Armchair-edge Graphene Nanoribbons Research Articles

Robust and controllable electronic local transports in armchair silicene nanoribbons under a perpendicular electric field

We study the electronic local distribution and transports in pristine armchair-edge silicene nanoribbons (ASiNRs) based on the tight-binding approximation. By calculating the local densities of states at different sites and the bond current between two adjacent sites, we show that comparing to the pristine armchair-edge graphene nanoribbons, a similar “[Formula: see text]” rule and multiple low-electron transport channels exist in the pristine [Formula: see text]-ASiNRs. However, differently, they are controllable to appear and disappear by applying an electric field perpendicular to the ribbon plane. Therefore, one can manipulate the semiconducting channels and realize the current switch “on/off,” unchanging their structures. Moreover, the results are robust against the edge-passivation and a few structural defects, which ensures their stability for the practical application in the silicene-based device.

Theoretical Study of Ballistic Transport in Silicon Nanowire and Graphene Nanoribbon Field-Effect Transistors Using Empirical Pseudopotentials

We present a theoretical study of the ballistic performance of gate-all-around field-effect transistors (FETs) with channels consisting of armchair-edge graphene nanoribbons (aGNRs) of various widths and silicon nanowires (SiNWs) with square cross sections. We apply an atomistic quantum transport formalism based on empirical pseudopotentials. Our results show that the turn-OFF behavior of aGNRFETs is seriously affected by source-to-drain tunneling at short channel lengths (e.g., below 10 nm for 3-aGNRs). Defining the threshold voltage ${\textit V}_{\textsf {th}}$ as the gate bias at which the current reaches a given $I_{ \mathrm{\scriptscriptstyle OFF}} = 0.4~\mu \text{A}/\mu \text{m}$ , devices with a 5-nm channel length can reach a maximum current of only $400~\mu \text{A}/\mu \text{m}$ at a gate overdrive ${\textit V}_{\textit {GS}}-{\textit V}_{\textsf {th}} = 0.25$ V (corresponding to an assumed ${V}_{\textsf DD}$ of about 0.4 V). In contrast, SiNWFETs remain immune from source-to-drain tunneling even at 5 nm and show an almost ideal subthreshold swing, a satisfactory current of $1000~\mu \text{A}/\mu \text{m}$ at the same gate overdrive of 0.25 V, and a reasonable ${\textit I}_{ \mathrm{\scriptscriptstyle ON}}/I_{ \mathrm{\scriptscriptstyle OFF}}$ ratio around $2\times 10^{3}$ .

Electronic states of coupled graphene nanoribbons

Electronic states of laterally coupled graphene nanoribbons (GNRs) have been calculated within a nearest-neighbor tight-binding approximation with varying inter-GNR coupling strength γ from γ = 0 to t (intra-GNR transfer integral). For a coupled zigzag-edge GNR array, both almost flat bands and anisotropic Dirac cones appear near the Fermi level. For a coupled armchair-edge GNR array with a ribbon width N = 3n or 3n + 1, the system is semiconducting with a finite bandgap at γ = 0, which decreases as γ increases. For N = 3n, it becomes metallic with a zero bandgap only at γ = t. On the other hand, for N = 3n + 1, it becomes metallic when γ ≥ t/2. At γ = t/2, a peculiar energy dispersion emerges; the energy dispersion is parabolic perpendicular to the GNR axis and linear parallel to the GNR axis. When N = 3n + 2, the system is always metallic regardless of γ.

Finite-size effects on electronic structure and local properties in passivated AA-stacked bilayer armchair-edge graphene nanoribbons

Based on the tight-binding model and dual-probe scanning tunneling microscopy technology, we theoretically investigate the electronic structure and local property in the passivated AA-stacked bilayer armchair-edge graphene nanoribbons (AABLAGNRs). We show that they are highly sensitive to the size of the ribbons, which is evidently different from the single-layer armchair-edge graphene nanoribbons. The ‘3p’ rule only applies to the narrow AABLGNRs. Namely, in the passivated 3p- and (3p + 1)-AABLGNRs, the narrow ribbons are semiconducting while the medium and wide ribbons are metallic. Although the passivated (3p + 2)-AABLGNRs are metallic, the ‘3j’ rule only applies to the narrow and medium ribbons. Namely, electrons are in the semiconducting states at sites of line 3j while they are in the metallic states at other sites. This induces a series of parallel and discrete metallic channels, consisting of lines 3j − 1 and 3j − 2, for the low-energy electronic transports. In the passivated wide (3p + 2)-AABLGNRs, all electrons are in the metallic states. Additionally, the ‘3p’ and ‘3j’ rules are controllable to disappear and reappear by applying an external perpendicular electric field. Resultantly, an electric filed-driven current switch can be realized in the passivated narrow and medium (3p + 2)-AABLGNRs.

Detecting CO, NO and NO2 gases by Boron-doped graphene nanoribbon molecular devices

Combining nonequilibrium Green’s function method and density functional theory, an azulene-like dipole molecule sandwiched between two graphene nanoribbon (GNR) electrodes are explored to gas sensors. Both the pristine zigzag edged GNR and Boron-doped armchair-edged GNR are considered in this study. It shows that certain specific toxic molecules CO, NO and NO2 would adsorb on the doped Boron atoms of the GNR, resulting in a dramatic change in the current–voltage profile. Changes in the subbands of electrodes, induced by gas adsorption, are responsible for the variation of current. The devices are thus demonstrated to be sensitive nanosensors for these toxic gases.

Magnetic structures and magnetic device properties of edge-modified armchair-edged graphene nanoribbons

The magnetic properties based on the armchair-edged graphene nanoribbons (AGNRs) are less studied in currently existing works. Here, the edge modification of AGNR by the transition metal Mn (AGNR-Mn) and further functionalized with monohydrogenation (AGNR-Mn-H) and dihydrogenation (AGNR-Mn-H2) are considered. Our calculations from first-principles method show that they posses three triplet antiferromagnetic (AFM) states, which are all semiconductoring for AGNR-Mn and metallic for AGNR-Mn-H2, but semiconductoring or metallic for AGNR-Mn-H. Interestingly, at ferromagnetic (FM) state AGNR-Mn-H and AGNR-Mn-H2 are a half-metal with bandgaps 0.46 and 0.97 eV, respectively. AGNR-Mn-H2 featuring both the stable magnetic ordering and the highest energy stability are predicted, as well as the calculated Gibbs free energy unveils that these three structures can stably exist in a different chemical potential range in experiment. Also shown is that the perfect spin polarization in a very large bias region and a spin-valve effect with a large magnetoresistance can be achieved in a magnetic device based on AGNR-Mn-H2.

Insight into negative differential resistance in polyphenylene molecular device with graphene electrodes

The organic molecule deposited between gaphene electrodes to form a molecular device has been demonstrated experimentally. Motivated by this case, devices consisting of the polyphenylene molecule bonded covalently with armchair-edged graphene nanoribbon (AGNR) electrodes are constructed and the selective doping with N atom is considered theoretically. Our modeling calculations show that such devices hold the nonlinear and doping-site-dependent transport properties, prominently with multi-peak NDR (negative differential resistance) effect. And, for a peculiar doping site, a very large NDR can be observed, which could be attributed to interactions of the molecular core and doped AGNR electrodes, namely, these hybridized wave functions hold distinctly different delocalization in different benzene rings of the polyphenylene molecule when the applied bias is altered. Also shown is that this large NDR is robust regarding the length increasing and rotation of molecule.

Armchair-edged nanoribbon as a bottleneck to electronic total transmission through a topologically nontrivial graphene nanojunction

It is currently a promising approach to experimentally realize the topological insulator phase transition of graphene by introducing the extrinsic spin–orbit coupling (SOC). Then, electronic total transmission through various topological nontrivial graphene nanojunctions (GNJs) is obtainable, if the electronic transport is supported by the helical edge states. Though the bulk graphene is a gapless semiconductor, the inter-valley scattering could introduce a topological trivial gap in semiconducting armchair-edged graphene nanoribbon (GNR). The SOC should be strong enough to reopen a topological nontrivial gap before close such a trivial gap. Therefore, our theoretical study indicates that a semiconducting armchair-edged graphene nanoribbon (GNR) can not develop the helical edge states when the SOC strength is lower than a threshold, though the bulk phase is topological nontrivial. This implies a competition between the SOC and the inter-valley scattering. However, for a metallic armchair-edged GNR, a small SOC can always open a nontrivial gap. Nevertheless, the helical edge state is much less localized than that in a zigzag-edged GNR of the same width. As a result, and by numerically calculating the electronic transmission spectrum of step- and L-shaped GNJs, we conclude that when an armchair-edged GNR is a part of a GNJ, it is the weak point to realize the electronic total transmission even though the bulk phase of graphene is topologically insulating.

Magnetic device properties for a heterojunction based on functionalized armchair-edged graphene nanoribbons

Graphene is predicted to hold a promising use for developing future miniaturized electronic devices. However, the magnetic transport properties based on the armchair-edged graphene nanoribbons (AGNRs) is less studied in currently existing work. So in this work the special chemical modified nanoribbons based on the edge of the AGNR bridged by the transition metal Mn atom and passivated subsequently by two F atoms or two H atoms (AGNR-Mn-F2 or AGNR-Mn-H2) are proposed theoretically. Our calculations from first-principle method based on the spin-polarized density functional theory combined with the non-equilibrium Green's function technique show that the heterojunction F2-AGNR-Mn-H2 consisting of such two types of nanoribbons possesses the excellent magnetic device features, namely, the spin polarization is able to reach almost 100% in a very large bias region, and under P magnetic configuration (the external magnetic fields applied perpendicularly to two electrodes are set to point to the same direction), the single spin filtering effects can be realized, while under the AP configuration (the external magnetic fields applied perpendicularly to two electrodes are set to point to the opposite directions), the dual spin filtering effects can be realized. It is also found that such a heterojunction features dual diode-like effect, and its rectification ratio is up to be 108. Additionally, changing the direction of switching magnetic field, namely, changing the magnetic configurations from one kind of case to another, would lead to an obvious spin valve effect, and the giant magnetoresistace approaches to 108%. These findings suggest that the excellent spin polarization, dual diode-like effect, and giant magnetoresistace effect can be realized simultaneously for this heterojunction, therefore, it holds good promise in developing spintronic devices.

Abnormally high oscillator strengths of the graphene nanoribbons electronic spectrum: quantum chemistry calculations

Armchair-edged narrow graphene nanoribbons (GNRs) are modelled by semi-empirical Hartree–Fock based quantum chemistry method ZINDO/S-CI. ​Abnormally high oscillator strengths of over 200 are found in long GNRs (length > 150 hexagonal carbon rings).

Strain-induced asymmetric modulation of band gap in narrow armchair-edge graphene nanoribbon

We investigate the band structure of narrow armchair-edge graphene nanoribbons (AGNRs) under tensile strain by means of an extension of the Extended Hückel method. The strain-induced band gap modulation presents asymmetric behavior. The asymmetric modulation of band gap is derived from the different changes of conduction and valence bands near Fermi level under tensile strain. Further analysis suggests that the asymmetric variation of band structure near Fermi level only appear in narrow armchair-edge graphene nanoribbons.

Theoretical study of quantum capacitance and associated delay in armchair-edge graphene nanoribbons

This work presents a comprehensive investigation of the quantum capacitance and the associated effects on the carrier transit delay in armchair-edge graphene nanoribbons (A-GNRs) based on semi-analytical method. We emphasize on the realistic analysis of bandgap with taking edge effects into account by means of modified tight binding (TB) model. The results show that the edge effects have significant influence in defining the bandgap which is a necessary input in the accurate analyses of capacitance. The quantum capacitance is discussed in both nondegenerate (low gate voltage) and degenerate (high gate voltage) regimes. We observe that the classical capacitance limits the total gate (external) capacitance in the degenerate regime, whereas, quantum capacitance limits the external gate capacitance in the nondegenerate regime. The influence of gate capacitances on the gate delay is studied extensively to demonstrate the optimization of switching time. Moreover, the high-field behavior of a GNR is studied in the degenerate and nondegenerate regimes. We find that a smaller intrinsic capacitance appears in the channel due to high velocity carrier, which limits the quantum capacitance and thus limit the gate delay. Such detail analysis of GNRs considering a realistic model would be useful for the optimized design of GNR-based nanoelectronic devices.

First principles design of divacancy defected graphene nanoribbon based rectifying and negative differential resistance device

We have studied using density functional theory and non-equilibrium Green’s function based approach, the electronic structures of 555-777 divacancy (DV) defected armchair edged graphene nanoribbons (AGNR) as well as the transport properties of AGNR based two-terminal devices constructed with one defected electrode and one N doped electrode. Introduction of 555-777 DV defect into AGNR results in shifting of the π and π∗ bands towards the higher energy value indicating a downward shift of the Fermi level. Formation of a potential barrier, analogous to that of conventional p-n junction, has been observed across the junction of defected and N-doped AGNR. The two terminal devices show diode like property with high rectifying efficiency for a wide range of bias voltages. The devices also show robust negative differential resistance with very high peak-to-valley ratio. Shift of the electrode energy states and modification of the transmission function with applied bias have been analyzed, in order to gain an insight into the nonlinear and asymmetric behavior of the current-voltage characteristics. Variation of the transport properties on the width of the ribbons has also been discussed.

Non-PGM ORR Catalyst Active-Site Screening

Non-platinum group metal (non-PGM) oxygen reduction reaction (ORR) catalysts, based on pyrolyzed C, N, and transition metal systems, may replace precious metal catalysts in Pt-based fuel cell cathodes in the future. Major barriers to the adoption of these catalyst materials include much needed improvements in both ORR activity as well as durability under operating conditions. Detailed knowledge of the atomic-scale structure of the active site(s) would aid in strategically synthesizing materials with a higher density of desired active sites (those with simultaneously high ORR activity and practical stability). Density functional theory (DFT) modeling has previously been utilized to consider both relative stability of atomic scale structures,1 as well as to evaluate ORR activity2-5. Through the use of easily calculated descriptors, high-throughput DFT-based materials searches are possible. Relative formation energies of structures can be calculated via DFT with uniform atomic reference states. These formation energies may serve as a descriptor of relative active-site structure stability. This approach also allows for consideration of shifts in chemical potentials that can affect relative stability. Once the relaxed structure has been simulated, calculation of an active site with a bound ORR intermediate allows for an estimate of the binding energy of that intermediate using a computational hydrogen electrode approach2. Previously, O binding has been utilized as an activity descriptor for ORR catalysts2 but for non-PGM catalysts, OH binding is often the potential determining step in the ORR reaction pathway. As such, for screening purposes, we utilize the OH binding energy as a descriptor for ORR activity. Ideally, this value should be close to 1.23 eV with values less than this indicating OH overbinding and values greater indicating underbinding. Scaling law relations6suggest that both O and OH binding energies should be equally valid activity descriptors. We have performed an initial screening of 22 proposed active-site structures containing C, Fe, N, and H, as suggested through previously established structural motifs7. Three graphene-like C host materials, a “bulk” periodic graphene sheet, a zig-zag edge graphene nanoribbon, and an arm-chair edge graphene nanoribbon were considered. Active-site structures with one and two N-coordinated Fe atoms were considered as embedded in each of the three host materials, acting as C substitutional defects. In some cases, iron replaced two carbons to give different Fe coordination symmetry. Direct comparisons between similar structures in different C host materials were made where possible. For the nanoribbon hosted active site structures, the N-coordinated Fe moiety was only considered at the nanoribbon edge. The outcomes of this study will be the focus of this talk. In addition to the previously reported Fe2N5(*OH) zig-zag edge active site structure,8 preliminary studies suggest that bulk and zig-zag edge Fe2N6 structures (Figure 1) also have favorable OH binding characteristics, suggesting further investigation of their full ORR reaction pathways is merited and will be discussed. In all three cases, Fe2N5(*OH) zig-zag edge, Fe2N6 bulk, and Fe2N6 zig-zag edge, Fe is five-fold coordinated with N and a neighboring Fe, suggesting such coordination is an important aspect for tuning OH binding energy. Fe-Fe bond distances of 2.20 Å, 2.25 Å, and 2.24 Å were calculated for the Fe2N5(*OH) zig-zag, Fe2N6 bulk, and Fe2N6 zig-zag structures, respectively. In the case of the Fe2N5(*OH) zig-zag structure, the Fe atoms are initially out of the graphene plane whereas in the Fe2N6structures, they are initially in the graphene plane. Both multi-metal structures show enhanced stability, compared to their single-Fe counterpart structures for a wide range of Fe chemical potentials. By combining these simulation data, all calculated using the same methodology, direct comparison of structure-function relations for activity and structural stability for a wide range of structures, including similar structures at bulk and multiple edges, will be available for comparison. References 1 Holby, E. F. & Taylor, C. D. Applied Physics Letters 101, 064102, (2012). 2 Norskov, J. K. et al. Journal of Physical Chemistry B 108, 17886-17892, (2004). 3 Studt, F. Catalysis Letters 143, 58-60, (2013). 4 Kattel, S., Atanassov, P. & Kiefer, B. The Journal of Physical Chemistry C 116, 17378-17383, (2012). 5 Kattel, S., Atanassov, P. & Kiefer, B. Physical Chemistry Chemical Physics 15, 148-153, (2013). 6 Abild-Pedersen, F. et al. Physical Review Letters 99, 016105, (2007). 7 Holby, E. F., Wu, G., Zelenay, P. & Taylor, C. D. The Journal of Physical Chemistry C 118, 14388-14393, (2014). 8 Holby, E. F. & Taylor, C. D. Scientific Reports 5, (2015). Figure 1. Model Fe2N6 structure at a zig-zag edge nanoribbon with bound OH ORR intermediate. Figure 1

Suppression of localized transmission peaks and deviation from Matthiessen's rule in disordered armchair graphene nano-ribbons due to acoustic phonons

The electronic properties of armchair edge graphene nanoribbons (AGNRs) in the presence of disorders such as edge roughness, and vacancy are studied based on the non-equilibrium Green's function formalism. Elastic electron–phonon scattering is also included in our study. AGNRs with disorder exhibit localized transmission peaks in the transport band-gap. These localized peaks contribute to unwanted electron transport and conductivity of AGNRs. Results of our numerical simulations show that these states are extremely unstable and sensitive to lattice vibrations and are suppressed by acoustic phonons at room temperature. It was also found that the resistivity of AGNRs due to acoustic phonons and disorders, especially in the case of off-states, deviates from the values predicted using Matthiessen's rule where the effect of each scattering mechanism is considered separately. According to the results, deviation from Matthiessen's rule become more severe with the increase in the strength of disorder and AGNR length.

Effect of edge chemistry doping on the transport and optical properties for asymmetric armchair-edge graphene nanoribbons under a uniaxial strain

The effect of edge n-type nitrogen (N) and p-type oxygen (O) doping on the transport and optical properties for asymmetric 6- and 8-armchair-edge graphene nanoribbons (AGNRs) under a uniaxial strain has been demonstrated in the method of the first-principles density functional theory combined with nonequilibrium Green’s function. The transmission spectra and current–voltage (I–V) curves of the semiconducting 6- and metallic 8-AGNRs have been found to be qualitatively unchanged as the uniaxial strain along the armchair edge (Ac-strain), while the quantitative variations come from the mutual actions of the strain and edge doping. However, the observed semiconductor–metal transition and metal–semiconductor transition in the transport properties for 6- and 8-AGNRs should be mainly from the resonance scattering of the doping edges when the external strain is applied along the zigzag edge (Zz-strain). The distinct variation trends of the system transport properties are well confirmed from the optical absorption spectra. It is believed that the obtained results are of importance in exploiting chemistry at the reactive edges of the asymmetric AGNRs and strain engineering of the nanoelectronic/optoelectronic devices based on AGNRs.

Spin-dependent Seebeck effects in a graphene nanoribbon coupled to two square lattice ferromagnetic leads

We theoretically investigate spin-dependent Seebeck effects for a system consisting of a narrow graphene nanoribbon (GNR) contacted to square lattice ferromagnetic (FM) electrodes with noncollinear magnetic moments. Both zigzag-edge graphene nanoribbons (ZGNRs) and armchair-edge graphene nanoribbons (AGNRs) were considered. Compared with our previous work with two-dimensional honeycomb-lattice FM leads, a more realistic model of two-dimensional square-lattice FM electrodes is adopted here. Using the nonequilibrium Green's function method combining with the tight-binding Hamiltonian, it is demonstrated that both the charge Seebeck coefficient SC and the spin-dependent Seebeck coefficient SS strongly depend on the geometrical contact between the GNR and the leads. In our previous work, SC for a semiconducting 15-AGNR system near the Dirac point is two orders of magnitude larger than that of a metallic 17-AGNR system. However, SC is the same order of magnitude for both metallic 17-AGNR and semiconducting 15-AGNR systems in the present paper because of the lack of a transmission energy gap for the 15-AGNR system. Furthermore, the spin-dependent Seebeck coefficient SS for the systems with 20-ZGNR, 17-AGNR, and 15-AGNR is of the same order of magnitude and its maximum absolute value can reach 8 μV/K. The spin-dependent Seebeck effects are not very pronounced because the transmission coefficient weakly depends on spin orientation. Moreover, the spin-dependent Seebeck coefficient is further suppressed with increasing angle between the relative alignments of magnetization directions of the two leads. Additionally, the spin-dependent Seebeck coefficient can be strongly suppressed for larger disorder strength. The results obtained here may provide valuable theoretical guidance in the experimental design of heat spintronic devices.

Transport properties and localization of electronic states in graphene nanoribbons with edge absorptions

We study the effect of edge methylene on transport properties in graphene nanoribbons (GNRs) using the recursive green’s function method. The concentration of methylene (w) is defined as the substituted probability of edge dangling bonds. Due to the antiresonance of quasilocalized states, some conductance dips are found when single absorption (w = 0.005 in this work) sits on the edge. Localization analyses of wave functions also confirm this. With w increasing, the conductance is suppressed significantly and transport gap develops near E = 0.0 eV. Conductance suppression is induced by antiresonances between edge scattering centers. Meanwhile, these scattering centers prevent the formation of edge extended states which play an important role in the electronic transport at low energy and consequently the transport gaps develop. We found that a stable gap can be obtained at w = 0.3 and it becomes small as the width of the GNRs increases. An interesting oscillation at transport gaps for armchair edge GNRs is observed and it relates to the geometric symmetry of sample. The physical mechanisms behind the novel phenomenon are still unclear.

Graphene nanopores as negative differential resistance devices

We present graphene nanopores as new negative differential resistance (NDR) devices, and study their quantum transport properties using non-equilibrium Green's function and the density functional tight binding method. The proposed device structure is created on intrinsic armchair-edged graphene nanoribbons with uniform widths, where the central scattering region has a nanopore in the interior, and the two ends of the nanoribbon act naturally as connecting electrodes. We show that nitrogen-passivated scattering regions generally result in pronounced NDR properties, while hydrogen-passivated ones do not. This NDR effect occurs at low bias voltages, below 1 V, and achieves extraordinarily high peak-to-valley current ratio, while still attaining very high peak current densities. In addition, very sharp current peaks in the μA range can occur in the I-V curves, and through varying structural dimensions of the proposed structure multiple NDR regions can be realized. These results suggest that the device has promising potential in applications such as high frequency oscillators, memory devices, and fast switches.

Reversible switching in gold-atom–organic-molecule complex induced by reversible bond formation

By applying nonequilibrium Green’s functions in combination with the density-functional theory, we investigate the electronic transport properties of a molecule PTCDA sandwiched between two Au electrodes. The results show that the adsorption of an Au atom can modulate the conductance of the Au–PTCDA–Au junction remarkably. The reversible switching behavior can be found in this device, arising from the reversible bond formation between the Au adatom and the PTCDA molecule, which is in line with the experimental results. However, the switching behavior of the molecule PTCDA induced by reversible bond formation disappeared when the Au electrode is replaced by the armchair-edge graphene nanoribbons. That means the graphene electrodes with armchair termination deeply affect the electronic transport properties of the molecule PTCDA at small bias voltages.