I'm working with thin films of organic molecules on quartz/silicon substrates. When I expose a film composed of molecule A to molecule B (vapors), I see a new peak in the UV-vis spectra. I want to know if that peak belongs to molecule B or to a new material AB.
How can I take the UV-vis of the powder B? The way I deposit molecule A on the slide is by PVD (physical vapor deposition), but this technique is not an option for molecule B. Exposing the slide to vapors of B doesn't result in its deposition on the surface.
Thank you!
As in many cases, there is too little information to give a helpful answer. What Kind of molecule is B? Is it soluble in any solvent? Could it be deposited by spin-coating?
Can you perform other techniques of thin film analysis besides only UV-Vis? Do you expect the thin film to be crystalline (which would make X-ray diffraction and hence the recognition of new phases an option)? Tell us more.
As I understood, you need to take UV_Vis spectra of a powder!!
You have to make a solid dilution with a powder that do not absorb in UV_Vis range (generally you can use MgO), make a pellet and take a Diffuse Reflectance Spectroscopy spectra. Your background will be MgO pellet without your powder.
B is a halogenated molecule of benzene. It is soluble in chloroform. I've tried to drop-cast its solution on the quartz slide, but the spectra looks really bad, without any defined peaks. I'm going to try spin-coating, maybe it's a better way.
I'm planning to use other techniques such as MS and HPLC to the dissolved film. I just want to be able to analyze the UV-vis spectra, to know what contributed to the emerged new peak.
You can use micro-Raman spectra to check the solutions. They can be different from A, B and AB solutions.
KBr is an other material besides MgO which is often used to make pressed pellets for such optical measurements. Alternatively, you could measure in the liquid phase using cuvettes that can be sealed. In this case you'd rather measure transmittance than diffuse reflectance.
if you have an integrating sphere UV-VIS you can measure a powder directy with diffuse reflectance. This should give you a longer path length of light, compounded with scattering to emphasize the peaks. You will likely not be able to get an absorption coeficient as the path length is not carefully controlled, but it will highlight a new peak.
Another option for the new peak may be due to an interaction of chemical A or B to your surface. are you changing A or B in any way when you use the PVD system?
Solutions of B could also be an option. To enhance the peak you would want to maximize the concentration of B and choose a solvent with no peak in the region of interest. If this sample is non scattering a typical dual beam UV-Vis should work. If it scatters an integrating sphere system is better and again path length will be lost so absorption coeficient cannot be had.
Good Luck.
@sami rtimi: Even i need help with DRS. Could you please tell me how to make the pellet. Thank you.
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