III−V Nanocrystals Capped with Molecular Metal Chalcogenide Ligands: High Electron Mobility and Ambipolar Photoresponse Wenyong Liu, †,§ Jong-Soo Lee, †,§ and Dmitri V. Talapin* ,†,‡ † Department of Chemistry and James Frank Institute, University of Chicago, Illinois 60637, United States ‡ Center for Nanoscale Materials, Argonne National Lab, Argonne, Illinois 60439, United States * S Supporting Information ABSTRACT: In this work, we synthesized InP and InAs nanocrystals (NCs) capped with different inorganic ligands, including various molecular metal chalcogenide complexes (MCCs) and chalcogenide ions. We found that MCCs and chalcogenide ions can quantitatively displace organic ligands from the surface of III−V NCs and serve as the inorganic capping groups for III−V NC surfaces. These inorganic ligands stabilize colloidal solutions of InP and InAs NCs in polar solvents and greatly facilitate charge transport between individual NCs. Charge transport studies revealed high electron mobility in the films of MCC-capped InP and InAs NCs. For example, we found that bridging InAs NCs with Cu 7 S 4 − MCC ligands can lead to very high electron mobility exceeding 15 cm 2 /(V s). In addition, we observed unprecedented ambipolar (positive/negative) photoresponse of MCC-capped InAs NC solids that changed sign depending on the ligand chemistry, illumination wavelength, and doping of the NC solid. For example, the sign of photoconductance of InAs NCs capped with Cu 7 S 4 − or Sn 2 S 6 4− ions converted from positive at 0.80 and 0.95 eV to negative at 1.27 and 1.91 eV. We propose an explanation of this unusually complex photoconductivity of InAs NC solids. 1. INTRODUCTION III−V semiconductors such as GaAs, InP, and InAs combine direct band gap with very high mobility, reliable p- and n-type doping, and other characteristics that make them excellent materials for various electronic and photonic applications. 1−3 The fastest commercial transistors and the most efficient solar cells employ III−V semiconductors. At the same time, despite all the benefits, technological difficulties associated with growing and processing single crystals do not allow III−V materials to successfully compete with Si and II−VI semi- conductors for large consumer markets. As a possible solution, colloidal nanocrystals (NCs) such as InAs and InP can be used as a cost-efficient alternative to III−V single crystals for applications in photovoltaics, photo detectors, field-effect transistors (FETs), 4,5 and light-emitting diodes (LEDs). 6,7 Moreover, many III−V nanomaterials also show relatively low toxicity (e.g., InP) that provides an important advantage over Cd- and Pb-based NCs. From this perspective, switching to InP and InAs NCs instead of CdSe and PbSe NCs would constitute a significant step forward in the manufacturability of functional nanomaterials and their transitioning from laboratory to real- world applications. Most device applications of semiconductor NCs require efficient injection or extraction of charge carriers. Traditional colloidal synthesis of III−V NCs requires the use of surface ligands with long hydrocarbon tails (e.g., tri-n-octylphosphine (TOP) or myristate) that form insulating shells around each NC and negatively affect the charge transport. 8 Removal of organic surfactants via thermal or chemical treatments often leads to surface traps and uncontrollable sintering. 9 A more promising approach is to chemically treat NCs with small or conductive ligands. 10 These techniques have been originally developed for II−VI (e.g., CdSe) 11−14 and IV−VI (e.g., PbSe) 15−17 NCs. There are only a couple of reported charge transport studies for InAs NCs that used either postdeposition ligand exchange treatment of InAs NCs with ethanedithiol 18 or solution ligand exchange of TOP capped InAs NCs with aniline followed by postdeposition cross-linking with ethylenediamine (EDA). 5 Such treatments converted highly insulating organic- capped InAs NCs into semiconducting NC solids with the charge carrier mobility on the order of 10 −5 cm 2 /(V s). 12 Such mobilities show certain promise but are not yet sufficient for practical applications. Equally important, small organic ligands often are chemically unstable or volatile and therefore impart instability to the electronic properties of NC solids. It has been recently shown that insulating organic ligands can be replaced with the inorganic ligands, such as molecular metal chalcogenide complexes (MCCs; e.g., Sn 2 S 6 4− , In 2 Se 4 2− , etc.), 19,20 chalcogenide ions (S 2− , Se 2− , and Te 2− ), 17 and other charged small anions such as SCN − . 21 These develop- ments in the surface chemistry of II−VI and IV−VI NCs enabled strong electronic coupling between individual NCs and opened new prospects for electronic and optoelectronic applications of NC solids. 22,23 In this contribution, we explore the application of inorganic ligands to more covalent III−V Received: August 17, 2012 Published: December 26, 2012 Article pubs.acs.org/JACS © 2012 American Chemical Society 1349 dx.doi.org/10.1021/ja308200f | J. Am. Chem. Soc. 2013, 135, 1349−1357