Elettra | Xpress Tuning surface enhanced Raman spectroscopy effect in porous ZIF-67/R6G system by external pressure Surface Enhanced Raman Spectroscopy (SERS) is powerful tool for the ultrasensitive detection of specific analytes especially of suitable matrices with specific properties. Its growing appeal lies in its simplicity, effectiveness, and potential for on-site analysis. Traditional SERS substrates are noble metals (Au, Ag, Cu), where the enhancement in the Raman intensity is understood as through electromagnetic mechanism (EM). However, costeffective alternatives such as 2D materials and MOFs have emerged, where enhancement is attributed to the chemical mechanism (CM) via charge transfer (CT) between substrate and probe molecules. External pressure is one of the most effective tools to tune the energy levels of such SERS substrates and analyte molecules and thus have an immense potential to tune the SERS chemistry. However, systematic studies are lacking till date. Recently, we have explored the pressure effect of the SERS using porous semiconductor ZIF-67 as substrate and Rhodamine 6G (R6G) as an analyte molecule. Interestingly, the porous ZIF-67 exhibits breathing and gate opening phenomena in presence of guest molecules under external pressure due to its structural flexibility. These aspects make it an ideal system for the investigations of the SERS activity under high-pressure (HP), which was undertaken by us recently. We have conducted high-pressure SERS and photoluminescence (PL) studies at the Xpress beamline of Elettra in absence of pressure transmitting medium, combining these data with that of the HP X-ray diffraction (XRD) studies under identical conditions. Figs. 1 (a, b) show the compression and decompression pressure cycles of SERS of R6G on ZIF-67. Fig. 1 (c) shows the variation of Raman intensity of high intense mode at 612 cm-1 of R6G adsorbed on ZIF-67 substrates with pressure. A gradual rise in the SERS intensity is observed at 0.12 GPa due to admission of R6G molecules inside pores of ZIF-67 and thus greatly facilitate the electronic interaction between the two. Above 0.12 GPa, SERS intensity decreases and our HP-XRD measurements also demonstrated a typical compression behavior expected for a porous system (Figs. 1 d, e). The rise in SERS intensity at 0.12 GPa can be explained to be due to the initial expansion of ZIF-67 unit-cell volume (Fig. 1 e). With further rise in pressure, an additional enhancement of R6G Raman signal is observed at 0.38 GPa which may be resulting from the structural phase transition that was evidenced in our HP-XRPD analysis at similar pressures (Fig. 1 c). As a result, a rearrangement of the electronic energy levels leading to a change in band gap is expected. This change in the band gap can facilitate the charge transfer process between ZIF-67 substrate and R6G analyte molecules. Upon increasing the pressure to 1 GPa, the Raman enhancement decreases slowly and then shows a further enhancement at 1.12 GPa (Fig. 1 c). The change at 1.12 GPa might be due to a local distortion in the pore size of ZIF-67 prior to the initiation of the amorphization process. Thereafter, the enhancement decreases gradually till the maximum pressure of 6.42 GPa (Fig. 1 c). Fig. 1 (b) also includes the pressureinduced-SERS spectra of the ZIF-67/ R6G on decompression. In the pressure release cycle, the Raman spectra have enhanced intensity and reaches the maximum at 0.06 GPa and stable even after 24 hrs. The Raman peak intensity of 612 cm-1 shows huge enhancement at ambient pressure on decompression in comparison to the higher pressures in compression cycles and also that of the initial ambient spectrum (Fig. 1 c). The above enhancement at ambient pressure on decompression might be due to the trapping of R6G molecules inside the pore cavity. Although the crystal lattice seems to collapse irreversibly with high pressure, our Raman investigations indicated the recovery of the phonon modes upon pressure release. This indicates an important role of the local atomic arrangements in the SERS enhancement upon the pressure release. The HP-PL measurements were performed to further understand the pressure evolution of the band gap (Fig. 1 f). Raman enhancement of the ZIF-67/ R6G system under ambient conditions is attributed to the resonances like the exciton (ex) or inter-band and molecule resonances (mol). The inter-band transition or exciton (ex) resonance of MOFs is associated with the direct bandedge transition from HOMO to LUMO. The band gap of the ZIF-67 system is 2.03 eV (which is the resonance energy in this case) is close to the laser excitation wavelength 2.33 eV. With an increase in pressure up to 0.12 GPa, the band gap of ZIF-67 system might be mostly increasing due to increased unit-cell volume which helps it to come to in resonance with laser excitation at 0.12 GPa, thus activating the resonance mechanism for the SERS. With further increase in pressure, the enhancement observed at 0.38 GPa is mostly due to the structural transition which rearrange the energy levels leading to a favourable charge-transfer process. In conclusion, our present investigation thus clearly establish that the external pressure plays a crucial role in the modification of the surface and porestructure of ZIF-67 on the adsorption of R6G and thus affect significantly the SERS. Our current study underscores the importance of structural flexibility in unravelling the HP chemistry of MOFs based SERS substrate and provide an efficient strategy to design better preforming SERS substrates under pressure. Elettra Highlights 2024–2025 134
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