Che R. Seabourne

Probing the bonding and electronic structure of single atom dopants in graphene with electron energy loss spectroscopy

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations reveal striking electronic structure differences between two distinct single substitutional Si defect geometries in graphene. Optimised acquisition conditions allow for exceptional signal-to-noise levels in the spectroscopic data. The near-edge fine structure can be compared with great accuracy to simulations and reveal either an sp(3)-like configuration for a trivalent Si or a more complicated hybridized structure for a tetravalent Si impurity.

Electronic Structure Modification of Ion Implanted Graphene: The Spectroscopic Signatures of p- and n-Type Doping.

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations is used to describe the electronic structure modifications incurred by free-standing graphene through two types of single-atom doping. The N K and C K electron energy loss transitions show the presence of π* bonding states, which are highly localized around the N dopant. In contrast, the B K transition of a single B dopant atom shows an unusual broad asymmetric peak which is the result of delocalized π* states away from the B dopant. The asymmetry of the B K toward higher energies is attributed to highly localized σ* antibonding states. These experimental observations are then interpreted as direct fingerprints of the expected p- and n-type behavior of graphene doped in this fashion, through careful comparison with density functional theory calculations.

A systematic approach to choosing parameters for modelling fine structure in electron energy-loss spectroscopy.

A potential methodology is presented for the systematic prediction of EELS edges using DFT, suitable for codes that calculate ELNES for a specific atom in a unit cell. The method begins with the selection of a unit cell, chosen as the smallest cell that still provides a physically valid representation of the bulk material. Within this small cell, a single electron core-hole is included in the atom for which the EELS ionisation edge is to be calculated. The basis-set size and k-point mesh of the DFT calculation are converged specifically against the predicted EELS result. Subsequently, the cell size is increased until the theoretical core-holes no longer interfere. At this point one can then modify the exact core-hole approximation. This methodology was applied to the new EELS module of the CASTEP pseudopotential DFT code, as well as the all-electron code Wien2k. Aluminium K edges were investigated for various aluminium metal systems. It was observed that as the cell size was increased the predicted EELS result became less sensitive to the exact core-hole approximation used. It was noted however that due to high screening in metals a ground state single cell calculation is often acceptable. The semiconductor aluminium nitride (wurtzite form) was also investigated. It was observed that for both Wien2k and CASTEP, with careful convergence of the key DFT code parameters, single cell ground state calculations gave a reasonable agreement with experiment, contrary to what might be expected for a semiconductor with a large band gap. This was particularly true of the Wien2k result. Given the greater computational effort required for supercell calculations, these results are likely to form the beginnings of a detailed investigation into accepted methods of ELNES predictions.

Incisive probing of intermolecular interactions in molecular crystals: core level spectroscopy combined with density functional theory.

The α-form of crystalline para-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding.

Atomic-Scale Surface Roughness of Rutile and Implications for Organic Molecule Adsorption

Crystal surfaces provide physical interfaces between the geosphere and biosphere. It follows that the arrangement of atoms at the surfaces of crystals profoundly influences biological components at many levels, from cells through biopolymers to single organic molecules. Many studies have focused on the crystal-molecule interface in water using large, flat single crystals. However, little is known about atomic-scale surface structures of the nanometer- to micrometer-sized crystals of simple metal oxides typically used in batch adsorption experiments under conditions relevant to biogeochemistry and the origins of life. Here, we present atomic-resolution microscopy data with unprecedented detail of the circumferences of nanosized rutile (α-TiO2) crystals previously used in studies of the adsorption of protons, cations, and amino acids. The data suggest that one-third of the {110} faces, the largest faces on individual crystals, consist of steps at the atomic scale. The steps have the orientation to provide undercoordinated Ti atoms of the type and abundance for adsorption of amino acids as inferred from previous surface complexation modeling of batch adsorption data. A remarkably uniform pattern of step proportions emerges: the step proportions are independent of surface roughness and reflect their relative surface energies. Consequently, the external morphology of rutile nanometer- to micrometer-sized crystals imaged at the coarse scale of scanning electron microscope images is not an accurate indicator of the atomic smoothness or of the proportions of the steps present. Overall, our data strongly suggest that amino acids attach at these steps on the {110} surfaces of rutile.

Ab-initio modelling, polarity and energetics of clean rutile surfaces in vacuum and comparison with water environment

All terminations of the (1x1) rutile (110), (101), (001), (100) and (111) surfaces are classified according to their electrostatic polarity. Six are found to be non-polar. The plane-wave density functional theory code CASTEP is used with a GGA-PBE exchange-correlation functional and a vacuum/material slab supercell method to calculate the surface energy density of symmetric thin rutile films with the six non-polar terminations in vacuum. The ratio of the surface energy densities of a rutile crystal with {111} and {110} facets in water is deduced using Lagrange multipliers and found to be consistent with the DFT vacuum results.

EELS modelling using a pseudopotential DFT code

EELS, when performed in the (S)TEM, enables chemical state and coordination information to be obtained with a potential spatial resolution on the atomic scale [1].

Local Plasmon Engineering in Doped Graphene

Single-atom B or N substitutional doping in single-layer suspended graphene, realized by low-energy ion implantation, is shown to induce a dampening or enhancement of the characteristic interband π plasmon of graphene through a high-resolution electron energy loss spectroscopy study using scanning transmission electron microscopy. A relative 16% decrease or 20% increase in the π plasmon quality factor is attributed to the presence of a single substitutional B or N atom dopant, respectively. This modification is in both cases shown to be relatively localized, with data suggesting the plasmonic response tailoring can no longer be detected within experimental uncertainties beyond a distance of approximately 1 nm from the dopant. Ab initio calculations confirm the trends observed experimentally. Our results directly confirm the possibility of tailoring the plasmonic properties of graphene in the ultraviolet waveband at the atomic scale, a crucial step in the quest for utilizing graphenes properties toward the development of plasmonic and optoelectronic devices operating at ultraviolet frequencies.

Robust theoretical modelling of core ionisation edges for quantitative electron energy loss spectroscopy of B- and N-doped graphene

Electron energy loss spectroscopy (EELS) is a powerful tool for understanding the chemical structure of materials down to the atomic level, but challenges remain in accurately and quantitatively modelling the response. We compare comprehensive theoretical density functional theory (DFT) calculations of 1s core-level EEL K-edge spectra of pure, B-doped and N-doped graphene with and without a core-hole to previously published atomic-resolution experimental electron microscopy data. The ground state approximation is found in this specific system to perform consistently better than the frozen core-hole approximation. The impact of including or excluding a core-hole on the resultant theoretical band structures, densities of states, electron densities and EEL spectra were all thoroughly examined and compared. It is concluded that the frozen core-hole approximation exaggerates the effects of the core-hole in graphene and should be discarded in favour of the ground state approximation. These results are interpreted as an indicator of the overriding need for theorists to embrace many-body effects in the pursuit of accuracy in theoretical spectroscopy instead of a system-tailored approach whose approximations are selected empirically.

VEELS Study of Boron and Nitrogen Doped Single Layer Graphene

1. SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury, WA4 4AD, U.K. 2. Institute for Materials Research, SCAPE, University of Leeds, Leeds, LS2 9JT, UK 3. Department of Physics, Faculty of Arts and Sciences, Niğde University, Niğde 51000, Turkey 4. II Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany 5. Department of Physics and Energy, University of Limerick, Limerick, Ireland