Abstract
The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high-temperature BEC transition. The ...material realization of EI has been challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the preformed exciton gas phase is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking. Here we report a distinct correlated phase beyond the 2×2 CDW ground state emerging in monolayer 1T-ZrTe
2
and its investigation by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The results show novel band- and energy-dependent folding behavior in a two-step process, which is the signatures of an exciton gas phase prior to its condensation into the final CDW state. Our findings provide a versatile two-dimensional platform that allows tuning of the excitonic effect.
Controlling inorganic structure and dimensionality through structure-directing agents is a versatile approach for new materials synthesis that has been used extensively for metal-organic frameworks ...and coordination polymers. However, the lack of 'solid' inorganic cores requires charge transport through single-atom chains and/or organic groups, limiting their electronic properties. Here, we report that strongly interacting diamondoid structure-directing agents guide the growth of hybrid metal-organic chalcogenide nanowires with solid inorganic cores having three-atom cross-sections, representing the smallest possible nanowires. The strong van der Waals attraction between diamondoids overcomes steric repulsion leading to a cis configuration at the active growth front, enabling face-on addition of precursors for nanowire elongation. These nanowires have band-like electronic properties, low effective carrier masses and three orders-of-magnitude conductivity modulation by hole doping. This discovery highlights a previously unexplored regime of structure-directing agents compared with traditional surfactant, block copolymer or metal-organic framework linkers.
Integrating solid-state quantum emitters with photonic circuits is essential for realizing large-scale quantum photonic processors. Negatively charged tin-vacancy (SnV–) centers in diamond have ...emerged as promising candidates for quantum emitters because of their excellent optical and spin properties, including narrow-linewidth emission and long spin coherence times. SnV– centers need to be incorporated in optical waveguides for efficient on-chip routing of the photons they generate. However, such integration has yet to be realized. In this Letter, we demonstrate the coupling of SnV– centers to a nanophotonic waveguide. We realize this device by leveraging our recently developed shallow ion implantation and growth method for the generation of high-quality SnV– centers and the advanced quasi-isotropic diamond fabrication technique. We confirm the compatibility and robustness of these techniques through successful coupling of narrow-linewidth SnV– centers (as narrow as 36 ± 2 MHz) to the diamond waveguide. Furthermore, we investigate the stability of waveguide-coupled SnV– centers under resonant excitation. Our results are an important step toward SnV–-based on-chip spin-photon interfaces, single-photon nonlinearity, and photon-mediated spin interactions.
Effective stabilization of lithium metal has been hindered by the exacting requirements for the protection layer. Among all materials, the mechanical strength and electrochemical inertness of diamond ...is a prime candidate for lithium stabilization. Herein, we successfully rendered this desirable material compatible as lithium metal interface, which strictly satisfied the critical requirements. Our interface possessed the highest modulus among all the lithium coatings (>200 GPa), which can effectively arrest dendrite propagation. Since pinholes are the major failure mechanisms of artificial interfaces, a novel double-layer design was proposed to enhance the defect tolerance, enabling uniform ion flux and mechanical properties as confirmed by both simulation and experiments. Thanks to the multifold advantages of our interface design, high Coulombic efficiency of >99.4% was obtained at 1 mA cm−2; and more than 400 stable cycles were realized in prototypical lithium-sulfur cells with limited lithium, corresponding to an average anode Coulombic efficiency of >99%.
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•Nanodiamond thin film was fabricated as interfacial protection for Li metal anode•A unique double-layer nanodiamond design was proposed to ensure uniform ion flux•The nanodiamond thin film possess >200 GPa modulus for dendrite suppression•Significantly improved battery performance was realized in half and Li-S full cells
The Li metal anode holds great promise for next-generation battery systems. However, its practical applications are severely hindered by the low efficiency and potential safety hazards, largely due to the high reactivity of metallic Li toward liquid electrolytes. This work demonstrates the utilization of nanodiamond thin film as surface protection for metallic Li, where Li can be electroplated solely underneath the film and shielded from parasitic reactions with electrolyte. The nanodiamond thin film possesses not only excellent electrochemical stability but also extremely high modulus for dendrite suppression. Importantly, since pinholes in the surface protection layer undermine the uniformity of ion flux, a unique double-layer structure was proposed to enhance the defect tolerance of the design, where defects in one layer can be screened by the other intact layer. The nanodiamond interface enables efficient cycling of Li metal anode, paving the way for viable Li metal batteries in the future.
The stability of the Li-electrolyte interface is critical to the practical applications of Li metal anodes. Correspondingly, we developed a high-quality nanodiamond protection layer to reinforce the native solid-electrolyte interphase on Li metal. A double-layer film design was proposed to enhance the defect tolerance of the artificial interface, improving the macroscopic uniformity of the Li-ion flux; the exceptional mechanical property of a modulus of >200 GPa can be realized, which effectively arrested dendrite propagation, resulting in controlled Li deposition and significantly improved cycling efficiency.
Li2MnO3 has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li2MnO3, we ...clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li2MnO3, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li2MnO3 surface, which inspires Li-CO2/air batteries with Li2MnO3 acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li2MnO3 questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions.
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•Charge plateau of Li2MnO3 is from oxygen release and surface carbonate reactions•Mn(III/IV) redox is solely responsible for the reversible bulk Li2MnO3 cycling•Oxygen redox shares common nature in both Li-rich and conventional cathodes•Li2MnO3 and alkali-rich materials could be superior catalysts for Li–CO2/air batteries
In the debates on how to achieve high energy density battery cathodes, Li-rich compounds are often considered superior over conventional materials due to their high capacity associated with the oxygen redox reactions. Here, we clarify both the bulk and surface reaction mechanisms of Li2MnO3 during the initial and later cycles. Our results reveal that the initial charge plateau is from two types of surface activities, followed by predominating Mn redox reactions with no sign of reversible lattice oxygen redox. The surface chemistry of Li2MnO3 indicates a highly reactive surface to facilitate carbonate formation and decomposition, inspiring a Li-CO2/air battery with Li2MnO3 as a superior electrocatalyst. The comparison between Li-rich, conventional, and Li2MnO3 suggests that the oxygen redox in Li-rich and conventional materials is of the same nature and origin.
Li2MnO3 is a parent compound of Li-rich materials whose electrochemical activities are under debate. Mn and O redox reactions are analyzed both in the bulk and on the surface during the initial and later cycles. Mn(III/IV) redox dominates the bulk reversible reactions in Li2MnO3 with no lattice oxygen redox involved. The initial charge plateau is from various surface activities. Oxygen redox observed in Li-rich materials displays commonality in spectroscopic features compared with conventional materials. The highly reactive Li2MnO3 surface enables an efficient catalytic reaction in Li-CO2/air batteries.
Mechanical stimuli can modify the energy landscape of chemical reactions and enable reaction pathways, offering a synthetic strategy that complements conventional chemistry. These mechanochemical ...mechanisms have been studied extensively in one-dimensional polymers under tensile stress using ring-opening and reorganization, polymer unzipping and disulfide reduction as model reactions. In these systems, the pulling force stretches chemical bonds, initiating the reaction. Additionally, it has been shown that forces orthogonal to the chemical bonds can alter the rate of bond dissociation. However, these bond activation mechanisms have not been possible under isotropic, compressive stress (that is, hydrostatic pressure). Here we show that mechanochemistry through isotropic compression is possible by molecularly engineering structures that can translate macroscopic isotropic stress into molecular-level anisotropic strain. We engineer molecules with mechanically heterogeneous components-a compressible ('soft') mechanophore and incompressible ('hard') ligands. In these 'molecular anvils', isotropic stress leads to relative motions of the rigid ligands, anisotropically deforming the compressible mechanophore and activating bonds. Conversely, rigid ligands in steric contact impede relative motion, blocking reactivity. We combine experiments and computations to demonstrate hydrostatic-pressure-driven redox reactions in metal-organic chalcogenides that incorporate molecular elements that have heterogeneous compressibility, in which bending of bond angles or shearing of adjacent chains activates the metal-chalcogen bonds, leading to the formation of the elemental metal. These results reveal an unexplored reaction mechanism and suggest possible strategies for high-specificity mechanosynthesis.
Abstract
The field of two-dimensional (2D) ferromagnetism has been proliferating over the past few years, with ongoing interests in basic science and potential applications in spintronic technology. ...However, a high-resolution spectroscopic study of the 2D ferromagnet is still lacking due to the small size and air sensitivity of the exfoliated nanoflakes. Here, we report a thickness-dependent ferromagnetism in epitaxially grown Cr
2
Te
3
thin films and investigate the evolution of the underlying electronic structure by synergistic angle-resolved photoemission spectroscopy, scanning tunneling microscopy, x-ray absorption spectroscopy, and first-principle calculations. A conspicuous ferromagnetic transition from Stoner to Heisenberg-type is directly observed in the atomically thin limit, indicating that dimensionality is a powerful tuning knob to manipulate the novel properties of 2D magnetism. Monolayer Cr
2
Te
3
retains robust ferromagnetism, but with a suppressed Curie temperature, due to the drastic drop in the density of states near the Fermi level. Our results establish atomically thin Cr
2
Te
3
as an excellent platform to explore the dual nature of localized and itinerant ferromagnetism in 2D magnets.
The edges of a two-dimensional electron gas (2DEG) in the quantum Hall effect (QHE) regime are divided into alternating metallic and insulating strips, with their widths determined by the energy gaps ...of the QHE states and the electrostatic Coulomb interaction. Local probing of these submicrometer features, however, is challenging due to the buried 2DEG structures. Using a newly developed microwave impedance microscope, we demonstrate the real-space conductivity mapping of the edge and bulk states. The sizes, positions, and field dependence of the edge strips around the sample perimeter agree quantitatively with the self-consistent electrostatic picture. The evolution of microwave images as a function of magnetic fields provides rich microscopic information around the ν=2 QHE state.
Ultrathin topological insulator nanostructures, in which coupling between top and bottom surface states takes place, are of great intellectual and practical importance. Due to the weak van der Waals ...interaction between adjacent quintuple layers (QLs), the layered bismuth selenide (Bi2Se3), a single Dirac-cone topological insulator with a large bulk gap, can be exfoliated down to a few QLs. In this paper, we report the first controlled mechanical exfoliation of Bi2Se3 nanoribbons (>50 QLs) by an atomic force microscope (AFM) tip down to a single QL. Microwave impedance microscopy is employed to map out the local conductivity of such ultrathin nanoribbons, showing drastic difference in sheet resistance between 1−2 QLs and 4−5 QLs. Transport measurement carried out on an exfoliated (≤5 QLs) Bi2Se3 device shows nonmetallic temperature dependence of resistance, in sharp contrast to the metallic behavior seen in thick (>50 QLs) ribbons. These AFM-exfoliated thin nanoribbons afford interesting candidates for studying the transition from quantum spin Hall surface to edge states.
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