Magnetic Properties of Fe/Cu Codoped ZnO Nanocrystals

Free-standing ZnO nanocrystals simultaneously doped with Fe and Cu with varying Fe/Cu compositions have been synthesized using colloidal methods with a mean size of ∼7.7 nm. Interestingly, while the Cu-doped ZnO nanocrystal remains diamagnetic and Fe-doped samples show antiferromagnetic interactions between Fe sites without any magnetic ordering down to the lowest temperature investigated, samples doped simultaneously with Fe and Cu show a qualitative departure in exhibiting ferromagnetic interactions, with suggestions of ferromagnetic order at low temperature. XAS measurements establish the presence of Fe²⁺ and Fe³⁺ ions, with the concentration of the trivalent species increasing in the presence of Cu doping, providing direct evidence of the Fe²⁺ + Cu²⁺ ⇌ Fe³⁺ + Cu⁺ redox couple being correlated with the ferromagnetic property. Using DFT, the unexpected ferromagnetic nature of these systems is explained in terms of a double exchange between Fe atoms, mediated by the Cu atom, in agreement with experimental observations

High-Index Topological Insulator Resonant Nanostructures from Bismuth Selenide

Topological insulators (TIs) are a class of materials characterized by an insulting bulk and high mobility topologically protected surface states, making them promising candidates for future optoelectronic and quantum devices. Although their electronic and transport properties have been extensively studied, their optical properties and prospective photonic capabilities have not been fully uncovered. Here, we use a combination of far-field and near-field nanoscale imaging and spectroscopy, to study CVD grown Bi2Se3 nanobeams (NBs). We first extract the mid-infrared (MIR) optical constants of Bi2Se3, revealing refractive index values as high as n ~6.4, and demonstrate that the NBs support Mie-resonances across the MIR. Local near-field reflection phase mapping reveals domains of various phase shifts, providing information on the local optical properties of the NBs. We experimentally measure up to 2{\pi} phase-shift across the resonance, in excellent agreement with FDTD simulations. This work highlights the potential of TI Bi2Se3 for quantum circuitry, non-linear generation, high-Q metaphotonics, and IR photodetection

Large-Velocity Saturation in Thin-Film Black Phosphorus Transistors

A high saturation velocity semiconductor is appealing for applications in electronics and optoelectronics. Thin-film black phosphorus (BP), an emerging layered semiconductor, shows a high carrier mobility and strong mid-infrared photoresponse at room temperature. Here, we report the observation of high intrinsic saturation velocity in 7 to 11 nm thick BP for both electrons and holes as a function of charge-carrier density, temperature, and crystalline direction. We distinguish a drift velocity transition point due to the competition between the electron-impurity and electron–phonon scatterings. We further achieve a room-temperature saturation velocity of 1.2 (1.0) × 10⁷ cm s^–1 for hole (electron) carriers at a critical electric field of 14 (13) kV cm^–1, indicating an intrinsic current-gain cutoff frequency ∼20 GHz·μm for radio frequency applications. Moreover, the current density is as high as 580 μA μm^–1 at a low electric field of 10 kV cm^–1. Our studies demonstrate that thin-film BP outperforms silicon in terms of saturation velocity and critical field, revealing its great potential in radio-frequency electronics, high-speed mid-infrared photodetectors, and optical modulators