arXiv subjects

Roland Gillen

Publications and source records attributed to Roland Gillen.

32 records · Page 2Linked to original sources

arXiv2018

Interlayer excitons in MoSe$_2$/WSe$_2$ heterostructures from first-principles

Based on \emph{ab initio} theoretical calculations of the optical spectra of vertical heterostructures of MoSe$_2$ (or MoS$_2$) and WSe$_2$ sheets, we reveal two spin-orbit-split Rydberg series of excitonic states below the \textsl{A} excitons of MoSe$_2$ and WSe$_2$ with a significant binding energy on the order of 250\,meV for the first excitons in the series. At the same time, we predict crystalographically aligned MoSe$_2$/WSe$_2$ heterostructures to exhibit an indirect fundamental band gap. Due to the type-II nature of the MoSe$_2$/WSe$_2$ heterostructure, the indirect transition and the exciton Rydberg series corresponding to a direct transition exhibit a distinct interlayer nature with spatial charge separation of the coupled electrons and holes. The experimentally observed long-lived states in photoluminescence spectra of MoX$_2$/WY$_2$ heterostructure are attributed to such interlayer exciton states. Our calculations further suggest an effect of stacking order on the peak energy of the interlayer excitons and their oscillation strengths.

cond-mat.mes-hall↗

arXiv2016

Light-Matter Interactions in Two-Dimensional Transition Metal Dichalcogenides: Dominant Excitonic Transitions in mono- and few-layer MoX$_2$ and Band Nesting

We report ab initio calculations of the dielectric function of six mono- and bilayer molybdenum dichalcogenides based in a Bethe Salpether equation+G$_0$W$_0$ ansatz, focussing on the excitonic transitions dominating the absorption spectrum up to an excitation energy of 3\,eV. Our calculations suggest that switching chalcogen atoms and the strength of interlayer interactions should affect the detailed composition of the high 'C' peaks in experimental optical spectra of molybdenum dichalcogenides and cause a significant spin-orbit-splitting of the contributing excitonic transitions in monolayer MoSe$_2$ and MoTe$_2$. This can be explained through changes in the electronic dispersion around the Fermi energy along the chalcogen series S$\rightarrow$Se$\rightarrow$Te that move the van-Hove singularities in the density of states of the two-dimensional materials along the \textit{$Γ$}-\textit{K} line in the Brillouin zone. Further, we confirm the distinct interlayer character of the '\textsl{C}' peak transition in few-layer MoS$_2$ that was predicted before from experimental data and show that a similar behaviour can be expected for MoSe$_2$ and MoTe$_2$ as well.

cond-mat.mtrl-sci↗

arXiv2015

Beyond double-resonant Raman scattering: UV Raman spectroscopy on graphene, graphite and carbon nanotubes

We present an analysis of deep-UV Raman measurements of graphite, graphene and carbon nanotubes. For excitation energies above the strong optical absorption peak at the $M$ point in the Brillouin zone ($\approx 4.7\,\text{eV}$), we partially suppress double-resonant scattering processes and observe the two-phonon density of states of carbon nanomaterials. The measured peaks are assigned to contributions from LO, TO, and LA phonon branches, supported by calculations of the phonon dispersion. Moreover, we gain access to the infrared-active $E_{1u}$ mode in graphite. By lowering the excitation energy and thus allowing double-resonant scattering processes, we demonstrate the rise of the \textit{2D} mode in graphite with ultra-short phonon wave vectors.

cond-mat.mtrl-sci↗

arXiv2015

Splitting of the monolayer out-of-plane A'1 Raman mode in few-layer WS2

We present Raman measurements of mono- and few-layer WS2. We study the monolayer A'1 mode around 420 cm(-1) and its evolution with the number of layers. We show that with increasing layer number there is an increasing number of possible vibrational patterns for the out-of-plane Raman mode: in N-layer WS2 there are N Gamma-point phonons evolving from the A'1 monolayer mode. For an excitation energy close to resonance with the excitonic transition energy we were able to observe all of these N components, irrespective of their Raman activity. Density functional theory calculations support the experimental findings and make it possible to attribute the modes to their respective symmetries. The findings described here are of general importance for all other phonon modes in WS2 and other layered transition metal dichalcogenide systems in the few layer regime.

cond-mat.mes-hall↗

arXiv2015

Newly observed first-order resonant Raman modes in few-layer MoS$_2$

We report two new first-order Raman modes in the spectra of few-layer MoS$_2$ at 286~cm$^{-1}$ and 471~cm$^{-1}$ for excitation energies above 2.4~eV. These modes appear only in few-layer MoS$_2$; therefore their absence provides an easy and accurate method to identify single-layer MoS$_2$. We show that these modes are related to phonons that are not observed in the single layer due to their symmetry. Each of these phonons leads to several nearly degenerate phonons in few-layer samples. The nearly degenerate phonons in few-layer materials belong to two different symmetry representations, showing opposite behavior under inversion or horizontal reflection. As a result, Raman active phonons exist in few-layer materials that have nearly the same frequency as the symmetry forbidden phonon of the single layer. We provide here a general treatment of this effect for all few-layer two-dimensional crystal structures with an inversion center or a mirror plane parallel to the layers. We show that always nearly degenerate phonon modes of different symmetry must occur and, as a result, similar pseudo-activation effects can be excepted.

cond-mat.mtrl-sci↗

arXiv2014

Indirect doping effects from impurities in MoS$_2$/BN heterostructures

We performed density functional theory calculations on heterostructures of single layers of hexagonal BN and MoS$_2$ to assess the effect of doping in the BN sheet and of interstitial Na atoms on the electronic properties of the adjacent MoS$_2$ layer. Our calculations predict that $n$-doping of the boron nitride subsystem by oxygen, carbon and sulfur impurities causes noticeable charge transfer into the conduction band of the MoS$_2$ sheet, while $p$-doping by beryllium and carbon leaves the molybdenum disulphide layer largely unaffected. Intercalated sodium atoms lead to a significant increase of the interlayer distance in the heterostructure and to a metallic ground state of the MoS$_2$ subsystem. The presence of such $n$-dopants leads to a distinct change of valence band and conduction band offsets, suggesting that doped BN remains a suitable substrate and gate material for applications of $n$-type MoS$_2$.

cond-mat.mes-hall↗

arXiv2014

Photoluminescence of freestanding single- and few-layer MoS2

We present a photoluminescence study of freestanding and Si/SiO2 supported single- and few-layer MoS2. The single-layer exciton peak (A) is only observed in freestanding MoS2. The photoluminescence of supported single-layer MoS2 is instead originating from the A- (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate is decreasing the indirect band gap energy by up to ~ 80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects.

cond-mat.mtrl-sci↗

arXiv2013

Accurate screened exchange band structures for transition metal monoxides MnO, FeO, CoO and NiO

We report calculations of the band structures and density of states of the four transition metal monoxides MnO, FeO, CoO and NiO using the hybrid density functional sX-LDA. Late transition metal oxides are prototypical examples of strongly correlated materials, which pose challenges for electronic structure methods. We compare our results with available experimental data and show that our calculations yield accurate predictions for the fundamental band gaps and valence bands of FeO, CoO and NiO. For MnO, the band gaps are underestimated, suggesting additional many-body effects that are not captured by our screened hybrid functional approach.

cond-mat.mtrl-sci↗

arXiv2013

The nature of the electronic band gap in lanthanide oxides

Accurate electronic structures of the technologically important lanthanide/rare earth sesquioxides (Ln2O3, with Ln=La,...,Lu) and CeO2 have been calculated using hybrid density functionals HSE03, HSE06 and screened-exchange (sX-LDA). We find that these density functional methods describe the strongly correlated Ln f-electrons as well as the recent G0W0@LDA+U results, generally yielding the correct band gaps and trends across the Ln-period. For HSE, the band gap between O 2p states and lanthanide 5d states is nearly independent of the lanthanide, while the minimum gap varies as filled or empty Ln 4f states come into this gap. sX-LDA predicts the unoccupied 4f levels at higher energies, which leads to a better agreement with experiments for Sm2O3, Eu2O3 and Yb2O3.

cond-mat.mtrl-sci↗

arXiv2011

Hybrid functional calculations of the Al impurity in silica: Hole localization and electron paramagnetic resonance parameters

We performed first-principle calculations based on the supercell and cluster approaches to investigate the neutral Al impurity in smoky quartz. Electron paramagnetic resonance measurements suggest that the oxygens around the Al center undergo a polaronic distortion which localizes the hole being on one of the four oxygen atoms. We find that the screened exchange hybrid functional successfully describes this localization and improves on standard local density approaches or on hybrid functionals that do not include enough exact exchange such as B3LYP. We find a defect level at about 2.5 eV above the valence band maximum, corresponding to a localized hole in a O 2p orbital. The calculated values of the g tensor and the hyperfine splittings are in excellent agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2011

Band structure calculations of CuAlO2, CuGaO2, CuInO2 and CuCrO2 by screened exchange

We report density functional theory (DFT) band structure calculations on the transparent conducting oxides CuAlO2, CuGaO2, CuInO2 and CuCrO2. The use of the hybrid functional sX-LDA leads to considerably improved electronic properties compared to standard local density approximation (LDA) and generalized gradient approximation (GGA) approaches. We show that the resulting electronic band gaps compare well with experimental values and previous quasiparticle calculations and show the correct trends with respect to the atomic number of the cation (Al, Ga, In). The resulting energetic depths of Cu d and O p levels and the valence band widths are considerable improvements compared to LDA and GGA and in good agreement with available x-ray photoelectron spectroscopy (XPS) data. Lastly, we show the calculated imaginary part of the dielectric function for all four systems.

cond-mat.str-el↗

arXiv2010

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.

cond-mat.mes-hall↗

arXiv2010

Symmetry properties of vibrational modes in graphene nanoribbons

We present symmetry properties of the lattice vibrations of graphene nanoribbons with pure armchair (AGNR) and zigzag edges (ZGNR). In non-symmorphic nanoribbons the phonon modes at the edge of the Brillouin zone are twofold degenerate, whereas the phonon modes in symmorphic nanoribbons are non-degenerate. We identified the Raman-active and infrared-active modes. We predict 3N and 3(N+1) Raman-active modes for N-ZGNRs and N-AGNRs, respectively (N is the number of dimers per unit cell). These modes can be used for the experimental characterization of graphene nanoribbons. Calculations based on density functional theory suggest that the frequency splitting of the LO and TO in AGNRs (corresponding to the E2g mode in graphene) exhibits characteristic width and family dependence. Further, all graphene nanoribbons have a Raman-active breathing-like mode, the frequency of which is inversely proportional to the nanoribbon width and thus might be used for experimental determination of the width of graphene nanoribbons.

cond-mat.mes-hall↗

arXiv2009

Vibrational properties of graphene nanoribbons by first-principles calculations

We investigated the vibrational properties of graphene nanoribbons by means of first-principles calculations on the basis of density functional theory. We confirm that the phonon modes of graphene nanoribbons with armchair and zigzag type edges can be interpreted as fundamental oscillations and their overtones. These show a characteristic dependence on the nanoribbon width. Furthermore, we demonstrate that a mapping of the calculated Gamma-point phonon frequencies of nanoribbons onto the phonon dispersion of graphene corresponds to an "unfolding" of nanoribbons' Brillouin zone onto that of graphene. We consider the influence of spin states with respect to the phonon spectra of zigzag nanoribbons and provide comparisons of our results with past studies.

cond-mat.mtrl-sci↗