Chinping Chen

Synthesis, optical, and magnetic properties of diluted magnetic semiconductor Zn1−xMnxO nanowires via vapor phase growth

Diluted magnetic semiconductor Zn1−xMnxO nanowires were synthesized via an in situ doping of manganese in ZnO nanowires using vapor phase growth at 500 °C. The maximum content of the manganese in the ZnO is around 13 at. %, approaching the maximum thermal equilibrium limit of Mn solubility in ZnO at the growth temperature. Structure and composition analysis revealed that the manganese was doped into the lattice structure, forming solid solution instead of precipitation. Magnetic property measurements revealed that the as-doped Zn1−xMnxO nanowires exhibit ferromagnetic behavior with Curie temperature around 37 K.

Low-temperature growth and Raman scattering study of vertically aligned ZnO nanowires on Si substrate

High-density ZnO nanowires (ZnONWs) were aligned onto Au-catalyzed Si substrate through a simple low-temperature physical vapor deposition method. Scanning electron microscope (SEM) observations, x-ray diffraction (XRD) analysis, and photoluminescence spectra showed that the ZnONWs were single-crystalline, with a hexagonal wurzite structure. All of the results inferred from the SEM observations, the XRD rocking curves, and the Raman spectra for the investigated samples confirm that the ZnONWs are well aligned and c-axis oriented. The Raman spectra also indicated that the ZnONWs on Si substrates are under the biaxial compressive stress. Since it takes the advantage of low-cost, easily controlled deposition spot (due to the selective deposition trait of the Au layer), potential for scale-up production, and ability to integrate with Si substrate, this technique has a potential in future for fabricating the ZnONW array-based optoelectronic devices.

Porous nanotubes of Co3O4: Synthesis, characterization, and magnetic properties

Stoichiometric Co3O4 porous nanotubes have been synthesized through a simple modified microemulsion method. The structural and the chemical information of the as-grown nanotubes have been investigated by means of x-ray diffraction, electron microscopy, electron energy loss spectroscopy, and dynamic force microscopy. The results reveal that the as-grown materials are formed by concentric stacking of Co3O4 (111) planes or weaved porous nanotubes with diameters ranging from tens to ∼200nm and sidewall thickness ranging from 2to∼20nm. Magnetic property of the sample demonstrates a magnetic transition temperature at 8.4K, indicating macroscopic quantum confinement effects from the sidewall thickness of the porous nanotube.

Ni/Ni3C core-shell nanochains and its magnetic properties: one-step synthesis at low temperature.

One-dimensional Ni/Ni3C core-shell nanoball chains with an average diameter by around 30 nm were synthesized by means of a mild chemical solution method using a soft template of trioctylphosphine oxide (TOPO). It was revealed that the uniform Ni nanochains were capped with Ni3C thin shells by about 1-4 nm in thickness and each Ni core consists of polygrains. The coercivity of the core-shell nanochains is much enhanced (600 Oe at 5 K) and comparable with single Ni nanowires due to the one-dimensional shape anisotropy. Deriving from the distinctive structure of Ni core and Ni 3C shell, this architecture may possess a possible bifunctionality. This unique architecture is also useful for the study on the magnetization reversal mechanism of one-dimensional magnetic nanostructure.

Finite size effect on Néel temperature with Co3O4 nanoparticles

The finite size effect on the antiferromagnetic transition temperature, TN, has been investigated with Co3O4 nanoparticles of 75, 35, and 16 nm in diameter. Along with the results from the previous experiments on the Co3O4 nanoparticles with average diameter of 8 and 4.3 nm, the variation of TN with the particle diameter, d, appears to follow the finite size scaling relation. The shift exponent is determined as λ=1.1±0.2 and the correlation length ξ0=2.8±0.3 nm. The value of the shift exponent is consistent with the description by the mean field theory.

Size-dependent magnetic properties of nickel nanochains

Magnetic properties with three different sizes of Ni nanochains, synthesized by a technique of wet chemical solution, have been investigated experimentally. The sample sizes (average diameter of the nano-particles) are 50, 75, and 150 nm, with a typical length of a few microns. The characterizations by XRD and TEM reveal that the samples consist of Ni nano-particles forming a one-dimensional (1D) chain-like structure. Magnetic properties have been investigated by temperature dependent magnetization M(T) and field dependent magnetization M(H) measurements. The results are explained within the context of the core–shell model. First, the freezing of disordered spins in the shell layer has resulted in a peak structure on the zero-field-cooled (ZFC) M(T) curve. The peak position is identified as the freezing temperature TF. It is well described by the de Almeida–Thouless (AT) equation for the surface spin glass state. Second, the shape anisotropy of the 1D structure has caused a wide separation between the field-cooled (FC) and ZFC M(T) curves. This is mainly attributed to the blocking of the core magnetism by an anisotropy barrier, EA. Third, by the M(H) measurement in the low field region, the open hysteresis loop measured at T = 5 K TF. This indicates that a significant part of the contribution to the magnetic irreversibility at T<TF is arising from the disordered spins in the shell layer. Last, with the reduced sample size, the coercivity, HC, increases whereas the saturation magnetization goes down substantially. These imply that, as the sample size reduces, the effect of shape anisotropy becomes larger in the magnetization reversal process and the contribution to the magnetism from the ferromagnetically ordered core becomes smaller.

Monodispersed, ultrathin NiPt hollow nanospheres with tunable diameter and composition via a green chemical synthesis

Monodispersed NiPt hollow nanospheres with citric acid as surfactant were synthesized by a one-pot wet chemical method in water at room temperature. The diameter is adjustable from 13 to 100 nm with the shell thickness of 2–3 nm, roughly the size of nanocrystallites. The composition is also tunable with the ratio of Ni : Pt from 1 : 9 to 9 : 1. A phase formation mechanism is proposed for the controllability of the diameter from the sacrificial templates of intermediate Ni–B amorphous nanocompounds. Molecular dynamic calculations show that 2 nm is the lower-bound shell thickness for a stable hollow spherical structure to avoid distortion or collapse. Electrocatalytic properties for the oxidation of ethylene glycol are measured for 35 nm Ni50Pt50 hollow nanospheres. Its performance is apparently better than that using commercial Pt/C catalyst. This makes it a potential application as a fuel cell catalyst.

Effect of temperature-dependent shape anisotropy on coercivity for aligned Stoner-Wohlfarth soft ferromagnets

The temperature variation effect of shape anisotropy on the coercivity, HC(T), for the aligned Stoner-Wohlfarth (SW) soft ferromagnets, such as fcc Ni, fcc Co and bcc Fe, are investigated within the framework of Neel-Brown (N-B) analysis. An extended N-B equation is thus proposed,by introducing a single dimensionless correction function, the reduced magnetization, m(\tao) = MS(T)/MS(0), in which \tao = T/TC is the reduced temperature, MS(T) is the saturation magnetization, and TC is the Curie temperature. The factor, m(\tao), accounts for the temperature-dependent effect of the shape anisotropy. The constants, H0 and E0, are for the switching field at zero temperature and the potential barrier at zero field, respectively. According to this newly derived equation, the blocking temperature above which the properties of superparamagnetism show up is described by the expression, TB = E0m^2(\tao)/[kBln(t/t0)], with the extra correction factor m^2(\tao). The possible effect on HC(T) and the blocking temperature, TB, attributed to the downshift of TC resulting from the finite size effect has been discussed also.

Controlled synthesis of high-quality nickel sulfide chain-like tubes and echinus-like nanostructures by a solution chemical route

Chain-like NiS tubes and echinus-like nickel sulfide (Ni3S2) nanostructures were synthesized successfully by a solution chemical route. The growth models of these two materials and the most possible mutation process between these two different morphologies are discussed. The temperature and field dependent magnetizations, M(T) and M(H), show that both the samples have a very weak ferromagnetism and exhibit a strong paramagnetic response in a high applied field.

Strain-assisted tunneling current through TbMnO3∕Nb-1 wt %-doped SrTiO3 p–n junctions

Simple oxide heterostructures have been fabricated by growing multiferroic TbMnO3 thin film on Nb-1 wt %-doped SrTiO3 substrate. In addition to the beneficial rectifying characteristics in a temperature range from 350 to 30 K, the intriguing observation is that at constant reverse bias the current increases with decreasing temperature below 275 K. By analyzing the band diagram, the anomalous increasing current with decreasing temperature was ascribed to the strain-assisted tunneling current through TbMnO3∕Nb-doped p–n junctions.