Rebecca J. Nicholls

Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials

Layered transition metal dichalcogenides, such as tungsten disulfide, are exfoliated into atomically thin flakes. If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.

Achieving sub-nanometre particle mapping with energy-filtered TEM

A combination of state-of-the-art instrumentation and optimized data processing has enabled for the first time the chemical mapping of sub-nanometre particles using energy-filtered transmission electron microscopy (EFTEM). Multivariate statistical analysis (MSA) generated reconstructed datasets where the signal from particles smaller than 1 nm in diameter was successfully isolated from the original noisy background. The technique has been applied to the characterization of oxide dispersion strengthened (ODS) reduced activation FeCr alloys, due to their relevance as structural materials for future fusion reactors. Results revealed that most nanometer-sized particles had a core-shell structure, with an Yttrium-Chromium-Oxygen-rich core and a nano-scaled Chromium-Oxygen-rich shell. This segregation to the nanoparticles caused a decrease of the Chromium dissolved in the matrix, compromising the corrosion resistance of the alloy.

Probing the bonding in nitrogen-doped graphene using electron energy loss spectroscopy.

Precise control of graphene properties is an essential step toward the realization of future graphene devices. Defects, such as individual nitrogen atoms, can strongly influence the electronic structure of graphene. Therefore, state-of-the-art characterization techniques, in conjunction with modern modeling tools, are necessary to identify these defects and fully understand the synthesized material. We have directly visualized individual substitutional nitrogen dopant atoms in graphene using scanning transmission electron microscopy and conducted complementary electron energy loss spectroscopy experiments and modeling which demonstrates the influence of the nitrogen atom on the carbon K-edge.

Identifying suboxide grains at the metal-oxide interface of a corroded Zr-1.0%Nb alloy using (S)TEM, transmission-EBSD and EELS.

Here we report a methodology combining TEM, STEM, Transmission-EBSD and EELS to analyse the structural and chemical properties of the metal-oxide interface of corroded Zr alloys in unprecedented detail. TEM, STEM and diffraction results revealed the complexity of the distribution of suboxide grains at the metal-oxide interface. EELS provided accurate quantitative analysis of the oxygen concentration across the interface, identifying the existence of local regions of stoichiometric ZrO and Zr3O2 with varying thickness. Transmission-EBSD confirmed that the suboxide grains can be indexed with the hexagonal ZrO structure predicted with ab initio by Nicholls et al. (2014). The t-EBSD analysis has also allowed for the mapping of a relatively large region of the metal-oxide interface, revealing the location and size distribution of the suboxide grains.

Crystal structure of the zro phase at zirconium/zirconium oxide interfaces

Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and structural components. Under this harsh environment, the main factor limiting the service life of zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process has recently been linked to the presence of a sub-oxide phase with well-defined composition but unknown structure at the metal–oxide interface. In this paper, the combination of first-principles materials modeling and high-resolution electron microscopy is used to identify the structure of this sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys.

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.

The near edge structure of cubic boron nitride.

We compare the near edge structure (NES) of cubic boron nitride (cBN) measured using both electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) with that calculated using three commonly used theoretical approaches. The boron and nitrogen K-edges collected using EELS and XAS from cBN powder were found to be nearly identical. These experimental edges were compared to calculations obtained using an all-electron density functional theory code (WIEN2k), a pseudopotential density functional theory code (CASTEP) and a multiple scattering code (FEFF). All three codes were found to reproduce the major features in the NES for both ionisation edges when a core-hole was included in the calculations. A partial core hole (1/2 of a 1s electron) was found to be essential for correctly reproducing features near the edge threshold in the nitrogen K-edge and to correctly obtain the positions of all main peaks. CASTEP and WIEN2k were found to give almost identical results. These codes were also found to produce NES which most closely matched experiment based on χ² calculations used to qualitatively compare theory and experiment. This work demonstrated that a combined experimental and theoretical approach to the study of NES is a powerful way of investigating bonding and electronic structure in boron nitride and related materials.

OptaDOS - a new tool for EELS calculations

Many modern (Scanning) Transmission Electron Microscopes ((S)TEMs) are equipped with an energy loss spectrometer. The Electron Energy Loss (EEL) spectra collected provide an experimental method of probing the bonding within a material. With the extra addition of monochromators, the energy resolution obtainable means that even more information is revealed within the fine structure of the spectra. Interpreting the fine structure can often be aided by simulation. Density-functional theory (DFT) is one method of simulating EEL spectra. DFT allows us to simulate DOS and EEL spectra from different structures. This comparison between simulation and experiment enables us to explore how changes in the spectrum are related to changes within the sample. CASTEP is a pseudopotential DFT code which can simulate both low-loss and core-loss EEL spectra. Recent developments have resulted in a separate analysis tool, OptaDOS. This package computes various spectral properties including DOS, projected DOS, joint DOS, core-loss and low-loss EEL spectra and optical spectra. One of the important aspects of the code is the way in which the DOS is calculated. This is done via linear extrapolation or adaptive smearing, methods which are not currently available within CASTEP (or indeed any other code) and which allow detailed analysis of spectral properties. This paper summarises these developments and what they mean for the interpretation of EEL spectra.

Effects of temperature and ammonia flow rate on the chemical vapour deposition growth of nitrogen-doped graphene

We doped graphene in situ during synthesis from methane and ammonia on copper in a low-pressure chemical vapour deposition system, and investigated the effect of the synthesis temperature and ammonia concentration on the growth. Raman and X-ray photoelectron spectroscopy was used to investigate the quality and nitrogen content of the graphene and demonstrated that decreasing the synthesis temperature and increasing the ammonia flow rate results in an increase in the concentration of nitrogen dopants up to ca. 2.1% overall. However, concurrent scanning electron microscopy studies demonstrate that decreasing both the growth temperature from 1000 to 900 °C and increasing the N/C precursor ratio from 1/50 to 1/10 significantly decreased the growth rate by a factor of six overall. Using scanning tunnelling microscopy we show that the nitrogen was incorporated mainly in substitutional configuration, while current imaging tunnelling spectroscopy showed that the effect of the nitrogen on the density of states was visible only over a few atom distances.