Non-centrosymmetric topological phase probed by non-linear Hall effect.

Non-centrosymmetric topological material has attracted intense attention due to its superior characteristics as compared with the centrosymmetric one, although probing the local quantum geometry in non-centrosymmetric topological material remains challenging. The non-linear Hall (NLH) effect provides an ideal tool to investigate the local quantum geometry. Here, we report a non-centrosymmetric topological phase in ZrTe5, probed by using the NLH effect. The angle-resolved and temperature-dependent NLH measurement reveals the inversion and ab-plane mirror symmetries breaking at <30 K, consistently with our theoretical calculation. Our findings identify a new non-centrosymmetric phase of ZrTe5 and provide a platform to probe and control local quantum geometry via crystal symmetries.

Water binding and hygroscopicity in π-conjugated polyelectrolytes.

The presence of water strongly influences structure, dynamics and properties of ion-containing soft matter. Yet, the hydration of such matter is not well understood. Here, we show through a large study of monovalent π-conjugated polyelectrolytes that their reversible hydration, up to several water molecules per ion pair, occurs chiefly at the interface between the ion clusters and the hydrophobic matrix without disrupting ion packing. This establishes the appropriate model to be surface hydration, not the often-assumed internal hydration of the ion clusters. Through detailed analysis of desorption energies and O-H vibrational frequencies, together with OPLS4 and DFT calculations, we have elucidated key binding motifs of the sorbed water. Type-I water, which desorbs below 50 °C, corresponds to hydrogen-bonded water clusters constituting secondary hydration. Type-II water, which typically desorbs over 50-150 °C, corresponds to water bound to the anion under the influence of a proximal cation, or to a cation‒anion pair, at the cluster surface. This constitutes primary hydration. Type-III water, which irreversibly desorbs beyond 150 °C, corresponds to water kinetically trapped between ions. Its amount varies strongly with processing and heat treatment. As a consequence, hygroscopicity-which is the water sorption capacity per ion pair-depends not only on the ions, but also their cluster morphology.

Giant g-factor in Self-Intercalated 2D TaS2.

Central to the application of spintronic devices is the ability to manipulate spins by electric and magnetic fields, which relies on a large Landé g-factor. The self-intercalation of layered transitional metal dichalcogenides with native metal atoms can serve as a new strategy to enhance the g-factor by inducing ferromagnetic instability in the system via interlayer charge transfer. Here, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are performed to extract the g-factor and characterize the electronic structure of the self-intercalated phase of 2H-TaS2 . In Ta7 S12 , a sharp density of states (DOS) peak due to the Ta intercalant appears at the Fermi level, which satisfies the Stoner criteria for spontaneous ferromagnetism, leading to spin split states. The DOS peak shows sensitivity to magnetic field up to 1.85 mV T-1 , equivalent to an effective g-factor of ≈77. This work establishes self-intercalation as an approach for tuning the g-factor.

Phase stability of monolayer Si1-xGex alloys with a Dirac cone.

The phase stability and electronic properties of two-dimensional Si1-xGex alloys are investigated via the first-principles method in combination with the cluster expansion and Monte Carlo simulations. The calculated composition-temperature phase diagram indicates that at low temperatures (below 200 K) monolayer Si1-xGex alloys energetically favor phase separation, whereas when the temperature is increased above 550 K, Si1-xGex alloys can be stabilized and thereby form solid solutions across the whole composition range. Special quasi-random structures were constructed to model the monolayer Si1-xGex. The Si1-xGex alloys are found to possess a robust Dirac cone against composition variation. These results provide a guideline for the experimental realization of Si1-xGex alloys and monolayer Si1-xGex alloys are believed to hold great potential for realization of applications in nanoelectronics and nano-optoelectronics.

High oscillator strength interlayer excitons in two-dimensional heterostructures for mid-infrared photodetection.

The development of infrared photodetectors is mainly limited by the choice of available materials and the intricate crystal growth process. Moreover, thermally activated carriers in traditional III-V and II-VI semiconductors enforce low operating temperatures in the infrared photodetectors. Here we demonstrate infrared photodetection enabled by interlayer excitons (ILEs) generated between tungsten and hafnium disulfide, WS2/HfS2. The photodetector operates at room temperature and shows an even higher performance at higher temperatures owing to the large exciton binding energy and phonon-assisted optical transition. The unique band alignment in the WS2/HfS2 heterostructure allows interlayer bandgap tuning from the mid- to long-wave infrared spectrum. We postulate that the sizeable charge delocalization and ILE accumulation at the interface result in a greatly enhanced oscillator strength of the ILEs and a high responsivity of the photodetector. The sensitivity of ILEs to the thickness of two-dimensional materials and the external field provides an excellent platform to realize robust tunable room temperature infrared photodetectors.

Synergizing Mo Single Atoms and Mo2 C Nanoparticles on CNTs Synchronizes Selectivity and Activity of Electrocatalytic N2 Reduction to Ammonia.

Previous research of molybdenum-based electrocatalysts for nitrogen reduction reaction (NRR) has been largely considered on either isolated Mo single atoms (MoSAs) or Mo carbide particles (e.g., Mo2 C) separately, while an integrated synergy (MoSAs-Mo2 C) of the two has never been considered. The theoretical calculations show that the Mo single atoms and Mo2 C nanoparticles exhibit, respectively, different catalytic hydrogen evolution reaction and NRR selectivity. Therefore, a new role-playing synergistic mechanism can be well enabled for the multistep NRR, when the two are combined on the same N-doped carbon nanotubes (NCNTs). This hypothesis is confirmed experimentally, where the MoSAs-Mo2 C assembled on NCNTs (MoSAs-Mo2 C/NCNTs) yields an ammonia formation rate of 16.1 µg h-1 cmcat -2 at -0.25 V versus reversible hydrogen electrode, which is about four times that by the Mo2 C alone (Mo2 C/NCNTs) and 4.5 times that by the MoSAs alone (MoSAs/NCNTs). Moreover, the Faradic efficiency of the MoSAs-Mo2 C/NCNTs is raised up to twofold and sevenfold of the Mo2 C/NCNTs and MoSAs/NCNTs, respectively. The MoSAs-Mo2 C/NCNTs also demonstrate outstanding stability by the almost unchanged catalytic performance over 10 h of the chronoamperometric test. The present study provides a promising new prototype of synchronizing the selectivity and activity for the multistep catalytic reactions.

Perpendicular Magnetic Anisotropy and Dzyaloshinskii-Moriya Interaction at an Oxide/Ferromagnetic Metal Interface.

We report on the study of both perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interaction (DMI) at an oxide/ferromagnetic metal (FM) interface, i.e., BaTiO_{3} (BTO)/CoFeB. Thanks to the functional properties of the BTO film and the capability to precisely control its growth, we are able to distinguish the dominant role of the oxide termination (TiO_{2} vs BaO) from the moderate effect of ferroelectric polarization in the BTO film, on the PMA and DMI at an oxide/FM interface. We find that the interfacial magnetic anisotropy energy of the BaO-BTO/CoFeB structure is 2 times larger than that of the TiO_{2}-BTO/CoFeB, while the DMI of the TiO_{2}-BTO/CoFeB interface is larger. We explain the observed phenomena by first principles calculations, which ascribe them to the different electronic states around the Fermi level at oxide/ferromagnetic metal interfaces and the different spin-flip process. This study paves the way for further investigation of the PMA and DMI at various oxide/FM structures and thus their applications in the promising field of energy-efficient devices.

Imprinting Ferromagnetism and Superconductivity in Single Atomic Layers of Molecular Superlattices.

Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer-space-confined chemical design (ICCD) is reported for the synthesis of dilute single-atom-doped TaS2 molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS2 layers. The intercalation of 2H-TaS2 crystal with bulky organic ammonium molecule expands its van der Waals gap for single-atom doping via co-intercalated cobalt ions, resulting in the formation of quasi-monolayer Co-doped TaS2 superlattices. Isolated Co atoms are decorated in the basal plane of the TaS2 via substituting the Ta atom or anchoring at a hollow site, wherein the orbital-selected p-d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal-size TaS2 molecular superlattices.

Serendipity of a topological nontrivial band gap in the 2D borophene subunit lattice with broken mirror symmetry.

The exotic electronic band structures featured by Dirac cones and topological phases in two-dimensional (2D) materials are regarded as the holy grail of the next-generation electronic devices. Here we propose a 2D tungsten boride (WB4) lattice to concurrently host these interesting properties. Based on first-principles calculations, we demonstrate that in the absence of spin-orbit coupling (SOC), the mirror symmetry protects the WB4 lattice to spawn multiple Dirac bands around the Fermi level with high velocities. However, the broken mirror symmetry induces one cone to be opened with a small band gap, and gives rise to a nontrivially topological phase characterized by the non-zero Z2 topological invariant. Interestingly, topologically nontrivial states of the lattice without mirror symmetry are robust within external biaxial tension, which is confirmed from the appearance of gapless edge states in their nanoribbon structure. Our results provide a versatile platform for hosting nontrivial topological states usable for important nanoelectronic device applications.

A Facile and Effective Method for Patching Sulfur Vacancies of WS2 via Nitrogen Plasma Treatment.

Although transition metal dichalcogenides (TMDs) are attractive for the next-generation nanoelectronic era due to their unique optoelectronic and electronic properties, carrier scattering during the transmission of electronic devices, and the distinct contact barrier between the metal and the semiconductors, which is caused by inevitable defects in TMDs, remain formidable challenges. To address these issues, a facile, effective, and universal patching defect approach that uses a nitrogen plasma doping protocol is developed, via which the intrinsic vacancies are repaired effectively. To reveal sulfur vacancies and the nature of the nitrogen doping effects, a high-resolution spherical aberration corrected scanning transmission electron microscopy is used, which confirms the N atoms doping in sulfur vacancies. In this study, a typical TMD material, namely tungsten disulfide, is employed to fabricate field-effect transistors (FETs) as a preliminary paradigm to demonstrate the patching defects method. This doping method endows FETs with high electrical performance and excellent contact interface properties. As a result, an electron mobility of up to 184.2 cm2 V-1 s-1 and a threshold voltage of as low as 3.8 V are realized. This study provides a valuable approach to improve the performance of electronic devices that are based on TMDs in practical electronic applications.

Kane Fermion in a Two-Dimensional π-Conjugated Bis(iminothiolato)nickel Monolayer.

Massless Kane fermions revealed in zinc-blende semiconductors have recently gained interest in the broad study of relativistic materials. In particular, two-dimensional (2D) Kane fermions were expected to be hybrids of pseudospin-1 and -1/2 Dirac fermions. Based on first-principles calculations, we demonstrated that 2D Kane fermions can be realized in a recently synthesized metal-organic framework, namely, bis(iminothiolato)nickel monolayer. A slight compression takes the system from a semimetal to a semiconductor. At the critical strain of ∼1%, the upper and lower conical bands linearize and touch at a single point intersecting a flat band, showing the same dispersion as the pseudospin-1 Dirac-Weyl systems. We adopted a tight-binding Hamiltonian of a line-centered honeycomb lattice to reveal the origins and topology of the electronic band structure. The coexistence of Kane-type and Dirac-type spectra in the bis(iminothiolato)nickel monolayer is expected to benefit the study of multi quasiparticle effects.

Chern Insulator and Chern Half-Metal States in the Two-Dimensional Spin-Gapless Semiconductor Mn2C6S12.

Two-dimensional metal-organic frameworks (2D-MOFs) with exotic electronic structures are drawing increasing attention. Here, using first-principles calculations, we demonstrate a spin-gapless MOF, namely, Mn2C6S12, with the coexistence of a spin-polarized Dirac cone and parabolic degenerate points. The Curie temperature evaluated from Monte Carlo simulations implies Mn2C6S12 possessing stable ferromagnetism at room temperature. Taking the spin-orbit coupling into account, the Dirac cone is gapped and the degenerate points are lifted, giving rise to multiple topologically nontrivial states with nonzero Chern number, which imply the possibility of Mn2C6S12 to be a Chern insulator and a Chern half-metal. Our results offer versatile platforms for achieving spin filtering or a quantum anomalous Hall effect with promising application in spintronics devices.

Layer-dependent semiconductor-metal transition of SnO/Si(001) heterostructure and device application.

As the downscaling of electronic devices continues, the problems of leakage currents and heat dissipation become more and more serious. To address these issues, new materials and new structures are explored. Here, we propose an interesting heterostructure made of ultrathin SnO layers on Si(001) surface. Our first-principle calculations show that a single layer of SnO on Si(001) surface is a semiconductor, but a bilayer SnO on the same surface is metallic. This metal-semiconductor dichotomy allows construction of single-2D-material-based electronic devices with low contact resistance and low leakage currents. In particular, due to the interaction between Sn and the Si substrate, the semiconducting monolayer-SnO/Si(001) has a highly anisotropic band structure with a much lighter hole effective mass along one direction than that of Si and most other 2D materials, indicating a high carrier mobility. Furthermore, by combining density functional theory and nonequilibrium Green's function method, we directly investigate the transport characteristics of a field effect transistor based on the proposed heterostructures, which shows very low contact resistance, negligible leakage current, and easy gate control at a compact channel length.

Orbital dependent ultrafast charge transfer dynamics of ferrocenyl-functionalized SAMs on gold studied by core-hole clock spectroscopy.

Understanding the charge transport properties in general of different molecular components in a self-assembled monolayer (SAM) is of importance for the rational design of SAM molecular structures for molecular electronics. In this study, we study an important aspect of the charge transport properties, i.e. the charge transfer (CT) dynamics between the active molecular component (in this case, the ferrocenyl moieties of a ferrocenyl-n-alkanethiol SAM) and the electrode using synchrotron-based core-hole clock (CHC) spectroscopy. The characteristic CT times are found to depend strongly on the character of the ferrocenyl-derived molecular orbitals (MOs) which mediate the CT process. Furthermore, by systemically shifting the position of the ferrocenyl moiety in the SAM, it is found that the CT characteristics of the ferrocenyl MOs display distinct dependence on its distance to the electrode. These results demonstrate experimentally that the efficiency and rate of charge transport through the molecular backbone can be modulated by resonant injection of charge carriers into specific MOs.

Optimized growth of graphene on SiC: from the dynamic flip mechanism.

Thermal decomposition of single-crystal SiC is one of the popular methods for growing graphene. However, the mechanism of graphene formation on the SiC surface is poorly understood, and the application of this method is also hampered by its high growth temperature. In this study, based on the ab initio calculations, we propose a vacancy assisted Si-C bond flipping model for the dynamic process of graphene growth on SiC. The fact that the critical stages during growth take place at different energy costs allows us to propose an energetic-beam controlled growth method that not only significantly lowers the growth temperature but also makes it possible to grow high-quality graphene with the desired size and patterns directly on the SiC substrate.

Growth intermediates for CVD graphene on Cu(111): carbon clusters and defective graphene.

Graphene growth on metal films via chemical vapor deposition (CVD) represents one of the most promising methods for graphene production. The realization of the wafer scale production of single crystalline graphene films requires an atomic scale understanding of the growth mechanism and the growth intermediates of CVD graphene on metal films. Here, we use in situ low-temperature scanning tunneling microscopy (LT-STM) to reveal the graphene growth intermediates at different stages via thermal decomposition of methane on Cu(111). We clearly demonstrate that various carbon clusters, including carbon dimers, carbon rectangles, and 'zigzag' and 'armchair'-like carbon chains, are the actual growth intermediates prior to the graphene formation. Upon the saturation of these carbon clusters, they can transform into defective graphene possessing pseudoperiodic corrugations and vacancies. These vacancy-defects can only be effectively healed in the presence of methane via high temperature annealing at 800 °C and result in the formation of vacancy-free monolayer graphene on Cu(111).

Strain-engineered surface transport in Si(001): complete isolation of the surface state via tensile strain.

By combining density functional theory, nonequilibrium Green's function formulism and effective-Hamiltonian approaches, we demonstrate strain-engineered surface transport in Si(001), with the complete isolation of the Si surface states from the bulk bands. Our results show that sufficient tensile strain can effectively remove the overlap between the surface valence state and the bulk valence band, because of the drastically different deformation potentials. Isolation of the surface valence state is possible with a tensile strain of ∼1.5%, a value that is accessible experimentally. Quantum transport simulations of a chemical sensing device based on strained Si(001) surface confirm the dominating surface conductance, giving rise to an enhanced molecular sensitivity. Our results show promise for using strain engineering to further our ability to manipulate surface states for quantum information processing and surface state-based devices.

First principles study of bismuth alloying effects in GaAs saturable absorber.

First principles hybrid functional calculations have been carried out to study electronic properties of GaAs with Bi alloying effects. It is found that the doping of Bi into GaAs reduces the bandgap due to the intraband level repulsions between Bi induced states and host states, and the Bi-related impurity states originate from the hybridization of Bi-6p and its nearest As-4p orbitals. With the increase of Bi concentration in GaAs, the bandgap decreases monotonously. The calculated optical properties of the undoped and Bi-doped GaAs are similar except the shift toward lower energy of absorption edge and main absorption peaks with Bi doping. These results suggest a promising application of GaBi(x)As(1-x) alloy as semiconductor saturable absorber in Q-switched or mode-locked laser.