Transport Properties Of Graphene Nanoribbons Research Articles

Small Size, Big Impact: Recent Progress in Bottom-Up Synthesized Nanographenes for Optoelectronic and Energy Applications.

Bottom‐up synthesized graphene nanostructures, including 0D graphene quantum dots and 1D graphene nanoribbons, have recently emerged as promising candidates for efficient, green optoelectronic, and energy storage applications. The versatility in their molecular structures offers a large and novel library of nanographenes with excellent and adjustable optical, electronic, and catalytic properties. In this minireview, recent progress on the fundamental understanding of the properties of different graphene nanostructures, and their state‐of‐the‐art applications in optoelectronics and energy storage are summarized. The properties of pristine nanographenes, including high emissivity and intriguing blinking effect in graphene quantum dots, superior charge transport properties in graphene nanoribbons, and edge‐specific electrochemistry in various graphene nanostructures, are highlighted. Furthermore, it is shown that emerging nanographene‐2D material‐based van der Waals heterostructures provide an exciting opportunity for efficient green optoelectronics with tunable characteristics. Finally, challenges and opportunities of the field are highlighted by offering guidelines for future combined efforts in the synthesis, assembly, spectroscopic, and electrical studies as well as (nano)fabrication to boost the progress toward advanced device applications.

Edge Engineered Graphene Nanoribbons as Nanoscale Interconnect: DFT Analysis

Density functional theory (DFT) with non-equilibrium Green's function (NEGF) formalism and HSPICE simulator have been used to analyse the effect of elemental oxygen atom passivation on the structural, electronic and transport properties of Graphene Nanoribbons (GNRs). The analysis of delay, power dissipation, crosstalk effect, stability and frequency analysis has also been performed to understand its interconnect application. The present study includes all the possible morphologies of zigzag and armchair edge states of GNRs with oxygen and hydrogen passivation. The structural stability of GNRs, analysed in terms of binding energy <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(E_{binding})$</tex-math></inline-formula> observed that the Zigzag graphene nanoribbon (ZGNR) with both edge oxygen passivated, is the most stable configuration, where stability of the configuration increases with Oxygen concentration. Further, fro <uri xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">text</uri> m the bandstructure and density-of-states calculations, it has been observed that considered 6 atom wide and hydrogen passivated Armchair GNR (AGNR) is semiconducting in nature, whereas, with same width hydrogen passivated ZGNR is metallic. On the other hand, passivation of AGNR with increasing concentration of oxygen form them to metallic in nature. Based on the enhanced metallicity and increment in fermi velocity due to passivation of GNRs with oxygen, these structures may be a potential candidate for interconnects. Their computed electron transport properties, dynamical parameters, delay, power delay product and crosstalk induced delay confirms that the zigzag GNR with both the edges passivated with oxygen (O-ZGNR-O) can be considered as best contender for interconnect application due to its remarkable electrical and thermal transport in comparison to other GNR's. The O-ZGNR-O shows lowest value of kinetic inductance and quantum capacitance of. 01032H/m and 2.21 nF/m respectively with higher stability and higher immunity to crosstalk effect in comparison to other proposed GNRs, which is required for nanoscale interconnects. The relative stability of GNRs have been analyzed in terms of nyquist plot with different interconnect lengths. The results suggest that O-ZGNR-O have lowest delay and power dissipation with higher stability, hence defends its application for interconnect application.

Edge Doping Engineering of High-Performance Graphene Nanoribbon Molecular Spintronic Devices.

We study the quantum transport properties of graphene nanoribbons (GNRs) with a different edge doping strategy using density functional theory combined with nonequilibrium Green’s function transport simulations. We show that boron and nitrogen edge doping on the electrodes region can substantially modify the electronic band structures and transport properties of the system. Remarkably, such an edge engineering strategy effectively transforms GNR into a molecular spintronic nanodevice with multiple exceptional transport properties, namely: (i) a dual spin filtering effect (SFE) with 100% filtering efficiency; (ii) a spin rectifier with a large rectification ratio (RR) of 1.9 ×; and (iii) negative differential resistance with a peak-to-valley ratio (PVR) of 7.1 ×. Our findings reveal a route towards the development of high-performance graphene spintronics technology using an electrodes edge engineering strategy.

Bipolar Magnetic and Thermospin Transport Properties of Graphene Nanoribbons with Zigzag and Klein Edges

Magnetic nanoribbons based on one-dimensional materials are potential candidates for spin caloritronics devices. Here, we constructed ferromagnetic graphene nanoribbons with zigzag and Klein edges (N-ZKGNRs, N = 4–21) and found that the N-ZKGNRs are in the indirect-gap bipolar magnetic semiconducting state (BMS). Moreover, when a temperature difference is applied through the nanoribbons, spin-dependent currents with opposite flow directions and opposite spin directions are generated, indicating the occurrence of the spin-dependent Seebeck effect (SDSE). In addition, the spin-dependent Seebeck diode effect (SDSD) also appeared in these devices. More importantly, we found that the BMS with a larger bandgap is promising for generating the SDSD, while the BMS with a smaller bandgap is promising for generating the SDSE. These findings show that ZKGNRs are promising candidates for spin caloritronics devices.

Metallization-Induced Quantum Limits of Contact Resistance in Graphene Nanoribbons with One-Dimensional Contacts.

Graphene has attracted a lot of interest as a potential replacement for silicon in future integrated circuits due to its remarkable electronic and transport properties. In order to meet technology requirements for an acceptable bandgap, graphene needs to be patterned into graphene nanoribbons (GNRs), while one-dimensional (1D) edge metal contacts (MCs) are needed to allow for the encapsulation and preservation of the transport properties. While the properties of GNRs with ideal contacts have been studied extensively, little is known about the electronic and transport properties of GNRs with 1D edge MCs, including contact resistance (RC), which is one of the key device parameters. In this work, we employ atomistic quantum transport simulations of GNRs with MCs modeled with the wide-band limit (WBL) approach to explore their metallization effects and contact resistance. By studying density of states (DOS), transmission and conductance, we find that metallization decreases transmission and conductance, and either enlarges or diminishes the transport gap depending on GNR dimensions. We calculate the intrinsic quantum limit of width-normalized RC and find that the limit depends on GNR dimensions, decreasing with width downscaling to ~21 Ω∙µm in 0.4 nm-wide GNRs, and increasing with length downscaling up to ~196 Ω∙µm in 5 nm-long GNRs. We demonstrate that 1D edge contacts and size engineering can be used to tune the RC in GNRs to values lower than those of graphene.

Electronic and Transport Properties of Graphene Nanoribbons Based on Super-Heptazethrene Molecular Blocks

Electronic and Transport Properties of Graphene Nanoribbons Based on Super-Heptazethrene Molecular Blocks

Improvement of electronic and transport properties of graphene nanoribbon by simultaneous substitution of graphane and fluorographane

We investigate the electronic and transport properties of the graphene heterostructure nanoribbons terminated by H/H and F/F atoms, which are formed by partially substituting graphane and fluorographane nanoribbons into both edge parts of graphene nanoribbons, using the nonequilibrium Green's functions (NEGF) combined with the density functional theory (DFT). The calculated structural stability shows that all hybrid systems are stable than pristine GNRs. In particular, the hybrid systems terminated by F/F atoms are structurally more favorable than that of H/H ones. Our results indicate that armchair hybrid systems are nonmagnetic semiconductors with wide direct band gaps regardless of their spin polarizations, whereas zigzag systems have the half-metal or half-semiconducting behavior according to their spin polarizations. With the increase of nanoroad, band gaps increase (decrease) to different attributes. Interestingly, h-ZGAx/G8-2xNR/FGxNRs may produce from half-metal to half-semiconducting behavior transition, due to chemical potential change according to ribbon widths and nanoroads. Moreover, the calculated I–V curve exhibits negative differential resistance, which can be used for application in molecular spin electronic device.

Spin-dependent transport properties in heavy-metal adatom decorated graphene nanoribbons

We apply the non-equilibrium Green’s function (NEGF) method to study spin transport properties of graphene nanoribbons decorated with Gold (Au) and Thallium (Tl) adatoms. Both the enhanced intrinsic SOC and Rashba SOC induced by adatoms makes a huge impact and changes the spin-dependent transport properties such as spin relaxation, charge distribution, and electrical conductivity in the adatom armchair graphene nanoribbons (AGNR) system. Our results examine the Quantum Spin Hall phase in Tl adatom AGNR, and also support that the Rashba effect induced by Au adatom is harmful for the stabilizing of QSH phase. Furthermore, a faster spin-relaxation in Au adatom AGNR system is also demonstrated in our study. The comparison of these spin-dependent effects in different adatoms systems would help us to clarify the influences of the enhanced intrinsic SOC and Rashba SOC in graphene, and pave the way for manipulating graphene-based spintronic devices. In the end, we summarized and discussed the spin-dependent transport properties of several heavy-metal decorated graphene systems.

Electron transport properties of graphene nanoribbons with Gaussian deformation

Gaussian deformation in graphene structures exhibits an interesting effect in which flower-shaped confinement states are observed in the deformed region [Carrillo-Bastos et al., Phys. Rev. B 90 041411 (2014)]. To exploit such a deformation for various applications, tunable electronic features including a bandgap opening for semi-metallic structures are expected. Besides, the effects of disorders and external excitations also need to be considered. In this work, we present a systematic study on quantum transport of graphene ribbons with Gaussian deformation. Different levels of deformation are explored to find a universal behavior of the electron transmission. Using a tight-binding model in combination with Non-Equilibrium Green Functions formalism, we show that Gaussian deformation influences strongly the electronic properties of ribbons in which the electron transmission decreases remarkably in high energy regions even if small deformations are considered. Interestingly, it unveils that the first plateau of the transmission of semi-metallic armchair ribbons is just weakly affected in the case of small deformations. However, significant large Gaussian bumps can induce a strong drop of this plateau and a transport gap is formed. The transmission at the zero energy is found to decrease exponentially with increasing the size of the Gaussian bump. Moreover, the gap of semi-conducting ribbons is enlarged with large deformations. The opening or the widening of the transport gap in large deformed armchair structures is interpreted by a formation of a three-zone behavior along the transport direction of the hopping profile. On the other hand, a transport gap is not observed in zigzag ribbons regardless of the size of Gaussian bumps. This behavior is due to the strong localization of edge states at the energy point E = 0...

Intrinsic electronic and transport properties of graphene nanoribbons with different widths.

The intrinsic electronic and transport properties of a series of zigzag graphene nanoribbons (ZGNRs) have been investigated systematically using density functional theory coupled with the non-equilibrium Green's function (NEGF) method. It is found that ZGNRs with various ribbon widths have completely different transport properties under bias, depending on whether the number of zigzag chains is odd or even. ZGNRs with odd number except 1-ZGNR possess small current regardless of the bias applied. In contrast, ZGNRs with even number have much larger current and behave as a resistor having a linear current-voltage relationship. The results also reveal that the narrow ZGNRs, e.g., 1-ZGNR and 2-ZGNR, have different transport behaviors, which are governed by the edge-effect and the unique electronic structure.

Effects of Divacancy and Extended Line Defects on the Thermal Transport Properties of Graphene Nanoribbons.

The effects of divacancy, including isolated defects and extended line defects (ELD), on the thermal transport properties of graphene nanoribbons (GNRs) are investigated using the Nonequilibrium Green’s function method. Different divacancy defects can effectively tune the thermal transport of GNRs and the thermal conductance is significantly reduced. The phonon scattering of a single divacancy is mostly at high frequencies while the phonon scattering at low frequencies is also strong for randomly distributed multiple divacancies. The collective effect of impurity scattering and boundary scattering is discussed, which makes the defect scattering vary with the boundary condition. The effect on thermal transport properties of a divacancy is also shown to be closely related to the cross section of the defect, the internal structure and the bonding strength inside the defect. Both low frequency and high frequency phonons are scattered by 48, d5d7 and t5t7 ELD. However, the 585 ELD has almost no influence on phonon scattering at low frequency region, resulting in the thermal conductance of GNRs with 585 ELD being 50% higher than that of randomly distributed 585 defects. All these results are valuable for the design and manufacture of graphene nanodevices.

Strained effects on the thermal conductance of flexural and in-plane modes in graphene nanoribbons

Ballistic thermal transport properties in graphene nanoribbon modulated with strain are investigated by non-equilibrium Green’s function approach. The results show that the strain can suppress the phonon transport of flexural phonon mode (FPM) and enhance the phonon transport of in-plane mode (IPM) in low-frequency region, leading to the reduction in the thermal conductance of FPM and the enhancement in the thermal conductance of IPM. The total thermal conductance is decreased by strain as the reduction in the thermal conductance of FPM overcomes the enhancement in the thermal conductance of IPM.

Spectrally-resolved thermal transport in graphene nanoribbons

Thermal transport properties of graphene nanoribbons (GNRs) are investigated using phonon transport studies. Ribbons of varying widths are considered to investigate the transition of key thermal properties with width. The lattice structure of the ribbons is fully resolved, and phonon transport is modeled by accounting for all three-phonon scattering processes using a solution of the linearized Boltzmann transport equation. A 3× reduction in intrinsic thermal conductivity is observed compared to bulk graphene arising from increased strength of three-phonon scattering due to the additional nondegenerate phonon modes that appear due to the finite edges of confined nanoribbons. Strong dependence of thermal conductivity on ribbon width is also observed. The underlying mechanisms for thermal conductivity reduction and width dependence are presented by analyzing frequency- and polarization-resolved phonon transport. The additional scattering pathways present in 1D GNRs lead to a significant reduction in the thermal conductivity of otherwise highly conducting flexural phonons in bulk graphene. In contrast, confinement-induced changes to the density of states, specific heat or group velocity, and the subsequent impact on lattice thermal conductivity are found to be relatively small.

Spin transport properties of armchair graphene nanoribbons doped with Fe and B atoms

Spin-polarized density functional theory combined with non-equilibrium Green's function (NEGF) formalism is used to study the electronic, magnetic and transport properties of hydrogen-terminated armchair graphene nanoribbons doped with iron and boron atoms. We consider hydrogen terminated graphene nanoribbons with width up to seven C–C dimmers (7-AGNR-H). Our main focus is to investigate the effects of Fe and B doping on the spin transport properties of graphene nanoribbons. The substituted boron atoms at the nanoribbon edges suppress the semiconductor to metal transition. This substitutional B and Fe atoms respectively act as electrode and scattering region for the electronic transport along the nanoribbons. It is revealed that negative differential resistance (NDR) behavior and 86% spin filtering effect can be realized, which suggest that doped AGNR devices as the promising candidates in nanospintronics.

Spin filter effect in the zigzag graphene nanoribbons

The spin-related transport properties of graphene nanoribbons with zigzag edges which are partially coupled to Au linear-chain electrodes have been investigated by using non-equilibrium Green’s function in combination with the density-functional theory. The results show that the partial contact with the electrodes will break the spin degeneracy, leading to the completely spin-polarized transport. Meanwhile, the spin-related properties are obviously dependent on the electrodes contact positions. For the boron doped graphene nanoribbons, the spin polarized effect is enhanced.

Effect of randomly distributed asymmetric stone-wales defect on electronic and transport properties of armchair graphene nanoribbon

By applying tight binding model, we investigate the electronic and transport properties of randomly distributed Stone-Wales (SW) defects on an armchair graphene nanoribbon (AGNR). We use four different functions, as distribution functions, to generate our SW defected nanoribbons. It is found that defect density can have a major effect on the conductance of our defected system, whilst other configurations such as defect orientation will contribute less. In our investigations, some special geometries are found which shows interesting electronic and transport properties. These special cases along with the other data provided can be used to engineer band gap, electronic properties and transport properties of graphene nanoribbons to meet a desired purpose.

Finding the hidden valence band of N = 7 armchair graphene nanoribbons with angle-resolved photoemission spectroscopy

To understand the optical and transport properties of graphene nanoribbons, an unambiguous determination of their electronic band structure is needed. In this work we demonstrate that the photoemission intensity of each valence sub-band, formed due to the quantum confinement in quasi-one-dimensional (1D) graphene nanoribbons, is a peaked function of the two-dimensional (2D) momentum. We resolve the long-standing discrepancy regarding the valence band effective mass () of armchair graphene nanoribbons with a width of N = 7 carbon atoms (7-AGNRs). In particular, angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy report ≈0.2 and ≈0.4 of the free electron mass (me), respectively. ARPES mapping in the full 2D momentum space identifies the experimental conditions for obtaining a large intensity for each of the three highest valence 1D sub-bands. Our detail map reveals that previous ARPES experiments have incorrectly assigned the second sub-band as the frontier one. The correct frontier valence sub-band for 7-AGNRs is only visible in a narrow range of emission angles. For this band we obtain an ARPES derived effective mass of 0.4 me, a charge carrier velocity in the linear part of the band of 0.63 × 106 m s−1 and an energy separation of only ≈60 meV to the second sub-band. Our results are of importance not only for the growing research field of graphene nanoribbons but also for the community, which studies quantum confined systems.

Effect of ribbon width on electrical transport properties of graphene nanoribbons

There has been growing interest in developing nanoelectronic devices based on graphene because of its superior electrical properties. In particular, patterning graphene into a nanoribbon can open a bandgap that can be tuned by changing the ribbon width, imparting semiconducting properties. In this study, we report the effect of ribbon width on electrical transport properties of graphene nanoribbons (GNRs). Monolayer graphene sheets and Si nanowires (NWs) were prepared by chemical vapor deposition and a combination of nanosphere lithography and metal-assisted electroless etching from a Si wafer, respectively. Back-gated GNR field-effect transistors were fabricated on a heavily p-doped Si substrate coated with a 300 nm-thick SiO2 layer, by O2 reactive ion etching of graphene sheets using etch masks based on Si NWs aligned on the graphene between the two electrodes by a dielectrophoresis method. This resulted in GNRs with various widths in a highly controllable manner, where the on/off current ratio was inversely proportional to ribbon width. The field-effect mobility decreased with decreasing GNR widths due to carrier scattering at the GNR edges. These results demonstrate the formation of a bandgap in GNRs due to enhanced carrier confinement in the transverse direction and edge effects when the GNR width is reduced.

Electrical Transport Properties of Graphene Nanoribbons with Green Function

Electrical Transport Properties of Graphene Nanoribbons with Green Function

Disorder-driven insulator to semi-metallic transition in a graphene nanoribbon

We investigate the effects of disorder on the electron transport properties of graphene nanoribbons (GNs) with armchair edges described in tight-binding (TB) model. We try to compare the electronic properties of armchair graphene nanoribbons (AGNs) subjected to two types of disorder with different distributions. In this regard, the tight-binding model of electron transport in the presence of hopping to nearest and next-nearest neighbors is investigated. With the aid of the spectral and multifractal analyses, we find that for the case of ordered distribution of disorder the AGNs acts as an insulator and for disordered distribution AGN behaves as a semi-metal. Our results can report different regimes of conductivity and addresses the transition between semi-metallic and insulator phases.