Single-Molecule Detection-Enabled Plasmonic Ag Nanogap for Unmasking Vibrational Properties in 0D SnO₂

The strong electric field created by plasmons in the surface or tip-enhanced Raman scattering (SERS or TERS)-based detection of ultralow organic and biological molecules finds limited success in the case of inorganic molecules such as SnO2. Our report highlights the fabrication of a highly efficient Ag SERS substrate as the first direct evidence of the Raman A1g mode in rutile SnO2 quantum dots (QDs). SnO2 QDs show an uncharacteristic and broad Raman feature around 575 cm–1. First, the fabrication of an ultraefficient SERS substrate with a huge electric field enhancement is demonstrated with a display of single-molecule detection of rodamine6G (R6G) molecules (∼10–18 M), which is supported by finite-difference time-domain (FDTD) calculations. These efficient SERS substrates as nanonets help reveal, for the first time, the A1g mode at 627 cm–1 in QDs with a red shift from 633 cm–1 of the bulk SnO2. This observation is further supported by a high-pressure Raman study. Notably, these SERS substrates have allowed further observation of an intense IR-active Eu (TO) mode as a Raman mode for crystalline SnO2 of 25 nm size. Such an exclusive appearance of an IR is explained by the gradient-field Raman (GFR) effect instead of the previously employed mutual exclusion principle breakdown theory. This proposition is well supported by Fourier transfer infrared (FTIR) and temperature-dependent Raman spectroscopic measurements.