Electron Microscopy Characterization Research

Understanding the properties and behaviour of nanomaterials requires accurate knowledge of the atomic structure. The group uses state-of-the-art aberration-corrected transmission electron microscopy (AC-TEM) to directly image the structure and dynamics of materials with single atom sensitivity. Electron energy loss spectroscopy is used to probe elemental composition and bonding. We access a wide range of electron microscopes in Oxford and internationally to provide a diverse range of experimental techniques and results. The Oxford-JEOL 2200 TEM with both image and probe correctors has been the main microscope used by the group over the past few years and is shown below to the right. The image below to the left shows the JEOL ARM300 located at the new electron Physical Science Imaging Centre (ePSIC) in the Diamond light source at Harwell.

A wide variety of materials are studied with particular emphasis on using low accelerating voltages to minimize electron beam damage to sensitive samples. We use both phase contrast high resolution TEM and annular dark field scanning TEM (ADF-STEM) to image the atomic structure with sub-Angstrom spatial resolution. We use both real space and reciprocal space image analysis methods to extract displacement maps and strain fields in both 2D and 3D. Accurate image simulations based on density functional theory relaxed atomic models enable unambiguous determination of structure and bonding. Post-image processing methods such as multi-frame averaging of low dose images are utilized to improve signal to noise and atomic coordinate determination.

Defects, dislocations, grain boundaries, edges, dopants and interfaces are studied with the aim of providing accurate structural models that form the basis of simulated properties. The image below shows a line of S vacancies in the semiconducting monolayer 2D transition metal dichalcogenide MoS2 imaged by AC-TEM at 80kV. Reduced contrast and bond reconstruction is observed where the S atoms are missing.

We use in-situ holders to introduce heating and electrical biasing to instigate material transformations and this provides insights that help improve the quality of materials for electronic devices and energy applications. The image below shows a Si TEM chip with a thin SiN membrane that has slits cut into it using a focussed ion beam system. These chips enable heating of suspended 2D materials and other nanomaterials up to 900oC with minimal drift.

We can track single atoms in graphene as they are heated from room temperature up to 900oC. We have revealed new behaviour of dislocations and defects in 2D materials are elavated temperatures using in situ heating holders within AC-TEM. The image below shows the atomic structure of a partial dislocation line in graphene at 800oC, revealing pentagons and heptagon structure with bridging atoms stabilizing the defect. Bond length variations within the high resolution image of the pentagon are easily seen by eye.

The image below shows an in-situ biasing holder used to probe the electrical:structure correlations in materials. 4inch wafers of TEM chips are fabricated using photolithography and electron beam lithography and slits are cut into the electrodes using focussed ion beams.

ADF-STEM imaging of ultrathin and small materials 2D and monolayer crystals are ultrathin and provide contrast from single atoms, enabling the analysis atom by atom. Monolayer transition metal dichalcogenides are direct band gap semiconductors and we study their structure at sub-Angstrom level. This has involved the bonding of S in vacancies of MoS2, the dynamics of line vacancies at high temperature, the reconstructions at edges, and the interface step with bilayers, shown below.

Electron Energy Loss Spectroscopy (EELS)

@Inelastic scattering of the high energy electrons with the sample causes some energy loss that can be quantitatively measured. The group uses STEM imaging combined with EELS to probe the elemental composition and bonding of materials at the single atom level. ADF-STEM imaging can be coupled with 2D EELS maps with single atom precision to examine structure. In graphene, the C k-edge EELS is sensitive to variations in the covalent bonding and we examine peak shifts and broadening due to charge transfer and electron density distributions across bonds. We showed how map out variations in carbon bonding within defective regions of graphene that contain non-hexagonal rings (see image below)

Individual subsitutional dopants, such as N in graphene have been studied and the influence on the C-bonding detected. EELS maps of the N and C k-edge signals proves the N is replacing a single C in the graphene lattice. Analysis of the C-k-edge signal for atoms bonded to the N show shifts compared to atoms in the bulk. In-situ heating experiments show that N dopants are stable in graphene to at least 500oC and can be found in both hexagonal and pentagonal rings.