Nanoparticle layer deposition for highly controlled multilayer formation based on high-coverage monolayers of nanoparticles

Abstract

This paper establishes a strategy for chemical deposition of functionalized nanoparticles onto solid substrates in a layer-by-layer process based on self-limiting surface chemical reactions leading to complete monolayer formation within the multilayer system without any additional intermediate layers – nanoparticle layer deposition (NPLD). This approach is fundamentally different from previously established traditional layer-by-layer deposition techniques and is conceptually more similar to well-known atomic and molecular – layer deposition processes. The NPLD approach uses efficient chemical functionalization of the solid substrate material and complementary functionalization of nanoparticles to produce a nearly 100% coverage of these nanoparticles with the use of “click chemistry”. Following this initial deposition, a second complete monolayer of nanoparticles is deposited using a copper-catalyzed “click reaction” with the azide-terminated silica nanoparticles of a different size. This layer-by-layer growth is demonstrated to produce stable covalently-bound multilayers of nearly perfect structure over macroscopic solid substrates. The formation of stable covalent bonds is confirmed spectroscopically and the stability of the multilayers produced is tested by sonication in a variety of common solvents. The 1-, 2- and 3-layer structures are interrogated by electron microscopy and atomic force microscopy and the thickness of the multilayers formed is fully consistent with that expected for highly efficient monolayer formation with each cycle of growth. This approach can be extended to include a variety of materials deposited in a predesigned sequence on different substrates with a highly conformal filling.

Building high-coverage monolayers of covalently bound magnetic nanoparticles

“…Following addition of the nanoparticles, this peak diminishes drastically, indicating that the azide group participates in a chemical reaction. The alkyne C≡C stretch is also expected to appear in this spectral region; however, its intensity is normally very low and in previous studies it was not detected by IR for similar systems [17,29]. In addition, previous studies [17] showed that following the formation of the triazole ring, any unreacted azide can be titrated resulting in the complete disappearance of any absorption features in this spectral region, despite the presence of alkyne groups.…”

“…Spectrum (a) in the N 1 s region shows two distinct features that can be deconvoluted into three components at 404.4 eV, 400.9 eV, and 399.4 eV. These components can be readily assigned to the nitrogen atoms of the azide group: N= N =N-, N =N=N- and N=N= N -, respectively (bolded and italicized for clarity) [17,38,40,41]. Density functional theory (DFT) calculations were performed to predict the binding energies of nitrogen atoms in the azide group and are represented by the colored lines beneath the experimental spectrum.…”

“…More importantly, the spectra in the N 1 s region agree with the expected results, as the azide reacts to form the triazole ring and support evidence of the “click” reaction. It should be mentioned that alternative reactions resulting in the azide group oxidation leading to the formation of N=O, NO 2 , and –N-OH functionalities can be ruled out because it has been shown previously that all these reaction pathways would result in higher binding energy features [17,42–44] not observed in the experimental spectra of the N 1 s region.…”

“…In an approach fundamentally similar to ALD and MLD methods, the covalent attachment of mono- and multilayers of silica nanoparticles through the formation of a triazole ring has been proposed previously [17–21]. The copper (I)-catalyzed formation of a triazole ring through an azide-alkyne Huisgen cycloaddition has been the focus of several studies aimed at building single and multiple layered nanostructures for a diverse range of applications.…”

“…The method reported here highlights vital alterations that may provide insight into the parameters that affect coverage and complete monolayer formation. It is based on the nanoparticle layer deposition (NPLD) method that necessitates that the supporting solid substrate be functionalized with the azide moiety to produce a high-coverage nanoparticle monolayer [17]. Furthermore, it involves direct functionalization of substrate and particles with terminal complementary alkyl-azide and -alkyne groups, rather than more complicated structures.…”