Deformation Induced Semiconductor-Metal Transition in Single Wall Carbon Nanotubes Probed by Electric Force Microscopy

We report the direct experimental observation of the semiconductor-metal transition in single-wall carbon nanotubes (SWNTs) induced by compression with the tip of an atomic force microscope. This transition is probed via electric force microscopy by monitoring SWNT charge storage. Experimental data show that such charge storage is different for metallic and semiconducting SWNTs, with the latter presenting a strong dependence on the tip-SWNT force during injection. Ab initio calculations corroborate experimental observations and their interpretation.

Figure 1

(a) Schematic drawing representing the experiment: a SWNT (in orange) on top a silicon oxide layer (in blue) is charged through the contact with an AFM tip (in green) biased at $V_{\mathrm{INJ}}$ while it is pressed with a controlled force per unit length $F$. (b) 3D AFM image of a (14,6) semiconducting SWNT atop the ${\mathrm{SiO}}_x$ layer. (c) 3D EFM image of the same nanotube after it has been charged ($V_{\mathrm{INJ}}=6\mathrm{V}$, $\mathrm{\text{lift height}}=50\mathrm{nm}$) evidencing the negative frequency shift of the cantilever.

Figure 2

(a) Plot of the charge density $\lambda$ (in electrons/nm) as a function of injection bias $V_{\mathrm{INJ}}$ for a (10,7) metallic nanotube (black squares) and a (14, 6) semiconducting nanotube (red triangles). The inset shows an I(V) curve acquired with the tip in contact with a thin metallic (Mo) film. (b) Plot of the charge density $\lambda$ as function of the applied compressive force per unit length for (12,6) metallic nanotube (black squares) and (18,4) semiconducting nanotube (red triangles). The evolution of the apparent height (diameter) of the (18,4) semiconducting SWNT with applied force is also plotted in this graph (green circles). The dashed lines are guides for the eye.

Figure 3

(a) Calculated optimized geometry of a flattened (20,0) SWNT, characterized by a compressed diameter $d=0.66\mathrm{nm}$. (b) Calculated bandgap of the (20,0) SWNT as a function of its compressed diameter $d$ showing a semiconductor-metal transition at $d=0.55\mathrm{nm}$. The inset shows the compressive force per unit length, calculated both from the fitting of the total energy to $E=E_0+a/d$ (black line) and directly from the constraint forces (red circles).