Electronic properties of TaAs2topological semimetal investigated by transport and ARPES.

We have performed electron transport and angle-resolved photo-emission spectroscopy (ARPES) measurements on single crystals of transition metal dipnictide TaAs2cleaved along the (2¯01) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov-de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between magnetic field and the [2¯01] direction. The results indicate elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved (2¯01) surface and it was found that bulk states pockets at constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level (FL) is increased, which is due to a small n-doping of the samples. This shifts the FL closer to the Dirac point, allowing investigating the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.

Near-infrared emission from spatially indirect excitons in type II ZnTe/CdSe/(Zn,Mg)Te core/double-shell nanowires.

ZnTe/CdSe/(Zn, Mg)Te core/double-shell nanowires are grown by molecular beam epitaxy by employing the vapor-liquid-solid growth mechanism assisted with gold catalysts. A photoluminescence study of these structures reveals the presence of an optical emission in the near infrared. We assign this emission to the spatially indirect exciton recombination at the ZnTe/CdSe type II interface. This conclusion is confirmed by the observation of a significant blue-shift of the emission energy with an increasing excitation fluence induced by the electron-hole separation at the interface. Cathodoluminescence measurements reveal that the optical emission in the near infrared originates from nanowires and not from two-dimensional residual deposits between them. Moreover, it is demonstrated that the emission energy in the near infrared depends on the average CdSe shell thickness and the average Mg concentration within the (Zn, Mg)Te shell. The main mechanism responsible for these changes is associated with the strain induced by the (Zn, Mg)Te shell in the entire core/shell nanowire heterostructure.

Growth and optical properties of ZnO/Zn1-xMgxO quantum wells on ZnO microrods.

While synthesis methods for pure ZnO nanostructures are well established, an efficient technique for the growth of ZnO-based nanowires or microrods that incorporate any type of quantum structure is yet to be established. Here, we report on the fabrication and optical properties of axial Zn1-xMgxO/ZnO/Zn1-xMgxO quantum wells that were deposited by molecular beam epitaxy on ZnO microrods obtained using a hydrothermal method. Using the emission energy results found in cathodoluminescence measurements and the results of a numerical modeling process, we found the quantum well width to be 4 nm, as intended, at the growth stage. The emission of quantum well-confined excitons persists up to room temperature. We used the fabricated structures to determine the carrier diffusion length (>280 nm) in ZnO using spatially resolved cathodoluminescence. The micro-photoluminescence results suggest an increase in the electron-phonon coupling strength with increasing microrod size.

Fragility of the Dirac Cone Splitting in Topological Crystalline Insulator Heterostructures.

The "double Dirac cone" 2D topological interface states found on the (001) faces of topological crystalline insulators such as Pb1-xSnxSe feature degeneracies located away from time reversal invariant momenta and are a manifestation of both mirror symmetry protection and valley interactions. Similar shifted degeneracies in 1D interface states have been highlighted as a potential basis for a topological transistor, but realizing such a device will require a detailed understanding of the intervalley physics involved. In addition, the operation of this or similar devices outside of ultrahigh vacuum will require encapsulation, and the consequences of this for the topological interface state must be understood. Here we address both topics for the case of 2D surface states using angle-resolved photoemission spectroscopy. We examine bulk Pb1-xSnxSe(001) crystals overgrown with PbSe, realizing trivial/topological heterostructures. We demonstrate that the valley interaction that splits the two Dirac cones at each X̅ is extremely sensitive to atomic-scale details of the surface, exhibiting non-monotonic changes as PbSe deposition proceeds. This includes an apparent total collapse of the splitting for sub-monolayer coverage, eliminating the Lifshitz transition. For a large overlayer thickness we observe quantized PbSe states, possibly reflecting a symmetry confinement mechanism at the buried topological interface.

Facile synthesis of core/shell ZnO/ZnS nanofibers by electrospinning and gas-phase sulfidation for biosensor applications.

This study describes a new method of passivating ZnO nanofiber-based devices with a ZnS layer. This one-step process was carried out in H2S gas at room temperature, and resulted in the formation of core/shell ZnO/ZnS nanofibers. This study presents the structural, optical and electrical properties of ZnO/ZnS nanofibers formed by a 2 nm ZnS sphalerite crystal shell covering a 5 nm ZnO wurtzite crystal core. The passivation process prevented free carriers from capture by oxygen molecules and significantly reduced the impact of O2 on nanostructure conductivity. The conductivity of the nanofibers was increased by three orders of magnitude after the sulfidation, the photoresponse time was reduced from 1500 s to 30 s, and the cathodoluminescence intensity increased with the sulfidation time thanks to the removal of ZnO surface defects by passivation. The ZnO/ZnS nanofibers were stable in water for over 30 days, and in phosphate buffers of acidic, neutral and alkaline pH for over 3 days. The by-products of the passivation process did not affect the conductivity of the devices. The potential of ZnO/ZnS nanofibers for protein biosensing is demonstrated using biotin and streptavidin as a model system. The presented ZnS shell preparation method can facilitate the construction of future sensors and protects the ZnO surface from dissolving in a biological environment.

Low-Temperature Cathodoluminescence Investigations of High-Quality Zinc Oxide Nanorods.

We present results of cathodoluminescence (CL) investigations of high-quality zinc oxide (ZnO) nanorods obtained by an extremely fast hydrothermal method on a silicon substrate. A scanning electron microscopy (SEM) system equipped with CL allows direct comparison of SEM images and CL maps, taken from exactly the same areas of samples. Investigations are performed at a temperature of 5 K. An interlink between sample microstructure and emission properties is investigated. CL confirms a very high quality of ZnO nanorods produced by our method. In addition, the presence of super radiation effects in ZnO nanorod arrays is suggested.