Letter pubs.acs.org/NanoLett
Gated Hall Effect of Nanoplate Devices Reveals Surface-StateInduced Surface Inversion in Iron Pyrite Semiconductor Dong Liang, Miguel Cabán-Acevedo, Nicholas S. Kaiser, and Song Jin* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: Understanding semiconductor surface states is critical for their applications, but fully characterizing surface electrical properties is challenging. Such a challenge is especially crippling for semiconducting iron pyrite (FeS2), whose potential for solar energy conversion has been suggested to be held back by rich surface states. Here, by taking advantage of the high surface-to-bulk ratio in nanostructures and effective electrolyte gating, we develop a general method to fully characterize both the surface inversion and bulk electrical transport properties for the first time through electrolyte-gated Hall measurements of pyrite nanoplate devices. Our study shows that pyrite is n-type in the bulk and p-type near the surface due to strong inversion and yields the concentrations and mobilities of both bulk electrons and surface holes. Further, solutions of the Poisson equation reveal a high-density of surface holes accumulated in a 1.3 nm thick strong inversion layer and an upward band bending of 0.9−1.0 eV. This work presents a general methodology for using transport measurements of nanostructures to study both bulk and surface transport properties of semiconductors. It also suggests that high-density of surface states are present on surface of pyrite, which partially explains the universal p-type conductivity and lack of photovoltage in polycrystalline pyrite. KEYWORDS: Gated Hall effect, nanoplate devices, surface states, surface inversion, iron pyrite, solar energy conversion
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the unique opportunity to study both surface and bulk transport properties and explore the limits and concepts for applications.8 An intriguing semiconductor hypothesized to have a large density of surface states is iron pyrite (cubic FeS2). As an earth abundant and inexpensive semiconductor with low toxicity, iron pyrite has attracted resurgent attention as a candidate for solar energy conversion9,10 owing to its suitable band gap (reported to be 0.8−0.95 eV), high absorption coefficient (∼6 × 105 cm−1), and good semiconductor properties.9,11 However, the best reported solar conversion efficiency remains below 3% for single crystal pyrite photoelectrochemical (PEC) cells,9 which is limited by a low open circuit voltage Voc (
