Conductivity and photoconductivity in a two-dimensional zinc bis(triarylamine) coordination polymer.

We report the synthesis and characterization of a 2D semiconductive and photoconductive coordination polymer. [Zn(TPPB)(Cl2)]·H2O (1) (TPPB = N 1,N 1,N 4,N 4-tetrakis(4-(pyridin-4-yl)phenyl)benzene-1,4-diamine) consists of a TPPB redox-active linker with bis(triarylamine) as the core. It consists of two redox sites connected with a benzene ring as a bridge. Thus, this forms an extended conjugation pathway when the TPPB ligand is coordinated with the Zn2+ metal ions. The single crystal conductivity measurement revealed conductivity of 1 to be in the range of 0.83 to 1.9 S cm-1. Band structure analysis predicted that 1 is a semiconductor from the delocalization of electronic transport in the network. The computational calculations show the difference in charge distribution between holes and electrons, which led to spatial separation. This implies a long charge carrier lifetime as indicated by lifetime measurement. Incorporating a bis(triarylamine)-based redox-active linker could lead to a new semiconductive scaffold material with photocatalytic applications.

Non-conventional superconductivity in magnetic In and Sn nanoparticles.

We report on experimental evidence of non-conversional pairing in In and Sn nanoparticle assemblies. Spontaneous magnetizations are observed, through extremely weak-field magnetization and neutron-diffraction measurements, to develop when the nanoparticles enter the superconducting state. The superconducting transition temperature TC shifts to a noticeably higher temperature when an external magnetic field or magnetic Ni nanoparticles are introduced into the vicinity of the superconducting In or Sn nanoparticles. There is a critical magnetic field and a critical Ni composition that must be reached before the magnetic environment will suppress the superconductivity. The observations may be understood when assuming development of spin-parallel superconducting pairs on the surfaces and spin-antiparallel superconducting pairs in the core of the nanoparticles.

Charge transfer enhanced magnetic correlations in type-II multiferroic Co3TeO6.

Magnetic structure of the Co ions in monoclinic Co3TeO6 in the antiferroelectric state at 16 K has been determined by neutron powder together with single-crystal diffractions. The indices of the magnetic reflections that appear at the incommensurate positions were determined by diffractions from a single crystal, which allow to uniquely identify the magnetic modulation vector. There are two crystallographically distinct Co layers. Magnetic incommensurability appears in the Co spins in the layers comprising zig-zag chains, with a magnetic modulation vector of (0.357, 0.103, 0.121) at 3 K but changes to (0.4439, 0, 0.137) at 16 K, while the Co ions in the honeycomb webs form a collinear antiferromagnetic structure. Thermal reduction rate of the Co moments in the honeycomb webs was found to be much smaller than those in the zigzag chains. Shifting of large amounts of electronic charge into the Co─O bonds in the honeycomb webs on warming is used to understand the behavior.

Largely Enhanced Ferromagnetism in Bare CuO Nanoparticles by a Small Size Effect.

Magnetic properties of fully oxygenated bare CuO nanoparticles have been investigated using magnetization, X-ray diffraction, neutron diffraction, and Raman scattering measurements. The Langevin field profile is clearly revealed in the isothermal magnetization of 8.8 nm CuO nanoparticle assembly even at 300 K, revealing a 172 times enhancement of the ferromagnetic responses over that of bulk CuO. Surface magnetization of 8.8 nm CuO reaches 18% of the core magnetization. The Cu spins in 8.8 nm CuO order below 400 K, which is 1.7 times higher than the 231 K observed in bulk CuO. A relatively simple magnetic structure that may be indexed using a modulation vector of (0.2, 0, 0.2) was found for the 8.8 nm CuO, but no magnetic incommensurability was observed in bulk CuO. The Cu spins in 8.8 nm CuO form spin density waves with length scales of 5 chemical unit cells long along the crystallographic a- and c-axis directions. Considerable amounts of electronic charge shift from around the Cu lattice sites toward the interconnecting regions of two neighboring Cu-Cu ions, resulting in a stronger ferromagnetic direct exchange interaction for the neighboring Cu spins in 8.8 nm CuO.

Impact of oxygen defects on a ferromagnetic CrI3 monolayer.

Natural oxygen defects play a vital role in the integrity, functional properties, and performance of well-known two-dimensional (2D) materials. The recently discovered chromium triiodide (CrI3) monolayer is the first real 2D magnet. However, its interaction with oxygen remains an open fundamental question, an understanding of which is essential for further exploration of its application potentials. Employing the quantum first-principles calculation method, we investigated the influence of oxygen defects on the structural, electronic, and magnetic properties of the CrI3 monolayer at the atomic level. We considered two oxygen-defective CrI3 monolayers with either a single O-attached or single O-doped structure, comparing them with an un-defective pristine monolayer. The two different oxygen defects significantly affect the original architecture of the CrI3 monolayer, being energetically favorable and increasing the stability of the CrI3 monolayer. Moreover, these point defects introduce either deep band lines or middle gap states in the band structure. As a result, the bandgap of oxygen-defective monolayers is reduced by up to 58%, compared with the pristine sheet. Moreover, the magnetic property of the CrI3 monolayer is drastically induced by oxygen defects. Importantly, O-defective CrI3 monolayers possess robust exchange coupling parameters, suggesting relatively higher Curie temperature compared with the un-defective sheet. Our findings reveal that the natural oxygen defects in the CrI3 monolayer enrich its structural, electronic, and magnetic properties. Thus, the controlled oxidation can be an effective way to tune properties and functionalities of the CrI3 monolayer and other ultrathin magnetic materials.

Photoenhanced Ferromagnetism in High-K+-Containing K-Ni-Cr Prussian Blue Analogues Coated on Rb-Co-Fe Nanocubes.

A large enhancement of the Ni and Cr ferromagnetic moments under UV-light irradiation has been detected in 55 nm thick K0.98Ni[Cr(CN)6]0.70[(H2O)6]0.30·0.11H2O Prussian blue analogues coated on 240 nm Rb0.76Co[Fe(CN)6]0.74[(H2O)6]0.26·0.56H2O nanocubes. Two separate magnetic transitions were found. The one at 72 K marks the magnetic ordering of the Ni and Cr ions on the shell. A higher degree of electronic connection along the Ni-N-C-Cr-C-N-Ni chains was achieved by the incorporation of a larger amount of K+ ions into the voids enclosed by the NiN6 and CrC6 octahedra, which was used to understand the appearance of photoenhanced ferromagnetism in the K-Ni-Cr network. A weak moment developed in the core below 10 K, corresponding to separate ordering of the Co and Fe ions in the Rb-Co-Fe network. Photoinduced ferromagnetism of the Co and Fe ions in the Rb-Co-Fe was also detected.

Development of Ferromagnetic Superspins in Bare Cu Nanoparticles by Electronic Charge Redistribution.

We report on the results of investigating the ferromagnetic properties of bare Cu nanoparticles. Three sets of bare Cu nanoparticle assemblies with mean particle diameters of 6.6, 8.1, and 11.1 nm were fabricated, employing the gas condensation method. Curie-Weiss paramagnetic responses to a weak driving magnetic field were detected, showing the appearance of particle superspins that overcomes the diamagnetic responses from the inner core. The isothermal magnetization displays a Langevin field profile together with magnetic hysteresis appearing even at 300 K, demonstrating the existence of ferromagnetic superspins in the Cu nanoparticles. Shifting of a noticeable amount of electronic charge from being distributed near the lattice sites in bulk form toward their neighboring ions in nanoparticles was found. The extended 3d and 4s band mixture are the main sources for the development of localized 3d holes for the development of ferromagnetic particle superspins in Cu nanoparticles.