Modification of HOMO-LUMO gap of “zigzag” single-walled carbon nanotubes under hydrogen chemisorption

Background. The main interest in carbon nanotubes in the field of micro- and nanoelectronics is associated with their potential applications in creating various metal-semiconductor and semiconductor-semiconductor structures. To implement such structures in practice it is necessary to have a mechanism of effective control over the width of HOMO-LUMO energy gap of nanotubes. One of the most promising among these mechanisms is the hydrogen chemisorption on the nanotubes, which can be considered as surface doping. The aim of this work is to theoretically study the dependence of nanotube's HOMO-LUMO gap on the hydrogen coating's magnitude for both outer and inner types of regular hydrogen chemisorption on “zigzag” single-walled carbon nanotubes. Materials and methods. The range of finite “zigzag” single-walled carbon nanotubes of different radius covered with hydrogen was simulated by the quantum-chemical semi-empirical method AM1 (Austin Model 1). Different densities and patterns of internal and external hydrogen coating of nanotubes were studied. Results. The dependence of nanotubes' HOMO-LUMO gaps on hydrogen coating's magnitude was found for both internal and external types of regular hydrogen chemisorption on “zigzag” single-walled carbon nanotubes. Noticeably nonmonotonic character of the found dependence was explained by the large crosscut deformation of nanotubes under the regular chemisorption of hydrogen adatoms and by the strong dependence of the HOMO-LUMO gap on nanotube's diameter. The latter was demonstrated for few finite “zigzag” single-walled carbon nanotubes by means of quantum-chemical simulation. It was found that all considered “zigzag” nanotubes can be completely covered with chemisorbed hydrogen outside, whereas the regular internal chemisorption of hydrogen is possible only for nanotubes with chirality index more than n = 7. Conclusions. The regular hydrogen chemisorption on single-walled carbon nanotube can be regarded as an effective mechanism for controlling the nanotube's HOMO - LUMO gap. Herewith the HOMO-LUMO gap can be either increased or decreased depending on the diameter of the nanotube and the density and pattern of hydrogen coating.