Nanoparticle Clusters: Assembly and Control Over Internal Order, Current Capabilities, and Future Potential.

The current state of the art in the use of colloidal methods to form nanoparticle assemblies, or clusters (NPCs) is reviewed. The focus is on the two-step approach, which exploits the advantages of bottom-up wet chemical NP synthesis procedures, with subsequent colloidal destabilization to trigger assembly in a controlled manner. Recent successes in the application of functional NPCs with enhanced emergent collective properties for a wide range of applications, including in biomedical detection, surface enhanced Raman scattering (SERS) enhancement, photocatalysis, and light harvesting, are highlighted. The role of the NP-NP interactions in the formation of monodisperse ordered clusters is described and the different assembly processes from a wide range of literature sources are classified according to the nature of the perturbation from the initial equilibrium state (dispersed NPs). Finally, the future for the field and the anticipated role of computational approaches in developing next-generation functional NPCs are briefly discussed.