Molecular diode overlayers for enhancing the electronic properties of topological insulators.

Advancements in technology often require the coalescence of materials with a variety of properties in hybrid structures. Interface design plays a key role across many interdisciplinary fields such as thermoelectrics, photonics, and spintronics. One approach utilizes molecular films, capitalizing on the ability to design systems with specific optical, thermal, and spin physics. Similarly, topological insulators (TIs) have attracted experimental interest due to their unique band structure. Charge transport is dominated by spin-filtered edge states with suppressed scattering. These edge states represent a fascinating opportunity for research in low-loss electronics. Bismuth selenide (Bi₂Se₃) is one such material that is a well-developed TI system. Modifying TIs via electric field gating and doping has been shown to have a variety of applications. Increasing the bandgap (in order to inhibit bulk transport) can result in tuneable surface states and enhanced thermoelectric figure of merits. Here, we report on the growth of Bi₂Se₃/C₆₀/MnPc (TI-n-p) and Bi₂Se₃/MnPc/C₆₀ thin-film heterostructures grown by van der Waals epitaxy. The molecular diode junction formed between the fullerene and phthalocyanine molecule electrically gates the surface of the TI leading to large changes in the measured Hall carrier concentration and mobility. Our findings demonstrate the ability of functional molecular overlayers in tuning the electronic properties of 2-D materials for a range of applications.