Micropatterning by Non‐Densely Packed Interfacial Colloidal Crystals

Abstract

Methods for patterning solid surfaces are the focus of significant scientific efforts because micropatterned surfaces are important materials for modern technologies, such as microelectronics, optoelectronics, optics, and biological microanalysis.[1] Among the many surface-patterning options, nanosphere lithography [2] is unique in that it uses a self-assembled mask with hexagonal symmetry, consisting of densely packed monodispersed colloidal particles deposited on a substrate by either a Langmuir-type technique [3] or by direct evaporation of the aqueous particle suspension.[4] Non-densely packed 2Dcolloidal crystals also form at air/water or oil/water interfaces. [5] In these cases, charged particles with a hydrophobic surface self-organize into large 2D hexagonal lattices directed by electrostatic repulsion and capillary attraction between neighboring particles. [6] The interparticle spacing in the lattice is adjustable by varying the surface pressure of the film. Such interfacial films would be very attractive for surface patterning if they could be transferred intact from the aqueous interface onto solid substrates. Although densely packed particle films are routinely transferred, the transfer of non-densely packed films, where the particles are not physically touching each other and can easily be moved out of their lattice positions, is yet to be accomplished. We report on a methodology for the transfer of non-densely packed particle films at an air/water interface onto silicon substrates (Scheme 1) and also demonstrate that the transferred particle arrays can be used as an etching mask to generate hexagonal arrays of divot-like holes in the surface of the silicon-wafer substrate. In this work, monodispersed polystyrene latex particles were used with an average diameter of 2.7 lm and a sulfatefunctionalized surface having a charge density of 8.9 lC cm -2 . The particles were suspended in isopropyl alcohol (IPA) and allowed to age for 10 days before use. During this time, the water/particle surface contact angle progressively increased until approaching an asymptote at ca. 84°(see Supporting Information). The high contact angle was critical for irreversible particle attachment to the interface [5] and also strengthens the interparticle repulsion at the aqueous/air interface. [6a,b] After aging in IPA, the particles had readily spread over the air/ water interface with minimal particle flocculation and virtually no loss to the bulk aqueous phase. A continuous nondensely packed hexagonal particle array formed when the particle coverage on the surface approached 1.1 × 10 4 particles mm -2 .Under the pragmatic assumption that strong attractive forces between the substrate surface and the particle surface were necessary for the successful transfer of the interfacial film onto the solid substrate, we modified the surface of the silicon wafer to have a compatible degree of hydrophobicity and an opposite charge to that of the particle surface. This was achieved by treating the silicon wafer with tetradecyldimethy...

Fabrication of large scale two-dimensional colloidal crystal of polystyrene particles by an interfacial self-ordering process

“…Although it is successful for fabricating 2D colloidal crystals, the shortcoming of the LB deposition is obvious, such as the expensive apparatus and the complex processing steps. In contrast to the LB deposition, an analogous air-water interfacial floating method [36][37][38][39][40]41] and the oil-water interfacial trapping process [42] have been developed for the 2D colloidal crystals. This process, generally, consists of coating particle suspension on a hydrophilic surface, floating particles at the interface, solidifying particle film using a surfactant and transferring film on another substrate.…”

Non-equilibrium nature of two-dimensional isotropic and nematic coexistence in amyloid fibrils at liquid interfaces

“…The mica was then dipped into ethanol (Z99.8% v/v) for phase exchange and removal of any unabsorbed material before slowly drying the sample under a weak clean air flow. Ethanol was chosen because its lower surface tension prevents the formation of artefacts at the receding contact line of the droplet during drying under a weak clean air flow, as shown for the case of non-densely packed 2D colloidal crystals 32 .…”

Interaction between like-charged particles at a liquid interface: Electrostatic repulsion vs. electrocapillary attraction

“…Lateral capillary forces have been investigated during the last 15 years in relation to their importance for creation of two-dimensional structures of colloids (including protein molecules and nanoparticles) with various applications [2][3][4][5][6][7][8][9][10][11][12][13][14]. Comprehensive reviews can be found in Refs.…”