Martin Simson

A pnCCD-based, fast direct single electron imaging camera for TEM and STEM

We report on a new camera that is based on a pnCCD sensor for applications in scanning transmission electron microscopy. Emerging new microscopy techniques demand improved detectors with regards to readout rate, sensitivity and radiation hardness, especially in scanning mode. The pnCCD is a 2D imaging sensor that meets these requirements. Its intrinsic radiation hardness permits direct detection of electrons. The pnCCD is read out at a rate of 1,150 frames per second with an image area of 264 x 264 pixel. In binning or windowing modes, the readout rate is increased almost linearly, for example to 4000 frames per second at 4× binning (264 x 66 pixel). Single electrons with energies from 300 keV down to 5 keV can be distinguished due to the high sensitivity of the detector. Three applications in scanning transmission electron microscopy are highlighted to demonstrate that the pnCCD satisfies experimental requirements, especially fast recording of 2D images. In the first application, 65536 2D diffraction patterns were recorded in 70 s. STEM images corresponding to intensities of various diffraction peaks were reconstructed. For the second application, the microscope was operated in a Lorentz-like mode. Magnetic domains were imaged in an area of 256 x 256 sample points in less than 37 seconds for a total of 65536 images each with 264 x 132 pixels. Due to information provided by the two-dimensional images, not only the amplitude but also the direction of the magnetic field could be determined. In the third application, millisecond images of a semiconductor nanostructure were recorded to determine the lattice strain in the sample. A speed-up in measurement time by a factor of 200 could be achieved compared to a previously used camera system.

Rapid low dose electron tomography using a direct electron detection camera

We demonstrate the ability to record a tomographic tilt series containing 3487 images in only 3.5 s by using a direct electron detector in a transmission electron microscope. The electron dose is lower by at least one order of magnitude when compared with that used to record a conventional tilt series of fewer than 100 images in 15–60 minutes and the overall signal-to-noise ratio is greater than 4. Our results, which are illustrated for an inorganic nanotube, are important for ultra-low-dose electron tomography of electron-beam-sensitive specimens and real-time dynamic electron tomography of nanoscale objects with sub-ms temporal resolution.

4D STEM: High efficiency phase contrast imaging using a fast pixelated detector

Phase contrast imaging is widely used for imaging beam sensitive and weak phase objects in electron microscopy. In this work we demonstrate the achievement of high efficient phase contrast imaging in STEM using the pnCCD, a fast direct electron pixelated detector, which records the diffraction patterns at every probe position with a speed of 1000 to 4000 frames per second, forming a 4D STEM dataset simultaneously with the incoherent Z-contrast imaging. Ptychographic phase reconstruction has been applied and the obtained complex transmission function reveals the phase of the specimen. The results using GaN and Ti, Nd- doped BiFeO3 show that this imaging mode is especially powerful for imaging light elements in the presence of much heavier elements.

Electron ptychographic phase imaging of light elements in crystalline materials using Wigner distribution deconvolution

Recent development in fast pixelated detector technology has allowed a two dimensional diffraction pattern to be recorded at every probe position of a two dimensional raster scan in a scanning transmission electron microscope (STEM), forming an information-rich four dimensional (4D) dataset. Electron ptychography has been shown to enable efficient coherent phase imaging of weakly scattering objects from a 4D dataset recorded using a focused electron probe, which is optimised for simultaneous incoherent Z-contrast imaging and spectroscopy in STEM. Therefore coherent phase contrast and incoherent Z-contrast imaging modes can be efficiently combined to provide a good sensitivity of both light and heavy elements at atomic resolution. In this work, we explore the application of electron ptychography for atomic resolution imaging of strongly scattering crystalline specimens, and present experiments on imaging crystalline specimens including samples containing defects, under dynamical channelling conditions using an aberration corrected microscope. A ptychographic reconstruction method called Wigner distribution deconvolution (WDD) was implemented. Experimental results and simulation results suggest that ptychography provides a readily interpretable phase image and great sensitivity for imaging light elements at atomic resolution in relatively thin crystalline materials.

4D-STEM Imaging With the pnCCD (S)TEM-Camera

4D-STEM imaging describes a powerful imaging technique where a two-dimensional image is recorded for each probe position of a two-dimensional STEM image. For typical STEM images of 256x256 probe positions, a total of 65,536 2D images needs to be recorded. This amount of data can be recorded with the pnCCD (S)TEM camera in a practical timeframe. This camera uses a direct detecting, radiation hard pnCCD with a minimum readout speed of 1000 full frames per second (fps) [1]. With the pnCCD (S)TEM camera a 4D data cube consisting of 256x256 probe positions with a 264x264 pixel detector image for each probe position can be recorded in less than 70 s. Several measurements have been performed to prove the capability of the camera for 4D-STEM imaging, among them strain analysis, magnetic domain mapping and most recently electron ptychography.

Extending the dynamic range of fully depleted pnCCDs

pnCCDs are a special type of charge coupled devices (CCD) which were originally developed for applications in X-ray astronomy. At X-ray Free Electron Lasers (XFEL) pnCCDs are used as imaging X-ray spectrometers due to their outstanding characteristics like high readout speed, high and homogeneous quantum efficiency, low readout noise, radiation hardness and high pixel charge handling capacity. They can be used both as single-photon counting detectors for X-ray spectroscopy and as integrating detectors for X-ray imaging with count rates up to 104 photons of 1 keV per pixel and frame. However, extremely high photon intensities can result in pixel saturation and charge spilling into neighboring pixels. Because of this charge blooming effect, spatial information is reduced. Based on our research concerning the internal potential distribution we can enhance the pixel full well capacity even more and improve the quality of the image. This paper describes the influence of the operation voltages and space charge distribution of the pnCCD on the electric potential profile by using 2D numerical device simulations. Experimental results with signal injection from an optical laser confirm the simulation models.

High Efficiency Phase Contrast Imaging In STEM Using Fast Direct Electron Pixelated Detectors

Phase contrast imaging using electron elastic scattering has been shown to provide the highest efficiency for imaging weak phase objects for a given amount of radiation damage, compared to inelastic electron scattering as well as X-ray and neutron scattering [1]. Phase contrast imaging in scanning transmission electron microscopy (STEM) is attractive because of its flexibility in detector geometries without modifying the main optics of the electron column, and for the ability to simultaneously record analytical signals. Current bright-field STEM detector geometries include annular bright field (ABF) and differential phase contrast (DPC) imaging. As most phase information from weak scattering objects is contained within the bright field (BF) disc of the convergent beam electron diffraction (CBED) pattern, a detector geometry that makes full use of the variations in intensity of the BF disc is desirable to maximize the image contrast. We demonstrate high efficiency phase contrast imaging using a fast pixelated detector. By recording the CBED pattern at every probe position forming a 4-dimensional dataset, the phase can be reconstructed using a ptychographic phase reconstruction method based on one developed described by Rodenburg et al. [2,3] Taking the Fourier transform of the 4D dataset with respect to probe position allows interference between the BF discs and specific diffraction angles to be isolated. The interference inside the disc-overlapping region contains phase information, as shown in Figure 1. A phase image can be reconstructed by integrating such disc-overlapping regions for each spatial frequency contained in the final image. Through simulations, we find that for imaging weak phase objects, phase contrast imaging using ptychography with pixelated detectors gives the optimal phase contrast transfer function (PCTF), and generates the best phase contrast and low dose performances compared to existing imaging modes in STEM including BF, ABF and DPC. [4] Experiments on phase reconstruction using a fast pixelated detectors have been performed, and here we show results of an Au nanoparticle with five-fold twinning (Figure 2a-d), and gallium nitride GaN viewed along (Figure 2e-h). The experiments were performed using the pnCCD (S)TEM camera, a direct electron pixelated detector from PNDetector, mounted on the JEOL ARM200-CF aberration corrected microscope. The detector has a grid of 264x264 pixels and operates at a speed of 1000 framesper-second (fps). The detector can achieve a speed of up to 20,000 fps through binning/windowing. ADF images can be recorded simultaneously. Using the 4D dataset which records the diffraction patterns that contains the entire BF and part of DF regions, synthetic BF, ABF and DPC can be obtained along with the ptychographic phase reconstruction. The reconstructed phase of the Au nanoparticle (Figure 2c,d) shows a clear contrast on every atomic column. In comparison, ABF, as a nonlinear imaging mode, shows contrast that decreases towards the edge of the Au particle (Figure 2b). In the case of GaN, the N columns are hardly visible in the ABF image (Figure 2f), but are clearly resolved in the reconstructed phase (Figure 2g,h).[5] Paper No. 115

Development of Fast Pixelated STEM Detector and its Applications using 4-Dimensional Dataset

In scanning transmission electron microscopy (STEM), one can obtain a variety of STEM images such as annular bright-field (ABF), annular dark-field (ADF) and differential phase contrast (DPC) images by changing the shapes of the scintillators of the detectors [1,2]. However, the intensity distribution in the convergent beam electron diffraction (CBED) pattern, which is projected on the plane of scintillator, is not fully utilized using the conventional detectors as they integrate the intensity over the scintillator. Meanwhile, direct electron detectors with fast frame rate and several ten thousand pixels have recently been commercialized and used in scanning electron microscopy [3-6]. Such detectors, when used for recording CBED patterns for each STEM probe position, are called pixelated STEM detectors. With the obtained 4-dimensional (4D) dataset, any type of STEM image can be synthesized in a post or real time processing using a user-defined selection of the integration area.

Influence of distortions of recorded diffraction patterns on strain analysis by nano-beam electron diffraction

Images acquired in transmission electron microscopes can be distorted for various reasons such as e.g. aberrations of the lenses of the imaging system or inaccuracies of the image recording system. This results in inaccuracies of measures obtained from the distorted images. Here we report on measurement and correction of elliptical distortions of diffraction patterns. The effect of this correction on the measurement of crystal lattice strain is investigated. We show that the effect of the distortions is smaller than the precision of the measurement in cases where the strain is obtained from shifts of diffracted discs with respect to their positions in images acquired in an unstrained reference area of the sample. This can be explained by the fact that diffraction patterns acquired in the strain free reference area of the sample are distorted in the same manner as the diffraction patterns acquired in the strained region of interest. In contrast, for samples without a strain free reference region such as nanoparticles or nanoporous structures, where we evaluate ratios of lattice plane distances along different directions, the distortions are usually not negligible. Furthermore, two techniques for the detection of diffraction disc positions are compared showing that for samples in which the crystal orientation changes over the investigated area it is more precise to detect the positions of many diffraction discs simultaneously instead of detecting each disc position independently.

Towards a Direct Visualization of Charge Transfer in Monolayer Hexagonal Boron Nitride using a Fast Pixelated Detector in the Scanning Transmission Electron Microscope

Recent developments of fast pixelated detectors, such as the pnCCD (S)TEM camera developed by PNDetector, have enabled the acquisition of 4D-datasets that contain the full convergent beam electron diffraction pattern for each probe position in a scanning transmission electron microscope (STEM) experiment [1]. These type of datasets enable electron ptychography to be performed, which has been shown to be a powerful tool for phase imaging of a wide range of materials [2, 3]. Moreover, when using methods such as the Wigner Distribution Deconvolution (WDD) algorithm [4], aberration free phase imaging of light element materials can be achieved [3]. Electron phase imaging techniques, such as high resolution transmission electron microscopy (HRTEM), can provide information on the atomic potential and the electronic charge density distribution [5], opening the possibility to acquire information about chemical bonds and ionization of atoms. However, these measurements can be affected by residual lens aberrations, which affect the image contrast and can overwhelm small signals due to charge transfer. In this work, we profit from aberration free phase imaging using WDD electron ptychography to explore the direct visualization of charge transfer in monolayer hexagonal boron nitride (hBN).