Semiconducting Graphene Nanoribbons Research Articles

Bending and buckling of narrow armchair graphene nanoribbons via STM manipulation

Semiconducting graphene nanoribbons (GNRs) are envisioned to play an important role in future electronics. This requires the GNRs to be placed on a surface where they may become strained. Theory predicts that axial strain, i.e. in-plane bending of the GNR, will cause a change in the band gap of the GNR. This may negatively affect device performance. Using the tip of a scanning tunneling microscope we controllably bent and buckled atomically well-defined narrow armchair GNR and subsequently probed the changes in the local density of states. These experiments show that the band gap of 7-ac-GNR is very robust to in-plane bending and out-of-plane buckling.

Bottom-up Synthesis of Sub-10 Nm Semiconducting Graphene Nanoribbons with Smooth Armchair Edges on Ge(001)

The rational synthesis of graphene nanoribbons that are semiconducting with sub-10 nm width, controlled crystallographic orientation, and well-defined edges on non-metallic substrates has been a significant challenge. The growth of nanoribbons on metal substrates precludes their direct use in semiconducting electronics due to the conducting substrate, and the direct synthesis of nanoribbons in solution is complicated by challenges of their post-synthetic assembly. In this talk, we demonstrate the scalable synthesis of graphene nanoribbons from the bottom-up via chemical vapor deposition (CVD) on Ge(001). Low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) show that the ribbons are self-orienting ±2.9° from the Ge[110] directions and are self-defining. The nanoribbons have predominately smooth armchair edges that give rise to electron interference patterns that are indicative of the high quality of the edges. By tuning the precursor flux, growth time, and growth temperature, the ribbon anisotropy and growth kinetics can be tailored to yield ribbons with controlled width < 10 nm and aspect ratio > 60. Compared to previous reports of the growth of low aspect ratio crystals of graphene on Ge, we find that in order to realize high aspect ratio nanoribbons, it is critical to operate in a regime in which the growth rate is especially slow, on the order of 5 nm/h for growth in the width direction. Scanning tunneling spectroscopy shows that the ribbons have electronic structures that are consistent with semiconductors with bandgaps that are > 500 meV and that vary inversely with width. This work is important because unlike continuous two-dimensional graphene, which is semimetallic, one-dimensional graphene nanoribbons can be semiconducting, allowing for the substantial modulation of their conductance and enabling their application in semiconductor logic, optoelectronics, photonics, and sensors. Moreover, the direct synthesis of ultranarrow and smooth graphene nanoribbons on Ge demonstrated here provides a scalable, high throughput pathway for integrating semiconducting graphene directly on conventional large-area semiconductor wafer platforms that are compatible with planar processing.

Optical field terahertz amplitude modulation by graphene nanoribbons.

In this study, first-principles time-dependent density functional theory calculations were used to demonstrate the possibility to modulate the amplitude of the optical electric field (E-field) near a semiconducting graphene nanoribbon. A significant enhancement of the optical E-field was observed 3.34 Å above the graphene nanoribbon sheet, with an amplitude modulation of approximately 100 fs, which corresponds to a frequency of 10 THz. In general, a six-fold E-field enhancement could be obtained, which means that the power of the obtained THz is about 36 times that of incident UV light. We suggest the use of semiconducting graphene nanoribbons for converting visible and UV light into a THz signal.

Ultrafast Photoconductivity of Graphene Nanoribbons and Carbon Nanotubes

We present a comparative study of the ultrafast photoconductivity in two different forms of one-dimensional (1D) quantum-confined graphene nanostructures: structurally well-defined semiconducting graphene nanoribbons (GNRs) fabricated by a "bottom-up" chemical synthesis approach and semiconducting carbon nanotubes (CNTs) with a similar bandgap energy. Transient photoconductivities of both materials were measured using time-resolved terahertz spectroscopy, allowing for contact-free measurements of complex-valued photoconductivity spectra with subpicosecond time-resolution. We show that, while the THz photoresponse seems very different for the two systems, a single model of free carriers experiencing backscattering when moving along the long axis of the CNTs or GNRs provides a quantitative description of both sets of results, revealing significantly longer carrier scattering times for CNTs (ca. 150 fs) than for GNRs (ca. 30 fs) and in turn higher carrier mobilities. This difference can be explained by differences in band structures and phonon scattering and the greater structural rigidity of CNTs as compared to GNRs, minimizing the influence of bending and/or torsional defects on the electron transport.

Electronic properties of carbon nanotubes partially unzipped by oxygenation or fluorination

Unzipping carbon nanotubes has recently attracted interest as a promising route to synthesizing semiconducting graphene nanoribbons. Here, the band structures of O- and F-driven partially unzipped armchair carbon nanotubes (PUCNTs) are computed using the ab initio pseudopotential method. Although the model structures exhibit a similar pinhole with a length of ∼1nm along the tube axis, the band structures differ distinctly. The O-driven PUCNT has many localized states in the valence band arising mainly from the O 2p orbitals, and preserves metallic properties. In contrast, the F-driven PUCNT shows properties of a semiconductor with a bandgap of ∼0.15eV, and its localized states occur in the conduction band. When the unzipping process continues further, the O-driven PUCNT also shows the semiconducting behavior.

Electronic Structure and Optical Property of Phosphorus Doped Semiconducting Graphene Nanoribbons

The electronic structure and optical property of phosphorus doped semiconducting graphene nanoribbons were calculated by using the density functional theory. Energy band structure and optical spectra were considered to show the special electronic structure of phosphorus doped semiconducting graphene nanoribbons. Our results showed that the Fermi energy of phosphorus doped semiconducting graphene nanoribbons entered in the conduction bands, and that the optical coefficient depend on the width of armchair graphene nanoribbons. It is concluded that the phosphorus doped semiconducting graphene nanoribbons behave p type semiconducting. Therefore, our results have a great significance in developing nanomaterial for fabricating the nanophotovoltaic devices.

Magnetic moment of a single vacancy in graphene and semiconducting nanoribbons

Calculations of single-vacancy-induced magnetism in graphene have found different values within a large range, which restricts the understanding of magnetism in defected graphene. Here we report first-principles simulations to examine the accuracy of magnetic moment calculations of a single vacancy with respect to the smearing width and $k$-points sampling in calculations of total density of states, based on which the magnetic moment is calculated. We show that the magnetic moment of a single vacancy arises from the asymmetry between spin-up and spin-down electrons in a small energy window of 1 eV that contains a sharp resonance close to the Fermi level. A small smearing width is important to accurately describe the sharp resonance. However, we find that even with a very large number of $k$ points (thousands per unit cell), the magnetic moment value is still sensitive to the smearing width, and that the total density of states still shows spikes, which leads to unreliable evaluation of the magnetic moment. Calculations of a single vacancy in semiconducting graphene nanoribbons further illustrate the balance between smearing width and $k$-points sampling, and indicate a 2 ${\ensuremath{\mu}}_{B}$ ground state as in semiconducting graphene nanoflakes.

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.

Semiconducting graphene nanoribbon retains band gap on amorphous or crystalline SiO2

Electronic properties of a semiconducting armchair graphene nanoribbon on SiO2 are examined using first-principles calculations and taking into account the van der Waals interaction. Unlike semiconducting carbon nanotubes, which exhibit variations in band gap on SiO2, the nanoribbon is found to retain its band gap on SiO2, regardless of the separation distance or the dielectric’s surface type—crystalline or amorphous. The interfacial interaction leads to electron-transfer from the nanoribbon to the dielectric. Moreover, for crystalline SiO2, the quantity of electron-transfer and the binding energy depend strongly on the type of surface termination and weakly on the binding sites.

Electronic structure and optical property of boron doped semiconducting graphene nanoribbons

We present a system study on the electronic structure and optical property of boron doped semiconducting graphene nanoribbons using the density functional theory. Energy band structure, density of states, deformation density, Mulliken popular and optical spectra are considered to show the special electronic structure of boron doped semiconducting graphene nanoribbons. The C-B bond form is discussed in detail. From our analysis it is concluded that the Fermi energy of boron doped semiconducting graphene nanoribbons gets lower than that of intrinsic semiconducting graphene nanoribbons. Our results also show that the boron doped semiconducting graphene nanoribbons behave as p-type semiconducting and that the absorption coefficient of boron doped armchair graphene nanoribbons is generally enhanced between 2.0 eV and 3.3 eV. Therefore, our results have a great significance in developing nano-material for fabricating the nano-photovoltaic devices.

EFFECTIVE MOBILITY MODEL OF GRAPHENE NANORIBBON IN PARABOLIC BAND ENERGY

In this letter, we investigate the transport properties of one-dimensional semiconducting Graphene nanoribbons (GNRs) with parabolic band structure near the Dirac point. The analytical model of effective mobility is developed by using the conductance approach, which differs from the conventional method of extracting the effective mobility using the well-known Matthiessen rule. Graphene nanoribbons conductance model developed was applied in the Drude model to obtain the effective mobility, which then gives nearly close comparison with the experimental data.

Field-enhanced electron mobility by nonlinear phonon scattering of Dirac electrons in semiconducting graphene nanoribbons

The calculated electron mobility for a graphene nanoribbon as a function of applied electric field has been found to have a large threshold field for entering a nonlinear transport regime. This field depends on the lattice temperature, electron density, impurity scattering strength, nanoribbon width and correlation length for the line-edge roughness. An enhanced electron mobility beyond this threshold has been observed, which is related to the initially-heated electrons in high energy states with a larger group velocity. However, this mobility enhancement quickly reaches a maximum due to the Fermi velocity in graphene and the dramatically increased phonon scattering. Super-linear and sub-linear temperature dependence of mobility seen in the linear and nonlinear transport regimes. By analyzing the calculated non-equilibrium electron distribution function, this difference is attributed separately to the results of sweeping electrons from the right Fermi edge to the left one through the elastic scattering and moving electrons from low-energy states to high-energy ones through field-induced electron heating. The threshold field is pushed up by a decreased correlation length in the high field regime, and is further accompanied by a reduced magnitude in the mobility enhancement. This implies an anomalous high-field increase of the line-edge roughness scattering with decreasing correlation length due to the occupation of high-energy states by field-induced electron heating.

Surface Energy Modification by Spin-Cast, Large-Area Graphene Film for Block Copolymer Lithography

We demonstrate a surface energy modification method exploiting graphene film. Spin-cast, atomic layer thick, large-area reduced graphene film successfully played the role of surface energy modifier for arbitrary surfaces. The degree of reduction enabled the tuning of the surface energy. Sufficiently reduced graphene served as a neutral surface modifier to induce surface perpendicular lamellae or cylinders in a block copolymer nanotemplate. Our approach integrating large-area graphene film preparation with block copolymer lithography is potentially advantageous in creating semiconducting graphene nanoribbons and nanoporous graphene.

Hydrogenated graphene nanoribbons for spintronics

We show how hydrogenation of graphene nanoribbons at small concentrations can open venues toward carbon-based spintronics applications regardless of any specific edge termination or passivation of the nanoribbons. Density-functional theory calculations show that an adsorbed H atom induces a spin density on the surrounding $\ensuremath{\pi}$ orbitals whose symmetry and degree of localization depends on the distance to the edges of the nanoribbon. As expected for graphene-based systems, these induced magnetic moments interact ferromagnetically or antiferromagnetically depending on the relative adsorption graphene sublattice, but the magnitude of the interactions are found to strongly vary with the position of the H atoms relative to the edges. We also calculate, with the help of the Hubbard model, the transport properties of hydrogenated armchair semiconducting graphene nanoribbons in the diluted regime and show how the exchange coupling between H atoms can be exploited in the design of novel magnetoresistive devices.

Electronic structure and Peierls instability in graphene nanoribbons sculpted in graphane

Graphene nanoribbons are semiconductor nanostructures with great potentials in nanoelectronics. Their realization particularly with small lateral dimensions below a few nanometers, however, remains challenging. Here we theoretically analyze zigzag graphene nanoribbons created in a graphane substrate (a fully saturated two-dimensional hydrocarbon with formula CH) and predict that they are stable down to the limit of a single carbon chain. We exploit density functional theory with B3LYP functional that accurately treats exchange and correlation effects and demonstrate that at small widths below a few chains these zigzag nanoribbons are semiconducting due to the Peierls instability similar to the case of polyacetylene. Graphene nanoribbons in graphane might represent a viable strategy for the realization of ultranarrow semiconducting graphene nanoribbons with regular edges and controlled chemical termination and open the way for the exploration of the competition between Peierls distortion and spin effects in artificial one-dimensional carbon structures.

Comparisons of Performance Potentials of Silicon Nanowire and Graphene Nanoribbon MOSFETs Considering First-Principles Bandstructure Effects

In this paper, we investigate the performance potentials of silicon nanowire (SNW) and semiconducting graphene nanoribbon (GNR) MOSFETs by using first-principles bandstructures and ballistic current estimation based on the ¿top-of-the-barrier¿ model. As a result, we found that SNW-MOSFETs display a strong orientation dependence via the atomistic bandstructure effects, and SNW-MOSFETs provide smaller intrinsic device delays than Si ultrathin-body MOSFETs when the wire size is scaled smaller than 3 nm. Furthermore, GNR-MOSFETs are found to exhibit promising device performance if the ribbon width is designed to be larger than a few nanometers and a finite band gap can be established.

A theory for the high-field current-carrying capacity of one-dimensional semiconductors

It is shown that current saturation in semiconducting carbon nanotubes is indistinguishable from metallic nanotubes if the carrier density is above a critical value determined by the bandgap and the optical phonon energy. This feature stems from the higher number of current-carrying states in the semiconducting tubes due to the van Hove singularity at the band edge. Above this critical carrier density, the ensemble saturation velocity at high fields is found to be independent of the bandgap, but strongly dependent on the carrier density, explaining recent observations. The results derived are valid in the limit of ultrafast electron-optical phonon interaction and diffusive transport at high electric fields. The analytical results derived are then applied to one-dimensional (1D) semiconducting graphene nanoribbons as well as semiconductor nanowires with parabolic bandgap. A generalized concept of phonon-limited saturation currents in high-field transport in 1D structures emerges from these considerations.

I−Vcurve signatures of nonequilibrium-driven band gap collapse in magnetically ordered zigzag graphene nanoribbon two-terminal devices

Motivated by the very recent fabrication of sub-10-nm-wide semiconducting graphene nanoribbons [X. Li et al., Science 319, 1229 (2008)], where some of their band gaps extracted from transport measurements were closely fitted to density-functional theory predictions for magnetic ordering along zigzag edges that is responsible for the insulating ground state, we compute current-voltage $(I\text{\ensuremath{-}}V)$ characteristics of finite-length zigzag graphene nanoribbons (ZGNRs) attached to metallic contacts. The transport properties of such devices, at source-drain bias voltages beyond the linear-response regime, are obtained using the nonequilibrium Green's function formalism combined with the mean-field version of the Hubbard model fitted to reproduce the local spin-density approximation description of magnetic ordering. Our results indicate that magnetic ordering and the corresponding band gap in ZGNR can be completely eliminated by passing large enough direct current through it. The threshold voltage for the onset of band gap collapse depends on the ZGNR length and the contact transparency. If the contact resistance is adjusted to experimentally measured value of $\ensuremath{\simeq}60\text{ }\text{k}\ensuremath{\Omega}$, the threshold voltage for sub-10-nm-wide ZGNR with intercontact distance of $\ensuremath{\simeq}7\text{ }\text{nm}$ is $\ensuremath{\approx}0.4\text{ }\text{V}$. For some device setups, including $60\text{ }\text{k}\ensuremath{\Omega}$ contacts, the room-temperature $I\text{\ensuremath{-}}V$ curves demonstrate steplike current increase by one order of magnitude at the threshold voltage and can exhibit a hysteresis as well. On the other hand, poorly transmitting contacts can almost completely eliminate abrupt jump in the $I\text{\ensuremath{-}}V$ characteristics. The threshold voltage increases with the ZGNR length (e.g., reaching $\ensuremath{\approx}0.8\text{ }\text{V}$ for $\ensuremath{\simeq}13\text{-nm}$-long ZGNR) which provides possible explanation of why the recent experiments [Wang et al., Phys. Rev. Lett. 100, 206803 (2008)] on $\ensuremath{\sim}100\text{-nm}$-long GNR field-effect transistors with bias voltage $&lt;1\text{ }\text{V}$ did not detect the $I\text{\ensuremath{-}}V$ curve signatures of the band gap collapse. Thus, observation of predicted abrupt jump in the $I\text{\ensuremath{-}}V$ curve of two-terminal devices with short ZGNR channel and transparent metallic contacts will confirm its zigzag edge magnetic ordering via all-electrical measurements, as well as a current-flow-driven magnetic-insulator--nonmagnetic-metal nonequilibrium phase transition.

A Computational Study on the Device Performance of Graphene Nanoribbon Resonant Tunneling Diodes

Device performance of semiconducting graphene nanoribbon resonant tunneling diodes (GNR RTDs) with different shapes and dimensions was investigated using the real space, π-orbital tight-binding approach embedded in non-equilibrium Green's function formalism. The robustness of the device operating mechanism of GNR RTDs was demonstrated with their peak currents occurring at a similar bias (Vpeak) regardless of the shapes and temperatures. Furthermore, the impact of different ribbon widths at the contact/channel regions, which resulted in the different electronic structures, on electron transport was investigated at low temperature. A decrease in the channel width was found to increase the drive current while an increase in the contact width degraded it. The peak to valley ratio was degraded for both cases, while the Vpeak was increased. This study suggests that the device performance of GNR RTDs can be tuned by varying the ribbon width at the different sections, thereby providing great flexibility in future circuit designs.

Novel Band Structures and Transport Properties from Graphene Nanoribbons with Armchair Edges

Using an empirical linear combination of atomic orbitals method (LCAO) based on the sp3 tight-binding scheme, we studied the band structures of graphene nanoribbons with armchair edges terminated by H atoms and found that the band gap exhibits the size dependence and the ‘family’ behaviors, in which a metallic graphene nanoribbon and two semiconducting graphene nanoribbons coexist in a ‘family’. On the basis of Pichard’s transfer matrix technique, we calculated the conductance of the graphene nanoribbons and interestingly found that the steplike curves of conductance display conductance plateaus in the integer multiples of G0 = 2e2/h and the conductance gap emerges in the semiconducting graphene nanoribbon. Meanwhile, a side needle is located on the conductance curve at zero energy in the metallic graphene nanoribbon, which is attributed to degeneration of eigenstates at zero energy. Additionally, we showed that edge states do not have a notable contribution to the conductance because of the strong locali...