We report a systematic investigation of interfacial thermal conductance (Gk) between few to tens-layered mechanical exfoliated molybdenum disulfide (MoS2) and crystalline silicon (c-Si). Based on Raman spectroscopy, we find Gk at room temperature increases with increased layer numbers of MoS2 from 0.974 MW m-2 K-1 to 68.6 MW m-2 K-1. The higher Gk of thicker samples reveals their better interface contact with the substrate, leading to accordingly improved interfacial energy coupling. Molecular dynamics (MD) simulations are conducted to interpret and compare with the experimental observations. MD simulations predict a thermal conductance in the range of 53~77 MW m-2 K-1 , which agrees well with the upper bound Gk measured in our work. The thickness dependence of measured Gk reflects the improved interface spacing for thicker MoS2 samples. This phenomenon is further confirmed by the Raman intensity enhancement study by the interface spacing and local optical interference calculations.

This article is protected by copyright. All rights reserved.Soft thermoelectric materials, including conjugated polymers and organic-inorganic hybrids, now demonstrate figures of merit approaching those of inorganic materials. These materials development breakthroughs enable the design of thermoelectric devices that exhibit appropriate efficiencies for commercial use, while simultaneously leveraging the unique processing and mechanical advantages of soft materials. Such technology opens the door to a suite of new thermoelectric applications, including power generation for biomedical implants and the Internet of Things, or wearable heating and cooling devices. In order to realize deployment of such technologies, there is a fundamental need for deeper understanding of the complex transport physics underlying thermoelectric transport in soft materials. This progress report discusses the current state-of-the-art in soft thermoelectrics materials and highlight outstanding challenges specific to organic and organic-inorganic hybrid systems.

The thermal conductivity of supported MoS2 is discovered to first decrease with thickness (<9.2 nm), then increase with thickness.

Quantitative understanding of 2D atomic layer interface thermal resistance (R) based on Raman characterization is significantly hindered by unknown sample-to-sample optical properties variation, interface-induced optical interference, off-normal laser irradiation, and large thermal-Raman calibration uncertainties. In this work, we develop a novel energy transport state resolved Raman (ET-Raman) to resolve these critical issues, and also consider the hot carrier diffusion, which is crucial but has been rarely considered during interface energy transport study. In ET-Raman, by constructing two steady heat conduction states with different laser spot sizes, we differentiate the effect of R and hot carrier diffusion coefficient (D). By constructing an extreme state of zero/negligible heat conduction using a picosecond laser, we differentiate the effect of R and material's specific heat. In the end, we precisely determine R and D without need of laser absorption and temperature rise of the 2D atomic layer. Seven MoS 2 samples (6.6−17.4 nm) on c-Si are characterized using ET-Raman. Their D is measured in the order of 1.0 cm 2 /s, increasing against the MoS 2 thickness. This is attributed to the weaker in-plane electron−phonon interaction in thicker samples, enhanced screening of long-range disorder, and improved charge impurities mitigation. R is determined as 1.22−1.87 × 10 −7 K•m 2 /W, decreasing with the MoS 2 thickness. This is explained by the interface spacing variation due to thermal expansion mismatch between MoS 2 and Si, and increased stiffness of thicker MoS 2 . The local interface spacing is uncovered by comparing the theoretical Raman intensity and experimental data, and is correlated with the observed R variation.

This work reports on the discovery of a high thermal conductivity (κ) switch-on phenomenon in high purity graphene paper (GP) when its temperature is reduced from room temperature down to 10 K. The κ after switch-on (1732 to 3013 W m −1 K −1 ) is 4-8 times that before switch-on. The triggering temperature is 245-260 K. The switch-on behavior is attributed to the thermal expansion mismatch between pure graphene flakes and impurity-embedded flakes. This is confirmed by the switch behavior of the temperature coefficient of resistance. Before switch-on, the interactions between pure graphene flakes and surrounding impurity-embedded flakes efficiently suppress phonon transport in GP. After switch-on, the structure separation frees the pure graphene flakes from the impurity-embedded neighbors, leading to a several-fold κ increase. The measured κ before and after switch-on is consistent with the literature reported κ values of supported and suspended graphene. By conducting comparison studies with pyrolytic graphite, graphene oxide paper and partly reduced graphene paper, the whole physical picture is illustrated clearly. The thermal expansion induced switch-on is feasible only for high purity GP materials. This finding points out a novel way to switch on/off the thermal conductivity of graphene paper based on substrate-phonon scattering.

Nanosecond ET-Raman measures the thermal conductivity of 2D materials without temperature calibration and laser absorption evaluation and features the highest accuracy.

We report a novel approach for non-contact simultaneous determination of the hot carrier diffusion coefficient (D) and interface thermal resistance (R) of sub-10 nm virgin mechanically exfoliated MoS nanosheets on c-Si. The effect of hot carrier diffusion in heat conduction by photon excitation, diffusion, and recombination is identified by varying the heating spot size from 0.294 μm to 1.14 μm (radius) and probing the local temperature rise using Raman spectroscopy. R is determined as 4.46-7.66 × 10 K m W, indicating excellent contact between MoS and c-Si. D is determined to be 1.18, 1.07, 1.20 and 1.62 cm s for 3.6 nm, 5.4 nm, 8.4 nm, and 9.0 nm thick MoS samples, showing little dependence on the thickness. The hot carrier diffusion length (L) can be determined without knowledge of the hot carrier's life-time. The four samples L is determined as 0.344 (3.6 nm), 0.327 (5.4 nm), 0.346 (8.4 nm), and 0.402 μm (9.0 nm). Unlike previous methods that are implemented by making electrical contact and applying an electric field for D measurement, our technique has the advantage of being truly non-contact and non-invasive, and is able to characterize the electron diffusion behavior of virgin 2D materials. Also it points out that hot carrier diffusion needs to be taken into serious consideration in Raman-based thermal property characterization of 2D materials, especially under very tightly focused laser heating whose spot size is comparable to the hot carrier diffusion length.

The facile pyrolysis of a bipyridyl metal-organic framework, MOF-253, produces N-doped porous carbons (Cz-MOF-253), which exhibit excellent catalytic activity in the Knoevenagel condensation reaction and outperform other nitrogen-containing MOF-derived carbons. More importantly, by virtue of their high Lewis basicity and porous nature, Cz-MOF-253-supported Pd nanoparticles (Pd/Cz-MOF-253-800) show excellent performance in a one-pot sequential Knoevenagel condensation-hydrogenation reaction.