Since the first discovery of atomically thin sheets of carbon, two-dimensional (2D) materials have captured the interest from scientific community to expand the understanding in fundamental physics and chemistry at low dimensional systems. With extraordinary phenomena only possible at atomically thin limits, there has been high demand to reveal new and unique 2D materials and manipulate their structures and properties. Structural tunability of superatomic solids motivates us to control dimentionality of the materials and construct layered structures which could be exfoliated to 2D materials. The layered crystal [Co6Se8(PEt2phen)6][C60]5 can be used as a template to create a 2D C60-based material with an optical gap in mid-infrared. Re6Se8Cl2 and Mo6S3Br6, are presented as the first examples of covalently linked 2D superatomic solids built from nanoscale building blocks with hierarchical structures and semiconducting properties. We further demonstrate the emergence of hierarchical coherent phonons in a 2D superatomic semiconductor Re6Se8Cl2. Lastly, we explore complex magnetic phases in 2D ferromagnetic semiconductor CrSBr using second harmonic generation and Raman spectroscopy. 2D superatomic semiconductors and 2D magnetic materials provide additional sets of design principles to manipulate structural, electronic, phononic, and magnetic properties at the atomically thin limits. These materials hold promises as model systems to study fundamental physical principles as well as platform for applications with phonon engineering and magnetic optoelectronic devices.

Bibliography: p. 65-66. / ASTIA 110028

Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Physics and Astronomy." Includes bibliographical references (p. 86-91).

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We present a study of the synthesis, structural and magnetic properties of bulk and nanosized (Zn, Cd, Cu)0:5Ni0:5Fe2O4 and NiFe2O4 compounds. The e®ects of electronic con¯guration and atomic sizes of Zn, Cd, Cu and Ni on the magnetic properties of the ferrites are the primary focus of the study. Di®erent synthesis routes, preparation conditions and how they a®ect single phase formation are explored. The synthesis was undertaken by solid{state reaction, combustion, hydrothermal and glycothermal techniques. The structure determination was by Xray di®raction. The magnetic measurements were performed using MÄossbauer spectroscopy (from 79 K to about 850 K) and a vibrating sample magnetometer (at about 300 K). The bulk densities of the sintered pellets were deduced by Archimedes principle. The bulk oxides were produced by solid{state reaction and combustion techniques. Fine powders with grain sizes of about 10 nm were produced from bulk compounds by a Retsch planetary ball mill and by the hydrothermal and glycothermal processes. The e®ects of the applied pressure used to make pellets (related to green density of the raw pellets) and the sintering temperature on the properties were investigated. An anomalous variation of bulk densities of (Zn, Cd)0:5Ni0:5Fe2O4 oxides with increase in pelletizing pressure was observed which appears to suggest evidence for trapped porosity. Di®erent states of pelletizing the samples appear to be related to a systematic change of the hyper¯ne ¯eld distributions derived from the MÄossbauer spectra. The temperature dependence of the magnetic hyper ¯ne ¯elds at tetrahedral (A) and octahedral (B) sites were observed to vary with temperature according to the equations Bhf (T) = Bhf (0)[1 ¡ (T=TC)n]¯n where n = 1 (based on the Landau{Ginzburg theory) and n = 2 (based on the Stoner theory). The equation Bhf (T) = Bhf (0)[1¡(T=TC)2]¯2 appears to ¯t the hyper¯ne ¯eld data over a wider temperature range. The Zn{ and Cd{based oxides were found to be ferrimagnetic with Curie temperature TC = 548 § 3 K (measured by zero velocity technique). The Cu{based compound exhibited antiferromagnetic behavior with a magnetic transition temperature of 825 § 3 K. The di®erence in behavior between Zn{, Cd{ and Cu{based compounds is due to di®erence in electronic con¯guration and atomic or ionic sizes. The stronger magnetic coupling between spins in the Cu{based sample can be explained by the presence of RKKY interactions in addition to superexchange interactions. The larger ionic size for Cd appears to favour smaller grain sizes in Cd{based oxides. An anomalous increase in TC is obtained in the Zn0:5Ni0:5Fe2O4 compound with reduction in grain size. This increase in TC is attributed to a distribution of Zn ions on both A and B sites. The MÄossbauer spectra of the milled nanosized samples show a combination of ferrimagnetic and paramagnetic behavior. The coercive ¯eld (HC) at room temperature was found to increase with reduction in grain size (G) according to the equation HC = am+bm=G, which is consistent with multidomain particles. With further reduction in grain sizes, the coercive ¯eld reduced according to the equation HC = as ¡bs=G2. This equation is associated with the onset of single domain particles. The samples produced by hydrothermal and glycothermal processes show evidence of transformation from single domain to multidomain structure with increasing sintering temperature. The ease of single{phase formation in the compounds studied is shown to depend on the technique used to prepare the samples. Single phase formation of the spinel structure was easier to achieve in samples prepared by wet chemical methods because lower sintering temperatures (T < 1000 oC) were required. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2007.

The synthesis, structure and physical properties of a series of Mnx(Co, Mg)₁ˍxFe₂O₄, (Mg, Sr)₀.₂ Mn₀.₁Co₀.₇Fe₂O₄ and Mg₀.₅Mn₀.₅(RE)₀.₁Fe₁.₉O₄ (where RE are rare earth elements) nanoferrites have been studied. These compounds were synthesized at low reaction temperature of about 200 ⁰C using the glycol-thermal method. The starting materials were high-purity metal chlorides or nitrates which were precipitated by NH₄OH and KOH respectively. In addition, MnxCo₁₋xFe₂O₄ (x = 0, 0.5 and 1) samples were produced directly from high-purity metal oxides by high-energy ball milling technique. Single-phase cubic spinel structure and nanoparticle structure of the synthesized samples were confirmed by X-ray diffraction (XRD) and transmission electron microscope (TEM). The results show that the produced powders of the asprepared samples have average grain sizes ranging from 7 to 16 nm. Filtering the precipitates by Whatman glass microfiber filters (GF/F) appears to be important in obtaining the small particle sizes. We suspect higher stability of the MnxCo₁₋xFe₂O₄ at x = 0 and 0.5 where complete symmetry in the proportion of the atoms on tetrahedral (A) and octahedral (B) sites would tend to favour larger nanoparticles. The evolutions of the magnetic properties as a function of composition, annealing temperature under air and argon atmospheres or measuring temperature have been investigated by ⁵⁷Fe Mössbauer spectroscopy, vibration sample magnetometer (VSM) and superconducting quantum interference device (SQUID). Significant changes in magnetic properties are observed across the composition ranges studied. The Mössbauer spectra indicate ferrimagnetic, superparamagnetic and paramagnetic behaviours of the compounds. The results show evidence of transformation from single-domain to multi-domain structure with thermal annealing in our samples. Temperature dependence of magnetization shows differences between field cooling (FC) and zero field cooling (ZFC) which we attribute to spin-freezing and thermal relaxation for typical nanoparticles. Significant increase in coercive field with reduction in measuring temperature is obtained in Co- based compounds. Mn₀.₅Co₀.₅Fe₂O₄, Sr₀.₂Mn₀.₁Co₀.₇Fe₂O₄ and Mg₀.₂Mn₀.₁Co₀.₇Fe₂O₄ have large coercive fields of 1.45, 3.02 and 10.70 kOe at 4 K compared to 0.17, 0.05 and 0.05 kOe at room temperature respectively. Variation of coercive fields (Hc) with measuri ing temperature for MnxCo₁₋xFe₂O₄ (x = 0.1 and 0.05), (Mg, Sr)₀.₂Mn₀.₁Co₀.₇Fe₂O₄ nanoferrites follow the Kneller's law for uniaxial non-interacting single domain particles of the form Hc(T) = Hc(0)[1-( T/Tβ)α]. The observed temperature dependences are consistent with α = 1/2. We also find evidence of the departure from this law at lower temperature. The temperature dependence of the saturation magnetizations were observed to vary with temperature according to the modified Bloch's law Ms(T) = Ms(0)[1 - ( T/T₀)ᵝ] where β is at least 1.5. This is attributed to the confinement effects of the spin-wave spectrum for magnetic clusters. The equation appears to fit the saturation magnetization data over the entire temperature range with values of β from 2.1 to 2.4 for the samples studied. These results are consistent with the nanoparticle nature of the compounds. In Mg₀.₅Mn₀.₅(RE)₀.₁Fe₁.₉O₄ nanoferrites, the grain sizes, lattice parameters and saturation magnetizations increase with RE substitution which we attribute to larger RE ions substituting smaller Fe ions. The results show evidence of superparamagnetic behaviour of the nanoparticles. The highest grain size and magnetizations are obtained for the Gd substituted sample. We find strong correlation between the saturation magnetizations, grain sizes and microstrains with de Gennes factor G. The correlation with grain sizes and microstrains appear to be unique and characteristic of the nanoparticle nature of the compounds. Bulk samples in the form of pellets were also produced from the as-prepared samples of MnxCo₁₋xFe₂O₄ for resistivity measurements. The temperature dependence of the electrical resistivity for samples sintered from 600 - 1100 ⁰C under argon atmosphere were studied using the four-probe method from room temperature to about 110 ⁰C. Two possible mechanisms for resistivity involving Tˉ¹ and Tˉ¹/² dependences were investigated which we associated with semiconducting and inter-grain conductivity respectively. The Tˉ¹/² dependence is found to fit the data better and predicts higher activation energies. The resistivity was observed to be sensitive to the surface of the pellet being probed and the annealing temperature. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2012.

A series of para-dioxolene bridging ligands with bis-tridentate or bis-bidentate coordination pockets have been prepared. These ligands were designed to serve as building blocks in the preparation of functional metallosupramolecular structures. The bis-tridentate dipyridyl-diazaanthraquinones have topologies suitable for the preparation of [2 x 2] grids or larger extended structures. Employing a double condensation reaction between 1,3-diamino-4,6-benzenedicarboxaldehyde and 2-acetyl pyridine successfully afforded a cisoid binding diazaanthraquinone. The redox activity of the latter revealed it is more easily reduced than structurally similar 1,5- and 1,8-diazaanthraquinones. The synthesis of an analogous ligand displaying transoid coordination pockets proved challenging. Several approaches were attempted however the preparation of key intermediates 1,4-dinitro-2,5-benzenedicarboxaldehyde and 2,5-diamino-1,4-(hydroxyI-methyl) benzene were unsuccessful. A collection of bis-bidentate 2,5-bis(phosphino)1,4-dioxolenes were prepared with diphenyl, diisopropyl and diethoxy substituents at the phosphorus center. 2,5- Dibromo-1,4-dimethoxybenzene was reacted with the appropriate chlorophosphine under lithiation conditions to afford the dimethoxy compound, which was subsequently deprotected to the hydroquinone state with a Lewis acid. The diphenylphosphino hydroquinone was oxidized using phenyliodonium bisacetate, however efforts to oxidize other hydroquinone precursors to the targeted quinone state proved challenging. Diphenyl and diisopropyl phosphino hydroquinones were successfully coordinated with diamagnetic palladium hexafluoroacetylacetonate precursors. The resulting bimetallic bis(phosphino) dianion complexes were subsequently oxidized to the semiquinone state using silver (I) hexafluorophosphate and studied to determine the distinctive features of the semiquinone ligand.