Vincent Mauchamp

Site-projected electronic structure of two-dimensional Ti3C2 MXene: the role of the surface functionalization groups

The role of the surface groups T (T = OH, O or F) in the chemical bonding in two-dimensional Ti3C2Tx MXene is directly evidenced combining electron energy-loss spectroscopy in a transmission electron microscope and simulations based on density functional theory. By focusing on the 1s core electrons excitations of the C and (F, O) atoms, the site projected electronic structure is resolved. The Electron Energy-Loss Near Edge Structures (ELNES) at the C-K edge are shown to be sensitive to the chemical nature and the location of the T-groups on the MXenes surface and thereby allow for the characterization of the MXenes functionalization on the nanometre scale. In addition, the ELNES at the C and F-K edges are shown to be determined by the hybridizations of these atoms with the Ti d bands: these edges are thus relevant probes of the Ti d density of states close to the Fermi level which is of particular interest since it drives most of the Ti3C2Tx electronic properties. Finally, the crucial role in the MXenes functionalization of the etchant used for its synthesis is evidenced by locally determining the [O]/[F] concentration ratio using the corresponding K edges. This ratio is shown to be drastically increased from 1.4 to 3.5 when using HF or LiF/HCl respectively.

Anisotropic effects in ELNES of the O-K edge in rutile: a case of trichroism

When probing atomic species with non-isotropic atomic environments in low-symmetry materials by electron energy loss spectroscopy (EELS), simulation of ionization edges, as measured in a Transmission Electron Microscope (TEM), requires to consider experimental parameters that can affect the near-edge structures (ELNES): the energy, orientation and convergence of the electron beam, and the collecting angle aperture. The well-known case of dichroism (a situation where a threefold, fourfold or sixfold main rotation axis exists through the probed atom) has been extensively studied and elucidated (see [1–2] and references within). Within the dipole approximation, the current of inelastic electrons emitted from a particular atomic core level can be expressed as a linear combination of two particular electron inelastic scattering cross sections (the intrinsic components), respectively calculated for orientations of the transferred wave vector parallel and perpendicular to the main rotation axis c. In the situation of lower symmetries around the probed atom, different cases of trichroism [3] have to be considered. It has recently been shown that the collected current can then be expressed as a linear combination of three, four or six intrinsic components with adequate weights [4]. This analytical theory provides the mean of calculating the collected current for any beam orientation and any collector aperture from these intrinsic components.