Lithographically directed self-assembly of nanostructures
(a) Origin of flotation capillary forces. The meniscus is deformed leading to a difference in the meniscus slope angles and . Note that these forces become negligible for particles smaller than a few micrometers in size. (b) Origin of immersion capillary forces. Integration of the horizontal component of the surface tension around the contact lines produces a net attractive force between the particles. Note that these remain significant, even for particles as small as two nanometers. [After Kralchevsky and Nagayama (Ref. 19)].
SEM micrograph showing the assembly of Au particles into , pitch holes in KRS-XE resist. The inset shows the typical arrangement of particles within a single hole. The particles are concentrated by a factor of relative to the concentration of the suspension. Note that there are essentially no particles deposited between the lithographically defined features.
(a) Illustration of vapor concentration as a function of position (white-high, black-low) over an oblate spheroid. This solution does not exhibit the divergence of the solution for a lenticular drop at the contact line. The vapor flux is highest in the regions of the highest concentration gradient, i.e., at the edges. (b) Illustration of the contraction of an evaporating drop in the absence of contact line pinning. In this case the higher evaporation rate at the contact line is matched by a larger volume of liquid lost as the edge retreats. (c) The contraction and change in shape of an evaporating drop in the case that the contact line is pinned. Here there must be a flow of fluid from the center of the drop to the periphery to compensate for the accelerating rate of evaporation from the increasingly shallow-angled contact line.
Calculated profiles for a water contact line passing a pinning defect for effective defect sizes of 1, 3, and . The defect, and hence the pinning force, effectively grows as the defect emerges from the contact line. The inset is a schematic of the contact line motion as the fluid passes the particle. The fluid flow to the pinning point occurs as a result of the mechanism illustrated in Fig. 3.
SEM micrographs showing the effect of the particle-to-hole diameter ratio on the number of particles in each hole. (a) Two particles/hole. (b) Three particles/hole. (c) Many particles/hole. In this case the holes are deep enough to permit the particles to stack (inset). Note that the orientation of the particle dimers is random and therefore independent of the direction of the fluid flow.
Au particles in (a) a trench, (b) a trench, and (c) a hole. The organization of the particles is less controlled than for the larger particles, where the ratio of particle diameter to feature size can be more precisely controlled.
SEM micrographs of Au particles in (a) holes and (b) trenches. Note the separation of the particle aggregate from the trench walls. This is indicative of a final drying stage occurring after the liquid contact line has broken free of the lithographically defined feature.
SEM micrographs of Au particles arranged in extended templates. The high degree of ordering exhibited, particularly in (b), is indicative that the particles are still able to reconfigure within the features for some time after they have arrived.
(a) SEM micrograph of Au particles arranged around the perimeter of a trench in resist (hydrophobic) on an surface (hydrophilic). (b) Schematic illustrating the contact line tilt that causes the nanoparticles to decorate the sides of the trench. If there is no difference in hydrophilicity between the bottom and sides of the trench then the particles are distributed randomly within it.
(a) SEM micrograph of nanotetrapods assembled in trenches in resist. The trenches become wider from top to bottom and the tetrapod orientation becomes correspondingly less well controlled. (b) Schematic of proposed assembly mechanism: The tetrapods first slide across the surface until one foot is trapped and then rotate until a second is also captured. The tetrapods were dispersed in toluene.
(a) Sequence of SEM micrographs illustrating the assembly of single Au particles between pairs of prepatterned Au electrodes. The electrodes provide sufficient topography to pin the fluid contact line and cause assembly. (b) However, in this case, because the electrodes are raised above the surface, the assembly only works when the fluid flow direction permits mechanical pinning of the particles. (c) Preliminary electrical measurement of the current/voltage characteristics of an electrode/nanoparticle assembly.
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