Chiral Graphene Research Articles

ChemInform Abstract: Chirality Transfer from Graphene Quantum Dots

AbstractReview: 113 refs.

Chiral Graphene Quantum Dots.

Chiral nanostructures from metals and semiconductors attract wide interest as components for polarization-enabled optoelectronic devices. Similarly to other fields of nanotechnology, graphene-based materials can greatly enrich physical and chemical phenomena associated with optical and electronic properties of chiral nanostructures and facilitate their applications in biology as well as other areas. Here, we report that covalent attachment of l/d-cysteine moieties to the edges of graphene quantum dots (GQDs) leads to their helical buckling due to chiral interactions at the "crowded" edges. Circular dichroism (CD) spectra of the GQDs revealed bands at ca. 210-220 and 250-265 nm that changed their signs for different chirality of the cysteine edge ligands. The high-energy chiroptical peaks at 210-220 nm correspond to the hybridized molecular orbitals involving the chiral center of amino acids and atoms of graphene edges. Diverse experimental and modeling data, including density functional theory calculations of CD spectra with probabilistic distribution of GQD isomers, indicate that the band at 250-265 nm originates from the three-dimensional twisting of the graphene sheet and can be attributed to the chiral excitonic transitions. The positive and negative low-energy CD bands correspond to the left and right helicity of GQDs, respectively. Exposure of liver HepG2 cells to L/D-GQDs reveals their general biocompatibility and a noticeable difference in the toxicity of the stereoisomers. Molecular dynamics simulations demonstrated that d-GQDs have a stronger tendency to accumulate within the cellular membrane than L-GQDs. Emergence of nanoscale chirality in GQDs decorated with biomolecules is expected to be a general stereochemical phenomenon for flexible sheets of nanomaterials.

Open tubular-capillary electrochromatography: Developments and applications from 2013 to 2015.

Open tubular CEC (OT-CEC) separates analyte mixtures by a combination of electrophoretic, electro-osmotic, and/or chromatographic effects. OT-CEC research is an active and growing field, with studies encompassing a wide range of investigations related to new strategies for chemical modification of the inner surface of the capillary, leading to the introduction of novel stationary phase coatings. This review has examined the literature on OT-CEC from 2013 to August 2015 and highlights the developments in the fabrication of highly selective stationary phases, based on materials that include cyclodextrin chiral selectors, graphene and graphene oxide, metal-organic frameworks, molecularly imprinted polymers, nanoparticles, nanolatex particles, nanocomposites, in situ generated polymers, block polymers, tentacle-type polymers, polyelectrolyte multilayers, polysaccharides, phospholipids, and proteins. This review, while considering the development of novel OT-CEC coating materials, specifically examines different immobilization or coating methodologies and approaches and also discusses the separation mechanisms that occur with these new materials. These OT-CEC coatings are intended mainly to separate low molecular weight molecules relevant to the pharmaceutical, agricultural, and food industries as well as for use in environmental monitoring.

Chirality transfer from graphene quantum dots

Chiral graphene quantum dots were prepared by acidic exfoliation and oxidation of graphite, dialysis, and esterification with enantiomerically pure (R) or (S)-2-phenyl-1-propanol. Circular dichroism studies support the formation of supramolecular aggregates with pyrene molecules, where a transfer of chirality occurs from the chiral graphene quantum dots to the pyrene.

Role of geometry and topological defects in the one-dimensional zero-line modes of graphene

Breaking inversion symmetry in chiral graphene systems, \textit{e.g.}, by applying a perpendicular electric field in chirally-stacked rhombohedral multilayer graphene or by introducing staggered sublattice potentials in monolayer graphene, opens up a bulk band gap that harbors a quantum valley-Hall state. When the gap size is allowed to vary and changes sign in space, a topologically-confined one-dimensional (1D) zero-line mode (ZLM) is formed along the zero lines of the local gap. Here we show that gapless ZLM with distinguishable valley degrees of freedom K and K$'$ exist for every propagation angle except for the armchair direction that exactly superpose the valleys. We further analyze the role of different geometries of top-bottom gated device setups that can be realized in experiments, discuss the effects of their edge misalignment, and analyze three common forms of topological defects that could influence the 1D ZLM transport properties in actual devices.

Detection of graphene chirality using achiral liquid crystalline platforms

Monolayer graphene flakes were dispersed at low concentrations into two achiral liquid crystals (LCs) alkoxyphenylbenzoate (9OO4) and 4-cyano-4′-pentylbiphenyl (5CB), separately. The presence of graphene resulted in two types of chiral signatures in the LCs: an electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase of 9OO4, and a macroscopic helical twist of the LC director in the nematic phase of 5CB. Graphene flakes generally possess strain chirality and edge chirality. The non-covalent interactions between the LC molecules and chiral graphene flakes induce molecular conformational deracemization in the LC, exhibiting a bulk electroclinic effect and a macroscopic helical twist.

Mechanical Properties of Hydrogen Edge–Passivated Chiral Graphene Nanoribbons

AbstractUniaxial tension of chiral graphene nanoribbons (GNR) with and without edge hydrogen passivation are simulated using molecular dynamics (MD) simulations to study their mechanical properties. The results demonstrate that hydrogen saturation generally weakens chiral GNRs, although its influence on armchair GNRs is almost negligible. Mechanical properties of GNRs depend on chiral angles. Zigzag GNRs (chiral angle 0°) are always the strongest, whereas armchair GNRs (chiral angle 30°) are weaker. The mechanical properties of other chiral GNRs evolve gradually from these two distinct cases from chiral angles of 30° to 0°, with the smallest value of failure stress and failure strain happening around a chiral angle of 20°. As for the width size effect, wider GNRs always have lower failure strains and failure stress regardless of having edge hydrogen passivation or not.

Optical properties of chiral graphene nanoribbons: a first principle study

Recent successes in unzipping carbon nanotubes have introduced a new member of graphene nanoribbons (GNRs) family known as chiral GNRs. In this paper, optical properties of chiral GNRs both in longitudinal and transverse polarizations of incident beam have been studied using Density Functional Theory. We have shown that the selection rules, which were previously reported for armchair and zigzag GNRs, are no longer valid due to breaking symmetries in this new family. However, we have demonstrated that the edge states play a critical role in the dielectric constant. It has been shown that, depending on the polarization of incident beam the absorption peaks are different, although they are spread in the same energy range. We also suggested that the absorption energy range is sensitive to the chiral vectors and the light polarization. Due to breaking symmetries in chiral GNRs, absorption peaks are changed becoming approximately 1000 nm, thus a new potential of GNRs for optoelectronic devices is introduced.

Phonon Dispersions of Graphene from Unzipping Carbon Nanotubes

We calculate the phonon spectra of chiral single-walled carbon nanotubes and graphene within a mass and spring model which includes up to third neighbour interactions together with a radial bond-bending interaction. Firstly, the classical Hamiltonian for lattice vibrations of chiral single-walled carbon nanotubes is derived and then it is quantized. The resultant Hamiltonian is diagonalized under a unitary transformation scheme to obtain phonon modes as a function of momentum vector. Using resolvent formalism we analytically obtain phonon dispersions of these structures. We show that our calculated results for chiral SWCNTs reproduce well the results found for achiral SWCNTs. Finally, we introduce an unzipping technique to obtain full analytical phonon dispersion curves of graphene using phonon dispersions of chiral SWCNTs. We compare our analytical results for the phonon dispersions of graphene to the available experimental data and show that they agree well with the experiment. Our analytical results not only provide a basis for understanding phonon dispersions of carbon nanotubes, but also illuminate the bare phonon structure of graphene.

Effect of non-uniform exchange field in ferromagnetic graphene

We have presented here the consequences of the non-uniform exchange field on the spin transport issues in spin chiral configuration of ferromagnetic graphene. Taking resort to the spin–orbit coupling (SOC) term and non-uniform exchange coupling term we are successful to express the expression of Hall conductivity in terms of the exchange field and SOC parameters through the Kubo formula approach. However, for a specific configuration of the exchange parameter we have evaluated the Berry curvature of the system. We also have paid attention to the study of SU(2) gauge theory of ferromagnetic graphene. The generation of anti damping spin–orbit torque in spin chiral magnetic graphene is also briefly discussed.

Band gap oscillation and novel transport property in ultrathin chiral graphene nanoribbons

We report a numerical study of the edge states in chiral graphene nanoribbons (cGNRs) within the tight-binding approximation. For ultrathin ribbons, band gap oscillations are found as a function of the ribbon width, with a period of 3a, where a is the lattice constant of cGNRs. With the increasing ribbon width, the oscillation is found to be dampened at a critical width, depending on the chiral angle of the cGNRs. In addition, the chiral angle and the width of the cGNR also influence the generation of the edge states. Through the transfer matrix technique, we demonstrate that the energy dependence of the conductance exhibits an unusual step-like feature for cGNRs.

Mechanical properties of graphene nanoribbons with disordered edges

Using density functional theory (DFT), we investigate mechanical properties and failure characteristics of chiral and achiral graphene nanoribbons. Specifically, we study the dependence of maximum strength, Young’s modulus and failure pattern of nanoribbons on their widths, edge chirality and passivation. Besides investigating nanoribbons with perfect edges, nanoribbons with defective edges are considered and the impacts of the presence of Klein or edge pentagonal defects on the properties of nanoribbons are studied. Our DFT results show that as the width of achiral ribbons are reduced, their strength increases. Moreover, significant size effects on the Poisson’s ratio of armchair ribbons and Young’s modulus of zigzag ribbons are observed. The DFT modeling predicts that the presence of defects reduce the strength of chiral nanoribbons while defects might counter intuitively increase the Young’s modulus of chiral nanoribbons. The first-principle simulations show that achiral nanoribbons always fail along a zigzag surface, however the failure pattern of chiral nanoribbons can be more complicated and be accompanied by the nucleation of other kind of defects such as pentagonal–heptagonal rings prior to failure.

Edge magnetization and local density of states in chiral graphene nanoribbons

We study the edge magnetization and the local density of states of chiral graphene nanoribbons using a $\ensuremath{\pi}\text{-orbital}$ Hubbard model in the mean-field approximation. We show that the inclusion of a realistic next-nearest hopping term in the tight-binding Hamiltonian changes the graphene nanoribbon band structure significantly and affects its magnetic properties. We study the behavior of the edge magnetization upon departing from half-filling as a function of the nanoribbon chirality and width. We find that the edge magnetization depends very weakly on the nanoribbon width, regardless of chirality as long as the ribbon is sufficiently wide. We compare our results to recent scanning tunneling microscopy experiments reporting signatures of magnetic ordering in chiral nanoribbons and provide an interpretation for the observed peaks in the local density of states, that does not depend on the antiferromagnetic interedge interaction.

Particle-hole symmetry and bifurcating ground-state manifold in the quantum Hall ferromagnetic states of multilayer graphene

The orbital structure of the quantum Hall ferromagnetic states in the zero-energy Landau level in chiral multilayer graphene (AB, ABC, ABCA, etc. stackings) is determined by the exchange interaction with all levels, including deep-lying states in the Dirac sea. This exchange field favors orbitally coherent states with a U(1) orbital symmetry if the filling factor $\ensuremath{\nu}$ is not a multiple of the number of layers. If electrons fill the orbital sector of a fixed spin/valley component to one-half, e.g., at $\ensuremath{\nu}=\ifmmode\pm\else\textpm\fi{}3,\ifmmode\pm\else\textpm\fi{}1$ in the bilayer and at $\ensuremath{\nu}=\ifmmode\pm\else\textpm\fi{}2,\ifmmode\pm\else\textpm\fi{}6$ in the ABCA four-layer, there is a transition to a Z${}_{2}\phantom{\rule{0.28em}{0ex}}\ifmmode\times\else\texttimes\fi{}$ U(1) manifold. For weak interaction, the structure in the zero-energy Landau band compensates for the different exchange interaction on the sublattices in the Landau orbitals; on the other side, the ground state comes in two copies that distribute charge on the sublattices differently. We expect a sequence of similar bifurcations in multilayers of Bernal stacking.

Charge and spin Hall effect in spin chiral ferromagnetic graphene

We predict a specific type of charge Hall effect in undoped ferromagnetic graphene that is generated by the spin Hall mechanism in the absence of an external magnetic field. The essential feature is the so-called spin chiral configuration of the spin subbands in such a magnetic material where carriers with opposite spin direction are of different type of electron-like or hole-like. Within the semiclassical theory of spin-orbital dynamics of electrons, we obtain that a longitudinal electric field can produce a spin-orbit transverse current of pure charge with no polarization of the spin and the valley.

Bandgap opening by patterning graphene.

Owing to its remarkable electronic and transport properties, graphene has great potential of replacing silicon for next-generation electronics and optoelectronics; but its zero bandgap associated with Dirac fermions prevents such applications. Among numerous attempts to create semiconducting graphene, periodic patterning using defects, passivation, doping, nanoscale perforation, etc., is particularly promising and has been realized experimentally. However, despite extensive theoretical investigations, the precise role of periodic modulations on electronic structures of graphene remains elusive. Here we employ both the tight-binding modeling and first-principles electronic structure calculations to show that the appearance of bandgap in patterned graphene has a geometric symmetry origin. Thus the analytic rule of gap-opening by patterning graphene is derived, which indicates that if a modified graphene is a semiconductor, its two corresponding carbon nanotubes, whose chiral vectors equal graphene's supercell lattice vectors, are both semimetals.

Flat bands with Berry curvature in multilayer graphene

We demonstrate that flat bands with local Berry curvature arise naturally in chiral (ABC) multilayer graphene placed on a boron nitride (BN) substrate. The degree of flatness can be tuned by varying the number of graphene layers N. For N = 7 the bands become nearly flat, with a small bandwidth of 3.6 meV. The two nearly flat bands coming from the K and K' valleys cross along lines in the reduced zone. Weak intervalley tunneling turns the bandcrossing into an avoided crossing, producing two nearly flat bands with global Chern number zero, but with local Berry curvature. The flatness of the bands suggests that many body effects will dominate the physics, while the local Berry curvature of the bands endows the system with a nontrivial quantum geometry. The quantum geometry effects manifest themselves through the quantum distance (Fubini-Study) metric, rather than the more conventional Chern number. Multilayer graphene on BN thus provides a platform for investigating the effect of interactions in a system with a non-trivial quantum distance metric, without the complication of non-zero Chern numbers. We note in passing that flat bands with non-zero Chern number can also be realized by making use of magnetic adatoms, and explicitly breaking time reversal symmetry.

Interplay between symmetry and spin-orbit coupling on graphene nanoribbons

We study the electronic structure of chiral and achiral graphene nanoribbons with symmetric edges, including curvature and spin-orbit effects. Curved ribbons show spin-split bands, whereas flat ribbons present spin-degenerate bands. We show that this effect is due to the breaking of spatial inversion symmetry in curved graphene nanoribbons, while flat ribbons with symmetric edges possess an inversion center, regardless of their having chiral or achiral edges. We find an enhanced edge-edge coupling and a substantial gap in narrow chiral nanoribbons, which is not present in zigzag ribbons of similar width. We attribute these size effects to the mixing of the sublattices imposed by the edge geometry, yielding a distinct behavior of chiral ribbons from those with pure zigzag edges.

Quantum Monte Carlo studies of edge magnetism in chiral graphene nanoribbons

We investigate chiral graphene nanoribbons using projective quantum Monte Carlo simulations within the local Hubbard model description and study the effects of electron-electron interactions on the electronic and magnetic properties at the ribbon edges. Static and dynamical properties are analyzed for nanoribbons of varying width and edge chirality, and compared to a self-consistent Hartee-Fock mean-field approximation. Our results show that for chiral ribbons of sufficient width, the spin correlations exhibit exceedingly long correlation lengths, even between zigzag segments that are well separated by periodic armchair regions. Characteristic enhancements in the magnetic correlations for distinct ribbon widths and chiralities are associated with energy gaps in the tight-binding limit of such ribbons. We identify specific signatures in the local density of states and low- energy modes in the local spectral function which directly relate to enhanced electronic correlations along graphene nanoribbons and which can be accessed scanning tunneling spectroscopy.

Surface polar optical phonon interaction induced many-body effects and hot-electron relaxation in graphene

We theoretically study various aspects of the electron-surface optical phonon interaction effects in graphene on a substrate made of polar materials. We calculate the electron self energy in the presence of the surface phonon-mediated electron-electron interaction focusing on how the linear chiral graphene dispersion is renormalized by the surface phonons. The electron self energy as well as the quasiparticle spectral function in graphene are calculated, taking into account electron-polar optical phonon interaction by using a many-body perturbative formalism. The scattering rate of free electrons due to polar interaction with surface optical phonons in a dielectric substrate is calculated as a function of the electron energy, temperatures, and carrier density. Effects of screening on the self energy and scattering rate are discussed. Our theory provides a comprehensive quantitative (and qualitative) picture for surface phonon interaction induced many-body effects and hot-electron relaxation in Dirac materials.