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Luis Artus - One of the best experts on this subject based on the ideXlab platform.
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High-Pressure Softening of the Out-of-Plane A2u(Transverse-Optic) Mode of Hexagonal Boron Nitride Induced by Dynamical Buckling
Journal of Physical Chemistry C, 2019Co-Authors: A Segura, Ramon Cusco, Guillaume Cassabois, Bernard Gil, T. Taniguchi, Kanji Watanabe, Luis ArtusAbstract:We investigate the highly anisotropic behavior of the in-plane and out-of-plane infrared-active phonons of hexagonal boron nitride by means of infrared reflectivity and absorption measurements under high pressure. Infrared reflectivity spectra at normal incidence on high-quality single Crystals show strict fulfillment of selection rules and an unusually long E1u[transverse-optic (TO)] phonon lifetime. Accurate values of the dielectric constants at ambient pressure ε⊥0 = 6.96, ε⊥∞ = 4.95, ε∥0 = 3.37, and ε∥∞ = 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A2u phonon reflectivity band is revealed in measurements on an inclined facet, and absorption measurements at an incidence angle of 30° allow us to observe both the transverse- and longitudinal-optic A2u modes. Pressure coefficients and Grüneisen parameters for all infrared-active modes are determined and compared with ab initio calculations. While Grüneisen parameters are generally small in this Layered Crystal, the A2u(TO) displays an exceptionally large and negative Grüneisen parameter that results in widening of the type I hyperbolic region with increasing pressure. Softening of the A2u(TO) mode is induced by dynamical buckling of the flat honeycomb layers.
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Effects of isotopic substitution on the phonons of van der Waals Crystals: the case of hexagonal boron nitride
Journal of Physics D: Applied Physics, 2019Co-Authors: Ramon Cusco, Song Liu, Guillaume Cassabois, Bernard Gil, J. Edgar, Luis ArtusAbstract:The recent availability of isotopically pure samples of hexagonal boron nitride (h-BN) has allowed the isotopic substitution effects to be studied in this highly interesting Layered Crystal. Here, we review the application of Raman scattering to investigate phonon anharmonic decay and its particularities in Layered Crystals, exemplified by h-BN. The modification of the phonon spectrum and anharmonic phonon decay paths in isotopically pure samples is specifically addressed. Detailed information about phonon lifetimes and decay channels is obtained for h-BN from a thorough analysis of temperature-dependent Raman scattering measurements in the light of density functional theory and perturbation theory calculations. The phonon lifetime is substantially increased in isotopically pure Crystals, which may have important implications for the development of low-loss h-BN based phonon-polariton devices. On account of the low cation mass, isotopic substitution substantially alters the dominant phonon decay pathways and changes the strength of phonon anharmonic interactions.
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Isotopic effects on phonon anharmonicity in Layered van der Waals Crystals: Isotopically pure hexagonal boron nitride
Physical Review B: Condensed Matter and Materials Physics, 2018Co-Authors: Ramon Cusco, Luis Artus, James H. Edgar, Song Liu, Guillaume Cassabois, Bernard GilAbstract:Hexagonal boron nitride (h-BN) is a Layered Crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this Crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in Layered Crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state 10BN neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-Crystal h-BN. Phonon frequency and lifetime are measured in the 80–600-K temperature range for 10B-enriched, 11B-enriched, and natural composition high quality Crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of Crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in 10B-enriched Crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.
Bernard Gil - One of the best experts on this subject based on the ideXlab platform.
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High-Pressure Softening of the Out-of-Plane A2u(Transverse-Optic) Mode of Hexagonal Boron Nitride Induced by Dynamical Buckling
Journal of Physical Chemistry C, 2019Co-Authors: A Segura, Ramon Cusco, Guillaume Cassabois, Bernard Gil, T. Taniguchi, Kanji Watanabe, Luis ArtusAbstract:We investigate the highly anisotropic behavior of the in-plane and out-of-plane infrared-active phonons of hexagonal boron nitride by means of infrared reflectivity and absorption measurements under high pressure. Infrared reflectivity spectra at normal incidence on high-quality single Crystals show strict fulfillment of selection rules and an unusually long E1u[transverse-optic (TO)] phonon lifetime. Accurate values of the dielectric constants at ambient pressure ε⊥0 = 6.96, ε⊥∞ = 4.95, ε∥0 = 3.37, and ε∥∞ = 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A2u phonon reflectivity band is revealed in measurements on an inclined facet, and absorption measurements at an incidence angle of 30° allow us to observe both the transverse- and longitudinal-optic A2u modes. Pressure coefficients and Grüneisen parameters for all infrared-active modes are determined and compared with ab initio calculations. While Grüneisen parameters are generally small in this Layered Crystal, the A2u(TO) displays an exceptionally large and negative Grüneisen parameter that results in widening of the type I hyperbolic region with increasing pressure. Softening of the A2u(TO) mode is induced by dynamical buckling of the flat honeycomb layers.
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Effects of isotopic substitution on the phonons of van der Waals Crystals: the case of hexagonal boron nitride
Journal of Physics D: Applied Physics, 2019Co-Authors: Ramon Cusco, Song Liu, Guillaume Cassabois, Bernard Gil, J. Edgar, Luis ArtusAbstract:The recent availability of isotopically pure samples of hexagonal boron nitride (h-BN) has allowed the isotopic substitution effects to be studied in this highly interesting Layered Crystal. Here, we review the application of Raman scattering to investigate phonon anharmonic decay and its particularities in Layered Crystals, exemplified by h-BN. The modification of the phonon spectrum and anharmonic phonon decay paths in isotopically pure samples is specifically addressed. Detailed information about phonon lifetimes and decay channels is obtained for h-BN from a thorough analysis of temperature-dependent Raman scattering measurements in the light of density functional theory and perturbation theory calculations. The phonon lifetime is substantially increased in isotopically pure Crystals, which may have important implications for the development of low-loss h-BN based phonon-polariton devices. On account of the low cation mass, isotopic substitution substantially alters the dominant phonon decay pathways and changes the strength of phonon anharmonic interactions.
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Isotopic effects on phonon anharmonicity in Layered van der Waals Crystals: Isotopically pure hexagonal boron nitride
Physical Review B: Condensed Matter and Materials Physics, 2018Co-Authors: Ramon Cusco, Luis Artus, James H. Edgar, Song Liu, Guillaume Cassabois, Bernard GilAbstract:Hexagonal boron nitride (h-BN) is a Layered Crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this Crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in Layered Crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state 10BN neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-Crystal h-BN. Phonon frequency and lifetime are measured in the 80–600-K temperature range for 10B-enriched, 11B-enriched, and natural composition high quality Crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of Crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in 10B-enriched Crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.
Ramon Cusco - One of the best experts on this subject based on the ideXlab platform.
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High-Pressure Softening of the Out-of-Plane A2u(Transverse-Optic) Mode of Hexagonal Boron Nitride Induced by Dynamical Buckling
Journal of Physical Chemistry C, 2019Co-Authors: A Segura, Ramon Cusco, Guillaume Cassabois, Bernard Gil, T. Taniguchi, Kanji Watanabe, Luis ArtusAbstract:We investigate the highly anisotropic behavior of the in-plane and out-of-plane infrared-active phonons of hexagonal boron nitride by means of infrared reflectivity and absorption measurements under high pressure. Infrared reflectivity spectra at normal incidence on high-quality single Crystals show strict fulfillment of selection rules and an unusually long E1u[transverse-optic (TO)] phonon lifetime. Accurate values of the dielectric constants at ambient pressure ε⊥0 = 6.96, ε⊥∞ = 4.95, ε∥0 = 3.37, and ε∥∞ = 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A2u phonon reflectivity band is revealed in measurements on an inclined facet, and absorption measurements at an incidence angle of 30° allow us to observe both the transverse- and longitudinal-optic A2u modes. Pressure coefficients and Grüneisen parameters for all infrared-active modes are determined and compared with ab initio calculations. While Grüneisen parameters are generally small in this Layered Crystal, the A2u(TO) displays an exceptionally large and negative Grüneisen parameter that results in widening of the type I hyperbolic region with increasing pressure. Softening of the A2u(TO) mode is induced by dynamical buckling of the flat honeycomb layers.
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Effects of isotopic substitution on the phonons of van der Waals Crystals: the case of hexagonal boron nitride
Journal of Physics D: Applied Physics, 2019Co-Authors: Ramon Cusco, Song Liu, Guillaume Cassabois, Bernard Gil, J. Edgar, Luis ArtusAbstract:The recent availability of isotopically pure samples of hexagonal boron nitride (h-BN) has allowed the isotopic substitution effects to be studied in this highly interesting Layered Crystal. Here, we review the application of Raman scattering to investigate phonon anharmonic decay and its particularities in Layered Crystals, exemplified by h-BN. The modification of the phonon spectrum and anharmonic phonon decay paths in isotopically pure samples is specifically addressed. Detailed information about phonon lifetimes and decay channels is obtained for h-BN from a thorough analysis of temperature-dependent Raman scattering measurements in the light of density functional theory and perturbation theory calculations. The phonon lifetime is substantially increased in isotopically pure Crystals, which may have important implications for the development of low-loss h-BN based phonon-polariton devices. On account of the low cation mass, isotopic substitution substantially alters the dominant phonon decay pathways and changes the strength of phonon anharmonic interactions.
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Isotopic effects on phonon anharmonicity in Layered van der Waals Crystals: Isotopically pure hexagonal boron nitride
Physical Review B: Condensed Matter and Materials Physics, 2018Co-Authors: Ramon Cusco, Luis Artus, James H. Edgar, Song Liu, Guillaume Cassabois, Bernard GilAbstract:Hexagonal boron nitride (h-BN) is a Layered Crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this Crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in Layered Crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state 10BN neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-Crystal h-BN. Phonon frequency and lifetime are measured in the 80–600-K temperature range for 10B-enriched, 11B-enriched, and natural composition high quality Crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of Crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in 10B-enriched Crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.
Ali Erdemir - One of the best experts on this subject based on the ideXlab platform.
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Formation of ultralow friction surface films on boron carbide
Applied Physics Letters, 1996Co-Authors: Ali Erdemir, Cuma Bindal, George R. FenskeAbstract:In this letter, we describe the formation and ultralow friction mechanisms of a surface film on boron carbide (B4C). This film results from sequential reactions between B4C and oxygen and between the resulting boron oxide (B2O3) and moisture; it can afford friction coefficients of 0.03 to 0.05 to sliding steel surfaces. At temperatures above 600 °C, B4C undergoes oxidation and forms a layer of boron oxide (B2O3) in the upper surface. During cooling to room temperature, the B2O3 reacts with moisture in the air to form a secondary film, boric acid (H3BO3). The sliding friction coefficient of 440C steel balls against this film is 0.04, compared to 0.7 against the bare B4C surfaces. Mechanistically, we propose that the ultralow friction behavior of the heat‐treated B4C surface is due mainly to the Layered‐Crystal structure of the H3BO3 film that forms on the sliding surface.
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Ultralow friction behavior of borided steel surfaces after flash annealing
Applied Physics Letters, 1996Co-Authors: Cuma Bindal, Ali ErdemirAbstract:In this letter, we describe the ultralow friction mechanism of borided steel surfaces subjected to a short‐duration, or ‘‘flash,’’ annealing procedure. In this procedure, a borided steel surface is exposed to high temperature (600 to 800 °C) for a short time (3 to 5 min) and then cooled to room temperature in open air. During the high‐temperature exposure, boron atoms within the borided layer diffuse to the surface and react spontaneously with oxygen in air. The reaction product is a thin boron oxide film. During cooling, the boron oxide reacts spontaneously with moisture in the surrounding air to form a thin boric acid film. The sliding friction coefficient of a Si3N4 ball against this flash‐annealed surface is about 0.06, but is 0.5 and higher against the unborided or borided‐only surfaces. Mechanistically, we propose that the ultralow friction behavior of the borided and flash‐annealed surface is due mainly to the Layered‐Crystal structure of the boric acid film that forms on the sliding surface.
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Self-lubricating boric acid films for tribological applications
1990Co-Authors: Ali Erdemir, R. A. Erck, F.a. Nichols, George R. Fenske, D.e. BuschAbstract:Because of its Layered Crystal structure, boric acid, has been found to be lubricious. Its self-lubricating mechanism is related to the easy shear of atomic layers over one another. Moreover, laser-Raman spectroscopy and electron microscopy analyses have confirmed that thin boric acid films can form on surfaces containing boron and boric oxides. To study the lubricity and self lubricating mechanism of boric acid, pin-on-disk tests were performed on pairs of boric acid compacts and steel disks, boric oxide films and steel pins, boron films and steel pins, and boron-implanted steel disks and steel pins. The mean steady-state friction coefficients of these tribosystems ranged from 0.04 to 0.12. 22 refs., 4 figs., 4 tabs.
Kirill Kovnir - One of the best experts on this subject based on the ideXlab platform.
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geas highly anisotropic van der waals thermoelectric material
Chemistry of Materials, 2016Co-Authors: Kathleen Lee, Saeed Kamali, Tore Ericsson, Maverick Bellard, Kirill KovnirAbstract:GeAs and Sn-doped GeAs were synthesized from elements. Both Crystallize in a Layered Crystal structure in the C2/m space group (No. 12) in the GaTe structure type. The Crystal structure consists of As-terminated layers separated by van der Waals gaps. 119Sn Mossbauer spectroscopy reveals that in the doped compound, Sn atoms are situated in a symmetric and homogeneous environment, most probably in the form of Sn2 dumbbells. The anisotropic Crystal structure of GeAs leads to highly anisotropic transport properties. High electrical and thermal conductivities were determined along the Crystallographic layers. For the perpendicular direction across the layers, a sharp drop of more than an order of magnitude was observed for the transport properties of the GeAs single Crystal. As a result, an order of magnitude difference in the figure of merit, ZT, was achieved. High-temperature thermoelectric characterization of the Sn-doped compound reveals a remarkable ZT with a maximum of 0.35 at 660 K.