Phonons In Monolayers Research Articles

Ultrafast optical absorption and coherent phonon generation in monolayer and bilayer transition metal dichalcogenides

When ultrashort laser pulses are applied to the solid-state material within a time scale less than the period of a particular phonon mode, the energy and momentum of the absorbed light are used by photo-excited carriers to interact collectively with the crystal lattices and generate lattice oscillations coherently, known as the coherent phonons. The Fourier transform of the absorption modulation due to the coherent phonon oscillations gives the coherent phonon intensity. In this work, we theoretically investigate the shape of coherent phonon intensity for transition metal dichalcogenides as a function of the polarization angle of the laser pulses. In particular, we focus on the E2g and A1g phonons in monolayer and bilayer molybdenum disulfides. Within the simplified theory of displacive excitation of coherent phonons, we find that the laser pulses uniquely select the electronic states in monolayer and bilayer molybdenum disulfides, resulting in a rich feature of the polar plots of the energy-dependent coherent phonon intensity of these materials.

First-principles study on the anisotropic transport of electrons and phonons in monolayer and bulk GaTe: a comparative study

Recently, monoclinic-phase GaTe has attracted much attention due to its potential applications in nanoelectronics. Despite the experimental research, theoretical studies on the thermal and transport properties, which are necessary to provide information for future applications, are still absent. We have systematically investigated the electronic, phonon and electron transporting, and thermoelectric properties of monolayer and bulk GaTe using first-principles calculations plus the Boltzmann transport equation. At the valence band maximum and conduction band minimum, the effective mass shows large anisotropy as the band dispersions are along different k-paths. The group velocity of acoustic modes also shows large anisotropy owing to the in-plane low-symmetry. Our calculations reveal that the in-plane thermal conductivities, κa and κb, take 3.5 and 8.9 W m-1 K-1, respectively, for the bulk at 300 K, compared to κa = 5.5 and κb = 10.4 W m-1 K-1 of the monolayer. Due to the van der Waals interactions between interlayers, the out-of-plane thermal conductivity is very small, κc = 1.8 W m-1 K-1. The difference between the in-plane thermal conductivities of the bulk and the monolayer can be attributed to the strengthened Umklapp scattering, which is caused by the stiffening of the lowest-frequency optical mode in the bulk. The hole mobilities of the bulk is found to be about 12-35 cm2 V-1 s-1 at 300 K, in good agreement with the experimental results. The monolayer is found to have smaller mobility but larger anisotropy than those of the bulk. Interestingly, the out-of-plane conductivity is anomalously larger than the in-plane one for the bulk, which is attributed to the orbital overlaps between the interlayer Te atoms. Moreover, n-type GaTe is found to have much larger mobility and anisotropy than the p-type one, which is useful for future applications. Compared with the case of monolayer GaTe, thermoelectric performance can be enhanced by one order of magnitude for the bulk GaTe by exploiting the out-of-plane thermal and electrical conductivities.

Robust high-temperature trion emission in monolayers of Mo(SySe1−y)2 alloys

Atomically thin semiconducting transition-metal dichalcogenides enable new insight into physics of many-body effects mediated by Coulomb and electron-phonon interactions. We report photoluminescence (PL) and reflectivity contrast (RC) measurements of excitons ($X$) and trions ($T$) and Raman spectra of phonons in monolayers (MLs) of $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys with sulfur mole content up to $y=0.5$. We find the phonon energy crossing the trion binding energy with the sulfur mole content. Binary ${\mathrm{MoSe}}_{2}$ and ternary $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys exhibit contrasting behavior in the temperature evolution of excitons and trions PL intensity from $T=7$ to 295 K. In ${\mathrm{MoSe}}_{2}$, the trion dominates PL spectra at low temperatures but the exciton dominates PL at higher temperature. In contrast, in ternary $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys trions dominate PL spectra at all measured temperatures, with the trion to exciton PL intensity ratio increasing with sulfur content. We attribute the strong increase of the trion PL intensity in $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ MLs with increase of sulfur mole content to two effects: (i) strong increase of exciton-trion coupling mediated by the optical phonon, which is realized by tuning phonon energy through trion binding energy, and (ii) significant increase of two-dimensional electron gas concentration. We also demonstrate that increasing sulfur content in $\mathrm{Mo}{({\mathrm{S}}_{y}{\mathrm{Se}}_{1\ensuremath{-}y})}_{2}$ alloys increases total PL intensity at high temperature. With the temperature growth from 7 to 295 K, the total PL in ${\mathrm{MoSe}}_{2}$ decreases about three orders of magnitude more than in $\mathrm{Mo}{({\mathrm{S}}_{0.5}{\mathrm{Se}}_{0.5})}_{2}$.

Surface-Enhanced Infrared Absorption by Optical Phonons in Nanocrystal Monolayers on Au Nanoantenna Arrays

We report on a study of surface-enhanced infrared absorption (SEIRA) by optical phonons in monolayers (MLs) of CdSe, CdS, and PbS nanocrystals (NCs) deposited on arrays of linear nanoantennas the optimized structural parameters of which allow coupling between the localized surface plasmon resonance (LSPR) and diffraction modes in the far-infrared spectral region. The Langmuir–Blodgett technique was used for homogeneous deposition of the NCs. The structural parameters of the arrays and the NC MLs were determined by scanning electron microscopy. According to the three-dimensional electrodynamic simulations of the electromagnetic field distribution around the antennas, the maximal SEIRA enhancement is realized for an array period of about 15 μm when the energy of a diffraction mode coincides with that of the LSPR mode. SEIRA experimental results are in perfect quantitative agreement with the simulation. The maximal SEIRA enhancement is observed for the nanoantenna length and transverse periodicity predicted ...

Hot-electron cooling by acoustic and optical phonons in monolayers ofMoS2and other transition-metal dichalcogenides

We study hot-electron cooling by acoustic and optical phonons in monolayer ${\mathrm{MoS}}_{2}$. The cooling power $P$ (${P}_{e}=P/n$) is investigated as a function of electron temperature ${T}_{e}$ (0--500 K) and carrier density $n\phantom{\rule{0.28em}{0ex}}({10}^{10}--{10}^{13} {\mathrm{cm}}^{\ensuremath{-}2}$) taking into account all relevant electron-phonon (el-ph) couplings. We find that the crossover from acoustic phonon dominated cooling at low ${T}_{e}$ to optical phonon dominated cooling at higher ${T}_{e}$ takes place at ${T}_{e}\ensuremath{\sim}50--75$ K. The unscreened deformation potential (DP) coupling to the TA phonon is shown to dominate $P$ due to acoustic phonon scattering over the entire temperature and density range considered. The cooling power due to screened DP coupling to the LA phonon and screened piezoelectric (PE) coupling to the TA and LA phonons is orders of magnitude lower. In the Bloch-Gr\"uneisen (BG) regime, $P\ensuremath{\sim}{T}_{e}^{4}\phantom{\rule{4pt}{0ex}}({T}_{e}^{6})$ is predicted for unscreened (screened) el-ph interaction and $P\ensuremath{\sim}{n}^{\ensuremath{-}1/2}\phantom{\rule{4pt}{0ex}}({P}_{e}\ensuremath{\sim}{n}^{\ensuremath{-}3/2})$ for both unscreened and screened el-ph interaction. The cooling power due to optical phonons is dominated by zero-order DP couplings and the Fr\"ohlich interaction, and is found to be significantly reduced by the hot-phonon effect when the phonon relaxation time due to phonon-phonon scattering is large compared to the relaxation time due to el-ph scattering. The ${T}_{e}$ and $n$ dependence of the hot-phonon distribution function is also studied. Our results for monolayer ${\mathrm{MoS}}_{2}$ are compared with those in conventional two-dimensional electron gases (2DEGs) as well as monolayer and bilayer graphene.

Ultrafast relaxation of hot optical phonons in monolayer and multilayer graphene on different substrates

Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor deposition (CVD) grown graphene transferred onto a glass substrate was investigated by transient absorption spectroscopy and imaging. Coupling to the substrate was found to play a critical role in charge carrier cooling. For both multilayer epitaxial graphene and monolayer CVD graphene, charge carriers transfer heat predominantly to intrinsic in-plane optical phonons of graphene. At high pump intensity, a significant number of optical phonons are accumulated, and the optical phonon lifetime presents a bottleneck for charge carrier cooling. This hot phonon effect did not occur in few-layer epitaxial graphene because of strong coupling to the substrate, which provided additional cooling channels. The limiting charge carrier lifetimes at high excitation densities were 1.8 ± 0.1 ps and 1.4 ± 0.1 ps for multilayer epitaxial graphene and monolayer CVD graphene, respectively. These values represent lower limits on the optical phonon lifetime for the graphene samples.

Electron-phonon heat transfer in monolayer and bilayer graphene

We calculate the heat transfer between electrons to acoustic and optical phonons in monolayer and bilayer graphene (MLG and BLG) within the quasiequilibrium approximation. For acoustic phonons, we show how the temperature-power laws of the electron-phonon heat current for BLG differ from those previously derived for MLG and note that the high-temperature (neutral-regime) power laws for MLG and BLG are also different, with a weaker dependence on the electronic temperature in the latter. In the general case we evaluate the heat current numerically. We suggest that a measurement of the heat current could be used for an experimental determination of the electron-acoustic-phonon coupling constants, which are not accurately known. However, in a typical experiment heat dissipation by electrons at very low temperatures is dominated by diffusion and we estimate the crossover temperature at which acoustic-phonon coupling takes over in a sample with Joule heating. At even higher temperatures optical phonons begin to dominate. We study some examples of potentially relevant types of optical modes, including, in particular, the intrinsic in-plane modes and additionally the remote surface phonons of a possible dielectric substrate.

Phonons in solid argon monolayers.

Inelastic neutron scattering studies of excitations in {sup 36}Ar solid monolayers show a large thermal broadening of the scattered intensity between {ital T}=4 and 45 K. We find this thermal broadening can be readily explained in terms of anharmonic interactions between the phonons in the monolayers. The phonon energies and lifetimes, calculated using the self-consistent phonon theory, are also very sensitive to the nearest-neighbor spacing {ital a}{sub NN} as well as temperature. For {ital a}{sub NN}{approx gt}3.98 A, the floating monolayer is predicted to be mechanically unstable to vibration.