RESUMEN
The Te8 ring molecule (cluster) is poorly investigated due to the lack of experimental data. Here, we report an experimental and theoretical study of a regular array of oriented Te8 rings formed in the â¼1.14 nm diameter cavities of zeolite LTA, which are arranged in a cubic lattice with a spacing of â¼1.2 nm. Single crystals of LTA with encapsulated tellurium (LTA-Te) were studied using Raman spectroscopy (RS) and optical absorption spectroscopy (OAS). The experimental LTA-Te spectra were found to be in agreement with those calculated using density functional theory (PBE0 hybrid functional and def2-TZVP basis sets) for the crown-shaped Te8 ring molecule with D4d symmetry. Using polarization-orientation RS, we show that the Te8 rings are oriented by their major axes along the 4-fold axes of cubic LTA. We also show that the site symmetry of Te8 in LTA-Te is lower than D4d. Te8 bond-bending modes are well described in the harmonic approximation, while bond-stretching modes are mixed due to the reduced ring symmetry and, probably, anharmonicity. Importantly, OAS data of LTA-Te display dependence on the Te8 concentration, implying the interaction of the rings from neighbouring LTA cavities with the generation of the valence and conduction electron bands of such a cluster crystal.
RESUMEN
Single tellurium (Te) chains attract much attention as extreme nanowires with unique electronic and spintronic properties. Here, we encapsulate Te from a melt into channels of zeolites AFI (â¼0.73 nm-channel diameter) and mordenite (MOR, â¼0.67 × 0.7 nm2 channel cross-section) via high-pressure injection. Using polarized Raman and optical absorption spectra (RS and OAS) of zeolite single crystals with Te (AFI-Te and MOR-Te), we discriminate between features of Te chains and rings formed in the zeolites. We demonstrate good agreement of AFI-Te-chain RS and OAS with the calculated single Te-helix phonon and electron spectra. This suggests a very weak interaction of the AFI-Te-chain with the zeolite and its nearly perfect helix structure lacking inversion/mirror symmetry. An AFI-Te OAS feature, attributed to the electron transitions between Te-helix-Rashba-split valence and conduction bands confirms its 1D-electron-band origin with predicted possibilities of identifying Majorana fermions, manipulating spin transport and realizing topological superconductivity.
RESUMEN
Nanoelectronics require semiconductor nanomaterials with high electron mobility like Ge nanolayers. Phonon and electron states in nanolayers undergo size-dependent changes induced by confinement and surface effects. Confined electrons and acoustic phonons determine layer optical, electric and thermal properties. Despite scientific and practical significance, their experimental studies in individual nanolayers are still lacking. Thanks to recent progress in the fabrication of high-quality nanolayers, here, we report the thickness dependencies of Raman spectra of acoustic phonons and optical spectra of electrons confined in germanium-on-insulator (GeOI) nanolayers with thicknesses TGeOI = 1-20 nm. We show that for TGeOI > 5 nm, both GeOI acoustic phonon Raman spectra and the E1 electron energy gap display dependencies on TGeOI which are reasonably described by the corresponding phonon and electron confinement theories. Accordingly, TGeOI can be probed using acoustic phonon Raman spectra at TGeOI > 5 nm. However, both confinement theories fail to describe GeOI thickness dependencies at TGeOI < 5 nm. We attribute this discrepancy to an increased influence of the Ge-GeO2 interface disorder with TGeOI reduction. The acoustic phonon data suggest a decrease of Ge normal-to-the-layer longitudinal sound velocity. Generation of interface-disorder-induced dispersionless phonons might contribute to this. The change in GeOI phonon properties at TGeOI < 5 nm might influence E1(TGeOI) dependence via a change in the GeOI electron-phonon interaction. We demonstrate that the Al2O3 coating improves the agreement between experimental and confinement theories, probably, via reduction of disorder at the Ge-GeOx-Al2O3-interface. Our results are important for control of nanolayer-confined electrons and phonons with benefits for modern and future nanoelectronic devices.
RESUMEN
All-dielectric photonics is a rapidly developing field of optics and material science. The main interest at visible and near-infrared frequencies is light management using high-refractive-index Mie-resonant dielectric particles. Most work in this area of research focuses on exploiting Si-based particles. Here, we study monocrystalline Mie-resonant particles made of Ge-rich SiGe alloys with refractive index higher than that of Si. These islands are formed via solid state dewetting of SiGe flat layers by using two different processes: (i) dewetting of monocrystalline SiGe layers (60%-80% Ge content) obtained via Ge condensation of SiGe on silicon on insulator; and (ii) dewetting of a SiGe layer deposited via molecular beam epitaxy on silicon on insulator and ex situ Ge condensation, forming a Ge-rich shell surrounding a SiGe-core. Using high-spatial-resolution Raman microscopy we monitor Ge content x and strain ϵ of flat layers and SiGe-islands. We observe strain relaxation associated with formation of trading dislocations in the SiGe islands compared to the starting SiGe layers, as confirmed by TEM images. For initial high Ge concentration in the flat layers, the corresponding Ge content in the dewetted islands is lower, owing to diffusion of Si atoms from Si or SiO2 into SiGe islands. The Ge content also varies from particle to particle on the same sample. Size and shape of the dewetted particles depend on the fabrication process: thicker initial SiGe layers lead to larger particles. Samples with narrow island size distribution display rather sharp Mie resonances in the 1000-2500 nm spectral range. Larger islands display Mie resonances at longer wavelength. Positions of the resonances are in agreement with the theoretical calculations in the discrete dipole approximation.
