Resonance Raman spectroscopy | EPFL Graph Search

Resonance Raman spectroscopy (RR spectroscopy or RRS) is a variant of Raman spectroscopy in which the incident photon energy is close in energy to an electronic transition of a compound or material under examination. This similarity in energy (resonance) leads to greatly increased intensity of the Raman scattering of certain vibrational modes, compared to ordinary Raman spectroscopy. Resonance Raman spectroscopy has much greater sensitivity than non-resonance Raman spectroscopy, allowing for the analysis of compounds with inherently weak Raman scattering intensities, or at very low concentrations. It also selectively enhances only certain molecular vibrations (those of the chemical group undergoing the electronic transition), which simplifies spectra. For large molecules such as proteins, this selectivity helps to identify vibrational modes of specific parts of the molecule or protein, such as the heme unit within myoglobin. Resonance Raman spectroscopy has

Raman Spectroscopy of GaAs Nanowires

Bernt Ketterer

Semiconductor nanowires offer a wide range of opportunities for newgenerations of nanoscale electronic and optic devices. For these applications to become reality, deeper understanding of the fundamental properties of the nanowires is required. In this thesis, Raman spectroscopy has been applied to examine the characteristics of GaAs nanowires facing the following challenges in current nanowire research: (i) understanding of the doping mechanisms in catalyst-free GaAs nanowires grown byMBE, (ii) examination of the electronic band structure of the wurtzite polytype of GaAs, and (iii) probing the properties of free carrier systems in nanowire based quantum-heterostructures. The Si-doping of GaAs nanowires was studied in the first part of the thesis. The investigation of the local vibrational modes of the silicon dopants in the GaAs host lattice allowed to identify the incorporation pathways the dopants take during the nanowire growth and to determine the limitations of the doping process due to compensation effects. Important information on the concentration and mobility of the free carriers in the doped material has been obtained by analyzing the coupled longitudinal optical phonon-plasmon modes. The second part of the thesis focused on the electronic band structure in wurtzite GaAs. Here, resonant Raman scattering from first and second order longitudinal optical phonons was used in oder to determine the band-gap, the position of the light-hole band as well as the temperature dependence of the crystal-field split-off band. As important parameters the spin-orbit coupling and crystal-field splitting have been obtained. The photoexcited electron-hole plasma and the confined optical phonons in nanowire-based GaAs/AlAs multi-quantum-well structures were studied in the last part of the thesis. Structural parameters could be deduced from the position of the confined phononmodes. The coupling of the plasmon to the phonon gives important information on the system of photogenerated carriers within the quantum structure.

EPFL2011

High sensitivity, incoherent differential imaging

Chiara Bonati

Microscopic visualisation of optically transparent samples has been a topic of interest for several decades. Features such as density or chemical composition can influence the optical phase of transmitted light, and phase contrast can reveal these structures. Several methods of phase contrast have been developed, which can be categorised as either interferometric or non-interferometric, based on the type of coherence properties of the light used. In this work, I focus on incoherent based phase contrast, in particular Differential Phase Contrast (DPC). The choice of incoherent light brings benefits such as the absence of distortions like speckle patterns and ringing patterns, increased spatial resolution, and a simpler setup that can be used for in-vivo applications. Moreover, DPC allows reconstruction of quantitative phase maps of the samples. On the other hand, coherent-based techniques demonstrate superior performance in terms of phase sensitivity. The first part of this thesis offers a quantitative analysis of the phase sensitivity of DPC and investigates the influence of optical parameters and sample characteristics. With simulations and experiments, a relation between numerical aperture and phase sensitivity is demonstrated, and the concept of spectral matching is introduced to enhance the contrast. The methods can be generalised to any DPC setup, and allow a-priori investigation of the sensitivity of a DPC microscope at the design stage rather than through testing. Comparison between the best sensitivity that can be achieved in DPC and state-of-the-art interferometric techniques, shows that it is not possible to reach comparable single-shot performances. In this thesis, it is shown that the reason for this limitation is the strong background in DPC images, which degrades the dynamic range. DPC images are obtained with mirrored illuminations, for which the background is identical and the phase term switches in sign. The difference between these pairs of images is computed digitally, which does not improve the limited dynamic range. Lock-in DPC is proposed as a solution: instead of sampling different illumination states, lock-in DPC demodulates the phase signal when the illumination is switched, and the background is never encoded. This is enabled by periodical switching of sources coupled with a smart pixel detector, the so-called "lock-in camera". Part of this work is dedicated to the theoretical description of this method, and the analysis of the expected benefit. Experiments are also described, that demonstrate a factor of 8 improvement in single-shot sensitivity compared to standard DPC. DPC is not the only imaging technique to suffer from high intensity background: it is easy to see how the use of a lock-in camera for differential imaging can be generalised to any situation where weak modulations can be induced over a strong background. Here, an example is presented with lock-in Shifted Excitation Raman Difference Spectroscopy (SERDS). SERDS is an established Raman spectroscopy technique that takes advantage of the Raman emission spectrum being relative to the excitation wavelength to remove unwanted fluorescence emission. Two spectra with shifted excitation wavelengths are measured, their difference is computed, and the fluorescence is thus digitally removed. The parallel with DPC is immediately apparent. Both simulations and experiments are used to demonstrate the advantage of analog demodulation.

EPFL2022

Design of Optimized PEDOT‐Based Electrodes for Enhancing Performance of Living Photovoltaics Based on Phototropic Bacteria

Alessandra Antonucci, Ardemis Anoush Boghossian, Sara Politi, Melania Reggente

Living photovoltaics represent a growing class of microbial devices that are based on whole cell–electrode interactions. The limited charge transfer at the cell–electrode interface represents a significant bottleneck in realizing an efficient technology. This study focuses on the development of poly(3,4‐ethylenedioxythiophene) (PEDOT)‐based electrodes that are electrosynthesized in the presence of a sodium dodecyl sulphate (SDS) dopant. Potentiodynamic and potentiostatic electrochemical techniques, as well as scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, and theoretical modelling of the electropolymerization transient, are employed to create and characterize PEDOT electrodes under various conditions. The electrodes are able to capture photosynthetically derived current under multiple light–dark cycles when interfaced with Synechocystis sp. PCC 6803. In the presence of the Synechocystis, the PEDOT electrodes show a sixfold and twofold enhancement over conventional graphite electrodes for both mediatorless and K3Fe(CN)6‐mediated conditions, respectively. The ability of these electrodes to enhance extracted photocurrent for both direct and indirect electron transfer mechanisms provides a versatile platform for improving various microbial devices.

2020