Using non-equilibrium molecular dynamics simulations, we investigate thermal rectification (TR) in pristine/branched graphene nanoribbon (GNR) junctions. The results indicate that the TR ratio of such junctions can reach 470% under small temperature bias, which has distinct superiority over asymmetric GNR and many other junctions. Moreover, the TR ratio decreases rapidly as the applied temperature bias increases. It seems to be against common sense that the TR ratio generally increases with temperature bias. Phonon spectra analyses reveal that the observed phenomena stem from the local resonance of longitudinal phonons in branched GNR region under negative temperature bias. Furthermore, the influence of ambient temperature, system length, branch number, and defect density is studied to obtain the optimum conditions for TR. This work extends local resonance mechanism to GNR for thermal signal manipulation.

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The rapid development of synthesis and fabrication techniques has opened up a research upsurge in two-dimensional material heterostructures, which have received extensive attention due to their superior physical and chemical properties. Currently, thermoelectric energy conversion is an effective means to deal with the energy crisis and increasingly serious environmental pollution. Therefore, an in-depth understanding of thermoelectric transport properties in two-dimensional heterostructures is crucial for the development of micro-nano energy devices. In this review, the recent progress of two-dimensional heterostructures for thermoelectric applications is summarized in detail. Firstly, we systematically introduce diverse theoretical simulations and experimental measurements of the thermoelectric properties of two-dimensional heterostructures. Then, the thermoelectric applications and performance regulation of several common two-dimensional materials, as well as in-plane heterostructures and van der Waals heterostructures, are also discussed. Finally, the challenges of improving the thermoelectric performance of two-dimensional heterostructures materials are summarized, and related prospects are described.

Enhanced thermoelectric performance is restricted greatly by the interaction of various transport parameters, and this bottleneck urgently requires a solution. In this paper, first-principles calculations and Boltzmann transport theory are used to study the thermoelectric performance of two-dimensional [Formula: see text] [Formula: see text] monolayers, and it is found that the thermoelectric performance can be enhanced significantly by applying a biaxial tensile strain. The room-temperature ZT values of the p-type [Formula: see text], and [Formula: see text] in zigzag (armchair) directions are boosted as high as 1.97 (1.35), 2.26 (1.31), and 2.45 (1.59), respectively. The results show that it is mainly attributed to the significantly reduced phonon thermal conductivity. Moreover, the sharply reduced phonon thermal conductivity is mainly due to the enhancement of the phonon scattering rate caused by strong phonon anharmonicity. In addition, the excellent ZT value of the p-type [Formula: see text] [Formula: see text] monolayer exhibits their potential application in the thermoelectric field, and the external strain has a good prospect in enhancing the thermoelectric performance.

Recently, a ternary-layered material BiOCl has elicited intense interest in photocatalysis, environmental remediation, and ultraviolet light detection because of its unique band gap of around 3.6 eV, low toxicity, and earth abundance. In particular, Gibson et al. reported a measurement of the in-plane thermal conductivity of BiOCl experimentally using a four-point-probe method [Science, 373, 1017–1022 (2021)], which is only 1.25 W/m K at 300 K. Motivated by the work, we studied the thermoelectric property of monolayer BiOCl using first-principles calculations combined with the Boltzmann transport equation. The calculated phonon thermal conductivity of monolayer BiOCl is 3 W/m K at 300 K, which is far below that of other promising 2D thermoelectric materials like graphyne and MoS2. A comprehensive analysis of phonon modes is conducted to reveal the low thermal conductivity. Moreover, the maximal ZT value is as high as 1.8 at 300 K and 5.7 at 800 K for the p-type doping with the 2 × 1015 cm–2 concentration. More importantly, we found that the thermoelectric efficiency of such 2D materials is significantly enhanced to 8 at 800 K by applying 1.5% tensile strain, which clearly outperforms that of the reported 2D thermoelectric material SnSe. The results shed light on the promising application in medium-temperature (600–900 K) thermoelectric devices.