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Made available in DSpace on 2014-10-09T12:31:03Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:06Z (GMT). No. of bitstreams: 1 01363.pdf: 907419 bytes, checksum: 885bb5e2ea74ebae7dd9dd5a5d958871 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

I measured the transport properties of graphene nanoribbons as a function of the charged impurity density in ultra-high vacuum to directly probe the effect of dimensional confinement. Coulomb impurities create charge puddles in graphene sheets, which dominate transport properties. My results shed light on recent predictions about the fundamental mechanisms behind gaps in conductance that appear in transport measurements.

Calcium lanthanum sulfide (CLS) has been identified as a candidate material for various optical applications due to its wide transmission range, high refractive index, desirable thermo-optic properties, and refractory behavior. Implementation of CLS in optical systems has remained unrealized due to various difficulties in the synthesis and bulk fabrication of this material. The key challenge facing the development of CLS is the incorporation of oxygen and the presence of unwanted secondary phases related to the lack of control of stoichiometric variations across the ternary phase diagram. Though these difficulties have been previously discussed in the processing of CLS, the exact form, mechanisms of formation and impact of these attributes have not been systematically evaluated. Each of these problems must be meticulously elaborated upon to understand what processing conditions are needed to avoid these common pitfalls which have been shown to degrade the ultimate optical quality of the resulting ceramic. This thesis investigated the role of oxygen in CLS ceramics as it applies to the resulting optical quality of the processed ceramic. The research done specifically examined ceramic materials with high La- content, prepared by hot pressing and hot isostatic pressing. The materials chosen for this work displayed varying levels of optical quality and phase purity. Determination of the desired cubic crystal structure as the main phase present was confirmed using x-ray diffraction and transmission electron microscopy. Through the correlation of data from multiple material metrology tools, the concentration and bond environment of oxygen in the ceramics and starting powders has been determined. It has been shown that the role of oxygen does not necessarily induce local phase transformation in the crystalline material, but rather can be dissolved into the CLS crystal lattice in low levels. Raman spectroscopy has been identified as a simple, useful tool that can be used to aid future synthesis routes due to its ability to differentiate the different types of O present. This thesis demonstrates how low levels of oxygen impurities can be incorporated into the CLS lattice utilizing simple IR and Raman spectroscopy techniques.

Baigiamąjį magistro darbą sudaro įvadas, 3 medžiagos dėstymo skyriai, pagrindinių rezultatų bei išvadų sąrašas, literatūros sąrašas. Įvade aprašytas temos aktualumas, tyrimo problemos, iškelta hipotezė, suformuluoti darbo tikslai ir uždaviniai. Pirmajame skyriuje aprašytos silicio kristalo savybės. Antrajame skyriuje apžvelgiamos pagrindinės difuzijos lygtys ir mechanizmai. Trečiajame skyriuje, remiantis bandinių elektrinio laidumo tyrimais bei teoriniu difuzijos sužadintose sistemose modeliu, rastos eksperimentinės vakansijų difuzijos koeficientų reikšmės. / This master work consists of introduction, 3 material enunciation chapters, basic results and conclusions list, literature list. In the introduction subject topicality, research problems and hypothesis are stated, work purposes and tasks are formulated. In chapter one features of the silicon mono-crystal are described. In chapter two basic diffusion equations and mechanisms are overviewed. In chapter three, with reference to samples’ electrical conductivity researches, experimental values of the vacancies diffusion coefficients are founded.

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