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Metal oxide nanocrystals as optically driven dynamic manipulators of local (opto)electronic properties

Periodic Reporting for period 2 - MOPTOPus (Metal oxide nanocrystals as optically driven dynamic manipulators of local (opto)electronic properties)

Reporting period: 2018-09-01 to 2019-08-31

Nanoscale structuring can add exciting new functionality to materials altering their way of interacting with light. This might be beneficially exploited for light harvesting systems in energy applications, such as photovoltaics, 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, as used in our daily lives, impacts the nanomaterials' structural and optoelectronic properties. Within this project we aimed to establish a contactless approach to their manipulation controlled entirely by light. The exploitation of such a contactless approach is fundamentally new and allows accessing and dynamically controlling material properties at nanometer length scales previously hampered due to their connection to bulky devices. To this aim we 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 and serve as light-driven charge injection sources. This light-triggered and contactless hybrid nano-device enabled us to very locally manipulate the layered two dimensional materials in the nanometer range, of enormous importance for quantum optics application. Finally, the results of this project lead to the development of novel nanocapacitors that are charged only by light, hence taking a step towards the direct light energy storage. This is a first step towards the development of storage devices that are charged up directly by the sun and of fundamental importance for the development of green and sustainable energies.
All together the outcome of this project has allowed me to receive the ERC Starting Grant 2019 and ultimately a Tenure Track Researcher position at the Italian Institute of Technology, Genova, Italy.
In the second reporting period we targeted two different scientific questions: first a profound understanding of the role of the metal oxide (MO) nanocrystal surface and second the hybrid interaction of the nanomaterials with the 2D material. The role of the nanocrystal surface was studied by implementing nanocrystals with systematically grown oxide shell around them. In this way an energy band alignment is created that favors the storage of electrons in the core and the transfer of holes towards the nanocrystal surface as well as the creation of an artificial depletion layer that allows storing additional electrons in the nanocrystal after light absorption. Transient absorption spectroscopy revealed a slower recombination dynamics in the presence of a thicker shell layer, hence, supporting the enhanced spatial separation of the electron and hole pair due to the presence of a thicker shell. On the other hand, the time dependent photodoping in unison with optical modeling revealed that the thickness of the shell plays a major role on the charge storage capability of the nanocrystals.
In the hybrid system, two major outcomes were reported: first, we identified that the underlying 2D material serves as efficient, solid state and permanent hole acceptor and, hence, we were able to show for the first time all solid state photodoping of MO nanocrystals; and second that the amount of charges that were transferred is on the order of magnitude of the electrostatic gating of 2D materials. Together this means that the MO nanocrystals serve as light-driven charge injection sources and contactless nano-gates driven only by light. Such contactless, and localized charge injection has been implemented to observe on the microscale carrier migration in the 2D material extracting values up to tens of micrometers far from the micrometer sized injection laser spot. Furthermore, the amount of charges that were separated highlights that the ultrathin hybrid structure can serve as a nanoscale capacitor with capacitance values reaching values established for similar materials in solution.
The results of this reporting period have been published in 3 peer reviewed articles and one article pre-print on arxiv. The results of this work have been presented in two invited presentations at the NT19, International Conference on the Science and Application of Nanotubes and Low-Dimensional Materials, Würzburg, Germany (2019) and at the 41st PIERS conference, Rome, Italy (2019). A total of 10 peer reviewed articles was published.
Within MOPTOPus we have developed a novel solid-state nanoscale material system displaying light-driven capacitive charging dynamics. This system is capable of both harvesting optical energy and quasi-permanently storing it, thus demonstrating a new paradigm of nanoscale energy systems for autonomous miniature energy storage. The novel system is composed of a 2D material overlaid with a monolayer of doped indium tin oxide nanocrystals. By achieving direct coupling between the nanocrystals and the 2D semiconductor for the first time, we have discovered a method to quasi-permanently change the charge states of the two components: upon UV illuminationphotodoping, photo-generated holes in the nanocrystal efficiently transfer to the 2D semiconductor forming a nanoscale, capacitor-like hybrid structure that remains charged even after the UV illumination is terminated, for more than 72 days (we have only measured up to 72 days so far). This phenomenon, which is altogether distinct from transient charge transfer effects that diminish on ultrafast timescales, opens up new routes for micro/nanoscopic solid-state elements that generate and store energy via remote and local, all-optical charging. The temporal variation of the charge density in this 0D-2D hybrid system lead us to evaluate the observed charging dynamics with a capacitor model. Our estimates indicate that energy storage at levels of ~1 μJ cm-2 are possible in this system that has an overall thickness of only ~12 nm. Based on our current findings, we predict that even better performance can be achieved through, for instance, strain engineering or vertical heterostructure engineering of the 2D material, opening up an entirely new parameter space for follow-up works and catalyzing the development of a fully light-driven nanocapacitor system as a new form of autonomous energy storage. Moreover, we see the potential of this hybrid system to locally inject carriers on the micrometer scale as a step towards contactless light-driven nanoelectronics. The findings of this work reveal a new foundational building block for next-generation nanodevices that can enable remote-control of local charge density, novel light-driven energy storage schemes, and light-activated nanocircuitry. . (1,2)
(1) Kriegel, I. et al. J. Phys. Chem. C 2020, 124 (14), 8000–8007.
(2) Kriegel, I. et al. Arxiv 2018, 1810.05385.
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