TUNABLE ADSORPTION ON CARBON NANOTUBES

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INTRODUCTION

Novel mechanical, electrical and chemical properties of carbon nanotubes have been explored actively with a motivation of finding a new technological application. In particular, single wall carbon nanotubes (SWNT) provide a system where the electronic properties can be controlled by the structure (i.e. radius and the helicity defined by two integers (n,m)) of the nanotubes. It is therefore desirable to have a good understanding of their electronic and mechanical properties and the interrelations between them. In this project, we study the close relationship between mechanical deformation and the chemical properties of SWNT's from the state-of-the-art first-principles calculations.

Recently we have shown that the electronic properties of SWNTs can be modified by elliptical deformation. The energy gap of an insulating SWNT can decrease and eventually vanish at an insulator-metal transition with increasing applied radial strain (where the circular cross section of the nanotube is distorted to elliptical cross section). 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 and rectifying junctions.

Most noticeable, we have found that the elliptical distortion disturbs the uniformity of charge distribution of SWNTs. This, in turn, impose changes in the chemical reactivity and hence in the interaction of tube surface with foreign atoms and molecules. It is therefore anticipated that not only the electronic properties but also chemical reactions taking place on the surface of a SWNT can be engineered through elliptical deformation. This is shown in the following animation of the conduction charge reconstruction with the elliptical deformation.

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The reconstruction of the first-conduction band charge density (which are chemically the most active ones) with the elliptical deformation, indicating that electrons are pushed away from flat to sharp site of the distorted tube.

In this paper, we explore this feature by using the predictive power of density functional theory and demonstrate that indeed adsorption of foreign atoms on carbon nanotubes and associated properties can be modified continuously and reversible. Furthermore, we showed that there is a simple scaling of the adsorption energies with the elliptical of the SWNT. We believe that the tunable adsorption can have important implications for metal coverage and selective adsorption of foreign atoms and molecules on the carbon nanotubes, and can lead to a wide variety of technological applications, ranging from hydrogen storage to new materials.

RESULTS AND DISCUSSION

In order to investigate the effect of the elliptical deformation we first consider adsorption of H and a simple metal, Al, on undeformed SWNT's. In Fig.1, we present the binding energies of H and Al adsorped on the (n,0) zigzag nanotubes calculated for n=7,8,9,10, and 12. H is absorbed at the top site, i.e. directly above the C atoms of the tube. Al favors the hollow site, i.e. above the center of the hexagon as in the graphite. The binding energy Eb is calculated with respect to separate nanotube and absorbent systems in the same supercell. The positive value of Eb indicates that the adsorption is exothermic and hence stable.

BE CNT FIGURE 1. Binding energies Eb of single hydrogen and aluminum atom adsorped on the zigzag SWNTs versus the elliptical of the tubes (after fully optimized). The solid line is the fit to Eb = E0 + C/R.

Surprisingly the binding energy follows a rather simple 1/R relation and therefore decreases with increasing elliptical (or decreasing curvature) of the tube, and eventually saturates at a value corresponding to that on graphene plane. This finding suggests that by creating different curvature on a single SWNT by elliptical deformation one can attain different values of binding energies.

The elliptical deformation that we consider in this study is generated by applying uniaxial compressive stress on a narrow strip on the surface of the SWNT. In practice such a deformation can be realized by pressing the tube between two rigid flat surfaces. As a result of this elliptical deformation, the circular cross section of the tube is distorted to an elliptical one with major and minor axis a and b respectively. Figure 2 shows the variation of the binding energy of a single H (a) and Al (b) adsorbed on the (8,0) surface for the flat and high curvature region of the tube.

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- FIGURE 2. Binding energies Eb of single hydrogen (a) and aluminum (b) atom adsorped on the flat and sharp regions of a (8,0) nanotube versus elliptical deformation.

Figure 2a shows that the binding energy of H adsorbed on the high curvature site is increased by 0.85 eV by elliptical deformation. On the other hand, Eb, for the adsorption on the low curvature (flat) site first decreases with increasing deformation and then saturates at an energy 0.25 eV less than that corresponds to undeformed tube. The difference of binding energies of the sharp and flat sites is substantial and is equal to ~1.1 eV.

The binding energy of Al shown in Fig.2b exhibits a behavior similar to that of H, despite H and Al favor different sites on the (8,0) tube: the binding energy of the sharp site increases with increasing deformation. For the Al absorbed on the flat site, Eb first decreases with increasing deformation, then gradually increases.

The variation of Eb with the elliptical deformation is consistent with the results illustrated in Fig.1. In general, the higher is the curvature under deformation, the higher the binding energy is. Explanation of this remarkable and significant change of the binding energy with elliptical deformation is sought in the electronic energy structure and the charge density. This is nicely demonstrated by the animation shown at the top. In this animation, we show the lowest unoccupied molecular orbital (i.e. the first conducting band) as a function of the elliptical deformation. We note that the charge density is pushed away from the flat site to the sharp site. Since LUMO or the first conducting band is the most important band for the electrons which takes place in the chemical reaction, we expect that reorganization of the conduction band increases the absorption energy at the sharp site and decreases it at the flat site.

CONCLUSION

We believe that the tunable adsorption reported in this paper can have important implications for metal coverage and selective adsorption and desorption of foreign atoms and molecules on the carbon nanotubes. This could allow the "fine tuning" of the properties of SWNTs via reversible deformation and ultimately lead to a wide variety of technological applications such as variable and reversible quantum devices, catalysts, hydrogen storage, chemical sensors, magnetic tubes, etc.

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