Real time observation of GeSi/Si(001) island shrinkage due to surface alloying during Si capping.

The Si capping of Ge/Si(001) islands was observed by in situ time-resolved transmission electron microscopy. During the initial stages of the Si deposition, islands were observed not only to flatten but also to shrink in volume. This unexpected shrinkage is explained by taking into account the intermixing of the deposited Si with the wetting layer and a consequently induced diffusion of Ge from the islands into the wetting layer. A model of the capping process which takes into account Ge diffusion is presented which is in good agreement with the experimental data.

Simulated electron energy loss spectra from a C70 crystal.

Electron energy loss spectra are simulated for a C70 crystalline structure. It is found that the simulated spectrum is similar to the unoccupied density of states of a C70 molecule, indicating that the crystalline structure has only a small effect on the spectrum. Unlike the case of a single molecule, however, the main contribution to the second peak in the spectrum cannot be ascribed as being due to the equatorial atoms.

Self-assembled periodical polycrystalline-ZnO/a-C nanolayers on Zn nanowire.

Zn nanowires with an epitaxial thin surface layer of zinc oxide were dispersed onto amorphous carbon films and stored at room temperature. After 1500 h, a self-organized equal-spaced zinc oxide (approximately 2 nm)/carbon (approximately 2.5 nm) multilayer structure was found to form outside the Zn nanowire, taking the place of the original ZnO surface layer. We carried a systematic study to clarify the self-formation mechanism of the periodical multilayers outside the Zn nanowire and found out that such a configuration originated from a chemical reaction between Zn and CO2 and were formed via a gas phase diffusion-interfacial chemical reaction-phase separation process.

[001] zone-axis bright-field diffraction contrast from coherent Ge(Si) islands on Si(001).

Coherent Ge(Si)/Si(001) quantum dot islands grown by solid source molecular beam epitaxy at a growth temperature of 700 degrees C were investigated using transmission electron microscopy working at 300kV. The [001] zone-axis bright-field diffraction contrast images of the islands show strong periodicity with the change of the TEM sample substrate thickness and the period is equal to the effective extinction distance of the transmitted beam. Simulated images based on finite element models of the displacement field and using multi-beam dynamical diffraction theory show a high degree of agreement. Studies for a range of electron energies show the power of the technique for investigating composition segregation in quantum dot islands.

Electron diffraction from nanovolumes of amorphous material using coherent convergent illumination.

Using a conventional transmission electron microscope that incorporates a field emission gun it is possible to focus an electron beam to form a small probe (<1nm full-width at half-maximum). Such a probe can then be used to perform high spatial resolution diffraction experiments. The high spatial resolution allows technologically interesting amorphous volumes, such as those found in glassy intergranular phases or in semiconductor implantations, to be investigated directly. In order to achieve the probe characteristics necessary to investigate nanovolumes of material the probe must be highly convergent which results in it being highly coherent. In this paper we examine the effect of coherent convergent illumination on electron diffraction data taken from nanovolumes of amorphous material. It is shown that, for amorphous volumes as small as 1.2nm in diameter, the additional interference effects induced in the diffraction data by the use of coherent convergent illumination are largely suppressed by the lack of order in amorphous materials. This allows the use of deconvolution techniques, developed for the correction of broadening of the diffraction pattern in the case of incoherent illumination, and the subsequent application of reduced density function (G(r)) analysis, to also be used for coherent illumination.

Deconvolution of electron diffraction patterns of amorphous materials formed with convergent beam.

To perform reduced density function (G(r)) analysis on electron diffraction patterns of amorphous materials formed with convergent beams, the effects of convergence must be removed from the diffraction data. Assuming electrons incident upon the sample in different directions are incoherent, this can be done using deconvolution (Ultramicroscopy 76 (1999) 115). In this letter we show that the combination of an energy filtering transmission electron microscope with an image plate, increases the accuracy with which diffraction data can be measured and, subsequently, the accuracy of the deconvolution.

Limitations on the s-state approach to the interpretation of sub-angstrom resolution electron microscope images and microanalysis.

The s-state approach is useful for analysing transmission electron microscope images of a thin crystalline foil consisting of well-separated atomic columns. It assumes that the signal collected (e.g., the annular dark-field image, the EELS spectrum) can be attributed to only the lowest-energy bound eigenstate of the two-dimensional projected potential of a single column. When, however, columns are close, the form of the bound states depends on more than one column, which implies that interpretation of the signal may not be so simple. For closely spaced columns we show that the simple s-state approach fails for the case of a sub-Angstrom probe initially centred on one column of a pair, because two bound states are excited. The energy in the probe is almost completely transferred to the neighbouring column after it has propagated some tens of nanometres through the foil and then is transferred back. Signals which relate directly to the local probe intensity (e.g. annular dark-field formed by thermal diffuse scattering, EELS) must be analysed in terms of the two bound states. Accurate calculations of bound states of pairs of columns are more demanding than for a single column but sufficient accuracy can be achieved from knowledge of the 1s-states of isolated columns. We provide formulae for the bound states of a column pair. These can be used to determine if image analysis requires the extension to the s-state approach described in this paper.