Accurate determination of the spin Hamiltonian parameters for Mn2+ ions in cubic ZnS nanocrystals by multifrequency EPR spectra analysis.

Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagnetic resonance (EPR) spectra of paramagnetic impurity ions in wide band gap semiconductor nanocrystals, is essential for determining their localization and quantum properties. Here we present a procedure, based on publicly available software, for determining with higher accuracy the SH parameters of isolated Mn(2+) impurity ions in small cubic ZnS nanocrystals. The procedure, which can be applied to other cubic II-VI semiconductor nanocrystals as well, is based on the analysis of both low and high frequency EPR spectra with line shape simulation and fitting computing programs, which include the hyperfine forbidden transitions and line broadening effects. The difficulties, limitations and errors which can affect the accuracy in determining some of the SH parameters are also discussed.

Local structure at Mn2+ ions in vacuum annealed small cubic ZnS nanocrystals self-assembled into a mesoporous structure.

A mesoporous structure of self-assembled nanocrystals of cubic ZnS doped with Mn2+ ions with a homogeneous distribution of pores of similar size was synthesized at room temperature by a surfactant-assisted liquid-liquid reaction. The component nanocrystals exhibit a high crystallinity and a tight size distribution centered at 2 nm, as well as the narrowest Electron Paramagnetic Resonance (EPR) spectra linewidth and the best resolution reported so-far, effects attributed to self-assembling. The observed EPR spectra consist of lines from the substitutional Mn2+(I) and surface Mn2+(II) and Mn2+(III) centers. Here we show that, in contrast with previous reports, our EPR spectra are highly sensitive to structural changes during pulse annealing in vacuum up to 500 degrees C. The changes are related to the transformation of the surface Mn2+ centers in new Mn2+ centers, attributed to an oxidation process in which the thermal decomposition of the Tween 20 additive, also observed by EPR, seems to be involved. We have also been able to observe, for the first time by EPR spectroscopy, the formation of the ZnO phase and the nanocrystals size increase, which occur during annealing up to 500 degrees C, structural changes confirmed by XRD and TEM observations on the samples previously investigated by EPR.

Localization of Mn2+ ions in mesoporous NnS.

Nanocrystalline cubic ZnS doped with 0.2% mol manganese, exhibiting a stable mesoporous structure, was synthesized at room temperature by a non toxic surfactant-assisted liquid-liquid reaction. The X-ray diffraction measurements demonstrate the formation of a sponge-like mesoporous material built from cubic ZnS nanocrystals of 1.8 nm average sizes, with a tight distribution of pores of 1.8 nm mean diameter. The transmission electron microscopy images confirm the formation of the mesoporous structure with walls of 3.1 nm mean thickness built from cubic ZnS nanocrystallites of 2.1 nm average size. The resulting tight distribution of crystallites and pores yields a well resolved Electron Paramagnetic Resonance spectrum, with the narrowest reported component lines attributed to three types of isolated Mn2+ centers, called Mn2+(I), Mn2+(II) and Mn2+(III). From the analysis of the spin Hamiltonian parameters it is shown that in the Mn2+(I) centers the paramagnetic ion is situated at substitutional Zn sites in the ZnS nanocrystals, being also subjected to a small axial distortion. The relative concentration changes under thermal treatment experiments strongly suggest that in both Mn2+(II) and Mn2+(III) centers the Mn2+ ion is localized on the surface of the ZnS nanocrystallites, being bond to an oxygen ion in the first case and to an additional water molecule in the second case.

Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals.

A multifrequency electron paramagnetic resonance (EPR) investigation of Nd(3+) impurities in PbWO(4) single-crystals at the conventional microwave frequency (MF) 9.43 GHz, and at the 95, 190, and 285 GHz high frequencies was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective electron spin of one-half and to a tetragonal symmetry. For the magnetic field along the tetragonal axis, the g(parallel)-factor and the hyperfine constant A(parallel) of the lowest doublet of the ground multiplet decreases with frequency increase. For the magnetic field perpendicular to the tetragonal axis, the g(perpendicular)-factor exhibits a small azimuthal angular dependence that increases with increasing the frequency due to the S(4) site symmetry. The azimuthal angular dependence allows to clearly distinguish between different local axial symmetries. These properties are interpreted as high field/frequency (HF) effects associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. We show that from the combined analysis of the multifrequency MF- and HF-EPR spectra and of the optical data, an accurate description of the ground multiplet of the Kramers rare earth ions in solid matrices can be derived.

In-depth investigation of EPR spectra of Mn(2+) ions in ZnS single crystals with pure cubic structure.

The X (9.8 GHz)-band electron paramagnetic resonance (EPR) properties of substitutional Mn(2+) ions in high quality cubic ZnS single crystals grown from PbCl(2) flux have been thoroughly investigated. Accurate spin Hamiltonian (SH) parameters: g = 2.002 25 ± 0.000 06; a = (7.987 ± 0.008) × 10(-4) cm(-1) and A = -(63.88 ± 0.02) × 10(-4) cm(-1) were obtained by simulation and fitting to the experimentally allowed transitions recorded for the magnetic field aligned within ± 0.25° along the main crystal axes. The normally forbidden hyperfine [Formula: see text], Δm = ± 1 transitions were also observed. Their position was found to be in agreement, within the experimental accuracy of ΔH = ± 0.01 mT, with calculations using the same SH parameters. The angular variation of the ratios of the intensities of the central forbidden to the allowed transitions could be accounted for only by including an additional constant contribution. The observed line broadening of the [Formula: see text] and [Formula: see text] fine structure transitions and their line width variation in a (110) plane have been quantitatively described by considering a random distribution of lattice strains at the Mn(2+) impurity ions. The influence of the forbidden transitions and line broadening on the EPR spectra line shape of the Mn(2+) ions in cubic ZnS crystalline powders is also examined.

Calorimetric absorption coefficient measurements using pulsed CO(2) lasers.

An experimental arrangement is devised by which the absorption coefficient of low-loss materials can be measured by calorimetry using pulsed CO(2) lasers. The values obtained for long rod KCl samples are compared with those inferred when a cw CO(2) laser was used as radiation source. The values measured in the pulsed regime are little affected by the quality of the end sample surfaces and are close to the values corresponding to bulk absorption. Such behavior is explained as due to the essential role of the radiation process in dissipating the surface absorbed heat.