State of Silanes

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

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