Topologically Localized Vibronic Excitations in Second-Layer Graphene Nanoribbons.

It is of fundamental importance to characterize the intrinsic properties, like the topological end states, in the on-surface synthesized graphene nanoribbons (GNRs), but the strong electronic interaction with the metal substrate usually smears out their characteristic features. Here, we report our approach to investigate the vibronic excitations of the topological end states in self-decoupled second-layer GNRs, which are grown using an on-surface squeezing-induced spillover strategy. The vibronic progressions show highly spatially localized distributions at the second-layer GNR ends, which can be ascribed to the decoupling-extended lifetime of charging through resonant electron tunneling at the topological end states.

Artificial kagome lattices of Shockley surface states patterned by halogen hydrogen-bonded organic frameworks.

Artificial electronic kagome lattices may emerge from electronic potential landscapes using customized structures with exotic supersymmetries, benefiting from the confinement of Shockley surface-state electrons on coinage metals, which offers a flexible approach to realizing intriguing quantum phases of matter that are highly desired but scarce in available kagome materials. Here, we devise a general strategy to construct varieties of electronic kagome lattices by utilizing the on-surface synthesis of halogen hydrogen-bonded organic frameworks (XHOFs). As a proof of concept, we demonstrate three XHOFs on Ag(111) and Au(111) surfaces, which correspondingly deliver regular, breathing, and chiral breathing diatomic-kagome lattices with patterned potential landscapes, showing evident topological edge states at the interfaces.

On-Surface Synthesis of Graphene Nanoribbons with Atomically Precise Structural Heterogeneities and On-Site Characterizations.

Graphene nanoribbons (GNRs) are strips of graphene, with widths of a few nanometers, that are promising candidates for future applications in nanodevices and quantum information processing due to their highly tunable structure-dependent electronic, spintronic, topological, and optical properties. Implantation of periodic structural heterogeneities, such as heteroatoms, nanopores, and non-hexagonal rings, has become a powerful manner for tailoring the designer properties of GNRs. The bottom-up synthesis approach, by combining on-surface chemical reactions based on rationally designed molecular precursors and tip-based microscopic and spectroscopic techniques, promotes the construction of atomically precise GNRs with periodic structural modulations.

Revealing Intramolecular Isotope Effects with Chemical-Bond Precision.

Isotope substitution of a molecule not only changes its vibrational frequencies but also changes its vibrational distributions in real-space. Quantitatively measuring the isotope effects inside a polyatomic molecule requires both energy and spatial resolutions at the single-bond level, which has been a long-lasting challenge in macroscopic techniques. By achieving ångström resolution in tip-enhanced Raman spectroscopy (TERS), we record the corresponding local vibrational modes of pentacene and its fully deuterated form, enabling us to identify and measure the isotope effect of each vibrational mode.

Dynamic Metal-Ligand Coordination Boosts CO Electroreduction.

The interfacial structure of heterogeneous catalysts determines the reaction rate by adjusting the adsorption behavior of reaction intermediates. Unfortunately, the catalytic performance of conventionally static active sites has always been limited by the adsorbate linear scaling relationship. Herein, we develop a triazole-modified Ag crystal (Ag crystal-triazole) with dynamic and reversible interfacial structures to break such a relationship for boosting the catalytic activity of CO electroreduction into CO.

Remote-Triggered Domino-like Cyclodehydrogenation in Second-Layer Topological Graphene Nanoribbons.

Cyclodehydrogenation reactions in the on-surface synthesis of graphene nanoribbons (GNRs) usually involve a series of C-C and/or C-C couplings and just happen on uncovered metal or metal oxide surfaces. It is still a big challenge to extend the growth of second-layer GNRs in the absence of necessary catalytic sites. Here, we demonstrate the direct growth of topologically nontrivial GNRs via multistep C-C and C-C couplings in the second layer by annealing designed bowtie-shaped precursor molecules over one monolayer on the Au(111) surface.

Self-Limited Embedding Alternating 585-Ringed Divacancies and Metal Atoms into Graphene Nanoribbons.

Because of their theoretically predicted intriguing properties, it is interesting to embed periodic 585-ringed divacancies into graphene nanoribbons (GNRs), but it remains a great challenge. Here, we develop an on-surface cascade reaction from periodic hydrogenated divacancies to alternating 585-ringed divacancies and Ag atoms via intramolecular cyclodehydrogenation in a seven-carbon-wide armchair GNR on the Ag(111) surface. Combining scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy combined with first-principles calculations, we in-situ-monitor the evolution of the distinct structural and electronic properties in the reaction intermediates.

Step-Assisted On-Surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies.

The bottom-up approach through on-surface synthesis of porous graphene nanoribbons (GNRs) presents a controllable manner for implanting periodic nanostructures to tune the electronic properties of GNRs in addition to bandgap engineering by width and edge configurations. However, owing to the existing steric hindrance in small pores like divacancies, it is still difficult to embed periodic divacancies with a nonplanar configuration into GNRs. Here, we demonstrate the on-surface synthesis of atomically precise eight-carbon-wide armchair GNRs embedded with periodic divacancies (DV8-aGNRs) by utilizing the monatomic step edges on the Au(111) surface.

On-surface cyclodehydrogenation reaction pathway determined by selective molecular deuterations.

Understanding the reaction mechanisms of dehydrogenative C-C coupling is the key to directed formation of π-extended polycyclic aromatic hydrocarbons. Here we utilize isotopic labeling to identify the exact pathway of cyclodehydrogenation reaction in the on-surface synthesis of model atomically precise graphene nanoribbons (GNRs). Using selectively deuterated molecular precursors, we grow seven-atom-wide armchair GNRs on a Au(111) surface that display a specific hydrogen/deuterium (H/D) pattern with characteristic Raman modes.

Reversible Superconductor-Insulator Transition in (Li, Fe)OHFeSe Flakes Visualized by Gate-Tunable Scanning Tunneling Spectroscopy.

Electric field control of charge carrier density provides a key technology to continuously tune the ground states and map out the phase diagram of correlated electron systems in one device. This technique is highly expected to be combined with the modern state-of-the art spectroscopic probes, such as angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy (STM/S), to efficiently address these states and the underlying physics. However, it is extremely difficult and not successful so far, mainly because the fabrication process of such devices makes them prohibitive for surface probes.

Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence.

Solid-state narrow-band light emitters are on-demand for quantum optoelectronics. Current approaches based on defect engineering in low-dimensional materials usually introduce a broad range of emission centers. Here, we report narrow-band light emission from covalent heterostructures fused to the edges of graphene nanoribbons (GNRs) by controllable on-surface reactions from molecular precursors.

Step edge-mediated assembly of periodic arrays of long graphene nanoribbons on Au(111).

The influence of substrate steps on the bottom-up synthesis of atomically precise graphene nanoribbons (GNRs) on an Au(111) surface is investigated. Straight surface steps are found to promote the assembly of long and compact arrays of polymers with enhanced interchain π-π stacking interactions, which create a steric hindrance effect on cyclodehydrogenation to suppress the H passivation of polymer ends. The modified two-stage growth process results in periodic arrays of GNRs with doubled average length near step edges.

Landau Quantization of a Narrow Doubly-Folded Wrinkle in Monolayer Graphene.

Folding can be an effective way to tailor the electronic properties of graphene and has attracted wide study interest in finding its novel properties. Here we present the experimental characterizations of the structural and electronic properties of a narrow graphene wrinkle on a SiO/Si substrate using scanning tunneling microscopy/spectroscopy. Pronounced and nearly equally separated conductance peaks are observed in the d I/d V spectra of the wrinkle.

Tuning the Doping Types in Graphene Sheets by N Monoelement.

The doping types of graphene sheets are generally tuned by different dopants with either three or five valence electrons. As a five-valence-electrons element, however, nitrogen dopants in graphene sheets have several substitutional geometries. So far, their distinct effects on electronic properties predicted by theoretical calculations have not been well identified.

Seamless Staircase Electrical Contact to Semiconducting Graphene Nanoribbons.

Electrical contact to low-dimensional (low-D) materials is a key to their electronic applications. Traditional metal contacts to low-D semiconductors typically create gap states that can pin the Fermi level (E). However, low-D metals possessing a limited density of states at E can enable gate-tunable work functions and contact barriers.

Controllable conversion of quasi-freestanding polymer chains to graphene nanoribbons.

In the bottom-up synthesis of graphene nanoribbons (GNRs) from self-assembled linear polymer intermediates, surface-assisted cyclodehydrogenations usually take place on catalytic metal surfaces. Here we demonstrate the formation of GNRs from quasi-freestanding polymers assisted by hole injections from a scanning tunnelling microscope (STM) tip. While catalytic cyclodehydrogenations typically occur in a domino-like conversion process during the thermal annealing, the hole-injection-assisted reactions happen at selective molecular sites controlled by the STM tip.

Differentiation of Surface and Bulk Conductivities in Topological Insulators via Four-Probe Spectroscopy.

We show a new method to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators using a four-probe transport spectroscopy in a multiprobe scanning tunneling microscopy system. We derive a scaling relation of measured resistance with respect to varying interprobe spacing for two interconnected conduction channels to allow quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 against a pure 2D conductance of graphene on SiC substrate.

Structural and electronic properties of an ordered grain boundary formed by separated (1,0) dislocations in graphene.

We present an investigation of the structural and electronic properties of an ordered grain boundary (GB) formed by separated pentagon-heptagon pairs in single-layer graphene/SiO2 using scanning tunneling microscopy/spectroscopy (STM/STS), coupled with density functional theory (DFT) calculations. It is observed that the pentagon-heptagon pairs, i.e.

Evidence of van Hove singularities in ordered grain boundaries of graphene.

It has long been under debate whether the electron transport performance of graphene could be enhanced by the possible occurrence of van Hove singularities in grain boundaries. Here, we provide direct experimental evidence to confirm the existence of van Hove singularity states close to the Fermi energy in certain ordered grain boundaries using scanning tunneling microscopy. The intrinsic atomic and electronic structures of two ordered grain boundaries, one with alternative pentagon and heptagon rings and the other with alternative pentagon pair and octagon rings, are determined.