122 research outputs found
DFT screened-exchange approach for investigating electronical properties of graphene-related materials
We present ab initio calculations of the bandstructure of graphene and of
short zigzag graphene nanoribbons by the screened-exchange-LDA method (sX-LDA)
within the framework of density functional theory (DFT). The inclusion of
non-local electron-electron interactions in this approach results in a
renormalization of the electronic bandstructure and the Fermi velocity compared
to calculations within local density approximation (LDA) gives good agreement
with experiment. Similarly, the band gaps in zigzag nanoribbons (ZGNR) are
widened by more than 200%, being of similar magnitude than bandgaps from past
studies based on quasiparticle bandstructures. We found a noticeable effect of
non-local exchange on the spin-polarization of the electronic ground state of
ZGNRs, compared to LDA and GGA-PW91 calculations.Comment: 5 pages, 3 figure
Comprehensive Raman study of orthorhombic κ/ε-Ga2O3 and the impact of rotational domains
Gallium oxide (Ga2O3) is an ultra-wide bandgap material, which has recently attracted widespread attention for holding promising applications in power electronics and solar blind UV photodetectors, outclassing GaN or SiC in terms of a larger bandgap and higher breakdown voltages. The orthorhombic κ phase (also referred to as ε) has sparked particular interest for offering higher symmetry than β, while featuring ferroelectric behavior paired with a large predicted spontaneous polarization, paving the way to fabricating high-quality two-dimensional electron gases for application in heterostructure field effect transistors. The presently available κ phase samples are characterized by a domain structure, in which orthorhombic domains are rotated 120° against each other within the c-plane forming a pseudo-hexagonal structure, which has previously often been ascribed to ε-Ga2O3 and incorrectly been viewed as this polymorph's true crystal structure. A detailed investigation into the phonon modes of orthorhombic κ-Ga2O3 provides insights into fundamental material properties such as crystal structure and orientation as well as the vibrational symmetries of Raman active modes. We investigate the Raman active phonon modes of an MBE-grown orthorhombic κ-Ga2O3 thin film featuring the domain structure deposited on (0001)-Al2O3 by experiment and theory: Polarized micro-Raman spectroscopy measurements in conjunction with density functional perturbation theory (DFPT) calculations enable the identification of both the frequencies and vibrational symmetries of the Raman active phonons. Presenting comprehensive Raman spectra of the orthorhombic κ phase, the experimental frequencies of more than 90 Raman modes are determined and correlated with the 117 modes predicted by the calculations. Angular-resolved Raman measurements are utilized to provide an experimental verification of phonon mode symmetries. We present an analytical tool to deal with the domain structure and its effect on the obtained Raman spectra
Oxidation and phase transition in covalently functionalized MoS2
We present a Raman study of MoS2 powders and MoS2 individual layers covalently functionalized with organic molecules. In MoS2 powders, the defect-induced “LA” Raman mode shows evidence for successful functionalization. Increasing temperature induces oxidation of both functionalized and nonfunctionalized MoS2 into MoO3. In contrast, mechanically exfoliated individual MoS2 layers do not transfer into MoO3 under the same conditions. Instead, the Raman spectra show that the procedure of covalent functionalization leads to a partial transition from the 2H into the 1T' crystallographic phase in few-layer MoS2. We support the identification of the 1T' phase by DFT calculations of the corresponding vibrational modes in the mono- and bilayer 1T'-MoS2
Unveiling the oxidation behavior of liquid-phase exfoliated antimony nanosheets
Antimonene, a monolayer of β-antimony, is increasingly attracting considerable attention, more than that of other monoelemental two-dimensional materials, due to its intriguing physical and chemical properties. Under ambient conditions, antimonene exhibits a high thermodynamic stability and good structural integrity. Some theoretical calculations predicted that antimonene would have a high oxidation tendency. However, it remains poorly investigated from the experimental point of view. In this work, we study the oxidation behavior of antimonene nanosheets (ANS) prepared by ultrasonication-assisted liquid-phase exfoliation. Using a set of forefront analytical techniques, a clear effect of sonication time on the surface chemistry of prepared ANS is found. A dynamic oxidation behavior has been observed, which upon annealing at moderate temperature (210 °C) resulted in a semiconducting behavior with a bandgap of approximately 1 eV measured by ultraviolet photoelectron spectroscopy. This study yields valuable information for future applications of antimonene and paves the way towards novel modification approaches in order to tailor its properties and complement its limitations
Hybridized intervalley moiré excitons and flat bands in twisted WSe2 bilayers
The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe2 bilayers. In particular, we show that the strong hybridization of electrons at the Λ point leads to a drastic redshift of the momentum-dark K–Λ exciton, accompanied by the emergence of flat moiré exciton bands at small twist angles. We directly compare theoretically predicted and experimentally measured optical spectra allowing us to identify photoluminescence signals stemming from phonon-assisted recombination of layer-hybridized dark excitons. Moreover, we predict the emergence of additional spectral features resulting from the moiré potential of the twisted bilayer lattice
Interlayer excitonic spectra of vertically stacked MoSe/WSe heterobilayers
The optical spectra of vertically stacked MoSe/WSe heterostructures
contain additional 'interlayer' excitonic peaks that are absent in the
individual monolayer materials and exhibit a significant spatial charge
separation in out-of-plane direction. Extending on a previous study, we used a
many-body perturbation theory approach to simulate and analyse the excitonic
spectra of MoSe/WSe heterobilayers with three stacking orders,
considering both momentum-direct and momentum-indirect excitons. We find that
the small oscillator strengths and corresponding optical responses of the
interlayer excitons are significantly stacking-dependent and give rise to high
radiative lifetimes in the range of 5-200\,ns (at T=4\,K) for the 'bright'
interlayer excitons. Solving the finite-momentum Bethe-Salpeter Equation, we
predict that the lowest-energy excitation should be an indirect exciton over
the fundamental indirect band gap (KQ), with a binding energy of
220\,meV. However, in agreement with recent magneto-optics experiments and
previous theoretical studies, our simulations of the effective excitonic
Land\'e g-factors suggest that the low-energy momentum-indirect excitons are
not experimentally observed for MoSe/WSe heterostructures. We further
reveal the existence of 'interlayer' C excitons with significant exciton
binding energies and optical oscillator strengths, which are analogous to the
prominent band nesting excitons in mono- and few-layer transition-metal
dichalcogenides.Comment: 20 pages, 14 figures, 3 table
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