Group III−V nanowires offer the exciting possibility of epitaxial growth on a wide variety of substrates, most importantly silicon. To ensure compatibility with Si technology, catalyst-free growth schemes are of particular relevance, to avoid impurities from the catalysts. While this type of growth is well-documented and some aspects are described, no detailed understanding of the nucleation and the growth mechanism has been developed. By combining a series of growth experiments using metal−organic vapor phase epitaxy, as well as detailed in situ surface imaging and spectroscopy, we gain deeper insight into nucleation and growth of self-seeded III−V nanowires. By this mechanism most work available in literature concerning this field can be described.

Core flood and field tests have demonstrated that decreasing injection water salinity increases oil recovery from sandstone reservoirs. However, the microscopic mechanism behind the effect is still under debate. One hypothesis is that as salinity decreases, expansion of the electrical double layer decreases attraction between organic molecules and pore surfaces. We have developed a method that uses atomic force microscopy (AFM) in chemical force mapping (CFM) mode to explore the relationship between wettability and salinity. We functionalised AFM tips with alkanes and used them to represent tiny nonpolar oil droplets. In repeated measurements, we brought our “oil” close to the surface of sand grains taken from core plugs and we measured the adhesion between the tip and sample. Adhesion was constant in high salinity solutions but below a threshold of 5,000 to 8,000 ppm, adhesion decreased as salinity decreased, rendering the surface less oil wet. The effect was consistent, reproducible and reversible. The threshold for the onset of low salinity response fits remarkably well with observations from core plug experiments and field tests. The results demonstrate that the electric double layer force always contributes at least in part to the low salinity effect, decreasing oil wettability when salinity is low.

The material properties of III-V semiconductors are in many ways superior to those of Si. Examples of these properties are the possibilities for high electron mobilities and the direct bandgap in most III-V semiconductors. Even so, Si remains the standard material in the electronics industry. A successful combination of these two material systems would add new functionality and increased performance compared to standard Si technology.[1] Although these advantages have long been recognized, the monolithic integration of devicequality III-V materials on Si remains a major challenge. In this work, we report on heteroepitaxial growth of InAs nanowires (NWs) directly on Si substrates by employing self-assembled organic coatings to create an oxide template that guides NW nucleation. Such a template resembles the growth masks used in selective-area epitaxy of nanostructures on III-V substrates. [2][3][4] Importantly, Au, which is commonly used for NW synthesis but not compatible with modern complementary metal oxide semiconductor (CMOS) processing, is avoided. The described nucleation method presents clear advantages in terms of epitaxial quality and control of NW size and density distributions compared to previous results achieved on Si using catalyst-free NW growth. The control demonstrated in the fields of self-assembled [5] and printed organic nanostructures [6,7] illustrates how the method may be extended to more complex patterning in future work. NWs have been the subject of much study and have great potential as a future technology platform. [8,9] One unique feature of the NW geometry is the small NW/substrate interface, which could provide a path to monolithic integration of highperformance materials, such as heterostructure III-V device structures, onto the mainstream Si platform. The small interface helps to mitigate antiphase domain formation, and accommodates a considerable mismatch in lattice constants (ca. 12 % for InAs on Si) and thermal expansion coefficients. [10][11][12] InAs has been used in a number of NW devices (see the literature for examples [13][14][15][16] ), and is one of the most promising materials for high-speed electronics [17] due to its high electron mobility, high electron saturation velocity, and low resistance contacts; still, it has proven difficult to combine InAs directly with Si. At the same time there is a strong drive from the electronics industry to integrate high-performance nanomaterials with Si [1] using methods compatible with existing Si processing. Contamination from the commonly used Au particles in the vapor-liquid-solid (VLS) mechanism [18] for NW growth is a concern because Au is an impurity in Si, which traps electrons and holes by deep-level recombination centers. [19] Inevitably, some traces of Au will contaminate process equipment or end up in the nanostructures grown. [20,21] For this reason, Au is not compatible with modern CMOS processing. NW growth without foreign metal catalyst particles thus offers an attractive alternative to the VLS method. Approaches such...

Laboratory core flood and field scale tests have demonstrated that about 5 to 40% more oil can be released from sandstone reservoirs by injecting low salinity water, rather than high salinity fluids such as seawater or formation water. The effect has been explained by a change in wettability of the minerals that form the pore wall, as a result of the decrease in divalent cation concentration. Using X-ray photoelectron spectroscopy, we have demonstrated that even for solvent cleaned core samples, mineral surfaces retain a significant quantity of carbon containing material. Thus, pore wall wettability is more likely dominated by tightly adsorbed organic material than by the character of the underlying minerals. To test this hypothesis, we used the chemical force mapping (CFM) mode of atomic force microscopy (AFM) to directly measure adhesion forces on individual quartz grains that were plucked from core plugs. We functionalized AFM tips with model oil compounds so they would represent tiny oil droplets, and we measured their ability to adhere to surfaces as salinity changed. We examined grains from a sandstone core plug that had been cut into segments, which had been stored in kerosene or solvent cleaned. On all samples, surfaces were more oil wet (higher adhesion) in artificial seawater (ASW; 35,600 ppm) than in ASW diluted with ultrapure deionized water to ∼1,500 ppm. XPS demonstrated that solvent cleaned surfaces had less adsorbed organic material than the kerosene stored sample. AFM measurements showed that the low salinity effect, namely the change in adhesion caused by decreasing salinity, was twice as high on kerosene stored samples as on solvent cleaned surfaces. The organic material that is adsorbed on the pore surfaces in the preserved sandstone offer very sticky anchor points for adhering oil molecules. This suggests that in reservoirs, even hydrophilic minerals located at the pore-fluid interface have tightly adhering hydrocarbons and the low salinity response depends on the behavior of this adsorbed material.

We have succeeded in direct atomic scale imaging of the exterior surfaces of III-V nanowires by scanning tunneling microscopy (STM). By using atomic hydrogen, we expose the crystalline surfaces of InAs nanowires with regular InP segments in vacuum while retaining the wire morphology. We show images with atomic resolution of the two major types of InAs wurtzite nanowire surface facets and scanning tunneling spectroscopy (STS) data. Ab initio calculations of the lowest energy surface structures and simulated STM images, agree very well with experiments.

We establish a new mechanism for self-propelled motion of droplets, in which ordering of the nanoscale step morphology by sublimation beneath the droplets themselves acts to drive them perpendicular and up the surface steps. The mechanism is demonstrated and explored for Ga droplets on GaP(111)B, using several experimental techniques allowing studies of the structure and dynamics from micrometers to the atomic scale. We argue that the simple assumptions underlying the propulsion mechanism make it relevant for a wide variety of materials systems.

We report the first experiments carried out on a new imaging setup, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with the temporal resolution of extreme ultraviolet (XUV) attosecond pulse trains. The very short pulses were provided by high-harmonic generation and used to illuminate lithographic structures and Au nanoparticles, which, in turn, were imaged with a PEEM resolving features below 300 nm. We argue that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment. Problems with extensive space charge effects, which can occur due to the low probe pulse repetition rate and extremely short duration, are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging. Finally, a powerful femtosecond infrared (IR) beam was combined with the XUV beam in a pump-probe setup where delays could be varied from subfemtoseconds to picoseconds. The IR pump beam could induce multiphoton electron emission in resonant features on the surface. The interaction between the electrons emitted by the pump and probe pulses could be observed.

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