グラフェンナノリボンの前駆体設計と構造制御

グラフェンナノリボン（GNR）はグラフェンを細長いリボン状にした擬1次元ナノカーボン材料であり，その化学構造に依存して特異な電気的，光学的，磁気的な性質を示す．近年，特定の有機分子を前駆体として用いたボトムアップ合成により，さまざまな構造をもつGNRの合成が次々と報告されている．本稿では，金属表面上でのGNRボトムアップ合成について解説し，新しい前駆体の設計とGNRの構造制御に関して最近の研究成果を紹介する．直近ではアームチェア型とジグザグ型の異なるエッジ構造を併せもつGNRを合成することで，特異なトポロジカル電子状態が発現することが明らかとなった

カーボン系高分子のボトムアップ合成

有機・高分子化学の手法を用いたボトムアップ合成により、 構造の明確な擬一次元カーボン系高分子、グラフェンナノリ ボンが得られる。さまざまな構造のグラフェンナノリボンが 報告され、その多彩な性質や機能が明らかとなってきた。本 項では、グラフェンナノリボンのボトムアップ合成研究とそ の機能や応用可能性について概説する。Bottom-up synthesis can provide graphene nanoribbons (GNRs) with well defined structures. In addition to solution synthesis based on conventional methods of organic and polymer chemistry, on-surface synthesis based on the modern methods of surface science enabled fabrication of various GNRs with different structures and unique properties. This article provides an overview of the field, including introduction of different polymerization methods for the solution synthesis of GNRs, edge functionalization of GNRs for achieving novel properties, and the latest developments in the on-surface synthesis of GNRs

Synthesis of Circumpyrene by Alkyne Benzannulation of Brominated Dibenzo[hi,st]ovalene

A transition-metal catalyzed alkyne benzannulation allowed an unprecedented synthesis of circumpyrene, starting from 3,11-dibromo-6,14-dimesityldibenzo[hi,st]ovalene (DBOV). The circumpyrene was characterized by a combination of NMR, mass spectrometry, and single-crystal X-ray diffraction analysis, revealing its multizigzag-edged structure. Two newly introduced C═C bonds in circumpyrene strongly perturbed the electronic structures of DBOV, as evidenced by increased optical and electrochemical energy gaps. This is in good agreement with an increased number of Clar’s sextets as well as a decreased number of π-electrons in the conjugation pathway of circumpyrene, according to anisotropy of the induced current density (ACID) calculations. The present approach opens a new avenue to multizigzag-edged nanographenes and offers insights into their (opto)electronic properties

Coupled spin states in armchair graphene nanoribbons with asymmetric zigzag edge extensions

Carbon-based magnetic structures promise significantly longer coherence times than traditional magnetic materials, which is of fundamental importance for spintronic applications. An elegant way of achieving carbon-based magnetic moments is the design of graphene nanostructures with an imbalanced occupation of the two sublattices forming the carbon honeycomb lattice. According to Lieb's theorem, this induces local magnetic moments that are proportional to the sublattice imbalance. Exact positioning of sublattice imbalanced nanostructures in graphene nanomaterials hence offers a route to control interactions between induced local magnetic moments and to obtain graphene nanomaterials with magnetically non-trivial ground states. Here, we show that such sublattice imbalanced nanostructures can be incorporated along a large band gap armchair graphene nanoribbon on the basis of asymmetric zigzag edge extensions, which is achieved by incorporating specifically designed precursor monomers during the bottom-up fabrication of the graphene nanoribbons. Scanning tunneling spectroscopy of an isolated and electronically decoupled zigzag edge extension reveals Hubbard-split states in accordance with theoretical predictions. Investigation of pairs of such zigzag edge extensions reveals ferromagnetic, antiferromagnetic or quenching of the magnetic interactions depending on the relative alignment of the asymmetric edge extensions. Moreover, a ferromagnetic spin chain is demonstrated for a periodic pattern of zigzag edge extensions along the nanoribbon axis. This work opens a route towards the design and fabrication of graphene nanoribbon-based spin chains with complex magnetic ground states

Synthesis of Giant Dendritic Polyphenylenes with 366 and 546 Carbon Atoms and Their High-vacuum Electrospray Deposition

Dendritic polyphenylenes (PPs) can serve as precursors of nanographenes (NGs) if their structures represent 2D projections without overlapping benzene rings. Here, we report the synthesis of two giant dendritic PPs fulfilling this criteria with 366 and 546 carbon atoms by applying a "layer-by-layer" extension strategy. Although our initial attempts on their cyclodehydrogenation toward the corresponding NGs in solution were unsuccessful, we achieved their deposition on metal substrates under ultrahigh vacuum through the electrospray technique. Scanning probe microscopy imaging provides valuable information on the possible thermally induced partial planarization of such giant dendritic PPs on a metal surface

Structure-dependent electrical properties of graphene nanoribbon devices with graphene electrodes

Graphene nanoribbons (GNRs) are a novel and intriguing class of materials in the field of nanoelectronics, since their properties, solely defined by their width and edge type, are controllable with high precision directly from synthesis. Here we study the correlation between the GNR structure and the corresponding device electrical properties. We investigated a series of field effect devices consisting of a film of armchair GNRs with different structures (namely width and/or length) as the transistor channel, contacted with narrowly spaced graphene sheets as the source-drain electrodes. By analyzing several tens of junctions for each individual GNR type, we observe that the values of the output current display a width-dependent behavior, indicating electronic bandgaps in good agreement with the predicted theoretical values. These results provide insights into the link between the ribbon structure and the device properties, which are fundamental for the development of GNR-based electronics.Comment: Published in Carbon (2019

On-surface synthesis of polyazulene with 2,6-connectivity

Azulene, the smallest neutral nonalternant aromatic hydrocarbon, serves not only as a prototype for fundamental studies but also as a versatile building block for functional materials because of its unique opto(electronic) properties. Here, we report the on-surface synthesis and characterization of the homopolymer of azulene connected exclusively at the 2,6-positions using 2,6-diiodoazulene as the monomer precursor. As an intermediate to the formation of polyazulene, a gold-(2,6-azulenylene) chain is observed