Native defects in monolayer GaS and GaSe: Electrical properties and thermodynamic stability

in Physical Review Materials (2022), 6(11), 114002

Structural, electronic, and thermodynamic properties of native defects in GaS and GaSe monolayers are investigated by means of accurate ab initio calculations. Based on their charge transition levels we ... [more ▼]

Structural, electronic, and thermodynamic properties of native defects in GaS and GaSe monolayers are investigated by means of accurate ab initio calculations. Based on their charge transition levels we assess the influence of the studied defects on the electrical properties of the monolayers. Specifically, we show that native defects do not behave as shallow dopants and their presence cannot account for the experimentally observed intrinsic doping. In addition, we predict that native defects are efficient compensation and recombination centers. Besides pointing out their detrimental nature, we also calculate the corresponding finite-temperature formation energies and provide a window of growth conditions able to reduce the concentration of all relevant native defects. [less ▲]

The impact of strain on growth mode in chemical vapor deposited mono- and few-layer MoS2

in AIP Adv. (2022), 12(6), 065010

The development of high-quality chemical vapor-deposited mono- and few-layer MoS2 is of high relevance for future applications in functional devices. Consequently, a detailed understanding of the growth ... [more ▼]

The development of high-quality chemical vapor-deposited mono- and few-layer MoS2 is of high relevance for future applications in functional devices. Consequently, a detailed understanding of the growth mode and the parameters affecting it is important. Here, we show for the case of mono- and few-layer MoS2 grown on Muscovite mica, how strain and temperature impact the growth mode. We show how misleading the determination of the number of MoS2 layers is, solely based on Raman spectroscopy due to the occurrence of strain and changes in the growth mode. A combination of atomic force microscopy, Raman spectroscopy, and ab initio calculations reveal that that the growth at 500 dgree C synthesis temperature exhibits a strained layer-by-layer growth of up to three mono-layers, whereas at 700 degree C, a strain release occurs and layer-by-layer growth is confined to the first mono-layer only. We relate the occurrence of strain to the formation of gas bubbles below the MoS2 film, escaping the mica sheets during high temperature synthesis. Our analysis shows that mica substrates can be used to study strain in 2D materials without the need to apply external stress and that a detailed knowledge of the MoS2 morphology is necessary to correctly interpret the Raman results. [less ▲]

Collective electronic excitations in charge density wave systems: The case of CuTe

in Physical Review. B (2021), 104(12), 125101

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW ... [more ▼]

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative. [less ▲]

Time-Dependent Screening Explains the Ultrafast Excitonic Signal Rise in 2D Semiconductors

in ACS Nano (2021), 15(1), 1179--1185

We calculate the time evolution of the transient reflection signal in an MoS2 monolayer on a SiO2/Si substrate using first-principles out-of-equilibrium real-time methods. Our simulations provide a simple ... [more ▼]

We calculate the time evolution of the transient reflection signal in an MoS2 monolayer on a SiO2/Si substrate using first-principles out-of-equilibrium real-time methods. Our simulations provide a simple and intuitive physical picture for the delayed, yet ultrafast, evolution of the signal whose rise time depends on the excess energy of the pump laser: at laser energies above the A- and B-exciton, the pump pulse excites electrons and holes far away from the K valleys in the first Brillouin zone. Electron–phonon and hole–phonon scattering lead to a gradual relaxation of the carriers toward small Active Excitonic Regions around K, enhancing the dielectric screening. The accompanying time-dependent band gap renormalization dominates over Pauli blocking and the excitonic binding energy renormalization. This explains the delayed buildup of the transient reflection signal of the probe pulse, in excellent agreement with recent experimental data. Our results show that the observed delay is not a unique signature of an exciton formation process but rather caused by coordinated carrier dynamics and its influence on the screening. [less ▲]

Theory of resonant Raman scattering: Towards a comprehensive ab initio description

in Physical Review. B (2019), 99(17), 174312-15

We develop a general, fully quantum mechanical theory of Raman scattering from first principles in terms of many-body correlation functions. In order to arrive at expressions that are practically useful ... [more ▼]

We develop a general, fully quantum mechanical theory of Raman scattering from first principles in terms of many-body correlation functions. In order to arrive at expressions that are practically useful in the context of condensed matter physics, we adopt the Lehmann-Symanzik-Zimmermann reduction formula from high-energy physics and formulate it in the language of many-body perturbation theory. This enables us to derive a general and practically useful expression for the Raman scattering rate in terms of quantities that can be computed ab initio. Our work paves the way toward a comprehensive computational approach to the calculation of Raman spectra that goes beyond the current state of the art by capturing both excitonic and nonadiabatic effects. [less ▲]

Photoinduced Phase Transitions in Ferroelectrics

in PHYSICAL REVIEW LETTERS (2019), 123(8), 087601-6

Ferroic materials naturally exhibit a rich number of functionalities, which often arise from thermally, chemically, or mechanically induced symmetry breakings or phase transitions. Based on density ... [more ▼]

Ferroic materials naturally exhibit a rich number of functionalities, which often arise from thermally, chemically, or mechanically induced symmetry breakings or phase transitions. Based on density functional calculations, we demonstrate here that light can drive phase transitions as well in ferroelectric materials such as the perovskite oxides lead titanate and barium titanate. Phonon analysis and total energy calculations reveal that the polarization tends to vanish under illumination, to favor the emergence of nonpolar phases, potentially antiferroelectric, and exhibiting a tilt of the oxygen octahedra. Strategies to tailor photoinduced phases based on phonon instabilities in the electronic ground state are also discussed. [less ▲]

Searching for materials with high refractive index and wide band gap: A first-principles high-throughput study

in PHYSICAL REVIEW MATERIALS (2019), 3(4), 044602-12

Materials combining both a high refractive index and a wide band gap are of great interest for optoelectronic and sensor applications. However, these two properties are typically described by an inverse ... [more ▼]

Materials combining both a high refractive index and a wide band gap are of great interest for optoelectronic and sensor applications. However, these two properties are typically described by an inverse correlation with high refractive index appearing in small gap materials and vice versa. Here, we conduct a first-principles high-throughput study on more than 4000 semiconductors (with a special focus on oxides). Our data confirm the general inverse trend between refractive index and band gap but interesting outliers are also identified. The data are then analyzed through a simple model involving two main descriptors: the average optical gap and the effective frequency. The former can be determined directly from the electronic structure of the compounds, but the latter cannot. This calls for further analysis in order to obtain a predictive model. Nonetheless, it turns out that the negative effect of a large band gap on the refractive index can be counterbalanced in two ways: (i) by limiting the difference between the direct band gap and the average optical gap which can be realized by a narrow distribution in energy of the optical transitions and (ii) by increasing the effective frequency which can be achieved through either a high number of transitions from the top of the valence band to the bottom of the conduction band or a high average probability for these transitions. Focusing on oxides, we use our data to investigate how the chemistry influences this inverse relationship and rationalize why certain classes of materials would perform better. Our findings can be used to search for new compounds in many optical applications both in the linear and nonlinear regime (waveguides, optical modulators, laser, frequency converter, etc.). [less ▲]

Direct and indirect excitons in boron nitride polymorphs: A story of atomic configuration and electronic correlation

in Physical Review. B, Condensed Matter (2018), 98(12), 125206

We present a detailed discussion of the electronic band structure and excitonic dispersion of hexagonal boron nitride (hBN) in the single layer configuration and in three bulk polymorphs (usual AA′ ... [more ▼]

We present a detailed discussion of the electronic band structure and excitonic dispersion of hexagonal boron nitride (hBN) in the single layer configuration and in three bulk polymorphs (usual AA′ stacking, Bernal AB, and rhombohedral ABC). We focus on the changes in the electronic band structure and the exciton dispersion induced by the atomic configuration and the electron-hole interaction. Calculations are carried out at the level of ab initio many-body perturbation theory (GW and Bethe Salpeter equation) and of a purposely developed tight-binding model. We confirm the change from direct to indirect electronic gap when going from single layer to bulk systems and we give a detailed account of its origin by comparing the effect of different stacking sequences. We emphasize that the inclusion of the electron-hole interaction is crucial for the correct description of the momentum-dependent dispersion of the excitations. As a result the electron-hole dispersion is flatter than the one obtained from the band structure. In the AB stacking this effect is particularly important as the lowest-lying exciton is predicted to be direct despite the indirect electronic band gap. [less ▲]

Optical control of polarization in ferroelectric heterostructures

in Nature Communications (2018), 9

In the ferroelectric devices, polarization control is usually accomplished by application of an electric field. In this paper, we demonstrate optically induced polarization switching in BaTiO3-based ... [more ▼]

In the ferroelectric devices, polarization control is usually accomplished by application of an electric field. In this paper, we demonstrate optically induced polarization switching in BaTiO3-based ferroelectric heterostructures utilizing a two-dimensional narrow-gap semiconductor MoS2 as a top electrode. This effect is attributed to the redistribution of the photo-generated carriers and screening charges at the MoS2/BaTiO3 interface. Specifically, a two-step process, which involves formation of intra-layer excitons during light absorption followed by their decay into inter-layer excitons, results in the positive charge accumulation at the interface forcing the polarization reversal from the upward to the downward direction. Theoretical modeling of the MoS2 optical absorption spectra with and without the applied electric field provides quantitative support for the proposed mechanism. It is suggested that the discovered effect is of general nature and should be observable in any heterostructure comprising a ferroelectric and a narrow gap semiconductor. [less ▲]

Impact of many-body effects on Landau levels in graphene

in Physical Review Letters (2018), 120(18), 187701

We present magneto-Raman spectroscopy measurements on suspended graphene to investigate the charge carrier density-dependent electron-electron interaction in the presence of Landau levels. Utilizing gate ... [more ▼]

We present magneto-Raman spectroscopy measurements on suspended graphene to investigate the charge carrier density-dependent electron-electron interaction in the presence of Landau levels. Utilizing gate-tunable magnetophonon resonances, we extract the charge carrier density dependence of the Landau level transition energies and the associated effective Fermi velocity vF. In contrast to the logarithmic divergence of vF at zero magnetic field, we find a piecewise linear scaling of vF as a function of the charge carrier density, due to a magnetic-field-induced suppression of the long-range Coulomb interaction. We quantitatively confirm our experimental findings by performing tight-binding calculations on the level of the Hartree-Fock approximation, which also allow us to estimate an excitonic binding energy of ≈6  meV contained in the experimentally extracted Landau level transitions energies. [less ▲]