Pressure effects on the surface enhanced Raman spectroscopy of a ZIF-67/R6G system.

The high-pressure (HP) effect on surface-enhanced Raman spectroscopy (SERS) is a new horizon due to the importance of pressure, an external stimulus that can tune the lattice and electronic band structure of the SERS substrate and the analyte molecule. Here, we have used nano-ZIF-67 as the pressure induced (PI)-SERS substrate and R6G molecules as the analyte to explore the SERS effect in both the compression and decompression cycles. Our results demonstrate SERS signal enhancement at 0.12, 0.38 and 1.12 GPa. The initial SERS enhancement at 0.12 GPa is found to be due to inter-band transition resonance. A HP synchrotron diffraction study on the ZIF-67/R6G system illustrated an initial expansion of the ZIF-67-unit cell due to the infiltration of R6G molecules inside the pores and a structural phase transition around 0.38 GPa followed by an irreversible amorphization around 3 GPa. A significant SERS enhancement is also observed when decompressed to ambient pressure after a maximum pressure of 6.42 GPa. Although the crystal lattice seems to collapse irreversibly with HP, our Raman investigations indicated the recovery of the phonon modes upon pressure release. This observation indicates an important role of the local atomic arrangements in the SERS enhancement upon the pressure release.