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 MoS₂ 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].

[1] Slater et al, Nano Letters, 14, 1921-1926, (2014)

[2] Slater et al, Microscopy and Microanalysis, in press, (2016)

[3] Slater et al, Ultramicroscopy, 162, 61-73, (2016).

[4] Haigh et al, Nature Materials 11, 764-767, (2012); Georgiou et al Nature nanotechnology 8, 100-103 (2013)

[5] Withers et al, Nature Materials, 14, 301-306 (2015)

[6] Cao et al, Nano Letters, 15, 4914-4921 (2015)

[7] Zaluzec et al, Microscopy and Microanalysis 20 (2), 323 (2014)

[8] Lewis et al, Chemical Communications, 50, 10019-10022 (2014), Lewis et al Nanoscale, 6 (22), 13598-13605 (2014).

Figures:

Figure 1: Elemental tomographic 3D imaging of nanoparticles achieved using STEM EDX. For further information see Slater et al Correlating catalytic activity of Ag-Au nanoparticles with 3D compositional variations Nano Letters, 14 (4), 1921-1926 (2014)

Figure 2: Cross Sectional STEM images and EELS elemental mapping of a WS2 quantum well light emitting diode structure. Similar to structures reported in Withers et al Light-emitting diodes by band-structure engineering in the van der Waals heterostructures, Nature Materials, 14, 301-306 (2015)

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EMC Abstracts - https://emc-proceedings.com/abstract/high-resolution-stem-imaging-and-analysis-of-2d-crystal-heterostructure-devices-and-nanoparticle-catalysts/