Structurally Analogous Degradable Version of Fluorene-Bipyridine Copolymer with Exceptional Selectivity for Large-Diameter Semiconducting Carbon Nanotubes.

Separation of electronically pure, narrowly dispersed, pristine, semiconducting single-walled carbon nanotubes (CNTs) from a heterogeneous as-synthesized mixture is essential for various semiconducting technologies and biomedical applications. Although conjugated polymer wrappers are often utilized to facilitate electronic-type sorting, it is highly desirable to remove organic residues from the resulting devices. We report here the design and synthesis of a mild acid-degradable π-conjugated polyimine polymer, poly[(9,9-di-n-octyl-2,7-fluoren-dinitrilomethine)-alt-co-(6,6'-{2,2'-bipyridyl-dimethine})] (PFO-N-BPy), that is structurally analogous to the commonly used and commercially available poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-(2,2'-bipyridine))] (PFO-BPy). An acid cleavable imine link (-HC═N-) was introduced in the PFO-N-BPy backbone to impart degradability, which is absent in PFO-BPy. PFO-N-BPy was synthesized via a metal catalyst-free aza-Wittig reaction in high yields. PFO-N-BPy with a degree of polymerization of just ∼10 showed excellent (>99% electronic purity) selectivity for both large-diameter (1.3-1.7 nm) arc-discharge semiconducting CNTs (S-CNTs) and smaller diameter (0.8-1.2 nm) high-pressure carbon monoxide disproportionation reaction S-CNTs. Overall, the selectivity for the semiconducting species is similar to that of PFO-BPy but with an advantage of complete depolymerization under mild acidic conditions into recyclable monomers. We further show by ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy that the PFO-N-BPy-wrapped S-CNTs can be aligned into a monolayer array on gate dielectrics using a floating evaporative self-assembly process from which the polymer can be completely removed. Short channel field effect transistors were fabricated from the polymer-stripped aligned S-CNT arrays, which further confirmed the semiconducting purity on the order of 99.9% or higher.

Polymer-Free Electronic-Grade Aligned Semiconducting Carbon Nanotube Array.

Conjugated polymers are used commonly to selectively sort semiconducting carbon nanotubes (S-CNTs) from their metallic counterparts in organic solvents. The polymer-wrapped S-CNTs can be easily processed from organic solvents into arrays of CNTs for scalable device fabrication. Though the conjugated polymers are essential for sorting and device fabrication, it is highly desirable to remove them completely as they limit the electronic properties of the device. Here, we use a commercially available polymer, namely, poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-(2,2'-bipyridine))] (PFO-BPy), to sort large-diameter S-CNTs with ultrahigh selectivity and fabricate CNT-array-based field effect transistors (FETs) via a floating evaporative self-assembly (FESA) process. We report quantitative removal of the polymer wrapper from the FESA aligned S-CNT arrays using a metal-chelation-assisted polymer removal (McAPR) process. The implementation of this process on FESA films requires the selective thermal degradation of the polymer into oligomers, combined with optimization of the solvent type and temperature of the metal complexation reaction. Resulting S-CNT array FET devices show that the electronic properties of pristine CNT are preserved through this process. Optical microscopy, UV-vis spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to characterize the quantitative polymer removal. We quantitatively describe the FET devices to analyze the fundamental characteristics of FETs (mobility (µ), on-conductance (Gon), and contact resistance (2Rc)) by comparing before and after polymer removal. The ability to completely remove the polymer wrapper in aligned CNT arrays without adversely affecting the device properties opens up applications beyond FETs into photovoltaics and biosensing.

Ultrafast Exciton Hopping Observed in Bare Semiconducting Carbon Nanotube Thin Films with Two-Dimensional White-Light Spectroscopy.

We observe ultrafast energy transfer between bare carbon nanotubes in a thin film using two-dimensional (2D) white-light spectroscopy. Using aqueous two-phase separation, semiconducting carbon nanotubes are purified from their metallic counterparts and condensed into a 10 nm thin film with no residual surfactant. Cross peak intensities put the time scale for energy transfer at <60 fs, and 2D anisotropy measurements determine that energy transfer is most efficient between parallel nanotubes, thus favoring directional energy flow. Lifetimes are about 300 fs. Thus, these results are in sharp contrast to thin films prepared from nanotubes that are wrapped by polymers, which exhibit picosecond energy transfer and randomize the direction of energy flow. Ultrafast energy flow and directionality are exciting properties for next-generation photovoltaics, photodetectors, and other devices.

Isolation of Pristine Electronics Grade Semiconducting Carbon Nanotubes by Switching the Rigidity of the Wrapping Polymer Backbone on Demand.

Conjugated polymers are among the most selective carbon nanotube sorting agents discovered and enable the isolation of ultrahigh purity semiconducting singled-walled carbon nanotubes (s-SWCNTs) from heterogeneous mixtures that contain problematic metallic nanotubes. The strong selectivity though highly desirable for sorting, also leads to irreversible adsorption of the polymer on the s-SWCNTs, limiting their electronic and optoelectronic properties. We demonstrate how changes in polymer backbone rigidity can trigger its release from the nanotube surface. To do so, we choose a model polymer, namely poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,60-(2,20-bipyridine))] (PFO-BPy), which provides ultrahigh selectivity for s-SWCNTs, which are useful specifically for FETs, and has the chemical functionality (BPy) to alter the rigidity using mild chemistry. Upon addition of Re(CO)5Cl to the solution of PFO-BPy wrapped s-SWCNTs, selective chelation with the BPy unit in the copolymer leads to the unwrapping of PFO-BPy. UV-vis, XPS, and Raman spectroscopy studies show that binding of the metal ligand complex to BPy triggers up to 85% removal of the PFO-BPy from arc-discharge s-SWCNTs (diameter = 1.3-1.7 nm) and up to 72% from CoMoCAT s-SWCNTs (diameter = 0.7-0.8 nm). Importantly, Raman studies show that the electronic structure of the s-SWCNTs is preserved through this process. The generalizability of this method is demonstrated with two other transition metal salts. Molecular dynamics simulations support our experimental findings that the complexation of BPy with Re(CO)5Cl in the PFO-BPy backbone induces a dramatic conformational change that leads to a dynamic unwrapping of the polymer off the nanotube yielding pristine s-SWCNTs.

Polyfluorene-sorted, carbon nanotube array field-effect transistors with increased current density and high on/off ratio.

Challenges in eliminating metallic from semiconducting single-walled carbon nanotubes (SWCNTs) and in controlling their alignment have limited the development of high-performance SWCNT-based field-effect transistors (FETs). We recently pioneered an approach for depositing aligned arrays of ultra-high-purity semiconducting SWCNTs, isolated using polyfluorene derivatives, called dose-controlled floating evaporative self-assembly. Here, we tailor FETs fabricated from these arrays to achieve on-conductance (G(on)) per width and an on-off ratio (G(on)/G(off)) of 261 µS/µm and 2 × 10(5), respectively, for a channel length (L(ch)) of 240 nm and 116 µS/µm and 1 × 10(6), respectively, for an L(ch) of 1 µm. We demonstrate 1400× greater G(on)/G(off) than SWCNT FETs fabricated by other methods, at comparable G(on) per width of ∼250 µS/µm and 30-100× greater G(on) per width at comparable G(on)/G(off) of 10(5)-10(7). The average G(on) per tube reaches 5.7 ± 1.4 µS at a packing density of 35 tubes/µm for L(ch) in the range 160-240 nm, limited by contact resistance. These gains highlight the promise of using ultra-high-purity semiconducting SWCNTs with controlled alignment for next-generation semiconductor electronics.

Experimental Measurement of the Binding Configuration and Coverage of Chirality-Sorting Polyfluorenes on Carbon Nanotubes.

Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) exhibits exceptional (n,m) chirality and electronic-type selectivity for near-armchair semiconducting carbon nanotubes. To better understand and control the factors governing this behavior, we experimentally determine the surface coverage and binding configuration of PFO on nanotubes in solution using photoluminescence energy transfer and anisotropy measurements. The coverage increases with PFO concentration in solution, following Langmuir-isotherm adsorption behavior with cooperativity. The equilibrium binding constant (PFO concentration in solution at half coverage), KA, depends on (n,m) and is 1.16 ± 0.30, 0.93 ± 0.12, and 1.13 ± 0.26 mg mL(-1) for the highly selected (7,5), (8,6), and (8,7) species, respectively, and the corresponding PFO wrapping angle at low coverage is 12, 17, and 14 ± 2°, respectively. In contrast, the inferred KA for metallic nanotubes is nearly an order of magnitude greater, indicating that the semiconducting selectivity increases with decreasing PFO concentration. This understanding will quantitatively guide future experimental and computational efforts on electronic type-sorting carbon nanotubes.