P. Plochocka

Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites

Direct measurement of the exciton binding energy shows that the impressive performance of perovskite solar cells arises from the spontaneous generation of free electrons and holes after light absorption.

Approaching the dirac point in high-mobility multilayer epitaxial graphene.

Multilayer epitaxial graphene is investigated using far infrared transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance-like absorption is observed at low temperature in magnetic fields below 50 mT, probing the nearest vicinity of the Dirac point. The carrier mobility is found to exceed 250,000 cm2/(V x s). In the limit of high temperatures, the well-defined Landau level quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unusual in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated.

Carrier relaxation in epitaxial graphene photoexcited near the Dirac point.

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.

Determination of the exciton binding energy and effective masses for methylammonium and formamidinium lead tri-halide perovskite semiconductors

The family of organic–inorganic halide perovskite materials has generated tremendous interest in the field of photovoltaics due to their high power conversion efficiencies. There has been intensive development of cells based on the archetypal methylammonium (MA) and recently introduced formamidinium (FA) materials, however, there is still considerable controversy over their fundamental electronic properties. Two of the most important parameters are the binding energy of the exciton (R*) and its reduced effective mass μ. Here we present extensive magneto optical studies of Cl assisted grown MAPbI3 as well as MAPbBr3 and the FA based materials FAPbI3 and FAPbBr3. We fit the excitonic states as a hydrogenic atom in magnetic field and the Landau levels for free carriers to give R* and μ. The values of the exciton binding energy are in the range 14–25 meV in the low temperature phase and fall considerably at higher temperatures for the tri-iodides, consistent with free carrier behaviour in all devices made from these materials. Both R* and μ increase approximately proportionally to the band gap, and the mass values, 0.09–0.117m0, are consistent with a simple k.p perturbation approach to the band structure which can be generalized to predict values for the effective mass and binding energy for other members of this perovskite family of materials.

Optical manipulation of the exciton charge state in single-layer tungsten disulfide

Raman scattering and photoluminescence (PL) emission are used to investigate a single layer of tungsten disulfide (WS

High-energy limit of massless Dirac fermions in multilayer graphene using magneto-optical transmission spectroscopy.

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Slowing hot-carrier relaxation in graphene using a magnetic field

) obtained by exfoliating n-type bulk crystals. Direct gap emission with both neutral and charged exciton recombination is observed in the low temperature PL spectra. The ratio between the trion and exciton emission can be tuned simply by varying the excitation power. Moreover, the intensity of the trion emission can be independently tuned using additional sub band gap laser excitation.

Optical Investigation of Monolayer and Bulk Tungsten Diselenide (WSe2) in High Magnetic Fields

We have investigated the absorption spectrum of multilayer graphene in high magnetic fields. The low-energy part of the spectrum of electrons in graphene is well described by the relativistic Dirac equation with a linear dispersion relation. However, at higher energies (>500 meV) a deviation from the ideal behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation from linearity is approximately 40 meV. This result is in good agreement with the theoretical model, which includes trigonal warping of the Fermi surface and higher-order band corrections. Polarization-resolved measurements show no observable electron-hole asymmetry.

Unintentional High-Density p-Type Modulation Doping of a GaAs/AlAs Core–Multishell Nanowire

A degenerate pump-probe technique is used to investigate the nonequilibrium carrier dynamics in multilayer graphene. Two distinctly different dynamics of the carrier relaxation are observed. A fast relaxation

Magnetoexcitons in large area CVD-grown monolayer MoS2 and MoSe2 on sapphire

(\ensuremath{\sim}50\text{ }\text{fs})