Nanocrystals of thermodynamically stable alpha-MnAs (hexagonal NiAs-type) and metastable beta-MnAs (orthorhombic MnP-type) have been synthesized by the reaction of triphenylarsine oxide (Ph(3)AsO) and dimanganesedecacarbonyl (Mn(2)CO(10)) at temperatures ranging from 250 to 330 degrees C in the presence of the coordinating solvent trioctylphosphine oxide (TOPO). Morphologically, both alpha- and beta-MnAs nanoparticles adopt a core-shell type structure with a crystalline core and low-contrast noncrystalline shell. In contrast to prior studies on MnAs particles, disks, and films, the present bottom-up synthesis yields discrete, dispersible MnAs nanoparticles without a structural support. Even in the absence of epitaxial strain, the lattice parameters of the nanocrystals are decreased relative to bulk MnAs, resulting in a volume decrease of 0.35% in alpha-MnAs and 0.38% in beta-MnAs nanoparticles. In contrast to bulk MnAs, where the ferromagnetic phase transition upon warming through 313-317 K is concomitant with a structure change from ferromagnetic alpha- to paramagnetic beta-MnAs, powder X-ray diffraction studies suggest there is no conversion of alpha-MnAs to beta over the temperature range 298-343 K. Moreover, magnetic measurements suggest that both alpha- and beta-MnAs are ferromagnetic with T(C) approximately 315 K. Partial phase transformation of beta-MnAs nanoparticles into thermodynamically stable alpha-MnAs occurs slowly over time (i.e., months) at room temperature. However, there is no associated change in magnetization, suggesting the ferromagnetism observed in beta-MnAs is intrinsic and cannot be attributed to alpha-MnAs impurities.

Excellent quality superparamagnetic iron oxide coated iron nanoparticles were prepared via one pot synthesis with narrow size distribution. These particles were subsequently coated with Fe x O y , FePt or Pt shell to give long term protection to the highly reactive zero-valent iron core. Clear core-shell morphology was observed in the TEM images of Fe x O y and FePt coated particles and the magnetic properties suggested that these shells provided good protection to the core from oxidation, in particular iron oxide coated particles retained their magnetism for well over a year. FePt coated particles show superior magnetic properties to FePt particles alone and were found to be promising in biological applications. The magnetic studies further revealed that the saturation moments of the resultant particles were dependent on the type of shell used for encapsulation. Surface modifications of these particles were done with suitable hydrophilic ligands and they showed excellent stability and solubility in water and different buffer solutions which reveals the suitability of these nanoparticles for biomedical research.

III-V based diluted magnetic semiconductor (DMS) nanoparticles of In (1-x) Mn x P (x e 0.0135) have been prepared by slow heating of the reagents in trioctylphosphine oxide (TOPO) or by high-temperature injection of reagents dissolved in trioctylphosphine (TOP) into hot TOPO. The materials were prepared using either Mn(II) or Mn(III) salts as dopants and the resulting nanoparticles have diameters ranging from 2.95 ( 0.39 to 4.77 ( 0.73 nm, as determined from transmission electron micrographs. Chemical analysis of surface-exchanged samples revealed the incorporation of Mn into the crystal lattice with up to 6 Mn atoms per 3.4-nm diameter particle, or the equivalence of ∼10 20 Mn atoms/cm 3 in a zinc blende bulk lattice. The InP:Mn nanoparticles exhibited a red shift in the room-temperature photoluminescence of 0.02-0.03 eV relative to that for pure InP nanoparticles. Electron paramagnetic resonance studies suggest that the Mn atoms mostly reside near the surface and are Mn 2+ , regardless of the oxidation state of the precursor. The magnetic susceptibility of surface-exchanged nanoparticles doped with Mn(III) exhibited a paramagnetic behavior with a magnetic moment of 5.9 µ B /Mn atom, consistent with 5 unpaired spins (S ) 5/2 state). The successful incorporation of isovalent Mn to produce Mn 2+ with a corresponding hole may represent a valuable strategy for production of ferromagnetic DMS nanoparticles based on arsenide systems, where the hole is coupled to the metal center and delocalized through the pnictide framework.

Removal of CuI from Cu(2)P(1.8)As(1.2)I(2) results in a novel P--As allotrope. Although the product is X-ray amorphous, lattice fringes are observed in the transmission electron micrographs with spacings reflecting the diameter of the linear pnicogen polymer in Cu(2)P(1.8)As(1.2)I(2), suggesting the pnicogen chains remain intact upon loss of CuI. The straight needles present in Cu(2)P(1.8)As(1.2)I(2) appear to be kinked in the P--As phase due to lattice mismatch between the liberated polymers, ultrasonic treatment, or a combination of these effects. This new P--As modification is semiconducting with a band gap of 1.05 eV.