Structure, magnetic, and magnetocaloric properties of amorphous and crystalline La0.4Ca0.6MnO3+δ nanoparticles

We report a systematic study of the effects of size reduction on the magnetic and magnetocaloric properties of amorphous and crystalline La_0.4Ca_0.6MnO_3+δ nanoparticles. The materials were synthesized using a modified wet chemical Pechini route, starting with nitrate precursors to produce the perovskite structure. Phase purity, structure, size, and crystallinity were investigated using XRD and TEM. Thermal treatments resulted in nanocrystals with average diameters of 25 nm, 50 nm, and 130 nm, as well as amorphous particles ∼10 nm in diameter. Magnetic measurements revealed broad, second order ferromagnetic transitions in the nanocrystals. As particle size increased from 10 nm to 130 nm, the Curie temperature shifted from 40 K to 255 K. Magnetization, magnetic entropy change (ΔS_M), and refrigerant capacity (RC) also increased with size in the nanocrystalline samples. For a field change of 5 T, the 130 nm particles exhibit a magnetic entropy change of 2.8 J/kg K and a large refrigerant capacity of ∼240 J/kg at 250 K. Interestingly, the 10 nm amorphous particles undergo the sharpest magnetic transition, leading to a larger value of ΔS_M than in the 25 nm or 50 nm crystalline particles. These results reveal that size reduction has a significant impact on the magnetic and magnetocaloric properties of La_0.4Ca_0.6MnO_3+δ.     

Synthesis and optical studies of gamma irradiated Eu doped nanocrystalline CaF2

Europium (Eu) doped Calcium fluoride (CaF₂) nanoparticles are synthesized by co-precipitation method and characterized by powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Also, optical absorption and photoluminescence (PL) results on gamma irradiated Eu doped CaF₂ nanoparticles are presented. The PXRD patterns confirmed the cubic crystallinity of the samples and the particle size is found to be ∼25 nm. The purity of the synthesized nanoparticles is confirmed from FTIR spectrum. The morphological features studied using SEM revealed that nanoparticles are agglomerated and porous. Optical absorption spectrum shows that γ-rayed Eu doped CaF₂ nanoparticles exhibit absorption bands at ∼279, 360 and 434 nm. The PL studies showed emission at ∼420, 525 and 552 nm.     

Deposition and super liquid repellency of fluorinated ZnO nanoparticles on carbon fabrics

The liquid-repellent behavior of fluorinated zinc oxide (ZnO) nanoparticles deposited onto carbon fabric (CF) by a pulse microwave-assisted (MA) method followed by surface fluorination treatment was investigated. The MA process is performed at 80 °C within 10 min with different pH values of 5.5, 8 and 12. The hexagonal ZnO nanoparticles with an average size of 100 nm exhibit a well-defined wurtzite crystal structure without any heat treatment. The ZnO nanoparticles produced by MA synthesis at pH = 8 display the maximal density over CF substrate. The fluorination coating effectively imparts super water and oil repellencies on the ZnO–CF surface; i.e., the contact angles are 163° (water) and 153° (ethylene glycol, EG). The liquid repellencies toward water and EG droplets show an increasing function of surface density of ZnO nanoparticles. This result can be attributed to the fact that an air layer is confined in the nanoparticles, thereby inducing a rougher gas–vapor–solid contact line, referred to as the Cassie state. Based on the Young–Duprè equation incorporated with the Cassie parameter, the lowest work of adhesion (W_ad) values of the ZnO–CF surface for water and EG repellencies are estimated to be 3.16 and 4.93 mJ/m², respectively. Accordingly, this work sheds some light on the creation of a two-tier texture by an efficient MA route and on how the surface density of ZnO nanoparticles strongly affects the repellent behavior of the resultant ZnO–CF composites.     

Tunable deep-level emission in ZnO nanoparticles via yttrium doping

Yttrium-doped ZnO nanoparticles (Zn_1−xY_xO, x = 0, 0.03, 0.05) were synthesized by sol-gel technique. The effects of yttrium doping concentration on the structures, morphologies and optical properties of as-synthesized Zn_1−xY_xO nanoparticles were investigated in detail. The results from structural characterizations clearly demonstrated that yttrium ions were successfully doped into the crystal lattice of ZnO matrix. Besides a UV emission centered at ∼383 nm, the PL spectra of all the samples exhibited a broad deep-level emission, which can be deconvoluted into two Gauss peaks centered at 539 nm (P1) and 598 nm (P2), respectively. As the concentration of Y doping increased from 0% to 5%, the peak position with maximum intensity in deep-level emission band was gradually tuned from 539 nm to 598 nm and the relative intensity ratio of I_P1/I_P2 also decreased step by step, which revealed a unique optical property of yttrium-doped ZnO nanoparticles.     

Particle size effect on the magnetic properties of NiO nanoparticles prepared by a precipitation method

Nickel oxide (NiO) nanoparticles of nominal size range 16 nm and 25 nm were obtained by controlling the calcination temperature. These particles were prepared by the precipitation method. Structural, optical and morphological characterizations were done by X-ray powder diffraction, UV-vis diffuse reflectance spectroscopy and scanning electron microscopy. Studies of magnetic measurements (up to 60 kOe) and temperature variations from 2 K to 300 K of the NiO nanoparticles were investigated. Particles of 16 nm exhibited a weak ferromagnetic component and hysteresis loop. There is a increase in coercivity H_c and the remanence M_r at 8 K accompanied by an exchange bias H_E. H_E monotonically tends to zero as the particles size varied from 16 nm to 25 nm. The hysteresis loop and the size dependent χ are interpreted with the uncompensated surface spins, whereas the transition at 30 K is suggested to be Néel temperature T_N of the spins in the core of the 16 nm particles. In addition, the increasing temperature cannot showed an approach to saturation in the magnetization curve, it indicates the possibility of an asperomagnetism and/or spin glass behavior of the NiO nanoparticles.     

One-step synthesis of MFe2O4 (M = Fe, Co) hollow spheres by template-free solvothermal method

Monodispersed magnetic MFe₂O₄ (M = Fe, Co) hollow spheres were synthesized by simple template free solvothermal method in ethylene glycol (EG) solution. The hollow spheres were in the same size with an average diameter of about 360 nm and the shells of these spheres were about 80 nm, consisted of closely packed nanocrystallines due to Ostwald ripening. EG plays the key role in the synthesis of hollow spheres in contrast with octahedral crystals synthesized in aqueous solution. The products synthesized in aqueous solution were calcined at 800 °C and 1000 °C. The amount of spinel ferrite products increased monotonically with the increase of temperature and appeared as a single phase at 1000 °C. The saturation magnetization (M_s), remanent magnetization (M_r) and coercivity (H_c) for Fe₃O₄ hollow spheres was 74.47 emu/g, 2.59 emu/g and 32.503 Oe respectively whereas the reading of the same indicators for CoFe₂O₄ hollow spheres was 69.07 emu/g, 14.46 emu/g and 242.79 Oe, respectively. The magnetic variation between Fe₃O₄ and CoFe₂O₄ hollow spheres was caused by the radius difference of Fe²⁺ (3d⁶) and Co²⁺ (3d⁷) ions and it was also relevant with nanocrystal sizes of the spin disorder of crystal surface.     

Synthesis and magnetic properties of nickel nanoparticles deposited on the silicon nanowires

Nickel (Ni) nanoparticles with sizes of ∼35 nm were deposited on the surface of silicon nanowires (SiNWs) by electroless plating technique. The magnetic properties of Ni/SiNWs were investigated. The blocking temperature (T_B) of 370 K was obtained and confirmed by field-cooled (FC) and zero-field-cooled (ZFC) plots. The M-H hysteresis loops from 5 K to 400 K were measured. The saturation magnetization value was ∼4.5 emu/g and the coercivity was ∼375.3 Oe for the loop at 5 K, respectively. While for the loop at 400 K, these values were of ∼2.6 emu/g and ∼33.3 Oe, respectively. The temperature dependence of coercivity followed by the relation H_C(T) = H_C0[1 − (T/T_B)^1/2], indicating a superparamagnetic behavior. The magnetization of superparamagnetic grains in a magnetic field H was better described by Langevin function at 400 K. These novel magnetic properties of Ni/SiNWs were possibly attributed to the paramagnetic defects on the surface of SiNWs.     

Water-soluble Yb, Tm codoped NaYF4 nanoparticles: Synthesis, characteristics and bioimaging

Water soluble NaYF₄ nanocrystals codoped with 20 mol% Yb³⁺, 0.5 mol% Tm³⁺ were prepared by a facile solvothermal approach using polyvinylpyrrolidone (PVP) as a surfactant. The upconversion NaYF₄ nanocrystals were pure cubic phase with an average size of ∼40 nm. They could be well redispersed in water to form a clearly transparent solution without obvious precipitation. With the excitation of a 980-nm diode laser, the nanocrystal solution presents bright violet and blue upconversion luminescence. These upconversion nanoparticles (UCNPs) were incubated with HeLa cells at 37 °C for 24 h, and bright blue upconversion luminescence were observed from the UCNPs endocytosed into the HeLa cells on a microscope equipped with a 980-nm fiber laser. These results indicated that the UCNPs had potential applications for biological imaging as luminescent probes.     

Internal structure of clusters of partially coherent nanocrystallites in Cr-Al-N and Cr-Al-Si-N coatings

Nano-sized clusters consisting of strongly preferentially oriented, partially coherent nanocrystallites were observed in Cr-Al-N and Cr-Al-Si-N coatings deposited using cathodic arc evaporation. Microstructure analysis of the coatings, which was done using the combination of X-ray diffraction (XRD) and transmission electron microscopy with high resolution (HRTEM), revealed furthermore stress-free lattice parameters, size and local disorientation of crystallites within the nano-sized clusters in dependence on the aluminium and silicon contents, mean size of these clusters and the kind of structure defects. Within the face-centred cubic (fcc) Cr_{1 − x − y}Al_xSi_yN phase, the stress-free lattice parameter was described by the equation a = (0.41486 − 0.00827 · x + 0.034 · y) nm. The size of individual crystallites decreased from ∼ 11 nm in Cr_0.92Al_0.08N to ∼ 4 nm in Cr_0.24Al_0.65Si_0.10N. These nanocrystallites formed clusters with the mean size between 36 and 56 nm. The mutual disorientation of the partially coherent nanocrystallites forming the clusters increased with increasing aluminium and silicon contents from 0.5° to several degrees. The disorientation of neighbouring nanocrystallites was explained by the presence of screw dislocations and by presence of phase interfaces in coatings containing a single fcc phase and several phases, respectively.     

Microstructure, mechanical, EPR and optical properties of lithium disilicate glasses and glass-ceramics doped with Mn ions

The microstructure, mechanical, EPR and optical properties of transparent MnO₂ doped lithium disilicate (LDS) glass-ceramics prepared by melt quenching and controlled crystallization, have been studied. The microstructure of the glass-ceramics has been characterized using FE-SEM, TEM, FT-IR and XRD techniques. FE-SEM micrographs show elongated, highly interlocked, dense (∼80 vol.%) nanocrystals of LDS with an average size ∼100 nm. XRD and FT-IR studies reveal that the only crystalline phase formed after heat-treatment at 700 °C for 1 h is LDS. A good combination of average microhardness ∼5.6 GPa, high fracture toughness ∼2.8 MPa m^1/2, 3-point flexural strength ∼250 MPa and moderate elastic modulus 65 GPa has been obtained. The EPR spectra of both LDS glasses and glass-ceramics exhibit resonance signals with effective g values at g = 4.73, g = 4.10, g = 3.3, and g = 1.98. The resonance signal at g = 1.98 is found to be more intense than the other signals and exhibits hyperfine structure at lower concentration of manganese. From the observed spectrum, the spin-Hamiltonian parameters have been evaluated. In glass samples the optical absorption spectrum exhibits a broad band around ∼20,320 cm⁻¹ which has been assigned to the transition ⁶A_1g(S) → ⁴A_1g(G) ⁴E_g(G)-of Mn²⁺ ions. The cerammed samples upon 394 nm excitation emit a green luminescence (565 nm, ⁴T_1g → ⁶A_1g(G) transition of Mn²⁺ ions), and a weak red emission (710 nm). From the ultraviolet absorption edges, the optical bandgap energies (E_opt) were evaluated and are discussed.     

Magnetic properties of mechanochemically synthesized γ-Fe2O3 nanoparticles

The synthesis of mono-dispersed γ-Fe₂O₃ nanoparticles by mechanochemical processing was demonstrated for the first time, via the solid-state exchange reaction Fe₂(SO₄)₃ + 3Na₂CO₃ → Fe₂(CO₃)₃ + 3Na₂SO₄ → Fe₂O₃ + 3Na₂SO₄ + 3CO₂(g) and subsequent heat treatment at 673 K. The nanoparticles had a volume-weighted mean diameter of 6 nm and a narrow size distribution with the standard deviation of 3 nm. The particles showed a superparamagnetic nature with the superparamagnetic blocking temperature of 56.6 K. The anisotropy constant was 6.0 × 10⁶ erg/cm³, two orders of magnitude larger than the magnetocrystalline anisotropy constant of bulk γ-Fe₂O₃. The detailed analysis of the magnetic properties indicated that the γ-Fe₂O₃ nanoparticles had a core-shell structure, consisting of a ferrimagnetic core of ∼4 nm in diameter having a collinear spin configuration and a magnetically disordered shell of ∼1.2 nm in thickness.     

Kinetics of picosecond laser treatment of silver nanoparticles on ITO substrate

In this work, the effect of ITO substrate on Ag NP size, morphology and photoinduced absorption depending on the time of 30 ps laser treatment were explicitly studied. Silver nanoparticles with an average diameter of ∼40 nm supported on indium tin oxide (ITO) were irradiated with a tightly focused pulsed laser (doubled frequency beam) at 532 nm. The size transformation of silver nanoparticles induced by a single pulse of Nd:YAG laser (λ = 532 nm, pulse width = 30 ps) was directly observed by an electron scanning microscopy (FE-SEM) on indium tin oxide surface. Simultaneously the change in the absorption and the corresponding derivatives are also presented. These morphological changes are accompanied by a significant change in the optical absorption properties of the array. This study demonstrates that picosecond laser irradiation is an excellent technique to operate and control the properties of nanostructured materials on solid supports.     

Structural and optical characterization of BaSnO3 nanopowder synthesized through a novel combustion technique

Nanocrystalline Barium stannate (BaSnO₃) was synthesized through auto-ignited combustion technique. The X-ray diffraction studies of BaSnO₃ nanoparticles reveals that the nanopowder is single phase, crystalline, and has a cubic perovskite structure with a lattice constant a = 4.115 Å. The average particle size calculated from full width half maximum (FWHM) using Scherer formula is ∼25 nm. The phase purity of the powder was further examined using Fourier transform infrared spectroscopy, Raman spectroscopy and transmission electron microscopic techniques. XRD pattern of BaSnO₃ was refined for atomic coordinates, lattice parameters and occupancies using Rietveld analysis. Vibrational analysis of sample shows that there is a phase transition from distorted cubic to ideal cubic structure during heat treatment. The thermal stability of BaSnO₃ nanopowder has been confirmed using thermo gravimetric analysis (TGA) and differential thermal analysis (DTA). The particle size of the as-prepared powder from transmission electron microscopy was found to be in the range 20-30 nm. The absorption spectra and photoluminescence spectra of the sample were also studied. The band gap determined was 2.887 eV and found to be a semiconductor.     

Cu and CuO nanoparticles immobilized by silica thin films as antibacterial materials and photocatalysts

CuO nanoparticles with average diameter of about 20 nm were accumulated on surface of sol-gel silica thin films heat treated at 300 °C in air. Heat treatment of the CuO nanoparticles at 600 °C in a reducing environment resulted in effective reduction of the nanoparticles and penetration of them into the film. While the thin films heat treated at 300 °C exhibited a strong antibacterial activity against Escherichia coli bacteria, the reducing process decreased their antibacterial activity. However, by definition of normalized antibacterial activity (antibacterial activity/surface concentration of coppers) it was found that Cu nanoparticles were more toxic to the bacteria than the CuO nanoparticles (by a factor of ∼ 2.1). Thus, the lower antibacterial activity of the reduced thin films was assigned to diffusion of the initially accumulated copper-based nanoparticles into the film. The CuO nanoparticles also exhibited a slight photocatalytic activity for inactivation of the bacteria (∼ 22% improvement in their antibacterial activity). Instead, the normalized antibacterial activity of the Cu nanoparticles covered by a thin oxide layer highly increased (∼ 63% improvement) in the photocatalytic process. A mechanism was also proposed to describe the better antibacterial activity of the Cu than CuO nanoparticles in dark and under light irradiation.     

Spectroscopic and dielectric studies of meso-tetrakis(p-sulfonatophenyl) porphyrin doped hybrid borate glasses

Hybrid borate glasses containing different concentrations of meso-tetrakis(p-sulfonatophenyl) porphyrin sodium salt (TPPS₄) were prepared. The obtained glass samples were found to be transparent and homogeneous. Formation of TPPS₄-J-aggregates in borate glass was investigated by means of optical absorption, steady state and time resolved fluorescence spectroscopy. The hybrid glasses exhibit a strong S₂→S₀ emission at ∼473 nm and J-aggregates show emission at ∼733 nm. Time resolved fluorescence show two exponential decay with lifetime of τ₁ = 65 ± 10 ps (∼80%) and τ₂ = 3.87 ± 0.1 ns (∼20%) respectively. Dielectric properties such as dielectric constant (?′), dielectric loss (tan δ) and ac conductivity (σ_ac) over a range of frequency and temperature of these glasses were studied. The ac conductivity was found to be proportional to ω^s (where s < 1). The observed change in dielectric parameters due to different concentrations of TPPS₄ has been analyzed in light of different polarization mechanisms.     

Nanocrystals of a new complex perovskite dielectric Ba2TmSbO6

Nanocrystals of a new complex perovskites ceramic oxide, barium thulium antimony oxide - Ba₂TmSbO₆, were synthesized using a single step auto-ignition combustion process. The combustion product was single phase and composed of aggregates of nanocrystals of sizes in the range 20-50 nm. Ba₂TmSbO₆ crystallized in cubic perovskite structure with lattice parameter, a = 8.4101 Å. The polycrystalline fluffy combustion product was sintered to high density (∼97%) at ∼1450 °C for 4 h. Resistivity of the sintered specimen was ∼5 MΩ/cm. The Ba₂TmSbO₆ has dielectric constant (?′) and dielectric loss (tan δ) of 17 and ∼10⁻⁴ at 5 MHz; the new material would probably be developed as a low-loss dielectric material.     

Smart magnetodielectric nano-materials for the very high frequency applications

Cobalt and copper doped Ni-Zn nano-ferrite with a composition of Ni_0.4Zn_0.4Co_0.2Cu_0.02Fe_1.98O₄ are prepared by a coprecipitation method. The structural, electromagnetic and magnetic properties are investigated by means of X-ray diffraction, impedance analyzer and vibrating sample magnetometer, respectively. Samples are calcinated at 600 °C and then subjected to different sintering temperatures. After sintering at 900 °C for 5 h, the average crystalline size is found to be 38 nm. The material shows almost constant permeability and permittivity, in the frequency range from 10 to 200 MHz, equal to ∼10.8 (loss tangent ∼ 0.04) and ∼6.5 (loss tangent ∼ 0.006), respectively. Relaxation phenomenon takes place beyond 200 MHz. The refractive index n is close to 8.3, and the reduced impedance Z/Z₀ is close to 1.3. The persistent and higher value of permeability than that of permittivity along with low losses enables this material useful for the very high frequency applications.     

In situ synthesis of nanocrystalline intermetallic layer during surface plastic deformation of zirconium

By means of a surface plastic deformation method a nanocrystalline (NC) intermetallic compound was in situ synthesized on the surface layer of bulk zirconium (Zr). Hardened steel shots (composition: 1.0C, 1.5Cr, base Fe in wt.%) were used to conduct repetitive and multidirectional peening on the surface layer of Zr. The microstructure evolution of the surface layer was investigated by X-ray diffraction and scanning and transmission electron microscopy observations. The NC intermetallic layer of about 25 μm thick was observed and confirmed by concentration profiles of Zr, Fe and Cr, and was found to consist of the Fe_100 − xCr_x compound with an average grain size of 22 nm. The NC surface layer exhibited an extremely high average hardness of 10.2 GPa. The Zr base immediately next to the compound/Zr interface has a grain size of ∼ 250 nm, and a hardness of ∼ 3.4 GPa. The Fe_100 − xCr_x layer was found to securely adhere to the Zr base.     

Corrosion inhibition during synthesis of Cu2O nanoparticles by 1,3-diaminopropylene in solution

Corrosion inhibition for Cu₂O nanoparticles in solution using the regulators of 1,3-dimorpholinpropylene and 1,3-diethylaminopropylene was studied by experimental and theoretical calculation methods. The inhibition mechanism of regulators was related to the arrangement of cetyltrimethylammonium (CTAB) molecules on the surface of Cu₂O nanoparticles. Control of the type and amount of regulators has been demonstrated to produce well-dispersed and active Cu₂O nanoparticles (∼100 nm). The oxidation temperature of Cu₂O nanoparticles decreased from 297.9-404.4 °C (blank) to 232.3-334.3 °C (containing regulator). For 1,3-dimorpholinpropylene, its arc structure and active sites (C_4C_5, N_2 and O_2) facilitate the formation of stable protective film over Cu₂O surface.     

Impedance spectroscopy and dielectric properties of Ce and La substituted Pb0.7Sr0.3(Fe0.012Ti0.988)O3 nanoparticles

We present the structural, microstructural, dielectric and impedance behavior of Pb_0.7Sr_0.3[(Fe_2/3Ce_1/3)_0.012Ti_0.988]O₃ (PSFCT) and Pb_0.7Sr_0.3[(Fe_2/3La_1/3)_0.012Ti_0.988]O₃ (PSFLT) nanoparticles. These nanoparticles were prepared by a chemical synthesis route using polyvinyl alcohol as surfactant. The X-ray diffraction pattern shows polycrystalline nature with coexistence of tetragonal and cubic phase in both PSFCT and PSFLT nanoparticles. The average particle size has been measured using Scherer's relation. The average particle sizes also measured by TEM are 10 and 11 nm, and by SEM 9 and 12 nm, respectively, of PSFCT and PSFLT nanoparticles. By measuring the value of relative permittivity (?′) and loss (tan δ) at lower frequency, the dielectric properties show Maxwell-Wagner type interfacial polarization. However, due to nano size effect of PSFCT and PSFLT, dispersionless dielectric response has been observed up to higher frequency of 15 MHz. The frequency dependent real (Z′) and imaginary (Z″) parts of impedance confirmed the variation which was observed in dielectric properties. The values of resistance of grain boundaries, R_gb is higher than grains, R_g indicates that the effect of grain boundaries is dominant on electrical properties when the size of nanoparticles is quite small.