Novel carbon nanotube iron oxide magnetic nanocomposites

A novel magnetic nanocomposite of γ-Fe₂O₃ nanoparticles decorated multiwalls carbon nanotubes (MWNTs) was synthesized for the first time by a simple chemistry precipitation method. The structure and morphology of the composite was characterized by X-ray powder diffractometer (XRD), TEM and EDS. The results of XRD and TEM show that γ-Fe₂O₃ nanoparticles is immobilized on the side wall of the MWNTs, the size of most of the particle is <5 nm.The EDS analysis shows that the atomic ratio of Fe to O is 2:3. The magnetization curves of the MWNTs and γ-Fe₂O₃ decorated MWNTs were measured by VSM at room temperature, which indicate that the saturated magnetization (Ms), remanence (Mr) and coercivity (Hc) of the decorated MWNTs are much larger than those of MWNTs, and the decorated MWNTs exhibit well magnetic properties.     

Laser-driven synthesis and magnetic properties of iron nanoparticles

Nanoparticles of iron have been prepared by laser-driven decomposition of iron pentacarbonyl vapor. In this method, an infrared laser rapidly heats a dilute mixture of precursor vapors to decompose the precursor and initiate particle nucleation. It was found that when using SF₆ as a photosensitizer during the synthesis, ferrous fluoride (FeF₂) was produced as an undesired byproduct in the product powder. The particle size, composition, and crystalline structure have been characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X-ray photoelectron spectroscopy (XPS). Results of magnetization measurements for small iron nanoparticles (about 5 nm diameter) are also presented, showing superparamagnetic behavior at room temperature, and a blocking temperature near 125 K.     

Size-dependent magnetic properties of iron oxide nanoparticles

Uniform iron oxide nanoparticles in the size range from 10 to 24 nm and polydisperse 14 nm iron oxide particles were prepared by thermal decomposition of Fe(III) carboxylates in the presence of oleic acid and co-precipitation of Fe(II) and Fe(III) chlorides by ammonium hydroxide followed by oxidation, respectively. While the first method produced hydrophobic oleic acid coated particles, the second one formed hydrophilic, but uncoated, nanoparticles. To make the iron oxide particles water dispersible and colloidally stable, their surface was modified with poly(ethylene glycol) and sucrose, respectively. Size and size distribution of the nanoparticles was determined by transmission electron microscopy, dynamic light scattering and X-ray diffraction. Surface of the PEG-functionalized and sucrose-modified iron oxide particles was characterized by Fourier transform infrared (FT-IR) and Raman spectroscopy and thermogravimetric analysis (TGA). Magnetic properties were measured by means of vibration sample magnetometry and specific absorption rate in alternating magnetic fields was determined calorimetrically. It was found, that larger ferrimagnetic particles showed higher heating performance than smaller superparamagnetic ones. In the transition range between superparamagnetism and ferrimagnetism, samples with a broader size distribution provided higher heating power than narrow size distributed particles of comparable mean size. Here presented particles showed promising properties for a possible application in magnetic hyperthermia.     

Magneto-Coulomb effect in carbon nanotube quantum dots filled with magnetic nanoparticles

Electrical transport measurements of carbon nanotubes filled with magnetic iron nanoparticles are reported. Low-temperature (40 mK) magnetoresistance measurements showed conductance hysteresis with sharp jumps at the switching fields of the nanoparticles. Depending on the gate voltage, positive or negative hysteresis was observed. The results are explained in terms of a magneto-Coulomb effect: The spin flip of the iron island at a nonzero magnetic field causes a shift of the chemical potential induced by the change of Zeeman energy; i.e., an effective charge variation is detected by the nanotube quantum dot.     

Synthesis,structure, and magnetic properties of iron and nickel nanoparticles encapsulated into carbon

Nanocomposites based on iron and nickel particles encapsulated into carbon (Fe@C and Ni@C), with an average size of the metal core in the range from 5 to 20 nm and a carbon shell thickness of approximately 2 nm, have been prepared by the gas-phase synthesis method in a mixture of argon and butane. It has been found using X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy that iron nanocomposites prepared in butane, apart from the carbon shell, contain the following phases: iron carbide (cementite), α-Fe, and γ-Fe. The phase composition of the Fe@C nanocomposite correlates with the magnetization of approximately 100 emu/g at room temperature. The replacement of butane by methane as a carbon source leads to another state of nanoparticles: no carbon coating is formed, and upon subsequent contact with air, the Fe₃O₄ oxide shell is formed on the surface of nanoparticles. Nickel-based nanocomposites prepared in butane, apart from pure nickel in the metal core, contain the supersaturated metastable solid solution Ni(C) and carbon coating. The Ni(C) solid solution can decompose both during the synthesis and upon the subsequent annealing. The completeness and degree of decomposition depend on the synthesis regime and the size of nickel nanoparticles: the smaller is the size of nanoparticles, the higher is the degree of decomposition into pure nickel and carbon. The magnetization of the Ni@C nanocomposites is determined by several contributions, for example, the contribution of the magnetic solid solution Ni(C) and the contribution of the nonmagnetic carbon coating; moreover, some contribution to the magnetization can be caused by the superparamagnetic behavior of nanoparticles.     

Coverage dependent magnetic properties of Co chain-coated carbon nanotube

Magnetic properties of Co chain-coated carbon nanotube (CNT) were investigated using a first-principles calculation. Binding energy between Co chain and CNT increased with the coverage ratio, and the adsorption of Co chains on CNT enhanced the conductance channel. Total magnetic moment of Co chains coated on CNT increased with the coverage ratio, while the magnetic moment per Co atom decreased due to spin flip of majority spin states in Co atoms. Spin polarization at the Fermi level of the Co chains was calculated to converge to that of bulk fcc Co.     

Magnetic properties of core-shell catalyst nanoparticles for carbon nanotube growth

Two types of core-shell nanoparticles have been prepared by laser pyrolysis using Fe(CO)₅ and C₂H₂ or [(CH₃)₃Si]₂O as precursors and C₂H₄ as sensitizer. The first type (about 4 nm diameter) - produced by the decomposition of Fe(CO)₅ in the presence of C₂H₄ and C₂H₂ - consists of Fe cores protected by graphenic layers. The second type (mean particle size of about 14 nm) consists also of Fe cores, yet covered by few nm thick γ-Fe₂O₃/porous polycarbosiloxane shells resulted from the [(CH₃)₃Si]₂O decomposition and superficial oxidation after air exposure. The hysteresis loops suggest a room temperature superparamagnetic behavior of the Fe-C nanopowder and a weak ferromagnetic one for larger particles in the Fe-Fe₂O₃-polymer sample. Both types of nanoparticles were finally used as a catalyst for the carbon nanotube growth by seeding Si(100) substrates via drop-casting method. CNTs were grown by Hot-Filament Direct.Current PE CVD technique from C₂H₂ and H₂ at 980 K. It is suggested that the increased density and orientation degree observed for the multiwall nanotubes grown from Fe-Fe₂O₃-polymer nanoparticles could be due to their magnetic behavior and surface composition.     

Structure and magnetic properties of nanoparticles encapsulated in carbon shells

Ni₂Y and Nd–Fe–Nb–B catalysts were used for the processing of nanoparticles by arc discharge between graphite electrodes. The products were collected from the cathode (deposit and collar) and reactor walls (soot). The ferromagnetic nanoparticles have size in the range of 10–50 nm and are encapsulated in carbon shells. The chemical composition, structure and magnetic properties of the nanoparticles have been studied. For the Ni₂Y catalyst we found that the arc discharge results in decomposition of the intermetallic Ni₂Y phase and formation of Ni nanoparticles encapsulated in carbon shells in the collar and soot, whereas yttrium oxide was found in the deposit. For the Nd–Fe–Nb–B catalysts the magnetic properties depend on the collection place and erosion rate. Fe and Fe–Nd–Nb nanoparticles were found in the soot and deposit, respectively.     

外加轴向磁场下碳纳米管场效应晶体管的电学性质

在紧束缚理论的基础上推导出轴向磁场下碳纳米管的能带公式,研究外加磁场下碳纳米管场效应晶体管的电学特性.说明磁场可使碳管的导电性质在金属型和半导体型之间转变,转变的磁场周期为0.5₀.进一步应用场效应晶体管Natori理论模拟计算了外加磁场对碳纳米管场效应晶体管的电流-电压特性的影响,研究结果显示zigzag管和armchair管的电流随外电压和磁场都有振荡行为,而且两类管的振荡行为有明显差别. 关键词： 碳纳米管 紧束缚理论 费米能 能带结构     

Phase transition and magnetic properties of Mg-doped hexagonal close-packed Ni nanoparticles

Mg-doped Ni nanoparticles with the hexagonal close-packed (hcp) and face-centered cubic (fcc) structure have been synthesized by sol-gel method sintered at different temperatures in argon atmosphere. The sintering temperature played an important role in the control of the crystalline phase and the particle size. The pure hcp Mg-doped Ni nanoparticles with average particle size of 6.0 nm were obtained at 320 °C. The results indicated that the transition from the hcp to the fcc phase occurred in the temperature range between 320 °C and 450 °C. Moreover, the VSM results showed that the hcp Mg-doped Ni nanoparticles had unique ferromagnetic and superparamagnetic behavior. The unsaturation even at 5000 Oe is one of the superparamagnetic characteristics due to the small particle size. From the ZFC and FC curves, the blocking temperature T_B of the hcp sample (6.0 nm) was estimated to be 10 K. The blocking temperature was related to the size of the magnetic particles and the magnetocrystalline anisotropy constant. By theoretical calculation, the deduced particle size was 6.59 nm for hcp Mg-doped Ni nanoparticles which was in agreement with the results of XRD and TEM.     

Solid state synthesis and room temperature magnetic properties of iron phosphide nanoparticles

Room temperature magnetic properties have been achieved for nano-crystalline iron phosphide synthesized from the direct solid state reaction of iron chloride and tri-octylphosphine (TOP). The magnetization continuously increased with higher magnetic fields, indicating a super-paramagnetic behavior. It is observed that room temperature magnetism is possible for the material showing antiferromagnetic nature at low temperatures. In the present synthesis, TOP acted as a source of phosphorus as well as a surfactant. X-ray diffraction (XRD) studies revealed that the black powder is a mixture of FeP and Fe₂P. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed elongated as well spherical particles. Energy dispersion X-ray analysis (EDAX) confirmed a non-stoichiometric iron phosphide. Presence of TOP was confirmed by infra-red (IR) spectroscopy, and thermo-gravimetric analysis (TGA) indicated about 6% wt. loss due to presence of organics.     

Effect of iron doping concentration on magnetic properties of ZnO nanoparticles

The ZnO:Fe nanoparticles of mean size 3-10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe-Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mössbauer spectroscopy investigation showed Fe³⁺ nature of the iron atom in ZnO matrix.     

Controlling magnetic properties of iron oxide nanoparticles using post-synthesis thermal treatment

Changes in morphological and magnetic properties of Fe₃O₄ nanoparticles before and after annealing are investigated in the present work. The nanoparticles are synthesized in a standard capacitively coupled plasma enhanced chemical vapour deposition system with two electrodes using ferrocene as the source compound. Post annealing, due to the sintering process, the particles fuse along with recrystallization. This results in increased size of the nanoparticles and the interparticle interaction, which play a major role in deciding the magnetic properties. X-ray diffraction patterns of the samples before and after annealing indicate a phase change from Fe₃O₄ to Fe₂O₃. Annealing at 200 ^°C causes the apparent saturation magnetization to increase from 6 emu?g^?1 to 15 emu?g^?1. When annealed at 500 ^°C, the magnetic properties of the nanoparticles resemble those of the bulk material. The evidence for the transition from a superparamagnetic state to a collective state is also observed when annealed at 500 ^°C. Variation of the magnetic relaxation data with annealing also reflects the change in the magnetic state brought about by the annealing. The correlation between annealing temperature and the magnetic properties can be used to obtain nanocrystallites of iron oxide with different sizes and magnetic properties.     

Characterization of magnetic iron oxide nanoparticles

Magnetic nanoparticles have promising applications in many areas, for example optics, electronics, biology, medicine, etc. The main goal of this study is to synthezise and characterize ε-Fe₂O₃ magnetic nanoparticles embedded in amorphous SiO₂ matrix. The Mössbauer spectroscopy analysis of the samples was complemented by the study of X-ray powder diffraction and high-resolution transmission electron microscopy.     

Pressure control of conducting channels in single-wall carbon nanotube networks

We measure electrical transport on networks of single-wall nanotube ropes as a function of temperature T, voltage V, and pressure up to 22 GPa. We observe Luttinger liquid (LL) behavior, a conductance proportional to T(alpha), and a dynamic conductance proportional to V(alpha). With pressure, conductance increases while alpha decreases, enabling us to test the theoretical prediction for LL behavior on the alpha dependence of the T and V independent coefficient of the tunneling conductance, and to obtain the high frequency cutoff of LL modes. The possible transition to a Fermi liquid at alpha-->0 is unattainable, as nanotubes collapse to an insulating state at high pressures.     

Effects of electromigration on copper atoms in carbon nanotube channels

The effects of electromigration on copper in carbon nanotube (CNT) channels are investigated using molecular dynamics simulations. The study shows that the potential energy of copper and the resistive forces on copper are dependent on the shape of the CNT junction, and the increase in bias voltages magnifies these effects. Bias voltages affect the density of copper in the downstream CNT. The velocity of copper in the downstream CNT is relatively lower than that in the upstream CNT when the biased voltage is high.     

Functionalization of carbon encapsulated iron nanoparticles

Carbon-encapsulated magnetic nanoparticles are a new class of materials where the core magnetic nanoparticle is protected from reactions with its environment by graphite shells. Having a structure similar to carbon nanotubes, these nanoparticles could be potentially functionalized using methods which are already applied to those structures. We present the effects of acidic treatments based on HCl, HNO₃, and H₂SO₄ on these nanoparticles highlighting the impact on their magnetic and surface properties. We show that acidic treatments based on HNO₃ can be successfully applied for the generation of carboxylic groups on the surface of the nanoparticles. Using methylamine as a model, we demonstrate that these functional groups can be used for further functionalization with amino-containing biomolecules via diimide-activated amidation.     

Polyoxometalates nanoparticles: synthesis, characterization and carbon nanotube modification

In this paper, we described a large-scale synthesis method of the polyoxometalates (POMs) nanoparticles and the modification of carbon nanotube (CNTs) through a chemical modified approach. Four types of POMs nanoparticles were prepared by a one-step solid-state reaction at room temperature and characterized by IR, elemental analyses, XRD and TEM. These uniform nanoparticles have an average size of 8-10 nm. Furthermore, based on chemical adsorption between POMs and carboxylic acid groups, which were introduced to the CNTs by adding dilute nitric acid, POMs nanoparticles were successfully located on the CNTs as the modifier.     

Acoustic and acoustoelectronic properties of carbon nanotube films

The effect of spontaneous nanotube doping by substrate atoms was discovered using acoustic resonator microwave spectroscopy. The acoustoelectric effect involving surface acoustic waves was observed in nanotube films. The possibility of using nanotube films for efficient acoustic-wave excitation in solids by electrostriction was analyzed.     

New nanocomposites containing metal nanoparticles, carbon nanotube and polymer

Metal-carbon nanotube-graft-polymer (MCNT-g-P) nanocomposites were synthesized and characterized successfully. In this work, multiwall carbon nanotubes (MWCNT) were opened using HNO₃/H₂SO₄ mixture and filled by metal nanoparticles such as silver nanoparticles through wet chemistry method. Then MWCNT containing metal nanoparticles were used as macroinitiator for ring opening polymerization of ε-caprolactone and MCNT-g-P nanocomposites were obtained. Length of grafted polymer arms onto the MWCNT was controlled using MWCNT/ε-caprolactone ratio. Structure and properties of nanocomposites were evaluated by TEM, DSC, TGA, and spectroscopy methods.