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Magnetism in multiferroics and low dimensional metal-organic complexesHan, Shou January 2016 (has links)
Multiferroics and magnetic metal-organic complexes are candidates for sophisticated applications in the future. In thisthesis, the magnetism in BiFeO3 (a multiferroic material with an incommensurate spin cycloidal structure), copper guanidiniam formate (a multiferroic metal-organic complex with a one-dimensional magnetic structure) and CP -RE-COT (a series of \zero-dimensional" single molecule magnets) are discussed. A radio-frequency plasma sputtering thin lm deposition system and a ferroelectric characterisation system were developed for the study of BiFeO3 epitaxial thin lms. A large leakage current was observed in BiFeO3 thin lms, which hindered the investigations on the ferroelectric properties and magnetoelectric coupling in them. An evidence of the spin cycloid in a BiFeO3 thin lm was observed by grazing-incidence small angle neutron scattering. The magnetism of a multiferroic metal-organic complex with a one-dimensional magnetic chain, [C(NH2)3][Cu(HCOO)3], was studied by magnetometry and muon spin spectroscopy. A spin-canted antiferromagnetic order and critical phenomenon in this material were investigated. It was shown that this material possessed an 3D Heisenberg long-range order below 4.6K. The one-dimensional magnetic chain was also studied by muon spin spectroscopy. The correlation length was measured with a eld dependence of H 1. Magnetisation relaxations of a series of single molecule magnets CP -RE-COT (COT = C8H8- CP = C5Me, which show "zero-dimensional" magnetism, were studied using an AC magnetometer and muon spin spectroscopy. Three possible relaxation pathways, including a quantum tunnelling process and two Orbach relaxation processes, were suggested by the relaxation behaviour. The suppression of the quantum tunnelling effect resulting from the entanglement of the ground states, which probably arises from the exchange interactions in CP -RE-COT, was also observed with a 1000 Oe applied magnetic eld. Data that were consistent with long-range magnetic ordering was observed in CP -Dy-COT, which would be the fi rst ever report of long-range magnetic order in a single ion magnet.
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Transient absorption spectroscopy of metal complexes : dithizonatophenylmercury(II) and derivativesBosman, Gurthwin Wendell 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The availability of laser systems capable of generating ultrashort optical pulses in the visible
spectral range have made it possible to study molecular species with a temporal resolution
in the order of intra-molecular dynamics which previously were not accessible. Henceforth,
in this study a popular pump-probe technique called ultrafast transient absorption spectroscopy
(UTAS) is employed to study the initial photochromic reaction of metal complex, dithizonatophenylmercury
(II) (DPM) under various conditions. These conditions include the use of
different solvents and electronically altering DPM by the addition of substituents with specific
electron affinity properties.
For this study the photochromic behaviour of DPM dissolved in methanol is used as standard.
Using this standard, the initial photochromic reaction after ultrafast excitation within less than
100 fs caused a radiationless photoreaction with a time constant of about 2 ps, which is interpreted
as C=N isomerization along the twist coordinate. It is found that during this isomerization
reaction an orthogonally twisted intermediate state was formed and observed through
its excited-state absorption, which funnels through a conical intersection onto the ground state
potential. Once here, bifurcation along pathways towards the ground states of the reactant and
product configurations occurs. This was the first observation of photochromism for DPM in a
strong polar solvent such as methanol.
From the results of the test compound in methanol, comparisons to the photo-behaviour of
DPM dissolved in deuterated methanol and dichloromethane are made. What is established is
that changing the solvent resulted in slightly different decay times as well as spectral shifts in
the absorption profile which suggested that the potential energy surface of the excited state is
modified. This is similar to the results found from the electronically altered DPM species. Here
the entire photoreaction is found to either be sped up (1 ps) or slowed down (4 ps) depending
on the sample investigated. Previous studies on the photochromicity of DPM like systems concluded that apart from the
photo-activated forward reaction, a back reaction occurs in the absence of light. Therefore a
detailed look at the back reaction of DPM and its derivatives are also studied. This is done by
temporally monitoring the absorption change of the photo-product as it converts back to the
reactant. In doing so the lifetime of the photo-product is determined and found to differ greatly
depending on the solvent and substituent used. Lastly, in assuming that the back reaction
follows first order kinetics, an Arrhenius type measurement on the photo-product of DPM is
conducted. The result of this measurement is that a potential energy barrier of 64.8 kJ/mol is
overcome in the back reaction. The measurements performed and the results obtained from the photochromicity of DPM contribute
to the understanding of photo-induced cis-trans isomerization reactions about a C=N
double bond. / AFRIKAANSE OPSOMMING: Die beskikbaarheid van laser sisteme in staat om ultrakort optiese pulse te genereer in die sigbare
spektrale gebied, het die studie van molekulˆere spesies met ’n tydresolusie in die orde van
intra-molekulˆere dinamika wat voorheen nie toeganklik was nie. Voortaan, in hierdie studie
word ’n populˆere aktiveer-interogeer tegniek genaamd ultravinnige tydafhanklike absorpsie
spektroskopie (UTAS) aangewend om die aanvanklike fotochromiese reaksie van die metaal
kompleks, ditisoonfenielkwik (II) (DFK) onder verskillende kondisies te bestudeer. Hierdie
kondisies sluit in die gebruik van verskillende oplosmiddels en die elektroniese verstelling van
DFK deur die toevoeging van substituente met spesifieke elektron affiniteit eienskappe.
Vir hierdie studie word die fotochromiese gedrag van DFK, opgelos in metanol, as standaard
gebruik. Met hierdie standaard, word gevind dat die aanvanklike fotochroom reaksie na ultravinnige
opwekking binne minder as 100 fs, ’n stralingslose fotoreaksie met ’n tydkonstante van
2 ps veroorsaak. Hierdie fotoreaksie word verklaar as die C=N isomerisasie met betrekking tot
die draai ko¨ordinaat. Daar is ook gevind dat gedurende hierdie isomerisasie reaksie, ’n ortogonaal
verdraaide oorgangstoestand gevorm word wat waargeneem is deur sy absorpsie in die
opgewekte toestand wat tregter deur ’n koniese kruispunt na die potensiaal van die grondtoestand.
By hierdie punt vurk die proses na die grondtoestande van die reaktant en die produk
vorme. Di´e was die eerste waarneming van fotochromisme van DFK opgelos in ’n sterk polˆere
oplosmiddel soos metanol.
Vanuit die resultate vir die toets mengsel in metanol word vergelykings getrek tot die fotogedrag
van DFK opgelos in gedeureriese metanol en dichlorometaan. Wat hieruit vasgestel is, is
dat die verandering in oplosmiddel wel die fotoreaksie be¨ınvloed deurdat verskillende vervaltye
en spekrale verskuiwings in die apsorpsie profiel waargeneem word.
Hierdie verskille dui aan dat die potensie¨ele energie oppervlakte van die opgewekte toestand
wel gemodifiseer word. Dit is ook soortgelyk aan die resultate vir die elektronies verstelde BFK
spesies. Hier is gevind dat die tempo van die totale fotoreaksie `of toeneem (1 ps) `of afneem (4
ps) afhanklik van die spesifieke spesie wat bestudeer word. Vorige studies van die fotochromiese gedrag van sisteme soortgelyk aan DFK het gevind dat behalwe
vir die fotogeaktiveerde voorwaartse reaksie, daar ook ’n keersy reaksie plaasvind in die
afwesigheid van lig. Dus word die keersy reaksie vir DFK en sy derivate ook in detial bestudeer.
In hierdie studie word die absorpsie verskille van die fotoproduk tydelik waargeneem soos dit
terugkeer na die reaktant. Op hierdie manier word die leeftyd van die fotoproduk bepaal en
gevind dat dit grootliks afhang van die tipe oplosmiddel en tipe substituent wat gebruik word. Ten slote, wanneer daar aanvaar word dat di´e keersy reaksie eerste orde kinetika volg, word ’n
Arrhenius tipe meting op die fotoproduk uitgevoer. Die resultaat van hierdie meting is dat ’n
potensie¨ele enrgie versperring van 64.8 kJ/mol oorkom word in die keersy reaksie.
Die metings wat uitgevoer is en die resultate wat verkry is van die fotochromiese gedrag van
DFK dra by tot die verstandhouding van lig-ge¨ınduseerde cis-trans isomerisasie reaksies rondom
’n C=N dubbelbinding.
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Density functional calculation of simple moleculesOlaoye, Olufemi Opeyemi. 03 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2012. / AFRIKAANSE OPSOMMING: Berekeninge met Density Functional Theory (DFT) is ’n nuttige tegniek om die dinamika
van molekules op potensiële energievlakke te verstaan. Beginnende met ’n
prototipe molekuul formaldimien, wat die kern vorm van die groter fotochromiese
molekuul dithizonatophenyl kwik (DPM), word die modellering van die molekuul
meer ingewikkeld tot laasgenoemde bestudeer kan word asook sy fotochromiese
afgeleides wat vervanging van elektronryk en elektronarm radikale by orto, meta en
para posisies van die phenyl ringe insluit. DFT berekeninge word met spektra van
Absorpsiespektroskopie met UV en sigbare lig asook tyd opgeloste spektra, verkry
dmv femtosekondespektroskopie, vergelyk. In pol^ere aprotiese, pol^ere protiese en
nie-pol^ere oplosmiddels, isomeriseer die molekuul om die C=N dubbelbinding. Daar
kan tussen die twee isomere onderskei word deur dat die een in oplossing in sy
grondtoestand blou en die ander een oranje voorkom. Die isomerisering is’n fotogeinduseerde
proses. Die optimering van die molekul^ere struktuur, absorpsiespektra,
oplosmiddel-afhanklikheid, en potensiële energievlak metings van die molekuul word
bestudeer. Die sterk/swak wisselwerking wat in pol^ere protiese/aprotiese oplosmiddels
verskyn word geopenbaar deur die hoe/lae absorpsie van die sekond^ere bande
van die molekules. Daar is gevind dat die absorpsiespektra van DPM bathochromies
in oplosmiddels met hoë diëlektriese konstantes is. Vir die potensiële energievlak
berekeninge van die grondtoestand word rigiede en ontspanne metodes gebruik waar
laasgenoemde met gebroke simmetrie berekeninge verkry word. Van alle metodes
wat vir berekeninge gebruik was, gee die B3LYP/CEP-31G metode die beste benadering
aan eksperimentele data. Alle berekeninge word gedoen met twee bekende
sagteware pakkette; Amsterdam Density Functional (ADF) en Gaussian, wat op twee
verskillende DFT metodes gebaseer is. / ENGLISH ABSTRACT: Density functional theory is a useful computational tool in the understanding of
molecular dynamics on potential energy surfaces. Starting with a prototype molecule
formaldimine, the photochromic molecule dithizonatophenylmercury II (DPM) and a
set of its photochromic derivatives, (involving substitutions of electron donating and
electron withdrawing substituents at ortho, meta and para positions of the dithizonato
phenyl rings), are studied through density functional calculation in comparison
with steady state absorption spectra obtained from UV-Visible and femto second
spectroscopy experiments. In polar aprotic, polar protic and non-polar solvents these
molecules isomerise around C=N double bond chromophore, from orange electronic
ground states to blue electronic ground states upon photo-excitation. We investigate
the structural optimisations, the absorption spectra, the solvent dependence and the
potential energy surface (PES) of these molecules. The strong (weak) interactions
exhibited by the polar protic (aprotic) solvents used are revealed through high (low)
absorbance in the secondary bands of these molecules. The absorption spectra of
DPM are found to be bathochromic in solvents with high dielectric constants. For
the ground state PES calculation we make use of rigid and relaxed methods, and the
latter is obtained through broken symmetry calculation. Of all the methods used in
calculation, B3LYP/CEP-31G method gives the best approximation to the experimental
data. All calculations are done using the two renown software, Amsterdam
Density Functional (ADF) and Gaussian, availing their different density functional
methods.
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Photophysical and Photosensitizing Properties of Dimetal Quadruply Bonded Paddlewheel Complexes Probed Through Ultrafast SpectroscopyBrown-Xu, Samantha E. 10 October 2014 (has links)
No description available.
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Magnetic properties of Mn, Ni and Fe based metal-organic complexesParameswaran, Anupama 08 March 2011 (has links) (PDF)
This dissertation presents the investigation of magnetic exchange and anisotropy in novel metal-organic complexes containing minimum number of magnetic ions. Such complexes can serve as a model system to understand the exciting magnetic phenomena in such class of materials and also can put forward as candidates for the so called molecular nanomagnets.
A direct assessment of the effective magnetic moment and the effective interaction between the metal ions in the complex can be done using magnetization measurements. Here the magnetization studies are performed as a function of temperature and field using a SQUID magnetometer. Yet another powerful tool to characterize and determine the spin levels, the ESR spectroscopic methods, has also been exploited. The study of the dynamical properties of this class of materials was relevant to understand the relaxation mechanism in the low temperatures. For this a new ac susceptometer has been built in house which was another main objective of this dissertation work. The design, fabrication, calibration and automation done on this device is presented in this thesis. The device has been tested using the known molecular magnet Mn12 acetate, and the antiferromagnet Dy2PdSi3.
The present work is mainly focused on the magnetic properties of Mn, Ni and Fe based organometallic complexes. The studied Mn dimer with different acceptor and donor ligands exhibit the fine tuning of the electron density at the core of molecular complex by variation in ligands. This in turn shows that the change in peripheral ligands can control the magnetism of the molecule. The influence of the change in Ni-S-Ni bond angle in the magnetic exchange interaction is studied in a Ni(2) dimer and a Ni(2) trimer complex. The Ni dimer complex shows a ferromagnetic interaction (J = -42K) whereas trimer shows an antiferromagnetic interaction (J = 140K). Another Ni based complex bridged via phosphorous has been studied which shows the existence of glassy nature at low temperature. Also a polymeric chain compound based on Fe is studied and presented. All these phosphorous or sulphur bridged complexes are novel materials and these are the first data on these complexes.
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Root-Driven Weathering Impacts on Mineral-Organic Associations Over Pedogenic Time ScalesGarcia Arredondo, Mariela 19 March 2019 (has links)
Plant roots are critical weathering agents in deep soils, yet the impact of resulting mineral transformations on the vast deep soil carbon (C) reservoir are largely unknown. Root-driven weathering of primary minerals may cause the formation of reactive secondary minerals, which protect mineral-organic associations (MOAs) for centuries or millennia. Conversely, root-driven weathering may also transform secondary minerals, potentially enhancing the bioavailability of C previously protected in MOAs. Here we examined the impact of root-driven weathering on MOAs and their capacity to store C over pedogenic time scales. I compared soil that experienced root-driven weathering, resulting in the formation of discrete rhizosphere zones in deep soil horizons (100-160 cm) of the Santa Cruz Marine Terrace chronosequence (65 ka-226 ka), with adjacent soil that experienced no root growth. Using a combination of radiocarbon, mass spectrometry, Mössbauer spectroscopy, and X-ray spectromicroscopy approaches, we characterized MOA transformations in relation to changes in C content, turnover and chemistry across four soils ranging in age (65 ka-226 ka). We found that the onset of root-driven weathering (65-90 ka) increased the amount of C associated with poorly crystalline iron (Fe) and aluminum (Al) phases, particularly highly-disordered nano-goethite. The increase in C coincided with greater overall C concentrations, longer C residence times, and a greater abundance of microbially-derived C. Continued root-driven weathering (137-226 ka) did not significantly change the amount of C associated with crystalline Fe and Al phases, but resulted in a decline in the amount of C associated with poorly crystalline metal phases. This decline in C associated to poorly crystalline phases coincided with a decrease in C concentrations and potential turnover rates, and a shift toward plant-derived C. In contrast, soil not affected by root-driven weathering showed low amounts of C bound to poorly crystalline Fe and Al phases regardless of soil age and, correspondingly, lower C concentrations and estimated residence times. My results demonstrate that root-driven formation and disruption of poorly crystalline Fe and Al phases directly controls both C accrual and loss in deep soil. Hence root impacts on soil C storage are dependent on soil weathering stage, a consideration critical for predictions of the vulnerability of deep soil C to global change.
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Magnetic properties of Mn, Ni and Fe based metal-organic complexesParameswaran, Anupama 03 March 2011 (has links)
This dissertation presents the investigation of magnetic exchange and anisotropy in novel metal-organic complexes containing minimum number of magnetic ions. Such complexes can serve as a model system to understand the exciting magnetic phenomena in such class of materials and also can put forward as candidates for the so called molecular nanomagnets.
A direct assessment of the effective magnetic moment and the effective interaction between the metal ions in the complex can be done using magnetization measurements. Here the magnetization studies are performed as a function of temperature and field using a SQUID magnetometer. Yet another powerful tool to characterize and determine the spin levels, the ESR spectroscopic methods, has also been exploited. The study of the dynamical properties of this class of materials was relevant to understand the relaxation mechanism in the low temperatures. For this a new ac susceptometer has been built in house which was another main objective of this dissertation work. The design, fabrication, calibration and automation done on this device is presented in this thesis. The device has been tested using the known molecular magnet Mn12 acetate, and the antiferromagnet Dy2PdSi3.
The present work is mainly focused on the magnetic properties of Mn, Ni and Fe based organometallic complexes. The studied Mn dimer with different acceptor and donor ligands exhibit the fine tuning of the electron density at the core of molecular complex by variation in ligands. This in turn shows that the change in peripheral ligands can control the magnetism of the molecule. The influence of the change in Ni-S-Ni bond angle in the magnetic exchange interaction is studied in a Ni(2) dimer and a Ni(2) trimer complex. The Ni dimer complex shows a ferromagnetic interaction (J = -42K) whereas trimer shows an antiferromagnetic interaction (J = 140K). Another Ni based complex bridged via phosphorous has been studied which shows the existence of glassy nature at low temperature. Also a polymeric chain compound based on Fe is studied and presented. All these phosphorous or sulphur bridged complexes are novel materials and these are the first data on these complexes.
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Development of Metal Oxide/Composite Nanostructures via Microwave-Assisted Chemical Route and MOCVD : Study of their Electrochemical, Catalytic and Sensing ApplicationsJena, Anirudha 07 1900 (has links) (PDF)
No description available.
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