NANO LETTERS
Electrostatic Force Microscopy Study of Single Au-CdSe Hybrid Nanodumbbells: Evidence for Light-Induced Charge Separation
2009 Vol. 9, No. 5 2031-2039
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-semiconductor9,10 and metal-magnet7,11-14 have been demonstrated. Nanodumbbells (NDBs) are such hybrid nanoparticles, consisting of semiconductor nanorod bodies and selectively grown metal tips, usually made of cadmium selenide and gold, respectively.1 The synthesis of these hybrid nanostructures introduced a unique model system for nanometric metalsemiconductor 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 semiconductors, such as TiO217-19 and ZnO,20 with precious metals such as gold19 or silver.17,18 The need to take advantage of a wider part of the solar spectrum for photocatalysis has led to an ongoing search for systems composed * To whom correspondence should be addressed. E-mail:
[email protected]. huji.ac.il. † The Hebrew University. ‡ Tel Aviv University. 10.1021/nl900301v CCC: $40.75 Published on Web 03/25/2009
2009 American Chemical Society
of lower-gap semiconductors such as CdSe21,22 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 nanocomposites, 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 C6019 or methylene blue dye molecule.21,22 A previous photocatalytic study of CdSe-Au NDBs demonstrated 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
Figure 1. (a) A scheme of the EFM setup used demonstrating a two pass scan of each line with bias application in the interleaved scan. (b) TEM image of hybrid CdSe-Au nanodumbbells used in this work. (c) AFM tapping mode topography image of nanodumbbells with the corresponding phase image (d) showing contrast difference between the gold tips (white arrows) and the CdSe rods.
preirradiated NDBs in the presence of ethanol as a hole acceptor reduced methylene blue molecules even after the photoexcitation was turned off was also observed. Another experiment used CdSe-Pt hybrid nanoparticles that demonstrated catalytic activities that depended on the morphology of the hybrid systems.22 The photocatalytic properties of the hybrid NDBs raise the question of the physical properties of charge separation in NDBs including its mechanism and extent. It is possible to sense the surface charges, to measure the electrostatic force fields created by them, and to quantify the charging effects of a single nanodumbbell by using electrostatic force microscopy (EFM). This method utilizes an atomic force microscopy (AFM) conductive tip to sense the long-range electrostatic forces at a set distance from the sample. Different approaches to the measurement of electrostatic forces exist,30-34 all using an AFM tip oscillated and raised above the sample. In this study we utilize a method developed by Brus and co-workers30,32,35,36 that was previously applied to different semiconductor quantum dot systems. 2032
In the method chosen, AC bias at frequency ω is applied to the elevated tip and the contribution to the electrostatic forces arising from polarizability and from charges can be separated; the signal measured at 2ω is mainly attributed to polarizability while the signal at ω is attributed to the charge. Quantification of the amount of charge carriers in the sample can be performed by calibrating the tip-substrate capacitance in advance. Previous EFM studies on CdSe quantum dots have shown a positive charging under illumination due to photoionization.35,36 EFM studies on CdSe rods without light excitation were also reported.37 The goal of the present study is to understand the role of hybrid systems, and in particular the effect of the gold tips, on the charging properties of the nanoparticles. This will aid in developing systems where photocatalysis is controlled by the composition of the semiconducting and conducting materials. The EFM experimental setup is portrayed in Figure 1a. AFM and EFM images were obtained at room temperature under N2 flow leading to
