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BAND GAP ENGINEERING BY ELLIPTICAL DEFORMATION OF NANOTUBE

In order to answer this question, we investigate the band gaps and
electronic structures of nanotubes as a function of elliptical deformation
using the state-of-the-art first-principles calculations. Based on
the idea of band gap engineering in nanotubes by elliptical deformation,
we propose
variable and reversible quantum structures on a single SWNT.
** FIGURE 1. ***
(a) Top and side view of (7,0) SWNT under different degrees of elliptical
deformation. (b) Variation of the energy band gap E*_{g}
and (c) density of the states at the Fermi energy as a function of
elliptical deformation from first-principles calculations.
For deformations a/b~1.2 the band gap is
almost zero, indicating insulating-metal transition. Increasing
the deformation further increases the density of states at
the Fermi level. (d) and (e) show the band gap and the valence
and conduction bands obtained from tight-binding calculations.

Our first-principles calculations summarized in Fig.1 indicate that the
electronic properties of SWNTs can be indeed modified by elliptical deformation.
The energy gap of an insulating SWNT can decrease and eventually vanish at
the insulator-metal transition with increasing applied radial strain.
More interestingly, the elliptical deformation necessary to induce metalicity was
found to be in the elastic range. This could allow the *``fine-tuning''*
of the properties of SWNTs via reversible deformation and ultimately lead to
variable and reversible quantum devices, such as metal-insulator junctions
as shown below.

** *** ``If we apply elliptical deformation locally on a nanotube,
we can realize a reversible metal insulator junctions! Using this idea,
we propose to construct various quantum devices on a single nanotube with
a reversible and variable electronic properties as discussed below.''
**
*

__BAND GAP ENGINEERING: __

Since nanotubes can display metallic or semiconducting character depending on their chiralities and diameters, joining different nanotubes together can produce quantum structures. However, constructing the quantum devices in this way would require fabricating single wall carbon nanotube junctions for each application. In this project, we
explored a practical alternative, namely that various quantum structures can easily be realized on an individual carbon nanotube, and the properties of these structures can be controlled by applied transverse compressive stress. If such a deformation is not uniform but has different magnitude at different positions along the nanotube then the different regions will have different band gap and quantum wells of the desired electronic character can be formed. We performed first-principles calculations to demonstrate various nanodevices on a single nanotube using this idea.

** FIGURE 2. ***
A schematic description of A*_{8} B_{8}
heterostructure generated from (7,0) nanotube by elliptical
deformation.

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