  We have designed three-dimensional models of topological insulator thin
films, showing a tunability of the odd number of Dirac cones on opposite
surfaces driven by the atomic-scale geometry at the boundaries. This enables
creation of a single Dirac cone at the $\Gamma$-point as well as possible
suppression of quantum tunneling between Dirac states at opposite surfaces (and
gap formation), when opposite surfaces are geometrically differentiated. The
spin texture of surface states was found to change from a spin-momentum-locking
symmetry to a progressive loss in surface spin polarization upon the
introduction of bulk disorder, related to the penetration of boundary states
inside the bulk. These findings illustrate the richness of the Dirac physics
emerging in thin films of topological insulators and may prove utile for
engineering Dirac cones and for quantifying bulk disorder in materials with
ultraclean surfaces.
