"Silicene on copper": monoatomic silicon surface layer with silicene-like arrangement on Cu3+xSi prepared by chemical and mechanical exfoliation
Final Report Abstract
The main idea of the project was to produce surfaces that are silicon terminated and have a Silicene like arrangement of Si atoms on the surface by mechanical exfoliation. To achieve this goal the project consisted of different successive parts. The first part with the synthetization of the Cu 3+xSi phase turned out to be very challenging. The initial step with the production of the desired η'-Cu3+xSi. Various methods were tested and the synthesis products were analyzed to identify the phases it contained. The approach to grow Cu 3+xSi by CVD from different precursor gases resulted in formation of different shaped crystalline nano objects (plates, ribbons, rods). The η'-Cu3+xSi was found in larger species, most of the synthesized material contained also η''-Cu3Si or γ-Cu83Si17. High resolution TEM was used to characterize the nano objects and to determine the exact orientation by SAED for the proposed mechanical splitting. The nano-objects turned out to oxidize in air and change morphology to bamboo like micro rods. The focused ion beam technique in combination with micromanipulators were used to prepare and to orient the electron transparent TEM samples. The micro-rods have a complex microstructure with dark bands within the crystals. The TEM investigations were the basis for a corrected crystal model of the Cu3+xSi phase. The expected η-Cu3+xSi phase was not found at room temperature nor during in-situ heating experiments. The second production route was for bulk samples with arc melting. The phase transitions temperatures are strongly composition dependent. Due to the larger amount of material a detailed study of the different phases and their transition up to 700 °C was possible with in-situ heating diffractometer. The phase transition for in total 6 individual phases were observed. The coexistence of three phases were observed during heating and cooling. This contradicts the normal understanding of phase formation in thermal equilibrium for chemical homogenous material, where only two phase should occur at the same time. A clear explanation for the observation was not found. Finally a third production route with magnetron sputtering and subsequent annealing was tested. The deposition process produced smooth amorphous layers of Si and Cu. The phase formation was achieved by an annealing process. Depending on the process parameters different phases and microstructure have been observed. If the heat treatment produced the desired phase, the film became very rough, with porous and inhomogeneous crystal distribution. A preferred orientation that is suitable for the intended cleaving experiments was not found. The detailed structural characterization of the Cu3+xSi phases resulted in the correction of the crystal model. The Cu3+xSi structures can be conveniently described in terms of layers stacked along c axis. The average structure contains 12 mostly hexagonal layers per unit-cell length, but only four layers are symmetry-independent. The difference between the phases are different level of supercell formation. In conclusion the assumption that the structure is fully ordered had to be rebutted. The correct assignment has a mixture of Cu and Si with random distribution of atoms in the layer, thus a splitting in a purely Si terminated layer is not possible with the η'-Cu3+xSi phase crystals. Therefore the fallback option of the proposal for investigation of alternative CaSi 2 phase was used. An insitu methodology was developed to cleave Silicene-terminated CaSi2 phase inside TEM. Again the phase was investigated by TEM for composition and orientation. A dedicated FIB procedure was developed to prepare a thin sample from bulk crystal and transfer it with reduced electron and ion beam damage to the push-to-pull MEMS device of the TEM in-situ sample holder. The in-situ observation of a splitting experiment resulted in a rough edge which could not be attributed to a Silicene terminated surface. The sample preparation has to be optimized to achieve atomic resolution image for the broken edges. It seems it is not practical to produce a large sheet of Silicene using mechanical exfoliation method due to strong sp 3 bonding with foreign atoms. However the experimental procedure was proven be feasible, which opens the path to new applications for future in-situ experiments.
Publications
- In-situ mechanically cleave Silicene-terminated CaSi2 phase inside TEM; 6th Dresden Nanoanalysis Symposium 2018; poster P17
Zhongquan Liao, Jürgen Gluch, Yvonne Standke, Lukas Palatinus, Ehrenfried Zschech
