**4. Conclusions**

Our results confirm that Cu/gCN nanomaterial composites can be useful for the development of SERS/SERRS applications. Our Cu NPs on sp2-bonded carbon nitride supports were fabricated using pulsed laser ablation of a Cu plate immersed in acetonitrile solvent. We carried out chemical, structural, and spectroscopic analyses to study the nature of the Cu NPs and their gCN support. The N:C ratio of the gCN material determined using bulk analysis was ≈1.3, significantly lower than the semiconducting carbon nitride compounds based on heptazine- to triazine-based structures, but it contained close-to-layered gCN materials containing mixed C–N/C–C bonding prepared using vapor deposition techniques. Such materials might contain locally electronically conducting domains. The degree of layer polymerization was incomplete, as shown by strong IR and Raman signals from terminal C≡N bonds. The Cu/gCN hybrid nanocomposites exhibited strong visible absorption extending toward the near-IR region with an SPR signal from the metal NPs at 580 nm. SERS/SERRS enhancement activity was tested using three analyte molecules (CV, MB, and R6G) prepared with initial solution concentrations of 10−3–10−<sup>7</sup> mol/L in acetonitrile and adsorbed onto the Cu/gCN nanocomposite surfaces. The observed SERS/SERRS enhancement factors were on the order of 107, comparable with *EF* values found for nanoparticle arrays of Cu produced using sputtering onto templates and other noble metal NPs supported by gCN materials. Further studies are now needed to investigate the possibility of cooperative interactions between the gCN support and the metal NPs that could enhance the optoelectronic effects leading to SERS/SERRS enhancement, as well as in stabilizing the Cu NPs.

**Supplementary Materials:** The Supplementary Materials are available online at http://www.mdpi.com/2079-4991/ 9/9/1223/s1.

**Author Contributions:** The study was devised by R.M. and synthesis, Raman, and SERS experiments were performed by H.D.-A. TEM and XPS characterization was performed by T.S.M. All authors participated in the interpretation of the results. H.D.-A. wrote a first draft of the manuscript that was completed and edited by P.F.M. and T.S.M.

**Funding:** R.M. thanks the Iran Nanotechnology Initiative Council (INIC) for financial support of the pulsed laser ablation setup. P.F.M. acknowledges support from the EU Graphene Flagship under Horizon 2020 Research and Innovation program gran<sup>t</sup> agreemen<sup>t</sup> No. 696656—Graphene Core2. T.S.M. thanks the UK Engineering Physical Research Council for support via the EPSRC Postdoctoral Fellowship EP/P023851/1.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
