Dipartimento di Fisica
SEMINARIO DI STRUTTURA
DR. A. Sindona
(∗ 1Dip. di Fisica, Universit`a della Calabria, INFN, sezione LNF, Gruppo collegato di Cosenza, Rende (CS), Italy)
“Interband π-like plasmon in silicene grown on silver”
Abstract
Following isolation of graphene sheets by mechanical exfoliation of its parent crystal graphite [1], an enormous effort has been directed towards two-dimensional (2D) crystals made of group-IV elements other than Carbon. A particularly noteworthy example is silicene, the siliconequivalent of graphene with a natural compatibility with current semiconductor technology. First-principles calculations of the structural properties report an intrinsic stability of a honeycomb arrangement of Si atoms in slightly buckled form, with mixed sp2 -sp3 hybridization [2]. This system, referred to as freestanding silicene, presents a graphene-like, semimetallic electronic structure characterized by linearly dispersing π and π ∗ bands around the Fermi energy, which allows charge-carriers to mimic massless relativistic particles. On the practical side, silicenelike nanostructures are synthesized by epitaxial growth of silicon on silver [3] and a few other metal substrates [4], very recently including gold [5]. In particular, a number of well-ordered domains have been observed on Ag(111), the formation and coexistence of which depend on the substrate temperature, during silicon growth, and the silicon deposition rate [6]. Most of these domains have a crystalline morphology closely commensurate with the (4×4), (2√ 3×2 √ 3)R30◦ , (√ 13× √ 13)R13.9 ◦ and (√ 7× √ 7)R19.1 ◦ phases of Ag(111). The different orientation, out-of-plane atomic buckling and lattice constant of the Si atoms in these superstructures are re- flected in markedly distinct electronic properties of the silicene overlayer, which, even without the supporting Ag substrate below, may or may not preserve the Dirac cones of its (1×1) freestanding form [7]. As for the interaction with Ag(111), silicene seems to lose such unique properties, as confirmed by angular-resolved photoemission spectroscopy (ARPES) measurements combined with density functional calculations [8]. Nonetheless, some evidences indicate that forms of Dirac cones exist in the (4×4) phase , which have been reinterpreted as originating from the s and p states of bulk Ag, or the strongly hybridized sp states of Ag and Si [9]. Even in this latter case, however, the π-like bands of freestanding silicene are destroyed by its interaction with Ag, and the transfer of the silicene to other substrates is impractical. The major goal of silicene technology is still to grow a honeycomblike structure of Si atoms with semimetallic, π-like electronic features. A new possible direction is outlined here, where, rather than looking at the band structure of silicene on Ag(111), we consider exploring the intrinsic plasmonics of the interface, i.e., the coherent charge-density oscillations of its valence electrons (plasmons). In particular, we present an electron energy loss spectroscopy (EELS) characterization of two surface reconstructions of silicene on Ag(111), due the (2√ 3×2 √ 3)R30◦ phase and a mixture of the (4×4), (2√ 3×2 √ 3)R30◦ and (√ 13× √ 13)R13.9 ◦ superstructures, respectively. The measurements are consistent with time dependent (TD) density functional theory (DFT) calculations [10] of the EL function of freestanding silicene [11], showing a well resolved loss peak at 1.75 eV for a momentum transfer of the order of 10−2˚A −1 . Our results indicate that the π or π ∗ character of the lowest-lying energy bands in silicene grown on Ag(111) may be maintained under specific geometric conditions. On the other hand, the π-like mode is absent in each single pure phase of the above mentioned types, which confirms that the semimetallic character of freestanding silicene is lost in favor of the hybridized band structure.
1 2 3456 Energy Loss ω(eV) Intensity (arb. units) (i) (iii)×4 Silicene on Ag(111) Mixed Phase (iii) clean Ag(111) (ii) Silicene on Ag(111) 2√3x2√3R30o Phase TDDFT 450 460 exp. geom. silicene (1x1) π-like Plasmon A. Sindona et al, arXiv:1708.03858 [cond-mat.mtrl-sci] ∗ [1] K. S. Novoselov et al, Science 306, 666 (2004) [2] K. Takeda and K. Shiraishi, Phys. Rev. B 50, 14916 (1994); S. Cahangirov et al, Phys. Rev. Lett. 102, 236804 (2009) [3] P. Vogt et al, Phys. Rev. Lett. 108, 155501 (2012) [4] A. Fleurence et al, Phys. Rev. Lett. 108 245501 (2012); L. Meng et al, Nano Lett. 13 685 (2013)
Giovedi’ 19 Ottobre 2017 ore 14,30
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