Silicene, like graphene — a carbon lattice of one atom — is a two-dimensional material composed of silicon atoms. It has aroused interest in researchers worldwide, especially for its potential applications in the semiconductor industry.
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Image Credit: Slobodenyuk Alexandr/Shutterstock.com
Bruno Grandidier, researcher at the IEMN in Lille, France, investigated the spectral composition of this new "wonder material."
Two-dimensional atomic crystals have very high specific surface areas (SSA) and remarkable electronic properties, making them unique for studying novel physical phenomena.
Additionally, their two-dimensional geometry matches with the established design and processing techniques already in use in the semiconductor industry. This asset opens the way to several new applications in electronics, sensors, photonics, catalysis, and energy storage.
Graphene is the most widely-known material of this type, though other two-dimensional atomic crystals also exist. In order to overcome graphene’s limitations and pave the way for the conception of new innovative devices, one of the main and current focuses of research in this field is to study the unexplored physical properties of these materials.
Silicene is the silicon (Si) analog of graphene. It consists of a single sheet of sp2-hybridized Si atoms, and is now emerging as a two-dimensional material with extremely attractive electronic properties.
Determining the Atomic Structure of Silicene
Recently, new specific physical properties of silicone were revealed by the IEMN research team.
In this study, a silicene sample was grown on a silver surface by evaporating atomic Si onto an Ag(111) surface at a deposition rate of 0.02 monolayer/min-1. The temperature of the silver surface was maintained at 230-250 °C.
The silicene sample was investigated using three different methodologies to gain insight into the true nature of the Si-Si bonds and the interplay between Si and Ag atoms. The three methodologies were all integrated in the same ultra-high vacuum system. These were:
- Raman spectroscopy
- scanning tunneling microscopy (STM) and
- low energy electron diffraction (LEED)
Similar experiments were performed on the surface of a Si(111) crystal for comparative purposes. The structure and composition of the silicene surface were visualized and analyzed by employing MountainsMap® software to compare it with the silicon crystal. The study confirmed the differences in spectroscopic properties.
MountainsMap®: A Multiphysics Analysis Tool
After the deposition of one monolayer of Si atoms onto the Ag(111) surface, the obtained atomic-scale STM image shows the typical (4x4) atomic structure consisting of a honeycomb lattice. Two terraces are visible and separated by an atomic step with a height of 2.4 Angströms. Tunneling conditions: sample bias of -1.0 V, tunneling current of 50 pA.
The structure is validated by the analysis of the LEED pattern. In the structure, the most intense diffraction one-four order spot (yellow) corresponds to the (4x4) structure. Other spots arise from the partial formation of a second layer with the (√3x √3) structure (blue spot) and the bare silver surface (red integer spot).
Raman spectroscopy was carried out with the sample on the STM head in ultra-high vacuum. However, the measurement illustrates a peak, followed by a tail toward lower frequencies in the wavenumber range 450-550 cm-1.
The decomposition yields two contributions, based on the reference spectrum of silicon. The major one is attributed to the TO mode of silicon. The second one at lower frequencies is caused by vibrational modes related to the particular hybridization of the Si orbitals with the Ag(111) surface.
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3D view of the silicene surface generated in MountainsMap® showing an atomic step and typical "honeycomb" structure.
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LEED analysis
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Spectrum analysis performed on the silicene surface shows two peaks including one at 494 cm-1, specific to this material and not present in the spectral response of silicon. Data obtained using the new Peak detection feature in MountainsMap®.
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