Dose-limited spectroscopic imaging of soft materials by low-loss EELS in the scanning transmission electron microscope Sergey Yakovlev, Matthew Libera * Department of Chemical, Biomedical, and Materials Engineering, Stevens Institute of Technology, 1 Castle Point Terrace, Hoboken, NJ 07030, USA Abstract Spectroscopic imaging in the scanning transmission electron microscope (STEM) using spatially resolved electron energy-loss spectroscopy (EELS) provides one of the few ways to quantitatively measure the real-space nanoscale morphology of soft materials such as polymers and biological tissue. This paper describes the basic principles of this technique and outlines some of the important attributes that define the achievable spatial resolution. Many soft materials can be differentiated from each other as well as from solvents based on their EELS fingerprints. Applying a multiple least squares (MLS) fitting algorithm using such spectral fingerprints to analyze spatially resolved spectrum datasets enables the quantitative mapping of the different components in a specimen. However, in contrast to TEM studies of many inorganic materials where the spatial resolution is limited principally by the spherical aberration of the objective lens, the spatial resolution associated with the imaging of radiation- sensitive soft materials is limited by the total electron dose to which they can be exposed before suffering irrevocable chemical or structural damage. The Rose criterion provides a simple guide to enhance the so-called dose-limited spatial resolution relevant to soft-materials imaging. By using the low-loss portion of an EELS spectrum where the inelastic scattering cross-sections are highest together with improvements in data- collection efficiency and post-acquisition data processing, the dose-limited resolution in spectrum images of solvated polymers has moved into the sub 10 nm regime. This resolution is sufficient to solve important applications-oriented problems associated with hetero interfaces, nanoscale mixing, and nanophase separation. # 2007 Elsevier Ltd. All rights reserved. 1. Introduction The transmission electron microscope (TEM) has long been recognized as an essential tool for nanoscale materials characterization, and developments in TEM imaging have paralleled the development of novel engineering materials for much of the modern era of Materials Science and Engineering (Marburger and Kvamme, 2005). However, soft materials such as synthetic polymers and biological structures bring two important challenges that differentiate the study of their nanoscale structure by electron microscopy from that of their hard-materials counterparts. First, generating contrast between different morphological features in soft materials using traditional TEM methods based on differential elastic electron scattering is typically difficult, because many soft materials are amorphous and have similar densities and similar elastic scattering cross-sections. Second, most soft materials are susceptible to significant chemical or structural change induced by the high-energy radiation imposed by exposure to intermediate-energy (50–300 keV) electrons. The first of these challenges is increasingly being met by spectroscopic imaging approaches that are based on differential inelastic scattering where the rich electronic structure of soft materials can often provide significant contrast. The second of these challenges imposes a very different set of constraints on the problem of improving resolution, since imaging radiation- sensitive materials is limited by the allowable incident dose whereas imaging radiation-resistant hard materials is typically limited by the quality of the electron optics. Contrast and resolution are closely related because the incident dose required to achieve a threshold level of signal above noise decreases as the intrinsic contrast between different features within a specimen increases. This paper outlines dose-limited spectro- scopic imaging of soft materials in the scanning transmission electron microscope, highlighting recent developments and ongoing challenges to improving spatial resolution. The spatial resolution has improved already over the past decade to the point where spectroscopic imaging can increasingly be used to answer important morphological questions associated with the www.elsevier.com/locate/micron Available online at www.sciencedirect.com Micron 39 (2008) 734–740 * Corresponding author. E-mail addresses: Siakovle@stevens.edu (S. Yakovlev), mlibera@stevens.edu (M. Libera). 0968-4328/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2007.10.019