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Reference : UPR2940-ELOBER-048
Workplace : GRENOBLE
Date of publication : Thursday, July 22, 2021
Scientific Responsible name : Martien den Hertog, with Jean-Luc Rouviere (CEA-Grenoble) and potentially Christophe Gatel (CEMES Toulouse)
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Description of the thesis topic
The aim of this PhD is to quantitatively measure the electrical properties of materials using transmission electron microscopy (TEM) based techniques at nm and atomic length scales, with a strong focus on developing new 4D-STEM methods with high potential and related data treatment. In-situ biasing will be a key experimental method to remove all material related contrast/signal by subtraction of a suitable reference image. 4D-STEM will be used to map the displacement of the transmitted disk, which is sensitive to the presence of local electric fields. This method, generally called Differential Phase Constrast (DPC) will be optimized (WP1) and applied to 2D materials and electrically contacted Nanowire p-n junctions, and then compared to ptychography, a technique where the electron phase is reconstructed from the same 4D-STEM data, using an iterative algorithm (WP2). On the best samples, these 2 techniques will be compared with electron holography (WP3). The PhD will provide the optimized conditions and data treatment for 4D STEM experiments of two kinds, and indicate which technique should be preferentially used as a function of the sample geometry.
This PhD is enabled by an increasingly strong collaboration between TEM specialists at CEA Grenoble and at CNRS Institut Néel.
In the framework of the ERC project e-See (PI Martien den Hertog (Néel), including CEA as a partner), we have started to carry out in-situ and high resolution 4D STEM experiments on electrically biased p-n junction nanowires and to characterize the electric field distribution at atomic length scales in 2D materials. The final goal of the ERC project is to electrically control and spatially locate a single charge with nm or atomic spatial resolution. In this context, we realized that data treatment of 4D STEM images is challenging but essential. Additionally, the prospect of ptychography is very promising for both types of samples. Presently, ptychography reconstruction programs are available. In particular, the MRS group at Institut Neel in the person of Julio Cesar da Silva has a strong expertise on ptychography for X-ray diffraction and Jean-Luc Rouvière (CEA) is in contact with several groups developing ptychography algorithms and plans within this PhD to develop new algorithms and methods. Indeed, current ptychography software only works for very thin objects. For this reason, 2D materials have been included in this PhD to validate existing and new reconstruction algorithm's. For p-n nanowires, new ptychography software including the dynamic propagation of electrons will have to be developed. We realized that, given the different know how in our consortium (for instance, sample fabrication for in-situ experiments on contacted nanowires, dedicated imaging conditions for 2D materials, theoretical knowledge on simulations, state-of-art TEM and new pixelated detector) we have the opportunity to go rapidly beyond state-of-the-art results. This PhD will allow us to collaborate much more efficiently then otherwise possible, and combine the resources of our two institutes in the best way, contributing to and benefitting from both the ERC e-See project as well as ongoing projects on 2D materials, in particular the ANR Magicvalley. In the ERC project, different kinds of p-n junction nanowires are studied consisting of Ge, GaN and InP. These nanowires are grown or fabricated in collaborating research groups (SINAPS and NPSC in CEA/Néel and TU Eindhoven). We fully master all cleanroom processing to contact these nanowires on TEM compatible silicon nitride membranes. For 2D materials, the optimal growth, transfer to TEM compatible substrates and imaging conditions are all mastered in the IRIG group thanks to Hanako Okuno, who has a unique state-of-the-art knowledge. Moreover, 2D materials can be accessed through a collaboration with the society Nanomegas. Therefore, this PhD project does not need to be concerned about obtaining relevant and high-quality samples, and can focus on developing the TEM technique, and simultaneously obtain valuable information about the material properties.
The PhD candidate will be hired at the Institut Néel, but spend quite some time also at CEA-Grenoble. The candidate will be in the MRS team in Néel, and also integrate the microscopy group in CEA: LEMMA.
Constraints and risks
Potential risks can be the unavailability of samples, as well as technical problems of the (complicated) TEM equipment. These risks are mitigated by the presence of several researchers (both PhD, postdoc and permanent staff) working on the sample fabrication. Moreover, due to the collaboration between the two institutes, multiple TEM instruments are accessible to this project.
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