Charge density | EPFL Graph Search

In electromagnetism, charge density is the amount of electric charge per unit length, surface area, or volume. Volume charge density (symbolized by the Greek letter ρ) is the quantity of charge per unit volume, measured in the SI system in coulombs per cubic meter (C⋅m−3), at any point in a volume. Surface charge density (σ) is the quantity of charge per unit area, measured in coulombs per square meter (C⋅m−2), at any point on a surface charge distribution on a two dimensional surface. Linear charge density (λ) is the quantity of charge per unit length, measured in coulombs per meter (C⋅m−1), at any point on a line charge distribution. Charge density can be either positive or negative, since electric charge can be either positive or negative. Like mass density, charge density can vary with position. In classical electromagnetic theory charge density is idealized as a continuous scalar function of position \boldsymbol{x}, like a fluid, and \rho(\boldsymbol{x}

Spin processes and charge dynamics in organic light emitting diodes

Francesco Comandè

In this thesis work we explore spin dependent processes and charge dynamics in α-NPD/Alq₃ based LEDs. Studies on polymer light emitting diodes based on PFO, which are fabricated by us, are also reported. Our objective is to shed light on the fundamental mechanisms that cause large magnetic field effects in these devices. First, we explore charge dynamics using both steady state and time resolved charge modulation spectroscopy. These techniques are able to monitor variations in the charge density by measuring the absorption induced by an external bias voltage. In spite of the presence of several bands in our charge modulation spectra, no effect of the magnetic field is detected. This is in contrast with the large magneto-electroluminescence we observe in our samples. This discrepancy can be explained by assigning these spectral features to free polarons and not to singlet and triplet excitons. These absorption bands are of strong interest in the field of OLED charge dynamics. In fact, to the best of our knowledge, previous works on charge modulation spectroscopy reported only on polaron absorption in the hole transport layer. In our work instead, there is evidence of bands that can be ascribed to polarons in the emission band. This discovery is important from a technical point of view since it proves that a more complete description of charge dynamics is possible using modulation spectroscopy. Moreover, by way of time resolved measurements, we show that a direct exploration of the absorption time evolution is technically possible. In the second part of the thesis we investigate spin processes using pulsed and continuous wave electroluminescence detected magnetic resonance (pELDMR and cwELDMR) and continuous wave electrically detected magnetic resonance (cwEDMR). As a first result, we confirm that the cwEDMR spectrumis composed by at least two bands, as it was previously hypothesized in the literature. However, this is not the same when we look at the cwELDMR spectrum. Here, the linewidth of the resonance peak is reduced and fits with one of the bands composing the cwEDMR spectrum. This observation suggests that our optical approach can selectively detect one part of the overall phenomenology detected by cwEDMR. Interestingly, we do not observe any dependence of the pELDMR temporal evolution on the bias voltage. This behavior cannot be explained by models that predict an effect of magnetic field on charge mobility. To support this statement, we carry out admittance spectroscopy measurements and electroluminescence frequency dependence measurements to estimate the device reaction time. We find this to be inconsistent with the pELDMR measurements. In addition to that, we also performad hoc numerical simulations, proving that, in a space charge limited regime, a voltage dependence from spin dependent mobility changes has to be expected. On the other hand, spin dependent recombination models are able to describe the pELDMR post pulse decay at different temperatures, and to explain the fast reaction time of our measurements.

EPFL2014

Lipid membrane characterization with second harmonic scattering

Cornelis Ulrich Lütgebaucks

Membranes, composed of a variety of lipids and other biomolecules, mediate signaling processes between cells and their aqueous environment. To fulfill this function, membranes can vary their composition leaflet-specific and thus alter their surface properties. To fully understand the impact of these processes on the molecular level, it is necessary to develop tools that can access the molecular properties of free-floating model membranes label-free. These tools are ideally surface-specific. In this thesis, we apply the nonlinear optical techniques second harmonic scattering (SHS) and vibrational sum-frequency scattering (SFS) together with electrokinetic measurements to label-free characterize the interfacial properties, hydration structure and surface potentials of liposomes in aqueous solutions. First, we generalize the nonlinear optical theory to describe the second-order surface response from interfaces with aqueous solutions independent of the ionic strength for reflection, transmission and scattering geometries. We demonstrate that interference effects from oriented water molecules in the bulk aqueous solution alter the probing depth and the expected second-order response at low ionic strengths. Then, we apply this theory to demonstrate that SHS patterns of liposomes and oil droplets contain all necessary information to extract the absolute surface potential of the respective particles without assuming a model for the interfacial structure. By analyzing scattering patterns that capture the orientational distribution of water around the particles, we find surface potentials of -38 mV for bare oil-droplets and -11 mV for zwitterionic liposomes in water. For anionic liposomes the surface potential varies between -150 mV and -23 mV in solutions containing different amounts of NaCl ranging from

\sim

0 mM to 10 mM. These values are remarkably different for solutions to the Gouy-Chapman model considering a fixed surface charge density. Next, we characterize the hydration and lipid asymmetries in binary mixed membranes using SHS and SFS. The liposomes exhibit hydration asymmetry between the inner and outer leaflet. The lipid number density between the inner and outer leaflet is the same, although geometrical packing arguments would suggest a different density. However, an asymmetric lipid distribution between the leaflets can be induced by fine tuning specific intermolecular interactions between the lipids. This is shown with dipalmitoylphosphoserine and dioleoylphosphocholine mixtures creating a membrane structure that allows intermolecular H-bonding between the phosphate and amine groups of the lipids. Finally, we quantify the surface properties of membranes composed of lipids containing phosphoserine and phosphocholine headgroups. Surprisingly, we find a very high degree of counterion condensation on anionic membranes in pure water: only 1 % of all lipids are ionized. This indicates a tightly packed layer of ions around the membrane that needs to be considered when modelling the interfacial structure around membranes.

EPFL2017