Molybdenum disulfide (MoS2) is systematically studied using Raman spectroscopy with ultraviolet and visible laser lines. It is shown that only the Raman frequencies of $ E_{2{\rm g}}^1 $ and $ ...A_{{\rm 1g}}^{} $ peaks vary monotonously with the layer number of ultrathin MoS2 flakes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons are found to become weaker when the layer number of MoS2 decreases, attributed to the increased electronic transition energies or elongated intralayer atomic bonds in ultrathin MoS2. The asymmetric Raman peak at 454 cm−1, which has been regarded as the overtone of longitudinal optical M phonons in bulk MoS2, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode ($ A_{{\rm 2u}}^{} $). Our findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS2 is scaled down from three‐ to two‐dimensional geometry.
Raman frequencies of $ E_{2{\rm g}}^1 $ and $ A_{{\rm 1g}}^{} $ peaks can be used to identify the layer number of ultrathin molybdenum disulfide (MoS2) flakes. The systematic Raman characterizations using various laser lines suggest a clear evolution of the coupling between the electronic transition and phonon when MoS2 is scaled down from three‐ to two‐dimensional geometry.
Abstract
Molybdenum disulfide (MoS
2
) is systematically studied using Raman spectroscopy with ultraviolet and visible laser lines. It is shown that only the Raman frequencies of
$ E_{2{\rm g}}^1 $
...and
$ A_{{\rm 1g}}^{} $
peaks vary monotonously with the layer number of ultrathin MoS
2
flakes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons are found to become weaker when the layer number of MoS
2
decreases, attributed to the increased electronic transition energies or elongated intralayer atomic bonds in ultrathin MoS
2
. The asymmetric Raman peak at 454 cm
−1
, which has been regarded as the overtone of longitudinal optical M phonons in bulk MoS
2
, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode (
$ A_{{\rm 2u}}^{} $
). Our findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS
2
is scaled down from three‐ to two‐dimensional geometry.
•Physical mechanisms of femtosecond generation of coherent acoustic phonons in solids.•Deformation potential, thermoelasticity, inverse-piezoelectricity, electrostriction.•High frequency acoustic ...phonons driven by electron–hole deformation potential.•Light-controlled piezoelectricity with surface, junction or the Dember electric field.•Recent photogeneration processes in ferroelectrics are discussed.
In this review we address the microscopic mechanisms that are involved in the photogeneration processes of GHz–THz coherent acoustic phonons (CAP) induced by an ultrafast laser pulse. Understanding and describing the underlying physics is necessary indeed for improving the future sources of coherent acoustic phonons useful for the non-destructive testing optoacoustic techniques. Getting more physical insights on these processes also opens new perspectives for the emerging field of the opto-mechanics where lattice motions (surface and/or interfaces ultrafast displacements, nanostructures resonances) are controlled by light. We will then remind the basics of electron–phonon and photon-phonon couplings by discussing the deformation potential mechanism, the thermoelasticity, the inverse piezoelectric effect and the electrostriction in condensed matter. Metals, semiconductors and oxide materials will be discussed. The contribution of all these mechanisms in the photogeneration process of sound will be illustrated over several examples coming from the rich literature.
In this work we present theoretical calculations and analysis of the vibronic structure of the spin-triplet optical transition in diamond nitrogen-vacancy (NV) centres. The electronic structure of ...the defect is described using accurate first-principles methods based on hybrid functionals. We devise a computational methodology to determine the coupling between electrons and phonons during an optical transition in the dilute limit. As a result, our approach yields a smooth spectral function of electron-phonon coupling and includes both quasi-localized and bulk phonons on equal footings. The luminescence lineshape is determined via the generating function approach. We obtain a highly accurate description of the luminescence band, including all key parameters such as the Huang-Rhys factor, the Debye-Waller factor, and the frequency of the dominant phonon mode. More importantly, our work provides insight into the vibrational structure of NV centres, in particular the role of local modes and vibrational resonances. In particular, we find that the pronounced mode at 65 meV is a vibrational resonance, and we quantify localization properties of this mode. These excellent results for the benchmark diamond (NV) centre provide confidence that the procedure can be applied to other defects, including alternative systems that are being considered for applications in quantum information processing.
The EPW (Electron-Phonon coupling using Wannier functions) software is a Fortran90 code that uses density-functional perturbation theory and maximally localized Wannier functions for computing ...electron–phonon couplings and related properties in solids accurately and efficiently. The EPW v4 program can be used to compute electron and phonon self-energies, linewidths, electron–phonon scattering rates, electron–phonon coupling strengths, transport spectral functions, electronic velocities, resistivity, anisotropic superconducting gaps and spectral functions within the Migdal–Eliashberg theory. The code now supports spin–orbit coupling, time-reversal symmetry in non-centrosymmetric crystals, polar materials, and k and q-point parallelization. Considerable effort was dedicated to optimization and parallelization, achieving almost a ten times speedup with respect to previous releases. A computer test farm was implemented to ensure stability and portability of the code on the most popular compilers and architectures. Since April 2016, version 4 of the EPW code is fully integrated in and distributed with the Quantum ESPRESSO package, and can be downloaded through QE-forge at http://qe-forge.org/gf/project/q-e.
Program title: EPW
Catalogue identifier: AEHA_v2_0
Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEHA_v2_0.html
Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland
Licensing provisions: GNU General Public Licence 3
No. of lines in distributed program, including test data, etc.: 1635099
No. of bytes in distributed program, including test data, etc.: 22533187
Distribution format: tar.gz
Programming language: Fortran 90, MPI.
Computer: Non-specific.
Operating system: Unix/Linux.
RAM: Typically 2GB/core
Classification: 7.3, 7.8, 7.9.
External routines: LAPACK, BLAS, MPI, FFTW, Quantum- ESPRESSO package 1
Does the new version supersede the previous version?: Yes
Nature of problem:
Calculation of electron and phonon self-energies, linewidths, electron–phonon scattering rates, electron–phonon coupling strengths, transport spectral functions, electronic velocities, resistivity, anisotropic superconducting gaps and spectral functions within the Migdal–Eliashberg theory.
Solution method:
The code relies on density-functional perturbation theory and maximally localized Wannier functions.
Reasons for new version:
New features (listed in the paper) and optimization of the code.
Summary of revisions:
Recent developments and new functionalities are described in Section 2 of the paper.
Running time:
Up to several hours on several tens of processors.
References:1P. Giannozzi, et al., J. Phys. Condens. Matter 21 (2009), 395502, http://www.quantum-espresso.org/.
2D Dion−Jacobson (DJ) perovskite single crystals (PSCs) usually demonstrate better X‐ray detection performance than Ruddlesden‐Popper (RP) PSCs. However, the mechanism of the improved performance is ...still elusive. Here, by the aid of strong interactions between dimethylbiguanide (DGA) and PbI2, a novel DJ‐perovskitoid (DGA)PbI4 is designed. From the comparison of (DGA)PbI4 to other 2D PSCs, it is discovered that the tiniest lattice distortion and increased hydrogen bonds in the atom‐scaled analysis strengthen lattice rigidity and weaken electron‐phonon coupling to suppress disordered scattering of carriers, resulting in significantly improved carrier transport and stability. Therefore, high carrier mobility (78.1 cm2 V−1 s−1) and a pronounced sensitivity of 4869.0 µC Gyair−1cm−2 are achieved using (DGA)PbI4, which are the best in 2D Pb‐based PSC devices to date. Finally, the (DGA)PbI4 devices exhibit good spatial resolution in X‐ray imaging and excellent long‐term stability to work as a promising candidate for medical diagnostics and nondestructive determination.
A novel 2D perovskite single crystal is designed by introducing a diamine molecule DGA into the lead‐iodine octahedron. Compared with the monoamine o‐F‐PEA, the DGA strongly interacts with the PbI64‐, improving the lattice stiffness, reducing electron‐phonon coupling and lattice distortion, and making the corresponding device to achieve high carrier mobility (78.1 cm2 V−1 s−1) and high X‐ray sensitivity (4869.0 µC Gyair−1 cm−2).
Vertically stacked transition metal dichalcogenide-graphene heterostructures provide a platform for novel optoelectronic applications with high photoresponse speeds. Photoinduced nonequilibrium ...carrier and lattice dynamics in such heterostructures underlie these applications but have not been understood. In particular, the dependence of these photoresponses on the twist angle, a key tuning parameter, remains elusive. Here, using ultrafast electron diffraction, we report the simultaneous visualization of charge transfer and electron–phonon coupling in MoS2-graphene heterostructures with different stacking configurations. We find that the charge transfer timescale from MoS2 to graphene varies strongly with twist angle, becoming faster for smaller twist angles, and show that the relaxation timescale is significantly shorter in a heterostructure as compared to a monolayer. These findings illustrate that twist angle constitutes an additional tuning knob for interlayer charge transfer in heterobilayers and deepen our understanding of fundamental photophysical processes in heterostructures, of importance for future applications in optoelectronics and light harvesting.
The interaction of ultra-fast sub-picosecond laser pulses with solids is a very broad area of research. The boundaries for research fields covered by this review are defined as follows. A laser pulse ...in the context of the review is of ultra-short duration if the pulse is shorter than all major relaxation times. Such pulses excite only electrons, leaving the lattice cold for the time required for the transfer of the absorbed laser energy from the heated electrons to the lattice. For this reason, any laser-induced phase transformations occur in non-equilibrium conditions, making properties of the material drastically different from their equilibrium counterparts. We study laser interaction with matter in a broad range of intensities from those inducing subtle atomic excitations (∼10
10 W/cm
2) up to high intensity (∼10
16 W/cm
2), when solid is swiftly transformed into hot and dense plasma. The phenomena emerging in succession in response to increasing laser intensity, namely, the excitations of coherent phonons, phase transitions, ablation, and transformation of material into plasma, are described in consecutive chapters. Two interaction geometries are investigated: the interaction of a laser pulse with a surface, and confined interaction when a laser is focused inside a transparent solid. The highest intensity in all these studies is well below the relativistic limit. Therefore, super-intense laser–matter interactions are beyond the scope in this review.
All phenomena involved in laser–matter interaction are considered from the first principles using explicit approximations, eventually aiming to establish the analytical scaling relations, which link the parameters of the laser and the material and allow comparison with experiments. We compare theory to experiments in all intensity ranges. The applications of some studies are described in a separate chapter. The prospects of these studies are indicated in the conclusion.
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