Temperature programmed desorption (TPD) spectroscopy can be one of the most powerful tools a surface scientist has at his/her disposal. At its best, TPD elucidates the strentgh of interfacial chemical bonds. For a research group like ours that is leveraging chemical modifications to stabilize semiconductor surfaces, quantifying the strength of those interactions is a critical aspect of our analysis!
TPD is an ultrahigh vacuum technique in which a sample is heated below a mass spectrometer that can “catch” surface-desorbed species. Typically, samples are heated linearly, say 1 °C s‑1 because that makes the math easier. Weakly bonded adsorbates require less heat to break their substrate-adsorbate bonds, and they desorb at a low temperature. More strongly boned adsorbates require more energy to break their bonds, and thus they desorb at a higher temperature.
The cartoon illustrates the desorption process. If water was adsorbed onto a surface with both hydrophilic and hydrophobic species, we might imagine that the water would desorb off of the hydrophobic parts of the surface at a lower temperature than the water desorbs from the hydrophilic parts. We can generate a TPD spectrum from the intensity of the m/z = 18 amu signal at the mass spectrometer vs the surface temperature. From this TPD spectrum we can acquire information about the interaction energy between the water and that particular surface.
Our application of temperature programmed desorption
We telegraphed that one pretty heavily: we use TPD to quantify the strength of the interaction between the chemical functionalization we do on a semiconductor surface and the surface itself. This tells us a lot about what the chemical interaction actually looks like, and whether that functionalization is viable for passivating the semiconductor. If we produced a beautiful self-assembled monolayer on the semiconductor surface, but it only interacted weakly with that surface, then it probably isn’t all that passivating, is it?
TPD and MQP’s
You get carte blance access to the phrase “adsorbate-substrate interaction”. That’s almost as great for scaring people off as “advanced ligand field theory”.
Temperature Programmed Desorption