Browsing by Subject "Nanotube"
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Item Open Access Formation of quantum structures on a single nanotube by modulating hydrogen adsorption(American Physical Society, 2003) Gülseren, O.; Yildirim, T.; Çıracı, SalimUsing first-principles density functional calculations we showed that quantum structures can be generated on a single carbon nanotube by modulating the adsorption of hydrogen atoms. The band gap of the hydrogen-free zone of the tube widens in the adjacent hydrogen covered zone. The sudden variation of the band gap leads to band offsets at the conduction- and valence-band edges. At the end, the band gap of the whole system is modulated along the axis of the tube, which generates quantum wells or quantum dots. Specific electronic states are confined in these quantum wells. The type and radius of the nanotube and the extent and sequence of hydrogen-free and hydrogen-covered zones can provide several options to design a desired optoelectronic nanodevice.Item Open Access High-conducting magnetic nanowires obtained from uniform titanium-covered carbon nanotubes(American Physical Society, 2004) Daǧ, Sefa; Durgun, Engin; Çıracı, SalimWe have shown that a semiconducting single-wall carbon nanotube (SWNT) can be covered uniformly by titanium atoms and form a complex but regular atomic structure. The circular cross section changes to a squarelike form, and the system becomes metallic with high state density at the Fermi level and with high quantum ballistic conductance. Metallicity is induced not only by the metal-metal coupling, but also by the band-gap closing of SWNT's at the corners of the square. Even more interesting is that uniform titanium-covered tubes have magnetic ground state with significant net magnetic moment. Our results have been obtained by the first-principles pseudopotential plane-wave calculations within the density-functional theory.Item Open Access Theoretical study of crossed and parallel carbon nanotube junctions and three-dimensional grid structures(American Physical Society, 2004) Dag, S.; Senger, R. T.; Çıracı, SalimThis work presents a first-principles study of parallel and crossed junctions of single-wall carbon nanotubes (SWNT). The crossed junctions are modeled by two-dimensional grids of zigzag SWNTs. The atomic and electronic structure, stability, and energetics of the junctions are studied for different magnitudes of contact forces pressing the tubes towards each other and hence inducing radial deformations. Under relatively weak contact forces the tubes are linked with intertube bonds which allow a significant conductance through the junction. These interlinking bonds survive even after the contact forces are released and whole structure is fully relaxed. Upon increasing contact force and radial deformation the tube surfaces are flattened but the interlinking bonds are broken to lead to a relatively wider intertube spacing. The intertube conductance through such a junction diminish because of finite potential barrier intervening between the tubes. The linkage of crossing tubes to form stable junctions is enhanced by a vacancy created at the contact. The three-dimensional grid structure formed by SWNTs is also investigated as a possible framework in device integration.Item Open Access A theoretical study on silicon and III-V compound nanotubes(TÜBİTAK, 2005) Durgun, Engin; Çıracı, SalimIn this paper we present a theoretical study on single-wall silicon and III-V compound nanotubes. First principles plane wave calculations within density functional theory are used to predict energetics and electronic structures of armchair and zigzag nanotubes. The stability of tubular structures is further investigated at finite temperature by ab initio molecular dynamics calculations. Our results indicate that (n,0) zigzag and (n,n) armchair single-wall Si nanotubes are stable for n ≥ 6. Mechanically, the Si nanotubes are radially soft, however they are strong against axial deformations. Electronic analysis showed that zigzag nanotubes are metallic for n ≤ 11, but they show semiconducting behavior for larger radii. On the other hand, all armchair nanotubes are metallic. (8,0) single wall nanotube has been chosen as prototypes for AlP, GaN, and GaAs compounds and we found that they are semiconducting and stable at room temperature. © TÜBİTAK.