Optical properties of cerium dioxide nanocrystals doped with 3d and 4f elements.

У овој дисертацији проучаван је утицај дефекaтa на структурна, вибрациона, оптичка и електронска својства чистог и допираних нанокристала церијум диоксида. Узорци чистог CeO2-y су синтетисани хидротермалном (H‒CeO2-y), самопропагирајућом синтезом на собној температури (S‒CeO2-y) и методом преципитације (P‒CeO2-y). Температурски третман S‒CeO2-y узорка је извршен на неколико различитих температура (400–800 oC). S‒CeO2-y узорак је допиран са 3d и 4f елементима. Испитивање синтетисаних узорака је извршено методама дифракције рендгенског зрачења (XRD), микроскопије на бази атомских сила, Раман и инфрацрвене спектроскопије, спектроскопске елипсометрије и скенирајуће тунелске микроскопије/спектроскопије. Карактеризацијом структурних својстава CeO2-y нанокристала XRD техником, установљено је да средња величина кристалита зависи од процеса синтезе нанопрахова. Температурски третман S‒CeO2-y узорака узрокује раст величине кристалита уз смањење параметра решетке и микронапрезања, што је последица смањења концентрације кисеоничних ваканција и Ce3+ јона у решетки церијум диоксида. Узорци допирани 3d и 4f елементима су чврсти раствори. Анализа резултата Раман спектроскопије је показала да метода синтезе има значајан утицај како на присуство тако и на концентрацију кисеоничних ваканција у недопираним CeO2-y нанокристалима. Температурски третман је проузроковао раст кристалита и смањење концентрације кисеоничних ваканција што је утврђено ишчезавањем Раман мода који потиче од комплекса Ce3+ јона и кисеоничних ваканција. Допирање 3d и 4f елементима је довело до формирања додатног Раман мода, који потиче од различитих комплекса (кисеонична ваканцијаметални јон) који настају у решетки CeO2-y. Изразито ширење F2g Раман мода код S‒CeO2-y и узорака допираних Fe2+(3+) јонима и додатно померање F2g мода ка нижим фреквенцијама услед допирања Fe3+ јонима, указују на значајан утицај слободних носилаца наелектрисања на понашање F2g мода преко механизма електрон‒молекулског спаривања. Раман спектри испитиваних узорака су нумерички фитовани применом модела фононске локализације који је омогућио да се урачуна допринос електрон-молекулског спаривања на положај и ширину F2g мода...The main objective of this thesis was to investigate the influence of defects on structural, vibrational, optical and electronic properties of pure and doped cerium dioxide (CeO2-y) nanocrystals. Pure CeO2-y samples were synthesized by hydrothermal (H–CeO2-y), self-propagating room temperature synthesis (S–CeO2-y) and precipitation (P–CeO2-y) method. High-temperature treatment of S–CeO2-y samples was performed at several temperatures (400 – 800 oC). S–CeO2-y samples were doped with 3d and 4f elements. The synthesized samples were characterized by X–ray diffraction (XRD) technique, atomic force microscopy, Raman and infrared spectroscopy, spectroscopic ellipsometry and scanning tunneling microscopy/spectroscopy. Characterization of structural properties of CeO2-y nanocrystals by XRD technique revealed that average crystallite size significantly depends on the synthesis method. Temperature treatment of the S–CeO2-y samples caused the crystallite size growth followed by decrease in the lattice parameter and microstrain, because of lower concentration of oxygen vacancies and Ce3+ ions presence in ceria lattice. Samples doped with 3d and 4f elements are solid solutions. Analysis of the Raman spectra revealed that synthesis method has significant influence on the appearance and concentration of oxygen vacancies formed in undoped CeO2- y nanocrystals. Temperature treatment caused the crystallite size growth and decreases of oxygen vacancies concentration which was confirmed by the disappearance of the Raman mode which is characteristic for Ce3+–oxygen vacancy defect complexes. Doping with 3d и 4f elements has caused the appearance of additional Raman mode, which originates from different oxygen vacancy-metal ion complexes formed in ceria lattice. Pronounced broadening of F2g Raman mode in S–CeO2-y and Fe2+(3+)–doped samples and additional softening of F2g mode with Fe3+ doping, indicate significant influence of free charge carriers on the F2g mode behavior through the mechanism of electron-molecular vibrational coupling. Raman spectra of the investigated samples were fitted with the phonon confinement model which enabled to take into account the contribution of the electron-molecular vibrational coupling on the F2g mode position and bandwidth. The electron–molecular vibration coupling constant λ for F2g mode in pure and Fe–doped samples and the density of states near the Fermi level were determined within the framework of Allen’s theory. The obtained results pointed out that delocalization of electrons in highly oxygen–deficient pure and Fe2+(3+) – doped CeO2−y samples caused a semiconductor–to–metallic state transition..

Effect of Fe2+ (Fe3+) Doping on Structural Properties of CeO2 Nanocrystals

We have measured the Raman scattering and magnetization of pure and Fe2+(Fe3+) doped CeO2 nanopowders at room temperature. The Raman scattering spectra revealed the existence of CeO2 fluorite cubic structure for all investigated samples. The Raman active mode at about 600 cm(-1), seen in all samples, can be ascribed to the CeO2 intrinsic oxygen vacancies. Additional Raman modes at 720 cm(-1), 1320 cm(-1) and 1600 cm(-1), which appear in the spectra of doped samples, can be assigned to maghemite (gamma-Fe2O3) cation deficient structure, to 2 omega(LO) IR-allowed overtone and two magnon structure, respectively. This implies that our powders are composed of mixed valence states and have defective structure. Presence of oxygen defect states and magnetic ions can be responsible for the observed ferromagnetism at room temperature in both pure and Fe doped samples.Symposium on Raman Scattering in Materials Science, Sep 15-19, 2008, Warsaw, Polan

Hydrothermal Synthesis of CeO2 and Ce0.9Fe0.1O2 Nanocrystals

Pure and 10 mol% Fe3+ doped CeO2 nanocrystals were synthesized by hydrothermal method using two different basic solutions (NH4OH and NaOH). All the samples were calcinated at 140 degrees C and 200 degrees C. The characterization of crystalline structure, vibrational and optical properties was performed using X-ray diffraction, Raman spectroscopy and spectroscopic ellipsometry. The obtained results showed that the Fe-doped samples are solid solutions with different size of nanocrystals, very dependent on the synthesis temperature and type of basic solution. The Raman measurements demonstrated electron molecular vibrational coupling and increase of oxygen vacancy concentration whereas doping provokes a small decrease of optical absorption edge in comparison with pure ceria.International School and Conference on Photonics (PHOTONICA09), Aug 24-28, 2009, Belgrade, Serbi

Raman study of Ba-doped ceria nanopowders

A series of Ce1-xBaxO2-y (5 LT = x LT = 0.20) nanometric powders were synthesized by self-propagating room temperature synthesis. XRD and Raman scattering measurements were used to characterize the samples. at room temperature. All the samples are solid solutions with fluorite type structure with an average crystallite size about 5 nm. The redshift and asymmetric broadening of the Raman F g mode can be well explained with combined confinement and strain effects because of the nanocrystalline powders nature. The appearance of the additional peaks at similar to 560 cm(-1) and similar to 600 cm(-1), are attributed to extrinsic and intrinsic O2- vacancies in ceria lattice. Raman spectra of temperature treated Ce0.80Ba0.20O2-delta sample revealed the instability of this system

Genetic Differentiation of the Western Capercaillie Highlights the Importance of South-Eastern Europe for Understanding the Species Phylogeography

The Western Capercaillie (Tetrao urogallus L.) is a grouse species of open boreal or high altitude forests of Eurasia. It is endangered throughout most mountain range habitat areas in Europe. Two major genetically identifiable lineages of Western Capercaillie have been described to date: the southern lineage at the species' southernmost range of distribution in Europe, and the boreal lineage. We address the question of genetic differentiation of capercaillie populations from the Rhodope and Rila Mountains in Bulgaria, across the Dinaric Mountains to the Slovenian Alps. The two lineages' contact zone and resulting conservation strategies in this so-far understudied area of distribution have not been previously determined. The results of analysis of mitochondrial DNA control region sequences of 319 samples from the studied populations show that Alpine populations were composed exclusively of boreal lineage; Dinaric populations of both, but predominantly (96%) of boreal lineage; and Rhodope-Rila populations predominantly (>90%) of southern lineage individuals. The Bulgarian mountains were identified as the core area of the southern lineage, and the Dinaric Mountains as the western contact zone between both lineages in the Balkans. Bulgarian populations appeared genetically distinct from Alpine and Dinaric populations and exhibited characteristics of a long-term stationary population, suggesting that they should be considered as a glacial relict and probably a distinct subspecies. Although all of the studied populations suffered a decline in the past, the significantly lower level of genetic diversity when compared with the neighbouring Alpine and Bulgarian populations suggests that the isolated Dinaric capercaillie is particularly vulnerable to continuing population decline. The results are discussed in the context of conservation of the species in the Balkans, its principal threats and legal protection status. Potential conservation strategies should consider the existence of the two lineages and their vulnerable Dinaric contact zone and support the specificities of the populations

Temperature Measurement Using Silicon Piezoresistive MEMS Pressure Sensors

In industrial processes, as well as in many other fields from vehicles to healthcare, temperature and pressure are the most common parameters to be measured and monitored. Silicon microelectromechanical (MEMS) piezoresistive pressure sensors are the first and the most successful MEMS sensors, widely used in the industry in various measurement configurations. The inherent temperature dependence of the output signal of such sensors adversely affects their pressure measurement performance. However, it can be utilized for temperature measurement, thus enabling new sensor applications. In this paper a method is presented for temperature measurement using MEMS piezoresistive pressure sensors

New synthetic route for nanocrystalline boron nitride powder

Nanocrystalline hexagonal boron nitride powder (h-BN) was synthesized by sol-gel polycondensation of resorcinol and formaldehyde in the presence of boric acid followed by freeze drying. Pyrolysis and subsequent heat treatment of these cryogels resulted in formation of boron nitride powder. Characterization by nitrogen adsorption showed that precomposite cryogels and the BN powders were micro and mesoporous materials with high surface areas. Materials have been analyzed by means of X-ray diffraction, Raman scattering and electron microscopy methods. (C) 2010 Elsevier B.V. All rights reserved

Synthesis, calcination and characterization of Nanosized ceria powders by self-propagating room temperature method

Nanometric ceria powders with fluorite-type structure were obtained by applying self-propagating room temperature method. The obtained powders were subsequently thermally treated (calcined) at different temperatures for different times. Powder properties such as specific surface area, crystallite size, particle size and lattice parameter have been studied. Roentgen diffraction analysis (XRD), BET and Raman scattering measurements were used to characterize the as-obtained (uncalcined) powder as well as powders calcined at different temperatures. It was found that the average diameter of the as-obtained crystallites is in the range of 3-5 nm whereas the specific surface area is about 70 m(2)/g. The subsequent, 15 min long, calcination of as-obtained powder at different temperatures gradually increased crystallite size up to similar to 60 nm and reduced specific surface down to 6 m(2)/g. Raman spectra of synthesized CeO2-y depicts a strong red shift of active triply degenerate F-2 (g) mode as well as additional peak at 600 cm(-1). The frequency of F-2 g mode increased while its line width decreased with an increase in calcination temperature. Such a behavior is considered to be the result of particle size increase and agglomeration during the calcination. After the heat treatment at 800 degrees C crystallite size reached value larger than 50 nm. Second order Raman mode, which originates from intrinsic oxygen vacancies, disappeared after calcination. (c) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Synthesis and characterization of the nanometric Pr-doped ceria

In this paper nanometric powders of solid solution of the host compound ceria (CeO(2)) with Pr dopant in the lattice were synthesized by self-propagating room temperature (SPRT) synthesis with composition (Ce(0.9)Pr(0.1)O(2-delta)). Powder properties such as specific surface area crystallite and particle size and lattice parameters have been studied. BET, TEM, X-ray diffraction (XRD) analysis and Raman scattering measurements were used to characterize the samples at room temperature. Obtained solid solution exhibits a fluorite-type crystal structure. The average crystallite size is about 3-4 nm. Williamson-Hall plots were used to separate the effect of the size and strain in the nanocrystals. It is noticed the redshift and asymmetric broadening of the Raman F(2g) mode which is explained with nanocrystalline nature of powders. Color characters of solid solution depending on calcinations temperature and their position in the chromaticity diagram were studied by UV-vis spectrophotometry (diffuse reflectance). (C) 2010 Elsevier B.V. All rights reserved

Infrared study of plasmon-phonon coupling in pure and Nd-doped CeO2-y nanocrystals

Plasmon-longitudinal-optical (LO) phonon interaction in pure and Nd-doped CeO2-y nanocrystals was investigated by measuring far-infrared reflectivity spectra in the 100-700 cm(-1) spectral range at room temperature. Analysis of the obtained results revealed that the presence of free charge carriers becomes significant with the particle size decrease to nanometer range and increase of lattice defects. The free charge carriers were found to be responsible for a plasmon mode which coupled strongly with two LO phonon modes of ceria. The presence of more pronounced low-frequency Drude tail and the screening of the phonon modes in Nd-doped CeO2-y nanocrystals implied that the Plasmon-LO phonon interaction increased with doping. Factorized and additive dielectric function models were applied to deduce about the coupled and decoupled LO phonon frequencies and the structure of the decoupled plasmon mode in pure and Nd-doped CeO2-y nanocrystals. These models were combined with Bruggeman effective medium approximation in order to properly describe the influence of porosity on the infrared reflectivity spectra. With increasing dopant content, the decoupled plasmon mode exhibited redshift and damping decrease implying that doping induced semiconductor-to-metalic state transition took place