Fast and controllable cooling at nanoscales requires a combination of highly efficient passive cooling and active cooling. Although passive cooling in graphene-based devices is quite effective due to graphene’s extraordinary heat conduction, active cooling has not been considered feasible due to graphene’s low thermoelectric power factor. Here, we show that the thermoelectric performance of graphene can be significantly improved by using hexagonal boron nitride (hBN) substrates instead of SiO₂. We find the room temperature efficiency of active cooling in the device, as gauged by the power factor times temperature, reaches values as high as 10.35 W⋅m⁻¹⋅K⁻¹, corresponding to more than doubling the highest reported room temperature bulk power factors, 5 W⋅m⁻¹⋅K⁻¹, in YbAl₃, and quadrupling the best 2D power factor, 2.5 W⋅m⁻¹⋅K⁻¹, in MoS₂. We further show that the Seebeck coefficient provides a direct measure of substrate-induced random potential fluctuations and that their significant reduction for hBN substrates enables fast gate-controlled switching of the Seebeck coefficient polarity for applications in integrated active cooling devices.

The best thermoelectric materials are believed to be heavily doped semiconductors. The presence of a band gap is assumed to be essential to achieve large thermoelectric power factor and figure of merit. In this work, we propose semi-metals with large asymmetry between conduction and valence bands as an alternative class of thermoelectric materials. To illustrate the idea, we study semi-metallic HgTe in details experimentally and theoretically. We employ ab initio calculations with hybrid exchange-correlation functional to accurately describe the electronic band structure in conjunction with the Boltzmann Transport theory to investigate the electronic transport properties. We calculate the lattice thermal conductivity using first principles calculations and evaluate the overall figure of merit. To validate our theoretical approach, we prepare semi-metallic HgTe samples and characterize their transport properties. Our first-principles calculations agree well with the experimental data. We show that intrinsic HgTe, a semimetal with large disparity in its electron and hole masses, has a high thermoelectric power factor that is comparable to the best known thermoelectric materials. Finally, we propose other possible materials with similar band structures as potential candidates for thermoelectric applications.

Heavily doped semiconductors are by far the most studied class of materials for thermoelectric applications in the past several decades. They have Seebeck coefficient values which are 2–3 orders of magnitude higher than metals, making them attractive for thermoelectric applications. Conventional wisdom suggests that the optimal band gap of a semiconductor to achieve the largest power factor should be between 6 and 10 $k_{B}T$, yet the highest power factor materials known up to now do not have a band gap. In this paper, we show that semimetals with very small or no band gap, but high asymmetry between their conduction and valence bands, can also have large Seebeck coefficient values on the order of 200 $\mu \text{V/K}$, which is near the optimum value for thermoelectric applications. We have studied the band structure of a class of 18 semimetals using first principles calculations and calculated their Seebeck coefficient using the linearized Boltzmann equation within the constant relaxation time approximation. We conclude that despite the absence of the band gap, semimetals with band asymmetry can have good thermoelectric performance. We analyze the metrics often used to describe thermoelectric properties of materials and show that the ratio of electron and hole mass of density of states is a key parameter resulting in high Seebeck coefficient values in semimetals. This work is therefore suggesting some high thermoelectric performance materials could be of semimetallic nature.

Active cooling systems use electrical work to bring a hot device (such as an integrated circuit) down to near ambient temperature by draining heat from the hot area of the device to a passive heat sink. Commercial thermoelectric modules are optimized for refrigeration, and are not ideal for active cooling. Refrigeration maintains a temperature that is below the ambient temperature in a device (such as a kitchen refrigerator) by pumping heat from the cold area of the device to a heat sink. The thermoelectric figure of merit zT is used traditionally to evaluate the performance of thermoelectric modules, including refrigeration modules. But it is not a good indicator of the performance of active cooling materials. Here, we describe an efficient, all-solid-state active cooler based on the Peltier effect in metals with high thermoelectric power factor due to electron correlation effects (${\mathrm{Ce}\mathrm{Pd}}_3$) or magnon drag ($\mathrm{Co}$) and high passive thermal conductivity. We show theoretically and experimentally that the effective thermal conductance under applied current can exceed the limits imposed by Fourier’s heat conduction law. The designed device measures an effective thermal conductance that is an order of magnitude larger than the passive thermal conductance at ΔT = 1 K with the dynamic response of 4 s.

In early 90s, Hicks and Dresselhaus proposed that low dimensional materials are advantages for thermoelectric applications due to the sharp features in their density-of-states, resulting in a high Seebeck coefficient and, potentially, in a high thermoelectric power factor. Two-dimensional (2D) materials are the latest class of low dimensional materials studied for thermoelectric applications. The experimental exfoliation of graphene, a single-layer of carbon atoms in 2004, triggered an avalanche of studies devoted to 2D materials in view of electronic, thermal, and optical applications. One can mix and match and stack 2D layers to form van der Waals hetero-structures. Such structures have extreme anisotropic transport properties. Both in-plane and cross-plane thermoelectric transport in these structures are of interest. In this short review article, we first review the progress achieved so far in the study of thermoelectric transport properties of graphene, the most widely studied 2D material, as a representative of interesting in-plane thermoelectric properties. Then, we turn our attention to the layered materials, in their cross-plane direction, highlighting their role as potential structures for solid-state thermionic power generators and coolers.

Using first-principles density functional theory (DFT) calculations combined with the Boltzmann transport theory, we investigate the effect of strain on the electronic and thermoelectric transport properties of the 1T-TiSe ₂ monolayer, a two-dimensional (2D) material, and compare it with the bulk phase within the PBE, LDA+ U and HSE exchange–correlation functionals.