Presence Of Metallic Nanotubes Research Articles

High Performance Field-Effect Transistors Via Aligned Polyfluorene-Sorted Single-Walled Carbon Nanotube Arrays

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising materials for charge transport in field-effect transistor (FET) devices. In order to develop short channel s-SWCNT FETs with high on/off ratios relevant for logic applications, ultra-pure s-SWCNTs of 99.99999% semiconducting purity and higher are needed. There have been many recent studies on the sorting of carbon nanotubes, but most reports have only reached semiconducting purity levels near 99%. In addition, aligned arrays of s-SWCNTs have the potential to improve the mobility and current output of FETs. However, unacceptable semiconducting purity levels (~99%), inter-nanotube charge screening, and inter-nanotube bundling have limited the optimization of such technologies in the short channel regime.1,2 Polyfluorene polymers have been shown to selectively wrap s-SWCNTs on the basis of electronic type, chirality, and diameter. Here we assess the purity and electronic transport properties of aligned arrays of polyfluorene-sorted s-SWCNTs in FET devices. We obtained high purity s-SWCNTs by dispersing nanotube powder in solutions of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6’-{2,2’-bipyridine)] (PFO-BPy) in toluene following procedures similar to Mistry et al.3 We fabricated well-aligned s-SWCNT arrays by dispersing droplets of of s-SWCNTs at an air water interface, leading to diffusion and self-assembly of nanotubes into well-aligned “stripes” spanning the entire substrate. Finally, we implemented the aligned films as the channel material in field-effect transistors at varying channel lengths ranging from 9 μm to 300 nm. We characterized the films using Raman spectroscopy, AFM and SEM, and found a high degree of alignment, monolayer thickness, and packing density of 50 tubes/micrometer. Previous works on aligned nanotube FETs have used nanotube densities of 200 tubes/micrometer; however, the proximity of nanotubes in these films cause inter-nanotube screening effects resulting lower conductance and mobility values than expected.1 The polyfluorene polymer plays an important role in isolating s-SWCNTs by preventing direct contact, preventing inter-nanotube screening effects.4 The electronic properties of the aligned film FETs were measured demonstrating high values of on/off ratio, mobility, and conductance. In previous studies, device on/off ratios degraded substantially to less than 103 at channel lengths shorter than 1 micrometer, indicating the presence of metallic nanotubes.5 Here we observe a different trend; as the channel length decreases from 9 μm to 300 nm the on/off ratio only decreases from 3x107 to 2x105. This data suggests that m-SWCNTs are far scarcer in our films than in other short channel s-SWCNT FET devices. We estimated purity by counting the number of nanotubes per channel in direct transport regime devices that have channel lengths of 500 nm and shorter. In fact, of the devices measured, encompassing approximately 1,000 nanotubes, zero exhibited metallic character (defined as having an on/off ratio of at least 103), suggesting a s-SWCNT purity of greater than 99.9%.We achieved consistently high performance devices indicated by high mobility in devices that also have high on/off ratios. The highest mobility was 45 cm2/Vs for a 2 μm channel length device with on/off ratio 2x105. We observed high mobility values ranging from 16-45 cm2/Vs across a range of channel lengths. Previous works were limited by purity, leading to trade offs between high on/off ratio and mobility, especially in the direct transport regime.5 Here we demonstrate a route to achieve both a large on/off ratio and high mobility, which is particularly significant for short channel devices in which nanotubes directly span the channel.References 1 Q. Cao, et al., Nature Nanotechnology 8 (3), 180 (2013). 2 M. Engel, et al., ACS Nano 2 (12), 2445 (2008). 3 K S. Mistry, et. al., ACS Nano 7 (3), 2231 (2013). 4 F. Léonard, Nanotechnology 17 (9), 2381 (2006). 5 Nima Rouhi, et al., ACSNANO 5 (11), 8471 (2011).

Physical removal of metallic carbon nanotubes from nanotube network devices using a thermal and fluidic process

Electronic and optoelectronic devices based on thin films of carbon nanotubes are currently limited by the presence of metallic nanotubes. Here we present a novel approach based on nanotube alkyl functionalization to physically remove the metallic nanotubes from such network devices. The process relies on preferential thermal desorption of the alkyls from the semiconducting nanotubes and the subsequent dissolution and selective removal of the metallic nanotubes in chloroform. The approach is versatile and is applied to devices post-fabrication.

General Rules for Selective Growth of Enriched Semiconducting Single Walled Carbon Nanotubes with Water Vapor as in Situ Etchant

The presence of metallic nanotubes in as-grown single walled carbon nanotubes (SWNTs) is the major bottleneck for their applications in field-effect transistors. Herein, we present a method to synthesize enriched, semiconducting nanotube arrays on quartz substrate. It was discovered that introducing appropriate amounts of water could effectively remove the metallic nanotubes and significantly enhance the density of SWNT arrays. More importantly, we proposed and confirmed that the high growth selectivity originates from the etching effect of water and the difference in the chemical reactivities of metallic and semiconducting nanotubes. Three important rules were summarized for achieving a high selectivity in growing semiconducting nanotubes by systematically investigating the relationship among water concentration, carbon feeding rate, and the percentage of semiconducting nanotubes in the produced SWNT arrays. Furthermore, these three rules can be applied to the growth of random SWNT networks on silicon wafers.

Fundamental Limits on the Mobility of Nanotube‐Based Semiconducting Inks

[∗] N. Rouhi , D. Jain , K. Zand Prof. P. J. Burke Integrated Nanosytems Research Facility Department of Electrical Engineering and Computer Science University of California-Irvine Irvine, CA, USA; pburke@uci.edu Carbon-nanotube-based semiconducting inks offer great promise for a variety of applications including fl exible, transparent, and printed electronics and optics. A critical drawback of such inks has been the presence of metallic nanotubes, which causes high-mobility inks to suffer from poor on/off ratios, preventing their applications in a wide variety of commercial settings. Here, we report a comprehensive study of the relationship between mobility, density, and on/off ratios of solution-based, deposited semiconducting nanotube ink used as the channel in fi eld effect transistors. A comprehensive spectrum of the density starting from less than 10 tubes μ m − 2 to the high end of more than 100 tubes μ m − 2 have been investigated. These studies indicate a quantitative trend of decreasing on/off ratio with increasing density and mobility, starting with mobilities over 90 cm 2 V − 1 s − 1 (approaching that of p-type Si MOSFETs) but with on/off ratios ∼ 10, and ending with on/off ratios > 10 5

Technological dispersion in CNTFET: Impact of the presence of metallic carbon nanotubes in logic circuits

This paper investigates the impact of the metallic nanotube (M-NT) presence within CNTFET circuits: circuit yield and performances are analyzed using calibrated compact models for both the CNTFET and the M-NT. The major cause of technological dispersion in CNTFET technology comes from the control of the carbon nanotube chirality. This lack of control may lead to the presence of metallic nanotubes in the transistor. These M-NTs create shorts which dramatically increase the source–drain leakage current. The random presence and position of M-NT in the ring oscillator circuit is analyzed using Monte Carlo simulation. Two inverter layout configurations are considered in the study and we deduce that a strong improvement in term of yield and power consumption can be obtained using a specific layout configuration where the tubes are shared for the P-CNTFET and the N-CNTFET.