Van Marum Colloquium: Guilty as charged: Electrochemical control over the charge carrier density in colloidal semiconductor nanomaterials
- Date
- Tuesday 8 October 2024
- Time
- Location
-
Gorlaeus Building
Einsteinweg 55
2333 CC Leiden - Room
- CE.0.18
Abstract
Colloidal semiconductor nanomaterials, such as quantum dots (QDs) and nanoplatelets (NPLs), are important materials for opto-electronic applications. They are already commercialized as phosphors in displays, and intensely investigated as active materials for e.g. LEDs, lasers and photodetectors. A common element in these applications is that charging of the materials is involved, either through charge injection, intentional electronic doping, or photoexcitation. However, charging of colloidal nanomaterials is not always easy, nor is it innocent.
In this talk I will discuss how electrochemistry can be used to both control the charge density and study the effect of charging on colloidal nanomaterials. The porous nature of nanomaterials allows efficient electrochemical doping, accommodated by charge compensation with electrolyte ions. This can be used to control the Fermi-level and study its effect on the electronic and optical properties of the materials, using steady state or (ultrafast) time-resolved spectroscopy.
Control over the Fermi level is useful to study the position of band edges, the occurrence of traps in the band gap and to controllable remove the threshold for optical gain via doping. I will show that it is possible to fix the Fermi level after electrochemical doping so that this method can be used to permanently dope films of nanocrystal. This allows the creation of e.g. pn junctions making electrochemical doping an interesting alternative technique for the formation of semiconductor devices.
However, changing the Fermi level is not always innocent and may induce structural changes that lead to trap formation and even decomposition. Understanding the nature of the electrochemical reactions that occur on the surface of colloidal nanomaterials is key in controlling their efficiency and stability. Subtle surface reactions, like the formation of dimers or the local reduction of surface ions on II-VI and III-V semiconductor nanocrystals, can already lead to quenching of the photoluminescence. For other materials, most notably lead based perovskites, progressive reduction of Pb ions results in the complete cathodic dissolution. I will discuss how this is governed by the solubility of surface Pb complexes, which form the weakest link in the system.
Biography
Arjan Houtepen heads the NCFun research group in the Chemical Engineering department at TU Delft. His group studies the FUNdamental properties of semiconductor nanomaterials, develops FUNctional nanomaterials and has FUN in the process. To this end, the group combines synthesis, advanced spectroscopy and electrochemistry. Arjan received his PhD from Utrecht University in 2007, under supervision of prof. Daniel Vanmaekelbergh and prof. John Kelly. He then joined Delft University of Technology as tenure track assistant professor. In 2020 he was promoted to full professor of Colloidal Nanomaterials at TU Delft. Arjan spent time as a visiting scientist at LMU Munich (2008-2009) and was guest professor at Ghent University (2014-2019). From 2017-2022 he was a (board member) of the Young Academy (DJA) of the Royal Dutch Academy of Sciences (KNAW).