Chun-Rong Lin

Magnetic properties of monodisperse iron oxide nanoparticles

We have synthesized a set of monodisperse iron oxide nanoparticles ranging from 7.8to17.9nm by thermal decomposition methods. Based on the evidence of high-resolution transmission electron microscopy, the iron oxide nanoparticles appear as spherical dots with size standard deviations of less than 5%. Blocking temperatures of the set of nanoparticles were measured by the zero-field-cooled magnetization measurements. The anisotropy energy constants are estimated from the measured blocking temperatures. The contribution from the surface anisotropy is the dominant factor of the higher anisotropy energy found. The saturation magnetization and coercive force HC (77K) are functions of the particle size and increase with the particle size.

The vibrational dephasing and relaxation of CH and CD stretches on diamond surfaces: An anomaly

The temperature dependence of infrared absorption spectra of CH and CD on diamond nanocrystal surfaces has been investigated. Phase relaxation was closely examined by analyzing frequency shifts and line broadening in the spectra. Based on the model of Persson and Ryberg [Phys. Rev. B 40, 10 273 (1989)], coupling phonons responsible for the pure dephasing process were found to resonate at ω0≊1200 cm−1 for the CH stretch. By including both the phase and energy relaxation in the linewidth analysis and assuming that energy relaxes via three‐phonon emission, we estimate a pure dephasing time of T*2≊340 ps at room temperature. This value is one order of magnitude larger than the energy relaxation time, T1≊19 ps, measured by Chin et al. [Europhys. Lett. 30, 399 (1995)] on a C(111) single crystal surface. We interpret the anomalous observation to be the result of the high frequency of the coupling phonons. For the CD stretches, however, severe line broadening due to exceedingly rapid energy relaxation disallows a...

Synthesis and characterization of magnetic nanoparticles embedded in polyvinyl pyrrolidone nanofiber film by electrospinning method

We fabricated magnetic nanofiber films by manufacturing the nanoparticles with the polyol process followed by the electrospinning process to combine them into a thin film. The magnetite (Fe3O4) nanoparticles with mean crystallite size of 6–8 nm were synthesized through reduction of iron (II) acetate in the polyols and using polyvinyl pyrrolidone (PVP) as the protecting agent. The PVP-coated Fe3O4 nanoparticles were dispersed into PVP ethanol solution and then electrospun directly to make nanofiber films. The diameters of fibers range between 200–400 nm, and the film thickness is about 50 μm. The Fe3O4 nanoparticles show a saturation magnetization of 36.6 emu/g, and together with the nanofiber films display a superparamagnetic behavior.

Vibrational dephasing dynamics at hydrogenated and deuterated semiconductor surfaces: Symmetry analysis

Raman scattering has been performed on single crystal hydrogenated silicon and germanium surfaces to investigate the temperature dependence of their vibrational spectral profiles. Based on a single-mode dephasing model for pure vibrational dephasing, the frequency shift and line broadening were analyzed to extract the following dephasing parameters: exchange mode frequency, coupling strength, and friction parameter. The exchange modes for the XH stretches on hydrogenated X(100) surfaces (X=Ge, Si, and C) are found to match their respective bending frequencies. The corresponding ones for hydrogenated X(111) surfaces, on the other hand, are located within the bulk phonon. This surface dependence of the exchange mode in surface vibrational dephasing dynamics is correlated with the structural relaxation and its associated symmetry variation at surfaces. It is further confirmed by the experiments performed on deuterated semiconductor surfaces. A site-symmetry induced representation method is exploited to analy...

Interaction of atomic hydrogen with a Ge(111) surface: low-energy electron diffraction and surface Raman studies

Abstract We report the preparation and characterization of a sufficiently ordered Ge(111)-1×1:H surface by prolonged hydrogenation of Ge(111)-c(2×8) at elevated temperatures. For both annealed and sputtered/annealed c(2×8) surfaces, a (1×1) pattern with distinct primary-order spots was observed by low-energy electron diffraction (LEED) after extensive hydrogenation treatment. We demonstrated that a surface Raman spectroscopic method based on polarization effects can be used successfully to characterize such a prepared Ge(111)-1×1:H surface, which is flat enough to yield a single prominent peak of the monohydride GeH stretch. The possible mechanism for surface smoothing by atomic hydrogen is also discussed. The smoothness of this surface makes various spectroscopic characterization methods feasible.

Synthesis and magnetic behavior of silica-coated cobalt ferrite hollow spheres

We have used functional polymer poly(methyl methacrylate-co-methacrylic acid) latex (500nm) as a core template to prepare magnetic hollow SiO2∕CoFe2O4 spheres. The mean crystallite sizes of coated CoFe2O4 nanoparticles, depending on the calcined temperature, are in the range from 2.2to10.1nm. The thickness of the CoFe2O4∕SiO2 composite shell is between 40 and 50nm and the content of CoFe2O4 in the composites is 73wt%. The magnetic properties of the hollow spherical particles can go from superparamagnetic to ferromagnetic behavior depending on the sizes of CoFe2O4 nanoparticles.

Magnetic and Mössbauer spectroscopy studies of hollow microcapsules made of silica-coated CoFe 2 O 4 nanoparticles

The hollow microcapsules made of silica-coated CoFe2O4 nanoparticles were synthesized using chemical co-precipitation, followed by the sol–gel method. Poly(MMA-co-MAA) microspheres were used as a core template which can be completely removed after annealing at 450 °C. The microcapsules are monodisperse with the outer diameter of about 450 nm and the thickness of the shell is about 50 nm. The nanoparticles of Co-ferrite are single crystalline. The size of the nanoparticles and magnetic properties of CoFe2O4/SiO2 hollow spheres can be tuned with high accuracy at the annealing stage. The Mossbauer data indicate that CoFe2O4 ferrite is an inverse spinel, in which Fe3+ and Co2+ ions are distributed in both octahedral and tetrahedral sites with the inversion degree close to the bulk ferrite value. At low temperature the CoFe2O4/SiO2 nanoparticles are in antiferromagnetic (AFM) state due to the canted or triangular magnetic structure. Under heating in the applied field, AFM structure transforms to the ferrimagnetic (FM) structure, that increases the magnetization. The Mossbauer data revealed that the small size CoFe2O4/SiO2 particles do not show superparamagnetic behavior, but they transit to the paramagnetic state by the jump-like first order magnetic transition (JMT). This effect is a specific property of the magnetic nanoparticles isolated by inert material. The suggested method of synthesis can be modified with various bio-ligands on the silane surface, and such materials can find many applications in diagnostics and bio-separation.

Synthesis and magnetic properties of iron sulfide nanosheets with a NiAs-like structure

We have synthesized the hexagonal pyrrhotite Fe1−xS nanodisks with the NiAs-type structure via thermal decomposition of ferrous chloride and thiourea in oleylamine. Mossbauer spectra analysis shows that the main part of iron of hexagonal pyrrhotite is in the Fe2+ valence state, and the spectrum has broadened half-maximum width due to the vacancy distribution. Magnetic measurements indicate that the magnetic moment configuration of nanodisks transforms from the ferrimagnetic to the ferromagnetic state at 400 K and magnetic moment disordering takes place at the Curie temperature, 580 K, at which ferromagnetic state transforms to paramagnetic structure. This magnetic transition behavior can be explained in terms of the iron vacancy rearrangement.

Preparation and magnetic properties of monodisperse Permalloy hollow spheres

The ionic poly(methyl methacrylate-co-methacrylic acid) copolymer sphere (500nm) was used as a core template to prepare monodisperse Ni80Fe20∕SiO2 hollow spheres. The hollow Ni80Fe20∕SiO2 spherical structures were synthesized by coprecipitating metallic oxides of nickel and iron on the surface of the ionic polymer spheres and reducing in H2∕Ar mixed gas following heat treatment in air. The Ni80Fe20 nanoparticles involved in hollow spheres have face-centered-cubic structure and mean crystallite sizes ranging from 5.5to22.1nm. In addition, these hollow spheres have uniform size with shell thickness of 80nm and contain 71wt% of Ni80Fe20 alloy. Magnetic measurements show that the hollow spheres containing Ni80Fe20 nanoparticles with crystallite size d⩾10.9nm reveal ferromagnetic property, while those with d⩽6.7nm display a superparamagnetic behavior.The ionic poly(methyl methacrylate-co-methacrylic acid) copolymer sphere (500nm) was used as a core template to prepare monodisperse Ni80Fe20∕SiO2 hollow spheres. The hollow Ni80Fe20∕SiO2 spherical structures were synthesized by coprecipitating metallic oxides of nickel and iron on the surface of the ionic polymer spheres and reducing in H2∕Ar mixed gas following heat treatment in air. The Ni80Fe20 nanoparticles involved in hollow spheres have face-centered-cubic structure and mean crystallite sizes ranging from 5.5to22.1nm. In addition, these hollow spheres have uniform size with shell thickness of 80nm and contain 71wt% of Ni80Fe20 alloy. Magnetic measurements show that the hollow spheres containing Ni80Fe20 nanoparticles with crystallite size d⩾10.9nm reveal ferromagnetic property, while those with d⩽6.7nm display a superparamagnetic behavior.

Iron Sulfide Nanoparticles: Preparation, Structure, Magnetic Properties

The series of iron sulfide nanoparticles (NPs) were synthesized with the polyol mediated process which exploits high-boiling polyalcohol solvents at different boiling temperatures (TB) what determined the NPs phase state from Fe3S4 to FeS. The XRD and HRTEM revealed the content of the Fe3S4 cubic phase to reduce linearly with the TB increase, and at TB=320 ◦C the FeS phase became predominant. Non monotonous coercivity dependence on the NPs phase state is revealed and interpreted.