YUAN ET AL. VOL. 6 ’ NO. 1 ’ 176 –182 ’ 2012 www.acsnano.org 176 December 02, 2011 C 2011 American Chemical Society Single-Particle Studies of Band Alignment Effects on Electron Transfer Dynamics from Semiconductor Hetero-nanostructures to Single-Walled Carbon Nanotubes Chi-Tsu Yuan, † Yong-Gang Wang, † Kuo-Yen Huang, † Ting-Yu Chen, † Pyng Yu, † Jau Tang, †, * Amit Sitt, ‡ Uri Banin, ‡, * and Oded Millo §, * † Research Center for Applied Sciences, Academia Sinica, 128 Academia Road, Taipei 115-29, Taiwan, and ‡ Institute of Chemistry and § Racah Institute of Physics and the Center for Nanoscience and Nanotechnology, The Hebrew University, Jerusalem 91904, Israel I norganic colloidal semiconductor quan- tum dots (QDs) exhibit unique optical properties, including large absorption cross sections, broad absorption bands, tun- able band gap by control of size and shape, high photo- and chemical stability, possibi- lity for multiple exciton generation, and hot- electron transfer. 1À3 These properties are beneficial to photovoltaic (PV) operation, and consequently, they are being investi- gated for use as sensitizers for light-harvesting applications. 4,5 In addition, colloidal QDs share all of the main advantages of conven- tional organic dyes, for example, low-cost fabrication, solution processing, and well- controlled synthesis methods. Therefore, colloidal QDs show potential for integration in future generations of solar cell devices. Unfortunately, low efficiencies were re- ported so far for QD-based PV devices, and this is commonly attributed mainly to low electron-injection rates as compared to other competing processes, including radiative and nonradiative recombination. 6 Obviously, further investigation is required in order to gain better understanding of the above fun- damental issues, which may improve the performance of QD-based PV and other optoelectronic devices. The utilization of colloidal QDs in optoe- lectronics was restricted also by poor trans- port properties owing to their insulating surface. For PV operation, the photogener- ated excitons need to be dissociated into free charge carriers, then extracted and redirected to respective electrodes. Unlike pÀn junction-type solar cells, no built-in electric fields can be exploited to separate electronÀhole pairs. In addition, direct charge transport among QDs is difficult. Therefore, additional materials serving as electron acceptors and conductors need to be in- troduced along with the QDs to form PV devices. 1,7 Single-walled carbon nanotubes (SW- CNTs) that have a high aspect ratio and large surface area can accommodate a large amount of sensitizers and at the same time provide direct electrical pathways for charge-carrier transport owing to their one- dimensional nature. 8 CNTs have high elec- tron affinity and exhibit excellent electrical properties, including high electron mobility, * Address correspondence to jautang@gate.sinica.edu.tw, banin@chem.ch.huji.ac.il, milode@vms.huji.ac.il. Received for review August 21, 2011 and accepted November 29, 2011. Published online 10.1021/nn2036957 ABSTRACT We utilize single-molecule spectroscopy combined with time-correlated single-photon counting to probe the electron transfer (ET) rates from various types of semiconductor hetero-nanocrystals, having either type-I or type-II band alignment, to single-walled carbon nanotubes. A significantly larger ET rate was observed for type-II ZnSe/CdS dot-in-rod nanostructures as compared to type-I spherical CdSe/ZnS core/shell quantum dots and to CdSe/CdS dot-in-rod structures. Furthermore, such rapid ET dynamics can compete with both Auger and radiative recombination processes, with significance for effective photovoltaic operation. KEYWORDS: single-molecule spectroscopy . colloidal nanocrystals . carbon nanotubes ARTICLE