Heterogeneous Surfaces with Nanosized Channel Lattices
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Microscopically engineered surface structures allow the local control of physical properties such as adhesion, friction, and wettability. These properties are related both to molecular interactions and surface topology. If the structure features are small enough—till nanometer scale—and well ordered, they may be applicable to electron confinement for light emission or novel electronic devices. Commonly used techniques to create laterally structured surfaces are photolithography and electron beam lithography. Based on these structures, the feature size can be reduced down to 30 nm using soft lithography (microcontact printing). A newly developed technique named as Nanoimprinting can be used to create surface features down to 5 nm over macroscopic areas. Another way to create structured surface with nanometer-sized features is to introduce surface coating layers with defined structures, for instance, by using block copolymer lithography and surface-induced patterning using polymers.
Ultrathin organic films prepared, for example, with the Langmuir–Blodgett (LB) and the self-assembly (SA) techniques are other promising candidates for surface modification. By using these techniques, organic molecules with great variability can be incorporated into organized assemblies in a well-controlled manner. During the condensation procedure of molecules into densely packed states, lateral structures with adjustable shapes and arrangements may occur, which can be understood in terms of dipole–dipole interaction and line tension. The size of structures induced in this way is usually in the range of 1–100 µm, and many of them (especially the anisotropic structures) are randomly arranged. Less addressed are the nanoscopic structures created in these quasi two-dimensional systems, although the existence of “micelle” clusters in organic monolayers has been suggested even at the very early stage by Langmuir. This was partially because of lack of high-resolution real space approach in detection methods for this size range, until the advent of scanning probe microscopy (SPM) applicable on thin organic films.
A different approach to create microscopically patterned surface is based on the wetting instability during LB transfer, as first reported by Riegler. By withdrawing a hydrophilic solid substrate (Si-wafer) through a monolayer-covered [l-α-dipalmitoyl-phosphatidylcholine (DPPC)] water surface under suitable conditions, parallel stripe patterns can be created.