Investigation of Size Specific Fe2O3 nanoparticles: Towards Single Nanoparticle Resolved Spectro-ptychography

Abstract

Ptychography is a scanning coherent diffraction imaging technique, where an unlimited field of view can be probed with a high spatial resolution. This technique relies on the increased coherent flux from third-generation synchrotron sources to measure the absorption and phase of a sample. Since ptychography is not limited by the properties of a focusing lens as is the case in many imaging techniques, it has the potential to reach the atomic regime. In practice, there are barriers to overcome to reach the analytical limit of ptychography. For example, the data process required to reconstruct diffraction images to real space images can be computationally intensive.

In this thesis, experiments were designed to understand these barriers and propose solutions to work towards achieving single nanoparticle resolved spectro-ptychography. Iron oxide nanoparticles, specifically 𝛼-Fe2O3 were used because they are well characterized, have a sharp core edge, and the ptychography imaging detector is sensitive to X-rays at this energy. Therefore, iron oxide nanoparticles were ideal to study with spectro-ptychography and to identify challenges within the acquisition, dataflow, and analysis. Here, we first compare conventional Scanning Transmission X-ray Microscopy (STXM) imaging with ptychographic imaging and then use ptychography images to obtain spectra. Both STXM and ptychography are spectro-microscopy techniques based on Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, which provides elemental and chemical sensitivity. In this thesis, NEXAFS spectro-ptychography was used to obtain the Fe L2,3 NEXAFS absorption and phase spectra.

The phase signal is theoretically related to the absorption spectrum through Kramers- Kronig integral. This thesis will use the Fe L2,3 NEXAFS absorption spectra of iron oxide nanoparticles to discuss the relationship between absorption and phase. The measured phase spectra obtained from ptychography are compared with the calculated phase spectra. Lastly, this thesis will summarize some of the critical experimental factors that will play a role in improving spectro-ptychography to realize the broad goal of obtaining well-resolved single nanoparticle spectra.