Intervalley Excitonic Hybridization, Optical Selection Rules, and Imperfect Circular Dichroism in Monolayer h-BN.

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.

On-Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition.

[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.

Interlayer Quasi-Bonding Interactions in 2D Layered Materials: A Classification According to the Occupancy of Involved Energy Bands.

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 Synthesized on Au(111) Surface via Selective [2+2] Cycloaddition.

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.

Strain-driven phase transition and spin polarization of Re-doped transition-metal dichalcogenides.

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.

Polarity and Spin-Orbit Coupling Induced Strong Interfacial Exchange Coupling: An Asymmetric Charge Transfer in Iridate-Manganite Heterostructure.

Charge transfer is of particular importance in manipulating the interface physics in transition-metal oxide heterostructures. In this work, we have fabricated epitaxial bilayers composed of polar 3d LaMnO3 and nonpolar 5d SrIrO3. Systematic magnetic measurements reveal an unexpectedly large exchange bias effect in the bilayer, together with a dramatic enhancement of the coercivity of LaMnO3. Based on first-principle calculations and X-ray absorption spectroscopy measurements, such a strong interfacial magnetic coupling is found closely associated with the polar nature of LaMnO3 and the strong spin-orbit interaction in SrIrO3, which collectively drive an asymmetric interfacial charge transfer and lead to the emergence of an interfacial reentrant spin/superspin glass state. Our study provides a new insight into the charge transfer in transition-metal oxide heterostructures and offers a novel means to tune the interfacial exchange coupling for a variety of device applications.

Probe Type II Band Alignment in One-Dimensional Van Der Waals Heterostructures Using First-Principles Calculations.

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.

Highly in-plane anisotropic 2D semiconductors ß-AuSe with multiple superior properties: a first-principles investigation.

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 (ß-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 (~105 cm-1) within the whole visible light range, and high/ultrahigh carrier mobility (103-105 cm2 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 ß-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 ß-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 ß-AuSe have great potential for multifunctional applications and thus stimulate immediately experimental interests.

Multifunctional two-dimensional semiconductors SnP3: universal mechanism of layer-dependent electronic phase transition.

Two-dimensional (2D) semiconductors SnP3 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 (∼104 cm2 V-1 s-1 for bilayer), strong absorption coefficients (∼105 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 SnP3 are smaller than those of the common arsenic-type honeycomb structures GeP3 and InP3. 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 (GeP3, InP3 and SnP3).

Parity-breaking in single-element phases: ferroelectric-like elemental polar metals.

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 P63 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 P63/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.

A random rotor molecule: Vibrational analysis and molecular dynamics simulations.

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.