Electronic Structure Of Graphene Nanoribbons Research Articles

On the nanoscale interface, electronic structure, and optical properties of nanocarbon-reinforced calcium silicate hydrates

This study presents results from quantum chemical simulations of the synergetic interaction, electronic structure, and optical properties of calcium-silicate hydrates (C-S-H) reinforced by graphene-nanoribbons and single-walled carbon nanotubes (SWCNT). The calculations show that C-S-H/graphene-nanoribbon and C-S-H/SWCNT composites are stabilized by electrostatic interaction due to the charge transfer from Ca ions at the interface of C-S-H to the nearby C atoms of the graphene-nanoribbon and SWCNT. Removing Ca ions at the interface drastically decreases the strength of interaction into a weak van der Waals type. The Bader charge transfer analysis and electron distribution topology further confirm these results. Generally, the electronic states of the graphene-nanoribbon and SWCNT are shifted to lower energy in the complex. The electronic structure of graphene-nanoribbon and SWCNT is susceptible to the Ca ions-rich C-S-H environment. The composites’ overall absorption spectra can be considered superimposed of the isolated nanocarbon and C-S-H except in the lower energy region due to charge transfer and realignment of energy states. The results presented here reveal the bonding mechanism of the C-S-H with nanocarbon at the fundamental level. This work serves as a reference for the nanoengineering cement-based material with nanocarbon for the next-generation smart infrastructure.

Examining the Long-Range Effect in Very Long Graphene Nanoribbons: A First-Principles Study.

The role of long-range effect on the modulation of the electronic structure of graphene nanoribbons has been little studied due to the limitations of existing theoretical and computational methods. By splitting a molecule top-down and calculating and jointing the Fock matrix of fragments, we developed a computational method suitable for large-size molecules with random doping and arbitrary geometry. Utilizing this method, we achieved the study of the effects of dopants and curvature on graphene nanoribbons (GNRs). It reveals that both dopants and curvature can change the charge distribution of GNRs, while the influence of dopants is more significant and can extend up to 1-3 nm. The electronic excitation properties of GNRs are also largely modified by the doping state or nonuniform curvature. Our findings provide not only a feasible approach for studying the electronic structure of large-size molecules but also the possibility to improve the properties of graphene-based materials by dopants and local curvature.

Tuning the carrier mobility and electronic structure of graphene nanoribbons using Stone–Wales defects

In the last decade, surface-assisted reactions involving well-defined molecular precursors have led to the controlled assembly of several clean-edged and defect-free graphene nanoribbons. The recent realization of a bisanthene-like quantum dot with pairs of pentagon–heptagon, or Stone–Wales (SW), defects has been successfully achieved. Based on the similarity between the pristine and SW-defective bisanthene blocks, we propose a set of systems based on the concatenation of SW-bisanthene motifs and study their electronic properties using density functional theory. We demonstrate that nanoribbons with SW-defects preserve the semiconducting character of their pristine counterparts. Furthermore, noteworthy behaviors involving the frontier levels emerge, which affect carrier mobilities of both electrons and holes. We also investigate the electronic transport properties of nanojunctions composed by graphene nanoribbons with a localized distribution of SW-defects compatible with the geometry of the corresponding bisanthene-like blocks. Our simulations shown that the transmission spectrum is sensitive to the position and concentration of SW-defects.

Tuning the Carrier Mobility and Electronic Structure of Graphene Nanoribbons Using Stone-Wales Defects

Tuning the Carrier Mobility and Electronic Structure of Graphene Nanoribbons Using Stone-Wales Defects

Electronic structure of graphene nanoribbons under external electric field by density functional tight binding

<sec> In recent years, the rapid development of electronic information technology has brought tremendous convenience to people’s lives, and the devices used have become increasingly miniaturized. However, due to the constraints of the process and the material itself, as the size of the devices made of silicon materials is further reduced, obvious short channel effects and dielectric tunneling effects will appear, which will affect the normal operations of these devices. In order to overcome this development bottleneck, it is urgent to find new materials for the devices that can replace silicon. Carbon has the same outer valence electron structure as silicon. Since 2004, Geim [Novoselov K S, Geim A K, Morozov S V, et al. 2005 <i>Nature</i> <b>438</b> 197] prepared two-dimensional graphene with a honeycomb-like planar structure formed by sp<sup>2</sup> hybridization, graphene has received extensive attention from researchers and industrial circles for its excellent electronic and mechanical properties. However, graphene is not a true semiconductor, and it has no band gap in its natural state. The energy gap can be opened by preparing graphene nanoribbons. On this basis, the electronic structure of the nanoribbons can be further controlled by using an external electric field to destroy the symmetric structure of the nanoribbons. </sec><sec>In this paper, the tight-binding method based on density functional theory is used to calculate and study the influence of external transverse electric field on the electronic structure and electron population of un-hydrogenated/hydrogenated armchair graphene nanoribbons. The calculation results show that whether there is hydrogen on the edge of the graphene nanoribbons or not, the energy gap changed at the Г point shows a three-group periodic oscillation decreasing law, and as N increases, the energy gap will disappear. Under the external electric field, the band structure and the density of states of the nanoribbons will change greatly. For un-hydrogenated nanoribbons with semiconducting properties, as the intensity of the external electric field increases, a semiconductor-metal transition occurs. At the same time, the electric field will also have a significant influence on the energy level distribution, resulting in significant changes in the peak height and peak position of the density of states. The external electric field causes the electron population distribution on the atoms in the nanoribbons to break its symmetry. The greater the electric field strength, the more obvious the population asymmetry is. The edge hydrogenation passivation can significantly change the population distribution of atoms in nanoribbons.</sec>

Regulation of Electronic Structure of Graphene Nanoribbon by Tuning Long-Range Dopant-Dopant Coupling at Distance of Tens of Nanometers.

Long-range dopant-dopant coupling in graphene nanoribbon (GNR) has been under intensive study for a very long time. Using a newly developed dopant central insertion scheme (DCIS), we performed first-principles study on multiple H, O, OH, and FeN4 dopants in long (up to 1000 nm) GNRs and found that, although potential energy of the dopant decays exponentially as a function of distance to the dopant, GNR's electronic density of states (DOS) exhibits wave-like oscillation modulated by dopants separated at a distance up to 100 nm. Such an oscillation strongly infers the purely quantum mechanical resonance states constrained between double quantum wells. This has been unambiguously confirmed by our DCIS study together with a one-dimensional quantum well model study, leading to a proof-of-principle protocol prescribing on-demand GNR-DOS regulation. All these not only reveal the underlining mechanism and importance of long-range dopant-dopant coupling specifically reported in GNR, but also open a novel highway for rationally optimizing and designing two-dimensional materials.

A patterning-free approach for growth of free-standing graphene nanoribbons using step-bunched facets of off-oriented 4H-SiC(0 0 0 1) epilayers

The tunable electronic structure of graphene nanoribbons (GNRs) has attracted much attention due to the great potential in nanoscale electronic applications. Most methods to produce GNRs rely on the lithographic process, which suffers from the process-induced disorder in the graphene and scalability issues. Here, we demonstrate a novel approach to directly grow free-standing GNRs on step-bunched facets of off-oriented 4H-SiC epilayers without any patterning or lithography. First, the 4H-SiC epilayers with well-defined bunched steps were intentionally grown on 4 degree off-axis 4H-SiC substrates by the sublimation epitaxy technique. As a result, periodic step facets in-between SiC terraces were obtained. Then, graphene layers were grown on such step-structured 4H-SiC epilayers by thermal decomposition of SiC. Scanning tunneling microscopy (STM) studies reveal that the inclined step facets are about 13–15 nm high and 30–35 nm wide, which gives an incline angle of 23–25 degrees. LEEM and LEED results showed that the terraces are mainly covered by monolayer graphene and the buffer layer underneath it. STM images and the analysis of their Fourier transform patterns suggest that on the facets, in-between terraces, graphene is strongly buckled and appears to be largely decoupled from the surface.

Energetics and electronic structures of N-doped graphene nanoribbons with pyridinic and graphitic edges

Using the density functional theory, we study the energetics and electronic structures of graphene nanoribbons of which edges are substituted by N atoms. Our calculations showed that the edge N atoms with pyridinic structure mimic the π environment of C atoms, participating the π electron network of C atoms. In contrast, N atoms act as impurities for the π electron network of the nanoribbons when they are terminated by H atoms, leading to versatile edge localized state those are hardly synthesized using C and H atoms. The total energy of N-doped nanoribbons is the same as that of the pristine graphene nanoribbons, irrespective of the edge hydrogenation, and gradually decreases with increasing nanoribbon width. We also found that the edge hydrogenation decrease and increase the stability of the N-doped graphene nanoribbons with armchair and zigzag edges, respectively.

Energetics and electronic structure of graphene nanoribbons under uniaxial torsional strain

We investigate the energetics and electronic structure of graphene nanoribbons (GNRs) with structural torsion in terms of their torsion angles and edge shapes using the density functional theory with the generalized gradient approximation. The structural torsion with the period of nanometer scale does not cause the increase of the total energy of GNRs up to the torsion angle of 20°. On the other hand, the total energy with respect to the torsion is weakly dependent on the edge angle. The GNR with zigzag edges is the most flexible against the structural torsion, while those with armchair and chiral are robust. Electronic band structures of GNRs are also insensitive to the structural torsion, because the local atomic structure retains its flat conformation up to the largest torsion.

Two-dimensional electronic spectroscopy of graphene nanoribbons in organic solution

We unravel the electronic structure of graphene nanoribbons in solution using two-dimensional electronic spectroscopy. We identify different excitons, their vibrational couplings and find that exciton diffusion or relaxation among the exiton sublevels takes place in ~300 fs.

Electronic structure of graphene nanoribbons on hexagonal boron nitride

Hexagonal boron nitride is an ideal dielectric to form two-dimensional heterostructures due to the fact that it can be exfoliated to be just few atoms thick and its a very low density of defects. By placing graphene nanoribbons on high quality hexagonal boron nitride it is possible to create ideal quasi one dimensional (1D) systems with very high mobility. The availability of high quality one-dimensional electronic systems is of great interest also given that when in proximity to a superconductor they can be effectively engineered to realize Majorana bound states. In this work we study how a boron nitride substrate affects the electronic properties of graphene nanoribbons. We consider both armchair and zigzag nanoribbons. Our results show that for some stacking configurations the boron nitride can significantly affect the electronic structure of the ribbons. In particular, for zigzag nanoribbons, due to the lock between spin and sublattice degree of freedom at the edges, the hexagonal boron nitride can induce a very strong spin-splitting of the spin polarized, edge sates. We find that such spin-splitting can be as high as 40~meV.

Energetics and electronic structures of corrugated graphene nanoribbons

On the basis of the density functional theory with the generalized gradient approximation, we investigated the energetics and electronic structure of graphene nanoribbons (GNRs) with structural corrugation in terms of their edge shape. Although the corrugation gradually increases, the corrugation energy remains constant under small corrugation angles for zigzag GNRs. In contrast, for armchair GNRs, the energy rapidly increases with increasing corrugation angle. The structural corrugation does not affect the local atomic structure of GNRs except that with armchair edges. The electronic structure of GNRs is insensitive to the structural corrugation up to the corrugation angle of 20° for all GNRs.

Concentration Dependence of Dopant Electronic Structure in Bottom-up Graphene Nanoribbons.

Bottom-up fabrication techniques enable atomically precise integration of dopant atoms into the structure of graphene nanoribbons (GNRs). Such dopants exhibit perfect alignment within GNRs and behave differently from bulk semiconductor dopants. The effect of dopant concentration on the electronic structure of GNRs, however, remains unclear despite its importance in future electronics applications. Here we use scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of bottom-up synthesized N = 7 armchair GNRs featuring varying concentrations of boron dopants. First-principles calculations of freestanding GNRs predict that the inclusion of boron atoms into a GNR backbone should induce two sharp dopant states whose energy splitting varies with dopant concentration. Scanning tunneling spectroscopy experiments, however, reveal two broad dopant states with an energy splitting greater than expected. This anomalous behavior results from an unusual hybridization between the dopant states and the Au(111) surface, with the dopant-surface interaction strength dictated by the dopant orbital symmetry.

Влияние адсорбции атомов и молекул кислорода на электронное строение графеновой наноленты

Влияние адсорбции атомов и молекул кислорода на электронное строение графеновой наноленты

Electronic structure and electric polarity of edge-functionalized graphene nanoribbons

On the basis of the density functional theory combined with the effective screening medium method, we studied the electronic structure of graphene nanoribbons with zigzag edges, which are terminated by functional groups. The work function of the nanoribbons is sensitive to the functional groups. The edge state inherent in the zigzag edges is robust against edge functionalization. OH termination causes the injection of electrons into the nearly free electron states situated alongside the nanoribbons, resulting in the formation of free electron channels outside the nanoribbons. We also demonstrated that the polarity of zigzag graphene nanoribbons is controllable by the asymmetrical functionalization of their edges.

Spatially resolving and energy splitting of edge state in zigzag edged triangle graphene quantum dots on Cu(111) surface

The electronic structure of graphene nanoribbons (GNRs) and graphene quantum dots (GQDs) has been predicted to depend sensitively on the crystallographic orientation of their edges. However, direct observation of edge state for triangle graphene quantum dots (TGQDs) has not been verified experimentally. Here we explore, using the scanning tunneling spectroscopy (STS), the zigzag edged electronic property of varisized TGQDs. Predominantly zigzag-edged TGQDs exhibit edge-localized states with the energy splittings of about 0.2–0.3V when its lateral dimension is less than 7nm. The measured energy splittings agree with theoretical calculations, and show that these edge states originate from a hybridization effect of the substrate, and not from a magnetic splitting of the edge state.

Controlling the electronic structure of graphene nanoribbon by controlling its shape and strain distribution

Controlling the electronic structure of graphene nanoribbon by controlling its shape and strain distribution

Energetics and electronic structure of graphene nanoribbons under a lateral electric field

We studied the energetics and electronic structure of graphene nanoribbons with hydrogenated and clean edges with respect to the detailed edge shapes using density functional theory. Our calculations showed that the stability of graphene edges strongly depends on the length of the zigzag edge portion. Near-zigzag edges are less stable than near-armchair edges because of the large number of states at the Fermi level (EF) in nanoribbons with near-zigzag edges. The edge formation energy retains a constant value up to the edge angle of 16°, after which it monotonically increases with increasing zigzag portion or edge angle. We also found that the edge stability strongly correlates with the electronic structures near the EF of graphene nanoribbons. Nanoribbons with a small zigzag portion already possess edge localized π electronic states near the EF. We also demonstrate that a lateral electric field increases the energy gap of ribbons with/without hydrogen termination, resulting in a decrease in edge formation energy.

Electronic structure of graphene nanoribbons doped with nitrogen atoms: a theoretical insight.

The electronic structure of graphene nanoribbons doped with a graphitic type of nitrogen atoms has been studied using B3LYP, B2PLYP and CAS methods. In all but one case the restricted B3LYP solutions were unstable and the CAS calculations provided evidence for the multiconfigurational nature of the ground state with contributions from two dominant configurations. The relative stability of the doped nanoribbons depends mostly on the mutual position of the dopant atoms and notably less on the position of nitrogen atoms within the nanoribbon. N-graphitic doping affects cationic states much more than anionic ones due the participation of the nitrogen atoms in the stabilization of the positive charge, resulting in a drop in ionization energies (IPs) for N-graphitic doped systems. Nitrogen atoms do not participate in the negative charge stabilization of anionic species and, therefore, the doping does not affect the electron affinities (EAs). The unrestricted B3LYP method is the method of choice for the calculation of IPs and EAs. Restricted B3LYP and B2PLYP produces unreliable results for both IPs and EAs while CAS strongly underestimates the electron affinities. This is also true for the reorganization energies where restricted B3LYP produces qualitatively incorrect results. Doping changes the reorganization energy of the nanoribbons; the hole reorganization energy is generally higher than the corresponding electron reorganization energy due to the participation of nitrogen atoms in the stabilization of the positive charge.

Electromechanical properties of armchair graphene nanoribbons under local torsion

While graphene nanoribbons are prone to twist intrinsically, the effect of local twist on the electromechanical properties remains unexplored. By using the density functional theory in combination with the nonequilibrium Green’s function method, we investigate the responses of structural evolution and electrical transport of armchair graphene nanoribbons to local torsion. We show that local twist can alter their transport properties significantly. The current at a given bias can switch on/off or change many times with twist angle, which is related with twist-induced changes in electronic structures of graphene nanoribbons. Our results can provide a valuable guideline for design and implementation of graphene nanoribbons in nanoelectromechanical systems and devices.