Seeded Growth of Highly Luminescent CdSe/CdS Nanoheterostructures with Rod and Tetrapod Morphologies

Abstract

We have demonstrated that seeded growth of nanocrystals offers a convenient way to design nanoheterostructures with complex shapes and morphologies by changing the crystalline structure of the seed. By using CdSe nanocrystals with wurtzite and zinc blende structure as seeds for growth of CdS nanorods, we synthesized CdSe/CdS heterostructures nanorods and nano-tetrapods, respectively. Both of these structures showed excellent luminescent properties, combining high photoluminescence efficiency (~80% and ~50% for nanorods and nano-tetrapods, correspondingly), giant extinction coefficients (~2·10 7 M -1 cm -1 and ~1.5·10 8 M -1 cm -1 at 350 nm for nanorods and nano-tetrapods, correspondingly) and efficient energy transfer from the CdS arms into the emitting CdSe core. † Current address:

Low Threshold, Amplified Spontaneous Emission from Core‐Seeded Semiconductor Nanotetrapods Incorporated into a Sol–Gel Matrix

“…Radiative recombination typically takes place at the CdSe core at the centre of four rod-like CdS arms, and extensive leakage of the CdSe electron wavefunction into the CdS shell results in a dramatic redshift in its optical spectra relative to the original CdSe NC seed. [ 9 ] The ensuing reduced spatial overlap between the CdSe electron and hole wavefunctions is expected to yield suppressed Auger recombination rates, as was previously observed in a related system of CdSe seeded CdS nanorods exhibiting relatively long biexciton lifetimes of ∼ 300 ps. [ 10 ] While it has been suggested that CdSe seeded CdS nanotetrapods hold promise as good NC-based optical gain materials, [ 9 , 11 ] however, ASE from such structures has to the best of our knowledge not yet been demonstrated.…”

“…Atomic Force Microscope (AFM) measurements on the resulting tetrapodsilica composite fi lm, as exemplifi ed in Figure 1 (b), yielded a smooth morphology with a typical surface roughness (RMS) of about ∼ 7.5 nm, over a 1 μ m 2 scan area, suggesting a uniform incorporation of tetrapods into the sol-gel derived silica matrix. Absorbance measurements, profi lometry analysis of the fi lm thickness and the molar absorptivity of CdSe seeded CdS tetrapods determined via a previously reported method [ 9 ] provided an estimate of the volume fraction of tetrapods in the silica fi lm, which was about ∼ 6-10% for all the fi lms presented in this work. It is important, however, to note that the volume fraction with respect to the CdSe core is less than 1% given the dominant contribution of CdS to the overall volume of the CdSe seeded CdS tetrapod structure.…”

“…[ 9 ] Indeed, due to their unique tetrahedrallike arrangement, the tetrapod arms can capture photons from a wide range of incident angles and polarization more effectively than nanorods of the same volume, thus leading to a very large absorption cross section of the entire nanostructure. Radiative recombination typically takes place at the CdSe core at the centre of four rod-like CdS arms, and extensive leakage of the CdSe electron wavefunction into the CdS shell results in a dramatic redshift in its optical spectra relative to the original CdSe NC seed.…”

Colloidal Quantum Nanostructures: Emerging Materials for Display Applications

“…In 2003 a dot‐in‐rod architecture of a CdSe dot within a CdS rod was introduced,82 and later a seeded growth synthesis for this interesting system was reported 83, 84. This CdSe/CdS dot‐in‐rod heterostructure system has a mixed dimensionality as it integrates a 0D confined seed within a 1D confined system.…”

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

“…We can conclude that, by pumping well above the CdS band-edge, hot carrier trapping in the shell (at a rate τ tr ) may still limit the number of excitons reaching the lowest energy state in the CdSe core. Further support for this notion stems from PL excitation spectroscopy in CdSe/CdS Qdot-in-rods, showing a reduced PL QE when pumping the material above the CdS absorption edge, [ 20 ] and from transient absorption measurements on CdSe Qdots, in which fast (ps-timescale) hot exciton trapping has also been observed. [ 21 ] For such bare nanocrystals it lead to strongly suppressed gain when pumping them at 400 nm.…”