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PhD position: Puzzling out h-BN optics by high space, energy and momentum resolution spectro-microscopy

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Context

Atomic defects play a key role in defining the ultimate optical properties of semi­conducting materials but the precise correlation of their structure and chemistry with spe­cific spectral features remains a challenging task. Recent developments in aberration corrected scanning transmission electron microscopes (STEM) have provided new tools to tackle this problem allowing to achieve the sub-Angstrom space resolution both in imaging and spec­troscopy. Still, spectro-microscopy of defects in standard 3D semi­conductors is strongly hin­dered by deep embedding into the bulk matrix. On the contrary, bi-di­mensional layered semi-conductors represent opti­mal model systems since they associate re­markable optical properties to a reduced di­mensionality which allows to address individual atoms. Among this class of materials, hexagonal boron nitride (h-BN) presents a unique com­bination of a layered wide indi­rect band gap and a strong luminescence in the far UV due to excitons strongly cou­pled with phonon modes. The h-BN luminescence can be further modu­lated at the nanometric scale by structural deformations but the mechanism beneath this be­havior remains still unclear. Furthermore, h-BN have been shown to host very bright and room temperature stable single photon sources but no precise structural and chemical identif­ication of the related defects has been reported. The complex spectrum of h-BN make it an ideal candidate to prove the ultimate capabilities of state-of-the-art spec­tro-microscopy techniques applied to defects optics.

Thesis description

To address the open questions on the optics of h-BN it will be neces­sary to employ an unprecedented methodology which combines different spectroscopies in a wide en­ergy range, from the IR to the UV, to a high space selectivity. Within this PhD thesis, this goal will be pursued by employing a world unique instrumentation which combines within an aberration corrected and monochromated scanning transmission electron microscope (STEM) both electron energy loss spectroscopy (EELS) and cathodolumi­nescence (CL). The sys­tem provides a sub-Angstrom or high momentum resolution and it permits to investigate at the same time excitation processes in IR-X energy range by EELS and recombination pro­cesses in the visible-far UV by CL. Several aspects of h-BN optics will be tackled such as the momentum dependence of excitons and phonons and their inter-couplings down to the sin­gle layer limit, the correlation be­tween specific absorption/emission lines and well character­ized atomic defects, the identification of possible new single-photon sources in the visible and UV spectral do­main.

Working environment

The STEM group at the Solid State Physics Lab is a world leading electron microscopy team well recognized for its work on the struc­tural, optical and electronic characterization of nanomaterials combining experimen­tal spectro-microscopy and numerical modeling. Located south of Paris, the Paris-Saclay University is at the core of the biggest scien­tific pole in France and it has been recently ranked first European university in physics by the Academic Ranking of World Universities.

Profile

Candidates must have a M.Sc (or equivalent) in Physics or Materials science and good experimental skills. Good working knowledge of both written and spoken English is required while French is not mandatory. The starting date is flexible (preferably in 2020), the position will be filled as soon as a suitable candidate will be found.

Contacts

Alberto Zobelli

alberto.zobelli@universite-paris-saclay.fr

Odile Stéphan

odile.stephan@universite-paris-saclay.fr

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