Michele Ghini, Italian Institute of Technology

Written by  Michele Ghini, Nicola Curreli, Ilka Kriegel.

Doped metal oxide (MO) nanocrystals (NCs) are gaining the attention of the scientific community thanks to the combination of unique properties, such as high electron mobility, the tunability of their carrier density, chemical stability, and low toxicity, as well as suitable operating temperature, which makes them appropriate for a large plethora of applications, ranging from nanoelectronics and plasmonics to the next-generation energy storage. In MO NCs, Fermi level pinning results into the formation of electronically depleted layers, which affect their optical and electronic properties. For precise control over functionality, it is important to understand the role of several structural and electronic parameters, with the aim to engineer the NC’s electronic band profiles at the nanoscale. 

Our research group, led by Dr. I. Kriegel at the Italian Institute of Technology, investigated the impact of depletion layers on doped model oxide NCs (Figure 1). For instance, the presence of surface states and the combination of various materials and doping levels in diverse architectures lead to different energy band profiles and depletion layer widths. Exemplified by the case study of ITO-In₂O₃ core-shell NCs, we show that the introduction of more than one electronic interface induces a double bending of the energy bands accompanied by a distinct carrier density profile and multi-modal plasmonic responses. This induces a multiple bending within the nanoparticle effectively separating the NC into three distinct electronic regions: an active core region with specific carrier density, a transition region with an order of magnitude lower carrier density, and an undoped depletion region. Notably, the electronic band profile does not correspond to the physical core-shell boundaries (Figure 2a).

Such bands are modulated upon the introduction of extra electrons after light absorption and we found that the electronic rearrangement of these three distinct regions has a fundamental role in the photodoping of MO NCs. Specifically, the light-induced bending of the bands close to the surface of the nanocrystal is the main mechanism responsible for the storage of extra photogenerated electrons (Figure 2b). This modulation leads to the suppression of the depletion layer, which is filled with the extra photogenerated carriers and results in a significant rise of carrier density in the superficial regions of the NC (Figure 2c). Our theoretical findings are supported by experiments in which we implemented depletion layer engineering to enhance the light-driven charge storage capability of MO NCs, resulting in the storage of hundreds of electrons per nano-unit. In this way have been able to compare numerical simulations with empirical modelling and experiments. This allowed to extract the main mechanism of photodoping in MO NCs, a process so far not understood from the electronic point of view.

These results are transferable to other core-shell and core-multishell systems as well, opening up a novel direction to control the optoelectronic properties of nanoscale MOs by designing their energetic band profiles through depletion layer engineering, and ultimately their implementation in smart optoelectronics, photocatalysis, or energy storage applications.

For more details, please check out our paper “Control of electronic band profiles through depletion layer engineering in core–shell nanocrystals” in Nature Communications (www.nature.com/articles/s41467-022-28140-y) and our Functional Nanosystem group website (www.iit.it/it/web/functional-nanosystems).