Abundance of Second Order Topology in Two-dimensional Insulators

We have screened 71 two-dimensional (2D) materials with $C_3$ symmetry for non-trivial second order topological order and find that 28 compounds exhibit an obstructed atomic limit (OAL). In the case of $C_3$ symmetry, the second order topology can be calculated from bulk symmetry indicator invariants, which predict the value of fractional corner charges in symmetry conserving nanoflakes. The procedure is exemplified by MoS$_2$ in the H-phase, which constitutes a generic example of a 2D OAL material and the predicted fractional corner charges is verified by direct calculations of nanoflakes with armchair edges. We also determine the bulk topological polarization, which always lead to gapless states at zigzag edges and thus deteriorates the concept of fractional corner charges in nanoflakes with zigzag edges that are typically more stable that armchair flakes. We then consider the case of TiCl$_2$, which has vanishing polarization as well as an OAL and we verify that the edge states of nanoflakes with zigzag edges may indeed by passivated such that the edges remain insulating and the corner charges are well defined. For the 28 OAL materials we find that 16 have vanishing polarization and these materials thus constitute a promising starting point for experimental verification of second order topology in a 2D material.Comment: 5 pages plus supplementar

Imaging the Electric Field with X-Ray Diffraction Microscopy

The properties of semiconductors and functional dielectrics are defined by their response in electric fields, which may be perturbed by defects and the strain they generate. In this work, we demonstrate how diffraction-based X-ray microscopy techniques may be utilized to image the electric field in insulating crystalline materials. By analysing a prototypical ferro- and piezoelectric material, BaTiO$_{3}$, we identify trends that can guide experimental design towards imaging the electric field using any diffraction-based X-ray microscopy technique. We explain these trends in the context of dark-field X-ray microscopy, but the framework is also valid for Bragg scanning probe X-ray microscopy, Bragg coherent diffraction imaging and Bragg X-ray ptychography. The ability to quantify electric field distributions alongside the defects and strain already accessible via these techniques offers a more comprehensive picture of the often complex structure-property relationships that exist in many insulating and semiconducting materials

Triggering Cation Exchange Reactions by Doping

Cation exchange (CE) reactions have emerged as a technologically important route, complementary to the colloidal synthesis, to produce nanostructures of different geometries and compositions for a variety of applications. Here it is demonstrated with first-principles simulations that an interstitial impurity cation in CdSe nanocrystals weakens nearby bonds and reduces the CE barrier in the prototypical exchange of Cd2+ ions by Ag+ ions. A Wannier function-based tight binding model is employed to quantify microscopic mechanisms that influence this behavior. To support our model, we also tested our findings in a CE experiment: both CdSe and interstitially Ag-doped CdSe nanocrystals (containing 4% of Ag+ ions per nanocrystal on average) were exposed to Pb2+ ions at room temperature and it was observed that the exchange reaction proceeds further in doped nanocrystals. The findings suggest doping as a possible route to promote CE reactions that hardly undergo exchange otherwise, for example, those in III–V sem..

Shift current photovoltaic efficiency of 2D materials

Shift current photovoltaic devices are potential candidates for future cheap, sustainable, and efficient electricity generation. In the present work, we calculate the solar-generated shift current and efficiencies in 326 different 2D materials obtained from the computational database C2DB. We apply, as metrics, the efficiencies of monolayer and multilayer samples. The monolayer efficiencies are generally found to be low, while the multilayer efficiencies of infinite stacks show great promise. Furthermore, the out-of-plane shift current response is considered, and material candidates for efficient out-of-plane shift current devices are identified. Among the screened materials, MXY Janus and MX2 transition metal dichalchogenides (TMDs) constitute a prominent subset, with chromium based MXY Janus TMDs holding particular promise. Finally, in order to explain the band gap dependence of the PV efficiency, a simple gapped graphene model with a variable band gap is established and related to the calculated efficiencies.M.O.S., A.T., K.S.T., and T.G.P. are supported by the CNG center under the Danish National Research Foundation, project DNRF103. U.P. acknowledges funding from the European Union’s Next Generation EU plan through the María Zambrano programme (MAZAM21/19). T.O. is supported by the Villum foundation, Grant No. 00028145. K.S.T. acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program Grant No. 773122 (LIMA) and Grant agreement No. 951786 (NOMAD CoE). K.S.T. is a Villum Investigator supported by the Villum foundation (Grant No. 37789)

Temperature Driven Transformation of CsPbBr3_3 Nanoplatelets into Mosaic Nanotiles in Solution through Self-Assembly

Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light emitting applications. In addition, they can be used as components to create a variety of materials through physical and chemical transformations. Recent studies focused on nanoplatelets that are able to self-assemble and transform on solid substrates. Yet, the mechanism behind the process and the atomic arrangement of their assemblies remain unclear. Here, we present the transformation of self-assembled stacks of CsPbBr$_3$ nanoplatelets in solution, capturing the different stages of the process by keeping the solutions at room temperature and monitoring the nanocrystal morphology over a period of a few months. Using ex-situ transmission electron microscopy and surface analysis, we demonstrate that the transformation mechanism can be understood as oriented attachment, proceeding through the following steps: i) desorption of the ligands from the particles surfaces, causing the merging of nanoplatelet stacks, which first form nanobelts; ii) merging of neighboring nanobelts that form more extended nanoplates; and iii) attachment of nanobelts and nanoplates, which create objects with an atomic structure that resemble a mosaic made of broken nanotiles. We reveal that the starting nanoplatelets merge seamlessly and defect-free on an atomic scale in small and thin nanobelts. However, aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr. In this case, the atomic columns of neighboring perovskite lattices shift by a half-unit-cell, forming Ruddlesden-Popper planar faults.Comment: 28 pages, 5 Figure

Recent Progress of the Computational 2D Materials Database (C2DB)

The C2DB is a highly curated open database organizing a wealth of computed properties for more than 4000 atomically thin two-dimensional (2D) materials. Here we report on new materials and properties that were added to the database since its first release in 2018. The set of new materials comprise several hundred monolayers exfoliated from experimentally known layered bulk materials, (homo)bilayers in various stacking configurations, native point defects in semiconducting monolayers, and chalcogen/halogen Janus monolayers. The new properties include exfoliation energies, Bader charges, spontaneous polarisations, Born charges, infrared polarisabilities, piezoelectric tensors, band topology invariants, exchange couplings, Raman- and second harmonic generation spectra. We also describe refinements of the employed material classification schemes, upgrades of the computational methodologies used for property evaluations, as well as significant enhancements of the data documentation and provenance. Finally, we explore the performance of Gaussian process-based regression for efficient prediction of mechanical and electronic materials properties. The combination of open access, detailed documentation, and extremely rich materials property data sets make the C2DB a unique resource that will advance the science of atomically thin materials.Comment: 30 pages, 26 figure

Polarization switching induced by domain wall sliding in two-dimensional ferroelectric monochalcogenides

The ability to switch between distinct states of polarization comprises the defining property of ferroelectrics. However, the microscopic mechanism responsible for switching is not well understood and theoretical estimates based on coherent monodomain switching typically overestimate experimentally determined coercive fields by orders of magnitude. In this work we present a detailed first principles characterization of domain walls (DWs) in two-dimensional ferroelectric GeS, GeSe, SnS and SnSe. In particular, we calculate the formation energies and migration barriers for 180 and 90 DWs, and then derive a general expression for the coercive field assuming that polarization switching is mediated by DW migration. We apply our approach to the materials studied and obtain good agreement with experimental coercive fields. The calculated coercive fields are up to two orders of magnitude smaller than those predicted from coherent monodomain switching in GeSe, SnS and SnSe. Finally, we study the optical properties of the compounds and find that the presence of 180 DWs leads to a significant red shift of the absorption spectrum, implying that the density of DWs may be determined by means of simple optical probes.Comment: 14 pages, 6 figure

Oxygen vacancies nucleate charged domain walls in ferroelectrics

We study the influence of oxygen vacancies on the formation of charged 180$^\circ$ domain walls in ferroelectric BaTiO$_3$ using first principles calculations. We show that it is favorable for vacancies to assemble in crystallographic planes, and that such clustering is accompanied by the formation of a charged domain wall. The domain wall has negative bound charge, which compensates the nominal positive charge of the vacancies and leads to a vanishing density of free charge at the wall. This is in contrast to the positively charged domain walls, which are nearly completely compensated by free charge from the bulk. The results thus explain the experimentally observed difference in electronic conductivity of the two types of domain walls, as well as the generic prevalence of charged domain walls in ferroelectrics. Moreover, the explicit demonstration of vacancy driven domain wall formation implies that specific charged domain wall configurations may be realized by bottom-up design for use in domain wall based information processing.Comment: 10 pages, 7 figure

Anisotropic properties of monolayer 2D materials: An overview from the C2DB database

We analyze the occurrence of in-plane anisotropy in the electronic, magnetic, elastic and transport properties of more than one thousand 2D materials from the C2DB database. We identify hundreds of anisotropic materials and classify them according to their point group symmetry and degree of anisotropy. A statistical analysis reveals that a lower point group symmetry and a larger amount of different elements in the structure favour all types of anisotropies, which could be relevant for future materials design approaches. Besides, we identify novel compounds, predicted to be easily exfoliable from a parent bulk compound, with anisotropies that largely outscore those of already known 2D materials. Our findings provide a comprehensive reference for future studies of anisotropic response in atomically-thin crystals and point to new previously unexplored materials for the next generation of anisotropic 2D devices.Comment: 19 pages, 8 figure