Optimizing experimental parameters for the projection requirement in HAADF-STEM tomography

Abstract

Tomographic reconstruction algorithms offer a means by which a tilt-series of transmission images can be combined to yield a three dimensional model of the specimen. Conventional reconstruction algorithms assume that the measured signal is a linear projection of some property, typically the density, of the material. Here we report the use of multislice simulations to investigate the extent to which this assumption is met in HAADF-STEM imaging. The use of simulations allows for a systematic survey of a range of materials and microscope parameters to inform optimal experimental design. Using this approach it is demonstrated that the imaging of amorphous materials is in good agreement with the projection assumption in most cases. Images of crystalline specimens taken along zone-axes are found to be poorly suited for conventional linear reconstruction algorithms due to channelling effects which produce enhanced intensities compared with off-axis images, and poor compliance with the projection requirement. Off-axis images are found to be suitable for reconstruction, though they do not strictly meet the linearity requirement in most cases. It is demonstrated that microscope parameters can be selected to yield improved compliance with the projection requirement.

Automatic correction of nonlinear damping effects in HAADF–STEM tomography for nanomaterials of discrete compositions

“…pears in a concave "cup" shape, while ideally it should be flat. The cupping artifacts are caused by the strong damping effects of Au at large thickness, which is illustrated by the simulated relationships between measurement intensity and sample thicknesses using the multislice simulation method [5] in Fig. 2 .…”

“…One example is that when projections of a crystalline material are acquired at zone-axis orientations, fringes and large overall intensity differences can be observed. Thus the tilts at zone-axis are usually excluded from the tomographic reconstruction step [5] . Another example is that the image intensity damps at high sample thickness due to the multiple scattering events redirecting electrons outside the annular detector, which can occur in all projection orientations.…”

“…If we look at the line-profile of the 2D image ( Fig. 1 (b)), the curve ap- Normalized HAADF signal intensity w.r.t the thickness of Au slabs mistilted 10 degrees from the [100] zone axis about the < 100 > axis, simulated using the multislice method [5] . The accelerating voltage is 200 kV, the convergence angle is 10 mrad and the detector angular range is 50-250 mrad.…”

“…Despite these shortcomings of using uncorrected data, there are few publications addressing the nonlinearity issue in ET [5,6] . Nonlinear effects are usually ignored or mitigated during image acquisition by increasing the inner angle of the HAADF detector but at the cost of losing signal strength [5] .…”

“…Nonlinear effects are usually ignored or mitigated during image acquisition by increasing the inner angle of the HAADF detector but at the cost of losing signal strength [5] . An alternative to adjusting the acquisition parameters is to correct the measured data in a post-processing step by linearizing the projection data, provided that the incident beam intensity is known [6] .…”