Monolayer SnP3: an excellent p-type thermoelectric material

Pd-Adsorbed SiN3 Monolayer as a Promising Gas Scavenger for SF6 Partial Discharge Decomposition Components: Insights from the First-Principles Study

Gas-insulated switchgear (GIS) equipment must be protected by detecting and eliminating the toxic SF6 partial discharge decomposition components. This study employs first-principles calculations to thoroughly investigate the interaction between a Pd-adsorbed SiN3 monolayer (Pd-SiN3) and four typical SF6 decomposition gases (H2S, SO2, SOF2, and SO2F2). The study also investigates the associated geometric, electrical, and optical characteristics along with the sensing sensitivity and desorption efficiency. The ab initio molecular dynamics (AIMD) simulations demonstrated the favorable stability of the Pd-SiN3 monolayer. Furthermore, the Pd-SiN3 monolayer exhibited strong chemisorption behavior toward H2S, SO2, SOF2, and SO2F2 gases because of the higher adsorption energies of -2.717, -2.917, -2.457, and -2.025 eV, respectively. Furthermore, significant changes occur in the electronic and optical characteristics of the Pd-SiN3 monolayer following the adsorption of these gases, resulting in remarkable sensitivity of the Pd-SiN3 monolayer in relation to electrical conductivity and optical absorption. Meanwhile, all of these gas adsorption systems exhibited extremely long recovery times. The aforementioned theoretical findings suggest that the Pd-SiN3 monolayer has the potential to be an effective gas scavenger for the storage or removal of the SF6 decomposition components. Additionally, it might function as a reliable one-time sensor for detecting these gases. The results potentially provide valuable theoretical guidance for maintaining the normal operation of the SF6 insulation devices.

Stacking-induced phonon transport engineering of siligene

Tunable phonon transport properties of two-dimensional materials are desirable for effective heat management in various application scenarios. Here, we demonstrate by first-principles calculations and Boltzmann transport theory that the lattice thermal conductivity of siligene could be efficiently engineered by forming various stacking configurations. Unlike few-layer graphene, the stacked siligenes are found to be covalently bonded along the out-of-plane direction, which leads to unique dependence of the thermal conductivity on both the stacking order and layer number. Due to the restricted flexural phonon scattering induced by the horizontal reflection symmetry, the AA stacking configuration of bilayer siligene exhibits obviously higher thermal conductivity compared with the AB stacking. In addition, we observe increasing thermal conductivity with the layer number, as evidenced by the reduced phonon scattering phase space and Grüneisen parameter. Interestingly, the Fuchs-Sondheimer model works well for the thickness-dependent thermal conductivity of stacked siligenes.

Experimental Characterization of Defect-Induced Phonon Lifetime Shortening

We present the first direct experimental measurement of defect-induced lifetime shortening of acoustic surface phonons. Defects are found to contribute a temperature-independent component to the linewidths of Rayleigh wave phonons on a Ni(111) surface. We also characterized the increase in phonon scattering with both surface defect density and phonon wave vector. A quantitative estimate of the scattering rate between phonon modes and surface line defects is extracted from the experimental data for the first time.

First principles study of strain effects on prospective 2D photocatalysts Sn2Se2X4 (X = P, As) with ultra-high charge carrier mobility

Ab initio calculations were employed to investigate the properties of Sn2Se2P4 and Sn2Se2As4, which are new semiconductors formed based on the 2D SnP3 structure. A comprehensive analysis was conducted to examine the structural characteristics and stability of both compounds. It was observed that both Sn2Se2P4 and Sn2Se2As4 exhibit notable toughness and ductility, characterized by a Poisson's ratio ranging from 0.16 to 0.20 and a Young's modulus ranging from 42.12 to 49.84 N m-1. The investigation focused on the examination of the electronic characteristics of the two compounds, as well as their correlation with optical properties, charge transport, and potential as photocatalysts. Being ductile semiconductors, the effects of strains on the properties of Sn2Se2P4 and Sn2Se2As4 were also investigated. The charge carrier mobility in the y-direction ranges from 103 to 104 cm2 V-1 s-1. Moreover, the electron-hole separation is expected to be very high as the difference in the mobilities of holes and electrons is really large. Moreover, it is worth noting that both Sn2Se2P4 and Sn2Se2As4 exhibit a significantly high absorption rate of 106 cm-1 in the visible region. The observed features of Sn2Se2P4 and Sn2Se2As4 indicate their potential as effective photocatalysts for the process of water splitting through the utilization of solar energy.

DFT Study of Zn-Modified SnP3: A H2S Gas Sensor with Superior Sensitivity, Selectivity, and Fast Recovery Time

The adsorption properties of Cu, Ag, Zn, and Cd-modified SnP3 monolayers for H2S have been studied using density functional theory (DFT). Based on phonon spectrum calculations, a structurally stable intrinsic SnP3 monolayer was obtained, based on which four metal-modified SnP3 monolayers were constructed, and the band gaps of the modified SnP3 monolayers were significantly reduced. The adsorption capacity of Cu, Zn-modified SnP3 was better than that of Ag, Cd-modified SnP3. The adsorption energies of Cu-modified SnP3 and Zn-modified SnP3 for H2S were -0.749 eV and -0.639 eV, respectively. In addition, Cu-modified SnP3 exhibited chemisorption for H2S, while Zn-modified SnP3 exhibited strong physisorption, indicating that it can be used as a sensor substrate. Co-adsorption studies showed that ambient gases such as N2, O2, and H2O had little effect on H2S. The band gap change rate of Zn-modified SnP3 after adsorption of H2S was as high as -28.52%. Recovery time studies based on Zn-modified SnP3 showed that the desorption time of H2S was 0.064 s at 298 K. Therefore, Zn-modified SnP3 can be used as a promising sensor substrate for H2S due to its good selectivity, sensitivity, and fast recovery time.

Layer-dependent excellent thermoelectric materials: from monolayer to trilayer tellurium based on DFT calculation

Monoelemental two-dimensional (2D) materials, which are superior to binary and ternary 2D materials, currently attract remarkable interest due to their fascinating properties. Though the thermal and thermoelectric (TE) transport properties of tellurium have been studied in recent years, there is little research about the thermal and TE properties of multilayer tellurium with interlayer interaction force. Herein, the layer modulation of the phonon transport and TE performance of monolayer, bilayer, and trilayer tellurium is investigated by first-principles calcuations. First, it was found that thermal conductivity as a function of layer numbers possesses a robust, unusually non-monotonic behavior. Moreover, the anisotropy of the thermal transport properties of tellurium is weakened with the increase in the number of layers. By phonon-level systematic analysis, we found that the variation of phonon transport under the layer of increment was determined by increasing the phonon velocity in specific phonon modes. Then, the TE transport properties showed that the maximum figure of merit (ZT) reaches 6.3 (p-type) along the armchair direction at 700 K for the monolayer and 6.6 (p-type) along the zigzag direction at 700 K for the bilayer, suggesting that the TE properties of the monolayer are highly anisotropic. This study reveals that monolayer and bilayer tellurium have tremendous opportunities as candidates in TE applications. Moreover, further increasing the layer number to 3 hinders the improvement of TE performance for 2D tellurium.

Intrinsic and strain dependent ultralow thermal conductivity in novel AuX (X = Cu, Ag) monolayers for outstanding thermoelectric applications

A large power factor and ultralow lattice thermal conductivity in 2D-monolayers of AuX (X = Cu and Ag) are achieved via first principles calculations. Low phonon frequency, small Debye temperature and high Gruneisen parameter limit the intrinsic thermal conductivity of both the studied materials. An ultra-low lattice thermal conductivity of 0.13 (0.30) W m-1 K-1 and 0.66 (1.59) W m-1 K-1 is obtained for unstrained AuCu and AuAg monolayers, respectively, at 700 (300) K, which further reduces to 0.04 (0.09) and 0.26 (0.63) W m-1 K-1 at 6% biaxial tensile strain. Such values of thermal conductivity are lower than the critical thermal conductivity for the state-of-art thermoelectric materials (kl < 2 W m-1 K-1). The peak values of ZT for unstrained monolayers are 2.20 and 1.40, which enhances to 3.61 and 2.91 at 6% strain for AuCu and AuAg monolayers, respectively. Interestingly pudding-mold band textures are found to be responsible for this unusual thermoelectric behaviour. The stability concerns (chemical/dynamic/mechanical) of these monolayers are ensured to stimulate experimental determinations for novel synthesis and possible applications.

Boosting thermoelectric performance of HfSe2 monolayer by selectivity chemical adsorption

In this work, a strategy to boosting thermoelectric (TE) performance of 2D materials is explored. We find that, appropriate chemical adsorption of atoms can effectively increase the TE performance of HfSe₂ monolayer. Our results show that the adsorption of Ni atom on HfSe₂ monolayer (Ni-HfSe₂) can improve the optimal power factor PF and ZT at 300 K, increased by more than ∼67% and ∼340%, respectively. The PF and ZT of Ni-HfSe₂ at 300 K can reach 85.06 mW m^-1 K^-2 and 3.09, respectively. The detailed study reveal that the adsorption of Ni atom can induce additional conductional channels of electrons, enhance the coupling of acoustic-optical phonons and the phonon anharmonicity, resulting in an obvious increment of electrical conductivity (increased by more than ∼89%) in n-type doped system and an ultralow phonon thermal conductivity (1.17 W/mK at 300 K). The high electrical conductivity and ultralow phonon thermal conductivity results in the significant increments of PF and ZT. Our study also shows that, Ni-HfSe₂ is a thermal, dynamic and mechanical stable structure, which can be employed in TE application. Our research indicates that selectivity chemical adsorption is a promising way to increase TE performance of 2D materials.

High thermoelectric performance of two-dimensional SiPGaS/As heterostructures

Thermoelectric technology holds great promise as a green and sustainable energy solution, generating electric power directly from waste heat. Herein, we investigate the thermoelectric properties of SiPGaS/As van der Waals heterostructures by using computations based on density functional theory and semiclassical Boltzmann transport theory. Our results show that both models of SiPGaS/As van der Waals heterostructures have low lattice thermal conductivity at room temperature (300 K). Applying 4% tensile strain to the models leads to a significant enhancement in the figure of merit (ZT), with model-I and model-II exhibiting ZT improvements of up to 24.5% and 14.8%, respectively. Notably, model-II outperforms all previously reported heterostructures in terms of ZT value. Additionally, we find that the maximum thermoelectric conversion efficiency (η) for model-II at 4% tensile strain reaches 23.98% at 700 K. Our predicted ZT_avg > 1 suggests that these materials have practical potential for thermoelectric applications over a wide temperature range. Overall, our findings offer valuable insights for designing better thermoelectric materials.

Strain Tunable Thermoelectric Material: Janus ZrSSe Monolayer

Thermoelectric (TE) performance of the Janus ZrSSe monolayer under biaxial strain is systematically explored by the first-principles approach and Boltzmann transport theory. Our results show that the Janus ZrSSe monolayer has excellent chemical, dynamical, thermal, and mechanical stabilities, which provide a reliable platform for strain tuning. The electronic structure and TE transport parameters of the Janus ZrSSe monolayer can be obviously tuned by biaxial strain. Under 2% tensile strain, the optimal power factor PF of the n-type-doped Janus ZrSSe monolayer reaches 46.36 m W m^-1 K^-2 at 300 K. This value is higher than that of the most classical TE materials. Under 6% tensile strain, the maximum ZT values for the p-type- and n-type-doped Janus ZrSSe monolayers are 4.41 and 4.88, respectively, which are about 3.83 and 1.49 times the results of no strain, respectively. Such high TE performance can be attributed to high band degeneracy and short phonon relaxation time under strain, causing simultaneous increase of the Seebeck coefficient and suppression of the phonon thermal transport. Present work demonstrates that the Janus ZrSSe monolayer is a promising candidate as a strain-tunable TE material and stimulates further experimental synthesis.

The enhanced effect of magnetism on the thermoelectric performance of a CrI3 monolayer

The effect of magnetism on the thermoelectric (TE) transformation efficiency has recently attracted a lot of attention. A CrI₃ monolayer is a two-dimensional (2D) ferromagnetic (FM) semiconductor with a Curie temperature of 45 K. In this work, we employed first-principles calculations within the framework of density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method and Landauer-Buttiker theory to study the effect of magnetism on the TE performance of a CrI₃ monolayer. The stability, electronic structures, density of states (DOS) and TE parameters of a CrI₃ monolayer are calculated. Our calculation results indicate that the TE performance of a CrI₃ monolayer in a FM state is superior to that in a non-magnetic (NM) state. Namely, magnetism is beneficial to improving the TE performance. To further investigate the physical mechanism, the phonon group velocity, the electronic and phonon transmission spectra and the effective mass of a CrI₃ monolayer in FM and NM states are analyzed in detail. For a CrI₃ monolayer in a NM state, the maximum ZT value at 40 K is 0.09 and 0.16 for p-type and n-type doping, respectively. Relative to that in a NM state, the maximum ZT of a CrI₃ monolayer in a FM state is largely improved, and can reach 0.23 and 1.58 for p-type and n-type doping. Our research provides a valuable reference by showing that magnetism is a possible factor for improving the TE efficiency.

High thermoelectric performance of two-dimensional layered AB2Te4 (A = Sn, Pb; B = Sb, Bi) ternary compounds

The thermoelectric properties of two-dimensional layered ternary compounds AB₂Te₄, in which A (Sn, Pb) and B(Sb, Bi) are group-IV and group-V cations, respectively, were investigated by using first-principles based transport theory. These septuple-atom-layer monolayers have wider band gaps with respect to their bulks, which extend their operating temperature and inhibit the bipolar carrier conduction and thermal conductivity, and more importantly, their energy bands exhibit multiple valence band convergence to a narrow energy range near the Brillouin zone center, which induces an optimal p-type power factor up to 10.94-32.11 W m^-1 K^-2 at room temperature. Moreover, these monolayers contain heavy atomic masses and high polarizability of some chemical bonds, leading to small group velocities of phonons and anharmonic phonon behavior that produce an intrinsic lattice thermal conductivity as low as 0.79-3.13 W m^-1 K^-1 at room temperature. Thus, these monolayers act as p-type thermoelectric materials with thermoelectric figure of merit of up to 2.6-5.5 for SnSb₂Te₄, 0.7-2.2 for PbSb₂Te₄, and 1.6-4.2 for PbBi₂Te₄ in the temperature range of 300 to 750 K, and 4.5-5.9 for SnBi₂Te₄ in the temperature range of 300 to 450 K.

High thermoelectric performance of a Sc2Si2Te6 monolayer at medium temperatures: an ab initio study

Thermoelectric (TE) materials have attracted great attention in solving the problems in the waste heat field, while low figure of merit and poor material stability drastically limit their practical applications. In this work, a two-dimensional (2D) Sc₂Si₂Te₆ monolayer was systematically explored as a promising TE material via ab initio methods. The results reveal that the Sc₂Si₂Te₆ monolayer possesses an indirect band gap with a rhombohedral crystal phase and exhibits excellent dynamic stability. The lower electronic/lattice thermal conductivity and higher electron carrier mobility result in good n-type power factor parameters between 6.24 × 10¹⁰ and 1.5 × 10¹¹ W m^-1 s^-1 K^-2 from 300 to 700 K. Such combined merits of low thermal conductivity and high power factor parameters endow the Sc₂Si₂Te₆ monolayer with superior thermoelectric properties with figure of merit (ZT) values of 1.41 and 3.81 at 300 K and 700 K, respectively. This study presented here can shed light on the future design of various 2D materials for thermoelectric applications.

First-principles study on bilayer SnP3 as a promising thermoelectric material

The bilayer SnP₃ is recently predicted to exfoliate from its bulk phase, and motivated by the transition of the metal-to-semiconductor when the bulk SnP₃ is converted to the bilayer, we study the thermoelectric performance of the bilayer SnP₃ using first-principles combined with Boltzmann transport theory and deformation potential theory. The results indicate that the bilayer SnP₃ is an indirect band gap semiconductor and possesses high carrier mobility. The high carrier mobility results in a large Seebeck coefficient observed in both n- and p-doped bilayer SnP₃, which is helpful for acquiring a high figure of merit (ZT). Moreover, by analyzing the phonon spectrum, relaxation time, and joint density of states, we found that strong phonon scattering makes the phonon thermal conductivity extremely low (∼0.8 W m^-1 K^-1 at room temperature). Together with a high power factor and a low phonon thermal conductivity, the maximum ZT value can reach up to 3.8 for p-type doping at a reasonable carrier concentration, which is not only superior to that of the monolayer SnP₃, but also that of the excellent thermoelectric material SnSe. Our results shed light on the fact that bilayer SnP₃ is a promising thermoelectric material with a better performance than its monolayer phase.

Two-dimensional IV-VA3 monolayers with enhanced charge mobility for high-performance solar cells

High-performance photovoltaics (PVs) constitute a subject of extensive research efforts, in which silicon (Si)-based solar cells (SCs) have been widely commercialized. However, the low carrier mobility of Si-based SCs can limit the effective charge separation, thereby negatively impacting the device performance. Here, via calculating the physicochemical and PV performance based on density functional theory, we demonstrate SCs based on two-dimensional (2D) group IV and V compounds with an AX₃ configuration. Firstly, the cleavage energies of AX₃ (A = Si, Ge; X = P, As, and Sb) are calculated to be less than 1 J m^-2, providing an experimental feasibility to be exfoliated from the corresponding bulk. Secondly, electronic and optical properties have been systematically investigated. To be specific, the band gap of monolayer AX₃ falls in the range of 1.11-1.27 eV, which is comparable with that of Si. Significantly, the electron mobility of monolayer AX₃ can reach as high as ∼30 000 cm² V^-1 s^-1, which is one order of magnitude higher than that of Si. Furthermore, the optical absorbance of monolayer SiAs₃, SiP₃ and GeAs₃ exhibits high coefficients in visible light. Therefore, we believe that our designed AX₃-based PV systems with power conversion efficiency of 20% can offer great potential in the application of high-performance two-dimension-based PVs.

Strain-Enhanced Thermoelectric Performance in GeS2 Monolayer

Strain engineering has attracted extensive attention as a valid method to tune the physical and chemical properties of two-dimensional (2D) materials. Here, based on first-principles calculations and by solving the semi-classical Boltzmann transport equation, we reveal that the tensile strain can efficiently enhance the thermoelectric properties of the GeS₂ monolayer. It is highlighted that the GeS₂ monolayer has a suitable band gap of 1.50 eV to overcome the bipolar conduction effects in materials and can even maintain high stability under a 6% tensile strain. Interestingly, the band degeneracy in the GeS₂ monolayer can be effectually regulated through strain, thus improving the power factor. Moreover, the lattice thermal conductivity can be reduced from 3.89 to 0.48 W/mK at room temperature under 6% strain. More importantly, the optimal ZT value for the GeS₂ monolayer under 6% strain can reach 0.74 at room temperature and 0.92 at 700 K, which is twice its strain-free form. Our findings provide an exciting insight into regulating the thermoelectric performance of the GeS₂ monolayer by strain engineering.

Monolayer SnI2: An Excellent p-Type Thermoelectric Material with Ultralow Lattice Thermal Conductivity

Using density functional theory and semiclassical Boltzmann transport equation, the lattice thermal conductivity and electronic transport performance of monolayer SnI₂ were systematically investigated. The results show that its room temperature lattice thermal conductivities along the zigzag and armchair directions are as low as 0.33 and 0.19 W/mK, respectively. This is attributed to the strong anharmonicity, softened acoustic modes, and weak bonding interactions. Such values of the lattice thermal conductivity are lower than those of other famous two-dimensional thermoelectric materials such as MoO₃, SnSe, and KAgSe. The two quasi-degenerate band valleys for the valence band maximum make it a p-type thermoelectric material. Due to its ultralow lattice thermal conductivities, coupled with an ultrahigh Seebeck coefficient, monolayer SnI₂ possesses an ultrahigh figure of merits at 800 K, approaching 4.01 and 3.34 along the armchair and zigzag directions, respectively. The results indicate that monolayer SnI₂ is a promising low-dimensional thermoelectric system, and would stimulate further theoretical and experimental investigations of metal halides as thermoelectric materials.

First-Principles Study of Electronic Properties of Substitutionally Doped Monolayer SnP3

SnP3 has a great prospect in electronic and thermoelectric device applications due to its moderate band gap, high carrier mobility, absorption coefficients, and dynamical and chemical stability. Doping in two-dimensional semiconductors is likely to display various anomalous behaviors when compared to doping in bulk semiconductors due to the significant electron confinement effect. By introducing foreign atoms from group III to VI, we can successfully modify the electronic properties of two-dimensional SnP3. The interaction mechanism between the dopants and atoms nearby is also different from the type of doped atom. Both Sn7BP24 and Sn7NP24 systems are indirect bandgap semiconductors, while the Sn7AlP24, Sn7GaP24, Sn7PP24, and Sn7AsP24 systems are metallic due to the contribution of doped atoms intersecting the Fermi level. For all substitutionally doped 2D SnP3 systems considered here, all metallic systems are nonmagnetic states. In addition, monolayer Sn7XP24 and Sn8P23Y may have long-range and local magnetic moments, respectively, because of the degree of hybridization between the dopant and its adjacent atoms. The results complement theoretical knowledge and reveal prospective applications of SnP3-based electrical nanodevices for the future.

Ultralow lattice thermal conductivity at room temperature in 2D KCuSe from first-principles calculations

Ultralow lattice thermal conductivity is crucial for achieving a high thermoelectric figure of merit for thermoelectric applications. In this work, using first-principles calculations and the phonon Boltzmann transport theory, we investigate the phonon thermal transport properties of 2D KCuSe. Our calculations indicate that the strong acoustic-optical coupling, the low-lying acoustic phonon modes and the strong lattice anharmonic effect with a large Grüneisen parameter and phase space volume result in an ultralow lattice thermal conductivity of 0.021 W m^-1 K^-1 at 300 K for monolayer KCuSe, which is lower than those of recently reported KAgSe (0.26 W m^-1 K^-1 at 300 K) and TlCuSe (0.44 W m^-1 K^-1 at 300 K). Importantly, although the Coulomb interactions and the tensile biaxial strain lead to the increase of lattice thermal conductivity due to the increasing relaxation time (0.056 and 0.28 W m^-1 K^-1 at 300 K without and with 6% tensile strain, respectively), it is still lower than those of most 2D thermoelectric materials. The advantages of being cheap, environmentally friendly and having low lattice thermal conductivity compared to the KAgSe and TlCuSe derivatives make KCuSe a promising candidate for thermoelectric applications, which will stimulate more efforts toward theoretical and experimental studies on this class of 2D ternary semiconductors.

Low-cost pentagonal NiX2 (X=S, Se, and Te) monolayers with strong anisotropy as potential thermoelectric materials

Pentagonal compounds, as a new family of 2D materials, have recently been extensively studied in the fields of electrocatalysis, photovoltaics, and thermoelectrics. Encouraged by the successful synthesis of pentagonal PdSe2,...

The Thermal Stability of Janus Monolayers SnXY (X, Y = O, S, Se): Ab-Initio Molecular Dynamics and Beyond

In recent years, the Janus monolayers have attracted tremendous attention due to their unique asymmetric structures and intriguing physical properties. However, the thermal stability of such two-dimensional systems is less known. Using the Janus monolayers SnXY (X, Y = O, S, Se) as a prototypical class of examples, we investigate their structure evolutions by performing ab-initio molecular dynamics (AIMD) simulations at a series of temperatures. It is found that the system with higher thermal stability exhibits a smaller difference in the bond length of Sn-X and Sn-Y, which is consistent with the orders obtained by comparing their electron localization functions (ELFs) and atomic displacement parameters (ADPs). In principle, the different thermal stability of these Janus structures is governed by their distinct anharmonicity. On top of these results, we propose a simple rule to quickly predict the maximum temperature up to which the Janus monolayer can stably exist, where the only input is the ADP calculated by the second-order interatomic force constants rather than time-consuming AIMD simulations at various temperatures. Furthermore, our rule can be generalized to predict the thermal stability of other Janus monolayers and similar structures.

Thermoelectric performance of ZrNX (X = Cl, Br and I) monolayers

A low thermal conductivity and a high power factor are essential for efficient thermoelectric materials. The lattice thermal conductivity can be reduced by reducing the dimensions of the materials, thus improving the thermoelectric performance. In this work, the electronic, carrier and phonon transport and the thermoelectric properties of ZrNX (X = Cl, Br, and I) monolayers were investigated using density functional theory and Boltzmann transport theory. The electronic and phonon transport show anisotropic properties. The thermal conductivities are 20.8, 14.6 and 12.4 W m^-1 K^-1 at room temperature along the y-direction for the ZrNCl, ZrNBr, and ZrNI monolayers, respectively. Combining the low lattice thermal conductivity and the high power factor results in an excellent thermoelectric performance of the ZrNX monolayers. The thermoelectric figure of merit of ZrNX monolayers can reach magnitudes of ∼0.49-3.15 by optimal hole and electron concentrations between 300 and 700 K. ZrNX monolayers with high ZT values for n- and p-type materials would thus be novel, promising candidate 2D thermoelectric materials for heat-electricity conversion.

The thermoelectric properties of α-XP (X = Sb and Bi) monolayers from first-principles calculations

Thermoelectric (TE) materials as one of the effective solutions to the energy crisis are gaining more and more interest owing to their capability to generate electricity from waste heat without generating air pollution. In this work, the TE properties of α-XP monolayers such as the stability, electronic structure, electrical and phonon transport were thoroughly studied in combination with the first-principles calculations and Boltzmann transport equations. We found that α-SbP and α-BiP have indirect bandgaps of 0.85 eV and 0.73 eV, respectively, which are suitable for thermoelectric materials. Furthermore, due to the multiple valleys at the energy band edges and the high carrier mobility, α-XP possesses both large Seebeck coefficients and high electrical conductivities. It is also found that the lattice thermal conductivity of α-BiP is smaller than that of α-SbP due to lower phonon frequencies, smaller phonon group velocities, larger Grüneisen parameters and higher phonon relaxation times. High TE performance was achieved with the ZT values reaching 4.59 (for α-BiP at 500 K) and 1.34 (for α-SbP at 700 K). Our results quantify α-XP monolayers as promising candidates for building outstanding thermoelectric devices.

Unique Arrangement of Atoms Leads to Low Thermal Conductivity: A Comparative Study of Monolayer Mg2C

Two-dimensional Mg₂C, one of the typical representative MXene materials, is attracting lots of attention due to its outstanding properties. In this study, we find the thermal conductivity of monolayer Mg₂C is more than 2 orders of magnitude lower than graphene and is even lower than MoS₂ despite the relatively lighter atoms of Mg and C. Based on the comparative analysis with graphene, silicene, and MoS₂, the underlying mechanism is found lying in the unique arrangement of atoms (lighter atoms in the middle plane) and large electronegativity difference in Mg₂C. The phonon anharmonicity is strong due to the resonant bonding. In addition, dual band gaps emerge in the phonon dispersion of Mg₂C, which limit the phonon-phonon scattering and reduce the phonon relaxation time. This study reveals a new mechanism responsible for low thermal conductivity, which would be helpful for designing thermal functional materials and pave the way for applications in thermoelectrics.

Promising thermoelectric candidate based on a CaAs3 monolayer: A first principles study

The CaAs₃ monolayer is a newly predicted two-dimensional material with attractive properties, such as a moderate direct bandgap, high carrier mobility, prominent visible-light absorption, etc. To evaluate its potential applications in thermoelectric (TE) fields, herein, the thermoelectric properties of CaAs₃ monolayers were comprehensively investigated by a first-principles method in combination with Boltzmann transport theory. Our calculated results indicate that the CaAs₃ monolayer has an exceptionally low lattice thermal conductivity of 0.44 W m^-1 K^-1 at 300 K, mainly because of the small group velocity and strong phonon-phonon scattering. The CaAs₃ monolayer also exhibits a high power factor due to the large Seebeck coefficient and electrical conductivity. Therefore, large ZT values of 1.72/1.58 were achieved for the n-type/p-type CaAs₃ monolayer at 800 K. Compared with conventional 2D TE materials, the CaAs₃ monolayer does not contain expensive heavy elements, which is beneficial for its practical applications as a TE material. Our results qualify the CaAs₃ monolayer as a promising candidate for building excellent 2D TE devices.

Excellent Room-Temperature Thermoelectricity of 2D GeP3: Mexican-Hat-Shaped Band Dispersion and Ultralow Lattice Thermal Conductivity

Although some atomically thin 2D semiconductors have been found to possess good thermoelectric performance due to the quantum confinement effect, most of their behaviors occur at a higher temperature. Searching for promising thermoelectric materials at room temperature is meaningful and challenging. Inspired by the finding of moderate band gap and high carrier mobility in monolayer GeP_3, we investigated the thermoelectric properties by using semi-classical Boltzmann transport theory and first-principles calculations. The results show that the room-temperature lattice thermal conductivity of monolayer GeP₃ is only 0.43 Wm⁻¹K⁻¹ because of the low group velocity and the strong anharmonic phonon scattering resulting from the disordered phonon vibrations with out-of-plane and in-plane directions. Simultaneously, the Mexican-hat-shaped dispersion and the orbital degeneracy of the valence bands result in a large p-type power factor. Combining this superior power factor with the ultralow lattice thermal conductivity, a high p-type thermoelectric figure of merit of 3.33 is achieved with a moderate carrier concentration at 300 K. The present work highlights the potential applications of 2D GeP₃ as an excellent room-temperature thermoelectric material.

Ultra-low thermal conductivity and high thermoelectric performance of monolayer BiP3: a first principles study

The thermoelectric properties of monolayer triphosphide BiP₃ are studied via first principles calculations and Boltzmann transport equation. First, the Seebeck coefficient, electrical conductivity and electron thermal conductivity at different temperatures are calculated using the Boltzmann transport equation with relaxation time approximation. It has been observed that BiP₃ has a large power factor (265 × 10^-4 W K^-2 m^-1, 700 K). Then, by analyzing the second-order interatomic force constant (IFCS), the atomic structure and phonon dispersion were studied, and the thermal conductivity of monolayer BiP₃ was predicted in the temperature range of 300-800 K, and it was found that it had a very low thermal conductivity (2.13 W m^-1 K^-1) at room temperature. The thermal conductivity is mainly contributed by the branches of acoustics along in-plane transverse (TA). Finally, the maximum ZT value of monolayer BiP₃ is 3.06 at 700 K, when the electron doping concentration is 2.35 × 10¹¹ cm^-2, which indicates that it is a promising thermoelectric material.

Conflux of tunable Rashba effect and piezoelectricity in flexible magnesium monochalcogenide monolayers for next-generation spintronic devices

The coupling of piezoelectric properties with Rashba spin-orbit coupling (SOC) has proven to be the limit breaker that paves the way for a self-powered spintronic device (ACS Nano, 2018, 12, 1811-1820). For further advancement in next-generation devices, a new class of buckled, hexagonal magnesium-based chalcogenide monolayers (MgX; X = S, Se, Te) have been predicted which are direct band gap semiconductors satisfying all the stability criteria. The MgTe monolayer shows a strong SOC with a Rashba constant of 0.63 eV Å that is tunable to the extent of ±0.2 eV Å via biaxial strain. Also, owing to its broken inversion symmetry and buckling geometry, MgTe has a very large in-plane as well as out-of-plane piezoelectric coefficient. These results indicate its prospects for serving as a channel semiconducting material in self-powered piezo-spintronic devices. Furthermore, a prototype for a digital logic device can be envisioned using the ac pulsed technology via a perpendicular electric field. Heat transport is significantly suppressed in these monolayers as observed from their intrinsic low lattice thermal conductivity at room temperature: MgS (9.32 W m-1 K-1), MgSe (4.93 W m-1 K-1) and MgTe (2.02 W m-1 K-1). Further studies indicate that these monolayers can be used as photocatalytic materials for the simultaneous production of hydrogen and oxygen on account of having suitable band edge alignment and high charge carrier mobility. This work provides significant theoretical insights into both the fundamental and applied properties of these new buckled MgX monolayers, which are highly suitable for futuristic applications at the nanoscale in low-power, self-powered multifunctional electronic and spintronic devices and solar energy harvesting.

2D Nb2SiTe4and Nb2GeTe4: promising thermoelectric figure of merit and gate-tunable thermoelectric performance

Recently, the experimentally synthesized Nb₂SiTe₄was found to be a stable layered narrow-gap semiconductor, and the fabricated field-effect transistors (FETs) based on few-layers Nb₂SiTe₄are good candidates for ambipolar devices and mid-infrared detection (Zhaoet al2019ACS Nano1310705-10). Here, we use first-principles combined with Boltzmann transport theory and non-equilibrium Green's function method to investigate the thermoelectric transport coefficients of monolayer Nb₂XTe₄(X = Si, Ge) and the gate voltage effect on the thermoelectric performance of the FET based on monolayer Nb₂SiTe₄. It is found that both monolayers have largep-type Seebeck coefficients due to the 'pudding-mold-type' valence band structure, and they both exhibit anisotropic thermoelectric behavior with optimal thermoelectric figure of merit of 1.4 (2.2) at 300 K and 2.8 (2.5) at 500 K for Nb₂SiTe₄(Nb₂GeTe₄). The gate voltage can effectively increase the thermoelectric performance for the Nb₂SiTe₄-based FET. The high thermoelectric figure of merit can be maintained in a wide temperature range under a negative gate voltage. Furthermore, the FET exhibits a good gate-tunable Seebeck diode effect. The present work suggests that Nb₂XTe₄monolayers are promising candidates for 2D thermoelectric materials and thermoelectric devices.

Quadruple-layer group-IV tellurides: low thermal conductivity and high performance two-dimensional thermoelectric materials

Through first-principles calculations, we report the thermoelectric properties of two-dimensional (2D) hexagonal group-IV tellurides XTe (X = Ge, Sn and Pb), with quadruple layers (QL) in the Te-X-X-Te stacking sequence, as promising candidates for mid-temperature thermoelectric (TE) materials. The results show that 2D PbTe exhibits a high Seebeck coefficient (∼1996 μV K^-1) and a high power factor (6.10 × 10¹¹ W K^-2 m^-1 s^-1) at 700 K. The lattice thermal conductivities of QL GeTe, SnTe and PbTe are calculated to be 2.29, 0.29 and 0.15 W m^-1 K^-1 at 700 K, respectively. Using our calculated transport parameters, large values of the thermoelectric figure of merit (ZT) of 0.67, 1.90, and 2.44 can be obtained at 700 K under n-type doping for 2D GeTe, SnTe, and PbTe, respectively. Among the three compounds, 2D PbTe exhibits low average values of sound velocity (0.42 km s^-1), large Grüneisen parameters (∼2.03), and strong phonon scattering. Thus, 2D PbTe shows remarkable mid-temperature TE performance with a high ZT value under both p-type (2.39) and n-type (2.44) doping. The present results may motivate further experimental efforts to verify our predictions.

GeP3/NbX2 (X=S, Se) Nano-Heterostructures: Promising Isotropic Flexible Anodes for Lithium-Ion Batteries with High Lithium Storage Capacity

A new trend is emerging that flexible batteries will play an indispensable role in the progress of social science and technology. However, flexibility exists only in a single direction for the existing electrode material. Searching for flexible battery materials has attracted more and more attention from researchers. In this article, the lattice structural stability, electronic structure modulation, and the Li adsorption properties of the heterostructures designed by assembling GeP3 and NbX2 (X = S, Se) together were methodically explored based on van der Waals. We found that diffusion barrier of the GeP3/NbS2 heterostructure with metallic properties is 0.21 eV for Li. It greatly improves the charge and discharge performance of the battery. The predicted heterostructure shows quite high theoretical specific capacity with 540.24 mA h/g, which is higher than the traditional graphite anode (372 mA h/g). It demonstrates superior isotropic flexibility with a considerable small Young's modulus (151.98-159.02 N/m), which has promising application as flexible electrodes for rechargeable battery equipment.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

The Thermoelectric Properties of Monolayer MAs2 (M = Ni, Pd and Pt) from First-Principles Calculations

The thermoelectric property of the monolayer MAs₂ (M = Ni, Pd and Pt) is predicted based on first principles calculations, while combining with the Boltzmann transport theory to confirm the influence of phonon and electricity transport property on the thermoelectric performance. More specifically, on the basis of stable geometry structure, the lower lattice thermal conductivity of the monolayer NiAs₂, PdAs₂ and PtAs₂ is obtained corresponding to 5.9, 2.9 and 3.6 W/mK. Furthermore, the results indicate that the monolayer MAs₂ have moderate direct bang-gap, in which the monolayer PdAs₂ can reach 0.8 eV. The Seebeck coefficient, power factor and thermoelectric figure of merit (ZT) were calculated at 300, 500 and 700 K by performing the Boltzmann transport equation and the relaxation time approximation. Among them, we can affirm that the monolayer PdAs₂ possesses the maximum ZT of about 2.1, which is derived from a very large power factor of 3.9 × 10¹¹ W/K²ms and lower thermal conductivity of 1.4 W/mK at 700 K. The monolayer MAs₂ can be a promising candidate for application at thermoelectric materials.

Theoretical model for predicting thermoelectric properties of tin chalcogenides

The global energy crisis demands the search for new materials for efficient thermoelectric energy conversion. Theoretical predictive modelling with experiments can expedite the global search of novel and ecoconscious thermoelectric materials. The efficiency of thermoelectric materials depends upon the thermoelectric figure of merit (ZT). In this perspective, we discuss the theoretical model to calculate thermoelectric properties. Different scattering mechanisms of electrons and phonons are calculated using a simple model for the fast prediction of thermoelectric properties. Thermoelectric properties based on the simple model have shown more than 90% agreement with the experimental values. Possibility to optimize the figure of merit by alloying, defects, nanostructuring and band convergence is also discussed for layered chalcogenides of tin. In the case of doped materials, ion-impurity scattering is found to be dominating over electron-phonon scattering and the power factor can be optimized by tuning the former scattering rate. For phonon transport, alloy scattering is found to be the most dominating among all other scattering mechanisms. Theoretically, it is found that in the temperature range between 300 K and 800 K, SnSe_0.70S_0.30 has the highest ZT with an efficiency of 17.20% with respect to Carnot efficiency. There could be 53.8% enhancement of the device efficiency in SnSe_0.70S_0.30 compared to experimentally reported SnSe_0.50S_0.50 in the medium temperature range (300 K to 800 K). Possible routes to achieve the best ZT in the medium temperature range are also discussed in this perspective.

The Effect of Janus Asymmetry on Thermal Transport in SnSSe

Several ternary "Janus" metal dichalcogenides such as {Mo,Zr,Pt}-SSe have emerged as candidates with significant potential for optoelectronic, piezoelectric, and thermoelectric applications. SnSSe, a natural option to explore as a thermoelectric given that its "parent" structures are SnS₂ and SnSe₂ has, however, only recently been shown to be mechanically stable. Here, we calculate the lattice thermal conductivities of the Janus SnSSe monolayer along with those of its parent dicalchogenides. The phonon frequencies of SnSSe are intermediate between those of SnSe₂ and SnS₂; however, its thermal conductivity is the lowest of the three and even lower than that of a random Sn[S_0.5Se_0.5]₂ alloy. This can be attributed to the breakdown of inversion symmetry and manifests as a subtle effect beyond the reach of the relaxation-time approximation. Together with its low favorable power factor, its thermal conductivity confirms SnSSe as a good candidate for thermoelectric applications.

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.

First-principle study on monolayer and bilayer SnP3 sheets as the potential sensors for NO2, NO, and NH3 detection

Layered SnP₃ sheets have recently been predicted as interesting 2D material with many potential applications. In the present work, first-principles calculations were performed to investigate the possibility of layered SnP₃ sheets as a candidate for detecting pollutant gases (CO, CO₂, H₂S, NH₃, NO, and NO₂). Our results indicate that CO, CO₂, H₂S molecules are all physisorbed on SnP₃ sheets with an adsorption energy of 0.116-0.363 eV. On the other hand, the strong interactions of NH₃, NO, and NO₂ and SnP₃ are found based on the moderate adsorption energy (around 1 eV for NO₂) and large charge transfer. The monolayer SnP₃ shows a higher affinity to these molecules than bilayer one. The chemisorption of NH₃, NO, and NO₂ molecules on layered SnP₃ sheets could efficiently evoke the electrical signal, and show short recovery time for NH₃, NO, and NO₂ capture. The work function calculations exhibit significant responses to the NH₃ and NO₂ molecules. Our results proposed that SnP₃ sheets could be utilized as a gas sensor for NO, NO₂, and NH₃, and extend the potential applications of 2D SnP₃ sheets.

Thermoelectric properties of two-dimensional magnet CrI3

The thermoelectric, phonon transport, and electronic transport properties of two-dimensional magnet CrI₃ are systematically investigated by combining density functional theory with Boltzmann transport theory. A low lattice thermal conductivity of 1.355 W m^-1K^-1 is presented at 300 K due to the low Debye temperature and phonon group velocity. The acoustic modes dominate the lattice thermal conductivity, and the longitudinal acoustic mode has the largest contribution of 42.31% on account of its relatively large phonon group velocity and phonon lifetime. The high band degeneracy and the peaky density of states near the conduction band minimum appear for the CrI₃ monolayer, which is beneficial for forming a significantly increased Seebeck coefficient (1561 μV K^-1). Furthermore, the thermoelectric figure of merit is calculated reasonably, and the value is 1.57 for the optimal n-type doping level at 900 K. N-type doping maintains a higher thermoelectric conversion efficiency than p-type doping throughout the temperature range, while the difference gradually increases as the temperature rises. Our investigation may provide some theoretical support for the application of the CrI₃ monolayer in the thermoelectric field.

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.

Superhigh flexibility and out-of-plane piezoelectricity together with strong anharmonic phonon scattering induced extremely low lattice thermal conductivity in hexagonal buckled CdX (XS, Se) monolayers

Although CdX (X = S, Se) has been mostly studied in the field of photocatalysis, photovoltaics, their intrinsic properties, such as, mechanical, piezoelectric, electron and phonon transport properties have been completely overlooked in buckled CdX monolayers. Ultra-low lattice thermal conductivity [1.08 W m-1K-1(0.75 W m-1K-1)] and high p-type Seebeck coefficient [1300μV K-1(850μV K-1)] in CdS (CdSe) monolayers have been found in this work based on first-principles DFT coupled to semi-classical Boltzmann transport equations, combining both the electronic and phononic transport. The dimensionless thermoelectric figure of merit is calculated to be 0.78 (0.5) in CdS (CdSe) monolayers at room temperature, which is comparable to that of two-dimensional (2D) tellurene (0.8), arsenene and antimonene (0.8), indicating its great potential for applications in 2D thermoelectrics. Such a low lattice thermal conductivity arise from the participation of both acoustic [91.98% (89.22%)] and optical modes [8.02% (10.78%)] together with low Debye temperature [254 K (187 K)], low group velocity [4 km s-1(3 km s-1)] in CdS (CdSe) monolayers, high anharmonicity and short phonon lifetime. Substantial cohesive energy (∼4-5 eV), dynamical and mechanical stability of the monolayers substantiate the feasibility in synthesizing the single layers in experiments. The inversion symmetry broken along thezdirection causes out-of-plane piezoelectricity. |d33| ∼ 21.6 pm V-1, calculated in CdS monolayer is found to be the highest amongst structures having atomic-layer thickness. Superlow Young's modulus ∼41 N m-1(31 N m-1) in CdS (CdSe) monolayers, which is comparable to that of planar CdS (29 N m-1) and TcTe2(34 N m-1), is an indicator of its superhigh flexibility. Direct semiconducting band gap, high carrier mobility (∼500 cm2V-1s-1) and superhigh flexibility in CdX monolayers signify its gigantic potential for applications in ultrathin, stretchable and flexible nanoelectronics. The all-round properties can be synergistically combined together in futuristic applications in nano-piezotronics as well.

First-principles study on photovoltaic properties of 2D Cs2PbI4-black phosphorus heterojunctions

Both 2D perovskite Cs₂PbI₄ and phosphorus are significant optoelectronic semiconductor materials, the optical-electrical characters between both contact interfaces are interesting topics. In present work, we demonstrate comparative investigation of optoelectronic properties for two kinds of electrical contact interfaces. i.e. Pb-I and Cs-I interfaces with black phosphorus contacts. The carrier transport, charge transferring and optical properties for both cases are investigated by using first principle calculation. Both contact interfaces exhibit type II band alignment with direct band gap. Charge carrier migration from Cs-I interface to black phosphorus is more strong than that of Pb-I interface by considering differential charge density and bader charge between distinct electrical contact interfaces. Besides, electron-hole effective masses of heterojunctions for both cases along different direction are investigated. Optical absorption coefficients of both cases are compared with those of free-standing Cs₂PbI₄ and black phosphorus in the visible spectrum. We systematically compared advantages and disadvantages of two kinds of contact interfaces for photovoltaic application, and the results reveal interfacial engineering of 2D heterojunction plays a important role in tuning optoelectronic properties.

Tweaking the Physics of Interfaces between Monolayers of Buckled Cadmium Sulfide for a Superhigh Piezoelectricity, Excitonic Solar Cell Efficiency, and Thermoelectricity

Interfaces of heterostructures are routinely studied for different applications. Interestingly, monolayers of the same material when interfaced in an unconventional manner can bring about novel properties. For instance, CdS monolayers, stacked in a particular order, are found to show unprecedented potential in the conversion of nanomechanical energy, solar energy, and waste heat into electricity, which has been systematically investigated in this work, using DFT-based approaches. Moreover, stable ultrathin structures showing strong capabilities for all kinds of energy conversion are scarce. The emergence of a very high out-of-plane piezoelectricity, |d₃₃| ≈ 56 pm/V, induced by the inversion symmetry broken in the buckled structure helps to supersede the previously reported bulk wurzite GaN, AlN, and Janus multilayer structures of Mo- and W-based dichalcogenides. The piezoelectric coefficients have been found to be largely dependent on the relative stacking between the two layers. CdS bilayer is a direct band gap semiconductor, with its band edges straddling the water redox potential, thereby making it thermodynamically favorable for photocatalytic applications. Strain engineering facilitates its transition from type I to type II semiconductor in CdS bilayer stacked over monolayer boron phosphide, and the theoretically calculated power conversion efficiency (PCE) in the 2D excitonic solar cell exceeds 27% for a fill factor of 0.8, which is much higher than that in ZnO/CdS/CuInGaSe solar cell (20% efficiency). Thermoelectric properties have been investigated using semi classical Boltzmann transport equations for electrons and phonons within the constant relaxation time approximation coupled to deformation potential theory, which reveal ultralow thermal conductivity (κ_l ≈ 0.78 W m^-1 K^-1) at room temperature because of the presence of heavy element Cd, strong anharmonicity (high mode Gruneisen parameter at long wavelength, phonon lifetime <5 ps), low phonon group velocity (4 km/s), and low Debye temperature (260 K). Such a low thermal conductivity is lower than that of dumbbell silicene (2.86 W m^-1 K^-1), SnS₂ (6.41 W m^-1 K^-1) and SnSe₂ (3.82 W m^-1 K^-1), and SnP₃ (4.97 W m^-1 K^-1). CdS bilayer shows a thermoelectric figure of merit (ZT) ≈ 0.8 for p-type and ∼0.7 for n-type doping at room temperature. Its ultrahigh carrier mobility (μ_e ≈ 2270 cm² V^-1 s^-1) is higher than that of single-layer MoS₂ and comparable to that in InSe. The versatile properties of CdS bilayer together with its all-round stability supported by ab initio molecular dynamics simulation, phonon dispersion, and satisfaction of Born-Huang stability criteria highlight its outstanding potential for applications in device fabrication and applications in next-generation nanoelectronics and energy harvesting.

Tuning the electronic structures of monolayer triphosphides MP3 (M = Sn and Ge) for CO2 electroreduction through interface engineering: a theoretical prediction

Interface engineering by integrating various two-dimensional materials to form heterostructures can not only preserve the desired properties of individual components, but also induce new functions. Herein, by means of density functional theory (DFT) computations, we have investigated the effects of interface engineering of the graphene substrate on the electronic structures of monolayer triphosphides MP3 (M = Sn and Ge) and their catalytic performance for the electroreduction of carbon dioxide (CO2ER). Our results revealed that the MP3/graphene interfaces exhibit good structural stability, enhanced electrical conductivity, superior CO2ER performance, and obvious suppressing effects on hydrogen evolution due to the charge transfer at the interface. Thus, our results suggested that SnP3/graphene and GeP3/graphene heterostructures can be utilized as promising CO2ER catalysts with high-efficiency and high-selectivity, offering cost-effective opportunities to convert CO2 for renewable energy supply via interface engineering.

High-performance III-VI monolayer transistors for flexible devices

Group III-VI family MX (M = Ga and In, and X = S, Se, and Te) monolayers have attracted global interest for their potential applications in electronic devices due to their unexpectedly high carrier mobility. Herein, via density functional theory calculations as well as ab initio quantum transport simulations, we investigated the performance limits of MX monolayer metal oxide semiconductor field-effect transistors (MOSFETs) at the sub-10 nm scale. Our results highlighted that the MX monolayers possessed good structural stability and mechanical isotropy with large ultimate strains and low Young's modulus, which are intensely anticipated in the next-generation flexible devices. More importantly, the MX monolayer MOSFETs show excellent device performance under optimal schemes. The on-state current, delay time, and power dissipation of the MX monolayer MOSFETs satisfy the International Technology Roadmap for Semiconductors (ITRS) 2013 requirements for high-performance devices. Interestingly, the sub-threshold swings were in a very low range from 68 mV dec-1 to 108 mV dec-1, which indicated the favorable gate control ability for fast switching. Therefore, we believe that our findings shed light on the design and application of MX monolayer-based MOSFETs in next-generation flexible electronic devices.

Ultra-low thermal conductivity and high thermoelectric performance of two-dimensional triphosphides (InP3, GaP3, SbP3 and SnP3): a comprehensive first-principles study

By performing first-principles calculations combined with the Boltzmann transport equation, we report a comprehensive study of the thermal and thermoelectric properties of monolayer triphosphides InP3, GaP3, SbP3 and SnP3. Firstly, we studied the structure and phonon dispersion, and discussed the long-range atomic interactions by analyzing the second-order interatomic force constants (IFCs). Next, we predicted the corresponding thermal conductivities of monolayer InP3, GaP3, SbP3 and SnP3 at 300 K to be 0.64 W m-1 K-1, 3.02 W m-1 K-1, 1.04 W m-1 K-1 and 0.48 W m-1 K-1, respectively. To study the thermoelectric properties, the carrier mobility and electron relaxation time of the four materials were predicted by the deformation potential theory method and explained by analyzing their energy band structures. Then, the Seebeck coefficient, electrical conductivity and thermoelectric figure of merit (ZT) at different temperatures were calculated by using the Boltzmann transport equation with relaxation time approximation. Finally, we predicted the maximum ZT values of InP3, GaP3, SbP3 and SnP3 to be up to 2.6, 0.9, 1.9 and 3.7 at 300 K and up to 4.6, 1.6, 3.5 and 6.1 at 500 K, respectively. With ultra-low thermal conductivity and high thermoelectric performance, monolayer triphosphides are considered as potential candidates for thermoelectric materials.

Phonon transport and thermal conductivity of diamond superlattice nanowires: a comparative study with SiGe superlattice nanowires

Due to the coupling of a superlattice's longitudinal periodicity to a nanowire's radial confinement, the phonon transport properties of superlattice nanowires (SLNWs) are expected to be radically different from those of pristine nanowires. In this work, we present the comparative investigation of phonon transport and thermal conductivity between diamond SLNWs and SiGe SLNWs by using molecular dynamics simulations. In the case of period length ∼ 25 Å, the thermal conductivities of diamond SLNWs and SiGe SLNWs both increase linearly with increasing the period number, which implies the wave-like coherent phonons dominate the heat transport of SLNWs. In the case of period length ∼ 103 Å, the thermal conductivity of SiGe SLNWs is length-independent with increasing the period number, indicating that the particle-like incoherent phonons in SiGe SLNWs control the heat transport, because the phonon–phonon scattering causes phonons to not retain their phases and the coherence is destroyed before the reflection at interfaces. However in diamond SLNWs the coherent phonons still dominate heat conduction and the thermal conductivity is length-dependent, because the mean free path of phonon–phonon scattering in diamond SLNWs is much longer. The spatial distribution of phonon localized modes further supports these opinions. These results are helpful not only to understand the coherent and incoherent phonon transport, but also to modulate the thermal conductivity of SLNWs.

We present the comparative investigation of phonon transport and thermal conductivity between diamond SLNWs and SiGe SLNWs by molecular dynamics simulations.

Thermal transport properties of monolayer MoSe2 with defects

The defects in monolayer MoSe₂ have a significant effect on its lattice thermal conductivity.