Structural and electronic properties of bilayer and trilayer graphdiyne

Stimulated by the recent experimental synthesis of a new layered carbon allotrope-graphdiyne film, we provide the first systematic ab initio investigation of the structural and electronic properties of bilayer and trilayer graphdiyne and explore the possibility of tuning the energy gap via a homogeneous perpendicular electric field. Our results show that the most stable bilayer and trilayer graphdiyne both have their hexagonal carbon rings stacked in a Bernal way (AB and ABA style configuration, respectively). Bilayer graphdiyne with the most and the second most stable stacking arrangements have direct bandgaps of 0.35 eV and 0.14 eV, respectively; trilayer graphdiyne with stable stacking styles have bandgaps of 0.18-0.33 eV. The bandgaps of the semiconducting bilayer and trilayer graphdiyne generally decrease with increasing external vertical electric field, irrespective of the stacking style. Therefore, the possibility of tuning the electronic structure and optical absorption of bilayer and trilayer graphdiyne with an external electric field is suggested.     

First principle calculations of the electronic properties of nitrogen-doped carbon nanoribbons with zigzag edges

Calculations have been performed for carbon nanoribbons (CNRs) with zigzag edges containing one substitutional nitrogen atom per 154 carbon atoms, using ab initio density functional theory. It is found that the formation energies of these nanoribbons depend on the nitrogen doping site, as do the electrical properties. The doping nitrogen atom energetically prefers to distribute near the nanoribbon edges, and there is an impurity state below or above the Fermi level for the nitrogen-doped CNR, which depends on the nitrogen doping site. Also, the distribution of non-bonding electrons of nitrogen atom depends on the nitrogen doping site.     

First-principles prediction of the transition from graphdiyne to a superlattice of carbon nanotubes and graphene nanoribbons

Graphdiyne is a recently-synthesized carbon allotrope with a framework of sp- and sp²-hybridized carbon atoms. From first-principles calculations, we propose a possible transition of graphdiyne to a novel carbon allotrope (h-carbon) whose structure is a superlattice of carbon nanotubes and graphene nanoribbons. The energy barrier of this endothermic transition was estimated to be 4.30 kcal/mol at zero pressure, which is much lower than that of the graphite–diamond transition at high pressure. First-principles calculations on the phonon spectrum and the elastic constants of the h-carbon revealed that it is kinetically and mechanically stable. This unique framework of sp²- and sp³-hybridized carbon atoms is energetically neutral versus diamond. The hardness of the h-carbon (35.52 GPa) is 1/3 that of diamond and very close to β-SiC crystal. Accurate electronic structure calculations based on the Heyd, Scuseria, and Ernzerhof approach and GW approximation indicate that the h-carbon is a semiconducting material with a band gap of 2.20–2.56 eV.     

Probing the chemical and electronic properties of the core-shell architecture of transition metal trisulfide nanoribbons

Ultraviolet and X-ray photoelectron spectroscopies are used to probe the chemical and electronic structure of an amorphous, 2-20 nm-thick shell that encases the crystalline core in core-shell nanoribbons of TaS(3). The shell is chemically heterogeneous, containing elemental sulfur and a with a notable (S(2))(2-) deficiency over the crystalline TaS(3) core. We find nanoribbon stability to be substrate-dependent; whilst the ribbons are stable on the native oxide of a silicon surface, mass transport of sulfur species between the amorphous shell and a gold substrate leads to a significant change in the electronic properties of the nanomaterials. Our observations may have general implications for the incorporation of nanostructured transition metal chalcogenides into electronic devices.     

The effects of interlayer mismatch on electronic properties of bilayer armchair graphene nanoribbons

We investigate the impact of interlayer mismatch on the electronic properties of bilayer graphene nanoribbons (BGNRs) with armchair-edges in terms of the total energy and electronic structures by first principle calculations. Simulation results show that in-plane misalignments require little energy and a large variation in the energy bandgap (E_G) can be observed. Based on the resulting atomic configurations due to the misalignments, the details of the observed relationship between bandgap and the lattice mismatch are investigated. It is observed that in general, misalignment in the transverse direction results in a decrease in the interaction between the two layers, giving rise to a larger E_G. On the other hand, misalignment in the longitudinal direction, i.e. along the edges, leads to an oscillation in E_G due to the periodic change of the GNR stacking order. A combination of these movements results in a complex variation of E_G, which introduces great uncertainty in electronic devices. However, such a phenomenon could also be used in various kinds of nanoelectromechanical systems as it provides a large change in electronic properties with a small movement.     

Edge effect on electronic transport properties of graphene nanoribbons and presence of perfectly conducting channel

Numerical calculations have been performed to elucidate unconventional electronic transport properties in disordered nanographene ribbons with zigzag edges (zigzag ribbons). The energy band structure of zigzag ribbons has two valleys that are well separated in momentum space, related to the two Dirac points of the graphene spectrum. The partial flat bands due to edge states make the imbalance between left- and right-going modes in each valley, i.e. appearance of a single chiral mode. This feature gives rise to a perfectly conducting channel in the disordered system, i.e. the average of conductance 〈g〉 converges exponentially to 1 conductance quantum per spin with increasing system length, provided impurity scattering does not connect the two valleys, as is the case for long-range impurity potentials. Ribbons with short-range impurity potentials, however, through inter-valley scattering, display ordinary localization behavior. Symmetry considerations lead to the classification of disordered zigzag ribbons into the unitary class for long-range impurities, and the orthogonal class for short-range impurities. The electronic states of graphene nanoribbons with general edge structures are also discussed, and it is demonstrated that chiral channels due to the edge states are realized even in more general edge structures except for armchair edges.     

Defect symmetry influence on electronic transport of zigzag nanoribbons

The electronic transport of zigzag-edged graphene nanoribbon (ZGNR) with local Stone-Wales (SW) defects is systematically investigated by first principles calculations. While both symmetric and asymmetric SW defects give rise to complete electron backscattering region, the well-defined parity of the wave functions in symmetric SW defects configuration is preserved. Its signs are changed for the highest-occupied electronic states, leading to the absence of the first conducting plateau. The wave function of asymmetric SW configuration is very similar to that of the pristine GNR, except for the defective regions. Unexpectedly, calculations predict that the asymmetric SW defects are more favorable to electronic transport than the symmetric defects configuration. These distinct transport behaviors are caused by the different couplings between the conducting subbands influenced by wave function alterations around the charge neutrality point.     

Stacking dependent electronic structure and transport in bilayer graphene nanoribbons

The stacking-dependent electronic structure and transport properties of bilayer graphene nanoribbons suspended between gold electrodes are investigated using density functional theory coupled with non-equilibrium Green’s functional method. We find substantially enhanced electron transmission as well as tunneling currents in the AA stacking of bilayer nanoribbons compared to either single-layer or AB stacked bilayer nanoribbons. Interlayer separation between the nanoribbons appears to have a profound impact on the conducting features of the bilayer nanoribbons, which is found to be closely related to the topology and overlap between the edge-localized π orbitals.     

Excitonic properties of hydrogen saturation-edged armchair graphene nanoribbons

First-principle density functional theory calculations with quasiparticle corrections and many body effects are performed to study the electronic and optical properties of armchair graphene nanoribbons (AGNRs) with variant edges saturated by hydrogen atoms. The "effective width" method associated with the reported AGNR family effect is introduced to understand the electronic structures. The method is further confirmed by analyses of the optical transition spectra and the exciton wavefunctions. The optical properties, including the optical transition spectra, exciton binding energies and the distribution of exciton wavefunctions, can be tuned with the hydrogen saturation edge, thus providing an effective way to control the features of the AGNRs.     

Spectral phonon thermal properties in graphene nanoribbons

This work provides a comprehensive investigation on the spectral phonon properties in graphene nanoribbons (GNRs) by the normal mode decomposition (NMD) method, considering the effects of edge chirality, width, and temperature. We find that the edge chirality has no significant effect on the phonon relaxation time but has a large influence to the phonon group velocity. As a result, the thermal conductivity of around 707 W/(m K) in the 4.26 nm-wide zigzag GNR at room temperature is higher than that of 467 W/(m K) in the armchair GNR with the same width. As the width decreases or the temperature increases, the thermal conductivity reduces significantly due to the decreasing relaxation times. Good agreement is achieved between the thermal conductivities predicted from the Green–Kubo method and the NMD method. We find that optical phonons dominate the thermal transport in the GNRs while the relative contribution of acoustic phonons to the thermal conductivity is only 10.1% and 13% in the zigzag GNR and the armchair GNR, respectively. Interestingly, the ZA mode is found to contribute only 1–5% to the total thermal transport in GNRs, being much lower than that of 30–70% in single layer graphene.     

The properties of BiSb nanoribbons from first-principles calculations

The structural, electronic and magnetic properties of BiSb nanoribbons (BSNRs) with different widths and edge configurations are investigated via the first-principles pseudopotential method. It is found that the pristine BSNRs with armchair edges (ABSNRs) are semiconductors and the band gaps exhibit a width dependent odd-even oscillation. In contrast, the pristine BSNRs with zigzag edges (ZBSNRs) are found to be metallic. When all the edge atoms are passivated by hydrogen, both the ABSNRs and ZBSNRs become semiconducting and the corresponding band gaps decrease monotonically with the increasing width. If, however, the edge atoms are partially passivated, the ABSNRs can be either semiconducting or metallic. Moreover, local magnetism appears when all the edge Sb atoms are passivated and there are one or more unsaturated Bi atoms. Using the nonequilibrium Green's function (NEGF) approach, we find that all the investigated odd-numbered ABSNRs have almost the same peak value of the power factor around the Fermi level. This is not the case for the even-numbered ABSNRs, where the peaks are twice that of when they are n-type doped. Our calculations indicate that BSNRs can have a very high room temperature figure of merit (ZT value), which makes them very promising candidates for thermoelectric applications.     

A theoretical investigation on the transport properties of overlapped graphene nanoribbons

The transport characteristics of overlapped junctions of Zigzag Graphene NanoRibbons (ZGNRs) are simulated and analyzed using Non-Equilibrium Green’s function combined with the Density Functional Theory. It is found that the carriers pass through an overlapped junction via several different energy states by tunneling process. In general, the current passing across the junction is mainly due to tunneling of carriers between many quasi-bound states. Meanwhile, few transmissions are observed between individual states. The latter behaviors cannot be explained by quasi-bound states; however, they are interpreted as the states which show long range resonance phenomenon. A combination of these states results in complex variations of transmission characteristics. These variations introduce several negative differential resistances by increasing the voltage across the junction of ZGNRs. In other words, misalignment in the states along the junction leads to periodic changes in the coupling of the quasi-bound states and long range resonant states. This makes an oscillation in the current voltage characteristics of the overlapped junction in ZGNRs. Consequently, the overlapped junction in ZGNR has a lower electrical transport comparing to that of an ideal (non-overlapped) ZGNR.     

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.     

A single particle Hamiltonian for electro-magnetic properties of graphene nanoribbons

Graphene zigzag edges are known to show the spin polarized ferromagnetic states, which are well described by the mean field treatment of Hubbard model. The parameter of onsite Coulomb interaction U is estimated to be comparable to the kinetic hopping parameter t so as to fit the electronic band structures obtained by the spin–polarized density functional theory (DFT). In this paper, we propose a simple way to transfer the electronic band structures obtained by DFT onto the mean-field Hubbard Hamiltonian by adopting site-dependent U parameter, which is taken as the decaying function from the edge. This approach is applicable to both anti-ferromagnetic and ferromagnetic states between two edges of graphene nanoribbons and will serve to perform the further large-scale simulation of electro-magnetic transport properties of graphene-based nanodevices.     

Electronic and magnetic properties of pristine and chemically functionalized germanene nanoribbons

We perform a spin polarized density-functional theory (DFT) study of the electronic and magnetic properties of pristine and chemically doped germanene nanoribbons (GeNRs) with different widths. It is found that the Ge atom at the ribbon edge always prefers to be substituted by an impurity atom. Our study reveals that a single N or B atom substitution induces a semiconducting-metal transition in armchair oriented germanene nanoribbons (AGeNRs) as evidenced by the appearance of a half-filled band with less dispersion; however, N and B co-doping at the ribbon edges only modifies their band gaps, due to the accomplishment of an effective charge compensation. A single N or B atom substitution usually turns antiferromagnetic (AFM) semiconducting zigzag germanene nanoribbons (ZGeNRs) into ferromagnetic (FM) semiconductors. This AFM-FM transition is attributed mainly to the perturbation of π and π states localized at the doped edge. Double atom substitutions (regardless of N-N, B-B or N-B configurations) at the edges of ZGeNRs removes the spin-polarization at both edges and transforms them into non-magnetic (NM) semiconductors. Moreover, it is interesting that some single atom doped ZGeNRs can exhibit a FM half-metallic character with 100% spin-polarization at the Fermi level. Our results suggest that doped AGeNRs and ZGeNRs have potential applications in Ge-based nanoelectronics, such as field effect transistors (FETs), negative differential resistance (NDR) and spin filter (SF) devices.     

Influence of the curvature of deformed graphene nanoribbons on their electronic and adsorptive properties: theoretical investigation based on the analysis of the local stress field for an atomic grid

Electronic and adsorptive properties of deformed graphene are investigated in the current work. Armchair and zigzag nanoribbons are the subject of the study. The axial compression was a deforming load. A calculation method for the local stress field was developed. This method was based on the quantum model of the finite graphene nanoribbon and empirical calculation method of the single atom energy. The stress field of the deformed ribbon was calculated by means of the suggested methodology. The effects of the atomic grid curvature on the adsorptive capacity of graphene and the hydrogenation process was investigated by means of the developed method. The prediction of the appearance of defects on covalent C-C bond breakdown is also performed.     

Effect of vacancy defects on phonon properties of hydrogen passivated graphene nanoribbons

The phonon properties of hydrogen-passivated armchair graphene nanoribbons (AGNRs) with different vacancy concentrations are investigated theoretically. We calculate the change in the phonon density of states (PDOSs) due to a broad range of vacancies and hydrogen passivation effects using forced vibrational method. A large downshift of prominent Raman active Г point LO mode phonons with an increase of vacancy concentration or decrease of ribbon widths are observed. We find an increasing peak intensities for the C–H stretching mode with the decrease of ribbon width or the increase of defect density. An inserted vacancy concentration of 10% and higher induce the broadening and distorting of the PDOS peaks significantly. The localization properties of phonon due to defects were also studied. The typical mode pattern of K point iTO mode phonons show the spatial localized vibrations persuaded by armchair edges or vacancies, which are in conceptually good agreement with the large D band of the Raman spectra comes from the armchair-edges or the imperfections of crystal. The typical displacement pattern for C–H stretching mode shows a random displacement of H atoms in contrast to C atoms. Our simulation results show the significant impact of vacancy defects on the vibrational properties of GNRs.     

Preparation, characterization, and electrochemical properties of lithium vanadium oxide nanoribbons

Highly uniform lithium vanadium oxide nanoribbons were successfully prepared in large quantities using a facile hydrothermal approach without employing any surfactants or templates. The as-prepared products were up to hundreds of micrometers in length, about 200 nm in width, and 20 nm in thickness. These nanoribbons and nafion composite were employed to modify glassy carbon electrode, which displayed excellent electrochemical sensitivity and rapid response in detecting dopamine in phosphate buffer solution. Lithium ions can greatly increase the electron transfer between the electrode and biological materials, and significantly increase the reversibility of electrochemical process. A linear relationship between the concentrations of dopamine and its oxidation peak currents was obtained. The linear range for the detection of dopamine was 2.0 × 10⁻⁶ to 1.0 × 10⁻⁴ M with a detection limit of 1.0 × 10⁻⁷ M. In addition, the good reproducibility and long-term stability of the sensor make it valuable for further application.     

Half metallicity in BC2)N nanoribbons: stability, electronic structures, and magnetism

Nanoribbons are suggested to be among the most promising candidates being considered as building blocks in future electronics. In this study, we use density functional calculations to examine the structures and electronic properties of BC(2)N nanoribbons with bare zigzag-shaped edges (zz-BC(2)NNRs). Four different types of atomistic edge configurations are considered, including ribbons terminated with two C edges, B and N edges, B an C edges, and C and N edges. We find the existence of half-metallicity in the ground state of the zz-BC(2)NNRs with two bare C edges and with bare C and N edges. The other two configurations of the zz-BC(2)NNRs can be either semiconducting or metallic, depending on the specific configuration. We also find that the stability of the zz-BC(2)NNRs are largely dependent on ribbon width. The zz-BC(2)NNRs become energetically more stable when the nanoribbon width exceeds 3.3 nm. It is interesting to find that half-metallic zz-BC(2)NNRs with a width of 0.7 nm are thermodynamically more stable than either metallic or semiconducting counterparts. Therefore, the possibility of synthesizing half-metallic zz-BC(2)NNRs exists.