2D Materials

2D Materials, sometimes referred to as single layer materials, are crystalline materials consisting of a single layer of atoms. Since the isolation of graphene, a single-layer of graphite, in 2004, a large amount of research has been directed at isolating other 2D materials due to their unusual characteristics and for use in applications such as photovoltaics, semiconductors, electrodes, catalysis and water purification.

2D Materials, sometimes referred to as single layer materials, are crystalline materials consisting of a single layer of atoms. Since the isolation of graphene, a single-layer of graphite, in 2004, a large amount of research has been directed at isolating other 2D materials due to their unusual characteristics and for use in applications such as photovoltaics, semiconductors, electrodes, catalysis and water purification.

Carbon Materials

The various carbon allotropes available; diamond, graphite, graphene, nanotube, and buckyball mean this material is one of the most studied material. The electronic properties of graphene, the most well-known two-dimensional material, were the basis of the 2010 Nobel Prize in Physics.

People working in Carbon Materials

  • Prof Ursel Bangert
  • Dr Andy Stewart
  • Dr Jennifer Cookman
  • Mr Gearóid Mangan
  • Mr Eoghan O' Connell
  • Mr Kalani Moore

Carbon exists is many forms and is a constituent of the majority of materials. Carbon materials have risen specific interest in the nano-materials sector, following the discovery of carbon nanotubes (Iijima) and the demonstration of graphene (Geim and Novoselov), with promising applications in materials strengthening and conductivity enhancement, as components in batteries, as antennae and filters, to name a few. Electron microscopy has been and carries on being instrumental in the research and development of these materials, for which knowledge of their structure at the atomic level is an utmost requirement.

Nano-materials

Nanomaterial constituents are of the size of a few up to several hundreds of nanometers. They encompass single or poly-crystalline materials, as well as pieces of composite, amorphous and organic materials. Nanotechnology is a fast and globally expanding area, enabling immense developments not only in the ICT, but also in environmental, medical and generally all sectors impacting on daily life.

Nanomaterial constituents are of the size of a few up to several hundreds of nanometers. They encompass single or poly-crystalline materials, as well as pieces of composite, amorphous and organic materials. Nanotechnology is a fast and globally expanding area, enabling immense developments not only in the ICT, but also in environmental, medical and generally all sectors impacting on daily life. Since the fundamentals shaping the properties of nano-materials differ from those of 3-D materials, the former assume new and unknown forms and shapes; hence TEM is indispensable in establishing and understand these properties and enable the development of nano-materials.

Semiconductors

Semiconductors dominate current electronic technologies and exist in every aspect of our lives from phones to lighting to modern life defining computer technologies. To increase the capacity of IT circuits, semiconductor components require constant size reduction, and the stage is reached, where employment of two-dimensional materials such as MoS2 is aspired. These are now under investigation regarding their potential in future electronics.

TEM and STEM imaging

Most materials research, including in-organic and organic materials requires structural imaging. Imaging via electron microscopy (either in diffraction or phase contrast or in scanning TEM), down to the atomic level is carried out on a variety of materials, including nano- and 2D-materials, composites, ceramics, semiconductors, organic crystals and pharmaceuticals, to assess structural perfection/imperfections, boundaries and interfaces.

People working in TEM and STEM imaging

  • Prof Ursel Bangert
  • Dr Alan Harvey
  • Dr Andy Stewart
  • Mr Eoghan O' Connell
  • Mr Kalani Moore
  • Mr Michael Hennessy
  • Mr Gearóid Mangan
  • Ms Eileen Courtney
  •  Eoin Moynihan
  • Dr Jennifer Cookman
  • Dr Robbie O'Connell
  • Dr Michele Conroy

Most materials research, including in-organic and organic materials requires structural imaging. Imaging via electron microscopy, down to the atomic level, is carried out on a variety of materials, including nano- and 2D-materials, composites, ceramics, semiconductors, organic crystals and pharmaceuticals, to assess structural perfection/imperfections, boundaries and interfaces. This is done in stationary TEM (diffraction contrast and phase contrast) imaging mode as well as in scanning TEM (STEM) mode. The aberration correctors (for the condenser and objective lenses) and the monochromator allow for high angle annular dark field (HAADF) STEM imaging -also called Z-contrast imaging- to be carried out with atomic resolution. Using the latter method, the dependence of the intensity on the atomic number (as ~Z2) of the elements in the sample then enables determination of the elemental nature of the material under investigation down to the single atom level. In 2-D materials this has proven extremely useful, since elemental occupancy of lattice sites, including elemental nature and site of individual of dopant and impurity atoms, can be directly determined.

Electron Energy Loss Spectroscopy

Electron Energy Loss Spectroscopy (EELS) is an absorption spectroscopy technique wherein the energy absorbed from the incoming electron beam when passing through the sample is measured with high energy and spatial resolution. The energy losses are characteristic for the elements in a material (characteristic absorption edges), and in the low loss regime they reflect the electronic bandstructure (BG energies and density of states) of the material.

People working in Electron Energy Loss Spectroscopy

  • Prof Ursel Bangert
  • Dr Alan Harvey
  • Mr Kalani Moore
  • Mr Eoghan O' Connell
  • Ms Eileen Courtney
  • Dr Andy Stewart
  • Dr Jennifer Cookman
  • Dr Michele Conroy
  •  Eoin Moynihan

Electron Energy Loss Spectroscopy (EELS) is an absorption spectroscopy technique, where the energy, absorbed from the incoming electron beam when passing through the sample, is measured with high energy and spatial resolution. The achievable energy resolution is <100 meV (for an electron beams with energies from ranging from 80 to 300 keV), and the spatial resolution down to the Angstrom level. The energy losses are characteristic for the elements in a material (-> characteristic absorption edges, similar to EDX absorption peaks); in the low loss regime they reflect the electronic bandstructure (-> bandgap energies and density of states) of the material. EELS can be carried out in energy-filtered imaging mode (EFTEM), where an image is taken with an electron which has lost a certain amount of energy due to interaction with the material, revealing, e.g., elemental maps. It can also be carried out in spectrum imaging mode, where an entire energy loss spectrum is taken in each pixel, when the electron beam is scanned across the sample. The method is applied to a variety of materials, including nano- and 2D-materials, complex oxides, alloys, semiconductors…. to reveal composition and elemental distributions down to the atomic level. EELS can furthermore reveal bonding states, energy level distributions and phenomena related to optical properties. It has been extensively employed to investigate elemental nature, local bonding and effects of defects on the electronic structure, as well as to reveal plasmonic properties in semiconductors, nano- and especially 2D-materials.

Plasmonics

Plasmons are collective excitations in materials exhibiting metallic and semiconducting properties, and, depending on the material, occur over an energy range from tens of eV down to the meV regime. Based on their collective nature, their confinement and enhancement of the optical near field with sub-wavelength dimensions, they have immense potential for applications in the field of nano-photonics and plasmonics.

Plasmons are collective excitations in materials exhibiting metallic and semiconducting properties, and, depending on the material, occur over an energy range from tens of eV down to the meV regime. Based on their collective nature, their confinement and enhancement of the optical near field with sub-wavelength dimensions, they have immense potential for applications in the field of nano-photonics and plasmonics, and in the development of nano-photonic devices. Research into the plasmonic properties of materials together with assessment of the related nano-structural properties can be carried out via electron energy loss spectroscopy in energy-filtered or scanning spectrum imaging mode.

EDS Mapping

From EDS spectra taken in specific regions of a sample the chemical composition of these regions can be determined. When carried out in scanning mode elemental maps over larger regions can be acquired. The spatial resolution of these maps can go down to the atomic level. The method is widely applied to reveal materials' chemical compositions and modulations thereof, e.g., in complex oxides, alloys, semiconductors and nano- and 2D-materials.

Diffraction Tomography

In diffraction tomography, a series of diffraction patterns of the same object is taken at varying angles.  Data reconstruction then enables the determination of complex nanocrystal- and cell-structures and of the disorder, e.g., polycrystallinity, twinning, within. Due to the short, one-shot exposures, it can also be used for unit cell determination of organic crystals, enabling characterisation of crystals as small as 10nm.

People working in Diffraction Tomography

  • Mr Gearóid Mangan
  • Dr Andy Stewart

In diffraction tomography, a series of diffraction patterns of the same object is taken at varying angles.  Data reconstruction then enables the determination of complex nanocrystal- and cell-structures and of the disorder, e.g., polycrystallinity, twinning, within. Due to the short, one-shot exposures, it can also be used for unit cell determination of organic crystals, enabling characterisation of crystals as small as 10nm.

Nano-Crystallography

Nanocrystallography is a technique used to determine the structure of materials objects smaller than 100 nm. This structure is revealed in great detail on the atomic scale, showing up defects, facets, twinning, phase and domain boundaries in these objects. The technique is particularly aimed at investigations of nano-crystalline structures.

People working in Nano-Crystallography

  • Dr Andy Stewart
  • Prof Ursel Bangert
  • Dr Jennifer Cookman
  • Mr Gearóid Mangan

High Resolution Imaging

Knowledge of the atomic-scale structure of materials in all application sectors, especially concerning the impact of structural perfection/defects on the strength and on interfacing of materials, as well as on their performance in the electronics and nano-material development sector is of paramount importance. The methods of HREM as well as atomic resolution STEM have proven to be instrumental and unique methods for these investigations.

People working in High Resolution Imaging

  • Dr Andy Stewart
  • Prof Ursel Bangert
  • Ms Eileen Courtney
  • Mr Eoghan O' Connell
  • Mr Kalani Moore
  • Dr Michele Conroy

Knowledge of the atomic-scale structure of materials in all application sectors, especially concerning the impact of structural perfection/defects on the strength and on interfacing of materials, as well as on their performance in the electronics and nano-material development sector is of paramount importance. The methods of high resolution (phase contrast) imaging (HREM) as well as atomic resolution high angle annular dark field imaging in scanning mode (STEM) have proven to be instrumental and unique methods for these investigations.

Imaging Tomography

In a tomographic series images of a particular feature are taken at varying angles. The images can then be used to create a 3-D model of the object and its specific properties, which are being imaged. So for example 3-D plasmon (EELS), and chemical (EDX) distributions in materials can be reconstructed. Tomography of nano-particles is an example of the state-of-the-art tomography being currently carried out by researchers around the world.

People working in Imaging Tomography

  • Mr Kalani Moore
  • Prof Ursel Bangert
  • Mr Gearóid Mangan
  • Dr Andy Stewart

Modelling

Calculations are essential in support of experimental observations. Energetically favourable atom arrangements are calculated and used for image simulations to predict HREM and atomic resolution HAADF images. DFT calculations are carried out using, e.g., WIEN2K and AIMPRO and image simulations using, e.g., TEMSIM and Dr Probe. Furthermore, calculations of core loss and low loss EEL spectra are carried out for a wide range of materials.

In-situ TEM/STEM

In-situ electron microscopy allows for the imaging and physical characterization of a sample's response to a stimulus in real time down to atomic resolution. The stimulus can range from applied heat, voltage, changing gases and liquids. Capturing the dynamic changes of samples in these different states of matter allows for microscopy to move beyond just static vacuum studies.

People working in In-situ TEM/STEM

  • Dr Michele Conroy
  • Dr Jennifer Cookman
  • Dr Andy Stewart
  • Prof Ursel Bangert

Dissolution and reactivity of nanomaterials in an aqueous environment is vitally important to a wide range of disciplines including pharmaceutical chemistry, geochemistry, battery science etc. In the image above AlOOH mineral nanoparticles suspended in water within a liquid cell are imaged by TEM at 200kV. Here the imaging of the dissolution of boehmite reveals it is in fact a much more complex multi-step process than previously inferred. The electron beam irradiation destabilises the hydrogen bonding mineral inter-layer network resulting in internal delamination of the particle forming 2D exfoliated boehmite nanosheets prior to the complete dissolution.