No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Magnetism of Nanographene-Based Microporous Carbon and Its Applications: Interplay of Edge Geometry and Chemistry Details in the Edge State
Abstract
This paper is a contribution to the Physical Review Applied collection in memory of Mildred S. Dresselhaus.
... The elusive reconciliation [60] of the 'physics' (top-down view) and the 'chemistry' (bottom-up view) of graphene becomes closer than ever when the vast accumulated evidenceboth experimental and theoretical, and including both novel (single-sheet) graphene-based materials and the more traditional sp 2 -hybridized carbon materialsis not considered selectively. As in heterogeneous catalysis, when both electronic and geometric factors [61] are taken into account (and chemistry is reduced to physics, as Dirac would say), the graphene edges become the focus of attention. ...
There is a crucial difference between the structure of a polycyclic aromatic hydrocarbon (PAH) molecule and that of a single-layer sp²-hybridized carbon cluster; its misunderstanding leads to unresolvable inconsistencies when attempting to rationalize the behavior of graphenes across their impressively wide spectrum of existing or potential applications. The PAHs are not ‘nanographenes’, as is too often assumed, because not all the edges in graphenes are H-saturated. This increasingly obvious fact has profound implications for both ‘physical’ (e.g., magnetism, luminescence, thermoelectric power) and ‘chemical’ properties (e.g., functionalization, adsorption, reactivity, electrocatalysis) of graphene-based carbon materials. As revealed by computational quantum chemistry, the triplet ground state of graphenes, containing carbene-like active sites at the zigzag edge or in basal-plane vacancies, offers a consistent explanation for their wide-ranging physicochemical properties and behavior. The most salient consequences of such σ-π coupling of valence electrons, as well as of tailorable singlet-triplet and HOMO-LUMO gaps, account not only for the creation of holes in the conduction band, and thus positive thermoelectric power, but also for the pH-sensitive basicity of oxygen-free graphene-based materials, as well as the ubiquitous (electro)chemical interactions with molecular oxygen including the transition from a peroxy to dioxiranyl intermediate.
... properties [20][21][22]. Therefore, ZGNRs offer opportunities for efficient spin manipulation and for the creation of a full spectrum of spintronic nanodevices [23][24][25]. To date, many unique spin dependent electronic properties such as spin-based switching [26][27][28][29], spin filtering [30][31][32][33], spinbased rectification [34][35][36][37], spin-based negative differ ential resist ance (NDR) [38][39][40][41] had been found in ZGNRs or the molecular junctions with ZGNRs electrode. ...
Using nonequilibrium Green's function combined with density functional theory, we investigate the spin-resolved transport properties of a single porphycene molecular device with the zigzag graphene nanoribbon electrodes from first-principles calculations. The calculated results show that the width and the relative position of the electrode play an important factor in the spin-resolved I-V characteristics of this molecular device. When a single porphycene molecule connects to electrodes straightly, the device exhibits a significant spin-filtering behavior with a nearly 100% spin filtering efficiency. In addition, negative differential resistance behavior can be observed in its β-spin I-V characteristic. The electrode widening can enhance the negative differential resistance behavior of the β-spin I-V characteristics, but weaken the spin filtering efficiency of the device at high bias voltages. When a single porphycene molecule connects to electrodes un-straightly, the β-spin currents increase linearly in the whole bias range and its I-V characteristics no longer show the negative differential resistance behavior. Moreover, the electrode widening can enhance both α-spin currents and β-spin currents evidently leading to the absent of the spin filtering characteristic.
By using the high-level quantum chemistry method, we investigate the electronic and magnetic properties of Ni substituted polyacene structures as well as the laser-induced ultrafast spin dynamics and related spin logic gates. It is found that among the 61 lowest many-body states for each structure, the states with spin localized on a single magnetic atom mainly concentrate in the lower-energy region, while the states with spin localized on fused benzene rings are located in the higher-energy region. As the acene length becomes longer, the amount of states with spin localization on Ni atoms increases and the possibility of finding states with spin localization on benzene rings gets larger. Through the exploration of laser-induced spin dynamics in these structures, we find that different structures achieve different types of spin-transfer scenarios. Especially, although there exist several states with spin localization on different magnetic atoms for all the structures, the transfer between the two Ni atoms via one Λ process is only achieved in Ni2(C18H10), Ni2(C22H12), and Ni2(C38H20). However, the indirect transfer between two Ni atoms is possible through several bridging Λ processes. Based on the achieved spin scenarios and by appropriately defining the bit information according to the spin localization and directions of the initial and final states, two types of ultrafast spin logic gates, i.e., SWAP and XOR, are constructed. The results obtained in these carbon-based magnetic structures provide valuable insights into their electronic structures and ultrafast spin manipulation and show their potential applications in future molecular spintronic devices and quantum computing.
Electrons near the Fermi level behaving as massless Dirac fermions in graphene in (1+2)-D relativistic spacetime have been confirmed by an experiment. Using this aspect, a myriad of novel and interesting devices can be sought. In this paper, we laid out the theory for using a monolayer graphene sheet as an electron diffractometer, aiming at the determination of surface properties in materials. The key ingredient is the Mott scattering of electrons by screened Coulomb scatterers in (1+2)-D spacetime. The specific array of scatterers provided by a given surface placed in contact with a graphene sheet will induce an angular distribution for the electron scattering events, which can be properly measured through the electric current flowing to external electrodes. It can provide an in situ technique for characterizing quantum dot superlattices with a resolution of a few nanometers.
Graphene is considered as a promising material in spintronics due to its long spin relaxation time and long spin relaxation length. However, its spin transport properties have been studied at low carrier density only, beyond which much is still unknown. In this study, we explore the spin transport and spin precession properties in multilayer graphene at high carrier density using ionic liquid gating. We find that the spin relaxation time is directly proportional to the momentum relaxation time, indicating that the Elliott-Yafet mechanism still dominates the spin relaxation in multilayer graphene away from the charge neutrality point.
Graphene sheet is a single-atom thick material and attracts great interest due to its extraordinary physical properties, but it has zero bandgap. At the boundary of graphene sheet, two types of stable edges were the zigzag edge and armchair edge. The electronic states of graphene largely depend on the edge structures. In this research, we used Quantum ESPRESSO package to calculate the band gap of armchair and zigzag graphene due to the increase of their number of cells. The results show that the graphene band gaps are influenced by the number of cells in the armchair and zigzag configuration.
Graphene-based nanostructures exhibit a vast range of exciting electronic
properties that are absent in extended graphene. For example, quantum
confinement in carbon nanotubes and armchair graphene nanoribbons (AGNRs) leads
to the opening of substantial electronic band gaps that are directly linked to
their structural boundary conditions. Even more intriguing are nanostructures
with zigzag edges, which are expected to host spin-polarized electronic edge
states and can thus serve as key elements for graphene-based spintronics. The
most prominent example is zigzag graphene nanoribbons (ZGNRs) for which the
edge states are predicted to couple ferromagnetically along the edge and
antiferromagnetically between them. So far, a direct observation of the
spin-polarized edge states for specifically designed and controlled zigzag edge
topologies has not been achieved. This is mainly due to the limited precision
of current top-down approaches, which results in poorly defined edge
structures. Bottom-up fabrication approaches, on the other hand, were so far
only successfully applied to the growth of AGNRs and related structures. Here,
we describe the successful bottom-up synthesis of ZGNRs, which are fabricated
by the surface-assisted colligation and cyclodehydrogenation of specifically
designed precursor monomers including carbon groups that yield atomically
precise zigzag edges. Using scanning tunnelling spectroscopy we prove the
existence of edge-localized states with large energy splittings. We expect that
the availability of ZGNRs will finally allow the characterization of their
predicted spin-related properties such as spin confinement and filtering, and
ultimately add the spin degree of freedom to graphene-based circuitry.
Gas adsorption is an important tool for the characterisation of porous solids and fine powders. Major advances in recent years have made it necessary to update the 1985 IUPAC manual on Reporting Physisorption Data for Gas/Solid Systems. The aims of the present document are to clarify and standardise the presentation, nomenclature and methodology associated with the application of physisorption for surface area assessment and pore size analysis and to draw attention to remaining problems in the interpretation of physisorption data.
The optoelectronic nature of two-dimensional sheets of sp(2)-hydridized carbons (for example, graphenes and nanographenes) can be dramatically altered and tuned by altering the degree of π-extension, shape, width and edge topology. Among various approaches to synthesize nanographenes with atom-by-atom precision, one-shot annulative π-extension (APEX) reactions of polycyclic aromatic hydrocarbons hold significant potential not only to achieve a 'growth from template' synthesis of nanographenes, but also to fine-tune the properties of nanographenes. Here we describe one-shot APEX reactions that occur at the K-region (convex armchair edge) of polycyclic aromatic hydrocarbons by the Pd(CH3CN)4(SbF6)2/o-chloranil catalytic system with silicon-bridged aromatics as π-extending agents. Density functional theory calculations suggest that the complete K-region selectivity stems from the olefinic (decreased aromatic) character of the K-region. The protocol is applicable to multiple APEX and sequential APEX reactions, to construct various nanographene structures in a rapid and programmable manner.
Edge contacts to graphene can offer excellent contact properties. Role of different chemical terminations is examined by using ab initio density functional theory and quantum transport simulations. It is found that edge termination by group VI elements O and S offers considerably lower contact resistance compared to H and group VII element F. The results can be understood by significantly larger binding energy and shorter binding distance between the metal contact and these group VI elements, which results in considerably lower interface potential barrier and larger transmission. The qualitative conclusion applies to a variety of contact metal materials.
Bandgap engineering is used to create semiconductor heterostructure devices that perform processes such as resonant tunnelling and solar energy conversion. However, the performance of such devices degrades as their size is reduced. Graphene-based molecular electronics has emerged as a candidate to enable high performance down to the single-molecule scale. Graphene nanoribbons, for example, can have widths of less than 2 nm and bandgaps that are tunable via their width and symmetry. It has been predicted that bandgap engineering within a single graphene nanoribbon may be achieved by varying the width of covalently bonded segments within the nanoribbon. Here, we demonstrate the bottom-up synthesis of such width-modulated armchair graphene nanoribbon heterostructures, obtained by fusing segments made from two different molecular building blocks. We study these heterojunctions at subnanometre length scales with scanning tunnelling microscopy and spectroscopy, and identify their spatially modulated electronic structure, demonstrating molecular-scale bandgap engineering, including type I heterojunction behaviour. First-principles calculations support these findings and provide insight into the microscopic electronic structure of bandgap-engineered graphene nanoribbon heterojunctions.
We investigated the thermal oxidation process of nanographene using activated carbon fibers (ACFs) by thermogravimetry (TG), X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and electrical conductance measurements. The oxidation process started from the edge of nanographene with the formation of phenol (-OH) or ether (C-O-C) groups attached to edge carbon atoms, as verified by the XPS and NEXAFS results. While the TG results indicated a decrease in the size of the nanographene sheet during the oxidation process, the intensity of the edge-state peak, i.e., the signature of the zigzag edge, decreased in the C K-edge NEXAFS spectra. This suggests that the zigzag edge preferentially reacted with oxygen and that the nanographene terminated with the thermodynamically unstable zigzag edges converted to one terminated with stable armchair edges. As the oxidation temperature increased, the activation energy for the electron hopping transport governed by the Coulomb gap variable range hopping between the nanographene sheets increased, and the tunneling barrier decreased. This change can be understood on the basis of the decrease in the size of the nanographene sheets together with the preferential etching of nanogarphene edges and the decrease in the inter-nanographene-sheet distance.
Understanding how foreign chemical species bond to atomic vacancies in
graphene layers can advance our ability to tailor the electronic and magnetic
properties of defective graphenic materials. Here we use ultra-high vacuum
scanning tunneling microscopy (UHV-STM) and density functional theory to
identify the precise structure of hydrogenated single atomic vacancies in a
topmost graphene layer of graphite and establish a connection between the
details of hydrogen passivation and the electronic properties of a single
atomic vacancy. Monovacancy-hydrogen complexes are prepared by sputtering of
the graphite surface layer with low energy ions and then exposing it briefly to
an atomic hydrogen environment. High-resolution experimental UHV-STM imaging
allows us to determine unambiguously the positions of single missing atoms in
the defective graphene lattice and, in combination with the ab initio
calculations, provides detailed information about the distribution of
low-energy electronic states on the periphery of the monovacancy-hydrogen
complexes. We found that a single atomic vacancy where each sigma-dangling bond
is passivated with one hydrogen atom shows a well-defined signal from the
non-bonding pi-state which penetrates into the bulk with a (\sqrt 3 \times
\sqrt 3)R30^ \circ periodicity. However, a single atomic vacancy with full
hydrogen termination of sigma-dangling bonds and additional hydrogen
passivation of the extended pi-state at one of the vacancy's monohydrogenated
carbon atoms is characterized by complete quenching of low-energy localized
states. In addition, we discuss the migration of hydrogen atoms at the
periphery of the monovacancy-hydrogen complexes which dramatically change the
vacancy's low-energy electronic properties, as observed in our low-bias
high-resolution STM imaging.
The electronic structure and spin magnetism for few-layer-graphene nanoribbons synthesized by chemical vapor deposition have been investigated using near-edge x-ray absorption fine structure (NEXAFS) and electron-spin resonance (ESR). For the pristine sample, a prepeak was observed below the π∗ peak close to the Fermi level in NEXAFS, indicating the presence of additional electronic states close to the Fermi level. The intensity of this prepeak decreased with increasing annealing temperature and disappeared after annealing above 1500 °C. The ESR spectra, which proved the presence of localized spins, tracked the annealing-temperature-dependent behavior of the prepeak with fidelity. The NEXAFS and ESR results jointly confirm the existence of a magnetic edge state that originates from open nanographene edges. The disappearance of the edge state after annealing at higher temperatures is explained by the decrease in the population of open edges owing to loop formation of adjacent graphene edges.
Nanographene has electronic structure which depends crucially on its edge shape; that is, the zigzag edge has an edge state having a localized spin in spite of the absence of such a state in the armchair edge. Nanoporous activated carbon fibers, which consist of a three-dimensional random network of nanographite domains (stacked nanographene sheets), are investigated to clarify the magnetism of the edge-state spins in relation to Ar physisorption. In addition to the deviation of the susceptibility from the Curie-Weiss behavior, the electron spin resonance linewidth is found to increase abruptly below the boiling point (87 K) of Ar condensed in the nanopores. This is the evidence of mechanically induced magnetic short-range ordering of edge-state spins in the nanographite upon the condensation of Ar molecules.
The zigzag edge of a graphene nanoribbon possesses a unique electronic state that is near the Fermi level and localized at the edge carbon atoms. The authors investigate the chemical reactivity of these zigzag edge sites by examining their reaction energetics with common radicals from first principles. A “partial radical” concept for the edge carbon atoms is introduced to characterize their chemical reactivity, and the validity of this concept is verified by comparing the dissociation energies of edge-radical bonds with similar bonds in molecules. In addition, the uniqueness of the zigzag-edged graphene nanoribbon is further demonstrated by comparing it with other forms of sp2 carbons, including a graphene sheet, nanotubes, and an armchair-edged graphene nanoribbon.
In graphene edges or nanographene, the presence of edges strongly affects the electronic structure depending on their edge shape (zigzag and armchair edges) as observed with the electron wave interference and the creation of non-bonding π-electron state (edge state). We investigate the edge-inherent electronic features and the magnetic properties of edge-sate spins in nanographene/graphene edges. Graphene nanostructures are fabricated by heat-induced conversion/fabrication of nanodiamond particles/graphite step edges; single-layer nanographene islands (mean size 10 nm) and armchair-edged nanographene ribbons (width 8 nm). Scanning tunneling microscopy (STM)/scanning tunneling spectroscopy observations demonstrate that edge states are created in zigzag edges in spite of the absence of such states in armchair edges. In addition, zigzag edges tend to be short and defective, whereas armchair edges are long and continuous in general. These findings suggest that a zigzag edge has lower aromatic stability than an armchair edge, consistent with Clar's aromatic sextet rule. The manner in which electron wave scattering takes place is different between zigzag and armchair edges. In the vicinity of an armchair edge, a honeycomb superlattice is observed in STM images together with a fine structure of threefold symmetry, in spite of the superlattice at a zigzag edge. The honeycomb lattice is a consequence of the intervalley K–K' transition that accompanies the electron wave interference taking place at the armchair edge. The Raman G-band is also affected by the interference, showing polarization angle dependence specifically at armchair edges. The magnetism of a three-dimensional disordered network of nanographene sheets is understood on the basis of the ferrimagnetic structure of the edge-state spins in individual constituent nanographene sheets. The strengthening of the inter-nanographene-sheet magnetic interaction brings about a spin glass state.
We consider the low-energy magnetic excitations of nanographite ribbons with zigzag edges. The zigzag ribbons possess almost flat bands at the Fermi level which cause a ferrimagnetic spin polarization localized at the edge sites. The spin wave mode of this magnetic state is investigated by a random phase approximation of the corresponding Hubbard model. This result is used to derive an effective Heisenberg model with ladder structure. Although this system has a spin gap (Haldane type), our analysis shows that the gap is small and the tendency towards ferrimagnetic correlation at the edges is strong.
We study the electronic states of graphite ribbons with edges of two typical shapes, armchair and zigzag, by performing tight binding band calculations, and find that the graphite ribbons show striking contrast in the electronic states depending on the edge shape. In particular, a zigzag ribbon shows a remarkably sharp peak of density of states at the Fermi level, which does not originate from infinite graphite. We find that the singular electronic states arise from the partly flat bands at the Fermi level, whose wave functions are mainly localized on the zigzag edge. We reveal the puzzle for the emergence of the peculiar edge state by deriving the analytic form in the case of semi-infinite graphite with a zigzag edge. Applying the Hubbard model within the mean-field approximation, we discuss the possible magnetic structure in nanometer-scale micrographite.
In Chapter 2, graphene is discussed from the viewpoint of physics. When a graphene sheet is cut into nano-size fragments, nano-size graphene, which is called nanographene, is created. Nanographene is a two-dimensional material with a hexagonal network of sp2-carbons. In nanographene, one can see the structures of many polycyclic aromatic hydrocarbons (PAHs) such as hexaben- zocoronene, perylene, anthracene, and naphthalene, and ultimately benzene. This means that the property of PAHs would be related to that of nanographene. Accordingly, in this chapter, we discuss nanographene from the viewpoint of PAHs.
Spintronic systems exploit the fact that the electron current is composed of spin-up and spin-down carriers, which are more easily disturbed than electronic systems. Here, we investigate the spin transport properties of a single phenalenyl or pyrene molecule connected to zigzag graphene nanoribbon electrodes by using the non-equilibrium Green's function formalism with density functional theory. We found the difference of the symmetry on these two molecules will bring a remarkable effect on the spin transport properties of the devices. The spin-resolved currents of the single pyrene molecular device are much lower than that of the single phenalenyl molecular device when they all connected to electrodes symmetrically. In addition, we found the change of the connected site will decrease the spin-resolved currents of the phenalenyl-based molecular device drastically, but had no longer any influence with the pyrene-based molecular device. The results will be helpful for us to further understand the transfer of the spin-carriers in the spintronic systems.
Zigzag edges of the honeycomb structure of graphene exhibit magnetic polarization making them attractive as building blocks for spintronic devices. Here, we show that devices with zigzag edged triangular antidots perform essential spintronic functionalities, such as spatial spin-splitting or spin filtering of unpolarized incoming currents. Near-perfect performance can be obtained with optimized structures. The device performance is robust against substantial disorder. The gate-voltage dependence of transverse resistance is qualitatively different for spin-polarized and spin-unpolarized devices, and can be used as a diagnostic tool. Importantly, the suggested devices are feasible within current technologies.
The electronic and magnetic properties of chemically modified graphene armchair edges are studied using a combination of tight-binding calculations, first-principles modelling, and low temperature scanning tunneling microscopy (STM) experiments. The atomically resolved STM images of the hydrogen etched graphitic edges suggest the presence of localized states at the Fermi level for certain armchair edges. We demonstrate theoretically that the topological zero-energy edge mode may emerge at armchair boundaries with asymmetrical chemical termination of the two outermost atoms in the unit cell. We particularly focus our attention on armchair edges terminated by various combinations of the hydrogen (H, H2) and methylene (CH2) groups. The inclusion of the spin component in our calculations reveals the appearance of π-electron-based magnetism at the armchair edges under consideration.
Semiconductor technology is currently based on the manipulation of electronic charge; however, electrons have additional degrees of freedom, such as spin and valley, that can be used to encode and process information. Over the past several decades, there has been significant progress in manipulating electron spin for semiconductor spintronic devices, motivated by potential spin-based information processing and storage applications. However, experimental progress towards manipulating the valley degree of freedom for potential valleytronic devices has been limited until very recently. We review the latest advances in valleytronics, which have largely been enabled by the isolation of 2D materials (such as graphene and semiconducting transition metal dichalcogenides) that host an easily accessible electronic valley degree of freedom, allowing for dynamic control.
Electron paramagnetic resonance (EPR) study of air-physisorbed defective carbon nano-onions evidences in favor of microwave assisted formation of weakly-bound paramagnetic complexes comprising negatively-charged O2- ions and edge carbon atoms carrying pi-electronic spins. These complexes being located on the graphene edges are stable at low temperatures but irreversibly dissociate at temperatures above 50-60 K. These EPR findings are justified by density functional theory (DFT) calculations demonstrating transfer of an electron from the zigzag edge of graphene-like material to oxygen molecule physisorbed on the graphene sheet edge. This charge transfer causes changing the spin state of the adsorbed oxygen molecule from S = 1 to S = 1/2 one. DFT calculations show significant changes of adsorption energy of oxygen molecule and robustness of the charge transfer to variations of the graphene-like substrate morphology (flat and corrugated mono- and bi-layered graphene) as well as edges passivation. The presence of H- and COOH- terminated edge carbon sites with such corrugated substrate morphology allows formation of ZE-O2- paramagnetic complexes characterized by small (<50 meV) binding energies and also explains their irreversible dissociation as revealed by EPR.
High surface area graphene monoliths consist mainly of single graphene layers wider than 10 nm. The interlayer porosity of high temperature treated nanoporous graphene monoliths with tuned intergraphene layer structures is evaluated by hybrid analysis of Ar adsorption at 87 K, N2 adsorption at 77 K, high resolution transmission electron microscopic observation, and small-angle X-ray scattering (SAXS) measurements. SAXS analysis results in surface areas that are 1.4 and 4.5 times larger than those evaluated by Ar adsorption for graphene monoliths nontreated and treated at 2273 K, respectively. A distorted graphene sheet structure model is proposed for the high surface area graphene monoliths on the basis of the hybrid analysis.
The selective functionalization of graphene edges is driven by the chemical reactivity of its carbon atoms. The chemical reactivity of an edge, as an interruption of the honeycomb lattice of graphene, differs from the relative inertness of the basal plane. In fact, the unsaturation of the pz orbitals and the break of the π conjugation on an edge increase the energy of the electrons at the edge sites, leading to specific chemical reactivity and electronic properties. Given the relevance of the chemistry at the edges in many aspects of graphene, the present Review investigates the processes and mechanisms that drive the chemical functionalization of graphene at the edges. Emphasis is given to the selective chemical functionalization of graphene edges from theoretical and experimental perspectives, with a particular focus on the characterization tools available to investigate the chemistry of graphene at the edge.
Nanographene, nanosized fragments of graphene, has edge-shape-dependent electronic structures different from those of infinite size graphene. Around the zigzag-shaped edges of a nanographene sheet, a nonbonding edge state of pi-electron origin is created, which is the origin of electronic and chemical activities of nanographene. In addition, the strongly spin-polarized edge state has localized spin, giving magnetic activity. We investigate the magnetic properties of nanoporous activated carbon fibers consisting of a three-dimensional disordered network of nanographite domains, each of which is a stack of 3-4 nanographene sheets, in relation to host-guest interaction. Chemically inert guest species such as water, organic solvent, or argon, physisorptively condensed into the nanopores mechanically compress the nanographite domains, resulting in high spin/low spin magnetic switching of the edge-state spins. The presence of an energy gap and the nanographene edges decorated by oxygen-containing functional groups make nanographene less amphoteric, modifying charge-transfer mechanisms in the host-guest interaction. HNO3 shows a two-step charge-transfer process. In Br adsorption, physisorbed Br species cooperate with charge-transfer Br species, giving an interesting interplay of charge transfer and magnetic switching phenomena. In the K adsorption, physisorbed K atoms form antiferromagnetic K clusters in the nanopores, while a portion of K species are responsible for charge transfer. The reaction with fluorine creates magnetic sigma-dangling bond states, at the expense of the pi-conjugated electron system. The interplay between the edge-state spins and the sigma-dangling bond spins gives a variety of magnetism depending on the fluorine concentration.
The geometry and chemistry of graphene nanostructures significantly affect their electronic properties. Despite a large number of experimental and theoretical studies dealing with the geometrical shape-dependent electronic properties of graphene nanostructures, experimental characterisation of their chemistry is clearly lacking. This is mostly due to the difficulties in preparing chemically modified graphene nanostructures in a controlled manner and in identifying the exact chemistry of the graphene nanostructure on the atomic scale. Herein, we present scanning probe microscopic and first-principles characterisation of graphene nanostructures with different edge geometries and chemistry. Using the results of atomic scale electronic characterisation and theoretical simulation, we discuss the role of the edge geometry and chemistry on the electronic properties of graphene nanostructures with hydrogenated and oxidised linear edges at graphene boundaries and the internal edges of graphene vacancy defects. Atomic-scale details of the chemical composition have a strong impact on the electronic properties of graphene nanostructures, i.e., the presence or absence of nonbonding π states and the degree of resonance stability.
The heat treatment effect on the electronic and magnetic structures of a disordered network of nanographene sheets has been investigated by in situ measurements of X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and electrical conductance, together with temperature-programmed desorption measurements. Oxygen-containing functional groups bonded to nanographene edges in the pristine sample are almost completely decomposed under heat treatment up to 1300-1500 K, resulting in the formation of edges primarily terminated by hydrogen. The removal of the oxygen-containing groups enhances the conductance owing to the decrease in the electron transport barriers between nanographene sheets. Heat treatment above 1500 K removes also the hydrogen atoms from the edges, promoting the successive fusion of nanographene sheets at the expense of edges. The decrease in the π* peak width in NEXAFS indicates the progress of the fusion reaction, that is, the extension of the π-conjugation, which agrees with the increase in the orbital susceptibility previously reported. The fusion leads to the formation of local π/sp(2) bridges between nanographene sheets and brings about an insulator-to-metal transition at 1500-1600 K, at which the bridge network becomes infinite. As for the magnetism, the intensity of the edge state peak in NEXAFS, which corresponds to the number of the spin-polarized edge states, decreases above 1500 K, though the effective edge-state spin density per edge state starts decreasing at approximately 200 K lower than the temperature of the edge state peak change. This disagreement indicates the development of antiferromagnetic short range ordering as a precursor of a spin glass state near the insulator-metal transition, at which the random network of inter-nanographene-sheet exchange interactions strengthened with the formation of the π/sp(2) bridges becomes infinite.
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.
The magnetism and its dynamical behavior is investigated in relation to
electron-localization effect for the edge-state spins of
three-dimensional randomly networked nanographene sheets which interact
weakly with each other. The electron transport is governed by
Coulomb-gap variable-range hopping between nanographene sheets. At high
temperatures, the electron spin resonance (ESR) signal with a feature of
homogeneous spin system reveals the bottleneck effect in the spin
relaxation to the lattice for a strongly coupled system of edge-state
spins and conduction π electrons, in a given nanographene sheet.
Below 20 K, a discontinuous ESR line broadening accompanied by
hole-burning proves the formation of an inhomogeneous spin state,
indicating a static spatial distribution of on-resonance fields. This
inhomogeneity originates from a distribution of the strengths of the
ferrimagnetic moments on the individual nanographene sheets, taking into
account that the constituent nanographene sheets with their shapes
randomly varying have different strengths of ferrimagnetic moments.
Strong electron localization below 20 K in the internanographene
electron hopping is responsible for the crossover from the homogeneous
spin state to the inhomogeneous one, in the latter of which
ferrimagnetic short-range ordering is evident in the edge-state spin
system.
The relation between electronic structure and hydrogen content at the edges of nanosized holes (nanoholes) in graphite single layer is studied by means of atomically resolved scanning tunneling microscopy and first-principles calculations. Repeatable production of nanoholes with predominantly zigzag hydrogenated edges was realized by bombardment of the top graphene layer of graphite with low-energy (100 eV) Ar+ ions and its further treatment (etching) in an atomic hydrogen environment. Two main types of nanohole zigzag edge, with a striking contrast to each other, are identified: (i) monohydrogenated zigzag edge supporting edge-localized π state on its boundary; (ii) zigzag edge with repeating two monohydrogenated sites and one dihydrogenated site, without features of the edge-localized state and characterized by a prominent standing wave pattern. Absence of the localized state at a general (chiral) hydrogenated graphitic edge consisting of a mixture of zigzag and armchair fragments is also discussed.
Combined scanning tunneling microcopy (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 oxidization 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 groups act as additional pi sites at the edges; thus, the topology of the pi electron network changes from "zigzag" to "Klein" type, leading to drastic modification of the edge states at the oxidized edges.
Activated carbon fibers (ACF) consist of microporous carbon with a huge specific surface area (SSA) ranging from 700 m2 g-1 to 3000 m2 g-1, and a having random structures consisting of an assembly of micrographites with a dimension of ca. 20 × 20 Å2. The electrical conductivity and magnetic susceptibility were investigated for ACFs with SSA = 1000 and 2000 m2 g-1 in order to clarify the relation between the electronic properties and the structure of ACF having a random network of micrographites. The electrical conductivity is explained by the two-dimensional variable-range hopping conduction at lower temperatures and thermally activated conduction at higher temperatures. The introduction of N2 or O2 gas to a sample induces a change in the conductivity, which is considered to be caused by a structural change and a charge transfer between dangling bonds and O2 gas. The observed value of the orbital diamagnetic susceptibility is considerably small compared with that of a condensed polycyclic aromatic hydrocarbon having the same dimensions as that of the micrographite in ACF. This implies that the micrographitic domains have a deformed planar structure with the presence of defects.
We propose that to modify zigzag edges of nanographite structures by hydrogenation, fluorination or oxidation is a method to create magnetic materials made only from light elements. These reactions and methylene addition are compared with each other in several aspects by considering a graphene ribbon having mono-hydrogenated zigzag edges as a starting material. A local-spin-density approximation was applied to the electronic band-structure calculation of nanographite ribbon structures and stability of each ribbon was tested by the first-principles manner. Among possible reactions for graphene ribbon, hydrogenation produces the largest magnetic moment per a carbon atom. Since the hydrogenation is exothermic, however, fluorination has advantage, where the reaction is endothermic. The possible maximum moment is 1/3 of that for the ideal hydrogenationed graphene ribbon. A graphene ribbon with an oxidized zigzag edge and a monohydrogenated zigzag edge possesses a partially spin-polarized flat band similar to the fluorinated ribbon. A magnetic moment appears at the monohydrogenated zigzag edge but not at the oxidized edge. No evidence of spin polarization, however, has been found for a methylene-substituted graphene ribbon.
Activated carbon fibers (ACFs) are microporous carbonsconsisting of a three-dimensional disordered network of nano-graphiteswith a mean in-plane size of about 30 Å.We investigated the structure,electronic properties and iodine doping effectsfor ACF samples heat-treated up to 2800° C.The samples heat-treated below 1000° Cexhibit Coulomb gap variable-range hopping conductionand the presence of localized spins,suggesting the importance of charging effectsand the edge-inherited non-bonding states in nano-graphites,the latter being predicted theoretically.Iodine doping reduces the charging effectdue to the dielectric constant enhanced by the iodinethat is accommodated in the micropores.Heat treatment above 1300° C changes ACFsfrom an Anderson insulator to a disordered metalby the development of an infinite inter-nano-graphite percolation path networkfor electron transport,accompanied by a change from localized-spin magnetismto itinerant electron magnetism.In the metallic regime,carrier scattering is subjected to nano-graphite boundariesin terms of a short range random potential.Iodine-doping introduces ionized impurity scattering,which is caused by the I3- ionsgenerated by the charge transfer from iodine to nano-graphite.
The dynamical properties of two spin systems composed of conduction electron spins and localized d-spins interacting by exchange are investigated in connection with the electron-spin resonance of d-spins with the free electron g-value. It seems to us that some basic onsiderations are necessary for the problem along the line of the widely accepted idea that the s-d interaction itself does not lead to any relaxation time and shift. The usual adiabatic approximation is applied to this idea. A modified damping equation of the Bloch type as well as Bloch's relaxation theory are also useful, but these are not enough for a complete solution of the problem. A certain controversy involved in recent experimental and theoretical work at Berkeley concerning the electron spin resonancein Cu-Mn dilute alloy is pointed out and examined. We believe that the possibility could not be eliminated that the spin-lattice relaxation time of conduction electrons in the alloy due to mechanisms other than the s-d interaction is longer than previous authors have assumed.
The Hückel band structure is analytically investigated for a family of arbitrary-width π-network polymers which may be viewed as being cut from the graphite lattice. For wider strips a portion of two nearly nonbonding bands on either side of the Fermi energy are found to be localized to opposite edges of the strip. Novel consequences of these edge-localized band orbitals are considered and correlated to a simple resonating valence bond picture.
Various kinds of pores in solid malerials are classified into intraparticle pores and interparticle pores according to the origin of the pores. The structural factors of the pores are discussed as well as the methods for evaluation of the pore size distribution with molecular adsorption (molecular resolution porosimetry), small angle X-ray scattering, mercury porosimetry, nuclear magnetic resonance, and thermoporositmetry. Recent progresses in molecular resolution porosimetry for micropores are stressed; this review describes new approaches such as high resolution N2 adsorption, He adsorption at 4.2 K, and molecular simulation for the characterization of porous solids.
Microporous activated carbon fibers with huge specific surface area (3000 m2/g) have dangling bond spins at the peripheries of micro-graphitic domains. Spin-lattice relaxation of the dangling bond spins was studied by ESR measurements in the presence of various gases, He, Ne, Ar, H2, N2, O2. The introduction of helium gas strongly enhances the spin-lattice relaxation rate 1/T1, suggesting the contribution of the collisional process to the spin-lattice relaxation mechanism. The introduction of a simple model on the basis of the electric dipole-dipole interaction makes the calculation of 1/T1 possible and the obtained result can explain the experimentally observed result.
The spin-lattice relaxation mechanism of dangling bond spins in activated carbon fibers (ACF) with huge specific surface areas (˜3000m2/g) was investigated by ESR measurements in the presence of the gases, He, Ne, Ar, H2, N2, and O2. The introduction of helium gas remarkably enhances the spin-lattice relaxation rate, suggesting the participation of the collisional process of helium atoms in the spin relaxation from the dangling bond spins to the lattice. Taking into account an exceptionally large condensation of helium gas, this proves that ACF has ultra micropores which can accommodate only the small diameter helium atoms, resulting in a novel molecular sieve effect.
The electronic structure of nanographene having open edges around its circumference crucially depends on its edge shape. The circumference of an arbitrary shaped nanographene sheet is described in terms of a combination of zigzag and armchair edges. According to theoretical suggestions, nanographene has a non-bonding pi-electron state (edge state) localized in zigzag edges. This is reminiscent of the non-bonding pi -electron state appearing in non-Kekulé-type aromatic molecules. The localized spins of the edge states can give rise to unconventional magnetism in nanographene such as carbon-only ferromagnetism, magnetic switching phenomenon, spin glass state, etc. Nanographene can be prepared by heat-induced conversion of nanodiamond particles. Nanographene ribbons are found by chance around step edges of graphite. The detailed structures of individual nanographene ribbons thus found can be characterized by resonance Raman experiments in which the graphitic G-band is used as a fingerprint. A nanographene sheet inclined along a direction is found to show an interference superperiodic pattern with a varying periodicity. The stacking of sheets also gives an interference effect on the dislocation network created by rhombohedral stacking faults. STM/STS investigations of well defined graphene edges which are hydrogen terminated in ultra-high vacuum condition confirm the presence of edge states around zigzag edges in good agreement with theoretical works. Armchair edges are generally long and defect free whereas zigzag edges tend to be short and defective. This suggests that the armchair edge is energetically more stable than the zigzag edge that has an edge state at the Fermi level. The feature of the edge state depends on the detailed geometry of the edge structures. The edge state in a short zigzag edge embedded between armchair edges becomes less localized due to state mixing with the adjacent armchair edges. The intersheet interaction modifies the spatial distribution of the local density of states of the edge states. The electrons in the edge state in a finite-length zigzag edge are subjected to an electron confinement effect. Nanographene sheets are tailored by cutting along the direction which is chosen intentionally for designing functionality. Well defined edges can be prepared by chemical modifications with foreign atoms or functional groups. A combination of an atomic-resolution electron lithography technique and chemical modifications of the nanographene edges is expected to give nanographene-based molecular devices in the development of nanotechnology. Recent works on the preparations structural and electronic characterizations of graphene edges and nanographene are reviewed.
Activated carbon fibers (ACF) are microporous carbon consisting of a three-dimensional network of micrographites having the size of ∼3 nm. We investigate structural and electronic properties of ACFs in relation to the adsorption of helium, nitrogen, and oxygen gases, by means of adsorption isotherm, ESR (electron spin resonance), magnetic susceptibility, and electrical conductivity measurements. The linewidth of ESR associated with dangling bond spins on micrographites decreases with gas uptake at low pressures below 0.1 kPa regardless of gas species. The similar behavior of oxygen to that of nonmagnetic helium and nitrogen demonstrates that adsorbed oxygen molecules having chemisorption feature are stabilized in the singlet ground state in the low-pressure range. Taking into account that the linewidth is governed by dipole–dipole interaction between dangling bond spins, the reduction in the ESR linewidth proves the swelling of micropores induced by gas uptake. The conductivity decreases with gas uptake in the same pressure range regardless of gas species. This behavior is also explained by the modification of microstructure of ACFs. At higher gas pressures 0.1–10 kPa where adsorption is characterized as physisorption for all gaseous species, nitrogen, and helium do not change the ESR linewidth, whereas the linewidth increases with introduced oxygen pressure, due to the dipolar field of paramagnetic oxygen molecules. The mean distance between a dangling bond spin and an oxygen molecule is estimated at ∼0.8 nm. © 1998 American Institute of Physics.
Carbon K-edge X-ray absorption spectra of nanographene have been simulated by density functional theory calculations to obtain information on the edge termination by hydrogen. Such information is crucially important to understand and predict functions such as transport and catalysis. Our results show that different edge terminations significantly affect the binding energy of the 1s core-level of C atoms in the vicinity of edges because of the change in chemical bonding and the localized edge states. We find that a shoulder or a peak appears below the π* peak at relatively different positions with respect to the π* peak position in the theoretical spectra of zigzag graphene nanoribbons, depending on the ratio of monohydrogen- to dihydrogen-terminations. We also point out that the two additional features observed between the π* and σ* peaks of an ideal graphene originate from the σ* states of C−H bonding and C−H2 bonding at the edges.
Hueckel molecular orbital (HMO) theory has been used to calculate energy level densities, bond orders, electron distributions, free valence, resonance energies, and heats of formation for several homologous series of large, hexagonally symmetric benzenoid polyaromatic molecules with well-defined edge structures containing up to 2300 carbon atoms. When extrapolated to the infinite limit, values for all properties converge to reasonable values. This is in contrast to several other ..pi..-electron theories that do not yield correct graphite limits. Carbon atoms at the edge of such large molecules are predicted to behave like those in small polynuclear aromatic molecules, with properties strongly dependent on local structure. Regardless of edge structure, interior carbons several bond lengths from an edge have properties similar to those in an infinite graphite sheet. Edge structure has a larger influence on heats of formation than that predicted by group additivity methods. Only a weak correlation was found between the energy of the highest occupied molecular orbital and the reactivity of the most reactive position.
To evaluate the average layer size of carbonaceous material, it is essential to have its accurate elemental composition data. However, accurate determination of the hydrogen content is not easy. In the previous paper, we reported that a temperature-programmed oxidation (TPO) technique is suitable to accurately determine the amounts of carbon and hydrogen of the organic portion of carbonaceous material, using three Chinese anthracites as examples. In the present study, we attempt to determine the structure of the same set of anthracites on the basis of the elemental analysis data obtained by a TPO method. The contents of other components, such as oxygen, sulfur, nitrogen, and free radicals, were estimated by referring to the literature. The average layer sizes of the three anthracites range from 2.1 to 3.7 nm, and these values were compared with those estimated from XRD and HRTEM analysis. The size estimated from the TPO technique was larger than those from other techniques, and this discrepancy was attributed to the fact that these techniques are based on different theoretical and mathematical assumptions. The combination of these three techniques provided a more comprehensive understanding about the structure of these materials. In addition to the average structure, an attempt was made to obtain more detailed information through the deconvolution of TPO peaks of H2O, CO, and CO2 in the temperature region between 500 and 800 °C. The peaks could be reasonably resolved into at least two peaks. The H/C atomic ratio determined from the lower temperature peaks was smaller than that for the higher temperature peaks. This is likely because the oxidation of smaller size molecules takes place at lower temperatures.
The Coulomb interaction between localized electrons is shown to create a 'soft' gap in the density of states near the Fermi level. The new temperature dependence of the hopping DC conductivity is the most important manifestation of the gap. The form of the density of states within the gap is discussed.
The adsorption/desorption processes of oxygen are investigated in nanoporous carbon ( activated carbon fiber (ACF)) consisting of a disordered network of nanographene sheets. The heat-induced desorption at 200 degrees C shows the decomposition of oxygen-including functional groups weakly bonded to nanographene edges. The removal of these oxygen-including negatively charged functional groups brings about a change in the type of majority carriers, from holes to electrons, through charge transfer from the functional groups to the interior of nanographene sheets. The oxygen adsorption brings ACF back to the electronic state with holes being majority carriers. In this process, a large concentration of negatively charged O(2)(delta-) molecules with delta similar to 0.1 are created through charge transfer from nanographene sheets to the adsorbed oxygen molecules. The changes in the thermoelectric power and the electrical resistance in the oxygen desorption process is steeper than that in the oxygen adsorption process. This suggests the irreversibility between the two processes.
Recent work on activated carbon fibers with specific surface area SSA ≥ 1000 m2/g is reviewed. Because of their heterogeneity, it is necessary to characterize activated carbon fibers by multiple characterization techniques, such as x-ray diffraction, Raman scattering, transport properties, magneto-resistance, photoconductivity, and electron spin resonance. Further insight into the structure-property relationships in activated carbon fibers is achieved through their study as a function of heat treatment temperature, using these same characterization techniques. Whereas transport-related properties are most sensitive to the specific surface area, the structure and Raman spectra are most sensitive to the state of graphitization.
A detailed theory is given of the nuclear relaxation time in metals. The 1/T law of H e i t l e r and T e l l e r, which is imposed by the statistics of the electrons, is obtained by using the Bloch approximation of metals. An approximate relation is derived between the relaxation time τ and the relative line shift ΔH/H, viz. where g is the nuclear g-factor in Bohr-units.The experimental results on 7Li, 27Al and 63Cu are discussed with special reference to the influence of the correlation between the electrons. The deviations from the 1/T law for Li are all or nearly all attributed to the influence of paramagnetic impurities.
Two kinds of one-dimensional graphite series starting from polyacene and polyphenanthrene have been studied on the basis of the tight-binding crystal orbital (CO) calculations. Emphasis has been put on decrease of the band gap, according to the development of the polymer skeleton toward graphite, and distribution of the CO patterns being important to the path of charge carriers.
Ambiguities in extracting a value for the superlocalization exponent from variable-range conductivity data of fractal systems are pointed out. In particular it is shown that by allowing for a pre-exponential temperature-dependent factor, recent data taken on carbon-black-polymer composites may support percolation as well as non-percolation exponents.