  One-dimensional inorganic nanotubes hold promise for technological
applications due to their distinct physical/chemical properties, but so far
advancements have been hampered by difficulties in producing single-wall
nanotubes with a well-defined radius. In this work we investigate, based on
Density Functional Theory (DFT), the formation mechanism of 135 different
inorganic nanotubes formed by the intrinsic self-rolling driving force found in
asymmetric 2D Janus sheets. We show that for isovalent Janus sheets, the
lattice mismatch between inner and outer atomic layers is the driving force
behind the nanotube formation, while in the non-isovalent case it is governed
by the difference in chemical bond strength of the inner and outer layer
leading to steric effects. From our pool of candidate structures we have
identified more than 100 tubes with a preferred radius below 35 {\AA}, which we
hypothesize can display unique properties compared to their parent 2D
monolayers. Simple descriptors have been identified to accelerate the discovery
of small-radius tubes and a Bayesian regression approach has been implemented
to assess the uncertainty in our predictions on the radius.
