Mobility In Graphene Nanoribbons Research Articles

Role of Edge Engineering in Photoconductivity of Graphene Nanoribbons.

The effect of edge engineering of graphene nanoribbons (GNRs) on their ultrafast photoconductivity is investigated. Three different GNRs were fabricated by bottom-up synthesis in the liquid phase, where structure, width, and edge planarity could be controlled chemically at the atomic level. The charge carrier transport in the fabricated GNRs was studied on the ultrafast, sub-picosecond time scale using time-resolved terahertz spectroscopy, giving access to the elementary parameters of carrier conduction. While the variation of the side chains does not alter the photoconductive properties of GNRs, the edge structure has a strong impact on the carrier mobility in GNRs by affecting the carrier momentum scattering rate. Calculations of the ribbon electronic structure and theoretical transport studies show that phonon scattering plays a significant role in microscopic conduction in GNRs with different edge structures. A comparison between theory and experiment indicates that the mean free path of charge carriers in the nanoribbons amounts to typically ∼20 nm.

Magnetic-Phase Dependence of the Spin Carrier Mean Free Path in Graphene Nanoribbons.

We show theoretically that the intrinsic (phonon-limited) carrier mobility in graphene nanoribbons is considerably influenced by the presence of spin-polarized edge states. When the coupling between opposite edges switches from antiferromagnetic to ferromagnetic with increasing carrier density, the current becomes spin polarized and the mean free path rises from 10nm to micrometers. In the ferromagnetic state, the current flows through one majority-spin channel which is ballistic over micrometers and several minority-spin channels with mean free paths as low as 1nm. These features predicted in technology-relevant conditions could be nicely exploited in spintronic devices.

Performance Improvement in SC-MLGNRs Interconnects Using Interlayer Dielectric Insertion

Due to their higher resistance, single layer graphene nanoribbons (GNRs) are not suitable for high-speed on-chip interconnect applications. Hence, we use multilayer GNRs (MLGNRs) that offer multiple conduction channels and lower resistance. However, MLGNRs turn into graphite as the number of layers increase, which reduces the mean-free path of each layer. Insertion of a dielectric between GNR layers prevents its conversion into graphite, thereby improving its mean-free path and scattering rate. In this paper, we are proposing an analytical model for the time-domain analysis of side-contact MLGNRs (SC-MLGNRs) with intermediate dielectric insertion. The proposed model computes scattering rate, mean-free path, and carrier mobility in GNRs where dielectric has been inserted between individual layers. Our analytical results for mobility and scattering rate have been verified with the existing experimental data that exhibit excellent accuracy (maximum of error 4% for mobility and 16% for scattering time). Based on our analysis, we have found that the electron mean-free path in GNRs strongly depends on surrounding dielectric environment. In that, the mean-free path increases with interlayer insertion of high- $k$ dielectrics. Equivalent $RLC$ parameters, delay, energy-delay product, and bandwidth density are calculated for our proposed GNR interconnects using our model. We observe that these performance metrics significantly improve due to the presence of dielectric between GNR layers. When compared with Cu interconnects, insertion of HfO2 between GNR layers results in reduction in both propagation delay and energy-delay product by $2\times $ for interconnect lengths of $1400 ~\mu \text{m}$ . In addition, zigzag SC-MLGNR interconnect with $N=10$ and $\varepsilon _{2}=20$ gives nearly 35% higher bandwidth density than that of Cu interconnects for all interconnect lengths. In our analysis, we propose a new performance metric, bandwidth density/energy-delay product to determine the performance limits of our proposed interconnect structure. Finally, we compare the performance of SC-MLGNR interconnect structure with copper and optical interconnects to exhibit its application in local and global interconnects.

Entanglement of CeO<sub>2</sub> Nanorods and Graphene Nanoribbons and their Properties Studies of Nanocomposites

Nano/Micro-structured CeO2and their nanocomposites have been received considerable attention in basic research and commercial applications, such as, new energy fields, photocatalysts, environmental fields, et al. To extend its visible light response and pave the effective conductive channels for charge transfer and separation in nanoscale is still facing great challenges. To explore these key issues of materials chemistry and physics, CeO2nanorods were prepared with aid of soft templates by wet chemical approach. Graphene nanoribbons were obtained with unzipping method of carbon nanotube (CNTs). Entanglement of CeO2nanorods and graphene nanoribbons oxides was realized based on the supermolecular interactions between surface active groups of CeO2nanorods and graphene nanoribbons oxides and excellent flexibility of graphene nanoribbons. A series of characterizations were examined by SEM (scanning electron microscopy), TEM (transmission electron microscopy), XRD (X-ray diffraction), the Fourier-Transform Infrared (FTIR) spectra, ultraviolet-visible spectroscopy (UV-Vis) and so on. Photocatalytic efficiency was examined by selecting typical organic pollutants. The results indicated that the entanglement of a small amount of graphene nanoribbons on the surface of CeO2nanorods not only expanded the light response of nanocomposite to visible light, but also enhanced the adsorption properties to organic pollutants. Because of excellent charge transfer properties and high mobility of graphene nanoribbons, the nanocomposites of CeO2/graphene nanoribbons are favor for electron-holes pairs generated by visible light, separation, and transfer, which would be important potential applications in photocatalysts, artificial photosynthesis system, nano/micro-devices, et al.

Interpretation of graphene mobility data by means of a semiclassical Monte Carlo transport model

In this paper we compare experimental data and simulations based on a semiclassical model in order to investigate the relative importance of a several scattering mechanisms on the mobility of graphene nano-ribbons. Furthermore, some new experimental results complementing the range of ribbon widths available in the literature are also reported.We show that scattering with remote phonons originating in the substrate insulator can appreciably reduce the mobility of graphene and it should not be neglected in the interpretation of graphene mobility data. In fact by accounting for remote phonon scattering we could reproduce fairly well the experimentally observed dependence of the mobility on the ribbon width, the temperature and the inversion density, whereas the agreement with experiments is much worse when remote phonons are not included in the calculations.

Influence of substrate type and quality on carrier mobility in graphene nanoribbons

We report the results of a thorough numerical study on carrier mobility in graphene nanoribbons (GNRs) with the widths from ∼250 nm down to ∼1 nm, with a focus on the influence of substrate type (SiO2, Al2O3, HfO2, and h-BN) and substrate quality (different interface impurity densities) on GNR mobility. We identify the interplay between the contributions of Coulomb and surface optical phonon scattering as the crucial factor that determines the optimum substrate in terms of carrier mobility. In the case of high impurity density (∼1013 cm−2), we find that HfO2 is the optimum substrate irrespective of GNR width. In contrast, for low impurity density (1010 cm−2), h-BN offers the greatest enhancement, except for nanoribbons wider than ∼200 nm for which the mobility is highest on HfO2.

Investigation of ripple-limited low-field mobility in large-scale graphene nanoribbons

Combining molecular dynamics and quantum transport simulations, we study the degradation of mobility in graphene nanoribbons caused by substrate-induced ripples. First, the atom coordinates of large-scale structures are relaxed such that surface properties are consistent with those of graphene on a substrate. Then, the electron current and low-field mobility of the resulting non-flat nanoribbons are calculated within the Non-equilibrium Green's Function formalism in the coherent transport limit. An accurate tight-binding basis coupling the σ- and π-bands of graphene is used for this purpose. It is found that the presence of ripples decreases the mobility of graphene nanoribbons on SiO2 below 3000 cm2/Vs, which is comparable to experimentally reported values.

Atomistic Investigation of Low-Field Mobility in Graphene Nanoribbons

We have investigated the main scattering mechanisms affecting mobility in graphene nanoribbons using detailed atomistic simulations. We have considered carrier scattering due to acoustic and optical phonons, edge roughness, single defects, and ionized impurities, and we have defined a methodology based on simulations of statistically meaningful ensembles of nanoribbon segments. Edge disorder heavily affects mobility at room temperature in narrower nanoribbons, whereas charged impurities and phonons are hardly the limiting factors. Results are favorably compared to the few experiments available in the literature.

Simple and efficient modeling of the E–k relationship and low-field mobility in Graphene Nano-Ribbons

This paper proposes a simple analytical formulation for the dispersion relationship of extended electronic states in Graphene Nano-Ribbons (GNRs). The model has been validated by comparison with Tight-Binding calculation of GNRs in the presence of edge disorder. The model is suited for inclusion in semiclassical models for GNRs featuring widths down to approximately 2 nm. Monte-Carlo simulations accounting for phonons and edge roughness scattering are then used to understand the ribbon width of the low-field mobility. The mechanisms responsible for the low mobility values measured in narrow ribbons compared to graphene sheets are the increased phonon scattering rate and mobility effective mass due to the strong band structure modification induced by the reduced lateral dimensions and the increased scattering with the edges. However, scattering with phonons and with edges is not sufficient to reproduce the experimental mobility on insulating substrates, suggesting that the effect of remote polar phonons originating in the substrate can be significant in graphene based devices.

Impact of Size Effect on Graphene Nanoribbon Transport

Graphene has shown impressive properties for nanoelectronics applications including a high mobility and a width-dependent bandgap. Use of graphene in nanoelectronics would most likey be in the form of graphene nanoribbons (GNRs) where the ribbon width is expected to be less than 20 nm. Many theoretical projections have been made on the impact of edge-scattering on carrier transport in GNRs - most studies point to a degradation of mobility (of GNRs) as well as the on/off ratio (of GNR FETs). This study provides the first clear experimental evidence of the onset of size-effect in patterned GNRs; it is shown that for W<60 nm, carrier mobility in GNRs is limited by edge-scattering.