Nicola Bonini

Northeast Institute of Geography and Agroecology, Beijing, Beijing Shi, China

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Publications (44)205.81 Total impact

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    ABSTRACT: We present a first-principles study of the temperature- and density-dependent intrinsic electrical resistivity of graphene. We use density-functional theory and density-functional perturbation theory together with very accurate Wannier interpolations to compute all electronic and vibrational properties and electron-phonon coupling matrix elements; the phonon-limited resistivity is then calculated within a Boltzmann-transport approach. An effective tight-binding model, validated against first-principles results, is also used to study the role of electron-electron interactions at the level of many-body perturbation theory. The results found are in excellent agreement with recent experimental data on graphene samples at high carrier densities and elucidate the role of the different phonon modes in limiting electron mobility. Moreover, we find that the resistivity arising from scattering with transverse acoustic phonons is 2.5 times higher than that from longitudinal acoustic phonons. Last, high-energy, optical, and zone-boundary phonons contribute as much as acoustic phonons to the intrinsic electrical resistivity even at room temperature and become dominant at higher temperatures.
    Nano Letters 02/2014; · 13.03 Impact Factor
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    ABSTRACT: We perform a comparative experimental and theoretical study of the temperature dependence up to 700 K of the frequency and linewidths of the graphite E1u and E2g optical phonons (~1590 and 1580 cm-1) by infra-red (IR) and Raman spectroscopy. Despite their similar character, the temperature dependence of the two modes is quite different, being, e.g., the frequency shift of the IR-active E1u mode is almost twice as big as that of the Raman active E2g mode. Ab initio calculations of the anharmonic properties are in remarkable agreement with measurements and explain the observed behavior.
    Physical review. B, Condensed matter 07/2012; 86(12). · 3.77 Impact Factor
  • Nicola Bonini, Jivtesh Garg, Nicola Marzari
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    ABSTRACT: We use first-principles methods based on density functional perturbation theory to characterize the lifetimes of the acoustic phonon modes and their consequences on the thermal transport properties of graphene. We show that using a standard perturbative approach, the transverse and longitudinal acoustic phonons in free-standing graphene display finite lifetimes in the long-wavelength limit, making them ill-defined as elementary excitations in samples of dimensions larger than ∼1 μm. This behavior is entirely due to the presence of the quadratic dispersions for the out-of-plane phonon (ZA) flexural modes that appear in free-standing low-dimensional systems. Mechanical strain lifts this anomaly, and all phonons remain well-defined at any wavelength. Thermal transport is dominated by ZA modes, and the thermal conductivity is predicted to diverge with system size for any amount of strain. These findings highlight strain and sample size as key parameters in characterizing or engineering heat transport in graphene.
    Nano Letters 05/2012; 12(6):2673-8. · 13.03 Impact Factor
  • Nicola Bonini, Jivtesh Garg, Nicola Marzari
    Nano Letters. 05/2012;
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    ABSTRACT: The dominant phonon wave vectors q* probed by the 2D Raman mode of pristine and uniaxially strained graphene are determined via a combination of ab initio calculations and a full two-dimensional integration of the transition matrix. We show that q* are highly anisotropic and rotate about K with the polarizer and analyzer condition relative to the lattice. The corresponding phonon-mediated electronic transitions show a finite component along K-Γ that sensitively determines q*. We invalidate the notion of “inner” and “outer” processes. The characteristic splitting of the 2D mode of graphene under uniaxial tensile strain and given polarizer and analyzer setting is correctly predicted only if the strain-induced distortion and red-shift of the in-plane transverse optical (iTO) phonon dispersion as well as the changes in the electronic band structure are taken into account.
    Physical review. B, Condensed matter 03/2012; 85(11). · 3.77 Impact Factor
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    ABSTRACT: The quest for materials capable of realizing the next generation of electronic and photonic devices continues to fuel research on the electronic, optical and vibrational properties of graphene. Few-layer graphene (FLG) flakes with less than ten layers each show a distinctive band structure. Thus, there is an increasing interest in the physics and applications of FLGs. Raman spectroscopy is one of the most useful and versatile tools to probe graphene samples. Here, we uncover the interlayer shear mode of FLGs, ranging from bilayer graphene (BLG) to bulk graphite, and suggest that the corresponding Raman peak measures the interlayer coupling. This peak scales from ~43 cm(-1) in bulk graphite to ~31 cm(-1) in BLG. Its low energy makes it sensitive to near-Dirac point quasiparticles. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions.
    Nature Material 02/2012; 11(4):294-300. · 35.75 Impact Factor
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    ABSTRACT: The quest for materials capable of realizing the next generation of electronic and photonic devices continues to fuel research on the electronic, optical and vibrational properties of graphene. Few-layer graphene (FLG) flakes with less than ten layers each show a distinctive band structure. Thus, there is an increasing interest in the physics and applications of FLGs. Raman spectroscopy is one of the most useful and versatile tools to probe graphene samples. Here, we uncover the interlayer shear mode of FLGs, ranging from bilayer graphene (BLG) to bulk graphite, and suggest that the corresponding Raman peak measures the interlayer coupling. This peak scales from �43 cm􀀀1 in bulk graphite to �31 cm􀀀1 in BLG. Its low energy makes it sensitive to near-Dirac point quasiparticles. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions.
    Nature Material 02/2012; · 35.75 Impact Factor
  • Jivtesh Garg, Nicola Bonini, Nicola Marzari
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    ABSTRACT: The thermal conductivity of ideal short-period superlattices is computed using harmonic and anharmonic force constants derived from density-functional perturbation theory and by solving the Boltzmann transport equation in the single-mode relaxation time approximation, using silicon-germanium as a paradigmatic case. We show that in the limit of small superlattice period the computed thermal conductivity of the superlattice can exceed that of both the constituent materials. This is found to be due to a dramatic reduction in the scattering of acoustic phonons by optical phonons, leading to very long phonon lifetimes. By variation of the mass mismatch between the constituent materials in the superlattice, it is found that this enhancement in thermal conductivity can be engineered, providing avenues to achieve high thermal conductivities in nanostructured materials.
    Nano Letters 12/2011; 11(12):5135-41. · 13.03 Impact Factor
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    ABSTRACT: We investigate the dominant phonon wavevectors q* and the associated dominant phonon-assisted electronic transitions implied by the 2D Raman mode of graphene by combining ab initio calculations with a full two-dimensional integration over the graphene Brillouin zone. We find that q* are highly anisotropic and rotate with the polarizer:analyzer condition, providing access to the entire angular extent around K. The resonant electronic transitions do not lie along the line and can be transformed from being apparently “inner” to “outer” with the addition of a reciprocal lattice vector, showing that both are equivalent. We thus invalidate the notion of “inner” and “outer” processes completely.
    physica status solidi (b) 10/2011; 248(11):2635 - 2638. · 1.49 Impact Factor
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    ABSTRACT: We uncover the interlayer shear mode of multi-layer graphene samples, ranging from bilayer-graphene (BLG) to bulk graphite, and show that the corresponding Raman peak measures the interlayer coupling. This peak scales from~43cm-1 in bulk graphite to~31cm-1 in BLG. Its low energy makes it a probe of near-Dirac point quasi-particles, with a Breit-Wigner-Fano lineshape due to resonance with electronic transitions. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions
    06/2011;
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    ABSTRACT: The dominant phonon wavevectors $q^{*}$ probed by the 2D Raman mode of graphene are highly anisotropic and rotate with the orientation of the polarizer:analyzer direction relative to the lattice. The corresponding electronic transitions connect the electronic equibandgap contours where the product of the ingoing and outgoing optical matrix elements is strongest, showing a finite component along $\bm{K}-\bm{\Gamma}$ that sensitively determines $q^{*}$. We revoke the notion of 'inner' and 'outer' processes. Our findings explain the splitting of the 2D mode of graphene under uniaxial tensile strain. The splitting originates from a strain-induced distortion of the phonon dispersion; changes in the electronic band structure and resonance conditions are negligeable for the 2D Raman spectrum.
    02/2011;
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    ABSTRACT: The thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For Si(x)Ge(1-x), more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1   μm, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying. Intriguingly, mass disorder is found to increase the anharmonic scattering of phonons through a modification of their vibration eigenmodes, resulting in an increase of 15% in thermal resistivity.
    Physical Review Letters 01/2011; 106(4):045901. · 7.94 Impact Factor
  • Nicola Bonini, Jivtesh Garg, Nicola Marzari
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    ABSTRACT: Carbon nanostructures, such as graphene and carbon nanotubes, are particularly promising materials for thermal management applications because of their very high thermal conductivity. A clear understanding of the transport properties of these materials is a key step in view of their possible integration into future devices. Here, we present a first-principles study of the thermal transport properties of graphene. We use density-functional theory and density-functional perturbation theory to determine both harmonic and cubic anharmonic terms in the crystalline potential---the key ingredients to calculate phonon frequencies and phonon lifetimes. Our results show that the long-wavelenght longitudinal and transverse in-plane acoustic phonon modes of graphene have an anomalously small lifetime, that leads to a significant underestimation of the thermal conductivity computed within the single mode relaxation time approximation. We will discuss the effect of strain and graphene-substrate interaction on the lifetime of the acoustic modes, and we will present results for the thermal conductivity determined by directly solving the linearized Boltzmann transport equation.
    03/2010;
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    ABSTRACT: Phonon-mediated el-el interactions are the microscopic basis of low temperature superconductivity. The Eliashberg-Migdal theory formulates the problem in terms of el-ph matrix elements and corresponding el-ph linewidths. In principle, ab-initio DFT codes based on pseudo-potentials and plane-waves can provide an accurate prediction of the el-ph linewidth spectrum over the entire Brillouin zone in bulk materials. In practice, fully converged calculations are often unattainable because of the required dense samplings of the Fermi surfaces and limitations in the available CPU time. In many cases, the reformulation of the el-ph matrix problem using maximally localized Wannier functions reduces the time-consuming part of the calculation to the determination of the phonon frequencies via linear perturbation DFT and leads to a fully-physical based interpolation technique. Here, we present results on the superconducting critical temperature Tc of Al and Nb crystals under various mechanically-strained configurations and show that even in such simple bulk systems the use of the Wannier basis approach is necessary to ensure accurate and reasonably fast calculations.
    03/2010;
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    ABSTRACT: QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
    Journal of Physics Condensed Matter 08/2009; 21(39):395502. · 2.36 Impact Factor
  • Nicola Bonini, Nicola Marzari
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    ABSTRACT: Low-frequency phonon modes play an important role in the electronic and thermal transport properties of carbon nanotubes and ultrathin graphitic films. Not only they determine the very high thermal conductivity of these materials, but they also affect the electrical transport: at low bias they weakly scatter electrons, while at high bias they concur to determine the population of those optical phonon modes that most strongly limit the electrical conductivity. Quite interestingly, these low frequency phonons are also expected to couple to the vibrational modes of a surrounding medium more efficiently than high frequency phonons, providing an effective channel for the exchange of vibrational energy between the nanostructure and the environment. Here we use density functional theory and density functional perturbation theory to characterize the inelastic relaxation mechanisms---phonon-phonon and electron-phonon interactions---that determine the lifetime of these phonon modes. We will discuss the relevance of these results to estimate the transport properties of carbon nanomaterials.
    03/2009;
  • Jivtesh Garg, Nicola Bonini, Nicola Marzari
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    ABSTRACT: Thermoelectric materials will become commercially viable for converting heat into electricity and for refrigeration once their figure of merit (ZT) is improved. One key approach to increase performance is to reduce thermal conductivity - e.g. in alloys it is lower than the binary endpoints due to increased scattering induced by strain and disorder. Understanding the thermal conductivity of complex materials is also important in other applications from reducing hot-spot temperatures in electronic chips to better thermal-insulation materials. Here, we have calculated the thermal conductivity of silicon-germanium alloys using ab-initio density functional perturbation theory. The electronic structure of the alloy is studied with the virtual crystal approximation and the single mode relaxation time approximation; perturbation theory up to the third order provides phonon lifetimes, and disorder effects are taken into account by ensemble averages over configurations with random mass disorder. We find that first-principles calculations lead to excellent qualitative agreement with experiments. The thermal conductivity of Si/Ge superlattices has been measured to be lower than Si/Ge alloys. Here we present the first principle calculations of the thermal conductivity of Si/Ge superlattices as a function of layer thickness.
    03/2009;
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    ABSTRACT: In the past 20 years, efforts have been devoted to predict the critical current density Jc of superconducting magnets based on the Nb3Sn compound. The use of Nb3Sn magnets for high-field applications has highlighted the dependence of Jc on strain. We present calculations of the Tc-dependence of Al and Nb crystals on pressure, uni-axial and shear strains using the DFT PWscf package from the Quantum-ESPRESSO distribution to evaluate the phonon linewidth and the el-ph coupling parameter using very dense k-space samplings of the IBZ. The superconducting critical Tc is calculated by using the McMillan formula as a fit to the solution of the Migdal-Eliashberg equations. Favourable comparisons with available experimental data have been obtained and will be presented. The modelling of the Tc-dependence on strain in Nb3Sn crystals is an ongoing effort. The potential for modelling the Tc-dependence on strain in Nb3Sn is discussed. In this regard, recent advances in the implementation of the Wannier formalism give access to the sampling of the dense k-point grids required to calculate fully-converged electron-phonon coupling quantities. This approach opens the possibility to extend the study of the Tc-dependence on strain to unit cells characterized by a higher number of atoms or electronic complexity.
    03/2009;
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    ABSTRACT: We uncover the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E[subscript 2g] optical mode splits in two components: one polarized along the strain and the other perpendicular. This splits the G peak into two bands, which we call G+ and G−, by analogy with the effect of curvature on the nanotube G peak. Both peaks redshift with increasing strain and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The 2D and 2D′ bands also redshift but do not split for small strains. We study the Grüneisen parameters for the phonons responsible for the G, D, and D′ peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance University of Palermo Sultan Qaboos University MITRE Interconnect Focus Center European Research Council Royal Society
    Physical review. B, Condensed matter 01/2009; · 3.77 Impact Factor
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    ABSTRACT: The anharmonic properties of low-dimensional carbon crystal lattices are reviewed. The energy and crystal momentum conservation rules in two- and one-dimensional crystals lead to a drastic reduction of the phase space available for anharmonic phonon decay. This is illustrated with first principles calculations of the anharmonic properties of graphite and graphene. Experimental Raman linewidth data for the Radial Breathing Mode (RBM) in suspended single-walled carbon nanotubes are also interpreted in terms of a simple model in which a phonon decay bottleneck induced by the low dimensionality leads to a population time dependence in which a fast initial decay is followed by a slow decay determined by the decay rate of a large population of secondary phonons. These results are key to understanding the combined dynamics of electrons and phonons that determines the electrical transport properties in low-dimensional carbon nanostructures. In the case of the RBM in carbon nanotubes, they raise the intriguing possibility of using the linewidth of the Raman peak to determine the chirality of the nanotube. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (b) 09/2008; 245(10):2149 - 2154. · 1.49 Impact Factor

Publication Stats

818 Citations
205.81 Total Impact Points

Institutions

  • 2012
    • Northeast Institute of Geography and Agroecology
      • State Key Laboratory for Superlattices and Microstructures
      Beijing, Beijing Shi, China
    • King's College London
      • Department of Physics
      London, ENG, United Kingdom
  • 2011–2012
    • University of Oxford
      • Department of Materials
      Oxford, ENG, United Kingdom
  • 2006–2011
    • Massachusetts Institute of Technology
      • • Department of Mechanical Engineering
      • • Department of Materials Science and Engineering
      Cambridge, MA, United States
  • 2003–2006
    • Scuola Internazionale Superiore di Studi Avanzati di Trieste
      Trst, Friuli Venezia Giulia, Italy
  • 2001–2003
    • Università degli Studi di Milano-Bicocca
      • Department of Materials Science
      Milano, Lombardy, Italy
  • 2000
    • University of Milan
      • Department of Physics
      Milano, Lombardy, Italy