Photodetection is one of the most important applications of colloidal QDs currently being studied, where they are used to absorb light and transport photogenerated charge carriers. For infrared light in particular, lead sulphide (PbS) QDs provide a competitive and cheap alternative to existing detectors. However, carrier transport is hampered by the electrically insulating organic ligands that are needed to cap the QDs during synthesis and stabilize the QD dispersion. Replacement of these organic ligands by short inorganic moieties is a crucial step to achieve performing photodetectors, yet no method was available for the patterning of films of QDs stabilized by small inorganic ligands.

Micropatterning

A multidisciplinary team of the Centre of Nano- and Biophotonics (NB-Photonics) of Ghent University – including the Physics and Chemistry of Nanostructures Group , the Photonics Research Group (PRG) and the Liquid Crystals and Photonics Group – have demonstrated the first technique to form high-quality micropatterns of QDs stabilized by inorganic ligands. In the first step, smooth and crack-free PbS QD films are formed through a layer-by-layer approach where each cycle involves the deposition of a QD layer by dip-coating, the replacement of the native organic ligands by metal-free inorganic ligands, followed by a thorough cleaning of the resulting film. Next, the micropatterns are defined by a positive photoresist spun on the QD layer, followed by the removal of uncovered QDs by wet etching using an HCl/H3PO4 mixture.

Photoluminescent properties

The resulting micropatterns can have submicron features and the process applies to 3D substrates as well. In addition, patterned films of highly luminescent CdSe/CdS core/shell QDs can be formed with the same technique, where the combination of photolithography and wet etching maintains more than 90% of the photoluminescence quantum yield. This indicates that this approach provides a new way to implement colloidal QDs in large-scale optoelectronic integration, without affecting the properties of the QDs, and can ultimately lead to integrated PbS QD-based photodetectors.

More information can be found in the journal Nanotechnology 25 175302.

Further reading

Colloidal zinc phosphide for photovoltaics (Sept 2013)
Mild post-deposition treatments benefit colloidal nanocrystal solar cells (Aug 2012)