Exciton Dynamics in CdS-Ag 2 S Nanorods with Tunable Composition Probed by Ultrafast Transient Absorption Spectroscopy Paul Peng, † Bryce Sadtler, †,‡ A. Paul Alivisatos, †,‡ and Richard J. Saykally* ,†,§ Department of Chemistry, UniVersity of California, Berkeley, California 94720, Materials Sciences DiVision, Lawrence Berkeley National Laboratory, California 94720, and Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, California 94720 ReceiVed: December 9, 2009; ReVised Manuscript ReceiVed: February 11, 2010 Electron relaxation dynamics in CdS-Ag 2 S nanorods have been measured as a function of the relative fraction of the two semiconductors, which can be tuned via cation exchange between Cd 2+ and Ag + . The transient bleach of the first excitonic state of the CdS nanorods is characterized by a biexponential decay corresponding to fast relaxation of the excited electrons into trap states. This signal completely disappears when the nanorods are converted to Ag 2 S but is fully recovered after a second exchange to convert them back to CdS, demonstrating annealing of the nonradiative trap centers probed and the robustness of the cation exchange reaction. Partial cation exchange produces heterostructures with embedded regions of Ag 2 S within the CdS nanorods. Transient bleaching of the CdS first excitonic state shows that increasing the fraction of Ag 2 S produces a greater contribution from the fast component of the biexponential bleach recovery, indicating that new midgap relaxation pathways are created by the Ag 2 S material. Transient absorption with a mid-infrared probe further confirms the presence of states that preferentially trap electrons on a time scale of 1 ps, 2 orders of magnitude faster than that of the parent CdS nanorods. These results suggest that the Ag 2 S regions within the heterostucture provide an efficient relaxation pathway for excited electrons in the CdS conduction band. I. Introduction The size, shape, and chemical composition of semiconductor nanocrystals control their resulting optoelectronic properties. For instance, in both semiconductor alloys and heterostructures, the band gap absorption and emission energies can be widely tuned through the spatial variation of composition. 1-8 The composition of colloidal nanocrystals is typically mediated by the time- dependent variation of cation and anion precursor concentrations during solution-phase nucleation and growth. 1,2 Alternatively, nanocrystals of a desired shape and size can be chemically transformed, postsynthesis, through a solid-state reaction into a new composition. Notably, in ionic nanocrystals, cation exchange can be used to partially or fully replace the cations within the lattice of the crystal with substitutional cations from solution. 6-12 Such solid-state reactions can occur with remark- able efficiency, owing to molecular-like reaction kinetics of nanoscale materials possessing high surface-to-volume ratios. 8-10,13 Furthermore, when the exchange is limited to occur through only a portion of the nanocrystal, heterostructures are produced containing distinct regions of the secondary and host crystals connected by a continuous anion framework. Several recent reports have characterized the structural and static optical properties of semiconductor nanocrystals produced by cation exchange. 7-11 Transmission electron microscopy (TEM) and X-ray diffraction (XRD) show that the crystals retain their size and shape and are highly crystalline after the exchange reaction. However, such techniques are rather insensitive to local defects that may be introduced, and so far, there has not been a systematic determination of how such chemical transforma- tions affect exciton dynamics in the nanocrystals. There is evidence from related systems that the structural disorder (such as vacancies or substitutional impurities) introduced by the interdiffusion of ions can lead to additional relaxation pathways for photoexcited charge carriers. 14,15 We report pump-probe measurements in CdS-Ag 2 S nano- rods to characterize the effect that cation exchange has on the relaxation of excited carriers. We have chosen this material system as the relative fraction of the two semiconductors can be readily controlled by the amount of Ag + that is added to the CdS nanorods (or conversely the amount of Cd 2+ added to Ag 2 S nanorods). 8,16 Moreover, the combination of semiconductors that possess a type I (sandwiched) electronic band alignment can lead to the transfer of excited electrons and holes in the wide band gap material (CdS) to the smaller band gap material (Ag 2 S), provided that competing relaxation pathways are inhibited. 8 We study carrier relaxation dynamics by monitoring the transient bleaching of the first excitonic transition (1S e ), and the intraband absorption (1S e -1P e ) of electrons in the conduc- tion band of CdS nanorods. The change in time constants of the relaxation dynamics following complete and cyclic conver- sion between CdS and Ag 2 S nanorods (i.e., forward conversion from CdS to Ag 2 S and back conversion to CdS) is used to characterize how the process of cation exchange can lead to new relaxation pathways through introduction of defect states. Partial cation exchange to make CdS-Ag 2 S heterostructures demonstrates how the presence of Ag 2 S affects the relaxation of excited carriers in the CdS material. II. Experimental Methods A. CdS Synthesis. Colloidal CdS nanorods were synthesized according to previously published procedures. 8,10 A detailed description of the reaction conditions is provided in the * To whom correspondence should be addressed. E-mail: saykally@ berkeley.edu. † University of California, Berkeley. ‡ Materials Sciences Division, Lawrence Berkeley National Laboratory. § Chemical Sciences Division, Lawrence Berkeley National Laboratory. J. Phys. Chem. C 2010, 114, 5879–5885 5879 10.1021/jp9116722  2010 American Chemical Society Published on Web 03/04/2010