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Understanding and EXPloiting dielectric REsponse in novel Semiconducting nanoSheets

Final Report Summary - EXPRESS (Understanding and EXPloiting dielectric REsponse in novel Semiconducting nanoSheets)

The end of the last century has seen a progressive interest in materials and devices with reduced size and dimensionality where nanoscale appeared as the new frontier of technological innovation. Nanostructures and nanomaterials have been considered the key development for the next generation technology, due to their ease of processing, unique properties and compatibility with the existent microelectronics. In this context two-dimensional (2D) nanosheets (NSs), which possess atomic or molecular thickness and infinite planar dimensions, are the strongest candidate to provide the change of paradigm needed by the new generation of electronic and optoelectronic devices, either by replacing or, more realistically, integrating the existing Complementary Metal-Oxide-Semiconductor (CMOS) technology. NSs can be extracted from their bulk counterparts – as in the case of graphite – using mechanical or solvent-based exfoliation methods whose advances in the last fifteen years have represented a cutting-edge research topic in nanoscience.
The long term aim of the EU funded project EXPRESS was to obtain a deep understanding of the electronic and optical properties of novel semiconducting nanosheets. To absolve this task, theoretical and experimental aspects of Electron Energy Loss Spectroscopy (EELS) have been developed and joined to study the dielectric response in visible-near UV range of such materials.
The project has been initially funded for 24 months. However, due the recruitment of the principal investigator (Dr Michele Amato) as Assistant Professor at Universite Paris Sud, it has last only 6 months. It is worth to note that the award of this fellowship has played a crucial role in the career of Dr Amato and has represented one of the most important stage of his path towards a permanent position. During these 6 months, noticeable and remarkable results have been obtained, focusing the attention on the theoretical-experimental study of the dielectric properties hexagonal boron nitride based systems.
Hexagonal boron nitride (h-BN) is one of the most promising candidates for light emitting devices in the far UV region. Indeed h-BN presents a strong emission line at 5.8 eV that has been attributed to Frenkel excitons. However, a single emission line appears only in extremely pure mono-crystals that can be obtained in a limited amount only through complex high pressure and temperature synthesis processes. Common h-BN samples present more complex emission spectra with a series of sharp peaks below the free exciton and an additional broad emission band and a series of three sharp lines within the electronic band gap. An equivalent spectral richness has been confirmed by optical absorption experiments. The presence of these additional emission lines have been attributed generally to structural defects. It has been shown for instance that the sharp series in the free exciton region can appear on a pristine perfect crystal after applying a reduced mechanical strain. On the same way the diffused band is generally associated with impurities and low quality crystals. Despite a large number of experimental studies it has not been possible yet to attribute specific emission features to well identified defective structures.
The primary objective of the project has been to provide fundamental insights on the electronic structure and spectroscopic response of hexagonal boron nitride in the presence of extended morphological modifications. In particular it has been considered how absorption spectra and excitonic effects can be affected by confinement effects in few layers systems and by symmetry distortions induced by crystal stacking faults.
The aim of this theoretical study was to provide a more in-deep understanding to experiments of spatially resolved cathodoluminescence in few layer h-BN. Main objectives of the project can be summarized as follows:
- A complete comprehension of the role of stacking faults on the electronic structure of bulk h-BN, with particular emphasis on the different structural and electronic changes that different stacking sequences can induce;
- To calculate the optical absorption and EELS spectra for the over described systems, in order to give a valid reference for the experimental measurements;
- To investigate the role of excitonic effects in both bulk and few layered h-BN systems, aiming to describe how the screening is affected by confinement and if it possible to predict a transition from dark to bright exciton due to symmetry lowering.
The study has been carried out within the framework of the Density Functional Theory (DFT) for the ground state properties and using also the Many Body Perturbation Theory (MBPT) and Time Dependent DFT (TDDFT) for the evaluation of quasi-particle corrections and optical responses respectively.
We can state that the main objectives of this first part of the project have been successfully reached. We have performed first-principles calculations on the bulk boron nitride with different stacking in the framework of many body perturbation theory (namely GW approximation and Bethe-Salpeter equation). In particular our calculations show that absorption spectra and excitonic effects can be strongly affected by symmetry distortions induced by crystal stacking faults. Indeed calculations have demonstrated that a strong red shift of the first excitonic peak can be obtained by changing the atomic configuration stacking. This shift is strongly related to the atomic interactions between the planes and to the excitonic localization.
Moreover crystal stacking faults (in particular translation of one atomic plane with respect to the other) has been demonstrated to be responsible of the splitting of the first excitonic peak in two peaks of minor intensity.
Our theoretical results have been then used to correctly interpret and explain the additional emission lines in the h-BN spectra measured in cathodoluminescence experiments performed by the Scanning Transmission Electronic Microscope Group (STEM Group) of Université Paris Sud. In particular high resolution transmission electron microscopy has allowed to associate the additional emission lines with extended crystal deformation such as stacking faults and folds of the planes which correspond to the different crystal stacking configuration evaluated theoretically.
This study suggests that deliberatly introducing stacking faults in a perfect h-BN crystal can be used as a way to tune the emission spectrum in the far UV region. From a complementary point of view, the analysis of the excitonic response can be employed as a powerful tool to precisely map structural deformations in h-BN crystallites and in the new nanoelectric systems constituted by h-BN/graphene heterostructures.
We can say that this project has allowed the state-of-the-art methods in many-body solid-state physics, which have been proved to be extremely accurate, to be applied to the new class of layered materials which have a great technological potential. Therefore, it will provide a significant step towards predictive modelling in integration with experiments which is highly demanded by the nano and materials science scientific community. The excellent scientific quality of the project is highlighted by the combination as closely as feasible between a primarily theoretical approach with advanced EELS experiments conducted by one of the most recognized groups in the field.