Influence of Quantum Dot Structure on the Optical Properties of Group IV Materials Fabricated by Ion Implantation

In nanostructures (NSs), to acquire a fundamental understanding of the electronic states by studying the optical properties is inherently complicated. A widely used simplification to this problem comes about by developing a model for a small scale representation of types of NSs and applying it to a hierarchy of fabrication methods. However, this methodology fails to account for structural differences incurred by the fabrication method that lead to differences in the optical properties. Proper modelling is realized by first considering the proper range of experimental parameters individually as inputs to a theoretical model and applying the correct parameters to the corresponding fabrication method. This thesis studies the connection between the structural and optical properties of NSs as a function of the fabrication method, using, principally, x-ray photoemission, Rutherford backscattering, photoluminesence, and Raman spectroscopy. Ion implanted Si and Ge quantum dots (QDs) in dielectric matrix were prepared to study the optical and structural properties, and compared against several other preparation methods. Ge QDs are known to exhibit a high concentration of defect states. The cause of these states was studied for QDs in a sapphire matrix and attributed to diffusion and desorption of Ge during annealing. Optical studies of Si QDs fabricated using an implantation mask revealed that state-filling and excitation transfer are important parameters in densely packed QD arrays. Structural analysis of Si QDs in silica revealed a well defined interface composed of Si$_2$O$_3$ and no stress was detected. Furthermore, the valence level was pinned at its bulk position possibly due to interface states. This information was used to refine our theoretical model of QDs and then compared with a range of crystalline and amorphous Si and Ge NSs. Stronger confinement effects were observed in amorphous Si and Ge NSs, possibly due to the nature of the interface or re-normalization of the effective mass as a function of NS size. These results establish a framework for proper parameter control in theoretical modelling

NEURONAL TRANSDIFFERENTIATION: UNCOVERING THE ROLE OF MLL1 AND MLL2 DURING LINEAGE CONVERSION

Neuronal transdifferentiation entails the direct conversion between mouse embryonic fibroblasts (MEFs) and induced neuronal cells (iNs), through the expression of the neuronal-specific transcription factors Brn2, Ascl1 and Myt1l (i.e., BAM factors). To date it is still unclear how BAM factors guide such epigenetic remodelling necessary to drive transdifferentiation. Some hints suggest the involvement of MLL1 and MLL2, two H3K4 trimethylases belonging to the Trithorax protein family, in both in vivo and in vitro neuronal differentiation. Therefore, I studied their role during MEF-to-iNs transdifferentiation. I showed that the absence of MLL1 does not affect either transdifferentiation efficiency or iNs neuronal morphology, but only the survival rate. On the contrary, transdifferentiation efficiency and neurite elongation are compromised in Mll2-/- iNs. The co-deletion of Mll1 and Mll2 impinges on cell viability as the knock-out of Mll1 and further exacerbates the Mll2-/- transdifferentiation defect, causing iNs to have very short neurites. These results suggest a role for MLL2-mediated H3K4 methylation in the control of transdifferentiation. Therefore, I defined the direct and indirect MLL2 targets through the integrative analysis of: i) the RNA-seq on iNs, ii) the ChIP-seq for MENIN, the specific common subunit of MLL1 and MLL2, and iii) the H3K4me3 ChIP-seq. I showed that in absence of Mll2 a conspicuous fraction of the transcriptome is down-regulated and/or looses the H3K4me3 mark, therefore highlighting the absence of MLL1-mediated compensation. Moreover, many deregulated genes (either differentially expressed or differentially marked by H3K4me3) are linked to neuronal differentiation and maturation, as expected by the phenotype analysis

Photoluminescence transient study of surface defects in ZnO nanorods grown by chemical bath deposition

Two deep level defects (2.25 and 2.03 eV) associated with oxygen vacancies (V$_o$) were identified in ZnO nanorods (NRs) grown by low cost chemical bath deposition. A transient behaviour in the photoluminescence (PL) intensity of the two V$_o$ states was found to be sensitive to the ambient environment and to NR post-growth treatment. The largest transient was found in samples dried on a hot plate with a PL intensity decay time, in air only, of 23 and 80 s for the 2.25 and 2.03 eV peaks, respectively. Resistance measurements under UV exposure exhibited a transient behaviour in full agreement with the PL transient indicating a clear role of atmospheric O$_2$ on the surface defect states. A model for surface defect transient behaviour due to band bending with respect to the Fermi level is proposed. The results have implications for a variety of sensing and photovoltaic applications of ZnO NRs

Influence of interface potential on the effective mass in Ge nanostructures

The role of the interface potential on the effective mass of charge carriers is elucidated in this work. We develop a new theoretical formalism using a spatially dependent effective mass that is related to the magnitude of the interface potential. Using this formalism we studied Ge quantum dots (QDs) formed by plasma enhanced chemical vapour deposition (PECVD) and co-sputtering (sputter). These samples allowed us to isolate important consequences arising from differences in the interface potential. We found that for a higher interface potential, as in the case of PECVD QDs, there is a larger reduction in the effective mass, which increases the confinement energy with respect to the sputter sample. We further understood the action of O interface states by comparing our results with Ge QDs grown by molecular beam epitaxy. It is found that the O states can suppress the influence of the interface potential. From our theoretical formalism we determine the length scale over which the interface potential influences the effective mass

Female Circumcision

Health care professionals have a duty to provide culturally competent care to individuals of diverse backgrounds. Despite the legal and ethical questions surrounding female circumcision, every individual is entitled to the same care and respect. Learning about the cultural beliefs and practices behind this tradition is mandatory to providing culturally competent care. The purpose of this paper is to educate the healthcare workforce about female circumcision and establish a culturally competent foundation for serving patients. Understanding the beliefs of a particular culture allows for the establishment of a respectful, nonjudgmental and trusting relationship between the provider and their patient

Quantum confinement in Si and Ge nanostructures: Effect of crystallinity

We look at the relationship between the preparation method of Si and Ge nanostructures (NSs) and the structural, electronic, and optical properties in terms of quantum confinement (QC). QC in NSs causes a blue shift of the gap energy with decreasing NS dimension. Directly measuring the effect of QC is complicated by additional parameters, such as stress, interface and defect states. In addition, differences in NS preparation lead to differences in the relevant parameter set. A relatively simple model of QC, using a `particle-in-a-box'-type perturbation to the effective mass theory, was applied to Si and Ge quantum wells, wires and dots across a variety of preparation methods. The choice of the model was made in order to distinguish contributions that are solely due to the effects of QC, where the only varied experimental parameter was the crystallinity. It was found that the hole becomes de-localized in the case of amorphous materials, which leads to stronger confinement effects. The origin of this result was partly attributed to differences in the effective mass between the amorphous and crystalline NS as well as between the electron and hole. Corrections to our QC model take into account a position dependent effective mass. This term includes an inverse length scale dependent on the displacement from the origin. Thus, when the deBroglie wavelength or the Bohr radius of the carriers is on the order of the dimension of the NS the carriers `feel' the confinement potential altering their effective mass. Furthermore, it was found that certain interface states (Si-O-Si) act to pin the hole state, thus reducing the oscillator strength.Comment: arXiv admin note: substantial text overlap with arXiv:1111.201

Role of Quantum Confinement in Luminescence Efficiency of Group IV Nanostructures

Experimental results obtained previously for the photoluminescence efficiency (PL$_{eff}$) of Ge quantum dots (QDs) are theoretically studied. A $\log$-$\log$ plot of PL$_{eff}$ versus QD diameter ($D$) resulted in an identical slope for each Ge QD sample only when $E_{G}\sim (D^2+D)^{-1}$. We identified that above $D\approx$ 6.2 nm: $E_{G}\sim D^{-1}$ due to a changing effective mass (EM), while below $D\approx$ 4.6 nm: $E_{G}\sim D^{-2}$ due to electron/ hole confinement. We propose that as the QD size is initially reduced, the EM is reduced, which increases the Bohr radius and interface scattering until eventually pure quantum confinement effects dominate at small $D$