Cu- and Al-Decorated Monolayer TiSe2 for Enhanced Gas Detection Sensitivity: A DFT Study.

The rapid industrial development has contributed to worsening global pollution, necessitating the urgent development of highly sensitive, cost-effective, and portable gas sensors. In this work, the adsorption of CO, CO2, H2S, NH3, NO, NO2, O2, and SO2 gas molecules on pristine and Cu- and Al-decorated monolayer TiSe2 has been investigated based on first-principles calculations. First, the results of the phonon spectrum and ab initio molecular dynamics simulations demonstrated that TiSe2 is dynamically stable. In addition, compared to pristine TiSe2 (-0.029 to -0.154 eV), the adsorption energy of gas molecules (excluding CO2) significantly decreased after decorated with Cu or Al (-0.212 to -0.977 eV in Cu-decorated TiSe2, -0.438 to -2.896 eV in Al-decorated TiSe2). Among them, NH3 and NO2 have the lowest adsorption energies in Cu and Al-decorated TiSe2, respectively. Further research has shown that the decrease in adsorption energy of gas molecules is mainly due to orbital hybridization and charge transfer between decorated Cu and Al atoms and gas molecules. These findings suggest that TiSe2 decorated with Cu and Al can effectively improve its sensitivity to NH3 and NO2, respectively, making it promising in gas sensing applications.

Extremely promising monolayer materials with robust ferroelectricity and extraordinary piezoelectricity: δ-AsN, δ-SbN, and δ-BiN.

Low-dimensional ferroelectric materials, the mainstay of current high-density non-volatile memory devices, sensors, and nanoscale electronics, have attracted tremendous attention recently. Through employing an evolutionary algorithm and first-principles calculations, we report three novel and stable two-dimensional (2D) ferroelectric materials δ-AsN, δ-SbN, and δ-BiN with spontaneous polarization of up to 5.72 × 10-10, 5.20 × 10-10, and 4.45 × 10-10 C m-1, respectively. The ab initio molecular dynamics (AIMD) simulations further show that the failure temperature of ferroelectricity of δ-AsN, δ-SbN, and δ-BiN is as high as 2000 K, 2000 K, and 1700 K, demonstrating their strong robustness. More interestingly, the three novel materials also exhibit extraordinary piezoelectricity with relaxation ion piezoelectric coefficients d11 of 5.39, 19.55, and 43.87 pm V-1, respectively. The external strain effect found can effectively modulate their spontaneous polarization, ferroelectric switching energy barrier, and piezoelectric properties. These fascinating ferroelectricity and piezoelectricity features endow δ-AsN, δ-SbN, and δ-BiN with significant potential application in future miniaturized and integrated multi-functional electronic devices.

Strain induced magnetic hysteresis in MoS2 and WS2 monolayers with symmetric double sulfur vacancy defects.

It has been found that magnetism in two-dimensional (2D) transition metal dichalcogenides can be realized by properly introducing vacancies and applying strain. However, no work has clearly clarified the modulation of such 2D magnetism under a sweeping strain. Thus we were motivated in this work to investigate the mechanical and electronic properties of the monolayer MS2 (M = Mo, W) with symmetric S vacancy defects under sweeping strain. The results show that the local structure of the M atoms in MS2 around the defect undergoes a reversible phase transition from a triangular shape (Tri-3M) with short M-M bonds, to a circular one (Cir-6M-12S) with larger M-M bonds as the planar strain increases. The critical tensile strain for the transition from Tri-3M to Cir-6M-12S are 12.53% for MoS2 and 11.46% for WS2, while the critical compressive strain for the reversal from Cir-6M-12S to Tri-3M are -3.60% and -2.16%, respectively. In particular, we find that the magnetism can be continuously modulated and undergoes a hysteresis loop behavior under the sweeping strains, with the residual magnetism being 2 µB. Our work theoretically predicts the promising prospect for exploring low-dimensional semiconductor spintronic devices working without applying a magnetic field.

Robust pure spin current induced by the photogalvanic effect in half-silicane with spatial inversion symmetry.

The spin-dependent photogalvanic (PG) effect in low-dimensional spin semiconductors has attracted great interest recently. Here, we have studied the spin semiconducting feature and spin-dependent photocurrent in a two-dimensional (2D) silicene-based device with spatial inversion symmetrical half-hydrogenation, in which half of the silicene is hydrogenated on the upper surface and half is hydrogenated on the lower surface. Because of the unique spin semiconductor properties and symmetry of the system, pure spin current can be robustly produced in both the zigzag and armchair directions for linearly and elliptically polarized light. The behavior of the spin-dependent photoresponse in the spin PG effect is highly anisotropic and can be tuned by the polarization/phase angles or photon energy (Eph). Moreover, the produced pure spin current in such a half-silicane device with spatial inversion symmetry via the PG effect is several orders of magnitude larger than that obtained in metal/semiconductor/metal systems. These findings suggest a promising approach for generating pure spin current by the PG effect and provide a new possibility for the application of 2D half-silicane in spintronics.

High-Throughput Screening of Two-Dimensional Planar sp2 Carbon Space Associated with a Labeled Quotient Graph.

The configurational space of two-dimensional planar sp2 carbon has been systematically scanned by a random strategy combined with group and graph theory, and 1114 new carbon allotropes have been identified. These allotropes are energetically more favorable than most of the previously predicted 120 carbon allotropes. By fitting the HSE06 band structures of six old structures, we optimize the parameters for a general and transferable tight-binding model for high-throughput band structure calculations. We identified that there are 190 Dirac semimetals, 241 semiconductors, and 683 normal metals among the new allotropes. Interestingly, several stable low-energy carbon systems with exotic electronic properties are proposed, such as type III, type I/II mixed, and type I/III mixed semimetals, which are very rare in planar carbon systems. In particular, one nodal-line semimetal has been discovered among these thousands of allotropes, which is the first nodal-line semimetal in sp2 carbon systems. Our discoveries greatly enrich our knowledge of the structures and electronic properties of the two-dimensional carbon family.

New Two-Dimensional Wide Band Gap Hydrocarbon Insulator by Hydrogenation of a Biphenylene Sheet.

Based on first-principles calculations, the ground state configuration (Cmma-CH) of a hydrogenated biphenylene sheet ( Science 2021, 372, 852) is carefully identified from hundreds of possible candidates generated by RG2 code ( Phys. Rev. B. 2018, 97, 014104). Cmma-CH contains four inequivalent benzene molecules in its crystalline cell due to its Cmma symmetry. Hydrogen atoms bond to carbon atoms in each benzene with a boat-like (DDUDDU) up/down sequence and reversed boat-1 (UUDUUD) sequence in adjacent benzene rings. Cmma-CH is energetically less stable than the proposed allotropes of hydrogenated graphene, but the formation energy for hydrogenating a biphenylene sheet is remarkably lower than that for hydrogenating graphene to graphane. Our results of mechanical and dynamical stability also confirm that Cmma-CH is a stable 2D hydrocarbon, which is expected to be realized experimentally. Especially, biphenylene undergoes a transition from normal metal to a wide band gap insulator (4.645 eV) by hydrogenation to Cmma-CH, which has potential applications in nanodevices at elevated temperatures and high voltages.

Tunable topologically nontrivial states in newly discovered graphyne allotropes: from Dirac nodal grid to Dirac nodal loop.

By means of quotient-graph associated crystal prediction method, a new graphyne allotrope with unique Dirac nodal grid state is reported in this work. It is named as 191-E24Y24-1 according to its hexagonal lattice (with P6/mmm symmetry, No. 191) containing 24 sp2-hybridized carbon atoms and 24 sp-hybridized ones. The first-principles results show that the total energy of 191-E24Y24-1 is more favorable than that of recent synthesizedß-graphdiyne and carbon ene-yne. It is also demonstrated to be dynamically, thermally, and mechanically stable. Interestingly, the 191-E24Y24-1 harbors intrinsic semimetal features showing intriguing hexagonal Dirac nodal grid state in the reciprocal space. Such unique electronic state is stable against small external tensile strains, and it is tunable under compression strains which will transform to new triangle Dirac nodal grid state. Moreover, a new metastable graphyne allotrope named 191-E12Y36-4 with Dirac nodal loop state is also observed in the process of stretching 191-E24Y24-1 with large tensile strains. The results presented in this work reveal two novel graphyne allotropes with exotic electronic properties. These discoveries are not only physical interesting, but also provide potential material candidates for carbon-based high performance electronic nanodevices.

Newly discovered graphyne allotrope with rare and robust Dirac node loop.

Two-dimensional (2D) carbon allotropes with topologically nontrivial states are drawing considerable attention owing to their unique physical properties and great potential applications in the next generation of micro-nano devices. In contrast to the numerous Dirac points predicted in 2D carbon allotropes, systems featuring Dirac nodal lines (loops) are still quite rare. Here, by means of first-principles calculation, we report our newly discovered carbon monolayer 123-E8Y24-1 with robust Dirac nodal line states, which possesses a tetragonal lattice with P4/mmm symmetry and contains 8 sp2 carbon atoms (graphene: E8) and 24 sp carbon atoms (grapheyne: Y24) in the crystalline cell. This 2D material is as energetically stable as the recently experimentally synthesized ß-graphdiyne, and it is further predicted to be dynamically, mechanically, and also thermodynamically stable. Owing to its intrinsic geometric characteristics, 123-E8Y24-1 also exhibits obvious Young's modulus anisotropy, with a sizable ratio between the maximum and minimum value of up to 5.8. Remarkably, 123-E8Y24-1 presents a semimetal nature and possesses Dirac nodal line states in the electronic band structure, and such behavior could be kept well under external strain between -10.0% and 8.0%. The electronic properties of 123-E8Y24-1 can be carefully confirmed by constructing a tight-binding (TB) model. The findings presented in this paper reveal a novel 2D Dirac nodal loop carbon sheet, providing a new candidate for carbon-based high-speed electronic devices.

Two-Dimensional Carbon Allotropes and Nanoribbons based on 2,6-Polyazulene Chains: Stacking Stabilities and Electronic Properties.

The previously predicted phagraphene [Wang et al., Nano Lett. 15, 6182 (2015)] and a recently proposed TPH-graphene have been synthesized from fusion of 2,6-polyazulene chain (5-7 chain) in a recent experiment [Fan et al., J. Am. Chem. Soc., 141, 17713 (2019)]. Theoretically, phagraphene and TPH-graphene can be considered as the combinations of the 5-7 chains with distinct 6-6-6 and 4-7-7 interfacial stacking manners, respectively. In this work, we propose another new graphene allotrope, named as penta-hex-hepta-graphene (PHH-graphene), which can be constructed by coupling the synthesized 5-7 chains with a new type of 5-7-6 stacking interface. It is found that the PHH-graphene is dynamically and thermally stable, and especially notable, the total energy of PHH-graphene is lower than that of synthesized TPH-graphene. Thus, it is highly possible that PHH-graphene can be realized through assembly of 5-7 chains. We have systematically investigated the electronic properties of these three graphene allotropes and their nanoribbons. The results show that PHH-graphene is a type-I semimetal with a highly anisotropic Dirac cone similar to phagraphene, while TPH-graphene is a metal. Their nanoribbons exhibit different electronic band structures as the number (n) of 5-7 chains increases. For TPH-graphene nanoribbons, they become metal rapidly as n ≥ 2. The nanoribbons of the semimetallic phagraphene and PHH-graphene are narrow band gap semiconductors with gaps decreasing as n increases, which are similar to the graphene nanoribbons. We also find that the band gaps of PHH-graphene nanoribbons exhibit two distinct families with n = 2i and n = 2i + 1, which can be understood by the width-dependent symmetries of the system.

Type-II lateral SnSe/GeTe heterostructures for solar photovoltaic applications with high efficiency.

Recently, lateral heterostructures based on two-dimensional (2D) materials have provided new opportunities for the development of photovoltaic nanodevices. In this work, we propose a novel lateral SnSe/GeTe heterostructure (LHS) with high photovoltaic performance and systematically investigate the structural, electronic and optical properties of the lateral heterostructure by using first-principles calculations. Our results show that this type of heterostructure processes excellent stability due to the small lattice mismatch and formation energy and also covalent bonding at the interface, which is greatly beneficial for the epitaxial growth of heterostructures. These heterostructures are semiconductors with type-II band alignment and their electronic properties can be effectively tuned by the size and composition ratio of the heterostructures. More importantly, it is found that these heterostructures possess high absorption over a wide range of visible light and high power conversion efficiency (up to 22.3%). These extraordinary properties make the SnSe/GeTe lateral heterostructures ideal candidates for photovoltaic applications.

The electronic and magnetic properties of h-BN/MoS2 heterostructures intercalated with 3d transition metal atoms.

We performed density functional theory calculations to investigate the electronic and magnetic properties of h-BN/MoS2 heterostructures intercalated with 3d transition-metal (TM) atoms, including V, Cr, Mn, Fe, Co, and Ni atoms. It was found that metal and magnetic semiconductor characteristics are induced in the h-BN/MoS2 heterostructures after intercalating TMs. In addition, the results demonstrate that h-BN sheets could promote charge transfer between the TMs and the heterogeneous structure. Specifically, the h-BN/MoS2 heterostructure transforms from an indirect semiconductor to a metal after intercalating V or Cr atoms in the interlayers. For Mn, Fe, and Co atoms, the bandgaps of the intercalated heterojunction systems become smaller when the spin polarization is 100% at the highest occupied molecular orbital level. However, the system intercalated with Ni atoms exhibits no spin polarization and non-magnetic character. Strong covalent-bonding interactions emerged between the intercalated TMs and the nearest S atom of the h-BN/MoS2 heterostructure. In addition, the magnetic moments of the TM atoms show a decreasing trend for all the interstitial intercalated heterostructures compared with their free-standing states. These results reveal that h-BN/MoS2 heterostructures with intercalated TMs are promising candidates for application in multifarious spintronic devices.

Space-confined and substrate-directed synthesis of transition-metal dichalcogenide nanostructures with tunable dimensionality.

Atomically thin transition-metal dichalcogenide (TMDC) nanostructures are predicted to exhibit novel physical properties that make them attractive candidates for the fabrication of electronic and optoelectronic devices. However, TMDCs tend to grow in the form of two-dimensional nanoplates (NPs) rather than one-dimensional nanoribbons (NRs) due to their native layered structure. Herein, we have developed a space-confined and substrate-directed chemical vapor deposition strategy for the controllable synthesis of WS2, WSe2, MoSe2, MoS2, WS2(1-x)Se2x NPs and NRs. TMDC NRs with lengths ranging from several micrometers to 100 µm have been obtained and the widths of TMDC NRs can be effectively tuned. Moreover, we found that TMDC NRs show different growth behaviors on van der Waals (vdW) and non-vdW substrates. The micro-nano structures, optical and electronic properties of synthesized TMDC NRs have been systematically investigated. This approach provides a general strategy for controllable synthesis of TMDC NRs, which makes these materials easily accessible as functional building blocks for novel optoelectronic devices.

Band offsets engineering in asymmetric Janus bilayer transition-metal dichalcogenides.

Using the first-principles calculation, we systematically studied the electronic properties of the bilayer transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se, Te) with replacing one, two, three or four layers of X atoms as Y atoms (X ≠ Y = S, Se, Te). By comparison, it is found that when the inner two layers of chalcogenide atoms are different, the system has both valence band offset (VBO) and conduction band offset (CBO). Among them, values of the band offsets reach maxima when the inner one layer of X atoms is replaced by Y atoms, namely forming the asymmetric Janus bilayer XMX/YMX. We take SMoS/SeMoS as an example to analyze the formation of the band offsets and the improvement of optoelectronic properties. Importantly, it is also found that both external electric field and biaxial strain can regulate electronic structures of asymmetric Janus bilayer TMDs with noticeable modulation of the values of band offsets. When the external electric field changes from negative to positive continually, CBO decreases and VBO increases. While when the biaxial strain changes from compression to stretch continually, CBO increases and VBO decreases. These findings enrich the study of bilayer TMDs that can be used as optoelectronic, nanoelectronic and valleytronic devices.

Quasi-bonding driven abnormal isotropic thermal transport in intrinsically anisotropic nanostructure: a case of study of a phosphorus nanotube array.

Thermal anisotropy/isotropy is one of the fundamental characteristics of the thermal properties of a material, playing a significant role in the high-performance thermal management in micro-/nanoelectronics. It has been well documented in the literature that the symmetry of geometric structures governs the anisotropy/isotropy of thermal transport. However, the fundamental correlation and the underlying mechanism remain unclear. In this paper, using a new two-dimensional (2D) van der Waals (vdW) phosphorus nanotube array as a case study, we show that the lattice thermal conductivity can be abnormally almost isotropic although the geometric structure presents remarkable anisotropy, which contradicts the previous consensus. The key factor for the abnormal isotropic thermal conductivity is mainly the essentially analogous group velocities along the intratube and intertube directions. Compared with a carbon-nanotube array, a traditional vdW system, a microscopic picture is established to underpin the underlying mechanism. The quasi-bond (non-covalent bonding, but far stronger than the vdW interatomic interaction) between the phosphorus nanotubes is found to be responsible for such diverse isotropic transport phenomena. The findings in this paper are expected to deepen our understanding of the anisotropy/isotropy thermal transport of materials and are also helpful for future thermal management technology.

First principles study of semihydrogenated graphene and topological insulator heterojunction.

Based on first principles calculations, we study the electronic properties of heterostructures formed by a 2D ferromagnetic insulator semihydrogenated graphene (SG) and topological insulator Bi2Se3 thin films of a few quintuple layers (QLs). It is found that the unsaturated C atoms in SG form bonds with Se atoms in Bi2Se3 thin film and the top surface states (at the interface) are strongly hybridized with SG. Due to breaking of time-reversal symmetry, the surface states open gaps of 40 meV and 150 meV for SG/3QL-Bi2Se3 and SG/5QL-Bi2Se3 heterostructures, respectively. Furthermore, a giant Rashba spin splitting is found induced by the SG layer.

First-principles prediction of two atomic-thin phosphorene allotropes with potentials for sun-light-driven water splitting.

Based on first-principles, the structures, stabilities, electronic and optical properties of two new atomic-thin phosphorene allotropes, named as stair-P and zipper-P, are systematically investigated. Stair-P and zipper-P are constructed based on the previously proposed stair-graphane and zipper-graphane, respectively. They are confirmed to be dynamically stable phosphorene allotropes with energetic stabilities comparable to the experimentally synthesized black-P and blue-P. Stair-P and zipper-P are all semiconductors with indirect band gaps of 2.32 eV and 2.00 eV, respectively. These band gaps can be effectively modulated by in-layer compressive and stretching strains. The band edges of stair-P and zipper-P are proper for water splitting at both acidic (PH = 0) and neutral (PH = 7) environments and their sun-light adsorbing abilities are better than black-P and blue-P in the visible range.

Complex Low Energy Tetrahedral Polymorphs of Group IV Elements from First Principles.

The energy landscape of carbon is exceedingly complex, hosting diverse and important metastable phases, including diamond, fullerenes, nanotubes, and graphene. Searching for structures, especially those with large unit cells, in this landscape is challenging. Here we use a combined stochastic search strategy employing two algorithms (ab initio random structure search and random sampling strategy combined with space group and graph theory) to apply connectivity constraints to unit cells containing up to 100 carbon atoms. We uncover three low energy carbon polymorphs (Pbam-32, P6/mmm, and I4[over ¯]3d) with new topologies, containing 32, 36, and 94 atoms in their primitive cells, respectively. Their energies relative to diamond are 96, 131, and 112 meV/atom, respectively, which suggests potential metastability. These three carbon allotropes are mechanically and dynamically stable, insulating carbon crystals with superhard mechanical properties. The I4[over ¯]3d structure possesses a direct band gap of 7.25 eV, which is the widest gap in the carbon allotrope family. Silicon, germanium, and tin versions of Pbam-32, P6/mmm, and I4[over ¯]3d also show energetic, dynamical, and mechanical stability. The computed electronic properties show that they are potential materials for semiconductor and photovoltaic applications.

Transition metal atoms absorbed on MoS2/h-BN heterostructure: stable geometries, band structures and magnetic properties.

We have studied the stable geometries, band structures and magnetic properties of transition-metal (V, Cr, Mn, Fe, Co and Ni) atoms absorbed on MoS2/h-BN heterostructure systems by first-principles calculations. By comparing the adsorption energies, we find that the adsorbed transition metal (TM) atoms prefer to stay on the top of Mo atoms. The results of the band structure without spin-orbit coupling (SOC) interaction indicate that the Cr-absorbed systems behave in a similar manner to metals, and the Co-absorbed system exhibits a half-metallic state. We also deduce that the V-, Mn-, Fe-absorbed systems are semiconductors with 100% spin polarization at the HOMO level. The Ni-absorbed system is a nonmagnetic semiconductor. In contrast, the Co-absorbed system exhibits metallic state, and the bandgap of V-absorbed system decreases slightly according to the SOC calculations. In addition, the magnetic moments of all the six TM atoms absorbed on the MoS2/h-BN heterostructure systems decrease when compared with those of their free-standing states.

Enhancing the thermoelectric performance of gamma-graphyne nanoribbons by introducing edge disorder.

Structure disorder especially edge disorder is unavoidable during the fabrication of nanomaterials. In this paper, using the non-equilibrium Green's function method, we investigate the influence of edge disorder on the thermoelectric performance of gamma(γ)-graphyne nanoribbons (GYNRs). Our results show that the high Seebeck coefficient in pristine γ-GYNR could still be preserved although edge disorder is introduced into the structure. Meanwhile, in these edge-disordered nanoribbons the suppression of thermal conductance including electronic and phononic contributions outweighs the reduction of electronic conductance. These two positive effects combine together, and finally boost the thermoelectric conversion efficiency of γ-GYNRs. The thermoelectric figure of merit ZT in the edge-disordered γ-GYNRs (the length and width are about 55.68 and 1.41 nm) could approach 2.5 at room temperature, and can even reach as high as 4.0 at 700 K, which is comparable to the efficiency of conventional energy conversion methods. The findings in this paper indicate that the edge-disordered γ-GYNRs are a promising candidate for efficient thermoelectric energy conversion and thermal management of nanodevices.

Effect of hydrogen passivation on the decoupling of graphene on SiC(0001) substrate: First-principles calculations.

Intercalation of hydrogen is important for understanding the decoupling of graphene from SiC(0001) substrate. Employing first-principles calculations, we have systematically studied the decoupling of graphene from SiC surface by H atoms intercalation from graphene boundary. It is found the passivation of H atoms on both graphene edge and SiC substrate is the key factor of the decoupling process. Passivation of graphene edge can weaken the interaction between graphene boundary and the substrate, which reduced the energy barrier significantly for H diffusion into the graphene-SiC interface. As more and more H atoms diffuse into the interface and saturate the Si dangling bonds around the boundary, graphene will detach from substrate. Furthermore, the energy barriers in these processes are relatively low, indicating that these processes can occur under the experimental temperature.