**4. Conclusions**

In summary, a highly active silver aggregates SERS material was directly synthesized on the ITO–solution interface via a facile in situ photochemical reduction method. The morphologies of these aggregates were e ffectively controlled by laser power and irradiation time. By correlating the morphologies with their SERS signals, the best SERS aggregates were obtained under the synthesis parameters: power = 0.9 mW, time = 60 s. The average SERS EF was as large as 2.0 × 107. Importantly, the morphology features of optimal SERS aggregates were identified. Aggregates composed of packed nanorices and flakes with abundant surface roughness would possess better SERS ability. An understanding of the relation between morphology and SERS performance would be beneficial for controlled synthesis of optimal SERS materials with a high density of hot spots, and the development of practical SERS techniques.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/11/1529/s1. Contents: SERS enhancement factor estimation. Figure S1. The absorption spectrum of 10−<sup>4</sup> M CV in water; Figure S2. The morphological evolutions of silver aggregates; Figure S3. Monitoring the silver aggregates growths under di fferent citrate concentrations; Figure S4. The SERS spectra measured on the silver aggregates synthesized under di fferent parameters; Figure S5. The magnified images of aggregates i, x and xii; Figure S6. The numerical simulations of the local field distributions around the two coupled roughened nanorices.

**Author Contributions:** Z.L. and L.Y. supervised the experiments; J.Y.; X.C. and P.L. performed the experiments; Y.L. performed the numerical simulations; L.Y. and Z.L. analyzed the data; L.Y. and Z.L. wrote the manuscript. All authors discussed the results and commented on the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant Nos. 11774245 and 11704266), the Fok Ying Tung Education Foundation, China (Grant No. 151010), the General Foundation of Beijing Municipal Commission of Education (Grant No. KM201810028006), the Beijing Natural Science Foundation (Z190006), the Training Program of the Major Research Plan of Capital Normal University, Yanjing Scholar Foundation of Capital Normal University, and the Scientific Research Base Development Program of Beijing Municipal Commission of Education.

**Acknowledgments:** This work was supported by the National Natural Science Foundation of China (Grant Nos. 11774245 and 11704266), the Fok Ying Tung Education Foundation, China (Grant No. 151010), the General Foundation of Beijing Municipal Commission of Education (Grant No. KM201810028006), the Beijing Natural Science Foundation (Z190006), the Training Program of the Major Research Plan of Capital Normal University, Yanjing Scholar Foundation of Capital Normal University, and the Scientific Research Base Development Program of Beijing Municipal Commission of Education.

**Conflicts of Interest:** The authors declare no conflict of interest.
