<b><i>Ab initio</i></b>lattice dynamics of BN and AlN: Covalent versus ionic forces

Karch, K.; Bechstedt, F.

doi:10.1103/physrevb.56.7404

Cited by 238 publications

(107 citation statements)

References 70 publications

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“…The Raman spectra from two different regions (1&2) contain all characteristic TO and LO peaks of cBN, where there is very small peak of hBN (region 1) and is completely gone in region 2 (lower curve), showing a complete conversion of hBN into phase-pure cBN. Table II shows a detailed comparison of our experimental results of first-order Raman peaks with previous experimental results from second-order peaks [30] and theoretical ab-initio calculations of Raman active modes in cBN. [31] Such a fine structure in the Raman spectra, besides well known TO(C) at 1060 cm À1 and LO(C) at 1310 cm À1 , is indicative of very high-quality structure of cBN.…”

Section: R E T R a C T E D A R T I C L Ementioning

confidence: 89%

RETRACTED ARTICLE: Fundamental Discovery of New Phases and Direct Conversion of Carbon into Diamond and hBN into cBN and Properties

Narayan

2016

Metall Mater Trans A

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We review the discovery of new phases of carbon (Q-carbon) and BN (Q-BN) and address critical issues related to direct conversion of carbon into diamond and hBN into cBN at ambient temperatures and pressures in air without any need for catalyst and the presence of hydrogen. The Q-carbon and Q-BN are formed as a result of quenching from super undercooled state by using high-power nanosecond laser pulses. We discuss the equilibrium phase diagram (P vs T) of carbon, and show that by rapid quenching, kinetics can shift thermodynamic graphite/diamond/liquid carbon triple point from 5000 K/12 GPa to super undercooled carbon at atmospheric pressure in air. Similarly, the hBN-cBN-Liquid triple point is shifted from 3500 K/9.5 GPa to as low as 2800 K and atmospheric pressure. It is shown that nanosecond laser heating of amorphous carbon and nanocrystalline BN on sapphire, glass, and polymer substrates can be confined to melt in a super undercooled state. By quenching this super undercooled state, we have created a new state of carbon (Q-carbon) and BN (Q-BN) from which nanocrystals, microcrystals, nanoneedles, microneedles, and thin films are formed depending upon the nucleation and growth times allowed and the presence of growth template. The large-area epitaxial diamond and cBN films are formed, when appropriate planar matching or lattice matching template is provided for growth from super undercooled liquid. The Q-phases have unique atomic structure and bonding characteristics as determined by high-resolution SEM and backscatter diffraction, HRTEM, STEM-Z, EELS, and Raman spectroscopy, and exhibit new and improved mechanical hardness, electrical conductivity, and chemical and physical properties, including room-temperature ferromagnetism and enhanced field emission. The Q-carbon exhibits robust bulk ferromagnetism with estimated Curie temperature of about 500 K and saturation magnetization value of 20 emu g À1 . We have also deposited diamond on cBN by using a novel pulsed laser evaporation of carbon and obtained cBN/diamond composites, where cBN acts as template for diamond growth. Both diamond and cBN grown from super undercooled liquid can be alloyed with both p-and n-type dopants. This process allows carbon to diamond and hBN to cBN conversions and formation of useful nanostructures and microstructures at ambient temperatures in air at atmospheric pressure on practical and heat-sensitive substrates in a controlled way without need for any catalysts and hydrogen to stabilize sp 3 bonding for diamond and cBN phases.

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Section: R E T R a C T E D A R T I C L Ementioning

confidence: 89%

RETRACTED ARTICLE: Fundamental Discovery of New Phases and Direct Conversion of Carbon into Diamond and hBN into cBN and Properties

Narayan

2016

Metall Mater Trans A

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“…[65]. By adjusting our model to results of ab initio calculations [67,68] we obtained corresponding results. Figure 12 displays the phonon dispersion of hexagonal GaN and AlN.…”

Section: Zone-boundary Phonons In Gan and Alnmentioning

confidence: 86%

“…Figure 15 exhibits a room temperature Raman spectrum of hexagonal AlN (upper curve) together with a calculated density of two-phonon states [67] of cubic AlN (bottom curve).…”

Section: C)mentioning

confidence: 99%

Lattice dynamics in GaN and AlN probed with first‐ and second‐order Raman spectroscopy

Haboeck

Siegle

Hoffmann

et al. 2003

phys. stat. sol. (c)

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We present a selection of our contributions to basic research on the lattice dynamical properties of group-III nitrides and their alloys. We used first-order Raman scattering to determine the zone-center phonons and their dependence on structural attributes such as stress, chemical composition, impurities, and doping. Results on the angular dispersion of the polar modes, strain distribution, coupled LO-phonon plasmon modes, multi-mode behavior in Al x Ga 1-x N, and the quantitative determination of the phase purity of cubic and hexagonal GaN are shown. Second-order Raman-scattering experiments on GaN and AlN provide information on the vibrational states throughout the entire Brillouin zone. Based on a comparison of experimental data and calculated phonon-dispersion curves we assigned the observed structures to particular phonon branches and points in the Brillouin zone. We also discuss the behavior of the optical modes under large hydrostatic pressure. 1 Introduction During the last decade group-III nitrides have been an object of increasing research because of their interesting physical properties such as an adjustable band gap (1.9 eV to 6.2 eV) by varying the composition of the alloys [1] and application as basic materials for optoelectronic devices working in the blue and ultraviolet spectral region [2][3][4]. In particular GaN and AlN can withstand high temperatures and have large piezoelectric constants which makes them suitable for applications in highfrequency devices, sensors and low-dimensional structures [5,6].Although basic information about group-III nitrides e.g. crystal-and band structure have been available for a decade several details still remained unclear. In the following review of our work we present results of first-and second-order Raman investigations on various GaN, AlN, and Al x Ga 1-x N samples and point out some remarkable details of the structural and optical properties of these materials.

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“…Table II shows a detailed comparison of our experimental results of first-order Raman peaks with previous experimental results 16 and theoretical ab initio calculations of Raman active modes in c-BN. 17 Such a fine structure in the Raman spectra, besides well known TO(Γ) at 1060 cm −1 and LO(Γ) at 1310 cm −1 , is indicative of very high-quality structure of c-BN. The Raman shift (∆ω) is related to ∆ω (in cm…”

Section: Boron Nitride (Bn) Exists In Four Polymorphs Namely Hexagomentioning

confidence: 99%

Research Update: Direct conversion of h-BN into pure c-BN at ambient temperatures and pressures in air

Narayan

Bhaumik

2016

APL Materials

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We report a direct conversion of hexagonal boron nitride (h-BN) into pure cubic boron nitride (c-BN) by nanosecond laser melting at ambient temperatures and atmospheric pressure in air. According to the phase diagram, the transformation from h-BN into c-BN can occur only at high temperatures and pressures, as the hBN-cBN-Liquid triple point is at 3500 K/9.5 GPa. Using nanosecond laser melting, we have created super undercooled state and shifted this triple point to as low as 2800 K and atmospheric pressure. The rapid quenching from super undercooled state leads to formation of super undercooled BN (Q-BN). The c-BN phase is nucleated from Q-BN depending upon the time allowed for nucleation and growth.

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Ab initiolattice dynamics of BN and AlN: Covalent versus ionic forces

Cited by 238 publications

References 70 publications

RETRACTED ARTICLE: Fundamental Discovery of New Phases and Direct Conversion of Carbon into Diamond and hBN into cBN and Properties

RETRACTED ARTICLE: Fundamental Discovery of New Phases and Direct Conversion of Carbon into Diamond and hBN into cBN and Properties

Lattice dynamics in GaN and AlN probed with first‐ and second‐order Raman spectroscopy

Research Update: Direct conversion of h-BN into pure c-BN at ambient temperatures and pressures in air

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