Gang Xiang

Nanostructured magnetic materials

The saga of nanostructured magnetic materials (NMMs) has prevailed since the discovery of the first giant magnetoresistance (GMR) effect in metals in 1988. NMMs represent a unique system that incorporates the interplay between the properties associated with spin degrees of freedom and the nanoscaled structures, which provide a very strong platform for exploring both basic science and technical applications in the fields of solid-state physics, chemistry, materials science, and engineering.

Magnetism in transition-metal-doped ZnO: A first-principles study

The magnetic properties and clustering behaviors of the transition-metal (TM)-doped ZnO have been studied by first-principles calculations. We demonstrate the relationship between the types of TM (Co, Fe, Ni, and Cu) pairs and the energetic most stable configurations in ZnO:TM. Further calculations show that Co pairs have no tendency to form clusters, showing paramagnetic (PM) state in ZnO:Co samples in an equilibrium state and without excessive defects, as evidenced by various experiments, while ferromagnetic (FM) state is possible for the samples prepared under non-equilibrium conditions. Finally, systematical studies of a series of TM elements reveal different types of TM pairs in terms of clustering behaviors determined by spin-polarized and non-spin-polarized contributions: Co pairs and Fe pairs have no tendency to form clusters, showing PM and FM states, respectively, while Ni pairs and Cu pairs have a clustering tendency and show intrinsic FM ordering.

Homostructured negative differential resistance device based on zigzag phosphorene nanoribbons

The electronic transport properties of zigzag phosphorene nanoribbon (ZPNR) homostructures are investigated using density functional theory calculations and the nonequilibrium Green’s function technique. The proposed devices have simple constructions but exhibit robust negative differential resistance (NDR) as well as quite large current, which implies great potential for building nanoelectronic devices. Through the analysis of the electronic structures and transmission spectra, we give a clear physical picture of the NDR mechanism, in which such current–voltage (I–V) behaviors originate from the bias-dependent interaction between the discrete density of states (DOS) peaks of the electrodes and the narrow states around the Fermi level in the scattering region.

Vacancy dependent structural, electronic, and magnetic properties of zigzag silicene nanoribbons:Co

We study the structure and properties of perfect and defect zigzag silicene nanoribbons (SiNRs) with cobalt (Co) atom adsorbed on different sites of SiNRs using density-functional theory (DFT) calculations. A variety of electronic and magnetic behaviors have been demonstrated, which are found to be strongly dependent on both vacancy configurations and Co atom sites in the system. Detailed analysis reveals that vacancy configurations dramatically influence the structure and electronic band dispersions, and Co atom plays a key role in magnetic properties of the system by raising the magnetic moment of its neighbor Si atoms and suppressing the magnetism of its nearer ribbon-edge. Our results suggest potential applications of doped SiNRs in silicon-based nanodevices.

Cm-size free-standing self-organized buckypaper of bucky-onions filled with ferromagnetic Fe3C

Novel cm-size free-standing buckypapers of bucky-onions filled with a single-phase of ferromagnetic Fe3C single crystals were serendipitously discovered. These buckypapers are obtained directly in situ as the dominant product of the pyrolysis of ferrocene. Vibrating sample magnetometry also revealed an extremely large coercivity of 0.120 tesla and a saturation magnetization of 41 emu g−1.

Zinc vacancy-induced room-temperature ferromagnetism in undoped ZnO thin films

Undoped ZnO thin films are prepared by polymer-assisted deposition (PAD) and treated by postannealing at different temperatures in oxygen or forming gases (95% Ar + 5% H2). All the samples exhibit ferromagnetism at room temperature (RT). SQUID and positron annihilation measurements show that post-annealing treatments greatly enhance the magnetizations in undoped ZnO samples, and there is a positive correlation between the magnetization and zinc vacancies in the ZnO thin films. XPS measurements indicate that annealing also induces oxygen vacancies that have no direct relationship with ferromagnetism. Further analysis of the results suggests that the ferromagnetism in undoped ZnO is induced by Zn vacancies.

The static and dynamic magnetic properties of monolayer iron dioxide and iron dichalcogenides

The electronic structures, and static and dynamic magnetic properties of monolayer iron dioxide and iron dichalcogenides (FeX2 (X = O, S, Se, Te)) are investigated using first-principle calculations in conjunction with Monte Carlo (MC) simulation and atomic spin dynamics (ASD) simulation. In this rarely studied family of monolayer binary compounds a variety of possible phases are discovered, including narrow bandgap a semiconductor (FeO2), half-metal (FeS2) and metal (FeSe2 and FeTe2), and all the ground states are ferromagnetic (FM). Based on the magnetic exchange interactions, the temperature dependence of the average magnetic moment per unit cell and magnetic susceptibility of monolayer FeX2 are predicted. The Curie temperatures (TCs) are estimated and magnon dispersions as a function of temperature are demonstrated, revealing a new family of pristine monolayers with transition temperatures (96–168 K) above the liquid-nitrogen temperature.

Observation of large coercivities in radial carbon nanotube structures filled with Fe3C and FeCo single-crystals by viscous boundary layer pyrolysis of ferrocene and cobaltocene

Viscous boundary layer chemical vapor synthesis is a novel technique that uses the viscous boundary layer between a laminar pyrolysed metallocene/Ar vapor flow and a rough surface to induce the nucleation and growth of radial carbon nanotube (CNT) structures highly filled with ferromagnetic materials. Here we report the synthesis and characterization of radial structures comprising multiwall CNTs filled with large quantities of Fe3C and FeCo alloys and low quantities of γ-Fe in the forms of small single crystals. Surprisingly high saturation magnetizations up to 80 emu g−1 and a very high coercivity of 1400 Oe at room temperature are found. Such values of magnetization suggest that no room-temperature magnetic interaction is present between γ-Fe and the ferromagnetic crystals in our samples. The presence of such large coercivity values may be associated with the small size of the encapsulated particles which is strongly dependent on the high evaporation temperature of the precursors for fixed pyrolysis temperatures and vapour flow rate. The addition of Cl-species is also considered in the attempt to slow down the growth-rate of the radial CNT-structure and further investigate their growth mechanism.

The structures and diffusion behaviors of point defects and their influences on the electronic properties of 2D stanene

During the synthesis of stanene monolayers, defects are inevitably present and always affect the properties. Here we used ab initio calculations to systemically investigate the structures, diffusion behaviors and related properties of several kinds of typical point defects, including the Stone–Wales (SW) defect, single vacancy (SV-1(55|66) and SV-2(3|555)) and double vacancy (DV-1(5|8|5) and DV-2(555|777)) defects. Scanning tunneling microscopy (STM) images were also simulated to help experimentalists identify these defects in stanene. The investigation of structures and diffusion behaviors of the defects revealed that SW can be easily recovered by annealing due to its low reverse barrier, both SV-1(55|66) and SV-2(3|555) are the most stable SVs, the energetically favored DV-1(5|8|5) can be formed from two SVs coalescing together, and DV-2(555|777) can arise from DV-1(5|8|5) via bond rotation by overcoming a diffusion barrier of 0.89 eV. The point defects exhibit nontrivial influences on the electronic properties of stanene: SW can open a direct gap in the energy band without harm to the high-velocity carriers, SV-1(55|66) makes stanene metallic, and SV-2(3|555), DV-1(5|8|5) and DV-2(555|777) may change stanene to an indirect or direct band gap semiconductor. Spin orbit coupling (SOC) effects have visible influences on the electronic bands, specifically the band gaps. Our theoretical results may provide valuable insights into the identification of point defects in further experiments and the understanding of their effects on the electronic properties and potential applications of stanene.

Micrometre-length continuous single-crystalline nm-thin Fe3C-nanowires with unusual 010 preferred orientation inside radial few-wall carbon nanotube structures: the key role of sulfur in viscous boundary layer CVS of ferrocene

A key challenge in the fabrication of carbon nanotubes filled with ferromagnetic nanowires is the control of the number of nanotube-walls together with the nanowire continuity, composition and crystallinity. We report the serendipitous observation of novel radial carbon nanotube structures with few walls (2–5 walls) filled with nm-thin and many-micrometres long continuous single-crystalline Fe3C nanowires. These are the dominant reaction products in chemical vapour synthesis experiments involving the pyrolysis of ferrocene/sulfur mixtures in the viscous boundary layer between a rough surface and a laminar Ar flow. These nanowires are found with an unusual preferred 010 orientation along the nanotube capillary. The properties of these structures are investigated through the use of multiple techniques: SEM, TEM, HRTEM, EDX, STEM, XRD, Raman spectroscopy, FT-IR spectroscopy and VSM.