Development of 4D-STEM with variable tilts
The development of 4D-STEM (Scanning Transmission Electron Microscopy) has profoundly transformed transmission electron microscopy (TEM) by enabling the simultaneous recording of spatial (2D) and diffraction (2D) information at each probe position. These so-called “4D” datasets make it possible to extract a wide variety of virtual contrasts (bright-field imaging, annular dark-field imaging, ptychography, strain and orientation mapping) with nanometer-scale spatial resolution.
In this context, 4D-STEM with variable beam tilts (4D-STEMiv) is an emerging approach that involves sequentially acquiring electron diffraction patterns for different incident beam tilts. Conceptually similar to precession electron diffraction (PED), this method offers greater flexibility and opens new possibilities: improved signal-to-noise ratio, faster two-dimensional imaging at higher spatial resolution, access to three-dimensional information (orientation, strain, phase), and optimized coupling with spectroscopic analyses (EELS, EDX).
The development of 4D-STEMiv thus represents a major methodological challenge for the structural and chemical characterization of advanced materials, particularly in the fields of nanostructures, two-dimensional materials, and ferroelectric systems.
Fabrication of Metasurfaces by Self-Assembly of Block Copolymers
Block copolymers (BCP) are an industrial technology in full expansion, offering promising perspectives for material nanostructuring. These polymers, composed of chemically distinct block chains, self-assemble to form ordered structures at the nanometric scale. However, their current use is limited to specific nanostructuring per product (1 product = 1 nanostructuring), thus restricting their application potential.
This PhD proposes to develop an innovative method to create multiple patterns in a single BCP self-assembly step using a mixture of two products. The student will also focus on controlling the localization of these patterns using chemoepitaxy, a technique combining chemical and morphological guidance to precisely control the position of patterns at the micrometric and nanometric scales.
The work will proceed in several steps: understanding the mechanisms of mixed block copolymers, developing functionalized substrates for chemoepitaxy using advanced lithography techniques, and conducting BCP self-assembly experiments on these substrates. The resulting structures will be analyzed using the metrology equipment available at CEA-Leti.
The targeted applications include the creation of nanostructures capable of interacting with light, reducing diffraction, and controlling polarization. The expected results include demonstrating the ability to generate multiple types of patterns in a single self-assembly step, with precise control over their position and dimensions.