Rearrangement of π‑Electron Network and Switching of Edge-Localized π State in Reduced Graphene Oxide

Introduced defects can modulate the intrinsic electronic structure of graphene, causing a drastic switch in its electronic and magnetic properties, in which defect-induced localized π states near the Fermi level play an important role. Accordingly, considerable effort has been directed toward detailed characterization of the defect-induced state; however, identification of the chemical nature of the defect-induced state remains a challenge. Here, we demonstrate a method for reliable identification of the localized π states of oxidized vacancy edges in reduced graphene oxide. Depending on the dynamic changes in the oxygen-binding modes, <i>i.e.</i>, between carbonyl and ether forms in the vacancy edges, the π-electron network near the edges can rearrange, leading to drastic on–off switching of the localized π state. This switching can be manipulated <i>via</i> scanning-probe-induced local mechanical force. This study provides fundamental guidance toward understanding how oxidized defect structures contribute to the unique electronic state of graphene oxide and its potential future applications in electronic devices

Nanographene and Graphene Edges: Electronic Structure and Nanofabrication

Graphene can be referred to as an infinite polycyclic aromatic hydrocarbon (PAH) consisting of an infinite number of benzene rings fused together. However, at the nanoscale, nanographene’s properties lie in between those of bulk graphene and large PAH molecules, and its electronic properties depend on the influence of the edges, which disrupt the infinite π-electron system. The resulting modulation of the electronic states depends on whether the nanographene edge is the armchair or zigzag type, corresponding to the two fundamental crystal axes. In this Account, we report the results of fabricating both types of edges in the nanographene system and characterizing their electronic properties using a scanning probe microscope.We first introduce the theoretical background to understand the two types of finite size effects on the electronic states of nanographene (i) the standing wave state and (ii) the edge state which correspond to the armchair and zigzag edges, respectively. Most importantly, characterizing the standing wave and edge states could play a crucial role in understanding the chemical reactivity, thermodynamic stability and magnetism of nanosized grapheneimportant knowledge in the design and realization of promising functionalized nanocarbon materials.In the second part, we present scanning probe microscopic characterization of both edge types to experimentally characterize the two electronic states. As predicted, we find the armchair-edged nanographene to have an energetically stable electronic pattern. The zigzag-edged nanographene shows a nonbonding (π radical) pattern, which is the source of the material’s electronic and magnetic properties and its chemical activity. Precise control of the edge geometry is a practical requirement to control the electronic structure. We show that we can fabricate the energetically unstable zigzag edges using scanning probe manipulation techniques, and we discuss challenges in using these techniques for that purpose

Electrically Controlled Adsorption of Oxygen in Bilayer Graphene Devices

We investigate the chemisorptions of oxygen molecules on bilayer graphene (BLG) and its electrically modified charge-doping effect using conductivity measurement of the field effect transistor channeled with BLG. We demonstrate that the change of the Fermi level by manipulating the gate electric field significantly affects not only the rate of molecular adsorption but also the carrier-scattering strength of adsorbed molecules. Exploration of the charge transfer kinetics reveals the electrochemical nature of the oxygen adsorption on BLG

Electrically Controlled Adsorption of Oxygen in Bilayer Graphene Devices

Electrically Controlled Adsorption of Oxygen in Bilayer Graphene Device

Carrier Control of Graphene Driven by the Proximity Effect of Functionalized Self-assembled Monolayers

We demonstrated the carrier control of graphene by employing the electrostatic potential produced by several types of self-assembled monolayer (SAM) formed on SiO2 substrates. For single layer graphene on perfluoroalkylsilane-SAM, the stiffening of the Raman G-band indicates a large down shift of the Fermi level (∼−0.8 eV) by accumulated hole carriers. Meanwhile, aminoarylsilane-SAM accumulated electron carriers, which compensate the hole carriers doped by adsorbed molecules under the ambient atmosphere, in graphene. The present results and their theoretical analysis reveal that the use of the dipole moments of SAM molecules can systematically modulate the electrostatic potential affecting graphene without destroying its intrinsic electronic structure and let us know that the proximity effect of the SAMs is a promising way in developing graphene-based solid state electronics

Synthesis, Crystal and Network Structures, and Magnetic Properties of a Hybrid Layered Compound: [K(18-cr)(2-PrOH)₂][{Mn(acacen)}₂{Fe(CN)₆}] (18-cr = 18-Crown-6-ether, acacen = <i>N</i>,<i>N</i>‘-Ethylenebis(acetylacetonylideneiminate))

A hybrid layered compound [{K(18-cr)(2-PrOH)2}{Mn(acacen)}2{Fe(CN)6}] has been prepared by the reaction of [Mn(acacen)(Cl)] with [K(18-cr)(H2O)2]3[Fe(CN)6]·3H2O in an ethanol/2-propanol mixed solvent (18-cr = 18-crown-6-ether, acacen = N,N‘-ethylenebis(acetylacetonylideneiminate)). It crystallizes in the monoclinic space group P21/a with cell dimensions of a = 13.272(3) Å, b = 15.768 (2) Å, c = 14.771(2) Å, β = 105.64(1)°, Z = 2. It assumes a hybrid layered structure of alternating arrays of two types of layers. One of the layers is formed by the anionic part [{Mn(acacen)}2{Fe(CN)6}]nn-, where [Fe(CN)6]3- coordinates through its four cyanide groups on a plane to the axial sites of four [Mn(acacen)]+ entities. The two-dimensional layer consists of the cyclic octamer [−Mn−NC−Fe−CN−]4 having the Fe ions at the corners and the Mn ions on the edges of a deformed square. Another layer is formed by the cationic part [K(18-cr)(2-PrOH)2]+ that has a hexagonal-bipyramidal geometry about the metal with two 2-PrOH molecules at the apexes of the nearly planar [K(18-cr)]+. The anionic and cationic layers are combined by the hydrogen bond between the cyanide groups (free from coordination) of the anionic layer and the 2-propanol groups of the cationic layer with bond distance of N···O = 2.861(5) Å. Magnetic studies (magnetic susceptibility vs T, field-cooled magnetization vs T, saturation magnetization vs H) indicate that the compound is a metamagnet with a Néel temperature TN = 5.0 K, showing the onset of ferromagnetic ordering within the anionic layer and an antiferromagnetic interlayer interaction. Magnetization as a function of the applied magnetic field indicates a spin-flipping from antiferromagnetic arrangement to ferromagnetic arrangement between the layers around 1200 Oe and exhibits hysteresis behavior

Electronic State of Oxidized Nanographene Edge with Atomically Sharp Zigzag Boundaries

Combined scanning tunneling microscopy (STM) and density functional theory (DFT) characterizations of the electronic state were performed on the zigzag edge of oxidized nanographene samples. The oxidized zigzag edge with atomically sharp boundaries was prepared by electrochemical oxidation of the graphite surface in aqueous sulfuric acid solution. Bias-dependent STM measurements demonstrated the presence of the edge state at the zigzag edges with local density of states (LDOS) split into two peaks around the Fermi level. Our DFT-based analysis showed that the two-peak structure of the edge state was due to the termination of the zigzag edge by carbonyl functional groups. The LDOS arising from the edge states was slowly dampened in the bulk at the carbonyl-terminated zigzag edges (∼1.5 nm). This result is in clear contrast to the strongly localized edge states at hydrogenated zigzag edges in previous reports. The oxygen atoms in the carbonyl functional groups act as additional π sites at the edges; thus, the topology of the π electron network changes from “zigzag” to “Klein” type, leading to drastic modification of the edge states at the oxidized edges

Synthesis, Crystal and Network Structures, and Magnetic Properties of a Hybrid Layered Compound: [K(18-cr)(2-PrOH)₂][{Mn(acacen)}₂{Fe(CN)₆}] (18-cr = 18-Crown-6-ether, acacen = <i>N</i>,<i>N</i>‘-Ethylenebis(acetylacetonylideneiminate))

A hybrid layered compound [{K(18-cr)(2-PrOH)2}{Mn(acacen)}2{Fe(CN)6}] has been prepared by the reaction of [Mn(acacen)(Cl)] with [K(18-cr)(H2O)2]3[Fe(CN)6]·3H2O in an ethanol/2-propanol mixed solvent (18-cr = 18-crown-6-ether, acacen = N,N‘-ethylenebis(acetylacetonylideneiminate)). It crystallizes in the monoclinic space group P21/a with cell dimensions of a = 13.272(3) Å, b = 15.768 (2) Å, c = 14.771(2) Å, β = 105.64(1)°, Z = 2. It assumes a hybrid layered structure of alternating arrays of two types of layers. One of the layers is formed by the anionic part [{Mn(acacen)}2{Fe(CN)6}]nn-, where [Fe(CN)6]3- coordinates through its four cyanide groups on a plane to the axial sites of four [Mn(acacen)]+ entities. The two-dimensional layer consists of the cyclic octamer [−Mn−NC−Fe−CN−]4 having the Fe ions at the corners and the Mn ions on the edges of a deformed square. Another layer is formed by the cationic part [K(18-cr)(2-PrOH)2]+ that has a hexagonal-bipyramidal geometry about the metal with two 2-PrOH molecules at the apexes of the nearly planar [K(18-cr)]+. The anionic and cationic layers are combined by the hydrogen bond between the cyanide groups (free from coordination) of the anionic layer and the 2-propanol groups of the cationic layer with bond distance of N···O = 2.861(5) Å. Magnetic studies (magnetic susceptibility vs T, field-cooled magnetization vs T, saturation magnetization vs H) indicate that the compound is a metamagnet with a Néel temperature TN = 5.0 K, showing the onset of ferromagnetic ordering within the anionic layer and an antiferromagnetic interlayer interaction. Magnetization as a function of the applied magnetic field indicates a spin-flipping from antiferromagnetic arrangement to ferromagnetic arrangement between the layers around 1200 Oe and exhibits hysteresis behavior

Origin of Current Enhancement through a Ferrocenylundecanethiol Island Embedded in Alkanethiol SAMs by Using Electrochemical Potential Control

Electroactive ferrocenylundecanethiol islands embedded in an n-decanethiol SAM matrix were studied under potential control using in situ scanning tunneling microscopy (STM). Contrary to previous reports, the positive charges on ferrocene moieties are not a prerequisite to enhancing the current through ferrocenylundecanethiol on gold. Rather, conduction paths are opened at determined potentials for both neutral and monocationic ferrocene moieties. In addition, although stable and reproducible images were obtained under potential control, conventional STM measurements under N2 atmosphere were sometimes unstable and irreversibly changed the sample, indicating that current measurements of ferrocenylundecanethiol are much easier under an electrochemical environment

Aromatic Character of Nanographene Model Compounds

Superaromatic stabilization energy (SSE) defined to estimate the degree of macrocyclic aromaticity can be used as a local aromaticity index for individual benzene rings in very large polycyclic aromatic hydrocarbons (PAHs) and finite-length graphene nanoribbons. Aromaticity patterns estimated using SSEs indicate that the locations of both highly aromatic and reactive rings in such carbon materials are determined primarily by the edge structures. Aromatic sextets are first placed on the jutting benzene rings on armchair edges, if any, and then on inner nonadjacent benzene rings. In all types of nanographene model compounds, the degree of local aromaticity varies markedly near the edges