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Highly uniform arrays of site-controlled GaN nanowires are synthesized by selective area growth using a Si3N4 mask and molecular beam epitaxy. Systematic modulation of the emission along the nanowire axis is observed in spectrally resolved cathodoluminescence linescans. We show that this intensity change is an indicator of unintentional Si incorporation during growth resulting from the interaction between the impinging Ga atoms and the mask material. The gradual reduction of the cathodoluminescence intensity along the nanowire highlights the important role of the growth geometry within the synthesis reactor, with shadowing from the elongating nanowires inhibiting the reaction with the mask. This gradient in Si doping is confirmed by the quenching of the longitudinal optical phonon line measured in Raman spectra along the nanowire axis. The corresponding carrier density is derived from the frequency of the coupled phonon–plasmon mode. The spectroscopic identification of inversion domain boundaries in the majority of the nanowires is also attributed to the Si incorporation. From temperature dependent cathodoluminescence experiments, we derive the activation energy for excitons bound to these defects. [ABSTRACT FROM AUTHOR]

Rutile GeO2 has been predicted to be an ultra-wide bandgap semiconductor suitable for future power electronic devices, while quartz-like GeO2 shows piezoelectric properties. To explore these crystalline phases for application and fundamental materials investigations, molecular beam epitaxy (MBE) is a well-suited thin film growth technique. In this study, we investigate the reaction kinetics of GeO2 during plasma-assisted MBE using elemental Ge and plasma-activated oxygen fluxes. The growth rate as a function of oxygen flux is measured in situ by laser reflectometry at different growth temperatures. A flux of the suboxide GeO desorbing off the growth surface is identified and quantified in situ by the line-of-sight quadrupole mass spectrometry. Our measurements reveal that the suboxide formation and desorption limits the growth rate under Ge-rich or high temperature growth conditions and leads to etching of the grown GeO2 layer under a Ge flux in the absence of oxygen. The quantitative results fit the sub-compound mediated reaction model, indicating the intermediate formation of the suboxide at the growth front. This model is further utilized to delineate the GeO2-growth window in terms of oxygen-flux and substrate temperature. Our study can serve as a guidance for the thin film synthesis of GeO2 and defect-free mesa etching in future GeO2-device processing. [ABSTRACT FROM AUTHOR]

We investigate the buildup of strain in InP quantum dots with the addition of shells of the lower-lattice constant materials ZnSe and ZnS by Raman spectroscopy. Both materials induce compressive strain in the core, which increases with increasing shell volume. We observe a difference in the shell behavior between the two materials: the thickness-dependence points toward an influence of the material stiffness. ZnS has a larger Young's modulus and requires less material to develop stress on the InP lattice at the interface, while ZnSe requires several layers to form a stress-inducing lattice at the interface. This hints at the material stiffness being an additional parameter of relevance for designing strained core/shell quantum dots. [ABSTRACT FROM AUTHOR]

Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light. Optical excitation of transition metal dichalcogenide monolayers mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here, the authors demonstrate efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. [ABSTRACT FROM AUTHOR]

Holding promising applications in power electronics, the ultra-wide band gap material gallium oxide has emerged as a vital alternative to materials like GaN and SiC. The detailed study of phonon modes in β-Ga2O3 provides insights into fundamental material properties such as crystal structure and orientation and can contribute to the identification of dopants and point defects. We investigate the Raman active phonon modes of β-Ga2O3 in two different oxygen isotope compositions (16O,18O) by experiment and theory: By carrying out polarized micro-Raman spectroscopy measurements on the (010) and (2¯01) planes, we determine the frequencies of all 15 Raman active phonons for both isotopologues. The measured frequencies are compared with the results of density functional perturbation theory (DFPT) calculations. In both cases, we observe a shift of Raman frequencies towards lower energies upon substitution of 16O with 18O. By quantifying the relative frequency shifts of the individual Raman modes, we identify the atomistic origin of all modes (Ga–Ga, Ga–O or O–O) and present the first experimental confirmation of the theoretically calculated energy contributions of O lattice sites to Raman modes. The DFPT results enable the identification of Raman modes that are dominated by the different, inequivalent O- or Ga-atoms of the unit cell. We find that oxygen substitution on the OIII site leads to an elevated relative mode frequency shift compared to OI and OII sites. This study presents a blueprint for the future identification of different point defects in Ga2O3 by Raman spectroscopy. [ABSTRACT FROM AUTHOR]

Anti‐Stokes photoluminescence excitation of a WS2 monolayer flake between 10 and 300 K is reported herein. Even with continuous‐wave lasers at low power, the emission of the exciton at excitation 100 meV below its emission energy at room temperature is observed. A mechanism which involves the trions as the intermediate state is proposed, leading to an efficient up‐conversion process. In addition, it is demonstrated that phonons are the source of the additional energy needed by the system. Overall, the results provide evidence that anti‐Stokes luminescence in transition metal dichalcogenides is very efficient. [ABSTRACT FROM AUTHOR]

Anti‐Stokes photoluminescence excitation of a WS2 monolayer flake between 10 and 300 K is reported herein. Even with continuous‐wave lasers at low power, the emission of the exciton at excitation 100 meV below its emission energy at room temperature is observed. A mechanism which involves the trions as the intermediate state is proposed, leading to an efficient up‐conversion process. In addition, it is demonstrated that phonons are the source of the additional energy needed by the system. Overall, the results provide evidence that anti‐Stokes luminescence in transition metal dichalcogenides is very efficient. [ABSTRACT FROM AUTHOR]

In this Letter, we present photoluminescence measurements with different excitation energies on single-layer MoS2 and MoSe2 in order to examine the resonance behavior of the conservation of circular polarization in these transition metal dichalcogenides. We find that the circular polarization of the emitted light is conserved to 100% in MoS2 and 84%/79% (A/A-peaks) in MoSe2 close to resonance. The values for MoSe2 surpass any previously reported value. However, in contrast to previous predictions, the degree of circular polarization decreases clearly at energies less than the two-phonon longitudinal acoustic phonon energy above the resonance. Our findings indicate that at least two competing processes underly the depolarization of the emission in single-layer transition metal dichalcogenides. [ABSTRACT FROM AUTHOR]

A perpendicularly magnetized spin injector with a high Curie temperature is a prerequisite for developing spin optoelectronic devices on two-dimensional (2D) materials working at room temperature (RT) with zero applied magnetic field. Here, we report the growth of Ta/CoFeB/MgO structures with large perpendicular magnetic anisotropy (PMA) on full-coverage monolayer (ML) molybdenum disulfide (MoS 2 ). A large perpendicular interface anisotropy energy of 0.975 mJ/m 2 has been obtained at the CoFeB/MgO interface, comparable to that observed in magnetic tunnel junction systems. It is found that the insertion of MgO between the ferromagnetic (FM) metal and the 2D material can effectively prevent the diffusion of the FM atoms into the 2D material. Moreover, the MoS 2 ML favors a MgO(001) texture and plays a critical role in establishing the large PMA. First-principles calculations on a similar Fe/MgO/MoS 2 structure reveal that the MgO thickness can modify the MoS 2 band structure, from a direct band gap with 3ML-MgO to an indirect band gap with 7 ML-MgO. The proximity effect induced by Fe results in splitting of 10 meV in the valence band at the Γ point for the 3ML-MgO structure, while it is negligible for the 7 ML-MgO structure. These results pave the way to develop RT spin optoelectronic devices based on 2D transition-metal dichalcogenide materials.

How can ion-exchange process occur in nanocrystals without the size and shape changing and why is the ion transport much faster than in classical interdiffusion processes in macrocrystalline solids? We have investigated these processes at the molecular level by means of high-resolution and analytical electron microscopy in temperature-dependent kinetic experiments for several model reactions. The results clearly show a diffusion process that proceeds exclusively through the interstitial lattice positions with a subsequent "kick out" to remove individual ions from lattice sites without the formation of vacancies. This mechanism has not been observed in nanocrystalline systems before., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)