Temperature dependences of the magnetic susceptibility and heat capacity C p of CdSe quantum dots with size d = 2.8, 4.1, and 5.6 nm are compared to those of bulk CdSe to determine the size-dependent effects. With decreasing size d, the following effects are observed: ͑i͒ room temperature optical absorption shows a blueshift of the band gap; ͑ii͒ room temperature x-ray diffraction show wurtzite structure but with smaller lattice constants; ͑iii͒ magnetic susceptibility changes from negative ͑diamagnetic͒ for the bulk to positive with magnitude increasing with decreasing d; and ͑iv͒ the Sommerfeld constant ␥ determined from the C p / T vs T 2 data increases. Possible explanations for these size-dependent properties are presented.

No abstract

The strategy for shape control of alloy FePt nanocrystal was studied systematically. By the careful adjustments of reaction parameters in a solution reaction, surfactant-facet bindings on the growth seed were controlled delicately. FePt octapod, cuboctahedron, truncated cube, and nanocube were successfully prepared from cuboctahedral seed and examined by high-resolution transmission electron microscopy. The formations of FePt nanostructures were mainly attributed to the differences in the growth rate between the {111} and {100} planes of cuboctahedral seeds. The magnetic measurements showed that the order of volume, V (nanocube) >V (octapod) >V (cuboctahedron) obviously reflected the order of saturated magnetization (M s ), M s (nanocube) >M s (octapod) >M s (cuboctahedron) . Furthermore, the measurements of octapod exhibited the highest coercivity and blocking temperature because of its higher surface to volume ratio and more structural facets.

No abstract

For nominal 3 and 9 nm FePt nanoparticles coated with oleylamine/oleic acid and having a face-centred-cubic (fcc) structure, temperature variations (5–300 K) of magnetization M, ac susceptibility χ′ and χ″ for the frequency range fm = 0.1–1000 Hz and electron magnetic resonance (EMR) spectra at 9.28 GHz are reported. X-ray diffraction of the samples shows fcc structure with a lattice constant a = 3.84 Å and TEM characterization yields log-normal distributions of the particle sizes with average D = 3.15(0.16) nm and D = 8.70(0.12) nm for the 3 nm and 9 nm samples, respectively. M versus T data for the zero-field-cooled and field-cooled modes yield a blocking temperature TB = 15 K (85 K) for the 3 nm (9 nm) samples whereas the hysteresis loops at 5 K yield a coercivity Hc = 0 Oe (1.4 kOe). Analysis of the data of TB at different fm determined from the peaks in χ″ in ac susceptibility and the temperature variation of the EMR spectra are used to determine the following parameters of the Vogel–Fulcher relaxation for the 3 nm (9 nm) samples respectively: the attempt frequency fo = 8 × 1010 Hz (2 × 1012 Hz); inter-particle interaction temperature To = 3 K (33 K) and anisotropy Ka = 1.96 × 106 ergs cm−3 (4.3 × 105 ergs cm−3). The use of the above parameters for the calculations of the optimum size for magnetic hyperthermia is analysed and discussed.

The potential of exploiting the spin of the electron (in addition to its charge) in novel new electronic devices and the associated opportunities for new science has led to extensive search for viable magnetic semiconductors with room temperature ferromagnetism (RTFM). [1][2][3] Some success for ferromagnetism has been reported in dilute magnetic semiconductors (DMS) and dilute magnetic oxides (DMO) containing a few percent of transition metal ions such as (Ga,Mn)As, [4,5] (Zn,M)O [6][7][8][9] , and (Ti,M)O 2 [10][11][12] with M ¼ V,Cr, Mn, Co, Ni, and Cu. Despite the numerous reports on the successful observation of room temperature ferromagnetism (RTFM) in a number of these systems in apparent agreement with computations based on density functional theory, the issue remains unsettled and contentious for a number of reasons including lingering doubt of the possible role of undetected ferromagnetic impurities such as Fe, Co, and Ni. In some systems such as (Zn,Ni)O [13] and (Zn,Cr)Te [14] the observed RTFM has been linked to clustering of Ni and Cr nanocrystals, respectively. In this paper, we report the observation of RTFM in CdSe quantum dots (QD) capped with TOPO (tri-n-octylphosphine oxide). This RTFM is labeled as ex-nihilo since the RTFM is due to the marriage of two diamagnetic materials viz. CdSe and TOPO, possibly resulting from charge transfer from Cd d-band to the oxygen atoms of TOPO. We further show that the RTFM varies inversely with the size of the QD, in agreement with our calculations.CdSe is a II-VI semiconductor with direct band gap E g ¼ 1.74 eV at 300 K. Semiconductors nanocrystals (NCs) such as CdSe are often called quantum dots (QD) when the size of the first Bohr radius exceeds the crystallite size D, leading to quantum confinement effects such as increase in the band-gap E g with decrease in D. For the CdSe NCs, E g can be tuned to cover the whole visible range by change in D, thus making this system potentially useful for solar energy applications. [15,16] Attempts to dope CdSe with Mn to obtain a magnetic semiconductor have not been successful partly because of the limited solubility of Mn in CdSe. However, Magana et al. have reported superparamagnetism with a blocking temperature T B ' 40 K in thermally annealed Mn/CdSe QD's. [17] As the size of the nanocrystals is decreased, the fraction of atoms on the surface to the total number of atoms in NC increases as 1/D. Since the atoms on the surface experience broken symmetry and often higher anisotropy, new surface states are formed. For the CdSe QDs, a number of interesting optical properties such as band edge luminescence [18] and excitonic radiative decay [19] have been related to the surface states. Since CdSe NCs are often passivated by capping with TOPO (tri-n-octylphosphine oxide) to avoid aggregation and surface oxidation, it is important to understand the effect of TOPO on the surface atoms and on the measured properties. XPS [15] and EXAFS [20] In CdSe QDs, we report here the observation of size-dependent RTFM whose strength, wi...

Iron disilicide in a bulk form is practically nonmagnetic. In contrast, nanoparticles of FeSi 2 exhibit superparamagnetism with blocking temperatures ranging from 8 K ͑15 nm͒ to 34 K ͑55 nm͒. Their relatively low saturation magnetization suggests that the magnetic behavior is associated with only a small fraction of Fe ions, which have a sufficient number of other Fe as nearest neighbors. The chemical disorder is presumably induced in the formation of nanoparticles. A spin glass-type anomaly below 10 K observed in specific heat data gives a further evidence for the compositional heterogeneity.

No abstract