687 research outputs found
Substitutional Si impurities in monolayer hexagonal boron nitride
We report the first observation of substitutional silicon atoms in
single-layer hexagonal boron nitride (h-BN) using aberration corrected scanning
transmission electron microscopy (STEM). The medium angle annular dark field
(MAADF) images reveal silicon atoms exclusively filling boron vacancies. This
structure is stable enough under electron beam for repeated imaging. Density
functional theory (DFT) is used to study the energetics, structure and
properties of the experimentally observed structure. The formation energies of
all possible charge states of the different silicon substitutions
(Si, Si and Si) are calculated. The
results reveal Si as the most stable substitutional
configuration. In this case, silicon atom elevates by 0.66{\AA} out of the
lattice with unoccupied defect levels in the electronic band gap above the
Fermi level. The formation energy shows a slightly exothermic process. Our
results unequivocally show that heteroatoms can be incorporated into the h-BN
lattice opening way for applications ranging from single-atom catalysis to
atomically precise magnetic structures
Fractional Charge Methods for Correcting Approximate Kohn-Sham Potentials
The Kohn-Sham density functional theory relies on approximating the exchange-correlation energy functional or the corresponding potential. The behavior of the exchange-correlation potential as a function of position in a system can be used to detect and correct deficiencies of the parent functional. The too-fast decay of the potentials derived from common density functionals is a major problem, because it causes inaccurate Rydberg excitation energies and erroneous fractional charges in dissociating molecules. An efficient method to correct the shape of the exchange-correlation potential was proposed by Gaiduk et al. [A. P. Gaiduk, D. S. Firaha, and V. N. Staroverov, Phys. Rev. Lett. 108, 253005 (2012)]. In that method, the potential of an auxiliary system with a fractionally occupied frontier orbital is used to model a potential for the neutral system of interest. In this thesis, we investigate a method to eliminate unphysical partial charges in dissociating polar molecules via the use of the fractional occupation technique. The method proves successful not only for enforcing correct integer charges in the dissociation limit, but also for predicting how atomic charges change at intermediate separations. We also test the hypothesis that a system with an integral number of electrons but fractional nuclear charge may be used to correct Rydberg excitation energies. Our findings show that, although this approach gives rise to some improvements, the optimal charge to be added depends on the system. In contrast, the advantage of the fractional occupation technique is that the parameter required to correct excitation energies is system-independent
High-throughput computation of Raman spectra from first principles
Raman spectroscopy is a widely-used non-destructive material characterization
method, which provides information about the vibrational modes of the material
and therefore of its atomic structure and chemical composition. Interpretation
of the spectra requires comparison to known references and to this end,
experimental databases of spectra have been collected. Reference Raman spectra
could also be simulated using atomistic first-principles methods but these are
computationally demanding and thus the existing databases of computational
Raman spectra are fairly small. In this work, we developed an optimized
workflow to calculate the Raman spectra more efficiently compared to existing
approaches. The workflow was benchmarked and validated by comparison to
experiments and previous computational methods for select technologically
relevant material systems. Using the workflow, we performed high-throughput
calculations for a large set of materials (5099) belonging to many different
material classes, and collected the results to a database. Finally, the
contents of database are analyzed and the calculated spectra are shown to agree
well with the experimental ones.Comment: 19 pages, 7 figure
Screening 0D materials for 2D nanoelectronics applications
As nanoelectronic devices based on two-dimensional (2D) materials are moving
towards maturity, optimization of the properties of the active 2D material must
be accompanied by equal attention to optimizing the properties of and the
interfaces to the other materials around it, such as electrodes, gate
dielectrics, and the substrate. While these are usually either 2D or 3D
materials, recently K. Liu et al. [Nat. Electron. 4, 906 (2021)] reported on
the use of zero-dimensional (0D) material, consisting of vdW-bonded SbO
clusters, as a highly promising insulating substrate and gate dielectric. Here,
we report on computational screening study to find promising 0D materials for
use in nanoelectronics applications, in conjunction with 2D materials in
particular. By combining a database and literature searches, we found 16
materials belonging to 6 structural prototypes with high melting points and
high band gaps, and a range of static dielectric constants. We carried out
additional first-principles calculations to evaluate selected technologically
relevant material properties, and confirmed that all these materials are van
der Waals-bonded, thus allowing for facile separation of 0D clusters from the
3D host and also weakly perturbing the electronic properties of the 2D material
after deposition.Comment: 10 pages, 3 figure
Do fractionally incremented nuclear charges improve time-dependent density functional theory excitation energies as reliably as fractional orbital populations?
Gaiduk et al. (Phys Rev Lett 108:253005, 2012) showed that one can improve local, semilocal, and hybrid approximations to the Kohn–Sham effective potentials of atoms and molecules by removing a system-independent fraction of electron charge from the highest occupied molecular orbital (HOMO); if the corrected Kohn–Sham potential is used for adiabatic linear-response time-dependent density functional theory (TDDFT) calculations, accurate Rydberg excitation energies are obtained. One may ask whether the same effect could also be achieved by fractionally increasing the positive charges of the nuclei. We investigate this question and find that a small increase in nuclear charges can indeed substantially reduce errors in TDDFT Rydberg excitation energies. However, the optimal magnitude of the charge increase is system-dependent. In addition, the procedure is ambiguous for molecules, where one has to decide how to distribute the additional charge among individual nuclei. These two drawbacks of the fractional nuclear charge method make it disadvantageous compared to the HOMO depopulation technique
Raman Spectra of Titanium Carbide MXene from Machine-Learning Force Field Molecular Dynamics
MXenes represent one of the largest class of 2D materials with promising
applications in many fields and their properties tunable by the surface group
composition. Raman spectroscopy is expected to yield rich information about the
surface composition, but the interpretation of measured spectra has proven
challenging. The interpretation is usually done via comparison to simulated
spectra, but there are large discrepancies between the experimental and earlier
simulated spectra. In this work, we develop a computational approach to
simulate Raman spectra of complex materials that combines machine-learning
force-field molecular dynamics and reconstruction of Raman tensors via
projection to pristine system modes. The approach can account for the effects
of finite temperature, mixed surfaces, and disorder. We apply our approach to
simulate Raman spectra of titanium carbide MXene and show that all these
effects must be included in order to properly reproduce the experimental
spectra, in particular the broad features. We discuss the origin of the peaks
and how they evolve with surface composition, which can then be used to
interpret experimental results
Inbreeding depression and segregation distortion in Chinook salmon: Conservation implications of genetic load
Strong evidence that shows inbreeding in many plants and animals can lead to detrimental effects at the level of the organism as well as the population. Inbreeding and its effects have been the focus of much attention in terms of conservation and captive rearing and breeding programs for many endangered species. Inbreeding depression (ID) is the fitness loss associated with individuals and populations that have experienced inbreeding. Surprisingly, ID studies within salmonids are not extensive. Here, I used inbred offspring of self-fertilized hermaphrodite parents to study the effects of ID in Chinook salmon ( Oncorhynchus tshawytscha ) at an extreme level of inbreeding. High levels of ID were found after a single generation of self-crossing, resulting in substantial fitness losses. The potential to purge genetic load from a population was shown to take place and the major genetic mechanism driving ID was due to expression of deleterious recessive alleles
Charged Point Defects in the Flatland: Accurate Formation Energy Calculations in Two-Dimensional Materials
Impurities and defects frequently govern materials properties, with the most prominent example being the doping of bulk semiconductors where a minute amount of foreign atoms can be responsible for the operation of the electronic devices. Several computational schemes based on a supercell approach have been developed to get insights into types and equilibrium concentrations of point defects, which successfully work in bulk materials. Here, we show that many of these schemes cannot directly be applied to two-dimensional (2D) systems, as formation energies of charged point defects are dominated by large spurious electrostatic interactions between defects in inhomogeneous environments. We suggest two approaches that solve this problem and give accurate formation energies of charged defects in 2D systems in the dilute limit. Our methods, which are applicable to all kinds of charged defects in any 2D system, are benchmarked for impurities in technologically important h-BN and MoS2 2D materials, and they are found to perform equally well for substitutional and adatom impurities.Peer reviewe
- …