CNR NANO - Istituto Nanoscienze Consiglio Nazionale delle Ricerche

TRANSMISSION ELECTRON MICROSCOPY

 

People

Vincenzo Grillo

Stefano Frabboni

 

The TEM group works in two main directions: nanostructure characterization and microscopy methodological development. Methodological research has been devoted to extract quantitative information on strain, composition and morphology at atomic scale. The main technical developments have been in the Convergent Beam Electron Diffraction, Scanning Transmission Electron Microscopy with and annular dark field detector (STEM-ADF) and High resolution phase contrast TEM (HREM).

In the field of Convergent Beam Electron Diffraction the group has developed methods for the determination of strain from HOLZ line patterns an static displacement from the analysis of Rocking Curves of high-angle diffracted beam extracted from Large Angle Convergent Beam Electron Diffraction patterns.

In the field of HRTEM the group is working by one side on the extension of strain analysis like the classic geometric phase algorithm (GPA) and on the application and optimization of exit wave reconstruction (EWR) algorithm to overcome the resolution limit imposed by lens aberration and obtain an information on the phase. These two techniques together are a very effective tool for chemical and structural analysis of nanoparticles. Fig. 1 is an example of application to icosahedral multi-twinned nanoparticles of Ni where EWR permits to directly correlate with the complex structure and identify NiO at surfaces without artifacts. The inset in particular show the 5fold symmetry of the diffractogram, which can be related to the theory of Quasi-crystal. The interest of quasi-crystal is proved by the Nobel Prize in this field in 2011.

In the field of ADF, the TEM group is developing methods to simulate the effect of strain and the detailed analysis of quantitative composition analysis. In particular the group is maintaining the first parallel computing multislice simulation of the STEM process and is actively collaborating with international research groups on quantitative analysis based on ADF. On the other hand a lot of research is being devoted to the creation of approximate methods for a qualitative image interpretation of strain related effects in ADF. Fig. 2 shows the comparison of approximate simulation and experiments for a screw dislocation in GaN.

All methodologies have been implemented in a freely available code named STEM_CELL maintained by the group. The advanced TEM methodologies have been successfully applied to nanoparticles grown by physical and chemical methods, nanowires and semiconductor heterostructures.

 

 

Fig 1. Exit wave reconstruction (phase) of icosahedral multitwinned Ni particles along different directions. The presence of NiO is highlighted. The inset shows the Fourier transform of a simulated and experimental HREM image in the same direction.

Fig 2. a) Schematic of the strain field about a screw dislocation touching a surface. b) Simulations and experiments of ADF contrast of a screw dislocation as in a), a small mistilt was applied in the apparent dipole direction.