Interplay of Quenching and Enhancement Effects in Apertureless Near

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 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 nearfield 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.

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 topography 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,2
4 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 subdiffraction limit imaging techniques with nanoscale resolution,7
18 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 quenched7,10,14
18 or enhanced8,9,11
13,16,18 by the tip. The specific effect induced by the tip depends on several parameters; the important ones are the tip material and geometry,19
21 the excitation and emission wavelength and polarization,22
27 the particle quantum 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 experimental—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,12,13,18,20,21,23,24,28,30
34 r 2011 American Chemical Society

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 tip
molecule 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 experimental 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
39 Received: April 16, 2011 Revised: July 6, 2011 Published: July 07, 2011 15834

dx.doi.org/10.1021/jp2035604 | J. Phys. Chem. C 2011, 115, 15834–15844

The Journal of Physical Chemistry C In a recent paper, we used distance-dependent, lifetime ANSOM imaging to directly map the emission of these structures and determine its origin with high precision along different types of seeded-NRs.16 It was shown that the fluorescence was emanating from a sub-20 nm region, correlated to the seed location. When examining the tip
particle distance-dependent interaction, we observed that, at this small region, interplay between quenching and enhancement was taking place. Interestingly, the enhancement was confined to the core area rather than spread along the rod, and quenching was not observed at the shortest distances, as could be expected. This nontrivial outcome under the chosen experimental conditions provided motivation to investigate in depth the interaction of the tip with different types of NPs, and the intriguing interplay between quenching and enhancement therein. Here, we present a systematic study of tip
sample interactions for two families of NPs: spherical CdSe (core)/CdS (shell) quantum dots (QDs) and CdSe (core)/CdS (shell) seeded-NRs. The choice of QDs serves here for studying the interaction of the tip with a nanoparticle that is approximately spherical-symmetric, and as a comparison basis for understanding the interaction of the tip with the seeded-NRs, which are more complex in their structure and optical behavior. More specifically, we explored the two opposing effects in the NP fluorescence: enhancement and quenching and the experimental conditions that lead to one or the other. Furthermore, we use unique experimental analysis to characterize the emission enhancement as originating either from enhanced excitation or from enhanced emission. Unlike previous works, we clearly distinguish between the two types of enhancement in the fluorescence of a single particle. We find that the dominant enhancement effect induced by the tip is the enhanced excitation rate, although we also show a clear indication for increased radiative rate indicative of emission enhancement for the very same particle. Uniquely, we experimentally show the increase in emission enhancement correlated in time with the decrease of the quantum yield of the particle. 1.1. Tip
Particle Interaction Principles. The initial theoretical framework used to describe quenching and enhancement of the fluorescence intensity and the accompanying changes in the radiative and nonradiative decay rates of an emitter near a scanning probe was based on models developed for a molecule, represented as an emitting dipole, near a dielectric or metallic interface.40 More recently, the antenna theory was adopted to model the interaction of the emitting molecule with the nanostructure. In this work, we will generally follow the approach taken in the work performed by Sandoghdar and co-workers.20,21,24 This work takes into account the dimensions and, to a certain extent, the geometry of the nanostructures, using Mie Theory. A similar approach, occasionally with some variations, was used in many other recent works.18,27,30,35,41 In the quenching pathway, the emitter relaxes from its excited state back to its ground state by energy transfer to the nanostructure through dipole
dipole interaction. The energy transferred to the nanostructure dissipates nonradiatively. As for the enhancement, within the antenna framework, the nanostructure serves both as a receiver and as a transmitter. As a receiver, it locally enhances the incoming excitation field, thereby increasing the flux of photons in the vicinity of the emitter, resulting in higher excitation rate. As a transmitter, it couples to the electromagnetic field of the emitter transition dipole, accelerating photon radiation and, therefore, increasing the emitter’s radiative rate. This antenna concept was further explored to optimize the

ARTICLE

antenna parameters in order to gain better control of the emitter radiation, and to achieve stronger emission.42
50 The emitter’s fluorescence (S) depends on two factors, the excitation rate (γex) and the particle quantum yield (η): S µ γex 3 η

ð1Þ

The excitation rate depends both on the local field at the emitter location and on the relative orientation of the emitter’s dipole and the excitation field polarization. The emitter excitation rate enhancement factor is, therefore, defined by r Þ ¼ j dB 3 B E loc ð B r Þj2 =j dB 3 B E inc ð B r Þj2 Kex ð dB, B ¼ γex;tip =γex;0

ð2Þ

where d is the emitter transition dipole moment, Eloc and Einc are the local field in the presence of the tip and the incident field without the tip, respectively, r is the distance from the emitter, and γex,tip and γex,0 are the excitation rates in the presence and absence of the tip, respectively. Within the lightening rod effect, when the incident field is polarized parallel to tip long axis, it is expected that the local excitation field at the tip apex will be enhanced. This increased local field is significantly larger when the incident field frequency is close and red shifted relative to the plasmon frequency of the tip material.23,30 The emitter’s quantum yield, η, is given by γr ð3Þ η¼ γr þ γnr where γr and γnr are the radiative and nonradiative decay rates of the emitter, respectively. The quantum yield in the presence of the tip, ηtip, may be smaller than the unperturbed quantum yield, η0, in the case of quenching, or larger than η0 where there is emission enhancement. For the latter, the emission enhancement factor is defined as26,29 ηtip Kem ¼ ð4Þ η0 As the tip approaches the emitter, both the radiative and the nonradiative decay rates increase under nonsaturation conditions. Only when the unperturbed quantum yield is significantly less than one, the overall fluorescence of the emitter may be enhanced due to the increase in the radiative rate. Theory predicts that, in short nanostructure
emitter distances, typically less than 5 nm, quenching dominates over enhancement. The overall observed enhancement factor is the multiplication of the different effects that might oppose each other and, therefore, may be much lower than Kex when quenching occurs and Kem < 1. The experimental work shows good agreement with the above theory, mostly for dye molecules emission proximal to a nanostructure.12,18,22,24,32,35 In previous work, focused on the interaction of the tip with single molecules, attention was given to the orientation of the molecule and its transition dipole. As long as the excitation rate is below saturation, a single nanoparticle can be well represented as a single emitting dipole. However, unlike molecules, the transition dipole in CdSe spherical semiconductor NPs is 2D degenerate perpendicular to the c-axis plane of the particle crystal at low temperature51 and may be considered spherical at room temperature. Moreover, when exciting the QDs with a wavelength much shorter than that of their band-gap optical transition, as done in our experiments, the density of states is higher and there are many optional absorbing states with different polarizations. 15835

dx.doi.org/10.1021/jp2035604 |J. Phys. Chem. C 2011, 115, 15834–15844

The Journal of Physical Chemistry C

ARTICLE

Figure 1. (a) The experimental setup; see text for details. (b) Near-field distance-dependent imaging; see text for details. (c) Excitation light polarization: Excitation light is linearly polarized and passes through a polarization converter to produce a radially polarized beam (upper line). Using a mask for annular illumination and a high N.A. objective (not shown), a strong out-of-plane component is created. Alternatively, by changing the voltage applied to the converter, an azimuthally polarized beam is produced (lower line), which is then focused in the same optical path to an in-plane excitation spot. Linearly polarized light may also be directly focused on the sample plane without the converter for in-plane excitation.

We, therefore, expect the system to be insensitive to the orientation of the QD, since the dipole has a component parallel to the tip axis in both the absorption and the emission. For seeded-NRs, the situation is different due to the mixed dimensionality of the NP. In polarization experiments carried on ensemble and single seeded-NRs, the emission was found to be polarized parallel to the rod long axis.37,38 However, considering again the high excitation energy relative to the lower-energy band gap, and the 0D nature of the spherical core, a polarization component parallel to the tip long axis exists in this system as well. Another noteworthy difference in this study performed on NPs rather than molecules is the typically longer lifetimes of the QDs. It is interesting to see if these differences between an emitting molecule and the two types of particles produce dissimilarity in the effect induced by the tip. In this work, we experimentally control the excitation polarization and measure the fluorescence intensity and lifetime as a function of the distance from the tip in three dimensions. This provides us with data that directly relate to many of the theoretical variables mentioned above. Therefore, it allows us to analyze the results in light of the antenna theory and decipher the observed interplay between quenching, emission enhancement, and enhanced excitation, induced by the tip.

2. EXPERIMENTAL METHODS We provide a short description of the system setup and data acquisition process, as the results strongly depend on the experimental conditions. A more detailed description was published elsewhere.16,17 The experimental setup (Figure 1a) consists of an AFM (Veeco, Bioscope) mounted on an inverted optical microscope (Zeiss, Axiovert 100). A highly diluted solution of NPs is spin-cast on a glass coverslip, such that the average distance between the particles is ∼1 μm or more, as required for single particle measurements. The sample is excited by a pulsed laser at 400 nm (second harmonic of a Coherent Mira Ti-sapphire laser with ∼150 fs pulses at 76 MHz), tightly focused on the sample using an epi-illumination configuration with a 1.4 N.A. oil immersed objective (Zeiss, X100). Different batches of particles synthesized by published procedures, with emission peaks ranging between 580 and 630 nm, were used for the measurements. Details of the particle compositions and dimensions are given in the Results and Discussion section. In a typical experiment, a single particle is positioned in the laser spot using a scanning stage (Nanonics, FlatScan), where its

fluorescence count rate is maximized. The AFM tip is then scanned in tapping mode over an area of 1  0.25 μm2 around the emitting particle. The emission is collected by the same objective, filtered for residues of the excitation light and the AFM diode laser, and focused onto an avalanche photodiode detector (PerkinElmer, SPCM-ARQ 14) with a 200 μm aperture that serves also as a pinhole for confocal collection. We use a timecorrelated single-photon counter (TCSPC) card (PicoQuant, TimeHarp 200) to time-tag each collected photon. Using these time tags, the arrival of each photon at the detector is recorded relative to the lateral position of the tip and to the tip vertical motion. This way, by knowing the tip frequency and tapping amplitude, each photon is attributed to a specific lateral pixel and a specific tip
surface distance. In parallel, the photon arrival time is recorded relative to the laser pulse, enabling us to build a measured lifetime curve of the particle. The topographic data are collected simultaneously, resulting in fully synchronized topographic-optical images. Figure 1b demonstrates the method for building correlated topography-optical images. When the tip is far from the particle, there is no interaction between the particle and the tip, and the unperturbed intensity and lifetime (black curve) are recorded. As the tip approaches the particle, the measured fluorescence intensity and lifetime are modified. Fluorescence intensity may either increase (enhancement) or decrease (quenching, as demonstrated in this figure), while the lifetime will be shortened in both cases due to the increase in the nonradiative decay rate (quenching) and possibly also in the radiative decay rate (enhancement). During the scan, all collected photons, corresponding to different tip
particle distances in each pixel, are recorded, enabling one to produce images and to perform distance-dependence analysis. Lifetime curves at each pixel are built for any chosen tip
sample vertical distance. The effective decay rate (1/τ) of the particle when the tip was at this chosen height is calculated for each pixel to construct the lifetime image simultaneously with the intensity image sectioned for the same tip
sample distance. This analysis may be repeated for different heights, as well as for constructing vertical lifetime and intensity approach curves at a single position over the complete tip amplitude. To explore the dependence of the observed effect on the excitation conditions, the particles were excited in two modes (Figure 1c), using a polarization converter (Arcoptix). Using a tightly focused radially polarized beam, the particles were excited with a dominant out-of-plane polarization component, parallel to the tip long axis. Alternatively, the linearly polarized beam (for 15836

dx.doi.org/10.1021/jp2035604 |J. Phys. Chem. C 2011, 115, 15834–15844

The Journal of Physical Chemistry C

ARTICLE

Figure 2. Near-field imaging of the same single quantum dot in 3D using in-plane polarized excitation light (left column) and out-of-plane polarized excitation light (right column). (a, b) AFM topography. The scale bar is the same for (a)
(f). (c, d) Intensity images, consisting of photons collected at the smallest tip
sample separation, taken simultaneously with topography. Panel (c) shows quenching only, while panel (d) shows a central enhancement spot, surrounded by a darkened area. (e, f) Composite images, including the particle topography (bottom), smallest-height sectioned intensity image at the sample plane (center, (c) and (d) rotated horizontally), and a vertical intensity slice along a line marked with arrows in (c) and (d). The color scale is the same as in (c) and (d), respectively. (g, h) Vertical (left) and horizontal (right) cross sections for (e) and (f), respectively. Curves were taken at the arrow position (vertical) and along the dashed line (horizontal) marked on each image.

seeded-NRs) or the azimuthally polarized beam (for QDs) created an excitation field characterized by in-plane polarization, perpendicular to the tip long axis.

3. RESULTS AND DISCUSSION 3.1. Quenching and Enhancement of Single Quantum Dots. We begin by exploring the effect of the excitation

polarization on the observed near-field effect. Figure 2 shows the results of two successive measurements of the same single QD excited with azimuthally (left column) and radially (right column) polarized light. The TEM image of these particles is shown in Figure 3a. The two topographic images, panels a and b in Figure 2, confirm that the particle was not moved or changed by the tip and that the tip was not deformed during or between the scans. Panels c and d in Figure 2 show intensity images of the particle, composed only of photons that arrived at small tip
particle distances (