Trevor P. Hardcastle

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.

Universal synthesis method for mixed phase TiO2(B)/anatase TiO2 thin films on substrates via a modified low pressure chemical vapour deposition (LPCVD) route

A universal method for the synthesis of mixed phase TiO2 bronze (B)/anatase titania thin films by Low Pressure Chemical Vapour Deposition (LPCVD) onto any substrate is presented. General LPCVD conditions were titanium isopropoxide (TTIP) and N2 gas as the precursor and carrier gas respectively, 600 °C nominal reaction temperature, and 15 min reaction time; a range of different substrates were investigated including: a silicon wafer, fused quartz, highly ordered pyrolytic graphite (HOPG) and pressed graphite flake (grafoil). X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy were used to characterise the thin films which exhibited a columnar morphology together with smaller equi-axed particles. Pre-treatment of substrates by spraying with a Na-containing solution was found to encourage the crystallization of TiO2(B) during the LPCVD process. Increasing the concentration of Na in the pre-treatment process resulted in a higher proportion of TiO2(B) in the thin films up to an optimum condition of 0.75% w/v of Na. Na diffusion from the substrate surface into the adjacent TiO2 is the proposed mechanism for promoting TiO2(B) formation as opposed to the anatase phase with Density Functional Theory (DFT) modelling suggesting the presence of Na stabilises the TiO2(B) phase. Dye degradation tests indicate an increased photocatalytic activity for mixed phase anatase/TiO2(B) thin films.

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.

Single-atom spectroscopy of phosphorus dopants implanted into graphene

One of the keys behind the success of modern semiconductor technology has been the ion implantation of silicon, which allows its electronic properties to be tailored. For similar purposes, heteroatoms have been introduced into carbon nanomaterials both during growth and using post-growth methods. However, due to the nature of the samples, it has been challenging to determine whether the heteroatoms have been incorporated into the lattice as intended. Direct observations have so far been limited to N and B dopants, and incidental Si impurities. Furthermore, ion implantation of these materials is challenging due to the requirement of very low ion energies and atomically clean surfaces. Here, we provide the first atomic-resolution imaging and electron energy loss spectroscopy (EELS) evidence of phosphorus atoms in the graphene lattice, implanted by low-energy ion irradiation. The measured P L 2,3-edge shows excellent agreement with an ab initio spectrum simulation, conclusively identifying the P in a buckled substitutional configuration. While advancing the use of EELS for single-atom spectroscopy, our results demonstrate the viability of phosphorus as a lattice dopant in sp 2-bonded carbon structures and provide its unmistakable fingerprint for further studies.

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.

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.

Structure and electronic states of a graphene double vacancy with an embedded Si dopant

Silicon represents a common intrinsic impurity in graphene, bonding to either three or four carbon neighbors, respectively, in a single or double carbon vacancy. We investigate the effect of the latter defect (Si-C4) on the structural and electronic properties of graphene using density functional theory. Calculations based both on molecular models and with periodic boundary conditions have been performed. The two-carbon vacancy was constructed from pyrene (pyrene-2C) which was then expanded to circumpyrene-2C. The structural characterization of these cases revealed that the ground state is slightly non-planar, with the bonding carbons displaced from the plane by up to ±0.2 Å. This non-planar structure was confirmed by embedding the defect into a 10 × 8 supercell of graphene, resulting in 0.22 eV lower energy than the previously considered planar structure. Natural bond orbital analysis showed sp3 hybridization at the silicon atom for the non-planar structure and sp2d hybridization for the planar structure. Atomically resolved electron energy loss spectroscopy and corresponding spectrum simulations provide a mixed picture: a flat structure provides a slightly better overall spectrum match, but a small observed pre-peak is only present in the corrugated simulation. Considering the small energy barrier between the two equivalent corrugated conformations, both structures could plausibly exist as a superposition over the experimental time scale of seconds.

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

Electronic Structure Modification of Boron and Nitrogen Ion-Implanted Graphene Fingerprinted by STEM-EELS

In the past few years, graphene, boron nitride and other 2D materials have stirred a research frenzy in materials science. In addition to their very promising application prospects, the field of 2D materials has also become a remarkably fruitful ‘playground’ for physicists, allowing them to use newly developed capabilities of electron microscopes [1] to study the structure-property relationships of materials literally one atom at a time. From imaging, to single dopant chemical and electronic structure determination, atom-by-atom fingerprinting of 2D materials is now possible thanks in large parts to advances in aberration correction and to ‘gentle’ observation conditions such as low-dose techniques [1-7].