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Methods and Tools



Liquid cell transmission electron microscopy

A novel sample holder (Protochips Inc.) for transmission electron microscopy (TEM) allows for the imaging of objects in liquid environments such as minerals forming from supersaturated ionic solutions or biogenic materials that undergo significant changes when dried or frozen to be imaged in TEM. A schematic shown in Figure 1 This versatile technique gives access to many aspects of liquid phase processes with an unprecedented spatial resolution of less than 1 nm, especially if scanning TEM (STEM) is used for imaging. We are using this technique to study the dynamics of the formation of minerals such as calcium carbonate or hydroxyapatite from solution. An important aspect in this context is the role of amorphous precursor phases for the growth rates, polymorph selection and shape evolution. Moreover, this method allows for the investigation of biological materials such as bacteria in liquids, which we aim to develop.

Atmospheric scanning electron microscopy

The examination of processes in liquids using a scanning electron microscope (SEM) offers a huge scope of questions to be addressed regarding mineral formation in liquids. Such a unique system (JEOL Clairscope available at the Bioscience Technology Facility), is available in York and consists of an inverted SEM combined with a confocal optical microscope and allows for the imaging of processes and objects in liquids using back-scattered electron detection as shown in Figure 2. The object remains in liquid and under atmospheric condition during the observation. Our studies focus on the precipitation from supersaturated solutions, and in particular the effect of the presence of organic additives - important in many biological systems - on this process. Results of these investigations have been published in the Journal of Structural Biology (Journal of Structural Biology 2013).

Atomic level strain analysis of core/shell nanoparticles

Aberration corrected transmission and scanning transmission electron microscopy offer exciting opportunities for the study of materials with a spatial resolution of better than 1 Å. We use this tool (JEOL 2200 FS) to investigate the oxidation of iron nanoparticles deposited by a cluster source. The unprecedented resolution enables the detailed study of strain in the oxide shell formed around a cubic iron nanoparticle. Using atomic level strain-state analysis it was possible to determine the strain within the oxide layer forming on the iron core as shown in Figure 3. It revealed that extreme strains can exist on the nanoscale due to the fact that dislocations, which would normally relax this strain in bulk systems, are energetically unfavorable at these length scales and hence strains of approx. 10% can be sustained. This effect was found to be important with e.g. in the accelation of otoxidation process in iron nanoparticles (Nature Materials 2013).

Raman microscopy

We are using a Horiba XploRA Raman Microscope to study the structure of minerals (e.g. carbonates and phosphate), mineral/organic composites (e.g. corals, bones and teeth) and organic microfibers such as collagen and cellulose. The system is equipped with three laser sources (with wavelengths of 532 nm, 633 nm, 785 nm) allowing for material specific analyses.
    
Schematic of liquid cell holder
Figure 1: Schematic of the liquid-cell transmission electron microscopy setup allowing for imaging of liquids  through a pair of silicon nitride membranes.


Schematic of ASEM
Figure 2: Schematic of the atmospheric SEM (ASEM). The microscope consists of an inverted SEM with the object placed into a petri-dish with a silicon nitride window at the bottom. This setup allows for direct imaging of processes in liquids. Additionally the system is equipped with a confocal optical microscope.


Iron oxid strainFigure 3: Atomic-level strain state analysis of an iron nanoparticle which has undergone oxidation in air.









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