First-principles prediction of the thermal conductivity of two configurations of difluorinated graphene monolayer.

Lattice thermal conductivity (κL) plays a crucial role in the thermal management of electronic devices. In this study, we systematically investigate the thermal transport properties of monolayer fluorinated graphene using a combination of machine learning-based interatomic potentials and the phonon Boltzmann transport equation. At a temperature of 300 K, we find that the κL values for chair-configured fluorinated graphene monolayers are 184.24 W m-1 K-1 in the zigzag direction and 205.57 W m-1 K-1 in the armchair direction. For the boat configuration, the κL values are 120.45 W m-1 K-1 and 64.26 W m-1 K-1 in the respective directions. The disparities in κL between these two configurations predominantly stem from differences in phonon relaxation times, which can be elucidated by examining the Grüneisen parameters representing the degree of anharmonicity. A more in-depth analysis of bond strengths, as assessed by the crystal orbital Hamiltonian population, reveals that the stronger in-plane CC bonds in chair-configured fluorinated graphene monolayers are the primary contributors to the observed variations in anharmonicity.

Anisotropic thermoelectric properties in hydrogenated nitrogen-doped porous graphene nanosheets.

By using density functional theory calculations combined with the nonequilibrium Green's function method and machine learning, we systematically studied the thermoelectric properties of four kinds of porous graphene nanosheets (PGNS) before and after nitrogen doping. The results show that the thermoelectric performance of porous graphene nanosheets along the armchair or zigzag chiral direction is improved due to the dramatically enhanced power factor caused by nitrogen doping. The calculated ZT values of nitrogen-doped porous graphene nanosheets are boosted by about one order of magnitude compared with those of undoped porous graphene nanosheets at room temperature. More importantly, an anisotropic thermoelectric transport is found in the nitrogen-doped porous graphene nanosheets. The results show that the ZT values of nitrogen-doped porous graphene nanosheets along the zigzag transport direction are nearly 11 times larger than those of them along the armchair transport direction. These results reveal that the thermoelectric properties of porous graphene nanosheets can be well regulated by nitrogen doping, and provide a good theoretical guidance for their application in thermoelectric devices.

Nano-phononic metamaterials enable an anomalous enhancement in the interfacial thermal conductance of the GaN/AlN heterojunction.

Improving the interfacial thermal conductance (ITC) is very important for heat dissipation in microelectronic and optoelectronic devices. In this work, taking GaN-AlN contact as an example, we demonstrated a new mechanism to enhance the interfacial thermal conductance using nano-phononic metamaterials. First, how a superlattice affects the ITC is investigated, and it is found that with decreasing superlattice periodic length, the ITC first decreases and then increases, because of the coherent phonon interference effect. However, although constructing a superlattice is effective for tuning the ITC, it cannot enhance the ITC. We suggest that the ITC can be enhanced by 9% through constructing an interfacial nano phononic metamaterial, which is contributed by the additional phonon transport channels for high-frequency phonons with a wide incidence-angle range. These results not only establish a deep understanding of the fundamental physics of the interfacial thermal conductance, but also provide a robust and scalable mechanism, which provides a degree of freedom for efficient thermal management.

Tuning interfacial thermal conductance of GaN/AlN heterostructure nanowires by constructing core/shell structure.

The ability to tune the interfacial thermal conductance of GaN/AlN heterojunction nanowires (NWs) with a core/shell structure is shown using molecular dynamics and non-equilibrium Green's functions method. In particular, an increase in the shell thickness leads to a significant improvement of interfacial thermal conductance of GaN/AlN core/shell NWs. At room temperature (300 K), the interfacial thermal conductance of NWs with specific core/shell ratio can reach 0.608 nW K-1, which is about twice that of GaN/AlN heterojunction NWs due to the weak phonon scattering and phonon localization. Moreover, changing the core/shell type enables one to vary interfacial thermal conductance relative to that of GaN/AlN heterojunction NWs. The results of the study provide an important guidance for solving the thermal management problems of GaN-based devices.

Recent progress of two-dimensional heterostructures for thermoelectric applications.

The rapid development of synthesis and fabrication techniques has opened up a research upsurge in two-dimensional (2D) 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 2D heterostructures is crucial for the development of micro-nano energy devices. In this review, the recent progress of 2D heterostructures for thermoelectric applications is summarized in detail. Firstly, we systematically introduce diverse theoretical simulations and experimental measurements of the thermoelectric properties of 2D heterostructures. Then, the thermoelectric applications and performance regulation of several common 2D materials, as well as in-plane heterostructures and van der Waals heterostructures, are also discussed. Finally, the challenges of improving the thermoelectric performance of 2D heterostructures materials are summarized, and related prospects are described.

Excellent Medium-Temperature Thermoelectric Performance of Monolayer BiOCl.

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.

Enhancement of thermoelectric performance in graphenylene nanoribbons by suppressing phonon thermal conductance: the role of phonon local resonance.

Based on the method of non-equilibrium Green's function, we investigate the thermal transport and thermoelectric properties of graphenylene nanoribbons (GRNRs) with different width and chirality. The results show that the thermoelectric (TE) performance of GRNRs significantly increases with decreasing ribbon width, which stems from the reduction of thermal conductance. In addition, by changing the ribbon width and chirality, the figure of merit (ZT) can be controllably manipulated and maximized up to 0.45 at room temperature. Moreover, it is found that theZTvalue of GRNRs with branched structure can reach 1.8 at 300 K and 3.4 at 800 K owing to the phonon local resonance. Our findings here are of great importance for thermoelectric applications of GRNRs.

KAgX (X = S, Se): High-Performance Layered Thermoelectric Materials for Medium-Temperature Applications.

Monolayer KAgX are a class of novel two-dimensional (2D) layered materials with efficient optical absorption and superior carrier mobility, signifying their potential application prospect in photovoltaic (PV) and thermoelectric (TE) fields. Motivated by the recent theoretical studies on the KAgX monolayer, we carried out systematic investigations on the TE performance of KAgS and KAgSe monolayers, employing density functional theory (DFT) and semiclassical Boltzmann transport equation (BTE). For both KAgSe and KAgS monolayers, large Grüneisen parameters, low group velocities, and short phonon scattering time greatly hinder their heat transport and result in an ultralow thermal conductivity, 0.26 and 0.33 W m-1 K-1 at 300 K, respectively. A twofold degeneracy appearing at the Γ point and the abrupt slope of the density of states (DOS) near the Fermi level give rise to high Seebeck coefficients of KAgX monolayers. Due to the ultralow thermal conductivity and excellent electronic transport performance, the ZT values as high as 4.65 (3.11) and 4.05 (2.63) at 500 (300) K in the n-type doping for KAgSe and KAgS monolayers are obtained. The exceptional performance of KAgX monolayers sheds light on their immense potential applications in the medium-temperature (around 300-500 K) thermoelectric devices and greatly stimulates further experimental synthesis and validation.

α-Ag2S: A Ductile Thermoelectric Material with High ZT.

Using first-principles calculation and Boltzmann electron/phonon transport theory, we present an accurate theoretical prediction of thermoelectric properties of the α-Ag2S crystal, a ductile inorganic semiconductor reported experimentally [Nat. Mater. 2018, 17, 421]. The semiconductor α-Ag2S has ultralow thermal conductivity associated with high anisotropy, which can be attributed to the complex crystalline structure and weak bonding. The optimal values of the Seebeck coefficient are 0.27 × 10-3 V/K for n-type and 0.21 × 10-3 V/K for p-type α-Ag2S, respectively, which are comparable to those of many promising thermoelectric materials. As a consequence, a maximum ZT value of 0.97/1.12 can be realized for p-type/n-type α-Ag2S at room temperature. More interestingly, the value of ZT can be further enhanced to 1.65 at room temperature by applying 5% compressive strain. Moreover, we find that the electronic thermal conductivity is a major factor limiting the ZT, which is several times the lattice thermal conductivity for n-type α-Ag2S. Our work demonstrates the great advantage of the α-Ag2S crystal as a ductile thermoelectric material and sparks new routes to improve its figure of merit.

Thermal transport properties in monolayer group-IV binary compounds.

New classes of two-dimensional (2D) materials beyond graphene are now attracting intense interest owing to their unique properties and functions. By combining first-principle calculation and the Boltzmann transport equation, we investigated the thermal transport properties of monolayer honeycomb structures of group-IV (C, Si, Ge, Sn) binary compounds. It is found that the thermal conductivity (κ) of these compounds span an enormously large range from 0.04 to 144.29 W m-1 K-1, demonstrating promising applications to nanoscale thermoelectrics and thermal management. The κ of low-buckled structures such as SiGe, SiSn and GeSn is lower than that of planar structures such as SiC, GeC and SnC, which can be ascribed to heavy atomic mass and broken in-plane reflection symmetry. Moreover, the κ of planar or low-buckled compounds with Sn atom is much lower than others, and the detailed origin for this phenomenon and contribution of different phonon modes to the κ are investigated. This work has fully studied the diversity of the thermal phenomenon and provides more options for application on thermal transport.

Excellent thermoelectric performance induced by interface effect in MoS2/MoSe2 van der Waals heterostructure.

Herein, thermoelectric properties of MoS2/MoSe2 lateral and van der Waals heterostructure are investigated by using density functional theory calculations and non-equilibrium Green's function method. Compared with pure MoS2, the thermoelectric performance of MoS2/MoSe2 lateral heterostructure is significantly improved due to the sharply decreased thermal conductance and slightly reduced power factor. Moreover, the thermoelectric performance can be further improved by constructing MoS2/MoSe2 van der Waals heterostructure. The room temperature ZT can reach 3.5, which is about 3 and 6 times greater than MoS2/MoSe2 lateral heterostructure and pure MoS2, respectively. This is because the strongly local electron and phonon states result in an ultralow thermal conductance in MoS2/MoSe2 van der Waals heterostructure. Furthermore, we also find that the thermoelectric performance of MoS2/MoSe2 van der Waals heterostructure is insensitive to contact areas due to the competing influence of PF and total thermal conductance. The current study presents an effective strategy to improve the thermoelectric performance of 2D heterostructures, which can be extended to a variety of materials for different applications.

Monolayer SnP3: an excellent p-type thermoelectric material.

Monolayer SnP3 is a novel two-dimensional (2D) semiconductor material with high carrier mobility and large optical absorption coefficient, implying its potential applications in the photovoltaic and thermoelectric (TE) fields. Herein, we report on the TE properties of monolayer SnP3 utilizing first principles density functional theory (DFT) together with semiclassical Boltzmann transport theory. Results indicate that it exhibits a low lattice thermal conductivity of ∼4.97 W m-1 K-1 at room temperature, mainly originating from its small average acoustic group velocity (∼1.18 km s-1), large Grüneisen parameters (∼7.09), strong dipole-dipole interactions, and strong phonon-phonon scattering. A large in-plane charge transfer is observed, which results in a non-ignorable bipolar effect on the lattice thermal conductivity. The exhibited mixed mode between in-plane and out-of-plane vibrations enhances the complexity of the phonon phase space, which enhances the possibility of phonon scattering processes and results in suppression of thermal conductivity. A highly twofold degeneracy appearing at the K point gives a high Seebeck coefficient. Our calculated figure of merit (ZT) for optimal p-type doping at 500 K can approach 3.46 along the armchair direction, which is better than the theoretical value of 1.94 reported in the well-known TE material SnSe. Our studies here shed light on monolayer SnP3 in use as a TE material and supply insights to further optimize the TE properties in similar systems.

Phonon transport in periodically and quasi-periodically modulated cylindrical nanowires.

Phonon transport in periodically modulated cylindrical nanowire (PMCN) and quasi-periodically modulated cylindrical nanowire (QPMCN) is comparatively studied. It is shown that the transmission coefficient and thermal conductance for PMCN is greater than the corresponding values for QPMCN. At low frequencies, a wide stop-frequency gap due to the destructive interference between the incoming and back waves can be clearly observed here. For PMCN, such stop-frequency gap seems to be insensitive to the change of N (the periodic number). For QPMCN, however, its breadth increases with the increase of N (the Fibonacci number). When N is increased, the thermal conductance for PMCN presents a distinct change from the decrease to the constant, while QPMCN has a tendency of monotonous decrease. A brief discussion on these results is made.

Effect of electrophilic substitution and destructive quantum interference on the thermoelectric performance in molecular devices.

Using density function theory combined with the non-equilibrium Green's function method, the thermoelectric properties of para-Xylene-based molecular devices are investigated. It is found that destructive quantum interference can be triggered in n-type of para-connected para-Xylene-based molecular device and can obviously enhance the thermoelectric performance of the devices. Moreover, bridge atom electrophilic substitution can significantly improve the thermoelectric properties of p-type monolayer molecular device. The ZT value of p-type monolayer molecular device with doped electrodes can be optimized to 2.2 at 300 K and 2.8 at 500 K, and n-type bilayer molecular device can achieve the value of 1.2 at 300 K and 2.0 at 500 K. These results offer the information to design the complete molecular thermoelectric device with p-type and n-type of components and to promote the thermoelectric properties of bilayer molecular junctions by employing destructive quantum interference effects.

Thermoelectric Properties of Hexagonal M2C3 (M = As, Sb, and Bi) Monolayers from First-Principles Calculations.

Hexagonal M2C3 compound is a new predicted functional material with desirable band gaps, a large optical absorption coefficient, and ultrahigh carrier mobility, implying its potential applications in photoelectricity and thermoelectric (TE) devices. Based on density-functional theory and Boltzmann transport equation, we systematically research the TE properties of M2C3. Results indicate that the Bi2C3 possesses low phonon group velocity (~2.07 km/s), low optical modes (~2.12 THz), large Grüneisen parameters (~4.46), and short phonon relaxation time. Based on these intrinsic properties, heat transport ability will be immensely restrained and therefore lead to a low thermal conductivity (~4.31 W/mK) for the Bi2C3 at 300 K. A twofold degeneracy is observed at conduction bands along Γ-M direction, which gives a high n-type electrical conductivity. Its low thermal conductivity and high Seebeck coefficient lead to an excellent TE response. The maximum thermoelectric figure of merit (ZT) of n-type can approach 1.41 for Bi2C3. This work shows a perspective for applications of TE and stimulate further experimental synthesis.

Ab initio study of the moisture stability of lead iodine perovskites.

The stability of hybrid lead iodine perovskite in a humid environment has been a major obstacle to developing long-term photovoltaic devices. However, understanding the detailed degradation mechanism of lead iodine perovskite in moisture is still challenging. Herein, using first-principles calculations, we show that embedded water molecules will facilitate the decomposition of lead iodine perovskite. Alloying FAPbI3 and CsPbI3 to form mixed-cation lead iodine perovskites not only can optimize the tolerance factor to obtain better phase stability, but also can improve the moisture stability of them. With the accumulation of water molecules in the perovskite lattice, the optical absorption spectra show a blue-shift and decreased intensity, and the moisture stabilities of lead iodine perovskites are further lowered. The iodine vacancy in lead iodine perovskites can facilitate the water molecule migration and thus is a disadvantage in improving the moisture stability of them, which should be minimized during perovskite growth. These findings provide new insight in understanding the poor moisture stability of lead iodine perovskites, which should be helpful for the future design and optimization of stable perovskite solar cells.

Excellent thermoelectric properties induced by different contact geometries in phenalenyl-based single-molecule devices.

We investigated the thermoelectric properties of phenalenyl-based molecular devices by using the non-equilibrium Green's function method combined with density function theory. The results show that the thermoelectric performance of molecular device can be significantly improved by different contact geometries. The ZT value of the device can reach 1.2 at room temperature, which is two orders of magnitude higher than that of graphene. Moreover, the change of the coupling between molecule and electrodes can also enhance the ZT value. The ZT value can be further optimized to 1.4 at 300 K and 5.9 at 100 K owing to the decrease of electronic thermal conductance and almost unchanged power factor.

Excellent Thermoelectric Properties in monolayer WSe2 Nanoribbons due to Ultralow Phonon Thermal Conductivity.

By using first-principles calculations combined with the nonequilibrium Green's function method and phonon Boltzmann transport equation, we systematically investigate the influence of chirality, temperature and size on the thermoelectric properties of monolayer WSe2 nanoribbons. The results show that the armchair WSe2 nanoribbons have much higher ZT values than zigzag WSe2 nanoribbons. The ZT values of armchair WSe2 nanoribbons can reach 1.4 at room temperature, which is about seven times greater than that of zigzag WSe2 nanoribbons. We also find that the ZT values of WSe2 nanoribbons increase first and then decrease with the increase of temperature, and reach a maximum value of 2.14 at temperature of 500 K. It is because the total thermal conductance reaches the minimum value at 500 K. Moreover, the impact of width on the thermoelectric properties in WSe2 nanoribbons is not obvious, the overall trend of ZT value decreases lightly with the increasing temperature. This trend of ZT value originates from the almost constant power factor and growing phonon thermal conductance.

Conjunction of standing wave and resonance in asymmetric nanowires: a mechanism for thermal rectification and remote energy accumulation.

As an important way to control and manage heat transport, thermal rectification has become an elementary issue in the field of phononics and plays a key role in the designing of thermal devices. Here we investigate systematically the standing wave and the accompanying resonance process in asymmetric nanowires to understand the standing wave itself and its great effect on thermal rectification. Results show that the standing wave is sensitive to both the structural and thermal properties of the material, and its great effect on enhancing the thermal rectification is realized not only by the energy-localization nature of the standing wave, but also by the resonance-caused large amplitude and high energy of the standing wave.

First-Principles Determination of Ultralow Thermal Conductivity of monolayer WSe2.

By using first-principles calculations combined with the phonon Boltzmann transport equation, we systematically investigate the phonon transport of monolayer WSe2. Compared with other 2D materials, the monolayer WSe2 is found to have an ultralow thermal conductivity due to the ultralow Debye frequency and heavy atom mass. The room temperature thermal conductivity for a typical sample size of 1 µm is 3.935 W/m K, which is one order of magnitude lower than that of MoS2. And the room temperature thermal conductivity can be further decreased by about 95% in 10 nm sized samples. Moreover, we also find the ZA phonons have the dominant contribution to the thermal conductivity, and the relative contribution is almost 80% at room temperature, which is remarkably higher than that for monolayer MoS2. This is because the ZA phonons have longer lifetime than that of LA and TA phonons in monolayer WSe2.