Transmission Electron Microscopy 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.

We are currently using 3 aberration-corrected TEMs in Oxford: JEOL ARM300, JEOL ARM200 and JEOL 2200MCO. The Oxford-JEOL 2200 TEM has CEOS image and probe correctors, and has been the main microscope used by the group over the past 8 years and is shown below to the right. It has monochromation of the electron source for ultra-high spatial resolution at the low accelerating voltage of 80kV. Over the past two years, we have been using the ARM300 and ARM200 for annular dark field scanning transmission electron microscopy (ADF-STEM). The ARM200 is located in the David Cockayne Center for Electron Microscopy and has cold-FEG with CEOS probe corrector and EELS capability at 80kV accelerating voltage. The image below to the left shows the JEOL ARM300 located at the electron Physical Science Imaging Centre (ePSIC) in the Diamond light source at Harwell. It has ultra-stable JEOL image and probe correctors and operates down to 60kV. It is also equipped with a fast frame rate pixelated detector for 4D STEM acquisition.

EM techniques and methods

The main approach is using low accelerating voltage (60-80kV) to probe 2D materials at the atomic scale. Both phase contrast high resolution TEM and annular dark field scanning TEM (ADF-STEM) are used to image the atomic structure with sub-Angstrom spatial resolution. Research using atomically resolved electron energy loss spectroscopy by STEM has also been undertaken. We were one of the first groups to utilize monochromated electron sources for ultra-high spatial resolution imaging, demonstrating 80pm resolution detection of single C atoms in graphene. This was published in Science. Phase contrast imaging has been used to capture a wide range of defect dynamics in 2D materials, and we have used post-imaging real space and reciprocal space 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 frame-to-frame alignments, and multi-frame averaging of low dose images are utilized to improve signal to noise and atomic coordinate determination.

Recent focus has been on using ADF-STEM to examine 2D transition metal dichalcogenide layered materials, as shown below.

Our newest activity involves the high spatial resolution 4D STEM data acquisition using high frame rate, high sensitivty pixelated array detectors. This is done in collaboration with ePSIC). We have been measuring electrostatic field distributions in monolayer MoS2 with single atom resolution and undertaking scattering angular dependent imaging reconstructions to similatenously observe low Z and high Z scattering atoms.

Structural studies of defects and dopants in 2D materials

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. We spent many years invsetigating graphene defects and dopants, and more recently we have focused on TMDs. 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.


In-situ holders for electron microscopy

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 heating 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 900 C. 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.

Monolayer-Bilayer step edges in 2D TMDs

Dislocation pair in graphene

Si bonding to graphene

S vacancies at 60kV ADF-STEM imaging in MoS2 monolayers

Structure of Graphene and other 2D crystals

The structure of the synthetic graphene has been studied primarily using atomic-resolution TEM imaging using aberration-corrected machines. We primarily use the Oxford-JEOL 2200MCO FEG-TEM in the Department of Materials for graphene imaging. When running in optimized alignment, we have achieved imaging of monolayer graphene with 80 picometer spatial resolution. This ultra-high resolution enabled the first report of measuring bond length changes between two carbon atoms reported in Science, Vol. 337 no. 6091 pp. 209-212 (2012) and further refined in ACS Nano, Vol. 7 (11), pp 98609866 (2013).

We have conducted extensive studies into the variety of defects that exist in graphene and their stability, dynamics and transformations. These include the Stone-Wales bond rotation, vacancy defects (mono to tetra to larger), dislocation pairs and their resulting structural deformations, to excess C atoms forming self-interstitual defects. Our results how revealed that bond rotations are active in defects at room temperature to lower the total energy in the system through strain relief. This work is built upon previous work we did on understanding defects in carbon nanotubes. Understanding the behaviour of defects in 2D materials is now under investigation.

Aberration corrected TEM image of a 6-atom vacancy linear arm-chair defect in graphene

We have studied the dynamics of edge atoms in graphene, shown below. The edges of graphene are unique in that they are not fully bonded and therfore highly reactive with motion confined to 1D. Using HRTEM image combined with image simulations we were able to prove that graphene edges could be hydrogen free and armchair edges can have triple-bond-like terminations. This was reported in Nature Communications 5, Article number: 3040 doi:10.1038/ncomms4040 (2013)

The image below is a series of images 10 seconds apart showing an extra atom arriving at the edge in triangular bonding form, then ejecting.

The image below shows a single atom edge, as well as bond breaking and reforming after 10 seconds of electron beam irradiation.

Stone-Wales bond rotations were captured at the edge. (10 s between images).

Atomic Resolution Imaging of Dopants in Graphene

We study impurity atoms, such as Fe and Si, and their interaction with vacancies in graphene (See HRTEM movies below). Sequences of HRTEM images taken every 10 seconds can be compiled into a time-series that reveals dynamics of these heavier atoms. These movies how single Fe atoms are trapped in vacancies intentionally created in graphene by our focussed electron beam irradiation technique. We have demonstrated our ability to add a single metal dopant into the lattice of graphene with a 10nm spatial precision. This was published as Robertson et al. Nano Lett., 2013, 13 (4), pp 14681475 and as He at al. Nano Lett., 2014, 14 (7), pp 37663772.

Structural Studies of TMDs.

Large WS2 single crystals are transferred onto SiN TEM grids to study their atomic structure. We map our the crystal continuity by taking selected area electron diffraction patterns from each hole in the TEM grid to obtain a spatial map of crystal orientation. Our measurements show long range crystallinity.

In-situ Studies of MoS2 edges

A SiN grid with Pt wire electrical coils was used for in-situ heating during ADF-STEM studies of MoS2 monolayers. At high temperatures of 800 C, edges became atomically flat along the zigzag direction, see figure below. The S terminated zigzag edge of MoS2 is S depleted at these high temperaturs and the Mo reconstructs to form a double Mo terminated edge. This work was reported in Nano Letters.

ADF-STEM imaging of Point Defects in TMDs