R. Magalhães-Paniago

General equation for the determination of the crystallite size La of nanographite by Raman spectroscopy

This work presents a systematic study of the ratio between the integrated intensities of the disorder-induced D and G Raman bands (ID∕IG) in nanographite samples with different crystallite sizes (La) and using different excitation laser energies. The crystallite size La of the nanographite samples was obtained both by x-ray diffraction using synchrotron radiation and directly from scanning tunneling microscopy images. A general equation for the determination of La using any laser energy in the visible range is obtained. Moreover, it is shown that ID∕IG is inversely proportional to the fourth power of the laser energy used in the experiment.

X-ray method to study temperature-dependent stripe domains in MnAs∕GaAs(001)

MnAs films grown on GaAs (001) exhibit a progressive transition between hexagonal (ferromagnetic) and orthorhombic (paramagnetic) phases at wide temperature range instead of abrupt transition during the first-order phase transition. The coexistence of two phases is favored by the anisotropic strain arising from the constraint on the MnAs films imposed by the substrate. This phase coexistence occurs in ordered arrangement alternating periodic terrace steps. We present here a method to study the surface morphology throughout this transition by means of specular and diffuse scattering of soft x rays, tuning the photon energy at the Mn2p resonance. The results show the long-range arrangement of the periodic stripe-like structure during the phase coexistence and its period remains constant, in agreement with previous results using other techniques.

Direct observation of the coexistence of coherent and incoherent InAs self-assembled dots by x-ray scattering

In this letter, grazing incidence x-ray scattering is employed as a method to identify relaxed islands in an ensemble of partially coherent self-assembled InAs quantum dots. A simple model of strained pyramidal islands enables the association of the local lattice parameter of an island to its lateral size. A comparison between the island side length and its strain state allows the identification of coherent and incoherent nanostructures, revealing the size–strain interplay during growth.

Determination of the epitaxial growth of zinc oxide nanowires on sapphire by grazing incidence synchrotron x-ray diffraction

This letter shows that aligned zinc oxide (ZnO) nanowires growth on sapphire substrates is epitaxial and demonstrates the crystallographic relation between the two using grazing incidence synchrotron x-ray diffraction (XRD). The in-plane lattice match between the sapphire and the nanowires was directly probed by using XRD at grazing angles of incidence, where the lattice match between the (0001) plane of the sapphire and the (11−20) plane of the ZnO were observed simultaneously. It will also be shown that gold acts as a catalyst to initiate ZnO nanowire growth, but it does not interfere with the epitaxial mechanism between the nanowires and the sapphire substrate.

Temperature-Induced Coexistence of a Conducting Bilayer and the Bulk-Terminated Surface of the Topological Insulator Bi2Te3

Topological insulators such as Bi2Se3 and Bi2Te3 have extremely promising transport properties, due to their unique electronic behavior: they are insulators in the bulk and conducting at the surface. Recently, the coexistence of two types of surface conducting channels has been observed for Bi2Se3, one being Dirac electrons from the topological state and the other electrons from a conventional two-dimensional gas. As an explanation for this effect, a possible structural modification of the surface of these materials has been hypothesized. Using scanning tunneling microscopy we have directly observed the coexistence of a conducting bilayer and the bare surface of bulk-terminated Bi2Te3. X-ray crystal truncation rod scattering was used to directly show the stabilization of this epitaxial bilayer which is primarily composed of bismuth. Using this information, we have performed density functional theory calculations to determine the electronic properties of the possible surface terminations. They can be used to understand recent angular resolved photoemission data which have revealed this dual surface electronic behavior.

Correlation between (in)commensurate domains of multilayer epitaxial graphene grown on SiC(

A systematic study of the evolution of the electronic behavior and atomic structure of multilayer epitaxial graphene (MEG) as a function of growth time was performed. MEG was obtained by sublimation of a 4H-SiC(0001(-)) substrate in an argon atmosphere. Raman spectroscopy and x-ray diffraction were carried out in samples grown for different times. For 30 min of growth the sample Raman signal is similar to that of graphite, while for 60 min the spectrum becomes equivalent to that of exfoliated graphene. Conventional x-ray diffraction reveals that all the samples have two different (0001) lattice spacings. Grazing incidence x-ray diffraction shows that thin films are composed of rotated (commensurate) structures formed by adjacent graphene layers. Thick films are almost completely disordered. This result can be directly correlated to the single layer electronic behavior of the films as observed by Raman spectroscopy. Finally, to understand the change in lattice spacings as a result of layer rotation, we have carried out first principles calculations (using density functional theory) of the observed commensurate structures.

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We investigated structural and optical properties of type-II InP/GaAs quantum dots using reflection high energy electron diffraction, transmission electron microscopy, atomic force microscopy, grazing incidence x-ray diffraction, and photoluminescence techniques. The InP dots present an efficient optical emission even when they are uncapped, which is attributed to the low surface recombination velocity in InP. We compare the difference in the optical properties between surface free dots, which are not covered by any material, with dots covered by a GaAs capping layer. We observed a bimodal dispersion of the dot size distribution, giving rise to two distinct emission bands. The results also revealed that the strain accumulated in the InP islands is slightly relieved for samples with large InP amounts. An unexpected result is the relatively large blue shift of the emission band from uncapped samples as compared to capped dots.

) and single layer electronic behavior

Three-dimensional strain mapping of InAs self-assembled nanowires on an InP substrate using grazing incidence x-ray diffraction is reported. A remarkable anisotropy was observed for the strain components, parallel [−220] and perpendicular [220] to the wire axis. The highest strain relaxation was measured along the [220] direction. The relationship between the interatomic distances along the [−220] and [220] directions, for each z position (height) in the nanostructure, was obtained by angular scans in the vicinity of the (040) reciprocal lattice point.

Structural and optical properties of InP quantum dots grown on GaAs(001)

The degree of vertical alignment of InAs quantum dots in InAs/GaAs(001) multilayers was studied using grazing incidence x-ray scattering. We show that it is necessary to access one of the weak (200) x-ray reflections to observe the modulation of the GaAs lattice periodicity produced by the stacking of the InAs dots. The degree of alignment of the dots was assessed by fitting the x-ray diffuse scattering profiles near a GaAs (200) reciprocal lattice point. By using a model of gaussian lateral displacement of the dots, we show that we can determine the average value of the mistake in stacking positions of the islands from one bilayer to the next.

Three-dimensional mapping of the strain anisotropy in self-assembled quantum-wires by grazing incidence x-ray diffraction

We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on GaAs(001) during the coexistence of hexagonal/orthorhombic phases using polarized resonant (magnetic) soft x-ray scattering and magnetic force microscopy. The results of the diffuse satellite x-ray peaks were compared to those obtained by magnetic force microscopy and suggest a reorientation of ferromagnetic terraces as temperature rises. By measuring hysteresis loops at these peaks we show that this reorientation is common to all ferromagnetic terraces. The reorientation is explained by a simple model based on the shape anisotropy energy. Demagnetizing factors were calculated for different configurations suggested by the magnetic images. We noted that the magnetic moments flip from an in-plane monodomain orientation at lower temperatures to a three-domain out-of-plane configuration at higher temperatures. The transition was observed when the ferromagnetic stripe width L is equal to 2.9 times the film thickne...