Charge-Transfer in Organic-TMDC Heterojunctions Probed by SERS*

Heterojunctions fabricated with 2D materials can give rise to unique effects at the interface of the composite layers. Organic molecules have the attraction of synthetically tunable electronic and optical properties and facile processing, which can lead to controllable opto-electronic coupling to 2D substrates such as 2D transition metal dichalcogenides (TMDCs). Phthalocyanines (Pc) are a class of organic dye molecules consisting of a planar macrocycle bonded to a metal core atom. These molecules can be configured with different metal cores to induce distinct phenomena such as metal-insulator transitions and ferromagnetism, potentially providing a mechanism to influence 2D heterostructures. We report surface-enhanced Raman scattering in TMDC/Pc heterojunctions, how it depends on the metal core, the underlying TMDC, layer number, and the excitation laser energy. Observed trends include a strong dependence on the metal core and laser energy, showing up to 40x enhancement in signal intensity. This Raman enhancement in organic/TMDC systems demonstrates the importance of band and energy level alignment on charge transfer across van der Waals interfaces.