Chenhan Liu

Thermal Transport in Quasi-1D van der Waals Crystal Ta2Pd3Se8 Nanowires: Size and Length Dependence

Van der Waals (vdW) crystals with covalently bonded building blocks assembled together through vdW interactions have attracted tremendous attention recently because of their interesting properties and promising applications. Compared to the explosive research on two-dimensional (2D) vdW materials, quasi-one-dimensional (quasi-1D) vdW crystals have received considerably less attention, while they also present rich physics and engineering implications. Here we report on the thermal conductivity of exfoliated quasi-1D Ta2Pd3Se8 vdW nanowires. Interestingly, even though the interatomic interactions along each molecular chain are much stronger than the interchain vdW interactions, the measured thermal conductivity still demonstrates a clear dependence on the cross-sectional size up to >110 nm. The results also reveal that partial ballistic phonon transport can persist over 13 μm at room temperature along the molecular chain direction, the longest experimentally observed ballistic transport distance with observable effects on thermal conductivity so far. First-principles calculations suggest that the ultralong ballistic phonon transport arises from the highly focused longitudinal phonons propagating along the molecular chains. These data help to understand phonon transport through quasi-1D vdW crystals, facilitating various applications of this class of materials.

Influence of coherent optical phonon on ultrafast energy relaxation

Ultrafast energy relaxation process in Bi2Te3 thin films is studied using a collinear two color pump-probe technique. The coherent optical phonon is enhanced and destroyed by changing the separation times of double pump pulses. The non-oscillatory component of the reflectivity trace after the second pump pulse shows a distinct difference with and without the presence of coherent optical phonons, thus providing a direct evidence of the effect of optical phonon on the hot carrier relaxation process. The deduced characteristic times are systematically smaller when coherent optical phonons are involved in the energy transfer process. Comparatively, the conventional relaxation process is relatively slow, which is explained by the screening effect of the incoherent optical phonon. This work suggests that the energy relaxation can be manipulated through the excitation of coherent optical phonons.

Axial tensile strain effects on the contact thermal conductance between cross contacted single-walled carbon nanotubes

The axial strain effects on the contact thermal conductance between two cross contacted single walled carbon nanotubes (SWCNTs) are assessed using nonequilibrium molecular dynamics simulation. The results show that the contact thermal conductance can be decreased by ∼44% as the axial strain increases from 0 to 10%. The calculated vibrational density of state reveals that the enhanced phonon scattering resulting from the blue shift of the low frequency phonon is the main factor leading to the reduction of the contact thermal conductance. We also studied the effect of the defects caused by hydrogenation and vacancy in SWCNTs on the contact thermal conductance and found that this effect can be neglected.

Cross-plane phonon transport properties of molybdenum disulphide

The cross-plane thermal conductivity of a molybdenum disulphide (MoS2) film is calculated from the nonequilibrium molecular dynamics simulation. The results show that, unlike graphite which has a slow convergent speed, the thermal conductivity of MoS2 tends to a convergent value when the film thickness is beyond about 40 nm. We also construct the cross-plane thermal conductivity of bulk MoS2 as an accumulation function of the phonon mean free path (MFP). It is found that phonons with MFPs below 40 nm contribute ~90% of the MoS2 cross-plane thermal conductivity at room temperature. This critical size of the phonon MFP is about two orders of magnitude smaller than that of graphite. Further calculations show that the shorter cross-plane phonon MFPs in bulk MoS2 may result from the lower phonon cut-off frequency and the mismatch of phonon density of state between Mo and S due to the mass difference. The phonon transport properties obtained would be helpful in the design and optimization of MoS2-based devices.

The contact area dependent interfacial thermal conductance

The effects of the contact area on the interfacial thermal conductance σ are investigated using the atomic Green’s function method. Different from the prediction of the heat diffusion transport model, we obtain an interesting result that the interfacial thermal conductance per unit area Λ is positively dependent on the contact area as the area varies from a few atoms to several square nanometers. Through calculating the phonon transmission function, it is uncovered that the phonon transmission per unit area increases with the increased contact area. This is attributed to that each atom has more neighboring atoms in the counterpart of the interface with the increased contact area, which provides more channels for phonon transport.

Interfacial Thermal Conductance Between Carbon Nanotubes From Nonequilibrium Green’s Function Method

In this paper, the interfacial thermal conductance between two single-wall carbon nanotubes (SWCNTs) is evaluated using the nonequilibrium Green’s function (NEGF) method. The calculation results show that, for offset parallel contact type, interfacial thermal conductance increases almost linearly with the overlap length. This is because the coupling atom number in overlap region is the main contributor to heat flow through interface. With the same overlap length, interfacial thermal conductance of the nested contact type is much higher than that of the offset parallel contact type. By comparing the phonon transmission function between the two contact types, it is found that the nested contact type has much larger transmission function than the offset parallel contact type due to more atoms involving in the interfacial coupling in the overlap region. By adjusting the chirality of SWCNTs in the offset parallel contact type, it is found that the difference of phonon spectrum can reduce interfacial thermal transfer. We also find the transmission function profiles with only different overlap length are quite similar, that is, changing in the overlap length will not change the phonon transmission probability at the interface. Moreover, acoustic phonon is the main contributor to the interfacial thermal conductance and the radical breathing mode is the vital mode of coupling modes for CNT-CNT system. The calculated results in this paper indicate that increasing the coupling atom number between CNTs would increase the heat energy transfer in CNT-based composites.Copyright

First Principles Study of Thermal Conductance Across Cu/Graphene/Cu Nanocomposition and the Effect of Hydrogenation

In this work, the interfacial thermal conductance across Cu/graphene/Cu interfaces is investigated using the density functional theory (DFT) and the nonequilibrium Green’s function (NEGF) method. In order to study how hydrogenation of graphene affects thermal transport behaviors at the interfaces of Cu/graphene/Cu, we also analyze the interfacial thermal conductance across Cu/hydrogenated-graphene/Cu (Cu/H-graphene/Cu) with both double-sided and single-sided hydrogenated graphene. Our results show that, the interfacial thermal conductance across Cu/H-graphene/Cu interfaces is almost twice of the value across Cu/graphene/Cu interfaces. For Cu/H-graphene/Cu with double-sided hydrogenated graphene (Cu/DH-graphene/Cu), the hydrogen atoms between graphene and Cu layers provide additional thermal transport channels. While for Cu/H-graphene/Cu with single-sided hydrogenated graphene (Cu/SH-graphene/Cu), the hydrogen atoms not only provide additional thermal transport channels at the hydrogenated side of graphene, but also reduce the equilibrium separation between graphene and Cu layers at the non-hydrogenated side of graphene due to the transfer of massive electrons, which enhances the interface coupling between graphene and Cu layers. The phonon transmission shows that both double-sided and single-sided hydrogenation of graphene can increase the heat transport across the interface. Our calculation indicates that the interfacial thermal conductance of Cu/graphene/Cu nanocomposition can be improved by hydrogenation.Copyright

The Cross-Plane Thermal Conductance of Multi-Layer Graphene Bundles

In this paper, the cross-plane thermal conductance σ of multi-layer graphene nanobundles (MLGNBs) is investigated using the non-equilibrium Green’s function method. For the normal MLGNBs, the σ has a positive dependence on the lateral area S due to more atoms involved in the heat transport in the larger S. However, the thermal conductance per unit area Λ is negative dependent on the S since high-frequency phonons contribute less to Λ with low transmission function and small number while the increased phonon branches are mainly located in the high-frequency range. Interestingly, as the S is larger than several square nanometers, the Λ converges to the macroscopic value, independently on the S. Then the staggered MLGNBs is investigated, the results show that increasing both staggering distance between neighboring graphene layers with each other and the graphene layer number in the central device can modulate the σ in a large scope due to the boundary scattering. Finally, in the MLGNBs junction, we found the variation of heat flux direction has an important effect on the σ while the layer number in the central device has weak effect on the cross-plane thermal transport. Our results help understand the cross-plane thermal transport of MLGNBs and provide a model to investigate the thermal property of layered material nanobundles.Copyright