Finally, we contrast the atomically sharp attachment of above aspect (ii) with the more commonly observed “loose” attachment of two particles on corners or faces during live video-recording, with orientation relationship remaining flexible as of Fig.
6 and
8. In this aspect (iii), the driving force for attraction is mainly expected to be a result of surface charges with field lines connecting the tips or spikes of neighbouring particles, not forming any chemical bond. Formation of particle chains or rod chains would minimise the free surface charge and stray field energies. Within liquid cell TEM research, chain-forming behaviour has been found for metallic NPs, such as Fe
3Pt [
38] or Au [
39] and motivated either by magnetic dipole interactions or electrostatic interactions. For Au nanoparticles, pair formation with a small equilibrium distance between non-touching particles has been described [
11], with a balance struck between attractive van der Waals forces and repulsive hydration double-layer forces, while entire Au nanorod chains are found in [
40] with an end-to-end local van der Waals attachment proposed after irradiation had weakened electrostatic repulsive forces. The particle–particle finite distance is considered transient before atomistic bonds eventually form, as observed in liquid cell TEM in [
11]. Furthermore, it is highly likely that all particles in the chain are attached to the underlying Si
3N
4 membrane and merely move in 2D. For ceramic nanoparticulate materials, e.g. in the ZnO or ZnS family, Wurtzite structure would contribute a natural orientational asymmetry. However, for the cubic sphalerite modification, net dipole moments have been identified to be necessary to be generated by surface occupancy defects, or subtle shape asymmetries [
41]; furthermore, embedding in water is shown to strengthen dipole moments [
41]. For fluorite instead of sphalerite, the [100] facets contribute unsaturated dipoles on the nanoscale even in the absence of water. Once a dipole moment is established, electric field interaction along the axis of a particle pair forces the 3rd particle on line [
12] rather than in a close-packed position, which would be expected [
42] in a general 2D colloidal particle assembly.
As the attachment events in this work happen alongside overall corrosive dissolution [
21], we remind of earlier examinations, including dry irradiation [
43] finding a “robust” (irradiation resistant and mechanically rigid) cerium oxide fluorite structure, which would exclude the formation of necks/bridges using diffusion and surface flow. Equivalently, radiation-induced fluidity or “quasi-melting”, as examined in glasses and ceramics, is un-realistic for liquid cell TEM, as most beam energy is absorbed by the water and the Si
3N
4 membranes.
The question when or whether particles during dissolution adopt roundening to a sphere (as found in Asghar et al. [
21]), elongation from isotropic to oval form (as found in Asghar et al. [
22]), or ultimately conversion to needle shape, as reported here, must be due to timing, as there is no deliberate change in sample geometry, liquid, or particle loading. Different amounts of pre-exposure to electrons will alter local pH and water dissociation products, and formation of surface reaction layers on raw particles. The option that any needle shapes are already in existence on the raw nanoparticles can be safely excluded due to our extensive studies of the dry particles [
24,
25,
44].