Seeded grown heterostrucutred semicondcutor nanorods are novel materials with unique characteristics, composed of a spherical seed, covered with a rod-shaped shell. These particles combine the characteristics of two distinctive structures leading to a dot in rod architecture and therefore attract a significant interest, especially in the field of optics, where they demonstrate bright and stable polarized emission. As such, they are promising candidates, both for fundamental research and for diverse applications.

Intriguing questions concerning these structures are the location and extent of confinement of the exciton in type-I and type-II rods, and the lateral origin of the emission in each type. In this work we explore the characteristics of the exciton in heterostructured type-I and type-II nanorods, and provide optical images with nanoscale resolution. Resolving this structure optically is impossible with far field measurements. In order to overcome the diffraction limit we developed in our lab Appertureless Near Field Scanning Microscopy (ANSOM) based method, combined with distance-dependent Time Correlated Single Photon Counting (TCSPC) capabilities. In this method, an AFM tip scans in tapping mode proximal to the emitting nanoparticle and a correlated AFM-fluorescence image can be obtained. Height sectioning of the distance dependent fluorescence lifetime data allows for sub 20nm resolution in the optical images with very good signal-to-noise characteristics. Based both on fluorescence quenching and fluorescence enhancement as our contrast mechanisms, we produce lifetime images of single nanorods. These images are fully synchronized with the AFM topography image, providing comprehensive data on the particle under investigation.

Our lifetime images show a clearly localized optical peak, for nanorods of both types, with different size of seeds and different lengths. The size of the optical feature, mainly determined by the size of the AFM tip, suggests that in type-I rods the exciton is confined within the seed. In type-II rods the localized optical feature suggests emission from the core-shell interface.  Correlating the optical image with the AFM topography, we find that the exciton is located in average at ~1/4 of the rod length, and moving towards lower ratio as the rod becomes longer, in agreement with structural characterization. Aside from providing unprecedented optical resolution to locate where ‘the light comes from’ within the rod heterostructure, fluorescence intensity and lifetime approach curves provide significant physical insight into the interplay of near-field enhancement effects and fluorescence quenching. Combining this with versatile seeded rod samples with either type I or type II band offsets, leads to a comprehensive view of the optical and electronic properties of such high quality heterostructured nanorods.