Luminescent devices operating at sub-250nm wavelength present a strong commercial interest. Applications such as antibacterial properties, high density optical storage or nanofabrication possibilies require reliable, portable and efficient devices. Actual Deep UltraViolet (DUV) sources are based on gaz active regions which are difficult to integrate in mobile devices, exhibit poor efficiencies and are harmful for environment. Thus, the development of solid state based DUV emitters is getting more and more relevant.

Hexagonal boron nitride is a wide band gap semiconductor (~ 6.5 eV), which meets a growing interest for DUV applications. In contrast to carbon nanotubes, Boron Nitride Nanotubes (BNNTs) are all semiconductors whatever their diameter and chirality and their luminescence emission occurs between 200 nm and 250 nm and is governed by strong excitonic effects. Until recently, the optical properties were poorly known due to both the scarcity of samples and suitable investigation tools. This situation has changed thanks to the development of dedicated photoluminescence (PL) and cathodoluminescence (CL) experiments running at 4K and adapted to the detection in the far UV range [1, 2, 3].

These previous studies on boron nitride nanotubes have mainly dealt with multi-wall BNNTs with a large number of walls (20-120 walls). These tubes luminesce between 226 and 234nm and this spectral range has been assigned, in hBN, to transitions involving defects. A critical point to further study the confinement effect on the excitonic transitions is therefore to elucidate the luminescence origin of these multiwalls. Furthermore it is important to investigate the luminescence of small diameter BNNTs (with a reduced number of walls), which actually appears to be very challenging.

Cathodoluminescence from a single BNNT with a large number of walls have been measured with a spatial resolution of about ten nanometers, thanks to an UV dedicated SEM system. Different areas along the tube were investigated, from which luminescence is detected at few wavelengths. From 224 to 228 nm, monochromatic cathodoluminescence images exhibit features, which can be linked to defects in the crystallographic structure, separately observed by Transmission Electron Microscopy (TEM) on the same tube.HRTEM observations and tomography experiments revealed that the BNNTs exhibit a peculiar shape. The section of the tube is polygonal with a number of facets between 6 and 9 (Fig 1). These facets forms an helix along the axis of the nanotube. An important consequence of this facetting is the formation of a large number of dislocations along the tube.

We will discuss the relations between these structural properties and the luminescence as shown on fig 2.

References

[1] P. Jaffrennou el al., J. Appl. Phys. 102  (2007) 116102

[2] P. Jaffrennou and al., Phys. Rev. B, 77 (2008), 235422.

[3] K. Watanabe and al., Phys. Rev. B, 79 (2009), 193104.

Figures:

Fig 1 : Upper image shows a MW-BNNT. Lower images show different reconstructed cross sections of the tube revealing the facets and the helicoidal shape of the tube.

Fig 2 : Correlation between cathodoluminescence (upper image), HR image of the tube (middle image) and our model (bottom scheme)

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