Quantification of electronic band gap and surface states on FeS2(100)

a r t i c l e i n f o The interfacial electronic properties and charge transfer characteristics of pyrite, FeS 2 , are greatly influenced by the presence of electronic states at the crystal free surface. We investigate the surface electronic structure of FeS 2 (100) using scanning tunneling spectroscopy (STS) and interpret the results using tunneling current simulations informed by density functional theory. Intrinsic, dangling bond surface states located at the band edges reduce the fundamental band gap E g from 0.95 eV in bulk FeS 2 to 0.4 ± 0.1 eV at the surface. Extrinsic surface states from sulfur and iron defects contribute to Fermi level pinning but, due to their relatively low density of states, no detectable tunneling current was measured at energies within the intrinsic surface E g. These findings help elucidate the nature of energy alignment for electron transfer processes at pyrite surfaces, which are relevant to evaluation of electrochemical processes including corrosion and solar energy conversion. Pyrite or FeS 2 is a semiconducting mineral for which the electronic structure has been intensively studied in relation to reactivity in geo-chemical [1–4] and bio-catalytic [5–7] processes, as well as for photovol-taic (PV) and photoelectrochemical properties [8–12]. Heterostructures of FeS 2 and perovskite oxides such as LaAlO 3 have recently been proposed as promising devices for spintronics applications [13]. FeS 2 is also known to form in anoxic, H 2 S-containing environments such as those encountered by the oil and gas industry, where it is typically incorporated into passive corrosion films on steel structures [14]. In the following, we review the literature and discuss the surface electronic structure of pyrite and its characterization by scanning tunneling microscopy and density functional theory calculations. 1.1. Surface electronic structure of pyrite Despite this wide ranging scientific interest in pyrite, important questions remain regarding the fundamental electronic properties of its free surface, which is critical towards understanding how energy levels align during interfacial charge exchange with reduction–oxidation (redox) species in the surrounding environment. For example, the reactivity of semiconducting materials can be significantly altered by surface states that are either intrinsic to the crystal termination or have arisen from the presence of crystalline defects at the surface, such as steps, kinks, dis-locations, impurities or vacancies [15,16]. Moreover, in the context of PV, low open circuit voltages (VOC) of b200 mV (or ~21% of the widely accepted bulk band gap …