Kurt W Kolasinski

We investigate the formation of porous silicon by electroless 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 IrCl₆^2- 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. Our newly discovered ReEtching process allows us to form por-Si powder with fully etched particles as well as porous particles with hierarchical nanostructures. Now we are applying advances developed for ReEtching to metal assisted catalytic etching (MACE).

As described below, we make macroporous silicon (porous silicon with very large pores) by etching pillar-covered Si substrates in alkaline solutions.

Read more on porous silicon.

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. 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.

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.

 

While making silicon and germanium pillars, we noticed that nanoscale 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. 

Solidification Driven Extrusion (Nanospikes)

Dave Mills, who now spends some of his time reading old scrolls with X-rays, noted that in some of the scanning electron microscopy (SEM) images that he took of our laser ablation created silicon pillars, that a little spike formed on the tip of the pillar. He also thought that these might be related to ice spikes. You can get ice spikes to form in your freezer simply by freezing pure distilled water. A major question in nanoscience is: what techniques to form structures scale from the macroscopic (for instance centimeter) range into the nanoscale regime? We showed that the physics behind ice spike formation scales down to the nanoscale. It works for silicon pillars just like it works for ice cubes.

In subsequent work with germanium performed with Barada Nayak and Mool Gupta, we found that nanospikes also form on germanium pillars, as shown on the right.

So what do water, silicon and germanium all have in common? They all form tetrahedrally co-ordinated solids phases and all of them expand when they freeze. This is an unusual property but that is what makes ice less dense than liquid water and why ice cubes float. It also means that if the surface of liquid water, silicon or germanium freezes that it compresses the liquid that is trapped inside. This leads to a build up of pressure. Eventually the pressure is too much and a jet of supercooled liquid shoots out and forms the ice spike.

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. 

This project was part of a TMR Network. Being part of a TMR network is often  hard work, including walking up snowy mountain roads and skiing. During this work, we built a rather unique machine to study ultrafast (about 1 ps or less) photochemistry in the vacuum ultraviolet regime. A schematic drawing of the apparatus appears below. The required photons were made through a laser-based technique: high harmonic generation with an Ar-ion-pumped Ti:sapphire laser. This laser produces roughly 80 fs pulses at a wavelength near 800 nm. The output of the Ti:sapphire laser is focused into a rare gas that flows out of a tube in a vacuum chamber, as shown in the photo on the right. A nonlinear interaction between the laser field and the atoms in the rare gas jet creates the photons that we are after: ~10-40 eV or 120-30 nm. We studied the photochemistry of  O₂ adsorbed on graphite. This was the first use of HHG to initiate surface photochemistry.