The energetics and electronic and magnetic properties of G/MS hybrid structures embedded with 3d transition metal atoms, TM@(G/MS) (G = graphene; M = W, Mo; TM = Sc-Ni), have been systematically studied using first-principles calculations. TM atoms were found to be covalently bound to two-sided graphene and MS layers with sizable binding energies of 4.35-7.13 eV. Interestingly, a variety of electronic and magnetic properties were identified for these TM@(G/MS) systems. Except for TM = Ni, all other systems were ferromagnetic, due to exchange splitting of the TM 3d orbitals. In particular, four TM@(G/MoS) systems (TM = V, Mn, Fe, Co) and three TM@(G/WS) systems (TM = Mn, Fe, Co) were half-metals or quasi half-metals, while Ni@(G/MoS) and Ni@(G/WS) were semiconductors with bandgaps of 33 and 37 meV, respectively. Further quasi-particle scattering theory analysis demonstrated that the origin of semiconducting or half-metallic properties could be well understood from the variation in on-site energy by the transition metal dichalcogenide substrate or the different on-site scattering potential induced by TM atoms. Our findings propose an effective route for manipulating the electronic and magnetic properties of graphene@MS heterostructures, allowing their potential application in modern spintronic and electronic devices.

The quasi-one-dimensional conductor Li0.9Mo6O17 has been of great interest because of its unusual properties. It has a conducting phase with properties different from a simple Fermi liquid, a poorly understood “insulating” phase as indicated by a metal-“insulator” crossover (a mystery for over 30 years), and a superconducting phase which may involve spin triplet Cooper pairs as a three-dimensional (p-wave) non-conventional superconductor. Recent evidence suggests a density wave (DW) gapping regarding the metal-“insulator” crossover. However, the nature of the DW, such as whether it is due to the change in the charge state or spin state, and its relationship to the dimensional crossover and to the spin triplet superconductivity, remains elusive. Here by performing 7Li-/95Mo-nuclear magnetic resonance (NMR) spectroscopy, we directly observed the charge state which shows no signature of change in the electric field gradient (nuclear quadrupolar frequency) or in the distribution of it, thus providing direct experimental evidences demonstrating that the long mysterious metal-“insulator” crossover is not due to the charge density wave (CDW) that was thought, and the nature of the DW gapping is not CDW. This discovery opens a parallel path to the study of the electron spin state and its possible connections to other unusual properties.

Two series of α-and β-Na x CoO 2 materials have been prepared by means of Na deintercalation from the parent α-NaCoO 2 and β-Na 0.6 CoO 2 phases, respectively. The α-Na x CoO 2 materials undergo a clear phase transition from the hexagonal to the β-phase like monoclinic structure along with Na deintercalation. Measurements of resistivity and magnetization demonstrated the presence notable charge ordering transitions in both α-and β-Na x CoO 2 with 0.4 < x < 0.5 below 100K. Bulk Superconductivity has been observed in the hydrated α-Na x CoO 2 ·1.3H 2 O at ~4.5K and in β-Na x CoO 2 ·1.3H 2 O at ~4.3K. Intercalation of D 2 O in α, β-Na 0.33 CoO 2 also yields superconductivity at slightly lower temperatures. It is worthy to note that, despite of the structural difference, the α-, β-and γ-Na x CoO 2 .yH 2 O materials show up notable commonalities in their essential physical properties, e.g. superconductivity and charge ordering transitions. Layered γ-Na x CoO 2 system have been investigated systematically in the past years due to its particular properties of large thermoelectric power coexisting with low electric resistivity [1-4]. Recently, superconductivity in the water intercalated Na 0.3 CoO 2 ·1.3H 2 O (γ-phase) and complex charge-ordering (CO) transitions in γ-Na 0.5 CoO 2 has been extensively investigated and discussed in connection with the strong electron correlation in present system [5-12]. Actually, there are three distinctive structural series of Na x CoO 2 materials corresponding with the CoO 6 octahedra stacking differently along c-axis direction, so called α-, β-and γ-phase respectively. In order to know the structural and physical properties, especially the superconductivity in α, β-Na x CoO 2 phases, we have performed an extensive study on samples with a variety of Na contents. Actually, a few works, concerning superconductivity in the hydrated α-and β-Na x CoO 2 ·yH 2 O samples, have reported that α-phase is a superconductor with T c ~ 4.6 K [13] and, on the other hand, the water-intercalated β-Na x CoO 2 ·yH 2 O compound is likely to be not a superconductor [14]. In this paper, we will perform a systematical study on the α-and β-Na x CoO 2 materials, certain notable properties are specially analyzed in comparison with the results obtained from γ-phase, i.e. structural transformation induced by Na-deintercalation, CO phenomenon at x ≅0.5, and superconductivity in the hydrated α-and β-Na x CoO 2 materials. Polycrystalline samples of α-NaCoO 2 were prepared by a conventional solid-state reaction [15]. Powdered cobalt (Co) metal (99.5%) and anhydrous NaOH pellets (Aldrich) in 10 molar excess (Na : Co = 1.1 : 1) were ground together under inert atmosphere and placed in an alumina boat under flowing O 2 for approximately 6 days at 500°C with one intermittent grinding. The parent material with nominal composition of β-Na 0.6 CoO 2 was prepared by a similar process as described above for the α-NaCoO 2 : Co powder and NaOH pellets were mixed in a molar ratio of 3 Na : Co = 0.7 : 1 and ground together under ...

Controlling the electronic and magnetic properties of G/TMD (graphene on transition metal dichalcogenide) heterostructures is essential to develop electronic devices. Despite extensive studies in perfecting G/TMDs, most products have various defects due to the limitations of the fabrication techniques, and research investigating the performances of defective G/TMDs is scarce. Here, we conduct a comprehensive study of the effects of 3d transition metal (TM ¼ Sc-Ni) atom-intercalated G/ WSe 2 heterostructures, as well as their defective configurations having single vacancies on graphene or WSe 2 sublayers. Interestingly, Ni-intercalated G/WSe 2 exhibits a small band gap of 0.06 eV, a typical characteristic of nonmagnetic semiconductors. With the presence of one single vacancy in graphene, nonmagnetic (or ferromagnetic) semiconductors with sizable band gaps, 0.10-0.51 eV, can be achieved by intercalating Ti, Cr, Fe and Ni atoms into the heterostructures. Moreover, V and Mn doped nondefective and Sc, V, Co doped defective G/WSe 2 can lead to sizable half metallic band gaps of 0.1-0.58 eV. Further analysis indicates that the significant electron transfer from TM atoms to graphene accounts for the opening of a large band gap. Our results provide theoretical guidance to future applications of G/TMD based heterostructures in (spin) electronic devices.

The structures and properties of one-dimensional (1D) sandwich molecular wires constructed with altering 3d transition metal (TM) and the metallofullerene (TM@C 60 ) entities, [TM&(TM@C 60 )] ∞ , are studied using density functional theory calculations. Different from the bonding character insensitivity to TM of previously reported 1D [TMBz] ∞ and [TMCp] ∞ analogues, the bonding characters of the investigated 1D [TM&(TM@C 60 )] ∞ molecular wires depend heavily on the identity of metal elements. In 1D [TM&(TM@C 60 )] ∞ , molecular wires with early TMs like Ti and V, TM-η 5 coordinate bonds are favored. In contrast, TM-η 6 bonding conformations are energetically preferred for those with later TMs, for example, Cr−Ni. Bader charge analysis reveals that valence electrons are transferred from both encapsulated and sandwiched TM atoms to the C 60 ligand. More importantly, all the molecular wires in ground states are robust antiferromagnetic semiconductors because of the peierls distortion of the configurations and the moderate binding energies. Therefore, the fabrication of endohedral metallofullerenes offers an effective route to regulate the magnetism and electronic properties of C 60 −ligand sandwich complexes.