High resolution STEM imaging and analysis of 2D crystal heterostructure devices and nanoparticle catalysts

Abstract

The new generation of aberration corrected scanning transmission electron microscope (STEM) instruments optimized for high spatial resolution energy dispersive x‐ray (EDX) spectroscopy provide exciting opportunities for elemental analysis of nanoscale objects. Here I will discuss recent example applications from our studies of nanoparticle catalysts and 2D device heterostructures where these analytical capabilities have provided new insights to interpret the electronic and catalytic properties of such systems. Elementally sensitive STEM EDX electron tomography provides a route to understanding full 3D morphology and chemistry with nanometre resolution. I will demonstrate results showing the effect of different elemental segregation on the catalytic performance of bimetallic nanoparticles [1]. I will also discuss the current limitations of this technique, including compensation of detector shadowing using a time varied acquisition scheme, and recent progress towards quantitative analysis [2,3]. I will also present work demonstrating that cross sectional STEM‐EDX spectrum imaging can be used to reveal the internal atomic structure of van der Waals heterostructure devices produced by layering together multiple 2D crystals [4]. Recently we have studied light emitting diode devices, produced by mechanical exfoliation and subsequent stacking of 13 different 2D crystals, including 4 MoS 2 monolayer quantum wells [5]. Using cross sectional STEM spectrum imaging we reveal that the crystal interfaces of such devices are atomically flat and provide detailed structural information to help to explain the electroluminescence results obtained. Other 2D crystal heterostructures will also be discussed including those incorporating air sensitive 2D crystals, such as black phosphorus, that require fabrication under an argon atmosphere to preserve the device performance [6]. Finally recent work where customised modification of an in situ STEM holder system [7] has allowed us to perform high spatial resolution STEM‐EDX spectrum imaging during in‐situ gas and liquid phase experiments and at elevated temperature [8].