Two-dimensional inorganic materials (2DIM): property simulations from band structure to devices
- Andras Kis (EPFL, Switzerland)
- Thomas Heine (Leipzig University, Germany)
The field of two-dimensional inorganic materials (2DIM) is rapidly increasing at the moment, reflected by the increased number of groups working on this topic, shown by the number of publications, but also in the recent interest of the FP7 Flagship Graphene to incorporate inorganic materials “beyond graphene”, in particular transition metal dichalcogenides. The field profits strongly from the close collaboration experiment-theory. The contributions of the theory groups are manifold, starting from DFT calculations on the structure, electronic and phonon band structure, over high-level calculations involving explicit electron correlation, non-equilibrium calculations of electron and heat transport, electron-phonon coupling, and also applied models to model device performances. To further develop this field it is required to accumulate knowledge of all those methods, and to explore how experiment and theory can closer interact. This is particularly important as both experiment and theory are forced to make compromises in their setups. For example, state-of-the-art transition electrom microscopy reaches atomic resolution and even atomic specification is possible, but only in two dimensions, and the electron beam may have destructive influence on the samples. Optical spectroscopy provides valuable information about the electronic structure of the samples, but lacks structural information. Electric measurements are always subject to contact resistance. In all cases samples may have impurities as defects or dopants, and finite size effects are appreciable. On the other hand, high-level theory is restricted to small model systems such as the unit cells of the lattices or small super cells. The influence of dopants or defects can only be treated by low-level methods as large structure models are needed. Edge effects can be studied, generally, only on relatively small models. Electronic structure methods are typically different for finite-size systems (clusters) and periodic models that make it difficult to compare results directly. Long-range effects are important and they are not restricted to electronic London dispersion interactions, they also include structural motifs such as rippling, and long-range phonon effects and charge density waves. To reach device characteristics, multi-scale simulations based on density-functional based band structures are needed. Optical excitations often require treatments beyond the single particle picture of DFT.
This workshop will bring leading experimentalists and theorists together. In lectures the state of the art and recent developments in experiment and theory will be provided to the participants. Two hands-on sessions will educate the students in simulation techniques. Method developers will gather in parallels sessions to the hands-on sessions in order to discuss next steps in method development and implementation, also based on suggestions of experimentalists who need support by theory. Finally, it is important to note that special attention is needed to work with nanoparticles. Computer simulations are an important tool to predict properties of new materials and in cases hence to avoid unnecessary experiments, or to motivate them. In any case, all researchers working in this field need to be aware of the risks of nanotechnology and a special lecture is devoted to that topic. A CECAM workshop appears to be the ideal platform for this undertaking, with having both experienced and young researchers participating.
Two-dimensional materials are extremely interesting as building blocks of next-generation nanoelectronic devices because it is much easier to fabricate circuits and other complex structures by tailoring 2D layers into desired forms, than to deposit or grow nanowires or nanotubes with predictable electrical properties in predefined positions. Due to their atomic scale thickness, 2D materials are the ultimate limit of miniaturization in the vertical dimension and allow shorter transistors before short channel effects set in. The most widely studied 2D material to date is graphene with its high room temperature mobility of at least 120,000 cm2/Vs. Pristine graphene has no band gap, resulting in small current on/off ratios in field effect transistors. This makes it very difficult to build logic circuits based on graphene that would operate at room temperature with low stand-by power dissipation. In fact, any potential replacement of silicon in CMOS-like logic devices is desired to have a current on/off ratio, Ion/Ioff between 104 and 107 and a band gap exceeding 400 meV. Band gaps of 400 meV induced by confinement result in significant mobility reduction while inducing a gap in bilayer graphene requires voltages exceeding 100V for a gap of 250 meV. All these has made the scientific community advance to semiconducting 2D materials beyond graphene.
One of the most promising materials are the transition metal dichalcogenides (TMDs) that have the common formula MX2 where M stands for a transition metal (M=Mo, W, Nb, Ta, Ti…) and X for chalcogenide element (X=Se, S or Te). The crystal structure of these materials is a succession of stacked layers weakly bonded to each other by Van der Waals forces. The band gap of these materials can be tuned by reducing the number of layers (e.g. bulk MoS2 is semiconducting with an indirect band gap of 1.2 eV, while single-layer MoS2 is a direct gap semiconductor with a band gap of 1.8 eV). The band gap can be further modulated cutting the layers into nanoribbons. Other features that make TMDs interesting for nanoelectronic applications include their chemical stability, the absence of dangling bonds and thermal stability up to 1100 °C.
Prof. Kis in École Polytechnique Fédérale de Lausanne has recently demonstrated that single-layer MoS2 can be used to fabricate transistors with high mobility (>200 cm2/Vs, sometimes as high as 780 cm2/Vs) and high current on/off ratios (>108). These transistors were based on scotch-tape exfoliated MoS2 using standard e-beam lithography and incorporate features such as gate oxide and local gates which make these devices suitable for building integrated circuits. Logic operations and integrated circuits based on single-layer MoS2 were also recently demonstrated. Additionally mechanical measurements have shown that this material has high stiffness and breaking strength, compatible with use in bendable electronics.
The TMDs exfoliation can be achieved mechanically on a small scale, moreover scalable liquid phase exfoliation methods are required for any realistic applications. It has been shown that TMDs can be exfoliated by ion intercalation. However, this method is time consuming, extremely sensitive to the environment and incompatible with the majority of solvents and so is unsuitable for most applications. Furthermore, removal of the ions results in re-aggregation of the layers. Recently it has demonstrated that graphite can be exfoliated into graphene by sonication in common organic solvents, provided their surface energy matched that of graphene. This means that exfoliation can proceed with a very small net energy cost. This breakthrough is facilitating research in many areas from composite formation to development of transparent conductors. Importantly, this method was recently applied to other inorganic layered materials such as boron nitride (BN). This procedure has a large number of advantages being the most important one that its simple and fast. Additionally it only requires mild sonication of the dispersed powder, followed by centrifugation to remove any un-exfoliated material. This procedure is not sensitive to the environment (inert atmosphere not required), requires no third phase dispersant (no ion intercalation needed) and is environmentally friendly as both the solvent and the centrifuged sediment can be recycled.
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