Surface Structure Determination of Black Phosphorus Using Photoelectron Diffraction

Atomic structure of single-crystalline black phosphorus was studied by high resolution synchrotron-based photoelectron diffraction (XPD). The results show that the topmost phosphorene layer in the black phosphorus is slightly displaced compared to the bulk structure and presents a small contraction in the direction perpendicular to the surface. Furthermore, the XPD results show the presence of a small buckling among the surface atoms, in agreement with previously reported scanning tunneling microscopy results. The contraction of the surface layer added to the presence of the buckling indicates an uniformity in the size of the sp3 bonds between P atoms at the surface

Determinação estrutural de ligas metálicas de superfície via difração de fotoelétrons

Orientadores: Richard Landers, George G. KleimanTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: As propriedades físico-químicas de um material, como uma liga metálica, podem ser entendidas como uma função da sua estrutura eletrônica e das posições relativas entre os átomos. A superfície representa um caso particular, e a quebra da periodicidade em uma direção pode provocar mudanças estruturais e eletrônicas importantes que afetam as propriedades do material. É particularmente importante conhecer a posição dos átomos na rede cristalina. No volume de um material isto pode ser feito através de técnicas consagradas como difração de raios-x; contudo, para a superfície esta tarefa é muito mais complicada. No caso de ligas de superfície estas apresentam-se como novos materiais e desenvolvem um grande interesse do ponto de vista acadêmico e aplicado. Este trabalho apresenta uma investigação da estrutura eletrônica e cristalográfica de ligas de superfície a partir de filmes finos crescidos por MBE sobre superfícies monocristalinas bem caracterizadas. A caracterização da dinâmica de crescimento dos filmes foi feita por XPS e RHEED; e na determinação estrutural das ligas utilizou-se LEED (qualitativo) e Difração de Fotoelétrons. Para o caso de liga ordenada de superfície também é apresentado a determinação estrutural por cálculos de primeiros princípios usando a Teoria do Funcional-Densidade. Nesta dissertação são apresentados os casos de Pd sobre Cu(111), Cu sobrePd(111) e Sb sobre Pd(111)Abstract: The physico-chemical properties of a material, such as a metallic alloy, can be understood as a function of its electronic structure and the relative positions between its atoms. The surface is a particular case in the material, due to the break of periodicity in the direction perpendicular to the surface this can affect the structural and electronic properties of the material. It is particularly important to know the position of the atoms in the crystalline lattice. In the bulk of a material this can be measured using techniques such as rays-x diffraction. For the surface this task is much more complicated. Surface alloys represent new materials and their development is of great interest both from the academic and technological point of view. The present body of work presents a study of the electronic and geometric structure of surface alloys; grown by MBE on well-characterized monocrystalline surfaces. XPS and RHEED were used to characterize the dynamics of growth, and LEED (qualitative) and Photoelectron Diffraction did the structural determination of surface alloys. For the particular case of ordered surface alloys we also present a theoretical first principles structural determination using Density Functional-Theory. This dissertation shows three systems: Pd on Cu(111), Cu on Pd(111) and Sb on Pd(111)DoutoradoFísicaDoutor em Ciência

Spin orientation in an ultrathin CoO/PtFe double-layer with perpendicular exchange coupling

International audienceWe studied by soft x-ray absorption spectroscopy the magnetization axis in an 4nm thin CoO (111) layer exchange-coupled to an ultra thin L1 0 PtFe layer with perpendicular magnetic anisotropy. The angular dependence of the linear magnetic dichroism at 10K and the relative variations of the spectral features provide a full description of the spin orientation in this antiferromagnetic layer. The spins are found in the film plane, pointing along the 110 direction. This results is discussed in relation to the film strain and preferential occupation of t 2g orbitals.The strong orthogonal coupling between Co and Fe spins should be at the origin of the robustness of the exchange bias effect foun in this bilayer system

Self-assembly Of Nitpp On Cu(111): A Transition From Disordered 1d Wires To 2d Chiral Domains.

The growth and self-assembling properties of nickel-tetraphenyl porphyrins (NiTPP) on the Cu(111) surface are analysed via scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). For low coverage, STM results show that NiTPP molecules diffuse on the terrace until they reach the step edge of the copper surface forming a 1D system with disordered orientation along the step edges. The nucleation process into a 2D superstructure was observed to occur via the interaction of molecules attached to the already nucleated 1D structure, reorienting molecules. For monolayer range coverage a 2D nearly squared self-assembled array with the emergence of chiral domains was observed. The XPS results of the Ni 2p(3/2) core levels exhibit a 2.6 eV chemical shift between the mono- and multilayer configuration of NiTPP. DFT calculations show that the observed chemical shifts of Ni 2p(3/2) occur due to the interaction of 3d orbitals of Ni with the Cu(111) substrate.1718344-1835

From quenched to unquenched orbital magnetic moment on metallic@oxide nanoparticles: dc magnetic properties and electronic correlation

FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOIn this study, the correlation between magnetic, structure, and electronic properties of Ag@Fe3O4 hetero nanostructures are presented. These nanostructures were prepared using a two-step new chemical approach. Three different nanoparticle systems with different Ag concentrations have been prepared and characterized using high resolution transmission electron microscopy, dc magnetization (magnetization and coercive field as a function of temperature), X-ray absorption near edge spectroscopy, and magnetic circular dichroism studies (XMCD). From the correlation between XMCD and dc magnetic measurements (Verwey transition) the presence of non-stoichiometric magnetite in Ag@Fe3O4 nanoparticle systems was confirmed. From the spin and orbital contribution to the total magnetic moment, we conclude that the sample with less Ag seeds particle concentration presents a non-quenched orbital contribution. These phenomena were analyzed based on the actual models and correlated with dc magnetic properties. From these, we conclude that the enhancement on the orbital contribution increases the spin orbital interaction, also increasing the magnetocrystalline anisotropy reflected on the dc magnetic properties.In this study, the correlation between magnetic, structure, and electronic properties of Ag@Fe3O4 hetero nanostructures are presented. These nanostructures were prepared using a two-step new chemical approach. Three different nanoparticle systems with different Ag concentrations have been prepared and characterized using high resolution transmission electron microscopy, dc magnetization (magnetization and coercive field as a function of temperature), X-ray absorption near edge spectroscopy, and magnetic circular dichroism studies (XMCD). From the correlation between XMCD and dc magnetic measurements (Verwey transition) the presence of non-stoichiometric magnetite in Ag@Fe3O4 nanoparticle systems was confirmed. From the spin and orbital contribution to the total magnetic moment, we conclude that the sample with less Ag seeds particle concentration presents a non-quenched orbital contribution. These phenomena were analyzed based on the actual models and correlated with dc magnetic properties. From these, we conclude that the enhancement on the orbital contribution increases the spin orbital interaction, also increasing the magnetocrystalline anisotropy reflected on the dc magnetic properties.15118FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOSem informaçãoThis study was financially supported by the Brazilian agency FAPESP. These studies were performed in the Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP. TEM data were acquired by Dr. L. Socolovsky at the LME-HRTEM (JEM-3010) of the Brazilian Synchrotron Light Laboratory (LNLS). Carlos Ramos, from Instituto Balseiro, is also acknowledged for fruitful discussions

Structural and electronic characterization of Co nanostructures on Au(332)

FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCo nanoislands were grown on (332) vicinal surface of Au in UHV using the e-beam evaporation technique. Scanning tunneling microscopy results reveal that Co deposition occurs following an islanding mode for θCo ranging from 0.17 to 0.64 ML. At low coverage nanoislands show a monolayer height, while at higher Co loadings, islands have a maximum bilayer height. XPS measurements rule out the possibility of alloy formation provided that binding energy of Co2p core lines remains unchanged as cobalt loading increases. Also, XPS data reveals that, when subjected to thermal annealing, Co atoms diffuse into Au crystal retaining its chemical nature as before the annealing. Finally, NO adsorption experiments show that Co nanostructures are partially oxidized upon adsorption, as evidenced by changes in core photoemission lineshapes of the Co2p lines. Also, NO adsorption seems to inhibit Co atom diffusion into Au crystal during moderate thermal treatment.Co nanoislands were grown on (332) vicinal surface of Au in UHV using the e-beam evaporation technique. Scanning tunneling microscopy results reveal that Co deposition occurs following an islanding mode for θCo ranging from 0.17 to 0.64 ML. At low coverage nanoislands show a monolayer height, while at higher Co loadings, islands have a maximum bilayer height. XPS measurements rule out the possibility of alloy formation provided that binding energy of Co2p core lines remains unchanged as cobalt loading increases. Also, XPS data reveals that, when subjected to thermal annealing, Co atoms diffuse into Au crystal retaining its chemical nature as before the annealing. Finally, NO adsorption experiments show that Co nanostructures are partially oxidized upon adsorption, as evidenced by changes in core photoemission lineshapes of the Co2p lines. Also, NO adsorption seems to inhibit Co atom diffusion into Au crystal during moderate thermal treatment.6178793FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP [Proc. 2011/12.566-3]CNPq [Proc. 160172/2011-0]2011/12.566-3160172/2011-