A testament to the extreme versatility of aberration-corrected analytical electron microscopy, the research projects tackled by SuperSTEM staff and users cover a very wide range of areas. From traditional microscopy subjects such as metals and oxides, to biological materials and electron irradiation-sensitive catalysts, you will find below a selection of some typical projects carried out at SuperSTEM.
2 Dimensional Materials
Revealing the structure and chemistry of novel 2-dimensional materials *
Since the first isolation of single layer graphene, a large body of research has been devoted to exploring the ever expanding family of two dimensional materials, studying their remarkable structural and electronic properties. SuperSTEM is one of only a handful of research groups able to apply so-called ‘gentle-STEM’, whereby the acceleration voltage is lowered to <60kV to prevent knock on damage and samples are placed in ultra-high vacuum to prevent contamination (or chemical etching). These unique observation conditions, coupled with the ultra-high environmental stability of the SuperSTEM site have enabled studies of novel 2-dimensional materials as varied as: functionalised graphene, hexagonal boron nitride, transition metal di-chalcogenides (MoS2, WS2, WSe2, TiS2…), GaS, black phosphorous, Cu3P, siloxene…
Publications on: (hBN)/(MoS2)/(GaS)/(BP)
* In collaboration with Prof. Valeria Nicolosi (Trinity College Dublin, Ireland).
Single atom spectroscopy: engineering the electronic structure of graphene
The introduction of substitutional dopant species into graphene is considered one of the most promising means of tailoring the ‘wonder’ material’s properties: Si dopants can alter the plasmonic response of graphene and act as nano-antennae, while B and N substitutional defects can shift the Fermi level in graphene and engineer p- or n-type behaviour, opening promising avenues for applications in nano-electronics. STEM-EELS is arguably the only technique capable of probing at the single atom level the electronic structure modifications induced by such dopants, providing a direct visulalisation of the atomic structure and a chemical fingerprint . Thanks to low-kV operation to prevent beam damage and utra-high vacuum conditions in the microscope’s sample chamber, SuperSTEM have been pioneering experimental single atom physical chemistry.
Interaction of metals with graphene
The fabrication of any graphene-based device involves the incorporation of metal contacts, to exploit its thermal or electrical conductivity for instance. Depending on the type of metal used to fabricate these contacts, the performance of the resulting devices can be dramatically altered, and it follows that the choice of metal is therefore key to a successful design. A comprehensive study of the graphene-metal interactions at SuperSTEM using STEM-EELS has provided some key insights into the phenomena at play. The high mobility of single metal atoms on ‘clean’ graphene promotes clustering at step edges or on contamination patches; these clusters can in turn catalyse a drilling process, creating perforations in the graphene sheet which can in some cases self-heal.
Oxides and Ceramics
Re-discovering thermoelectrics: complex oxides *
Thermoelectric devices convert heat into electricity and represent an important route for green technologies, from waste heat recovery to self-powered, portable devices. Complex oxides, such as perovskites, have emerged as new candidate materials for the old problem of heat recovery as they possess several advantages over traditional thermoelectrics including low price, non-toxicity, and chemical and thermal stability at high temperatures. The project lead by SuperSTEM’s long-standing collaborators Prof. R. Freer and Dr. F. Azough, from the University of Manchester, aims to understand the atomic scale effects, such as such as the local chemistry and structure that govern the transport properties of thermoelectric oxides. Thanks to the extreme stability and advanced analytical capabilities of the SuperSTEM instruments, the local distribution of cations as well as small distortions of the cation and oxygen sublattices can be directly visualised and correlated, while local bonding changes are observed in EELS core loss variations. These observations are indispensable input to ab initio DFT calculations of the electronic structure and transport property calculations of thermoelectric materials lead by Prof. S. Parker University of Bath and Dr. M. Molinari, University of Huddersfield.
* In collaboration with Prof. R. Freer and Dr F. Azough (University of the Manchester), Prof. Steve Parker (University of Bath) and Dr Marco Molinari (University of Huddersfied).
Oxide heterostructures & superlattices *
Complex oxide heterostructures can exhibit wide range of novel structural, electronic, and magnetic properties compared to their bulk counterpart materials. By careful materials selection and precise synthesis of the heterointerface chemistry, it is possible to introduce local structural distortions and strain as well as and charge transfer due to band alignment. These effects can be used to create and control local electric and magnetic fields with applications in photochemistry, solar cells and spintronics. Using high precision STEM-HAADF acquisition techniques and advanced image analysis it is possible to directly visualize and measure local the ionic displacements that correspond to local strain and ferroelectric polarization of the heterostructures. Large scale atomically resolved STEM EELS mappings and careful analysis of EELS ionization near edge fine structure are used to probe local chemistry and bonding of the heterostructures at the atomic scale.
* In collaboration with Dr. Scott Chambers, Dr. Ryan Comes and Dr. Steven Spurgeon (Pacific Northwest National Laboratory, USA) and Prof. Mitra Taheri (Drexel University, Philadelphia, USA).
Growth mechanisms of eutectic Si in an Al-Si alloy *
The addition of small amounts of dopants such as Na or Eu to Al-Si alloys transforms the morphology of eutectic Si from flake-like to fibrous – tremendously improving the castabiity and performance of the alloy. While this process was patented in 1921, and has been widely used by the foundry industry since, the exact mechanisms resulting in the desired modification are still debated. This project, led by SuperSTEM collaborators Dr. Jeihua Li and Prof. Peter Schumacher (Montanuniversität Leoben, Leoben, Austria) aims at providing a detailed, atomic -scale understanding of the roles of these dopants in the eutectic Si. High resolution STEM-EELS core loss maps and HAADF-STEM images acquired on the SuperSTEM2 microscope were key in demonstrating that solute entrainment of Eu occurs in eutectic Si during solidification, and, that the so-called “impurity induced twining” and “poisoning of the twin plane re-entrant edge growth” mechanisms are both active.
* In collaboration with Dr Jiehua Li and Prof. Peter Schumacher, Montanuniversität, Leoben, Austria.
Structure and chemistry of hardening precipitates in an Al-Mg-Si-Cu-Ag alloy *
Upon heat treatment, the growth of hardening precipitates significantly increases the strength of Al-Mg-Si alloys by preventing the propagation of cracks. Adding small amounts of Cu and Ag is known to increase the mechanical strength of the Al-Mg-Si alloy further by supressing the formation of Al-Mg-Si precipitates in favour of Cu and Ag containing ones. The structure and chemistry of these precipitates is hugely complex however, and a combination of high resolution STEM-EELS and HAADF-STEM imaging is required to provide a detailed atomic scale investigation to inform a further optimisation of the hardening treatments. Dr. Sigurd Wenner and Prof. Randi Holmestad (NTNU, Trondheim, Norway) used SuperSTEM2 to show that in Cu-rich precipitate regions, Cu atoms participate in so-called “C” and “Q’” phases. Ag rich clusters were found to be localised in Cu-free regions but did not exhibit a well-defined structural order.
* In collaboration with Dr Sigurd Wenner and Prof. Randi Holmestad, NTNU, Trondheim, Norway.
Nano-structure and chemistry of bone *
Brittle bone disease is a genetic disorder that results in changes in the structure and quantity of collagen and that affects not only bones but also tendons and joints. In order to design effective therapeutics, the effect of molecular alterations on the structure, chemistry and organisation of pathologic bone must be understood. Although much work has been done at the whole bone and tissue levels, little research has been conducted on pathologic bone and tendons at the fibril level. High resolution HAADF imaging in combination with monochromated EELS are being used to map the chemistry of the mineralised collagen fibrils in healthy and diseased bone, to probe variations between these tissues. Differences in fine structure at the carbon K and nitrogen K edges serve to elucidate the functional chemistry of proteins and to contribute to our poor understanding of how molecular defects affect the quality of the bone tissue and biomineralisation events.
* In collaboration with Prof. Alexandra Porter, Drs. Angela Goode, Catriona McGilvery and Michal Klosowski, Imperial College London.
Nano-scale structure and chemical pathways in polyamide reverse osmosis membranes *
As freshwater sources become increasingly scarce and global demands grow, it is necessary to develop alternative water supplies. Polyamide (PA) reverse osmosis (RO) membranes are cost-effective and commonly used for water desalination. Given the membrane’s complex hierarchical structure, which spans across different length scales, the controllability of ion selectivity remains unclear. Designing new RO membranes with enhanced performance requires knowledge of the membrane structures and of the chemical pathways controlling the salt selectivity and permeance at the molecular - nanoscale. We are characterising commercially available, as well as new RO membranes designed at Imperial which are additionally able to remove small molecules from organic liquids. High resolution STEM imaging and monochromated EELS are applied to the map the nanostructure and functional chemistry across PA nano-films to elucidate the presence and role of < 1nm pore like structures and the local functional chemistry on transport processes.
* In collaboration with Prof. Alexandra Porter, Drs. Catriona McGilvery and Michal Klosowski, Imperial College London.
Analytical electron microscopy of metal-organic frameworks
Metal-organic frameworks (MOFs) are highly porous, crystalline materials used for gas storage, separation and as heterogeneous catalysts. Formed by organic and inorganic components, MOFs can be functionalized using pre- and post-synthetic processing methods. As a result, their native composition is modified to expand the scope and functionality of the material. Correctly assigning the composition of these materials at the (sub)nanometer level is key for understanding the relation between pre/post-synthetic strategies and properties. EELS is being used to investigate the local chemistry of MOFs produced using different processing conditions to provide insights into metal incorporation processes.
Structure and edge chemistry of industrial-style MoS2 nano-catalysts *
Gasoline, diesel and other fossil fuels contain small amounts of sulphur and nitrogen which are emitted into the atmosphere during fuel combustion. These elements are harmful to human health and the environment, and are a major source of acid rain. Catalysts are required in oil refineries to reduce these harmful emissions. 2D nanoplatelets of single layer molybdenum disulphide (MoS2), is the active ingredient found in catalysts used by oil refineries across the world. As the world’s supply of crude oil is stretched, and low-sulphur crude oils become less available, demand for MoS2-based catalysts is increasing. But more efficient catalysts are urgently needed to keep up with this demand due to increasing global oil consumption and dirtier oil wells. A team of researchers, led by Dr Stig Helveg from Haldor Topsøe A/S in collaboration with SuperSTEM was able to image atom-by-atom the edge structure of these key industrial nanocatalysts and reveal the exact location of single cobalt promoter atoms, added to the material to boost its catalytic activity.
* In collaboration with Dr Stig Helveg (Haldor Topsøe A/S, Kgs. Lyngby, Denmark)
Nano-designed vanadium-oxide-based catalysts for clear-air technology *
Surface redox processes involving oxygen atom exchange are fundamental in catalytic reactions mediated by metal oxides. The VOx/TiO2 system is of particular interest as it can catalyse a variety of chemical reactions, including the selective reduction of NOx for emission abatement from power plants and diesel engines. This project is aimed at shedding light on the dynamics at the atomic scale of the redox reactions, using a combination of ex-situ STEM-EELS at high energy and spatial resolution, and in-situ imaging and spectroscopy in alternating reducing and oxidising atmosphere. In particular, observations at SuperSTEM and Haldor Topsøe A/S revealed a reversible transformation of the vanadium oxide surface between an ordered and disordered state. Crucially, these structural transformation are shown to be sensitive to the surface termination (and therefore the crystallography) of the support TiO2 particles.
* In collaboration with Drs. Martin Ek and Stig Helveg (Haldor Topsøe A/S, Kgs. Lyngby, Denmark).
Single-atom catalysts *
Single-atom catalysts (SACs) offer a means to achieve efficient and environmentally less harmful chemical conversions in processes that are used throughout modern society, such as industrial hydrogenations. The ‘holy-grail’ is to achieve stable single-atom dispersions where the active sites are precision-tuned for a target reaction. This requires the identification of atomic-scale structures that are capable of strongly anchoring catalytic metal atoms, and that provide structural and chemical environments that can choreograph a catalytic process. This project in collaboration with Dr Rowan Leary work has already shown that entrapping catalytic metal atoms in the cavities of a nano-porous polymeric graphitic carbon nitride (GCN) (that is cheap and readily synthesized) is a promising approach to stable SAC anchoring. Further work will take advantage of the capabilities offered by new ultra-high resolution monochromated instruments enabling vibrational spectroscopy at the nanoscale to probe single atom and molecular functionalisations of catalytic sites through vibrational spectroscopy.
* In collaboration with Dr Rowan Leary (University of Cambridge).
Microscopy Techniques and Instrumentation
Focused Ion Beam sample preparation for low-kV atomic resolution microscopy
While FIB sample preparation has become a standard technique for many TEM applications at 200 kV or higher, many such samples lack the properties needed for quantitative atomic resolution HAADF imaging and EELS analysis at low voltages, either because they are too thick ( >60 nm) or because a too large portion of the material has been seriously damaged during sample preparation.
However, by simulating the ion beam damage and carefully adjusting milling parameters and procedure, FIB in-situ lift-out sample preparation can be optimized to produce highly suitable samples without the need of additional preparation or cleaning steps. Samples as thin as 10 nm over several µm have already been prepared and atomic resolution HAADF images show no obvious artefacts due to sample damage. The technique is highly reproducible for different materials at a specimen thickness between 20 and 30 nm. The imminent purchase of an advanced Focused Ion Beam instrument and the significant expansion of SuperSTEM's sample preparation facilities in the next 12 months will provide renewed impetus to this project. Watch this space!
Discrete electron tomography of nanoparticles
Catalyst nanoparticles are of great promise in many fields of industries and science. As the particle size is reduced in an effort to reduce the catalyst usage without compromise in the activity, an understanding of the 3D structure of the particles is crucial. A novel discrete electron tomographic algorithm: multiplicative backprojection method, using five HAADF images from SuperSTEM2 is developed. In contrast to the other electron tomographic methods, which requires over 100 images, our algorithm greatly reduced the number of projections for reconstruction and hence data acquisition time and electron dose to the specimen. Using 2D atomic resolution images holds the possibility of improving 3D volume resolution towards atomic resolution. The algorithm demonstrated successful and accurate reconstructions of catalyst nanoparticles with a resolution of 4x10-3nm-3 voxels (0.16nm/pixel) and show potential applications to beam-sensitive specimens and particles whose properties are dominated by surface structure.
Sparse scanning imaging and spectroscopy for dose-controlled acquisition
Many nanoscale materials and processes investigated in the STEM are sensitive to the total electron dose and would benefit from new acquisition procedures that can provide the necessary information whilst drastically reducing the electron dose required for image and spectra formation. Inpainting techniques have been successfully applied in a number of areas outside the electron microscopy comunity, e.g. astronomy, MRI scanning, and allow for recovery of the essential image information from randomly undersampled datasets. In SuperSTEM, we have developed a scheme able to produce images and spectra from which quantifiable information from atomically resolved images of beam-sensitive materials and time-changing processes and spectroscopic datasets can be recovered. The implementation relies on custom-built hardware and software which controls an electrostatic beam shutter to blank the electron beam during all but a few randomly chosen pixels through a regular image (or indeed spectrum image) acquisition. Datasets acquired in this fashion down to a few % of the total incoming dose can be reconstructed to yield a truthful atomic resolution representation of the sample, using a variety of reconstruction algorithms.
Electrothermal cartridge adapter development
The Nion UltraSTEM 100M at SuperSTEM is equipped with an electrical cartridge which has provision for in-situ electrical and thermal analysis of samples. Although manufacturer specific solutions exit (a Protochips Fusion system is available at SuperSTEM), this project is aimed at adapting in-house the cartridge design to allow for a wider a range of e-chip configurations to be used, thereby providing greater flexibility for users whose own systems may not be compatible with the configuration available at SuperSTEM. Environmental chips employ micro-electro-mechanical-systems (MEMS) technology and are designed to replace conventional TEM grid supports for in-situ experiments. Adaptation of the existing sample support pcb is currently in progress, with prototype trials scheduled for early 2018. In terms of experimental capability, the adapted electrical cartridge will allow a range of dynamic measurements to be performed and analyses to be made; for example, the thermal treatment of nano-particles at elevated temperatures, annealing studies of alloys and metallic thin films, phase transformations occurring in catalysts while in service, the influence of electric fields on ferro-electric materials at elevated temperature (phase change memory materials) and the electrical properties of nano-electric structures such as nano-wires and solar cells.
Space and Momentum-resolved low loss spectroscopy
Localised surface plasmons in metallic nanostructures
The resonant interaction between electromagnetic radiation and localized surface plasmon (LSP) eigenmodes in metallic nanostructures is a phenomenon of ever-growing interest. LSPs have found applications in fields as diverse as ultrasensitive chemical detection and nanoscale chemical imaging, targeted drug delivery and therapeutics, photovoltaics, as well as photo-catalysis. STEM-EELS are capable of probing both bright and dark modes and of relating the spatial distribution of these modes with the atomic scale structure of the nanostructure or nanoplasmonic device. The fundamentals involved in plasmonic coupling between metallic nanostructures as well as the effect of local environment on the plasmonic response are being investigated by EELS. This will aid providing useful insights to be applied on the design of devices with specific optical functionalities (e.g. for efficient solar energy conversion).
Valence loss spectroscopy of carbon nano materials in real and momentum spaces
Resolving valence EEL spectra in momentum space can provide information about a material system often obscured in spectra acquired in conventional geometry (at high spatial resolution). This technique development project explores the intrinsic trade-off between the simultaneously achievable momentum resolution and electron probe size, and demonstrates that even with a modest momentum resolution a wealth of information about the vibrational response of a material can be extracted on the nanometre scale. Combining high resolution imaging with EELS resolved in both real and momentum space allowed for attributing confinement of collective valence electron excitations (plasmons) to the presence and local concentration of non-hexagonal carbon rings incorporated in walls of single wall carbon nanotubes and at apices of multilayer graphene nanocones.
Vibrational loss spectroscopy of low dimensional materials *
Phonons are collective vibrational modes in materials, affecting physical properties such as the conduction of sound and heat. Thanks to the high energy resolution of the SuperSTEM3 instrument, the momentum-resolved STEM-EELS approach developed at SuperSTEM can be applied in the phonon regime. Momentum resolved vibrational spectra in combination with ab initio modelling allows for unambiguous identification and analysis of acoustic and optical phonon modes across the entire first Brillouin zone of individual cubic and hexagonal boron nitride particles, using a ~1nm electron probe.
* In collaboration with R.J. Nicholls, J.R. Yates (University of Oxford, Oxford, UK), K. Refson (STFC Rutherford Appleton Lab., Harwell Science and Innovation Campus, Didcot, UK, and Royal Holloway, University of London, Egham, UK) T.C. Lovejoy, N. Dellby and O.L. Krivanek (Nion Company, Kirkland, WA, USA).
Distribution of Au atoms in Si nanowires *
Si nanowires are produced using the vapour-liquid-solid (VLS) method and Au-Si eutectic. The presence of metals such as Au is deleterious to the electrical properties of Si; hence it is important to characterise the distribution of any Au atoms on the surface or within the bulk of Si nanowires produced by the VLS technique. The unique optical sectioning properties of aberration corrected STEM was used to acquire HAADF images at various depths through a Si nanowire so that the 3D distribution of Au atoms could be determined for the first time.
* In collaboration with Prof. Lincoln Lauhon, Northwestern University.
Composition measurement of buried quantum dots
Quantum dots, such as In(Ga)As in a GaAs matrix, have many potential applications in optoelctronic devices and hence there is a need to characterise their size, shape and composition. A major obstacle is the fact that the quantum dot lies buried in a matrix containing identical chemical elements (e.g. Ga and As for the In(Ga)As/ GaAs system). The HAADF image was therefore first used to estimate the size and shape of the quantum dot from which the number of In atom sites can be estimated at any given position. From EELS measurements of the In areal density the In occupancy is easily determined and hence a 2D projection of the composition constructed for the entire quantum dot.