Nanoscale Near-Field Imaging of Excitons in Single Heterostructured Nanorods

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 sub-20 nm region, correlated to the seed location, clearly indicating exciton localization.

Tip induced fluorescence quenching for nanometer optical and topographical resolution

“…For sharp tips, we see a distinct dark dot with a full width at half maximum (FWHM) less than 5 nm located inside the diffraction limited fluorescence spot (Figure 2a). The FWHM can be used as measure for the resolution (Dertinger et al, 2010a;Gerton et al, 2004;Huang et al, 2009;Ma et al, 2006;Westphal et al, 2003;Yoskovitz et al, 2010). Typical spot sizes are under 8 nm (Figure 2b-d).…”

Strain in free standing CdSe/CdS core-shell nanorods

“…2 We may actually find a wide variety of shapes 3 from nearly spherical nanocrystals (NCs) to large aspect ratio nanorods (NRs) and tetrapods. 4 In addition, colloidal heteronanocrystals are also currently synthesized 5 as, e.g., core-shell dot-in-a-rod 6,7 and rod-in-a-rod. 8 Elongated nanocrystals have an interesting physics, as systems located between zerodimensional QDs and onedimensional quantum wires (QWs).…”

“…18 Thus, it has been reported that by varying the composition and size, CdSe/CdS heterostructured nanorods were tuned between type-I, quasi type-II, and type-II characteristics. 7,8 In the present paper, we study the main characteristic strain trends arising in free-standing coated NRs. We have chosen CdSe/CdS as representative of II-IV coated NRs.…”

Universal Length Dependence of Rod-to-Seed Exciton Localization Efficiency in Type I and Quasi-Type II CdSe@CdS Nanorods

“…15,16,29,36À41 Due to the large valence band (VB) offset (>0.45 eV) and small conduction band (CB) offset (<0.3 eV) between bulk wurtzite CdSe and CdS, 15,38 the band alignment of these CdSe@CdS NRs can be tuned by the seed size and rod diameter from type I (with the lowest energy electron and hole levels confined in the CdSe seed) to quasi-type II (with the lowest energy hole confined in the CdSe seed and the lowest energy electron level degenerate in CdSe and CdS). 37À48 While it is clear that in type I NRs the lowest energy exciton is confined in the CdSe seed, extensive recent studies have revealed that in quasi-type II NRs, the electron in the lowest energy exciton state is also localized at and near the CdSe seed, 41,42,49 due to strong electronÀhole bindings in 1D nanostructures. 50 Therefore, in these heterostructures, the CdS rods acts as nanoscale light-harvesting antennas and the photogenerated excitons in the rod can be transported and localized to the CdSe seed, 40 mimicking natural lightharvesting complexes.…”