Seminar: prof. Arjan Houtepen - Guilty as charged: Electrochemical control over the charge carrier density in colloidal semiconductor nanomaterials

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
followed by charge separation. 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 it’s effect on the
electronic and optical properties of the materials, using steady state or (ultrafast) time-resolved
spectroscopy.
Control over the Fermi level is very 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.