  We employ Monte Carlo simulations to investigate the self-assembly of patchy
colloidal dumbbells interacting via a modified Kern-Frenkel potential by
probing the system concentration and dumbbell shape. We consider dumbbells
consisting of one attractive sphere with diameter $\sigma_1$ and one repulsive
sphere with diameter $\sigma_2$ and center-to-center distance $d$ between the
spheres. For three different size ratios, we study the self-assembled
structures for different separations $l = 2d/(\sigma_1+\sigma_2)$ between the
two spheres. In particular, we focus on structures that can be assembled from
the homogeneous fluid, as these might be of interest in experiments. We use
cluster order parameters to classify the shape of the formed structures. When
the size of the spheres is almost equal, $q=\sigma_2/\sigma_1=1.035$, we find
that, upon increasing $l$, spherical micelles are transformed to elongated
micelles and finally to vesicles and bilayers. For size ratio $q=1.25$ we
observe a continuously tunable transition from spherical to elongated micelles
upon increasing the sphere separation. For size ratio $q=0.95$ we find bilayers
and vesicles, plus faceted polyhedra and liquid droplets. Our results identify
key parameters to create colloidal vesicles with attractive dumbbells in
experiments.
