Emergent Properties of the Organic Molecule-Topological Insulator Hybrid Interface: Cu-Phthalocyanine on Bi₂Se₃

We unravel the bidirectional influence of the adsorbate–substrate hybrid interface in the case of CuPc doping of the topological insulator (TI) surface Bi2Se3. Using ultrahigh vacuum scanning tunneling microscopy at low temperature (77 K), we observe that for a dilute concentration, single CuPc molecules are dispersed on terraces of Bi2Se3 as individual entities or as clusters. The site-dependent submolecular resolution images of CuPc on Bi2Se3 reveal three different sites for CuPc adsorption. Scanning tunneling spectroscopy (STS) measurements show a rigid shift of the Dirac point toward negative voltage by 336 meV upon CuPc deposition. This is a clear signature that the topological surface state experiences a charge transfer because of CuPc. The highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap of CuPc on Bi2Se3 is also measured using STS. It is found to be larger than that on the Au(111) substrate because of the reduced screening offered by the TI Bi2Se3 surface state as compared to the Au noble metal surface state. We also find that a higher concentration of CuPc results in an unconventional standing-up stacking of CuPc molecules at the step edge. This is suggestive of a magnetic coupling between the CuPc layers analogous to the observations in magnetic phthalocyanine thin films. Thus, the CuPc/Bi2Se3 system is a potent combination where CuPc induces a charge transfer, a change in the HOMO–LUMO gap on the TI surface of Bi2Se3, and the step edges of Bi2Se3 act as an anchor to guide the unique vertical alignment of CuPc molecules proposing a new route to harness the TI nature and the magnetic behavior of the system