̽��ֱ�� of Cambridge - Stephan Hofmann

Farming at nanoscale dimensions

sc604 — Fri, 18 Mar 2016 09:59:33 +0000

Building transistors today is done with lithography, which is a “top-down” process that uses patterning to create the complex layers that make up the transistor structure. It’s a bit like exposing a negative on photographic paper to get the pattern you want and then using this pattern as a template to place each material – metal, insulator or semiconductor – in exactly the right location.

This process has worked successfully since the 1950s, and scientists have even demonstrated the first working test chips with features approaching seven nanometres, the equivalent of placing more than 20 billion tiny switches on chips the size of a fingernail. But as we get to ever-smaller dimensions, new approaches to building nanoscale devices will be required.

At IBM’s T.J. Watson Research Center, and working with researchers from Lund ̽��ֱ�� in Sweden and Stephan Hofmann’s group at the ̽��ֱ�� of Cambridge, we use a technique called self-assembly to grow and directly control nanostructures that could one day form parts of integrated circuits. Self-assembly looks at chip building from the other end of the spectrum: a “bottom-up” approach that builds nanostructures in a way that is dictated by physics rather than by an imposed pattern. In some ways it’s like farming, in that that you plant seeds to grow a crop, and then support the growth with the right conditions to get the result you want.

But exploring self-assembly doesn’t mean we are ready to throw away today’s approach; instead, we want to use top-down strategies that we have already learned over many years, and combine them with new tricks that use self-assembly.

Think of water splashing onto a pane of glass. It spontaneously forms little hemispheres. ̽��ֱ��droplets are hemispherical because surface tension pulls the water molecules into this shape to minimize the surface area and energy of each droplet. But there is no reason for the droplets to form in any particular location or to be any particular size, so their positions and sizes are random. ̽��ֱ��spontaneous formation of the hemispherical shape is an example of self-assembly, but other aspects of the process (position, size) are not controlled.

Now imagine there is a scratch on the glass. Water droplets form on the scratch, because it is a good, low energy place for the water molecules to stick. We have now combined self-assembly – “make a hemispherical droplet on this surface” – with an imposed pattern – “make a droplet on this part of the surface by using carefully placed scratches.” ̽��ֱ��result is that we can build more complicated patterns. Flexible, customised patterns like this water example, but on the nanoscale, help us build integrated circuits.

̽��ֱ��more precisely we can direct this self-assembly, the more versatility we can achieve. We can choose different materials for our nanostructures, build them with different sizes, and control their chemical compositions in ways that allow them to be tuned to have the properties we need. ̽��ֱ��properties of some nanomaterials could include the ability to do the job of a transistor but with less power, or at extreme temperatures beyond what silicon can handle.

How to direct a nanowire

In order to direct self-assembly, we have to understand the physical stimuli that influence atoms to assemble in a certain way as they form a nanostructure. ̽��ֱ��particular nanostructures we find most interesting are called nanowires. These are long thin crystals whose amazing length-to-width ratio could help create very densely packed transistors. Using a combination of imposed patterning and self-assembly, we can grow nanowires spontaneously using the help of catalytic particles. And we can watch the nanowires as they grow, recording the process on video using a one-of-a-kind Ultra High Vacuum Transmission Electron Microscope in our lab.

We load a flat substrate into the microscope, place catalytic particles onto it (this is the directed part of the process), then heat it and add some reactive gases. We watch what happens to the catalytic particles (this is the self-assembly part of the process) by magnifying the image by 50,000 times or more. ̽��ֱ��reaction can be slow – it takes hours for the whole experiment to be finished – but the videos show how the nanostructures grow, one layer of atoms after another. Recording videos, for example at different temperatures or with different added gases, is central to understanding every step of the nanowires’ growth. We get to see cause and effect when the conditions change, so we can work out the laws of physics that control the growth.

Recently, we have become especially interested in growing nanowires made of gallium arsenide that form with the help of catalysts made of gold nanoparticles. For this we need two reactive gases, trimethylgallium and arsine. We chose these because they supply the two components needed to build the nanowire, gallium and arsenic. When we record our movies, the first reaction we see is between gallium and gold. This reaction turns the original gold nanoparticles into hemispherical liquid gold-gallium droplets. As we continue to watch, gallium and arsenic combine within each droplet to start growing a gallium arsenide nanowire beneath the droplet.

Gallium arsenide nanowires grown this way are particularly special because it is possible to change the way the gallium and arsenic atoms stack up within each nanowire. Two arrangements of the atoms are possible, and we can change from one to the other simply by altering the temperature of the reaction or even just varying the ratio of the two gases as they flow past the catalysts. ̽��ֱ��videos show how these changes in growth conditions modify the way the atoms arrange themselves at the junction between the nanowire and the catalyst. And that causes a change in how the atoms eventually stack up when they form the nanowire. We still have the same material, gallium arsenide, but the two possible arrangements of the atoms lead to different electrical properties for the whole nanowire. Understanding what drives atoms to take up one arrangement versus another gives us a better chance of growing nanowires that have the particular electrical properties that are needed for a device such as a nano-transistor. It’s akin to having more colors on your palette so that you can paint a better picture.

These special nanowires, composed of regions with different atomic arrangement, have applications in photonics or single electron transistors, both important building blocks for electronic circuits. And simply knowing that we can control the crystal arrangement in a nanowire will open up the microprocessor community’s imagination for new devices. In particular, optoelectronics, where light and electricity are combined in photonics structures, is a good bet. But that’s just the “tip of the crystal.”

̽��ֱ��latest results of the collaborative work between Cambridge, IBM and Lund ̽��ֱ�� are published in the journal Nature.

Adapted from a blog post by Dr Frances Ross, materials scientist at IBM Research. Originally published on the IBM Research blog.

Reference:
Daniel Jacobsson et. al. 'Interface dynamics and crystal phase switching in GaAs nanowires.' Nature (2016). DOI: 10.1038/nature17148.

Researchers from Cambridge, IBM and Lund ̽��ֱ�� have discovered how tiny 'nanowires' of a widely-used semiconductor self-assemble. Dr Frances Ross of IBM Research explains how the findings could lead to a new crop of nanodevices.

Aidan Sugano

Artistic rendering of self-assembled nanowires composed of different crystal structures that spontaneously grow with the help of a catalytic nanoparticle at the tip of each nanowire.

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New technique to synthesise nanostructured nanowires

sc604 — Thu, 16 Jul 2015 05:00:00 +0000

A new approach to self-assemble and tailor complex structures at the nanoscale, developed by an international collaboration led by the ̽��ֱ�� of Cambridge and IBM, opens opportunities to tailor properties and functionalities of materials for a wide range of semiconductor device applications.

̽��ֱ��researchers have developed a method for growing combinations of different materials in a needle-shaped crystal called a nanowire. Nanowires are small structures, only a few billionths of a metre in diameter. Semiconductors can be grown into nanowires, and the result is a useful building block for electrical, optical, and energy harvesting devices. ̽��ֱ��researchers have found out how to grow smaller crystals within the nanowire, forming a structure like a crystal rod with an embedded array of gems. Details of the new method are published in the journal Nature Materials.

“ ̽��ֱ��key to building functional nanoscale devices is to control materials and their interfaces at the atomic level,” said Dr Stephan Hofmann of the Department of Engineering, one of the paper’s senior authors. “We’ve developed a method of engineering inclusions of different materials so that we can make complex structures in a very precise way.”

Nanowires are often grown through a process called Vapour-Liquid-Solid (VLS) synthesis, where a tiny catalytic droplet is used to seed and feed the nanowire, so that it self-assembles one atomic layer at a time. VLS allows a high degree of control over the resulting nanowire: composition, diameter, growth direction, branching, kinking and crystal structure can be controlled by tuning the self-assembly conditions. As nanowires become better controlled, new applications become possible.

̽��ֱ��technique that Hofmann and his colleagues from Cambridge and IBM developed can be thought of as an expansion of the concept that underlies conventional VLS growth. ̽��ֱ��researchers use the catalytic droplet not only to grow the nanowire, but also to form new materials within it. These tiny crystals form in the liquid, but later attach to the nanowire and then become embedded as the nanowire is grown further. This catalyst mediated docking process can ‘self-optimise’ to create highly perfect interfaces for the embedded crystals.

To unravel the complexities of this process, the research team used two customised electron microscopes, one at IBM’s TJ Watson Research Center and a second at Brookhaven National Laboratory. This allowed them to record high-speed movies of the nanowire growth as it happens atom-by-atom. ̽��ֱ��researchers found that using the catalyst as a ‘mixing bowl’, with the order and amount of each ingredient programmed into a desired recipe, resulted in complex structures consisting of nanowires with embedded nanoscale crystals, or quantum dots, of controlled size and position.

“ ̽��ֱ��technique allows two different materials to be incorporated into the same nanowire, even if the lattice structures of the two crystals don’t perfectly match,” said Hofmann. “It’s a flexible platform that can be used for different technologies.”

Possible applications for this technique range from atomically perfect buried interconnects to single-electron transistors, high-density memories, light emission, semiconductor lasers, and tunnel diodes, along with the capability to engineer three-dimensional device structures.

“This process has enabled us to understand the behaviour of nanoscale materials in unprecedented detail, and that knowledge can now be applied to other processes,” said Hofmann.

Researchers have developed a new method for growing ‘hybrid’ crystals at the nanoscale, in which quantum dots – essentially nanoscale semiconductors – of different materials can be sequentially incorporated into a host nanowire with perfect junctions between the components.

̽��ֱ��key to building functional nanoscale devices is to control materials and their interfaces at the atomic level

Stephan Hofmann

Images recorded in the electron microscope showing the formation of a nickel silicide (NiSi2) nanoparticle (coloured yellow) in a silicon nanowire

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How carbon cousins shaped warfare and can electrify the future

lw355 — Fri, 13 Jun 2014 13:07:18 +0000

History’s deadliest swords – the ‘Damascene’ sabres forged in the Middle East from the 13th to the 18th centuries – were so sharp they could slice through falling silk, so legend has it. Their astonishing qualities are thought to have come from a combination of specific impurities in the iron ore and how hot and how long they were fired – a process that some scientists believe may have unwittingly created carbon nanotubes (CNTs) within them.

These thin, hollow tubes are only a single carbon atom in thickness. Like their carbon cousin, graphene – in which the atoms lie flat, in a two-dimensional sheet – they are among the strongest, most lightweight and flexible materials known.

“Fast-forward centuries,” said Dr Stephan Hofmann from the Department of Engineering, “and we now realise there is a whole family of these extraordinary origami forms of carbon… and how to make them.” In fact, the ̽��ֱ�� has over 25 years’ cutting-edge experience in carbon nanotechnology, from diamond to nanotubes, and from conducting polymers to diamond-like carbon and graphene.

What makes carbon nanoforms such as graphene and CNTs so exciting is their electrical and thermal properties. Their potential use in applications such as lighter electrical wiring, thinner batteries, stronger building materials and flexible devices could have a transformational impact on the energy, transport and healthcare industries. As a result, investment totalling millions of pounds is now underpinning research and development in carbon-based research across the ̽��ֱ��.

“But all of the superlatives attributed to the materials refer to an individual, atomically perfect, nanotube or graphene flake,” Hofmann added. “ ̽��ֱ��frequently pictured elephant supported by a graphene sheet epitomises the often non-realistic expectations. ̽��ֱ��challenge remains to achieve high quality on a large scale and at low cost, and to interface and integrate the materials in devices.”

These are the types of challenges that researchers in the Departments of Engineering, Materials Science and Metallurgy, Physics and Chemistry, and the Cambridge Graphene Centre have been working towards overcoming.

Professor Alan Windle from the Department of Materials Science and Metallurgy, for instance, has been using a chemical vapour deposition process to ‘spin’ very strong and tough fibres made entirely of CNTs. ̽��ֱ��nanotubes form smoke in the reactor but, because they are entangled and elastic, fibres can be wound continuously out of the reactor like nano candy floss. ̽��ֱ��yarn-like texture of the fibres gives them extraordinary toughness and resistance to cutting, making them promising alternatives to carbon fibres or high-performance polymer fibres like Kevlar, as well as for building tailored fibre-reinforced polymers used in aerospace and sports applications.

It is on the electrical front that they meet their greatest challenge, as Windle explained: “ ̽��ֱ��process of manufacture is being scaled up through a Cambridge spin-out, Q-Flo; however, electrical conductivity is the next grand challenge for CNT fibres in the laboratory. To understand and develop the fibre as a replacement for copper conductors will be world-changing, with huge benefits.”

In 2013, Windle’s colleague Dr Krzysztof Koziol succeeded in making electric wiring made entirely from CNT fibres and developing an alloy that can solder carbon wires to metal, making it possible to incorporate CNT wires into conventional circuits. ̽��ֱ��team now makes wires ranging from a few micrometres to a few millimetres in diameter at a rate of up to 20 metres per minute – no small feat when you consider each CNT is ten thousand times narrower than a human hair.

With funding from the Royal Society and the European Research Council (ERC), the research is aimed at using CNTs to replace copper and aluminium in domestic electrical wiring, overhead power transmission lines and aircraft. CNTs carry more current, lose less energy in heat and do not require mineral extraction from the earth.

Moreover, they can be made from greenhouse gases; Koziol’s team is working with spin-out company FGV Cambridge Nanosystems to become the world’s first company to produce high-grade CNTs and graphene directly from natural gas or contaminated biogas. ̽��ֱ��company is already operating at an industrial scale, with high-purity graphene being produced at 1 kg per hour. “ ̽��ֱ��aim is to produce high-quality materials that can be directly implemented into new devices, or used to improve other materials, like glass, metal or polymers,”
said Koziol.

Working directly with industry will be key to speeding up the transition from lab to factory for new materials. Hofmann is leading a large effort to develop the manufacturing and integrated processing technology for CNTs, graphene and related nanomaterials, with funding from the ERC and Engineering and Physical Sciences Research Council (EPSRC), and in collaboration with a network of industrial partners.

“ ̽��ֱ��field is at a very exciting stage,” he said, “now, not only can we ‘see’ and resolve their intricate structures, but new characterisation techniques allow us to take real-time videos of how they assemble, atom by atom. We are beginning to understand what governs their growth and how they behave in industrially relevant environments. This allows us to better control their properties, alignment, location and interfaces with other materials, which is key to unlocking their commercial potential.”

For high-end applications in the electronics and photonics industry, achieving this level of control is not just desirable but a necessity. ̽��ֱ��ability to produce carbon controllably in its many structural forms widens the ‘materials portfolio’ that a modern engineer has at their disposal. With carbon films or structures already found in products such as hard drives, razor blades and lithium ion batteries, the industrial use of CNTs is becoming increasingly widespread, driven, for instance, by the demand for new technologies such as flexible devices and our need to harvest, convert and store energy more efficiently.

Professor Andrea Ferrari, Director of the Cambridge Graphene Centre and doctoral training programme, which has been funded through a £17 million grant from the EPSRC, explained: “People can now make graphene by the tonne – it’s not an issue. ̽��ֱ��challenge is to match the properties of the graphene you produce with the final application. Our facilities and equipment have been selected to promote alignment with industry; we have collaborations with over 20 companies who share our agenda of advancing real-life applications, and many more are discussing their involvement with our activities.”

Cambridge has pioneered graphene engineering and technology from the very start and, with multiple spin-offs, has become a hub for graphene manufacturing and innovation. ̽��ֱ��Cambridge Graphene Centre aims to improve manufacturing techniques for graphene and related materials, as well as explore applications in the areas of energy storage and harvesting devices, high-frequency electronics, photonics, flexible and wearable electronics, and composites. Graphene is also the focus of large-scale European funding – the Graphene Flagship, a pan-European 10-year, €1 billion science and technology programme was launched in 2013. Ferrari was one of the key investigators who prepared the proposal, has led the development of the science and technology roadmap for the project, and now chairs the Flagship’s Executive Board.

Now, building work has begun on a £12.9 million bespoke facility that will host the Cambridge Graphene Centre, with additional spaces for large-area electronics. ̽��ֱ��facility is due to open in late spring 2015.

“We recognise that there is still much to be done before the early promise becomes reality, but there are major opportunities now,” said Ferrari. “We are at the beginning of a journey. We do not know the final outcome, but the potential of graphene and related materials is such that it makes perfect sense to put a large effort into this early on.”

What links legendarily sharp Damascene swords of the past with flexible electronics and high-performance electrical wiring of the future? They all owe their remarkable properties to different structural forms of carbon.

̽��ֱ��field is at a very exciting stage... we are beginning to understand what governs their growth and how they behave in industrially relevant environments

Stephan Hofmann

̽��ֱ��District

Carbon nanotechnology

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