Maciej Dendzik

Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor

The coupling between spin, valley and layer degrees of freedom in transition-metal dichalcogenides is shown to give rise to spin-polarized electron states, providing opportunities to create and manipulate spin and valley polarizations in bulk solids. Methods to generate spin-polarized electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices1. This is generally accepted to require breaking global structural inversion symmetry1,2,3,4,5. In contrast, here we report the observation from spin- and angle-resolved photoemission spectroscopy of spin-polarized bulk states in the centrosymmetric transition-metal dichalcogenide WSe2. Mediated by a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localized6, we show how spin splittings up to ∼0.5 eV result, with a spin texture that is strongly modulated in both real and momentum space. Through this, our study provides direct experimental evidence for a putative locking of the spin with the layer and valley pseudospins in transition-metal dichalcogenides7,8, of key importance for using these compounds in proposed valleytronic devices.

Van der Waals Epitaxy of Two-Dimensional MoS2-Graphene Heterostructures in Ultrahigh Vacuum.

In this work, we demonstrate direct van der Waals epitaxy of MoS2-graphene heterostructures on a semiconducting silicon carbide (SiC) substrate under ultrahigh vacuum conditions. Angle-resolved photoemission spectroscopy (ARPES) measurements show that the electronic structure of free-standing single-layer (SL) MoS2 is retained in these heterostructures due to the weak van der Waals interaction between adjacent materials. The MoS2 synthesis is based on a reactive physical vapor deposition technique involving Mo evaporation and sulfurization in a H2S atmosphere on a template consisting of epitaxially grown graphene on SiC. Using scanning tunneling microscopy, we study the seeding of Mo on this substrate and the evolution from nanoscale MoS2 islands to SL and bilayer (BL) MoS2 sheets during H2S exposure. Our ARPES measurements of SL and BL MoS2 on graphene reveal the coexistence of the Dirac states of graphene and the expected valence band of MoS2 with the band maximum shifted to the corner of the Brillouin zone at K̅ in the SL limit. We confirm the 2D character of these electronic states via a lack of dispersion with photon energy. The growth of epitaxial MoS2-graphene heterostructures on SiC opens new opportunities for further in situ studies of the fundamental properties of these complex materials, as well as perspectives for implementing them in various device schemes to exploit their many promising electronic and optical properties.

Electronic structure of epitaxial single-layer MoS2.

The electronic structure of epitaxial single-layer MoS2 on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a (1.39±0.05)  eV direct band gap at K[over ¯] with the valence band top at Γ[over ¯] having a significantly higher binding energy than at K[over ¯]. The moiré superstructure of the epitaxial system does not lead to the presence of observable replica bands or minigaps. The degeneracy of the upper valence band at K[over ¯] is found to be lifted by the spin-orbit interaction, leading to a splitting of (145±4)  meV. This splitting is anisotropic and in excellent agreement with recent calculations. Finally, it is shown that the potassium doping does not only give rise to a rigid shift of the band structure but also to a distortion, leading to the possibility of band structure engineering in single-layers of transition metal dichalcogenides.

Synthesis of Epitaxial Single-Layer MoS2 on Au(111)

We present a method for synthesizing large area epitaxial single-layer MoS2 on the Au(111) surface in ultrahigh vacuum. Using scanning tunneling microscopy and low energy electron diffraction, the evolution of the growth is followed from nanoscale single-layer MoS2 islands to a continuous MoS2 layer. An exceptionally good control over the MoS2 coverage is maintained using an approach based on cycles of Mo evaporation and sulfurization to first nucleate the MoS2 nanoislands and then gradually increase their size. During this growth process the native herringbone reconstruction of Au(111) is lifted as shown by low energy electron diffraction measurements. Within the MoS2 islands, we identify domains rotated by 60° that lead to atomically sharp line defects at domain boundaries. As the MoS2 coverage approaches the limit of a complete single layer, the formation of bilayer MoS2 islands is initiated. Angle-resolved photoemission spectroscopy measurements of both single and bilayer MoS2 samples show a dramatic change in their band structure around the center of the Brillouin zone. Brief exposure to air after removing the MoS2 layer from vacuum is not found to affect its quality.

Ultrafast Band Structure Control of a Two-Dimensional Heterostructure

The electronic structure of two-dimensional (2D) semiconductors can be significantly altered by screening effects, either from free charge carriers in the material or by environmental screening from the surrounding medium. The physical properties of 2D semiconductors placed in a heterostructure with other 2D materials are therefore governed by a complex interplay of both intra- and interlayer interactions. Here, using time- and angle-resolved photoemission, we are able to isolate both the layer-resolved band structure and, more importantly, the transient band structure evolution of a model 2D heterostructure formed of a single layer of MoS2 on graphene. Our results reveal a pronounced renormalization of the quasiparticle gap of the MoS2 layer. Following optical excitation, the band gap is reduced by up to ∼400 meV on femtosecond time scales due to a persistence of strong electronic interactions despite the environmental screening by the n-doped graphene. This points to a large degree of tunability of both the electronic structure and the electron dynamics for 2D semiconductors embedded in a van der Waals-bonded heterostructure.

Single-layer MoS2 on Au(111): Band gap renormalization and substrate interaction

The electronic structure of epitaxial single-layer MoS

Symmetry-Driven Band Gap Engineering in Hydrogen Functionalized Graphene

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Growth and electronic structure of epitaxial single-layer WS 2 on Au(111)

on Au(111) is investigated by angle-resolved photoemission spectroscopy, scanning tunnelling spectroscopy, and first principles calculations. While the band dispersion of the supported single-layer is close to a free-standing layer in the vicinity of the valence band maximum at

Facile electrochemical transfer of large-area single crystal epitaxial graphene from Ir(1 1 1)

\bar{K}

Spin and valley control of free carriers in single-layer WS2

and the calculated electronic band gap on Au(111) is similar to that calculated for the free-standing layer, significant modifications to the band structure are observed at other points of the two-dimensional Brillouin zone: At