Hugo Levy-Falk

Optical spectroscopy of graphene quantum dots and halide perovskite nanocrystals

I defended my thesis, the manuscript is available on HAL.

This work focuses on the optical spectroscopy of two classes of materials using fluorescence microscopy at room temperature.First, halide perovskites, a class of semiconductors that have known a surge in interest in the last ten years because of their outstanding optoelectronic properties, making them a promising platform for photovoltaic applications, but also light emission in diodes, lasers, and quantum devices. These crystalline materials consist of corner-sharing octahedra with a metallic ion at the center, often lead, and halide ions at the corners: Cl, Br, or I. A cation completes the structure. It is either organic, for example, methylammonium (MA) or formamidinium, or inorganic, for example, cesium. In the context of light emission, halide perovskites are an excellent choice to address the problem of the green gap, that is, the lack of efficient emitters in the green region of the optical spectrum, because of the possibility to tune their band gap thanks to an informed choice of the halide during the synthesis.Moreover, because the synthesis is done at room temperature and involves soft chemistry steps, they are promising for industrial applications. The synthesis and characterization of CsPbBr₃ nanocrystals emitting in the optical spectrum's green region using a new reprecipitation-based method is reported. In particular, the nanocrystals' high calibration and good stability are highlighted.The second part of this study is about graphene quantum dots. Those low-dimensional objects allow the opening of the band gap of graphene, making them fluorescent. These emitters are promising because their atomically-thin structure and tunability make them suitable for realizing nano-sensors. Building on the recently studied structure-properties relationship of rod-shaped graphene quantum dots, a thorough single-molecule study of highly fluorescent graphene quantum dots with 96 sp² carbon atoms is reported. The excellent purity of the samples was highlighted. The study of the time dynamics of those single-photon emitters in a polystyrene matrix allowed estimating the characteristic times of the transient dynamic of the quantum dots. Finally, the third part reports the study of the graphene quantum dots on a perovskite surface. The surface of perovskites is of peculiar interest for the realization of devices with these semiconductors, making it an interesting playground to use graphene quantum dots. To that end, the quantum dots were deposited on a millimetric MAPbBr₃ single-crystal surface.

• As thin films deposited on the perovskite, the graphene quantum dots present photophysics compatible with the formation of excimers.
• As the concentration of quantum dots on the surface is lowered, diffraction-limited spots are observed. The time-domain study of the photoluminescence reveals jumps between discrete states of the system.
• The frequency-domain investigation of the intensity of photoluminescence of these diffraction-limited emitters is dominated by 1/f noise, which highly contrasts the stable, shot-noise-dominated dynamics of the single emitters when studied in a polystyrene matrix.

Investigation of Rod-Shaped Single-Graphene Quantum Dot

Physica Status Solidi (b)

In recent years, there has been a resurgence of interest in graphene quantum dots (i.e., large polycyclic aromatic hydrocarbons) in particular due to their interesting properties as single-quantum emitters. Herein, a thorough study of a graphene quantum dot made of 96 carbon atoms in a D_2h symmetry is reported. In particular, experiments at the single-molecule level reveal that this graphene quantum dot structure is a bright and photostable single-photon emitter at room temperature. Finally, a statistical study of the emission wavelength as a function of the graphene quantum dot concentration highlights the high purity and degree of control on the samples.

Interplay of structure and photophysics of individualized rod-shaped graphene quantum dots with up to 132 sp² carbon atoms

Nature Communications

Nanographene materials are promising building blocks for the growing field of low-dimensional materials for optics, electronics and biophotonics applications. In particular, bottom-up synthesized 0D graphene quantum dots show great potential as single quantum emitters. To fully exploit their exciting properties, the graphene quantum dots must be of high purity; the key parameter for efficient purification being the solubility of the starting materials. Here, we report the synthesis of a family of highly soluble and easily processable rod-shaped graphene quantum dots with fluorescence quantum yields up to 94%. This is uncommon for a red emission. The high solubility is directly related to the design of the structure, allowing for an accurate description of the photophysical properties of the graphene quantum dots both in solution and at the single molecule level. These photophysical properties were fully predicted by quantum-chemical calculations.

Synthesis method of highly calibrated CsPbBr3 nanocrystals perovskites by soft chemistry

Chemical Communications (Royal Society of Chemistry)

A new synthesis method of highly calibrated CsPbBr3 nanocrystals perovskites is described and analyzed using High-Resolution Scanning Transmission Electron Microscopy. This new method based on soft chemistry leads to the large-scale production of nanocrystals. Such monodisperse nanocrystals allow for the deposition of homogeneous films which provides new opportunities for the next generation of optoelectronic devices.

Le Fediverse dans tous ses Meta

Ces dernières semaines ont été mouvementées sur le Fediverse. Les réseaux sociaux fédérés qui le composent, et Mastodon en tête, ont grossi soudainement depuis l’acquisition de Twitter par le controversé Elon Musk. Mais des concurrents commerciaux pourraient bien émerger.
Ce long article finit par un petit guide pratique pour se mettre au Fediverse. N’hésitez pas à vous y rendre si le reste vous intéresse moins !

Quelques outils pour le physicien avec Julia

Bien choisir les outils que l’on utilise au quotidien est important. Pour ma part, je veux des outils avec lesquels je suis à l’aise, qui soient suffisamment performants et qui produisent des rendus de bonne qualité.

Dans la vie de tous les jours, je suis physicien. Plus précisément, j’étudie la photo-physique de semi-conducteurs un peu exotiques. Concrètement, cela signifie que je passe beaucoup de temps en salle de manipe à collecter des données de spectroscopie. Je ne suis donc pas un physicien théoricien qui utiliserait l’informatique pour faire du calcul symbolique, ou un numéricien qui utiliserait de gros calculateurs pour faire tourner des simulations. Mon utilisation de l’informatique est double :

• Contrôler mon expérience finement;
• Traiter les données acquises, c’est-à-dire réaliser des ajustements de variable sur les données par rapport à des modèles relativement simples, et surtout afficher les données pour alimenter ma réflexion.

Aujourd’hui j’ai choisi de vous détailler un peu la manière dont je réalise la seconde partie. Ceci n’a absolument pas vocation à être une recommandation d’utilisation, ou à dénigrer d’autres manières de travailler. Cependant, si cela peut donner des idées pour s’inspirer, ou si vous pensez que je suis passé à côté d’un outil intéressant, n’hésitez pas à me contacter.

Jouons à implémenter une transformée de Fourier rapide !

Un algorithme que vous utilisez probablement au quotidien.

La transformée de Fourier est un outil essentiel dans de nombreux domaines, que ce soit en Physique, en traitement du signal, ou en Mathématiques. La méthode qui est probablement la plus connue pour la calculer numériquement s’appelle la FFT pour Fast Fourier Transform, ou Transformée de Fourier Rapide. Dans ce petit tutoriel, je vous propose d’essayer de comprendre et d’implémenter cet algorithme de manière efficace. J’utiliserais pour cela le langage Julia, mais il devrait vous être possible de suivre en utilisant d’autres langages tels que Python ou C. Nous comparerons les résultats obtenus avec ceux donnés par le portage en Julia de la bibliothèque FFTW.

Norg.jl

A Norg parser in Julia

Norg.jl is a library to parse the norg file format used in NeoVim's neorg.

wireguard-manager

Manage wireguard from waybar

This is a simple, stupid waybar extension for toggling wireguard. It uses rofi for password prompting, but you could use anything you like.

SPEFiles.jl

A library to use Lightfield SPE files in Julia.

SPEFiles is a library aiming at providing utilities to open Princeton instruments SPE 3.0 files with Julia.

vim-slime-cells

A plugin on top of vim-slime to enhance its cell feature.

It adds the possibility to jump between cells and to send the current cell then jump to the next one. There is also a nice syntax-highlighting feature for cell boundaries.

How I over-engineered a Fast Fourier Transform for Arduino.

The lengthy, excruciating, details.

Everything began with me wanting to implement the Fast Fourier Transform (FFT) on my Arduino Uno for a side project. The first thing you do in such case is asked your favorite search engine for existing solutions. If you google "arduino FFT" one of the first result will be related to this instructable: ApproxFFT: The Fastest FFT Function for Arduino. As you can imagine, this could only tickle my interest: there was an existing solution to my problem, and the title suggested that it was the fastest available! And thus, on April 18ᵗʰ 2021, I started a journey that would bring me to write my own tutorial on implementing the FFT in Julia, learn AVR Assembly and write a blog post about it, about one year and a half later.

A nice approximation of the norm of a 2D vector.

Some cool sunday project!

While wandering on the internet, I stumbled uppon Paul Hsieh's blog-post, where he demonstrates a way to approximate the norm of a vector without any call to the sqrt function. Let's see if I can reproduce the steps to derive this.

Modeling a honeycomb grid in FreeCAD

A small tutorial on FreeCAD

Someone asked me how to make a honeycomb grid in @FreeCADNews. Here's how I do it, and bonus it's parametric! ⬇️

Let's play at implementing a fast Fourier transform!

An algorithm you probably use on a daily basis.

The Fourier transform is an essential tool in many fields, be it in Physics, Signal Processing, or Mathematics. The method that is probably the most known to calculate it numerically is called the FFT for Fast Fourier Transform. In this little tutorial, I propose to try to understand and implement this algorithm in an efficient way. I will use the language Julia, but it should be possible to follow using other languages such as Python or C. We will compare the results obtained with those given by the Julia port of the FFTW library.