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Descripción
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| New forms of silicon may give rise to novel electronic properties. One striking case is that of Si clathrates, polymorphs of Si having fourfold connected frameworks with internal nanocavities which are known since decades [1] and have later received attention due to their superconducting and thermoelectric properties [2]; synthesis methods able to obtain them in thin film form have been recently reported [3]. Here we show that they can lead to materials presenting inside the bandgap an intermediate band (IB) of the kind claimed to lead to high efficiencies in photovoltaic (PV) cells by allowing electron excitation from the valence band to the conduction band in two steps, using two photons of sub-bandgap energy [4]. We study the type II clathrate, containing cavities made by 20- and 28-atom polyhedra (respectively 4 and 2 such cavities in each 34 Si atom primitive cell) and having, when free form metals, a bandgap of 1.9 eV [5], close to the optimum for efficient IB PV operation. We have carried out DFT quantum calculations on the system in which some Si atoms are substituted by vanadium; this system has not been reported to date, although substitutions by other metals like Ni, Cu, Pt or Au have been described [6]. The results show that vanadium, after forming four bonds with neighbouring Si atoms, keeps in one if its 3d orbitals an excess electron that forms a half-filled spin-polarized IB inside the gap. Another option is to insert a guest atom inside one cavity. This is a well-known situation for alkali and alkaline earth guests; in fact, clathrates are synthesized having always initially such inclusions. These metals donate their weakly bound s electrons to the conduction band of the Si lattice. Here we show that, on the contrary, if an atom with higher electron affinity, like Ag, is inserted in one of the large cavities its s-type electron is kept by the metal, forming an IB within the gap. The guest Ag atom spontaneously moves from the center of the cavity towards its walls, the lowest energy position staying near one of the sixfold windows that communicate the cavities among themselves. The electronic structures and overall features that result when inserting in the cavities other monovalent atoms like Cu and Au will be discussed as well. It is proposed that some of these structures, if prepared in thin film form, could lead to higher efficiency PV cells or to other interesting optical, magnetic or electronic devices. | |
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Internacional
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Si |
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Nombre congreso
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2012 MRS Spring Meeting & Exhibit |
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Tipo de participación
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960 |
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Lugar del congreso
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San Francisco, CA (EEUU) |
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Revisores
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Si |
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ISBN o ISSN
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000-0000-000-000 |
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DOI
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Fecha inicio congreso
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09/04/2012 |
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Fecha fin congreso
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13/04/2012 |
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Desde la página
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1 |
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Hasta la página
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2 |
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Título de las actas
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Proc. 2012 MRS Spring Meeting & Exhibit |