On surface Raman scattering and luminescence radiation in boron carbide

Boron carbide thin film surface characterization after graphitic carbon removal using low-pressure oxygen gas RF plasma

B ₄ C-coated thin film mirrors are used in high brilliance synchrotron and x-ray free electron laser beamlines due to their low absorption coefficient and high thermal stability. As in the case of gold, platinum, and other thin film mirrors, B ₄ C-coated mirrors also are affected due to synchrotron radiation-induced carbon contaminations in beamlines. In the present study, a graphitic carbon (C) layer deposited on top of boron carbide (B _x C) thin film surface is removed by five successive oxygen radio frequency (RF) plasma exposures (RF power, 10 W; O ₂ flow, 30 sccm; exposure time, 10 min each). Before and after the carbon layer removal, structural and compositional properties of the B _x C/C bilayer are characterized by soft x-ray reflectivity, x-ray photoelectron spectroscopy, grazing angle x-ray diffraction, and Raman spectroscopy techniques. Characterization results reveal that in the first four exposures the carbon layer thickness decreases continuously without affecting the B _x C layer properties; however, in the fifth exposure, the carbon layer is completely removed along with a partial etching of the B _x C layer too.

Systematic error in conventionally measured Raman spectra of boron carbide-A general issue in solid state Raman spectroscopy

Solid state Raman spectroscopy requires careful attention to the penetration depth of exciting laser radiation. In cases like semiconducting boron carbide and metallic hexaborides, high fundamental absorption above the bandgap and reflectivity R ≈ 1 beyond the plasma edge respectively prevent the excitation of bulk phonons largely. Thus, correspondingly measured spectra stem preferably from surface scattering. For decades, Raman spectra of boron carbide obtained with different equipment were controversially discussed. Now, common features of icosahedral phonons in Fourier transform-Raman spectra of B_4.3C and Raman spectra of α-rhombohedral boron evidence their bulk character, while refuting conventionally measured Raman spectra.

Isotopic effects on the phonon modes in boron carbide

The effect of isotopes ((10)B-(11)B; (12)C-(13)C) on the infrared- and Raman-active phonons of boron carbide has been investigated. For B isotopes, the contributions of the virtual crystal approximation, polarization vector and isotopical disorder are separated. Boron and carbon isotope effects are largely opposite to one another and indicate the share of the particular atoms in the atomic assemblies vibrating in specific phonon modes. Some infrared-active phonons behave as expected for monatomic boron crystals.

Raman effect in icosahedral boron-rich solids

We present Raman spectra of numerous icosahedral boron-rich solids having the structure of α-rhombohedral, β-rhombohedral, α-tetragonal, β-tetragonal, YB66, orthorhombic or amorphous boron. The spectra were newly measured and, in some cases, compared with reported data and discussed. We emphasize the importance of a high signal-to-noise ratio in the Raman spectra for detecting weak effects evoked by the modification of compounds, accommodation of interstitial atoms and other structural defects. Vibrations of the icosahedra, occurring in all the spectra, are interpreted using the description of modes in α-rhombohedral boron by Beckel et al. The Raman spectrum of boron carbide is largely clarified. Relative intra- and inter-icosahedral bonding forces are estimated for the different structural groups and for vanadium-doped β-rhombohedral boron. The validity of Badger's rule is demonstrated for the force constants of inter-icosahedral B-B bonds, whereas the agreement is less satisfactory for the intra-icosahedral B-B bonds.