J. Kästner

Ion beam induced destabilization of icosahedral structures in gas phase prepared FePt nanoparticles

Multiply twinned FePt nanoparticles with icosahedral structures were prepared by dc magnetron sputtering in argon. The icosahedral structure of these particles is known to be very stable against structural transformations into both the face-centered cubic phase (fcc, A1) and the chemically ordered tetragonal L10 phase upon in-flight or post-deposition thermal annealing. Irradiation of these multiply twinned FePt particles with 5keV He ions, however, resulted in a transformation into predominantly single crystalline fcc particles at high ion fluences of f>1017ions∕cm2. Adjacent particles were observed to coalesce under the effect of He irradiation, and the size of individual particles was found to be slightly reduced, which indicates a high atomic mobility owing to temporarily enhanced defect concentrations caused by the ion bombardment. Strikingly, there was no indication for the occurrence of L10 ordered FePt nanoparticles upon ion irradiation in these samples.

Composition control in metal-organic vapor-phase epitaxy grown InGaAs nanowhiskers

InGaAs nanowhiskers were grown by metal-organic vapor-phase epitaxy on (111)B GaAs substrates using the vapor-liquid-solid growth mode. The diameter of nanowhiskers was defined by monodisperse gold nanoparticles deposited on the GaAs substrate from the liquid phase. By adjusting the triethylgallium to trimethylindium flow ratio, InxGa1−xAs whiskers with various compositions were realized. The composition characterization of the grown whiskers was done by high-resolution x-ray diffractometry. A detailed analysis of measured spectra allowed resolving the presence of an InGaAs three-dimensional layer between whiskers. High-resolution transmission electron microscopy investigation revealed the lattice constant of the grown whisker structures, which agrees with the whisker composition defined by x-ray diffractometry. Finally, low-temperature photoluminescence measurements of the realized InGaAs whiskers were carried out.

Gaas whiskers grown by metal-organic vapor-phase epitaxy using Fe nanoparticles

GaAs nanowhiskers were grown by metal-organic vapor-phase epitaxy on (111)B GaAs substrates. The diameter of the nanowhiskers was defined by monodisperse Fe nanoparticles deposited on the GaAs substrate from the vapor phase. The growth temperature of the whiskers was investigated from 480to520°C. The whiskers are preferentially directed along the crystal orientations of ⟨001⟩, ⟨111⟩, and their equivalents. High-resolution transmission electron microscopy characterization including energy disperse x-ray spectroscopy measurements revealed not only iron oxide but also arsenic inside the seed particle at the top of the GaAs whiskers. This indicates that the particle stays at the top during the whisker growth.

Enhancement of L10 phase formation in FePt nanoparticles by nitrogenization

FePt particles 6 nm in size are produced by argon/nitrogen sputtering and gas-phase condensation. Marked changes in the atomic structure and morphology of the particles occur upon addition of nitrogen to the sputter gas. Electron energy loss spectroscopy and x-ray absorption spectroscopy show that nitrogen is incorporated in the particles in molecular and compound form. In situ sintering of the particles drives out the nitrogen causing enhanced diffusion leading to the preference of the L10 structure over the multiply twinned icosahedral structure, which forms in the absence of nitrogen in the sputter gas.

Low temperature specific heat of FePt Invar alloys

Abstract We determine γ-terms and Debye temperatures Θ D ( T ) on ordered and disordered Fe 100− x Pt x ( x = 26–30 at %). Both quantities are independent of x . Enhancement of γ with respect to theoretical DOS can be related to electron-phonon coupling, consistent with phonon softening observed with decreasing T in neutron spectra. This property also explains the decrease of Θ D ( T ) for T → 0.

Single InGaAs nanowhiskers characterized by analytical transmission electron microscopy

Single crystal In x Ga1−x As nanowhisker were grown by metal-organic vapour-phase epitaxy on (111)B GaAs substrates using the vapour–liquid–solid growth mode. Present study is aimed to the direct evaluation of the lattice parameter, the phase, and the composition of nanowhiskers with high spatial resolution by high-resolution transmission electron microscopy and analytical transmission electron microscopy including energy-dispersive X-ray spectroscopy in nano probe mode. The goal is the direct identification of the quantified concentrations and the local element distributions at characteristic whisker areas. In particular, chemical measurements provide insights into the segregation of the gallium and arsenic out of the gold seed particle and the presence of indium remaining in the Au particle of the as-prepared whiskers. For comparison, InGaAs nanowhiskers are characterized by high-resolution X-ray diffractometry providing the mean whisker composition information indirectly by substituting the X-ray data into the formula of the law of Vegard.