Published: July 07, 2011 r2011 American Chemical Society 15834 dx.doi.org/10.1021/jp2035604 | J. Phys. Chem. C 2011, 115, 15834–15844 ARTICLE pubs.acs.org/JPCC Interplay of Quenching and Enhancement Effects in Apertureless Near-Field Fluorescence Imaging of Single Nanoparticles Eyal Yoskovitz, Ido Hadar, Amit Sitt, Itai Lieberman, and Uri Banin* Institute of Chemistry and The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel 1. INTRODUCTION The ability to bring a nanometric probe into close proximity to nanostructures has paved the way for a wide range of applications and experiments. Scanning probe methods, such as atomic force microscopy (AFM) or scanning tunneling microscopy (STM), were used for the more common measurements, such as topo- graphy and conductivity, but also for high-end applications, such as molecular recognition, 1 manipulating and controlling the position of single particles in two and three dimensions, 2À4 writing nanocircuits, 5 controlling the emission of a single emitter, 6 and more. In the field of optics, the interaction of a nanoprobe with light and a single nanoparticle led to the development of subdi ffraction limit imaging techniques with nanoscale resolution, 7À18 generally known as apertureless near-field scanning optical microscopy (ANSOM). In these methods, the interaction of the AFM tip with the emitter modifies its fluorescence intensity accompanied by changes in the measured lifetime. The emission of the emitter may be either quenched 7,10,14À18 or enhanced 8,9,11À13,16,18 by the tip. The specificeffect in- duced by the tip depends on several parameters; the important ones are the tip material and geometry, 19À21 the excitation and emission wavelength and polarization, 22À27 the particle quan- tum yield, 24,28,29 and the distance between the tip and the emitter. 8,17,18,24 To provide a better understanding of the phenomena taking place, several papers and reviews—both theoretical and experi- mental—were published in the recent years, providing insight on the interaction of a single emitter with a nanostructure. In many of these cases, the emitter was a single dye molecule, usually embedded in a thin layer of a polymer, 12,13,18,20,21,23,24,28,30À34 interacting with a nanostructure that was either the tip itself or a single, spherical metal nanoparticle, connected to an AFM tip or a pointed fiber. By changing the distance between the metallic nanostructure and the emitter in these experiments, it was possible to record the emission intensity and lifetime at different tipÀmolecule separations. Similar experiments were carried out where a single semiconductor nanoparticle (NP) served as the emitter. 7,8,17,27,35,36 NPs possess unique optical characteristics, including size-tunable emission wavelength, photostability, shape-controlled polarizability, and high quantum yield. The possibility to manipulate, control, and optimize their emission by an adjacent nanostructure requires a deep theoretical and experi- mental understanding of the interaction between the two. The above-mentioned ANSOM experiments performed with single NPs showed imaging capabilities with nanoscale resolution and started to explore the effect of the tip on the NP fluorescence, under the specific experimental conditions. Controlling the dimensions, composition, and shape of NPs enables one to sculpture the electronic profile of the particle and, as a result, control its transition band gap, carrier localization, emission polarization, and lifetime. A large variety of NPs have already been synthesized and characterized for different optical applications. Seeded nanorods (seeded-NRs), heterostructured semiconductor nanorods with a mixed dimensionality, composed of a spherical seed covered with a rod-shaped shell, constitute an important family of nanocrystals with exceptional optical properties. 37 À39 Received: April 16, 2011 Revised: July 6, 2011 ABSTRACT: We systematically explore the interaction of an AFM tip with single CdSe/CdS quantum dots and seeded CdSe/CdS nanorods. Using distance-dependent intensity and lifetime near-field microscopy in 3D, we analyze the interplay between quenching and enhancement in proximity to the tip. Under tightly focused radially polarized excitation, a nanoscale, central enhancement spot is observed for both types of particles, revealing an identical physical mechanism underlying the near- field interaction in both cases. Furthermore, lifetime and intensity near-field images of both types of nanoparticles exhibit characteristics similar to those of a single molecule with a well-defined molecular dipole. We also investigate the origin of the observed enhancement effect. By exploring the dependence on excitation polarization and tip material, we conclude that the main contribution to the fluorescence enhancement is from excitation field enhancement at the apex of the tip, serving as a lightening rod. However, we also show clear correlation between the particle quantum yield and the measured enhancement factor, providing a direct proof to a limited contribution of emission enhancement as well.