Fabrication And Photoelectric Properties Of Nanotube Based Low-Dimensional Composite Structures

Tubular nanostructures have advantages of hollow structures, large surface-to-volume ratios and high adsorption capacities, which make them more sensitive to the surroundings and external fields, compared with other one-dimensional nano-materials. Basing on TiO₂ nanotube arrays prepared by electrochemical anodization and double-walled carbon nanotubes（CNTs） synthesized by chemical vapor deposition, we designed and fabricated several low-dimensional composite structures, and then further study their transport properties in optical and electric fields. Scanning Electron Microscopy was employed to characterize the morphologies of nanostructures.Firstly, highly ordered TiO₂ nanotube arrays were prepared by two-step anodic oxidation, with nanotube length controlled by tailoring electrochemical conditions, such as anodization voltage and reaction time. After thermal annealing, amorphous TiO₂ nanotube arrays transformed into an anatase TiO₂ nanoparticle film. CNT films were used as the top electrode, we fabricated a photodetector based on CNT film/TiO₂ nanotube array heterojunctions, and reported a new and simple method to improve the device performance. Via pre-electroforming, the device was switched from a high resistance state（HRS） to a low resistance state（LRS）. At zero bias, the photoresponsivities of the LRS device under 532 and 1064 nm laser illuminations were significantly improved, compared with those obtained from the HRS device under the same conditions.Secondly, the photoconductive properties of CNT films in both ambient air and vacuum were explored and found to be strongly dependent on light wavelength. In air, strong negative photoconductivity（NPC） phenomena were observed under laser irradiation at 405, 532 and 1064 nm, induced by photodesorption of oxygen molecules, the effect of which is more pronounced in the short wavelength region. In vacuum, the photoconductivity changed into positive under 405 and 532 nm laser illuminations; while for 1064 nm illumination, it remained negative, we propose that the interactions between surface plasmon polaritons and electrons may be responsible for such NPC behavior. Our results indicate that the photoconductivity of CNT films can be modulated by light wavelength, which may be of guiding significance in designing carbon nanotube based optoelectronic devices. Taking advantages of the ultra-broadband spectral ranges of CNTs, we fabricated a room-temperture THz detector composed of CNT/metal heterojunctions. This detector could work at zero bias for THz irradiation（frequency: 2.52 T）, with photovoltage and photocurrent responsivities up to 22 mV/W and 166.7 mA/W, respectively.Finally, we skillfully designed and fabricated a mixed ionic-electronic conductors composed of CNT films and superionic conductor RbAg4I5 films, in which CNT provides the electronic transport channel and RbAg4I5 provides the ionic transport channel. Due to Coulomb interactions, Ag ions in RbAg4I5 and the electrons in CNT near the interface may form ionic-electronic bound states（IEBSs）, leading to the remarkable decrease in conduction; while in external electric or optical fields, part of the IEBSs could be dissociated, causing the conduction to increase. The above results indicate that electron transport in CNTs can be modulated by Ag ions. Furthermore, we fabricated a new-type electronic device based on ion-electron interactions, showing similar functions as a field-effect transistor, with drain-source currents modulated linearly by gate voltage.