagonal phase (2H-phase) remains a great challenge. Here, we developed a new method, water-assisted exfoliation (WAE), for the scalable synthesis of carboxylated chitosan (CC)/2H-MoS2 nanocomposites. With facile hand grinding of the CC powder, bulk MoS2 and water followed by conventional liquid-phase exfoliation in water, this method can not only efficiently exfoliate the 2H-MoS2 nanosheets, but also produce two-dimensional (2D) CC/2H-MoS2 nanocomposites. Interestingly, the intercalated CC in MoS2 nanosheets increases the interlayer spacing of 2H-MoS2 to serve as good candidates for the semiconductor devices. 2D CC/2H-MoS2 nanocomposites show superior electronic rectification effects in nonvolatile write-once-read-many-times memory (WORM) behavior with an ON/OFF ratio over 103, which can be rationally controlled by the weight ratios of CC and MoS2. These findings by the WAE method would open tremendous potential opportunities to prepare commercially available semiconducting 2D nanocomposites for promising high-performance device applications.

This communication uncovers missing fundamental elements and an expanded model of gas phase electrodeposition; a relatively new and in large parts unexplored process, which combines particle generation, transport zone and deposition zone in an interacting setup. The process enables selected area deposition of charged nanoparticles that are dispersed and transported by a carrier gas at atmospheric pressure conditions. Two key parameters have been identified: carrier gas flow rate and spark discharge power. Both parameters affect electrical current carried by charged species, nanoparticle mass, particle size and film morphology. In combination, these values enable to provide an estimate of the gas flow dependent Debye length. Together with Langmuir probe measurements of electric potential and field distribution, the transport can be described and understood. First, the transport of the charged species is dominated by the carrier gas flow. In close proximity, the transport is electric field driven. The transition region is not fixed and correlates with the electric potential profile, which is strongly dependent on the deposition rate. Considering the film morphology, the power of the discharge turns out to be the most relevant parameter. Low spark power combined with low gas flow leads to dendritic film growth. In contrast, higher spark power combined with higher gas flow produces compact layers.

Controlling the surface formation of the group-V face of (111)-oriented III-V semiconductors is crucial for subsequent successful growth of III-V nanowires for electronic and optoelectronic applications. With a view to preparing GaP/Si(111) virtual substrates, we investigate the atomic structure of the MOVPE (metalorganic vapor phase epitaxy)-prepared GaP(111)B surface (phosphorus face). We find that upon high-temperature annealing in the H2-based MOVPE process ambience, the surface is phosphorus-depleted, as evidenced by X-ray photoemission spectroscopy (XPS). However, a combination of density functional theory calculations and scanning tunneling microscopy (STM) suggests the formation of a partially H-terminated phosphorus surface, where the STM contrast is due to electrons tunneling from non-terminated dangling bonds of the phosphorus face. Atomic force microscopy (AFM) reveals that a high proportion of the surface is covered by islands, which are confirmed as Ga-rich by Auger electron spectroscopy (AES). We conclude that the STM images of the samples after high-temperature annealing only reflect the flat regions of the partially H-terminated phosphorus face, whereas an increasing coverage with Ga-rich islands, as detected by AFM and AES, forms upon annealing and underlies the higher proportion of Ga in the XPS measurements.