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

Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.

Emerging Photoluminescence in Monolayer MoS2

Large-area MoS(2) atomic layers are synthesized on SiO(2) substrates by chemical vapor deposition using MoO(3) and S powders as the reactants. Optical, microscopic and electrical measurements suggest that the synthetic process leads to the growth of MoS(2) monolayer. The TEM images verify that the synthesized MoS(2) sheets are highly crystalline.

Synthesis of Large‐Area MoS2 Atomic Layers with Chemical Vapor Deposition

Two-dimensional (2D) materials, such as molybdenum disulfide (MoS2), have been shown to exhibit excellent electrical and optical properties. The semiconducting nature of MoS2 allows it to overcome the shortcomings of zero-bandgap graphene, while still sharing many of graphene’s advantages for electronic and optoelectronic applications. Discrete electronic and optoelectronic components, such as field-effect transistors, sensors, and photodetectors made from few-layer MoS2 show promising performance as potential substitute of Si in conventional electronics and of organic and amorphous Si semiconductors in ubiquitous systems and display applications. An important next step is the fabrication of fully integrated multistage circuits and logic building blocks on MoS2 to demonstrate its capability for complex digital logic and high-frequency ac applications. This paper demonstrates an inverter, a NAND gate, a static random access memory, and a five-stage ring oscillator based on a direct-coupled transistor logic...

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Integrated Circuits Based on Bilayer MoS2 Transistors

Bulk MoS2, a prototypical layered transition-metal dichalcogenide, is an indirect band gap semiconductor. Reducing its size to a monolayer, MoS2 undergoes a transition to the direct band semiconductor. We support this experimental observation by first principles calculations and show that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure at the nanoscale. We further studied the properties of related TmS2 nanolayers (Tm = W, Nb, Re) and show that the isotopological WS2 exhibits similar electronic properties, while NbS2 and ReS2 remain metallic independent on size.

Influence of quantum confinement on the electronic structure of the transition metal sulfide T S 2

The band structures of the semiconducting layered compounds ${\mathrm{MoSe}}_{2}$, ${\mathrm{MoS}}_{2}$, and ${\mathrm{WSe}}_{2}$ have been calculated self-consistently with the augmented-spherical-wave method. Angle-resolved photoelec- tron spectroscopy of ${\mathrm{MoSe}}_{2}$ using He i, He ii, and Ne i radiation, and photon-energy-dependent normal-emission photoelectron spectroscopy using synchrotron radiation, show that the calculational results give a good description of the valence-band structure. At about 1 eV below the top of the valence band a dispersionless state was measured, almost completely of Mo 4d character. Such a state, which is not predicted by band-structure calculations, has also been observed in metallic layered compounds. Suggestions are given for the explanation of this phenomenon.

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Electronic structure of MoSe2, MoS2, and WSe2. I. Band-structure calculations and photoelectron spectroscopy

Electronic band-structure calculations are presented for the ferromagnetic compounds CrTe, Cr3Te4 and Cr2Te4 and Cr2Te3. In these compounds the Cr3d-Te5p covalency and the Cr3d(z2)-Cr3d(z2)-overlap along the c axis are the most important interactions. The magnetic polarisation of Te is parallel to the Cr local moment in CrTe, antiparallel to it in Cr2Te3 and about zero in Cr2Te4. Measurements of electronic transport properties (resistivity, Hall effect and thermo-electric power) and magnetic properties of Cr1- delta Te( delta =0.1) and Cr3+xTe4(x=0.2) indicate that these chromium tellurides are p-type metals, with strong interaction between the holes in the Te5p band and the Cr magnetic moments. In the literature the variation of the magnetic properties of Cr3.2Te4 near Ts approximately=100 K has been attributed to a change from a canted antiferromagnetic to a collinear ferromagnetic structure. However, our Hall-effect measurements indicate that the spin structure is not collinear ferromagnetic above Ts.

https://iopscience.iop.org/article/10.1088/0953-8984/1/46/008/pdf

Band-structure calculations, and magnetic and transport properties of ferromagnetic chromium tellurides (CrTe, Cr3Te4, Cr2Te3)

Band-structure calculations of ferromagnetic Fe1/3TaS2 and Mn1/3TaS2 using the augmented spherical-wave method are reported. The calculations give a spin magnetic moment of 4.35 mu B per Fe for Fe1/3TaS2 and 4.18 mu B per Mn for Mn1/3TaS2. The local moments at Fe and Mn induce a magnetic polarisation of the Ta 5d(z2) band, which is parallel to the local moment for Fe1/3TaS2 and antiparallel for Mn1/3TaS2. Measurements of the transport properties of Fe0.28TaS2 show a large spin-disorder contribution to the resistivity and a large anomalous Hall effect. The anomalous Hall effect is analysed in terms of side jump and skew scattering contributions. The large magnetisation anisotropy indicates the presence of Fe2+, in agreement with the band-structure calculations.

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Band-structure calculations of Fe1/3TaS2 and Mn1/3TaS2, and transport and magnetic properties of Fe0.28TaS2

The electronic structures of the layered compound TiS2 and its intercalates LiTiS2, NaTiS2 and AgTiS2 are calculated using the ASW method. Various deviations from the rigid-band model are observed, of which the increase of the Ti-3d/S-3p gap and the variation of the inter-sandwich and intra-sandwich interactions between S-3p orbitals are the most pronounced. The effect of the flattening of the TiS2 slabs-such as occurs at the 3R(I)-3R(II) transition in NaxTiS2 for x approximately=0.7-on the electronic structure is calculated for Na1TiS2 and is shown to be coupled with an increased ionicity in the Ti-S bond.

https://iopscience.iop.org/article/10.1088/0953-8984/1/27/003/pdf

The electronic-structure of some monovalent-metal intercalates of tis2

The electronic structure of the layered compound KCrSe2 in the ferromagnetic spin arrangement is calculated using the augmented-spherical-wave method. For the minority-spin direction the Fermi level lies in a 1.34-eV-wide gap between the bottom of the Cr 3d ↓ band and the Se 4p ↓ band, while for the majority-spin direction the Fermi level crosses the Cr 3d ↑ bands. This means that KCrSe2 is a half-metallic ferromagnet.

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J. Dijkstra

Papers

Electronic structure of MoSe2, MoS2, and WSe2. I. Band-structure calculations and photoelectron spectroscopy

Band-structure calculations, and magnetic and transport properties of ferromagnetic chromium tellurides (CrTe, Cr3Te4, Cr2Te3)

Band-structure calculations of Fe1/3TaS2 and Mn1/3TaS2, and transport and magnetic properties of Fe0.28TaS2

The electronic-structure of some monovalent-metal intercalates of tis2

Electronic structure of the half-metallic ferromagnet KCrSe2