Electrostatic Force Microscopy Study of Single Au-CdSe Hybrid Nanodumbbells: Evidence for Light-Induced Charge Separation Ronny Costi, † Guy Cohen, ‡ Asaf Salant, † Eran Rabani, ‡ and Uri Banin* ,† Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew UniVersity, Jerusalem 91904, Israel, and The School of Chemistry, The Raymond and BeVerly Sackler Faculty of Exact Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel Received January 29, 2009; Revised Manuscript Received March 13, 2009 ABSTRACT Electrostatic force microscopy is used to study light-induced charging in single hybrid Au-CdSe nanodumbbells. Upon illumination, nanodumbbells show negative charging, which is in contrast with CdSe rods and Au particles that show positive charging. This different behavior is attributed to charge separation in the nanodumbbells, where after excitation the electron is transferred to the gold tips and the hole is subsequently filled through tunneling interactions with the substrate. The process of light-induced charge separation at the metal-semiconductor interface is key for the photocatalytic activity of such hybrid metal-semiconductor nanostructures. Hybrid nanoparticles, constructed from domains of different materials, have been studied with great interest over the past few years. Different material combinations such as metal-semiconductor, 1-8 semiconductor-semiconductor 9,10 and metal-magnet 7,11-14 have been demonstrated. Nano- dumbbells (NDBs) are such hybrid nanoparticles, consisting of semiconductor nanorod bodies and selectively grown metal tips, usually made of cadmium selenide and gold, respec- tively. 1 The synthesis of these hybrid nanostructures intro- duced a unique model system for nanometric metal- semiconductor interfaces, allowing a thorough investigation into their physical attributes. 15 In such metal-semiconductor hybrid nanoparticles the metal domains may serve for electrical connections and as anchoring sites for assembly of different structures. 16 Photocatalytic processes are also of interest in metal-semiconductor systems, due to the tendency of light-induced charge separation in the system and to the catalytic properties of the metal islands. Most studies of metal-semiconductor hybrid nanoparticles so far have focused on systems involving wide gap semi- conductors, such as TiO 2 17-19 and ZnO, 20 with precious metals such as gold 19 or silver. 17,18 The need to take advantage of a wider part of the solar spectrum for photo- catalysis has led to an ongoing search for systems composed of lower-gap semiconductors such as CdSe 21,22 or CdS. 23,24 The hybrid systems studied in the past consisted of either separated semiconductor and metal quantum dots brought together by bifunctional linker molecules, thus creating spatially separated pairs or small groups of the different quantum dots, 19 or a solution containing both semiconductor and metal quantum dots. More recently hybrid nanocom- posites, consisting of a single nanostructure with two or more areas of different materials, have arisen much interest in the catalytic and photocatalytic fields. 21,25-28 For example, ZnO nanoparticles with gold and silver islands were investigated and when illuminated above the bandgap in the presence of a hole acceptor, electron transfer to the metal islands was identified, leading to Fermi level equilibration between the semiconductor and the metal. 20 The photocatalytic effect in such systems consists of irradiating the particles followed by a charge separation between the metal and semiconductor islands. 29 For example, following light excitation the hole stays on the semiconductor while the electron is transferred to the metal. The hole can be scavenged by a hole acceptor in the solution, while the electron is transferred through the metal part to an oxidizing agent such as C 60 19 or methylene blue dye molecule. 21,22 A previous photocatalytic study of CdSe-Au NDBs demon- strated light-induced charge separation that allowed for a photocatalytic reduction of methylene blue dye molecules at low yields. 21 Light-induced charge retention in which * To whom correspondence should be addressed. E-mail: banin@chem.ch. huji.ac.il. † The Hebrew University. ‡ Tel Aviv University. NANO LETTERS 2009 Vol. 9, No. 5 2031-2039 10.1021/nl900301v CCC: $40.75  2009 American Chemical Society Published on Web 03/25/2009