Mar 7, 2014
The art of colloidal quantum dots: micropatterning
Colloidal quantum dots (QDs), as new optoelectronic materials, hold great potential in photovoltaic and photodetection applications. Their electrical and optical properties are easily tunable, thanks to the quantum size effect, and the high precision hot injection synthesis method makes QDs readily available as colloidal nanoparticles dispersed in solution. The simple deposition of the QDs, however, does not allow for their use as an active material in integrated electronic or photonic circuits. For this, the formation of well defined and aligned patterns of colloidal QD layers is required. Reporting in Nanotechnology, researchers from Ghent University have addressed this issue and demonstrated the first technique to form high-quality micropatterns of QDs stabilized by inorganic ligands.
(a) SEM images of a micro-patterned OH- terminated PbS QD film. (b) Fluorescence microscope image of a CdSe/CdS micropatterned film.
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.
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.
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.
About the author
Chen Hu is a PhD candidate at Ghent University, working within the Centre of Nano- and Biophotonics (NB-Photonics) under the supervision of Gunther Roelkens and Zeger Hens. Her research focusses on QD-based photodetectors for silicon-on-insulator integrated photonics.