Published: October 13, 2011 r2011 American Chemical Society 4555 dx.doi.org/10.1021/nl2030823 | Nano Lett. 2011, 11, 4555–4561 LETTER pubs.acs.org/NanoLett Chemical Transformation of Au-Tipped CdS Nanorods into AuS/Cd Core/Shell Particles by Electron Beam Irradiation Marijn A. van Huis,* ,†,‡,r Albert Figuerola, §,||,r Changming Fang, †,^ Armand B  ech  e, #,‡ Henny W. Zandbergen, † and Liberato Manna* ,†,§ † Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands ‡ EMAT, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium § Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy ) Departament de Química Inorg  anica, Universitat de Barcelona, Martí i Franqu es 1-11, 08028 Barcelona, Spain ^ Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands # FEI Company, Achtseweg Noord 5, 5600 KA Eindhoven, The Netherlands b S Supporting Information W et chemistry techniques nowadays enable the fabrication of a wide variety of heterogeneous nanostructures, consisting of two or more nanoscale materials which can be assembled in various three-dimensional configurations. Examples are coreÀ shell nanoparticles, dumbbells, nanorods with tips, and branched nanocrystals. 1À6 The plethora of possibilities to design nanoscale particles has led to new nanomaterials with a wide range of physical properties. 5À7 SemiconductorÀmetal heterostructures such as the CdS/Au system are promising materials for photo- voltaics, photocatalysis, and optoelectronics. 8À13 Their interest- ing optical properties stem from a different spatial distribution of electron and hole charge carriers. In the case of nanoscale Au/ CdS core/shell particles it is found that the electrons are mainly confined in the Au domain, while the holes are confined in the CdS domain, 14,13 which leads to long radiative lifetimes. De Paiva and Di Felice, who studied the electronic structure of the CdS/ Au interface by means of first-principles calculations, predicted orbital mixing effects between S and Au, 15 which was actually confirmed by Khon et al. who found experimentally that strong electronic coupling occurs between the CdS and Au domains in CdS/Au nanorods. 13 In a recent work, Chen et al. reported the synthesis of hybrid CdSÀAu 2 SÀAu nanostructures obtained starting from CdSÀAu nanocrystals via an inter-nanocrystal Cd 2+ ÀAu + cation exchange process. 16 The control over the selective location of metallic domains on the surface of semiconductor nanocrystals and the quality of the interface between them are also important issues when consider- ing the possible use of such types of heterostructures in electronic devices. In an earlier work, we studied the ripening process of Au dots on CdSe nanorods as a result of thermal heating, and found that ripening takes place through a combined process of atomic and Au cluster diffusion, and eventually resulted in a well-defined epitaxial orientation relationship between the CdSe nanorod and the Au tips. 11 Besides thermal treatments, there is an increasing number of examples in the literature of the so-called radiation- induced annealing, whereby photon or electron beams induce ripening processes leading to the selective nucleation and growth of Au domains in nanostructures. 9,10,17 For instance, in the speci- fic case of Au-tipped CdS nanorods with different seed materials (CdS, ZnTe, and CdSe), Menagen et al. studied the ripening process as a result of thermal heating and of light illumination. 9,10 Recently, Carbone et al. were able to grow large gold domains exclusively on one side of CdS or CdSe/CdS quantum rods via a photoreduction process of gold ions under anaerobic conditions. 18 Received: May 14, 2011 Revised: October 10, 2011 ABSTRACT: We demonstrate that electron irradiation of colloidal CdS nanorods carrying Au domains causes their evolution into AuS/Cd core/shell nanoparticles as a result of a concurrent chemical and morphological transformation. The shrinkage of the CdS nanorods and the growth of the Cd shell around the Au tips are imaged in real time, while the displace- ment of S atoms from the CdS nanorod to the Au domains is evidenced by high-sensitivity energy-dispersive X-ray (EDX) spectroscopy. The various nanodomains display different susceptibility to the irradiation, which results in nanoconfigurations that are very different from those obtained after thermal annealing. Such physical manipulations of colloidal nanocrystals can be exploited as a tool to access novel nanocrystal heterostructures. KEYWORDS: Colloidal heteronanocrystals, chemical transformations, structural transformations, electron irradiation, transmis- sion electron microscopy