Scientific publications

The Monte Carlo software CASINO has been expanded with new modules for the simulation of complex beam scanning patterns, for the simulation of cathodoluminescence (CL), and for the calculation of electron energy deposition in subregions of a three-dimensional (3D) volume. Two examples are presented of the application of these new capabilities of CASINO. First, the CL emission near threading dislocations in gallium nitride (GaN) was modeled. The CL emission simulation of threading dislocations in GaN demonstrated that a better signal-to-noise ratio was obtained with lower incident electron energy than with higher energy. Second, the capability to simulate the distribution of the deposited energy in 3D was used to determine exposure parameters for polymethylmethacrylate resist using electron-beam lithography (EBL). The energy deposition dose in the resist was compared for two different multibeam EBL schemes by changing the incident electron energy.

Lateral profiles of the electron probe of scanning transmission electron microscopy (STEM) were simulated at different vertical positions in a micrometers-thick carbon sample. The simulations were carried out using the Monte Carlo method in CASINO software. A model was developed to fit the probe profiles. The model consisted of the sum of a Gaussian function describing the central peak of the profile and two exponential decay functions describing the tail of the profile. Calculations were performed to investigate the fraction of unscattered electrons as a function of the vertical position of the probe in the sample. Line scans were also simulated over gold nanoparticles at the bottom of a carbon film to calculate the achievable resolution as a function of the sample thickness and the number of electrons. The resolution was shown to be noise limited for film thicknesses less than 1 µm. Probe broadening limited the resolution for thicker films. The validity of the simulation method was verified by comparing simulated data with experimental data. The simulation method can be used as quantitative method to predict STEM performance or to interpret STEM images of thick specimens.

We have measured the size of the localized electron emission sites on multiwalled carbon nanotubes (MWNTs) with caps closed by a fullerenelike structure. MWNTs were individually mounted on tungsten support tips and imaged with a field emission microscope (FEM). The magnification of the FEM was calibrated using electron ray tracing and verified by comparing transmission electron microscope images. The FEM image was also tested for effects of the lateral energy spread. We found ring-shaped emission areas with three flattened sides, of a radius of 1.7±0.3nm, and separated by 5±1nm.

Nickel containing amorphous hydrogenated carbon (Ni:a-C:H) thin films prepared by reactive sputtering have a high potential for use as piezoresistive sensors. Investigations by means of X-ray diffraction (XRD), transmission electron microscopy, energy-dispersive X-ray spectroscopy, and magnetic characterizations indicate that sputtering parameters and heat treatment influence the film composition, the microscopic structure, and some relevant macroscopic physical properties. The films are heterogeneous in nature and consist of either nanometer sized hcp nickel, nickel carbide (these phases being indistinguishable by XRD), or fcc nickel clusters encapsulated by graphite-like carbon shells. The nature of the metal clusters in the thin films has a strong effect on its magnetic properties. For approximately 55. at.% Ni the electrical resistivity of the film is nearly temperature independent over a broad temperature range from 100. K to 400. K. The strain sensitivity, with a gauge factor of 20, is up to ten times higher than conventional temperature independent strain sensitive films. Compared to industry standard NiCr functional layers used for pressure sensors, Ni:a-C:H films provide a ten fold higher output signal.

Immobilized gold nanoparticles were imaged in a liquid containing water and 50% glycerol with scanning transmission electron microscopy (STEM). The specimen was enclosed in a liquid compartment formed by two silicon microchips with electron transparent windows. A series of images was recorded at video frequency with a spatial resolution of 1.5 nm. The nanoparticles detached from their support after imaging them for several seconds at a magnification of 250,000. Their movement was found to be much different than the movement of nanoparticles moving freely in liquid as described by Brownian Motion. The direction of motion was not random-the nanoparticles moved either in a preferred direction, or radially outwards from the center of the image. The displacement of the gold nanoparticles over time was three orders of magnitude smaller than expected on the basis of Brownian Motion. This finding implies that nanoscale objects of flexible structure or freely floating, including nanoparticles and biological objects, can be imaged with nanoscale resolution, as long as they are in close proximity to a solid support structure.

ZnO with additions of Fe2O3 or In2O3 shows characteristic inversion domain structures. ZnO domains are separated by two types of inversion domain boundaries (IDBs): basal b-IDBs parallel to (0 0 0 1) planes, and complementary pairs of three possible variants of pyramidal p-IDBs parallel to {2 over(1, -) over(1, -) 5} lattice planes. The structure and composition of IDBs were investigated in a sophisticated aberration-corrected scanning transmission electron microscope (probe-corrected TEM/STEM). It is shown that Fe and In additions are essentially located in monolayers within the IDBs, and EELS electron spectroscopic imaging (ESI) as well as EDS spectroscopic imaging by X-rays (SIX) are capable of rapidly mapping the element distribution. With solid solubility of trivalent dopant species well below 1 at.% within ZnO domains, the lateral spacings of b-IDBs are inversely proportional to the dopant concentration. Quantification of data acquired by ESI and SIX from well defined sample regions in STEM both confirm the assumption of one full monolayer of dopants per IDB. Atom columns of cations are well resolved in HAADF STEM imaging; experimental contrast intensities are approximately proportional to Z1.6. Furthermore, annular bright-field (ABF)-STEM imaging is capable of resolving oxygen columns even in thick sample regions, thus providing highly localized information on atom positions and lattice distortions, and enables the construction of more reliable structure models of IDBs in doped ZnO.

Light emission due to discharge in air is detected during and after the detachment of microstructured PDMS samples from glass surfaces, showing contact charging of the surfaces. The light emission provides information about the detachment process, like the velocity of the peeling front, which is difficult to obtain otherwise. While the work of separation exhibits the dependence on pulling velocity typically found for viscoelastic materials, the emission intensity exhibits almost no velocity dependence. We present a model for the rate-dependent contribution of a mosaic of contact charges to the work of separation. Also, the work of separation increases as expected with increasing aspect ratio of the microstructure, while the emission intensity shows a maximum for intermediate structures. Based on their different dependencies and on an upper-bound estimate of the energy emitted as light, we conclude that for the given system the contribution of electrostatic attraction to the work of separation is minor.

Structural and elemental mapping analyses were applied to characterize the mesostructure in the bulk metallic glass-forming Zr60FexCu30−xAl10 and Zr60+xFe5Cu25−xAl10 alloys. It is found that the observed meso-/microstructure is adjustable by the addition of Fe and tunes the acoustic anelasticity of the samples, while the relaxation time derived from our ultrasound measurements inversely scales with the mechanical properties and ductility of the alloys. The measurements also give data for the thermal diffusivity and coefficient of thermal expansion. Together with mechanical compression tests the results are further discussed on the basis of the thermo-elastic effect and the related role of heat dissipation on the propagation of shear bands in metallic glasses during plastic deformation.

The present work represents phase transformations observed upon cyclic loading of Zr62.5Fe5Cu22.5Al10 bulk metallic glassy samples within reversible deformation regime prior to plastic deformation. The structure of the samples was studied by X-ray diffraction and transmission electron microscopy including high-resolution imaging and analysis of selected-area electron diffraction and nanobeam diffraction patterns. Thermal properties of the samples were characterized by differential scanning calorimetry. It is found that kinetically frozen anelastic deformation accumulates at room temperature and causes crystallization of metallic glassy phase forming precursors of metastable crystalline FCC Zr-based phase stabilized by the alloying elements.