  Carbon Nanotubes (CNTs) of sufficiently large diameter and a few layers
self-collapse into flat ribbons at atmospheric pressure, forming bundles of
stacked CNTs that maximize packing and thus CNT interaction. Their improved
stress transfer by shear makes collapsed CNTs ideal building blocks in
macroscopic fibers of CNTs with high-performance longitudinal properties,
particularly high tensile properties as reinforcing fibres. This work
introduces cross-sectional transmission electron microscopy of FIB-milled
samples as a way to univocally identify collapsed CNTs and to determine the
full population of different CNTs in macroscopic fibers produced by spinning
from floating catalyst chemical vapour deposition. We show that close proximity
in bundles is a major driver for collapse and that CNT stoutness (number of
layers/diameter), which dominates the collapse onset, is controlled by the
growth promotor. Despite differences in decomposition route, different C
precursors lead to similar distributions of the ratio layers/diameter. The
synthesis conditions in this study give a maximum fraction of collapsed CNTs of
70$\%$ when using selenium as promotor, corresponding to an average of $0.25
layer/nm$.
