S. A. Nepijko

The structure of Pt-aggregates on a supported thin aluminum oxide film in comparison with unsupported alumina: a transmission electron microscopy study

Abstract Platinum has been deposited from the vapor onto a sample consisting of a thin aluminum oxide film grown on a NiAl(110) substrate wedge. At the tip of the wedge small γ-alumina crystallites are formed which have also been platinum decorated. The size distribution of the Pt aggregates on the thin aluminum oxide film is almost identical to the one found for the support-free alumina crystallites. It is also identical to the size distribution observed with STM under UHV conditions in earlier experiments [Th. Bertrams et al., Surf. Sci. 331–333 (1995) 1515]. The present structural analysis reveals that after Pt decoration the structure of the substrate remains unchanged. The Pt forms flat, epitaxial islands on both substrates. The nearest neighbor distances within the platinum aggregates have been determined for the supported oxide as a function of particle size. For large particles with radii greater than 20 A the nearest neighbor distance of bulk Pt is observed. In the case of the smallest particles with radii close to 10 A a contraction of up to 10% is detected. The contraction of the interatomic distance appears not to be isotropic. The observed Moire patterns comply with the assumption that the substrate lattice underneath the Pt aggregates does not change.

Magnetic sensitivity of a dispersion of aggregated ferromagnetic carbon nanotubes in liquid crystals

Using carbon nanotubes filled with α-Fe, we have shown that aggregated ferronematic colloids demonstrate reliable and very effective response to a weak (<5 mT) magnetic field. The magnetic field realigns the aggregates of the particles which results in a non-threshold reorientation of the LC nearby, leading to the optically observed director distortions. The distortion regions expand with the increase of the magnetic field and achieve maximum size of several micrometres, comparable with the size of the agglomerates. In the non-distorted regions the reorientation of the director begins at the magnetic field reaching the Freedericksz transition value. Taking into account the extreme sensitivity of aggregated ferronematics to magnetic field, the following experimental and theoretical studies of the individual response of the aggregated nanoparticles to magnetic field may became the topical task of the physics and applications of ferronematics.

Heusler compounds as ternary intermetallic nanoparticles: Co2FeGa

This work describes the preparation of ternary nanoparticles based on the Heusler compound Co2FeGa. Nanoparticles with sizes of about 20?nm were synthesized by reducing a methanol impregnated mixture of CoCl2 ? 6H2O, Fe(NO3)3 ? 9H2O and Ga(NO3)3 ? xH2O after loading on fumed silica. The dried samples were heated under pure H2 gas at 900??C. The obtained nanoparticles?embedded in silica?were investigated by means of x-ray diffraction (XRD), transmission electron microscopy, temperature dependent magnetometry and M??bauer spectroscopy. All methods clearly revealed the Heusler-type L21 structure of the nanoparticles. In particular, anomalous XRD data demonstrate the correct composition in addition to the occurrence of the L21 structure. The magnetic moment of the particles is about 5?B at low temperature in good agreement with the value of bulk material. This suggests that the half-metallic properties are conserved even in particles on the 10?nm scale.

Formation of an intermolecular charge-transfer compound in UHV codeposited tetramethoxypyrene and tetracyanoquinodimethane

Ultrahigh vacuum UHV-deposited films of the mixed phase of tetramethoxypyrene and tetracyanoquinodimethane TMP1-TCNQ1 on gold have been studied using ultraviolet photoelectron spectroscopy UPS, x-ray diffraction XRD, infrared IR spectroscopy, and scanning tunneling spectroscopy STS. The formation of an intermolecular charge-transfer CT compound is evident from the appearance of new reflexes in XRD d1 = 0.894 nm and d2 = 0.677 nm. A softening of the CN stretching vibration redshift by 7 cm �1 of TCNQ is visible in the IR spectra, being indicative of a CT on the order of 0.3e from TMP to TCNQ in the complex. Characteristic shifts in the electronic level positions occur in UPS and STS that are in reasonable agreement with the prediction of density-functional theory DFT calculations GAUSSIAN03 with hybrid functional B3LYP. STS reveals a highest occupied molecular orbital HOMO-lowest unoccupied molecular orbital LUMO gap of the CT complex of about 1.25 eV being much smaller than the gaps 3.0 eV of the pure moieties. The electron-injection and hole-injection barriers are 0.3 eV and 0.5 eV, respectively. Systematic differences in the positions of the HOMOs determined by UPS and STS are discussed in terms of the different information content of the two methods.

Transmission electron microscopic investigation of an ordered Al2O3 film on NiAl(110)

Abstract We have prepared under ultrahigh vacuum conditions a NiAl(110) wedge which was oxidized such that, after oxidation, small support-free Al 2 O 3 crystallites were formed at the edge of the wedge, and the remaining part of the wedge was covered by a thin well-ordered Al 2 O 3 film. This film, which has been characterized in great detail in earlier studies [J. Libuda et al., Surf. Sci. 318 (1994) 61], was transferred in air to a transmission electron microscope (TEM) and studied with respect to its geometric structure. The particular sample preparation allows for a direct comparison between the structure of the film and the bulk aluminum oxide. We find that the supported oxide exhibits a γ-Al 2 O 3 -like structure similar to that observed previously under UHV conditions, indicating that exposure to air has only a limited influence on the film. The lattice constant of the oxide film agrees within ∼2% with that reported for bulk Al 2 O 3 . The domain structure of the alumina film as found in the ultrahigh vacuum experiments was also identified using TEM of the supported layer. The support-free oxide formed at the edge of the wedge exhibits the structure of the supported film formed under UHV conditions with deviations of the lattice constants of ∼7% in one direction and ∼1% in the other.

Investigation of a novel material for magnetoelectronics:Co2Cr0.6Fe0.4Al

Heusler compounds are promising candidates for future spintronics device applications. The electronic and magnetic properties of Co2Cr0.6Fe0.4Al, an electron-doped derivative of Co2CrAl, are investigated using circularly polarized synchrotron radiation and photoemission electron microscopy (PEEM). Element specific imaging reveals needle shaped Cr rich phases in a homogeneous bulk of the Heusler compound. The ferromagnetic domain structure is investigated on an element-resolved basis using x-ray magnetic circular dichroism (XMCD) contrast in PEEM. The structure is characterized by micrometre-size domains with a superimposed fine ripple structure; the lateral resolution in these images is about 100 nm. The domains look identical for Co and Fe giving evidence of a ferromagnetic coupling of these elements. No ferromagnetic contrast is observed at the Cr line. Magnetic spectroscopy exploiting XMCD reveals that the lack of magnetic moment, detected in a SQUID magnetometer, is mainly due to the moment of the Cr atom.

Incoherent magnetization rotation observed in subnanosecond time-resolving x-ray photoemission electron microscopy

We present recent results of time-resolved x-ray photoemission electron microscopy on permalloy microstructures. The stroboscopic experiments feature a time-resolution of Δτ⩽130ps. We observe a strong influence of incoherent magnetization rotation processes, leading to a significant transient stray-field formation at the edges of the microstructure.

Size-dependent luminescence of small palladium particles

Abstract We report cathodoluminescence spectra of small Pd clusters of various size deposited on a thin, well-ordered oxide film. The size distribution of the deposited metal aggregates can be chosen and deliberately varied. The luminescence spectra vary characteristically as a function of the variation of particle size. The resonance energy at the highest luminescence energy with decreasing average particle size vary as expected for electronic states in a quantum confined metal particle. For an average aggregate size of 10 A the luminescence is maximum.

Peculiarities of imaging one- and two-dimensional structures using an electron microscope in the mirror operation mode.

Measurements performed in an electron microscope with the mirror operation mode are most sensitive to local electric fields and geometrical roughness of any kind of the object being studied. The object with a geometrical relief is equivalent to a smooth surface with an effective distribution of microfields. Electrons forming the image interact with the local microfields for an extended time: during approach to the object, deceleration and acceleration away from the object. As a result, the electron trajectories can be strongly distorted, and the contrast changes essentially, leading to image deformation of details of the object under investigation and to lowering of the resolution. These effects are theoretically described and are illustrated by experiments. An analysis of these effects enables the real size and the shape of the object involved to be reconstructed.

TEM study of tantalum clusters on Al2O3/NiAl(110)

Employing transmission electron microscopy (TEM ), energy dispersive X-ray analysis ( EDX ) and X-ray photoelectron spectroscopy ( XPS) we have studied tantalum clusters on a thin Al 2 O 3 film epitaxially grown on NiAl(110). Our data reveal that the clusters are three dimensional, growing epitaxially on the oxide film with their [110] directions parallel to the surface normal and