Monolayer Borophene Formation on Cu(111) Surface Triggered by ⟨ 1 1 ¯ 0 ⟩ $\langle {1\bar{1}0} \rangle $ Step Edge.

Borophene, a promising material with potential applications in electronics, energy storage, and sensors, is successfully grown as a monolayer on Ag(111), Cu(111), and Au(111) surfaces using molecular beam epitaxy. The growth of two-dimensional borophene on Ag(111) and Au(111) is proposed to occur via surface adsorption and boron segregation, respectively. However, the growth mode of borophene on Cu(111) remains unclear. To elucidate this, scanning tunneling microscopy in conjunction with theoretical calculations is used to study the phase transformation of boron nanostructures under post-annealing treatments. Results show that by elevating the substrate temperature, boron nanostructures undergo an evolution from amorphous boron to striped-phase borophene (η = 1/6) adhering to the Cu ⟨ 1 1 ¯ 0 ⟩ $\langle {1\bar{1}0} \rangle $ step edge, and finally to irregularly shaped ß-type borophene (η = 5/36) either on the substrate surface or embedded in the topmost Cu layer. dI/dV spectra recorded near the borophene/Cu lateral interfaces indicate that the striped-phase borophene is a metastable phase, requiring more buckling and electron transfer to stabilize the crystal structure. These findings offer not only an in-depth comprehension of the ß-type borophene formation on Cu(111), but also hold potential for enabling borophene synthesis on weakly-binding semiconducting or insulating substrates with 1D active defects.

Dissociation dynamics of anionic carbon monoxide in dark states.

A resonant system consisting of an excess electron and a closed-shell atom or molecule, as a temporary negative ion, is usually in doublet-spin states that are analogous to bright states of photoexcitation of the neutral. However, anionic higher-spin states, noted as dark states, are scarcely accessed. Here, we report the dissociation dynamics of CO- in dark quartet resonant states that are formed by electron attachments to electronically excited CO (a3Π). Among the dissociations to O-(2P) + C(3P), O-(2P) + C(1D), and O-(2P) + C(1S), the latter two are spin-forbidden in the quartet-spin resonant states of CO-, while the first process is preferred in 4Σ- and 4Π states. The present finding sheds new light on anionic dark states.

Spectroscopic signature of obstructed surface states in SrIn2P2.

The century-long development of surface sciences has witnessed the discoveries of a variety of quantum states. In the recently proposed "obstructed atomic insulators", symmetric charges are pinned at virtual sites where no real atoms reside. The cleavage through these sites could lead to a set of obstructed surface states with partial electronic occupation. Here, utilizing scanning tunneling microscopy, angle-resolved photoemission spectroscopy and first-principles calculations, we observe spectroscopic signature of obstructed surface states in SrIn2P2. We find that a pair of surface states that are originated from the pristine obstructed surface states split in energy by a unique surface reconstruction. The upper branch is marked with a striking differential conductance peak followed by negative differential conductance, signaling its localized nature, while the lower branch is found to be highly dispersive. This pair of surface states is in consistency with our calculational results. Our finding not only demonstrates a surface quantum state induced by a new type of bulk-boundary correspondence, but also provides a platform for exploring efficient catalysts and related surface engineering.

A Case of Fulminant Right Heart Failure Owing to Tumoral Pulmonary Hypertension.

Tumoral pulmonary hypertension is a rare cause of pulmonary hypertension. We report a patient who was thought to have idiopathic pulmonary arterial hypertension, but later developed fulminant right heart failure ultimately leading to death. Autopsy revealed substantial pulmonary tumor embolism burden originating from liver adenocarcinoma. (Level of Difficulty: Advanced.).

Charge instability of topological Fermi arcs in chiral crystal CoSi.

Topological boundary states emerged at the spatial boundary between topological non-trivial and trivial phases, are usually gapless, or commonly referred as metallic states. For example, the surface state of a topological insulator is a gapless Dirac state. These metallic topological boundary states are typically well described by non-interacting fermions. However, the behavior of topological boundary states with significant electron-electron interactions, which could turn the gapless boundary states into gapped ordered states, e.g., density wave states or superconducting states, is of great interest theoretically, but is still lacking evidence experimentally. Here, we report the observation of incommensurable charge density wave (CDW) formed on the topological boundary states driven by the electron-electron interactions on the (001) surface of CoSi. The wavevector of CDW varies as the temperature changes, which coincides with the evolution of topological surface Fermi arcs with temperature. The orientation of the CDW phase is determined by the chirality of the Fermi arcs, which indicates a direct association between CDW and Fermi arcs. Our finding will stimulate the search of more interactions-driven ordered states, such as superconductivity and magnetism, on the boundaries of topological materials.

Arsenic Monolayers Formed by Zero-Dimensional Tetrahedral Clusters and One-Dimensional Armchair Nanochains.

One-dimensional (1D) arsenene nanostructures are predicted to host a variety of interesting physical properties including antiferromagnetic, semiconductor-semimetal transition and quantum spin Hall effect, which thus holds great promise for next-generation electronic and spintronic devices. Herein, we devised a surface template strategy in a combination with surface-catalyzed decomposition of molecular As4 cluster toward the synthesis of the superlattice of ultranarrow armchair arsenic nanochains in a large domain on Au(111). In the low annealing temperature window, zero-dimensional As4 nanoclusters are assembled into continuous films through intermolecular van der Waals and molecule-substrate interactions. At the elevated temperature, the subsequent surface-assisted decomposition of molecular As4 nanoclusters leads to the formation of a periodic array of 1D armchair arsenic nanochains that form a (2 × 3) superstructure on the Au(111) surface. These ultranarrow armchair arsenic nanochains are predicted to have a small bandgap of ∼0.50 eV, in contrast to metallic zigzag chains. In addition, the Au-supported arsenic nanochains can be flipped to form a bilayer structure through tip indentation and manipulation, suggesting the possible transfer of these nanochains from the substrate. The successful realization of arsenic nanostructures is expected to advance low-dimensional physics and infrared optoelectronic nanodevices.

Outer Valence Photoionization and Autoionization of Formaldehyde.

We investigate photoionization from the ground electronic state of the formaldehyde molecule. Both partial cross sections and asymmetry parameters leading to the X2B2, A2B1, C2B2, and D2A1 states of H2CO+ ions are studied in the photon region of 10-90 eV using a multichannel R-matrix method, which uses the configuration interaction (CI) to describe electronic correlation. We check the sensitivity of the results to change descriptions of the continuum, the different partial waves, and the active spaces in the theoretical model. And we obtain the convergent result of the present calculations. Extensive resonance structures near the ionization threshold are observed for the first time. Our predicted total cross sections and asymmetry parameters differ from these obtained by previous theoretical approaches, all of which neglected correlation effects. The present results were found to agree reasonably well with the available experimental results, suggesting the reliability of our calculations.

Pressure-Tuned Intralayer Exchange in Superlattice-Like MnBi2Te4/(Bi2Te3)n Topological Insulators.

The magnetic structures of MnBi2Te4(Bi2Te3)n can be manipulated by tuning the interlayer coupling via the number of Bi2Te3 spacer layers n, while the intralayer ferromagnetic (FM) exchange coupling is considered too robust to control. By applying hydrostatic pressure up to 3.5 GPa, we discover opposite responses of magnetic properties for n = 1 and 2. MnBi4Te7 stays at A-type antiferromagnetic (AFM) phase with a decreasing Néel temperature and an increasing saturation field. In sharp contrast, MnBi6Te10 experiences a phase transition from A-type AFM to a quasi-two-dimensional FM state with a suppressed saturation field under pressure. First-principles calculations reveal the essential role of intralayer exchange coupling from lattice compression in determining these magnetic properties. Such magnetic phase transition is also observed in 20% Sb-doped MnBi6Te10 because of the in-plane lattice compression.

Electrochemically Exfoliated Platinum Dichalcogenide Atomic Layers for High-Performance Air-Stable Infrared Photodetectors.

Platinum dichalcogenide (PtX2), an emergent group-10 transition metal dichalcogenide (TMD) has shown great potential in infrared photonic and optoelectronic applications due to its layer-dependent electronic structure with potentially suitable bandgap. However, a scalable synthesis of PtSe2 and PtTe2 atomic layers with controlled thickness still represents a major challenge in this field because of the strong interlayer interactions. Herein, we develop a facile cathodic exfoliation approach for the synthesis of solution-processable high-quality PtSe2 and PtTe2 atomic layers for high-performance infrared (IR) photodetection. As-exfoliated PtSe2 and PtTe2 bilayer exhibit an excellent photoresponsivity of 72 and 1620 mA W-1 at zero gate voltage under a 1540 nm laser illumination, respectively, approximately several orders of magnitude higher than that of the majority of IR photodetectors based on graphene, TMDs, and black phosphorus. In addition, our PtSe2 and PtTe2 bilayer device also shows a decent specific detectivity of beyond 109 Jones with remarkable air-stability (>several months), outperforming the mechanically exfoliated counterparts under the laser illumination with a similar wavelength. Moreover, a high yield of PtSe2 and PtTe2 atomic layers dispersed in solution also allows for a facile fabrication of air-stable wafer-scale IR photodetector. This work demonstrates a new route for the synthesis of solution-processable layered materials with the narrow bandgap for the infrared optoelectronic applications.

Superconductivity in a Hole-Doped Mott-Insulating Triangular Adatom Layer on a Silicon Surface.

Adsorption of one-third monolayer of Sn on an atomically clean Si(111) substrate produces a two-dimensional triangular adatom lattice with one unpaired electron per site. This dilute adatom reconstruction is an antiferromagnetic Mott insulator; however, the system can be modulation doped and metallized using heavily doped p-type Si(111) substrates. Here, we show that the hole-doped dilute adatom layer on a degenerately doped p-type Si(111) wafer is superconducting with a critical temperature of 4.7±0.3 K. While a phonon-mediated coupling scenario would be consistent with the observed T_{c}, Mott correlations in the Sn-derived dangling-bond surface state could suppress the s-wave pairing channel. The latter suggests that the superconductivity in this triangular adatom lattice may be unconventional.

On-Surface Synthesis of Graphene Nanoribbons on Two-Dimensional Rare Earth-Gold Intermetallic Compounds.

Here, we demonstrate two reliable routes for the fabrication of armchair-edge graphene nanoribbons (GNRs) on TbAu2/Au(111), belonging to a class of two-dimensional ferromagnetic rare earth-gold intermetallic compounds. On-surface synthesis directly on TbAu2 leads to the formation of GNRs, which are short and interconnected with each other. In contrast, the intercalation approach-on-surface synthesis of GNRs directly on Au(111) followed by rare earth intercalation-yields GNRs on TbAu2/Au(111), where both the ribbons and TbAu2 are of high quality comparable with those directly grown on clean Au(111). Besides, the as-grown ribbons retain the same band gap while changing from p-doping to weak n-doping mainly due to a change in the work function of the substrate after the rare earth intercalation. The intercalation approach might also be employed to fabricate other types of GNRs on various rare earth intermetallic compounds, providing platforms to tailor the electronic and magnetic properties of GNRs on magnetic substrates.

Symmetric Sodium-Ion Battery Based on Dual-Electron Reactions of NASICON-Structured Na3MnTi(PO4)3 Material.

Symmetric sodium-ion batteries possess promising features such as low cost, easy manufacturing process, and facile recycling post-process, which are suitable for the application of large-scale stationary energy storage. Herein, we proposed a symmetric sodium-ion battery based on dual-electron reactions of a NASICON-structured Na3MnTi(PO4)3 material. The Na3MnTi(PO4)3 electrode can deliver a stable capacity of up to 160 mAh g-1 with a Coulombic efficiency of 97% at 0.1 C by utilizing the redox reactions of Ti3+/4+, Mn2+/3+, and Mn3+/4+. This is the first time to investigate the symmetric sodium-ion full cell using Na3MnTi(PO4)3 as both cathode and anode in the organic electrolyte, demonstrating excellent reversibility and cycling performance with voltage plateaus of about 1.4 and 1.9 V. The full cell exhibits a reversible capacity of 75 mAh g-1 at 0.1 C and an energy density of 52 Wh kg-1. In addition, both ex situ X-ray diffraction (XRD) analysis and first-principles calculations are employed to investigate the sodiation mechanism and structural evolution. The current research provides a feasible strategy for the symmetric sodium-ion batteries to achieve high energy density.

Two-Dimensional Rare Earth-Gold Intermetallic Compounds on Au(111) by Surface Alloying.

Surface alloying is a straightforward route to control and modify the structure and electronic properties of surfaces. Here, we present a systematic study on the structural and electronic properties of three novel rare earth-based intermetallic compounds, namely, ReAu2 (Re = Tb, Ho, and Er), on Au(111) via directly depositing rare earth metals onto the hot Au(111) surface. Scanning tunneling microscopy/spectroscopy measurements reveal very similar atomic structures and electronic properties, e.g., electronic states and surface work functions, for all these intermetallic compound systems because of the physical and chemical similarities between these rare earth elements. Further, these electronic properties are periodically modulated by the moiré structures caused by the lattice mismatches between ReAu2 and Au(111). These periodically modulated surfaces could serve as templates for the self-assembly of nanostructures. In addition, these two-dimensional rare earth-based intermetallic compounds provide platforms to investigate rare earth-related catalysis, magnetisms, etc. in the lower dimensions.

Synthesis of nanoporous graphenes via decarboxylation reaction.

Two kinds of C-C bonded crystalline nanoporous graphenes (NPGs) have been synthesized by using a newly developed decarboxylation reaction. Both NPGs show good electrocatalytic oxygen evolution reaction (OER) activities. The clear pore-edge structures of the synthesized NPGs provide an ideal platform for further OER investigations.

Band Sharpening and Band Alignment Enable High Quality Factor to Enhance Thermoelectric Performance in n-Type PbS.

Low-cost and earth-abundant PbS-based thermoelectrics are expected to be an alternative for PbTe, and have attracted extensive attentions from thermoelectric community. Herein, a maximum ZT (ZTmax) ≈ 1.3 at 923 K in n-type PbS is obtained through synergistically optimizing quality factor with Sn alloying and PbTe phase incorporation. It is found that Sn alloying in PbS can sharpen the conduction band shape to balance the contradictory interrelationship between carrier mobility and effective mass, accordingly, a peak power factor of ∼19.8 µWcm-1K-2 is achieved. Besides band sharpening, Sn alloying can also narrow the band gap of PbS so as to make the conduction band position between Pb0.94Sn0.06S and PbTe well aligned, which can benefit high carrier mobility. Therefore, incorporating the PbTe phase into the Pb0.94Sn0.06S matrix can not only favorably maintain the carrier mobility at ∼150 cm2V-1s-1 but also suppress the lattice thermal conductivity to ∼0.61 Wm-1K-1 in Pb0.94Sn0.06S-8%PbTe, which contributes to a largely enhanced quality factor. Consequently, an average ZT (ZTave) ≈ 0.72 in 300-923 K is achieved in Pb0.94Sn0.06S-8%PbTe that outperforms other n-type PbS-based thermoelectric materials.

A two-dimensional ErCu2 intermetallic compound on Cu(111) with moiré-pattern-modulated electronic structures.

A rare-earth compound on a metal may form a two-dimensional (2D) intermetallic compound whose properties can be further modulated by the underlying substrate periodicity and coupling. Here, we present a combinational and systematic investigation using scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations on erbium (Er) on Cu(111). Experimentally, an intriguing growth mode transition from a branched island to a fractal-like island has been observed depending on whether the deposition process of Er is interrupted for a certain duration: post-deposition effects, such as nucleation and island growth controlled by diffusion, play an essential role in altering the Er island edge and its activity. Upon annealing, the branched Er islands become strands of amorphous surface alloy; in contrast, the fractal-like islands (with additional Er atoms on top) give rise to a monolayer thick 2D ErCu2 intermetallic compound and display a moiré pattern. Theoretically, using DFT calculations, we found that the characteristic energy states, particularly the state in the unoccupied region around 582-663 meV, of the 2D ErCu2 intermetallic compound are position-dependent, consistent with STS measurements. The moiré pattern originating from the mismatch of the periodicities of the ErCu2 layer and the Cu(111) surface was identified to be responsible for the observed periodic modulation on the coupling interaction that affects the electronic structures. Our further DFT calculations on a free-standing ErCu2 monolayer found it to be a 2D ferromagnet with topological band structures. Our work should stimulate further studies on such 2D rare-earth-based nanostructures and exploration of the use of the tunable electronic structures in such atomically-thin layers.

The shielding effects of a C60 cage on the magnetic moments of transition metal atoms inside the corner holes of Si(111)-(7 × 7).

The strong interaction between transition metal (TM) atoms and semiconductor surface atoms may diminish the magnetic moments of the TM atoms and prevent them from being used as single atom spin-based devices. A carbon cage that can encapsulate TM atoms and isolate them from interacting with surface atoms is considered to protect the magnetic moments of the TM atoms. We have studied the magnetic moments of Fe, Co, and Ni atoms adsorbed inside the corner hole of Si(111)-(7 × 7) by using first-principles calculations based on the density functional theory. The results show that when Co and Ni atoms are directly adsorbed inside the corner hole, the magnetic moments are 1.353µB and 0, respectively. However when a C60 cage is used to encapsulate the atoms, the magnetic moments increase to 1.849µB and 0.884µB, respectively. The results show a clear protecting effect of a carbon cage. For Fe with and without C60, the magnetic moments are 2.909µB and 2.825µB, respectively. The presence of a C60 cage can also maintain their magnetic moments. Further analysis shows that the TM atoms possess magnetic moments when the conduction electrons are localized around them. All the results can be well understood in the framework of the Anderson impurity model. Our results demonstrate that a carbon cage may effectively protect the magnetic moments of TM atoms. This provides a new strategy for developing single atom spin-based devices on semiconductors.

3D charge and 2D phonon transports leading to high out-of-plane ZT in n-type SnSe crystals.

Thermoelectric technology enables the harvest of waste heat and its direct conversion into electricity. The conversion efficiency is determined by the materials figure of merit ZT Here we show a maximum ZT of ~2.8 ± 0.5 at 773 kelvin in n-type tin selenide (SnSe) crystals out of plane. The thermal conductivity in layered SnSe crystals is the lowest in the out-of-plane direction [two-dimensional (2D) phonon transport]. We doped SnSe with bromine to make n-type SnSe crystals with the overlapping interlayer charge density (3D charge transport). A continuous phase transition increases the symmetry and diverges two converged conduction bands. These two factors improve carrier mobility, while preserving a large Seebeck coefficient. Our findings can be applied in 2D layered materials and provide a new strategy to enhance out-of-plane electrical transport properties without degrading thermal properties.

MRCI study on transition dipole moments and transition probabilities of 18 low-lying states of CP+ cation.

This study calculates the potential energy curves of 18 Λ-S and 50â€¯Ω states, which arise from the C(3Pg) + P+(3Pg) dissociation channel of the CP+ cation. The calculations are made using the CASSCF method, followed by the icMRCI approach with the Davidson correction. Core-valence correlation and scalar relativistic corrections, as well as extrapolation to the complete basis set limit are included. The transition dipole moments are computed for 25 pairs of Λ-S states. The spin-orbit coupling effect on the spectroscopic and vibrational properties is evaluated. The Franck-Condon factors and Einstein coefficients of emissions are calculated. Radiative lifetimes are obtained for several vibrational levels of some states. The transitions are evaluated and spectroscopic measurement schemes for observing these Λ-S states are proposed. The potential energy curves, spectroscopic constants, vibrational levels, transition dipole moments, and transition probabilities reported in this paper can be considered to be very accurate and reliable. Because no experimental observations are currently available, the results obtained here can be used as guidelines for the detection of these states in appropriate spectroscopy experiments, in particular for observations in stellar atmospheres and in interstellar space.

Growth Behavior of Pristine and Potassium Doped Coronene Thin Films on Substrates with Tuned Coupling Strength.

The growth of polycyclic aromatic hydrocarbon (PAH) molecular coronene film on various substrates and the subsequent doping of potassium under ultrahigh vacuum (UHV) conditions have been systematically investigated by low-temperature scanning tunneling microscopy and spectroscopy (STM/STS). The crystalline structures and molecular orientations of coronene thin films are both thickness-dependent and substrate-sensitive due to the competition between molecule-substrate interaction and intermolecular interaction. In mono- or bilayer films, coronene molecules are flat-lying on the surface with hexagonal lattice, whereas in multilayer films, the topmost molecules are in a standing-up but tilted configuration with rectangular lattice. In particular, a 2 × 1 superstructure with respect to that of bulk coronene is formed on thick KCl film. Furthermore, we have studied the potassium doped coronene monolayer and multilayer on Ag(100) and KCl/Ag(100) surface. For K-doped coronene monolayer, at certain doping ratio x = 3, the lowest unoccupied molecular orbital (LUMO) of coronene film moves to the Fermi level, and a splitting of the LUMO state is observed. Increased potassium doping would result in a filled LUMO state below the Fermi level. By contrast, no well-ordered structures are obtained in the K-doped coronene multilayers which are vulnerable to rather moderate annealing processes owing to their relatively weak bonding with the supporting substrates, implying a big challenge of growth of PAH thick films in vacuum. The differences in the crystal structures of coronene thin films compared with that in bulk crystals might shed insight on the controversies in the experimental results on the electronic properties of alkali-metal-doped PAHs.