A German-Dutch collaboration between Professors Ullrich Scherf and Maria Antonietta Loi is investigating carbon nanotubes. In an illuminating interview, the distinguished researchers talk about the synthesis, properties and functional application of these intriguing structures

How did your interest in nanotechnology develop?

US: Carbon nanoobjects are a fascinating interdisciplinary research topic. Fullerenes, single- and multiwall carbon nanotubes, and graphene may fin d a wide range of applications in electronics, optoelectronics and sensing devices. However, all these applications require proper nanomaterial handling, including their controlled deposition (vacuum or solution deposition) and positioning.

ML: Carbon nanotubes have a lot of potential in terms of fabricating high performance electronic devices, but their use in technology has so far been strongly hindered because of difficulties with handling them.

Can you elaborate on the term non-covalent functionalisation? How is this property advantageous?

US: Non-covalent functionalisation utilises weak binding forces (electrostatic, van der Waals and hydrogen bonds) without the formation of strong chemical (covalent) bonds. The main advantage of a non-covalent functionalisation strategy is its reversibility, which allows the break-up of aggregates without bond cleavage.

ML: Moreover, the weak interaction prevents damage of the nanotube’s wall, maintaining its physical properties. Damage to the walls causes charge scattering which is highly undesirable when you want to fabricate field-effect transistors.

What properties of single-wall carbon nanotubes (SWNTs) render them of such interest to researchers and industry?

ML: SWNTs are micrometre-sized carbon tubes with diameters of 1-2 nm and are composed of single, curved graphene sheets. Their properties strongly depend on the tube diameter and their chirality/handedness, and these factors determine whether they are metallic or semiconducting. SWNTs show outstanding electronic properties including very large (ballistic) charge carrier mobility (much higher than silicon) for semiconducting tubes.

How are SWNTs synthesised?

US: SWNTs are synthesised in metal-catalysed, solid-state template processes as a mixture of metallic and semiconducting tubes. The large-scale separation of these two tube types is one of the big challenges for their commercialisation and application in electronics.

In what context will the resulting nanotubes be used?

US & ML: Semiconducting, polymer-wrapped SWNTs will be used as an active component of field-effect nanotransistors. We have already demonstrated transistors with an on/off ratio greater than 107 and mobility higher than 5 cm2/Vs, using polymer-wrapped SWNTs. Carefully designed additional polymer functions will act as anchoring groups, thus allowing precise positioning of the tubes in electrode arrays.

How will the properties of nanotubes shape the way that they are used? What exciting properties are still to be tested?

US: With a high aspect (length/diameter) ratio, SWNTs are ideal objects for the realisation of nanoelectronic devices. The influence of the polymer shell wrapped around the tubes has to be investigated in detail. We observed that these polymers do not greatly compromise the electronic properties of SWNT/polymer nanohybrids, but we are sure that we can improve the quality of our devices.

Are you involved in any other nanotechnology projects?

US: Our group is indeed active in other fields of nanotechnology, including the self-organisation of rod-rod diblock copolymers into nanoassemblies, and nanometre-thick conjugated polyelectrolyte (CPE) interlayers for organic electronics devices.

ML: We also work on using colloidal semiconductors to make field-effect transistors and solar cells. We are investigating how to use polymer-wrapped SWNTs in solar cells; this research is in its early stages.

Have you collaborated with any other institutions or researchers over the course of this project? How has this helped to further enhance insight?

US: The project is a collaborative effort between mine and Loi’s groups. My group brings longstanding expertise in functional polymer synthesis, whereas Loi’s offers expertise in optoelectonics/electronics and the physics of SWNTs. The project is funded by the Technology Foundation STW (an agency of the Netherlands Organisation for Scientific Research – NWO) and the German Research Foundation (DFG).

Do you feel nanotechnology will develop in any unexpected directions in the coming years? Which areas are of notable significance?

ML: Nanotechnology will revolutionise many areas of our daily life – think about nanoelectronic devices, bio-inspired coatings, nanoparticle-based drugs, etc. Nanoelectronic devices will be the basis for further miniaturisation of electronic devices and modules.

Special effects

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PROFESSO

RS ULLRICH

SCHERF AN

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ARIA ANTO

NIETTA LO

I

The power of polymer wrappingResearchers from the University of Groningen and the University of Wuppertal are using conjugated polymers to facilitate the efficient selection and separation of carbon nanotubes with desirable characteristics

Artist’s impression of the polymer-wrapped SWNT’s random network. On the side (top): absorption of the starting solution showing the presence of a few semiconducting species (sharp peaks); (middle) transfer characteristics of the transistors showing an on/off ratio higher than 107; (bottom) atomic force microscopy of the SWNT’s network on the transistor channel.

A TRANSISTOR IS the fundamental building block of modern electronic devices. It is an electrical component that amplifies and switches electronic signals and electrical power. Since its development in the early 1950s, the transistor – made of semiconducting materials – has been widely incorporated into modern electronic systems, enabling the development of smaller and cheaper devices.

Across the globe, the ever-expanding electronics industry is largely dependent on the use of the semiconducting element silicon in integrated circuits. However, despite much research aiming to progress silicon-based electronic components, the downscalability of such materials is approaching its limit. In a situation where there is continual pressure for faster and more advanced technologies, with greater storage and capable of being shrunk into smaller and smaller devices, researchers are searching for alternative solutions. The size and unusual features of nanomaterials – such as a greatly enhanced surface area to volume ratio, or useful optical, electrical or physical properties – make them prime candidates for incorporation into electronic circuits.

CARBON CONSTITUENT

Carbon is increasingly being used in nanomaterials because, when arranged in complex allotropes, it can give rise to highly anisotropic semiconductors (or conductors) – a necessary and very useful property for application in electronic circuit transistors. For these reasons, carbon nanomaterial technology is receiving greater funding and attention from researchers to help realise its promise for use in smaller and more efficient transistors at the heart of modern technology.

Professors Ullrich Scherf and Maria Antonietta Loi from the University of Wuppertal, Germany and University of Groningen in The Netherlands, respectively, are leading research into the use of single-wall carbon nanotubes (SWCNTs) as a novel semiconducting component of transistors. SWCNTs are characterised as being an allotrope of carbon with a hollow, cylindrical nanostructure,

the wall of which consists of carbon atoms one layer thick. This construction offers the possibility for scattering-free transport with up to 1,000 times higher carrier mobilities than current silicon-based transistors, highlighting their enormous potential in terms of industrial application. Although the benefits of SWCNTs are clear to see, they are still underutilised in industry, primarily due to difficulties associated with their large-scale synthesis, purification and implementation.

LIMITATIONS OF NANOTUBES

The functional nature of nanotubes is dependent on their diameter and helicity, which affect their physical properties and determine whether they will behave as metals or semiconductors. To allow their wide-scale use in the electronics industry to be efficient and cost-effective, it is necessary to overcome issues associated with their purification and processing in large quantities. The most important remaining hurdles include: the synthesis of nanotubes with identical physical and electric properties; the efficient separation of synthesised tubes with metallic versus semiconducting properties; and the accurate placement of a large number of individual nanotubes in specific positions within electronic circuits with industrial precision and reproducibility.

To overcome these problems, studies carried out by the team led by Loi and Scherf look to wrap as-synthesised single-wall nanotubes (SWNTs) in specifically designed polymers that help in the selection of appropriate nanotubes according to their tube diameter and/or helicity. These wrapping polymers could also anchor SWNTs to specific positions in the final electronic device (eg. a transistor or transistor array), thus facilitating self-assembly into desired structures.

DISCRIMINATIVE WRAPPING

Previous methods for producing, sorting and utilising carbon nanotubes have made use of processes such as non-covalent functionalisation, eg. with surfactants, and ultracentrifugation to separate them out according to their various properties. However,

these techniques have not proved suitable for the necessary industrial scale-up.

Research involving wrapping SWNTs in conjugated polymers was first published in 2007, where it was shown how a coat of fluorene-based polymer surrounding the nanotube could enable efficient helicity discrimination between nanotubes in a mixed SWCNT sample, dependent on the interplay between the functional properties of the polymer chains and the physical/electrical characteristics of the individual SWNTs. Since then, further research has involved testing a wide variety of alternative polymer structures to identify varieties able to discriminate SWNTs with desirable physical properties.

COLLABORATION: THE KEY TO SUCCESS

The complementary skills, knowledge and expertise that the groups of Loi and Scherf bring

PROFESSORS ULLRICH SCHERF AND MARIA ANTONIETTA LOI

66 INTERNATIONAL INNOVATION

Professors Ullrich Scherf and

Maria Antonietta Loi are leading

research into the use of single-

wall carbon nanotubes as a novel

semiconducting component of

transistors

POLYMER-WRAPPED CARBON NANOTUBES FOR HIGH PERFORMANCE FIELD-EFFECT TRANSISTORS

OBJECTIVES

To fabricate high performance field-effect transistors by self-assembling semiconducting carbon nanotubes.

KEY PARTNERS

Macromolecular Chemistry Group (buwmakro), University of Wuppertal, Germany Photophysics and OptoElectronics Group, Zernike Institute for Advanced Materials, University of Groningen, The Netherlands

FUNDING

Stichting voor Technische Wetenschappen (STW), Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), The Netherlands • Deutsche Forschungsgemeinschaft (DFG), Germany

CONTACT

Professor Dr Ullrich Scherf

University of Wuppertal Macromolecular Chemistry Group Gauss-Strasse 20, D-42119 Wuppertal Germany

T +49 202 439 3871 E scherf@uni-wuppertal.de

www.makro.uni-wuppertal.de/en/ welcome.html

Professor Dr Maria Antonietta Loi

University of Groningen Faculty of Mathematics and Natural Sciences Zernike Institute for Advanced Materials Photophysics and OptoElectronics Nijenborgh 4, 9747 AG Groningen The Netherlands

T +31 50 363 4119 E m.a.loi@rug.nl

www.photophysics-optoelectronics.nl

PROFESSOR ULLRICH SCHERF received his PhD in 1988 from the Friedrich-Schiller-Universität Jena. After receiving his Habilitation from the University of Mainz In 1996, Scherf was Professor of Polymer Chemistry at the University of Potsdam (2000-02) and, since 2002, Professor of Macromolecular Chemistry at the University of Wuppertal. He currently heads the Macromolecular Chemistry Group. PROFESSOR MARIA ANTONIETTA LOI gained her PhD in Physics from the Università di Cagliari, Italy in 2001, after which followed postdoctoral positions at the Johannes Kepler University Linz, Austria and the Istituto per lo Studio di Materiali Nanostrutturati Consiglio Nazionale delle Ricerche, Bologna, Italy. In 2010, she became Associate Professor of Photophyics and OptoElectronics at the University of Groningen. She currently heads the Photophysics and OptoElectronics Group.

to the project are fundamental to its success. The team from the University of Groningen has demonstrated that the generation of hybrid materials by polymer wrapping enables an extraordinary level of control over nanotube functionalisation and assembly into high performance field-effect transistors. This has been achieved with high levels of precision using single-stranded DNA-based wrapping agents attached to the SWNT. The nanotube can consequently be accurately placed by positioning complementary DNA strands at the target site. “Although such a method is simple and efficient, the DNA moiety makes the production of this copolymer rather expensive for large-area applications,” warns Loi. “Consequently, the current project aims to solve this problem by identifying a simpler technique for the programmable immobilisation of SWNTs.”

Alongside this work, Scherf’s group is exploring the synthesis of conjugated polymers and their self-assembly onto SWNTs. During this project, the researchers are attempting to understand and characterise the potential of different conjugated polymer structures for obtaining particular nanotube properties. They hope to develop a library of highly selective polymer structures that can be drawn upon depending on the desired SWNT characteristics. Research will particularly focus on the efficient selection of polymers that target nanotubes with a relatively wide diameter – up to 1.6 nm – as polymer wrapping around smaller diameter tubes has shown much higher success. Moreover, large

diameter tubes hold more promise for electronic application because of their higher mobility.

To further support Scherf and Loi’s work, external collaborators working on molecular dynamics simulations – led by Professors Maria Cristina dos Santos and Siewert Jan Marrink from the University of São Paulo, Brazil and the University of Groningen in The Netherlands, respectively – are using theoretical approaches to model the role of single experimental parameters in the interaction between the polymer chain and SWNT. They do this by performing simulations on realistic systems to understand the mechanism of interaction between the polymer chain and the SWNT’s wall. More knowledge of this mechanism will allow design rules to be used to expand the polymer library that Loi and Scherf are constructing.

SPREADING THE WORD

Early successes from this ongoing project have already resulted in the publication of findings in the high-impact journal Advanced Materials, demonstrating how long alkyl chain-substituted polyfluorenes as conjugated polymers can be used to specifically target large diameter semiconducting SWNTs to separate and individualise them for processing into field-effect transistors.

In this article, Loi and Scherf demonstrated that transistors fabricated from solution with polymer-wrapped SWNTs achieve on/off ratio above 106 and mobilities for electrons and holes superior to 10 cm2/Vs. These are exceptional results highlighting the significant potential of polymer-wrapped SWNTs.

Since the project began in 2012, the collaborators have been working to realise self-assembling, polymer-wrapped semiconducting carbon nanotubes in the development of high performance field-effect transistors. Scherf concludes: “Achieving the project goals could be groundbreaking for nanoscience and electronics; hopefully providing the means to sort and place semiconducting nanotubes in specific substrate positions and opening the way to the integration of carbon nanotubes into electronic devices”.

(a) Chirality map of SWCNTs selected by polymer wrapping. In yellow, the SWNTs selected are underlined; the colour of the dots inside the hexagons indicates which of the polyfluorene derivatives (colour code used for the chemical structures) is able to select them. (b) Chemical structure of the polyfluorene derivatives used. (c) Structure as obtained by molecular dynamics simulations of three PF12 chains wrapped around a (12,10) nanotube after 10 ns at constant pressure in toluene solution.

INTELLIGENCE

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