Probing the Interfacial Charge-Transfer Process of Uniform ALD Semiconductor-Molecule-Metal Models: A SERS Study

Among all coating methods, atomic layer deposition (ALD), which can provide a precise thickness control at the angstrom or the monolayer level, appears to be one of the most promising techniques. To investigate the interfacial charge-transfer mechanism from semiconductor-molecule-metal systems, the order of different layers is very essential because the charge-transfer process can be affected by the interfacial contact order of different materials. Also, for TiO2/MBA/Ag charge-transfer (CT) investigation, homogeneous assembling of TiO2 with precisely controllable thickness is of great importance because the energy level of semiconductor is sensitive to its size at the nanoscale. Here, unlike previous 3D composite CT models, our semiconductor-molecule-metal interfacial CT models are fabricated with the ALD and e-beam evaporation techniques, which ensures the accuracy of the CT investigation. The surface-enhanced Raman scattering (SERS) technique is adopted in the investigation of the interfacial charge-transfer process through the changes of CT-sensitive bands. In TiO2/MBA/Ag, the SERS signal of MBA molecules and the Raman spectra of TiO2 phonon vibrational mode exhibit evident CT-driven changes. To confirm these phenomena, 4 nm thickness of wide-bandgap HfO2 and Al2O3 are inserted as isolated layers. Also, the possible CT mechanisms and the charge-transfer degree in different systems are discussed. This work not only suggests a role for ALD in fabricating CT models but also promotes the application of SERS in more intensive fields.