1,720,992 research outputs found
Optical excitations of quasi-one-dimensional systems: carbon nanotubes versus polymers and semiconductor wires
No full textWe review the main characteristics of optical excitations of semiconductor nanotubes, as obtained from accurate ab-initio theories and model calculations as well as experimental evidence, and discuss them in light of the previous understanding of other quasi-one-dimensional semiconducting systems. We point out striking similarities of nanotubes with III-V quantum wires and conjugated polymers, especially (i) the clear excitonic nature of absorption, very far from the single-particle behaviour; (ii) its manifestations in optical spectra, where excitonic peaks are accompanied by a strong intensity reduction at the onset of the free-particle continuum; (iii) the strategies that allow experimental access to exciton binding energies. The recent theoretical and experimental evidence obtained on semiconducting single-walled nanotubes converges quantitatively to a picture of strongly bound excitons (about 0.3-1.0 eV for nanotubes with 0.4-1.0 nm diameter). We discuss its implications and list a few open issues of relevance to fundamental understanding and optoelectronic applications. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Exact biexciton binding energy in carbon nanotubes using a quantum Monte Carlo approach
No full textWe performed quantum Monte Carlo (QMC) calculations for a model system of excitons and biexcitons in carbon nanotubes (CNTs) and compared the results with those of a variational approach [T.G. Pedersen, K. Pedersen, H.D. Cornean, P. Duclos, Nano Lett. 5 (2005) 291]. Due to their geometric properties, the shape of a hollow cylinder, CNTs can be treated as 2D objects. With decreasing diameter one expects them even to exhibit quasi-1D properties. In the present study the biexciton in its ground state is found to be more strongly bound than estimated before. Biexcitonic complexes are predicted to remain stable for all diameters even at room temperature. The binding energy grows significantly with decreasing diameter, showing indeed a transition from a quasi-2D system to a quasi-ID system. (c) 2007 Elsevier B.V. All rights reserved
Spin-transport selectivity upon Co adsorption on antiferromagnetic graphene nanoribbons
No full textWe investigate from first principles the electronic and transport properties of zigzag graphene nanoribbons in the presence of Co adatoms. Comparing different adsorption sites across the width, we find that the Co-C coupling is rather sensitive to the local environment. While a net spin polarization appears in all cases, the spin filtering effect is significantly enhanced when the Co adatom is at the edge, where the adsorption energy is maximized and a partial suppression of edge-associated transport channels occurs. We also probe the magnetic interaction in the nonbonding regime, for Co-graphene nanoribbon (GNR) distances ranging from adsorption to - typical configurations. Our results indicate that Co-GNR coupling is still appreciable in an intermediate range, whereas it becomes vanishingly small in the limit of - distances
Optical Properties of Bilayer Graphene Nanoflakes
No full textThe optical properties of coupled graphene nanoflakes are investigated theoretically within the framework of HartreeFock based semiempirical methods, with the aim of unraveling the role of pi pi interactions. Two different types of pi-stacking are considered, obtained either by coupling two identical flakes with different relative displacement or by coupling flakes having different width or edge functionalization, i.e., with different electronic gap or ionization potential. Our results indicate that a systematic red shift and broadening of lowest excitations occur: an overall widening of the optical absorption range can therefore be expected in an ensemble of flakes. However, the coupling prevents a strong enhancement of the absorption intensity. In the case of a heterogeneous ensemble of flakes, the possibility of introducing low-energy excitations with considerable charge transfer character is also demonstrated by properly exploiting the chemical edge functionalization
Optical properties of graphene nanoribbons: The role of many-body effects
No full textWe investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons (GNRs). We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination. © 2008 The American Physical Society
Biexciton Stability in Carbon Nanotubes
Full text linkWe have applied the quantum Monte Carlo method and tight-binding modeling to calculate the binding energy of biexcitons in semiconductor carbon nanotubes for a wide range of diameters and chiralities. For typical nanotube diameters we find that biexciton binding energies are much larger than previously predicted from variational methods, which easily brings the biexciton binding energy above the room temperature threshold
Optical properties of one-dimensional graphene polymers: the case of polyphenanthrene
No full textWe investigate from first principles the effect of many-body corrections on the optoelectronic properties of polyphenanthrene (PPh), a prototype system for carbon-based ladder polymers and I D nanographenes with cis-polyene edges. We show that the inclusion of many-body effects is essential to correctly describe both quasiparticle bandstructure and optical response. Consistently with the reduced dimensionality of the system, the inclusion of electron-hole interaction leads to strongly bound excitons which dominate the spectra. A complete characterization of the low-energy excitonic states is carried out, together with their optical activity. In particular, we find a dark exciton below the first optically active one, which is expected to crucially affect the luminescence efficiency. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.We investigate from first principles the effect of many-body corrections on the optoelectronic properties of polyphenanthrene (PPh), a prototype system for carbon-based ladder polymers and 1D nanographenes with cis-polyene edges. We show that the inclusion of many-body effects is essential to correctly describe both quasiparticle bandstructure and optical response. Consistently with the reduced dimensionality of the system, the inclusion of electron-hole interaction leads to strongly bound excitons which dominate the spectra. A complete characterization of the low-energy excitonic states is carried out, together with their optical activity. In particular, we find a dark exciton below the first optically active one, which is expected to crucially affect the luminescence efficiency. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA,
Electronics and Optics of Graphene Nanoflakes: Edge Functionalization and Structural Distortions
No full textThe effects of edge covalent functionalization on the structural, electronic, and optical properties of elongated armchair graphene nanoflakes (AGNFs) are analyzed in detail for a wide range of terminations, within the framework of Hartree-Fock-based semiempirical methods. The chemical features of the functional groups, their distribution, and the resulting system symmetry are identified as the key factors that determine the modification of strutural and optoelectronic features. While the electronic gap is always reduced in the presence of substituents, functionalization-induced distortions contribute to the observed lowering by about 35-55% This effect is paired with a red shift of the first optical peak, corresponding to about 75% of the total optical gap reduction. Further, the functionalization pattern and the specific features of the edge-substituent bond are found to influence the strength and the character of the low-energy excitations. All of these effects are discussed for flakes of different widths, representing the three families of AGNFs
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