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Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.

The excitonic recombinations in hexagonal boron nitride (hBN) are investigated with spatially resolved cathodoluminescence spectroscopy in the UV range. Cathodoluminescence images of an individual hBN crystallite reveals that the 215 nm free excitonic line is quite homogeneously emitted along the crystallite whereas the 220 nm and 227 nm excitonic emissions are located in specific regions of the crystallite. Transmission electron microscopy images show that these regions contain a high density of crystalline defects. This suggests that both the 220 nm and 227 nm emissions are produced by the recombination of excitons bound to structural defects.Hexagonal Boron Nitride (hBN) is a wide band gap semiconductor [1,2,3,4] isostructural with graphite. For a few years, much interest has been devoted to BN materials since hBN and more specifically BN nanotubes [5,6,7,8,9,10,11,12,13] are expected to be promising materials for optoelectronic applications. Recently, experimental and theoretical studies have been undertaken in order to investigate the optical properties of hBN. Using luminescence experiments, the near band-edge emission has been observed in the UV region [14,15,16,17,18]. Theoretical groups have interpreted this UV emission as due to unusually strong excitonic effects [3,4]. They propose that excitons in hBN are closer to Frenkel-type excitons with large binding energy (0.7 eV) than to the usual Wannier-like excitons of classical III-N semiconductors.The hBN near-band edge emission consists in several peaks at 215 nm (5.77 eV), 220 nm (5.63 eV) and 227 nm (5.46 eV) [15]. Watanabe et al. [16] recently observed that this UV band shows drastic changes after deformation of a single crystal [16,17]. By pressing a hBN single crystal between two fingers, the authors show that the relative intensities of the 215 nm and 227 nm bands are reversed. After deformation, the CL intensity of the 215 nm band becomes negligible as compared to the 227 nm band which is then predominant. According to recent calculations [3], the 215 nm luminescence band has been attributed to Frenkel-type free excitonic recombinations and Watanabe et al. tentatively assign the 227 nm band, which appears predominantly after deformation, to exci- * Electronic address: francois.ducastelle@onera.fr tons bound to stacking faults or to the shearing of lattice planes [16].In this work, we present a detailed investigation of the excitonic luminescence in hBN. By combining spatially resolved cathodoluminescence (SR-CL) spectroscopy with a structural analysis of the crystal by means of Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED) analyses, we investigate the nature of the 220 nm and 227 nm excitonic recombinations in terms of excitons bound to well identified structural defects.In these experiments, commercial Aldrich hBN powders are used. The samples were dispersed in ethanol and deposited on carbon coated TEM copper grids in order to isolate individual hBN crystallites. Commercial Starck hBN powders were also inve...

Individual multiwall boron nitride nanotubes with diameters from 30 nm to 110 nm are shown to be efficient UV emitters by cathodoluminescence. Their luminescence does not evolve much in this diameter range, with dominant UV recombinations at about 230 nm. As a result, single nanotube properties can be obtained from experiments performed on ensembles of nanotubes. Such ensembles are studied by photoluminescence as a function of temperature ͑5 K-300 K͒ and by photoluminescence excitation experiments at 9 K. The results are discussed and compared with the related bulk material, hexagonal boron nitride. The strong luminescence recorded around 230 nm is attributed to excitonic effects, more precisely to excitons bound to the structural defects: dislocations, facets, which are observed along the walls.

Optical properties of multiwall boron nitride nanotubes are investigated by means of luminescence and absorption spectroscopies. Cathodoluminescence imaging shows that boron nitride nanotubes are highly UV luminescent materials and that the luminescence is located all along the nanotube. In comparison with the related bulk material, hexagonal boron nitride, luminescence and absorption experiments point out the role of excitonic effects in this nanoobject.