Gentle STEM: ADF imaging and EELS at low primary energies $ Ondrej L. Krivanek a,n , Niklas Dellby a , Matthew F. Murfitt a , Matthew F. Chisholm b , Timothy J. Pennycook b , Kazutomo Suenaga c , Valeria Nicolosi d a Nion Co., 1102 8th St., Kirkland, WA 98033, USA b Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN 37831-6069, USA c National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan d Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK article info Keywords: STEM ADF EELS Aberration correction Nanotube Graphene abstract Aberration correction of the scanning transmission electron microscope (STEM) has made it possible to reach probe sizes close to 1 ˚ A at 60 keV, an operating energy that avoids direct knock-on damage in materials consisting of light atoms such as B, C, N and O. Although greatly reduced, some radiation damage is still present at this energy, and this limits the maximum usable electron dose. Elemental analysis by electron energy loss spectroscopy (EELS) is then usefully supplemented by annular dark field (ADF) imaging, for which the signal is larger. Because of its strong Z dependence, ADF allows the chemical identification of individual atoms, both heavy and light, and it can also record the atomic motion of individual heavy atoms in considerable detail. We illustrate these points by ADF images and EELS of nanotubes containing nanopods filled with single atoms of Er, and by ADF images of graphene with impurity atoms. & 2010 Elsevier B.V. All rights reserved. 1. Introduction The first field emission STEM designed and built in Crewe’s laboratory operated at 30 keV [1,2], most likely because a 30 keV instrument was easier to build from scratch than one operating at a higher energy. The final version of this instrument had an objective lens with a spherical aberration coefficient C s of 0.3 mm and also a low chromatic aberration coefficient C c , and it attained about 3 ˚ A resolution. It was the first electron microscope able to resolve single heavy atoms [3], and to produce high-quality electron energy-loss spectra from atomic-dimension sample areas [4]. Aiming for better resolution, Crewe’s lab embarked on building a series of aberration correctors, a 1 MeV STEM and an aberration-corrected 200 kV STEM that was supposed to give 0.5 ˚ A resolution, but these projects were not completed [5,6]. However, higher primary energy STEMs were built by others [7–9], and the standard STEM operating energy soon became 100 keV and later on 200 and 300 keV. 100 keV and higher energy operation brought advantages such as better resolution and an ability to look at thicker samples. It allowed researchers to examine materials and their interfaces at atomic or near-atomic resolution [10–12]. However, in low Z materials such as carbon and boron nitride, the higher operating energy produced significant knock-on damage (e.g. [13]), which limited the usable electron doses and hence the counting statistics of the experimental data. Theoretical estimates for knock-on threshold in carbon and boron nitride are close to 80 kV [14], and operating below this threshold should therefore either eliminate or considerably reduce the radiation damage. Prior to aberration correction, the loss of spatial resolution that would have resulted from lowering the operating energy was typically too large to permit atomic-resolution imaging of closely packed lattices. This situation has now changed: probe sizes of about 0.8 ˚ A have been available for some time at 120 keV [15,16], and close to 1 ˚ A probe size has become possible at 60 kV [17,18]. There are no near-neighbor atomic distances not involving hydrogen that are shorter than 1.2 ˚ A, and 60 kV operation is therefore sufficient for most structural investigations. This is opening up a new/old sub-field of scanning transmission electron microscopy that we like to call ‘‘gentle STEM’’. In this paper we illustrate what has now become possible in this field, and examine future possibilities. We concentrate on model samples that have become available in the last few years: single wall nanotubes filled with fullerene ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ultramic Ultramicroscopy 0304-3991/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2010.02.007 $ Dedication: This paper is dedicated to the memory of Professor Albert Crewe, who was the pioneer of both STEM and gentle STEM, and who recently passed away. n Corresponding author. E-mail addresses: krivanek@nion.com, krivanek.ondrej@gmail.com (O.L. Krivanek). Please cite this article as: O.L. Krivanek, et al., Ultramicroscopy (2010), doi:10.1016/j.ultramic.2010.02.007 Ultramicroscopy ] (]]]]) ]]]–]]]