Why did you choose to explore carbon nanotubes (CNTs) at the start of the century, and can you outline the most signifi cant discoveries you have made since then?

It is very diffi cult to fabricate atomically perfect nanoscale structures. However, in the case of CNTs, nature does the job for us. That said, it is important to understand how nanotubes interact with their environment, which includes their electrical contacts and the surfaces they contact.

We set out to explore these effects, fi rst by developing a method to deposit nanotubes onto atomically clean surfaces. We were then able to observe subtle effects; for example, what happens when you alter the alignment of a nanotube with the underlying atomic lattice of the substrate.

Can you give examples of carbon nanotube network (CNN) devices and provide an insight into what triggered your interest in fi nding a way to improve current conduction between nanotubes?

CNN devices are attractive in that they are quite easy to fabricate and have numerous potential applications; for example, in low-cost

wearable fl exible electronics. The key device is the transistor, whose speed and power effi ciency critically depends on how well electrons can fl ow through the nanotube network.

Unfortunately, in these nanotube networks the electrons must jump from nanotube to nanotube as they pass from one end of the transistor to the other. These ‘jumps’ are very ineffi cient and slow down the performance of the transistor by a factor of 10 to 100. It simply occurred to me one day that the nanotube-nanotube junction problem could be solved via nanosoldering.

Beyond CNT research, what other areas does the Lyding Research Group explore?

We are exploring the fabrication of devices based on the 2D atomically thin materials graphene and boron nitride, as well as the layered transition metal dichalcogenides like molybdenum disulphide. There is a worldwide effort to study these and related materials to replace silicon in future integrated circuit chips.

In addition, we are investigating the atomically precise fabrication of semiconducting graphene nanoribbons by combining bottom-up chemical synthesis with top-down fabrication driven by electrons from our scanning tunnelling microscope (STM). We are also developing nanometre-scale metal wires that can be used to make connections to ultra-small structures like CNTs and graphene nanoribbons.

An offshoot of our work in microscopy has been the development of a patented tip sharpening process that now serves as the basis for a startup company, Tiptek, LLC, which was founded to commercialise this process.

What skills and expertise are represented in the Lyding Group? Who are your collaborators and what do they add to your studies?

My group has expertise in scanning probe microscopy. We construct our STM systems from the ground up as well as synthesise many of the materials, such as CNTs and graphene,

which we use in our experiments. And we have expertise in fabricating these materials into electronic devices.

My collaborators include Professor Gregory Girolami (from the Chemistry Department of the University of Illinois at Urbana-Champaign – UIUC) whose group synthesises all of the molecules used in our nanosoldering experiments. We also collaborate with Professors John Rogers (UIUC Materials Science and Engineering) and Eric Pop (Stanford University Electrical Engineering). Their groups fabricate nanotube devices for the nanosoldering experiments.

To explain the subtleties of graphene-substrate interactions that we observe in our experiments, we use atomistic simulations performed by Professor Narayana Aluru (UIUC Mechanical Science and Engineering). And fi nally, we collaborate with Professor Salvador Barraza-Lopez (Physics Department of the University of Arkansas) who also conducts simulations to explain our atomistic-level experiments.

How do you involve undergraduate students in your nanotechnology projects? Is your aim to inspire the next generation of nanoscience thought leaders?

Undergraduates are heavily involved in our research. It is important to immerse them in open-ended problems that they do not see in their classroom curriculum. This is how I got my start in research and I found it to be enlightening and inspiring. A number of our undergraduate researchers have been authors on our papers and presented their research at major conferences.

In which areas do you see your investigations progressing in the next fi ve to 10 years?

We are just beginning to obtain some very interesting results in our graphene nanoribbon experiments, so I expect this effort to grow in the near future. We are also exploring the effect of the nanosoldering process on mechanical strength and thermal conductivity of nanotube networks.

Scanning tunnelling microscopy pioneer and co-founder of technology company Tiptek, Professor Joseph W Lyding discusses his work on 2D materials and carbon nanotubes

Carbon nanocreations PRO

FESSOR JO

SEPH W

LYDIN

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40 YEARS AGO, many scientists would most likely have thought that there was little, if anything, more to learn about carbon, seeing as it had been known, used and analysed throughout human history in the form of coal and diamonds. However, an unplanned experiment in 1985 led to the discovery of a new molecule made purely of carbon – the buckyball. Arranged in the shape of a football and consisting of 60 carbon atoms, the buckyball was actually the first of a brand new class of molecules, later named the fullerenes.

Numerous arrangements were subsequently proposed but one in particular caught the imagination of scientists worldwide – the carbon nanotube (CNT). Cylinders only a few nanometres wide, CNTs’ unique molecular structure furnishes high tensile strength, electrical conductivity, ductility, heat conductivity and relative chemical inactivity. Since the later isolation of graphene – carbon arranged into a one atom-thick honeycomb sheet – in 2004, single-walled CNTs have been described as 1D cylinders of monolayer graphene.

BREAKING NEWS: CNNs HAVE POTENTIAL

With such stunning properties, it is little surprise CNTs have been proposed as components for future nanoelectronics. However, primitive synthesised (as-grown) CNTs are a mixture of metallic and semiconducting types, an arrangement which often hinders their practical application.

Many have attempted, with limited success, to isolate metallic or semiconducting CNTs to form a more useful structure. But an alternative approach is to find ways to enhance the performance of random as-grown carbon nanotube networks

(CNNs) – made up of many CNTs randomly laid across one another. CNNs are easy to fabricate and transfer to arbitrary substrates, which make them attractive for applications in integrated circuits and display drivers on flexible or transparent substrates, particularly as transistors.

USEFUL BOND

Professor Joseph W Lyding heads the Lyding Research Group at the Beckman Institute within the University of Illinois, USA. One of the team’s primary research areas concerns how to improve the flow of electrons through CNNs in order to make them practicable for applications.

Despite CNNs’ potential as transistors in nanoelectronic applications, one key impediment remains: their inability to allow electrons to efficiently jump from one nanotube to the next as current passes through the transistor. Unlike many who thought this disadvantage fundamental, Lyding instead viewed it as a challenge and hit upon the idea of improving the contact between nanotubes in CNNs via an ingenious technique: nanosoldering.

“Nanosoldering takes advantage of the fact that current flow through nanotubes is highly efficient, but current flow across nanotube-nanotube junctions is quite inefficient,” summarises Lyding.

An electric bondThe Lyding Research Group within the University of Illinois brings together experienced, distinguished researchers with undergraduate students. Aiming to uncover atomistic-level information, the team provides insight into, and innovative new methods to improve, important nanotechnology systems

ATOMIC MAGNIFICATION

The multidisciplinary team works on diverse material surface problems, but all research threads are linked by the experimental technique scanning tunnelling microscopy (STM), which the scientists use to investigate surface chemical reactions at the atomic scale.

Not only is Lyding well-known for his discoveries using STM, he has also been instrumental in developing the STM technique itself: “The novel feature of my design was a thermally compensated sample positioning mechanism that dramatically reduced the effects of vibration and thermal drift,” he outlines. “We still use this design today, as do many groups worldwide.” Most recently, the Illinois researchers have been utilising STM in their groundbreaking studies into CNNs.

Animation demonstrating the process of nanosoldering, which improves nanotube transistors. Metal self-deposits onto hotspot junctions, healing gaps between nanotubes.

PROFESSOR JOSEPH W LYDING

38 INTERNATIONAL INNOVATION

NANOSOLDERING CARBON NANOTUBE JUNCTIONS BY LOCAL CHEMICAL VAPOR DEPOSITION FOR IMPROVED DEVICE PERFORMANCE

OBJECTIVES

• To determine atomistic-level information that provides insight into important nanotechnology systems, utilising the ultra-high vacuum scanning tunnelling microscope (STM) as an atomic-resolution imaging, spectroscopic and fabrication tool

• To enhance the performance of as-grown carbon nanotube networks through a novel nanosoldering technique

KEY COLLABORATORS

Professor Narayana Aluru; Jae Won Do; Professor Greg Girolami; Professor Martin Gruebele; Professor Karl Hess (emeritus); Professor Bill King; Professor John Rogers, University of Illinois • Professor Mark Hersam; Dr Joshua Wood, Northwestern University • Dr Phaedon Avouris, IBM T J Watson Research Center • Professor Salvador Barraza-Lopez, University of Arkansas • Professor David Estrada, Boise State University • Dr Scott Lockledge, CEO, Tiptek, LLC • Professor Eric Pop, Stanford University • Dr John Randall, Zyvex Laboratories • Dr Scott Schmucker, Naval Research Laboratory • Professor Yang Xu, Zhejiang University, China

FUNDING

US Offi ce of Naval Research

US National Science Foundation

CONTACT

Professor Joseph W LydingHead, Lyding Research Group

Electrical and Computer Engineering3065 Beckman Institute MC 251405 North MathewsUrbana, Illinois 61801, USA

T +1 217 333 8370E lyding@illinois.edu

http://bit.ly/1vmcB0Qhttp://bit.ly/1yJw6Dc

PROFESSOR JOSEPH W LYDING received his PhD in Electrical Engineering from Northwestern University in 1983. Since being recruited to the University of Illinois in 1984 by Professor John Bardeen, Lyding has risen to Professor of Electrical and Computer Engineering. Most recently, he has developed ultra-clean nanotube deposition and STM spectroscopic methodologies to understand the interaction of carbon nanotubes and graphene with technological substrates.

AN ILLUSTRIOUS HISTORY

1983 – receives his PhD in Electrical Engineering from Northwestern University

1984 – recruited by Professor John Bardeen (the only person to have won the Nobel Prize for Physics twice) to work on the 1D charge density wave problem

1985-86 – constructs the fi rst atomic-resolution scanning tunnelling microscope (STM) in the Midwest and one of the earliest STMs in the US

1987 – invents an ultra-stable STM that is subsequently patented and licensed by the University of Illinois to RHK and McAllister Technical Services. The design has been copied worldwide

1994 – creates a novel STM-based nanofabrication scheme and achieves atomic-resolution patterning of silicon surfaces through the selective desorption of hydrogen. The method was immediately recognised and copied globally

1995 – discovers a giant isotope effect when hydrogen is replaced by deuterium on silicon surfaces, in an experiment suggested by Dr Phaedon Avouris at IBM. STM desorption studies showed that deuterium was two orders of magnitude more diffi cult to desorb than hydrogen

1996 – proposes, with Porfessor Karl Hess at the University of Illinois, replacing hydrogen with deuterium in complementary metal-oxide-semiconductor (CMOS) technology, revealing a dramatic 50x reduction in hot-carrier degradation. Deuterium has been used in commercial production by several major chip manufacturers, most recently Samsung, under license from the University of Illinois for the ARM processors used in mobile phones and tablets

2001-07 – develops and applies a new method termed dry contact transfer (DCT) in which nanotubes can be brought into an ultra-high vacuum environment and then directly stamped onto atomically clean surfaces

2006-present – invents a novel process for sharpening conductive probes down to sub-5 nm radii, named fi eld-directed sputter sharpening (FDSS). High-performance STM and atomic force microscopy (AFM) tips with super hard coatings have now been demonstrated and patented, and Lyding has since become the Chief Technical Offi cer of Tiptek, LLC, a new start-up company to commercialise the technology

2007-present – extends the DCT method to the study of monolayer graphene on silicon and III-V substrates. Alongside many other notable discoveries, Lyding and graduate student Kyle Ritter demonstrate for the fi rst time the existence of the metallic zigzag edge state

“During current fl ow these junctions get hot, so we expose the CNN to a gaseous chemical that thermally decomposes at the hot junctions, leaving behind a metal deposit.” As the metal deposition continues to build up, it forms a conducting bridge between the nanotubes. Current then starts to fl ow effi ciently across the junction, which causes it to cool down and thus prevents further metal deposition, essentially acting as solder between the nanotubes.

This technique – developed by Lyding and colleagues, and led by graduate student Jae Won Do – has provided a staggering improvement in CNN conduction: “In our fi rst published experiments, we observed close to an order of magnitude improvement in the conduction effi ciency of our nanotube array transistors,” Lyding enthuses.

PUSHING TOWARDS THE MARKET

Lyding’s nanosoldering method relies on chemical vapour deposition (CVD), a process well-established in the semiconductor industry to produce high-purity, high-performance thin fi lms. Not only does this mean industry is accustomed to the technique, making nanosoldering more easily transferrable to market, but it also means a wide range of metal precursor molecules are available.

However, the Lyding Research Group is not one to rest on its laurels: “We are trying an alternative method to CVD, which requires a vacuum system and gas handling apparatus,” Lyding illuminates. “In this method, we simply coat the nanotube array transistors with a thin layer of the CVD chemical, either by simple drop casting or by a spin-on process.”

Asked how close the Group’s innovations are to making it into commercial technology, Lyding replies: “I could see this happening over the next three to fi ve years”. The Group is not currently working with commercial partners, but initial discussions with potential industrial collaborators are underway that Lyding hopes will lead to a development process.

HOLISTIC COLLABORATION

Lyding’s distinguished academic career began under the tutelage of twice Nobel Prize for Physics winner Professor John Bardeen: “At fi rst I was in complete awe just to be near someone of his stature, but then I realised that he was not at all egotistical about his accomplishments,” recalls Lyding. “Bardeen’s lasting infl uence on me has been to embrace challenges and take the risks that can lead to discoveries, but above all make sure that people come fi rst.”

Evidence of this continuing ethos can be seen in the makeup of the Group, which consists of graduate research assistants, PhD students and undergraduates, all engaged in research alongside Lyding and his world-leading collaborators.

INTELLIGENCE

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