pubs.acs.org/NanoLett
Nanoscale Near-Field Imaging of Excitons in Single Heterostructured Nanorods Eyal Yoskovitz, Gabi Menagen, Amit Sitt, Ella Lachman, and Uri Banin* Institute of Chemistry and The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel ABSTRACT The mixed 0D-1D dimensionality of heterostructured semiconductor nanorods, resulting from the dot-in-rod architecture, raises intriguing questions concerning the location and confinement of the exciton and the origin of the fluorescence in such structures. Using apertureless near-field distance-dependent lifetime imaging together with AFM topography, we directly map the emission and determine its location with high precision along different types of nanorods. We find that the fluorescence is emanating from a sub20 nm region, correlated to the seed location, clearly indicating exciton localization. KEYWORDS Near-field imaging, seeded nanorods, fluorescence quenching and enhancement, excitons, fluorescence lifetime
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platinum-tipped quasi type-II nanorods.19 However, the direct mapping of the exciton in these systems has not been performed, nor its correlation with the material and topographical substructure. The unique combination of this nanoscale optical and structural information is essential for further understanding of these systems, which are also promising building blocks for optical and optoelectronic devices utilizing their strong, stable, polarized emission. High-resolution exciton mapping requires the use of optical methods that overcome the diffraction limit. In Apertureless Near-Field Optical Microscopy (ANSOM), a nanometric AFM tip scans the area surrounding an emitting particle (Figures 1c,d). When the tip is in close proximity to the particle, the fluorescence is modified, enabling to construct a near-field optical image with high resolution.20 Such methods were applied to nanostructure imaging including mapping of emission from carbon nanotubes6,21 and imaging single molecules and quantum dots.4,5,8,22-28 However, the substructure imaging performed here presents a significant challenge for this approach. To perform this task, we employ a distance-dependent lifetime based ANSOM technique (Figure 1d).8 As the tip scans in tapping mode, the optical contrast mechanism is based on height-sectioning of the collected photons. Lifetime curves at each pixel are built for any chosen tip-sample separation, allowing us to generate decay curves for photons that arrived when the tip was proximal to the surface. The effective decay rate (1/τ) is calculated for each pixel to construct the lifetime image simultaneously with height sectioned intensity image. All measurements were performed using platinum-coated commercial tips (see Supporting Information for experimental details). Several different seeded rod samples were synthesized by the published procedures and were deposited for optical experiments at high dilution on precleaned glass coverslips. Figure 1e shows an AFM topography scan of a single type-I
ombining nanoscale optical imaging together with other high-resolution imaging techniques provides an essential toolbox for nanoscience and single molecule studies in material-science, optics and biology.1-9 This capability is of particular significance in the investigation of complex nanoparticles. Heterostructured semiconductor nanorods, composed of a spherical seed covered with a rodshaped shell, constitute an important family of emergent nanocrystals, with exceptional optical properties.10-13 Charge carrier localization in seeded grown heterostrucutred semiconductor nanorods is controlled by the dimensions and composition of the seed and shell materials, allowing the sculpting of the potential landscape.10-14 This can lead either to an enclosed type-I band offset profile (Figure 1a,b), where both the electron and the hole wave functions are confined to the seed, or to a staggered type-II band alignment, in which the electron and hole are separated to different regions (Supporting Information, Figure S1). This determining aspect of the optical and electronic properties of heterostructured nanorods is under active debate, in particular for the prototypical CdSe/CdS seeded rod system. While elegant optical measurements were interpreted as demonstrating electron delocalization,15,16 scanning tunneling spectroscopy (STS) mapping of the level structure and electron wave function along the nanorods17 provided evidence for localization in the seed, as expected for a type-I system. Size dependence studies of the multiexciton spectra showed that for small seed diameters the system transfers from a localized type-I behavior to a quasi-type-II behavior where the electron wave function delocalizes into the shell.18 This was exploited in recent photocatalysis experiments, showing improved performance due to facile charge separation in
* Prof. Uri Banin,
[email protected], Tel: 972-2-658-4515; Fax: 972-2-6584148. Received for review: 05/06/2010 Published on Web: 07/06/2010 © 2010 American Chemical Society
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DOI: 10.1021/nl101614s | Nano Lett. 2010, 10, 3068–3072
FIGURE 1. Distance-dependence near-field imaging of a type-I heterostructure nanorod. (a) TEM image of CdSe/CdS heterostructure nanorods with seed diameter of 3.9 nm (inset) and length of 80-100 nm. Scalebar is 50 nm. (b) A scheme of the electronic potential profile and the first few confined levels. (c) Schematic of the experimental setup. (d) Basics of distance dependent lifetime imaging; for each pixel in the AFM scan, the collected photons are height-sectioned, and a lifetime curve is built, consisting of photons that arrived at small tip-particle distances. When the tip is far, the particle lifetime is unperturbed (black curve), but as the tip approaches the particle, lifetime is shortened. The effective decay rate is calculated for every pixel and a lifetime image is constructed, synchronized with the simultaneously acquired topography scan. (e) Topography scan of a CdSe/CdS rod, with 3.9 nm core and length of 108 nm (fwhm). The inset shows the cross section marked with arrows on the image. (f,g), Height-sectioned near-field intensity image and lifetime image of the same particle with cross sections of 45 and 15 nm (fwhm), respectively, taken along the topography cross section. (h) Superposition of the optical image on top of the topography image. The optical peak indicates the seed location along the rod. Scalebar for (e-h) is 50 nm. Images (e-h) were processed with WSxM.37
CdSe/CdS nanorod. Figure 1 panels f and g are the near-field intensity and lifetime images of the same nanorod, respectively, height-sectioned for short tip-sample vertical distances. Both images show clear signatures of fluorescence quenching upon interaction with the tip, expressed in lower counts and shortened lifetime. While the intensity image displays a feature of fluorescence quenching with full-width at half-maximum (fwhm) of 45 nm, the lifetime image shows a remarkably sharp peak with 15 nm fwhm. As seen, the lifetime imaging is of significantly higher quality compared to the intensity-based imaging since the particle lifetime shows sharper response to the presence of the tip29 and is less sensitive to fluctuations in the excitation or emission intensities.8 A major advantage of this method is that the optical measurement is accompanied by a simultaneous synchronized topography scan,4-6,8,9,21,22 as shown in Figure 1h where the optical lifetime image is superimposed over the topography scan. It is clear from this image that the © 2010 American Chemical Society
quenching effect took place in a well-defined small area within the rod. The lifetime image peak is similar in shape and width to those measured for spherical core/shell quantum dots with CdSe seeds of comparable diameter.8 Therefore, the quenching peak width is limited by the present experimental resolution, which is dictated by the range of tip-particle interaction and the active region of the AFM tip.8,30 We conclude that the exciton in the seeded rod is spatially localized at a specific position, which is in fact the location of the CdSe seed, within an error related to a possible small shift between the topography image and the lifetime image (
