Electrical Properties of a Tube-in-a-Tube Semiconductor
Ng, Allen Lee
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Tube-in-a-tube (Tube^2) nanostructures were synthesized through the outer-wall selective covalent functionalization of double-walled carbon nanotubes (DWCNTs) at high functional densities. Upon functionalization, the properties of individual walls within the structure decouple resulting in an electrically insulating functional outer tube while the inner tube retains exceptional CNT properties. The exceptional electrical properties of Tube^2 semiconductor structures were demonstrated for applications that include molecular and biological sensors and patterning of CNTbased structures with electronic type specificity. Tube^2 thin film transistor (TFT) sensors exhibited simultaneous ultrahigh sensitivity and selectivity towards chemical and biological targets. Carboxylic acid terminated Tube^2 sensors displayed an NH3 sensitivity of 60 nM, which is comparable with small molecule aqueous solution detection using state-of-the-art TFT sensors while simultaneously attaining 6,000 times higher chemical selectivity towards a variety of amine containing analyte molecules over carboxylic acids. Similarly, 23-base ii oligonucleotide terminated Tube^2 sensors demonstrated concomitant sensitivity down to 5 nM towards their complementary sequence without amplification techniques and single mismatch selectivity without the use of a gate electrode. Unique sensor architectures can be designed with the requirement of a gate electrode, such as the creation of millimeter-scale point sensors. The optical features and unique structural features of Tube^2 thin films were also exploited to address the challenge of patterning CNT nanostructures with electronic type specificity. Patterned dot arrays and conductive pathways were created on an initially insulating Tube^2 thin film by tuning the resonance of the direct-writing laser with the electronic type of the inner tube (i.e., metallic or semiconducting). The successful patterning of Tube^2 thin films was unambiguously confirmed with in situ Raman spectral imaging and electrical characterization. Furthermore, a hybrid 2-D carbon nanostructure comprised of a functionalized graphene that covers a semiconducting (6,5) SWCNT network (fG/sSWCNT) was developed. The hybrid fG/sSWCNT nanostructure exhibits similar structural and electrical properties as a semiconducting Tube^2 thin film, but possesses a transconductance that is an order of magnitude larger than Tube^2 and ON/OFF ratios as high as 5400 without the useful of further processing steps such as electrical breakdown.