A. Hunt

A Re‐evaluation of How Functional Groups Modify the Electronic Structure of Graphene Oxide

The first 4 eV of the conduction band in graphene oxide is dominated by states from carbon sites that are in close proximity, but not directly bonded, to oxidizing functional groups. The carbon sites that are bonded directly to these groups, such as epoxide and hydroxyl groups, are much higher in energy.

The characterization of Co-nanoparticles supported on graphene

The results of density functional theory (DFT) calculations and measurements of X-ray photoelectron (XPS) and X-ray emission (XES) spectra of Co-nanoparticles dispersed on graphene/Cu composites are presented. It is found that for 0.02nm and 0.06nm Co coverage the Co atoms form islands which are strongly oxidized under exposure at the air. For Co (2nm) coverage the upper Co-layers is oxidized whereas the lower layers contacting with graphene is in metallic state. Therefore Co (2 nm) coverage induces the formation of protective oxide layer providing the ferromagnetic properties of Co nanoparticles which can be used as spin filters in spintronics devices.

The formation of Ti–O tetrahedra and band gap reduction in SiO2 via pulsed ion implantation

Titanium ions are implanted into amorphous SiO2 at two different fluences using pulsed ion implantation, and the resulting samples are annealed. Bulk sensitive soft X-ray absorption spectroscopy of the Ti L2,3 edge reveal strikingly different spectra for the two fluences. Spectral simulations using multiplet crystal field theory show clearly that for low fluence the Ti ions have a local octahedral coordination, while at higher fluence the formation of Ti4+–O tetrahedra dominates. Using O K-edge X-ray absorption and emission, the effect of the Ti states on the valence and conduction bands of the host SiO2 is revealed. With the introduction of Ti tetrahedra, the band gap reduces from about 8 eV to just over 4 eV, due entirely to the Ti 3d conduction band states. These results demonstrate the possibility to obtain Ti–O tetrahedra in silica by Ti ion implantation and a suitable thermal treatment, clarify the mechanism of band gap reduction with Ti doping in SiO2, and demonstrate the sensitivity of L-edge X-ra...

Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation.

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications.

Structural ordering in a silica glass matrix under Mn ion implantation

Mn(+)-implanted, amorphous SiO(2) samples were synthesized using pulsed-ion implantation without thermal annealing. The crystal and electronic structures have been studied using x-ray diffraction and synchrotron-based soft x-ray absorption and emission spectroscopy at the Si and Mn L(2,3) edges. We find a combination of small MnO clusters and Si crystallites at shallow depths while tetrahedral Mn coordination is found deeper in the host target. Through a combination of techniques, we find that the implantation process simultaneously decreases the long-range order in the near-surface region and increases order deeper in the SiO(2) host. Our results suggest Mn substitution into Si sites at deep levels catalyzes the formation of α-quartz, providing insight into the complex interactions that determine the local structure around the impurities as well as the overall changes to the crystallinity of implanted SiO(2).