We report the density-functional calculations of NO2 adsorption on single-walled carbon nanotube walls. A single molecular adsorption was endothermic with an activation barrier, but a collective adsorption with several molecules became exothermic without an activation barrier. We find that NO2 adsorption is strongly electronic structure- and strain-dependent. The NO2 adsorption on metallic nanotubes was energetically more favorable than that on semiconducting nanotubes and furthermore the adsorption became less stable with increasing diameters of nanotubes. The adsorption barrier height shows similar dependence on the electronic structure and diameter to the adsorption energy. Our theoretical model can be a good guideline for the separation of nanotubes by electronic structures using various adsorbates.

Nanostructures, especially nanowires with high aspect ratios, have attracted much attention due to their potential applications, such as interconnects in nanofabrication, [1] optoelectronics, [2] nanosensors, [3] nano-biotechnology, [4] and electron emitters. [5][6][7] It was reported that the properties of the nanostructures were affected by their size and morphology, [8][9][10] which subsequently prompted extensive efforts to control them. The synthesis of metal nanostructures, such as nanorods and nanowires of copper, silver, and gold, has been demonstrated using various methods. [7,[11][12][13][14][15][16][17][18][19][20] For these metals, face-centered cubic (fcc) structures were synthesized, particularly with fivefold twinned symmetry, which was done mostly in the solution phase. The randomness inherent to this solution-phase synthesis, however, has largely prevented the resulting nanostructures from being integrated into highdensity electronic and optoelectronic devices. In fact, welldefined five-twinned fcc nanostructures vertically grown on a substrate surface have not been possible except in a few cases involving copper nanowires (CuNWs) and copper nanobats. [7,21] In this Communication, we present a detailed structural analysis, based on a transmission electron microscopy (TEM) and electron diffraction (ED) study, of CuNWs grown by chemical vapor deposition (CVD) that not only have fivefold-twinned structures but are also suitable for integration into devices. The analysis unveils new details of the five-twinned structure and elucidates the growth mechanism of the CuNWs, specific to the special precursor used, which does not require templates or catalysts. The electron emission characteristics of the CuNWs were investigated and they indicate that the CuNWs are a promising electron emitter. An array of CuNWs was grown on a patterned silicon substrate to show that the present CuNW growth mechanism is suitable for practical application. The array was used in a proofof-principle experiment to demonstrate a field emission display. The CuNWs were prepared by a method previously described that uses Cu(etac)[P(OEt) 3 ] 2 as a precursor, where etac is ethyl 3-oxobutanoate and P(OEt) 3 is triethyl phosphite.[21] Typically, the CuNWs of 70-250 nm diameter were grown on Si at substrate temperatures of 200-300 8C under 0.1-1.0 Torr using argon as a carrier gas. The nanowires were also grown on other substrates, including glass, metal, metal oxide, and polymer. The diameters and lengths of the CuNWs were controlled by the processing conditions, such as the substrate material, substrate temperature, deposition time, and precursor feeding rate. Figures 1a-c show scanning electron microscopy (SEM) images of CuNWs grown vertically on a Si substrate at 250 8C, indicating that the nanowires have five side planes forming a pentagonal pyramid tip. Figure 1d shows the copper seed formation at an early stage of nanowire growth. The structure of the CuNWs was analyzed using TEM. Figure 2 shows TEM images of a CuNW and correspo...

We have investigated the electronic structure of capped carbon nanotubes under electric fields by density functional calculations. The calculated effective work function of the capped armchair nanotubes decreases linearly with increasing electric fields, whereas that of the metal tip decreases quadratically. We analyzed the density of states, highest occupied molecular orbitals ͑HOMO's͒ and lowest unoccupied molecular orbitals ͑LUMO's͒ for various charged states under electric fields. While the HOMO and the LUMO are not localized at the cap for a neutral state, the HOMO and LUMO of a negatively charged state are localized at the cap, particularly under electric fields. This localization plays an important role in the field emission. The Mulliken charge population shows that the charge accumulation is not dependent on the local atomic geometry but on the sharpness of the tip.