ROWLAND ET AL. VOL. 8 ’ NO. 1 ’ 977 –985 ’ 2014 www.acsnano.org 977 December 16, 2013 C 2013 American Chemical Society Thermal Stability of Colloidal InP Nanocrystals: Small Inorganic Ligands Boost High-Temperature Photoluminescence Clare E. Rowland, † Wenyong Liu, ‡ Daniel C. Hannah, † Maria K. Y. Chan, § Dmitri V. Talapin, ‡,§ and Richard D. Schaller †,§, * † Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States, ‡ Department of Chemistry and James Frank Institute, University of Chicago, Chicago, Illinois 60637, United States, and § Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States I nterest in semiconductor nanocrystals (NCs) arises from promise regarding their practical application in areas as diverse as light-emitting diodes (LEDs), 1À3 lasers, 4,5 concentrated solar cells, 6,7 bio- labels, 8 electronics, 9 and sensors. 10,11 How- ever, surface passivation methods that max- imize photoluminescence (PL) efficiency in some instances also impede utility in these very applications. For example, long-chain aliphatic capping ligands are commonly bound to a NC core, yet their presence creates an insulating barrier that encom- passes the particle, effectively impeding the injection and extraction of electrons and holes. The addition of a wide band- gap type-I shell (e.g. , ZnS) to the NC is often a more effective means of surface passivation (i.e. , results in higher PL quan- tum yields), yet it also often contributes a signi ficant barrier to electronic contact with the core. Thus, in both traditional core and core/shell NCs, surface passivation is typically accomplished by sacrificing car- rier transport. Additionally, a variety of practical appli- cations subject NCs to elevated operating temperatures, 2,3,5,6 and others require pro- cessing at elevated temperatures prior to use, both conditions under which signifi- cant loss in PL efficiency occurs for reasons that are the focus of current study. 12À20 Volatility or decomposition of organic com- pounds, and the associated loss of surface passivation, diminishes the utility of organi- cally capped NC cores under these cir- cumstances, 21,22 while core/shell structures appear less susceptible to the effects of elevated temperature. 12 Small inorganic ligands have recently garnered interest as a new surface passiva- tion option, one that achieves electronic passivation while providing a conductive rather than insulating surface and interstitial medium. 21,23À25 Among inorganic capping ligands, molecular metal chalcogenides * Address correspondence to schaller@anl.gov, schaller@northwestern.edu. Received for review November 8, 2013 and accepted December 16, 2013. Published online 10.1021/nn405811p ABSTRACT We examine the stability of excitons in quantum-confined InP nanocrystals as a function of temperature elevation up to 800 K. Through the use of static and time-resolved spectroscopy, we find that small inorganic capping ligands substantially improve the temperature dependent photoluminescence quantum yield relative to native organic ligands and perform similarly to a wide band gap inorganic shell. For this composition, we identify the primary exciton loss mechanism as electron trapping through a combination of transient absorption and transient photoluminescence measurements. Density functional theory indicates little impact of studied inorganic ligands on InP core states, suggesting that reduced thermal degradation relative to organic ligands yields improved stability; this is further supported by a lack of size dependence in photoluminescence quenching, pointing to the dominance of surface processes, and by relative thermal stabilities of the surface passivating media. Thus, small inorganic ligands, which benefit device applications due to improved carrier access, also improve the electronic integrity of the material during elevated temperature operation and subsequent to high temperature material processing. KEYWORDS: colloidal nanomaterials . thermal stability . inorganic ligands . transient absorption . static . time-resolved photoluminescence . PL quenching . semiconductors . InP ARTICLE