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Inorganic molecular crystal films, particularly α-Sb2O3, have emerged as promising van der Waals (vdW) dielectrics for the large-scale integration of two-dimensional (2D) semiconductors in field-effect transistors (FETs) [K. L. Liu, B. Fin, W. Han, X. Chen, P. L. Gong and L. Huang, et al., Nat. Electron., 2021, 4, 906.]. Nevertheless, a notable gap exists in understanding the electronic and dielectric characteristics of few-layer α-Sb2O3 and the underlying physics governing its interaction with common 2D semiconductors. Herein, we address such issues through first-principles calculations. As the layer number increases, the electronic properties (e.g., band gap and band edges) of α-Sb2O3 exhibit minimal variations, resembling the electronic decoupling behavior, while the out-of-plane high dielectric constant significantly rises, indicating significant surface ionic behavior. These features stem from the weak interlayer quasi-bonding interaction, small atomic Born effective charge at the surface, and the influence of surface-to-volume ratio. Furthermore, exploring device physics, with a focus on complementary metal-oxide-semiconductor FETs, demonstrates that the leakage currents between the N-layer α-Sb2O3 (N ≥ 4) and all our studied 2D semiconducting channels adhere to international standards and the dielectrics with 4 and 5 layers meet the criteria for small equivalent oxide thickness. Unlike other layered vdW dielectrics, few-layer α-Sb2O3 emerges as a novel high-k vdW dielectric with exceptional dielectric performance and distinctive electronic characteristics, inspiring further exploration of inorganic molecular crystals for vdW dielectrics in integrated 2D semiconductor devices. [ABSTRACT FROM AUTHOR]

Motivated by the recently discovered superconductivity in Sr-doped nickelate oxides NdNiO2 reduced from NdNiO3 in experiments, we predict a Mott multiferroic BiNiO2 that may be obtained from BiNiO3. There is a ferroelectric structural transition for BiNiO2 from the nonpolar phase to the polar phase with the P4mm space group, which is driven by the lone pair on Bi. The lowest energy ground-state for polar BiNiO2 is a Hubbard Mott insulator with the G-type antiferromagnetic spin configurations. The replacement of Nd by Bi serves as a connecting link between a parent compound of high-temperature superconductor and a Mott multiferroic. [ABSTRACT FROM AUTHOR]

Electronic contacts to two-dimensional (2D) semiconductors, e.g., MoS2, of both n- and p-type, are important for complementary metal-oxide–semiconductor logic circuitry. Here, via systematic first-principles density-functional theory calculations, we report that both n- and p-type ohmic contact to MoS2 can be obtained via different surfaces of the same material, the binary compound covellite (CuS). The weak metallicity is helpful to suppress the metal-induced gap states and hence suppress the Fermi-level pinning effect. Importantly, the work functions of different CuS surfaces varies a lot from 3.8 eV to 5.8 eV. The higher work function F(Cu–S) surface forms a p-type contact to MoS2, and the p-type Schottky barrier height (SBH) can be reduced by increasing the layer number of the MoS2. The origin of the p-type SBH reduction can be attributed to quasi-bonding both at the F(Cu–S)/MoS2 interface and between MoS2 layers, which synergistically shifts the valence band edge up. Due to the large work function variation of CuS surfaces and interface quasi-bonding, p-type ohmic contact to monolayer MoS2 can be obtained with the P(S) surface. Additionally, the P(Cu)/monolayer MoS2 junction forms an n-type ohmic contact because of the large work function variation. The widely tunable SBH and contact types of the binary compound CuS/MoS2 junctions make them promising for high-efficiency electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross‐conjugated π‐electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non‐contact atomic force microscopy, and time‐of‐flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides (TMDCs) are promising in spintronics due to their spin–orbit coupling, but their intrinsic non-magnetic properties limit their further development. Here, we focus on the energy landscapes of TMDC (MX2, M = Mo, W and X = S, Se, Te) monolayers by rhenium (Re) substitution doping under axial strains, which controllably drive 1H ↔ 1Td structural transformations. For both 1H and 1Td phases without strain, Re-doped TMDCs have an n-type character and are non-magnetic, but the tensile strain could effectively induce and modulate the magnetism. Specifically, 1H-Re0.5Mo0.5S2 gets a maximum magnetic moment of 0.69 μB at a 6% uniaxial tensile strain along the armchair direction; along the zigzag direction it exhibits a significant magnetic moment (0.49 μB) at a 2.04% uniaxial tensile strain but then exhibits no magnetism in the range of [5.10%, 7.14%]. By contrast, for 1Td-Re0.5Mo0.5S2 a critical uniaxial tensile strain along the zigzag direction reaches up to ∼9.18%, and a smaller uniaxial tensile strain (∼5.10%) along the zigzag direction is needed to induce the magnetism in 1Td-Re0.5M0.5Te2. The results reveal that the magnetism of Re-doped TMDCs could be effectively induced and modulated by the tensile strain, suggesting that strain engineering could have significant applications in doped TMDCs. [ABSTRACT FROM AUTHOR]

Molecular structures that permit intramolecular rotational motion have the potential to function as molecular rotors. We have employed density functional theory and vibrational frequency analysis to study the characteristic structure and vibrational behavior of the molecule (4′,4″″-(bicyclo[2,2,2]octane-1,4-diyldi-4,1-phenylene)-bis-2,2′:6′,2″-terpyridine. IR active vibrational modes were found that favor intramolecular rotation. To demonstrate the rotor behavior of the isolated single molecule, ab initio molecular dynamics simulations at various temperatures were carried out. This molecular rotor is expected to be thermally triggered via excitation of specific vibrational modes, which implies randomness in its direction of rotation. [ABSTRACT FROM AUTHOR]

High-performance nanodevices require fast and reversible tunability of electronic and optical properties under external stimuli. In the current work, using first-principles simulations and non-equilibrium Green function transport calculations, we demonstrate that bending can effectively and asymmetrically modulate the optoelectronic properties of Janus transition-metal dichalcogenides (J-TMDCs), due to their out-of-plane flexoelectric gating. The dynamic correlation of the electronic and optical behaviors is revealed by the bending-induced interplay between the quantum confined giant Stark effect and deformation potential. The nonsymmetric directional-information encoded in the concave and convex bending motions and the intrinsic dipole of the atomically thin film renders J-TMDCs promising for wearable motion sensors and chemical sensors. [ABSTRACT FROM AUTHOR]

Investigations of van der Waals (vdW) heterostructures based on distinct low-dimensional materials have attracted significant attention for higher performance devices. Here we use density functional theory computations to scrutinize the band alignment in one-dimensional (1D) vdW heterostructures. In particular, using nanoribbons (NRs) encapsulated inside nanotubes (NTs) based on ten binary-compounds of group IV–IV and group III–V elements, we identified both momentum-matched and -mismatched type II heterostructures with gaps varying from 0.56 eV to 4.37 eV. In addition, we demonstrate a substantial reduction (up to near 0.95 eV) in the staggered band gap of BN compounds by both transverse electric field and longitudinal tensile strain. These findings are favorable for enhancing light harvesting through a wide spectrum and reducing the carrier recombination; our designed heterostructures are expected to offer opportunities for photocatalytic water splitting with safe storage of H2 products inside the NTs. [ABSTRACT FROM AUTHOR]

Motivated by the recently discovered superconductivity in Sr-doped nickelate oxides NdNiO2 reduced from NdNiO3 in experiments, we predict a Mott multiferroic BiNiO2 that may be obtained from BiNiO3. There is a ferroelectric structural transition for BiNiO2 from the nonpolar phase to the polar phase with the P4mm space group, which is driven by the lone pair on Bi. The lowest energy ground-state for polar BiNiO2 is a Hubbard Mott insulator with the G-type antiferromagnetic spin configurations. The replacement of Nd by Bi serves as a connecting link between a parent compound of high-temperature superconductor and a Mott multiferroic. [ABSTRACT FROM AUTHOR]

Cobalt has the highest Curie temperature ( T c ) among the elemental ferromagnetic metals and has a hexagonal close-packed (HCP) structure at room temperature. In this study, HCP Co was thinned to the thickness of several ( n ) unit cells along the c -axis and then passivated by halogen atoms, thus being named Co 2 n X 2 (X = F, Cl, Br and I). For Co 2 X 2 and Co 3 X 2 , all of them are not only kinetically but also thermodynamically stable from the viewpoint of the phonon spectra and molecular dynamics. Similar to HCP Co, two-dimensional (2D) Co 2 F 2 , Co 2 Cl 2 and Co 3 X 2 (X = Cl, Br and I) are still ferromagnetic metals within the Stoner model but Co 2 X 2 (X = Br and I) is a ferromagnetic half-metal with the coexistence of the metallic behavior for one spin and the insulating behavior for the other spin. Taking into account the spin-orbital coupling (SOC), the easy-magnetization axis is within the plane where the magnetization is isotropic, making it look like a 2D XY magnet. Applying a critical biaxial strain could lead to an easy-magnetization axis changing from the in-plane to the out-of-plane direction. Finally, we use classical Monte Carlo simulations to estimate the Curie temperature ( T c ) which is as high as 957 and 510 K for Co 2 F 2 and Co 2 Cl 2 , respectively, because of the strong direct exchange interaction. Different from being obtained by mechanical or liquid exfoliation from van der Waals layered structures, our study opens up new possibilities to search for novel 2D ferromagnets from the elemental ferromagnets and provides opportunities for realizing realistic ultra-thin spintronic devices.

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross-conjugated π-electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non-contact atomic force microscopy, and time-of-flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction., (© 2022 Wiley-VCH GmbH.)

We perform first-principles GW plus Bethe-Salpeter equation calculations to investigate the photophysics of monolayer hexagonal boron nitride (h-BN), revealing excitons with novel k-space characteristics. The excitonic states forming the first and third peaks in its absorption spectrum are s-like, but those of the second peak are notably p-like, a first finding of strong co-occurrence of bright s-like and bright p-like states in an intrinsic 2D material. Moreover, even though the k-space wave function of these excitonic states are centered at the K and K^{'} valleys as in monolayer transition metal dichalcogenides, the k-space envelope functions of the basis excitons at one valley have significant extents to the basin of the other valley. As a consequence, the optical response of monolayer h-BN exhibits a lack of circular dichroism, as well as a coupling that induces an intervalley mixing between s- and p-like states.

Recent studies have revealed that the interlayer interaction in two-dimensional (2D) layered materials is not simply of van der Waals character but could coexist with quasi-bonding character. Herein, we classify the interlayer quasi-bonding interactions into two main categories (I: homo-occupancy interaction; II: hetero-occupancy interaction) according to the occupancy of the involved energy bands near the Fermi level. We then investigate the quasi-bonding-interaction-induced band structure evolution of several representative 2D materials based on density functional theory calculations. Further calculations confirm that this classification is applicable to generic 2D layered materials and provide a unified understanding of the total strength of interlayer interaction, which is a synergetic effect of the van der Waals attraction and the quasi-bonding interaction. The latter is stabilizing in main category II and destabilizing in main category I. Thus, the total interlayer interaction strength is relatively stronger in category II and weaker in category I.

Ladder phenylenes (LPs) composed of alternating fused benzene and cyclobutadiene rings have been synthesized in solution with a maximum length no longer than five units. Longer polymeric LPs have not been obtained so far because of their poor stability and insolubility. Here, we report the synthesis of linear LP chains on the Au(111) surface via dehalogenative [2+2] cycloaddition, in which the steric hindrance of the methyl groups in the 1,2,4,5-tetrabromo-3,6-dimethylbenzene precursor improves the chemoselectivity as well as the orientation orderliness. By combining scanning tunneling microscopy and noncontact atomic force microscopy, we determined the atomic structure and the electronic properties of the LP chains on the metallic substrate and NaCl/Au(111). The tunneling spectroscopy measurements revealed the charged state of chains on the NaCl layer, and this finding is supported by density functional theory calculations, which predict an indirect bandgap and antiferromagnetism in the polymeric LP chains.

Computational tools based on density-functional theory (DFT) enable the exploration of the qualitatively new, experimentally attainable nanoscale compounds for a targeted application. Theoretical simulations provide a profound understanding of the intrinsic electronic properties of functional materials. The goal of this protocol is to search for photocatalyst candidates by computational dissection. Photocatalytic applications require suitable band gaps, appropriate band edge positions relative to the redox potentials. Hybrid functionals can provide accurate values of these properties but are computationally expensive, whereas the results at the Perdew-Burke-Ernzerhof (PBE) functional level could be effective for suggesting strategies for band structure engineering via electric field and tensile strain aiming to enhance the photocatalytic performance. To illustrate this, in the present manuscript, the DFT based simulation tool VASP is used to investigate the band alignment of nanocomposites in combinations of nanotubes and nanoribbons in the ground state. To address the lifetime of photogenerated holes and electrons in the excited state, nonadiabatic dynamics calculations are needed.

Discovering highly in-plane anisotropic two-dimensional (2D) semiconductors with multiple superior properties (good stability, widely tunable bandgap and high mobility) are of great interest for fundamental studies and for developments of novel (opto)electronic devices. By means of state-of-the-art first-principles calculations, herein we present a thorough investigation on the stability, electronic properties and promising applications of previously unexplored 2D semiconductors-gold-selenium (β-AuSe) with strong in-plane anisotropy, whose layered bulk counterpart was synthesized fifty years ago. We show that they have stable structures, widely tunable bandgap varying from 1.66 eV in monolayer to 0.70 eV in five-layer, strong light absorption coefficient (~10 5 cm -1 ) within the whole visible light range, and high/ultrahigh carrier mobility (10 3 -10 5 cm 2 V -1 s -1 ). More importantly, they show highly in-pane anisotropic behaviors in absorption coefficients, photoconductance and carrier mobility. Especially, the anisotropic ratio of carrier mobility is much higher than the literature reported ones. The above findings show that the in-plane anisotropic 2D β-AuSe are promising candidates for developing polarization-sensitive photodetectors, synaptic devices and micro digital inverters based on multiple superior properties and highly anisotropic behaviors. Besides, few-layer β-AuSe systems can serve as channel materials in field-effect transistors with high mobility or be applied in solar cells with strong light absorption. Our findings demonstrate that few-layer 2D β-AuSe have great potential for multifunctional applications and thus stimulate immediately experimental interests.

Two-dimensional (2D) semiconductors SnP 3 are predicted, from first-principles calculations, to host moderate band gaps (0.72 eV for monolayer and 1.07 eV for bilayer), ultrahigh carrier mobility (∼10 4 cm 2 V -1 s -1 for bilayer), strong absorption coefficients (∼10 5 cm -1 ) and good stability. Moreover, the band gap can be modulated from an indirect character into a direct one via strain engineering. For experimental accessibility, the calculated exfoliation energies of monolayer and bilayer SnP 3 are smaller than those of the common arsenic-type honeycomb structures GeP 3 and InP 3 . More importantly, a semiconductor-to-metal transition is discovered with the layer number N  >  2. We demonstrate, in remarkable contrast to the previous understandings, that such phase transition is largely driven by the correlation between lone-pair electrons of interlayer Sn and P atoms. This mechanism is universal for analogues phase transitions in arsenic-type honeycomb structures (GeP 3 , InP 3 and SnP 3 ).

Polar metals based on binary and ternary compounds have been demonstrated in literature. Here, we propose a design principle for ferroelectric-like elemental polar metals and relate it to real materials. The design principle is that, to be an elemental polar metal, atoms should occupy at least two inequivalent Wyckoff positions in a crystal with a polar space group, where inversion symmetry is spontaneously broken. According to this rule, we propose the first class of potential ferroelectric-like elemental polar metals in a distorted α-La-like structure with a polar space group P6 3 mc in which two inequivalent Wyckoff positions 2a (0, 0, z) and 2b (1/3, 2/3, z) are occupied by group-V elements (phosphorus, arsenic, antimony, and bismuth). Analyses based on first-principles calculations indicate that the dynamically stable polar phase results from a lone pair driven polar distortion of the nonploar phase in P6 3 /mmc symmetry where two inequivalent Wyckoff positions 2a (0, 0, 0) and 2c (1/3, 2/3, 1/4) are occupied. This ferroelectric-like transition involves a transition from a metallic state to a semimetallic state. These predicted polar phases are metastable with respect to their corresponding ground phases. Moreover, ionic bonding characters are found due to the inequivalence in Wyckoff positions between group-V atoms. Our work opens a route to single-element parity-breaking phases.