Chemisorption of atomic aluminum on Si(111): Evidence for an adsorbate-induced relaxation based on ab initio cluster-model calculations

Interaction models of atomic Al with Si4H9, Si4H7, and Si6H9 clusters have been studied to simulate Al chemisorption on the Si(111) surface in the atop, fourfold atop, and open sites. Calculations were carried out using nonempirical pseudopotentials in the framework of the ab initio Hartree-Fock procedure. Equilibrium bond distances, binding energies for adsorption, and vibrational frequencies of the adatoms are calculated. Several basis sets were used in order to show the importance of polarization effects, especially in the binding energies. Final results show the importance of considering adatom-induced relaxation effects to specify the order of energy stabilities for the three different sites, the fourfold atop site being the preferred one, in agreement with experimental findings.

Ab initio cluster-model study of the on-top chemisorption of F and Cl on Si(111) and Ge(111) surfaces

The interaction of atomic F and Cl with Si4H9 and Ge4H9 cluster models has been studied by using ab initio pseudopotentials and basis sets of increasing complexity. The results show that the effect of d orbitals is important in order to reproduce the experimental findings. However, the use of polarization functions in the atoms which are directly involved in the chemisorption bond leads to results which are very close to those obtained using extended basis sets. The local nature of the chemisorption bond is also interpreted by means of a Mulliken population analysis. For F-Si4H9 and Cl-Si4H9 the present results are in good agreement with previous ab initio all-electron calculations, and for the chemisorption of Cl on Si(111) and Ge(111) surfaces, good agreement is found with respect to the available experimental results as well as with previous slab calculations based on the local-density-functional formalism.

Chemisorption of group-III metals on the Si(111) and Ge(111) surfaces: An ab initio study

Chemisorption of group-III metal adatoms on Si(111) and Ge(111) has been studied through the ab initio Hartree-Fock method including nonempirical pseudopotentials and using cluster models to simulate the surface. Three different high-symmetry sites (atop, eclipsed, and open) have been considered by using X4H9, X4H7, and X6H9 (X=Si,Ge) cluster models. In a first step, ideal surface geometries have been used. Metal-induced reconstruction upon chemisorption has also been taken into account. Equilibrium distances, binding energies, and vibrational frequencies have been obtained and compared with available experimental data. From binding-energy considerations, the atop and eclipsed sites seem to be the most favorable ones and thus a coadsorption picture may be suggested. Group-III metals exhibit a similar behavior and the same is true for Si(111) and Ge(111) surfaces when chemisorption is considered.

Evidence for two different bonding mechanisms of Al on Si(111)

By means of the ab initio cluster-model approach, we present theoretical evidence for two different mechanisms of bonding of atomic Al to Si(111). On the atop site (T1) the interaction of atomic Al to Si(111) is characteristic of an ionic bond whereas interaction above the threefold eclipsed site (T4) leads to the formation of a typical covalent bond. Moreover, both sites have a similar interaction energy if electronic correlation effects are included. While the conclusions regarding the nature of the chemisorption bond in the two sites do not depend either on the cluster-model size, the kind of embedding hydrogen atoms used, or the quality of the wave function (Hartree-Fock or configuration interaction), the chemisorption energy depends strongly on the wave function used. In fact, inclusion of correlation energy is necessary to properly describe the interaction energies.

Performance of correlation functionals in ab initio chemisorption cluster-model calculations: Alkali metals on Si(111)

The performance of different correlation functionals has been tested for alkali metals, Li to Cs, interacting with cluster models simulating different active sites of the Si(111) surface. In all cases, the ab initio Hartree-Fock density has been obtained and used as a starting point. The electronic correlation energy is then introduced as an a posteriori correction to the Hartree-Fock energy using different correlation functionals. By making use of the ionic nature of the interaction and of different dissociation limits we have been able to prove that all functionals tested introduce the right correlation energy, although to a different extent. Hence, correlation functionals appear as an effective and easy way to introduce electronic correlation in the ab initio Hartree-Fock description of the chemisorption bond in complex systems where conventional configuration interaction techniques cannot be used. However, the calculated energies may differ by some tens of eV. Therefore, these methods can be employed to get a qualitative idea of how important correlation effects are, but they have some limitations if accurate binding energies are to be obtained.

A construção de si pela atividade de trabalho: a socialização profissional

A sociologia funcionalista das profissões distingue dois tipos muito diferentes de atividades de trabalho: as profissões e as ocupações (ambas no sentido que os ingleses lhes dão). Como apenas as primeiras (a dos médicos, advogados, engenheiros, professores...) são consideradas escolhas e áreas autônomas que permitem a construção de uma carreira, as segundas (que constituem a maioria) acabam sendo desvalorizadas. Os sociólogos interacionistas (como os de Chicago: Hughes, Becker, Strauss etc.) e críticos (marxianos, weberianos etc.) contestam essa posição e consideram que todas as atividades de trabalho poderiam se tornar "profissionais" (no sentido francês), desde que resultassem de uma socialização que permitisse a aquisição de competências e o reconhecimento (inclusive monetário) de todos os que exercem e compartilham uma mesma atividade. A comparação da socialização de clínicos gerais e de auxiliares de enfermagem na França ilustra esta tese, ao tomar o trabalho como um processo de construção e de reconhecimento de si.

Escrita de resumos e estratégias de autorregulação da aprendizagem

Com este estudo, que se situa na articulação de áreas das ciências humanas, quer-se verificar se autores de resumos evidenciam em seus textos conhecer as características desse gênero discursivo e se mostram competência autorregulada ao fazê-lo. Primeiramente, foram analisados 24 resumos, realizados por candidatos ao mestrado de uma instituição de ensino federal. Para isso, construiu-se um instrumento de análise e de ponderações quantitativas, procurando melhor expressar os resultados. Dos 24 resumos analisados, cinco foram considerados compatíveis com uma produção escrita, por expressarem semanticamente propriedades de leitura e de organização discursiva; 13 não apresentaram as propriedades esperadas; outros seis apresentaram problemas graves, como cópia do trabalho original. As análises foram alicerçadas pelo construto da autorregulação da aprendizagem e por teorias linguísticas discursivas.

Ya Adimou Chani ; Ya Adimou Chani (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et perc.]

[Traditions. Afrique du Nord. Algérie]

Ya Adimou Chani ; El Hamed El Rab El Hanine / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et instr.]

[Traditions. Afrique du Nord. Algérie]

Billah Ya Ziar ; Billah Ya Ziar (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et instr.]

[Traditions. Afrique du Nord. Algérie]

Allah Ya Ramani ; Allah Ya Ramani (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc.voc et instr.]

[Traditions. Afrique du Nord. Algérie]

Salouala Has Brouhi ; Salouala Has Brouhi (suite) / Si Mahmoud Hadjadji El Ouargli, chant [acc. voc. et instr.]

[Traditions. Afrique du Nord. Algérie]