High-Resolution Imaging of Electric Field Enhancement and Energy

Jun 25, 2013 - In the experiment, a diffraction-limited spot (∼250 nm beam waist at the sample) is illuminated, and the tips are aligned and fixed a...
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Letter pubs.acs.org/JPCL

High-Resolution Imaging of Electric Field Enhancement and EnergyTransfer Quenching by a Single Silver Nanowire Using QD-Modified AFM Tips Zheng Liu,† Allen M. Ricks,† Haining Wang,‡ Nianhui Song,† Fengru Fan,§ Shengli Zou,*,‡ and Tianquan Lian*,† †

Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States Department of Chemistry, University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816, United States § State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡

S Supporting Information *

ABSTRACT: Plasmonic nanostructures can profoundly affect the optical properties of light absorbers and emitters. Using quantum dot-modified-atomic force microscopy (AFM) tips as a nanoscale light source, the topological features of a Ag nanowire and its effect on the fluorescence intensity and lifetime of the QDs can be simultaneously imaged with AFM spatial resolution. Modeling of the QD−nanowire interaction and the contrasts in the fluorescence intensity and lifetime images suggests that this novel method can be used for direct high-resolution mapping of the electric field distributions and energy-transfer quenching near metallic nanostructures.

SECTION: Physical Processes in Nanomaterials and Nanostructures

L

The design of plasmonic nanostructures often relies on computational modeling of field distributions.2−5 Direct experimental mapping of such field distribution on the nanometer scale is still challenging. Two-photon luminescence microscopy has been used for probing the near-field optical response of plasmonic nanostructures, but its spatial resolution is limited to hundreds of nanometers by optical diffraction.14 Diffraction-limited spatial resolution can be improved by using probes with short wavelength. For example, direct mapping of surface plasmon modes of metallic nanostructures with nanometer resolution has been achieved with electron energy loss spectroscopy (EELS).15−17 However, there are substantial differences in the photonic local density of states probed by EELS and optical techniques.16 To overcome the optical diffraction limit, scanning near-field optical microscopy (SNOM) has been employed to characterize surface plasmon polaritons with high spatial resolution, but the presence of the metallic tip may significantly alter the intrinsic response of the nanostructure under study.13,18 The use of photosensitive polymeric materials has enabled the mapping of optical nearfield strength around metallic nanostructures with high spatial

ocalized surface plasmon resonance has been shown to greatly enhance the local electromagnetic field strength near metallic nanostructures.1 The field enhancement has profound effects on the absorption, emission, and scattering properties of chromophores, which has been utilized in a broad array of applications, ranging from solar energy conversion to ultrasensitive detection.2−11 The enhancement of field strength depends on many factors, such as the shape and nature of metal nanostructures and their dielectric environments as well as the frequency of the electromagnetic field.11 The spatial variation of field strength in plasmonic nanostructures often leads to the formation of nanoscale “hot spots” with ultrahigh field enhancement factors. For example, vibrational spectra of single molecules in these hot spots can be measured by surfaceenhanced Raman spectroscopy, and the enhancements of Raman intensity by factors of 10 6 −10 14 have been reported.8,9,12,13 Therefore, the ability to determine the field distribution with nanometer spatial resolution is important for the design and application of plasmonic nanostructures.2,6,9,11 In addition to field enhancement, the presence of metallic plasmonic structures can also affect the excited-state dynamics of chromophores through energy- and charge-transfer quenching. Therefore, techniques that are sensitive to both the field enhancement and excited-state quenching are highly desired. © 2013 American Chemical Society

Received: May 21, 2013 Accepted: June 25, 2013 Published: June 25, 2013 2284

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resolution.19,20 In these novel approaches, the light-induced topographic modifications of the polymer films record the local field strength under light illumination and can be subsequently imaged with high spatial resolution by atomic force (AFM) or electron microscopy. However, the chemical modification (i.e., the polymerization step) may cause irreversible change of the nanostructure. In this Letter, we report a novel technique for directly imaging the nanoscale electric field strength distribution and energy-transfer quenching near a nanostructure by using simultaneous aperture-less SNOM and AFM. Unlike traditional SNOM, we use quantum dot (QD)-modified AFM tips as a nanoscale light source, which allows the simultaneous recording of AFM, fluorescence intensity, and lifetime images with high spatial resolution. Recent experimental and theoretical investigations show that the emission properties of spontaneous emitters (QDs or dye molecules) are significantly modified near metallic nanostructures due to the enhanced electric field strength and energy-transfer quenching.21−39 Enhancement in the electric field leads to increased absorption and emission rates of emitters, increasing the emission intensity and reducing the emission lifetime, while energy-transfer quenching reduces both the emission lifetime and intensity. Thus, by monitoring the spatial variation of the fluorescence properties (lifetime and/or intensity) of the emitters, the electric field distribution near the metallic nanostructure can be directly probed. Previous attempts by other groups have used near-field optical fibers attached with relatively large microcrystals or polystyrene beads doped with fluorescent molecules, whose sizes limit the spatial resolutions of those techniques.24,40 We demonstrate that AFM tips modified with CdSe/ZnS core/shell QDs can determine the topological features of a Ag metal nanowire (NW), and the wire’s effect on the QD fluorescence lifetime and intensity could be simultaneously recorded by AFM and confocal aperture-less near-field fluorescence microscopy, respectively. The presence of the NW leads to pronounced variation of the QD fluorescence intensity and lifetime. The Ag wire induced lifetime and intensity variation generate images with spatial resolution below 100 nm, similar to the AFM lateral resolution and significantly better than the optical diffraction limit. Modeling of the NW−QD interaction showed that the observed spatial modulation of the fluorescence intensity and lifetime could be attributed to the variation of the plasmonic field strength and the QD−wire energy-transfer quenching. Our findings suggest that the method reported in this study can be used to directly map the electric field distribution near metallic nanostructures with suboptical diffraction-limited spatial resolution. Functionalization of AFM Tips by QDs. To reduce the quenching of QD emission by the n-Si AFM tips (MIKROMASCH HQ:NSC35/Al BS), which is likely caused by energy and/or charge transfer from excited QDs to silicon,41 we coated a thin (∼10 nm) layer of SiO2 on the tip.42 The procedures for functionalizing silicon AFM tips by QDs have been reported previously and are described briefly here.43 SiO2coated AFM tips were dipped in concentrated sulfuric acid for 20 min to yield a clean hydroxyl-rich surface and then immersed into a 1 mM (aminopropyl)triethoxysilane (APTES, Aldrich)/toluene room-temperature solution for 30 min. After washing with ethanol and drying with a flow of nitrogen, the silanized AFM tip was incubated for 15 min in an aqueous solution of water-soluble CdSe/ZnS core/shell QDs (10 nM) that were capped with carboxylic acid functional groups (Ocean

NanoTech, LLC, U.S.A.). After incubation, the AFM tip was washed with deionized water to remove weakly bound QDs, dried with nitrogen flow, and stored in the dark. The presence of QDs could be verified by fluorescence emission from the functionalized tip. Sample Preparation. Ag NWs used for this study were synthesized by reducing AgNO3 with ethylene glycol (EG) in the presence of polyvinyl pyrrolidone (PVP), following a literature procedure.44 The solution of Ag NWs was diluted five-fold by volume with deionized water, centrifuged at 3000 rpm for 20 min, and decanted to remove excess PVP and other unwanted materials (such as Ag nanoparticles). To prepare samples for correlated AFM and fluorescence measurements, glass coverslips were sonicated in acetone for 30 min. A diluted solution of Ag NWs in H2O was then drop-casted on clean glass coverslips, and the samples were dried in air before use. Correlated AFM and Fluorescence Imaging. The schematic of the correlated AFM and confocal fluorescence microscopy setup used for this study is shown in Figure 1a. All

Figure 1. (a) Schematic of the experimental setup for correlated AFM and fluorescence imaging. The sample is placed on an AFM scanner to control its position relative to the tip in the x−y plane, while the tip can be moved in the z direction to control the tip−sample vertical distance. (b) Expanded view of the tip−sample region, showing a QDmodified AFM tip (consisting of SiO2-coated Si) positioned near a NW of interest.

measurements were performed at room temperature under ambient conditions. Briefly, an AFM head (Asylum MFP-3DBIO) was mounted on an inverted microscope (Olympus IX70) equipped with a 100× 1.4 NA oil immersion objective (Olympus). Femtosecond laser pulses (∼100 fs) with a repetition rate of 80 MHz were generated with a mode-locked Ti:Sapphire laser (Tsunami oscillator pumped by a 10 W Millennia Pro, Spectra-Physics). The output beam (at 820 nm) was passed through a pulse picker (Conoptics, U.S.A.) to reduce the repetition rate and then frequency-doubled in a BBO crystal to generate 410 nm excitation pulses. The glass coverslips containing Ag NWs were placed on the AFM (x−y) scanning stage. The excitation beam (∼200 nW) was focused through the objective down to a diffraction-limited spot on the sample. For correlated AFM topography and fluorescence measurements, the AFM tip was positioned within the diffraction-limited excitation spot (Figure 1b). The AFM images were recorded by intermittent contact mode (AC) with the tip−sample distance (z) oscillating around an average value of 10−100 nm and the sample raster scanned in the x and y directions by the AFM piezo-stage. At the same time, the resulting epifluorescence from the QD-coated AFM tip was detected by an avalanche photodiode (APD, Perkin-Elmer SPCM-AQR-16). To reduce Rayleigh scattering from Ag NWs, dark-field illumination was utilized by blocking the center part of the 410 nm beam. 2285

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For simultaneous AFM and fluorescence intensity imaging, the APD output was integrated, and a signal proportional to the photon count per second was sent to an analog input channel of the AFM. Each pixel in the fluorescence image could be uniquely assigned to a position in a specific line (y) and column (x) in the AFM image. The total image size was set at 256 lines by 300 columns, which, for a typical scan area of 6 × 6 μm2, corresponded to pixel dimensions of ∼Δx = 20 nm by Δy = 23 nm. From the line scan rate of 0.1 Hz (or 5 s per line in each direction), the average fluorescence detection time was estimated to be ∼16 ms per pixel. Because the AFM was operated in the AC mode, in which the AFM cantilever oscillated at a typical frequency of ∼100 kHz, the fluorescence intensity (and lifetime below) at each pixel represented an averaged intensity for all tip−sample distances z. For correlated AFM and fluorescence lifetime imaging, the APD output was analyzed by a time-correlated single-photon counting (TCSPC) board (Becker & Hickel SPC 600). For each detected photon, both the arrival time (t, relative to the start of the experiment) and the delay time (τ, relative to the excitation pulse) were recorded.45 To record the x and y positions in the lifetime image, TCSPC data collection was initiated at the beginning of each line in the AFM sample raster scan, and collected photons were binned based on their arrival time with respect to the start of the scan. This generated a 2-D map of photons based on arrival time relative to the start of a line scan (xi) and line number (yi). The fluorescence photons within each pixel (xi,yj) were used to construct a delay time histogram, which was then fitted by a single-exponential decay function to obtain an average lifetime. Because a larger number of photons were needed to determine the lifetime, we reduced the total number of lines to 128 and the line scan rate to 0.03 Hz for the same 6 × 6 μm2 image area. The effective pixel size in the fluorescence lifetime image was 47 × 47 nm2, and the average fluorescence collection time was ∼130 ms per pixel, corresponding to a total number of ∼300−1000 photons. Although a fluorescence intensity image can be simultaneously obtained with the lifetime image, the spatial resolution is significantly reduced in this case due to larger pixel size. For this reason, only the higher-resolution intensity images acquired separately are shown below. The simultaneous and correlated AFM and fluorescence imaging technique described above was used to image the electric field enhancement of a Ag NW. Figure 2a and b shows the simultaneously acquired AFM height and fluorescence intensity images, respectively, of a silver wire using a QDmodified AFM tip. The AFM height image shows a single bent Ag wire with a length of 4.6 ± 0.1 μm and a diameter of 55 ± 5 nm. The fluorescence lifetime image of the same sample area acquired with the same tip is shown in Figure 2c. Two typical fluorescence decay histograms at the two indicated positions in Figure 2c are compared in Figure 2d. These curves show clearly faster fluorescence decay for the tip on top of the silver wire compared to that at a position far away from the wire. These curves can be adequately fit by single-exponential decay functions with decay time constants of 15.6 ± 0.3 and 26.8 ± 0.4 ns, respectively. It should be pointed out that the decay kinetics are expected to be non-single-exponential due to multiple reasons, including the distributions of lifetimes of different QDs and QD−sample distances caused by tip−sample oscillation and the relative position of QDs on the tip. Surprisingly, single-exponential decay functions appear to fit these decay curves adequately, which is likely a result of limited

Figure 2. Correlated (a) AFM height, (b) fluorescence intensity (in units of detected photons per second), and (c) fluorescence lifetime image of a silver NW acquired using a QD-modified AFM tip. (d) Fluorescence decay curves (dots) and single-exponential fits (solid lines) measured at indicated sample positions in panel c, showing a lifetime faster decay when the tip is on top of the NW.

signal-to-noise ratio of the data. For these reasons, the lifetime obtained from the single-exponential fit should not be considered as a quantitative measure of the lifetime of a single exponential decay process. Instead, we use the fit to calculate a half-lifetime, which serves as a useful indicator of the qualitative changes in decay rates at different tip−sample positions. The fluorescence intensity and lifetime images (Figure 2b and c) show clear features that correlate with the topography (Figure 2a) of the silver NW. The average fluorescence intensity decreases significantly when the tip is on top of the wire; it is enhanced when the tip is near either side of the NW, although the enhancement factors are noticeably different. The fluorescence lifetime decreases monotonically as the tip approaches the NW. These spatial variations of QD fluorescence intensity and lifetime can be more clearly seen by a line scan plot across the NW shown in Figure 3a and b. The cross section scan of the AFM height image indicates a NW with a peak height of 55 ± 5 nm and a full width at halfmaximum (fwhm) of ∼100 nm. The peak height reflects the diameter of the Ag wire, and the fwhm reflects the lateral resolution of the AFM tip, limited by the tip size and sharpness. The comparison of the AFM and fluorescence intensity image (Figure 3a) shows that a similarly sharp feature can be observed in the fluorescence intensity image, indicating similar spatial resolution in these images and a suboptical diffraction-limited spatial resolution in the latter. The fluorescence intensity (measured by the rate of detected photons) is ∼4500−5500 counts/s (Hz) when the tip is far away from the NW. The intensity starts to increase when the tip is ∼1 μm away from the nanorwire until it reaches a peak rate of ∼7400 Hz at 100 nm from the wire, representing a factor of 1.3 enhancement of emission intensity. The emission intensity decreases rapidly to ∼2900 Hz when the tip is on top of the NW. There is also a slight enhancement when the tip is at the right side of the NW, although the enhancement factor is 2286

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and right sides of the wire can only be accounted for with modeling of the QD−wire−field interaction, which is presented below. To further confirm that the QD−wire interaction (via plasmonic field enhancement and energy transfer) is responsible for the contrast in fluorescence lifetime and intensity images, we have also examined the dependence of the fluorescence intensity image on the average QD−wire distance. The fluorescence intensity and lifetime images shown in Figure 2 were acquired with the AFM operating in the intermittent contact mode in which the tip−sample distance z oscillated at around a set average value of 10 nm. Thus, the collected fluorescence intensity was averaged over the tip−sample distance of ∼0−20 nm. We have also acquired a fluorescence intensity image of the sample at an average tip−sample distance of 40 nm, in which the distance oscillated between ∼0 and 80 nm. On average, the QD−NW distance has been increased 4fold compared to that in Figure 2b. As shown in Figure S1 (Supporting Information), the overall intensity has decreased noticeably. This can be better seen in a comparison of line scans along the dashed lines in Figure 2a and Figure S1 (Supporting Information), which are shown in Figure S2a (Supporting Information). The corresponding enhancement factors are compared in Figure S2b (Supporting Information). We have defined the enhancement factor as the intensity ratio between any points along the line scan and the average intensity far away from the wire. Due to the low intensity in the images acquired with large average z, the measured enhancement factors are considerably noisier. Within the signal-to-noise level of the measurement, the enhancement factors appear to be similar in Figure S1 (Supporting Information) and Figure 2b. It is known that the plasmonic field strength decreases exponentially with distance away from the wire, and the energy-transfer rate decays by either r−4 or r−6 with distance r.41,48−52 The effects of both processes decrease rapidly with distance. The similar contrasts shown in these figures suggest that the detected fluorescence may result from tip positions that are close to the sample. This can be attributed to the dark-field illumination scheme adopted in this measurement, where the illumination intensity decays exponentially with distance from the substrate and QDs absorb and emit photons mainly when the tip is close to the sample. It is likely that in images acquired with a large average z, the overall emission intensity is low due to the larger average tip−sample distance, but the intensity contrast is similar because QD emission at larger distance contributes negligibly to the total intensity. This comparison also suggests that the image contrast can be further improved with a shorter tip−sample distance. Although it is difficult to further reduce the average tip−sample distance below 10 nm in the AC mode, methods for separating the photons at different tip−sample distances z have been reported.41,45,53 These authors showed that by sorting the detected photons according to their tip−sample distance z (instead of averaging over z), fluorescence lifetime and intensity images at any tip−sample vertical distance z can be constructed. To understand the observed fluorescence intensity and lifetime images, we have simulated the fluorescence intensity and lifetime changes of a single QD around a silver nanorod using a coupled dipole method,54,55 which is based on a modification of a previously published discrete dipole approximation method.56 For a QD placed near a metallic object, the change of the emission intensity and lifetime can be calculated using eqs 1 and 2.

Figure 3. (a) Cross section line scans of AFM height (black line) and fluorescence intensity (blue line) of QDs and (b) cross section line scans of QD fluorescence intensity (blue line) and lifetime (red line) along the dashed line shown in Figure 2a. Inset in (a) An expanded view of the data near the NW.

much smaller. The average fluorescence lifetime shows a monotonic decrease from ∼18 ns when the tip is far away from the NW to ∼12 ns when the tip is on top of the wire, with the onset of the decrease occurring at ∼1 μm at either side of the wire. The observed dependence of the QD fluorescence lifetime and intensity on its relative position to the NW suggests that the contrast in Figure 2b and c is caused by QD−NW interactions. It has been well-established that in QD−metal hybrid nanostructures, the metallic structure can modify the fluorescence properties of QDs.46,47 The strong plasmonic field (E) of the metallic structure can enhance the absorption rate and radiative decay rate of the QD. In addition, there also exist energy-transfer and charge-transfer pathways between the excited QD and the metal. Energy transfer increases the nonradiative decay rate of the QD, reducing the emission lifetime and intensity. The charge-transfer rate depends exponentially on the QD−metal distance and can occur only when the QD is in close contact with the metal. Because of the AC mode employed in this study, the QDs are over 1 nm away from the metal surface most of the time. For this reason, we have neglected the effect of charge transfer. As the QD−NW distance decreases, both the enhanced local electric field (increasing the radiative decay rate) and energy-transfer rate (increasing the nonradiative decay rate) lead to the reduction of the QD fluorescence lifetime, as shown in Figure 3b. The change in the emission intensity is more complicated due to the opposite effects of the enhanced field strength and energytransfer rate on the emission intensity. At a short a tip−wire distance, the enhanced field strength increases the absorption and emission rates, increasing the emission intensity, while the increased energy-transfer rate (nonradiative) reduces the emission quantum yield, leading to a decrease of the emission intensity. The competition of these two processes leads to the net change of emission intensity shown in Figure 3a. The relative importance of these processes and the asymmetry in both the emission intensity and lifetime distribution on the left 2287

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f × k r /(fr × k r + kENT + k nr) η′ I =K =K r I0 η k r /(k r + k nr) =K

fr /(ft × k r + k nr) 1/(k r + k nr)

τ 1 = τ0 ft × η + 1 − η

=K

fr ft × η + 1 − η

(1)

(2)

Here, I (τ) and I0 (τ0) are the fluorescence intensities (lifetimes) of the QD in the presence and absence of the NW, respectively. K = |E|2/|E0|2 is the enhancement factor of the local electric field intensity (|E|2) relative to the incident field (|E0|2) at the incident wavelength (400 nm), which accounts for the enhancement of absorption rates by the QD. The emission quantum yield of a free QD is given by η = kr/(kr + knr), where knr and kr are its intrinsic nonradiative and radiative decay rate constants, respectively. η′ is the quantum yield of a QD near the NW, which differs from that of the free QD due to the enhanced radiative ( f r × kr) and nonradiative (kENT + knr) decay rate constants. kENT is the nonradiative energy-transfer rate between the QD and NW. We have also defined f t = f r + kENT/kr due to the convenience of calculating this quantity, as described below. In the experiment, a diffraction-limited spot (∼250 nm beam waist at the sample) is illuminated, and the tips are aligned and fixed at the center of the spot as the sample moves in the x−y plane. Regardless of the sample xy positions, the incident intensity on the tip (and the QD) is the same prior to accounting for the plasmonic field of the silver NW. To simulate the field experienced by the QD, we assume that the silver wire is illuminated by a field with constant incident intensity (|E0|2) and move the QDs to various positions relative to the wire to mimic the sample scanning. We first calculated the electric field distribution of the NW without the presence of the QD and the AFM tip using a discrete dipole approximation method.56 In the simulations, the silver nanorod was 50 nm in diameter and 200 nm long, which was long enough to represent a rod of several micrometer in length (in the y direction) because the incident polarization was perpendicular to the rod symmetric axis (and in the xz plane). The NW was divided into an array of polarizable cubes with a length of 1 nm, and the properties of the cube were defined by its dielectric constants. After considering the interaction between these polarizable cubes as well as their interaction with the incident light, the enhanced electric field, scattering, absorption, and extinction spectra of the NW can be calculated. For convenience, the intensity of the incident light (|E0|2) was assumed to be 1 in the calculations. The calculated electric field distribution in the x−z plane is shown in Figure 4a. The electric field strength is enhanced in the direction ∼50° from the z axis, reflecting the polarization of the incident beam. The field distribution shown in Figure 4a was used to calculate the enhancement factor K in eq 1. Other terms associated with changes in the emission properties of the QD along a line scan in the x−z plane across the NW are calculated at emission wavelength using strategies described in the following paragraphs. To mimic the experiment, we used a virtual tip with the same shape parameter as that in the experiment (tip radius of 8 nm, full tip cone angle of 40°, and tip height of 18 μm) and placed a QD at the apex of the tip. For each x position, the tip was placed at a fixed distance (5 nm) with the sample (glass surface or the silver wire) by finding the

Figure 4. (a) Contour plot of the electric field |E|2 in a xz plane near a Ag NW. The position of the QD in a cross section line scan shown in panel b is indicated by the red dotted line. (b) QD fluorescence intensity (blue solid line), lifetime (red solid line), f t (pink dotted line, scaled by a factor of 0.05), and f r (black dashed line, scaled by a factor of 0.5) as a function of the tip x position in a cross section line scan across a Ag wire in the x−z plane. In the calculation, one QD is fixed at the apex of the tip, and the tip−sample distance is set at 5 nm, from which the coordinates of the QD in the xz plane can be determined as indicated in panel a.

shortest contact distance between (any position of) the tip and the sample. After determining the coordinate of the QD (shown in Figure 4a) at the contact point, the tip was then removed, and the effect of the field distribution on the radiative and nonradiative decay rates of the QD was calculated without the explicit consideration of the tip. The QD was treated as a point dipole with an intrinsic decay rate of kr, and its interaction with the induced dipoles in the NW gave rise to the energytransfer quenching of the QD. The exact value of kr is not important because it is canceled out in the calculated ratio of the emission intensities and lifetimes. The intrinsic nonradiative decay of the QD (with rate constant knr) is assumed to be unaffected by the NW and is added at the end (in eqs 1 and 2) to account for the experimentally determined quantum yield of free QDs (η = 0.8) and lifetime. We compare the radiative rate of a QD−NW complex with that of a free QD and attribute the enhancement factor to f r. Similarly, we compare the energy decay rate of the QD subsystem in the QD−NW complex with that of the free QD. The enhancement factor in this decay rate is ascribed to f t (=f r + kENT/kr), which accounts for the increase in the radiative decay rate of the QD as well as its energytransfer rate to the NW. With the computed f t and f r, the enhancement in emission intensity and lifetime change as a function of tip x position along the line scan can then be calculated using eqs 1 and 2. Figure 4b shows the calculated fluorescence intensity enhancement (I/I0), lifetime enhancement (τ/τ0), f t, and f r as 2288

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a function of the tip x position in a line scan across the silver wire. The simulation reproduces qualitatively the observed spatial variations of the QD lifetime and intensity. It is clear that the spatial variation of f r (Figure 4b) closely follows the field amplitude experienced by the QD along the cross section line scan, as indicated by the red dashed line in Figure 4a. It shows large f r when the QD is at both sides of the wire and decreased amplitude when the QD is near the top of the wire. This spatial variation and asymmetry (at either side of the wire) in the enhancement factor are caused by the polarization direction of the incident field. The f t factor increases when the QD approaches the wire and becomes much larger than f r at shorter QD−wire distances. It suggests that as the QD approaches the wire, f t first increases because of increased f r (i.e., enhanced radiative decay rate), which is followed by much larger increases in nonradiative decay rates at shorter QD−wire distances. The combined effects lead to the enhancement in the total decay rate (and a decrease in the fluorescence lifetime). The changes in radiative and nonradiative decay rates have opposite effects on the fluorescence intensity. At either side of the wire, when the QD−wire distance is large, the enhancement in the radiative decay rate is larger than that in nonradiative decay rates, resulting in a net increase in the fluorescence intensity. At a shorter QD−wire distance, the enhancement factor in the nonradiative decay rate dominates, leading to a decrease in the fluorescence intensity. Although the calculated fluorescence intensity and lifetime distributions capture the overall features of the experimentally measured curves, the agreement is not quantitative. In the simulations, we consider only one QD placed at the apex of the tip, while multiple QDs are present on the tip in the experiment. Unfortunately, both the exact number and the spatial locations of the QDs are not known, which prevents a more detailed simulation. Qualitatively, the distribution of QDs likely broadens the features observed in the simulation. Although this difficulty can be solved by coating the tips with a monolayer of QDs, it will likely reduce the contrast and spatial resolution of this technique. Alternatively, it is possible to conduct the measurement with tips modified by a single QD, which should also improve the spatial resolution and image contrast. This experiment is ongoing in our laboratory. Another deficiency of the simulation is that the effects of the AFM tip and the glass coverslip on the electromagnetic field distribution have not been considered. AFM tip-induced fluorescence intensity enhancement and micromirror effects have been reported, and the possibility of asymmetry in intensity enhancement caused by these effects cannot be excluded.39,57 Despite these deficiencies, the qualitative agreement between the experiment and simulation suggests that the observed spatial variations of the fluorescence intensity and lifetime distributions are determined mostly by the electric field distribution of the NW as well as the energy transfer between QDs and the NW. It also suggests that using QD-modified AFM tip as a nanoscale probe is a viable way to map out the electric field distribution in plasmonic nanomaterials. Our result demonstrates that the lateral spatial resolution of this technique (∼100 nm) is limited by the AFM tip used in this measurement. Although similar resolution can also be achieved with SNOM, the spatial resolution of our method can be further improved by using sharper tips and tips that are modified by a single QD. Because plasmonic field enhancement is wavelength-dependent, it would also be informative to vary the excitation wavelength and/or the emission wavelength (via

the size of the QD), both of which can be achieved with this technique and will be examined in future studies. In summary, we have demonstrated that by using QDmodified AFM tips as a nanoscale light source, we can simultaneously record the topography image of a Ag NW and the fluorescence intensity or lifetime image of the QD−NW system. The fluorescence images show features that are wellcorrelated with the topography of the NW with a spatial resolution similar to that of the AFM image and below the optical diffraction limit. Computational modeling of the QD− NW system reveals that the contrasts in the fluorescence lifetime and intensity images arise from the spatial variation of the electric field strength enhancement and energy-transfer quenching caused by the NW. Our finding suggests that correlated fluorescence and AFM imaging using a QD-modified AFM tip can be a useful method for direct imaging of the effect of a plasmonic nanostructure on the emission properties of chormophores. Coupled with computational modeling, this technique can reveal the spatial distribution of both the electric field strength enhancement and energy-transfer quenching in plasmonic nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

Fluoroscence intensity image at a large average tip−sample distance and comparison of enhancement factors at different average z’s. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.L.); [email protected] (S.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.L. acknowledges financial support from the National Science Foundation (CHE-0848556 and CHE-1212907). S.Z. is grateful for support from the NSF and ONR. T.L. thanks Prof. U. Banin for helpful discussion on the experimental implementation of correlated AFM and fluorescence lifetime imaging.

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REFERENCES

(1) Novotny, L.; Hecht, B. Principles of Nano-optics, 2nd ed.; Cambridge University Press: Cambridge, U.K., 2012. (2) Giannini, V.; Fernández-Domínguez, A. I.; Heck, S. C.; Maier, S. A. Plasmonic Nanoantennas: Fundamentals and Their Use in Controlling the Radiative Properties of Nanoemitters. Chem. Rev. 2011, 111, 3888−3912. (3) Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111, 3913−3961. (4) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111, 3828−3857. (5) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Nanostructured Plasmonic Sensors. Chem. Rev. 2008, 108, 494−521. (6) Cang, H.; Labno, A.; Lu, C.; Yin, X.; Liu, M.; Gladden, C.; Liu, Y.; Zhang, X. Probing the Electromagnetic Field of a 15-Nanometre Hotspot by Single Molecule Imaging. Nature 2011, 469, 385−388. (7) Ferry, V. E.; Munday, J. N.; Atwater, H. A. Design Considerations for Plasmonic Photovoltaics. Adv. Mater. 2010, 22, 4794−4808. 2289

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(8) Cialla, D.; Maerz, A.; Boehme, R.; Theil, F.; Weber, K.; Schmitt, M.; Popp, J. Surface-Enhanced Raman Spectroscopy (SERS): Progress and Trends. Anal. Bioanal. Chem. 2012, 403, 27−54. (9) Le Ru, E. C.; Etchegoin, P. G. Single-Molecule Surface-Enhanced Raman Spectroscopy. Annu. Rev. Phys. Chem. 2012, 63, 65−87. (10) Fort, E.; Gresillon, S. Surface Enhanced Fluorescence. J. Phys. D: Appl. Phys. 2008, 41, 013001. (11) Johansson, P.; Xu, H. X.; Kall, M. Surface-Enhanced Raman Scattering and Fluorescence Near Metal Nanoparticles. Phys. Rev. B 2005, 72, 035427. (12) Fang, Y.; Wei, H.; Hao, F.; Nordlander, P.; Xu, H. RemoteExcitation Surface-Enhanced Raman Scattering Using Propagating Ag Nanowire Plasmons. Nano Lett. 2009, 9, 2049−2053. (13) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Visualization of Localized Intense Optical Fields in Single Gold− Nanoparticle Assemblies and Ultrasensitive Raman Active Sites. Nano Lett. 2006, 6, 2173−2176. (14) Ghenuche, P.; Cherukulappurath, S.; Taminiau, T. H.; van Hulst, N. F.; Quidant, R. Spectroscopic Mode Mapping of Resonant Plasmon Nanoantennas. Phys. Rev. Lett. 2008, 101, 116805. (15) Bosman, M.; Keast, V. J.; Watanabe, M.; Maaroof, A. I.; Cortie, M. B. Mapping Surface Plasmons at the Nanometre Scale with an Electron Beam. Nanotechnology 2007, 18, 165505. (16) Hohenester, U.; Ditlbacher, H.; Krenn, J. R. Electron-EnergyLoss Spectra of Plasmonic Nanoparticles. Phys. Rev. Lett. 2009, 103, 106801. (17) Mirsaleh-Kohan, N.; Iberi, V.; Simmons, P. D.; Bigelow, N. W.; Vaschillo, A.; Rowland, M. M.; Best, M. D.; Pennycook, S. J.; Masiello, D. J.; Guiton, B. S.; Camden, J. P. Single-Molecule Surface-Enhanced Raman Scattering: Can STEM/EELS Image Electromagnetic Hot Spots? J. Phys. Chem. Lett. 2012, 3, 2303−2309. (18) Zayats, A. V.; Smolyaninov, I. I.; Maradudin, A. A. Nano-Optics of Surface Plasmon Polaritons. Phys. Rep. 2005, 408, 131−314. (19) Hubert, C.; Rumyantseva, A.; Lerondel, G.; Grand, J.; Kostcheev, S.; Billot, L.; Vial, A.; Bachelot, R.; Royer, P.; Chang, S.h.; Gray, S. K.; Wiederrecht, G. P.; Schatz, G. C. Near-Field Photochemical Imaging of Noble Metal Nanostructures. Nano Lett. 2005, 5, 615−619. (20) Volpe, G.; Noack, M.; Aćimović, S. S.; Reinhardt, C.; Quidant, R. Near-Field Mapping of Plasmonic Antennas by Multiphoton Absorption in Poly(methyl methacrylate). Nano Lett. 2012, 12, 4864− 4868. (21) Akimov, A. V.; Mukherjee, A.; Yu, C. L.; Chang, D. E.; Zibrov, A. S.; Hemmer, P. R.; Park, H.; Lukin, M. D. Generation of Single Optical Plasmons in Metallic Nanowires Coupled to Quantum Dots. Nature 2007, 450, 402−406. (22) Fedutik, Y.; Temnov, V. V.; Schöps, O.; Woggon, U.; Artemyev, M. V. Exciton−Plasmon−Photon Conversion in Plasmonic Nanostructures. Phys. Rev. Lett. 2007, 99, 136802. (23) Truegler, A.; Hohenester, U. Strong Coupling between a Metallic Nanoparticle and a Single Molecule. Phys. Rev. B 2008, 77, 115403. (24) Frimmer, M.; Chen, Y.; Koenderink, A. F. Scanning Emitter Lifetime Imaging Microscopy for Spontaneous Emission Control. Phys. Rev. Lett. 2011, 107, 123602. (25) Guo, X.; Qiu, M.; Bao, J.; Wiley, B. J.; Yang, Q.; Zhang, X.; Ma, Y.; Yu, H.; Tong, L. Direct Coupling of Plasmonic and Photonic Nanowires for Hybrid Nanophotonic Components and Circuits. Nano Lett. 2009, 9, 4515−4519. (26) Li, Z.; Hao, F.; Huang, Y.; Fang, Y.; Nordlander, P.; Xu, H. Directional Light Emission from Propagating Surface Plasmons of Silver Nanowires. Nano Lett. 2009, 9, 4383−4386. (27) Wei, H.; Ratchford, D.; Li, X.; Xu, H.; Shih, C.-K. Propagating Surface Plasmon Induced Photon Emission from Quantum Dots. Nano Lett. 2009, 9, 4168−4171. (28) Yan, R.; Pausauskie, P.; Huang, J.; Yang, P. Direct Photonic− Plasmonic Coupling and Routing in Single Nanowires. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 21045−21050.

(29) Celebrano, M.; Lettow, R.; Kukura, P.; Agio, M.; Renn, A.; Goetzinger, S.; Sandoghdar, V. Efficient Coupling of Single Photons to Single Plasmons. Opt. Express 2010, 18, 13829−13835. (30) Andersen, M. L.; Stobbe, S.; Sorensen, A. S.; Lodahl, P. Strongly Modified Plasmon−Matter Interaction with Mesoscopic Quantum Emitters. Nat. Phys. 2011, 7, 215−218. (31) Schuller, J. A.; Barnard, E. S.; Cai, W.; Jun, Y. C.; White, J. S.; Brongersma, M. L. Plasmonics for Extreme Light Concentration and Manipulation. Nat. Mater. 2010, 9, 193−204. (32) Ridolfo, A.; Di Stefano, O.; Fina, N.; Saija, R.; Savasta, S. Quantum Plasmonics with Quantum Dot−Metal Nanoparticle Molecules: Influence of the Fano Effect on Photon Statistics. Phys. Rev. Lett. 2010, 105, 263601. (33) Ma, X. D.; Tan, H.; Kipp, T.; Mews, A. Fluorescence Enhancement, Blinking Suppression, and Gray States of Individual Semiconductor Nanocrystals Close to Gold Nanoparticles. Nano Lett. 2010, 10, 4166−4174. (34) Li, Z.; Bao, K.; Fang, Y.; Huang, Y.; Nordlander, P.; Xu, H. Correlation between Incident and Emission Polarization in Nanowire Surface Plasmon Waveguides. Nano Lett. 2010, 10, 1831−1835. (35) Jun, Y. C.; Pala, R.; Brongersma, M. L. Strong Modification of Quantum Dot Spontaneous Emission via Gap Plasmon Coupling in Metal Nanoslits. J. Phys. Chem. C 2010, 114, 7269−7273. (36) Giessen, H.; Lippitz, M. Directing Light Emission from Quantum Dots. Science 2010, 329, 910−911. (37) Dorfmueller, J.; Vogelgesang, R.; Khunsin, W.; Rockstuhl, C.; Etrich, C.; Kern, K. Plasmonic Nanowire Antennas: Experiment, Simulation, and Theory. Nano Lett. 2010, 10, 3596−3603. (38) Curto, A. G.; Volpe, G.; Taminiau, T. H.; Kreuzer, M. P.; Quidant, R.; van Hulst, N. F. Unidirectional Emission of a Quantum Dot Coupled to a Nanoantenna. Science 2010, 329, 930−933. (39) Hu, D. H.; Micic, M.; Klymyshyn, N.; Suh, Y. D.; Lu, H. P. Correlated Topographic and Spectroscopic Imaging beyond Diffraction Limit by Atomic Force Microscopy Metallic Tip-Enhanced NearField Fluorescence Lifetime Microscopy. Rev. Sci. Instrum. 2003, 74, 3347−3355. (40) Michaelis, J.; Hettich, C.; Miynek, J.; Sandoghdar, V. Optical Microscopy Using a Single-Molecule Light Source. Nature 2000, 405, 325−328. (41) Mangum, B. D.; Mu, C.; Gerton, J. M. Resolving Single Fluorophores within Dense Ensembles: Contrast Limits of TipEnhanced Fluorescence Microscopy. Opt. Express 2008, 16, 6183− 6193. (42) Liz-Marzán, L. M.; Giersig, M.; Mulvaney, P. Synthesis of Nanosized Gold−Silica Core−Shell Particles. Langmuir 1996, 12, 4329−4335. (43) Ebenstein, Y.; Mokari, T.; Banin, U. Quantum-Dot-Functionalized Scanning Probes for Fluorescence-Energy-Transfer-based Microscopy. J. Phys. Chem. B 2004, 108, 93−99. (44) Wiley, B.; Sun, Y.; Xia, Y. Synthesis of Silver Nanostructures with Controlled Shapes and Properties. Acc. Chem. Res. 2007, 40, 1067−1076. (45) Yoskovitz, E.; Menagen, G.; Sitt, A.; Lachman, E.; Banin, U. Nanoscale Near-Field Imaging of Excitons in Single Heterostructured Nanorods. Nano Lett. 2010, 10, 3068−3072. (46) Bharadwaj, P.; Novotny, L. Robustness of Quantum Dot PowerLaw Blinking. Nano Lett. 2011, 11, 2137−2141. (47) Ratchford, D.; Shafiei, F.; Kim, S.; Gray, S. K.; Li, X. Manipulating Coupling between a Single Semiconductor Quantum Dot and Single Gold Nanoparticle. Nano Lett. 2011, 11, 1049−1054. (48) Chance, R. R.; Prock, A.; Silbey, R. Molecular Fluorescence and Energy Transfer Near Interfaces; Advances in Chemical Physics; John Wiley & Sons, Inc.: New York, 2007, pp 1−65. (49) Anger, P.; Bharadwaj, P.; Novotny, L. Enhancement and Quenching of Single-Molecule Fluorescence. Phys. Rev. Lett. 2006, 96, 113002. (50) Kuhn, S.; Hakanson, U.; Rogobete, L.; Sandoghdar, V. Enhancement of Single-Molecule Fluorescence Using a Gold Nanoparticle as an Optical Nanoantenna. Phys. Rev. Lett. 2006, 97, 017402. 2290

dx.doi.org/10.1021/jz401051s | J. Phys. Chem. Lett. 2013, 4, 2284−2291

The Journal of Physical Chemistry Letters

Letter

(51) Jennings, T. L.; Singh, M. P.; Strouse, G. F. Fluorescent Lifetime Quenching near d = 1.5 nm Gold Nanoparticles: Probing NSET Validity. J. Am. Chem. Soc. 2006, 128, 5462−5467. (52) Li, M.; Cushing, S. K.; Wang, Q.; Shi, X.; Hornak, L. A.; Hong, Z.; Wu, N. Size-Dependent Energy Transfer between CdSe/ZnS Quantum Dots and Gold Nanoparticles. J. Phys. Chem. Lett. 2011, 2, 2125−2129. (53) Mangum, B. D.; Shafran, E.; Mu, C.; Gerton, J. M. ThreeDimensional Mapping of Near-Field Interactions via Single-Photon Tomography. Nano Lett. 2009, 9, 3440−3446. (54) Chhabra, R.; Sharma, J.; Wang, H.; Zou, S.; Lin, S.; Yan, H.; Lindsay, S.; Liu, Y. Distance-Dependent Interactions between Gold Nanoparticles and Fluorescent Molecules with DNA as Tunable Spacers. Nanotechnology 2009, 20, 485201. (55) Wang, Q.; Wang, H.; Lin, C.; Sharma, J.; Zou, S.; Liu, Y. Photonic Interaction between Quantum Dots and Gold Nanoparticles in Discrete Nanostructures through DNA Directed Self-Assembly. Chem. Commun. 2010, 46, 240−242. (56) Draine, B. T. The Discrete-Dipole Approximation and Its Application to Interstellar Graphite Grains. Astrophys. J. 1988, 333, 848−872. (57) He, Y.; Lu, M.; Cao, J.; Lu, H. P. Manipulating Protein Conformations by Single-Molecule AFM-FRET Nanoscopy. ACS Nano 2012, 6, 1221−1229.

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