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J. Phys. Chem. C 2008, 112, 16306–16311
Apertureless Near-Field Distance-Dependent Lifetime Imaging and Spectroscopy of Semiconductor Nanocrystals Eyal Yoskovitz, Dan Oron,† Itzhak Shweky, and Uri Banin* Institute of Chemistry, and the Center for Nanoscience and Nanotechnology, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: May 15, 2008
Apertureless near-field scanning optical microscopy, along with time-resolving capabilities, is used to produce optical imaging and spectroscopy measurements of single-semiconductor nanocrystals, in correlation with the AFM topography scan. The strongly distance-dependent energy transfer between the excited particle and silicon or metallic-coated AFM tips provides a contrast mechanism for subdiffraction-limited optical imaging. Fluorescence lifetime optical images show excellent contrast, sharpness, sensitivity, and resolution equivalent to that of the AFM topography images (sub 20 nm) and significantly improved over fluorescence intensity images. The sharper resolution of lifetime images is consistent with model predictions of energy transfer between an emitting dipole and a dielectric surface. Lifetime images also enable resolving multiple emitters located in the excitation spot. The comprehensive time and distance dependent data is used to study the imaging mechanism and the properties of silicon tips and platinum-coated tips as energy acceptors and quenchers. The findings provide a basis for use of lifetime imaging, in conjunction with apertureless near field microscopy, for simultaneous high-resolution topography and optical imaging. Introduction Near-field scanning optical microscopy (NSOM) is widely used to overcome the constraint of the diffraction limit in optical imaging. In apertureless near-field scanning optical microscopy (ANSOM), a sharp probe, usually an AFM tip, is brought to the proximity of an emitting fluorophore. The changes in the emission due to the proximity of the tip are translated into an optical image, and its resolution depends upon the range and strength of the interaction between the tip and the particle. Parameters such as the tip material and the system optical characteristics,1-5 tip, and particle geometry,6,7 the fluorophore type, size, and polarization8 are the main factors that determine this range for a specific pair. The tip-fluorophore interaction can result in two opposing effects. The first one is fluorescence enhancement as a consequence of field enhancement at the apex of the tip.6,9-13 The second effect, dominant at short distances, is fluorescence quenching by nonradiative energy transfer from the fluorophore to the tip,1-4,14 which is electrostatic and derived from dipole-dipole coupling. Both effects were used for highresolution optical imaging, below the diffraction limit.1,2,11,13,15 Here, we report on ANSOM imaging of semiconductor nanocrystals based on the distance-dependent fluorescence lifetime and intensity measurements in a quenching scheme. Optical resolution comparable to the AFM topography resolution is obtained from the lifetime images. Physical interaction mechanisms with the AFM tips are also investigated by distancedependent measurements. The results point out the strength and potential of use of such a scheme for high-resolution nanoscale optical probing of materials and biological systems. Several studies combined AFM topographic data with optical imaging of single quantum dots and single molecules.1,10-13,16-18 Adding continuous tip-to-sample distance-dependent information was first implemented with tapping-mode AFM in an enhancement scheme10 and later in a quenching scheme;1 both cases * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. † Current address: Dr. Dan Oron, Dept. of Physics of Complex Systems, Weizmann Institute of Science,Rehovot 76100, Israel
used intensity as the contrast mechanism. An alternative contrast mechanism is distance-dependent lifetime detection, where the particle lifetime is monitored continuously with respect to the tip position. As the tip approaches the particle, the measured lifetime is affected by the tip, resulting at very short distances in significant shortening due to the opening of nonradiative decay channels. These changes can be mapped into an optical image. Calculations within the CPS (Chance, Prock, Silbey) model show that the response of the fluorophore lifetime to the proximity of a dielectric mirror is significantly sharper at small distances than the response of its integrated intensity.19 Therefore, fluorescence lifetime imaging is expected to provide improved spatial resolution over the intensity images, which in our previous studies were limited to ∼60 nm. Furthermore, fluorescence intensity has experimental limitations. Changes in the excitation power induced by the presence of the tip,1 transitions of the particle between low and high emission states,4 and the reflection of the emission by the tip acting as a mirror directly influence the fluorescence intensity and therefore the image quality. Fluorescence lifetime imaging (FLIM)20,21 can bypass these latter difficulties as the decay rate is fairly insensitive to experimental variations such as excitation intensity fluctuations or misalignment. In a combined enhancementquenching scheme, lifetime yet has another advantage over intensity. In short tip-sample distances,the fluorescence quenching competes with the enhancement effect and the quality of the intensity image deteriorates, whereas both effects shorten the measured lifetime15,22 and can contribute to the FLIM image quality. FLIM was previously combined with near-field microscopy.23 Use of FLIM in contact-mode scanning required the sample to be embedded in a thin polymer film compromising the topographic data.2 Tapping mode AFM with FLIM16 was also reported, but the data was averaged over the entire tapping amplitude of the tip (∼30 nm), compromising the optical information. The effect of a nanostructure on the fluorescence quantum efficiency and lifetime of a single dye molecule was investigated in several works, either by connecting a nanoparticle
10.1021/jp8043253 CCC: $40.75 2008 American Chemical Society Published on Web 10/01/2008
Semiconductor Nanocrystals
Figure 1. A single particle lifetime, intensity, and topography images, taken with a silicon tip. The inset shows the experimental scheme. (A) is the AFM topography, and (B) is the original fluorescence image, integrating photons of all tip-surface distances. (C) and (D) show the photon counts vs time and photon counts vs tip-particle vertical distance respectively, measured in location 1, 100 nm laterally away from the particle. The black lifetime curve was measured when the tip was high above the particle (indicated by black circle) and the red curve when the tip was at the bottom of its oscillation (red circle). In this location, there is no interaction between the tip and the particle and no change in the particle fluorescence. The minus sign on the distance axis implies the tip is moving away from the surface. (E) and (F) show the same curves for location 2, where the tip is above the particle. A clear decrease in fluorescence intensity and lifetime is noted. (G) is the sliced lifetime image, consisting of photons arriving at tip-sample distances
