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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MOPTOPus (Metal oxide nanocrystals as optically driven dynamic manipulators of local (opto)electronic properties)

Teaser

Summary

Nanoscale structuring can add exciting new functionality to materials altering their way of interacting with light, which might be beneficially exploited for light harvesting systems in energy applications, such as photovoltaics or photocatalysis, but also for solid state lighting and new technologies such as quantum information. For example certain materials show enhanced light absorption and emission when structuring them as monolayers, with respect to their bulk counterpart. These so called two dimensional layered materials are only several atoms thick, which corresponds to a few tenth of a billionth of a meter, while their lateral dimensions can extend to several micrometers. This restricts the motion of their electrons to the two-dimensional plane, while the third direction is governed by quantum mechanics, largely influencing their electronic structure.
Due to the small size of the active material, the physical and electrical connection of such nanoscale functional material to devices of dimensions several orders of magnitude larger impact the nanomaterials\' structural and thus optoelectronic properties. Within this project we aim to establish a contactless approach to the manipulation of such layered two dimensional materials, controlled entirely by light. The exploitation of such a contactless approach to manipulate layered two dimensional materials is fundamentally new and allows accessing and dynamically controlling material properties at nanometer length scales previously hampered due to their connection to fabricated devices of several hundreds of nanometers in size.
To this aim we will interface the layered two dimensional materials with small semiconductor crystallites of several nanometers only. Such materials are grown bottom up in solution and stabilized to form floating nanocrystals. A specific type of this material, namely doped metal oxides, shows interesting properties, which we want to exploit to implement them as nano-manipulators driven by interaction with light only: the charging of the nano-manipulator upon light absorption (photo-doping), followed by charge storage, similar to a nano-battery, and the deliberate charge release with a second light pulse at a given point in time. This represents a fundamentally new application of doped semiconductor nanocrystals. The hybrid structures then are formed by physically contacting such nano-manipulators with the TMDCs by simple drop casting, exploiting the solution based approach of our nano-manipulators. We envisage that this nano-device will allow very local manipulation of the layered two dimensional materials in the nanometer range, of enormous importance for quantum optics applications, e.g. individually-addressable local single-photon sources at specified locations.
We furthermore foresee that a successful implementation and understanding will extend the application range of metal oxide nanocrystals to topics ranging from photocatalysis, to optically controlled nano-batteries, to all optical memory devices, giving way to new science in interdisciplinary research directions with new technological targets.

Work performed

In the first part of the MOPTOPus project we studied the possibility to accumulate extra charges in metal oxide nanocrystals (NCs) after light absorption beyond the bandgap of the material (in the ultraviolet, UV, spectral range). For the successful establishment of photo-doping and charge storage within such materials, we synthesized new metal oxide NCs, such as fluorine indium cadmium oxides (FICO) and tin doped indium oxide/indium oxide core-shell NCs (ITO/In2O3). Both materials were characterized with morphological and structural characterization tools, such as transmission electron microscopy or X-ray diffraction to identify their size (of several nanometers in diameter) and their crystal structure (compared to the bulk material). As a first set of experiments, we studied photo-doping in a solution of NCs and exposed them to UV light (around 310 nm). While in the FICO NC case, photo-doping was observed only when an additional hole reducing agent was added to the solution, in the ITO/In2O3 NCs, photo-doping occurred also in the absence of additional chemical species. This indicates that the surface, i.e., the shell of the ITO/In2O3 crystal acts as efficient hole scavenger. This is further confirmed by testing an ITO NC film deposited from the NCs in solution, which shows photo-doping effects without any additional hole scavenger after illumination with UV light. A profound understanding of the optical response of the nanomaterials and their dielectric function was extracted by implementing optical modeling of the plasmonic response of the materials under investigation. Models to be employed are based on the Drude model, the Mie theory, and the transfer matrix method (to account for thin film interference effects). Important aspects that were extracted indicate that depletion layers in NCs of dimensions of several nanometer in diameter play a crucial role and that after photo-doping the variation of the dielectric function is a result not only of an enhanced carrier density, but also due to a change in the high frequency dielectric constant.
As a final step in the first reporting period we went on to create the hybrid structures of the metal oxide NC/2D transition metal dichalcogenides hybrid structure. We implemented monolayer molybdenum disulfide (MoS2) as the underlying 2D material and spin coated the ITO/In2O3 NCs over them. In this way a hybrid structure was created for the further investigation of the effect of NC photo-doping on the underlying monolayer 2D material.

Final results

We were able to achieve photo-doping also in solid state architectures, an achievement so far not obtained in literature. We assign this effect to the special surface chemistry of the implemented ITO NCs, which is capable of accepting an additional hole after photo-doping. In our specific case we were fabricating one dimensional photonic crystals with alternating layers of photo-active ITO NCs and TiO2 NC films. By implementing this specific architecture we were able to tune the light transmission of the photonic crystal in two different regimes: first, the region of the plasmonic response, which changes as a result of the enhanced carrier density, i.e., plasmonic response, and, second, the region of the photonic bandgap as a result of the variation of the refractive index contrast between the two implemented layers. The latter work was published in Scientific Reports.[1] This latter achievement will have a direct impact on the way sunlight and heat energy is managed with smart windows for future low consumption buildings.
[1] G.M. PaternÃ², C. Iseppon, A. Dâ€™Altri, C. Fasanotti, G. Merati, M. Randi, A. Desii, E.A.A. Pogna, D. Viola, G. Cerullo, F. Scotognella, and I. Kriegel, Scientific Reports 8, 3517 (2018).