Dissertations / Theses on the topic 'Cross-sectional scanning tunneling microscopy'

This work investigates Cross-Sectional Scanning Tunnelling Microscopy applied to semiconductor laser structures which are biased whilst they are being scanned. Images are presented as a function of sample bias. Increasing the sample bias removes the built-in band bending across the device and causes the horizontal topographic gradient of the scan to change. The p-type side of the sample is held at ground whilst the n-type side is biased. When tunnelling out of a double quantum well structure the topographic height of the n-type side increases by around 0.2nm at 1V. Tunnelling in to the structure, the height decreases by 0.02nm under low tunnelling current conditions. A tunnelling current model is developed which confirms these changes. Tunnelling in to a buried heterostructure device the apparent topographic height of the n-type side is again found to decrease, by over 2nm. Biased-dependent spectroscopic shifts are also observed with this device which are again confirmed by modelling. In both devices the apparent height of the quantum wells is found to increase by a factor of 2.5 to 4 times, at 1V sample bias. This is caused by the effects of tip-induced band bending being altered by the applied bias. An experimental and modelled example of a superlattice structure which displays contrast enhancement through tip-induced band bending is given. Several other device-specific physical and irreversible changes occur as a result of sample bias. These are well-suited to give characterisation information not available from other techniques.

This thesis focuses on the characterization of In 0,4 Ga 0,6 Sb/InAs and InSb/InAs quantum dots using Cross-Sectional Scanning Tunneling Microscopy (X-STM). Quantum dots (QDs) are small and spatially confined semiconductor nanostructures with a size-dependent band gap. This property makes them very attractive for devices such as sensors, solar cells and lasers. The QDs analyzed in this thesis were grown using Metal-Organic Vapor Phase Epitaxy (MOVPE) and are meant to be utilized in long wavelength infrared (LWIR) (~8μm) detectors. To study buried QDs by X-STM, the sample has to be cleaved and measured in Ultra High Vacuum (UHV). In order to do this, a cleaving apparatus was built and installed on an STM system. A sample preparation methodology was worked out in order to make the samples ready for cleaving. An easy method for finding the QDs with the X-STM was also developed. Measurements resulted in a number of atomically resolved images, revealing the QD layer morphology. Furthermore, larger images were captured in order to study growth defects. Because of the high dot density, at low resolution the QDs were perceived as quantum wells. It was only at atomic resolution that QDs could be resolved. The observed dot sizes ranged between ~3 nm (InSb) and ~8 nm (In 0,4 Ga 0,6 Sb) in diameter.

Graphene and other two-dimensional materials can be stacked together to form vander Waals heterostructures: synthetic crystals composed of different atomically thin layers with a bespoke electronic band structure. Structural characterisation of vander Waals heterostructures is difficult using conventional methods as the properties are almost entirely defined by the nature of the buried interfaces between dissimilar crystals. These methods also fall short of resolving the atomic structure of buried defects in van der Waals materials such as graphite. This work demonstrates the refinement and successful application of ion beam specimen preparation to produce cross sectional slices through these unique crystals so that they can be characterised by high resolution scanning transmission electron microscopy (STEM). Cross sectional specimen were prepared using in situ lift-out in a focused ion beam (FIB) dual-beam instrument. The fine polishing steps were optimised to prevent damage to the core of the specimen. High resolution STEM imaging of twin defects in graphene, hexagonal boron ni-tride and MoSe2 revealed that the boundaries are not atomically sharp but extended across many atoms. Advanced processing and analysis of these images uncovered fundamental mechanics which govern their geometry. This technique was further applied to complex transition metal dichalcogenide heterostructures to quantitatively determine the properties of buried interfaces between atomically thin crystals.

This thesis describes the development and use of a computer-automated microscopy routine for characterization of respirable dust particles from coal mines. Respirable dust in underground coal mining environments has long been known to pose an occupational health hazard for miners. Typically following years of exposure, coal workers' pneumoconiosis (CWP) and silicosis are the most common disease diagnoses. Although dramatic reductions in CWP and silicosis cases were achieved across the US between about 1970-1999 through a combination of regulatory dust exposure limits, improved ventilation and dust abatement practices, a resurgence in disease incidence has been noted more recently – particularly in parts of Appalachia. To shed light on this alarming trend and allow for better understanding of the role of respirable dust in development of disease, more must be learned about the specific characteristics of dust particles and occupational exposures. This work first sought to develop an automated routine for the characterization of respirable dust using scanning electron microscopy with energy dispersive x-ray (SEM-EDX). SEM-EDX is a powerful tool that allows determination of the size, shape, and chemistry of individual particles, but manual operation of the instrument is very time consuming and has the potential to introduce user bias. The automated method developed here provides for much more efficient analysis – with a data capture rate that is typically 25 times faster than that of the manual method on which it was based – and also eliminates bias between users. Moreover, due to its efficiency and broader coverage of a dust sample, it allows for characterization of a larger and more representative number of particles per sample. The routine was verified using respirable dust samples generated from known materials commonly observed in underground coal mines in the central Appalachian region, as well as field samples collected in this region. This effort demonstrated that particles between about 1-9μm were accurately classified with respect to defined chemical categories, and suggested that analysis of 500 particles across a large area of a sample filter generally provides representative results. The automated SEM-EDX routine was then used to characterize a total of 210 respirable dust samples collected in eight Appalachian coal mines. The mines were located in three distinct regions (i.e., northern, mid-central and south-central Appalachia), which differed in terms of primary mining method, coal seam thickness and mining height, and coal and/or rock mineralogy. Results were analyzed to determine whether number distributions of particle size, aspect ratio, and chemistry classification vary between and within distinct mine regions, and by general sampling location categories (i.e., intake, feeder, production, return). Key findings include: 1) Northern Appalachian mines have relatively higher fractions of coal, carbonate, and heavy mineral particles than the two central Appalachian regions, whereas central Appalachian mines have higher fractions of quartz and alumino-silicate particles. 2) Central Appalachian mines tended to have more mine-to-mine variations in size, shape, and chemistry distributions than northern Appalachian mines. 3) With respect to particle size, samples collected in locations in the production and return categories have the highest percentages of very small particles (i.e., 0.94-2.0μm), followed by the feeder and then the intake locations. 4) With respect to particle shape, samples collected in locations in the production and return categories have higher fractions of particles with moderate (i.e., length is 1.5 to 3x width) to relatively high aspect ratios (i.e., length is greater than 3x width) compared to feeder and intake samples. 5) Samples with relatively high fractions of alumino-silicates have higher fractions of particles with moderate aspect ratios than samples with low alumino-silicate fractions. 6) Samples with relatively high fractions of quartz particles have higher fractions of particles with moderate aspect ratios and higher percentages of very small particles than samples with no identified quartz particles. 7) Samples with high fractions of carbonates have higher percentages of particles with relatively low aspect ratios (i.e., length and width are similar) than samples with no identified carbonate particles.
Master of Science

Os bisfosfonatos representam uma classe de drogas que agem sobre o metabolismo ósseo e são amplamente utilizadas na prevenção e tratamento de estados osteopênicos e osteoporóticos. Os objetivos do presente estudo foram avaliar, in vivo, o efeito do uso sistêmico de alendronato sódico: na densidade mineral óssea de ratos, por meio da densitometria óptica radiográfica e da técnica de absortometria radiológica de dupla energia (DXA); e nas estruturas dentárias mineralizadas de incisivos murinos, por meio da espectrometria na região do infravermelho, espectroscopia de fluorescência, microdureza transversal, microscopia eletrônica de varredura e microscopia de luz polarizada. Foram utilizados 45 ratos Wistar, com 36-42 dias de idade, pesando em média 200-230g, os quais foram divididos em dois grupos: experimental (n= 25) e controle (n= 20). No grupo experimental foram administradas duas doses semanais de 1mg/Kg de alendronato de sódio quimicamente puro diluído em água destilada, via gavagem, enquanto que os animais do grupo controle receberam apenas água destilada. Decorrido o período de 60 dias, os animais foram mortos por sobredose anestésica e, em seguida, foram extraídos os incisivos superiores e removidas as tíbias. As tíbias foram submetidas à avaliação da densidade mineral óssea por meio de análise radiográfica e da técnica de absortometria radiológica de dupla energia (DXA). Os incisivos superiores foram submetidos às seguintes avaliações: análise química por espectrometria na região do infravermelho e espectroscopia de fluorescência, microdureza transversal do esmalte e da dentina; microscopia eletrônica de varredura e microscopia de luz polarizada. Os resultados numéricos obtidos foram submetidos à análise estatística por meio do teste não-paramétrico de Kruskal-Wallis, utilizando o software SAS (Statistical Analysis System) for Windows versão 9.1.3. O nível de significância adotado foi de 5%. O grupo experimental apresentou valores de densidade mineral óssea superiores (p<0,05) em relação ao grupo controle, pelos métodos da densitometria óptica radiográfica e DXA. A análise química pelos métodos de espectrometria na região do infravermelho e espectroscopia de fluorescência permitiu detectar a presença do alendronato na estrutura dentária mineralizada do grupo experimental e que a porcentagem dessa incorporação foi de 0,0018% por elemento dental. Os resultados da microdureza transversal do esmalte e da dentina não revelaram diferença estatisticamente significante entre os grupos experimental e controle (p>0,05). Não foram observadas diferenças morfológicas significativas entre as amostras de ambos os grupos por meio da análise por microscopia eletrônica de varredura e microscopia de luz polarizada. Com base nos resultados obtidos, conclui-se que o tratamento com alendronato sódico provocou aumento na densidade mineral óssea da metáfise proximal da tíbia e que o alendronato incorporou-se nas estruturas dentárias mineralizadas, porém sem provocar efeitos significativos na microdureza e na morfologia do esmalte e da dentina de incisivos de ratos.
Bisphosphonates represent a class of drugs that act on bone metabolism and are widely used in the prevention and treatment of osteopenic and osteoporotic states. The objectives of this study were to evaluate, in vivo, the effect of the systemic use of sodium alendronate on: the mineral bone density of rats, by radiographic optical densitometry and dual-energy x-ray absorptiometry (DXA); the mineralized dental structures of murine incisors, by analysis of infrared (IR) spectrometry, fluorescence spectroscopy, cross-sectional microhardness (CSMH), scanning electron microscopy (SEM) and polarized light microscopy (PLM). Forty-five Wistar rats aged 36-42 days and weighing 200-230 g were assigned to two groups: experimental (n= 25) and control (n= 20). The experimental group received two weekly doses of 1 mg/kg of chemically pure sodium alendronate diluted in distilled water, via gavage, while the animals of the control group received only distilled water. After 60 days, the animals were killed by anesthetic overdose, and the maxillary incisors were extracted and the tibias were removed. The mineral bone density of the tibias was analyzed radiographically and by DXA. The maxillary incisors were subjected to the following evaluations: chemical analysis by IR spectrometry and fluorescence spectroscopy, enamel and dentin CSMH, SEM and PLM. The results were subjected to statistical analysis by the Kruskal-Wallis non-parametric test, using the SAS (Statistical Analysis System) software for Windows version 9.1.3. The significance level was set at 5%. The experimental group presented higher mineral bone density (p<0.05) than the control group, by radiographic optical densitometry and DXA. The chemical analysis by IR spectrometry and fluorescence spectroscopy revealed the presence of alendronate in the mineralized dental structure of the specimens of the experimental group, with a percentage of incorporation of 0.0018% per tooth. The results of enamel and dentin CSMH did not show statistically significant difference between the experimental and control groups (p>0.05). There were no significant morphological differences among the specimens of the groups by SEM and PLM. Based on the obtained results, it may be concluded that the treatment with sodium alendronate caused an increase in the mineral bone density of the proximal tibial metaphysis, and that alendronate was incorporated in the mineralized dental structures without causing significant effects in the enamel and dentin microhardness and morphology of rat incisors.

碩士
國立臺灣大學
物理學研究所
100
Aside from the bulk properties, interesting phenomena can be revealed in thin films of perovskite oxides, such as magnetoelectric switching effects in multiferroic BiFeO3 (BFO) films. Certain interesting perovskite surfaces (e.g., BFO(111)) normally pre- pared by methods such as pulsed laser deposition, but with a resulting morphology unsuitable for STM studies, can be made accessible by crystal cleavage. Instead of taking bottom-up way to get a surface, in this work, using the common substrate as our model system, we demonstrate the first observation of (110) polar surfaces of fractured Nb-doped SrTiO3 (Nb:STO) utilizing cross-sectional scanning tunneling microscopy (XSTM) at room temperature. A rich variety of surface morphologies have been observevd, but few are well characterised. Terraced STO(110) surfaces with two unit-cell height could be found by in situ cleaving. Comparing the results of atomically flat terraces with STO(100) non-polar surfaces, indicates the existence of polar instability on fractured STO(110) surfaces, in agreement with theoretical predictions. Future advances in understanding these surfaces will be accompanied by fracturing at low temperature.

碩士
國立清華大學
物理學系
85
In this thesis we model cross-sectional scanning tunneling spectroscopy studies of semiconductor superlattice electronic structures by computing tunneling current-voltage characteristics from calculated superlattice layer-local density of states. We demonstrate our approach for a type-II InAs/GaSb superlattice, and compare it with known experimental data.