Increasing Ribbon Width Research Articles

Bandgap tuning in armchair MoS2 nanoribbon

We report on the first-principles calculations of bandgap modulation in armchair MoS2 nanoribbon (AMoS2NR) by transverse and perpendicular electric fields respectively. In the monolayer AMoS2NR case, it is shown that the bandgap can be significantly reduced and be closed by transverse field, whereas the bandgap modulation is absent under perpendicular field. The critical strength of transverse field for gap closure decreases as ribbon width increases. In the multilayer AMoS2NR case, in contrast, it is shown that the bandgap can be effectively reduced by both transverse and perpendicular fields. Nevertheless, it seems that the two fields exhibit different modulation effects on the gap. The critical strength of perpendicular field for gap closure decreases with increasing number of layers, while the critical strength of transverse field is almost independent of it.

Dimensional crossover of thermal conductance in graphene nanoribbons: a first-principles approach

First-principles density-functional calculations are performed to investigate the thermal transport properties in graphene nanoribbons (GNRs). The dimensional crossover of thermal conductance from one to two dimensions (2D) is clearly demonstrated with increasing ribbon width. The thermal conductance of GNRs of a few nanometers width already exhibits an approximate low-temperature dependence of T1.5, like that of 2D graphene sheets which is attributed to the quadratic nature of the dispersion relation for the out-of-plane acoustic phonon modes. Using a zone-folding method, we heuristically derive the dimensional crossover of thermal conductance with the increase of ribbon width. Combining our calculations with the experimental phonon mean-free path, some typical values of thermal conductivity at room temperature are estimated for GNRs and for 2D graphene sheet. Our findings clarify the issue of the low-temperature dependence of thermal transport in GNRs and suggest a calibration range of thermal conductivity for experimental measurements in graphene-based materials.

The role of size in spin properties of zigzag graphene nanoribbon

The effects of ribbon size on the spin properties of zigzag graphene nanoribbon (ZGNR) with and without edge pentagon-heptagon reconstruction are investigated using spin-polarized density functional theory. It is found that when ribbon width is less than 26.31 A, the Coulomb and RKKY-like interaction between two ribbon-edges result in the edge state splitting. For a wider ZGNR, the interedge coupling is negligible. In addition to stabilizing the system and weakening the interedge coupling, the edge pentagon-heptagon reconstruction could demagnetize the ZGNR. In perfect and single-edge reconstructed ZGNR, the magnetic moment depends on ribbon width, and the change of magnetic moment becomes inconspicuous with increasing ribbon width. Double-edge reconstructed ZGNR shows nonmagnetic. It may be useful for fabricating graphene-based nanoelectronic and spintronic devices.

Electronic structures of the F-terminated AlN nanoribbons

Using the first-principles calculations, electronic properties for the F-terminated AlN nanoribbons with both zigzag and armchair edges are studied. The results show that both the zigzag and armchair AlN nanoribbons are semiconducting and nonmagnetic, and the indirect band gap of the zigzag AlN nanoribbons and the direct band gap of the armchair ones decrease monotonically with increasing ribbon width. In contrast, the F-terminated AlN nanoribbons have narrower band gaps than those of the H-terminated ones when the ribbons have the same bandwidth. The density-of-states (DOS) and local density-of-states (LDOS) analyses show that the top of the valence band for the F-terminated ribbons is mainly contributed by N atoms, while at the side of the conduction band, the total DOS is mainly contributed by Al atoms. The charge density contour analyses show that Al–F bond is ionic because the electronegativity of F atom is much stronger for F atom than for Al atom, while N–F bond is covalent because of the combined action of the stronger electronegativity and the smaller covalent radius.

Thermoelectric properties of armchair and zigzag silicene nanoribbons

Using the nonequilibrium Green's function method and nonequilibrium molecular dynamics simulations, we discuss the possibility of using silicene nanoribbons (SiNRs) as high performance thermoelectric materials. It is found that SiNRs are structurally stable if the edge atoms are passivated by hydrogen, and those with armchair edges usually exhibit much better thermoelectric performance than their zigzag counterparts. The room temperature ZT value of armchair SiNRs shows a width-dependent oscillating decay, while it decreases slowly with increasing ribbon width for the zigzag SiNRs. In addition, there is a strong temperature dependence of the thermoelectric performance of these SiNRs. Our theoretical calculations indicate that by optimizing the doping level and applied temperature, the ZT value of SiNRs could be enhanced to as high as 4.9 which suggests their very appealing thermoelectric applications.

First-principles study of the F-terminated Boron Nitride nanoribbons

Using first-principles calculations we study the electromagnetic properties of the F-terminated Boron Nitride nanoribbons (BNNRs) with both zigzag and armchair edges (ZBNNRs and ABNNRs). The results show that, compared with the indirect the H-terminated ZBNNRs, the F-terminated ZBNNRs are direct semiconductors and also the band gaps decrease monotonically with the increasing ribbon width, while the ABNNRs maintain the direct character. The band gap of the F-terminated BNNR is lower than that of the H-terminated one with the same ribbon width because of the mixing effects from the non-bonding 2p electrons of F and N atoms. Both the ZBNNRs and ABNNRs maintain the non-magnetic characters. Compared with the scarce contribution from H atom, the charge density of the highest valence band and the lowest conduction band analyses show that the contribution from F atom cannot be neglected. The N–B, B–F and B–H are ionic bonds with different bonding strength, while the N–F and N–H are all covalent bonds with different charge density accumulation.

Electronic and Magnetic Properties and Structural Stability of BeO Sheet and Nanoribbons

The novel electronic and magnetic properties of BeO nanoribbons (BeO NRs) as well as their stability are investigated through extensive density functional theory calculations. Different from semiconducting graphene nanoribbons and insulating BN ribbons, all zigzag edged BeO NRs are revealed to display ferromagnetic and metallic natures independent of the ribbon width and edge passivation. The polarized electron spins in H-passivated zigzag BeO NRs are from the unpaired electrons around the weakly formed Be-H bonds, while those of bare zigzag BeO NRs are due to the 2p states of edge O atoms. In sharp contrast, all armchair BeO NRs are nonmagnetic insulators regardless of the edge passivation. In particular, all bare armchair BeO NRs have a nearly constant band gap due to a peculiar edge localization effect. Interestingly, the band gaps of all armchair BeO NRs can be markedly reduced by an applied transverse electric field and even completely closed at a critical field. The critical electric field required for gap closing decreases with increasing ribbon width, thus the results have practical importance. Further stability analysis shows that bare BeO NRs are more stable than H-passivated BeO NRs of similar ribbon widths and bare armchair BeO NRs are energetically the most favorable among all the nanoribbons.

Structural and electronic properties of a single C chain doped zigzag AlN nanoribbon

The effects of a single C-chain on the structural and electronic properties of the H terminated zigzag AlN nanoribbons (ZAlNNRs) have been investigated systemically by using the first-principles. The results show that in perfect 7-ZAlNNR for example, the states of the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) at zone boundary Z are edge states their charges are localized at edge Al and N atoms, respectively. Introducing a single C-chain and changing its position lead to the LUCB and HOVB getting closer with each other. Similar to the edge states existing in perfect ZAlNNR, the flat dispersion border states also exist in a single C-chain decorated ZAlNNR, but their charges are localized at border C–N and C–Al for LUCB and HOVB, respectively. Furthermore, for N z -ZAlNNR-C( n) with ribbon width N z = 2, 3, 4, 5, 6, 7 and 10, only N z -ZAlNNR-C(1) has a direct band gap, while the other N z -ZAlNNR-C( n) has an indirect band gap. Variation of the band gap with C-chain position n shows that, for N z -ZAlNNR-C( n) of arbitrary width N z , the N z -ZAlNNR-C(1) and N z -ZAlNNR-C(2) have nearly identical the minimum band gap of 0.132 eV and nearly identical the maximum band gap of 1 eV, respectively, except the maximum band gap of 0.63 eV for 2-ZAlNNR-C(2) because it belongs to the group of C-chain substituting the right edge Al–N chain, the band gap of this group decreases linearly with increasing ribbon width N z . For 3-, 4-, 5-, 6-, 7- and 10-ZAlNNR-C( n), the band gap decreases successively for C-chain position n from 2 to 3, 4, 5, 6, 7 and 10, respectively.

Electronic and magnetic properties of perfect and defected germanium nanoribbons

The electronic and magnetic properties of both perfect and defected germanium nanoribbons (GeNRs) are investigated by using projector-augmented wave method based on density-functional theory. All the GeNRs with different edge shapes (armchair or zigzag) and widths are cut from the buckled Ge hexagonal sheet which is found to be semi-metallic as the planar graphene sheet. The results show that the perfect armchair GeNRs are nonmagnetic semiconductors and their band gaps exhibit three branches with decaying profiles, while the perfect zigzag GeNRs show the stable antiferromagnetic semiconducting ground state and their band gaps monotonously decrease with increasing ribbon width. These properties of the GeNRs are similar to graphene nanoribbons and should be important for designing new functional Ge-based nanodevices. The effects of the monovacancy or divacancy on the electronic and magnetic properties of the GeNRs are also considered. We found that the band gap of armchair GeNRs can be easily tuned by a monovancancy or divacancy at different positions, which provides a way of band gap engineering of armchair GeNRs for actual applications. Different from the defected armchair GeNRs, the metallization can be realized in zigzag GeNRs by a monovacancy or a divacancy, however, their magnetic properties depend closely upon the vacancy positions.

Structural, electronic, and magnetic properties of C-doped GaN nanoribbon

First-principles calculations are performed to study the structural, electronic, and magnetic properties of pure and C-doped GaN nanoribbon with both zigzag edge (ZGaNNR) and armchair edge (AGaNNR). With increasing ribbon width, both the indirect bandgap of ZGaNNR and the direct bandgap of AGaNNR decrease monotonically and become closer to each other approaching their asymptotic limit of a single layer of GaN sheet. One C atom substituting for one N atom causes a slight local expansion, while one C atom substituting for one Ga atom results in a large local contraction. Furthermore, the C atom is preferred to substitute for an edge N or Ga atom in either 6-ZGaNNR or 6-AGaNNR, especially edge Ga atoms in 6-AGaNNR. There exists about 0.65 µB magnetic moment, which arises mainly from doped C atom while a single N atom is substituted by one C atom in either 6-ZGaNNR or 6-AGaNNR, which may open a way to design magnetic nanodevices based on GaNNR.

Electronic Structures of BC2N Nanoribbons

We reveal a rich variety of electronic and magnetic properties of H-terminated BC2N nanoribbons (BC2NNRs) by using extensive first-principles calculations. Zigzag edged BC2NNRs (z-BC2NNRs) can be semiconducting or metallic depending on the alignment of edge atoms. In particular, magnetic and even half-metallic behaviors can appear in some edged z-BC2NNRs when the ribbon width is over a critical value. Armchair-edged BC2NNRs also can be semiconducting or metallic but determined by the proportion of carbon, nitrogen, and boron atoms in the ribbons. The band gaps of all semiconducting BC2NNRs can be explained by a universal mechanism that is due to the charge polarization between the opposite edges, which is impaired with increasing ribbon width.

Computational Investigation on Structural and Physical Properties of AlN Nanosheets and Nanoribbons

Through first-principles computations, we investigated the structural, electronic and magnetic properties of two-dimensional AIN single layer and one-dimensional AIN nanoribbons. AIN single layer and nanoribbons quit the Wurtzite configuration and adopt a graphitic-like structure after geometry optimization. Both hydrogen-terminated zigzag and armchair AIN nanoribbons have a direct band gap, which increases monotonically with increasing ribbon width. Bare zigzag AIN nanoribbons have a spin-polarized ground state and are magnetic semiconductors. The results may promote the experimental preparation of AIN nanosheets and nanoribbons and their applications to nanotechnology.

Configuration and electronic properties of graphene nanoribbons on Si(2 1 1) surface

AbstractWe perform first-principles calculations based on density functional theory to study the configuration and electronic properties of graphene nanoribbons (GNRs) on Si(211) surface. Both [011¯] and [1¯11] adsorption orientations of Si(211) surface are considered. We find that the adsorption energy is determined not only by the edge states of GNRs, but also by the ribbon width and the orientation of the substrate. Bridge and M-shape adsorption configurations appear gradually as the ribbon width increases. The substrate effectively affects the edge states of GNRs and tends to depress the metallic nature of zigzag GNRs (Z-GNRs) and metallize the armchair GNRs (A-GNRs).

Insights on the Atomic and Electronic Structure of Boron Nanoribbons

A study of the structural stability of boron nanoribbons is presented. Antiaromatic instabilities are found to destabilize boron nanoribbons. Our studies suggest that nanoribbons obtained from "αsheets" are more stable than those from reconstructed {1221} sheets and traditional triangular boron sheets. The stability of the nanoribbons increases with an increasing ribbon width resulting in an increased hole density (η) and, hence, an increased number of hexagonal motifs in the nanoribbon. The boron nanoribbons formed are mostly metallic; however, semiconducting structures have also been observed.

Electronic structure of oxygen-terminated zigzag graphene nanoribbons: A hybrid density functional theory study

The size-dependent electronic structure of oxygen-terminated zigzag graphene nanoribbons is investigated using standard density functional theory (DFT) with an exchange-correlation functional of the generalized gradient approximation form as well as hybrid DFT calculations with two different exchange-correlation functionals. Hybrid DFT calculations, which typically provide more accurate band gaps than standard DFT, are found to predict semiconducting behavior in oxygen-terminated zigzag graphene nanoribbons; this is in distinct contrast to standard DFT with (semi)local exchange-correlation functionals, which have been widely employed in previous studies and shown to predict metallic behavior. (Semi)local exchange-correlation functionals employed in standard DFT calculations cause unphysical delocalization of lone pairs from the oxygen atoms due to self-interaction errors and lead to metallic behavior. Hybrid DFT calculations do not suffer from this spurious effect and produce a clear size-dependent band gap. Appreciable fundamental band gaps $(\ensuremath{\sim}1\text{ }\text{eV})$ are found for the smallest ribbons (two zigzag rows); the band gap decreases rapidly with increasing ribbon width, resulting eventually in a zero band-gap semiconductor at about 4--5 zigzag rows. This finding could have useful implications for molecular electronics, in particular, since oxygen-terminated zigzag graphene nanoribbons are thermodynamically stable unlike their hydrogenated counterparts. More generally, through a concrete example, this study suggests caution when employing (semi)local functionals in DFT studies of functionalized graphene/graphene derivatives when the functional groups contain electron lone pairs.

First-principles study of the perfect and vacancy defect AlN nanoribbon

Under the generalized gradient approximation (GGA), the electronic and magnetic properties of the perfect and vacancy defect AlN nanoribbon with both zigzag edge (ZAlNNR) and armchair edge (AAlNNR) are studied using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. Both ZAlNNR and AAlNNR are semiconducting and nonmagnetic, and the indirect band gap of ZAlNNR and the direct band gap of AAlNNR decrease monotonically with increase in ribbon width. A single non-edge Al vacancy makes either ZAlNNR or AAlNNR a semi-metal and thus yields complete (100%) spin polarization as well as a significant magnetic moment. Hence a single non-edge Al vacancy defect AlNNR can be used to construct efficient spin-polarized transport devices. But a single non-edge N vacancy induces a small magnetic moment in AAlNNR only. The AlNNR with a single edge N or Al vacancy is still semiconducting and nonmagnetic, leading to additional states only within the gap region and thus reducing the band gap width, except for a single edge Al vacancy in AAlNNR.

Magnetism in armchair BC2N nanoribbons

We show by first-principles calculations that H-terminated armchair BC2N nanoribbons (a-BC2NNRs) have exceptional electronic and magnetic properties. In particular, a-BC2NNRs with the B and N atoms uncoordinated can be either p- or n-doped semiconductors, and the wide ones own spontaneous magnetization. It is shown that the magnetism is induced by the localization of acceptor or donor levels from unpaired B or N atoms, and the magnetic moment increases with further increasing ribbon width. The a-BC2NNRs with B and N atoms coordinated have band gaps decreasing with increasing ribbon width.

Tuning Magnetism in Zigzag ZnO Nanoribbons by Transverse Electric Fields

We show by first-principles calculations that the magnetic moments of zigzag ZnO nanoribbons can be efficiently modulated by transverse electric fields. Depending on the field direction, the total magnetic moment in a zigzag ZnO nanoribbon can be remarkably enhanced or reduced and even completely quenched with increasing field over a threshold strength. However, in weak electric fields below the threshold, the magnetic moment in the zigzag ZnO nanoribbons nearly remains unchanged, which can be explained in terms of intrinsic transverse electric polarization and quantum confinement effects. The threshold electric field required to modulate the magnetic moment decreases significantly with increasing ribbon width, showing practical importance.

First-principles study on electronic properties of SiC nanoribbon

Under the generalized gradient approximation (GGA), the electronic properties are studied for SiC nanoribbon with zigzag edge (ZSiCNR) and armchair edge (ASiCNR) by using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. Distinct variation behaviors in band gap are exhibited with increasing ribbon width. The ZSiCNR is metallic except for the thinner ribbons (N z = 2–4) with small direct band gaps, while the direct band gaps of ASiCNR exhibit sawtooth-like periodic oscillation features and quench to a constant value of 2.359 eV as width N a increases. The PDOS onto individual atom shows that a sharp peak appeared at the Fermi level for broader ZSiCNR comes from the edge C and Si atoms with H terminations. The charge density contours analysis shows the valence charges are strongly accumulated around C atom, reflecting a significant electron transfer from Si atom to C atom and thus an ionic binding feature. In addition, the Si–H bond is also ionic bond while the C–H bond is covalent bond. The dangling bonds give rise to one (two) flat extra band at the Fermi level for ZSiCNR with either bare C or bare Si edge (for ZSiCNR with bare C and Si edges as well as for ASiCNR with either bare C edge or bare Si edge), except for ASiCNR with bare C and Si edges in which two nearly flat extra bands appear up and below the Fermi level.

Electric-Field- and Hydrogen-Passivation-Induced Band Modulations in Armchair ZnO Nanoribbons

We report on the electric-field- and H chemical-absorption-induced band manipulations of armchair ZnO nanoribbons using first-principles calculations. It is shown that the band gap of a semiconducting armchair nanoribbon can be reduced monotonically with increasing transverse field strength, demonstrating a giant Stark effect. The critical field strength to completely close the band gap decreases with increasing ribbon width, while it is almost independent of the stacking thickness. On the other hand, the nanoribbon with the edges fully passivated shows an enhanced gap but a slightly weaker Stark effect. We also observe hydrogen-termination-induced metallization of the ribbons when only the edge O atoms are passivated, which results from an n-type doping effect. These findings suggest potential ways of band engineering in armchair ZnO nanoribbons.