Efficient phase contrast imaging in STEM using a pixelated detector. Part II: Optimisation of imaging conditions Hao Yang a,n , Timothy J. Pennycook a,b , Peter D. Nellist a,b a University of Oxford, Department of Materials. Parks Rd, Oxford OX1 3PH, UK b EPSRC SuperSTEM Facility, Daresbury Laboratory, WA4 4AD, UK article info Article history: Received 28 August 2014 Received in revised form 17 October 2014 Accepted 17 October 2014 Available online 5 November 2014 Keywords: Phase contrast Pixelated detectors Ptychography Contrast transfer function abstract In Part I of this series of two papers, we demonstrated the formation of a high efficiency phase-contrast image at atomic resolution using a pixelated detector in the scanning transmission electron microscope (STEM) with ptychography. In this paper we explore the technique more quantitatively using theory and simulations. Compared to other STEM phase contrast modes including annular bright field (ABF) and differential phase contrast (DPC), we show that the ptychographic phase reconstruction method using pixelated detectors offers the highest contrast transfer efficiency and superior low dose performance. Applying the ptychographic reconstruction method to DPC segmented detectors also improves the de- tector contrast transfer and results in less noisy images than DPC images formed using difference signals. We also find that using a minimum array of 16 Â 16 pixels is sufficient to provide the highest signal-to- noise ratio (SNR) for imaging beam sensitive weak phase objects. Finally, the convergence angle can be adjusted to enhance the contrast transfer based on the spatial frequencies of the specimen under study. & 2014 Elsevier B.V. All rights reserved. 1. Introduction In the accompanying Part I of this work [1], we showed that using a pixelated detector which records the detailed intensity variations within the bright-field disc in the scanning transmission electron microscope (STEM) detector plane, combined with a ptychographic reconstruction approach based on that previously described by Rodenburg et al. [2] can give rise to efficient phase contrast imaging at atomic resolution with zero aberrations in STEM. The ptychographic reconstruction method in this paper follows the accompanying Part I of this pair of papers [1] and is based on the weak phase approximation. Other ptychographic reconstruction methods that are applied to X-ray as well as elec- tron microscopy using iterative methods, for example [3–5], are not discussed in this paper. Detector geometries previously pro- posed to provide enhanced phase contrast in STEM include dif- ferential phase contrast (DPC) [5–8] which makes use of a quad- rant segmented detector, and annular bright-field (ABF) imaging [10,11] (see Fig. 1). In Part I of this work, we showed that pixelated detectors gave an apparently stronger signal to noise ratio than either DPC or ABF. A particular challenge for imaging biological samples is beam damage, with critical doses of 0.01–0.1 C/cm 2 (1 C/cm 2 ¼ 6.25 Â 10 4 e À /nm 2 ) being typical [12]. Furthermore, high defocus values are commonly used to provide a sufficient contrast at low spatial frequencies. An advantage of developing efficient phase contrast imaging using zero aberrations in STEM is that it can be used alongside annular dark-field (ADF) imaging, and indeed other STEM signals such as energy-dispersive X-ray spec- troscopy. The optical transfer function for annular dark-field (ADF) imaging [13] monotonically increases as the image spatial fre- quency decreases, thereby providing long-range information alongside the possibility of recording higher-resolution phase contrast information. Furthermore, such an approach detects scattering across a large range of angles from low to high and therefore makes use of as many of the transmitted electrons as possible. The contrast transfer function (CTF) describes the strength by which a particular spatial frequency in an object being examined is transferred to the image. Here we use a theoretical and modelling approach to compare the CTF for weak-phase imaging of ptycho- graphic reconstruction using pixelated detectors, ABF and DPC imaging. To demonstrate the low dose capability of this novel imaging model, simulated low dose images of weak phase objects using pixelated detectors are compared with images using current detector geometries including STEM bright-field (BF), ABF and DPC detectors. Pixelated detectors and DPC are closely compared, and we demonstrate that a better image SNR can be achieved by ap- plying the ptychographic reconstruction method to the signals obtained using the DPC detector geometry compared to images Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ultramic Ultramicroscopy http://dx.doi.org/10.1016/j.ultramic.2014.10.013 0304-3991/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: hao.yang@materials.ox.ac.uk (H. Yang). Ultramicroscopy 151 (2015) 232–239