We are developing new synthesis methods for the preparation of II-VI and III-V semiconductor nanocrystals with their absorption/emission covering the near UV, visible and near IR spectrum. For the last few years, our research focused on the development of cadmium-free quantum dots of controlled size, shape, optical and electronic properties. Examples of these are ZnSe, InP, CuInS2, and CuInSe2. The use of non-pyrophoric precursors, such as fatty acid complexes, carbamates or xanthates allows for an easy upscaling of the process to larger quantities.
The key parameters, which have to be controlled during nanocrystal synthesis are:
-size : emission colour
-size dispersion : photoluminescence line width 
-surface passivation : photoluminescence quantum yield (QY)

SIZE
The nanocrystal size depends on a large number of experimental conditions, including precursor types and concentrations, solvent, reaction time and temperature, etc.


The scheme on the left shows a typical experimental setup for the synthesis of monodisperse semiconductor nanocrystals by rapid injection of precursors into the hot solvent. Alternatively, in many cases all precursors, stabilizers and solvents can be mixed at low temperature and the heated to the reaction temperature ("heating up approach").

Main collaborations: F. Delpech, C. Nayral (LPCNO Toulouse), M. Kovalenko (ETH Zuerich), A. Cabot (IREC Barcelona)

MONODISPERSITY
Among the diverse methods for synthesis of semiconductor nanocrystals, the most efficient in terms of a small size dispersion are based on a temporal separation of nucleation and growth. To achieve this, the precursors are rapidly injected into the reaction flask containing the solvent and surfactants at elevated temperature (typically 250-350°C). Transient supersaturation of the precursors leads to a nucleation burst where all seeds are formed. At the same time the precursor concentration goes down below the nucleation threshold and the seeds grow into nanocrystals. This growth from solution is usually followed by a second growth process, called Ostwald ripening, where the smallest particles dissolve and the matter is deposited onto larger ones.
A sample is called 'monodisperse', if its size dispersions is equal or inferior to 5%. We are developing synthesis methods for semiconductor nanocrystals aiming at monodisperse samples without additional size sorting procedures.

Core/Shell nanocrystals
Without further surface passivation, the QY of the nanocrystals is small, typically around 10% at room temperature. However, by growth of a shell of larger bandgap semiconductor, the QY can be increased to above 50%. Our group has developed a method for coating CdSe nanocrystals with a ZnSe shell that leads to QYs above 80%, a value similar to the best organic dyes. In 2003, we have introduced the CdSe/ZnSe/ZnS core/double shell system. Here, an intermediate shell (ZnSe) assures a "smooth" passage between the crystallographically different core (CdSe) and outer shell (ZnS) materials. As a result, higher emission efficiencies in combination with thicker ZnS shells can be obtained with respect to standard CdSe/ZnS core/shell systems. In the same manner, a CdS intermediate shell can be used between the CdSe core and the ZnS shell. The photo below shows a sample of CdSe/ZnSe/ZnS nanocrystals, excited by sunlight. Reaching similar performance with alternative Cd-free materials is still a challenging task, in this field we are focusing on InP- and CuInS₂-based nanocrystals. 

SURFACE FUNCTIONALIZATION
Via exchange of the original surfactant molecules on their surface by new ligands, several important properties of the nanocrystals can be tuned, such as their processibility, reactivity and stability. The principal functions of the surface ligands are:
1) They prevent individual colloidal nanocrystals from aggregation.
2) They facilitate nanocrystals dispersion in a large variety of solvents. For the application of nanocrystals in biological labeling, ligands enabling their dispersion in aqueous solutions are of special interest.
3) Ligands containing appropriate functional groups may serve as bridging units for the coupling of molecules or macromolecules to nanocrystals or their grafting on substrates.
We are developing 'tailor-made' surface ligands with the goal to enhance nanocrystals' photostability and to introduce functional groups, while maintaining essentially their optical properties and size. An example are chelating bifunctional ligands derived from carbodithioic acids, which enable the grafting of electroactive oligomers or polymers on the nanocrystal surface.