Van der Waals heterostructures have recently been identified as providing many opportunities to create new two-dimensional materials, and in particular to produce materials with topologicallyinteresting states. Here we show that it is possible to create such heterostructures with multiple topological phases in a single nanoscale island. We discuss their growth within the framework of diffusion-limited aggregation, the formation of moiré patterns due to the differing crystallographies of the materials comprising the heterostructure, and the potential to engineer both the electronic structure as well as local variations of topological order. In particular we show that it is possible to build islands which include both the hexagonal β-and rectangular α-forms of antimonene, on top of the topological insulator α-bismuthene. This is the first experimental realisation of α-antimonene, and we show that it is a topologically non-trivial material in the quantum spin Hall class.
Using scanning tunneling microscopy, we report the observation of moiré patterns (MPs) on van der Waals heterostructures comprised of various 2D allotropes of bismuth and antimony grown on highly ordered pyrolytic graphite and MoS 2 . The spatial periods of the MPs range from λ ∼ 1 to ∼10 nm. For all the reported cases (α-bismuthene, α-antimonene, β-antimonene, and monolayer bismuthene), we model the observations using a simple superposition model (SSM). Where possible, the results obtained from the SSM are compared to analytical prediction. MPs emerging from mixed symmetry stacking (hexagonal on rectangular) are explained without requiring commensuration of the layers.
We report the observation of a new allotrope of two dimensional bismuth. Our Scanning Tunnelling Microscopy experiments show that the structure is clearly different than the previously synthesized allotropes β-and α-bismuthene. It has a rectangular symmetry similar to that of α-bismuthene, but is composed of a puckered single monolayer of Bi atoms (α-bismuthene is intrisincally a paired layer material similar to black phosphorous). Atomic resolution images and an observed moiré pattern show that the new allotrope has a significantly contracted surface unit cell. The electronic structure is dominated by high density of states at the Fermi level as measured using scanning tunneling spectrocopy (STS) and confirmed by calculations based on density functional theory (DFT) which reveal Dirac cones at three different points in the Brillouin zone.
We propose that the crystallinity of two-dimensional
(2D) materials
is a crucial factor for achieving highly effective work function (WF)
modification. A crystalline 2D MoO3 monolayer enhances
substrate WF up to 6.4 eV for thicknesses as low as 0.7 nm. Such a
high WF makes 2D MoO3 a great candidate for tuning properties
of anode materials and for the future design of organic electronic
devices, where accurate evaluation of the WF is crucial. We provide
a detailed investigation of WF of 2D α-MoO3 directly
grown on highly ordered pyrolytic graphite, by means of Kelvin probe
force microscopy (KPFM) and ultraviolet photoemission spectroscopy
(UPS). This study underlines the importance of a controlled environment
and the resulting crystallinity to achieve high WF in MoO3. UPS is proved to be suitable for determining higher WF attributed
to 2D islands on a substrate with lower WF, yet only in particular
cases of sufficient coverage. KPFM remains a method of choice for
nanoscale investigations, especially when conducted under ultrahigh
vacuum conditions. Our experimental results are supported by density
functional theory calculations of electrostatic potential, which indicate
that oxygen vacancies result in anisotropy of WF at the sides of the
MoO3 monolayer. These novel insights into the electronic
properties of 2D-MoO3 are promising for the design of electronic
devices with high WF monolayer films, preserving the transparency
and flexibility of the systems.
By considering a straightforward geometrical construction in reciprocal space we derive simple analytical equations that describe the geometry (period and angle) of the moiré patterns (MPs) formed by superposition of any 2D material on any substrate (including other 2D materials). These equations should be valuable tools for interpreting experimentally observed MPs and for designing van der Waals heterostructures in which MPs are used to modulate the topological and/or electronic states of a device.
Nonencapsulated CIGSSe solar cells, with a silver grid, were exposed to different temperatures for various periods in order to measure the effect of the heat exposure in CIGSSe modules. The heat treatment time and temperature were varied during the experiments, which were executed at atmospheric conditions. In all the cases, after reaching a temperature of about 300°C, theIVmeasurement showed a reduction of 2-3% in terms ofVOCandJSC. This is confirmed, respectively, by Raman and EQE measurements as well. The efficiency drop was −7%, −29%, and −48%, respectively, for 30 seconds, 300 seconds, and 600 seconds of exposure time. With temperatures larger than 225°C, the series resistance starts to increase exponentially and a secondary barrier becomes visible in theIVcurve. This barrier prevents the extraction of electrons and consequently reducing the solar cells efficiency. Lock-in thermography demonstrated the formation of shunts on the mechanical scribes only for 300 and 600 seconds exposure times. The shunt resistance reduction is in the range of 5% for all time periods.
A combined experimental and computational study is reported on a hitherto unrecognised single lanthanide catalyst for the breaking of molecular nitrogen and formation of ammonia at ambient temperature and low pressure.<br>We combine in situ electrical conductance and electron diffraction measurements to track the conversion from the lanthanide metals to the insulating lanthanide nitrides.<br>The efficiency of the conversion is then interpreted using DFT+U calculations, suggesting a molecular nitrogen dissociation pathway separate from that well-established for transition metals.<br>Finally, we show that exposure of the lanthanide surfaces to both molecular nitrogen and hydrogen results in the formation of ammonia.<br><br>
A combined experimental and computational study is reported on a hitherto unrecognised single lanthanide catalyst for the breaking of molecular nitrogen and formation of ammonia at ambient temperature and low pressure. We combine in situ electrical conductance and electron diraction measurements to track the conversion from the lanthanide metals to the insulating lanthanide nitrides. The eciency of the conversion is then interpreted using DFT+U calculations, suggesting a molecular nitrogen dissociation pathway separate from that well-established for transition metals.
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