State of Silanes

Professors Bernhard Hidding, Thomas Klapötke and Andreas Kornath, and Dr Xiaomin Zhu

outline the progression of their collaborative silicon hydride-related research in Europe SILANES

What is the importance of gaseous would then fuel the colonisation of the Moon monosilane to the semiconductor field? (and planets).

Monosilane is probably the most important With silane-focused research activity material in the photovoltaic industry today. increasing in Europe, how do you hope your It decomposes thermally into the elements own work will make a mark on silicon and hydrogen at temperatures of the field? 420 °C – a property utilised in chemical vapour decomposition (CVD) methods to produce Regional and national networks well-controlled, thin layers of silicon on a such as our ‘Silanes’ scientific photovoltaic panel. Electronic-grade silicon is network (www.silanet.de) are also produced from monosilane. excellent candidates as seed activities for broader, European initiatives. At the Why is there a need to create increasingly University of Strathclyde, UK, which is part longer-chain liquid silanes? of the Scottish Universities Physics Alliance (SUPA), space science and exploration is also Monosilane is gaseous in ambient conditions. a focus. Excitingly, this may allow for highly Generally speaking, gaseous and solid materials symbiotic research on liquid silanes by our are more difficult to handle than liquids. team and other collaborators in the future. As synthetic chemical compounds, could The density of a gas is much lower than that you explain how silicon hydrides (silanes) of a liquid and heating a gas to elevated Looking ahead to the next five to 10 years, can be produced and, moreover, why liquid temperatures of a few hundred degrees is how do you see your studies progressing? silanes are harder to synthesise? extremely energy-intensive. Recently, the Scottish Centre for the The good news is that silicon is one of the most In contrast, liquid silanes have a much higher Application of Plasma-based Accelerators abundant elements on Earth. For example, silicon density per volume, as well as a variety (SCAPA) was formed as part of SUPA and a sand (silicon dioxide) is used industrially to of additional decomposition methods. This £350 million investment into the University produce raw silicon via reduction with coke, results in dramatically improved epitaxial of Strathclyde in order to exploit particle and monosilane can be produced just as growth rates for electronic-grade silicon. and photon beams for free-electron lasers easily through the reduction of magnesium Increasing growth rates increases the panel and other applications. Additionally, a silicide with hydrochloric acid. A very similar output rate and can therefore decrease £100 million Technology and Innovation production method called Stock’s synthesis is product costs. At the same time, longer- Centre (TIC) with a focus on energy research used to produce a mixture of crude silanes in a chain silanes are easier to handle, transport is nearing completion on the University way that is reminiscent of crude oil produced and store, not only because of their liquid campus. It will be the first Fraunhofer in nature. state but also because of their decreased Institute in the UK and, excitingly, a pyrophoricity – that is, the tendency to ignite Fraunhofer UK umbrella organisation is This mixture contains longer-chained silanes and combust spontaneously when in contact now located in Strathclyde. Recently, such as disilane, trisilane and tetrasilane. with ambient air. It is important to remember the first Max Planck Institute Unfortunately, the longer the silane molecule, that monosilane, despite all of its advantages, partnership in the UK has also been the lower the fraction of it contained in is quite a dangerous substance. implemented. These organisations the liquid crude silane mix. This is because and collaborations make for an the inner energy of silanes increases as the What is the true potential reach of silane- ideal playground for R&D in these complexity of the silane increases. based applications? respective areas.

In our scientific network, we have therefore Some believe that the merging of carbon- analysed and developed novel production based structures with silicon-based structures strategies, such as a method for the synthesis will lead to a natural science which is much of liquid cyclopentasilane; a molecule that richer than we know today. One fascinating is especially interesting for photovoltaics idea along these lines originates from a much applications. We hope that the increased broader view: Earth is not the only silicate number of applications and demand for planet in the Solar System. As early as the liquid silanes will further novel large-scale 1950s, NASA considered using silicon on the production methods. Moon for in situ silane manufacture, which

WWW.RESEARCHMEDIA.EU 41 SILANES Spark of ingenuity

Researchers based in the University of Strathclyde in Scotland, UK, are collaborating with scientists across Europe to study the production and applications of silicon hydrides, as well as novel plasma accelerators

AS THE MOST widely used organic compounds, the properties of hydrocarbons have been extensively explored. With their diverse range of applications, they are often seen as the driving force behind modern society. Conversely, silicon hydrides (or silanes) – which are regarded as the silicon-based equivalents of hydrocarbons – have not been subjected to the same extent of investigation. However, many scientists believe there is an enormous number of silane-based

applications that are waiting to be discovered. Visualisation of the geometry of decasilane Si10H22, calculated with Gaussian.

Professor Bernhard Hidding, based in the Physics Department at the University of These characteristics lend themselves to a range of pulses of radiation. As a Chair of Experimental Strathclyde, UK, is a prominent silanes specific applications. For instance, their propensity Physics at the Scottish Centre for the Application researcher keen to realise this family of to self-ignite means liquid silanes could be highly of Plasma-Based Accelerators (SCAPA), directed synthetic compounds’ potential. In a project desirable as propellant fuel for rockets. At present, by Professor Dino Jaroszynski, Hidding engages funded by the German Ministry of Research and the enormous thrust rockets generate when in collaborative research on the use of laser- Education (BMBF), he worked with Professor they are launched from Earth into space requires plasma particle accelerators to build a number Andreas Kornath from the Ludwig Maximilian chemical propulsion. Hidding and his collaborators of applications in the fields of chemistry, biology, University of Munich and Dr Xiaomin Zhu from are working to design novel synthetic, reactive and material science and medicine. RWTH Aachen University to study polysilanes. high-energy density fuel from silanes: “In a rocket or Together, the researchers uncovered novel scramjet combustion chamber, the highly reactive While particle accelerators have grown and cheaper production methods for silanes, silanes would release chemical energy due to enormously in terms of size in the last 100 years, as well as studying applications of liquid decomposition and combustion,” Hidding explains. the technology behind the acceleration has silanes. The small group quickly expanded into “The largely gaseous combustion products would not significantly advanced since the 1950s. a broader research network funded by the then be converted into kinetic energy, the thrust Traditionally, higher energies have been achieved German Research Foundation (DFG) until 2014 from which would propel the vessel.” Initial studies by increasing the length of the accelerator, (www.silanet.de). The multidisciplinary team of have shown that liquid silanes are comparably but in a break from the past Hidding and his over 50 scientists – with expertise in chemistry, non-toxic, and thus attractive and environmentally collaborators have pioneered a novel approach physics and engineering – is currently working friendly potential space propellants for the future. to generating and harvesting electric fields to further advance knowledge about silanes. produced in plasma accelerators: “The ability Recent research projects have also focused on the of plasmas to produce such enormous fields to use of liquid silanes in the semiconductor industry. accelerate electrons is already an experimentally One goal in this area is the creation of a spray-on proven technology,” Hidding discloses. “New semiconducting paint that covers large areas – for accelerators would be capable of producing Their propensity to self-ignite example, the walls of buildings – with low-cost teravolts per metre, and in turn the accelerators photovoltaic layers. Doing so could pave the way could be shrunk down from tens of kilometres to means liquid silanes could be for the production of textile-based solar cells. the submetre size – concurrent with a dramatic decrease in the costs of these facilities.” highly desirable as propellant fuel In another research direction, Hidding and his collaborators have developed strategies to replace So far, the major challenge in realising the for rockets carbon atoms with silicon atoms in molecules. This advantages of plasma-based accelerators has seemingly small change can result in enormous been the relatively low quality of the electron differences, leading to the creation of a variety of beam. Indeed, greater electron beam brightness DIVERSE APPLICATIONS novel materials such as sila-explosives, hydrogen is one of the most important requirements for Silicon and carbon are the most significant carriers, silicon-based drugs and odourants. next-generation light sources because it could non-metallic substances on Earth and share Excitingly, it is likely that these initial avenues translate into greater photon beam brightness fundamental similarities – yet they also have of research represent only the beginning of a when, for instance, it is converted into photon major differences. For example, carbon bonds revolution in silicon-based applications. laser beams in X-ray free electron lasers. In much more tightly with hydrogen than silicon response to this challenge, Hidding and his does. This means that breaking carbon-hydrogen collaborators are currently attempting to PLASMA-BASED ACCELERATORS bonds requires energy while breaking silicon- develop an ‘underdense plasma photocathode’, hydrogen bonds releases energy. Additionally, One other important focus of Hidding’s work is which they hope will enable the production of because silanes are so energy-rich, they self- laser wakefield acceleration and beam-driven monoenergetic electron beams that exceed the ignite when they come into contact with oxidisers plasma wakefield acceleration, which are compact quality and brightness of conventional state-of- such as air. and inexpensive methods of producing short the-art particle accelerators. 42 INTERNATIONAL INNOVATION REPRODUCING KILLER ELECTRONS INTELLIGENCE An additional use of plasma-based accelerators is SILANES the reproduction of broadband space radiation in OBJECTIVES the laboratory. The so-called killer electrons and To facilitate the communication of knowledge protons in the Earth’s Van Allen belts – that is, the about silanes, their production methods, and radiation belts that encircle the Earth – represent applications between chemists, physicists and an enormous threat to space vessels. At present, engineers •To expand the use and applications the quality and safety of space equipment is of liquid silanes by outreach to industry •To tested by conventional particle accelerators, but develop a future with many silicon applications the resultant radiation beams are monoenergetic similar to the current carbon-based world rather than broadband. In contrast, with stochastic KEY COLLABORATORS AND PARTNERS modes of operation, the beams plasma accelerators produce are broadband and very similar to those www.silanet.de • European Space Agency found in space. Funded by the European Space • Strathclyde Space Institute • Centre for Agency (ESA), Hidding and his collaborators have Space Science and Applications • Professor Dino Jaroszynski; Professor Zheng-Ming conducted proof-of-concept experiments that Sheng; Professor Paul McKenna, Scottish demonstrate space radiation can be reproduced Centre for the the Application of Plasma-based by plasma accelerators. Promisingly, this could Accelerators (SCAPA) • University of Hamburg lead to more rigorous and reliable tests of space • RWTH Aachen • Ludwig-Maximilians- electronics in future. Universität MÜnchen (LMU Munich) FUNDING SYNERGETIC STUDIES German Research Foundation (DFG) • European Looking ahead, Hidding hopes that plasma-based Space Agency • German Ministry of Education particle accelerators will become much more and Research (BMBF) commonplace over the course of the next decade, CONTACT perhaps even overtaking the large X-ray facilities that are the current gold standard for material and Professor Bernhard Hidding natural sciences. Universität Hamburg Fachbereich Physik Institut für Experimentalphysik Hidding and his collaborators are also eager to Luruper Chaussee 149, 22761 Hamburg further develop their scientific and technological Germany knowledge of using liquid silanes in chemical T +49 40 8998 1531 propulsion and the semiconductor industry. E [email protected] They want to continue forming robust academic E [email protected] collaborations across Europe, as well as working closely with industry partners. Ultimately, the aim www.silanet.de http://bit.ly/UATt0a is to create a range of new and innovative silane- based applications that mirror those of the carbon- BERNHARD HIDDING joined the Physics based world. Department of the University of Strathclyde as Professor in 2013 and holds a Chair of Experimental Physics at SCAPA. Prior to this, Hidding was at the University of Hamburg/CFEL/ DESY, where he still maintains a group which is now closely collaborating with Strathclyde.

A NOVEL APPROACH THOMAS KLAPÖTKE has been Professor Using compact and cost-effective tools, of Inorganic Chemistry at LMU Munich since 1997. As well as leading a research group Hidding and his collaborators have in computational chemistry and advanced pioneered a three-step approach to materials, he has also been a Bavarian Elite producing short pulses of radiation: Academy Representative since 1998. SILANES • First, a light pulse is taken from a high- ANDREAS KORNATH was made Professor of power femtosecond laser system First synthesised in 1857 by the German Inorganic Chemistry at LMU Munich in 2007. chemist Friedrich Wöhler, silanes were His research specialisations include fluorine • Second, the laser pulse is focused into heavily investigated by another German chemistry and matrix spectroscopy. the laser-plasma wakefield accelerator chemist, Alfred Stock, in the early 20th DR XIAOMIN ZHU works as a senior lecturer at – a capillary channel containing plasma, Century. Stock developed innovative RWTH Aachen University, and at the same time which is just a few centimetres long and methods for chemical separation using leads a research group at the Leibniz Institute less than 1 mm wide. This produces high- high-vacuum manifolds. From these for Interactive Materials (DWI). His research energy electrons almost travelling at the investigations it became clear that silanes fields include polymer chemistry, molecular self- speed of light were one of the first groups of binary assembly and sol-gel technology. compounds to resemble the incredible • Finally, using either an array of magnets structural diversity of hydrocarbons. or the electromagnetic fields of the plasma itself, the electrons are forced to oscillate very rapidly. This in turn produces short- wavelength radiation pulses, such as ultraviolet light, X-rays or gamma rays