Chris Van Haesendonck

Field emission from vertically aligned few-layer graphene

The electric field emission behavior of vertically aligned few-layer graphene was studied in a parallel plate–type setup. Few-layer graphene was synthesized in the absence of any metallic catalyst by microwave plasma enhanced chemical vapor deposition with gas mixtures of methane and hydrogen. The deposit consists of nanostructures that are several micrometers wide, highly crystalline stacks of four to six atomic layers of graphene, aligned vertically to the substrate surface in a high density network. The few-layer graphene is found to be a good field emitter, characterized by turn-on fields as low as 1 V/μm and field amplification factors up to several thousands. We observe a clear dependence of the few-layer graphene field emission behavior on the synthesis parameters: Hydrogen is identified as an efficient etchant to improve field emission, and samples grown on titanium show lower turn-on field values and higher amplification factors when compared to samples grown on silicon.

Reversing the Training Effect in Exchange Biased CoO/Co Bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.

Thin film growth of semiconducting Mg2Si by codeposition

Ultrahigh vacuum evaporation of magnesium onto a hot silicon substrate (⩾200 °C), with the intention of forming a Mg2Si thin film by reaction, does not result in any accumulation of magnesium or its silicide. On the other hand, codeposition of magnesium with silicon at 200 °C, using a magnesium-rich flux ratio, gives a stoichiometric Mg2Si film which can be grown several hundreds of nm thick. The number of magnesium atoms which condense is equal to twice the number of silicon atoms which were deposited; all the silicon condenses while the excess magnesium in the flux desorbs. The Mg2Si layers thus obtained are polycrystalline with a (111) texture. From the surface roughness analysis, a self-affine growth mode with a roughness exponent equal to 1 is deduced.

Can atomic force microscopy tips be inspected by atomic force microscopy

An attempt has been made to image prospective tips by atomic force microscopy. The apex of mounted diamond fragments and of traditional metallic tips was investigated by the same diamond probe. The peculiar tip–tip configuration allowed to search for the effect of sample rotation on the images. Identical images were obtained when the diamond stylus scanned different etched tungsten tips, illustrating an interchange in the roles of tip and sample.

Topological transport and atomic tunnelling–clustering dynamics for aged Cu-doped Bi2Te3 crystals

Enhancing the transport contribution of surface states in topological insulators is vital if they are to be incorporated into practical devices. Such efforts have been limited by the defect behaviour of Bi2Te3 (Se3) topological materials, where the subtle bulk carrier from intrinsic defects is dominant over the surface electrons. Compensating such defect carriers is unexpectedly achieved in (Cu0.1Bi0.9)2Te3.06 crystals. Here we report the suppression of the bulk conductance of the material by four orders of magnitude by intense ageing. The weak antilocalization analysis, Shubnikov–de Haas oscillations and scanning tunnelling spectroscopy corroborate the transport of the topological surface states. Scanning tunnelling microscopy reveals that Cu atoms are initially inside the quintuple layers and migrate to the layer gaps to form Cu clusters during the ageing. In combination with first-principles calculations, an atomic tunnelling–clustering picture across a diffusion barrier of 0.57 eV is proposed.

Dependence of the NaCl/Au(111) interface state on the thickness of the NaCl layer

We investigated the growth and the electronic properties of crystalline NaCl layers on Au(111) surfaces by means of cryogenic scanning tunneling microscopy and spectroscopy under ultra-high vacuum conditions. Deposition of NaCl on Au(111) at room temperature yields bilayer NaCl islands, which can be transformed into trilayer NaCl islands by post-annealing. Upon NaCl adsorption, the Au(111) Shockley surface state becomes an interface state (IS) at the NaCl/Au(111) interface. Using Fourier-transform images of maps of the local density of states, the energy versus wave vector dispersions of the IS and the Au(111) bulk states are determined. The dispersion of both states is found to depend strongly on the thickness of the adsorbed NaCl layer.

Imaging of vortices in conventional superconductors by magnetic force microscopy

Abstract We have imaged vortices in the conventional superconductors NbSe2 (crystal) and Nb (thin film) with a low temperature magnetic force microscope (MFM). The MFM detection is based on commercially available piezoresistive cantilevers. A considerably improved sensitivity (0.2 pN/nm) at 4.3 K has been obtained by using a higher flexural mode of the cantilevers. The operation at higher (2nd or 3rd) mechanical resonances improves the signal-to-noise ratio by a factor of 5. The improved sensitivity allows us to reduce the heat dissipation down to 0.05 mW in the cantilevers without lowering their performance, which is highly desirable for MFM applications at liquid helium temperatures. The magnetic tip coating was optimized by relying on Co/Au multilayers grown by oblique incidence molecular beam epitaxy. A magnetic field of 0.5–3 mT was used to induce the vortices under field-cooled conditions. In thin Nb films, we observed an irregular vortex arrangement and the imaged vortices are attached to individual pinning centers. On the cleaved surface of NbSe2 crystals, we observed an evolution from a disordered towards an ordered state of the vortex lattice. The possibility to image the Abrikosov vortex lattice in NbSe2 can be understood in terms of collective pinning effects.

Band structure quantization in nanometer sized ZnO clusters

Nanometer sized ZnO clusters are produced in the gas phase and subsequently deposited on clean Au(111) surfaces under ultra-high vacuum conditions. The zinc blende atomic structure of the approximately spherical ZnO clusters is resolved by high resolution scanning transmission electron microscopy. The large band gap and weak n-type conductivity of individual clusters are determined by scanning tunnelling microscopy and spectroscopy at cryogenic temperatures. The conduction band is found to exhibit clear quantization into discrete energy levels, which can be related to finite-size effects reflecting the zero-dimensional confinement. Our findings illustrate that gas phase cluster production may provide unique possibilities for the controlled fabrication of high purity quantum dots and heterostructures that can be size selected prior to deposition on the desired substrate under controlled ultra-high vacuum conditions.

Amyloid beta oligomers induce neuronal elasticity changes in age-dependent manner: a force spectroscopy study on living hippocampal neurons

Small soluble species of amyloid-beta (Aβ) formed during early peptide aggregation stages are responsible for several neurotoxic mechanisms relevant to the pathology of Alzheimer’s disease (AD), although their interaction with the neuronal membrane is not completely understood. This study quantifies the changes in the neuronal membrane elasticity induced by treatment with the two most common Aβ isoforms found in AD brains: Aβ40 and Aβ42. Using quantitative atomic force microscopy (AFM), we measured for the first time the static elastic modulus of living primary hippocampal neurons treated with pre-aggregated Aβ40 and Aβ42 soluble species. Our AFM results demonstrate changes in the elasticity of young, mature and aged neurons treated for a short time with the two Aβ species pre-aggregated for 2 hours. Neurons aging under stress conditions, showing aging hallmarks, are the most susceptible to amyloid binding and show the largest decrease in membrane stiffness upon Aβ treatment. Membrane stiffness defines the way in which cells respond to mechanical forces in their environment and has been shown to be important for processes such as gene expression, ion-channel gating and neurotransmitter vesicle transport. Thus, one can expect that changes in neuronal membrane elasticity might directly induce functional changes related to neurodegeneration.

Size-Dependent Penetration of Gold Nanoclusters through a Defect-Free, Nonporous NaCl Membrane

Membranes and their size-selective filtering properties are universal in nature and their behavior is exploited to design artificial membranes suited for, e.g., molecule or nanoparticle filtering and separation. Exploring and understanding penetration and transmission mechanisms of nanoparticles in thin-film systems may provide new opportunities for size selective deposition or embedding of the nanoparticles. Here, we demonstrate an unexpected finding that the sieving of metal nanoparticles through atomically thin nonporous alkali halide films on a metal support is size dependent and that this sieving effect can be tuned via the film thickness. Specifically, relying on scanning tunneling microscopy and spectroscopy techniques, combined with density functional theory calculations, we find that defect-free NaCl films on a Au(111) support act as size-dependent membranes for deposited Au nanoclusters. The observed sieving ability is found to originate from a driving force toward the metal support and from the dynamics of both the nanoparticles and the alkali halide films.