Surfaces

Overview | Semiconductors| Catalysts | Protection | Structures | Surfaces | Devices| Systems | Sensors

Goal

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

Control of Band-Edge Positions by Functionalization of Si(111)

Functionalization of semiconductor surfaces with organic groups can change the charge distribution, surface dipole, and electric field at interfaces between the functionalized semiconductor surface and a contacting phase (such as an electrolyte or metal).  These changes shift the positions of the semiconductor energy bands relative to the energy levels of the contacting phase, and affect the voltages that solar cells made from the functionalized semiconductors provide.  Functionalization of Si(111) surfaces with methyl groups yields surfaces that are stable against oxidation and that can be interfaced with metals without the formation of energetically detrimental metal silicides at the surface; however, for p-type Si substrates, methyl termination produces a surface dipole that significantly lowers the attainable photovoltage (by about 400 mV). Functionalization of p-Si surfaces with fluorine-containing groups should produce a surface dipole in the direction opposite to that produced by methyl termination.

Effect of surface dipole on the band-edge positions and barrier height, Φ_b, for a p-type semiconductor. The partial δ+ and δ- charges show the orientation of the dipole moment at the interface. The relative energy positions of the valence band, E_V, the Fermi level, E_F, the conduction band, E_C, the vacuum level, E_Vac, and the average electron energy of the contacting phase, E(A/A-), are indicated.²

We prepared n-Si(111) and p-Si(111) surfaces that had varied coverage by covalently bonded 3,4,5-trifluorophenylacetylenyl (TFPA) groups, and that had the Si atoms not bonded to TFPA bound to methyl groups.  We characterized the modified surfaces using transmission infrared spectroscopy, X-ray photoelectron spectroscopy, and measured the charge-carrier recombination rates of the surfaces.  We additionally characterized junctions between the modified surfaces and Hg contacts, as well as junctions between the modified surfaces and redox couples in solution.

We found that the barrier heights for the junctions were a function of the fractional monolayer coverage of TFPA on the Si surfaces.  Relative to methyl-terminated surfaces, TFPA-terminated Si(111) samples with high coverages of TFPA produced shifts in barrier heights of ≥ 600 mV for n-Si/Hg contacts and ≥ 500 mV for p-Si/Hg contacts.  Consistently, the open-circuit potentials of TFPA-terminated Si(111) samples in contact with redox couples shifted relative to methyl-terminated Si(111) samples by +270 mV for n-Si and by up to +100 mV for p-Si.

Ethynyl- and Propynyl-Terminated Silicon Surfaces

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.³

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.; Ramachandran, A. A.; Lim, A.; Brunschwig, B. S.; Lewis, N. S., Control of the Band-Edge Positions of Crystalline Si(111) by Surface Functionalization with 3,4,5-Trifluorophenylacetylenyl Moieties. J. Phys. Chem. C 2016, 120, 14157-14169.
3. 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 (updated March 2018).}