EXPRESS

Understanding and EXPloiting dielectric REsponse in novel Semiconducting nanoSheets

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 Obiettivo del progetto (Objective)

'The revolutionary potential of nanoscience lies in the ability of designing new materials and functionalities for specific applications taking advantages of quantum nature of electrons. After the breakthrough discovery of graphene, the recent years have witnessed an explosive interest in two-dimensional Transition Metal Dichalcogenides nanosheets - diselenides and disulfides of transition metals, like MoS2, WS2, TiS2 and WSe2 - due their high technological potential for renewable energies, nanoelectronics and nanocatalysis. Major advances in this field heavily depend on the understanding of the electronic excitations sustained by these nanostructures under irradiation (by light, electron beams, synchrotrons or ultra-fast lasers) and on the ability to link the local chemical, electronic and structural modifications to changes in their macroscopic optical behavior. This project aims to provide a deep insight (through theory, simulation and experiments) to the electronic and optical properties of semiconducting Transition Metal Dichalcogenides nanosheets. By applying innovative computational modelling ab-initio techniques - within the framework of Time Dependent Density Functional Theory - the electronic structure and optical response of such systems will be investigated from the atomic scale up and then compared to experiments (i.e. electron energy loss spectroscopies). One key objective will focus on quantum confinement effects, evaluating the change in the electronic spectra when the size of the system is reduced. A second and fundamental aim is to investigate chemical modifications, e.g. the insertion of dopant atoms or molecules into the layered structure of the material. Due its high fundamental scientific content, enormous technological potential and strong multidisciplinary character, the present proposal will provide a significant step towards the understanding of optoelectronic properties of nanostructures through theory, simulation and material designing.'

Introduzione (Teaser)

Semi-Conducting sheets of nanostructured materials promise to form the basis of a new generation of electronic and optoelectronic devices. Novel insight into the electronic and optical properties of such materials should spur new areas of discovery.

Descrizione progetto (Article)

Graphene, a single-atom thick sheet of carbon atoms, has amazing strength to weight ratio and many other unusual properties. Graphene-like two-dimensional nanosheets have been in the research spotlight. In particular, compounds of transition metals - such as titanium - show great promise for use in renewable energy technologies, nano-optoelectronics and nanocatalysis.

Full exploitation depends on detailed characterisation of the electronic and optical properties of such materials. EU-funded scientists utilised theory, simulation and experiments to enhance insight from the atomic scale up with work on the project 'Understanding and exploiting dielectric response in novel semiconducting nanosheets' (EXPRESS).

Remarkable results were obtained regarding the dielectric properties of materials for light-emitting devices in the far-ultraviolet (UV) range using hexagonal boron nitride (hBN)-based systems. The main motivation of the work was related to the fact that emission at a single frequency occurs only in very pure samples and the multiple emission frequencies in common hBN are attributed to defects. However, the large number of experimental studies had not achieved correlation of specific emission features to well identified defective structures.

Advanced numerical methods for many-body solid-state physics have shown that opticalspectra and excitonic effects in bulk hBN can be strongly affected by symmetry distortions. These distortions are induced by crystal stacking faults and the associated atomic interactions between planes. Theories were then correlated with experimental results and used to explain them correctly.

EXPRESS findings suggest that deliberately inducing stacking faults in a perfect hBN crystal could be a way to tune the emission spectra in the far-UV region. Conversely, analysis of the complex spectra can be a way to map structural defects. Finally, outcomes show that many-body solid-state physics models are highly accurate when applied to the new class of layered materials with great technological potential.

These tools will thus be invaluable in predictive modelling and hypothesis formulation for effective design of future experiments. Knowledge contributed by EXPRESS will thus speed the discovery process and the development of exciting new products for a variety of applications.