Growth of diameter-modulated single-walled carbon nanotubes through instant temperature modulation in laser-assisted che
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Growth of diameter-modulated single-walled carbon nanotubes through instant temperature modulation in laser-assisted chemical vapor deposition M. Mahjouri-Samani, Y. S. Zhou, W. Xiong, Y. Gao, M. Mitchell, and Y. F. Lu Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0511 ABSTRACT The diameter of individual single-walled carbon nanotubes (SWNTs) was successfully modulated along their axes by instant temperature control in a laser-assisted chemical vapor deposition (LCVD) process. SWNTs were grown using different temperature profiles to investigate the effects of temperature variation on their growth. Due to the inverse relationship between SWNT diameter and growth temperature, SWNTs with ascending diameters were obtained by reducing the LCVD temperature from high to low. The diameter-modulated SWNTs were grown across a pair of Mo electrodes to form field-effect transistors (FETs) for investigation of their electronic transport properties. Fabricated devices demonstrated properties similar to Schottky diodes, implying different bandgap structures at the ends of the SWNTs. Raman spectroscopy, transmission electron microscopy, and electronic transport characteristics were studied to investigate the influence of temperature variation on the structural and electronic characteristics of SWNTs. INTRODUCTION Remarkable electronic and physical characteristics of single-walled carbon nanotubes (SWNTs) have made them an interesting material for nanoscale electronics and optical devices such as transistors and sensors [1]. One of the most fascinating characteristic of SWNTs is their ability to be either metallic or semiconducting with variable bandgaps depending on their diameters and chiral vectors [2, 3]. However, many researchers treat this as a challenging issue for fabrication of SWNT-based devices with uniform electronic properties. These variations of structures, electronic types, and bandgaps have been limiting the applications of SWNTs in electronic and optical devices. Thus, large amounts of research have been done to control and separate SWNTs with different chiralities and diameters in the pre and post synthesis processes [4-8]. Tremendous efforts and focus on the uniformity issue have prevented researchers from taking these variations as an advantage. Recently, diameter variation in the growth of ultralong SWNTs by altering the growth temperature has been reported [9]. However, the growth and characterization of diameter-modulated SWNTs have not been investigated for their applications in nanoscale devices. In this study, we took these variations in diameters and bandgaps as an advantage and were able to alter their diameters during the growth in a laser-assisted chemical vapor deposition (LCVD) process. SWNTs with continuous change of diameters along their length were grown, implying the possibility to form array of straddling bandgaps along the individual semiconducting SWNTs. Equation (1) clearly describes the inverse proportionality of the bandgaps of the semiconducting SW
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