Role of liquid indium in the structural purity of wurtzite InAs nanowires that grow on Si(111).

InAs nanowires that grow catalyst-free along the [111] crystallographic orientation are prone to wurtzite-zincblende polytypism, making the control of the crystal phase highly challenging. In this work, we explore the dynamic relation between the growth conditions and the structural composition of the nanowires using time-resolved X-ray scattering and diffraction measurements during the growth by molecular beam epitaxy. A spontaneous buildup of liquid indium is directly observed in the beginning of the growth process and associated with the simultaneous nucleation of InAs nanowires predominantly in the wurtzite phase. The highly arsenic-rich growth conditions that we used limited the existence of the liquid indium to a short time interval, which is defined as the nucleation phase. After their nucleation, the nanowires grow in the absence of liquid indium, and with a highly defective wurtzite structure. Complementary ex-situ diffuse X-ray scattering measurements and modeling revealed that this structural degradation is due to the formation of densely spaced stacking faults. Thus, high wurtzite phase purity is associated with the presence of liquid indium. This finding implies that pure wurtzite nanowires may be obtained only if the growth is performed under the continuous presence of liquid indium at the growth interface, that is, in the vapor-liquid-solid mode.

Element-specific surface X-ray diffraction study of GaAs(001)-c(4 x 4).

In situ structure analysis of GaAs(001)-c(4 x 4) has been carried out by synchrotron surface x-ray diffraction, which is sensitive to the three-dimensional structure and the atomic species. On the basis of 98 independent in-plane diffractions and 11 fractional-order rod profiles, the atomic coordinates and thermal vibration parameters were determined. X-ray diffraction results show the buckling of surface dimers and a strain field extending up to the sixth layer from the surface. An anomalous diffraction technique has been employed to specify the atomic species of the surface dimers. It has provided direct evidence of the formation of Ga-As heterodimers.

X-ray reflectivity and diffraction studies on lipid and lipopolymer Langmuir-Blodgett films under controlled humidity.

Synchrotron X-ray reflectivity and grazing incidence diffraction were applied to study the structures of phospholipid Langmuir-Blodgett films under controlled humidity. Distearoylphosphatidylethanolamine (DSPE) and DSPE-PEO lipopolymers, in which a poly(ethylene oxide) chain with n = 8, 17, or 45 EO units is covalently linked to the polar headgroup of DSPE, were used. When the relative humidity was changed from below 2 to 97%, the phosphate headgroup in a DSPE film was hydrated with a concomitant thickness increase of 4.2 A and an electron density decrease. The swelling of the monolayer was found to be a reversible process without any significant roughening of the film. Subtle differences in the film thickness could be detected for DSPE monolayers transferred under various lateral pressures. The degree of lateral ordering of the alkyl chains in DSPE monolayers increased considerably when the humidity was raised. In the case of DSPE-PEO with eight EO units, the hydration of the monolayer was also found to be reversible, but the water uptake was larger due to the presence of the hydrophilic polymer interlayer, which is located between the substrate and the DSPE moieties. Under high humidity, the lipopolymer monolayer with n = 17 exhibited a well-defined, layered structure similar to that of DSPE-PEO with n = 8. In the case of n = 45, however, there are indications for a significant intermixing of the polymer with the DSPE moieties, probably resulting in the formation of hydrophobic nanodomains of closely packed alkyl chains that are immersed into the hydrated polymer.