Control of PbSe Nanorod Aspect Ratio by Limiting Phosphine Hydrolysis Janice E. Boercker,* Edward E. Foos, Diogenes Placencia, and Joseph G. Tischler United States Naval Research Laboratory, Washington, D.C. 20375, United States * S Supporting Information ABSTRACT: The aspect ratio and yield of PbSe nanorods synthesized by the reaction of Pb-oleate with tris(diethylamino)- phosphine selenide are highly sensitive to the presence of water, making it critical to control the amount of water present in the reaction. By carefully drying the reaction precursors and then intentionally adding water back into the reaction, the nanorod aspect ratio can be controlled from 1.1 to 10 and the yield from 1 to 14% by varying the water concentration from 0 to 204 mM. 31 P{ 1 H} and 1 H NMR show that water reacts with tris- (diethylamino)phosphine to create bis(diethylamido)phosphorous acid. It was determined that bis(diethylamido)phosphorous acid is responsible for the observed aspect ratio and yield changes. Finally, it was found that excess oleic acid in the reaction can also react with tris(diethylamino)phosphine to create bis(diethylamido)phosphorous acid, and upon the removal of both excess oleic acid and water, highly uniform, nonbranching nanorods were formed. ■ INTRODUCTION Due to their exceptional physical properties, PbSe nanorods are attractive components for use in optoelectronic devices such as photovoltaics and light emitters. 1-3 Recently, Koh et al. 4 synthesized single-crystal, PbSe nanorods with homogeneous size distributions (5% diameter and 20% length) 5 using a catalyst-free, one-pot, solution chemistry method. The develop- ment of this synthesis has allowed PbSe shape-dependent optical properties to be explored which were previously unattainable due to the lack of high-quality materials. For example, it has been recently shown that PbSe nanorods have larger Stokes shifts, 6 more efficient multiple exciton gener- ation, 1,2,7 higher absorption coefficients, 1,7 and longer effective biexciton lifetimes 3,7 than PbSe nanocrystals. Additionally, PbSe nanorods are the first material to demonstrate the splitting of the Frö hlich mode in anisotropic structures. 5 While PbSe nanorods with aspect ratios of ∼1.5-12, 7 with diameters from ∼2.8 to 5.6 nm and lengths from 6 to 47 nm, 7,8 have been synthesized, it has been difficult for us as well as other groups to reproduce such a wide aspect ratio range. 3 Therefore, there is a need for a greater understanding of the formation mechanism of PbSe nanorods such that uniform nanorods with aspect ratios over a broad range can be controllably synthesized. In this paper, we show for the first time that the amount of water present in the reaction, either added intentionally or present as a contaminate, has a dramatic effect on both the PbSe nanorod morphology and yield. We found that if the amount of water in the reaction is not controlled, the small amount present in the nanorod precursors is sufficient to create a large irreproducibility from batch to batch. It is thus critical to control the amount of water in the reaction in order to control the nanorod morphology. If the precursors are first carefully dried and water is intentionally added to the reaction at concentrations from 0 to 204 mM, the nanorod aspect ratio can be precisely controlled from 1.1 to 10 and the yield from 1 to 14%. This result is particularly surprising since water has little effect on the common PbSe nanocrystal synthesis reaction. 9,10 We have found that the role of water is indirect; i.e., hydrolysis of tris(diethylamino)phosphine used in the reaction forms bis(diethylamido)phosphorous acid. By synthesizing this latter compound independently and adding it to the nanorod reaction, we have identified it as responsible for both the nanorod aspect ratio and yield variation. Our results add to the growing consensus 9,10 that drying metal oleates in situ by simple heating under an inert atmosphere is insufficient to obtain completely anhydrous reaction conditions. Additionally, we found that excess oleic acid in the reaction undergoes similar chemistry. Upon removal of both the excess oleic acid and water, the nanorod reaction slowed considerably, and highly uniform, nonbranching nanorods were formed. These homogeneous nanorods may allow for easier alignment of nanorods over large surface areas for devices. ■ MATERIALS AND METHODS General Considerations. Standard Schlenk-line techniques were used unless otherwise noted. Lead(II) oxide (PbO, 99.999%), 1- octadecene (90%), oleic acid (90%) tris(diethylamino)phosphine (97%), selenium powder (99.5%, 100 mesh), tetrachloroethylene (99%), and diethylamine (99.5%) were purchased from Sigma-Aldrich. Hexanes (98.5%), ethanol (200 proof), and tetrahydrofuran (THF, 99.9%) were purchased from commercial sources. PbO, Se, hexanes, ethanol, THF, and tetrachloroethylene were used without further Received: May 7, 2013 Published: September 13, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 15071 dx.doi.org/10.1021/ja404576j | J. Am. Chem. Soc. 2013, 135, 15071-15076