Improved Carrier Separation and Recombination by Ferroelectric Polarization in the CuBiP2Se6/C2N Heterostructure: A Nonadiabatic Molecular Dynamics Study.

The rapid recombination of photogenerated carriers heavily restricts the photocatalytic efficiency. Here, we propose a new strategy to improve catalytic efficiency based on the ferroelectric van der Waals heterostructure (CuBiP2Se6/C2N). Combining density functional theory and the nonadiabatic molecular dynamics (NAMD) method, we have systematically analyzed the ground-state properties and carrier dynamics images in the CuBiP2Se6/C2N heterostructure. Our calculations showed that the ferroelectric polarization of CuBiP2Se6 provides the internal driving force for the photogenerated carriers separation. NAMD results demonstrate that the excited-state carrier transfer and recombination processes in the CuBiP2Se6/C2N are consistent with a type II mechanism. Meanwhile, constructing the ferroelectric heterostructure can effectively prolong the carrier lifetime, from ∼65.98 to ∼124.54 ps. Moreover, the high quantum efficiency and tunable band edge positions mean that the CuBiP2Se6/C2N heterostructure is an excellent potential candidate material for photocatalytic water splitting.

Ferroelectric Polarization and Single-Atom Catalyst Synergistically Promoting CO2 Photoreduction of CuBiP2Se6.

Further improving the activity and selectivity of photocatalytic CO2 reduction remains a challenge. Herein, we propose a new strategy for synergistically promoting photocatalytic CO2 reduction by combining two-dimensional (2D) ferroelectric polarization and single-atom catalysis. Our calculations showed that the ferroelectric polarization of CuBiP2Se6 provides the internal driving force for the separation and migration of photogenerated carriers, which provides a prerequisite for enhancing the photocatalytic efficiency. In addition, the introduction of single Ag atoms can act as an electron reservoir to significantly modify the bonding configurations on the surface through proper static electron transfer, thus effectively promoting the adsorption and activation of CO2 molecules. More importantly, we found that switching the ferroelectric polarization can synergistically optimize the limiting potential as well as control the final products. This study provides a new approach for enhancing the catalytic activity and selectivity of photocatalytic CO2 reduction.

The effect of side substitution and quantum interference on the performance of molecular thermoelectric devices: a brief review.

In recent years, researchers have shown great interest in organic thermoelectric materials that are economical, efficient, lightweight, and environmentally friendly. With advancements in experimental measurement techniques and theoretical calculations, investigations of the thermoelectric properties of molecular devices have become feasible. To regulate the thermoelectric properties of molecular devices, many strategies have been proposed. In this work, we review the theoretical analytical and experimental research methods used to study these properties. We then focus on two tuning strategies, side substitution, and quantum interface effects, which have demonstrated significant improvements in the thermoelectric performance of molecular devices. Finally, we discuss the challenges faced in experimental and theoretical studies and the future prospects of molecular thermoelectric devices.

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

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

Nonrelativistic Spin-Momentum Coupling in Antiferromagnetic Twisted Bilayers.

Spin-momentum coupling, which depends strongly on the relativistic effect of heavy elements in solids, is the basis of many phenomena in spintronics. In this Letter, we theoretically predict nonrelativistic spin-momentum coupling in two-dimensional materials. By proposing magnetic symmetry requirements for spin splitting in two-dimensional systems, we find that a simple twisting operation can realize nonrelativistic spin splitting in antiferromagnetic bilayers. Through first-principles calculations, we demonstrate that momentum-dependent spin splitting exists extensively in antiferromagnetic twisted bilayers with different crystal structures and twist angles. The size of the spin splitting caused by twisting is of the same order of magnitude as that arising from spin-orbit coupling. In particular, a transverse spin current with an extremely high charge-spin conversion ratio can be generated in twisted structures under an external electric field. The findings demonstrate the potential for achieving electrically controlled magnetism in materials without spin-orbit coupling.

Fast carrier diffusion via synergistic effects between lithium-ions and polarons in rutile TiO2.

Carrier mobility in titanium dioxide (TiO2) systems is a key factor for their application as energy materials, especially in solar cells and lithium-ion batteries. Studies on the diffusion of Li-ions and polarons in rutile TiO2 systems have attracted extensive attention. However, how their interaction affects the diffusion of Li-ions and electron polarons is largely unclear and related studies are relatively lacking. By using first-principles calculations, we systematically investigate the interaction between the intercalated Li-ions and electron polarons in rutile TiO2 materials. Our analysis shows that the diffusion barrier of the electron polarons decreases around the Li-ion. The interaction between the Li-ions and polarons would benefit their synergistic diffusion both in the pristine and defective rutile TiO2 systems. Our study reveals the synergistic effects between the ions and polarons, which is important for understanding the carrier properties in TiO2 systems and in further improving the performance of energy materials.

Origin of Broadband Emission and Large Stokes Shift in Antimony Trisulfide.

The antimony trisulfide (Sb2S3) has been theoretically predicted to have various merits in exploiting high-performance thin-film solar cells and attracted intense attention. However, the power conversion efficiency of Sb2S3-based solar cells is yet to be satisfactory in experiments and the origin of large open circuit voltage (VOC) loss is still a controversial question. Based on first-principles calculations, we have systematically analyzed the excited state behavior and dynamics images of carriers in Sb2S3 materials. Our calculations showed that intrinsic defects like vacancy (VSb and VS) and antisites (SbS and SSb) are energetically accessible. More importantly, we found that the sulfide vacancy-bound excitons can produce a large Stokes shift of ∼0.66 eV, which could well rationalize the experimental observations like the reduction of VOC. These new findings suggest that the performance of Sb2S3-based solar cells might be largely enhanced by avoiding sulfide vacancy defects.

A first-principles study of exciton self-trapping and electric polarization in one-dimensional organic lead halide perovskites.

Revealing the origin of self-trapped excitons is a prerequisite for further improving the photoluminescence efficiency of low-dimensional organic perovskites. Here, the microscopic formation mechanism of intrinsic self-trapped excitons in one-dimensional (1D) C4N2H14PbX4 (X = Cl, Br and I) systems is investigated, and the polarization-luminescence relationship is established. Our results show that 1D-C4N2H14PbX4 has a low electronic dimension (flat band characteristics), which facilitates the formation of intrinsic self-trapped excitons. The potential well formed by local distortion of the [PbX6] octahedron is the origin of exciton self-trapping. Combined with the electronic density of states and partial charge density, we further confirmed the existence of intrinsic self-trapping excitons in 1D-C4N2H14PbX4. In addition, we found that the breaking of the central inversion symmetry will induce electric polarization, which greatly improves the transition probability of electrons. These results could potentially offer a new direction for improving the luminescence properties of 1D organic lead halide perovskites.

Perfect spin-filtering effect in molecular junctions based on half-metallic penta-hexa-graphene nanoribbons.

The design and control of spintronic devices is a research hotspot in the field of electronics, and pure carbon-based materials provide new opportunities for the construction of electronic devices with excellent performance. Using density functional theory in combination with nonequilibrium Green's functions method, we design spin filter devices based on Penta-hexa-graphene (PHG) nanoribbons-a carbon nanomaterial in which the intrinsic magnetic moments combines with edge effects leading to a half-metallic property. Spin-resolved electronic transport studies show that such carbon-based devices can achieve nearly 100% spin filtering effect at low bias voltages. Such SEF can resist the influence of hydrogen passivation at different positions, but hardly survive under a hydrogen-rich environment. Our analysis show that the perfect SEF transport properties are caused by the magnetic and electronic properties of PHG nanoribbons, especially the magnetic moments on the quasi-sp3carbons. These interesting results indicate that PHG nanomaterials have very prominent application prospects in future spintronic devices.

Nonequilibrium Green's function method for phonon heat transport in quantum system.

Phonon heat transport property in quantum devices is of great interesting since it presents significant quantum behaviors. In the past few decades, great efforts have been devoted to establish the theoretical method for phonon heat transport simulation in nanostructures. However, modeling phonon heat transport from wavelike coherent regime to particlelike incoherent regime remains a challenging task. The widely adopted theoretical approach, such as molecular dynamics, semiclassical Boltzmann transport equation, captures quantum mechanical effects within different degrees of approximation. Among them, Non-equilibrium Green's function (NEGF) method has attracted wide attention, as its ability to perform full quantum simulation including many-body interactions. In this review, we summarized recent theoretical advances of phonon NEGF method and the applications on the numerical simulation for phonon heat transport in nanostructures. At last, the challenges of numerical simulation are discussed.

Nuclear Quantum Effects on the Charge-Density Wave Transition in NbX2 (X = S, Se).

Understanding the origin of charge-density wave (CDW) instability is important for manipulating novel collective electronic states. Many layered transition metal dichalcogenides (TMDs) share similarity in the structural and electronic instability, giving rise to diverse CDW phases and superconductivity. It is still puzzling that even isostructural and isoelectronic TMDs show distinct CDW features. For instance, bulk NbSe2 exhibits CDW order at low temperature, while bulk NbS2 displays no CDW instability. The CDW transitions in single-layer NbS2 and NbSe2 are also different. In the classic limit, we investigate the electron correlation effects on the dimensionality dependence of the CDW ordering. By performing ab initio path integral molecular dynamics simulations and comparative analyses, we further revealed significant nuclear quantum effects in these systems. Specifically, the quantum motion of sulfur anions significantly reduces the CDW transition temperature in both bulk and single-layer NbS2, resulting in distinct CDW features in the NbS2 and NbSe2 systems.

A review of ultra-thin ferroelectric films.

Ultrathin ferroelectrics are of great technological interest for high-density electronics, particularly non-volatile memories and field-effect transistors. With the rapid development of micro-electronics technology, there is an urgent requirement for higher density electronic devices, which need ultra-thin ferroelectric materials films. However, as ferroelectric films have becomes thinner and thinner, electrical spontaneous polarization signals have been found in a few atomic layers or even monolayer structures. The mechanisms of detection and formation of these signals are not well understood and various controversial interpretations have emerged. In this review, we summarized the recent research progress in the ultra-thin film ferroelectric material, such as HfO2, CuInP2S6, In2Se3, MoTe2and BaTiO3. Various key aspects of ferroelectric materials are discussed, including crystal structure, ferroelectric mechanism, characterization, fabrication methods, applications, and future outlooks. We hope this review will offer ideas for further improvement of ferroelectric properties of ultra-thin films and promotes practical applications.

Tuning the Electrocatalytic Properties of Black and Gray Arsenene by Introducing Heteroatoms.

On the basis of density functional theory calculations, we explored the catalytic properties of various heteroatom-doped black and gray arsenene toward the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), and the hydrogen evolution reaction (HER). The calculation results show that pristine black (b-As) and gray arsenene (g-As) exhibit poor catalytic performance because of too weak intermediate adsorption. Heteroatom doping plays a key role in optimizing catalytic performance. Among the candidate dopants O, C, P, S, and Sb, O is the most promising one used in arsenene to improve the ORR and OER catalytic performance. Embedding O atoms could widely tune the binding strength of reactive intermediates and improve the catalytic activity. Single O-doped g-AsO 1 can achieve efficient bifunctional activity for both the OER and the ORR with optimal potential gap. b-AsO 1 and b-AsO 2 exhibit the optimal OER and ORR catalytic performance, respectively. For the HER, double C-doped g-AsC 2 could tune the adsorption of hydrogen to an optimal value and significantly enhance the catalytic performance. These findings indicate that arsenene could provide a new platform to explore high-efficiency electrocatalysts.

Giant gauge factor of Van der Waals material based strain sensors.

There is an emergent demand for high-flexibility, high-sensitivity and low-power strain gauges capable of sensing small deformations and vibrations in extreme conditions. Enhancing the gauge factor remains one of the greatest challenges for strain sensors. This is typically limited to below 300 and set when the sensor is fabricated. We report a strategy to tune and enhance the gauge factor of strain sensors based on Van der Waals materials by tuning the carrier mobility and concentration through an interplay of piezoelectric and photoelectric effects. For a SnS2 sensor we report a gauge factor up to 3933, and the ability to tune it over a large range, from 23 to 3933. Results from SnS2, GaSe, GeSe, monolayer WSe2, and monolayer MoSe2 sensors suggest that this is a universal phenomenon for Van der Waals semiconductors. We also provide proof of concept demonstrations by detecting vibrations caused by sound and capturing body movements.

Penta-Hexa-Graphene Nanoribbons: Intrinsic Magnetism and Edge Effect Induce Spin-Gapless Semiconducting and Half-Metallic Properties.

Two-dimensional materials with intrinsic long-range ordered magnetic moments have drawn a lot of attention. However, for practical applications, whether or not the magnetism is stable in their nanostructures has not been revealed. Here, based on the recently proposed magnetic penta-hexa-graphene, we study the electronic and magnetic properties of its nanoribbons (named PHGNRs). The results show that the PHGNRs have intrinsic robust magnetic moments that are different from zigzag graphene nanoribbons, where the magnetic moments caused by the edge effect are vulnerable. Moreover, the magnetic ground states, namely, ferromagnetic (FM) or antiferromagnetic (AFM), can be transformed by changing the width of PHGNRs. Most interestingly, under the FM ground state, the spin-polarized electronic properties reveal that the zigzag PHGNRs transform from spin-gapless semiconductors (SGSs) to half-metals, as the width of nanoribbons increases, while all the armchair PHGNRs are magnetic semiconductors. Furthermore, by considering different edge effects caused by the residual carbon atoms on the edges, the PHGNRs can further derive different types of SGSs, as well as half-metals. Our work suggests that the PHGNRs possessing intrinsic robust magnetic moments have potential applications in the field of spintronic devices.

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

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

Thermal transport properties in monolayer group-IV binary compounds.

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

Thermal Rectification in Asymmetric Graphene/Hexagonal Boron Nitride van der Waals Heterostructures.

Graphene/hexagonal boron nitride (h-BN) heterostructures assembled by van der Waals (vdW) interactions show numerous unique physical properties such as quantum Hall effects and exotic correlated states, which have promising potential applications in the design of novel electronic devices. Understanding thermal transport in such junctions is critical to control the performance and stability of prospective nanodevices. In this work, using nonequilibrium molecular dynamics simulations, we systematically investigate the thermal transport in asymmetric graphene/h-BN vdW heterostructures. It is found that the heat prefers to flow from the monolayer to the multilayer regions, resulting in a significant thermal rectification (TR) effect. To determine the optimum conditions for TR, the influences of sample length, defect density, asymmetric degree, ambient temperature, and vdW interaction strength are studied. Particularly, we found that the TR ratio could be improved by about 1 order of magnitude via increasing the coupling strength from 1 to 10, which clearly distinguishes from the commonly held notion that the TR ratio is practically insensitive or even decreasing with the interaction strength. Detailed spectral analysis reveals that this unexpected increase of the TR ratio can be attributed to heavily modified phonon properties of encased graphene due to enhanced interlayer coupling. Our results elucidate the importance of vdW interactions to heat conduction in nanostructures.

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

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

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.