Scanning Tunneling Spectroscopy Measurements Research Articles

Multiple Electronic Phases Coexisting under Inhomogeneous Strains in the Correlated Insulator.

Monolayer transition metal dichalcogenides (TMDs) can host exotic phenomena such as correlated insulating and charge-density-wave (CDW) phases. Such properties are strongly dependent on the precise atomic arrangements. Strain, as an effective tuning parameter in atomic arrangements, has been widely used for tailoring material's structures and related properties, yet to date, a convincing demonstration of strain-induced dedicate phase transition at nanometer scale in monolayer TMDs has been lacking. Here, a strain engineering technique is developed to controllably introduce out-of-plane atomic deformations in monolayer CDW material 1T-NbSe2 . The scanning tunneling microscopy and spectroscopy (STM and STS) measurements, accompanied by first-principles calculations, demonstrate that the CDW phase of 1T-NbSe2 can survive under both tensile and compressive strains even up to 5%. Moreover, significant strain-induced phase transitions are observed, i.e., tensile (compressive) strains can drive 1T-NbSe2 from an intrinsic-correlated insulator into a band insulator (metal). Furthermore, experimental evidence of the multiple electronic phase coexistence at the nanoscale is provided. The results shed new lights on the strain engineering of correlated insulator and useful for design and development of strain-related nanodevices.

Atomic structure and peculiarities of the electronic properties of Br layers on Ag(111) at different coverages

The structure and electronic properties of Br layers on Ag(111) at different coverage were studied by low-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy (STS) and density functional theory (DFT) calculations. The Br layers are found to form a hexagonal (3×3)R30∘ structure for coverage ⁓ 0.3 monolayer (ML) and a striped (61−12) structure for coverage ⁓ 0.4 ML. STS measurements reveal suppressed differential conductance for both structures at voltages ranging between − 1.2 and 1.4 V, and surprisingly intense peaks at 1.5 V and 2.0 V. The position of the high-intensity peak in the spectra for the striped structure shows lateral variations within the unit cell and is determined by the Br coverage. In our structural model for the striped structure, supported by DFT calculations, the hexagonal (3×3)R30∘ structure transforms to the striped structure due to uniaxial inhomogeneous compression of Br atomic rows. The local work function for the Br/Ag(111) surface, estimated from dz/dV(V) measurements, is 5.0 ± 0.2 eV, which is higher than for the Ag(111) surface (4.4 ± 0.1 eV). This suggests an increase in the electrostatic dipole barrier on Ag(111) due to Br adsorption. Combined with the suppressed local density of states around the Fermi level observed by STS, these results clarify the usefulness of Br layers in reducing the coupling between Ag(111) electrons and adsorbates.

Puddle formation and persistent gaps across the non-mean-field breakdown of superconductivity in overdoped (Pb,Bi)2Sr2CuO6+δ

The cuprate high-temperature superconductors exhibit many unexplained electronic phases, but the superconductivity at high doping is often believed to be governed by conventional mean-field Bardeen–Cooper–Schrieffer theory1. However, it was shown that the superfluid density vanishes when the transition temperature goes to zero2,3, in contradiction to expectations from Bardeen–Cooper–Schrieffer theory. Our scanning tunnelling spectroscopy measurements in the overdoped regime of the (Pb,Bi)2Sr2CuO6+δ high-temperature superconductor show that this is due to the emergence of nanoscale superconducting puddles in a metallic matrix4,5. Our measurements further reveal that this puddling is driven by gap filling instead of gap closing. The important implication is that it is not a diminishing pairing interaction that causes the breakdown of superconductivity. Unexpectedly, the measured gap-to-filling correlation also reveals that pair breaking by disorder does not play a dominant role and that the mechanism of superconductivity in overdoped cuprate superconductors is qualitatively different from conventional mean-field theory.

Mesoscopic 2D molecular self-assembly on an insulator

Two-dimensional (2D) nanostructures and nanomaterials offer potential for a wide range of technological applications in electronics, optoelectronics, data storage, sensing and catalysis. On-surface molecular self-assembly—where organic molecules act as building blocks and where surfaces play the role of supporting templates—allows for the bottom-up synthesis of such 2D systems with tuneable atomically precise morphologies and tailored electronic properties. These self-assembly protocols are well established on metal surfaces, but remain limited on electronically gapped substrates (insulators, semiconductors). The latter are useful for preventing electronic coupling (that is, hybridization between molecular assembly and underlying surface) and for avoiding quenching of optical processes, necessary for prospective electronic and optoelectronic applications. In particular, molecular self-assembly on surfaces other than weakly interacting metals can be challenging due to substrate reactivity, defects and inhomogeneities, resulting in intricate energy landscapes that limit the growth kinetically and hampers the synthesis of large-area defect-free 2D systems. Here, we demonstrate the self-assembly of a 2D, atomically thin organic molecular film on a model wide bandgap 2D insulator, single-layer hexagonal boron nitride (hBN) on Cu(111). The molecular film consists of flat, aromatic 9,10-di-cyano-anthracene (DCA) molecules. Our low-temperature scanning tunnelling microscopy and spectroscopy measurements revealed mesoscopic (>100 × 100 nm2), topographically homogeneous crystalline molecular domains resulting from flat molecular adsorption and noncovalent in-plane cyano-ring bonding, with electronically decoupled molecular orbitals (MOs) lying within the hBN electronic gap. These MOs exhibit an energy level spatial modulation (∼300 meV) that follows the moiré work function variation of hBN on Cu(111). These findings pave the way for large-area, atomically precise, highly crystalline 2D organic (and metal-organic) nanomaterials on electronically functional wide bandgap insulators.

Continuum effective Hamiltonian for graphene bilayers for an arbitrary smooth lattice deformation from microscopic theories

We provide a systematic real space derivation of the continuum Hamiltonian for a graphene bilayer starting from a microscopic lattice theory, allowing for an arbitrary inhomogeneous smooth lattice deformation, including a twist. Two different microscopic models are analyzed: first, a Slater-Koster like model and second, ab-initio derived model. We envision that our effective Hamiltonian can be used in conjunction with an experimentally determined atomic lattice deformation in twisted bilayer graphene in a specific device to predict and compare the electronic spectra with scanning tunneling spectroscopy measurements. As a byproduct, our approach provides electron-phonon couplings in the continuum Hamiltonian from microscopic models for any bilayer stacking. In the companion paper we analyze in detail the continuum models for relaxed atomic configurations of magic angle twisted bilayer graphene.

On-Surface Synthesis of Nanographenes and Graphene Nanoribbons on Titanium Dioxide.

The formation of two types of nanographenes from custom designed and synthesized molecular precursors has been achieved through thermally induced intramolecular cyclodehydrogenation reactions on the semiconducting TiO2(110)-(1×1) surface, confirmed by the combination of high-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) measurements, and corroborated by theoretical modeling. The application of this protocol on differently shaped molecular precursors demonstrates the ability to induce a highly efficient planarization reaction both within strained pentahelicenes as well as between vicinal phenyl rings. Additionally, by the combination of successive Ullmann-type polymerization and cyclodehydrogenation reactions, the archetypic 7-armchair graphene nanoribbons (7-AGNRs) have also been fabricated on the titanium dioxide surface from the standard 10,10'-dibromo-9,9'-bianthryl (DBBA) molecular precursors. These examples of the effective cyclodehydrogenative planarization processes provide perspectives for the rational design and synthesis of molecular nanostructures on semiconductors.

Temperature-Driven Twin Structure Formation and Electronic Structure of Epitaxially Grown Mg3Sb2 Films on Mismatched Substrates.

Mg3Sb2-based compounds are one type of important room-temperature thermoelectric materials and the appropriate candidate of type-II nodal line semimetals. In Mg3Sb2-based films, compelling research topics such as dimensionality reduction and topological states rely on the controllable preparation of films with high crystallinity, which remains a big challenge. In this work, high quality Mg3Sb2 films are successfully grown on mismatched substrates of sapphire (000l), while the temperature-driven twin structure evolution and characteristics of the electronic structure are revealed in the as-grown Mg3Sb2 films by in situ and ex situ measurements. The transition of layer-to-island growth of Mg3Sb2 films is kinetically controlled by increasing the substrate temperature (Tsub), which is accompanied with the rational manipulation of twin structure and epitaxial strains. Twin-free structure could be acquired in the Mg3Sb2 film grown at a low Tsub of 573 K, while the formation of twin structure is significantly promoted by elevating the Tsub and annealing, in close relation to the processes of strain relaxation and enhanced mass transfer. Measurements of scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) elucidate the intrinsic p-type conduction of Mg3Sb2 films and a bulk band gap of ~0.89 eV, and a prominent Fermi level downshift of ~0.2 eV could be achieved by controlling the film growth parameters. As elucidated in this work, the effective manipulation of the epitaxial strains, twin structure and Fermi level is instructive and beneficial for the further exploration and optimization of thermoelectric and topological properties of Mg3Sb2-based films.

Gate-Tunable Resonance State and Screening Effects for Proton-Like Atomic Charge in Graphene.

The ability to create a robust and well-defined artificial atomic charge in graphene and understand its carrier-dependent electronic properties represents an important goal toward the development of graphene-based quantum devices. Herein, we devise a new pathway toward the atomically precise embodiment of point charges into a graphene lattice by posterior (N) ion implantation into a back-gated graphene device. The N dopant behaves as an in-plane proton-like charge manifested by formation of the characteristic resonance state in the conduction band. Scanning tunneling spectroscopy measurements at varied charge carrier densities reveal a giant energetic renormalization of the resonance state up to 220 meV with respect to the Dirac point, accompanied by the observation of gate-tunable long-range screening effects close to individual N dopants. Joint density functional theory and tight-binding calculations with modified perturbation potential corroborate experimental findings and highlight the short-range character of N-induced perturbation.

Spatial segregation of substitutional B atoms in graphene patterned by the moiré superlattice on Ir(111)

Fabrication of ordered structures at the nanoscale limit poses a cornerstone challenge for modern technologies. In this work we show how naturally occurring moiré patterns in Ir(111)-supported graphene template the formation of 2D ordered arrays of substitutional boron species. A complementary experimental and theoretical approach provides a comprehensive description of the boron species distribution, bonding configurations, interfacial interaction with Ir(111) and the impact on graphene's electronic structure. Atomically-resolved scanning tunnelling microscopy images and density functional theory calculations reveal that boron preferably forms small aggregates of substitutional defects in geometrically low regions of the moiré superlattice of graphene, by inducing local bending of graphene towards the underlying Ir(111). Surprisingly, calculations reveal that the incorporation of electron deficient boron does not lead to an enhanced p-type doping, as the local rippling of the graphene layer prompts electron uptake from the iridium substrate that compensates the initial electron loss due to substitutional boron. Scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy measurements corroborate that the arrays of boron species do not modify the electronic structure of graphene near the Fermi level, hence preserving the slight p-type doping induced by Ir(111).

Aromatic Ring Fusion to Benzoporphyrin via γ-ortho Cyclodehydrogenation on a Ag(111) Surface.

Aromatic ring fusion to porphyrins and their derivatives represents an attractive route to tune the molecular conjugation and thus expand their functionalities. Here, we report the expansion of the aromatic π-system of palladium tetraphenyltetrabenzoporphyrins (Pd-TPTBP) via surface-assisted γ-ortho cyclodehydrogenation on Ag(111). The chemical transformation of Pd-TPTBP into different products at an elevated temperature of 600 K was revealed at the single-molecule level using bond-resolved scanning tunneling microscopy with a CO-functionalized tip. We captured a series of γ-ortho cyclodehydrogenation products, wherein the maximum extent to which the reaction can progress is associated with 7-fold C-C formation to afford nearly planar γ-ortho fused porphyrins with 66 conjugated π-electrons. In addition, a small number of molecules undergo C-C bond dissociation of meso-phenyl at elevated temperature, producing fully planar γ-ortho fused products lacking one or two phenyl moieties. Scanning tunneling spectroscopy measurements and DFT calculations suggest the electronic gap of the γ-ortho fused porphyrin decreases compared to that of the precursor. The HOMO and LUMO of the planar γ-ortho fused products are localized on the partially fused benzo moieties and the meso-position, respectively.

Surface and electronic structure at atomic length scales of the nonsymmorphic antiferromagnet Eu5In2Sb6

We performed scanning tunneling microscopy and spectroscopy (STM/STS) measurements to investigate the Zintl phase ${\mathrm{Eu}}_{5}{\mathrm{In}}_{2}{\mathrm{Sb}}_{6}$, a nonsymmorphic antiferromagnet. The theoretical prediction of a nontrivial Fermi surface topology stabilized by the nonsymmorphic symmetry motivated our research. On the cleaved (010) plane, we obtained striped patterns that can be correlated to the stacking of the [${\mathrm{In}}_{2}{\mathrm{Sb}}_{6}{]}^{10\ensuremath{-}}$ double chains along the crystallographic $c$ axis. The attempted cleavage along the $a$ axis revealed a more complex pattern. We combined the STS measurement on nonreconstructed (010) and (081) surfaces with DFT calculations to further elucidate the electronic structure of ${\mathrm{Eu}}_{5}{\mathrm{In}}_{2}{\mathrm{Sb}}_{6}$. From our investigations so far, direct experimental evidence of the predicted topological surface states remains elusive.

Interplay between π-Conjugation and Exchange Magnetism in One-Dimensional Porphyrinoid Polymers.

The synthesis of novel polymeric materials with porphyrinoid compounds as key components of the repeating units attracts widespread interest from several scientific fields in view of their extraordinary variety of functional properties with potential applications in a wide range of highly significant technologies. The vast majority of such polymers present a closed-shell ground state, and, only recently, as the result of improved synthetic strategies, the engineering of open-shell porphyrinoid polymers with spin delocalization along the conjugation length has been achieved. Here, we present a combined strategy toward the fabrication of one-dimensional porphyrinoid-based polymers homocoupled via surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituents on Au(111). Scanning tunneling microscopy and noncontact atomic force microscopy describe the thermal-activated intra- and intermolecular oxidative ring closure reactions as well as the controlled tip-induced hydrogen dissociation from the porphyrinoid units. In addition, scanning tunneling spectroscopy measurements, complemented by computational investigations, reveal the open-shell character, that is, the antiferromagnetic singlet ground state (S = 0) of the formed polymers, characterized by singlet–triplet inelastic excitations observed between spins of adjacent porphyrinoid units. Our approach sheds light on the crucial relevance of the π-conjugation in the correlations between spins, while expanding the on-surface synthesis toolbox and opening avenues toward the synthesis of innovative functional nanomaterials with prospects in carbon-based spintronics.

Nanopore-Patterned CuSe Drives the Realization of the PbSe-CuSe Lateral Heterostructure.

Monolayer PbSe has been predicted to be a two-dimensional (2D) topological crystalline insulator (TCI) with crystalline symmetry-protected Dirac-cone-like edge states. Recently, few-layered epitaxial PbSe has been grown on the SrTiO3 substrate successfully, but the corresponding signature of the TCI was only observed for films not thinner than seven monolayers, largely due to interfacial strain. Here, we demonstrate a two-step method based on molecular beam epitaxy for the growth of the PbSe-CuSe lateral heterostructure on the Cu(111) substrate, in which we observe a nanopore-patterned CuSe layer that acts as the template for lateral epitaxial growth of PbSe. This further results in a PbSe-CuSe lateral heterostructure with an atomically sharp interface. Scanning tunneling microscopy and spectroscopy measurements reveal a fourfold symmetric square lattice of such PbSe with a quasi-particle band gap of 1.8 eV, a value highly comparable with the theoretical value of freestanding PbSe. The weak monolayer-substrate interaction is further supported by both density functional theory (DFT) and projected crystal orbital Hamilton population, with the former predicting the monolayer's anti-bond state to reside below the Fermi level. Our work demonstrates a practical strategy to fabricate a high-quality in-plane heterostructure, involving a monolayer TCI, which is viable for further exploration of the topology-derived quantum physics and phenomena in the monolayer limit.

(Invited, Digital Presentation) Exciton Controlled-NOT Gate Realized By Carbon Nanotube Coupled Quantum Dots

Among various methods for constructing the quantum bits (qubits) required to perform the quantum computing, there is the concept to use the exciton in nano-structured material such as semiconductor quantum dot (QD), colloidal nanocrystal, or molecule. When the presence/absence of exciton is associated with two-level system of |1>/|0>, the Rabi oscillation of exciton serves as a basis for one-qubit operation. In order to realize the controlled-NOT (CNOT) gate which is essential for the quantum computing, one needs four-level system consisting of two qubits (i.e., |0>|0>, |1>|0>, |0>|1>, and |1>|1>). The construction of exciton CNOT gate is theoretically possible, however, the experimental demonstration has not been conducted. The optical processes of carbon nanotubes are governed by excitons. We attempted to realize the exciton CNOT gate by using the coupled QDs formed in a carbon nanotube.We fabricated two QDs by terminating both ends of a single-walled carbon nanotube (SWNT) with the collagen model peptides via peptide bonds. The scanning tunneling microscopy and spectroscopy measurements revealed that two QDs of ~ 3.1 nm and ~ 3.9 nm in widths, respectively, were formed in that SWNT with the inter-dot spacing of ~ 5.4 nm. It was confirmed by observing the Rabi oscillations that each QD acted as the exciton two-level system. When the excitons were produced in both QDs, additional exciton emission lines appeared, which may be originating from the coupled state of excitons across two QDs. We define the exciton four-level system as follows; |0>|0> when both QDs are empty, |1>|0>/|0>|1> when exciton is present in either of two QDs, and |1>|1> when excitons coupled across two QDs are generated. |1>|0>, |0>|1>, or |1>|1> could be prepared by irradiating the corresponding π pulse determined from the Rabi oscillation.The CNOT gate consists of a control qubit and a target qubit, and the logic of the target qubit is flipped only if the control qubit is in logic |1>. When we regard two QDs in our sample as a pair of control qubit and target qubit, the conditional flip of the target qubit corresponds to the π pulse-induced transition between |1>|1> and |1>|0> or between |1>|1> and |0>|1> depending on which QD we choose for the target qubit. The exciton CNOT gate operations were executed by irradiating that π pulse onto the states prepared as inputs. The input/output relations were measured in both ZZ and XX bases, which allowed us to estimate the quantum process fidelity. Our exciton CNOT gate actualized the expected operations whichever QD we used as the target qubit. By executing the CNOT gate operations three times, logic exchange is made between |1>|0> and |0>|1>, which is called the SWAP gate. It was also ascertained that the exchange of the exciton population took place between two QDs when conducting the SWAP operation by applying our CNOT gate.

(Invited) On-Surface Synthesis of Higher Acenes Using Precursors

Acenes are polycyclic aromatic hydrocarbons made up of linearly fused benzene rings. They are the smallest units of graphene and graphene nanoribbons, and have been extensively studied as organic semiconductor materials. Higher acenes are expected to have smaller band gaps required for practical use as organic electronic materials, but those with more than six benzene rings are poorly soluble in common organic solvents and unstable in aerobic condition to be synthesized and purified with traditional organic synthesis method. To overcome the obstacles, a precursor approach is a useful technique, where the soluble and stable precursors are purified enough and then converted to less-soluble and unstable target acenes in-situ.[1,2]Recently surface-assisted synthesis of higher acenes using precursors have attracted much attentions. Under ultra-high vacuum atmosphere, unstable acenes can be prepared from precursors by annealing, photoirradiation, or scanning tunneling microscopy (STM)-tip treatment on metal surface. We were successful to synthesize bis-diketone precursors of heptacene and nonacene. [3-6] The precursors were vacuum-deposited on Au(111) surface and converted to heptacene and nonacene by photoirradiation. Through combined STM, non-contact atomic force microscopy(nc-AFM) and scanning tunneling spectroscopy (STS) measurements, together with state-of-the-art first principles calculations, insight into the chemical and electronic structure of these elusive compounds was successful. It was revealed that nonacene has open-shell biradical structure in the ground state on Au(111). We were also successful to prepare tetraazaundecacene on Au(111) surface from bicyclo[2.2.2]octadiene (BCOD) precursor. [7] The tip-induced release of the protecting group can be employed to produce the targeted tetraazaundecacene species. The experimental frontier orbital gap, derived from the difference between PIR and NIR, is 1.35 eV. In this talk, the attraction and possibility of the “precursor approach” for the synthesis and application of acene compounds will be overviewed.References M. Suzuki, T. Aotake, Y. Yamaguchi, N. Noguchi, H. Nakano, K. Nakayama, H. Yamada, J. Photochem. Photobiol. C: Photochem. Rev. 2014, 18, 50-70.H. Yamada, D. Kuzuhara, M. Suzuki, H. Hayashi, N. Aratani, Bull. Chem. Soc. Jpn. 2020, 93, 1234-1267.J. I. Urgel, S. Mishra, H. Hayashi, J. Wilhelm, C. A. Pignedoli, M. Di Giovannantonio, R. Widmer, M. Yamashita, N. Hieda, P. Ruffieux, H. Yamada, R. Fasel, Nat. Commun. 2019, 10, 861–864.J. I. Urgel, H. Hayashi, M. Di Giovannantonio, C. A. Pignedoli, S. Mishra, O. Deniz, M. Yamashita, T. Dienel, P. Ruffieux, H. Yamada, R. Fasel, J. Am. Chem. Soc. 2017, 139, 11658–11661.J. I. Urgel, M. Di Giovannantonio, G. Gandus, Q. Chen, X. Liu, H. Hayashi, P. Ruffieux, S. Decurtins, A. Narita, D. Passerone, H. Yamada, S.-X. Liu, K. Müllen, C. A. Pignedoli, R. Fasel, ChemPhysChem 2019, 20, 2360-2366.C. G. Ayani, M. Pisarra, J. I. Urgel, J. J. Navarro, C. Díaz, H. Hayashi, H. Yamada, F. Calleja, R. Miranda, R. Fasel, F. Martín, A. L. Vázquez de Parga, Nanoscale Horiz. 2021, 6, 744-750.K. Eimre, J. I. Urgel, H. Hayashi, M. D. Giovannantonio, P. Ruffieux, S. Sato, S. Ohtomo, Y. S. Chan, N. Aratani, D. Passerone, O. Gröning, H. Yamada, R. Fasel, C. A. Pignedoli, Nat. Commun. 2021. in press.

Mott versus Hybridization Gap in the Low-Temperature Phase of 1T-TaS_{2}.

We address the long-standing problem of the ground state of 1T-TaS_{2} by computing the correlated electronic structure of stacked bilayers using the GW+EDMFT method. Depending on the surface termination, the semi-infinite uncorrelated system is either band insulating or exhibits a metallic surface state. For realistic values of the on-site and inter-site interactions, a Mott gap opens in the surface state, but it is smaller than the gap originating from the bilayer structure. Our results are consistent with recent scanning tunneling spectroscopy measurements for different terminating layers, and with our own photoemission measurements, which indicate the coexistence of spatial regions with different gaps in the electronic spectrum. By comparison to exact diagonalization data, we clarify the interplay between Mott insulating and band insulating behavior in this archetypal layered system.

Persistence of correlation-driven surface states in SmB6 under pressure

The proposed topological Kondo insulator SmB$_{6}$ hosts a bulk Kondo hybridization gap that stems from strong electronic correlations and a metallic surface state whose effective mass remains disputed. Thermopower and scanning tunneling spectroscopy measurements argue for heavy surface states that also stem from strong correlations, whereas quantum oscillation and angle-resolved photoemission measurements reveal light effective masses that would be consistent with a Kondo breakdown scenario at the surface. Here we investigate the evolution of the surface state via electrical and thermoelectric transport measurements under hydrostatic pressure, a clean symmetry-preserving tuning parameter that suppresses the Kondo gap and increases the valence of Sm from 2.6+ towards a 3+ magnetic metallic state. Electrical resistivity measurements reveal that the surface carrier density increases with increasing pressure, whereas thermopower measurements show an unchanged Fermi energy under pressure. As a result, the effective mass of the surface state charge carriers linearly increases with pressure as the Sm valence approaches 3+. Our results are consistent with the presence of correlation-driven surface states in SmB$_{6}$ and suggest that the surface Kondo effect persists under pressure to 2 GPa.

On-Surface Synthesis of C144 Hexagonal Coronoid with Zigzag Edges.

Coronoids as polycyclic aromatic macrocycles enclosing a cavity have attracted a lot of attention due to their distinctive molecular and electronic structures. They can be also regarded as nanoporous graphene molecules whose electronic properties are critically dependent on the size and topology of their outer and inner peripheries. However, because of their synthetic challenges, the extended hexagonal coronoids with zigzag outer edges have not been reported yet. Here, we report the on-surface synthesis of C144 hexagonal coronoid with outer zigzag edges on a designed precursor undergoing hierarchical Ullmann coupling and cyclodehydrogenation on the Au(111) surface. The molecular structure is unambiguously characterized by bond-resolved noncontact atomic force microscopy imaging. The electronic properties are further investigated by scanning tunneling spectroscopy measurements, in combination with the density functional theory calculations. Moreover, the values of the harmonic oscillator model of aromaticity are derived from calculations that suggest that the molecular structure is ideally represented by Clar's model. Our results provide approaches toward realizing a hexagonal coronoid with zigzag edges, potentially inspiring fabrication of hexagonal zigzag coronoids with multiple radical characters in the future.

Low-Dimensional Porous Carbon Networks Using Single-/Triple-Coupling Polycyclic Hydrocarbon Precursors.

Polycyclic hydrocarbons (PHs) share the same hexagonal structure of sp2 carbons as graphene but possess an energy gap due to quantum confinement effect. PHs can be synthesized by a bottom-up strategy starting from small building blocks covalently bonded into large 2D organic sheets. Further investigation of the role of the covalent bonding/coupling ways on their electronic properties is needed. Here, we demonstrate a surface-mediated synthesis of hexa-peri-hexabenzocoronene (HBC) and its extended HBC oligomers (dimers, trimers, and tetramers) via single- and triple-coupling ways and reveal the implication of different covalent bonding on their electronic properties. High-resolution low-temperature scanning tunneling microscopy and noncontact atomic force microscopy are employed to in situ determine the atomic structures of as-synthesized HBC oligomers. Scanning tunneling spectroscopy measurements show that the length extension of HBC oligomers narrows the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Furthermore, the energy gaps of triple-coupling HBC oligomers are smaller and decrease more significantly than that of the single-coupling ones. We hypothesize that the triple coupling promotes a more effective delocalization of π-electrons than the single coupling, according to density functional theory calculations. We also demonstrate that the HBC oligomers can further extend across the substrate steps to achieve conjugated polymers and large-area porous carbon networks.

Visualization of edge-modulated charge-density-wave orders in monolayer transition-metal-dichalcogenide metal

In two-dimensional materials with the many-body quantum states, edges become especially significant for realizing a host of physical phenomena and for potential applications in nanodevices. Here, we report the successful construction of ultra-flat monolayer 1H-phase niobium diselenide (NbSe2) with atomically sharp zigzag edges. Our scanning tunneling microscopy and spectroscopy measurements reveal that such zigzag edges hold intriguing one-dimensional edge states. Moreover, we observe an obvious energy-dependent charge-density-wave (CDW) modulation near the edge, highlighting the significant edge-CDW interference interactions. Our findings provide a comprehensive study of tunable structural and electronic properties at the edges in monolayer NbSe2. More importantly, the edge-CDW interference model can be feasible for other CDW metals, suggesting a promising direction of extending desired edge functionalities.