Nanoscale Thermal Transport in 2D Nanostructures from Cryogenic to Room Temperature

Abstract

2D materials with high in-plane thermal conductivity such as graphene, which is also highly electrically conductive, or hexagonal boron nitride (hBN), which is electrically insulating, have been proposed for heat management applications, [2] whereas MoS 2 has been used as an active channel on electronic devices due to its comparable bandgap to silicon. [3] 2D materials can also be excellent thermal insulators for cross-plane thermal transport, with WSe 2 known to possess one of the lowest thermal conductivity for a continuous solid-state material. [4] Furthermore, vertical, [5] lateral, [6] or composite [7] heterostructures of 2D materials are shown to be potential candidates for electronic applications. These unique properties open new possibilities for the development of highperformance thermoelectric and phase change memory structures.For thermoelectric applications, 2D materials are also highly attractive due to their high Seebeck coefficient values. For example, the Seebeck coefficient of graphene has been reported to be between 10 and 180 µV K −1 [8] while that of MoS 2 can be orders of magnitude higher (3 × 10 5 µV K −1 ). [9] Nevertheless, their implementation in such devices needs clever use of thermal anisotropy of the in-plane [10] and cross-plane [11] thermal conductivities. The efficiency of a thermoelectric device, which converts the waste heat to energy is determined by the dimensionless figure of merit ZT = S 2 σT/k, where S is the Seebeck coefficient, σ is the electrical conductivity, and k = k ph + k el is the thermal conductivity due to electrons (el) and phonons (ph). Therefore, a highly efficient thermoelectric device requires materials with high S and σ and low k in the direction of the electron flow.Recently, it has been proposed that stacking of 2D materials to create heterostructures is an efficient way of enhancing the efficiency of thermoelectric devices. [12] For graphene/MoS 2 stacks, high figure of merit values (up to 2.8) are predicted, due to the reduction of k and heat transport through the sharp edges of MoS 2 nanoribbons. [13] Additionally, the thermal conductivity of rippled graphene was predicted to notably drop compared to flat one [14] leading to more efficient thermoelectrics. It has also been predicted that reduced thermal conductivity can be achieved by engineering the thermal transport in graphene periodic phononic structures. [15] Furthermore, it has been reported that lateral confinement alone allows to change Nanoscale scanning thermal microscopy (SThM) transport measurements from cryogenic to room temperature on 2D structures with sub 30 nm resolution are reported. This novel cryogenic operation of SThM, extending the temperature range of the sample down to 150 K, yields a clear insight into the nanothermal properties of the 2D nanostructures and supports the model of ballistic transport contribution at the edge of the detached areas of exfoliated graphene which leads to a size-dependent thermal resistance of the detached material. The thermal resistance of graph...

Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

“…This implies that, even though the same trends are observed as with the NP-SThM method, this image analysis approach is perturbed by the interfacial thermal resistance, which more or less masks the actual values of the organic film thermal resistance. Previous works 27,28,49,50,54 also reported SThM tip-organic materials and organic-SiO 2 interfacial thermal resistances larger than 10 7 K W −1 (or >∼10 −8 m 2 K W −1 ), i.e., larger than R org of our C8-BTBT-C8 and BTBT films.…”

“…In order to determine the R org (t ) from this SThM voltage ratio, we need to estimate the various interfacial resistances (Kapitza resistance) 46 which cannot be directly measured here. Reported values for a large variety of interfaces 27,[47][48][49][50][51][52][53][54] range typically between 10 −9 and 10 −6 m 2 K W −1 . For simplicity, we consider the lower limit value for all interfaces.…”

Anomalous Low Thermal Conductivity of Atomically Thin InSe Probed by Scanning Thermal Microscopy

“…[ 38,42 ] Also, the measured SThM signal can be affected by the thermal resistance of the flake interface with other materials and/or supporting substrate, the thermal conductivity of the substrate, and the tip‐surface thermal resistance. [ 43–45 ] We examine these phenomena by thermal microscopy studies of different InSe‐based structures.…”

“…Previous studies of different 2D materials have demonstrated a size‐dependent phonon transport. [ 45,48–51 ] The value of κ decreases with decreasing the size of the sample as a result of an increase in phonon‐boundary scattering. For free‐standing graphene, κ decreases from 1700 to 250 W m −1 K −1 when the graphene domain size is reduced from 9 µm to 300 nm.…”

Theoretical Investigations of the BaRh2Ge4X6 (X = S, Se, Te) Compounds

“…Recently, 2D van der Waals materials have been shown to exhibit good thermoelectric properties owing to their specific structure that can enhance the electrical conductivity and lower the thermal one [21][22][23]. In a recent work, a series of new, layered compounds was discovered [24] that presents a variety of chemical bonding types.…”