The edge state of nanographene and the magnetism of the edge-state spins

Nanographene has unique edge-shape dependence of the electronic structure with non-bonding edge states being created in its zigzag edges. The presence of the edge state is experimentally confirmed in well-defined hydrogen-terminated zigzag edges by scanning tunneling microscopy/spectroscopy (STM/STS) observations. In the three-dimensional (3D) disordered network of nanographite domains in nanoporous carbon (activated carbon fibers), the localized edge-state spins are in a spin-glass-like ordered state at low temperatures with the aid of exchange interactions whose strengths varies randomly in space, when the strengths of inter-nanographene and nanographite interactions are tuned. Chemical and structural modifications of nanographene edges change the magnetism of edge-state spins through covalent bond formation and charge transfer.     

Magnetism of nanographene network influenced by the interaction with acid species

The magnetism of nanographite (stacked nanographene sheets)-based nanoporous carbon is investigated in relation to the interaction with acid guest species. The concentration of the localized spins of non-bonding π-electron state (edge state) localized in the nanographene edges decreases upon the sulfonation of nanographene edges through charge-transfer interaction with sulfonic groups. The sulfonation of nanographene edges enhances the hydrophilic nature of the edges, resulting in the easiness in the water adsorption into the nanopores. This enhances the mechanical compression effect of water molecules condensed in the nanopores on the nanographite domains, resulting in the decrease in the spin concentration. The change in the magnetism upon water uptake reveals ferrimagnetic nature of individual nanographene sheets. The adsorption of HCl having no oxidation ability shows a mechanical effect on the edge-state spins similar to water adsorption. The spin concentration is reduced in two-step manner by the charge-transfer interaction with guest concentrated HNO₃ that is strong oxidant. In the presence of H₂O molecules in diluted HNO₃ the cooperation of mechanical and charge-transfer interactions creates also a two-step change in the magnetism.     

STM observation of the quantum interference effect in finite-sized graphite

Superperiodic patterns were observed by STM on two kinds of finite-sized graphene sheets. One is nanographene sheets inclined from a highly oriented pyrolitic graphite (HOPG) substrate and the other is a several-layer-thick graphene sheets with dislocation-network structures against a HOPG substrate. As for the former, the in-plane periodicity increased gradually in the direction of inclination, and it is easily changed by attachment of a nanographite flake on the nanographene sheets. The oscillation pattern can be explained by the interference of electron waves confined in the inclined nanographene sheets. As for the latter, patterns and their corrugation amplitudes depended on the bias voltage and on the terrace height from the HOPG substrate. The interference effect by the perturbed and unperturbed waves in the overlayer is responsible for the patterns whose local density of states varies in space.     

Investigation of particle formation and superstructure development in FePt nanoparticles and their effect on magnetic properties

FePt nanoparticles with perpendicular magnetic anisotropy embedded in non-magnetic matrices M (M=Ag, C) have been fabricated by sputtering FePt/M multilayer films onto single crystal MgO [0 0 1] at temperatures above 300 °C. Particles with controlled particle size down to a few nanometers and tailored microstructure and magnetic properties can be obtained by varying the bilayer thicknesses, the substrate temperature, the type of substrate material and the post-annealing conditions. Ordered FePt nanoparticles have also been prepared directly by gas phase condensation techniques (cluster gun).The cluster gun allows a better control of particle size and distribution, and enables an in situ heat treatment of the particles to transform their structure into the desired phase before they are deposited onto the substrate, thus avoiding the undesirable effects of alloying and oxidation.     

Electronic specific heat of nanographite ribbons

The electronic specific heat of nanographite ribbons exhibits rich temperature dependence, mainly owing to the special band structures. The thermal property strongly depends on the geometric structures, the edge structure and the width. There is a simple relation between the ribbon width and the electronic specific heat for the metallic or semiconducting armchair ribbons. However, it is absent for the zigzag ribbons. The metallic armchair ribbons exhibit linear temperature dependence. The semiconducting armchair ribbons exhibit composite behavior of power and exponential functions. As for the zigzag ribbons, the temperature dependence of the specific heat is proportional to T^1−p. The value of p quickly increases from to 1 as the ribbon width gradually grows. The zigzag ribbons might be the first system which exhibits the novel temperature dependence. The nanographite ribbons differ from an infinite graphite sheet, which illustrates that the finite-size effects are significant.     

Oxygen guest molecules in the nanopores of nanographite-network

Magnetic properties of nanographite-network-based nanoporous carbon having high surface area are investigated in the presence of magnetic oxygen guest molecules. The nanographite domains (stacked nanographene sheets) of 2-3 nm average in-plane size have localized spins of non-bonding π-electron state (edge-state) in the zigzag-shaped periphery of individual nanographene sheets. The electron paramagnetic resonance signal of edge-state spins is found to be highly sensitive to the presence of magnetic oxygen molecules. The line-width variation in the limit of lower oxygen pressure (1-20 Pa) shows that the magnetic interaction of oxygen can be divided into two temperature regimes, viz., the regime below 100 K, where chemisorption of oxygen is effective and the regime above 100 K, where only physisorption of oxygen takes place. Here, 100 K marks the energy of physisorption of oxygen molecules on nanographene, which is higher than the condensation energy of oxygen molecules represented by the boiling point of oxygen, 90 K, owing to the capillary effect. Above 100 K, magnetically active physisorbed oxygen molecules work to increase the line-width, which is governed by the dipolar field with oxygen molecules. The presence of a maximum in the line-width around 150 K, which is higher than 100 K, suggests the diffusion motion of oxygen molecules reduces the strength of exchange interaction between the edge-state spins and the oxygen spins as the temperature is elevated.     

Photoluminescence and Raman spectroscopy of single diamond nanoparticle

The article reports techniques that we have devised for immobilizing and allocating a single nanodiamond on the electron beam (E-beam) lithography patterned semiconductor substrate. By combining the E-beam patterned smart substrate with the high throughput of a confocal microscope, we are able to overcome the limitation of the spatial resolution of optical techniques (~1 μm) to obtain the data on individual nano-object with a size range between 100 and 35 nm. We have observed a broad photoluminescence centered at about 700 nm from a single nanodiamond which is due to the defects, vacancies in the nanodiamonds, and the disordered carbon layer covered on the nanodiamond surface. We also observe red-shift in energy and broadening in linewidth of the sp³ bonding Raman peak when the size of the single nanodiamond is reduced due to the phonon-confinement effects.     

A new method for deaggregation of nanodiamond from explosive detonation: Graphitization-oxidation method

In this communication, a new method for the deaggregation of detonation nanodiamond (ND) and some preliminary results using this method are presented. ND is firstly graphitized in nitrogen at 1000°C and then oxidized by air at 450°C to remove the surface graphite layer formed. The sample after such treatment was suspended in water by ultrasonics, and the particle-size distributions were measured. It has been found that the diameters of more than 50% of the ND particles can be reduced to less than 50 nm.     

柱状磁光颗粒的局域表面等离激元共振及尺寸效应

柱状磁光颗粒的局域表面等离激元共振为二维磁光光子晶体的手征性边缘模的生成提供了重要的机制. 但目前对此类颗粒的局域表面等离激元共振效应的研究局限于长波长近似下的结果, 且缺乏对发生共振时的远场与近场特征的深入了解. 本文从散射理论出发, 计算并分析了柱状磁光颗粒发生局域表面等离激元共振的条件与特殊的场特征, 并讨论了颗粒尺寸对共振峰的影响. 计算结果解释了实验中观察到的二维磁光光子晶体的共振带隙与在长波长近似下得到的局域表面等离激元共振频率的明显偏移, 并展示了颗粒在较大尺寸下形成的高阶共振峰, 这可能有助于利用共振效应在磁光光子晶体中实现多模的手征边缘态.     

57Fe Mössbauer spectroscopic study of nanostructured zinc ferrite

Nanosize zinc ferrite particles, prepared by nitrate method, were investigated by XRD, TEM, ⁵⁷Fe Mössbauer spectroscopy and VSM. The average particle size in this system varies from 10 to 62 nm as the sintering temperature increases from 300°C to 1,000°C. The lattice parameters in this system are almost constant at a value of ～8.41 Å. The Mössbauer spectra of all the sintered samples show a single doublet. The Mössbauer hyperfine parameters show little change with the change of sintering temperature. The doublets are ascribed to the presence of superparamagnetism in this system, which is also corroborated by the VSM measurements.     

Zero-conductance resonances due to flux states in nanographite ribbon junctions

Electronic transport properties through junctions in nanographite ribbons are investigated using the Landauer approach. In the low-energy regime ribbons with zigzag boundary have a single conducting channel of edge states. The conductance as a function of the chemical potential shows a rich structure with sharp dips of zero conductance. Each zero-conductance resonance is connected with a resonant state which can be interpreted as the superposition of two degenerate flux states with Kekule-like current patterns. These zero-conductance dips are connected with a pronounced negative magnetoresistance.     

Spin‐dependent‐magnetoresistance control by regulation of heat treatment temperature for magnetite nano‐particle sinter

The control of spin‐dependent‐magnetoresistance by regulation of the heat treatment (HT) temperature for magnetite (Fe₃O₃) nano‐particle sinter (MNPS) has been studied. The average nano‐particle size in the MNPS is 30nm and the HT was carried out from 400°C to 800°C. The HT of the MNPS varies the coupling form between adjacent magnetite nano‐particles and the crystallinity of that. The measurements on electrical resistance (ER), magnetoresistance (MR) and magnetization were performed between 4K and 300K. The behavior of the ER and MR considerably changes at the HT temperature of ～600°C. Below ～600°C the ER indicates the variable‐range‐hopping conduction behavior and the MR shows the large intensity in a wide temperature region. Above ～600°C the ER shows the indication of the Verwey transition near 110K like a bulk single crystal and the MR designates the smaller intensity. We consider that below ～600°C the ER and MR are dominated by the grain‐boundary conduction and above ～600°C those are determined by the inter‐grain conduction. The magnetic field application to the grain‐boundary region is inferred to cause the large enhancement of the MR.     

Phosphor thermometry at the surface of single reacting large-diameter spherical coke particles to characterise combustion for packed bed furnaces

Packed-bed furnaces fired with large-diameter coke are important in high-temperature material processing industries such as lime and iron production. The combustion conditions are complicated by the presence of an ash layer surrounding the coke particle that remains intact during passage through the furnace and alters oxygen diffusion and heat transfer to the reacting particle core. The objective of this study is to determine the surface temperature of this ash layer using lifetime-based phosphor thermometry during combustion of single spherical 38 mm diameter coke particles in a high temperature tube furnace. Time traces of the coke particle core temperature and mass conversion rate do not significantly differ between experiments performed with and without the phosphor layer, indicating that the presence of the phosphor particles does not alter the overall combustion behaviour. Surface temperatures of up to 950 °C are measured and correlated with the fuel mass conversion rate. When the coke particle starts to react the surface temperature is up to 100 °C higher than that of the core. As the reaction front progresses toward the centre, the core temperature exceeds the surface temperature by 200 °C due to the insulating effect of the ash layer. The surface temperature of the ash layer decreases with time due to the steadily decreasing fuel mass conversion rate. The method and results can be used to provide key validation data for shrinking-core combustion models, for example by constraining the unknown transport properties of the ash layer, thereby assisting the development of complete packed-bed furnace simulations for process optimisation.     

Characterizing the coating and size-resolved oxidative stability of carbon-coated aluminum nanoparticles by single-particle mass-spectrometry

Aluminum nanoparticles are of significant interest in enhancing the rate of energy release from propellants. One of the major impediments to their use is that bare aluminum is highly reactive, while oxide coated aluminum significantly decreases overall performance. We investigate creating aluminum nanoparticles with a thin carbon coating using either a laser induced plasma or a DC plasma-arc. The carbon coating was created by injecting ethylene (C₂H₄) directly downstream of the plasma. The elemental composition of the coated aluminum nanoparticles was measured in real time with a recently developed quantitative single particle mass spectrometer (SPMS). We found that the aluminum nanoparticles were coated with a carbon layer of thickness around 1–3 nm.The thermal and oxidative stability of these particles was determined by passing the aerosols through a heated flow reactor in a carrier flow of either air or argon, and measuring the aluminum, carbon and oxygen content in the particles with the single particle mass spectrometer. We found that below 700°C the coating was stable, but that the coating oxidized above ∼ ∼800°C. In contrast the carbon coating was thermally stable above ∼ ∼900°C. These results indicate that a carbon coating may be a suitable passivating agent.     

Magnetic structure of graphite ribbon

Anoble mechanism of spin polarization is proposed for finite graphite sheet with edge. For graphite ribbon with zigzag edge, there appear peculiar ‘edge states’. These localized states comprise nearly flat band at the Fermi level, which easily causes magnetic instability. Magnetic structure is suggested from Hartree-Fock analysis of the Hubbard model, where huge magnetic moments are induced at around both of edges by weak HubbardU and are coupled antiferromagnetically with each other.     

EPR/FMR, FTIR, X-Ray and Raman Investigations of Fe-Doped SiCN Ceramics

SiCN magnetic ceramics doped with Fe ions were synthesized at different pyrolysis temperatures in the range from 600 to 1600°C. Several phases of ceramics were detected using the techniques of electron paramagnetic resonance/ferromagnetic resonance, Raman, Fourier-transform infrared and X-ray diffractometry, listed as follows: (a) transformation to the ceramic state from the polymer state, where the Fe ions are in the paramagnetic state, as the temperature is increased from 600 to 800°C; (b) formation of two different Fe species in the range of 950–1150°C: nanocrystalline particles in the ferromagnetic state and Fe ions incorporated into the free-carbon state in the superparamagnetic state; (c) diminution of the free-carbon content above 1150°C, and, as a consequence, diminution of the intensity of the broad Fe signal related to this phase; (d) appearance of a new Fe phase at about 1200°C; (e) disappearance of the ferromagnetic phase at about 1400°C; (f) disappearance of all Fe ions above 1530°C. The samples exhibiting superparamagnetic behavior are potentially useful in developing high-temperature magnetic sensor devices.     

Native defect changes in cds single crystal platelets induced by vacuum heat treatments at temperatures up to 600°C

Physical properties of selected CdS single crystal platelets as-grown and after vacuum heat treatments at temperatures up to 600°C have been studied using u.v. excited edge emission, mass spectrometry, electrical resistivity and electron paramagnetic resonance (EPR). It was found that sulfur leaves the crystal at temperatures as low as 100°C creating a depletion layer. The native defect changes were monitored by edge emission studies at 4.2°K in combination with etch treatments. The defect structure throughout the crystal is not only dependent upon the temperature and atmosphere of the treatments, but is also strongly dependent upon the cooling rate.     

Formation of small particles of gold on alumina support films and their behaviour in oxygen and hydrogen atmospheres

Attempt has been successfully made to disperse ultrafine gold particles on alumina support films by keeping the substrate at elevated temperatures during metal deposition under vacuum of ≈ 10^-5 Torr. It is found that reasonable dispersion of particles occurs for a mean thickness of 0.5 nm at a substrate temperature of 125 °C. The resulting samples turn out to be quite suitable for model studies of the behaviour of gold/alumina catalyst. Electron microscope observations coupled with diffraction have provided evidence for emergence of gold particles without any compound formation with the residual gases present during deposition at 10^-5 Torr. The nature of dispersion and average particle size are shown to depend on substrate temperature and the amount of metal evaporated. Heat treatments in hydrogen and oxygen at 200–500 °C for various lengths of time led to an increase in particle size with a simultaneous decrease in number density (i.e. coarsening of particles) following, in all probability, the Ostwald ripening mechanism. The oxidizing atmosphere has been shown to be more favourable to coarsening or deactivation of the catalyst. Finally, it is indicated that gold particles prefer to facet giving well-defined shapes (e.g. hexagonal, pentagonal or rhombohedral) with faces corresponding to planes of relatively smaller interfacial energies.     

Study of silica aerogel grain surfaces by using a positron age–momentum correlation technique

The effect of heat treatment on silica aerogel has been studied by a positron age–momentum correlation technique and infrared measurement. A difference was observed between the momentum distributions of the electrons on the first layers of the silica aerogel grain surfaces and the electrons in the grains in an as-supplied sample, but not in the sample heat treated at 800 °C. A large change in the S parameter for the momentum distribution of the electrons on the first layer occurs around 200 °C. This change correlates well with that of the infrared spectra, which show oxidation of the methoxyl groups at temperatures above around 200 °C. This correlation reveals that those groups are mainly located on the first layer of the silica grains. Received: 13 November 2000 / Accepted: 23 July 2001 / Published online: 30 October 2001     

Effect of deuterium on phase transformations in fullerenes at high temperatures and high pressures

The effect deuterium has on phase transformations is studied for amorphous and crystalline fullerenes C₆₀ and C₇₀ at high temperatures of up to 1300°C and high pressures (2–8 GPa). Amorphous fullerene phases are obtained via long grinding in a planetary mill. Structure is studied by means of neutron diffraction. In all cases, amorphous graphite (nanographite) forms in the temperature range of 800–1100°C. This material has different diffraction spectra distinguished by the heights of the halos observed on the graphite diffraction maxima and their relative intensities. These spectra (the structure of nanographite) are affected by preliminary amorphization, the number of carbon atoms in the fullerenes (C60 or C70), and the introduction of deuterium atoms. The different spectra of amorphous (disordered) graphite testify to its varying structure.