Particle size reduction and enhanced diffusion of Fe and Pt atoms in FePt-C granular films by N addition

N-doped FePt-C nanocomposite films were fabricated using facing-target sputtering method under different N₂ partial pressures (P_N) at room temperature. Annealing at 650 °C turns the amorphous films into ordered structures. Nitrogen doping not only make the ordering of FePt particles easier than the ordering in FePt-C films, due to the enhanced diffusivity of Fe and Pt atoms, but also effectively limits the growth of the FePt particles during the thermal induced ordering, especially for the annealed films fabricated at P_N = 40%, where the average size of well-isolated FePt particles is only ∼8 nm. The particle size reduction and the enhanced diffusion of Fe and Pt atoms can be ascribed to the desorption of doped N atoms and dissociation of FeN bonds during annealing. The room-temperature coercivity of the samples decreases with the P_N due to the particle size reduction and thus the enhancement of the thermal agitation for small particles during the magnetizing procedure.     

Optical second-harmonic generation by supported metal clusters: Size and shape effects

Optical Second Harmonic Generation (SHG) by metal clusters has been investigated. For this purpose clusters were generated by the deposition and nucleation of metal atoms on a LiF(100) single crystal surface under ultrahigh vacuum conditions. The size and shape of the metal particles was characterized by optical transmission spectroscopy. The SHG intensity was detected in situ as a function of cluster size during the nucleation. Fundamental wavelengths of =1064 and 532 nm were used and the SHG signal was measured for different polarization combinations of the incident and registered light. SH radiation is detectable for particles as small as approximately 1 nm. The signal grows monotonically as a function of particle size, passes a maximum and finally drops off. This behavior is discussed in terms of resonant enhancement of the signal by surface plasmon excitation and changes of ⁽²⁾ as a function of particle size and shape. In further experiments the chemisorption of oxygen on the surface of the metal particles was studied. The SH signal decreases as a function of oxygen coverage and amounts to only about 15% of the initial value upon chemisorption of one monolayer. This indicates that the SH signal originates almost exclusively from the surface of the clean clusters and that higher order bulk contributions are negligible.     

Synthesis of Aucore–Agshell type bimetallic nanoparticles for single molecule detection in solution by SERS method

This paper reports the evolution of a new class of core–shell type, that is, Au_core–Ag_shell bimetallic nanoparticles by seed mediated technique for surface enhanced Raman scattering (SERS) study. Here it is demonstrated how to control the thickness of Ag-shell with the variation of gold seed (15 nm) to Ag ion concentration which in turn control the particle size in the range from 50 to 100 nm with increase of shell thickness. For 50 nm core–shell particles the thickness of the shell was 17 nm, for 70 nm particles the thickness was 27 nm and for 100 nm the thickness was 42 nm. SERS study was performed on those particles using the analyte crystal violet (CV) to examine the impact of the size and field effects of the bimetallics on SERS spectra. A surprising finding is that a small particle as low as 50 nm have been found to be highly efficient for SERS, even it enables the detection of a selected dye molecule down to single molecular level. The sensitivity of the SERS detection limit has been improved further with an activating reagent like NaCl. The newly modeled bimetallic system establishes a relationship between the local electromagnetic (EM) field effect and chemical effect (CE) on the enhancement of SERS spectra, which provides further insight into the enhancement mechanism of SERS.     

Dynamic magnetization of <Emphasis Type="Bold">γ</Emphasis>- nanoparticles isolated in an amorphous matrix

We have studied the magnetization of a system of γ-Fe₂O₃ (0.68 vol.%) nanoparticles isolated in an SiO₂ amorphous matrix placed in an alternating magnetic field with a frequency of 640 Hz and in the temperature range of (77-300) K. Compared to temperatures closer to 300 K (where the system has a superparamagnetic behaviour), at lower temperatures, the magnetization has a dynamic hysteresis loop due to the magnetization's phase shift between the field and the magnetization. The delay of the magnetization (attributed to the Néel relaxation processes) increases with the decrease of temperature. It has been shown that the relaxation time resulting from the Néel theory is determined by an effective anisotropy constant ( K ) that takes into account the magnetocrystalline anisotropy, as well as the shape, surface and strain anisotropies. In the following we will show that the surface and strain anisotropy components have the most significant influence. When the temperature decreases from 300 to 77 K, the relative increase of the saturation magnetization of the nanoparticles is much higher than that of the (spontaneous) saturation magnetization of bulk γ-Fe₂O₃. This increase is due to the increase of the mean magnetic diameter of the particles attached to the core of aligned spins, from 10.16 nm to 11.70 nm, as a result of the modification of the superexchange interaction in the surface layer. Received 25 April 2002 / Received in final form 11 August 2002 Published online 14 February 2003 RID="a" ID="a"e-mail: ccaizer@physics.uvt.ro     

The Formation of Porous Membranes by Filtration of Aerosol Nanoparticles

Flame-generated aerosol particles of Al₂O₃ were deposited by gas filtration on two types of porous and ceramic tubes of -Al₂O₃ with mean pore diameters of 450 and 2700nm, respectively. The particles were aggregates with average mobility diameters in the range of 30–100nm and primary particle diameters of 4–8nm. The particles are characterized by differential mobility analysis, transmission electron microscopy, and by their specific surface area. The deposited membranes are characterized by gas permeability measurements, scanning electron microscopy, and by their pore size distribution from nitrogen capillary condensation. The particles form a distinct, homogeneous membrane layer with a porosity of 90% on top of the substrate surface and only penetrate slightly into the substrate structure. The mean pore sizes of the deposited membranes determined by nitrogen condensation agree approximately with those determined by gas permeation and the specific surface area. The mean pore diameter varies in the range of 30–70nm. The gas permeability of the deposited membranes is related to the specific surface area but influenced by the high porosity. The mean pore size and the permeability of the membranes are almost independent of the substrate structure.The development of a membrane with uniform properties is preceded by a short initial period in which the deposited particles, with an equivalent membrane thickness of roughly 2m, have a significantly lower permeability than the ultimately developed uniform membrane layer. This effect is particularly significant for the aerosol particles with the lowest mean size, probably due to particles deposited in the pore mouths of the substrate.The particles and the deposited membranes are X-ray amorphous but retain their specific surface area on heating to even high temperatures. When the membranes are heated to 1473K for 10h, X-ray diffraction shows a mixture of - and -alumina, accompanied by a partial disintegration of the membrane and a considerable loss of surface area.     

Magnetic Resonance in Gadolinium Nanoparticles near the Curie Point

The Influence of temperature in the range from 275 to 320 K on ESR spectra and magnetization m of ensembles of spherical gadolinium nanoparticles with the diameter from 89 to 18 nm was studied. The particles with d = 18 nm had a cubic face centered structure and no magnetic transition. At T > T_C all particles were paramagnetic, and their g factors were g = 1.98 ± 0.02 irrespective of their size and structure. At T = T_C the particles having 28 to 89 nm in size experienced a magnetic and orientation transition; at T < T_C their m(H) dependences were described by the Langevin function, and the FMR lines broadened and shifted towards H = 0. FMR lines of the Gd particle ensembles showed a hysteresis behavior during magnetization reversal, which did not correlate with the coercivity of the particles. Dependences of the Gd nanoparticles FMR linewidth ΔH(T) changed proportionally to |T–T_C|.     

Magnetic properties of 2 nm α-Fe particles

Ultrafine metallic iron particles with a mean diameter of 2 nm have been prepared on a carbon support and were studied by in situ Mössbauer spectroscopy. It is found that even in these extremely small particles, containing on the average about 350 atoms, most of the atoms are in environments which are quite similar to those of iron atoms in bulk -Fe. The surface hyperfine field is larger than the bulk value at 5 K, but decreases rapidly with increasing temperature. The magnetic energy constant was found to beK (3.0±1.0)×10⁵ J m^–3.     

Synthesis and magnetic properties of Sm–Y3Fe5O12 nanoparticles

Garnet nanoparticles Y_3−xSm_xFe₅O₁₂ were fabricated by a sol–gel method. The XRD patterns of all the samples have only peaks of the garnet structure and the sizes range from 34 to 67 nm. Results of VSM show that the saturation magnetization of Y_3−xSm_xFe₅O₁₂ (0<x3) decreases on increasing the Sm concentration x evidently. Meanwhile, it is observed that with the enhancement of the surface spin effects, the saturation magnetization rises as the nanoparticle size is increased.     

Theoretical prediction of a surface-induced spin-reorientation phase transition in BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanocrystals

The equation of the magnetization of a hexagonal crystal is derived for the first time for an arbitrary orientation of the external magnetic field relative to the crystallographic c axis. In order to clarify the magnetization mechanism for a real ensemble of small particles in the framework of the given problem, surface anisotropy (which is significant for nanosize objects) was taken into account along with crystalline magnetic anisotropy and anisotropy in the particle shape. Model computer experiments prove that the magnetization curves for nanocrystals oriented in a polar angle range of 65–90° exhibit an anomaly in the form of a jump, indicating a first-order spin-reorientation phase transition. This explains a larger steepness of the experimental curve reconstructed taking into account the interaction between particles as compared to the theoretical dependence obtained by Stoner and Wohlfarth [IEEE Trans. Magn. MAG 27 (4), 3469 (1991)]. An analysis of variation of the characteristic anisotropy surface and its cross section with increasing ratio |K₂|/K₁ of the crystalline magnetic anisotropy constants upon a transition from a macroscopic to a nanoscopic crystal shows that surface anisotropy leads to a change in the magnetic structure. As a result, an additional easy magnetization direction emerges in the basal plane apart from the easiest magnetization direction (along the c axis). The direction of hard magnetization emerges from the basal plane, the angle of its orientation relative to the c axis being a function of the ratio | K₂|/K₁.     

Preparation of ZnO–Al2O3 Particles in a Premixed Flame

Zinc oxide (ZnO) and alumina (Al₂O₃) particles are synthesized by the combustion of their volatilized acetylacetonate precursors in a premixed air–methane flame reactor. The particles are characterized by XRD, transmission electron microscopy, scanning mobility particle sizing and by measurement of the BET specific surface area. Pure (-)alumina particles appear as dendritic aggregates with average mobile diameter 43–93 nm consisting of partly sintered, crystalline primary particles with diameter 7.1–8.8 nm and specific surface area 184–229 m²/g. Pure zinc oxide yields compact, crystalline particles with diameter 25–40 nm and specific surface area 27–43 m²/g. The crystallite size for both oxides, estimated from the XRD line broadening, is comparable to or slightly smaller than the primary particle diameter. The specific surface area increases and the primary particle size decreases with a decreasing flame temperature and a decreasing precursor vapour pressure. The combustion of precursor mixtures leads to composite particles consisting of zinc aluminate ZnAl₂O₄ intermixed with either ZnO or Al₂O₃ phases. The zinc aluminate particles are dendritic aggregates, resembling the alumina particles, and are evidently synthesized to the full extent allowed by the overall precursor composition. The addition of even small amounts of alumina to ZnO increases the specific surface area of the composites significantly, for example, zinc aluminate particles increases to approximately 150 m²/g. The gas-to-particle conversion is initiated by the fast nucleation of Al₂O₃ or ZnAl₂O₃, succeeded by a more gradual condensation of the excess ZnO with a rate probably controlled by the cooling rate for the flame.     

Adsorption of parent nitrosamine on the nanocrystaline H-zeolite: A theoretical study

Adsorption of parent nitrosamine (NA) on 5T and 10T cluster models of H-ZSM-5 catalyst has been theoretically investigated using quantum chemical B3LYP and MP2 methods. Three stable complexes (A-C) were found on the potential energy surface of interaction between NA and cluster models of H-ZSM-5. NA can interact not only with acidic site of zeolite via the lone electron pair on nitrogen and oxygen atoms (O(N)?H_ZO) but also with the oxygen atoms of the framework via the hydrogen atoms of NH₂ group (NH?O) as well. However, the Lewis acidity of zeolite framework is the dominating factor in the interaction between NA and zeolite. The calculated adsorption enthalpy of NA on 5T and 10T clusters of H-ZSM-5 catalyst at ONIOM(MP2/6-311++G(d,p):HF/6-31+G(d)) level ranges from −19.73 to −40.33 and −63.81 to −73.73 kJ/mol, respectively. Adsorption energy for A-C complexes increases in going from B3LYP method to MP2 one. The results of atoms in molecules (AIM) calculations showed that NH5?O interactions have electrostatic character, whereas O(N)?H_Z interactions have partially covalent nature. The results of natural bond orbital (NBO) analysis showed that charge transfer occurs from NA to H-zeolite cluster.     

Magnetism of CrO overlayers on Fe(001)bcc surface: first principles calculations

Riva et al. [Surf. Sci. 621, 55 (2014)] as well as Calloni et al. [J. Phys.: Condens. Matter 26, 445001 (2014)] have studied the oxydation of Cr films deposited on Fe(001)bcc through low-energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy. In the present work we perform a density functional approach within Quantum Expresso code in order to study structural and magnetic properties of CrO overlayers on Fe(001)bcc. The calculations are performed using DFT+U. The investigated systems include O/Cr/Fe(001)bcc, Cr/O/Fe(001)bcc, Cr_0.25O_0.75/Fe(001)bcc, as well as the O coverage O _x /Cr/Fe(001)bcc (x = 0.25; 0.50). We have found that the ordered CrO overlayer presents an antiferromagnetic coupling between Cr and Fe atoms. The O atoms are located closer to the Fe atoms of the surface than the Cr atoms. The ground state of the systems O/Cr/Fe(001)bcc and Cr/O/Fe(001)bcc corresponds to the O/Cr/Fe(001)bcc system with a magnetic coupling c(2 × 2). The effect of the O monolayer on Cr/Fe(001)bcc changes the ground state from p(1 × 1) ↓ to c(2 × 2) and produces an enhancement of the magnetic moments. The O _x overlayer on Cr/Fe(001)bcc produces an enhancement of the Cr magnetic moments.     

Particle size effects on magnetic properties of yttrium iron garnets prepared by a sol–gel method

We present detailed measurements of field—and temperature—dependence of magnetization in nanocrystalline YIG (Y₃Fe₅O₁₂) particles. The fine powders were prepared using sol–gel method. Samples with particle sizes ranging from 45 to 450 nm were obtained. We observe that the saturation magnetization decreases as the particle size is reduced due to enhancement of the surface spin effects. Below a critical diameter D_s≅190 nm, the particles become single domains and the coercive forces reaches a maximum at diameters close to the critical value. As the particle size decreases the coercivity diminishes and at D_p≃35 nm diameters the upper limit of superparamagnetic behavior is reached. A quantitative comparison of temperature and particle size dependence of coercivity shows a satisfactory agreement that is expected for an assembly of randomly oriented particles.     

AC corona-discharge treatment of polypropylene foils effects of gaseous atmosphere

The AC corona-discharge treatment of PP foils has been studied in air, N₂, CO₂ gases at a pressure of 93·6 kPa and at ambient temperatures. The highest increase in surface energy _S of PP foil, by about 100% of initial value, was observed after treatment in CO₂. The best durability of induced enhancement of surface energy was also achieved after the treatment in carbon dioxide. The enhancement of polar component _S ^P and unsubstantial reduction of dispersion component _S ^D , and the results of other experiments indicate that the role of charged particles, namely the role of electrons in treatment process of PP is very important.Authors are much obliged to Mrs. L. Izoldova of Technical University, Bratislava, for technical assistance.     

Pulsed laser deposition of oriented barium ferrite (BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript>) thin films

Barium ferrite (BaFe₁₂O₁₉) thin films have been deposited by pulsed laser deposition at 900 °C in 250 mTorr oxygen on Si substrates with Pt(111) underlayers. The barium ferrite films contained platelet grains and a small amount of acicular grains, with sizes of 300 nm and 80×300 nm, respectively, and had a surface roughness of 11 nm. Vibrating-sample magnetometer measurements indicated that the BaFe₁₂O₁₉ films have some perpendicular orientation, with a perpendicular squareness of 0.64 and an in-plane squareness of 0.28. The saturation magnetization is about 190 emu/cc. The perpendicular and in-plane coercivities are 2.1 kOe and 1.6 kOe, respectively. PACS 75.50.Ss; 75.60.Ej; 75.70.Ak; 68.55.Ik     

Preparation of Ultrafine Colloidal Gold Particles using a Bioactive Molecule

Synthesis of nanometer-sized particles with new physical properties is an area of tremendous interest. In metal particles, the changes in size modify the electron density in the particles, which shifts the plasmon band. The most significant size effects occur when the particles are ultrafine (size is <10 nm). Thus the synthesis of ultrafine metal particles is enormously important to exploit their unique and selective application. Here we report a novel method for the synthesis of ultrafine gold particles in the size range of 0.5–3 nm using dopamine hydrochloride (dhc), an important neurotransmitter. This is the first time where such an important bioactive molecule like dhc has been used as a reagent for the transformation of Au(III) to Au(0). The synthesis is carried out, for the first time, either in simple aqueous or in a nonionic micellar (for example Triton X-100 (TX-100)) medium. The gold sol has a beautiful yellow–brown color showing _max at 470 nm. The appearance of the absorption peak at substantially shorter wavelength (usually gold sol absorbs at 520 nm) indicates that the particles are very small. The method discussed here is very simple, reproducible and does not involve any reagent, which contains 'P' or 'S' atoms. Also in this case no polymer or dendrimer or thiol-related stabilizer is used. The effects of different parameters (such as the presence or absence of O₂, temperature, TX-100 concentration and dhc concentration) on the formation of ultrafine gold particles are discussed. The effects of 3-mercapto propionic acid and pyridine on the ultrafine gold sol are also studied and compared with those on photochemically prepared gold sol. It is observed that 3-mercapto propionic acid dampens the plasmon absorption at 470 nm of ultrafine gold particles. Pyridine, on the other hand, has no effect on the particles.     

Chemical state of carbon atoms on the surface of nanodiamond particles

Auger electron spectroscopy study of the chemical state of carbon atoms on the surface of nanodiamond particles is performed. Auger spectra of nanodiamond particles indicate that carbon atoms in nanodiamond are in the same state as those in graphite, i.e., in the σ _s ¹ σ _p ² π¹ state, but the π band is displaced 1 eV in energy below the Fermi level. The surface of nanodiamond particles is inert with respect to the ambient medium.     

Silver Nanoparticles Synthesized by Vapor Deposition onto an Ice Matrix

As a strategy for synthesizing metal nanoparticles, thermally evaporated Ag was deposited onto a thin (1.2nm) crystalline ice layer on hafnia (HfO₂) at 100K. The Ag atoms penetrate into the ice matrix but do not reach the underlying HfO₂ substrate. After controlled thermal desorption of water by heating to 300K, atomic force microscopy reveals Ag particle formation. Their lateral dimensions are between 5 and 20nm and, in many cases, their heights exceed the thickness of the original water layer. Fewer, higher and more regular Ag particles are formed in the presence, as compared to the absence, of ice. This is discussed in terms of two factors – Ag atoms reaching HfO₂ are thermally colder when they arrive from the ice matrix and desorption of water involves formation of liquid droplets, a process that concentrates Ag into the volumes occupied by the water droplets.     

Magnetism of gadolinium nanoparticles near <Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>

Influence of temperature and magnetic field H on magnetism of spherical Gd nanoparticles of different sizes (89, 63, 47, 28, and 18 nm) was studied in the temperature range 250 K < T < 325 K. The particles were obtained by metal vapor condensation in the flow of helium. The particles with d = 18 nm did not show a magnetic transition; their structure is a combination of two cubic phases (FCC1 and FCC2). Large particles remained in the HCP phase and had an admixture of the FCC1 phase, the amount of which decreased as the particle sizes increased; magnetic transition took place at T _c = 293 K. The admixture of O₂ did not alter the structure but decreased the magnetization σ and magnetic permeability μ. An orientation transition in polycrystalline gadolinium initiated by the magnetic field H was proved in an experiment. The orientation transition in Gd particles smaller than 63 nm, the magnetic structure of which is close to the single-domain structure, occurred near T _c without the influence of H.     

Catalytically active gold clusters and nanoparticles for CO oxidation

Electronic states of gold nanoparticles in mordenite and their transformations under redox treatments have been studied by the methods of FTIR spectroscopy of adsorbed CO and diffuse reflectance UV-visible spectroscopy. Different states of ionic and metallic gold were detected in the zeolite channels and on the external surface of the zeolite - Au⁺ and Au³⁺ ions, charged clusters , and neutral nanoparticles Au_m. Catalytic tests of the samples revealed the existence of two types of active sites of gold in CO oxidation - gold clusters <2 nm (low-temperature activity) and gold nanoparticles (high temperature activity).