Browsing by Author "He, Xiaowei"

Now showing 1 - 4 of 4
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    Destabilization of Surfactant-Dispersed Carbon Nanotubes by Anions
    (Springer, 2017) Hirano, Atsushi; Gao, Weilu; He, Xiaowei; Kono, Junichiro
    The colloidal stability of surfactant-dispersed single-wall carbon nanotubes (SWCNTs) is determined by microscopic physicochemical processes, such as association, partitioning, and adsorption propensities. These processes can be controlled by the addition of solutes. While the effects of cations on the colloidal stability of SWCNTs are relatively well understood, little is known about the effects of anions. In this study, we examined the effects of anions on the stability of SWCNTs dispersed by sodium dodecyl sulfate (SDS) using sodium salts, such as NaCl and NaSCN. We observed that the intensity of the radial breathing mode Raman peaks rapidly decreased as the salts were added, even at concentrations less than 25 mM, indicating the association of SWCNTs. The effect was stronger with NaSCN than NaCl. We propose that the association of SWCNTs was caused by thermodynamic destabilization of SDS assemblies on SWCNT surfaces by these salts, which was confirmed through SWCNT separation experiments using aqueous two-phase extraction and gel chromatography. These results demonstrate that neutral salts can be used to control the colloidal stability of surfactant-dispersed SWCNTs.
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    p-n junction photodetectors based on macroscopic single-wall carbon nanotube films
    (2013-09-16) He, Xiaowei; Kono, Junichiro; Hauge, Robert H.; Kelly, Kevin F.
    Single-walled carbon nanotubes (SWCNTs) are promising for use in solar cells and photodetectors because of their strong optical absorption in most of the solar spectrum. There have been many reports about the photovoltaic effect in nanoelectronic devices based on individual SWCNTs, but they have been limited by complicated fabrication and miniscule absorption. There has been a growing trend for merging SWCNTs into micro-and macroscopic devices to provide more practical applications. Here we report the photoresponse of macroscopic SWCNT films with a p-n junction at room temperature. Photovoltage (PV) and photocurrent (PC) due to the photothermoelectric (PTE) effect were observed at the junction, and they were larger by one order of magnitude as compared with their values at the metal-SWCNT interfaces. Various factors affecting PV amplitude and response time have been studied, including junction length, substrate, and doping level. The maximal responsivity we observed was 1V/W with samples on Teflon tape, while a fast response time 80 S was observed with samples on AlN substrates. Hence an optimal combination of photoresponse time and amplitude can be found by proper choice of substrate. It was found that PV increased nonlinearly with increase in n-doping concentration, indicating the existence of an optimal doping level. This result also suggests the possibility to further improve photoresponse by changing p-doping level. Finally, we checked the photoresponse in wide wavelength range (360-900 nm), and PV was observed throughout, indicating that the device could potentially be used as a broadband photodetector.
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    Wafer-scale films of aligned single-wall carbon nanotubes: preparation, characterization, and optoelectronic applications
    (2015-11-20) He, Xiaowei; Kono, Junichiro; Kelly, Kevin; Hauge, Robert; Adams, Wade
    Single-wall carbon nanotubes (SWCNTs) are one-dimensional materials defined by a cylindrical and hollow structure with aspect ratios of up to 10^7:1. Individual SWCNTs have been shown to possess excellent electric, optical, thermal, and mechanical properties that are promising for electronic and optoelectronic device applications. However, when they are assembled into macroscopic objects such as films and fibers, these unique properties tend to vanish, primarily due to disorder. Hence, methods are being sought for fabricating ordered SWCNT assemblies for the development of high-performance devices based on SWCNTs. In this dissertation, we present two methods for preparing highly aligned SWCNT films with excellent optoelectronic properties. The first method is based on vertically aligned SWCNT arrays grown by water-assisted chemical vapor deposition. We transferred these arrays to desired substrates to form horizontally aligned SWCNT films and created p-n junction devices that worked as flexible, room-temperature-operating, and polarization-sensitive infrared and terahertz photodetectors. The second method is based on our discovery of spontaneous global alignment of SWCNTs that occurs during vacuum filtration of SWCNT suspensions. By carefully controlling critical factors during vacuum filtration, we obtained wafer-scale, monodomain films of strongly aligned SWCNTs. By measuring polarization-dependent terahertz transmittance, we demonstrated ideal polarizer performance with large extinction ratios. The universality of this method was confirmed by applying it to diverse types of SWCNTs, all of which showed exceptionally high degrees of alignment. Furthermore, we successfully fabricated aligned SWCNT films enriched in one specific chirality by combining our new method with an advanced nanotube sorting technique: aqueous two-phase extraction. Transistors fabricated using such films showed very high conductivity anisotropies and excellent on-off ratios.
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    Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes
    (Springer Nature, 2016) He, Xiaowei; Gao, Weilu; Xie, Lijuan; Li, Bo; Zhang, Qi; Lei, Sidong; Robinson, John M.; Hároz, Erik H.; Doorn, Stephen K.; Wang, Weipeng; Vajtai, Robert; Ajayan, Pulickel M.; Adams, W. Wade; Hauge, Robert H.; Kono, Junichiro
    The one-dimensional character of electrons, phonons and excitons in individual single-walled carbon nanotubes leads to extremely anisotropic electronic, thermal and optical properties. However, despite significant efforts to develop ways to produce large-scale architectures of aligned nanotubes, macroscopic manifestations of such properties remain limited. Here, we show that large (>cm2) monodomain films of aligned single-walled carbon nanotubes can be prepared using slow vacuum filtration. The produced films are globally aligned within ±1.5° (a nematic order parameter of ∼1) and are highly packed, containing 1 × 106 nanotubes in a cross-sectional area of 1 μm2. The method works for nanotubes synthesized by various methods, and film thickness is controllable from a few nanometres to ∼100 nm. We use the approach to create ideal polarizers in the terahertz frequency range and, by combining the method with recently developed sorting techniques, highly aligned and chirality-enriched nanotube thin-film devices. Semiconductor-enriched devices exhibit polarized light emission and polarization-dependent photocurrent, as well as anisotropic conductivities and transistor action with high on/off ratios.