Electron Energy Loss Spectroscopy Measurement of the Optical Gaps on Individual Boron Nitride Single-Walled and Multiwalled Nanotubes

Spatially resolved electron energy loss spectroscopy experiments have been performed in an electron microscope on several individual boron nitride (BN) single-, double-, and triple-walled nanotubes, whose diameters and number of shells have been carefully measured. In the low-loss region (from 2 to 50 eV) the spectra have been analyzed within the framework of the continuum dielectric theory, leading to the conclusion of a weak influence of out-of-plane contribution to the dielectric response of the tubes. The gap has been measured to be independent of the nanotubes geometry, and close to the in-plane gap value of hexagonal BN ($5.8\pm 0.2\mathrm{eV}$).

Figure 1

A series of deconvoluted spectra when the electron beam is moved across a BN TWNT, as sketched in the inset. Different spectral features are indicated by letters (A to D), and the two main excitation bands by numbers I and II.

Figure 2

From top to bottom: Experimental deconvoluted spectra for: (a) a TWNT, inside the tube and at grazing incidence, (b) a DWNT inside the tube and at grazing incidence, (c) a SWNT in the rastered mode, (d) ${ϵ}_{\perp ,2}$ as extracted from the bulk loss spectra in (e). (f), (g), (h): Bright field images of the tubes (a), (b), (c).

Figure 3

Top: Simulation of grazing incidence spectra for a SWNT (dashed line) and a TWNT (solid line) of radii equal to those of Fig. 2. Bottom: ${ϵ}_{\perp ,2}(\omega )$ (squares) and $\mathrm{Im}(-1/{ϵ}_{\parallel }(\omega ))$ (circles) computed with the dielectric tensor component of the h-BN used for the above simulations.

Figure 4

Gap measurement for (top to bottom) a TWNT (grazing incidence) and three SWNT.