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Preparation and study of 2-D semiconductors with Dirac type bands due to the honeycomb nanogeometry Kalesaki, Efterpi ; ; et al in Proceedings of SPIE (2014, March 07), 8981 The interest in 2-dimensional systems with a honeycomb lattice and related Dirac-type electronic bands has exceeded the prototype graphene [1]. Currently, 2-dimensional atomic [2,3] and nanoscale [4-8 ... [more ▼] The interest in 2-dimensional systems with a honeycomb lattice and related Dirac-type electronic bands has exceeded the prototype graphene [1]. Currently, 2-dimensional atomic [2,3] and nanoscale [4-8] systems are extensively investigated in the search for materials with novel electronic properties that can be tailored by geometry. The immediate question that arises is how to fabricate 2-D semiconductors that have a honeycomb nanogeometry, and as a consequence of that, display a Dirac-type band structure? Here, we show that atomically coherent honeycomb superlattices of rocksalt (PbSe, PbTe) and zincblende (CdSe, CdTe) semiconductors can be obtained by nanocrystal self-assembly and facet-to-facet atomic bonding, and subsequent cation exchange. We present a extended structural analysis of atomically coherent 2-D honeycomb structures that were recently obtained with self-assembly and facet-to-facet bonding [9]. We show that this process may in principle lead to three different types of honeycomb structures, one with a graphene type-, and two others with a silicene-type structure. Using TEM, electron diffraction, STM and GISAXS it is convincingly shown that the structures are from the silicene-type. In the second part of this work, we describe the electronic structure of graphene-type and silicene type honeycomb semiconductors. We present the results of advanced electronic structure calculations using the sp3d5s* atomistic tight-binding method10. For simplicity, we focus on semiconductors with a simple and single conduction band for the native bulk semiconductor. When the 3-D geometry is changed into 2-D honeycomb, a conduction band structure transformation to two types of Dirac cones, one for S- and one for P-orbitals, is observed. The width of the bands depends on the honeycomb period and the coupling between the nanocrystals. Furthermore, there is a dispersionless P-orbital band, which also forms a landmark of the honeycomb structure. The effects of considerable intrinsic spin-orbit coupling are briefly considered. For heavy-element compounds such as CdTe, strong intrinsic spin-‐orbit coupling opens a non-trivial gap at the P-orbital Dirac point, leading to a quantum Spin Hall effect [10-12]. Our work shows that well known semiconductor crystals, known for centuries, can lead to systems with entirely new electronic properties, by the simple action of nanogeometry. It can be foreseen that such structures will play a key role in future opto-electronic applications, provided that they can be fabricated in a straightforward way. [less ▲] Detailed reference viewed: 144 (1 UL)Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry Kalesaki, Efterpi ; ; et al in Physical Review X (2014), 4(1), 011010 We study theoretically two-dimensional single-crystalline sheets of semiconductors that form a honeycomb lattice with a period below 10 nm. These systems could combine the usual semiconductor properties ... [more ▼] We study theoretically two-dimensional single-crystalline sheets of semiconductors that form a honeycomb lattice with a period below 10 nm. These systems could combine the usual semiconductor properties with Dirac bands. Using atomistic tight-binding calculations, we show that both the atomic lattice and the overall geometry influence the band structure, revealing materials with unusual electronic properties. In rocksalt Pb chalcogenides, the expected Dirac-type features are clouded by a complex band structure. However, in the case of zinc-blende Cd-chalcogenide semiconductors, the honeycomb nanogeometry leads to rich band structures, including, in the conduction band, Dirac cones at two distinct energies and nontrivial flat bands and, in the valence band, topological edge states. These edge states are present in several electronic gaps opened in the valence band by the spin-orbit coupling and the quantum confinement in the honeycomb geometry. The lowest Dirac conduction band has S-orbital character and is equivalent to the π−π⋆ band of graphene but with renormalized couplings. The conduction bands higher in energy have no counterpart in graphene; they combine a Dirac cone and flat bands because of their P-orbital character. We show that the width of the Dirac bands varies between tens and hundreds of meV. These systems emerge as remarkable platforms for studying complex electronic phases starting from conventional semiconductors. Recent advancements in colloidal chemistry indicate that these materials can be synthesized from semiconductor nanocrystals. [less ▲] Detailed reference viewed: 153 (7 UL)Electronic structure of atomically coherent square semiconductor superlattices with dimensionality below two Kalesaki, Efterpi ; ; et al in Physical Review. B : Condensed Matter (2013), 88(11), 9 Detailed reference viewed: 144 (1 UL)Dielectric function of colloidal lead chalcogenide quantum dots obtained by a Kramers-Kronig analysis of the absorbance spectrum ; ; et al in Physical Review. B, Condensed Matter and Materials Physics (2010), 81(23), We combined the Maxwell-Garnett effective medium theory with the Kramers-Kronig relations to obtain the complex dielectric function epsilon of colloidal PbS, PbSe, and PbTe quantum dots (Qdots). The ... [more ▼] We combined the Maxwell-Garnett effective medium theory with the Kramers-Kronig relations to obtain the complex dielectric function epsilon of colloidal PbS, PbSe, and PbTe quantum dots (Qdots). The method allows extracting both real (epsilon(R)) and imaginary (epsilon(I)) parts of the dielectric function from the experimental absorption spectrum. This enables the quantification of the size-dependent oscillator strength of the optical transitions at different critical points in the Brillouin zone, strongly improving our understanding of quantum confinement effects in these materials. In addition, the static-limit sum rule yields the electronic dielectric constant from the epsilon(I) spectrum. Interestingly, values for lead chalcogenide Qdots remain close to the bulk dielectric constant. To verify these trends, we determined the dielectric constant of thin lead chalcogenide layers by ab initio calculations, and the results agree with the experimental data. [less ▲] Detailed reference viewed: 129 (3 UL)Near Kohn anomalies in the phonon dispersion relations of lead chalcogenides ; ; Wirtz, Ludger in Physical Review B (2009), 80(24), We present ab initio phonon dispersion relations for the three lead chalcogenides PbS, PbSe, and PbTe. The acoustic branches are in very good agreement with inelastic neutron-scattering data and ... [more ▼] We present ab initio phonon dispersion relations for the three lead chalcogenides PbS, PbSe, and PbTe. The acoustic branches are in very good agreement with inelastic neutron-scattering data and calculations of the specific heat give good agreement with experimental data. The pronounced minimum of the transverse-optical branch at Gamma due to the near ferroelectricity of the lead chalcogenides is qualitatively reproduced. In addition, we find a pronounced dip in the longitudinal-optical branch at Gamma. This dip was previously explained as the effect of "free carriers" (due to the presence of impurities). The calculations demonstrate that it persists also in the case of pure lead chalcogenides. We explain the dip as a "near Kohn anomaly" which is associated with the small electronic band gap at the high-symmetry point L. [less ▲] Detailed reference viewed: 102 (0 UL) |
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