Quantum control of polariton emission in a microcavity-quantum well system under magnetic field.

In this work, a quantum dissipative model is employed to investigate the influence of a perpendicular magnetic field on the photoluminescence (PL) spectrum of a quantum well embedded within a microcavity. This model incorporates both the exact electron-hole interaction within the semiconductor and the light-matter coupling between the fundamental photonic mode and the fermionic particles. The loss and pumping mechanisms are described using the quantum master equation, and the PL spectrum is determined via the quantum regression theorem. Our findings demonstrate that the magnetic field acts as a control mechanism in the polariton emission energy, the emission linewidth and the intensity distribution along the emission line. Finally, it is observed that the magnetic field can redistribute the density matrix occupations leading to modifications in the average number of polaritons in the system.

Tip-Induced and Electrical Control of the Photoluminescence Yield of Monolayer WS2.

The photoluminescence (PL) of monolayer tungsten disulfide (WS2) is locally and electrically controlled using the nonplasmonic tip and tunneling current of a scanning tunneling microscope (STM). The spatial and spectral distribution of the emitted light is determined using an optical microscope. When the STM tip is engaged, short-range PL quenching due to near-field electromagnetic effects is present, independent of the sign and value of the bias voltage applied to the tip-sample tunneling junction. In addition, a bias-voltage-dependent long-range PL quenching is measured when the sample is positively biased. We explain these observations by considering the native n-doping of monolayer WS2 and the charge carrier density gradients induced by electron tunneling in micrometer-scale areas around the tip position. The combination of wide-field PL microscopy and charge carrier injection using an STM opens up new ways to explore the interplay between excitons and charge carriers in two-dimensional semiconductors.

Shallow Donor Impurity States with Excitonic Contribution in GaAs/AlGaAs and CdTe/CdSe Truncated Conical Quantum Dots under Applied Magnetic Field.

Using the effective mass approximation in a parabolic two-band model, we studied the effects of the geometrical parameters, on the electron and hole states, in two truncated conical quantum dots: (i) GaAs-(Ga,Al)As in the presence of a shallow donor impurity and under an applied magnetic field and (ii) CdSe-CdTe core-shell type-II quantum dot. For the first system, the impurity position and the applied magnetic field direction were chosen to preserve the system's azimuthal symmetry. The finite element method obtains the solution of the Schrödinger equations for electron or hole with or without impurity with an adaptive discretization of a triangular mesh. The interaction of the electron and hole states is calculated in a first-order perturbative approximation. This study shows that the magnetic field and donor impurities are relevant factors in the optoelectronic properties of conical quantum dots. Additionally, for the CdSe-CdTe quantum dot, where, again, the axial symmetry is preserved, a switch between direct and indirect exciton is possible to be controlled through geometry.

Acoustically Driven Stark Effect in Transition Metal Dichalcogenide Monolayers.

The Stark effect is one of the most efficient mechanisms to manipulate many-body states in nanostructured systems. In mono- and few-layer transition metal dichalcogenides, it has been successfully induced by optical and electric field means. Here, we tune the optical emission energies and dissociate excitonic states in MoSe2 monolayers employing the 220 MHz in-plane piezoelectric field carried by surface acoustic waves. We transfer the monolayers to high dielectric constant piezoelectric substrates, where the neutral exciton binding energy is reduced, allowing us to efficiently quench (above 90%) and red-shift the excitonic optical emissions. A model for the acoustically induced Stark effect yields neutral exciton and trion in-plane polarizabilities of 530 and 630 × 10-5 meV/(kV/cm)2, respectively, which are considerably larger than those reported for monolayers encapsulated in hexagonal boron nitride. Large in-plane polarizabilities are an attractive ingredient to manipulate and modulate multiexciton interactions in two-dimensional semiconductor nanostructures for optoelectronic applications.

Coarse-Grained Quantum Theory of Organic Photovoltaic Devices.

Understanding the exciton dissociation process in organic solar cells is a fundamental issue for the design of high-performance photovoltaic devices. In this article, a parameterized quantum theory based on a coarse-grained tight-binding model plus non-local electron-hole interactions is presented, while the diffusion and recombination of excitons are studied in a square lattice of excitonic states, where a real-space renormalization method on effective chains has been used. The Hamiltonian parameters are determined by fitting the measured quantum efficiency spectra and the theoretical short-circuit currents without adjustable parameters show a good agreement with the experimental ones obtained from several polymer:fullerene and polymer:polymer heterojunctions. Moreover, the present study reveals the degree of polymerization and the true driving force at donor-acceptor interface in each analyzed organic photovoltaic device.

Síntesis y caracterización de las propiedades ópticas de puntos cuánticos de CdSe y CdSe/ZnS / Synthesis and characterization of optic properties of CdSe and CdSe/ZnS quantum dots / Síntese e caracterização das propriedades ópticas do pontos quânticos de CdSe e CdSe/ZnS

Resumen Se prepararon puntos cuánticos de CdSe y CdSe/ZnS (núcleo/capa) con ácido oleico como agente estabilizante en medio orgánico y se examinaron las propiedades ópticas de los nanocristales obtenidos. En la obtención de CdSe, se estudió la influencia del O2 en la cinética de crecimiento de los puntos cuánticos. Durante los primeros 90 s, el crecimiento de los nanocristales en presencia de O2 fue 1,6 veces mayor que en atmósfera inerte. A pesar de este rápido crecimiento, el O2 afectó las propiedades ópticas de los nanocristales, formando bandas de absorción anchas y espectros de fluorescencia de baja intensidad. En contraste, los puntos cuánticos de CdSe sintetizados en atmósfera inerte presentaron picos de absorción bien definidos y fluorescencia aguda e intensa. Estas propiedades se intensificaron con la formación de un 10% de la monocapa de ZnS: para un núcleo de 2,50 nm, el rendimiento cuántico de fluorescencia (ΦFl) en la región del verde se incrementó de 5,5 % a 42,3%. El procedimiento de síntesis de nanocristales de CdSe/ZnS desarrollado con baja concentración de Zn2+ y con un exceso de S2" puede emplearse en la obtención de materiales con excelentes propiedades fotoluminiscentes para aplicaciones como biomarcadores, sensores, catálisis y celdas solares.

Abstract CdSe and CdSe/ZnS (core/shell) quantum dots with oleic acid as stabilizing agent in organic medium were prepared and their optical properties were examined. For CdSe synthesis, the influence of O2 in the growth kinetics of quantum dots was determined. In the first 90 s, the nanocrystals growth was 1.6 higher in presence of O2 than when reaction was carried out in N2 atmosphere. However, the growth rate with O2 not favorable because the nanocrystal optical properties were affected: wider absorption band and lower fluorescence that those obtained in inert atmosphere. Properties of CdSe nanocrystals synthesized in inert atmosphere were intensified with 10% of monolayer. For a core with 2.5 nm diameter, the fluorescence quantum yield (ΦF1) in the green region increased from 5.5% to 42.3%. The synthesis process of CdSe/ZnS nanocrystals developed with low concentration of Zn2+ and an excess of S2- can be used to obtain materials with excellent photoluminescent properties for applications such as biomarkers, sensors, catalysis, and solar cells.

Resumo Pontos quânticos CdSe e CdSe/ZnS (núcleo/ couraça) com ácido oleico como um agente de estabilização em um meio orgânico foi preparado e as propriedades ópticas dos nanocristais obtidos são examinados Na obtenção de CdSe, a influência da O2 foi estudada em cinética de crescimento de pontos quânticos. Durante o primeiro crescimento de 90 s dos nanocristais na presença de O2 foi de 1,6 vezes mais elevado do que em atmosfera inerte. Apesar deste crescimento rápido, O2 afecta as propriedades ópticas dos nanocristais, formando bandas de absorção ampla e espectros de fluorescência de baixa intensidade. Em contraste, CdSe sintetizados em atmosfera inerte mostrou picos bem definidos de absorção e fluorescência nítida e intensa. Estas propriedades são intensificadas pela formação de 10% da monocamada de ZnS: para um núcleo de 2,50 nm, o rendimento quântico de fluorescência (ΦFl) na região do verde aumentou de 5,5% para 42,3%. O procedimento de síntese de nanocristais de CdSe/ZnS desenvolvidos com baixa concentração de Zn2+ e o excesso de S2" pode ser utilizada na obtenção de materiais fotoluminescentes com excelentes propriedades para aplicações como biomarcadores, sensores, catálise e células solares.

Atypical Exciton-Phonon Interactions in WS2 and WSe2 Monolayers Revealed by Resonance Raman Spectroscopy.

Resonant Raman spectroscopy is a powerful tool for providing information about excitons and exciton-phonon coupling in two-dimensional materials. We present here resonant Raman experiments of single-layered WS2 and WSe2 using more than 25 laser lines. The Raman excitation profiles of both materials show unexpected differences. All Raman features of WS2 monolayers are enhanced by the first-optical excitations (with an asymmetric response for the spin-orbit related XA and XB excitons), whereas Raman bands of WSe2 are not enhanced at XA/B energies. Such an intriguing phenomenon is addressed by DFT calculations and by solving the Bethe-Salpeter equation. These two materials are very similar. They prefer the same crystal arrangement, and their electronic structure is akin, with comparable spin-orbit coupling. However, we reveal that WS2 and WSe2 exhibit quite different exciton-phonon interactions. In this sense, we demonstrate that the interaction between XC and XA excitons with phonons explains the different Raman responses of WS2 and WSe2, and the absence of Raman enhancement for the WSe2 modes at XA/B energies. These results reveal unusual exciton-phonon interactions and open new avenues for understanding the two-dimensional materials physics, where weak interactions play a key role coupling different degrees of freedom (spin, optic, and electronic).

Vibrational circular dichroism (VCD), VCD exciton coupling, and X-ray determination of the absolute configuration of an α,ß-unsaturated germacranolide.

The absolute configuration of was deduced by vibrational circular dichroism together with the evaluation of the Flack and Hooft X-ray parameters. Vibrational circular dichroism exciton coupling, using the carbonyl group signals, confirmed the absolute configuration of . In addition, sodium borohydride reduction of the 11,13-double bond of 6-epi-desacetyllaurenobiolide () yields an almost equimolecular mixture of C11 epimers, while reduction of the same double bond of 6-epi-laurenobiolide () provided almost exclusively the (11S) diastereoisomer .