|My research concentrates on the study of dynamical processes at the surfaces of metals and semiconductors with a special
emphasis on structure formation and laser-surface interactions. Along these lines my co-workers and I are studying photochemical and thermal
reactive processes on surfaces. Of special interest are etching and growth reactions to form nanoscale and larger structures.
A great deal of my work has involved pointing lasers at or near surfaces and observing what happens. We use lasers to investigate what is occurring at the surface; to probe the properties of interfaces and porous materials; and to initiate chemical reactions and physical changes in interfaces and porous materials.
The red image at the right is a photo I took of an ultrafast dye laser that I used at NIST. The green image at the left is a photo of a femtosecond Ti:sapphire of the type I used in Birmingham and at UVa.
Photochemical & Chemical Modification of Si and Porous Si
|Si is an elemental semiconductor, the most widely used in integrated circuit applications. 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
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.
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 have already shown that the fluoride can be provided not only by HF but also by NH4HF2. 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-. The V(V) ion has been used to form uniform films that can be as much as 20 µm thick.
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.
More on pillars and macropores can be found here.
Anodic Porous Alumina
|This project just started. It involves electrochemically etching aluminum to created ordered arrays of pores while simultaneously oxidizing the aluminum to alumina (Al2O3). Read more about it here.|
Dynamics of Adsorption and Desorption
|I have long studied the simplest of surface chemical reactions, the adsorption and thermal desorption of small molecules from surfaces, particularly hydrogen on silicon. I have also reviewed stimulated desorption of hydrogen from silicon. While this work has laid the foundation for much of my research, currently these are not the types of studies that I'm performing in my lab in West Chester. Rather they are part of what of do when I work with, for instance, Eckart Hasselbrink in Essen, Germany.|
|Read more about the my studies of surface dynamics 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.|
Ultrafast Surface Photochemistry in the VUV
|This was a project I worked on while at the University of Birmingham that involves the use of femtosecond pulsed lasers to create vacuum
ultraviolet photons via high harmonic generation.
Read more about it here.