Structural transition and migration of incoherent twin boundary in diamond.

Grain boundaries (GBs), with their diversity in both structure and structural transitions, play an essential role in tailoring the properties of polycrystalline materials1-5. As a unique GB subset, {112} incoherent twin boundaries (ITBs) are ubiquitous in nanotwinned, face-centred cubic materials6-9. Although multiple ITB configurations and transitions have been reported7,10, their transition mechanisms and impacts on mechanical properties remain largely unexplored, especially in regard to covalent materials. Here we report atomic observations of six ITB configurations and structural transitions in diamond at room temperature, showing a dislocation-mediated mechanism different from metallic systems11,12. The dominant ITBs are asymmetric and less mobile, contributing strongly to continuous hardening in nanotwinned diamond13. The potential driving forces of ITB activities are discussed. Our findings shed new light on GB behaviour in diamond and covalent materials, pointing to a new strategy for development of high-performance, nanotwinned materials.

Superconductivity above 105 K in Nonclathrate Ternary Lanthanum Borohydride below Megabar Pressure.

Hydrides are promising candidates for achieving room-temperature superconductivity, but a formidable challenge remains in reducing the stabilization pressure below a megabar. In this study, we successfully synthesized a ternary lanthanum borohydride by introducing the nonmetallic element B into the La-H system, forming robust B-H covalent bonds that lower the pressure required to stabilize the superconducting phase. Electrical transport measurements confirm the presence of superconductivity with a critical temperature (Tc) of up to 106 K at 90 GPa, as evidenced by zero resistance and Tc shift under an external magnetic field. X-ray diffraction and transport measurements identify the superconducting compound as LaB2H8, a nonclathrate hydride, whose crystal structure remains stable at pressures as low as ∼ half megabar (59 GPa). Stabilizing superconductive stoichiometric LaB2H8 in a submegabar pressure regime marks a substantial advancement in the quest for high-Tc superconductivity in polynary hydrides, bringing us closer to the ambient pressure conditions.

Perylene-based molecular device: multifunctional spintronic and spin caloritronic applications.

Carbon-based magnetic molecular junctions are promising candidates for nanoscale spintronic applications because they are atomically thin and possess high stability and peculiar magnetism. Herein, based on first-principles and non-equilibrium Green's function, we designed a carbon-based molecular spintronic device composed of carbon atomic chains, zigzag-edged graphene nanoribbon (ZGNR), and a perylene molecule. Our results show that the device exhibits integrated spintronic and spin caloritronic functionalities, such as the bias-voltage driven spin filtering effect, negative differential resistance effect and giant magnetoresistance, temperature-gradient driven spin Seebeck effect, thermal spin filtering effect, high thermal magnetoresistance, and thermal colossal giant magnetoresistance. Furthermore, considering the phonon vibration effect, the spin and charge thermoelectric figure of merits (ZTsp and ZTch) can be enhanced and the peak of ZTsp is much larger than that of ZTch, indicating the excellent thermospin performance. The asymmetrical contact configuration between the carbon atomic chain and perylene/ZGNR inhibits the phonon thermal conductivity significantly, leading to the optimal ZTsp and ZTch of 2.4 and 0.5 at 300 K, respectively. These results suggest multifunctional spintronic and spin caloritronic applications for the perylene-based molecular device.

Structural, electronic phase transitions and thermal spin transport properties in 2D NbSe2 and NbS2: a first-principles study.

How to effectively tune 2D electronic and magnetic properties is key to developing novel spintronic materials and devices. Although the strain induced metal-to-half-metal electronic phase transition (EPT) has been studied in 2H NbSe2 and NbS2 monolayers, the 1T phase, the Coulomb interaction and the transport properties have not been explored. Here, using first-principles calculations in junction with nonequilibrium Green's function, we present a comprehensive and comparative study on the strain tuned structural, electronic, magnetic and thermal spin transport properties for NbSe2 and NbS2 monolayers with and without Coulomb interaction. It is found that the Coulomb interaction makes the strain induced 2H-to-1T structural phase transition easier. Similar to the 2H phase, there is also a strain induced metal-to-half-metal EPT for the 1T phase without Coulomb interaction, and the Coulomb interaction makes the ETP easier. Remarkably, the 2H-NbSe2 monolayer with Coulomb interaction is a bipolar spin gapless semiconductor (SGS), and novel Dirac half-metal and usually SGS can be obtained by the tensile strain. In addition, we predict the excellent spin Seebeck effect and thermal spin diode effect in the bipolar SGS of the 2H-NbSe2 monolayer with Coulomb interaction, and expect the spin filtering effect and high magnetoresistance in the half-metals driven by the strain. We also discuss the strength of Coulomb interaction by comparing the theoretical and available experimental electronic states, indicating the indispensability of Coulomb interactions. These results suggest that 2D NbSe2 and NbS2 are promising candidates for phase-change spintronic materials and devices, and will stimulate extensive studies on this class of 2D systems.

Nanocrystalline Cubic Silicon Carbide: A Route to Superhardness.

Superhard materials other than diamond and cubic boron nitride have been actively pursued in the past two decades. Cubic silicon carbide, i.e., ß-SiC, is a well-known hard material with typical hardness <30 GPa. Although nanostructuring has been proven to be effective in enhancing materials' hardness by virtue of the Hall-Petch effect, it remains a significant challenge to improve hardness of ß-SiC beyond the superhard threshold of 40 GPa. Here, the fabrication of nanocrystalline ß-SiC bulks is reported by sintering nanoparticles under high pressure and high temperature. These ß-SiC bulks are densely sintered with average grain sizes down to 10 nm depending on the sintering conditions, and the Vickers hardness increases with decreasing grain size following the Hall-Petch relation. Particularly, the bulk sintered under 25 GPa and 1400 °C shows an average grain size of 10 nm and an asymptotic Vickers hardness of 41.5 GPa. Boosting the hardness of ß-SiC over the superhard threshold signifies an important progress in superhard materials research. A broader family of superhard materials is in sight through successful implementation of nanostructuring in other hard materials such as BP.

A strain-induced considerable decrease of lattice thermal conductivity in 2D KAgSe with Coulomb interaction.

Based on first-principles calculations in combination with the Boltzmann transport theory, we investigate the effects of onsite Coulomb interaction and strain on the lattice thermal conductivity of the KAgSe monolayer, a recently discovered 2D thermoelectric system with a low lattice thermal conductivity when the onsite Coulomb interaction was not considered (X. Zhang, C. Liu, Y. Tao, Y. Li, Y. Guo, Y. Chen, X. C. Zeng and J. Wang, Adv. Funct. Mater., 2020, 30, 2001200). Our calculations reveal that the onsite Coulomb interaction leads to an increase in the lattice thermal conductivity from 1.22 to 1.82 W m-1 K-1 at room temperature due to the increased phonon group velocity and relaxation time. However, with onsite Coulomb interaction, small 3% biaxial tensile strain can give rise to a 75% considerable decrease in the lattice thermal conductivity at room temperature, from 1.82 to 0.45 W m-1 K-1, which is also much lower than the lattice thermal conductivity of 1.22 W m-1 K-1 without onsite Coulomb interaction and strain. The strain induced decrease of phonon group velocity and enhancement of lattice anharmonicity (large Grüneisen parameter and phase space volume) are responsible for the reduced lattice thermal conductivity. The present work highlights that the onsite Coulomb interaction is indispensable when determining the lattice thermal conductivity of 2D KAgSe, and small tensile strain can greatly decrease the lattice thermal conductivity.

A VSi2P4/FeCl2 van der Waals heterostructure: a two-dimensional reconfigurable magnetic diode.

Reconfigurable magnetic tunnel diodes have recently been proposed as a promising approach to decrease the base collector leakage currents. However, conventional bulk interfaces usually suffer from strong Fermi level pinning, making it difficult to miniaturize spintronic devices. Fortunately, 2D van der Waals (vdW) systems with ultra-clean interfaces and without dangling bonds can solve this problem. Inspired by the recently discovered novel electronic states of type-II spin gapless semiconductor in 2D VSi2P4 and half-metal in 2D FeCl2, we propose the VSi2P4/FeCl2 vdW heterostructure, and investigate the interface Schottky barrier and the bias-voltage-dependent spin transport properties by using density functional theory and the nonequilibrium Green's function. The most stable vdW interface is determined from the possible twelve interfaces with different stacking sequences and rotation angles. The interface Schottky barrier is beneficial to electrons moving in the spin-down channel due to the Ohmic contact. The heterostructure exhibits a huge rectification ratio (up to 2.9 × 105%) and an excellent spin filtering effect with zero threshold bias voltage, which are explained in terms of the spin-dependent band structure, transmission spectrum and transmission path. These results indicate the promising applications of the VSi2P4/FeCl2 vdW heterostructure as a 2D reconfigurable magnetic diode and a spin filter with miniaturization and low energy consumption.

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.

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.

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 GeP3, 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.

Sodium bismuth dichalcogenides: candidates for ferroelectric high-mobility semiconductors for multifunctional applications.

The combination of ferroelectricity with narrow-gap high-mobility semiconductors may not only entail both functions of nonvolatile memory and efficient manipulation of signals, but may also facilitate efficient ferroelectric photovoltaics and thermoelectrics. However, these applications are hindered by the wide gap and poor mobility of current ferroelectrics. A recent study (J. Am. Chem. Soc., 2018, 140, 3736) reported a facile, general, low-temperature, and size tunable solution phase synthesis of NaBiS2 and NaBiSe2 that are made of relatively abundant or biocompatible elements, which enables their large-scale practical applications. Herein we show first-principles evidence of their ferroelectricity with a large polarization (∼33 µC cm-2), a moderate bandgap (∼1.6 eV) and a high electron-mobility (∼104 cm2 V-1 s-1). Although they have a relatively small switching barrier, their ferroelectricity can be robust under ambient conditions with enhanced polarization upon either application of a small tensile strain or ion doping, where distortion can be increased and multiferroics may also be obtained, despite reduced mobility. Considering previous reports on photovoltaics and thermoelectrics of similar compounds, sodium bismuth dichalcogenides might be tuned for higher performance with the coexistence of these desirable properties.

Theoretical investigation of the valence states in Au via the Au-F compounds under high pressure.

In addition to the known Au3+ and Au5+, it has recently been shown that Au is likely to possess unusual valence states in compressed Au-F compounds. However, our simulations reveal that polymeric ground-state AuF4 shows an unexpected 6-fold coordination rather than a 4-fold one, indicating that more complete comprehending on the anomalous Au4+ is highly required. To fully understand the nature and origin of anomalous valence states in Au, we have extensively investigated the ground-state structures of Au-F compounds at high pressures using quantum mechanical computational methods. As a consequence, we identify several previously unreported (stable) AuF2, AuF3 and AuF4 structures. Our results extend the known polymorphism of AuFn compounds and offer a fundamental understanding of the origin of unusual valence states in Au that prevail at high pressure.

The half-metallicity and the spin filtering, NDR and spin Seebeck effects in 2D Ag-doped SnSe2 monolayer.

Two-dimensional SnSe2 has become more and more attractive due to the excellent electronic, optoelectronic, and thermoelectric properties. However, the study on magnetic properties is rare. Inspired by the recent experimental synthesis of SnSe2 monolayer and Ag-doped SnSe2 thin films, we use the first-principles calculations combined with the nonequilibrium Green's function method to investigate the structural, electronic, magnetic, and spin transport properties of an Ag-doped SnSe2 monolayer. It is found that the doped system exhibits half-metallic ferromagnetism with the energy gap of about 0.5 eV in the spin-down channel. The spin-polarized transport properties based on Ag-doped SnSe2 monolayers show an excellent spin filtering effect and a negative differential resistance effect under a bias voltage. Interestingly, under a temperature gradient, the spin Seebeck effect and the temperature-controlled reverse of spin polarization are also observed. These perfect spin transport properties can be understood from the calculated spin-polarized band structure and the spin-polarized transport spectrum. These studies indicate the potential spintronic and spin caloritronic applications for Ag-doped SnSe2 monolayer.

Strain-induced enhancement of thermoelectric performance of TiS2 monolayer based on first-principles phonon and electron band structures.

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI2-type) structural TiS2 monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS2 monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.

Novel carbon polymorphs with cumulative double bonds in three-dimensional sp-sp2 hybrid framework.

A conspicuous amount of theoretical study has been published on the properties of carbon allotropes with alternate single and triple bonds, (-C[triple bond, length as m-dash]C-)n. However, theoretical characterizations of carbon allotropes with cumulative double bonds ([double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash])n is almost non-existent in literature. Based upon first-principles calculations, two new three-dimensional (3D) microporous carbon allotropes consisting of whorl chains connected by cumulative double bonds in a sp-sp2 hybrid framework have been proposed in this study. One of these structures, namely, Trig-C9 was obtained by an evolutionary particle swarm structural search, while the other structure, denoted as Trig-C15, was obtained by inserting double bonds into Trig-C9. Both the 3D sp-sp2 hybridized carbons have a trigonal structure with 9 and 15 atoms in the hexagonal primitive cells. The calculated results demonstrate that these polymorphs are thermodynamically, mechanically, and dynamically feasible. Trig-C9 and Trig-C15 are indirect semiconductors with band gaps of 2.70 eV and 1.25 eV, respectively. Their unique frameworks render them mechanical ductility and significant elastic anisotropy. These results open up new horizons for the exploration of new carbon phases with unique structural, mechanical, and electronic properties.

Chemically Functionalized Phosphorene: Two-Dimensional Multiferroics with Vertical Polarization and Mobile Magnetism.

In future nanocircuits based on two-dimensional (2D) materials, the ideal nonvolatile memories (NVMs) would be based on 2D multiferroic materials that can combine both efficient ferroelectric writing and ferromagnetic reading, which remain hitherto unreported. Here we show first-principles evidence that a halogen-intercalated phosphorene bilayer can be multiferroic with most long-sought advantages: its "mobile" magnetism can be controlled by ferroelectric switching upon application of an external electric field, exhibiting either an "on" state with spin-selective and highly p-doped channels, or an "off" state, insulating against both spin and electron transport, which renders efficient electrical writing and magnetic reading. Vertical polarization can be maintained against a depolarizing field, rendering high-density data storage possible. Moreover, all those functions in the halogenated regions can be directly integrated into a 2D phosphorene wafer, similar to n/p channels formed by doping in a silicon wafer. Such formation of multiferroics with vertical polarization robust against a depolarizing field can be attributed to the unique properties of covalently bonded ferroelectrics, distinct from ionic-bonded ferroelectrics, which may be extended to other van der Waals bilayers for the design of NVM in future 2D wafers. Every intercalated adatom can be used to store one bit of data: "0" when binding to the upper layer and "1" when binding to the down layer, giving rise to a possible approach of realizing single atom memory for high-density data storage.

Control over the magnetism and transition between high- and low-spin states of an adatom on trilayer graphene.

Using density-functional theory, we investigate the electronic and magnetic properties of an adatom (Na, Cu and Fe) on ABA- and ABC-stacked (Bernal and rhombohedral) trilayer graphenes. In particular, we study the influence of an applied gate voltage on magnetism, as it modifies the electronic states of the trilayer graphene (TLG) as well as changes the adatom spin states. Our study performed for a choice of three different adatoms (Na, Cu, and Fe) shows that the nature of adatom-graphene bonding evolves from ionic to covalent in moving from an alkali metal (Na) to a transition metal (Cu or Fe). Applying an external electric field (EEF) to TLG systems with different stacking orders results in the transition between high- and low-spin states in the latter case (Cu, Fe) and induces a little of magnetism in the former (Na) without magnetism in the absence of an external electric field. Our study would be useful for controlled adatom magnetism and (organic) spintronic applications in nanotechnology.

Half-metallic YN2 monolayer: dual spin filtering, dual spin diode and spin Seebeck effects.

Most of the pristine graphene-like two-dimensional materials have been found to be non-magnetic, and the emergence of magnetism usually needs an external electric field, substrate, strain, vacancy, or doping, which is not easily controlled in an experiment, limiting the potential applications in spintronics. Very recently, layered transition-metal dinitrides were explored experimentally and theoretically, and a pristine YN2 monolayer was predicted to be a half-metallic ferromagnet with a graphene-like structure. To demonstrate the possible spintronic applications, herein, we designed spintronic devices based on the half-metallic YN2 monolayer, and found perfect dual spin filtering and dual spin diode effects when a bias voltage was applied. Moreover, the devices also exhibited excellent spin Seebeck effects under a temperature gradient, which make the YN2 monolayer a promising candidate for both spintronic and spin caloritronic applications. These peculiar spin transport properties were analyzed and explained from the calculated spin-resolved band structure and transmission spectrum based on first-principles combined with the non-equilibrium Green's function method.

Polymerization of Acetonitrile via a Hydrogen Transfer Reaction from CH3 to CN under Extreme Conditions.

Acetonitrile (CH3 CN) is the simplest and one of the most stable nitriles. Reactions usually occur on the C≡N triple bond, while the C-H bond is very inert and can only be activated by a very strong base or a metal catalyst. It is demonstrated that C-H bonds can be activated by the cyano group under high pressure, but at room temperature. The hydrogen atom transfers from the CH3 to CN along the CH⋅⋅⋅N hydrogen bond, which produces an amino group and initiates polymerization to form a dimer, 1D chain, and 2D nanoribbon with mixed sp(2) and sp(3) bonded carbon. Finally, it transforms into a graphitic polymer by eliminating ammonia. This study shows that applying pressure can induce a distinctive reaction which is guided by the structure of the molecular crystal. It highlights the fact that very inert C-H can be activated by high pressure, even at room temperature and without a catalyst.