Reverse-Engineering Strain in Nanocrystallites by Tracking Trimerons.

Although strain underpins the behavior of many transition-oxide-based magnetic nanomaterials, it is elusive to quantify. Since the formation of orbital molecules is sensitive to strain, a metal-insulator transition should be a window into nanocrystallite strain. Using three sizes of differently strained Fe3 O4 polycrystalline nanorods, the impact of strain on the Verwey transition and the associated formation and dissolution processes of quasiparticle trimerons is tracked. In 40 and 50 nm long nanorods, increasing isotropic strain results in Verwey transitions going from TV ≈ 60 K to 20 K. By contrast, 700 nm long nanorods with uniaxial strain along the (110) direction have TV ≈ 150 K-the highest value reported thus far. A metal-insulator transition, like TV in Fe3 O4 , can be used to determine the effective strain within nanocrystallites, thus providing new insights into nanoparticle properties and nanomagnetism.

Selective Growth of WSe2 with Graphene Contacts.

Nanoelectronics of two-dimensional (2D) materials and related applications are hindered with critical contact issues with the semiconducting monolayers. To solve these issues, a fundamental challenge is selective and controllable fabrication of p-type or ambipolar transistors with a low Schottky barrier. Most p-type transistors are demonstrated with tungsten selenides (WSe2) but a high growth temperature is required. Here, we utilize seeding promoter and low pressure CVD process to enhance sequential WSe2 growth with a reduced growth temperature of 800 °C for reduced compositional fluctuations and high hetero-interface quality. Growth behavior of the sequential WSe2 growth at the edge of patterned graphene is discussed. With optimized growth conditions, high-quality interface of the laterally stitched WSe2-graphene is achieved and characterized with transmission electron microscopy (TEM). Device fabrication and electronic performances of the laterally stitched WSe2-graphene are presented.

Remarkably enhanced photovoltaic effects and first-principles calculations in neodymium doped BiFeO3.

Remarkably enhanced photovoltaic effects have been observed in the heterostructures of p-type A-site Nd3+-doped BiFeO3 (Bi0.9375Nd0.0625)FeO3 (or BFONd) polycrystalline ceramics and the n-type ITO thin film. The maximum power conversion is ~0.82%, which is larger than 0.015% in BiFeO3 (BFO) under blue-ultraviolet irradiation of wavelength λ = 405 nm. The current-voltage (I-V) characteristic curve suggests a p-n junction interface between the ITO thin film and BFO (or BFONd) ceramics. The band gaps calculated from first-principles for BFO and BFONd are respectively 2.25 eV and 2.23 eV, which are consistent with the experimental direct band gaps of 2.24 eV and 2.20 eV measured by optical transmission spectra. The reduction of the band gap in BFONd can be explained by the lower electronic Fermi level due to acceptor states revealed by first-principles calculations. The optical calculations show a larger absorption coefficient in BFONd than in BFO.

Unusual ferromagnetism in CoSi nanowires from internal and interfacial defects.

The diamagnetic semimetal CoSi presents unanticipated ferromagnetism as CoSi/SiO2 nanowires (NWs). Using first-principles calculations, we offer physical insights into the origins of this unusual magnetism. Due to the distorted and dangling bonds near the NW surface with different bond lengths, the transition metal (Co) d-orbital electron spin up and spin down populations become asymmetric from the exchange interactions, providing the mechanism for some of the measured magnetization. However, the distorted and dangling bonds are clearly not the only factor contributing to the magnetization of the NWs. The transmission electron microscopy selected area electron diffraction analysis of the CoSi region suggested a superlattice structure existed in the cubic CoSi, and defects existing as ordered vacancies in the CoSi were present. The simulation's results for the Co moment in the CoSi NWs without these ordered vacancies, but incorporating the surface and internal spin moments, is only 0.1638 µ(B)/atom Co, which is a ∼80% shortfall compared to the experimental value of 0.8400 µ(B)/atom Co. When the effects of ordered vacancies are incorporated into the simulation, 0.7886 µ(B) per surface Co atom, a much better match with the experimental value (within ∼6%), indicating that the internal ordered vacancies in the CoSi NWs are the dominant mechanism of ferromagnetism.