References of "Kalesaki, Efterpi 50002066"
     in
Bookmark and Share    
Full Text
Peer Reviewed
See detailValence band splitting in Cu2(Sn,Ge, Si)S3: effect on optical absorption spectra
De Wild, Jessica UL; Kalesaki, Efterpi UL; Wirtz, Ludger UL et al

in Physica Status Solidi. Rapid Research Letters (2017)

We perform a detailed analysis of the valence band splitting (VBS) effect on the absorption spectra of monoclinic Cu2(Sn,Ge,Si)S3 combining theory and experiment. We cal- culate the imaginary part of the ... [more ▼]

We perform a detailed analysis of the valence band splitting (VBS) effect on the absorption spectra of monoclinic Cu2(Sn,Ge,Si)S3 combining theory and experiment. We cal- culate the imaginary part of the dielectric function for all three compounds using hybrid functionals and maximally lo- calized Wannier functions in remarkably dense k-meshes to ensure an accurate description of the low energy spectral regime. We find that the VBS will affect the absorption spectra of these materials leading to multiple absorption onsets. Our experimental spectra on Cu2(Sn,Ge)S3, analysed using both Tauc plots and inflection points, verify this prediction. A good agreement between theory and experiment in terms of VBS values is recorded. [less ▲]

Detailed reference viewed: 200 (10 UL)
Full Text
Peer Reviewed
See detailTopological states in multi-orbital ​HgTe honeycomb lattices
Beugeling, Wouter; Kalesaki, Efterpi UL; Delerue, Christophe et al

in Nature Communications (2015), 6(6316),

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. However, the quantum spin Hall effect predicted at the K point could not be observed in graphene and other ... [more ▼]

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. However, the quantum spin Hall effect predicted at the K point could not be observed in graphene and other honeycomb structures of light elements due to an insufficiently strong spin–orbit coupling. Here we show theoretically that 2D honeycomb lattices of ​HgTe can combine the effects of the honeycomb geometry and strong spin–orbit coupling. The conduction bands, experimentally accessible via doping, can be described by a tight-binding lattice model as in graphene, but including multi-orbital degrees of freedom and spin–orbit coupling. This results in very large topological gaps (up to 35 meV) and a flattened band detached from the others. Owing to this flat band and the sizable Coulomb interaction, honeycomb structures of ​HgTe constitute a promising platform for the observation of a fractional Chern insulator or a fractional quantum spin Hall phase. [less ▲]

Detailed reference viewed: 172 (47 UL)
Full Text
See detailTopological States in Multi-Orbital Honeycomb Lattices of HgTe (CdTe) Quantum Dots
Beugeling, Wouter; Kalesaki, Efterpi UL; Delerue, Christophe et al

in ECS Transactions (2015), 69(5), 81-88

We summarize recent theoretical works on artificial graphene realized by honeycomb lattices of semiconductor (CdSe, HgTe, CdTe) quantum dots forming a two-dimensional single-crystalline sheet. In the case ... [more ▼]

We summarize recent theoretical works on artificial graphene realized by honeycomb lattices of semiconductor (CdSe, HgTe, CdTe) quantum dots forming a two-dimensional single-crystalline sheet. In the case of CdSe, we predict conduction bands with Dirac cones at two distinct energies and nontrivial flat bands. An analogous behavior is found in HgTe but, in addition, the strong spin-orbit coupling opens large topologically nontrivial gaps, leaving a flattened band detached from the others. We deduce that honeycomb lattices of HgTe quantum dots may constitute promising platforms for the observation of a fractional Chern insulator or a fractional quantum spin Hall phase. Similar predictions are made for CdTe but with smaller nontrivial gaps. [less ▲]

Detailed reference viewed: 32 (0 UL)
Full Text
Peer Reviewed
See detailPreparation and study of 2-D semiconductors with Dirac type bands due to the honeycomb nanogeometry
Kalesaki, Efterpi UL; Boneschanscher, M. P.; Geuchies, J. J. 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: 72 (1 UL)
Full Text
Peer Reviewed
See detailDirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry
Kalesaki, Efterpi UL; Delerue, Christophe; Morais Smith, Cristiane 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: 104 (7 UL)
Full Text
Peer Reviewed
See detailElectronic structure of atomically coherent square semiconductor superlattices with dimensionality below two
Kalesaki, Efterpi UL; Evers, Wiel; Allan, Guy et al

in Physical Review. B : Condensed Matter (2013), 88(11), 9

Detailed reference viewed: 97 (1 UL)
Full Text
Peer Reviewed
See detailReconstructions and electronic structure of (11-22) and (11-2-2) semipolar AlN surfaces
Kalesaki, Efterpi UL; Lymperakis, Liverios; Kioseoglou, Joseph et al

in Journal of Applied Physics (2012), 112

The energetics, atomic geometry, and electronic structure of semipolar (11-22) and (11-2-2) AlN surfaces are investigated employing first principles calculations. For metal-rich growth conditions ... [more ▼]

The energetics, atomic geometry, and electronic structure of semipolar (11-22) and (11-2-2) AlN surfaces are investigated employing first principles calculations. For metal-rich growth conditions, metallic reconstructions are favoured on both polarity surfaces. For N rich to moderate Al rich conditions, the (11-22) planes promote semiconducting reconstructions having 2 × 2 or c(2 × 2) periodicity. In contrast, under the particular range of the Al chemical potential the (11-2-2) surfaces stabilize reconstructions with excess metal and it is only at the extreme N rich limit that the semiconducting c(2 × 2) N adatom structure prevails. The present study reveals that the reconstructed (11-22) surfaces do not contain steps in contrast to (11-2-2) where surface steps are inherent for N rich to moderate metal rich growth conditions and may result in intrinsic step-flow growth and/or growth of parasitic semipolar orientations. [less ▲]

Detailed reference viewed: 59 (2 UL)
Full Text
Peer Reviewed
See detailInterfaces between nonpolar and semipolar III-nitride semiconductor orientations: Structure and defects
Kioseoglou, Joseph; Lotsari, Antiopi; Kalesaki, Efterpi UL et al

in Journal of Applied Physics (2012), 111

Observations of easy transition between nonpolar and semipolar orientations during III-Nitride heteroepitaxy identify the 90o <-12-10> rotation relationship as being very important in defining this ... [more ▼]

Observations of easy transition between nonpolar and semipolar orientations during III-Nitride heteroepitaxy identify the 90o <-12-10> rotation relationship as being very important in defining this coexistence. A rigorous analysis of this relationship using the topological theory of interfaces showed that it leads to a high order of coincident symmetry and makes energetically favorable the appearance of the intergranular boundaries. Principal low-energy boundaries, that could also be technologically exploited, have been identified by high-resolution transmission electron microscopy (HRTEM) observations and have been studied energetically using empirical potential calculations. It is also shown that these boundaries can change their average orientation by incorporating disconnections. The pertinent strain relaxation mechanisms can cause such boundaries to act as sources of threading dislocations and stacking faults. The energetically favorable (10-10) // (0001) boundary was frequently observed to delimit m-plane crystallites in (-12-12) semipolar growth. [less ▲]

Detailed reference viewed: 44 (0 UL)
Full Text
Peer Reviewed
See detailScrew threading dislocations in AlN: Structural and electronic properties of In and O doped material
Kioseoglou, Joseph; Kalesaki, Efterpi UL; Belabbas, Imad et al

in Journal of Applied Physics (2011), 110

Density functional theory calculations were performed on undoped AlN screw threading dislocations (TDs) as well as TDs doped by indium and oxygen, prompted by integrated experiments through transmission ... [more ▼]

Density functional theory calculations were performed on undoped AlN screw threading dislocations (TDs) as well as TDs doped by indium and oxygen, prompted by integrated experiments through transmission electron microscopy and spectroscopic techniques demonstrating enhanced In and O concentrations in screw dislocation cores. It is revealed that screw TDs act as conduction pathways to charge carriers, introducing multiple levels in the bandgap due to overstrained, dangling, and “wrong” bonds formed even in the undoped cores. The presence of impurities and especially metallic In elevates the metal-like electronic structure of the distorted material and promotes the conductivity along the dislocation line. Hence screw dislocations in AlN are established as highly prominent conductive nanowires in semiconducting thin films and prospects for novel, highly functional nano-device materials through exploitation of screw TDs are attested. [less ▲]

Detailed reference viewed: 58 (0 UL)
Full Text
Peer Reviewed
See detailElectronic structure of 1/6⟨20-23⟩ partial dislocations in wurtzite GaN
Kioseoglou, Joseph; Kalesaki, Efterpi UL; Lymperakis, Liverios et al

in Journal of Applied Physics (2011), 109

The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {11-20} (a-plane) and {1-100} (m-plane) grown GaN. Their importance is established by recent ... [more ▼]

The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {11-20} (a-plane) and {1-100} (m-plane) grown GaN. Their importance is established by recent experimental results, which correlate the partial dislocations (PDs) bounding I1 BSFs to the luminescence characteristics of GaN. PDs are also found to play a critical role in the alleviation of misfit strain in hetero-epitaxially grown nonpolar and semipolar films. In the present study, the energetics and the electronic structure of twelve edge and mixed 1/6⟨20-23⟩ PD configurations are investigated by first principles calculations. The specific PD cores of the dislocation loop bounding the I1 BSF are identified for III-rich and N-rich growth conditions. The core structures of PDs induce multiple shallow and deep states, attributed to the low coordinated core atoms, indicating that the cores are electrically active. In contrast to edge type threading dislocations no strain induced states are found. [less ▲]

Detailed reference viewed: 53 (0 UL)
Full Text
Peer Reviewed
See detailEffect of edge threading dislocations on the electronic structure of InN
Kalesaki, Efterpi UL; Kioseoglou, Joseph; Lymperakis, Liverios et al

in Applied Physics Letters (2011), 98(7), 072103

The open issue of the n-type conductivity and its correlation to threading dislocations (TDs) in InN is addressed through first principles calculations on the electronic properties of a-edge TDs. All ... [more ▼]

The open issue of the n-type conductivity and its correlation to threading dislocations (TDs) in InN is addressed through first principles calculations on the electronic properties of a-edge TDs. All possible dislocation core models are considered (4-, 5/7-, and 8-atom cores) and are found to modify the band structure of InN in a distinct manner. In particular, nitrogen and indium low coordinated atoms in the eight-atom core induce states near the valence band maximum and above the conduction band minimum, respectively. The formation of a nitrogen–nitrogen “wrong” bond is observed at the 5/7-atom core resulting in a state inside the band gap. The 4- and 5/7-atom cores induce occupied states resonant in the conduction band due to In–In strain induced interactions and wrong bonds, respectively. These occupied states designate TDs as a source of higher electron concentrations in InN and provide direct evidence that TDs contribute to its inherent n-type conductivity. [less ▲]

Detailed reference viewed: 42 (0 UL)
Full Text
Peer Reviewed
See detailMorphology and strain of self-assembled semipolar GaN quantum dots in (11-22) AlN
Dimitrakopulos, George; Kalesaki, Efterpi UL; Kioseoglou, Joseph et al

in Journal of Applied Physics (2010), 108

GaN quantum dots (QDs) grown in semipolar (11-22) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy ... [more ▼]

GaN quantum dots (QDs) grown in semipolar (11-22) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques. The embedded (11-22)-grown QDs exhibited pyramidal or truncated-pyramidal morphology consistent with the symmetry of the nucleating plane, and were delimited by nonpolar and semipolar nanofacets. It was also found that, in addition to the (11-22) surface, QDs nucleated at depressions comprising {10-11} facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (11-22). Based on quantitative high-resolution TEM strain measurements, the three-dimensional QD strain state was analyzed using finite-element simulations. The internal electrostatic field was then estimated, showing small potential drop along the growth direction, and limited localization at most QD interfaces. [less ▲]

Detailed reference viewed: 42 (0 UL)