Search

Soil and lake sediments are important paleoclimate archives often forming a source-sink setting. To better understand magnetic properties in such settings, we studied red soil on low-magnetic bedrock and subrecent sediments of Caohai Lake (CL) in Heqing Basin, China. Red soil is the only important source material for the CL sediments, it is highly magnetic with susceptibilities (��) of ~10���5 m3/kg. The red soil is dominated by pedogenic nano-magnetite (~10���15 nm) arranged in aggregates of ~100 nm, with particle interaction that causes a wide effective grain size distribution in the superparamagnetic (SP) range tailing into stable single-domain behavior. Transmission electron microscopy and broadband frequency ��(f) suggest partial disintegration of the aggregates and increased alteration of the nanoparticles to hematite during transfer of red soil material to CL. This shifts the domain state behavior to smaller effective magnetic grain sizes, resulting in lower ��fd% and �� values, and a characteristic change of ��(f). The SP-stable single-domain distribution of the aggregates in red soil could be climate dependent, and the ratio of saturation remanence to �� is a potential bedrock-specific paleoclimate proxy reflecting it. Magnetic properties of the CL sediments are controlled by an assemblage of nanoparticle aggregates and larger-sized bedrock-derived magnetite. The results challenge the validity of the previous paleoclimate interpretation from the 168-m-long Core-HQ (900���30 ka) in Heqing Basin. Disintegration of aggregates could lead to SP behavior with low ��fd% without extinction of individual magnetite nanoparticles, and the ��fd%-based assumption of SP magnetite dissolution may be wrong.

We present an algorithm for retrieving three-dimensional domains of picometer-scale shifts in atomic positions from electron diffraction data, and apply it to simulations of ferroelectric polarization in BaTiO3. Our algorithm successfully and correctly retrieves polarization domains in which the Ti atom positions differ by less than 3 pm (0.4% of the unit cell diagonal distance) with 5 and 10nm depth resolution along the beam direction, and we also retrieve unit cell strain, corresponding to tetragonal-to-cubic unit cell distortions, for 10nm domains. Experimental applicability is also discussed.

I denne afhandling undersoges de faktorer, der pavirker elektronholografimalinger af det ”indre middelpotential” i materialer. I transmissionelektronmikroskopien (TEM) giver elektronholografi mulighed for kvantitativ udmaling af elektronernes amplitude og fase. Bade amplituden og fasen afspejler proveemnets egenskaber. Fasen giver en kvantitativ bestemmelse af de elektriske og magnetiske potentialer pa nanoskala. Et sadant elektrostatisk potential kaldes det ”indre middelpotential”. Det indre middelpotential er det gennemsnitlige elektrostatiske potential malt mellem det indre af materialet og vakuum langt fra proven; det er forskelligt fra nul for alle materialer. Imidlertid har tidligere malinger af det indre middelpotential udvist uoverensstemmelser for et givet materiale udmalt af forskellige grupper. Derudover er det velkendt at for hojoplosning TEM (HRTEM), men ikke for elektronholografi, afviger de eksperimentelle resultater og billedsimuleringsresultaterne fra hinanden med en storrelse, der kaldes Stobbs faktor - derfor ville elektronholografiske datasat der giver gode muligheder for sammenligninger mellem billedesimulation og eksperiment kunne lede frem til forbedringer i simuleringsalgoritmerne (softwaren).De faktorer, der pavirker det indre middelpotential undersoges gennem bade eksperimenter og simuleringer. Tykkelsesmalinger ved hjalp af forskellige tomografialgoritmer (algebraiske, geometriske, og diskret tomografi) og ikketomografiske metoder sammenlignes pa en InAs-nanotrad. En selvkalibrerende ”tilt”-serie af hologrammer er optaget pa den samme InAs-nanotrad og disse sammenlignet med billedesimuleringer for at analysere diffraktionseffekter pa amplituden og fasen. Der er relativt god overensstemmelse mellem resultaterne af billedesimulering og de eksperimentelle data, men den eksperimentelle absorptionsparameter ses at variere mellem tilfalde med stark og svage diffraktion. Tathedsfunktionalteori simuleringer af det indre middelpotential er udfort ved hjalp af et ”GPAW” program, der giver mulighed for beregninger af det indre middelpotentials overfladeafhangighed. Faktorer, herunder overfladefacet, strukturoptimering (graden af relaksation af de atomare positioner), adsorbater, og feltforvrangning omkring hjorner er alle undersogt. Endelig er det forsogt at foretage overflademodifikation af en InAs/InPnanotrad, men forsogene forer i stedet til destruktion (oplosning) af nanotraden - der gives en kortfattet beskrivelse af denne destruktion af GaAs-nanotrade. In this dissertation, factors affecting electron holographic measurements of the mean inner potential are explored. Electron holography in the transmission electron microscope (TEM) allows for quantitative retrieval of the amplitude and phase of the electron beam. Both the amplitude and the phase reflect properties of the specimen. The phase can yield quantitative measurements of nanoscale electric and magnetic potentials. One such electrostatic potential is called the mean inner potential. The mean inner potential is the average electrostatic potential measured between the bulk of a material and vacuum far from the specimen, and is non-zero for all materials. However, previous mean inner potential measurements have disagreed for the same material measured by different groups. Additionally, experiment and image simulation are known to differ for high-resolution TEM, but not for electron holography, and this difference is known as the Stobbs factor - therefore, a dataset that allows for good comparison between image simulation and experiment might highlight possible improvements in the simulation software.The factors that affect the mean inner potential are explored through both experiment and simulation. Thickness measurements using different tomographic algorithms (algebraic, geometric, and discrete tomography) and non-tomographic methods are compared on an InAs nanowire. A self-calibrating tilt-series of holograms on the same InAs nanowire is acquired and compared with image simulations to analyze diffraction effects on the amplitude and the phase. There is relatively good comparison between image simulation and experimental data, but the experimental absorption parameter is found to differ between strongly and weakly diffracting conditions. Density functional theory simulations of the mean inner potential are carried out using the GPAW program, allowing for exploration of the surface dependence of the mean inner potential. Factors including surface facet, structure optimization (atomic position relaxation), adsorbates, and fringing fields at corners are all examined. Finally, surface modification is attempted on an InAs/InP nanowire, but leads to nanowire dissolution instead - this dissolution is briefly characterized for GaAs nanowires.

We present a preliminary study of the in situ heating of InAs nanowires in a gaseous environment in an environmental transmission electron microscope Nanowire dissolution, accompanied by dynamic reshaping of crystalline Au-containing catalyst particles at the ends of the wires, is observed, accompanied by indium oxide crystallite formation nearby