Aurélie Champagne

In the present work, the electronic and vibrational properties of both pristine ${\mathrm{V}}_{2}\mathrm{C}$ and fully terminated ${\mathrm{V}}_{2}\mathrm{C}{T}_{2}$ (where $T=\mathrm{F},$ O, OH) two-dimensional monolayers are investigated using density functional theory. First, the atomic structures of ${\mathrm{V}}_{2}\mathrm{C}$-based MXene phases are optimized, and their respective dynamical stabilities are discussed. Second, electronic band structures are computed indicating that ${\mathrm{V}}_{2}\mathrm{C}$ is metallic as well as all the corresponding functionalized systems. Third, the vibrational properties (phonon frequencies and spectra) of ${\mathrm{V}}_{2}\mathrm{C}$-based MXenes are computed with the density functional perturbation theory and reported. Both Raman-active (${E}_{g}$, ${A}_{1g}$) and infrared-active (${E}_{u}$, ${A}_{2u}$) vibrational modes are predicted ab initio with the aim to correlate the experimental Raman peaks with the calculated vibrational modes and to assign them to specific atomic motions. The effect of the terminal groups on the vibrational properties is emphasized, along with the effect on the presence and position of the corresponding Raman peaks. Our results provide insights for the identification and characterization of ${\mathrm{V}}_{2}\mathrm{C}$-based samples using Raman spectroscopy.

Abstract The family of 2D materials has expanded quite rapidly, especially with the addition of transition metal carbides and nitrides called MXenes, in the last decade. Since their discovery in 2011, about 30 different MXenes have been synthesized, and the structure and properties of several dozens have been predicted by first-principles approaches. Given the outstanding advances in the MXene field, it is thus appropriate to review the most relevant properties of these MXenes and point out their potential applications. In this article, the structural, transport, magnetic, vibrational, mechanical, and electrochemical properties of MXenes are overviewed. The goal is to illustrate how the chemical versatility in the intrinsic composition and surface terminations combined with the potential addition of a fourth element enable to tune MXenes properties to meet the targeted applications.

Using first-principles calculations, we perform a comprehensive and systematic analysis to establish the role of van der Waals (vdW) interactions and anharmonicity in the vibrational properties of the low-temperature orthorhombic phase of the hybrid perovskite CH3NH3PbI3. To this end, we consider the most common approaches for including vdW effects in our phonon calculations: the semiempirical Grimme approximations, the Tkatchenko-Scheffler dispersion corrections, and the vdW density-functional method. The vibrational normal modes are first calculated within the harmonic approximation. We consider the LDA and GGA approximations to the exchange-correlation functional and include spin–orbit coupling (SOC) effects. On top of the harmonic calculations, we also evaluate the anharmonicity of the normal modes and the phonon–phonon coupling by solving one-dimensional and two-dimensional nuclear Schrödinger equations, respectively, via the finite-displacement method. We observe that both the LDA and GGA approximations work remarkably well in describing the vibrational properties of CH3NH3PbI3. We find that vdW effects and relativistic effects do not have any significant impact on the vibrational properties of CH3NH3PbI3. Our study also reveals that the spinning modes of the organic CH3NH3+ cations carry considerable anharmonicity but that the anharmonic coupling between different modes is relatively small.

The recent discovery of quaternary MAX phases with chemical in-plane order allowed the addition of nontraditional MAX phase elements, such as rare-earth elements. In the present study, first-principles calculations are performed to investigate the electronic structure, elastic and hardness response, and bonding strengths of the novel $\mathrm{RE}\text{\ensuremath{-}}i\text{\ensuremath{-}}\mathrm{MAX}$ phases with the formula ${({\text{Mo}}_{2/3}{\text{RE}}_{1/3})}_{2}\text{AlC}$. The Voigt-Reuss-Hill bulk, shear, and Young's moduli are compared along the series of RE = Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu, and the global trend is found to depend on the unit cell volume. Nanoindentation experiments on Ho-based single crystals result in moduli that are within 10% of predicted values and a hardness of $\ensuremath{\sim}10$ GPa. The computation of the projected density of states, projected crystal orbital Hamilton population, and integrated projected crystal orbital Hamilton population, reveals that the bonding of Mo and RE atoms with the Al atoms are weaker than those with the C atoms, suggesting the exfoliation of $\mathrm{RE}\text{\ensuremath{-}}i\text{\ensuremath{-}}\mathrm{MAX}$ into two-dimensional $\mathrm{RE}\text{\ensuremath{-}}i$-MXenes to be feasible. Such a possibility to form two-dimensional crystals is further confirmed by the computation of the exfoliation energies, which demonstrates the process to be easier as the RE atomic mass decreases.

Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances, which serve as a microscopic, noninvasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS2, MoSe2, WS2, and WSe2), but detailed exploration of excitonic phenomena has been lacking in the important TMDC material molybdenum ditelluride (MoTe2). Here, we report an experimental investigation of excitonic luminescence properties of monolayer MoTe2 to understand the excitonic Rydberg series, up to 3s. We report a significant modification of emission energies with temperature (4 to 300 K), thereby quantifying the exciton–phonon coupling. Furthermore, we observe a strongly gate-tunable exciton–trion interplay for all the Rydberg states governed mainly by free-carrier screening, Pauli blocking, and band gap renormalization in agreement with the results of first-principles GW plus Bethe–Salpeter equation approach calculations. Our results help bring monolayer MoTe2 closer to its potential applications in near-infrared optoelectronics and photonic devices.