Electronic properties and carrier mobilities of nanocarbons formed by non-benzoidal building blocks.

Exotic 1D and 2D carbon nanostructures have been grown in the laboratory in the last few years by means of surface-assisted chemical routes. In these processes, the strategical choice of a molecular precursor plays a dominant role in the determination of the synthesized nanocarbon. Further variations of these techniques are able to produce non-benzoidal carbon quantum-dots (QDs). Considering this experimental scenario as motivation, we propose a series of nanoribbon systems based on concatenating recently synthesized carbon QDs containing pentagonal, hexagonal, and heptagonal rings. We use density functional theory (DFT) simulations to reveal their properties can range from metallic to semiconducting depending on the concatenation hierarchy used to form the nanoribbons. This DFT implementation is based on a LCAO approach to describe valence wavefunctions and most of the simulations employ the PBE-GGA functional. Since this functional is known to underestimate band gaps, we also use the B3LYP functional in a plane-wave DFT approach for a selected case for comparison purposes. These systems show a different gap versus width relationship compared to conventional graphene nanoribbons setups and a particular set of carrier mobility values. We further discuss the interplay between the QD's frontier states and the electronic properties of the nanoribbons in light of their structural details.

Spin-polarized electronic properties of naphthylene-based carbon nanostructures.

The 2D naphthylene-ß structure is a theoretically proposed sp2 nanocarbon allotrope based on the assembly of naphthalene-based molecular building blocks, which features metallic properties. We report that 2D naphthylene-ß structures host a spin-polarized configuration which turns the system into a semiconductor. We analyze this electronic state in terms of the bipartition of the lattice. In addition, we study the electronic properties of nanotubes obtained from the rolling up of 2D naphthylene-ß. We show that they inherit the properties of the parent 2D nanostructure, such as the emergence of spin-polarized configurations. We further rationalize the results in terms of a zone-folding scheme. We also show that the electronic properties can be modulated using an external transverse electric field, including a semiconducting-to-metallic transition for sufficiently large field strength.

The electronic properties of non-conventional α-graphyne nanoribbons.

Graphyne nanocarbons are composed of a mixture of sp and sp2 hybridized atoms in different ratios and distributions. In addition to pure hexagonal systems, non-conventional graphynic structures can also accommodate non-hexagonal rings, as proposed recently on the basis of previously studied haeckelites. Here we use computational simulations to investigate the electronic properties emerging from quantum confinement when such 2D systems are cast into different families of nanoribbons. We show that the electronic behavior of these ribbons closely follow those of their 2D counterparts. However, we find that part of these quasi-1D systems become semiconductors due to the emergence of spin-polarized states at their edges. We further investigate how such multiple spin-configurations influence the electronic transport properties of nanojunctions involving these non-conventional graphyne nanoribbons. These findings highlight how details of these graphyne nanoribbons' atomic structure can be used to tune their electronic properties for targeted applications in nanoelectronics.

Electronic properties of 2D and 1D carbon allotropes based on a triphenylene structural unit.

Several bottom-up chemical routes have been developed in the last few years to find ways to grow new forms of nanocarbon by devising a strategical selection of molecular precursors. Here, theoretical calculations are performed on 2D nanocarbon allotropes obtained from the fusion of triphenylene-like units through tetragonal rings. This 2D triphenylene structure has a metallic character in a closed shell configuration, but it also features a spin-polarized semiconducting state. The behavior of the electronic properties of the system is investigated when the structure is cast into nanoribbon forms. It is found that to be metallic in the nonpolarized case, the ribbons must be sufficiently wide while narrow 1D systems are semiconducting. A lower threshold width is also needed for the emergence of a spin-polarized semiconducting configuration in these nanoribbons. These behaviors are robust as they do not depend on edge geometry and chirality, thus offering opportunities for their possible applications in nanoscale devices.

Electronic properties of N-rich graphene nano-chevrons.

Theoretical analysis based on density functional theory describes the microscopic origins of emerging electronic and magnetic properties in quasi-1D nitrogen-rich graphene nanoribbon structures with chevron-like (or wiggle-edged) configurations. The study focuses on systems with structural units composed of hexagonal graphitic units featuring one and two nitrogen atoms substituted in the graphitic structure, in positions contrasting with the more commonly considered pyridinic configurations. This type of substitution introduces nitrogen levels close to the Fermi level which in turn induce spin polarization depending on a number of structural features. We demonstrate that these systems present a broader set of electronic and magnetic behaviors relative to their pure hydrocarbon counterparts, with the possibility of engineering the electronic band gap strategically using different spin configurations and positions of the substituting nitrogen atoms.

Tripentaphenes: two-dimensional acepentalene-based nanocarbon allotropes.

Tripentaphenes are 2D nanocarbon lattices conceptually obtained from the assembly of acepentalene units. In this work, density functional theory is used to investigate their structural, electronic, and vibrational properties. Their bonding configuration is rationalized with a resonance mechanism, which is unique to each of the 2D assemblies. Their formation energies are found to lie within the range of other previously synthesized carbon nanostructures and phonon calculations indicate their dynamical stability. In addition, all studied tripentaphenes are metallic and display different features (e.g., Dirac cone) depending on the details of the atomic structure. The resonance structure also plays an important role in determining the electronic properties as it leads to delocalized electronic states, further highlighting the potential of the structures in nanoelectronics.

Improved all-carbon spintronic device design.

The discovery of magnetism in carbon structures containing zigzag edges has stimulated new directions in the development and design of spintronic devices. However, many of the proposed structures are designed without incorporating a key phenomenon known as topological frustration, which leads to localized non-bonding states (free radicals), increasing chemical reactivity and instability. By applying graph theory, we demonstrate that topological frustrations can be avoided while simultaneously preserving spin ordering, thus providing alternative spintronic designs. Using tight-binding calculations, we show that all original functionality is not only maintained but also enhanced, resulting in the theoretically highest performing devices in the literature today. Furthermore, it is shown that eliminating armchair regions between zigzag edges significantly improves spintronic properties such as magnetic coupling.

Electronic transport in three-terminal triangular carbon nanopatches.

The electronic transport properties of three-terminal graphene-based triangular patches are investigated using a combination of semi-empirical tight-binding calculations and Green's function-based transport theory within Landauer's framework. The junctions are composed of a triangular structure based on armchair edged graphene nanoribbons. We show how details of the central region influence the resonant electronic transport across the triangular patches and highlight the unique features of the current flow as a function of geometry. These properties indicate an array of functionalities for the development of carbon-based complex nanocircuits and operational devices at the nanoscale.

Electronic and magnetic structures of coronene-based graphitic nanoribbons.

We study the electronic properties of a series of coronene-derived graphitic nanoribbons recently synthesized in a pre-programmed, nanotube assisted, chemical route [Talyzin et al. Nano Lett., 2011, 11, 4352 and Fujihara et al. J. Phys. Chem. C, 2012, 116, 15141]. We employ a combination of density functional theory and spin-polarized tight-binding methods to show how details of the molecular building blocks and their assembly uniquely determine the electronic structure of the resulting ribbon. We demonstrate the onset of multiple magnetic states for these systems and a non-trivial dependence of the electronic bandgap with both atomic structure and spin configuration, which make these coronene-based ribbons potential candidates for applications in nanoelectronics.

Electronic transport properties of assembled carbon nanoribbons.

Graphitic nanowiggles (GNWs) are 1D systems with segmented graphitic nanoribbon GNR edges of varying chiralities. They are characterized by the presence of a number of possible different spin distributions along their edges and by electronic band-gaps that are highly sensitive to the details of their geometry. These two properties promote these experimentally observed carbon nanostructures as some of the most promising candidates for developing high-performance nanodevices. Here, we highlight this potential with a detailed understanding of the electronic processes leading to their unique spin-state dependent electronic quantum transport properties. The three classes of GNWs containing at least one zigzag edge (necessary to the observation of multiple-magnetic states) are considered in two distinct geometries: a perfectly periodic system and in a one-GNW-cell system sandwiched between two semi-infinite terminals made up of straight GNRs. The present calculations establish a number of elementary rules to relate fundamental electronic transport functionality, electronic energy, the system geometry, and spin state.

Emergence of atypical properties in assembled graphene nanoribbons.

Graphitic nanowiggles (GNWs) are periodic repetitions of nonaligned finite-sized graphitic nanoribbon domains seamlessly stitched together without structural defects. These complex nanostructures have been recently fabricated [Cai et al., Nature (London) 466, 470 (2010)] and are here predicted to possess unusual properties, such as tunable band gaps and versatile magnetic behaviors. We used first-principles theory to highlight the microscopic origins of the emerging electronic and magnetic properties of the main subclasses of GNWs. Our study establishes a road map for guiding the design and synthesis of specific GNWs for nanoelectronic, optoelectronic, and spintronic applications.

Electronic transport properties of carbon nanotoroids.

We investigate the electronic transport properties of carbon nanotori covalently connected to external electrodes made up of carbon nanotubes of various chiralities. The study is based on computing ballistic transport characteristics within the framework of Green's function theory using a simple π-orbital tight-binding model. The calculations focus on the effect of the relative angle made by the electrodes as they are placed at different positions along the nanoring. The conductance behavior is found to depend on the details of the atomic structure of the torus but also on the positions of the electrodes. Our findings are rationalized using an elementary quantum mechanical interference model, which reproduces well the main features of the numerical data.

A single molecule rectifier with strong push-pull coupling.

We theoretically investigate the electronic charge transport in a molecular system composed of a donor group (dinitrobenzene) coupled to an acceptor group (dihydrophenazine) via a polyenic chain (unsaturated carbon bridge). Ab initio calculations based on the Hartree-Fock approximations are performed to investigate the distribution of electron states over the molecule in the presence of an external electric field. For small bridge lengths (n=0-3) we find a homogeneous distribution of the frontier molecular orbitals, while for n>3 a strong localization of the lowest unoccupied molecular orbital is found. The localized orbitals in between the donor and acceptor groups act as conduction channels when an external electric field is applied. We also calculate the rectification behavior of this system by evaluating the charge accumulated in the donor and acceptor groups as a function of the external electric field. Finally, we propose a phenomenological model based on nonequilibrium Green's function to rationalize the ab initio findings.