New Materials for Future Applications
Two-dimensional silicon nanosheets have attracted the attention of researchers from different fields. They exhibit the structural properties of graphene - one of the key players in nanotechnology - but are made from silicon, which forms the basis of information technology. Silicon nanosheets also combine anisotropic structural characteristics with exceptional (opto)electronic properties. As such, they show photoluminescence and in first applications were used in photonic sensors as well as field-effect transistors and lithium ion batteries. In the future, the ease of their processing combined with their outstanding properties might enable the production of printable and flexible electronics.
Silicon is one of the most essential elements in industry as it forms the basis for electronics, tissue industry or pharmaceutical preparations. It is a metalloid and in its highly purified form a very attractive material for electronic systems: such as the field-effect transistors (FET) as central semiconductor device, chip-production in microelectronics and solar cells . Due to the strong innovative drive of making every-day use electronic devices ever smaller faster and cheaper, research was pushed towards finding new materials with comparable, or even more promising properties. Thus, the discovery of the field of two-dimensional (2D) nanomaterial science was a great possibility for industry to take a big step in the direction of flexible, transparent and at the same time robust electronics and sensors.
Inspired by graphene, the search for 2D nano silicon evoked the theoretical and experimental exploration of silicon nanosheets. The graphene counterpart silicene is a nearly planar 2D material with a zero-band gap and a Dirac cone-like electronic structure . In theoretical works silicene was determined to exhibit very high intrinsic carrier mobilities  with charge carriers behaving like massless Dirac fermions , emphasizing its potentials for electronic applications. Calculations additionally showed that the properties of silicene are dependent on external influences such as electric fields [5,6] and the underlying support [7,8].
Silicene can be synthesized by chemical vapor deposition (CVD). Thus, e.g., Ag  , Au  or Ir  substrates were used and the process could in the future allow a large-scale fabrication even on flexible surfaces. The drawbacks of this material are the necessity of the stabilizing substrate and the sensitivity against oxygen. Both of which could be tackled by Akinwande and co-workers when they fabricated the first silicene based FET by encapsulating the silicon layer in an Al2O3 cover .
Another type of 2D nano silicon is the so called silicane. This hydrogenated form of silicene is synthesized in a straight forward and scalable procedure via chemical exfoliation from CaSi2 [12,13]. The crystalline CaSi2 as the starting material consists of alternating layers of calcium ions and the characteristic silicane-type puckered silicon sheets (see Figure 1). In this process substantial amounts of free standing H-terminated silicon sheets can be exfoliated, possessing lateral sizes from hundreds of nanometers up to several micrometers .
Silicane possesses a band gap, due to its sp3-hybridization, which can potentially be tuned . Physical strain, external electric fields, or surface functionalizations are just a few examples which could enable precise manipulation of the material’s properties. Additionally, the material shows green photoluminescence (fig. 2) when irradiated with UV light.
The Si-H termination of silicane can be used for surface modification for example via hydrosilylation with unsaturated organic molecules. Different techniques were explored using for example Pt catalysts , diazonium salts, thermal radical formation , or the classic radical initiator AIBN (azobisisobutyronitrile) to induce hydrosilylations on the nanomaterial surface for stable Si-C bond formation . The radical initated surface modifications, which we explored, stick out due to a broad applicability of substrates with different functionalities. Due to a high surface to volume ratio, silicanes tend to stack easily, which leads to a formation of agglomerates in concentrated dispersions, resulting in the characteristic yellow color. The here mentioned functionalization steps allow not only a stabilization of silicon nanosheets (SiNSs) against external influences (e.g., UV light, oxygen), but also the control and modification of their properties for subsequent processing and fabrication steps.
First experimental studies exist, which show the potential of carbon coated silicanes in Li-ion battery anode material [18,19]. Furthermore, the material can be used as filler in nanocomposites . It has been shown that conventional monomers such as styrene, methyl methacrylate or acrylic acid can be used to functionalize the surface and form a polymer matrix, leading to a composite which possesses features of both components. This modification and at the same time preservation of the material’s properties facilitates processing of the composite and its incorporation into the already well established device fabrication techniques. For example, compounding, molding and hot press can be realized without affecting the features of the aimed for electronic device. In this context, we demonstrated for polystyrene-based nanocomposites the possibility of melting it on top of pre-patterned gold electrodes, forming a photonic sensor. It has been shown that with this device a very sensitive optical response can be detected when exposing it to a light source with wavelengths lower than 410 nm . Due to the possibility of composite shaping, the fabrication of the mentioned (opto)electronic device can be realized independent of the substrate. By tuning the polymer’s mechanical features, or exchanging the polymer, one might even extend the application area towards flexible substrates, or even wearable electronics.
In summary, both the graphene like silicene as well as its hydroganted form silicane show a great potential for applications in nano-sized electronics. Successful protection could enable their use in ambient conditions and preserve their outstanding properties. First steps towards successful application have already been taken by fabricating first working field-effect transistors and photonic sensors.
Alina Lyuleeva1, Tobias Helbich2, Paolo Lugli3, Markus Becherer1, Bernhard Rieger2
1Institute for Nanoelectronics, Technical University of Munich, Munich, Germany
2Catalysis Research Center, Wacker-Chair of Macromolecular Chemistry, Technical University of Munich, Garching, Germany
3Free University of Bozen – Bolzano, Faculty of Science and Technology, Bozen-Bolzano, Italy
20. Lyuleeva, A. et al. J. Phys. D. Appl. Phys. 50, 135106 (2017). doi:10.1088/1361-6463/aa5005