Layer-by-Layer Assembly of Thin Films of Mixed Nanoparticles
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In recent years, thin films of semiconductor nanoparticles have found an increasing number of applications in solar energy conversion, electronics, and light-emitting diodes (LEDs). Different approaches, such as Langmuir–Blodgett (LB), self-assembly (SA), and layer-by-layer assembly (LBL), were developed to fabricate thin films. Among these approaches, layer-by-layer assembly offers a simple scheme to fabricate films consisting of different layers. This may be potentially used to construct films with advanced functions, such as electron and energy transfer between component layers. Layer-by-layer assembly techniques which are based on electrostatic attraction between oppositely charged molecules in adjacent layers were successfully applied to the construction of multilayer films of molecules, nanoparticle/polymer mixtures, and other inorganic materials. This LBL assembly method is quite simple and versatile. In the preparation process, the substrate, functionalized with a charged layer, is first immersed in a solution containing materials of opposite charge to allow the deposition of the first layer. The resultant film is then rinsed and dried. To deposit the second layer, the same procedure is repeated for materials of opposite charges to the first layer. The deposition cycle can be repeated until the desired number of bilayers is reached. Because of the strong interactions between the oppositely charged layers, the LBL films have shown long-term mechanical stability in air, water, and other polar solvents. Many nanoparticles, such as TiO2, CdSe, and Au, were successfully assembled into films, for which a number of advanced electronic and photonic applications were demonstrated.
As an alternative method to electrostatic interaction-based LBL assembly, multilayer films based on hydrogen bonding have been reported by Stockton and Rubner using polyaniline, and by Wang et al. using poly(vinyl pyridine)/poly(acrylic acid) (PVP/PAA). Recently, we extended this hydrogen bonding-based LBL approach to assemble inorganic nanoparticles. In this report, we review our group's work on layer-by-layer assembly of metal and semiconductor nanoparticles. Both films of single- and multiple-particle types (size or chemical nature) have been fabricated. We will describe two approaches of LBL assembly based on hydrogen bonding interaction between adjacent layers. We will discuss the use of UV–visible absorption spectrum to monitor the buildup of the multilayers and Fourier transform infrared (FTIR) spectroscopy to investigate the hydrogen-bonding interaction between the layers.