Esterification-Induced\nSpontaneous Deposition of Nanoparticle\nMonolayers on Substrates with Percolation Network Morphology

Nanoparticle monolayers (NPMLs) exhibit\nunique collective\nproperties\nthat are highly desirable for applications in sensors, catalysts,\nand optics. However, their practical use is often hindered by structural\ninstability, especially when they are exposed to solvents. In this\nstudy, we developed a method for the spontaneous deposition of gold\nnanoparticle monolayers (AuNPMLs) on silicon substrates via covalent\nbonding interactions that provides excellent structural stability\nin solvents with varying polarities. The esterification reaction between\ncarboxyl-functionalized gold nanoparticles and alkyl-chloride-functionalized\nsilicon substrates spontaneously forms AuNPMLs on the substrate in\nwhich single-crystalline domains are interconnected, forming a percolation\nnetwork morphology. The <i>ex situ</i> scanning electron\nmicroscopy measurements show that the surface coverage of AuNPMLs\nrapidly increases up to ca. 60% during the initial 10 min, followed\nby gradual growth and saturation at ca. 70% at 360 min of deposition\ntime. On the other hand, the single-crystalline domain size reaches\nits maximum at 45 min and then gradually decreases, which may be attributed\nto the desorption of AuNPs by the hydrolysis of ester bonds. The reflectance\nspectra of AuNPMLs showed the red shift as the deposition time increases\nup to 45 min with a subsequent blue shift thereafter, which is consistent\nwith the change of the single-crystalline domain size with the deposition\ntime. The covalent bonding interaction-mediated nanoparticle deposition\nmethod can be used to form stable AuNPMLs with controlled surface\ncoverage and domain size, allowing for fine control of the optical\nproperties and possibly other properties. The excellent structural\nstability of AuNPMLs and their controlled properties may provide new\nopportunities for practical applications of NPMLs.