Cornell NanoScale Facility Papers, Research and Monographs

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https://hdl.handle.net/1813/3129

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.
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.
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.
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.
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.
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.
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.
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.
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.
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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