Plasmonic hot spots located among closely dispersed plasmonic nanoparticles (NPs) are intensively studied for efficient conversion of solar to chemical or electrical energy applications. Here, a successful method to synthesize high-density unaggregated plasmonic Ag or Au NPs (AgNPs or AuNPs) onto nanostructured semiconductors with 3D densely organized NPs is demonstrated. The densely dispersed plasmonic AgNPs or AuNPs are assembled chemically on the entire surface of the ZnO nanorods through bifunctional thioctic acid bridging linkers. The fabricated NPs possessing small interparticle gaps generate numerous plasmonic hot spots which boost catalytic activities of the photocatalysts. As depicted by exact 3D finite-difference time domain simulations, the electromagnetic fields are magnified exponentially among interparticle gaps, hot spots, due to the plasmonic coupling effects of the neighboring AgNPs. The electromagnetic fields are strengthened by decreasing the interparticle spacing of coupled AgNPs. It is consistent with the result that the photocatalytic reaction rate increases non-linearly with the Ag content under full-spectrum light irradiation. Using the spectral characterizations and electromagnetic field simulations, we unambiguously confirm the enhancement of photoactivity due to coupling of plasmonic hot spot effect to nanostructured semiconductors. Moreover, diverse heterostructures based on the plasmonic NPs on various ZnO nanostructures (films, nanorod arrays, branched nanowires, and mesoporous structures) or functional materials (CuInGaSe2 absorber films, multiferroic BiFeO3 films, visible-light photoactive Cu2S and CdS nanorods) are successfully fabricated using the present synthesis methodology.