Separation of electronically pure, narrow dispersed, pristine semiconducting single walled carbon nanotubes (S-CNT) from a heterogeneous as-synthesized mixture is essential for various semiconducting technologies and biomedical applications. While conjugated polymers are essential for this sorting step, it is highly desirable to remove any organic residues from the resulting devices. We report here the design and synthesis of a mild acid degradable π-conjugated polyimine polymer (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 S-CNTs and smaller diameter (0.8-1.2 nm) HiPCO S-CNTs. Overall, selectivity for 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 UV-Vis, X-ray photoelectron spectroscopy (XPS), and SEM 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 FETs were fabricated from the polymer-stripped aligned S-CNT arrays which further confirmed the semiconducting purity on the order of 99.9% or higher. We will present the recent optimized FET device results using this removable polymer.

The challenge of assembling semiconducting single-wall carbon nanotubes (s-SWCNTs) into densely packed, aligned arrays has limited the scalability and practicality of high-performance nanotube-based electronics technologies. The aligned deposition of s-SWCNTs via floating evaporative self-assembly (FESA) has promise for overcoming this challenge; however, the mechanisms behind FESA need to be elucidated before the technique can be improved and scaled. Here, we gain a deeper understanding of the FESA process by studying a stationary analogue of FESA and optically tracking the dynamics of the organic ink/water/substrate and ink/air/substrate interfaces during the typical FESA process. We observe that the ink/water interface serves to collect and confine the s-SWCNTs before alignment and that the deposition of aligned bands of s-SWCNTs occurs at the ink/water/substrate contact line during the depinning of both the ink/air/substrate and ink/water/substrate contact lines. We also demonstrate improved control over the interband spacing, bandwidth, and packing density of FESA-aligned s-SWCNT arrays. The substrate lift rate (5-15 mm min-1) is used to tailor the interband spacing from 90 to 280 μm while maintaining a constant aligned s-SWCNT bandwidth of 50 μm. Varying the s-SWCNT ink concentration (0.75-10 μg mL-1) allows the control of the bandwidth from 2.5 to 45 μm. A steep increase in packing density is observed from 11 s-SWCNTs μm-1 at 0.75 μg mL-1 to 20 s-SWCNTs μm-1 at 2 μg mL-1, with a saturated packing density of ∼24 s-SWCNTs μm-1. We also demonstrate the scaling of FESA to align s-SWCNTs on a 2.5 × 2.5 cm2 scale while preserving high-quality alignment on the nanometer scale. These findings help realize the scalable fabrication of well-aligned s-SWCNT arrays to serve as large-area platforms for next-generation semiconductor electronics.

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.

Recent advances in the solution-phase sorting and assembly of semiconducting single-walled carbon nanotubes (SWCNTs) have enabled significant gains in the performance of field-effect transistors (FETs) constructed from dense arrays of aligned SWCNTs. However, the channel length (LCH) downscaling behaviors of these arrays, which contain some organizational disorder (i.e., rotational misalignment and non-uniform pitch), have not yet been studied in detail below LCH of 100 nm. This study compares the behaviors of individualized SWCNTs with arrays of aligned, solution-cast SWCNTs in FETs with LCH ranging from 30 to 240 nm. The on-state conductance of both individual and array SWCNTs rises with decreasing LCH. Nearly ballistic transport is observed for LCH < 40 nm in both cases, reaching a conductance of 0.82 Go per SWCNT in arrays, where Go = 2e2/h is the quantum conductance. In the off-state, the off-current and subthreshold swing of the individual SWCNTs remain nearly invariant with decreasing LCH whereas array SWCNT FETs suffer from increasing off-state current and deteriorating subthreshold swing for LCH below 100 nm. We analyze array disorder using atomic force microscopy, which shows that crossing SWCNTs that arise from misoriented alignment raise SWCNTs off of the substrate for large portions of the channel when LCH is small. Electrostatics modeling analysis indicates that these raised SWCNTs are a likely contributor to the deteriorating off-current and subthreshold characteristics of arrays. These results demonstrate that improved inter-SWCNT pitch uniformity and alignment with minimal inter-SWCNT interactions will be necessary in order for solution processed SWCNT arrays to reach subthreshold performance on par with isolated SWCNTs. These results are also promising because they show that arrays of solution-processed SWCNTs can nearly reach ballistic conductance in the on-state despite imperfections in pitch and alignment.

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.

Single walled carbon nanotubes (SWCNTs) exhibit extraordinarily high current carrying capacity and suitable band gaps for logic and thin film field-effect transistors (FETs). Progress in honing the exceptional properties of SWCNTs at the multi-tube level, however, has been dampened by incremental advances in the materials science of electronic type sorting and assembly of SWCNTs. To address these challenges we leveraged the exceptional semiconducting sorting fidelity of polyfluorene polymers and we pioneered an alignment technique known as Floating Evaporative Self-Assembly (FESA). 1 Recently, we demonstrated individually placed and uniformly pitched purely semiconducting SWCNT arrays using these techniques, allowing for negligible current lost to scaling when the arrays were implemented as the channel in FETs. The SWCNT array FETs exhibited the highest on-state conductance of 261 μS/μm with a simultaneous on/off conductance ratio exceeding 10 5 for a multi-tube SWCNT FET. 2 Here we present our recent optimization of the contact resistance at the palladium-SWCNT interface, which enables a substantial increase in device on-current density compared to our previous report. For SWCNTs, the quality of the contact-semiconductor interface is highly dependent on the SWCNT packing density, film cleanliness, and surface energy, where any non-idealities may lead to poor palladium film adhesion, or even complete film delamination. 3 To address the contact challenge we perform surface treatments on the SWCNT array prior to depositing palladium contacts, which involves rinsing the SWCNT arrays in specific solvents and annealing the films in vacuum. In XPS and optical absorbance spectra, we observe significant mass reduction of the polyfluorene wrapper while maintaining the quality and alignment of the SWCNT array. AFM height maps of the palladium contacts deposited on top of the surface treated SWCNTs demonstrate nearly ideal conformation of the palladium on the SWCNT array. The surface-treated SWCNT array FETs exhibit more than 10-fold improvement in the device on-conductance compared to non-treated samples. [1] Y. Joo, G. J. Brady, M. S. Arnold, and P. Gopalan, Langmuir 30 (12), 3460 (2014). [2] G. J. Brady, Y. Joo, M. Y. Wu, M. J. Shea, P. Gopalan, and M. S. Arnold, ACS Nano 8 (11), 11614 (2014). [3] V. Perebeinos and J. Tersoff, Physical Review Letters 114 (8), 4 (2015).

In this paper, we report record radio frequency (RF) performance of carbon nanotube transistors based on combined use of a self-aligned T-shape gate structure, and well-aligned, high-semiconducting-purity, high-density polyfluorene-sorted semiconducting carbon nanotubes, which were deposited using dose-controlled, floating evaporative self-assembly method. These transistors show outstanding direct current (DC) performance with on-current density of 350 μA/μm, transconductance as high as 310 μS/μm, and superior current saturation with normalized output resistance greater than 100 kΩ·μm. These transistors create a record as carbon nanotube RF transistors that demonstrate both the current-gain cutoff frequency (ft) and the maximum oscillation frequency (fmax) greater than 70 GHz. Furthermore, these transistors exhibit good linearity performance with 1 dB gain compression point (P1dB) of 14 dBm and input third-order intercept point (IIP3) of 22 dBm. Our study advances state-of-the-art of carbon nanotube RF electronics, which have the potential to be made flexible and may find broad applications for signal amplification, wireless communication, and wearable/flexible electronics.

Carbon nanotubes (CNTs) are tantalizing candidates for semiconductor electronics because of their exceptional charge transport properties and one-dimensional electrostatics. Ballistic transport approaching the quantum conductance limit of 2G 0 = 4e (2)/h has been achieved in field-effect transistors (FETs) containing one CNT. However, constraints in CNT sorting, processing, alignment, and contacts give rise to nonidealities when CNTs are implemented in densely packed parallel arrays such as those needed for technology, resulting in a conductance per CNT far from 2G 0. The consequence has been that, whereas CNTs are ultimately expected to yield FETs that are more conductive than conventional semiconductors, CNTs, instead, have underperformed channel materials, such as Si, by sixfold or more. We report quasi-ballistic CNT array FETs at a density of 47 CNTs μm(-1), fabricated through a combination of CNT purification, solution-based assembly, and CNT treatment. The conductance is as high as 0.46 G 0 per CNT. In parallel, the conductance of the arrays reaches 1.7 mS μm(-1), which is seven times higher than the previous state-of-the-art CNT array FETs made by other methods. The saturated on-state current density is as high as 900 μA μm(-1) and is similar to or exceeds that of Si FETs when compared at and equivalent gate oxide thickness and at the same off-state current density. The on-state current density exceeds that of GaAs FETs as well. This breakthrough in CNT array performance is a critical advance toward the exploitation of CNTs in logic, high-speed communications, and other semiconductor electronics technologies.

We have pioneered a scalable approach for depositing aligned arrays of ultrahigh purity semiconducting SWCNTs (prepared using polyfluorene-derivatives) called floating evaporative self-assembly (FESA). In this talk, we will first present on the scaling and physics of FESA. We will then present on high performance field effect transistors (FETs) fabricated from the arrays. FESA is exploited to create FETs with exceptionally high combined on-conductance and on-off ratio of 261 μS/μm and 2x105, respectively, for a channel length of 240 nm. This is 1400x greater on-off ratio than SWCNT FETs fabricated by other methods, at comparable on-conductance per width of ~250 µS/µm, and 30-100x greater on-conductance per width, at comparable on-off ratio of 105-107.

Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribbons narrower than 10 nm with controlled crystallographic orientation and well-defined armchair edges. However, the scalable synthesis of nanoribbons with this precision directly on insulating or semiconducting substrates has not been possible. Here we demonstrate the synthesis of graphene nanoribbons on Ge(001) via chemical vapour deposition. The nanoribbons are self-aligning 3° from the Ge〈110〉 directions, are self-defining with predominantly smooth armchair edges, and have tunable width to <10 nm and aspect ratio to >70. In order to realize highly anisotropic ribbons, it is critical to operate in a regime in which the growth rate in the width direction is especially slow, <5 nm h −1 . This directional and anisotropic growth enables nanoribbon fabrication directly on conventional semiconductor wafer platforms and, therefore, promises to allow the integration of nanoribbons into future hybrid integrated circuits.

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.

Several schemes have been demonstrated to successfully extract semiconducting (s-) single-walled carbon nanotubes (SWCNTs) from heterogeneous as-grown mixtures containing metallic (m-) SWCNTs. The polyfluorene family of conjugated polymers, specifically poly(9,9-dioctylfluorene-2,7-diyl) (PFO), have been shown to extract s-SWCNTs from mixtures of s- and m-SWCNTs with high selectivity (>99% s-SWCNT) by preferentially wrapping the s-SWCNT components. In addition to electronic type, the selectivity is heavily dependent on the nanotube diameter and chiral angle. To date, there is an inadequate understanding of the polymer-CNT interactions that lead to the high preference for s- over m-SWCNTs in the wrapping process. It is vital to understand these parameters to design separation schemes to reduce the proportion of m-SWCNT impurities to technologically relevant ppm and ppb levels. In this work, we explore the chemistry of non-covalent nanotube-polymer wrapping processes and describe the effects of surface coverage and binding configuration on the separation efficiency. We disperse SWCNTs grown by the HiPCO process, which contain many chiral species of SWCNT with diameters near 1 nm, in toluene solutions of varying PFO concentration. The resulting dispersions are analyzed with optical absorption spectroscopy, excitation-emission photoluminescence spectroscopy, and photoluminescence anisotropy. First, we experimentally quantify the amount of PFO wrapping the SWCNTs by measuring exciton energy transfer from PFO to SWCNTs via photoluminescence spectroscopy. We characterize the dependence of the surface coverage on PFO concentration, finding that the surface coverage increases by a factor of 7 as PFO concentration increases from 0.2 to 2 mg/mL. Second, we demonstrate that the wrapping of nanotubes by PFO is described by a Langmuir adsorption isotherm, in which surface coverage of PFO on the nanotubes follows an S-shaped curve as a function of PFO concentration. The shape of the curve is related to the polymer-nanotube binding energy, which depends on the nanotube electronic type, diameter, chiral angle, solvent, and polymer molecular weight. Third, we determine the binding configuration of the PFO on the nanotube surface as a function of PFO concentration and demonstrate that the PFO becomes more ordered as the surface coverage decreases. In the highly-ordered PFO regime we estimate the wrapping angle of individual PFO strands around nanotubes in solution. Finally, we find that the metal-semiconductor separation efficiency is lowest when the concentration of PFO is high, surface coverage is high, and surface ordering is low. Understanding the PFO wrapping process will be an important step toward attaining high-purity s-SWCNT from this and similar sorting procedures for a range of polymer-CNT systems. These advances will enable further development of technologically relevant high purity s-SWCNTs for next-generation electronic devices.

We evaluate the performance of exceptionally electronic-type sorted, semiconducting, aligned single-walled carbon nanotubes (s-SWCNTs) in field effect transistors (FETs). High on-conductance and high on/off conductance modulation are simultaneously achieved at channel lengths which are both shorter and longer than individual s-SWCNTs. The s-SWCNTs are isolated from heterogeneous mixtures using a polyfluorene-derivative as a selective agent and aligned on substrates via dose-controlled, floating evaporative self-assembly at densities of ∼50 s-SWCNTs μm−1. At a channel length of 9 μm the s-SWCNTs percolate to span the FET channel, and the on/off ratio and charge transport mobility are 2.2 × 107 and 46 cm2 V−1 s−1, respectively. At a channel length of 400 nm, a large fraction of the s-SWCNTs directly span the channel, and the on-conductance per width is 61 μS μm−1 and the on/off ratio is 4 × 105. These results are considerably better than previous solution-processed FETs, which have suffered from poor on/off ratio due to spurious metallic nanotubes that bridge the channel. 4071 individual and small bundles of s-SWCNTs are tested in 400 nm channel length FETs, and all show semiconducting behavior, demonstrating the high fidelity of polyfluorenes as selective agents and the promise of assembling s-SWCNTs from solution to create high performance semiconductor electronic devices.

Arrays of aligned semiconducting single-walled carbon nanotubes (s-SWCNTs) with exceptional electronic-type purity were deposited at high deposition velocity of 5 mm min(-1) by a novel "dose-controlled, floating evaporative self-assembly" process with excellent control over the placement of stripes and quantity of s-SWCNTs deposited. This approach uses the diffusion of organic solvent on the water-air interface to deposit aligned s-SWCNT (99.9%) tubes on a partially submerged hydrophobic substrate, which is withdrawn vertically from the surface of water. By decoupling the s-SWCNT stripe formation from the evaporation of the bulk solution and by iteratively applying the s-SWCNTs in controlled "doses", we show through polarized Raman studies that the s-SWCNTs are aligned within ±14°, are packed at a density of ∼50 s-SWCNTs μm(-1), and constitute primarily a well-ordered monodispersed layer. The resulting field-effect transistor devices show high performance with a mobility of 38 cm(2) V(-1) s(-1) and on/off ratio of 2.2 × 10(6) at 9 μm channel length.

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.