Jieqiong Wang

Growth of highly oriented carbon nanotubes by plasma-enhanced hot filament chemical vapor deposition

Highly-oriented, multi-walled carbon nanotubes were grown on polished polycrystalline and single crystal nickel substrates by plasma enhanced hot filament chemical vapor deposition at temperatures below 666C. The carbon nanotubes range from 10 to 500 nm in diameter and 0.1 to 50 pm in length depending on growth conditions. Acetylene is used as the carbon source for the growth of the carbon nanotubes and ammonia is used for dilution gas and catalysis. The plasma intensity, acetylene to ammonia gas ratio and their flow rates, etc. affect the diameters and uniformity of the carbon nanotubes. In summary, we synthesized large-area highly-oriented carbon nanotubes at temperatures below 666C by plasma-enhanced hot filament chemical vapor deposition. Acetylene gas is used to provide carbon for nanotube growth and ammonia gas is used for dilution and catalysis. Plasma intensity is critical in determining the nanotube aspect ratios (diameter and length), and range of both site and height distributions within a given film.

Enhanced valley splitting in monolayer WSe2 due to magnetic exchange field

Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition-metal dichalcogenide (TMDC) with a broken inversion symmetry possesses two degenerate yet inequivalent valleys, which offers unique opportunities for valley control through the helicity of light. Lifting the valley degeneracy by Zeeman splitting has been demonstrated recently, which may enable valley control by a magnetic field. However, the realized valley splitting is modest (∼0.2u2005meV T-1). Here we show greatly enhanced valley spitting in monolayer WSe2, utilizing the interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A valley splitting of 2.5u2005meV is demonstrated at 1u2005T by magnetoreflectance measurements and corresponds to an effective exchange field of ∼12u2005T. Moreover, the splitting follows the magnetization of EuS, a hallmark of the MEF. Utilizing the MEF of a magnetic insulator can induce magnetic order and valley and spin polarization in TMDCs, which may enable valleytronic and quantum-computing applications.

Superconducting epitaxial (Tl,Bi)Sr1.6Ba0.4Ca2Cu3O9−δ film with high critical current in magnetic field

High‐quality 1‐μm‐thick (Tl,Bi)Sr1.6Ba0.4Ca2Cu3O9−δ films were synthesized by a simple two‐step procedure. A prefilm of TlxBiySr1.6Ba0.4Ca2Cu3Oy was first deposited on the (100) surface of LaAlO3 by laser ablation at 300–500u2009°C. This prefilm was placed on a gold plate sitting between two similarly placed unfired Tl0.95Bi0.22Sr1.6Ba0.4Ca2Cu3O8.76 pellets, wrapped in silver foil, and subsequently heated in air at 840–870u2009°C for 25–60 min. The resulting glossy film exhibited at 77 K a transport Jc of 2×106 A/cm2 at 0 T and 1.5×106 A/cm2 at 0.5 T with magnetic field parallel to the film. These observed values show the absence of weak links effect. X‐ray diffraction (XRD) reflections reveal the existence of only 1223 phase with nearly perfect c‐axis alignment. The half‐width of rocking angle monitored at (006) reflection is only 0.365°. XRD φ scan also shows no trace of intergranular misalignment along the a and b axes.

Synthesis and characterization of thallium-based 1212 films with high critical current density on LaAlO3 substrates

Growth of epitaxial Cr-doped (Tl,Bi)-1212 films on LaAlO3 substrates by pulsed laser deposition (PLD) and post-deposition annealing in static air has been achieved. The films exhibit a high transport Jc of over 1.5 × 106 A cm-2 at 77 K and self-field. Tc values measured by the four-probe method are in the range 94-100 K. ICP emission spectroscopy measurement shows a thallium deficiency, with about 0.15 atom bismuth per formula in the film. XRD -2 , and scans show that the dominant phase is 1212, with excellent epitaxy. The film surface has typical PLD morphology as analysed by SEM. TEM analysis found some inclusions in the film, and HREM found that three 1201 layers exist between the interface of 1212 phase and substrate.

Robust ferromagnetism in Mn-doped MoS2 nanostructures

Layered transition metal dichalcogenides (TMDs) have attracted extensive attention due to their interesting properties originating from an effective honeycomb lattice and strong spin-orbit coupling, and have potential applications in catalysis, lithium batteries, photonic, electronic, and valleytronic devices. Introducing magnetism in the TMDs can lead to the interesting functionalities such as magnetic order and carrier spin polarization with potential applications in spintronics. Here, we demonstrate an effective approach to induce robust ferromagnetism in MoS2 nanostructures by transition metal doping. After doping with a few percent Mn2+, the magnetism of MoS2 nanostructures is enhanced dramatically. Moreover, the magnetic properties are strongly temperature dependent, which is clearly different from the behavior of defect-induced magnetism. Our approach opens up the possibility for tuning the spin and magnetic properties in two-dimensional nanostructures.

The structural symmetry of epitaxial Tl2Ba2CuO6+δ thin films

Epitaxial films of Tl2Ba2CuO6+δ (Tl‐2201) have been prepared on SrTiO3 by sputtering and postdeposition annealing. The films were deposited onto (100)SrTiO3 single crystal substrates by rf magnetron sputtering and then annealed using a two‐step process. The annealed films typically exhibit transition temperatures of about 11 K. Subsequent reannealing to optimize the oxygen doping in the film results in an increase in the transition temperature, and optimally reannealed films exhibit transition temperatures of about 83 K. The crystal structure of these films has been studied by x‐ray diffraction and transmission electron microscopy. These results indicate that both the annealed (Tc=11 K) and optimally reannealed samples (Tc=83 K) have a tetragonal crystal structure. This structural information is crucial in evaluating recent measurements on these films to determine the electron pairing symmetry (d wave or s wave) in high‐Tc superconductors.

Morphotropic phase boundary and magnetoelastic behaviour in ferromagnetic Tb1−xGdxFe2 system

Morphotropic phase boundary (MPB), separating two ferroic phases of different crystal symmetries, has been studied extensively for its extraordinary enhancement of piezoelectricity in ferroelectrics. Based on the same mechanism, we have designed a magnetic MPB in the pseudobinary ferromagnetic system of Tb1−xGdxFe2 and the corresponding crystal structure, magnetic properties, and magnetostriction are explored. With the synchrotron x-ray diffractometry, the structure symmetry of TbFe2-rich compositions is detected to be rhombohedral (R) and that of GdFe2-rich compositions is tetragonal (T) below Tc. With the change of concentration, the value of magnetostriction of the samples changes monotonously, while the MPB composition Tb0.1Gd0.9Fe2, which corresponds to the coexistence of R and T phases, exhibits the maximum magnetization among all available compositions and superposition of magnetostriction behaviour of R and T phases. Our result of MPB phenomena in ferromagnets may provide an effective route to des...

Epitaxial superconducting thin films Tl0.78Bi0.22Sr1.6Ba0.4Ca2O9−δ on (001) YSZ synthesized by laser ablation and post-annealing in pure argon

Abstract Epitaxial superconducting thin films of Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ ((Tl,Bi)-1223) on (001) YSZ single crystal substrate have been successfully synthesized by laser ablation and post-annealing in a tube furnace with flowing pure argon through the tube all the time during annealing. X-ray diffraction (XRD) θ–2θ spectra showed that the films are c -axis aligned 1223 phase with a small amount of intergrown 1212 phase, while XRD φ-scans revealed that the films are also a - and b -axes aligned, with an epitaxy of the &.lang;100&.rang; of (Ti,Bi)-1223 film on the &.lang;110&.rang; of the YSZ substrate. Four-terminal electrical transport measurements showed that the zero-resistance transition temperature ( T c ) was 108.5 K, and the critical current density ( J c ) at 77 K and zero field was 1.1 × 10 6 A/cm 2 , both for the entire film width and for a patterned bridge. The magnetic field dependent transport J c suggest that (Tl,Bi)-1223 films on (001) YSZ have better flux-pinning properties than high- J c (Tl,Bi)-1223 films on (001) LaAlO 3 .

Superconductivity and microstructure of epitaxial (Tl,Bi)Sr1.6Ba0.4Ca2Cu3O9−δ thin films

Epitaxial (Tl,Bi)Sr{sub 1.6}Ba{sub 0.4}Ca{sub 2}Cu{sub 3}O{sub 9-{delta}} ((Tl,Bi)-1223) thin films on (100) single crystal LaAlO{sub 3} substrates were successfully synthesized by a two-step procedure. The (Tl,Bi)-1223 phase development in both air and argon was systematically investigated by X-ray diffraction. The microstructure and epitaxial relationship between films and substrates were measured by X-ray {phi} scan, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The DC resistance versus temperature transition, zero-field-cooled (ZFC) and field-cooled (FC) magnetization transition, hysteresis curve, and transport critical current density were measured. The zero-resistance transition temperature was 107 K, and the critical current density at 77 K and zero field was higher than 2 x 10{sup 6} A/cm2. (Tl,Bi)-1223 films have good flux-pinning properties as shown by the slow decrease of the critical current density in an external magnetic field.

Epitaxial superconducting Tl0.5Pb0.5Sr1.6Ba0.4Ca2Cu3O9 films on LaAlO3 by thermal spray and post-spray annealing

A simple, low-cost and easily scalable method - thermal spray followed by post-spray annealing - has been successfully demonstrated for fabricating high-Jc epitaxial superconducting films of Tl0.5Pb0.5Sr1.6Ba0.4Ca2Cu3O9 [(Tl, Pb)-1223]. The process was performed by spraying a stoichiometric solution containing Tl, Pb, Sr, Ba, Ca and Cu nitrates onto LaAlO3 substrates heated to 300-400 °C. Subsequently, the amorphous films were placed inside a silver containment along with two semicircular unfired pellets, and were reacted and crystallized by a thermal anneal at 870 °C in air. X-ray diffraction showed that the films consist mainly of c-axis aligned (Tl, Pb)-1223 phase with a small amount of (Tl, Pb)-1212 phase. X-ray -scans revealed very good epitaxial growth. Four-probe transport measurements on a 4 mm wide and 1.2 µm thick film showed a superconducting transition temperature of 106-109 K, with a zero-field critical current of 51 A at 77 K, which corresponds to a critical current density of 1.1 × 106 A cm-2.