Low-dimensional solution-processable electronics:from field-effect transistor to artificial synapse

Modern-day computing has irreversibly impacted our way of living. Nowadays, we are surrounded by computers that become smarter and more connected every day. The demand for more powerful hardware increases, while physical limitations already come into play. Pushing the boundaries of hardware development requires a new strategy, especially for (visual) pattern recognition. For this, a novel material candidate is semiconductive single-walled carbon nanotubes (s-SWCNTs). They promise inexpensive and scalable production methods for electronics. This thesis focuses on experimental research for the preparation and utilisation of these s-SWCNTs. We optimised s-SWCNT ink preparation by substituting the applicator-solvent in a colloidal dispersion of polymer-wrapped s-SWCNT. This resulted in improved shelf-life, field-effect transistor (FET) performances and device reproducibility. Also, we successfully demonstrated three newly designed low-bandgap polymers that wrap and select s-SWCNT. This resulted in FETs with improved energy-band alignment, potentially lowering energy consumption. More insight is provided into the relationship between the polymer structure and the dispersion capability for s-SWCNT, subsequently, the final inks' physical properties. Finally, we demonstrated that sufficient plasticity arises to obtain a functional artificial synapse using simple pulse-shapes by utilising the hysteresis commonly found in bottom-gate structure SWCNT FETs. As synapses are the functional connection between neurons and are believed to be the units enabling learning and computing, our finding enables usage in more energy-efficient computing architecture. We conclude that our improved inks can be easily industrially applied. The artificial synapse can potentially be used by neuromorphic computing for everyday pattern recognition tasks, enabling more powerful hardware

Field-Effect Transistors Based on Formamidinium Tin Triiodide Perovskite

To date, there are no reports of 3D tin perovskite being used as a semiconducting channel in field-effect transistors (FETs). This is probably due to the large amount of trap states and high p-doping typical of this material. Here, the first top-gate bottom-contact FET using formamidinium tin triiodide perovskite films is reported as a semiconducting channel. These FET devices show a hole mobility of up to 0.21 cm(2) V-1 s(-1), an I-ON/OFF ratio of 10(4), and a relatively small threshold voltage (V-TH) of 2.8 V. Besides the device geometry, the key factor explaining this performance is the reduced doping level of the active layer. In fact, by adding a small amount of the 2D material in the 3D tin perovskite, the crystallinity of FASnI(3) is enhanced, and the trap density and hole carrier density are reduced by one order of magnitude. Importantly, these transistors show enhanced parameters after 20 months of storage in a N-2 atmosphere

Real-Time Monitoring of Cellular Cultures with Electrolyte-Gated Carbon Nanotube Transistors

Cell-based biosensors constitute a fundamental tool in biotechnology, and their relevance has greatly increased in recent years as a result of a surging demand for reduced animal testing and for high-throughput and cost-effective in vitro screening platforms dedicated to environmental and biomedical diagnostics, drug development and toxicology. In this context, electrochemical/electronic cell-based biosensors represent a promising class of devices that enable long-term and real-time monitoring of cell physiology in a non-invasive and label-free fashion, with a remarkable potential for process automation and parallelization. Common limitations of this class of devices at large include the need for substrate surface modification strategies to ensure cell adhesion and immobilization, limited compatibility with complementary optical cell-probing techniques, and need for frequency-dependent measurements, which rely on elaborated equivalent electrical circuit models for data analysis and interpretation. We hereby demonstrate the monitoring of cell adhesion and detachment through the time-dependent variations in the quasi-static characteristic current curves of a highly stable electrolyte-gated transistor, based on an optically transparent network of printable polymer-wrapped semiconducting carbon-nanotubes

Efficient Selective Sorting of Semiconducting Carbon Nanotubes Using Ultra-Narrow-Band-Gap Polymers

[Image: see text] Conjugated polymers with narrow band gaps are particularly useful for sorting and discriminating semiconducting single-walled carbon nanotubes (s-SWCNT) due to the low charge carrier injection barrier for transport. In this paper, we report two newly synthesized narrow-band-gap conjugated polymers (PNDITEG-TVT and PNDIC8TEG-TVT) based on naphthalene diimide (NDI) and thienylennevinylene (TVT) building blocks, decorated with different polar side chains that can be used for dispersing and discriminating s-SWCNT. Compared with the mid-band-gap conjugated polymer PNDITEG-AH, which is composed of naphthalene diimide (NDI) and head-to-head bithiophene building blocks, the addition of a vinylene linker eliminates the steric congestion present in head-to-head bithiophene, which promotes backbone planarity, extending the π-conjugation length and narrowing the band gap. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest that inserting a vinylene group in a head-to-head bithiophene efficiently lifts the highest occupied molecular orbital (HOMO) level (−5.60 eV for PNDITEG-AH, −5.02 eV for PNDITEG-TVT, and −5.09 eV for PNDIC8TEG-TVT). All three polymers are able to select for s-SWCNT, as evidenced by the sharp transitions in the absorption spectra. Field-effect transistors (FETs) fabricated with the polymer:SWCNT inks display p-dominant properties, with higher hole mobilities when using the NDI-TVT polymers as compared with PNDITEG-AH (0.6 cm(2) V(–1) s(–1) for HiPCO:PNDITEG-AH, 1.5 cm(2) V(–1) s(–1) for HiPCO:PNDITEG-TVT, and 2.3 cm(2) V(–1) s(–1) for HiPCO:PNDIC8TEG-TVT). This improvement is due to the better alignment of the HOMO level of PNDITEG-TVT and PNDIC8TEG-TVT with that of the dominant SWCNT specie

Polar Side Chains Enhance Selection of Semiconducting Single-Walled Carbon Nanotubes by Polymer Wrapping

This paper describes the effectiveness of donor-acceptor (D-A) conjugated polymers to disperse and select for semiconducting single-walled carbon nanotubes (s-SWCNTs) when enhanced by the inclusion of polar oligoethylene glycol-based side chains, without altering the D-A backbone. We designed and synthesized two sets of naphthalenediimide(NDI)-alt-bithiophene(T2)-based conjugated polymers with one of two alkyl side chains (decyl and dodecyl chains) of different lengths and with or without polar triethylene glycol side chains. The resulting low-band-gap copolymers all effectively disperse and select for s-SWCNT, but the inclusion of polar side chains enhances the interactions between the polymer backbone and the walls of the s-SWCNTs relative to the polymers with only alkyl side chains. As a result, the wrapping and selection efficiency of the polymer-SWCNT systems with polar side chains are both significantly enhanced. We further optimized the binding energy and surface coverage by combining glycol ether and dodecyl side chains to maximize wrapping efficiency, leading to a field-effect mobility of 2.82 cm2 V-1 s-1 and on/off current ratios of ∼2 × 107 in polymer-wrapped SWCNTs. Our results provide insight into the role of the side-chain interactions in the polymer wrapping and dispersion technique, and, because we focus on manipulating side chains, they can be generalized for other conjugated polymer backbones

Customizing the Polarity of Single-Walled Carbon-Nanotube Field-Effect Transistors Using Solution-Based Additives

Polarity control in semiconducting single-walled carbon-nanotube field-effect transistors (s-SWNT FETs) is important to promote their application in logic devices. The methods to turn the intrinsically ambipolar s-SWNT FETs into unipolar devices that have been proposed until now require extra fabrication steps that make preparation longer and more complex. It is demonstrated that by starting from a highly purified ink of semiconducting single-walled carbon nanotubes sorted by a conjugated polymer, and mixing them with additives, it is possible to achieve unipolar charge transport. The three additives used are benzyl viologen (BV), 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), which give rise to n-type field-effect transistors, and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F-4-TCNQ) which gives rise to p-type transistors. BV and N-DMBI transform the s-SWNTs transistors from ambipolar with mobility of the order of 0.7 cm(2) V-1 s(-1) to n-type with mobility up to 5 cm(2) V-1 s(-1). F-4-TCNQ transform the ambipolar transistors in p-type with mobility up to 16 cm(2) V-1 s(-1)

Understanding the Selection Mechanism of the Polymer Wrapping Technique toward Semiconducting Carbon Nanotubes

Noncovalent functionalization of single-walled carbon nanotubes (SWNTs) using π-conjugated polymers has become one of the most effective techniques to select semiconducting SWNTs (s-SWNTs). Several conjugated polymers are used, but their ability to sort metallic and semiconducting species, as well as the dispersions yields, varies as a function of their chemical structure. Here, three polymers are compared, namely, poly[2,6-(4,4-bis-(2-dodecyl)-4H-cyclopenta[2,1-b;3,4b′]dithiophene)-alt-4,7(2,1,3-ben-zothiadiazole)] (P12CPDTBT), poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PF12), and poly(3-dodecylthiophene-2,5-diyl) (P3DDT) in their ability to select two types of carbon nanotubes comprising small (≈1 nm) and large (≈1.5 nm) diameters. P12CPDTBT is a better dispersant than PF12 for small diameter nanotubes, while both polymers are good dispersants of large diameter nanotubes. However, these dispersions contain metallic species. P3DDT, instead presents the best overall performance regarding the selectivity toward semiconducting species, with a dispersion yield for s-SWNTs of 15% for small and 21% for large diameter nanotubes. These results are rationalized in terms of electronic and chemical structure showing that: (i) the binding energy is stronger when more alkyl lateral chains adsorb on the nanotube surface; (ii) the binding energy is stronger when the polymer backbone is more flex-ible; (iii) the purity of the dispersions seems to depend on a strong polymer– nanotube interaction

Inducing ferromagnetism and Kondo effect in platinum by paramagnetic ionic gating

Electrically controllable magnetism, which requires the field-effect manipulation of both charge and spin degrees of freedom, has attracted growing interest since the emergence of spintronics. We report the reversible electrical switching of ferromagnetic (FM) states in platinum (Pt) thin films by introducing paramagnetic ionic liquid (PIL) as the gating media. The paramagnetic ionic gating controls the movement of ions with magnetic moments, which induces itinerant ferromagnetism on the surface of Pt films, with large coercivity and perpendicular anisotropy mimicking the ideal two-dimensional Ising-type FM state. The electrical transport of the induced FM state shows Kondo effect at low temperature, suggesting spatially separated coexistence of Kondo scattering beneath the FM interface. The tunable FM state indicates that paramagnetic ionic gating could serve as a versatile method to induce rich transport phenomena combining field effect and magnetism at PIL-gated interfaces

Heterostructure from PbS Quantum Dot and Carbon Nanotube Inks for High-Efficiency Near-Infrared Light-Emitting Field-Effect Transistors

Light-emitting field-effect transistors (LEFETs) are emerging optoelectronic devices able to display simultaneously electrical switching as transistors and electroluminescence emission as light emitting diodes. Lead chalcogenide colloidal quantum dots (CQDs) allow achieving light emission in a very broad spectral range, covering the near-infrared (NIR) and the short-wavelength infrared (SWIR) regions, which cannot be reached with other solution-processable materials. Therefore, the use of lead chalcogenide CQDs as active layer in LEFETs opens the possibility for very narrow and switchable light sources in the NIR and SWIR range. The recently reported, first fully solid-state lead chalcogenide (PbS) CQD based LEFET shows an electroluminescence (EL) quantum efficiency of 1.3 x 10(-5) at room temperature and of about 1% below 100 K. To overcome the limits of a previous report, an active material comprising two sequentially deposited layers is designed, the first of PbS CQDs displaying n-type transport and the second of polymer-wrapped semiconducting carbon nanotubes displaying p-type dominated transport. With this double layer system, LEFETs displaying a well-balanced ambipolar transport, charge carrier mobility of about 0.2 cm(2) V-1 s(-1) for both electrons and holes, and EL external quantum efficiency reaching 1.2 x 10(-4) at room temperature are obtained.ISSN:2199-160

Remarkably Stable, High-Quality Semiconducting Single-Walled Carbon Nanotube Inks for Highly Reproducible Field-Effect Transistors

In the past years, high-quality semiconducting single-walled carbon nanotube (s-SWCNT) inks obtained by conjugated polymer wrapping using toluene as solvent have been used for the fabrication of high-performance field-effect transistors. Charge-carrier mobilities up to 50 cm(2) V-1 s(-1) and on/off ratios above 10(8) have been reported for devices based on networks of s-SWCNT. However, devices fabricated from inks that are only a few weeks old generally show a marked decrease in performance, indicating the limited shelf life of toluene-based inks. The use of o-xylene as applicator solvent to obtain high quality and very stable s-SWCNT inks is reported. The charge carrier mobility of field-effect transistors fabricated with this new ink show a twofold increase in magnitude compared to devices prepared from a toluene solution. More importantly, the device-to-device performance shows improved reproducibility, which is ascribed to the higher degree of homogeneity of the s-SWCNT network deposited from o-xylene with respect to the one from toluene. Finally, the o-xylene inks maintain their initial properties for longer than one year. This very long shelf life is an important pre-condition for the industrial use of s-SWCNT inks