Jian-Min Li

White-light emission and weak antiferromagnetism from cubic rare-earth oxide Eu2O3 electrospun nanostructures

The authors report the synthesis of cubic europium oxide (C-Eu2O3) nanobelts (NBs) via an electrospinning method with a subsequent one-step calcination approach. Compared with the traditional red emission (612 nm) from the commercial bulk powders, the extended room temperature photoluminescence spectrum (with peaks ranging from ultraviolet (UV) to near infrared (NIR) at 727 nm) of the Eu2O3 NBs was first found to be responsible for the whole visible emission (white light). More excited electronic states were triggered in the Eu2O3 NBs. In addition, the synthesized Eu2O3 NBs are weakly antiferromagnetic with a Neel temperature ≈46 K, which can be due to the oxygen vacancy-related mechanism.

Exciton quenching and ferromagnetism-to-ferrimagnetism crossover in diluted magnetic semiconducting Zn1−xCoxO nanogranular nanofibers

The synthesis of Co-doped zinc oxide (Zn1−xCoxO) (x = 0, 1.8, 4.4, and 7.2 at.%) nanogranular nanofibers with a diameter of 70–250 nm and length up to several hundred micrometers by electrospinning and subsequent calcinations is reported. Novel nanoscale Co2+ doping-induced phenomena were found in as-calcined electrospun Zn1−xCoxO nanofibers including (i) thermal decomposition temperature of polyvinyl alcohol (PVA) in PVA/Zn1−xCoxO electrospun composite fibers which is quite different from that of pure PVA electrospun fibers and shows a linear dependence of dopant level, (ii) exciton quenching, (iii) dimensionality-related magnetization and (iv) a room-temperature weak ferromagnetism-to-ferrimagnetism crossover. Finally, an F-centered mechanism is proposed to explain the correlating exciton quenching and the occurrence of ferrimagnetism.

An “edge to edge” jigsaw-puzzle two-dimensional vapor-phase transport growth of high-quality large-area wurtzite-type ZnO (0001) nanohexagons

The authors reported the synthesis of large but thin transparent regularly shaped single-crystalline zinc oxide (ZnO) (0001) hexagonal nanodisks (or nanohexagons) (the most probable area scale of 95 μm2 and 40–70 nm in thickness) by an indium assisted vapor-phase transport (VPT) growth route at ambient pressure. These large-area ZnO nanohexagons exhibit excellent luminescent properties with a strong ultraviolet emission band centered at about 384 nm and a very weak green band shoulder emission, leading to the further creation of a promising low-defect-concentration nanohexagons-based nanooptoelectronics. Finally, a two-dimensional VPT growth mechanism was proposed to understand the formation of high-quality wurtzite-type ZnO nanohexagons.

Synthesis of ultrafine green-emitting BaCO3 nanowires with 18.5 nm-diameter by CO2 vapor-assisted electrospinning

The authors report the first preparation of high-density ultrafine green-emitting wool-like BaCO3 nanowires, having an average diameter of 18.5 nm, and an average crystallite size of 13.4 nm, by an electrospinning method with a subsequent two-step calcination approach. A possible vapor–solid interaction mechanism was proposed for the finest nanowires growth, where the reactive carbon dioxide vapors were generated during the static atmosphere ambient calcination and assisted oxidization nucleation at enhanced temperatures. Our work provides new insights into electrospun fiber diameter refinement via controlled calcination, and this simple post-electrospinning controlled strategy can be generally applied to other metal carbonate ultrafine NWs families.

A Cu(2+) ion-F center complex view on the photoluminescence quenching and correlating ferrimagnetism in (Cu2(+)/Cu1(2+))(0.044)Zn(0.956)O electrospun nanobelts.

Unlike to the most previous reports, mixed-cation Cu(+)/Cu(2+) doping-induced novel nanoscale phenomena, including photoluminescence quenching and a correlating ferrimagnetism with Néel temperature ≈ 14 K, were found in the as-calcined (Cu2(+)/Cu1(2+))0.044Zn0.956O electrospun nanobelts (NBs). There is also high strain (up to 1.98%) and shrunk lattice distortion (ΔV/V0 ∼ 0.127%) in the (Cu2(+)/Cu1(2+))0.044Zn0.956O NBs, leading to broken lattice symmetry in conjunction with nonstoichiometry (i.e., oxygen vacancies or accurate F centers), which could be possible origins of ferrimagnetism in the Cu-doped ZnO NBs. Electron paramagnetic resonance spectra reveal that there are giant and anisotropic g factors, suggesting that there is strong anisotropic spin-orbit interaction between the Cu(2+) ion and F center (i.e., forming Cu(2+)-F(+) complexes) in the (Cu2(+)/Cu1(2+))0.044Zn0.956O NBs. The above correlation enables the potential application of tuning of the optical and ferrimagnetic properties through strain and F-center engineering.

Non-layered wurtzite-type extralarge-area flexible ZnO (010) paper-like nanostructures grown by electrostatically induced vapor-phase transport

We report the synthesis of transparent, curling, extra large-area non-layered wurtzite-type low-defect-concentration ZnO (010) nanopapers with a maximum area of over 2000 μm2 and 30–60 nm in thickness via an indium-assisted vapor-phase transport (VPT) method. The ZnO nanopapers exhibit an intense ultraviolet emission band centered at about 382 nm. On reducing the substrate temperature, the morphological evolution of VPT-grown ZnO nanostructures from nanopapers to nanobelts and then to nanowires was sequentially observed, suggesting that the “one-dimensional branching and subsequent two-dimensional interspace filling” process was not necessary. Finally, based on the ZnO and Zn supersaturation state, a possible electrostatic interaction induced nanobricks alternatively interconnecting self-assembled growth mechanism of high-quality single-crystalline ZnO nanopapers is discussed.

Partial cationic inversion-induced magnetic hardening of densely packed 23-nm-sized nanocrystallite-interacting nickel ferrite electrospun nanowires

Although the average crystallite size (23 nm) is very close to a critical radius of superparamagnetism, a magnetic hardening (coercive field and saturation magnetization) was observed in interacting single-domain nanogranular nickel ferrite electrospun nanowires as compared with the bulk. The phenomena can be attributed to a small change in the cation occupancy on the spinel sites from (Fe3+)A(Ni2+Fe3+)BO4 to [F1−δ3+Niδ2+] A[Ni1−δ2+Fe1+δ3+] BO4 (δ = 3.67%), and an additive interaction of the “chain of spheres” type. The larger shape anisotropy contribution (Keff = 8.24 × 104 erg/cm3) is also as a factor leading to enhanced coercive field.

Morphological diversity and alternate evolution in tin-assisted vapor-transport-grown ZnO micro-nanocrystal tetrapods

The authors report the systematic shape-controlled synthesis of wurtzite-type tetrapod-like ZnO (T-ZnO) micro-nanocrystals (μNCs) using a tin-assisted vapor-phase transport (VPT) method. A morphological rebound in the VPT-grown T-ZnO μNCs was first observed at the narrow substrate temperature (Ts) window, 710 °C < Ts < 730 °C, which is found to be sensitive to a hidden local temperature gradient (∇T) distribution of the furnace. Except the well-known experimental factors (e.g., Ts and oxygen partial pressure), our studies demonstrate that the most-neglected environmental temperature (TE) should be considered in understanding of the morphology uncertainty in the VPT-grown nanostructures, thus potentially offering an insight into the VPT-grown T-ZnO μNCs.

Fabrication of cuprate superconducting La1.85Sr0.15CuO4 nanofibers by electrospinning and subsequent calcination in oxygen

The synthesis of high-temperature cuprate superconducting La1.85Sr0.15CuO4 (LSCO) flute-like nanofibers (FNFs) viaelectrospinning and subsequent calcination in oxygen is reported. A reduced superconducting critical temperature was found to be around 19.2 K, a considerable decrease in comparison to its bulk counterpart (∼ 37 K). The weakened superconductivity in the LSCO FNFs is relevant to the following observations that (i) the presence of disorder or the amount of structural defects in the FNFs, (ii) the ab-plane dilatation (∼ 0.41%) and c-axis-compression (∼ 0.16%) distortions, leads to the reduced in-plane hybridization, and (iii) the suppression of A1g Raman-active phonon mode ∼ 430 cm−1 and enhancement of B1g mode ∼ 370 cm−1, is associated with the weakened electron–phonon coupling interaction. Our work demonstrates a “clean” high-yield nanofabrication of high-temperature cuprate superconducting materials.

4 nm ZnO nanocrystals fabrication through electron beam irradiation on the surface of a ZnO nanoneedle formed by thermal annealing

A vertically-oriented growth of the monolithic ZnO nanocrystals (NCs)-on-nanoneedle (NN) single-crystalline (SC) hybrids is presented with intrinsic nanoscale complexity, consisting of spatially homogeneously distributed ultrahigh-density (2.0 × 104 μm−2) on-site homoepitaxial growth of 4 nm ZnO NCs on the surface of vertically oriented high-density (8 × 108 cm−2) SC [100] ZnO NNs. The electron-beam stimulated “self-additive on-surface oxidation-limited growth” at room temperature is proposed for the on-site growth of numerous ZnO NCs on the (0001) facets of a single ZnO NN. Compared with heterojunctions, the unprecedented compact monolithic ZnO NCs-on-NN SC hybrids, growing from the inside out, may hold great promise for enhanced photovoltaic performance (e.g., electron mobility) of ZnO NC-sensitized solar cells.