Photochemical & Chemical Modification of Si and Porous Si
Angew. Chem. Int. Ed. Eng., 52, 6731-6734 (2013)
Irradiation with laser light fundamentally
alters the surface chemistry of silicon. For instance, whereas clean crystalline Si is virtually inert to aqueous hydrofluoric acid,
irradiation of a Si crystal immersed in HF(aq) with a cw visible laser can lead to
the formation of porous Si. Once formed, the reactivity of porous Si can also be altered by irradiation. We are studying these processes in order to determine what factors affect the photochemical
reactivity of Si surface and to develop a mechanistic understanding of the photochemical reactions involved. An example can be found here in J. Amer. Chem. Soc.
We have also extended this work to investigate the formation of porous silicon by purely chemical methods, so-called stain etching.
In stain etching an oxidant is mixed with fluoride to form an aqueous solution that spontaneously produces porous silicon once a silicon crystal has
been dipped in it. We are now investigating the role of the oxidant and how it can be used to control both the photoluminescence spectrum and the morphology of
the por-Si film. We have demonstrated that several ions work well, including Fe(III), Ce(IV) and IrCl62- and we now have a quantitative understanding of charge transfer in terms of Marcus theory. The V(V) ion
has been used to form uniform films that can be as much as 20 µm thick and well as por-Si powder in collaboration with Vesta Ceramics.
As described below, we make macroporous silicon (porous silicon with very large pores) by etching pillar-covered Si substrates in alkaline solutions.
For more on porous silicon click here.
Pillar Formation & Sharpening
Laser irradiation of Si crystals under the appropriate conditions can lead to the
spontaneous formation of conical structures.
When made with a femtosecond laser, these pillars can be ten or so micrometers long. The
tips, however, are on the order of a few hundred nanometers or less.
Using a nanosecond laser, the pillars are much larger, up to 100
µm or more and a few micrometers at their tip. We have also shown
that we can make such pillars in
as well as titanium.
We have used alkaline solutions (concentrated KOH or tetramethylammonium hydroxide, TMAH) to etch silicon pillars. Short
etching times produce sharpended pillars. When the pillars are overetched, they disappear leaving behind macropores that are several
micrometers wide, such as the hexagon shown left.
More on pillars and macropores can be found here.
Anodic Titania Nanotubes & Porous Alumina
This project involves electrochemically etching Ti or Al to created arrays of nanotubes or pores while simultaneously
oxidizing the metal to its oxide (titania, TiO2, or alumina,Al2O3). Read more about it here.
Solidification Driven Extrusion (Nanospikes)
|While making silicon and germanium pillars, we noticed that nanoscales spikes form atop the pillars. We subsequently showed that the same physics that is behind this phenomenon is also active in your freezer and can result in the formation of centimeter long ice
spikes. Read more about this here.