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The effect of chemical functionalization on the electronic properties of graphene nanoribbon superlattices with zigzag and armchair terminations is investigated using the density functional theory. The calculated positive binding energies imply that all the considered structures are stable before and after chemical modifications. The superlattices with armchair edges are characterized by a wide energy gap while those with zigzag edges have a narrow energy gap. The energy gap in superlattices with armchair edges nearly independent of their length while it strongly decreases and almost closes in superlattices with zigzag edges. The energy gap is comparably sensitive to the width variations in both types of the superlattices. It was found that the electric dipole moment increases with increasing the width in the case of armchair while it oscillates in zigzag superlattices. The electric dipole moment can be enhanced by chemical functionalization with COOH and NH2 groups. The effect of this functionalization is moderate for the energy gap, but the adsorption of tetracyanoquinodimethane transforms the system from insulator (Eg = 2.66 eV) to a narrow band gap semiconductor (Eg = 0.25 eV). The adsorption energy of tetracyanoquinodimethane and tetrathiafulvalene molecules can be enhanced by chemical functionalization which makes graphene superlattices useful for sensor applications and wastewater treatment.

Density functional theory calculations are performed on phosphorene quantum dots having different shapes and edge terminations to investigate their structure stability, electronic properties, and gas sensing ability. All the selected phosphorene dots, namely hexagonal and triangular flakes with armchair and zigzag terminations, have positive binding energies which insure their stability even though the bond lengths are much longer than those in the infinite phosphorene layer. It is found that all the selected hydrogen passivated quantum dots have a wide energy gap. In contrast, the partial passivation with sulfur decreases the gap. Moreover, it transforms the system from antiferromagnetic to ferromagnetic state. The energy gap of hexagonal zigzag cluster can be additionally tuned by electric field: narrowed by about 1.7 eV for hydrogenated or broadened by 0.25 eV for partially sulfurated edges. It is shown that phosphorene quantum dots successfully adsorb H2S, CH4, CO, NH3 gas molecules either on their edge or surface. The highest adsorption energy is obtained for NH3 molecule, when it is placed over the surface. This adsorption is alleviated by in-plane electric field and hindered by perpendicular field Phosphorene quantum dot electronic properties and gas sensing © 2018. This is the authors’ accepted and refereed manuscript to the article. Locked until 12 November 2020 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Development of on-chip integrated carbon-based optoelectronic nanocircuits requires fast and non-invasive structural characterization of their building blocks. Recent advances in synthesis of single wall carbon nanotubes and graphene nanoribbons allow for their use as atomically precise building blocks. However, while cataloged experimental data are available for the structural characterization of carbon nanotubes, such an atlas is absent for graphene nanoribbons. Here we theoretically investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons continuously spanning the tube (ribbon) transverse sizes from 0.5(0.4) nm to 8.1(12.8) nm. We show that the linear mapping is guaranteed between the tube and ribbon bulk resonance when the number of atoms in the tube unit cell is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2N+4$$\end{document}2N+4, where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$N$$\end{document}N is the number of atoms in the ribbon unit cell. Thus, an atlas of carbon nanotubes optical transitions can be mapped to an atlas of zigzag graphene nanoribbons., The authors combine ab-initio density functional theory with tight-binding calculations to investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons, and show that an atlas of carbon nanotubes optical transitions can be mapped to an atlas of optical resonances of zigzag graphene nanoribbons.

The electronic and adsorption properties of chemically modified square hexagonal boron nitride quantum dots are investigated using density functional theory calculations. The free energy and frequency calculations show that all the boron nitride flakes are stable before/after modification and metal adsorption. Edge modification significantly enhances the stability and interactivity of the flake. For instance, the free energy of binding decreases from -6.5 eV in hydrogenated flake to -7.1 eV in pristine one and dipole moment increases from 4.5 D to 54.9 D, respectively. A wide spectrum of band gaps can also be achieved, where the band gap can be smoothly varied from ~ 6 eV in edge fluorinated flakes to 0.2 in sulfureted ones. Six hydrated metals, Cd, Co, Cr, Cu, Fe, and Zn, are considered for adsorption by the flakes. The transition metals are highly selected by the flakes while heavy metals are weakly adsorbed. All hydrated metals are physically adsorbed by the edge and surface of hydrogenated flakes except Cu which is chemically adsorbed. Chemical groups or elements attached to the flake strongly enhance the adsorption strength; the adsorption energy of hydrated Cr on surface increases from 0.6 eV to 8.6 eV after attaching two COOH groups to the surface. Hydrogen evolution has also been observed through the adsorption process. The calculated low overpotential for the oxygen evolution reaction (0.52 V) and hydrogen adsorption strength (0.11 eV) for the hydrogen evolution reaction indicate that boron nitride quantum dots are not only potential candidates for the removal of different metals from wastewater but also for efficient water splitting. © 2020. This is the authors' accepted and refereed manuscript to the article. Locked until 7.1.2021 due to copyright restrictions. The final authenticated version is available online at: http://dx.doi.org/10.1039/C9CP06823F

We use the robust nearest-neighbour tight-binding approximation to study on the same footing interband dipole transitions in narrow-bandgap carbon nanotubes and graphene nanoribbons. It is demonstrated that curvature effects in metallic single-walled carbon nanotubes and edge effects in gapless graphene nanoribbons not only open up bang gaps, which typically correspond to THz frequencies, but also result in a giant enhancement of the probability of optical transitions across these gaps. Moreover, the matrix element of the velocity operator for these transitions has a universal value (equal to the Fermi velocity in graphene) when the photon energy coincides with the band-gap energy. Upon increasing the excitation energy, the transition matrix element first rapidly decreases (for photon energies remaining in the THz range but exceeding two band gap energies it is reduced by three orders of magnitude), and thereafter it starts to increase proportionally to the photon frequency. A similar effect occurs in an armchair carbon nanotube with a band gap opened and controlled by a magnetic field applied along the nanotube axis. There is a direct correspondence between armchair graphene nanoribbons and single-walled zigzag carbon nanotubes. The described sharp photon-energy dependence of the transition matrix element together with the van Hove singularity at the band gap edge of the considered quasi-one-dimensional systems make them promising candidates for active elements of coherent THz radiation emitters. The effect of Pauli blocking of low-energy interband transitions caused by residual doping can be suppressed by creating a population inversion using high-frequency (optical) excitation., 22 pages, 6 figures

The electronic and adsorption properties of chevron and cove-edged graphene nanoribbons (GNRs) are studied using first principles calculations. The positive binding and adsorption energies in conjunction with the positive infrared frequencies insure the stability of the considered GNRs. The results show that the binding strength of coved-edged GNRs is higher than that of chevron ones because the morphology of the latter requires a higher number of C-atoms at the edges than the former. The edge atoms in chevron GNRs create interactive edge states that significantly decreases the energy gap (Eg = 0.03 eV) with respect to the wide gap between bulk states in cove-edged ones (Eg = 2.19 eV). The molecular orbitals distributions of these edge states are localized only on the arms of the nanoribbon making it a potential topological insulator. The energy gap between bulk states in cove-edged decreases by increasing the width due to quantum size effect, while in chevron GNRs the gap between edge states increases because of the interaction among these states. The adsorption of methylene blue shows interesting properties depending on the type of the nanoribbons, the interaction position, and the attached chemical group. The interactive edge states provide moderate adsorption on the arms of the nanoribbons and the attached chemical groups enhance the adsorption by adding new adsorption positions. The additional molecular orbitals from the physically adsorbed dye lower the band gap and create semimetal GNRs with zero or negative band gap.

International audience; This is the author accepted manuscript. The final version is available from Springer Verlag via the DOI in this record. ; The paper focuses on superlattices consisting of two coplanar fragments of one-layer graphene nanoribbons that have different width and are connected at an angle. Classification of such superlattices was carried out; their electronic properties were studied using the tight-binding method. It was demonstrated that in superlattices consisting of two fragments of graphene nanoribbons with armchair edges connected at an angle of 60°, the band gap can be regulated by the number of dimeric carbon atom chains of one of the fragments. In that case one can observe a periodic dependence of the band gap on the number of chains with a characteristic period equal to three dimeric chains. The number of dimeric chains of the second superlattice fragment regulates the average band gap value near which the periodic oscillations occur, as well as the amplitude of those oscillations. Therefore, one can accomplish a sufficiently precise band gap tuning for such structures. Such tuning can find its wide application in the booming carbon nanoelectronics industry when creating generators, amplifiers and sensors in the nanochains. ; This research was supported by projects FP7 ITN NOTEDEV(FP7-607521), CACOMEL(FP7-247007), FAEMCAR (FP7-318617) and CANTOR (FP7-612285); project H2020-MSCA-RISE-2014 CoExAN (SEP-210156718); European Graphene Flagship project (604391), as well as by the Belarusian Ministry of Education (grant 20140773), Belarusian State University (grant 11, 2014), and the international grant AFOSR “Nanosized CherenkovType Terahertz Light Emitter Based on Double-Walled Carbon Nanotubes and Bi-graphene Nanoribbons”.

We show that strong light-matter coupling can be used to overcome a long standing problem that has prevented efficient optical emission from carbon nanotubes. The luminescence from the nominally bright exciton states of carbon nanotubes is quenched due to the fast nonradiative scattering to the dark exciton state having a lower energy. We present a theoretical analysis to show that by placing carbon nanotubes in an optical microcavity the bright exctonic state may be split into two hybrid exciton-polariton states, while the dark state remains unaltered. For sufficiently strong coupling between the bright exciton and the cavity, we show that the energy of the lower polariton may be pushed below that of the dark exciton. This overturning of the relative energies of the bright and dark excitons prevents the dark exciton from quenching the emission. Our resutls pave the way for a new approach to band-engineering the properties of the nanoscale optoelectronic devices., 35 pages, 5 figures, 6 pages of supplementary materials, 1 supplementary figure

Density functional theory calculations are performed for the electronic band structures and optical absorption spectra of the zigzag nanoribbons and armchair nanotubes of graphene and hexagonal boron nitride as well as hybrid tubular structures obtained by embedding two dimer lines of B and N atoms into an armchair nanotube. Linear correlation coefficient analysis is carried out to quantitatively investigate relations between energies of absorption resonances in these tube-ribbon pairs. Despite the large disparity in the energy band gaps of some of these structures, our results show a high degree of correlation (r > 0.85 with >95% confidence level) between them. This is a post-peer-review, pre-copyedit version of an article. The final authenticated version is available online at: https://doi.org/10.1134/S1063782619140161

The stability and electronic properties of hexagonal and triangular silicene quantum dots are investigated under the effect of edge passivation by different elements and molecular groups. The structures experience a considerable alternation in shape depending on the attached elements or groups. The most noticeable alternations occur in zigzag triangular flakes passivated with sulfur and in all the selected flakes when OH groups are attached to their edge atoms. The resulting structure has a spherical shape with a large total dipole moment. All the studied clusters have been proven to be stable by the calculated positive binding energies. A flexible structure transformation from insulator (conductor) to conductor (insulator) is obtained in zigzag hexagonal-H (zigzag triangular-H) and zigzag hexagonal-S (zigzag triangular-OH), respectively. The magnetic properties of the triangular zigzag depend on the parity of the total number of Si atoms such that flakes with an even number of Si atoms will have antiferromagnetic properties while flakes with an odd number of Si atoms can have ferromagnetic or antiferromagnetic properties depending on the attached element or group. Thus, a proper choice of the attached functional groups or elements to silicene flakes allows tailoring of their properties to different application. In particular, hydrogenated or fluorinated flakes are highly interactive with the surrounding and can be used for sensor applications while clusters passivated with S or OH are insensitive to edge defects and have tunable electronic properties that make them promising in semiconductor device applications. Stability and electronic properties of edge functionalized silicene quantum dots: A first principles study © 2018. This is the authors’ accepted and refereed manuscript to the article. Locked until 18 July 2020 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Using a combination of the density functional theory and tight-binding calculations, we study electronic and optical properties of asymmetric chevron-type graphene nanoribbons recently synthesized with atomic precision. We demonstrate that the low-energy optical selection rules in such infinite chiral ribbons are more reminiscent of those for zigzag rather than for armchair ribbons. It is also shown that, starting from about 25 nm long ribbons, the low-energy absorption and therefore the selection rules of infinitely long ribbons are well reproduced in the finite cluster approach. Hence, ribbons longer than 25 nm (about 28 unit cells) can be treated as infinitely long ones.

In this paper we study the effect of absorption peak correlation in finite length carbon nanotubes and graphene nanoribbons. It is shown, in the orthogonal {\pi}-orbital tight-binding model with the nearest neighbor approximation, that if the ribbon width is a half of the tube circumference the effect takes place for all achiral ribbons (zigzag, armchair and bearded), and corresponding tubes, starting from lengths of about 30 nm. This correlation should be useful in designing nanoribbon-based optoelectronics devices fully integrated into a single layer of graphene., Comment: 20 pages, 4 figures

Interband dipole transitions are calculated in quasi-metallic single-walled carbon nan-otubes and armchair graphene nanoribbons. It is shown that the curvature effects for tubes and the edge effects for ribbons result not only in a small band gap opening, corresponding to THz frequencies, but also in a significant enhancement of the transition probability rate across the band gap. This makes these nanostructures perspective canditates for sources and detectors of THz radiation.

We study electronic and optical properties of single layer phosphorene quantum dots with various shapes, sizes, and edge types (including disordered edges) subjected to an external electric field normal to the structure plane. Compared to graphene quantum dots, in phosphorene clusters of similar shape and size there is a set of edge states with energies dispersed at around the Fermi level. These states make the majority of phosphorene quantum dots metallic and enrich the phosphorene absorption gap with low-energy absorption peaks tunable by the electric field. The presence of the edge states dispersed at around the Fermi level is a characteristic feature that is independent of the edge morphology and roughness., 14 pages, 13 figures

We present an analytical tight-binding theory of the optical properties of graphene nanoribbons with zigzag edges. Applying the transfer matrix technique to the nearest-neighbor tight-binding Hamiltonian, we derive analytical expressions for electron wave functions and optical transition matrix elements for incident light polarized along the structure axis. It follows from the obtained results that optical selection rules result from the wave function parity factor $(-1)^J$, where $J$ is the band number. These selection rules are that $\Delta J$ is odd for transitions between valence and conduction subbands and that $\Delta J$ is even for transitions between only valence (conduction) subbands. Although these selection rules are different from those in armchair carbon nanotubes, there is a hidden correlation between absorption spectra of the two structures that should allow one to use them interchangeably in some applications. The correlation originates from the fact that van Hove singularities in the tubes are centered between those in the ribbons if the ribbon width is about a half of the tube circumference. The analysis of the matrix elements dependence on the electron wave vector for narrow ribbons shows a smooth non-singular behavior at the Dirac points and the points where the bulk states meet the edge states., Comment: 20 pages, 11 figures, updated in accordance with Phys. Rev. B 96, 199901(E) (2017)

We study numerically the optical properties of low-buckled silicene and AB-stacked bilayer graphene quantum dots subjected to an external electric field, which is normal to their surface. Within the tight-binding model, the optical absorption is calculated for quantum dots, of triangular and hexagonal shapes, with zigzag and armchair edge terminations. We show that in triangular silicene clusters with zigzag edges a rich and widely tunable infrared absorption peak structure originates from transitions involving zero energy states. The edge of absorption in silicene quantum dots undergoes red shift in the external electric field for triangular clusters, whereas blue shift takes place for hexagonal ones. In small clusters of bilayer graphene with zigzag edges the edge of absorption undergoes blue/red shift for triangular/hexagonal geometry. In armchair clusters of silicene blue shift of the absorption edge takes place for both cluster shapes, while red shift is inherent for both shapes of the bilayer graphene quantum dots., Comment: 7 pages, 7 figures

We theoretically investigate the optical functionality of a semiconducting quantum ring manipulated by two electrostatic lateral gates used to induce a double quantum well along the ring. The well parameters and corresponding inter-level spacings, which lie in the THz range, are highly sensitive to the gate voltages. Our analysis shows that selection rules for inter-level dipole transitions, caused by linearly polarized excitations, depend on the polarization angle with respect to the gates. In striking difference from the conventional symmetric double well potential, the ring geometry permits polarization-dependent transitions between the ground and second excited states, allowing the use of this structure in a three-level lasing scheme., Comment: 7 pages, 6 figures