The team explains that the chemical etching of a silicon surface with hydrofluoric acid has been well studied. It is, after all, the chemical reaction commonly used to remove the normal oxide layer during typical processing steps to build a silicon chip, sensors or photovoltaic devices.
The etching process removes oxide and replaces it with a pristine bonded layer of outer hydrogen atoms. This is crucial to microelectronics but also underpins techniques that allow scientists to self-assemble functional monolayers on to a silicon surface for making biomedical and environmental sensors and diagnostics and in energy applications.
Hydrogen is not the only outcome for the termination of surface silicon atoms after HF etching. The French-US team has now demonstrated that it is possible to first nanopattern silicon with methoxy (OMe) groups and then to form a 30 percent coverage of silicon with a monolayer of stable Si-F bonds on atomically smooth Si(111) surfaces in the hydrofluoric acid etching reaction. Likewise they can achieve 30% coverage with hydroxyl groups (Si-OH termination) by immersion of the partially covered F-Si(111) surface in water; all these reactions occur without oxidising the underlying silicon substrate.
The formation of these monolayers with fluorine or hydroxyl groups also controls the reactivity of the surface. The results are in stark contrast to the received wisdom regarding silicon surfaces accrued during the last two decades. The team points out that what the approach highlights is that despite this received wisdom it is now possible to stabilize products at surfaces that cannot be produced on chemically homogeneous surfaces.
The research shows that a "snap" surface chemistry is possible simply by swapping Si-OCH3 for Si-F, Si-OH, and more generally Si-OR (where R could be almost any organic chemical group, such as alcohols) without damaging the silicon surface itself. The team suggests that organic self-assembled monolayers and various oxide materials, such as high-k
dielectrics might now be grafted on to an atomically smooth silicon surface.
The functionalization method is expected to extend to different sized alcohols to generate custom-sized nanopatterns for electronic devices, biological and sensor applications," the team concludes.