Klaus Leifer

Plastic deformation of gallium arsenide micropillars under uniaxial compression at room temperature

The authors have experimentally investigated the compressive strength of GaAs pillars with a diameter of 1μm by uniaxial compression tests. The tests were performed at room temperature and, contrary to macroscopic tests, the micropillars were found to exhibit ductile plasticity comparable to that of metal single crystal micropillars. The yield stress was 1.8±0.4GPa and, for one pillar that was more closely examined, a total deformation of 24% was observed. In the diffraction patterns from transmission electron microscopy studies of this pillar, a high density of twins was observed.

Ductile-brittle transition in micropillar compression of GaAs at room temperature

Experiments have been carried out on how compressive failure of <100> axis GaAs micropillars at room temperature is influenced by their diameter. Slip was observed in all micropillars, often on intersecting slip planes. Cracks could nucleate at these intersections and then grow axially in the sample, with bursts of crack growth. However, GaAs micropillars with diameters less than approximately 1 µm did not split, nor was splitting observed where slip occurred on only one plane. The conditions under which such splitting can occur have been estimated by modifying an existing analysis. This predicts a ductile–brittle transition at a micropillar diameter of approximately 1 µm, consistent with experimental observations.

Conductivity engineering of graphene by defect formation

Transport measurements have revealed several exotic electronic properties of graphene. The possibility to influence the electronic structure and hence control the conductivity by adsorption or doping with adatoms is crucial in view of electronics applications. Here, we show that in contrast to expectation, the conductivity of graphene increases with increasing concentration of vacancy defects, by more than one order of magnitude. We obtain a pronounced enhancement of the conductivity after insertion of defects by both quantum mechanical transport calculations as well as experimental studies of carbon nano-sheets. Our finding is attributed to the defect induced mid-gap states, which create a region exhibiting metallic behaviour around the vacancy defects. The modification of the conductivity of graphene by the implementation of stable defects is crucial for the creation of electronic junctions in graphene-based electronics devices.

Mild sonochemical exfoliation of bromine-intercalated graphite: a new route towards graphene

A method to produce suspensions of graphene sheets by combining solution-based bromine intercalation and mild sonochemical exfoliation is presented. Ultrasonic treatment of graphite in water leads ...

Development of Ni/Ti multilayer supermirrors for neutron optics

Artificial multilayer neutron supermirrors are composed of discrete sets of bilayer structures based on Ni and Ti layers with graded thicknesses and with high scattering contrast between individual layers. Various samples consisting of NiXxTiYy (X = C, N, dry air, Y = H, V) periodic multilayers and supermirrors, as well as NiXx and TiYy monolayers were deposited on normal float-glass, borkron glass and on silicon substrates using d.c. magnetron sputtering. The structure and stability of these thin films is the object of a detailed study mainly by means of X-ray diffraction, TEM, AFM and neutron reflectometry. The addition of N to the Ni layers yields a Ni lattice dilatation, a reorientation of the Ni grains from {111} to {200} parallel to the layer surface, a reduction of the grain size, a smoothing of the layer surfaces (Rr.m.s.∼0.3 nm for d = 10 nm), a sharpening of the multilayer interfaces and an enhancement of the scattering length density (nb=9.53×10−4nm−2 for Ni− N). Similar results are obtained with C and dry air, which in addition suppress the diffusion of Ni into Ti. The addition of V to the Ti layers shows a smoothing of the layer surfaces and helps to suppress interdiffusion in multilayers, leading also to sharp and stable interfaces. Large neutron supermirrors made of 40 bilayers of NiNO/TiV with a reflectivity in the range of 90% to 94% at glancing angles up to more than twice the critical angle of total reflection from an ordinary Ni surface have been achieved and are fabricated in mass production.

Improved gas sensing activity in structurally defected bilayer graphene

Graphene is a two-dimensional material with a capability of gas sensing, which is here shown to be drastically improved by inducing gentle disorder in the lattice. We report that by using a focused ion beam technique, controlled disorder can be introduced into the graphene structure through Ga(+) ion irradiation. This disorder leads to an increase in the electrical response of graphene to NO(2) gas molecules by a factor of three in an ambient environment (air). Ab initio density functional calculations indicate that NO(2) molecules bind strongly to Stone-Wales defects, where they modify electronic states close to the Fermi level, which in turn influence the transport properties. The demonstrated gas sensor, utilizing structurally defected graphene, shows faster response, higher conductivity changes and thus higher sensitivity to NO(2) as compared to pristine graphene.

Two-, three-, and four-component magnetic multilayer onion nanoparticles based on iron oxides and manganese oxides.

Magnetic multilayered, onion-like, heterostructured nanoparticles are interesting model systems for studying magnetic exchange coupling phenomena. In this work, we synthesized heterostructured magnetic nanoparticles composed of two, three, or four components using iron oxide seeds for the subsequent deposition of manganese oxide. The MnO layer was allowed either to passivate fully in air to form an outer layer of Mn(3)O(4) or to oxidize partially to form MnO|Mn(3)O(4) double layers. Through control of the degree of passivation of the seeds, particles with up to four different magnetic layers can be obtained (i.e., FeO|Fe(3)O(4)|MnO|Mn(3)O(4)). Magnetic characterization of the samples confirmed the presence of the different magnetic layers.

Quantitative magnetic information from reciprocal space maps in transmission electron microscopy

One of the most challenging issues in the characterization of magnetic materials is to obtain a quantitative analysis on the nanometer scale. Here we describe how electron magnetic circular dichroism (EMCD) measurements using the transmission electron microscope can be used for that purpose, utilizing reciprocal space maps. Applying the EMCD sum rules, an orbital to spin moment ratio of mL/mS=0.08+/-0.01 is obtained for Fe, which is consistent with the commonly accepted value. Hence, we establish EMCD as a quantitative element-specific technique for magnetic studies, using a widely available instrument with superior spatial resolution.

Decomposition, diffusion, and growth rate anisotropies in self-limited profiles during metalorganic vapor-phase epitaxy of seeded nanostructures

We present a model for the interplay between the fundamental phenomena responsible for the formation of nanostructures by metalorganic vapor phase epitaxy on patterned (001)/(111)B GaAs substrates. Experiments have demonstrated that V-groove quantum wires and pyramidal quantum dots form as a consequence of a self-limiting profile that develops, respectively, at the bottom of V-grooves and inverted pyramids. Our model is based on a system of reaction-diffusion equations, one for each crystallographic facet that defines the pattern, and include the group III precursors, their decomposition and diffusion kinetics (for which we discuss the experimental evidence), and the subsequent diffusion and incorporation kinetics of the group-III atoms released by the precursors. This approach can be applied to any facet configuration, including pyramidal quantum dots, but we focus on the particular case of V-groove templates and offer an explanation for the self-limited profile and the Ga segregation observed in the V-groove. The explicit inclusion of the precursor decomposition kinetics and the diffusion of the atomic species revises and generalizes the earlier work of Biasiol et al. [Biasiol et al., Phys.Rev. Lett. 81, 2962 (1998); Phys. Rev. B 65, 205306 (2002)] and is shown to be essential for obtaining a complete description of self-limiting growth. The solution of the system of equations yields spatially resolved adatom concentrations, from which average facet growth rates are calculated. This provides the basis for determining the conditions that yield self-limiting growth. The foregoing scenario, previously used to account for the growth modes of vicinal GaAs(001) and the step-edge profiles on the ridges of vicinal surfaces patterned with V-grooves during metalorganic vapor-phase epitaxy, can be used to describe the morphological evolution of any template composed of distinct facets.

A site-specific focused-ion-beam lift-out method for cryo Transmission Electron Microscopy.

The focused-ion-beam (FIB) is the method of choice for site-specific sample preparation for Transmission Electron Microscopy (TEM) in material sciences. A lamella can be physically lifted out from a specific region of a bulk specimen with submicrometer precision and thinned to electron transparency for high-resolution imaging in the TEM. The possibility to use this tool in life sciences applications has been limited by the lack of lift-out capabilities at the cryogenic temperatures often needed for biological samples. Conventional cryo-TEM sample preparation is mostly based on ultramicrotomy, a procedure that is not site-specific and known to produce artifacts. Here we demonstrate how a cooled nanomanipulator and a custom-built transfer station can be used to achieve cryo-preparation of TEM samples with the FIB, enabling high-resolution investigation of frozen-hydrated specimens in the TEM.