In contrast to spontaneous self-assembly processes discussed so far, directed self-assembly involves structure directing additives such as organic molecules that facilitate linkage between the nanoparticles and the substrate to form ordered arrays. A common example is to obtain nanoparticle assemblies on silicon substrates using siloxane chemistry where the organic molecule is bound to the substrate through the Si-O-Si bond. A simple procedure as shown by Liu et al. is to coat the silicon (111) surface with a aminopropyltriethoxylsilane (APTES) monolayer with the amino groups oriented away from the substrate so that nanoparticles with special affinity towards the amino group (e.g. Au or Ag nanoparticles) get attached, Figure . Nanoparticles bound by electrostatic attraction can then be functionalized, for instance thiol functionalization of Au nanoparticles, to weaken the electrostatic forces and form closed packed assemblies by van der Waal interactions. Instead of self-assembled molecular monolayers as templates, Yap et al. showed that a spin-coated film of polystyrene-blockpoly(2-vinylpyridine) (PS-b-P2VP) reverse micelles form positive zeta potential nanopatterns that act as templates for the negatively charged particles to attach electrostatically, Figure ). One interesting aspect of the nanoparticle assembly mediated by molecular monolayers (or multilayers) on a substrate is that it has shown the possibility of using the molecular layer as a resist layer in scanning probe lithography techniques. Scanning probe lithography, where an AFM tip is used for writing nanoscale patterns by some kind of chemical reaction instead of masks used in photolithography, can be a powerful future nanofabrication technique, Figure ). For instance, Ling et al. have shown the use of scanning probe oxidation of a monolayer of n-octadecyltrichlorosilane (OTS) on a silicon substrate into nanopatterns that can be etched and further modified with aminopropyltrimethoxysilane (APS) for Au nanoparticles to assemble into those patterns. Alternatively, nanopatterns can also be created by electrochemical oxidation on the OTS monolayer with an AFM tip to convert -CH3 groups of the base monolayer to -COOH sites. Subsequently, adsorption of Nonadecenyltrichlorosilane (NTS) on the tip-inscribed -COOH sites and chemical conversion of the terminal ethylenic groups of the NTS to -NH2 (amine) facilitates the selfassembly of Au nanoparticles on amines, Figure . In addition, in-situ generation of CdS nanoparticles is also possible by chemical oxidation of terminal ethylenic groups of the NTS region to -COOH groups which then react with cadmium salt and passing H2S gas. Figure . (a) Au nanoparticles assemble electrostatically on APTES monolayer grafted on Si substrate which can be converted to closed packed assemblies by thiol functionalization of nanoparticles. Reprinted with permission © 2002 Royal Society of Chemistry. (b) Au nanoparticle self-assembly on polystyrene-blockpoly(2-vinylpyridine) (PS-b-P2VP) reverse micelle patterns obtained by spincoating. Reprinted with permission © 2012, American Chemical Society. (c) Schematic view of the electrochemical process taking place between the AFM tip and the sample surface in oxidationscanning probe lithography. Reprinted with permission © 2019 Springer Nature (d) Au nanoparticle self-assembled patterns by oxidative scanning probe lithography on OTS monolayer on Si substrate and subsequent reactions to obtain -NH2 terminal groups. Reprinted with permission © 2004, American Chemical Society