Spatial variation in the electronic structures of carpetlike graphene nanoribbons and sheets

Graphene, when deposited on a supporting substrate with a step edge, may be deformed in the presence of the step edges of the substrate. In this study, we have investigated a spatial variation in the local electronic structure near the step region, by performing first-principles calculations for carpetlike armchair graphene nanoribbons (C-AGNR) and two-dimensional periodic carpetlike graphene sheets (PCGS). Our results indicate no practical difference in the local density of states (LDOS) between those of flat and step regions. Interestingly, however, the PCGS shows a remarkable variation in the LDOS with an external electric field (E-field). Furthermore, we also discuss the dependence of the direction and the magnitude of the applied E-field on the spatial variation in the LDOS.     

DFT study of the adsorption of 2, 3, 7, 8-tetrachlorodibenzofuran (TCDF) on vacancy-defected graphene doped with Mn and Fe

Dioxins are highly toxic to humans and environment, and developing the effective methods to control and detect the organic pollutant is particular important. Here we performed a density functional theory (DFT) study on the adsorption of 2, 3, 7, 8-tetrachlorodibenzofuran (TCDF) molecules on the modified graphene substrates. The results indicated that the introducing of vacancy-defect and dopants (Mn and Fe) significantly improves the sensitivity toward TCDF molecules. The impurity played a crucial role for interacting with TCDF molecules. Furthermore, the adsorption of TCDF induced band-gap open in defected graphene substrates, which could be seen as electric signal to detect TCDF pollutant. The present study is expected to be useful to explore effective materials to detect and remove dioxin pollutants based on graphene.     

Structural and electronic properties of armchair graphene nanoribbons functionalized with fluorine

The interaction of armchair graphene nanoribbons (AGNR) with F has been investigated by considering it as a passivating element as well as adatom impurity. The adsorption of F at three different sites viz. bridge (B), top (T) and hole (H) is examined to determine the most stable configuration. It is revealed that F passivation is slightly more favorable than the H passivation of AGNR and it also affects the band gap. Interestingly, F adsorbed AGNR exhibit magnetic ground state which is about 70 meV more favourable over nonmagnetic state. Further, F passivated AGNR exhibit linear I-V characteristic which indicates potential for interconnect applications.     

Theoretical investigation of structures and energetics of sodium adatom and its dimer on graphene: DFT study

Extensive ab initio calculations have been performed to study the energetics of a sodium (Na) atom and its dimer adsorbed on graphene using the SIESTA package Soler et al. (2002) [1] which works within a DFT(density functional theory)–GGA (generalized gradient approximation) pseudopotential framework. The adsorption energy, geometry, charge transfer, ionization potential and density of states (DOS), partial density states (PDOS) of adatom/dimer-graphene system have been calculated. After considering various sites for adsorption of Na on graphene, the center of a hexagonal ring of carbon atoms is found to be the preferred site of adsorption while the Na₂ dimer prefers to rest parallel to the graphene sheet. We find insignificant energy differences among adsorption configurations involving different possible sites in parallel orientation, which implies high mobility of the dimer on the graphene sheet. We also notice only a slight distortion of the graphene sheet perpendicular to its plane upon adatom adsorption. However, some lateral displacements seen are more perceptible.     

Specific heat of graphene nanoribbons

We studied the specific heat of graphene nanoribbons (GNRs) using an extended force constant model. We found that at low temperature, the specific heat decreases, and its variation with temperature increases with increasing GNR width. However, the specific heat increases with increasing GNR width after crossing a chaotic region. Free boundary conditions, -CHOH-terminated and armchair-edge-induced phonon nondegeneracy, shift and distortion and localized vibrational modes significantly influence GNR specific heat compared with periodic boundary conditions and bare and zigzag edges in GNRs. Finally, we found a uniform expression for specific heat vs. width at every temperature except for the chaotic region.     

A density functional theory study of formaldehyde adsorption on ceria

Molecular adsorption of formaldehyde on the stoichiometric CeO₂(1 1 1) and CeO₂(1 1 0) surfaces was studied using periodic density functional theory. Two adsorption modes (strong chemisorbed and weak physisorbed) were identified on both surfaces. This is consistent with recent experimental observations. On the (1 1 1) surface, formaldehyde strongly chemisorbs with an adsorption energy of 0.86 eV to form a dioxymethylene-like structure, in which a surface O lifts from the surface to bind with the C of formaldehyde. A weak physisorbed state with adsorption energy of 0.28 eV was found with the O of formaldehyde interacting with a surface Ce. On the (1 1 0) surface, dioxymethyelene formation was also observed, with an adsorption energy of 1.31 eV. The weakly adsorbed state of formaldehyde on the (1 1 0) surface was energetically comparable to the weak adsorption state on the (1 1 1) surface. Analysis of the local density of states and charge density differences after adsorption shows that strong covalent bonding occurs between the C of formaldehyde and surface O when dioxymethylene forms. Calculated vibrational frequencies also confirm dioxymethylene formation. Our results show that as the coverage increases, the adsorption of formaldehyde on the (1 1 1) surface becomes weak, but is nearly unaffected on the (1 1 0) surface.     

Numerical study of localization length in disordered graphene nanoribbons

In this work, we study quantum transport properties of a defective graphene nanoribbon (DGNR) attached to two semi-infinite metallic armchair graphene nanoribbon (AGNR) leads. A line of defects is considered in the GNR device with different configurations, which affects on the energy spectrum of the system. The calculations are based on the tight-binding model and Green’s function method, in which localization length of the system is investigated, numerically. By controlling disorder concentration, the extended states can be separated from the localized states in the system. Our results may have important applications for building blocks in the nano-electronic devices based on GNRs.     

Lithium adsorption on armchair graphene nanoribbons

Lithium adsorption on two dimensional graphene and armchair graphene nanoribbons is studied using advanced density functional theory calculations. The relative stability of different adsorption sites is investigated taking into account different ribbon widths, adsorbate densities, and spin states. We find the singlet spin state to be the true ground state of the systems considered. For this spin state, the binding energy increases with decreasing adatom density due to lower Coulomb repulsion between the partially charged Li atoms. At low adsorbate densities the favorable adsorption sites on the nanoribbons are found to be the hollow sites near the edges of the ribbon, whereas at higher densities, Li atoms tend to couple on next-nearest neighboring hexagons close to the ribbon's edge. Adsorption of the metal atoms is found to significantly decrease the bandgaps of all systems studied, turning them metallic for sufficiently large adatom densities. This suggests lithium doping as a possible route for bandgap engineering of graphitic systems.     

Hydrogen and deuterium adsorption on uranium decorated graphene nanosheets: A combined molecular dynamics and density functional theory study

In this study, the combined density functional theory (DFT) and molecular dynamics (MD) simulation methods were carried out to investigate the potential capability of uranium-decorated graphene (U–G) for the separation of deuterium from hydrogen gases. Graphene with hexagonal honeycomb lattice arrangement is suitable for adsorption of individual uranium atoms, with a high binding energy (?1.173 eV) and U-U distance longer than 7 Å. This U-G system has ability to hold up to six H₂ (5.16% wt) or seven D₂ (11.75% wt) molecules per U atoms. To gain further insights into these interactions, partial electronic density of states (PDOS) and the electron density distribution of the elements were analyzed. The MD results are in reasonable agreement with the results obtained by DFT method. Our calculated results indicate that at room temperature, D₂ molecule has higher affinity for U-G system than the H₂ molecule. In order to increase the D₂ separation factor from H₂, the effect of temperature was studied. The results indicated that adsorption ratio of D₂ to H₂ increases by decreasing the temperature.     

Perfect spin-filtering in p-aminophenol functionalized zigzag graphene nanoribbons: The role of sp3 hybridized nitrogen

Covalent functionalization of graphene is recently developed from the formation of sp³ hybridized carbon atom (sp³-C) to the sp³ hybridized nitrogen (sp³-N) at the anchoring site. Here, we investigated the electronic structures and transport properties of the zigzag graphene nanoribbons functionalized by covalently bonding of p-aminophenol (p-AP) molecule. First principles results demonstrate that the formed sp³-N plays a vital role in determining the electronic structure and transport properties of the system, resulting in a halfmetallic characteristic with a perfect spin-filtering behavior (100%). Interestingly, the performance of the spin-filtering is find to be insensitive to the sub-structures of the molecule. Our findings reveal the importance of sp³-N and suggest a new mechanism for realizing high-performance spin-filtering devices with functionalized graphene.     

Ab initio study of the electronic and transport properties of waved graphene nanoribbons

We apply the nonequilibrium Green's function method based on density functional theory to investigate the electronic and transport properties of waved zigzag and armchair graphene nanoribbons. Our calculations show that out-of-plane mechanical deformations have a strong influence on the band structures and transport characteristics of graphene nanoribbons. The computed I-V curves demonstrate that the electrical conductance of graphene nanoribbons is significantly affected by deformations. The relationship between the conductance and the compression ratio is found to be sensitive to the type of the nanoribbon. The results of our study indicate the possibility of mechanical control of the electronic and transport properties of graphene nanoribbons.     

Effect of oxidation of graphene nanoribbons on electronic and magnetic properties

The electronic properties, band gap and ionization potential as well as the energies of the singlet and triplet states of zigzag and armchair graphene nanoribbons are calculated as a function of the number of oxygen atoms on the ribbon employing density functional theory at B3LYP/6-31G* level. The calculated band gaps indicate that both structures are semiconducting. The band gap of the armchair ribbons initially decreases followed by an increase with oxygen number. For zigzag ribbons the band gap decreases with increasing oxygen number whereas the ionization potential increases with oxygen content. In both armchair and zigzag ribbons the ionization potential shows a gradual increase with the number of oxygen atoms. Some of the oxygenated ribbons calculated have triplet ground states and have the density of states at the Fermi level for spin down greater than spin up suggesting the possibility they may be ferromagnetic semiconductors.     

Detection of valley currents in graphene nanoribbons

There are two valleys in the band structure of graphene zigzag ribbons, which can be used to construct valleytronic devices. We studied the use of a T junction formed by an armchair ribbon and a zigzag ribbon to detect the valley-dependent currents in a zigzag graphene ribbon. A current flowing in a zigzag ribbon is divided by the T junction into the zigzag and armchair leads and this separation process is valley dependent. By measuring the currents in the two outgoing leads, the valley-dependent currents in the incoming lead can be determined. The method does not require superconducting or magnetic elements as in other approaches and thus will be useful in the development of valleytronic devices.     

Enhanced gas sensor based on nitrogen-vacancy graphene nanoribbons

We study the electron transport of nitrogen-vacancy zigzag graphene nanoribbons (ZGNRs) absorbing gas molecules. It is found that the nitrogen-vacancy ZGNRs are more sensitive to the gas molecules than the pristine ZGNRs. The gas molecules absorbed on the three-nitrogen vacancies lead to sharp resonant peaks on conductance, while those absorbed on the four-nitrogen vacancies lead to anti-resonant dips. Each kind of gas molecule can be detected by its own unique (different energy) resonant peaks (or dips). This indicates that the nitrogen vacancy can enhance the sensitivity to gas molecules, i.e., nitrogen-vacancy ZGNRs can serve as better gas sensors.     

Tunable band structure effects on ballistic transport in graphene nanoribbons

Graphene nanoribbons (GNR) in mutually perpendicular electric and magnetic fields are shown to exhibit dramatic changes in their band structure and electron transport properties. A strong electric field across the ribbon induces multiple chiral Dirac points, closing the semiconducting gap in armchair GNRs. A perpendicular magnetic field induces partially formed Landau levels as well as dispersive surface-bound states. Each of the applied fields on its own preserves the even symmetry Ek=E−k of the subband dispersion. When applied together, they reverse the dispersion parity to be odd and gives Ee,k=−Eh,−k and mix the electron and hole subbands within the energy range corresponding to the change in potential across the ribbon. This leads to oscillations of the ballistic conductance within this energy range.     

Resonant states in heterostructures of graphene nanoribbons

We study the transport properties of heterostructures of armchair graphene nanoribbons (AGNR) forming a double symmetrical barrier configuration. The systems are described by a single-band tight-binding Hamiltonian and Green's functions formalism, based on real-space renormalization techniques. We present results for the quantum conductance and the current for distinct configurations, focusing our analysis on the dependence of the transport with geometrical effects such as separation, width and transverse dimension of the barriers. Our results show the apparition of a series of resonant peaks in the conductance, showing a clear evidence of the presence of resonant states in the conductor. Changes in the barrier dimensions allow the modulation of the resonances in the conductance, making possible to obtain a complete suppression of electron transmission for determined values of the Fermi energy. The current–voltage curves show the presence of a negative differential resistance effect with a threshold voltage that can be controlled by varying the separation between the barriers and by modulating its confinement potential.     

Electronic properties of Li-doped zigzag graphene nanoribbons

Zigzag graphene nanoribbons (ZGNRs) are known to exhibit metallic behavior. Depending on structural properties such as edge status, doping and width of nanoribbons, the electronic properties of these structures may vary. In this study, changes in electronic properties of crystal by doping Lithium (Li) atom to ZGNR structure are analyzed. In spin polarized calculations are made using Density Functional Theory (DFT) with generalized gradient approximation (GGA) as exchange correlation. As a result of calculations, it has been determined that Li atom affects electronic properties of ZGNR structure significantly. It is observed that ZGNR structure exhibiting metallic behavior in pure state shows half-metal and semiconductor behavior with Li atom.     

Metal-semiconductor transition of graphene nanoribbons with different addends

Using a LCAO method, which is based on spinless sp³ scheme, we have studied the electronic properties of graphene nanoribbons with zigzag edges (ZGNRs) terminated partially by methylene groups. Metal-semiconductor transition is proved when the H atoms at both sides of ZGNRs are partially substituted by methylene groups. Furthermore, when one-third of H atoms are substituted and the distribution of methylenes is symmetric, the band gap comes to about 0.59 eV, which is the widest energy gap in this work. Otherwise, when the addends at both sides are of asymmetric distribution, a band gap of only 0.21 eV is obtained. These results suggest that the addends at the edge of ZGNRs play an important role in modifying the electronic properties.     

Electronic transport properties on V-shaped-notched zigzag graphene nanoribbons junctions

Using nonequilibrium Green?s functions in combination with the density functional theory, the spin-dependent electronic transport properties on V-shaped notched zigzag-edged graphene nanoribbons junctions have been calculated. The results show that the electronic transport properties are strongly depending on the type of notch and the symmetry of ribbon. The spin-filter phenomenon and negative differential resistance behaviors can be observed. A physical analysis of these results is given.     

A DFT study of carbon monoxide adsorption on a Si4 nano-cluster

Using the gradient-corrected hybrid density functional method of Predew, Burke, and Ernzerhof (PBEPBE) and the new hybrid meta-density functional method of Truhlar (MPW1B95), the geometry, adsorption energy, vibrational frequency, and charge distribution of carbon monoxide adsorption on a Si₄ nano-cluster has been studied. Taking into account spin multicipility in the calculations, a new stable structure of CO absorbed on the Si₄ cluster has been found, in addition to the previously reported structures. Exhaustive vibrational frequency analysis of optimized structures shows that some of the formerly reported structures have imaginary vibrational frequencies and are not proper stable structures. Thus, they do not represent real local energy minima. Also, CO vibrational frequency analysis shows that a significant change of vibrational frequency in the stable structures occurs.