  We derive a $2+1$ dimensional model with unconventional supersymmetry at the
boundary of an ${\rm AdS}_4$ $\mathcal{N}$-extended supergravity, generalizing
previous results. The (unconventional) extended supersymmetry of the boundary
model is instrumental in describing, within a top-down approach, the electronic
properties of graphene-like 2D materials at the two Dirac points, ${\bf K}$ and
${\bf K}'$. The two valleys correspond to the two independent sectors of the
${\rm OSp}(p|2)\times {\rm OSp}(q|2)$ boundary model in the $p=q$ case, which
are related by a parity transformation. The Semenoff and Haldane-type masses
entering the corresponding Dirac equations are identified with the torsion
parameters of the substrate in the model.
