Jingguo Liu

Preparation of MoO3 nanostructures and their optical properties

In this letter we report the synthesis of nanostructures of molybdenum trioxides by directly oxidizing a spiral coil of molybdenum, at ambient atmosphere, by passing a current through the coil. The advantage of this approach is that the temperature of the substrate is low (normally below 200?C), and that the nanostructures to be formed could be chosen, via controlling the current or, equivalently, the temperature of the coil. We show that, by adjusting the current through the coil, ?-MoO3 lamellas with a thickness of ~20?50?nm, and ?-MoO3 spheres of diameters down to nanometre scale can be synthesized at ambient atmosphere. These nanostructures exhibit a large optical band gap of ~3.05?eV, and room-temperature photoluminescence at ~395?nm. This study provides a simple, controllable way of fabricating metal oxide nanostructures of interest.

Correlation of lattice constant versus tungsten concentration of the Ni-based solid solution examined by molecular dynamics simulation

Abstract In order to investigate the Ni-enriched Ni–W solid solution at an atomic level, an n-body Ni–W potential was firstly derived based on the cohesion energies, lattice constants and bulk moduli of four Ni–W non-equilibrium solid phases obtained by ion mixing experiment or ab initio calculation. Based on the derived potential, molecular dynamics simulation was then performed to examine the correlation of the lattice constant of the Ni–W solid solution vs. the W concentration and the results were in good agreement with the experimental data.

From zinc nanowires to zinc oxide nanowires: a low substrate-temperature approach

We report here a simple two-step approach to produce ZnO nanowires, i.e. preparing Zn nanowires by evaporating Zn powder in a low vacuum and then oxidizing it to ZnO in air. This approach is very simple, catalyst free, and is capable of growing ZnO nanowires at low substrate temperatures; e.g. the substrate temperature for preparing Zn nanowires was ~200?C, and that for the oxidation step was 305 ? 2?C. The ZnO nanowires synthesized by this approach exhibited intense photoluminescence (PL) at ~380?nm and weak PL at ~450?nm at room temperature. This study provides an alternative method for preparing metal oxide nanostructures at relatively low substrate temperatures.

Interface assisted formation of a metastable hcp phase by ion mixing in an immiscible Ag-Ni system

A metastable Ag-Ni phase was formed by 200 keV xenon ion mixing at 77 K in the Ag80Ni20 multilayered films, in which the excess interfacial free energy provided partial driving force for alloying in an equilibrium immiscible Ag-Ni system characterized by a large positive heat of formation. The metastable phase was identified to be of D019 hcp structure with a stoichiometry of Ag3Ni and can therefore be considered as a Hume-Rothery 7/4 electron compound. A thermodynamic calculation showed that when the multilayers consisted of 8 or more bilayers, the interfacial free energy could elevate their initial energy levels up to a state higher than that of the hcp phase, and that while the multilayers composed of 6 or less bilayers, it was thermodynamically not favoured to form the hcp phase. Kinetically, the metastable hcp phase was grown from the fcc Ag lattice through a fast sliding mechanism. Furthermore, an ab initio calculation showed that a minimum total energy of the Ag3Ni phase did correspond to the above observed metastable state, indicating that the stability of the metastable Ag-Ni hcp phase was correlated with its electronic structure.

Ab initio calculation to predict the possible nonequilibrium A3B and AB3 states in the Co–Mo system

Ab initio calculations were carried out by means of the Vienna ab initio simulation package (VASP) to predict the possible nonequilibrium states in the Co–Mo system. At the composition of Co3Mo and CoMo3, the total energies are calculated for the four different structures, i.e. A15, D019, L12, and L60, as a function of the lattice constant and some possible nonequilibrium states in the Co–Mo system are predicted, i.e. at a stoichiometry of Co3Mo with D019 structure and at CoMo3 with an A15 structure. Experimentally, ion beam mixing of Co-enriched Co–Mo multilayers has resulted in the formation of a metastable hcp phase, which is in accordance with the predicted Co3Mo state and its crystalline structure as well as total energy.

Formation of metastable crystalline phases by solid state reaction in Ni-Nb multilayered films

Two new metastable crystalline (MX) phases were formed in the Ni-Nb system by solid state reaction (SSR) of multilayered films at medium temperatures. The first one was the Ni-rich hcp phase and the second one was the Nb-rich fcc-I MX phase. A Gibbs free energy diagram of the system was constructed by calculation of the free energy curves of the alloy phases, and the MX phase formation is correlated with the constructed diagram. The growth kinetics of the MX phases is also discussed.

Formation of a face-centered-cubic metastable phase in the Fe-Nb system by solid-state reaction

In the bcc-structured Fe–Nb system, two types of metastable crystalline phases, i.e., an fcc (namely fcc-I) and an enlarged fcc (namely fcc-II) were formed by solid-state reaction at medium temperatures in Fe–Nb multilayered films with 15–80 at% Nb. The fcc-I was only found in the Fe20Nb80 films. The fcc-II phase was found in all the multilayered films, i.e. Fe20Nb80, Fe35Nb65, Fe55Nb45 and Fe85Nb15. However, the lattice constant of the formed fcc-II decreased with increasing overall Fe atomic concentration in the as-deposited films. Thermodynamically, the formation of the fcc phases are favored in the range of 15–83 at% Nb, based on a constructed Gibbs free energy diagram of Fe–Nb system. Kinetically, the formation of the fcc phases can take place through a bcc–fcc transition by shearing and then sliding.

Metastable crystalline phases formed in the Fe–Ta system by solid-state reaction

Abstract Two metastable crystalline phases of face-centered-cubic and tetragonal structures were sequentially formed in both bcc structured Fe–Ta system by solid-state reaction at moderate temperatures. The heats of formation of the newly obtained phases were calculated based on Miedema’s model and Alonso’s method and a free energy diagram of the Fe–Ta system was established, which can give a relevant thermodynamic interpretation to the observed structural phase evolution.

Ion mixing to synthesize amorphous and metastable crystalline alloys in the Fe-Ta system

Formations of metastable phase in the Fe-Ta system upon ion irradiation are sensitive to the entire composition of the initial multilayered films and ion irradiation dose. Amorphization was achieved at high irradiation dose in the Fe35Ta65 multilayered films, which was in the vicinity of the deep eutectic point. A metastable face centred cubic (fcc) phase was, however, observed at low irradiation dose in the same sample. Detailed examination showed that the fcc phase had a composition of about Fe20Ta80.

Formation of non-equilibrium solid phases in Fe-Nb multilayered films by ion irradiation

In the Fe-Nb system, amorphous alloy, metastable crystalline fcc and hexagonal phases were formed in Nb-rich multilayered films by room temperature 200 keV xenon ion mixing. A heat of formation diagram of the system was constructed based on Miedemas model and Alonsos calculation method, and the diagram gave a relevant interpretation for the observed phase formation. The formation of fcc metastable phase can be interpreted by a reverse martensitic transition of bcc-fcc. The amorphous and hexagonal phases are thought to form through a traditional nucleation and growth mechanism with an additional consideration of the irradiation effect, which kinetically hindered the formation of a complicated intermediate compound.