Understanding Nanocrystals' Dopant Edge X-ray Absorption Fine Structure (XAFS) Spectra: An Iterative Fitting Model

X-ray absorption fine structure spectroscopy is sparsely employed in characterizing nanomaterials as opposed to bulk materials, primarily due to the dominating surface atoms and the lack of information on their bonding environment. Furthermore, dopant atoms add a layer of complexity in modeling due to the limited long-range crystal structure. In this work, we developed a two-step method that can be used to fit the dopant edge spectra that substitute host atoms in their lattice positions based on a modified lattice substitution model that requires only an educated guess of the guest bond length as a prerequisite. This model is tested for five different transition metal dopants (Mn, Fe, Co, Ni, Cu) in a CdS host nanomaterial and is expected to be successful for other substitutional dopants irrespective of the host. This model suggests that the dopants generally bind to their first nearest neighbors with bond lengths that are comparable to the long-range crystals of the dopant-anion lattices. However, the strain arising from the mismatch of bond lengths is generally absorbed largely by the second nearest neighbor and the third nearest neighbor. It also demonstrates the need for physically intuitive modeling to study the properties of doped nanomaterials.