Long-Range Hexagonal Pore Ordering as the Key to Controlling SERS Efficiency in Substrates Based on Porous Alumina

• Published in: Journal of physical chemistry. C 2020-11, Vol.124 (47), p.25931-25943
• Main Authors: Pashchanka, Mikhail, Okeil, Sherif, Schneider, Jörg J
• Format: Article
• Language: English
• Subjects: C: Plasmonics; Optical, Magnetic, and Hybrid Materials
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.0c02761

Self-organized porous anodic alumina (PAA) layers have been previously suggested as effective surface-enhanced Raman scattering (SERS) substrates, which exhibit high enhancement factors when coated with noble metals such as silver or gold. For the first time, we experimentally demonstrate that the long-range hexagonal ordering in PAA is a cornerstone of the topography-based SERS enhancement effect. The discovered correlation between structural order and enhancement creates a new possibility for cost-effective manufacturing of SERS substrates with highly tunable coherence-induced enhancement factors. This has been successfully achieved by applying different anodizing durations and flexible adjustment of the average well-ordered domain size. The currently available information about the impact of long-term potentiostatic anodizing on the degree of hexagonal ordering is rather unsystematic and sometimes even contradictory. Therefore, a considerable part of this work is devoted to the update of this knowledge and formulation of universal underlying principles describing the self-ordering evolution irrespective of the nature of the employed electrolyte. We have established a typical relationship between the average number of nanopores contained within a single coherent domain and the anodizing duration. Moreover, the strong correlation between such adjustable surface patterning and the observed SERS enhancement effect sheds light on Raman signal control via improved hexagonal arrangement of SERS-active hotspots.