Bingyan Qu

New-phase VO2 micro/nanostructures: investigation of phase transformation and magnetic property

New-phase VO2 micro/nanostructures built by nanoflakes have been first synthesized by a hydrothermal method with NH4VO3 as precursor in the presence of poly(vinyl pyrrolidone) (PVP). The combined structural analysis of X-ray powder diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy determined the crystal structure as a new-phase vanadium dioxide, which is the isostructure of monoclinic NiWO4 and designated as VO2(D). In particular, electron spin resonance (ESR) measurements provides the direct evidence of vanadium ion in the four oxidation state. The formation energy of VO2(D) was estimated and showed a very close value to rutile-type VO2(R), which guided the preparation of VO2(R/M) by making use of the structural transformation from VO2(D) to VO2(R) at 320 °C, which was a comparatively lower temperature than other vanadium oxide, such as VO2(B). The obtained VO2(R) shows the reversible metal-to-insulator transition (MIT) near critical temperature (Tc) which is associated with clear changes in differential scanning calorimetry (DSC) curves. In addition, the temperature-dependent DC electrical conductivity of the new-phased VO2(D) exhibits Arrhenius-type behaviour, which reveals its semiconducting character with a band gap of 0.33 eV. ESR and temperature-dependent magnetic susceptibility measurements were employed to obtain information on the magnetic properties of VO2(D), which correspond to one-dimentional Heisenberg system.

Highly depressed temperature-induced metal-insulator transition in synthetic monodisperse 10-nm V2O3 pseudocubes enclosed by {012} facets

Monodisperse 10-nm V(2)O(3) pseudocubes enclosed by {012} facets were successfully synthesized for the first time via a novel and facile solvothermal method, offering the first opportunity to elucidate the effect of finite-size and facet on the temperature-induced reversible metal-insulator transition (MIT) behavior of V(2)O(3). Very excitingly, the transition temperature of these V(2)O(3) pseudocubes drastically depressed from 133 K to 36 K and their corresponding hysteresis width highly narrowed from 17 K to 5 K, compared to the MIT behaviors of other irregular V(2)O(3) particles with average sizes of 10 nm, 20 nm, 40 nm, 170 nm and 2 μm. Notably, the size-related surface energy, grain boundary connectivity and volume expansion could be used to account for their strong size-dependent transition temperature and hysteresis width. Moreover, the improved grain boundary connectivity associated with well-defined {012} facets enabled these 10-nm V(2)O(3) pseudocubes to display a 10 times higher resistivity jump (at the order of 10(5)) and by nearly one-half smaller hysteresis width of 5 K than the irregular 10-nm V(2)O(3) particles with randomly exposed facets, directly evidencing the pronounced influence of facets on the MIT behavior. Briefly, the present work not only develops an effective strategy for synthesizing high-quality nanocrystals but also provides an excellent platform to investigate the size- and facet-dependent temperature-induced MIT behavior, enabling to design smart electrical switching nano-devices in the rapidly developing ultra-low temperature field.

Nature of the negative thermal expansion in antiperovskite compound Mn3ZnN

The magnetic structures of Mn3ZnN compound are theoretically studied, from which a new magnetic ground state (MGS) structure of Mn3ZnN is predicted. Comparison of the calculated volumes between different magnetic structures shows that the Mn3ZnN compound experiences a volume expansion from the high-temperature paramagnetic phase to the low-temperature antiferromagnetic phase Γ5g, and a volume contraction from the Γ5g phase to the MGS phase, in excellent agreement with the observation in experiment. Analysis of the exchange parameters between ions shows that the spin coupling between the Mn ions is responsible for the sudden expansion and contraction of the Mn3ZnN volume. Furthermore, we find that the existed N vacancies in the compound significantly lower the energy of Γ5g. When the concentration of N vacancies is large enough, Γ5g may become the ground state for the defective Mn3ZnN compound. This may be used to explain the experimental observation that the sudden change in volume of Mn3ZnN at about 127 ...

The dynamical process of the phase transition from VO2(M) to VO2(R)

The dynamical process of the metal-insulator transition, from VO2(M) to VO2(R), is studied in the framework of the dynamics theory. It is found that the thermal exciting of the Raman-active Ag mode with frequency of 212.7 cm-1 in the VO2(M) lattice drives the compound to be the VO2(R) lattice. The intermediate structures during the phase transition are revealed, from which we find that when the distortion of the atomic network away from its initial network in the M phase exceeds 60%, the system becomes metallic. At the moment, the monoclinic symmetry of the crystal remains still, but more V ions are dimerized. This strongly suggests that the dimerization of the V ions in the compound plays a critical role in the transition from the M phase to the R phase.

Persistent Luminescence Hole-Type Materials by Design: Transition-Metal-Doped Carbon Allotrope and Carbides

Electron traps play a crucial role in a wide variety of compounds of persistent luminescence (PL) materials. However, little attention has been placed on the hole-trap-type PL materials. In this study, a novel hole-dominated persistent luminescence (PL) mechanism is predicted. The mechanism is validated in the night pearl diamond (NPD) composed of lonsdaleite with ultralong persistent luminescence (PL) (more than 72 h). The computed band structures suggest that the Fe ion dopant in lonsdaleite is responsible for the luminescence of NPD due to the desired defect levels within the band gap for electronic transition. Other possible impurity defects in lonsdaleite, such as K, Ca, Mg, Zn, or Tl dopants, or C vacancy can also serve as the hole-trap centers to enhance the PL. Among other 3d transition-metal-ion dopants considered, Cr and Mn ions are predicted to give rise to PL property. The predicted PL mechanism via transition-metal doping of lonsdaleite offers an exciting opportunity for engineering new PL materials by design.

Origin of the Giant Negative Thermal Expansion in

The giant negative thermal expansion in the Ge-doped antiperovskite Mn3CuN compound is theoretically studied by using the first principles calculations. We propose that such a negative thermal expansion property is essentially attributed to the magnetic phase transition, rather than to the lattice vibration of the Ge-doped compound. Furthermore, we found that the doped Ge atoms in the compound significantly enhance the antiferromagnetic couplings between the nearest neighboring Mn ions, which effectively stabilizes the magnetic ground states. In addition, the nature of the temperature-dependent changes in the volume of the Ge-doped compound was revealed.

Au6S2 monolayer sheets: metallic and semiconducting polymorphs

Gold–sulfur interfaces, including self-assembled monolayers of thiol molecules on gold surfaces, thiolate-protected gold nanoclusters, and gold sulfide complexes, have attracted intensive interest due to their promising applications in electrochemistry, bioengineering, and nanocatalysis. Herein, we predict two hitherto unreported two-dimensional (2D) Au6S2 monolayer polymorphs, named as G-Au6S2 and T-Au6S2. The global-minimum G-Au6S2 monolayer can be viewed as a series of [–S–Au–]n and [–Au4–]n chains packed together in parallel. The metastable T-Au6S2 monolayer resembles the widely studied T-MoS2 monolayer structure with each Mo atom substituted with an octahedral Au6 cluster, while the S atom is bonded with three Au atoms in a μ3 bridging mode. The G-Au6S2 monolayer is predicted to be metallic. The T-Au6S2 monolayer is predicted to be a semiconductor with a direct bandgap of 1.48 eV and high carrier mobility of 2721 cm2 V−1 s−1, ∼10 times higher than that of semiconducting H-MoS2. Moreover, the T-Au6S2 monolayer can absorb sunlight efficiently over almost the entire solar spectrum. These properties render the G- and T-Au6S2 monolayers promising materials for advanced applications in microelectronics and optoelectronics.

Role of vacancies to p-type semiconducting properties of SiGe nanowires

Many experiments have shown that both composition-randomly-distributed Si1−xGex nanowires (NW) and the Ge/Si core/shell NW possess excellent p-type semiconducting properties without relying on any doping strategy. Vacancies in both NW are believed to play a key role in the p-type semiconducting properties. To gain deeper insights into the role of vacancies, we performed first-principle calculations to systematically study the effects of single Si or Ge vacancies in four distinct SiGe NW, namely, randomly-distributed triangular-prism (RTP) NW, fused triangular-prism (FTP) NW, the GecoreSishell NW and SicoreGeshell NW. We find that the tendency for vacancy formation depends strongly on the structures of the NW. The defective RTP, FTP and GecoreSishell NW show promising p-type semiconducting properties while the defective SicoreGeshell NW does not. The Si vacancies in the inner region are attributed to the p-type properties of the RTP NW, and both the Si and Ge vacancies at the core/shell interfaces are attributed to the p-type properties of the FTP and the GecoreSishell NW. Our results show how the vacancies affect the electronic structures and the semiconducting properties of different SiGe NW, and offer an explanation of why the synthesized Si1−xGex and GecoreSishell NW possess excellent p-type semiconducting properties without relying on any doping strategy.

The elastic properties of Mn3(Cu1−xGex)N compounds

We present an ab initio study of the elastic properties of the negative thermal expansion (NTE) compound Mn3(Cu1−x Ge x )N. The calculated energies show that the Ge atoms can be easily doped into the compound and, the distribution of the Ge atoms in the compound is very uniform. The elastic moduli of the compound in the form of polycrystalline are evaluated according to the Voigt-Reuss-Hill approximation, which show that the dopedGe enhances the ductile character of the compound, with fairly high elasticanisotropy. Furthermore, it is found that the bulk modulus and the Youngs modulus of the compound increase as the Ge content increases from 12.5 % to 50 %, being in agreement with experiments. Through analyzing the electronic structures, we propose that these elastic features are essentially stemmed from the valence states and the valence electrons of the dopedGe.

New phases of 3d-transition metal–cerium binary compounds: an extensive structural search

We perform a comprehensive study to explore the low-energy crystalline phases of 3d transitional metal–cerium (TM–Ce) binary compounds using an unbiased structural search method coupled with first-principles optimization. For Ce–Sc, Ce–Ti, Ce–V, Ce–Cr and Ce–Mn binary systems, no stable crystalline phases are found from the structural search, offering an explanation for why none of these binary compounds have been observed in experiments. For Ce–Fe, Ce–Co, Ce–Ni, Ce–Cu and Ce–Zn binary systems, in addition to the previously known experimental structures, we also find several new low-energy crystalline phases. The computed electronic structures show that Ce atoms are in different states in the predicted binary compounds. In the Ce–Fe, Ce–Co and Ce–Ni compounds, the Ce 4f electrons are partially itinerant so that Ce atoms tend to adopt intermediate valence states between Ce+4 and Ce+3 due to the hybridization among Ce-4f, Ce-5d states and 3d states of TM. In the Ce–Cu and Ce–Zn binary compounds, the Ce-4f states are more localized with the charge state of Ce being close to 3+. In particular, the ferromagnetic metal (FM)-rich phases of the Ce–Fe, Ce–Co and Ce–Ni compounds tend to exhibit FM ordering in their ground states, owing to the strong exchange interaction among metal elements, whereas the non-magnetic states are usually preferred for FM-deficient phases. Magnetic orderings are also found in some other TM-rich phases of Ce–Cu and Ce–Zn compounds, where the magnetic moments are located on the Ce atoms due to the Kondo effect. Mechanic properties of these compounds are also computed based on density functional theory methods. This systematic study offers significantly new data for Ce-based alloys and will be useful to understand the intriguing behavior of the Ce-4f electron, thereby calling for future experimental confirmation of the newly predicted phases of Ce–TM compounds.