Observation of the Full Exciton and Phonon Fine Structure in Colloidal Nanocrystals

Researchers from the HFML Nijmegen, in collaboration with scientists of the Debye Institute for Nanomaterials Science (Utrecht University), have performed high resolution optical spectroscopy experiments on semiconductor nanocrystals, revealing their full exciton and phonon fine structure. They have found that the nanocrystal light emission is a complex process, resulting from an intricate interplay between different exciton states, coupled to both acoustic and optical phonon modes. These findings enrich the understanding of the optical properties of nanocrystals, which have great potential in many nanoelectronic applications. The results are published in ACS Nano on 26 May 2014.

Colloidal semiconductor nanocrystals are organically capped nanoparticles with optical properties that are dramatically different from those of the bulk semiconductor material. They exhibit very efficient light emission that can be tuned in wavelength by varying their size, composition and shape, making them suitable for optoelectronic and photonic applications. However, light emission of nanocrystals is a complex process, depending on many factors, among which are the quantum mechanical size confinement of excitons (coupled electron–hole pairs) and the influence of confined phonon modes and the nanocrystal surface. Despite years of research, the nature of nanocrystal emission is still under debate.

For a detailed optical investigation CdSe dots were embedded in a rod-like CdS shell (dot-in-rod HNCs), which permitted to fabricate well-defined arrays with the long rod axes (the crystallographic c-axis) perpendicular to the sample surface (Figure left panel). These samples showed an unprecedented number of narrow emission lines upon resonant laser excitation (Figure middle panel). Application of high magnetic fields up to 30 T along the different crystallographic directions permitted to establish the origin of all these peaks.

A clear signature of an acoustic-phonon assisted transition, separated from the zero-phonon emission and optical-phonon replica, has been found. These results show that nanocrystal light emission results from an intricate interplay between bright (optically allowed) and dark (optically forbidden) exciton states, coupled to both acoustic and optical phonon modes (Figure, right panel). These findings enrich the understanding of the optical properties of nanocrystals.

Left: Schematic representation of a vertical assembly of CdSe/CdS nanorods, where the rod c-axes are oriented perpendicularly to the sample surface and parallel to the direction of the light emission k and magnetic field B. Middle: The optical spectrum consists of an unprecedented number of ultra-narrow peaks. Right: The resulting exciton and phonon fine structure of the CdSe/CdS dot-in-rod nanocrystals.

Reference

Observation of the Full Exciton and Phonon Fine Structure in CdSe/CdS Dot-in-Rod Heteronanocrystals, A. Granados del Águila, B. Jha, F. Pietra, E. Groeneveld, C. de Mello Donegá, J.C. Maan, D. Vanmaekelbergh and P.C.M. Christianen, ACS Nano 8, 5921–5931 (2014).