Abstracts Book - IMRC 2018

• SD4-O015 Invited Talk
UNDERSTANDING AMORPHOUS MESOPOROUS SILICA
SUPERSTRUCTURES BY ABERRATION-CORRECTED STEM
Sebastian Calderon 1 , Tânia Ribeiro 2 , José Paulo Farinha 2 , Carlos Baleizão 2 , Paulo Ferreira 1,3
1 International Iberian Nanotechnology Laboratory - INL, Advanced Electron Microscopy,
Imaging and Spectroscopy, Portugal. 2 IST, University of Lisbon, Centro de Química-Física
Molecular and Institute of Nanosciences and Nanotechnology, Portugal. 3 University of Texas at
Austin, Materials Science and Engineering, United States.
Mesoporous silica nanoparticles (NPs) have been intensively studied due to their
potential use in catalysis and biomedicine, including cancer treatment and drug
delivery applications. However, when mesoporous particles are produced at the
nanoscale, the pore configuration is modified. In order to determine the pore
morphology and structure, advanced microscopy techniques are required.
This work combines molecular dynamics techniques, electron microscopy
computer simulations and aberration-corrected STEM imaging to provide an
insight into the structure of amorphous SiO2 NPs. The amorphous silica model
is prepared using a simple melt-quench molecular dynamics (MD) method, while
the reconstruction of the mesoporous NPs is carried out using an isotropic unit
cell to avoid false symmetry in the final model. For the high-resolution STEM
simulations, the QSTEM software package is used based on a multisliced
technique. Finally, for comparison with the simulated images high angular
annular dark field (HAADF) STEM images were taken using an aberration
corrected FEI Titan ChemiSTEM microscope, operated at 200 kV.
The amorphous models are analyzed using the radial distribution function (RDF)
and mass density, demonstrating a good agreement with the experimental
results. Depending on the quenching model, the local density can be modified
obtaining isotropic values between 2.2 and 4 g cm-3, with radial distribution
function similar to the bulk values reported experimentally. The highest
probability of finding Si-O pairs is at 1.58 Å, O-O is at 2.62 Å and Si-Si 3.08 Å. The
multisliced STEM images demonstrate that the density of the models do not
have a significant impact on the STEM images for isolated SiO2 phases. However,
a detailed analysis reveals that the intensities of the systems shows that denser
SiO2 structures results in a more intense signal, due to the increase in the
scattering power.