Nanocrystals for mid-infrared photonics 

 

W. Heiss

 

 

We develop infrared optical devices based on colloidal nanocrystals (c-NCs). To improve photodetector performances charge separation will be improved by internal fields, charge transport through c-NCs films will be optimized, and device lifetimes will be prolonged. The longest cut-off wavelengths of c-NC detectors will be shifted to wavelengths exceeding 5 μm and the performance of c-NC based detectors will be compared to epitaxial quantum dot detectors, fabricated from the same materials and measured under identical conditions. The investigated model systems are: (i) PbS/CdS core/shell c-NCs covered by inorganic-organic hybrid ligands, which can be converted into a semiconducting matrix by thermal treatments. These conducting films will be combined with Si-nanowire electrodes to form depleted heterojunction photodiodes. (ii) Photoconductor films from semimetal c-NCs such as Bi and Sn with band gaps opened by quantum confinement. (iii) Epitaxial PbTe quantum dots in CdTe matrix and for comparison films of colloidal PbTe/CdTe core/shell NCs.

To obtain Pb-chalcogenide c-NC based lasers, colloidal solutions will be introduced into optical resonators formed by dielectric Bragg interference mirrors. Alternatively, the c-NCs will be deposited on the surface of GaAs based optical resonators. Of uppermost importance will be the reduction of the nonradiative Auger recombination in the c-NCs, which requires the growth of heterostructure c-NCs providing either a staggered type II band alignment (Klimov 2007) or at least a giant shell (Garcia-Santamaria 2009). Most promising for Pb-chalcogenide based c-NCs is, however, to avoid abrupt interfaces and to replace them by gradually changing potentials (Wang 2009), as can be obtained in annealed PbSe1-xSx c-NCs (Maikov 2010). The same materials developed for laser devices, with reduced Auger recombination, will be tested also as single photon infrared sources, which could show reduced blinking in respect to core only c-NCs. To facilitate detection of single photons the c-NC will be (i) placed on predefined positions on substrates or (ii), they will be embedded in cavities in the center of 2-dimensional photonic band gap structures, redirecting the emission of the c-NCs towards the collecting microscope objective.

Attaching inorganic-organic hybrid ligands to the PbS/CdS core/shell c-NCs, as is done for detector development, will open us another interesting route of research. Selfassembly of such c-NCs into 2 and 3 dimensional superlattices could allow the observation of miniband transport through such structures, similar as in epitaxial superlattice structures. For that purpose the properties of such superlattices will be investigated not only by transport but also by optical methods as well as by X-ray spectroscopy.

To achieve all these goals this project will strongly interact with a number of projects within the SFB (P02-P04, P06-P08, and P12 will be the main partners) and with two international collaborators at the ETH in Zürich, CH, and at the University in Groningen, NL.

 

Report 2005-2009 (.pdf)

 

Report 2009-2012 (.pdf)