RESUMEN
Excitonic insulators are long-sought-after quantum materials predicted to spontaneously open a gap by the Bose condensation of bound electron-hole pairs, namely, excitons, in their ground state. Since the theoretical conjecture, extensive efforts have been devoted to pursuing excitonic insulator platforms for exploring macroscopic quantum phenomena in real materials. Reliable evidence of excitonic character has been obtained in layered chalcogenides as promising candidates. However, owing to the interference of intrinsic lattice instabilities, it is still debatable whether those features, such as the charge density wave and gap opening, are primarily driven by the excitonic effect or by the lattice transition. Herein, we develop an intercalation chemistry strategy for obtaining a novel charge-transfer excitonic insulator in organic-inorganic superlattice interfaces that serves as an ideal platform to decouple the excitonic effect from the lattice effect. In this system, we observe a narrow excitonic gap, formation of a charge density wave without periodic lattice distortion, and metal-insulator transition, providing visualized evidence of exciton condensation occurring in thermal equilibrium. Our findings identify self-assembly intercalation chemistry as a new strategy for developing novel excitonic insulators.
RESUMEN
Electron transfer plays an important role in determining the energy conversion efficiency of energy devices. Nitrogen-coordinated single metal sites (M-N4) materials as electrocatalysts have exhibited great potential in devices. However, there are still great difficulties in how to directionally manipulate electron transfer in M-N4 catalysts for higher efficiency. Herein, we demonstrated the mechanism of electron transfer being affected by energy level structure based on classical iron phthalocyanine (FePc) molecule/carbon models and proposed an energy level engineering strategy to manipulate electron transfer, preparing high-performance ORR catalysts. Engineering molecular energy level via modulating FePc molecular structure with nitro induces a strong interfacial electronic coupling and efficient charge transfer from carbon to FePc-ß-NO2 molecule. Consequently, the assembled zinc-air battery exhibits ultrahigh performance which is superior to most of M-N4 catalysts. Energy level engineering provides a universal approach for directionally manipulating electron transfer, bringing a new concept to design efficient and stable M-N4 electrocatalyst.
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The quinoid structure, a resonance structure of benzenoid, gives rise to peculiar chemical reactivity and physical properties. A complete characterization of its geometric and electronic properties on the atomic scale is of vital importance to understand and engineer the chemical and physical properties of quinoid molecules. Here, we report a real-space structural and electronic characterization of quinoid poly(para-phenylene) (PPP) chains by using noncontact atomic force microscopy and scanning tunneling microscopy. Our results reveal that quinoid PPP chains adopt a coplanar adsorption configuration on Cu(111) and host in-gap states near Fermi level. In addition, intra- and interchain hopping of quinoid structure are observed, indicative of a quasiparticle behavior originating from charge-lattice interactions. The experimental results are nicely reproduced by tight-binding calculations. Our study provides a comprehensive understanding of the structural and electronic properties of quinoid PPP chains in real space and may be further extended to address the dynamics of nonlinear excitations in quinoid molecules.
RESUMEN
In this work, we demonstrate the bottom-up on-surface synthesis of poly( para-dibenzo[ bc, kl]-coronenylene) (PPDBC), a zigzag edge-encased analog of poly( para-phenylene) (PPP), and its lateral fusion into zigzag edge-extended graphene nanoribbons (zeeGNRs). Toward this end, we designed a dihalogenated di( meta-xylyl)anthracene monomer displaying strategic methyl groups at the substituted phenyl ring and investigated its applicability as precursor in the thermally induced surface-assisted polymerization and cyclodehydrogenation. The structure of the resulting zigzag edge-rich (70%) polymer PPDBC was unambiguously confirmed by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Remarkably, by further thermal treatment at 450 °C two and three aligned PPDBC chains can be laterally fused into expanded zeeGNRs, with a ribbon width of nine ( N = 9) up to 17 ( N = 17) carbon atoms. Moreover, the resulting zeeGNRs exhibit a high ratio of zigzag (67%) vs armchair (25%) edge segments and feature electronic band gaps as low as 0.9 eV according to gaps quasiparticle calculations.
RESUMEN
On-surface synthesis provides a convenient route to many kinds of conjugated molecular nanostructures, but it has remained challenging to precisely control the reaction pathway for using multicomponent precursors. Herein, we demonstrate a two-step strategy to synthesize iron phthalocyanine (FePc) molecules using metal-organic coordination for templating by using high-resolution scanning tunnelling microscopy and non-contact atomic force microscopy. In a first step, 1,2,4,5-tetracyanobenzene (TCNB) precursors and Fe atoms self-assembly into Fe(TCNB)4 coordination complexes on a clean Au(111) surface. The Fe(TCNB)4 complexes further undergo cyclic tetramerization upon thermal annealing, forming single FePc molecules. We expect that our demonstrated synthetic strategy may shed light on the design and synthesis of two-dimensional extended conjugated systems.
RESUMEN
The current strategies for building 2D organic-inorganic heterojunctions involve mostly wet-chemistry processes or exfoliation and transfer, leading to interface contaminations, poor crystallizing, or limited size. Here we show a bottom-up procedure to fabricate 2D large-scale heterostructure with clean interface and highly-crystalline sheets. As a prototypical example, a well-ordered hydrogen-bonded organic framework is self-assembled on the highly-oriented-pyrolytic-graphite substrate. The organic framework adopts a honeycomb lattice with faulted/unfaulted halves in a unit cell, resemble to molecular "graphene". Interestingly, the topmost layer of substrate is self-lifted by organic framework via strong interlayer coupling, to form effectively a floating organic framework/graphene heterostructure. The individual layer of heterostructure inherits its intrinsic property, exhibiting distinct Dirac bands of graphene and narrow bands of organic framework. Our results demonstrate a promising approach to fabricate 2D organic-inorganic heterostructure with large-scale uniformity and highly-crystalline via the self-lifting effect, which is generally applicable to most of van der Waals materials.
RESUMEN
Establishing a general model of heterogeneous ice nucleation has long been challenging because of the surface water structures found on different substrates. Identifying common water clusters, regardless of the underlying substrate, is one of the key steps toward solving this problem. Here, we demonstrate the presence of a common water cluster found on both hydrophilic Pt(111) and hydrophobic Cu(111) surfaces using scanning tunneling microscopy and non-contact atomic force microscopy. Water molecules self-assemble into a structure with a central flat-lying hexagon and three fused pentagonal rings, forming a cluster consisting of 15 individual water molecules. This cluster serves as a critical nucleus during ice nucleation on both surfaces: ice growth beyond this cluster bifurcates to form two-dimensional (three-dimensional) layers on hydrophilic (hydrophobic) surfaces. Our results reveal the inherent similarity and distinction at the initial stage of ice growth on hydrophilic and hydrophobic close-packed metal surfaces; thus, these observations provide initial evidence toward a general model for water-substrate interaction.
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Ultrathin van der Waals (vdW) magnets are heavily pursued for potential applications in developing high-density miniaturized electronic/spintronic devices as well as for topological physics in low-dimensional structures. Despite the rapid advances in ultrathin ferromagnetic vdW magnets, the antiferromagnetic counterparts, as well as the antiferromagnetic junctions, are much less studied owing to the difficulties in both material fabrication and magnetism characterization. Ultrathin CrTe3 layers have been theoretically proposed to be a vdW antiferromagnetic semiconductor with intrinsic intralayer antiferromagnetism. Herein, the epitaxial growth of monolayer (ML) and bilayer CrTe3 on graphite surface is demonstrated. The structure, electronic and magnetic properties of the ML CrTe3 are characterized by combining scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy and confirmed by density functional theory calculations. The CrTe3 MLs can be further utilized for the fabrication of a lateral heterojunction consisting of ML CrTe2 and ML CrTe3 with an atomically sharp and seamless interface. Since ML CrTe2 is a metallic vdW magnet, such a heterostructure presents the first in-plane magnetic metal-semiconductor heterojunction made of two vdW materials. The successful fabrication of ultrathin antiferromagnetic CrTe3 , as well as the magnetic heterojunction, will stimulate the development of miniaturized antiferromagnetic spintronic devices based on vdW materials.
RESUMEN
Polyacetylene (PA) comprises one-dimensional chains of sp2-hybridized carbon atoms that may take a cis or trans configuration. Owing to its simple chemical structure and exceptional electronic properties, PA is an ideal system to understand the nature of charge transport in conducting polymers. Here, we report the on-surface synthesis of both cis- and trans-PA chains and their atomic-scale characterization. The structure of individual PA chains was imaged by non-contact atomic force microscopy, which confirmed the formation of PA by resolving single chemical bond units. Angle-resolved photoemission spectroscopy suggests a semiconductor-to-metal transition through doping-induced suppression of the Peierls bond alternation of trans-PA on Cu(110). Electronically decoupled trans-PAs exhibit a band gap of 2.4 eV following copper oxide intercalation. Our study provides a platform for studying individual PA chains in real and reciprocal space, which may be further extended to study the intrinsic properties of non-linear excitons in conducting polymers.
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We demonstrated an approach to effectively apply in-plane pressures to molecular layers by utilizing the substrate confinement effect. The compressed crystal structure and mechanical behaviors of carbon monoxide (CO) monolayer subjected to the confinement of Cu(100) substrate were jointly investigated by low temperature scanning tunneling microscopy experiments and first-principles density functional theory calculations. By increasing molecular coverage, an exceptionally large Young's modulus of 33 GPa was derived for the constrained CO monolayer film. This extreme in-plane pressure leads to site-specific tilting geometries, polymeric-like electronic states, and vibrational behaviors of CO molecules in the compressed phases. These results provide an extended understanding of the physical and chemical properties of intermolecular interactions in this fundamental system.
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We report a real-space visualization of the formation of hydrogen bonding in 8-hydroxyquinoline (8-hq) molecular assemblies on a Cu(111) substrate, using noncontact atomic force microscopy (NC-AFM). The atomically resolved molecular structures enable a precise determination of the characteristics of hydrogen bonding networks, including the bonding sites, orientations, and lengths. The observation of bond contrast was interpreted by ab initio density functional calculations, which indicated the electron density contribution from the hybridized electronic state of the hydrogen bond. Intermolecular coordination between the dehydrogenated 8-hq and Cu adatoms was also revealed by the submolecular resolution AFM characterization. The direct identification of local bonding configurations by NC-AFM would facilitate detailed investigations of intermolecular interactions in complex molecules with multiple active sites.
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We have studied for the first time the structural change of high-purity metallofullerene (Gd@C(82)) upon heat treatment in an ultra-high vacuum system (10(-10) Torr) and examined the decomposition product through successive analysis with MS, IR, Raman, TEM, EDS and XPS. It was found that metallofullerene (Gd@C(82)) had fully collapsed at 580 °C which was lower than that for the complete destruction of C(60). The easier decomposition should be ascribed to the encapsulated metal in the carbon cage which could induce the deformation of the C-C bond. The analysis indicated that the broken metallofullerene (Gd@C(82)) became a kind of graphite-like material with a lot of defects. The Gd atoms leaked out from the carbon cage and aggregated together to form a regular arrangement.