My name is Carola Rickpraun and I am a professor in the Department of Chemistry at the Technical University of Berlin, here in the center of Berlin.

Our research area is organic chemistry and amongst our research projects are projects dealing with photo switches. Photo switches have experienced considerable interest for studying complex living systems using light and they are also promising building blocks for developing light-sensitive devices.

We are interested in the wet chemical functionalization of non-oxidized planar silicon substrates and of course we are already decorating these substrates with photo switches. Long term, the surface chemistry will be of importance for molecular functionalization of nano-electronic device elements, for instance, silicon nanowire field effect transistors.

So silicon remains to be one of the most important semiconductor materials for micro-electronic industry. Now the aim of our studies is to examine the fundamentals of silicon surface chemical reactions. Now the details will be provided by my co-workers.

For instance, they will let you know that we have to confirm each functionalization by infrared spectroscopy. My name is Andreas Hebert and I'm doing my PhD regarding research on semiconductor surface. Such a semiconductor surface can be silicon and silicon was in the past, is today and will be in the future

the subset of choice for the building of electronic components. But according to smaller and smaller devices, new concepts and ways are needed for the development of novel electronic devices. One novel concept could be the construction of a photo-switchable monolayer on such a silicon waver. As a concept, we can think about starting from the hydrogen-terminated silicon surface,

first an ester layer is established in a self-assembling process. After cleavage of the ester to an acid-terminated monolayer, then a photo-switch can be coupled on top of such a silicon waver. Such a photochromic unit can be the phalgemite shown here. Phalgemites belong to the p-type of photochromes,

which means that the different photochromic isomers are terminally stable. Up in the illumination with UV light, phalgemites undergo a ring-closing reaction to the so-called closed isomer. This closed isomer can only be opened again by using visible light. For analyzing of photo-switches on such a silicon surface or in solution,

different spectroscopic methods are known. Our appropriate choice of method is infrared spectroscopy. But also for anchoring of such a photo-switchable unit on such a silicon waver, these photo-switches have to be customized. My colleague Jana Liebmann will show the research regarding synthesis work on an example of naphtha pyran.

Here you can see a typical basic framework of naphtha pyran. Their photochromism is based on a 6-pi electrocyclic ring-opening, ring-closing reaction. So upon illumination of the colorless closed-form isomer with UV light, several interconvertible open-form isomers of the mirror cyanide type are formed.

Here you can see a model of a naphtha pyran. In the closed-form isomer, the aryl residues in this position are almost perpendicular to the chromine backbone. Upon illumination, this bond breaks. Now in the open form, these aryl residues are in plane with a chromine backbone.

Thus, the pi system is extended, leading to a bathochromic shift. In contrast to the fulgemites, naphtha pyrans belong to the T-type photochromic molecules. This means that the back reaction can take place either thermally or upon irradiation with visible light. The back reaction is typically very fast, but depends on the substitution pattern.

Here you can see one of the naphtha pyrans we are currently working on to examine if they are suitable for immobilization on silicon 111. With this particular substitution pattern, we were able to realize a suitable lifetime of several minutes of the open form isomers, which is necessary for further investigations.

As already mentioned before, our method of choice to characterize photoswitchable monolayers is infrared spectroscopy. To evaluate if our naphtha pyran is suitable for immobilization on silicon 111, we have to investigate if we can observe any significant changes in the IR spectra upon irradiation of the sample. Therefore, we have to perform preliminary studies in solution.

Here you can see the setup of our IR machine for measurements in solution. On the one side of the holder, you can place such a regular cuvette with potassium bromide windows, and on the other side, you can place this custom-made LED module, which allows us to irradiate the sample while measuring IR spectrum. In this case, we use 365 nm light.

I've already prepared a sample solution with a concentration of around 10 to the power of minus 2 mole per liter and filled the cuvette with it, and will now place it in the IR machine.

First, we measure an IR spectrum without illumination of the sample. Then we start irradiating the sample with 365 nm light, and while irradiating, we record IR spectra. Here you can see the results. The blue curve, which is our starting point, represents the closed-form photostationary state, and the red curve represents the open-form PSS.

There are obviously significant changes in the IR spectra. Even though there is no change in the CO stretching vibration of the Easter group, we can observe the formation of a characteristic absorption band, which is typical for the CO stretching vibrations of ketones, which can be clearly assigned to the open-form isomers.

Here you can see the different spectra. The blue curve indicates the switching process from the closed-form to the open-forms, and the red curve represents the thermal back reaction. The most significant changes you will find in this area. This is due to the change in the pi system, which was explained earlier.

These results prove that our nafta pyrin is a promising candidate for immobilization on silicon 111, and now my colleague Andreas Hebert will show you the steps of our on-chip satanic route on the way to photocontrollable monolayers. Starting for the immobilization of molecules, we start from the already shown commercial available silicon wafer,

where we broke smaller pieces from the whole wafer, followed by grinding of the edges in a certain degree of angle just by using ordinary sandpaper. After polishing with special diamond paste and lubricant, we obtain this special silicon wafer. Why the polished angles are needed will be shown and explained later.

Before hydrogen termination, all organic impurities have to be removed from the surface. Therefore, we are using a solution of hydrogen peroxide and sulfuric acid, also known as Perania solution. The clean hydrophilic wafer is then hydrogen terminated just by edging in fluoride solution,

which leads to a hydrophobic, well-defined surface as the starting point for immobilization. For our goal on oxide-free structures on silicon, all necessary reaction steps are conducted in a glove box to exclude any contaminations of oxygen and water. The assembly of the molecules is then done directly on a chip.

As we source from the concept, first the ester layer is established in a self-assembling process in a closed wild under thermal conditions. After transformation of the ester terminated monolayer to an ester terminated monolayer, the photo switch is coupled on top of the ester terminated monolayer by using a peptide synthesis protocol.

Due to the characteristic construction of the silicon surface, the number of immobilized molecules are in a concentration of 10 to the power of minus 10 mole per square centimeters, and therefore very low. So, for analyzing the photo switches on the silicon surface, a special spectroscopic method for IR spectroscopy is needed.

Our appropriate choice for this is Ethernet Total Reflection. The functionalized silicon wafer is mounted into a special holder and then placed onto RTR geometry in our IR spectrometer. Under Ethernet Total Reflection, the IR beam is coupled into one of the polished angle sides

of the silicon crystals. On the way of the IR beam through the whole crater, he is reflected a dozen of times on the surface. Every contact with the surface represents there by a measurement, which leads in the end to a very high signal-to-noise ratio. At the end, the IR beam is coupled out from the silicon crystal and directed over different mirrors to the detector.

As a result, we obtain an IR spectra, like it is here displayed for the Easter layer, which typical stretches in the alkyd region and also a sharp and narrow stretch for the carbonyl stretch of the Easter signal, which indicates that we have a closed and dense packed monolayer on top of the silicon surface. After transformation of the Easter layer

to an acid-terminated monolayer, we can clearly see a shift from the Easter signal to the acid signal. Last step is then the coupling of the photo switch on top of the acid-terminated monolayer. This is again monitored by IR spectroscopy and is obtained by shift in the appearance of characteristic signals for the imide stretches for the fuljamide.

Now having established our fuljamide on top of the silicon surface, last question would be, can we switch our monolayer? For this, we use again our RTL geometry and our special self-made LED setup for real-time measurements. With light of different wavelengths, we are able to alter between the different

photo stationary states of the fuljamide. As you can see here from the different spectra, where the blue line indicates the change in the ring closing and the red line indicates the change in the ring opening. With this analysis, we are able to open and close the fuljamide directly on top of the silicon surface. And as we can see here for repeatable cycles

for the carbonyl stretch, we are able to switch back and forwards over multiple cycles without any degradation. Which leads in the end that we can alter the surface property of silicon just by using light. Lessons learned from our research will have a major impact on developing light sensitive bottom-up devices.

Bottom-up devices have still the disadvantage of low reproducibility. This already in industry can be altered by applying our research.