DFG Final Report: Development and atomic and electronic structure characterization of functional Sn/SnOx surfaces for SERS-based misfolded protein analysis

Abstract

The main progress of the current project is a deepening of the understanding of the growth of tin-based thin films on silicon wafers during organometallic chemical vapor deposition using tin(IV) tert-butoxide as a source of tin and oxygen. A new phenomenon related to the homo-geneous distribution of tin-based phases, from Sn(0) to Sn(IV), along one-dimensional silicon nanowires turned out to be a complex result of the realization of the supercritical conditions of the organometallic molecule in the so-called nanocapillary and significant differences in ther-mal conductivity between planar and nanostructured silicon surfaces, which enable the reali-zation of the homogeneous distribution of tin phases. In addition, we found that the variation of the initial matrix of silicon nanowires geometry allows us to control the amount of deposited metallic tin phase. Another phenomenon observed in this project is related to the thermody-namic anomaly in the growth of the tin dioxide layer on silicon in the temperature range from 725°C to 735°C, which leads to the formation of new nano-composites based on localized nano-sized metallic tin nanoparticles in the silicon dioxide matrix. The thermodynamic anoma-ly is a result of the effective oxygen exchange between silicon and tin dioxide surfaces, in which the more electronegative silicon attracts oxygen from tin, leading to the reduction of tin(IV) dioxide (SnO2) and disproportionation of thermodynamically unstable tin(II) oxide (SnO) to metallic tin. Finally, the high flux of the molecular precursor leads to the creation of bulk and surface defects in undoped tin dioxide layers. The emission of blue and green light in the 450-520 nm range is well known in the literature and attributed to oxygen-related defects, but yellow-orange emission is hardly discussed in the literature and can only be achieved for doped tin dioxide layers. Our studies allowed us to realize and experimentally confirm the presence of defects of different nature in undoped tin oxide layers using surface-sensitive X-ray spectroscopy techniques at synchrotron large scale facilities. In addition, we found an effect on undoped SnO2 layers, which can be used in optical sensing, related to the activation of different light emission by applying the same excitation energy of the 325 nm system with different photon fluxes (Xe-lump and He-Cd laser). The initial hypothesis of a plasmon-active metallic tin surface for Raman spectroscopy in the deep ultraviolet region could only be confirmed to a limited extent, as only very weak SERS effect was observed at excitation wavelengths of 244-325 nm. However, thanks to the coop-eration with our USA partner from the University of Albany, Prof. Lednev, the postulated SERS activity of tin-based surfaces could be observed at excitation wavelengths below 220 nm. The experiments performed in the USA allowed us to detect bioanalytes, which con-firmed our initial scientific hypothesis about the metallic tin plasmonic activity in the deep UV range. These results prompted us to change the original project strategy by using a different metal. In fact, when using electroless wet-chemically deposited copper (Cu) nanoparticles on silicon as substrates, we observed a clear enhancement of the Raman signal at 229-325 nm. To the best of our knowledge, this spectral range has not yet been used to excite plasmons on Cu-based surfaces. In earlier studies, only excitation wavelengths in the visible to near-infrared region were reported. The Cu-based surfaces were characterized with an AFM and UV-SERS spectra of amyloid fibrils, dyes and their respective dye adducts were successfully recorded at different excita-tion wavelengths. In particular, the spectra of dye-containing samples indicated strong interac-tions with the metal surfaces. It is suggested that this is due to their electron-rich aromatic systems, which lead to a more effective enhancement of the Raman signal than that of the proteins. AFM imaging with different modes completed the studies and confirmed the accu-mulation of dyes on individual fibrils.