Surfaces

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Goal

methyl_si

Low-temperature scanning tunneling micrograph of a silicon surface that has been chemically modified to produce covalent bonds between each surface silicon atom and a methyl group (top). The methyl groups are apparent in the image, and low temperature allows resolution of each hydrogen atom in each methyl group. A model of a cross-section of the surface (bottom).1

Develop chemical methods to tune the properties of semiconductors such as reactivity, stability, and performance.

Strategy

Combine chemical modification of surfaces with rigorous characterization techniques to enable the development of relationships between semiconductor properties and the molecular-level structure of semiconductor surfaces.

We have developed chemical techniques for bonding carbon-based molecules to silicon covalently.  These techniques allow every silicon surface atom to bond covalently to methyl groups.  Relative to freshly etched silicon – where each surface atom is bonded to a hydrogen atom – the presence of a covalent bond to carbon at each surface site prevents rapid oxidation of the surface, and reduces the rate of loss of photogenerated carriers to charge-carrier recombination at the surface.  Bonding of functional groups other than methyl to the surface effectively positions chemical building blocks on the surface that could be used to bond other molecules, such as catalysts, or to provide desired reactivity not possessed by the semiconductor itself.

Two-dimensional materials, such as graphene, can be deposited on semiconductor surfaces, and have been shown to increase the electrochemical stability of silicon photoelectrodes.  Therefore, coatings of two-dimensional materials, or chemically-modified two-dimensional materials provide an additional means to control the reactivity, stability, and performance of semiconductors.

Highlights

Ethynyl- and Propynyl-Terminated Silicon Surfaces

hreels_noah

High-resolution electron energy-loss spectra for ethynyl-terminated silicon (top) and for propynyl-terminated silicon (bottom). Si-C covalent bonds are apparent in both spectra.2

Wet chemical methods offer a low-cost approach to controlling the physical and chemical properties of semiconductors.  Methyl-terminated silicon surfaces have been characterized extensively, and that methyl termination imparts favorable qualities to silicon surfaces, particularly resistance to oxidation.  However, methyl surfaces do not offer opportunities for further controlled building on the surface.  For this reason, ethynyl- and propynyl-termination of silicon has been of particular interest since each of these groups contains a carbon-carbon triple bond that might act as a chemical building block for further modification of the surface, and since each of these groups is small enough to allow nearly every surface silicon atom to bond to a carbon atom covalently.

We reacted halogenated Si(111) surfaces with ethynylsodium or with propynyllithium to obtain functionalized silicon surfaces.  We rigorously demonstrated that the ethynyl and propynyl functional groups were bonded covalently to the silicon surface using transmission infrared spectroscopy, high-resolution energy-loss spectroscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction.2  For the ethynyl-terminated surface, we showed that ~63% of the surface silicon atoms were bonded to carbon, and that the carbon-carbon triple bond was chemically accessible.  For the propynyl surface, we showed that ~100% of the surface silicon atoms were bonded to carbon, but the carbon-carbon triple bond was not accessible.

References

  1. Yu, H. B.; Webb, L. J.; Ries, R. S.; Solares, S. D.; Goddard, W. A.; Heath, J. R.; Lewis, N. S., Low-temperature STM images of methyl-terminated Si(111) surfaces. J. Phys. Chem. B 2005, 109 (2), 671-674.
  2. Plymale, N. T.; Kim, Y.-G.; Soriaga, M. P.; Brunschwig, B. S.; Lewis, N. S., Synthesis, characterization, and reactivity of ethynyl- and propynyl-terminated Si(111) surfaces. J. Phys. Chem. C 2015, 119 (34), 19847-19862.

by Kimberly Papadantonakis, June 2016.