Probing the chemical structure in diamond-based materials using combined low-loss and core-loss electron energy-loss spectroscopy.

We report the analysis of the changes in local carbon structure and chemistry caused by the self-implantation of carbon into diamond via electron energy-loss spectroscopy (EELS) plasmon energy shifts and core-edge fine structure fingerprinting. These two very different EELS energy and intensity ranges of the spectrum can be acquired under identical experimental conditions and nearly simultaneously using specially designed deflectors and energy offset devices known as "DualEELS." In this way, it is possible to take full advantage of the unique and complementary information that is present in the low- and core-loss regions of the EELS spectrum. We find that self-implanted carbon under the implantation conditions used for the material investigated in this paper creates an amorphous region with significant sp 2 content that varies across the interface.

Dynamics of single Fe atoms in graphene vacancies.

Focused electron beam irradiation has been used to create mono and divacancies in graphene within a defined area, which then act as trap sites for mobile Fe atoms initially resident on the graphene surface. Aberration-corrected transmission electron microscopy at 80 kV has been used to study the real time dynamics of Fe atoms filling the vacancy sites in graphene with atomic resolution. We find that the incorporation of a dopant atom results in pronounced displacements of the surrounding carbon atoms of up to 0.5 Å, which is in good agreement with density functional theory calculations. Once incorporated into the graphene lattice, Fe atoms can transition to adjacent lattice positions and reversibly switch their bonding between four and three nearest neighbors. The C atoms adjacent to the Fe atoms are found to be more susceptible to Stone-Wales type bond rotations with these bond rotations associated with changes in the dopant bonding configuration. These results demonstrate the use of controlled electron beam irradiation to incorporate dopants into the graphene lattice with nanoscale spatial control.

Characterization of multifunctional ß-NaEuF4/NaGdF4 core-shell nanoparticles with narrow size distribution.

The properties of ß-NaEuF4/NaGdF4 core-shell nanocrystals have been thoroughly investigated. Nanoparticles with narrow size distribution and an overall diameter of ∼22 nm have been produced with either small ß-NaEuF4 cores (∼3 nm diameter) or large ß-NaEuF4 cores (∼18 nm diameter). The structural properties and core-shell formation are investigated by X-ray diffraction, transmission electron microscopy and electron paramagnetic resonance, respectively. Optical luminescence measurements and X-ray photoelectron spectroscopy are employed to gain information about the optical emission bands and valence states of the rare earth constituents. Magnetic characterization is performed by SQUID and X-ray magnetic circular dichroism measurements at the rare earth M(4,5) edges. The characterization of the core-shell nanoparticles by means of these complementary techniques demonstrates that partial intermixing of core and shell materials takes place, and a significant fraction of europium is present in the divalent state which has significant influence on the magnetic properties. Hence, we obtained a combination of red emitting Eu(3+) ions and paramagnetic Gd(3+) ions, which may be highly valuable for potential future applications.

Spatial control of defect creation in graphene at the nanoscale.

Defects in graphene alter its electrical, chemical, magnetic and mechanical properties. The intentional creation of defects in graphene offers a means for engineering its properties. Techniques such as ion irradiation intentionally induce atomic defects in graphene, for example, divacancies, but these defects are randomly scattered over large distances. Control of defect formation with nanoscale precision remains a significant challenge. Here we show control over both the location and average complexity of defect formation in graphene by tailoring its exposure to a focussed electron beam. Divacancies and larger disordered structures are produced within a 10 × 10 nm(2) region of graphene and imaged after creation using an aberration-corrected transmission electron microscope. Some of the created defects were stable, whereas others relaxed to simpler structures through bond rotations and surface adatom incorporation. These results are important for the utilization of atomic defects in graphene-based research.