No abstract

We use atomic force microscopy to observe the structural changes in petroleum-asphaltene aggregates in air as a function of temperature. The aggregates are obtained by evaporating a toluene solution containing asphaltene. Increase in temperature leads to transition from self-assembled fractal structures to substantially larger mobile "liquid-like" domains that show distinct tendencies of substrate repulsion and self-coalescence. This new aggregation dynamics of asphaltene can be explained by temperature-induced transition of asphaltene from pure amorphous to liquid crystalline phase. Observation of this new phenomenon for asphaltene will have wide implications for asphaltene handling and separation.

Here we report mid infrared (mid-IR) photothermal response of multi layer MoS2 thin film grown on crystalline (p-type silicon and c-axis oriented single crystal sapphire) and amorphous substrates (Si/SiO2 and Si/SiN) by pulsed laser deposition (PLD) technique. The photothermal response of the MoS2 films was measured as changes in the resistance of MoS2 films when irradiated with mid IR (7 to 8.2 μm) source. We show that it is possible to enhance the temperature coefficient of resistance (TCR) of the MoS2 thin film by controlling the interface through proper choice of substrate and growth conditions. The thin films grown by PLD were characterized using XRD, Raman, AFM, XPS and TEM. High-resolution transmission electron microscopy (HRTEM) images show that the MoS2 films grow on sapphire substrate in a layerby-layer manner with misfit dislocations. Layer growth morphology is disrupted when grown on substrates with diamond cubic structure such as silicon due to growth twin formation. The growth morphology is very different on amorphous substrates such as Si/SiO2 or Si/SiN. TheMoS2 film grown on silicon shows a very high TCR (-2.9% K -1 ), mid IR sensitivity (∆R/R=5.2 %) and responsivity (8.7 V/W) as compared to films on other substrates.

A surface dominant sub-bandgap photo-carrier generation has been observed in multiferroic BiFeO3 (BFO) nanowires, which is mainly attributed to the depopulation of surface states that exist within the bandgap. Mapping of surface potential using Kelvin probe force microscopy (KPFM) further supports the depopulation of surface states in BFO nanowires under sub-bandgap illumination. The mechanism of photovoltage generation in BFO nanowires is investigated by measuring the photoresponse with local illumination of visible laser pulses at different positions of the BFO nanowires. Interestingly, large photovoltage signals were observed when the laser spot was focused close to contact electrodes, showing a position dependent effect of photoresponse in the BFO nanowires. The sub-bandgap excitation of surface states in multiferroic nanowires offers potential new strategies for application in photovoltaic devices.

No abstract