Self-Assembly of Nanocolloidal Gold Films
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The unique and new optical, electrical, and magnetic properties of colloidal superstructures as opposed to the bulk characteristics of the constituent materials is attracting the attention of an increasing number of both fundamental scientists and technology-oriented industry. The colloid size used in the assembled structures varies over approximately 3 orders of magnitude and is closely related to the specific application. For photonic band gap materials, the particle size is of the same order of magnitude as the wavelength of light, while for magnetic applications, such as ultrahigh density storage devices, the particle radius is in the low-nanometer range. A combination of the aforementioned physical properties of colloidal matter introduces even more exciting fields of research. Electron transport through monolayers of magnetic nanocrystals or tunable photonic band gap materials, both controlled by applying a magnetic field, are only two examples of the many possibilities.
Nanotechnology is characterized by a continuous decrease of feature sizes, e.g., in electronic devices. Top-down fabrication methods, such as the well-established photolithography techniques, are being pushed toward their physical limits. Increasingly more research effort is presently being devoted to bottom-up fabrication methods. Instead of reducing the size of much larger, bulk materials, as is done with photolithography in combination with etching techniques, the focus is turning toward building up superstructures of much smaller building blocks. Self-assembly of single molecules or nanocolloidal particles into larger arrangements, employing intrinsic, extrinsic, or even externally induced interactions, seems to be a most promising method, which is not hampered by problems related to scaling the processes to production scale. Much of the research activities in the field are focused on gold nanoparticles, as these systems are stable in a large number of environmental conditions and are relatively easy to prepare and control. In the first part of this contribution, we will summarize the different self-assembly methods with which nanocolloidal gold particles are composed into monolayers or multilayered superstructures.
In the second part of this contribution, we show how nonimaging, single wavelength reflectometry measurements can be applied to in situ study the formation of nanocolloidal gold monolayers at derivatized silicon surfaces. The kinetics of particles with dimensions in the low-nanometer range are investigated using a radial impinging jet setup, also referred to as a stagnation point flow geometry. We compare our results to similar adsorption experiments using micrometer-sized silica particles. For these large particles, the random sequential adsorption (RSA) model adequately describes the overall deposition kinetics. However, for considerably smaller particles in the 10–100 nm range, this relatively simple model fails. Here we show that a generalized adsorption model is in perfect agreement with deposition transients of particles in the low-nanometer range over the entire coverage range.