Nanoscale self-assembly is a concept that Nature has been making use of since the beginning of life, and we have only recently started realizing its potential for achieving better control over materials properties. An overwhelming number of self-assembly fabrication methods for the formation of nanocluster arrangements on surfaces have been found, and they feature processes having various timescales, complexities, and versatilities. The fabrication of such structures can be discussed in terms of two processing steps that are independent of the actual process sequence. First, nanocomponents must be formed, which can be accomplished in various ways from precursors in the liquid, solid, or gas phases employing either chemical or physical deposition processes. In the second step, the challenge is to organize the segregated and deposited nanoparticles into structures or patterns on surfaces. Several approaches that use either a serial-writing-type process or templates that allow fast parallel processing have been formulated. More simple are the template-free approaches based on self-organization, which is the autonomous organization of components into patterns without human intervention. Among all of these approaches, wet-chemical strategies utilizing fluid mechanics appear to be the simplest and most effective. Evaporation of drops on nonfunctionalized substrates has been used for the patterned deposition of solutes in DNA microarrays. Furthermore, the ring deposition of particles from colloidal dispersions deposited as droplets has also been shown. In addition to droplet drying, combined flow drying has been utilized for the deposition of semiconductor nanowires and nanotubes onto functionalized substrates. The self-organization of matter into regular sub-micrometerand nanoscale lines by using the wetting instability of a Langmuir–Blodgett film as a cost-effective method has also been discussed. However, structuring of nanocluster arrays or wirelike morphologies from a droplet still faces certain challenges. Yawahare et al. have demonstrated the room-temperature formation of lines from a drop if three prerequisites are met: evaporation between partially wetted surfaces, the presence of a pinning point, and the availability of a surfactant. In contrast to the above approaches, this paper presents a droplet-deposition-based, template-free, and rapid (only a few seconds) approach for fabricating nanostructures without the use of any surfactant. Our general setup can be understood as the so-called “anti-Lotus effect”. The Lotus effect is well known for its removal of dust particles from the surface of a lotus leaf by gathering them into a droplet that is moving over the surface, thus cleaning it. The effect is based on the ability of certain surfaces to form spherical droplets with contact angles near 180° (i.e., superhydrophobic), enabling the incorporation of surface particles as well as a reduction in friction. In contrast to this, our work makes use of an anti-Lotus effect, in which the droplet delivers material while moving over the surface. In this case, a reduction in friction is achieved by employing the Leidenfrost effect, thus enabling the use of normal surfaces, such as plain silicon, as substrates. If a liquid drop touches a hot solid having temperatures higher than the boiling point of the liquid, the lower part of the droplet will immediately evaporate and protect the rest of the droplet from further evaporating for a limited period of time. At this point, the drop is no longer in contact with the solid but levitates above its own vapor. Such a floating drop is called a Leidenfrost drop, after the German physicist who first reported the phenomenon some 250 years ago; it is only recently that the scientific potential of this phenomenon has been realized. However, it must be noted that a random deposition of a solution drop is totally ineffective in fathering any technologically useful nanoscale structures, such as arrangements of well-separated patterns prepared from monodisperse particles, which often demand a high degree of organization. In order to deposit material from a droplet in an organized manner, a deeper insight into the underlying mechanism of the fluid drop is necessary. A drop drying slowly on a polar surface will typically lead to ring-shaped depositions at room temperature rather than the thin films obtained at the Leidenfrost temperature, where the film homogeneity depends on the drop radius. To develop patterns at the Leidenfrost temperature, we made use of the reduced friction of the Leidenfrost droplet. This strategy is illustrated by the schematic in Figure 1. An C O M M U N IC A TI O N
Read full abstract
Jun 28, 2008
Shelley A Claridge + 6
Open Access
