SERS nanostructure imaging in NPN and npMgF2, with heat treatments

ByGreg Madejski

Over the last few months, I have been trying to reestablish my SERS measurements, and generate a paper based on SERS sensing with our nanomembranes. Previously, I have created magnesium fluoride coated substrates that seem to have better SERS performance than just NPN alone. Combining our TEM imaging with SERS data would be a powerful tool to understand the nanostructures that enable the SERS phenomena. To that end, I have been depositing various layers of gold (15-50 nm thick) on top of different substrates, then coating the substrates with 4 mM thiophenol, which is used as a benchmark spectra.

 

1257 NPN or npMgF2 coated with gold. The npMgF2 is necessarily narrower, due to infilling of the pores. The gold seems to be smaller and more densely packed on the MgF2 than the NPN.

 

Raw Raman traces. Green traces are 1257 npMgF2/NPN, with different amounts of Au. Yellow trace is 1257 NPN, etched to have wider pores (40s). Red trace is a glass coverslide, coated with the same 15 nm Au as the npMgF2 and NPN. The blue trace is 1236 npMgF2/NPN, measured 2 years ago. All were 10s integrations on Dr. Berger’s microscope, measured on the flat surface of the chips.

 

Background corrected Raman Spectra for thiophenol coated substrates. The npMgF2 is the best, NPN is middling, and the glass coverslip has the lowest number of peak counts.

 

If the nanostructures in the pores are responsible for the SERS performance, changing the spacing and size of the particles should impact the SERS signals we measure. To that end, we can heat up the substrates and melt the gold into new shapes and configurations.

Heat treatments and substrates change the morphology of gold in the nanopores. Heating tends to agglomerate the gold, but the npMgF2 appears to be more resistant to this than the NPN.

 

Coating the membranes with washing steps (applying thiophenol) does not appear to change the nanomorphology appreciably.

 

I will update this post with thiophenol spectra for these substrates soon.

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