Hydrogenation of Penta-Graphene Leads to Unexpected Large Improvement in Thermal Conductivity

The intrinsically low lattice thermal conductivity of monolayer T-Au6X2 (X = S, Se and Te)

Thermal conductivity (κ, which consists of electronic thermal conductivity κe and lattice thermal conductivity κl), as an essential parameter in thermal management applications, is a critical physical quantity to measure the heat transfer performance of materials. To seek low-κ materials for heat-related applications, such as thermoelectric materials and thermal barrier coatings. In this study, based on a complex cluster design, we report a new class of two-dimensional (2D) transition metal dichalcogenides (TMDs): T-Au6X2 (X = S, Se, and Te) with record ultralow κl values. At room temperature, the κl values of T-Au6S2, T-Au6Se2, and T-Au6Te2 are 0.25 (0.23), 0.30 (0.21), and 0.12 (0.10) W m-1 K-1 along the x-axis (y-axis) direction, respectively, exhibiting good thermal insulation. The ultralow κl originates from strong phonon softening and suppression, especially for the phonon with frequency 0-1 THz. In addition, T-Au6Te2 holds the lowest group velocity and phonon relaxation time among the three T-Au6X2 monolayers. Our study provides an alternative approach for achieving ultralow κl through complex cluster replacement. Meanwhile, this new class of TMDs is expected to shine in thermal insulation and thermoelectricity due to their ultralow κl values.

Thermal and electrical transport properties of two-dimensional Dirac graphenylene: a first-principles study

The development of high performance two-dimensional thermoelectric (TE) materials is crucial for enhancing the conversion of waste heat into electricity and for achieving the transition to new energy. In recent years, two-dimensional Dirac materials with high carrier mobility and non-trivial topological properties have been expected to extend the application of carbon-based materials in the TE field. However, research on the TE properties of two-dimensional Dirac materials is still scarce, and the relevant physical mechanisms that affect the TE figure of merit of the materials are still unclear. Therefore, we carefully selected a typical and experimentally synthesized Dirac structure, graphenylene, and systematically studied its thermal transport and electrical transport properties using density functional theory (DFT) and Boltzmann transport theory. The results show that the ZT value of graphenylene exhibits an extremely significant anisotropy. There is a significant discrepancy in the figure of merit (ZT) values of n-type and p-type systems at the optimum doping concentration, i.e., the ZT value of the n-type system (0.49) is one order of magnitude greater than that of the p-type system (0.06). Graphenylene exhibits excellent electronic performance due to its unique electronic band structure and has an extremely high conductivity (for the n-type system, electrical conductivity at room temperature is 109 S m-1). Interestingly, graphenylene has an unusually higher ZT at low temperature (0.5 at 300 K) than at high temperature (0.3 at 800 K) for n-type doping along the x-axis, contrary to the conventional view that higher ZT values exist in the high temperature range. This work provides a deep insight into the TE properties of two-dimensional Dirac carbon materials and offers new perspectives for enhancing the TE performance and application of carbon-based nanomaterials.

Efficient modulation of thermal transport in two-dimensional materials for thermal management in device applications

With the development of chip technology, the density of transistors on integrated circuits is increasing and the size is gradually shrinking to the micro-/nanoscale, with the consequent problem of heat dissipation on chips becoming increasingly serious. For device applications, efficient heat dissipation and thermal management play a key role in ensuring device operation reliability. In this review, we summarize the thermal management applications based on 2D materials from both theoretical and experimental perspectives. The regulation approaches of thermal transport can be divided into two main types: intrinsic structure engineering (acting on the intrinsic structure) and non-structure engineering (applying external fields). On one hand, the thermal transport properties of 2D materials can be modulated by defects and disorders, size effect (including length, width, and the number of layers), heterostructures, structure regulation, doping, alloy, functionalizing, and isotope purity. On the other hand, strain engineering, electric field, and substrate can also modulate thermal transport efficiently without changing the intrinsic structure of the materials. Furthermore, we propose a perspective on the topic of using magnetism and light field to modulate the thermal transport properties of 2D materials. In short, we comprehensively review the existing thermal management modulation applications as well as the latest research progress, and conclude with a discussion and perspective on the applications of 2D materials in thermal management, which will be of great significance to the development of next-generation nanoelectronic devices.

Effect of the co-adsorption of small molecules from air on the properties of penta-graphene and their proton transfer calculation

Penta-graphene has attracted considerable attention due to its unique structure and novel properties. Herein, we studied the effect of the co-adsorption of small molecules from air on the properties of penta-graphene using first-principles calculations. Our results show that oxygen molecules can be self-decomposed on the surface of penta-graphene and the process of O₂ decomposition is an exothermic reaction. On the contrary, the adsorption of H₂O or N₂ molecule on penta-graphene exhibits weak interaction characteristic. For co-adsorption systems, the adsorption of N₂ molecule has no effect on the electronic properties of penta-graphene because the N₂ molecule is more inert than other molecules. Hydrogen bonds (H-bonds) have been observed in the co-adsorption of H₂O and O₂ on penta-graphene. We find that shorter H-bonds lead to higher stability of the systems. We also explore the proton transfer process between H₂O and oxidized penta-graphene. Our results show that the proton transfer process is relatively difficult due to the high energy barrier. However, double-proton transfer is an exothermic process since the energy of the final state is 0.11 eV lower than that of the initial state. These results indicate that the configuration of oxidized penta-graphene is complicated. Our research provides a theoretical basis and important guidance for the experimental synthesis and functionalization of penta-graphene.

Effect of hydrogenation on the thermal conductivity of 2D gallium nitride

The indirect bandgap of two-dimensional GaN hinders its application in the optical field. Hydrogenation can convert the bandgap type of the GaN monolayer from an indirect to a direct one and also tune the bandgap size. The thermal transport, an important property in the application of two-dimensional materials, is also influenced by hydrogenation. By performing first-principles calculations and solving the phonon Boltzmann equation, we investigate the effect of hydrogenation on the thermal conductivity of the GaN monolayer. The results show that hydrogenation will slightly increase the thermal conductivity of the GaN monolayer from 70.62 Wm^-1 K^-1 to 76.23 Wm^-1 K^-1 at 300 K. The little effect of hydrogenation on thermal conductivity is mainly dominated by two competing factors: (1) the reduction of ZA mode lifetime due to the breaking of reflection symmetry after hydrogenation and (2) the increased contribution from TA and LA modes due to the reduction of anharmonic scattering caused by the enlarged phonon bandgap after hydrogenation. The results are compared with other two-dimensional materials with hexagonal monolayer structures.

Phonon Transport and Thermoelectric Properties of Imidazole-Graphyne

The pentagon has been proven to be an important structural unit for carbon materials, leading to different physical and chemical properties from those of hexagon-based allotropes. Following the development from graphene to penta-graphene, a breakthrough has very recently been made for graphyne—for example, imidazole-graphyne (ID-GY) was formed by assembling experimentally synthesized pentagonal imidazole molecules and acetylenic linkers. In this work, we study the thermal properties and thermoelectric performance of ID-GY by combining first principle calculations with the Boltzmann transport theory. The calculated lattice thermal conductivity of ID-GY is 10.76 W/mK at 300 K, which is only one tenth of that of γ-graphyne (106.24 W/mK). A detailed analysis of the harmonic and anharmonic properties, including the phonon group velocity, phonon lifetime, atomic displacement parameter, and bond energy curves, reveals that the low lattice thermal conductivity can be attributed to the low Young’s modulus, low Debye temperature, and high Grüneisen parameter. Furthermore, at room temperature, ID-GY can reach a high ZT value of 0.46 with a 5.8 × 10¹² cm⁻² hole concentration, which is much higher than the value for many other carbon-based materials. This work demonstrates that changing structural units from hexagonal to pentagonal can significantly reduce the lattice thermal conductivity and enhance the thermoelectric performance of carbon-based materials.

Effects of hydrogenation on the tensile and shear mechanical properties of defective penta-graphene

Penta-graphene (PG) is a new theoretical two-dimensional metastable carbon allotrope composed entirely of carbon pentagons. In this paper, molecular dynamics simulations are performed to investigate the effects of the hydrogenation on the tensile and shear mechanical properties, together with the failure mechanism of PG with vacancy defects. The results show that hydrogenation can effectively tune the mechanical properties and failure mechanism of PG with vacancy defects. The defective PG (DPG) with low hydrogenation coverages exhibits obvious plastic deformation features under tensile and shear loading, and pentagon-to-polygon structural transformation is observed, while complete hydrogenation can change the failure mechanism of DPG from plastic deformation to brittle fracture. Both the tensile and shear moduli and elastic limit of DPG first decrease dramatically and then increase slowly with the increase of hydrogenation coverage, while tensile and shear strain increases almost monotonically with rising hydrogenation coverage. Complete hydrogenation can result in large enhancement of tensile and shear elastic stress limit and strain. These results may provide an important guideline for effectively tuning the mechanical properties of PG and other two-dimensional nanomaterials.

Mechanics of penta-graphene with vacancy defects under large amplitude tensile and shear loading

Penta-graphene is a new two-dimensional metastable carbon allotrope composed entirely of carbon pentagons with unique electronic and mechanical properties. In this work we evaluate the mechanical properties of new classes of defective penta-graphene (DPG) subjected to tensile and shear loading by using molecular dynamics simulations. The types of defects considered here are monovacancy at either 4-coordinated C1 site or 3-coordinated C2 site, and double vacancy (DV). We focus in particular on the effects of the different topologies of defects and their concentrations on the elastic constants and the nonlinear mechanics of this allotropic form of carbon. The results indicate that DPG has a plastic behavior similar to pristine penta-graphene, which is caused by the irreversible pentagon-to-polygon structural transformation occurring during tensile and shear loading. The tensile and shear moduli decrease linearly with the concentration of defects. Monotonic reductions of the tensile yield and shear stresses are also present but less pronounced, while the yield strains are unaffected. Penta-graphene with 4-coordinated and DVs feature a change of the Poisson's ratio from negative to positive when the defect concentration rises to about 3% and 6%. Temperature can trigger structural reconstruction for free-standing DPG. The critical transition temperature increases due to the vacancy defects and the defects can delay the structure transition. These findings are expected to provide important guidelines for the practical applications of penta-graphene based micro/nano electromechanical systems.

Low lattice thermal conductivity of a 5-8-peanut-shaped carbon nanotube

5-8-defects are well-known in graphene and other 2D carbon structures, but not well-studied in one dimensional (1D) carbon materials. Here, we design a peanut-shaped carbon nanotube by assembling the 5-8-cage composed of carbon 5- and 8-membered rings, named 5-8-PSNT. Using first-principles calculations and molecular dynamics simulations, we find that 5-8-PSNT is not only thermally and dynamically stable, but also metallic. Moreover, its lattice thermal conductivity is only 95.87 W m-1 K-1, which is less than one tenth of the value of (6, 6) carbon nanotube that has a radius similar to that of 5-8-PSNT. A further analysis of the phonon properties reveals that the low lattice thermal conductivity of 5-8-PSNT arises from its low phonon group velocity, short relaxation time, large lattice vibrational mismatch and strong anharmonicity. These findings further suggest that a pentagon and an octagon as structural units can effectively modulate the properties of carbon materials.

Anisotropic lattice thermal conductivity in topological semimetal ZrGeX(XS, Se, Te): a first-principles study

Topological semimetals have attracted significant attentions owing to their potential applications in numerous fields such as low-power electron devices and quantum computation, which are closely related to their thermal transport properties. In this work, the phonon transport properties of topological Dirac nodal-line semimetals ZrGeX(X= S, Se, Te) with the PbClF-type structures are systematically studied using the first-principles calculations combined with the Boltzmann transport theory. The obtained lattice thermal conductivities show an obvious anisotropy, which is caused by the layer structures of ZrGeX(X= S, Se, Te). The room-temperature lattice conductivity of ZrGeTe alongcdirection is found to be as low as 0.24 W m^-1 K^-1, indicating that it could be of great significance in the fields of thermal coating materials and solar cell absorber. In addition, we extract each phonon branch from group velocities, phonon scattering rates, Grüneisen parameters, and phase space volumes to investigate the mechanism underlying the low thermal conductivity. It is concluded that the difference of thermal conductivities of three materials may be caused by the number of scattering channels and the effect of anharmonic. Furthermore, the phonon mean free path alongadirection is relatively longer. Nanostructures or polycrystalline structures may be effective to reduce the thermal conductivity and improve the thermoelectric properties.

Recent progress in the development of thermal interface materials: a review

Modern electronic devices are characterized by high-power and high-frequency with excessive heat accumulation. Thermal interface materials (TIMs) are of crucial importance for efficient heat dissipation to maintain proper functions and lifetime for these devices. The most promising TIMs are those polymer-based nanocomposites consisting of polymers and low-dimensional materials with high thermal conductivity (TC). This perspective summarizes the recent progress on the thermal transport properties of newly discovered one-dimensional (1D) nanomaterials and two-dimensional (2D) nanomaterials as well as three-dimensional (3D) nanostructures consisting of these 1D and 2D nanomaterials. Moreover, the applications of various nanomaterials in polymer nanocomposites for advanced TIMs are critically reviewed and the mechanism of TC enhancement is analysed. It is hoped that the present review could provide better understanding of the thermal transport properties of recently developed 2D nanomaterials and various 3D nanostructures as well as relevant polymer-based TIMs, shedding more light on the thermal management research.

First-principles calculations of phonon behaviors in graphether: a comparative study with graphene

Recently, a two-dimensional (2D) oxocarbon monolayer, graphether, has been arousing extensive attention owing to its excellent electrical properties. In this work, we calculate the lattice thermal conductivity (k) of graphether and graphene using first-principles calculations and the phonon Boltzmann transport equation. At 300 K, the lattice thermal conductivities of graphether and graphene along the armchair direction are 600.91 W m-1 K-1 and 3544.41 W m-1 K-1, respectively. Moreover, the electron localization function is employed to reveal the origin of the anisotropic k of graphether. Furthermore, the harmonic and anharmonic properties of graphether and graphene are analyzed. We attribute the lower k of graphether to the smaller phonon group velocity and shorter phonon lifetime. Finally, the size effects of phonon transport in graphether and graphene are studied, and the results show that the lattice thermal conductivities are significantly dependent on the system length. The analysis of phonon behaviors in our study contributes to an in-depth understanding of the thermal transport in graphether for the first time, which provides valuable guidelines for graphether-based phonon engineering applications and 2D nanoelectronic devices.

Structural property-induced different phonon-twin-boundary scattering in diamond

Twin boundaries (TBs), as one kind of crystal defect, have often been observed in various material systems, and the (111)/[110] TB has been verified to show weak phonon scattering. However, it's still not clear whether other TBs can show similar thermal properties to the (111)/[110] TB. To solve this issue, in this work, we perform a systematic study of heat transport across six kinds of twin boundaries in diamond, including the (111)/[110], (221)/[110], (331)/[110], (113)/[110], (112)/[110] and (310)/[001] TBs, by both molecular dynamics simulations and first-principles calculations. The results indicate that the thermal boundary resistance of the six TBs ranges from 1.01 × 10-11 to 6.35 × 10-10 m2 K W-1; specifically, the (111)/[110] TB shows much weaker phonon scattering than the others. The different phonon scattering at TBs mainly depends on the transmission coefficients across the twin boundaries for boundaries with the same symmetry, as well as the combined action of group velocity and phonon mean free path. Furthermore, by analyzing the structural properties of TBs, it can be observed that TB thermal resistance varies significantly with the TB structure, and is strongly correlated with TB energy and bond difference parameter. Our findings will provide useful guidelines for designing efficient thermoelectric and thermal management materials based on phonon-TB scattering.

Electronic and thermal properties of monolayer beryllium oxide from first principles

Monolayer beryllium oxide (BeO), a new graphene-like metal oxide material, has attracted tremendous interest since it was demonstrated to have high dynamic, thermal, kinetic and mechanical stabilities in recent years. This discovery enriches the catalogue of 2D materials and paves the way for the exploration of relevant properties. In this work, the electronic and thermal properties of monolayer BeO are predicted by first-principles calculations. Compared with graphene and monolayer hexagonal boron nitride (h-BN), the monolayer BeO is an insulator and its electrons are highly localized around O and Be atoms (ionic nature). More importantly, the thermal conductivity of monolayer BeO is found to be 266 Wm^-1K^-1 at 300 K, which is lower than that of graphene and h-BN but higher than most other 2D materials. Further spectrum analysis reveals that 75% of the thermal conductivity of monolayer BeO is contributed by phonons with a frequency from 0 to 5.4 THz. With the characteristics of wide bandgap and high thermal conductivity, monolayer BeO shows great potential for applications in electronic device packages and Li-ion batteries.

Ab-Initio Study of the Electronic and Magnetic Properties of Boron- and Nitrogen-Doped Penta-Graphene

First-principles calculations were performed to investigate the effects of boron/nitrogen dopant on the geometry, electronic structure and magnetic properties of the penta-graphene system. It was found that the electronic band gap of penta-graphene could be tuned and varied between 1.88 and 2.12 eV depending on the type and location of the substitution. Moreover, the introduction of dopant could cause spin polarization and lead to the emergence of local magnetic moments. The main origin of the magnetic moment was analyzed and discussed by the examination of the spin-polarized charge density. Furthermore, the direction of charge transfer between the dopant and host atoms could be attributed to the competition between the charge polarization and the atomic electronegativity. Two charge-transfer mechanisms worked together to determine which atoms obtained electrons. These results provide the possibility of modifying penta-graphene by doping, making it suitable for future applications in the field of optoelectronic and magnetic devices.

Thermal transport of carbon nanomaterials

The diversity of thermal transport properties in carbon nanomaterials enables them to be used in different thermal fields such as heat dissipation, thermal management, and thermoelectric conversion. In the past two decades, much effort has been devoted to study the thermal conductivities of different carbon nanomaterials. In this review, different theoretical methods and experimental techniques for investigating thermal transport in nanosystems are first summarized. Then, the thermal transport properties of various pure carbon nanomaterials including 1D carbon nanotubes, 2D graphene, 3D carbon foam, are reviewed in details and the associated underlying physical mechanisms are presented. Meanwhile, we discuss several important influences on the thermal conductivities of carbon nanomaterials, including size, structural defects, chemisorption and strain. Moreover, we introduce different nanostructuring pathways to manipulate the thermal conductivities of carbon-based nanocomposites and focus on the wave nature of phonons for controlling thermal transport. At last, we briefly review the potential applications of carbon nanomaterials in the fields of thermal devices and thermoelectric conversion.

Thermoelectric Penta-Silicene with a High Room-Temperature Figure of Merit

Silicon is one of the most frequently used chemical elements of the periodic table in nanotechnology (Goodilin et al., ACS Nano 2019, 13, 10879-10886). Two-dimensional silicene, a silicon analogue of graphene, has been readily obtained to make field-effect transistors since 2015 (Tao et al., Nat. Nanotechnol. 2015, 10, 227; Tsai et al., Nat. Commun. 2013, 4, 1500). Recently, as new members of the silicene family, penta-silicene and its nanoribbon have been experimentally grown on a Ag(110) surface with exotic electronic properties (Cerdá et al., Nat. Commun. 2016, 7, 13076; Sheng et al., Nano Lett. 2018, 18, 2937-2942). However, the thermoelectric performance of penta-silicene has not been so far studied, which would hinder its potential applications of electric generation from waste heat and solid-state Peltier coolers. Based on the Boltzmann transport theory and ab initio calculations, we find that penta-silicene shows remarkable room-temperature figures of merit ZT of 3.4 and 3.0 at the reachable hole and electron concentrations, respectively. We attribute this high ZT to the superior "pudding-mold" electronic band structure and ultralow lattice thermal conductivity. The discovery provides new insight into the transport property of pentagonal nanostructures and highlights the potential applications of thermoelectric materials at room temperature.

Structural stability of single-layer PdSe2 with pentagonal puckered morphology and its nanotubes

Two-dimensional (2D) materials have gained a lot of attention being a new class of materials with unique properties that could influence future technologies. Concomitant computational design and discovery of new two-dimensional materials have therefore become a significant part of modern materials research. The stability of these predicted materials has emerged as the main issue due to drawbacks of the periodic boundary condition approximation that allow one to pass common criteria of stability. Here, based on first-principle calculations, we demonstrate structural stability and instability of several recently proposed 2D materials with pentagonal morphology including the experimentally exfoliated single-layer PdSe2. It is found that an appropriate orientation of the central Pd sublattice with respect to Se2 dimers effectively compensates all mechanical stress and preserves the planar structure of the PdSe2 nanoclusters, while the flakes of all other materials having pentagonal morphology exhibit non-zero curvature induced by excessive interatomic forces. The relative energies of the PdSe2 monolayer and nanotubes per formula unit also confirm that the planar monolayer is a global energy minimum. Like the monolayer, (n,0) PdSe2 tubes are indirect band gap semiconductors with similar band gaps, while (n,n) tubes reveal indirect-direct band gap transitions following the increase of the tube diameter. Small strain energies of large diameter tubes propose their possible experimental realization for various optoelectronic applications.

Thermal transport properties of novel two-dimensional CSe

Recently, as a novel member of the IV-VI group compounds, two-dimensional (2D) buckled monolayer CSe has been discovered for use in high-performance light-emitting devices (Q. Zhang, Y. Feng, X. Chen, W. Zhang, L. Wu and Y. Wang, Nanomaterials, 2019, 9, 598). However, to date, the heat transport properties of this novel CSe is still lacking, which would hinder its potential application in electronic devices and thermoelectric materials that can generate electricity from waste heat. Here we systematically study the heat transport properties of monolayer CSe based on ab initio calculations and phonon Boltzmann transport theory. We find that the lattice thermal conductivity κlat of monolayer CSe is around 42 W m-1 K-1 at room temperature, which is much lower than those of black phosphorene, buckled phosphorene, MoS2, and buckled arsenene. Moreover, the longitudinal acoustic phonon mode contributes the most to the κlat, which is much larger than those of the out-of-plane phonon mode and transverse acoustic branches. The calculated size-dependent κlat shows that the sample size can significantly reduce the κlat of monolayer CSe and can persist up to 10 μm. These discoveries provide new insight into the size-dependent thermal transport in nanomaterials and guide the design of CSe-based low-dimensional quantum devices, such as thermoelectric devices.

Phonon and electron transport in Janus monolayers based on InSe

We systematically investigated the phonon and electron transport properties of monolayer InSe and its Janus derivatives including monolayer In₂SSe and In₂SeTe by first-principles calculations. The breaking of mirror symmetry produces a distinguishable A ₁ peak in the Raman spectra of monolayer In₂SSe and In₂SeTe. The long-range harmonic and anharmonic interactions play an important role in the heat transport of the group-III chalcogenides. The room-temperature thermal conductivity ([Formula: see text]) of monolayer InSe, In₂SSe and In₂SeTe are 44.6, 46.9, and 29.9 W (mK)^-1, respectively. There is a competition effect between atomic mass, phonon group velocity and phonon lifetime. The [Formula: see text] can be further effectively modulated by sample size for the purpose of thermoelectric applications. Meanwhile, monolayer In₂SeTe exhibits a direct band gap of 1.8 eV and a higher electron mobility than that of monolayer InSe, due to the smaller electron effective mass caused by tensile strain on the Se side and smaller deformation potential. These results indicate that 2D Janus group-III chalcogenides can provide a platform to design the new electronic, optoelectronic and thermoelectric devices.

Charge-induced electromechanical actuation of two-dimensional hexagonal and pentagonal materials

Using first-principles calculations, we investigate electromechanical properties of two-dimensional (2D) hexagonal and pentagonal materials as a function of electron and hole dopings, in which 2D materials including graphene, chair-like graphane, table-like graphane, penta-graphene (PG), hydrogenated penta-graphene (HPG), and penta-CN2 are considered. We find that the actuation responses such as actuation strain, stress generated, and work area-density per cycle of the 2D materials in the case of hole doping are substantially larger than those of electron doping. Moreover, the electromechanical properties of the 2D materials can be improved by hydrogenation. In particular, the actuation strain and work area-density per cycle of graphane and HPG are much larger than those of graphene and PG for hole doping, respectively. Interestingly, both the 2D hexagonal and pentagonal materials show an asymmetric dependence of theoretical strength (a maximum value of the stress that the materials can achieve by applying the strain) on the electron and hole dopings. These results provide an important insight into the electromechanical properties of the 2D hexagonal and pentagonal materials, which are useful for artificial muscle applications.

Penta-Graphene as a Potential Gas Sensor for NOx Detection

Two-dimensional (2D) penta-graphene (PG) with unique properties that can even outperform graphene is attracting extensive attention because of its promising application in nanoelectronics. Herein, we investigate the electronic and transport properties of monolayer PG with typical small gas molecules, such as CO, CO₂, NH₃, NO and NO₂, to explore the sensing capabilities of this monolayer by using first-principles and non-equilibrium Green's function (NEGF) calculations. The optimal position and mode of adsorbed molecules are determined, and the important role of charge transfer in adsorption stability and the influence of chemical bond formation on the electronic structure of the adsorption system are explored. It is demonstrated that monolayer PG is most preferred for the NO_x (x = 1, 2) molecules with suitable adsorption strength and apparent charge transfer. Moreover, the current-voltage (I-V) curves of PG display a tremendous reduction of 88% (90%) in current after NO₂ (NO) adsorption. The superior sensing performance of PG rivals or even surpasses that of other 2D materials such as graphene and phosphorene. Such ultrahigh sensitivity and selectivity to nitrogen oxides make PG a superior gas sensor that promises wide-ranging applications.

Symmetry-protected metallic and topological phases in penta-materials

We analyze the symmetry and topological features of a family of materials closely related to penta-graphene, derived from it by adsorption or substitution of different atoms. Our description is based on a novel approach, called topological quantum chemistry, that allows to characterize the topology of the electronic bands, based on the mapping between real and reciprocal space. In particular, by adsorption of alkaline (Li or Na) atoms we obtain a nodal line metal at room temperature, with a continuum of Dirac points around the perimeter of the Brillouin zone. This behavior is also observed in some substitutional derivatives of penta-graphene, such as penta-PC₂. Breaking of time-reversal symmetry can be achieved by the use of magnetic atoms; we study penta-MnC₂, which also presents spin-orbit coupling and reveals a Chern insulator phase. We find that for this family of materials, symmetry is the source of protection for metallic and nontrivial topological phases that can be associated to the presence of fractional band filling, spin-orbit coupling and time-reversal symmetry breaking.

Low Lattice Thermal Conductivity of a Two-Dimensional Phosphorene Oxide

A fundamental understanding of the phonon transport mechanism is important for optimizing the efficiency of thermoelectric devices. In this study, we investigate the thermal transport properties of the oxidized form of phosphorene called phosphorene oxide (PO) by solving phonon Boltzmann transport equation based on first-principles density functional theory. We reveal that PO exhibits a much lower thermal conductivity (2.42–7.08 W/mK at 300 K) than its pristine counterpart as well as other two-dimensional materials. To comprehend the physical origin of such low thermal conductivity, we scrutinize the contribution of each phonon branch to the thermal conductivity by evaluating various mode-dependent quantities including Grüneisen parameters, anharmonic three-phonon scattering rate, and phase space of three-phonon scattering processes. Our results show that its flexible puckered structure of PO leads to smaller sound velocities; its broken-mirror symmetry allows more ZA phonon scattering; and the relatively-free vibration of dangling oxygen atoms in PO gives rise to additional scattering resulting in further reduction in the phonon lifetime. These results can be verified by the fact that PO has larger phase space for three-phonon processes than phosphorene. Furthermore we show that the thermal conductivity of PO can be optimized by controlling its size or its phonon mean free path, indicating that PO can be a promising candidate for low-dimensional thermoelectric devices.

Ultra-low lattice thermal conductivity of monolayer penta-silicene and penta-germanene

We study the lattice thermal conductivity of two-dimensional (2D) pentagonal systems, such as penta-silicene and penta-germanene.

Disparate strain response of the thermal transport properties of bilayer penta-graphene as compared to that of monolayer penta-graphene

In this study, strain modulation of the lattice thermal conductivity of monolayer and bilayer penta-graphene (PG) at room temperature was investigated using first-principles calculations combined with the phonon Boltzmann transport equation.

The effect of oxidation on the electronic properties of penta-graphene: first-principles calculation

Herein, using first-principles calculations, we systematically studied the effect of oxidation on the structural and electronic properties of penta-graphene.

Versatile mechanical properties of novel g-SiC x monolayers from graphene to silicene: a first-principles study

Recently, a series of graphene-like binary monolayers (g-SiC _x ), where Si partly substitutes the C positions in graphene, have been obtained by tailoring the band gaps of graphene and silicene that have made them a promising material for application in opto-electronic devices. Subsequently, evaluating the mechanical properties of g-SiC _x has assumed great importance for engineering applications. In this study, we quantified the in-plane mechanical properties of g-SiC _x (x = 7, 5, 3, 2 and 1) monolayers (also including graphene and silicene) based on density function theory. It was found that the mechanical parameters of g-SiC _x , such as the ideal strength, Young's modulus, shear modulus, Poisson's ratio, as well as fracture toughness, are overall related to the ratio of Si-C to C-C bonds, which varies with Si concentration. However, for g-SiC₇ and g-SiC₃, the mechanical properties seem to depend on the structure because in g-SiC₇, the C-C bond strength is severely weakened by abnormal stretching, and in g-SiC₃, conjugation structure is formed. The microscopic failure of g-SiC _x exhibits diverse styles depending on the more complex structural deformation modes introduced by Si substitution. We elaborated the structure-properties relationship of g-SiC _x during the failure process, and in particular, found that the structural transformation of g-SiC₃ and g-SiC is due to the singular symmetry of their structure. Due to the homogeneous phase, all the g-SiC _x investigated in this study preserve rigorous isotropic Young's moduli and Poisson's ratios. With versatile mechanical performances, the family of g-SiC _x may facilitate the design of advanced two-dimensional materials to meet the needs for practical mechanical engineering applications. The results offer a fundamental understanding of the mechanical behaviors of g-SiC _x monolayers.

Tight-binding model for opto-electronic properties of penta-graphene nanostructures

We present a tight-binding parametrization for penta-graphene that correctly describes its electronic band structure and linear optical response. The set of parameters is validated by comparing to ab-initio density functional theory calculations for single-layer penta-graphene, showing a very good global agreement. We apply this parameterization to penta-graphene nanoribbons, achieving an adequate description of quantum-size effects. Additionally, a symmetry-based analysis of the energy band structure and the optical transitions involved in the absorption spectra is introduced, allowing for the interpretation of the optoelectronic features of these systems.

A C20 fullerene-based sheet with ultrahigh thermal conductivity

A new two-dimensional (2D) carbon allotrope, Hexa-C20, composed of C20 fullerene is proposed. State-of-the-art first principles calculations combined with solving the linearized phonon Boltzmann transport equation confirm that the new carbon structure is not only dynamically and thermally stable, but also can withstand temperatures as high as 1500 K. Hexa-C20 possesses a quasi-direct band gap of 3.28 eV, close to that of bulk ZnO and GaN. The intrinsic lattice thermal conductivity κlat of Hexa-C20 is 1132 W m-1 K-1 at room temperature, which is much larger than those of most carbon materials such as graphyne (82.3 W m-1 K-1) and penta-graphene (533 W m-1 K-1). Further analysis of its phonons uncovers that the main contribution to κlat is from the three-phonon scattering, while the three acoustic branches are the main heat carriers, and strongly coupled with optical phonon branches via an absorption process. The ultrahigh lattice thermal conductivity and an intrinsic wide band gap make the Hexa-C20 sheet attractive for potential thermal management applications.

Anisotropic intrinsic lattice thermal conductivity of borophane from first-principles calculations

Borophene (boron sheet) as a new type of two-dimensional (2D) material was grown successfully recently. Unfortunately, the structural stability of freestanding borophene is still an open issue. Theoretical research has found that full hydrogenation can remove such instability, and the product is called borophane. In this paper, using first-principles calculations we investigate the lattice dynamics and thermal transport properties of borophane. The intrinsic lattice thermal conductivity and the relaxation time of borophane are investigated by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. We find that the intrinsic lattice thermal conductivity of borophane is anisotropic, as the higher value (along the zigzag direction) is about two times of the lower one (along the armchair direction). The contributions of phonon branches to the lattice thermal conductivities along different directions are evaluated. It is found that both the anisotropy of thermal conductivity and the different phonon branches which dominate the thermal transport along different directions are decided by the group velocity and the relaxation time of phonons with very low frequency. In addition, the size dependence of thermal conductivity is investigated using cumulative thermal conductivity. The underlying physical mechanisms of these unique properties are also discussed in this paper.

The conflicting role of buckled structure in phonon transport of 2D group-IV and group-V materials

Controlling heat transport through material design is one important step toward thermal management in 2D materials. To control heat transport, a comprehensive understanding of how structure influences heat transport is required. It has been argued that a buckled structure is able to suppress heat transport by increasing the flexural phonon scattering. Using a first principles approach, we calculate the lattice thermal conductivity of 2D mono-elemental materials with a buckled structure. Somewhat counterintuitively, we find that although 2D group-V materials have a larger mass and higher buckling height than their group-IV counterparts, the calculated κ of blue phosphorene (106.6 W mK^-1) is nearly four times higher than that of silicene (28.3 W mK^-1), while arsenene (37.8 W mK^-1) is more than fifteen times higher than germanene (2.4 W mK^-1). We report for the first time that a buckled structure has three conflicting effects: (i) increasing the Debye temperature by increasing the overlap of the p_z orbitals, (ii) suppressing the acoustic-optical scattering by forming an acoustic-optical gap, and (iii) increasing the flexural phonon scattering. The former two, corresponding to the harmonic phonon part, tend to enhance κ, while the last one, corresponding to the anharmonic part, suppresses it. This relationship between the buckled structure and phonon behaviour provides insight into how to control heat transport in 2D materials.

Novel Two-Dimensional Silicon Dioxide with in-Plane Negative Poisson's Ratio

Silicon dioxide or silica, normally existing in various bulk crystalline and amorphous forms, was recently found to possess a two-dimensional structure. In this work, we use ab initio calculation and evolutionary algorithm to unveil three new two-dimensional (2D) silica structures whose thermal, dynamical, and mechanical stabilities are compared with many typical bulk silica. In particular, we find that all three of these 2D silica structures have large in-plane negative Poisson's ratios with the largest one being double of penta graphene and three times of borophenes. The negative Poisson's ratio originates from the interplay of lattice symmetry and Si-O tetrahedron symmetry. Slab silica is also an insulating 2D material with the highest electronic band gap (>7 eV) among reported 2D structures. These exotic 2D silica with in-plane negative Poisson's ratios and widest band gaps are expected to have great potential applications in nanomechanics and nanoelectronics.

Chemical Functionalization of Pentagermanene Leads to Stabilization and Tunable Electronic Properties by External Tensile Strain

Inspired by the unique geometry and novel properties of a newly proposed two-dimensional (2D) carbon allotrope called pentagraphene, we have performed first-principles calculations to study the structural stability and electronic properties of pentagermanene (pGe) modulated by chemical functionalization and biaxial tensile strain. It is observed that the 2D pGe is energetically unfavorable. However, the 2D pentagonal nanosheets can be stabilized by both hydrogenation and fluorination. Phonon dispersion spectrum and ab initio molecular dynamics simulations demonstrated that the dynamic and thermal stabilities of the two functionalized pGe nanostructures can be maintained even under a high temperature of 500 K. Our calculations revealed that both hydrogenated and fluorinated-pentagonal germanenes are semiconductors with indirect band gaps of 1.92 and 1.39 eV (2.60 and 2.09 eV by the hybrid functional), respectively. The electronic structures of the functionalized pGes can be effectively modulated by biaxial tensile strain, and an indirect to direct gap transition can be achieved for the hydrogenated pGe sheet by 6% biaxial strain. Moreover, the band gap of the hydrogenated pGe could be further tailored from 0.71 to 3.46 eV (1.16-4.35 eV by the hybrid functional) by heteroatom doping (C/Si/Sn/Pb), suggesting the semiconductor-insulator transition for differently doped nanostructures. As a result, the functionalized pGes are expected to have promising applications in nanoelectronics and nanomechanics.

Penta-graphene: A Promising Anode Material as the Li/Na-Ion Battery with Both Extremely High Theoretical Capacity and Fast Charge/Discharge Rate

Recently, a new two-dimensional (2D) carbon allotrope named penta-graphene was theoretically proposed ( Zhang , S. ; et al. Proc. Natl. Acad. Sci. U.S.A. 2015 , 112 , 2372 ) and has been predicted to be the promising candidate for broad applications due to its intriguing properties. In this work, by using first-principles simulation, we have further extended the potential application of penta-graphene as the anode material for a Li/Na-ion battery. Our results show that the theoretical capacity of Li/Na ions on penta-graphene reaches up to 1489 mAh·g^-1, which is much higher than that of most of the previously reported 2D anode materials. Meanwhile, the calculated low open-circuit voltages (from 0.24 to 0.60 V), in combination with the low diffusion barriers (≤0.33 eV) and the high electronic conductivity during the whole Li/Na ions intercalation processes, further show the advantages of penta-graphene as the anode material. Particularly, molecular dynamics simulation (300 K) reveals that Li ion could freely diffuse on the surface of penta-graphene, and thus the ultrafast Li ion diffusivity is expected. Superior performance of penta-graphene is further confirmed by comparing with the other 2D anode materials. The light weight and unique atomic arrangement (with isotropic furrow paths on the surface) of penta-graphene are found to be mainly responsible for the high Li/Na ions storage capacity and fast diffusivity. In this regard, except penta-graphene, many other recently proposed 2D metal-free materials with pentagonal Cairo-tiled structures may be the potential candidates as the Li/Na-ion battery anodes.

Band gap engineering in penta-graphene by substitutional doping: first-principles calculations

Using density functional theory, we study the structure, electronic properties and partial charges of a new carbon allotrope-penta-graphene (PG)-substitutionally doped by Si, B and N. We found that the electronic bandgap of PG can be tuned down to 0.2 eV due to carbon substitutions. However, the value of the band gap depends on the type and location of the dopants. For example, the strongest reduction of the band gap is obtained for Si substitutions on the top (bottom) plane of PG, whereas the substitution in the middle plane of PG has a smaller effect on the band gap of the material. Surface termination with fluorine and hydroxyl groups results in an increase of the band gap together with considerable changes in electronic and atomic partial charge distribution in the system. Our findings, which are robust against the use of different exchange-correlation functionals, indicate the possibility of tuning the bandgap of the material to make it suitable for optoelectronic and photovoltaic applications.

First-Principles Prediction of Ultralow Lattice Thermal Conductivity of Dumbbell Silicene: A Comparison with Low-Buckled Silicene

The dumbbell structure of two-dimensional group IV material offers alternatives to grow thin films for diverse applications. Thermal properties are important for these applications. We obtain the lattice thermal conductivity of low-buckled (LB) and dumbbell (DB) silicene by using first-principles calculations and the Boltzmann transport equation for phonons. For LB silicene, the calculated lattice thermal conductivity with naturally occurring isotope concentrations is 27.72 W/mK. For DB silicene, the calculated value is 2.86 W/mK. The thermal conductivity for DB silicene is much lower than LB silicene due to stronger phonon scattering. Our results will induce further theoretical and experimental investigations on the thermoelectric (TE) properties of DB silicene. The size-dependent thermal conductivity in both LB and DB silicene is investigated as well for designing TE devices. This work sheds light on the manipulation of phonon transport in two-dimensional group IV materials by dumbbell structure formed from the addition of adatoms.