A joint experimental and theoretical study on the electronic structure and photoluminescence properties of Al-2(WO4)(3) powders

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

A relationship between structural and electronic order-disorder effects and optical properties in crystalline TiO2 nanomaterials

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Local order and electronic structure of Pb1-xLaxZr0.40Ti0.60O3 materials and its relation with ferroelectric properties

The knowledge of electronic and local structures is a fundamental step towards understanding the properties of ferroelectric ceramics. X-ray absorption near-edge structure (XANES) of Pb1-xLaxZr0.40Ti0.60O3 ferroelectric samples was measured in order to know how the local order and electronic structure are related to their ferroelectric property, which was tailored by the substitution of lead by lanthanum atoms. The analysis of XANES spectra collected at Ti K- and L-edges XANES showed that the substitution of Pb by La leads to a decrement of local distortion around Ti atoms on the TiO6 octahedron. The analysis of O K-edge XANES spectra showed that the hybridization between O 2p and Pb 6sp states is related to the displacement of Ti atoms in the TiO6 octahedra. Based on these results, it is possible to determine that the degree of ferroelectricity in these samples and the manifestation of relaxor behavior are directly related to the weakening of O 2p and Pb 6sp hybridization. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4720472]

Electronic and magnetic properties of SnO2/CrO2 thin superlattices

In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2) n (SnO2) n superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2) n (SnO2) n SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2) n (SnO2) n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.

Spin Orbit Effects in the Electronic Transport Properties of Adsorbed Graphene Nanoribbons

Graphene has received great attention due to its exceptional properties, which include corners with zero effective mass, extremely large mobilities, this could render it the new template for the next generation of electronic devices. Furthermore it has weak spin orbit interaction because of the low atomic number of carbon atom in turn results in long spin coherence lengths. Therefore, graphene is also a promising material for future applications in spintronic devices - the use of electronic spin degrees of freedom instead of the electron charge. Graphene can be engineered to form a number of different structures. In particular, by appropriately cutting it one can obtain 1-D system -with only a few nanometers in width - known as graphene nanoribbon, which strongly owe their properties to the width of the ribbons and to the atomic structure along the edges. Those GNR-based systems have been shown to have great potential applications specially as connectors for integrated circuits. Impurities and defects might play an important role to the coherence of these systems. In particular, the presence of transition metal atoms can lead to significant spin-flip processes of conduction electrons. Understanding this effect is of utmost importance for spintronics applied design. In this work, we focus on electronic transport properties of armchair graphene nanoribbons with adsorbed transition metal atoms as impurities and taking into account the spin-orbit effect. Our calculations were performed using a combination of density functional theory and non-equilibrium Greens functions. Also, employing a recursive method we consider a large number of impurities randomly distributed along the nanoribbon in order to infer, for different concentrations of defects, the spin-coherence length.

Theoretical and experimental studies on the electronic, optical, and structural properties of poly-pyrrole-2-carboxylic acid films

A theoretical approach is used here to explain experimental results obtained from the electrosynthesis of polypyrrole-2-carboxylic acid (PPY-2-COOH) films in nonaqueous medium. An analysis of the Fukui function (reactivity index) indicates that the monomer (pyrrole-2-carboxylic acid, PY-2-COOH), and dimers and trimers are oxidized in the C4 or C5 positions of the heterocyclic ring of the PY-2-COOH structure. After calculating the heat of formation using semiempirical Austin Model 1 post-Hartree-Fock parameterization for dimer species, both C4 and C5 positions adjacent to the aromatic rings of PPY-2-COOH were considered the most susceptible ones to oxidative coupling reactions. The ZINDO-S/CI semiempirical method was used to simulate the electronic transitions typically seen in the UV-VIS-NIR range in monomer and oligomers with different conjugation lengths. The use of an electrochemical quartz crystal microbalance provides sufficient information to propose a polymerization mechanism of PY-2-COOH based on molecular modeling and experimental results.

Electronic transport properties of YBa 2 Cu 3 O 7-x

documentstyle[12pt,german]{article} pagestyle{empty} topmargin-1.5cm textheight24.5cm footskip-1.5cm % % begin{document} % begin{center} {Large {it Hern'{a}n Rodr'{i}guez}}\ vspace{24pt} {Large {bf Elektronische Transporteigenschaften von YBa$_{2}$Cu$_{3}$O$_{7-x}$/PrBa$_{2}$Cu$_{2.9}$Ga$_{0.1}$O$_{7-y}$ Dreifachschichten und "Ubergittern senkrecht zur Lagenstruktur}} end{center} vspace{24pt} noindent In der vorliegenden Arbeit wurden die Transporteigenschaften senkrecht zu den CuO$_{2}$--Ebenen von Hochtemperatur Supraleitern an YBa$_{2}$Cu$_{3}$O$_{7-x}$/\ PrBa$_{2}$Cu$_{2.9}$Ga$_{0.1}$O$_{7-y}$/ YBa$_{2}$Cu$_{3}$O$_{7-x}$ Dreifachschichten und [(YBa$_{2}$Cu$_{3}$O$_{7-x}$)$_{n}$\/(PrBa$_{2}$Cu$_{2.9}$Ga$_{0.1}$O$_{7-y}$)$_{m}$]$_{times M}$ "Ubergittern untersucht. Um die Transporteigenschaften senkrecht zu den Grenzfl"achen in Mehrlagenstrukturen messen zu k"onnen, ist ein Verfahren zur Herstellung von planaren Bauelemente verwendet worden. Die Untersuchungen an YBa$_{2}$Cu$_{3}$O$_{7-x}$/PrBa$_{2}$Cu$_{2.9}$Ga$_{0.1}$O$_{7-y}$ Dreifachschichten und "Ubergittern zeigen, da"s die Substrattemperatur w"ahrend des Wachstums die elektronischen Eigenschaften entlang der $c$--Achse stark beeinflusst. Bei Senkung der Abscheidetemperatur ergibt sich eine "Anderung von normalmetallischem zu tunnelkontaktartigem Verhalten. Die bei 840$^circ$C hergestellten Vielfachschichten weisen sowohl eine konstante Hintergrundleitf"ahigkeit als auch eine "Uberschu"sleitf"ahigkeit bei niedrigen Spannungen auf. Dies deutet darauf hin, da"s es sich um einen Supraleiter--Normalleiter--Supraleiter (S--N--S) Kontakt handelt. Dagegen zeigen Vielfachschichten, die bei 760$^circ$C deponiert wurden, deutlich unterschiedliches Verhalten verglichen mit den bei 840$^circ$C pr"aparierte Proben. Die Leitf"ahigkeit nimmt mit der Spannung zu, wobei der Leitf"ahigkeithintergrund eine ``V''--Form darstellt. Dar"uber hinaus zeigen die Leitf"ahigkeitskennlinien bei niedrigen Spannungen eine starke Abh"angigkeit sowohl von der Bias Spannung als auch von der Temperatur. Bei Dreifachschichten mit 20 nm PrBa$_{2}$Cu$_{2.9}$Ga$_{0.1}$O$_{7-y}$ tritt ein Leitf"ahigkeitmaximun bei Null--Spannung auf. Die Wechselwirkung zwischen tunnelnden Quasiteilchen und magnetischen Momenten in der Barriere ruft dieses Maximun hervor. Das "Ubergitter mit ($n/m$) = (4/5) Modulation zeigt Supraleiter--Isolator--Supraleiter (S--I--S) Tunnelkontakt--Verhalten mit Strukturen, die von der Energiel"ucke des Supraleiters hervorgerufen werden. Das S--N-- bzw., S--I--Kontaktverhalten der Heterostrukturen wurden ebenfalls mit Messungen der Leitf"ahigkeit bei tiefern Temperaturen weit au"serhalb der supraleitenden Energiel"ucke best"atigt. Diese Ergebnisse weisen auf die M"oglichkeit hin, durch Einstellen der Substrattemperaturen bei der Deposition das Auftreten von S--N--S und S--I--S Verhalten der Kontakte zu steuern. vspace{24pt} noindent Datum: 05.07.2004\ Betreuer: Prof. Dr. Hermann Adrian %Name des Betreuers, daneben dessen Unterschrift end{document}

Electronic structure and physical properties of Heusler compounds for thermoelectric and spintronic applications

This thesis focuses on synthesis as well as investigations of the electronic structure and properties of Heusler compounds for spintronic and thermoelectric applications.rnThe first part reports on the electronic and crystal structure as well as the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co2MnGe. The crystalline structure was examined in detail by extended X-ray absorption fine structure spectroscopy and anomalous X-ray diffraction. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab-initio calculations. Transport measurements and hard X-ray photoelectron spectroscopy (HAXPES) were performed to explain the electronic structure of the compound.rnA major part of the thesis deals with a systematical investigation of Heusler compounds for thermoelectric applications. Few studies have been reported on thermoelectric properties of p-type Heusler compounds. Therefore, this thesis focuses on the search for new p-type Heusler compounds with high thermoelectric efficiency. The substitutional series NiTi1−xMxSn and CoTi1−xMxSb (where M = Sc, V and 0 ≤ x ≤ 0.2) were synthesized and investigated theoretically and experimentally with respect to electronic structure and transport properties. The results show the possibility to create n-type and p-type thermoelectrics within one Heusler compound. The pure compounds showed n-type behavior, while under Sc substitution the system switched to p-type behavior. A maximum Seebeck coefficient of +230 μV/K (at 350 K) was obtained for NiTi0.26Sc0.04Zr0.35Hf0.35Sn, which is one of the highest values for p-type thermoelectric compounds based on Heusler alloys up to now. HAXPES valence band measurement show massive in gap states for the parent compounds NiTiSn, CoTiSb and NiTi0.3Zr0.35Hf0.35Sn. This proves that the electronic states close to the Fermi energy play a key role for the behavior of the transport properties. Furthermore, the electronic structure of the gapless Heusler compounds PtYSb, PtLaBi and PtLuSb were investigated by bulk sensitive HAXPES. The linear behavior of the spectra close to εF proves the bulk origin of Dirac-cone type density of states. Furthermore, a systematic study on the optical and transport properties of PtYSb is presented. The compound exhibits promising thermoelectric properties with a high figure of merit (ZT = 0.2) and a Hall mobility μh of 300 cm2/Vs at 350 K.rnThe last part of this thesis describes the linear dichroism in angular-resolved photoemission from the valence band of NiTi0.9Sc0.1Sn and NiMnSb. High resolution photoelectron spectroscopy was performed with an excitation energy of hν = 7.938 keV. The linear polarization of the photons was changed using an in-vacuum diamond phase retarder. Noticeable linear dichroism is found in the valence bands and this allows for a symmetry analysis of the contributing states. The differences in the spectra are found to be caused by symmetry dependent angular asymmetry parameters, and these occur even in polycrystalline samples without preferential crystallographic orientation.rnIn summary, Heusler compounds with 1:1:1 and 2:1:1 stoichiometry were synthesized and examined by chemical and physical methods. Overall, this thesis shows that the combination of first-principle calculations, transport measurements and high resolution high energy photoelectron spectroscopy analysis is a very powerful tool for the design and development of new materials for a wide range of applications from spintronic applications to thermoelectric applications.rn

A New Crystalline LiPON Electrolyte: Synthesis, Properties, and Electronic Structure

The new crystalline compound, Li2PO2N, was synthesized using high temperature solid state methods starting with a stoichiometric mixture of Li2O, P2O5, and P3N5. Its crystal structure was determined ab initio from powder X-ray diffraction. The compound crystallizes in the orthorhombic space group Cmc2(1) (# 36) with lattice constants a = 9.0692(4) angstrom, b = 53999(2) angstrom, and c = 4.6856(2) angstrom. The crystal structure of SD-Li2PO2N consists of parallel arrangements of anionic chains formed of corner sharing (PO2N2) tetrahedra. The chains are held together by Li+ cations. The structure of the synthesized material is similar to that predicted by Du and Holzwarth on the basis of first principles calculations (Phys. Rev. B 81,184106 (2010)). The compound is chemically and structurally stable in air up to 600 degrees C and in vacuum up to 1050 degrees C. The Arrhenius activation energy of SD-Li2PO2N in pressed pellet form was determined from electrochemical impedance spectroscopy measurements to be 0.6 eV, comparable to that of the glassy electrolyte LiPON developed at Oak Ridge National Laboratory. The minimum activation energies for Li ion vacancy and interstitial migrations are computed to be 0.4 eV and 0.8 eV, respectively. First principles calculations estimate the band gap of SD-Li2PO2N to be larger than 6 eV. (C) 2013 Elsevier B.V. All rights reserved.

Controlling electronic and magnetic properties of ultra narrow multilayered nanowires

Interest in the study of magnetic/non-magnetic multilayered structures took a giant leap since Grünberg and his group established that the interlayer exchange coupling (IEC) is a function of the non-magnetic spacer width. This interest was further fuelled by the discovery of the phenomenal Giant Magnetoresistance (GMR) effect. In fact, in 2007 Albert Fert and Peter Grünberg were awarded the Nobel Prize in Physics for their contribution to the discovery of GMR. GMR is the key property that is being used in the read-head of the present day computer hard drive as it requires a high sensitivity in the detection of magnetic field. The recent increase in demand for device miniaturization encouraged researchers to look for GMR in nanoscale multilayered structures. In this context, one dimensional(1-D) multilayerd nanowire structure has shown tremendous promise as a viable candidate for ultra sensitive read head sensors. In fact, the phenomenal giant magnetoresistance(GMR) effect, which is the novel feature of the currently used multilayered thin film, has already been observed in multilayered nanowire systems at ambient temperature. Geometrical confinement of the supper lattice along the 2-dimensions (2-D) to construct the 1-D multilayered nanowire prohibits the minimization of magnetic interaction- offering a rich variety of magnetic properties in nanowire that can be exploited for novel functionality. In addition, introduction of non-magnetic spacer between the magnetic layers presents additional advantage in controlling magnetic properties via tuning the interlayer magnetic interaction. Despite of a large volume of theoretical works devoted towards the understanding of GMR and IEC in super lattice structures, limited theoretical calculations are reported in 1-D multilayered systems. Thus to gauge their potential application in new generation magneto-electronic devices, in this thesis, I have discussed the usage of first principles density functional theory (DFT) in predicting the equilibrium structure, stability as well as electronic and magnetic properties of one dimensional multilayered nanowires. Particularly, I have focused on the electronic and magnetic properties of Fe/Pt multilayered nanowire structures and the role of non-magnetic Pt spacer in modulating the magnetic properties of the wire. It is found that the average magnetic moment per atom in the nanowire increases monotonically with an ~1/(N(Fe)) dependance, where N(Fe) is the number of iron layers in the nanowire. A simple model based upon the interfacial structure is given to explain the 1/(N(Fe)) trend in magnetic moment obtained from the first principle calculations. A new mechanism, based upon spin flip with in the layer and multistep electron transfer between the layers, is proposed to elucidate the enhancement of magnetic moment of Iron atom at the Platinum interface. The calculated IEC in the Fe/Pt multilayered nanowire is found to switch sign as the width of the non-magnetic spacer varies. The competition among short and long range direct exchange and the super exchange has been found to play a key role for the non-monotonous sign in IEC depending upon the width of the Platinum spacer layer. The calculated magnetoresistance from Julliere's model also exhibit similar switching behavior as that of IEC. The universality of the behavior of exchange coupling has also been looked into by introducing different non-magnetic spacers like Palladium, Copper, Silver, and Gold in between magnetic Iron layers. The nature of hybridization between Fe and other non-magnetic spacer is found to dictate the inter layer magnetic interaction. For example, in Fe/Pd nanowire the d-p hybridization in two spacer layer case favors anti-ferromagnetic (AFM) configuration over ferromagnetic (FM) configuration. However, the hybridization between half-filled Fe(d) and filled Cu(p) state in Fe/Cu nanowire favors FM coupling in the 2-spacer system.

Electronic tuning effects via cyano substitution of a fused tetrathiafulvalene–benzothiadiazole dyad for ambipolar transport properties

Electronic tuning effects of substituents at the 4- and 8-positions of benzothiadiazole (BTD) within the fused tetrathiafulvalene–BTD donor–acceptor dyad have been studied. The electron acceptor strength of BTD is greatly increased by replacing Br with CN groups, extending the optical absorption of the small dyad into the near-IR region and importantly, the charge transport can be switched from p-type to ambipolar behaviour.

First principles study of Bi dopen CdTe thin film solar cells: electronic and optical properties

Nowadays, efficiency improvement of solar cells is one of the most important issues in photovoltaic systems and CdTe is one of the most promising thin film photovoltaic materials we can found. CdTe reported efficiencies in solar energy conversion have been as good as that found in polycrystalline Si thin film cell [1], besides CdTe can be easily produced at industrial scale.

Electronic and photon absorber properties of Cr-Doped Cu2ZnSnS4

The Cu2ZnSnS4 (CZTS) semiconductor is a potential photovoltaic material due to its optoelectronic properties. These optoelectronic properties can be potentially improved by the insertion of intermediate states into the energy bandgap. We explore this possibility using Cr as an impurity. We carried out first-principles calculations within the density functional theory analyzing three substitutions: Cu, Sn, or Zn by Cr. In all cases, the Cr introduces a deeper band into the host energy bandgap. Depending on the substitution, this band is full, empty, or partially full. The absorption coefficients in the independent-particle approximation have also been obtained. Comparison between the pure and doped host's absorption coefficients shows that this deeper band opens more photon absorption channels and could therefo:e increase the solar-light absorption with respect to the host.