Probing Anisotropic Thermal Conductivity of Transition Metal Dichalcogenides MX₂ (M = Mo, W and X = S, Se) using Time‐Domain Thermoreflectance

Abstract

Transition metal dichalcogenides (TMDs) are a group of layered 2D semiconductors that have shown many intriguing electrical and optical properties. However, the thermal transport properties in TMDs are not well understood due to the challenges in characterizing anisotropic thermal conductivity. Here, a variable-spot-size time-domain thermoreflectance approach is developed to simultaneously measure both the in-plane and the through-plane thermal conductivity of four kinds of layered TMDs (MoS , WS , MoSe , and WSe ) over a wide temperature range, 80-300 K. Interestingly, it is found that both the through-plane thermal conductivity and the Al/TMD interface conductance depend on the modulation frequency of the pump beam for all these four compounds. The frequency-dependent thermal properties are attributed to the nonequilibrium thermal resistance between the different groups of phonons in the substrate. A two-channel thermal model is used to analyze the nonequilibrium phonon transport and to derive the intrinsic thermal conductivity at the thermal equilibrium limit. The measurements of the thermal conductivities of bulk TMDs serve as an important benchmark for understanding the thermal conductivity of single- and few-layer TMDs.

Ultralow thermal conductance of the van der Waals interface between organic nanoribbons

“…where T T and B T denote the temperatures of top and bottom nanoribbons, respectively. These equations are similar to the two-temperature model [31][32][33][34] and the two-channel thermal transport model [35]. Eq.…”

Quasi-Ballistic Thermal Transport Across MoS2 Thin Films

“…MJm -3 K -1 , the average sound velocity of cross-plane acoustic modes 16 ~2400 ms -1 , and the crossplane bulk conductivity 3,17 ~2 to 5 Wm -1 K -1 , gives a MFP of around 1.5 to 4 nm, which corresponds to a thickness of 2 to 6 layers. A similar calculation for graphite gives a gray MFP estimate of around 3 nm, corresponding to 9 layers.…”

Anisotropic thermal conductivity of 4H and 6H silicon carbide measured using time-domain thermoreflectance

“…25 For example, could be first measured using a large spot radius at a high modulation frequency 0 (as shown in Figure 1b), then can be measured using a small spot radius at a low modulation frequency (as shown in Figure 1c). However, we need to be cautious that the measured could depend on 0 when different phonon modes are out of thermal equilibrium [26][27][28][29][30][31] and could be underestimated if 0 is smaller than the mean free paths of heat carrying phonons. 29, 32-34 We perform the following measurements to make sure that the measured thermal conductivity of SiC samples is intrinsic and is not affected by the choices of operation parameters such as the laser spot radius or the modulation frequency.…”

“…We extended the error propagation formula by Yang et al 37 in our previous work for the case when multiple modulation frequencies are used. 31 The error propagation formula is written as:…”

“…The uncertainties (2 ) of the control parameters are estimated as follows: 10% for the thermal conductivity of Al, 5% for the heat capacity of Al and the substrate, 5% for the Al thickness, and 4% for the laser spot size. 23,31 We summarize the calculated 4 ⋅ [ ] for the SI 4H-SiC, n-type 4H-SiC and SI 6H-SiC in Table 1, so that the uncertainties 2 can be directly calculated as square root of the diagonal elements. The SI and n-type 4H-SiC have higher and than those of SI 6H-SiC sample, which agree well with the first principles predictions that the thermal conductivity of H-SiC ( = 2, 4, 6)…”