Size-induced exchange bias in single-phase CoO nanoparticles.

The tuning of exchange bias (EB) in nanoparticles has garnered significant attention due to its diverse range of applications. Here, we demonstrate EB in single-phase CoO nanoparticles, where two magnetic phases naturally emerge as the crystallite size decreases from 34.6 ± 0.8 to 10.8 ± 0.9 nm. The Néel temperature (TN) associated with antiferromagnetic ordering decreases monotonically with the reduction in crystallite size, highlighting the significant influence of size effects. The 34.6 nm nanoparticles exhibit magnetization irreversibility between zero-field cooled (ZFC) and field-cooled (FC) states belowTN. With further reduction in size this irreversibility appears well aboveTN, resulting in the absence of true paramagnetic regime which indicates the occurnace of an additional magnetic phase. The frequency-dependent ac-susceptibility in 10.8 nm nanoparticles suggests slow dynamics of disordered surface spins aboveTN, coinciding with the establishment of long-range order in the core. The thermoremanent magnetization (TRM) and iso-thermoremanent magnetization (IRM) curves suggest a core-shell structure: the core is antiferromagnetic, and the shell consists of disordered surface spins causing ferromagnetic interaction. Hence, the EB in these CoO nanoparticles results from the exchange coupling between an antiferromagnetic core and a disordered shell that exhibits unconventional surface spin characteristics.

Structural diversity, functional versatility and applications in industrial, environmental and biomedical sciences of polysaccharides and its derivatives - A review.

Recent efforts on the expansion of sustainable and commercial primal matters are essential to enhance the knowledge of their hazards and noxiousness to humans and their environments. For example, polysaccharide materials are widely utilized in food, wound dressing, tissue engineering, industry, targeted drug delivery, environmental, and bioremediation due to their attractive degradability, nontoxicity and biocompatibility. There are numerous easy, quick, and efficient ways to manufacture these materials that include cellulose, starch, chitosan, chitin, dextran, pectin, gums, and pullulan. Further, they exhibit distinctive properties when combined favourably with raw materials from other sources. This review discusses the synthesis and novel applications of these carbohydrate polymers in industrial, environmental and biomedical sciences.

Metal to insulator transition, colossal Seebeck coefficient and large violation of Wiedemann-Franz law in nanoscale granular nickel.

We report on the electrical and thermal transport properties of nickel nanoparticles with crystallite size from 23.1 ± 0.3 to 1.3 ± 0.3 nm. These nanoparticles show a systematic metal to insulator transition with the change in the conduction type fromn- to p-type, colossal Seebeck coefficient of 1.87 ± 0.07 mV K-1, and ultralow thermal conductivity of 0.52 ± 0.05 W m-1K-1at 300 K as the crystallite size drops. The electrical resistivity analysis reveals a dramatic change in the electronic excitation spectrum indicating the opening of an energy gap, and cotunneling and Coulomb blockade of the charge carriers. Seebeck coefficient shows transport energy degradation of charge carriers as transport level moves away from the Fermi level with decrease in crystallite size. The Lorenz number rising to about four orders of magnitude in the metallic regimes with decrease in crystallite size, showing a large violation of the Wiedemann-Franz law in these compacted nickel nanoparticles. Such an observation provides the compelling confirmation for unconventional quasiparticle dynamics where the transport of charge and heat is independent of each other. Therefore, such nanoparticles provide an intriguing platform to tune the charge and heat transport, which may be useful for thermoelectrics and heat dissipation in nanocrystal array-based electronics.

Ultralow Thermal Conductivity and Large Figure of Merit in Low-Cost and Nontoxic Core-Shell Cu@Cu2O Nanocomposites.

Identification of novel materials with enhanced thermoelectric (TE) performance is critical for advancing TE research. In this direction, this is the first report on TE properties of low-cost, nontoxic, and abundant core-shell Cu@Cu2O nanocomposites synthesized using a facile and cheap solution-phase method. They show ultralow thermal conductivity of nearly 10-3 of the copper bulk value, large thermopower of ∼373 µVK-1, and, consequently, a TE figure of merit of 0.16 at 320 K which is larger than those of many of the potential TE materials such as PbTe, SnSe, and SiGe, showing its potential for TE applications. The ultralow thermal conductivity is mainly attributed to the multiscale phonon scattering from intrinsic defects in Cu2O, grain boundaries, lattice-mismatched interface, as well as dissimilar vibrational properties. The large thermopower is associated with a sharp modulation in carrier density of states due to charge transfer between Cu and Cu2O nanoparticles and carrier energy filtering. They are tuned by varying the trioctylphosphine concentration.

Enhanced Thermoelectric Performance of Novel Reaction Condition-Induced Bi2S3-Bi Nanocomposites.

This is the first report on the enhanced thermoelectric (TE) properties of novel reaction-temperature (TRe) and duration-induced Bi2S3-Bi nanocomposites synthesized using a facile one-step polyol method. They are well characterized as nanorod composites of orthorhombic Bi2S3 and rhombohedral Bi phases in which the latter coats the former forming Bi2S3-Bi core-shell-like structures along with independent Bi nanoparticles. A very significant observation is the systematic reduction in electrical resistivity ρ with a whopping 7 orders of magnitude (∼107) with just reaction temperature and duration increase, revealing a promising approach for the reduction of ρ of this highly resistive chalcogenide and hence resolving the earlier obstacles for its thermoelectric application potentials in the past few decades. Most astonishingly, a TE power factor at 300 K of the highest Bi content nanocomposite pellet, made at 27 °C using ∼900 MPa pressure, is 3 orders of magnitude greater than that of hot-pressed Bi2S3. Its highest ZT at 325 K of 0.006 is over twice of that of similarly prepared CuS or Ag2S-based nanocomposites. A significantly improved TE performance potential near 300 K is demonstrated for these toxic-free and rare-earth element-free TE nanocomposites, making the present synthesis method as a pioneering approach for developing enhanced thermoelectric properties of Bi2S3-based materials without extra sintering steps.

The effect of lithium on structural and luminescence performance of tunable light-emitting nanophosphors for white LEDs.

Li+ incorporated tunable Y2O3:Eu3+ red-emitting nanophosphors were synthesized using a wet chemical method. The effect of Li+ on structural and luminescence properties of the nanophosphors were studied in detail. The structural results exhibited that nanophosphors have a body-centered cubic (I) phase with point group symmetry m3̄. No additional impurity peaks were observed within the range of the XRD pattern due to the Li+ ion. FTIR spectra reveal the formation of the pure and crystalline structure of the nanophosphors. TEM results show the prepared nanophosphors were highly crystalline and polycrystalline in nature. PL studies show the highly enhanced emission band due to the flux effect, greatly improved crystallinity caused by the Li+ ion, and the different excitation wavelengths. The most intense luminescence band was observed at 612 nm for red emission ascribed to the 5D0 → 7F2 transition of Eu3+ ion upon 254, 393, and 465 nm excitations in the C 3i and C 2 symmetry site of Y2O3 respectively. The highly enhanced emission band was observed under excitation at 254 nm and is 6.9 and 3.67 times higher than the emission band excited at 393 and 466 nm, respectively. The average lifetime also varies with different concentrations of Li+ ions. The chromaticity color coordinates, CCT values, were tuned in the red region of the color space. Hence, the results indicate that the prepared nanophosphor can be used as a red component to construct the white light for light-emitting diode applications.

Influence of surfactant, particle size and dispersion medium on surface plasmon resonance of silver nanoparticles.

Clear influence of particle size, surfactants and dispersion medium on surface plasmon resonance (SPR) features of Ag nanoparticles (NPs), synthesized in thermal decomposition method, in the broad range of ultraviolet (UV) radiation, critical for many potential applications such as a photocatalyst, UV-sensor and detector, has been demonstrated here. It involves adsorbate coverage, interparticle distance or agglomeration, surface charge density and solvent refractive index (µ). NP agglomeration and surface charge density in solvents of varying µ have been studied systematically through zeta-potential (ζ) and hydrodynamic diameter (HD) using dynamic light scattering (DLS). The main SPR feature found at 316 nm in 31.5 nm NPs shifts to 320 nm in 15.1 nm NPs. The peak at 320 nm in air shifts to 259, 261 and 277 nm in polar solvent methanol, deionized water and ethanol, respectively and to 255, 275 and 282 nm in non-polar solvent n-hexane, benzene and toluene, respectively. In general, the decrease in particle size and increase in µ of solvents show red-shift. Curiously, a number of peaks up to seven in these solvents that are attributed to charge-transfer mechanism and change in inter-particle interaction of the NPs turning from a single peak of SPR in air has been observed for the first time. A model for re-adjustment of Fermi level (E F) of Ag NP and the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to explain them has also been used. Moreover, the Drude model for shift in the position of SPR in these NPs is only applicable in non-polar solvents, not in polar solvents. Such novel features will be potential candidates for various applications.

Luminescence properties of Eu3+ doped CaMoO4 nanoparticles.

When Eu(3+) ions occupy Ca(2+) sites of CaMoO(4), which has a body centered tetragonal structure with inversion symmetry, only the magnetic dipole transition ((5)D(0)→(7)F(1)) should be allowed according to Judd-Ofelt theory. Even if there are a few distortions in the Eu(3+) environment, its intensity should be more than that of the electric dipole transition ((5)D(0)→(7)F(2)). We report here the opposite effect experimentally and ascribe this to the polarizability effect of the MoO(4) tetrahedron, which is neighboring to EuO(8) (symmetric environment). The contribution of the energy transfer process from the Mo-O charge transfer band to Eu(3+) and the role of Eu(3+) over the surface of the particle could be distinguished when luminescence decay processes were measured at two different excitations (250 and 398 nm). Further, the luminescence intensities and lifetimes increase significantly with increasing heat-treatment temperature of the doped samples. This is attributed to the reduction of H(2)O from the surface of the particles and a non-radiative process after heat treatment.