Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material

Atomically thin MoS2: a new direct-gap semiconductor

The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

Graphene-Like Two-Dimensional Materials

In this work the complete valence-band structure of the molybdenum dichalcogenides ${\mathrm{MoS}}_{2},$ ${\mathrm{MoSe}}_{2},$ and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{MoTe}}_{2}$ is presented and discussed in comparison. The valence bands have been studied using both angle-resolved photoelectron spectroscopy (ARPES) with synchrotron radiation, as well as ab initio band-structure calculations. The ARPES measurements have been carried out in the constant-final-state (CFS) mode. The results of the calculations show in general very good agreement with the experimentally determined valence-band structures allowing for a clear identification of the observed features. The dispersion of the valence bands as a function of the perpendicular component ${k}_{\ensuremath{\perp}}$ of the wave vector reveals a decreasing three-dimensional character from ${\mathrm{MoS}}_{2}$ to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{MoTe}}_{2}$ which is attributed to an increasing interlayer distance in the three compounds. The effect of this ${k}_{\ensuremath{\perp}}$ dispersion on the determination of the exact dispersion of the individual states as a function of ${k}_{\ensuremath{\Vert}}$ is discussed. By performing ARPES in the CFS mode the ${k}_{\ensuremath{\Vert}}$ component for off-normal emission spectra can be determined. The corresponding ${k}_{\ensuremath{\perp}}$ value is obtained from the symmetry of the spectra along the $\ensuremath{\Gamma}A,$ $KH,$ and $\mathrm{ML}$ lines, respectively.

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Band structure of MoS 2 , MoSe 2 , and α − MoTe 2 : Angle-resolved photoelectron spectroscopy and ab initio calculations

Ab initio calculations of structural and electronic properties of the C(001)-(2 \ifmmode\times\else\texttimes\fi{} 1) diamond surface are reported and discussed in direct comparison with Si(001) and Ge(001). Our results strongly favor a symmetric dimer reconstruction of C(001)-(2 \ifmmode\times\else\texttimes\fi{} 1) as opposed to an asymmetric dimer reconstruction of Si and Ge (001). The physical origin and quantitative nature of the dimer reconstructions are investigated systematically, and it is shown by analyzing chemical trends why Si(001) is the most subtle case for an unequivocal surface structure determination.

Dimer reconstruction of diamond, Si, and Ge (001) surfaces.

We report ab initio calculations of the lattice constants and the electronic band structure of the hexagonal wurtzite-structure semiconductors ZnO and ZnS. We employ the local-density approximation and solve the Kohn-Sham equations for nonlocal, separable, and norm-conserving pseudopotentials self-consistently. We use basis sets of localized Gaussian orbitals with s, p, d, and ${\mathit{s}}^{\mathrm{*}}$ symmetry. In particular, we investigate the influence of the Zn 3d electrons on the results for the lattice constants and the band structure. Results of calculations employing both ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Zn}}^{12+}$ ionic pseudopotentials are presented and discussed. For ZnS, both the cubic zinc blende and the hexagonal wurtzite polytype have been studied. The calculated lattice constants are found to be in excellent agreement with experiment for both semiconductors when the d electrons are explicitly taken into account as valence electrons. The agreement of the calculated bands of ZnS with experimental data and with the results of a plane-wave calculation from the literature using about 6.000 plane waves for the cubic crystal is very good except for the absolute energy position of the d bands. For ZnO the calculated bands agree better with angle-resolved photoemission data when the ${\mathrm{Zn}}^{12+}$ pseudopotential is employed. The agreement, however, is still far from satisfactory and the calculated absolute position of the d bands is off, again. The discrepancies seem to be related to correlation effects in the narrow d bands. We find the Zn 3d electrons to strongly interact with the O 2p electrons in ZnO. According to our results, the p-d mixing in ZnO is about twice as large as in ZnS.

First-principles calculation of the electronic structure of the wurtzite semiconductors ZnO and ZnS

We report state-of-the-art first-principles calculations of the quasiparticle energies of prototype homopolar and heteropolar covalent semiconductors described in terms of the electron self-energy operator. The wave functions are calculated within density-functional theory using the local-density approximation and employing nonlocal, norm-conserving pseudopotentials. The self-energy operator is evaluated in the GW approximation. Employing the plasmon-pole approximation for the frequency dependence of the dielectric matrix ${\mathrm{\ensuremath{\epsilon}}}_{\mathbf{G},\mathbf{G}\ensuremath{'}}$(q,\ensuremath{\omega}), its static part is fully calculated within the random-phase approximation (RPA) as well as by using a number of different models. All calculations are carried out employing localized Gaussian orbital basis sets. This will turn out to be very useful for detailed studies of the quasiparticle properties of more complex systems such as bulk defects including lattice relaxation and reconstructed surfaces with large unit cells or interfaces, which are otherwise computationally too demanding. Using an s,p,d,s* basis set of 40 Gaussian orbitals for Si, for example, yields already convergent results in excellent agreement with the results of a 350-plane-wave calculation in the corresponding plane-wave representation. Most of our results for Si, diamond, Ge, and GaAs are in very good agreement with experimental data and with available plane-wave GW calculations. To our knowledge, our results for SiC are the first quasiparticle energies reported so far for this important material of high current technological interest. Also in this case we find very good agreement with the available experimental data except for E(${\mathrm{L}}_{1\mathrm{c}}$). We believe that this deviation may be attributed to experimental uncertainties. In particular, we discuss and scrutinize the applicability of six different models for the static dielectric matrix ${\mathrm{\ensuremath{\epsilon}}}_{\mathbf{G},\mathbf{G}\ensuremath{'}}$(q,0) in the GW approximation ranging from the simple Hartree-Fock expression over diagonal models to nondiagonal models that take the local fields within the inhomogeneous electronic charge density into account. Some of the nondiagonal models are shown to yield results in very good agreement with the full RPA results.

Quasiparticle band-structure calculations for C, Si, Ge, GaAs, and SiC using Gaussian-orbital basis sets.

We present the results of a comparative ab initio study of single-walled SiC, BN, and BeO nanotubes (NTs) in zigzag and armchair configurations. Within density functional theory, we employ self-interaction-corrected pseudopotentials that were shown previously to yield reliable results for both structural and electronic properties of related bulk crystals. Using these pseudopotentials, we investigate the dependence of the atomic relaxation, strain energy, Young's modulus, and electronic structure on nanotube diameter and compound ionicity. Qualitatively, the NTs of all three wide-band-gap compounds show similar radially buckled geometries upon atomic relaxation, similar strain energy progressions with NT diameter and a saturation of Young's modulus as well as the band gap energy for large NT diameters. The band gap progression with NT diameter, which is of crucial importance for device applications, is presented and analyzed in detail. For SiC and BN, the calculated band gap energies of zigzag NTs vary much stronger for small and medium diameters than those of their armchair counterparts showing a significant narrowing of the band gaps. In contrast, the band gap progression in zigzag and armchair BeO NTs shows a very peculiar behavior for small diameters. No band gap breakdown occurs and the gap goes through a minimum for zigzag BeO NTs. The qualitative difference in the nature of the lower conduction band states in SiC and BN NTs, as compared to BeO NTs, and the increasing ionicity of these compounds are shown to be responsible for the observed effects.

Peter Krüger

Papers

Band structure of MoS 2 , MoSe 2 , and α − MoTe 2 : Angle-resolved photoelectron spectroscopy and ab initio calculations

Dimer reconstruction of diamond, Si, and Ge (001) surfaces.

First-principles calculation of the electronic structure of the wurtzite semiconductors ZnO and ZnS

Quasiparticle band-structure calculations for C, Si, Ge, GaAs, and SiC using Gaussian-orbital basis sets.

Structural, elastic, and electronic properties of SiC, BN, and BeO nanotubes