Adsorption and sensing of CO and NH₃ on chemically modified graphene surfaces

We have studied the electronic structure and adsorption characteristics of environmentally potent gaseous molecules like carbon monoxide (CO) and ammonia (NH₃) on chemically modified surfaces of graphene, employing ab initio density functional methods. An insight into the changes made in the electronic band structure due to intrinsic and extrinsic doping and through a combined effect of both is discussed. With this regard, the adsorption of these gaseous moieties is investigated on substitutionally p- and n- doped graphene surfaces, doped with various mole fractions and having different configurational patterns on the surface. Even though the electronic properties are modified with various mole fractions of doping they do not show a methodical increase with the increase in the dopant concentration. This is attributed to the sub-lattice induced symmetry breaking for the dopant configurations where equivalent lattice sites are occupied on the surface. An appreciable band gap opening of around 0.63 eV is observed on doping, due to sub-lattice symmetry breaking. This is further improved on molecular doping, with CO and NH₃, where an increase up to 0.83 eV is noted with adsorption of ammonia. While both the molecules are physisorbed on nitrogen doped surfaces, carbon monoxide is strongly physisorbed and ammonia molecules are chemisorbed on a few boron doped surfaces resulting in notable changes in the adsorption energy. Therefore, it is clear that changes in the transport properties can be brought about by adsorption of these molecules on such surfaces and this study clearly indicates the invaluable prospects of such doped surfaces as potential sensors for these molecules.