Photoluminescence | EPFL Graph Search

La photoluminescence (PL) est un processus par lequel une substance absorbe des photons puis ré-émet des photons. Principe Dans le cas d'un semi-conducteur, le principe est d'exciter des électrons de la bande de valence avec un photon d'une énergie supérieure à l'énergie de gap du composé, de telle sorte qu'ils se retrouvent dans la bande de conduction. L'excitation fait donc passer les électrons vers un état d'énergie supérieure avant qu'ils ne reviennent vers un niveau énergétique plus bas avec émission d'un photon. Après un temps très court (de l'ordre de 10 nanosecondes, qui peut cependant être allongé à des minutes voire des heures) , l'électron se recombine et retourne dans la bande de valence avec émission d'un phonon (lorsque le matériau est à gap indirect), d'un photon ou dans certains cas d'un électron Auger. La photoluminescence s'intéresse au cas d'un photon émis. Utilisation En science des matériaux, la photoluminescence est une méthode de spect

Exciton Dynamics in 2D Organic Assemblies for Realization of Next-generation Optoelectronic Devices

Surendra Babu Anantharaman

Supramolecular assemblies of cyanine dye molecules formed by weak van der Waals forces have gained immense attention for their distinct optical properties (high extinction coefficient, narrow absorption and photoluminescence line-width and small Stokes shift) compared to their monomer counterparts. Strong light-matter interaction in the J-aggregates causes coherent exciton migration and high color purity emission. This motivates to fabricate J-aggregate thin films for different opto-electronic devices such as solar cells, where J-aggregates serve as exciton transport channels, organic light emitting diodes (OLEDs) and polariton lasers. Exciton migration and photoluminescence quantum yield (PLQY) of the J-aggregates in thin films dictate the efficiency of energy harvesting and light emitting devices, respectively. The main focus of this thesis is the development of a new J-aggregate growth route which favors the formation of large, coherent domains of 2D J-aggregate thin films and to investigate the exciton dynamics. Furthermore, non-radiative decay channels suppressing the PLQY of J-aggregates in thin film and in solution were identified. The coherent crystalline domain size in thin films is typically limited to nanoscale length because dye molecules spontaneously self-assemble already in solution. This can be overcome by maintaining equilibrium between monomers and the critical nucleus of a J-aggregate in solution. This approach is shown here as a successful route to achieve large coherent domains on a functionalized substrate. The growth model was further confirmed with small-angle x-ray scattering (SAXS) studies. Using time-resolved photoluminescence (TRPL) spectroscopy, non-radiative decay channels limiting the exciton migration were identified. The results suggest that the increase in domain size and order is directly proportional to the increased radiative decay of the excitons. Furthermore, the PLQY of J-aggregates in water at room temperature is typically ~5%. Upon addition of alkylamine, the quantum yield was drastically improved while maintaining line-width and peak position. Using small-angle neutron scattering (SANS), a two-phase region was identified in the water-dye-alkylamine ternary phase diagram in which high PLQY J-aggregates form. From TRPL studies, an increase in radiative lifetime is in agreement with the increase in PLQY. As an application, ultra-narrowband photodetectors were explored owing to the narrow line-width of J-aggregate absorption. The concept proved advantageous over other strategies for narrowing the response width, with competing figure of merits. Realization of an inkjet printed prototype with higher frequency response compared to spin-coated devices highlights their potential for industrial applications. Excitonic channels using J-aggregate nanowires were formed by complexing J-aggregates with dendronized polymers. SAXS studies revealed a core-shell configuration of the DP-J-aggregate hybrid structure. A correlation between polymer conformation and J-aggregates allowed to assemble J-aggregate nanowires as thin films. Centrosymmetric cyanine dimers are known for their non-luminescent nature. Twisted dimer packing of dye molecules in a polymer matrix are confirmed from circular dichroism spectroscopy studies. This explains a strongly red-shifted photoluminescence with high quantum yield in the thin film.

EPFL2019

Copper(I) Based Photoredox Chemistry

Murat Alkan

Photoredox catalysis has become a powerful tool for chemists. However, the reliance on expensive Ir and Ru based photoredox catalysts (PC) motivates research for more sustainable options. Copper complexes based on favorable photophysical properties, high abundance and comparatively low prices offer a more sustainable alternative which is outlined in Chapter 1. This dissertation is dedicated to the development of new Cu(I) complexes and their investigation as PCs. Chapter 2 describes the synthesis of a new 2,2'-bipyridine (bpy) based ligand and the heteroleptic [Cu(N^N)(P^P)][PF6] complexes in combination with 4,5-Bis(diphenyl- phosphino)-9,9-dimethylxanthene (Xantphos, 1) and Bis[(2-diphenylphosphino)phenyl] ether (POP, 2). The complexes were thoroughly characterized and employed as PCs for the chlorotrifluoromethylation of activated alkenes with CF3SO2Cl. Complex 1 shows high activity with high yields and wide functional group tolerance in the chlorotrifluoromethylation of styrenes as well as alkenes adjacent to electron-withdrawing groups. The reaction with unactivated alkenes proved to be inefficient. Consequently, the design, synthesis and application of a new Cu(I) based photoredox catalyst is described. Chapter 3 details the use of complex 1 as PC for the chlorosulfonation of alkenes and alkynes with sulfonyl chlorides. The reactions occur under mild conditions and exhibit high functional group tolerance with broad scope in terms of both alkene and sulfonyl chloride. Notably, the reaction occurs with alkynes albeit in moderate to low yields to give exclusively the (E)âproduct. Chapter 4 reports the synthesis of new bpy based ligands and their heteroleptic [Cu(N^N)(xantphos)][PF6] complexes to investigate possible structure-activity relationships. The synthesized Cu(I) complexes are characterized and their activities as photoredox catalysts compared to different Cu(I) based complexes are investigated. Consequently, it is shown that PC 1 is the most active catalyst in the chlorosulfonation of styrenes based on a variety of substrates due its long excited state lifetime in combination with a relatively high photoluminescence quantum yield and photo-stability.

EPFL2020

Tailored-Potential Pyramidal Quantum Dot Heterostructures

Valentina Troncale

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

EPFL2010