Séminaire IPCMS/DCMI-DMONS, supporté par le labex NIE

Inferring the structure of amorphous materials from a joint experimental/ab initio approach and a novel extension of the Kubo-Greenwood formula

D. A. Drabold¹, B. Bhattarai, K. Subedi (Ohio University)
K. Prasai (Stanford University)

In this talk, we discuss two recent developments relevant to modeling amorphous materials. First, I describe a method “Force Enhanced Atomic Refinement²” as a means to “invert” structure factor data, guided by first principles interactions. While the method is akin to standard “Reverse Monte Carlo” (RMC) methods, chemical information is included in a non-biased way, and leads to realistic models of amorphous phases of silicon, carbon and chalcogenides. The idea is to determine atomic coordinates that jointly satisfy experimental information and are at the same time a suitable minimum of a DFT energy functional. We emphasize the case of low-density (0.95 gm/cc) amorphous carbon, and show that the material is a three-dimensional form of amorphous graphene (with wrapped, distorted graphene sheets with 5-, 6- and 7-member rings), and some sp³ and sp defects³.  Next, we point out that it is possible to adapt the standard Kubo-Greenwood formula to obtain a “space-projected conductivity” (or dielectric function), providing insight into what parts of an amorphous network (if any) participate in electronic conduction, or for finite frequency, what parts of the network absorb radiation of frequency ω. We apply the method to models of a conducting bridge memory material (Cu-doped alumina)⁴.

Mini symposium on two-dimensional semiconductors: optical spectroscopy and exciton dynamics

AFFICHE

ABSTRACTS

Soutenance d’Habilitation à Diriger des Recherches

Damien MERTZ

Titre :  « Conception de nanomatériaux hybrides multi-fonctionnels pour la délivrance d’agents thérapeutiques et l’imagerie médicale »

” Novelty of Nano-dispersed Solid Alloys and Thermal Fluids ”

Prof. Indranil Manna

Director of Indian Institute of Technology ITT,  Kharagpur, India

Abstract

” IV-group semiconductor nanoparticles: functionalities arising from surfaces ”

Katerina (Dohnalova) Newell

University of Amsterdam, Amsterdam, The Netherlands

In semiconductor nanoparticles (NPs), with decreasing core diameter, surface to volume ratio increases and larger fraction of the constituent atoms resides directly on the surface. For example, in 2 nm Silicon NPs, 50% of the Si atoms come from the surface. In the case of  Si, C and Ge, IV-group semiconductors, bonding chemistry is covalent, hence electrons from the surface moieties (attached elements or ligands) play important role in the electronic structure of the NP. In small NPs, quantised k-vector is not anymore a good quantum number defining available states in the NPs, as it was in bulk semiconductors, because of short range periodicity and quantum confinement that “blurs” the wavenumber values. Nevertheless, one can still plot so called “fuzzy” band-structures. From Tight Binding and Density Functional Theory approaches, we know that covalently bonded ligands strongly modify the band-gap, oscillator strength (and hence also radiative rate) and the overall density of states in the “bands”. Hence, ligands will also strongly modify optical and electronic properties of such material. In our work, we study theoretically and experimentally such emergent functionalities that are not native to the NP itself, but  are of extrinsic origin. Specifically we look on the IV-group semiconductors, such as Si, C and Ge NPs, materials with indirect band-gap, where novel functionalities contributing to more efficient light emission or absorption are welcomed.

“Facile Thioethers as Novel Class of Luminophores with Aggregation Induced Emission Properties”

Dr. Jens VOSKUHL

IInstitute of Organic Chemistry, University of Duisburg-Essen,

The need of novel efficient fluorophors for the recognition and labeling of biomolecules such as proteins enzymes and cells is one of the most challenging disciplines in modern biosupramolecular chemistry. Our group uses a phenomenon called aggregation induced emission. Molecules with this ability show fluorescence, contrary to normal fluorophores, when aggregated or in the solid state. Recently we found a novel class of facile thioethers with this remarkable characteristic. Our system can be easily modified and was used for the detection of proteins and bacteria. Furthermore we investigated the formation of fluorescent micelles using hydrogen bonding. Currently different compounds were investigated concerning their fluorescence properties. A range of emission in the visible region was found ranging from 430 to 590 nm. Interestingly delayed fluorescence was observed for specific compounds leading to the assumption, that and intersystem crossing to a triplet state occurs leading to a long lived fluorescence state, termed phosphorescence.

Quantum Photonic Engineering: from pure photon sources to integrated circuits

Dmytro O. Kundys

Heriot-Watt University, Edinburgh EH14 4AS, UK

In recent years, quantum technology has seen an immense development demonstrating the variety of platforms allowing coherent control of qubits – quantum bits of information. Quantum optics promises creation of superfast quantum computers and sensors, which can outperform classical counterparts thanks to the utilisation of photons, the ideal carriers of quantum information. Integrated optics is one of the promising routes to create such computers from waveguide networks on a chip with an ability to control the path and degree of freedom (such as polarization or frequency) of single photons, thus forming the quantum logic gates needed for linear optics quantum computation. In my talk I will review our recent progress on developing of sources of single photons for the above tasks using parametric down-conversion and group-velocity matching with tailored nonlinearities. Such sources of pure single photons are paramount for successful quantum information processing. Our engineered single photon sources exhibit simultaneously enhanced single photon purity and heralding efficiencies. I will describe how such sources, embedded into Sagnac interferometers can be used to generate ultrafast photon cluster states entangled in the polarisation degree of freedom. Also at the end of my talk I will review some of our previous results on non-classical multiphoton interference experiments of boson sampling and quantum teleportation performed on a photonic chip.

References:

[1] F. Graffitti et.al Optica 5, 514 (2018)
[2] J. Spring et.al, Science 339, 798 (2013)
[3] B Metcalf et al, Nat. Photonics 8, 770 (2014)