  Topological insulators are of interest for many applications in electronics
and optoelectronics, but harnessing their unique properties requires detailed
understanding and control of charge injection at electrical contacts. Here we
present large-scale ab initio calculations of the electronic properties of Au,
Ni, Pt, Pd, and graphene contacts to Bi2Se3. We show that regardless of the
metal, the Fermi level is located in the conduction band, leading to n-type
Ohmic contact to the first quintuplet. Furthermore, we find strong charge
transfer and band-bending in the first few quintuplets, with no Schottky
barrier for charge injection even when the topoplogical insulator is undoped.
Our calculations indicate that Au and graphene leave the spin-momentum locking
mostly unaltered, but on the other hand, Ni, Pd, and Pt strongly hybridize with
Bi2Se3 and relax spin-momentum locking. Our results indicate that judicious
choice of the contact metal is essential to reveal the unique surface features
of topological insulators.
