  The transfer of chirality across length-scales is an intriguing and universal
natural phenomenon. However, connecting the properties of individual building
blocks to the emergent features of their resulting large-scale structure
remains a challenge. In this work, we investigate the origins of mesophase
chirality in cellulose nanocrystal suspensions, whose self-assembly into chiral
photonic films has attracted significant interest. By correlating the ensemble
behaviour in suspensions and films with a quantitative morphological analysis
of the individual nanoparticles, we reveal an inverse relationship between the
cholesteric pitch and the abundance of laterally-bound composite particles.
These bundles thus act as colloidal chiral dopants, analogous to those used in
molecular liquid crystals, providing the missing link in the hierarchical
transfer of chirality from the molecular to the colloidal scale.
