Estados electrónicos de puntos cuánticos piramidales y cónicos

Abstract The electronic states confined in a quantum dot, of GaAs, of pyramidal and conical shape have been investigated through the quasi-analytical approach, valid for small angles, and the finite element method to include all angles and heights. The results of the confinement energy have been reported according to the shape and size of both structures and finally the approximate values have been compared with the exact values obtained from the finite element method.Resumo Os estados eletrônicos confinados em um ponto quântico, de GaAs, de forma piramidal e cônica foram investigados através da abordagem quase-analítica, válida para pequenos ângulos, e o método dos elementos finitos para incluir todos os ângulos e alturas. Os resultados da energia de confinamento foram relatados de acordo com a forma e tamanho de ambas estruturas e, finalmente, os valores aproximados foram comparados com os valores exatos obtidos pelo método dos elementos finitos.Resumen Los estados electrónicos confinados en un punto cuántico de GaAs, de forma piramidal y cónica, se han investigado a través del enfoque cuasi analítico válido para ángulos pequeños y el método exacto de elementos finitos para incluir todos los ángulos y alturas. Se han reportado los resultados de la energía de confinamiento en función de la forma y el tamaño de ambas estructuras y finalmente se han comparado los valores aproximados con los exactos provenientes del método de elementos finitos

Styrene oligomerization as a molecular probe reaction for Brønsted acidity at the nanoscale

The Brønsted acid-catalyzed oligomerization of 4-fluorostyrene has been studied on a series ofH-ZSM-5 zeolite powders, steamed under different conditions, with a combination of UV-Vismicro-spectroscopy and Scanning Transmission X-ray Microscopy (STXM). UV-Vis microspectroscopyand STXM have been used to monitor the relative formation of cyclic and lineardimeric carbocations as a function of the steaming post-treatment (i.e., parent vs. steaming at600, 700 and 800 1C). It was found that the UV-Vis band intensity ratios of linear to cyclicdimeric species increase from 0.79 (parent H-ZSM-5) over 1.41 (H-ZSM-5 steamed at 600 1C) and1.88 (H-ZSM-5 steamed at 700 1C) to 2.33 (H-ZSM-5 steamed at 800 1C). STXM confirms thistrend in reaction product selectivity, as the relative intensities of the transitions attributed to thepresence of the cyclic dimer in the carbon K-edge spectra decrease with increasing severity of thesteaming post-treatment. Furthermore, STXM reveals spatial heterogeneities in reaction productformation within the H-ZSM-5 zeolite powders at the nanoscale. More specifically, a shrinkingcarbon core–shell distribution was detected within the zeolite aggregates, in which the relativeamount of cyclic dimeric species is higher in the core relative to the shell of the zeolite aggregateand the relative amount of cyclic dimeric species in the zeolite core gradually decreases withincreasing severity of the steaming post-treatment. These differences are rationalized in terms ofspatial differences in Brønsted acidity within H-ZSM-5 zeolite powders as well as by changes inthe formation process of linear and dimeric carbocations within H-ZSM-5 micro- and mesopores

In-situ Scanning Transmission X-Ray Microscopy of Catalytic Solids and Related Nanomaterials

The present status of in-situ scanning transmission X-ray microscopy (STXM) is reviewed, with an emphasis on the abilities of the STXM technique in comparison with electron microscopy. The experimental aspects and interpretation of X-ray absorption spectroscopy (XAS) are briefly introduced and the experimental boundary conditions that determine the potential applications for in-situ XAS and in-situ STXM studies are discussed. Nanoscale chemical imaging of catalysts under working conditions is outlined using cobalt and iron Fischer-Tropsch catalysts as showcases. In the discussion, we critically compare STXM-XAS and STEM-EELS (scanning transmission electron microscopy-electron energy loss spectroscopy) measurements and indicate some future directions of in-situ nanoscale imaging of catalytic solids and related nanomaterials

In-situ Scanning Transmission X-Ray Microscopy of Catalytic Solids and Related Nanomaterials

The present status of in-situ scanning transmission X-ray microscopy (STXM) is reviewed, with an emphasis on the abilities of the STXM technique in comparison with electron microscopy. The experimental aspects and interpretation of X-ray absorption spectroscopy (XAS) are briefly introduced and the experimental boundary conditions that determine the potential applications for in-situ XAS and in-situ STXM studies are discussed. Nanoscale chemical imaging of catalysts under working conditions is outlined using cobalt and iron Fischer-Tropsch catalysts as showcases. In the discussion, we critically compare STXM-XAS and STEM-EELS (scanning transmission electron microscopy-electron energy loss spectroscopy) measurements and indicate some future directions of in-situ nanoscale imaging of catalytic solids and related nanomaterials