In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Synthetic pathways to water-soluble phthalocyanines and close analogs

Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

https://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004197.pdf

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

https://www.ucv.ro/pdf/invatamant/educatie/scoala_doctorala/mmureseanu/3.pdf

Modified SBA-15 mesoporous silica for heavy metal ions remediation.

Employing first principles calculations, we examined doped and imperfect ZnO sheets to reveal the electronic structure properties. We mainly investigated the imperfect sheets arising from the host atom vacancies and doped structures where group-III (Al, B, Ga) atoms were substitutionally placed. The energy band gap and electronic behavior were addressed. We revealed and confirmed that group-III dopants and a particular host atom vacancy played an essential role in the electronic structure behavior of a ZnO sheet. We observed that a Zn or O vacancy modified enormously the band gap of a ZnO sheet and that a single dopant in the supercell resulted in semiconductor-metal transition. Zero bias conductance of a group-III doped ZnO sheet was found to be sensitive to any group-III dopant when it replaced the O atom, implying the substantial role of the dopant plus a certain host atom vacancy in the principle electronic feature of a ZnO sheet.

Electronic Structure Properties of Doped and Imperfect ZnO Sheets

ESR Study on YBiPt compound doped by Er3+ and Gd3+

We made 50 periodic nano-layers of Si/Ge superlattice films with Au layers deposited on both sides as metal contacts. Each layer is 7.3 nm thick. The performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT. The purpose of this study is to improve the figure of merit of layered structures used as thermoelectric generators. The multi-layer Si/Ge films were then bombarded by 5 MeV Si ions at five different fluences to form nano-cluster structures. Rutherford Backscattering Spectrometry (RBS) was used to determine the total film thickness and stoichiometry. The total thickness was found as 364 nm. To get the figure of merit before and after MeV bombardments, we have measured the cross plane thermal conductivity by 3ω (3rd harmonic) method, cross plane Seebeck coefficient and electrical conductivity using Van der Pauw method. The electronic energy deposited due to ionization by the MeV Si beam in its track produces nano-scale structures. Micro-Raman spectra of the multilayered Si/Ge thin films were acquired using a surface sensitive LabRam spectrophotometer, equipped with a He–Ne laser with μ = 632.18 nm excitation wavelength to show nanoscale structure production effect on quantum well confinement by MeV Si on bombardment.

Surface modification of Si/Ge multi-layers by MeV Si ion bombardment

Thermoelectric properties of Zn4Sb3/CeFe(4−x)CoxSb12 nano-layered superlattices modified by MeV Si ion beam

The ternary chalcogenides AgBiTe2 and AgSbTe2 belong to the family of semiconductors with disordered NaCl cubic structure in which Ag and Sb occupy metal sublattices. Both compounds are very interesting due to their thermoelectric properties. We have grown single-layer AgBiTe and AgSbTe thin films on silicon (Si) and fused silica (Suprasil) substrates using electron beam deposition. High-energy (MeV) Si-ion bombardment was performed on the thin-film samples at five different fluences between 5 × 1013 ions/cm2 and 7 × 1015 ions/cm2. We have measured the thermoelectric efficiency (figure of merit, ZT) of the fabricated thermoelectric devices by measuring the cross-plane thermal conductivity using the third-harmonic (3ω) method, the cross-plane Seebeck coefficient, and the in-plane electrical conductivity using the van der Pauw method before and after MeV Si-ion bombardment. Rutherford backscattering spectrometry and the Rutherford Universal Manipulation Program (RUMP) simulation package were used to analyze the elemental composition and thickness of the deposited materials on the substrates. The RUMP simulation gave thicknesses for the AgBiTe and AgSbTe thin films of 270 nm and 188 nm, respectively. The figure of merit for AgBiTe started to decrease from the value of 0.37 for the virgin sample after bombardment. We saw similar decreasing behavior for the AgSbTe thin-film system. The figure of merit for AgSbTe started to decrease from the value of 0.88 for the virgin sample after bombardment. MeV Si-ion bombardment caused changes in the thermoelectric properties of the thin films.

Sadik Güner

Papers

Electronic Structure Properties of Doped and Imperfect ZnO Sheets

ESR Study on YBiPt compound doped by Er3+ and Gd3+

Surface modification of Si/Ge multi-layers by MeV Si ion bombardment

Thermoelectric properties of Zn4Sb3/CeFe(4−x)CoxSb12 nano-layered superlattices modified by MeV Si ion beam

Thermoelectric Generators from AgBiTe and AgSbTe Thin Films Modified by High-Energy Beam