Covalent grafting of aryls to modulate the electrical properties of phthalocyanine-based heterostructures: Application to ammonia sensing

Abstract

Covalent grafting of small organic molecules on the conducting surface is a smart strategy to tune the interfacial electrical properties of electronic devices. In this endeavor, the present work reports electrografting of a series of aryls, bearing different substituents on ITO electrode, which are further used to fabricate heterojunction devices for ammonia sensing. Electrografting is performed by electroreduction of respective aryldiazonium salt with synchronous measurement of surface coverage by electrochemical quartz crystal microbalance. It reveals a strong influence of substituents on the mass loading, such that methoxy-bearing aryl forms multilayers, while a hydrophobic or bulky group at para position prefers monolayer growth. The organic coatings possess high surface passivating properties, highlighted by the diminishing of electron transfer process of a redox probe over them and contain many structural defects in the form of azo and imine nitrogen functionalities. The electrografted aryls are incorporated into organic heterojunction devices in combination with lutetium bis-phthalocyanine (LuPc2), displaying accumulation of mobile charges at aryl-LuPc2 interface and a bias-dependent interfacial conductivity, while a bias-independent bulk conductivity. The heterojunction devices clearly show an improvement in ammonia sensing properties compared to LuPc2 resistor, demonstrating a reversible, repeatable and faster response with low interference from humidity fluctuation.