97 research outputs found
Crystalline and electronic structure of single-layer TaS
Single-layer TaS is epitaxially grown on Au(111) substrates. The
resulting two-dimensional crystals adopt the 1H polymorph. The electronic
structure is determined by angle-resolved photoemission spectroscopy and found
to be in excellent agreement with density functional theory calculations. The
single layer TaS is found to be strongly n-doped, with a carrier
concentration of 0.3(1) extra electrons per unit cell. No superconducting or
charge density wave state is observed by scanning tunneling microscopy at
temperatures down to 4.7 K.Comment: 6 pages, 4 figure
Direct observation of minibands in twisted heterobilayers
Stacking two-dimensional (2D) van der Waals materials with different
interlayer atomic registry in a heterobilayer causes the formation of a
long-range periodic superlattice that may bestow the heterostructure with
exotic properties such as new quantum fractal states [1-3] or superconductivity
[4, 5]. Recent optical measurements of transition metal dichalcogenide (TMD)
heterobilayers have revealed the presence of hybridized interlayer
electron-hole pair excitations at energies defined by the superlattice
potential [6-10]. The corresponding quasiparticle band structure, so-called
minibands, have remained elusive and no such features have been reported for
heterobilayers comprised of a TMD and another type of 2D material. Here, we
introduce a new X-ray capillary technology for performing micro-focused
angle-resolved photoemission spectroscopy (microARPES) with a spatial
resolution on the order of 1 m, enabling us to map the momentum-dependent
quasiparticle dispersion of heterobilayers consisting of graphene on WS at
variable interlayer twist angles (). Minibands are directly observed
for in multiple mini Brillouin zones (mBZs), while they
are absent for a larger twist angle of . These findings
underline the possibility to control quantum states via the stacking
configuration in 2D heterostructures, opening multiple new avenues for
generating materials with enhanced functionality such as tunable electronic
correlations [11] and tailored selection rules for optical transitions [12].Comment: Main manuscript: 14 pages, 4 figures. Supporting information: 8
pages, 5 figure
Quasi-free-standing single-layer WS2 achieved by intercalation
Large-area and high-quality single-layer transition metal dichalcogenides can
be synthesized by epitaxial growth on single-crystal substrates. An important
advantage of this approach is that the interaction between the single-layer and
the substrate can be strong enough to enforce a single crystalline orientation
of the layer. On the other hand, the same interaction can lead to hybridization
effects, resulting in the deterioration of the single-layer's native
properties. This dilemma can potentially be solved by decoupling the
single-layer from the substrate surface after the growth via intercalation of
atoms or molecules. Here we show that such a decoupling can indeed be achieved
for single-layer WS2 epitaxially grown on Ag(111) by intercalation of Bi atoms.
This process leads to a suppression of the single-layer WS2-Ag substrate
interaction, yielding an electronic band structure reminiscent of free-standing
single-layer WS2
Pnictogens Allotropy and Phase Transformation during van der Waals Growth
Pnictogens have multiple allotropic forms resulting from their ns2 np3
valence electronic configuration, making them the only elemental materials to
crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout
the group. Light group VA elements are found in the layered orthorhombic A17
phase such as black phosphorus, and can transition to the layered rhombohedral
A7 phase at high pressure. On the other hand, bulk heavier elements are only
stable in the A7 phase. Herein, we demonstrate that these two phases not only
co-exist during the vdW growth of antimony on weakly interacting surfaces, but
also undertake a spontaneous transformation from the A17 phase to the
thermodynamically stable A7 phase. This metastability of the A17 phase is
revealed by real-time studies unraveling its thickness-driven transition to the
A7 phase and the concomitant evolution of its electronic properties. At a
critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle
transition from AB to AA stacked alpha-antimonene followed by a gradual
relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of
this intermediate phase is found to be determined by surface self-passivation
and the associated competition between A7- and A17-like bonding in the bulk.
These results highlight the critical role of the atomic structure and
interfacial interactions in shaping the stability and electronic
characteristics of vdW layered materials, thus enabling a new degree of freedom
to engineer their properties using scalable processes
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