Vibrational Spectroscopy and Mass Spectrometry for Characterization of Soft Landed Polyatomic Molecules

We report implementation of two powerful characterization tools, in situ secondary ion mass spectrometry (SIMS) and ex situ surface enhanced Raman spectroscopy (SEAS), in analyzing surfaces modified by ion soft landing (SL). Cations derived from Rhodamine 6G are soft landed onto Raman-active silver colloidal substrates and detected using SERS. Alternatively and more conveniently, high-quality SERS data are obtained by spin coating a silver colloidal solution over the modified surface once SL is complete. Well-defined SERS features are observed for Rhodamine 6G in as little as 15 min of ion deposition. Deposition of similar to 3 pmo1 gave high-quality SERS spectra with characteristic spectroscopic responses being derived from just similar to 0.5 fmol of material. Confocal SERS imaging allowed the enhancement to be followed in different parts of deposited dried droplets on surfaces. Characteristic changes in Raman spectral features occur when Rhodamine 6G is deposited under conditions that favor gas-phase ion fragmentation. Simultaneous deposition of both the intact dye and its fragment ion occurs and is confirmed by SIMS analysis. The study was extended to other Raman active surfaces, including Au nanostar and Au coated Ni nanocarpet surfaces and to SL of other molecules including fluorescein and methyl red. Overall, the results suggest that combination of SERS and SIMS measurements are effective in the characterization of surfaces produced by ion SL with significantly enhanced molecular specificity.