Physics

Dr. Aparna Deshpande
IISER Pune

Abstract - Molecular assembly on a surface presents a rich testbed to probe intermolecular interactions and molecule-surface interactions. Such interactions are pertinent to understand the characteristics of the molecules and the host surfaces, which can then be used to functionalize 
surfaces for targeted applications. In this talk I will present a study of phthalocyanine molecules on metal surface - Au(111)  and topological insulator surface- Bi2Se3  using low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Starting with copper phthalocyanine – CuPc  and its cyano analogue – CuPc(CN)8 we observe the self-assembly characterized by an organizational chirality for both these molecules on Au(111). Superimposed on this is an orbital energy dependent chirality seen only in the case of CuPc(CN)8. STS measurements show an increment in the highest occupied–lowest unoccupied molecular orbital (HOMO–LUMO) gap from CuPc to CuPc(CN)8. Extensive ab initio calculations within density functional theory (DFT) match well with the experimental observations. For the topological insulator surface- 
Bi2Se3, known for its topologically protected surface state that prevents backscattering from nonmagnetic impurities, CuPc shows a peculiar self-assembly along with single CuPc adsorption. STS measurements show 
a shift of the Dirac point for this system that is suggestive of charge transfer. Finally, for the magnetic iron phthalocyanine – FePc, its adsorption on Bi2Se3 keeps the Dirac point intact indicating that the topological surface state remains protected.