Cornell NanoScale Facility Papers, Research and Monographs

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This is a collection of papers, research and monographs for the Cornell NonScale Facility.


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    Polar Effects on the Thermal Conductivity of Cubic Boron Nitride under Pressure
    Mukhopadhyay, Saikat; Stewart, Derek (American Physical Society, 2014-07-07)
    We report the lattice thermal conductivity (κ) of cubic boron nitride (c-BN) under pressure calculated using density functional theory. Pressure was used to manipulate the c-BN phonon dispersion and study its effect on thermal conductivity. These results were compared to c-BN’s mass-equivalent, nonpolar counterpart, diamond, in order to isolate the effect of polar bonds on thermal conductivity. Unlike diamond, the variation of κ at room temperature (κRT) with applied pressure in c-BN is nonlinear in the low pressure regime followed by a transition to a linear regime with a distinct change in the slope at P > 114 GPa. We find that the change in κ with pressure cannot be described with power law expressions commonly used for Earth mantle materials. The nonlinearity in the low-pressure regime can be related to the nonlinear change in LO-TO gap, group velocities, and specific heat with increasing pressure. In addition, we find that, although optical branch contributions to thermal conductivity are small (∼2% at RT), the rise in κRT for P > 114 GPa is due to (1) the decoupling of the longitudinal acoustic branch from the optical branches and (2) depopulation of the optical branches. These lead to a sharp reduction in acoustic-acoustic-optic (a-a-o) scattering and a discrete change in the acoustic phonon mean free paths. This study illustrates the importance of optical branches and their interactions with acoustic branches in determining the total thermal conductivity of polar materials. This finding is also relevant for current research in geologic minerals under pressure and the design of thermoelectrics.
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    Phonon Transport in Isotope-Disordered Carbon and Boron-Nitride Nanotubes: Is Localization Observable?
    Savic, Ivana; Mingo, Natalio; Stewart, Derek (American Physical Society, 2008-10-13)
    We present an ab initio study which identifies dominant effects leading to thermal conductivity reductions in carbon and boron-nitride nanotubes with isotope disorder. Our analysis reveals that, contrary to previous speculations, localization effects cannot be observed in the thermal conductivity measurements. Observable reduction of the thermal conductivity is mostly due to diffusive scattering. Multiple scattering induced interference effects were found to be prominent for isotope concentrations > 10%; otherwise, the thermal conduction is mainly determined by independent scattering contributions of single isotopes. We give explicit predictions of the effect of isotope disorder on nanotube thermal conductivity that can be directly compared with experiments.
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    Thermal conduction mechanisms in boron nitride nanotubes: Few-shell versus all-shell conduction
    Savic, Ivana; Stewart, Derek; Mingo, Natalio (American Physical Society, 2008-12-29)
    It has been argued that the experimentally observed limitation of heat transport through boron nitride nanotubes is due to intershell scattering rather than to inefficient heat transfer to inner shells. Using an atomistic Green’s function calculation, we present evidence that on the contrary, intershell or any other type of scattering along the nanotubes is not the main limiting mechanism, and heat conduction restricted to a few layers is responsible for the low thermal conductivities experimentally measured. Our results also indicate that anharmonic scattering in boron nitride is relatively weak, which might lead to considerably larger thermal conductivity for well-contacted nanotubes than previously reported.
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    Contribution of d-band electrons to ballistic transport and scattering during electron-phonon nonequilibrium in nanoscale Au films using an ab initio density of states
    Hopkins, Patrick; Stewart, Derek (American Institute of Physics (AIP), 2009-09-08)
    Electron-interface scattering during electron-phonon nonequilibrium in thin films creates another pathway for electron system energy loss as characteristic lengths of thin films continue to decrease. As power densities in nanodevices increase, excitations of electrons from sub-conduction-band energy levels will become more probable. These sub-conduction-band electronic excitations significantly affect the material’s thermophysical properties. In this work, the role of d-band electronic excitations is considered in electron energy transfer processes in thin Au films. The electronic structure and density of states for gold are calculated using a plane wave pseudopotential density function approach. In thin films with thicknesses less than the electron mean free path, ballistic electron transport leads to electron-interface scattering. The ballistic component of electron transport is studied by a ballistic-diffusive approximation of the Boltzmann transport equation with input from ab initio calculations. The effects of d-band excitations on electron-interface energy transfer are analyzed during electron-phonon nonequilibrium after short pulsed laser heating in thin films.
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    First principles study of the phonon dispersion and dielectric properties of wurtzite InP: Role of In 4d electrons
    Mukhopadhyay, Saikat; Stewart, Derek (American Physical Society, 2014-02-13)
    Although wurtzite InP nanowires have recently been grown, an accurate description of the wurtzite InP phonon dispersion is still missing. We calculate the ab initio phonon dispersion of wurtzite and zinc-blende InP using density-functional perturbation theory and a real space supercell approach. Our predicted optical phonon frequencies agree well with measured Raman data from InP nanowires. We find that treating In 4d electrons as valence electrons is required to accurately describe InP lattice dynamics and dielectric constants, but including spin-orbit coupling has little effect. We also compare the sound velocities and specific heat and find that any difference in the thermal conductivity of InP polytypes should be due to differences in phonon-scattering rates.
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    Ultrafast thermoelectric properties of gold under conditions of strong electron-phonon nonequilibrium
    Hopkins, Patrick; Bauer, Matthew; Duda, John; Smoyer, Justin; English, Timothy; Norris, Pamela; Beechem, Thomas; Stewart, Derek (American Institute of Physics (AIP), 2010-11-23)
    The electronic scattering rates in metals after ultrashort pulsed laser heating can be drastically different than those predicted from free electron theory. The large electron temperature achieved after ultrashort pulsed absorption and subsequent thermalization can lead to excitation of subconduction band thermal excitations of electron orbitals far below the Fermi energy. In the case of noble metals, which all have a characteristic flat d-band several electron volts well below the Fermi energy, the onset of d-band excitations has been shown to increase electron-phonon scattering rates by an order of magnitude. In this paper, we investigate the effects of these large electronic thermal excitations on the ultrafast thermoelectric transport properties of gold, a characteristic noble metal. Under conditions of strong electron-phonon nonequilibrium (relatively high electron temperatures and relatively low lattice temperatures, T_{e} >> T_{L}), we find that the Wiedemann–Franz law breaks down and the Seebeck coefficient is massively enhanced. Although we perform representative calculations for Au, these results are expected to be similar for the other noble metals (Ag and Cu) due to the characteristic large d-band separation from the Fermi energy.
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    Thermal conductivity of bulk and nanowire Mg2Si_{x}Sn_{1-x} alloys from first principles
    Li, Wu; Lindsay, Lucas; David, Broido; Stewart, Derek; Mingo, Natalio (American Physical Society, 2012-11-29)
    The lattice thermal conductivity (κ) of the thermoelectric materials, Mg2Si, Mg2Sn, and their alloys, are calculated for bulk and nanowires, without adjustable parameters. We find good agreement with bulk experimental results. For large nanowire diameters, size effects are stronger for the alloy than for the pure compounds. For example, in 200 nm diameter nanowires κ is lower than its bulk value by 30%, 20%, and 20% for Mg2Si0.6Sn0.4, Mg2Si, and Mg2Sn, respectively. For nanowires less than 20 nm thick, the relative decrease surpasses 50%, and it becomes larger in the pure compounds than in the alloy. At room temperature, κ of Mg2Si_{x}Sn_{1−x} is less sensitive to nanostructuring size effects than Si_{x}Ge_{1−x}, but more sensitive than PbTe_{x}Se_{1−x}. This suggests that further improvement of Mg2Si_{x}Sn_{1−x} as a nontoxic thermoelectric may be possible.
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    Chemical ordering in Cr3Al and relation to semiconducting behavior
    Boekelheide, Zoe; Stewart, Derek; Hellman, Frances (American Physical Society, 2012-08-15)
    Cr3Al shows semiconductor-like behavior which has been attributed to a combination of antiferromagnetism and chemical ordering of the Cr and Al atoms on the bcc sublattice. This article presents a detailed theoretical and experimental study of the chemical ordering in Cr3Al. Using density functional theory within the Korringa-Kohn-Rostoker (KKR) formalism, we consider five possible structures with the Cr3Al stoichiometry: a bcc solid solution, two-phase C11b Cr2Al+Cr, off-stoichiometric C11b Cr3Al, D03 Cr3Al, and X-phase Cr3Al. The calculations show that the chemically ordered, rhombohedrally distorted X-phase structure has the lowest energy of those considered and should, therefore, be the ground state found in nature, while the D03 structure has the highest energy and should not occur. While KKR calculations of the X phase indicate a pseudogap in the density of states, additional calculations using a full potential linear muffin-tin orbital approach and a plane-wave technique show a narrow band gap. Experimentally, thin films of Cr(1−x)Alx were grown and the concentration, growth temperature, and substrate were varied systematically. The peak resistivity (2400 μΩ-cm) is found for films with x=0.25, grown epitaxially on a 300 ∘C MgO substrate. At this x, a transition between nonmetallic and metallic behavior occurs at a growth temperature of about 400 ∘C, which is accompanied by a change in chemical ordering from X phase to C11b Cr3Al. These results clarify the range of possible structures for Cr3Al and the relationship between chemical ordering and electronic transport behavior.
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    Antiferromagnetism in Cr3Al and relation to semiconducting behavior
    Boekelheide, Zoe; Saerbeck, Thomas; Stampfl, Anton; Robinson, Robert; Stewart, Derek; Hellman, Frances (American Physical Society, 2012-03-09)
    Antiferromagnetism and chemical ordering have both been previously suggested as causes of the observed semiconductorlike behavior in Cr3Al. Two films of Cr3Al(001)/MgO(001) were grown under different conditions to achieve different types of chemical ordering and electronic properties: one X-phase structure (semiconducting) and one C11b structure (metallic). The films were investigated by x-ray and neutron diffraction. Both films show commensurate antiferromagnetic order, with a high Néel temperature greater than 578 K, showing that the antiferromagnetism in Cr3Al is quite robust. Density-functional theory calculations were performed and it was shown that the well-known antiferromagnetic pseudogap in the density of states occurs for all types of chemical ordering considered. The conclusion of these studies is that the antiferromagnetism causes a pseudogap in the density of states, which is a necessary condition for the semiconductorlike transport behavior; however, that antiferromagnetism is seen in both metallic and semiconducting Cr3Al samples shows that antiferromagnetism is not a sufficient condition for semiconducting behavior. Chemical ordering is equally important.
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    Thermal conductivity of diamond nanowires from first principles
    Li, Wu; Mingo, Natalio; Lindsay, Lucas; Broido, David; Stewart, Derek; Katcho, Nebil (American Physical Society, 2012-05-17)
    Using ab initio calculations we have investigated the thermal conductivity (k) of diamond nanowires, unveiling unusual features unique to this system. In sharp contrast with Si, k(T) of diamond nanowires as thick as 400 nm still increase monotonically with temperature up to 300 K, and room-temperature size effects are stronger than for Si. A marked dependence of k on the crystallographic orientation is predicted, which is apparent even at room temperature. [001] growth direction always possesses the largest k in diamond nanowires. The predicted features point to a potential use of diamond nanowires for the precise control of thermal flow in nanoscale devices.
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    Role of light and heavy embedded nanoparticles on the thermal conductivity of SiGe alloys
    Kundu, Anupam; Mingo, Natalio; Broido, David; Stewart, Derek (American Physical Society, 2011-09-09)
    We have used an atomistic ab initio approach with no adjustable parameters to compute the lattice thermal conductivity of Si0.5Ge0.5 with a low concentration of embedded Si or Ge nanoparticles of diameters up to 4.4 nm. Through exact Green's function calculation of the nanoparticle scattering rates, we find that embedding Ge nanoparticles in Si0.5Ge0.5 provides 20% lower thermal conductivities than embedding Si nanoparticles. This contrasts with the Born approximation, which predicts an equal amount of reduction for the two cases, irrespective of the sign of the mass difference. Despite these differences, we find that the Born approximation still performs remarkably well, and it permits investigation of larger nanoparticle sizes, up to 60 nm, not feasible with the exact approach.
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    Thermal conductivity of indium arsenide nanowires with wurtzite and zinc blende phases
    Zhou, Feng; Moore, Arden; Bolinsson, Jessica; Persson, Ann; Froberg, Linus; Pettes, Michael; Kong, Huijun; Rabenberg, Lew; Caroff, Philippe; Stewart, Derek; Mingo, Natalio; Dick, Kimberly; Samuelson, Lars; Linke, Heiner; Shi, Li (American Physical Society, 2011-05-19)
    The thermal conductivity of wurtzite and zinc blende indium arsenide nanowires was measured using a microfabricated device, with the crystal structure of each sample controlled during growth and determined by transmission electron microscopy. Nanowires of both phases showed a reduction of thermal conductivity by a factor of 2 or more compared to values reported for zinc blende indium arsenide bulk crystals within the measured temperature range. Theoretical models were developed to analyze the measurement results and determine the effect of phase on phonon transport. Branch-specific phonon dispersion data within the discretized first Brillouin zone were calculated from first principles and used in numerical models of volumetric heat capacity and thermal conductivity. The combined results of the experimental and theoretical studies suggest that wurtzite indium arsenide possesses similar volumetric heat capacity, weighted average group velocity, weighted average phonon-phonon scattering mean free path, and anharmonic scattering-limited thermal conductivity as the zinc blende phase. Hence, we attribute the differing thermal conductivity values observed in the indium arsenide nanowires of different phases to differences in the surface scattering mean free paths between the nanowire samples.
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    Band Gap and Electronic structure of an Epitaxial, Semiconducting Cr0.80Al0.20 Thin Film
    Boekelheide, Z.; Gray, A. X.; Papp, C.; Balke, B.; Stewart, D. A.; Ueda, S.; Kobayashi, K.; Hellman, F.; Fadley, C. S. (American Physical Society, 2010-12-03)
    Cr(1-x)Alx exhibits semiconducting behavior for x=0.15-0.26. This Letter uses hard x-ray photoemission spectroscopy and density functional theory to further understand the semiconducting behavior. Photoemission measurements of an epitaxial Cr0.80Al0.20 thin film show several features in the band region, including a gap at the Fermi energy (Ef) for which the valence band edge is 95 +- 14 meV below Ef. Theory agrees well with the valence band measurements, and shows an incomplete gap at Ef due to the hole band at M shifting almost below Ef.
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    Cluster scattering effects on phonon conduction in graphene
    Mingo, Natalio; Esfarjani, Keivan; Broido, David; Stewart, Derek (American Physical Society, 2010-01-13)
    The phonon-scattering cross section associated with isotopic clusters is evaluated from first principles and used to estimate the reduction in thermal conductance of wide graphene samples. A strong sensitivity of the thermal conductivity toward clustering is predicted for micrometer-sized samples at low-temperatures. Important differences are obtained between the atomistically computed cross section, and existing analytical approximations, emphasizing the importance of atomistic investigations of thermal transport. Finally, possible techniques are suggested for synthesizing graphene containing isotopic clusters.
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    Ab initio theory of the lattice thermal conductivity in diamond
    Ward, Alistair; Broido, David; Stewart, Derek; Deinzer, Gernot (American Physical Society, 2009-09-16)
    We present a first-principles theoretical approach to calculate the lattice thermal conductivity of diamond based on an exact solution of the Boltzmann transport equation. Density-functional perturbation theory is employed to generate the harmonic and thir-order anharmonic interatomic force constants that are required as input. A central feature of this approach is that it provides accurate representations of the interatomic forces and at the same time introduced no adjustable parameters. The calculated lattice thermal conductivities for isotopically enriched and naturally occurring diamond are both in very good agreement with experimental data. The role of the scattering of heat-carrying acoustic phonons by optic branch phonons is also investigated. We show that inclusion of this scattering channel is indispensable in properly describing the thermal conductivity of semiconductors and insulators. The accurate adjustable-parameter free results obtained herein highlight the promise of this approach in providing predictive descriptions of the lattice thermal conductivity of materials.
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    Ab initio investigation of phonon dispersion and anomalies in palladium
    Stewart, Derek (IOP Publishing Ltd and Deutsch Physikalische Gesellschaft, 2008-04-14)
    In recent years, palladium has proven to be a crucial component for devices ranging from nanotube field effect transistors to advanced hydrogen storage devices. In this work, I examine the phonon dispersion of fcc Pd using first principle calculations based on density functional perturbation theory (DFPT). While several groups in the past have studied the acoustic properties of palladium, this is the first study to reproduce the full phonon dispersion and associated anomaly in the [110]-direction with high accuracy and no adjustable parameters. I will show that the [110] anomaly is a Kohn anomaly due to electron-phonon interactions and that paramagnons play no significant role in the [110] phonon dispersion.
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    Phonon transmission through defects in carbon nanotubes from first principles
    Mingo, Natalio; Stewart, Derek A; Broido, David A; Srivastava, Deepak (American Physical Society, 2008-01-30)
    We compute the effect of different isolated defects on the phonon transmission through carbon nanotubes, using an ab initio density functional approach. The problem of translational and rotational invariance fulfillment in the nonperiodic system is solved via a Lagrange-multiplier symmetrization technique. The need for an ab initio approach is illustrated for the case of phonon transmission through a nitrogen substitutional impurity, for which no reliable empirical interatomic potentials exist. This opens an avenue for the accurate parameterfree study of phonon transport through general systems with arbitrary composition and structure, without any need for semiempirical potential descriptions.
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    Ab-initio study of polarizability and induced charge densities in multilayer graphene films
    Yu, Eric; Stewart, Derek; Tiwari, Sandip (American Physical Society, 2008-05-06)
    We present an ab initio analysis of polarization of multilayer graphene systems under applied electric fields. The effects of applied electric fields are calculated using a Berry phase approach within a plane-wave density functional formalism. We have determined polarizability values for graphene films and carbon nanotubes and found that the polarizability of graphene films follows a linear relationship with the number of layers. We also examined changes in the induced charge distribution as a function of graphene layers. We focus, in particular, on the bilayer graphene system. Under applied electric fields, we found the Mexican hat band structure near the K point reported by previous groups. We found that the induced charge primarily accumulated on the B sublattice sites. This observation is supported by additional calculations with a tight-binding Green's function model. By examining the local density of states at the Fermi energy, we found a high density of states at the B sites at the Fermi energy. In contrast, coupling between A sites in neighboring graphene layers leads to negligible density of states at the Fermi level. This high density of states at the B sites results in greater induced charge under applied electric fields. This scenario of preferential induced charge on the B sublattice sites under applied electric fields could impact the stability of atoms and molecules absorbed on bilayer graphene.
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    Intrinsic lattice thermal conductivity of semiconductors from first principles
    Broido, David; Malorny, Michael; Birner, Gerd; Mingo, Natalio; Stewart, Derek (American Institute of Physics, 2007-12-07)
    We present an ab initio theoretical approach to accurately describe phonon thermal transport in semiconductors and insulators free of adjustable parameters. This technique combines a Boltzmann transport formalism with density functional calculations of harmonic and anharmonic interatomic force constants. Without any fitting parameters, we obtain excellent agreement (<5% difference at room temperature) between calculated and measured intrinsic lattice thermal conductivities of silicon and germanium. As such, this method may provide predictive theoretical guidance to experimental thermal transport studies of bulk and nanomaterials as well as facilitating the design of new materials.
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    Magnetism in Coaxial Palladium Nanowires
    Stewart, Derek (American Institute of Physics, 2007-03-21)
    While bulk palladium is non-magnetic, several recent studies have shown that magnetism can occur in hcp Pd films, fcc twinned Pd nanoparticles, and Pd atomic strands. These studies show that small changes in Pd atomic configurations can induce magnetic properties. In this work, we examine coaxial palladium nanowires in an effort to determine the most stable configurations and their magnetic properties. Relaxed nanowire structures are found using density functional calculations in the plane wave basis. In several metallic systems such as gold, coaxial nanowires have proven to be the lowest energy configuration for ultrathin (< 1 nm) nanowires. We consider magnetism in these structures as a function of diameter and coaxial configuration. These results are also compared to fcc and hcp Pd nanowires of comparable diameters. We find that the (6,0) coaxial nanowire provides the most stable structure and exhibits a magnetic moment of 0.278 uB/atom.