Mapping phonon modes from reduced-dimensional to bulk systems

Kim, Sujeong; Parrish, Kevin D.; McGaughey, Alan J. H.

doi:10.1063/1.5119141

Cited by 3 publications

(3 citation statements)

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“…We now analyze the phonon mode properties to identify signatures of confinement, with a focus on the 5 to the film's free surfaces, which remove the degeneracy of the transverse dispersion branches along high-symmetry directions. 40 The peak in the DOS at a frequency of 4.9 Trad/s in the two unit cell film corresponds to a flattened phonon branch, as shown in Fig. S4.…”

Section: Signatures Of Confinementmentioning

confidence: 89%

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Phonon confinement and transport in ultrathin films

Parrish

Kim

et al. 2020

Phys. Rev. B

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1 arXiv:1907.00969v2 [cond-mat.mes-hall] Abstract Thermal transport by phonons in films with thicknesses of less than 10 nm is investigated in a soft system (Lennard-Jones argon) and a stiff system (Tersoff silicon) using two-dimensional lattice dynamics calculations and the Boltzmann transport equation. This approach uses a unit cell that spans the film thickness, which removes approximations related to the finite cross-plane dimension required in typical three-dimensional-based approaches. Molecular dynamics simulations, which make no assumptions about the nature of the thermal transport, are performed to obtain finite-temperature structures for the lattice dynamics calculations and to predict thermal conductivity benchmarks. Thermal conductivity decreases with decreasing film thickness for both the two-dimensional lattice dynamics calculations and the MD simulations, until the thickness reaches four unit cells (2.1 nm) for argon and three unit cells (1.6 nm) for silicon.With a further decrease in film thickness, thermal conductivity plateaus in argon while it increases in silicon. This unexpected behavior, which we identify as a signature of phonon confinement, is a result of an increased contribution from low-frequency phonons, whose density of states increases as the film thickness decreases. Phonon mode-level analysis suggests that confinement effects emerge below thicknesses of ten unit cells (5.3 nm) for argon and six unit cells (3.2 nm) for silicon. These transition points both correspond to approximately twenty atomic layers. Thermal conductivity predictions based on the bulk (i.e., three-dimensional) phonon properties combined with a boundary scattering model do not capture the low thickness behavior. To match the two-dimensional lattice dynamics and molecular dynamics predictions for larger thicknesses, the three-dimensional lattice dynamics calculations require a finite specularity parameter that in some cases approaches unity. These findings point to the challenges associated with interpreting experimental thermal conductivity measurements of ultrathin silicon films, where surface roughness and a native oxide layer impact phonon transport.

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Section: Signatures Of Confinementmentioning

confidence: 89%

“…Kim et al 40 recently proposed an algorithm to map between 2D and 3D phonon modes, which they applied to Lennard-Jones (LJ) argon and multi-layer graphene films. A near one-to-one correspondence was discovered for thicknesses greater than 5 nm for LJ argon and for all thicknesses of graphene.…”

Section: Introductionmentioning

confidence: 99%

Phonon confinement and transport in ultrathin films

Parrish

Kim

et al. 2020

Phys. Rev. B

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“…This work focuses on understanding how phonon transport is modified during the transition from 3D to 2D (or vice versa) by taking a van der Waals material, in this case graphite, and gradually pulling the layers apart until they are effectively isolated graphene. While comparisons between mono-/few-layer graphene and graphite already exist [17][18][19][20][21][22][23][24], this approach provides a different perspective on how the phonon and scattering characteristics continously evolve from bulk to monolayer. We selected graphite/graphene as a case study, since it is one of the most common and extensively explored layered materials , and has shown interesting properties such as hydrodynamic phonon transport [46][47][48][49][50] and second-sound [51].…”

Section: Introductionmentioning

confidence: 99%

Impact of dimensional crossover on phonon transport in van der Waals materials: a case study of graphite and graphene

Strongman,

Maassen

2020

Preprint

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Using first-principles modeling, we investigate how phonon transport evolves in layered/van der Waals materials when going from 3D to 2D, or vice versa, by gradually pulling apart the atomic layers in graphite to form graphene. Focus is placed on identifying the features impacting thermal conductivity that are likely shared with other layered materials. The thermal conductivity κ of graphite is found to be lower than that of graphene mainly due to changes in the phonon dispersion driven by van der Waals coupling. Specifically, as the atomic layers are brought closer together, the acoustic flexural phonons in graphene form low-energy optical flexural phonons in graphite that possess lower in-plane velocities, density-of-states and phonon occupation, thus reducing κ. Similar dispersion changes, and impact on thermal conductivity, can be expected in other van der Waals materials when transitioning from 2D to 3D. Our findings also indicate that the selection rules in graphene, which reduce phonon-phonon scattering and contribute to its large κ, effectively hold as the atomic layers are brought together to form graphite. While the selection rules do not strictly apply to graphite, in practice similar scattering behavior is displayed due in part to the weak inter-layer coupling. This suggests that van der Waals materials, in bulk 3D form, may have lower phonon-phonon scattering rates than other non-layered bulk materials.

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