13 research outputs found
Efficient Charge Separation in 2D Janus van der Waals Structures with Build-in Electric Fields and Intrinsic p-n Doping
Janus MoSSe monolayers were recently synthesised by replacing S by Se on one
side of MoS (or vice versa for MoSe). Due to the different
electronegativity of S and Se these structures carry a finite out-of-plane
dipole moment. As we show here by means of density functional theory (DFT)
calculations, this intrinsic dipole leads to the formation of built-in electric
fields when the monolayers are stacked to form -layer structures. For
sufficiently thin structures () the dipoles add up and shift the vacuum
level on the two sides of the film by eV. However, for
thicker films charge transfer occurs between the outermost layers forming
atomically thin n- and p-doped electron gasses at the two surfaces. The doping
concentration can be tuned between about e/cm and
e/cm by varying the film thickness. The surface charges
counteract the static dipoles leading to saturation of the vacuum level shift
at around 2.2 eV for . Based on band structure calculations and the
Mott-Wannier exciton model, we compute the energies of intra- and interlayer
excitons as a function of film thickness suggesting that the Janus multilayer
films are ideally suited for achieving ultrafast charge separation over atomic
length scales without chemical doping or applied electric fields. Finally, we
explore a number of other potentially synthesisable 2D Janus structures with
different band gaps and internal dipole moments. Our results open new
opportunities for ultrathin opto-electronic components such as tunnel diodes,
photo-detectors, or solar cells
Anomalous Non-Hydrogenic Exciton Series in 2D Materials on High- Dielectric Substrates
Engineering of the dielectric environment represents a powerful strategy to
control the electronic and optical properties of two-dimensional (2D) materials
without compromising their structural integrity. Here we show that the recent
development of high- 2D materials present new opportunities for
dielectric engineering. By solving a 2D Mott-Wannier exciton model for WSe
on different substrates using a screened electron-hole interaction obtained
from first principles, we demonstrate that the exciton Rydberg series changes
qualitatively when the dielectric screening within the 2D semiconductor becomes
dominated by the substrate. In this regime, the distance dependence of the
screening is reversed and the effective screening increases with exciton
radius, which is opposite to the conventional 2D screening regime.
Consequently, higher excitonic states become underbound rather than overbound
as compared to the Hydrogenic Rydberg series. Finally, we derive a general
analytical expression for the exciton binding energy of the entire 2D Rydberg
serie
The Computational 2D Materials Database: High-Throughput Modeling and Discovery of Atomically Thin Crystals
We introduce the Computational 2D Materials Database (C2DB), which organises
a variety of structural, thermodynamic, elastic, electronic, magnetic, and
optical properties of around 1500 two-dimensional materials distributed over
more than 30 different crystal structures. Material properties are
systematically calculated by state-of-the art density functional theory and
many-body perturbation theory (GW\!_0 and the Bethe-Salpeter Equation
for 200 materials) following a semi-automated workflow for maximal
consistency and transparency. The C2DB is fully open and can be browsed online
or downloaded in its entirety. In this paper, we describe the workflow behind
the database, present an overview of the properties and materials currently
available, and explore trends and correlations in the data. Moreover, we
identify a large number of new potentially synthesisable 2D materials with
interesting properties targeting applications within spintronics,
(opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily
accessible overview of the rapidly expanding family of 2D materials and forms
an ideal platform for computational modeling and design of new 2D materials and
van der Waals heterostructures.Comment: Add journal reference and DOI; Minor updates to figures and wordin
Engineering covalently bonded 2D layered materials by self-intercalation
10.1038/s41586-020-2241-9NATURE5817807171-