Evaporation‐Induced Self‐Assembly (EISA) at Its Limit: Ultrathin, Crystalline Patterns by Templating of Micellar Monolayers

Abstract

Patterning of surfaces at the nanoscale is important in many fields of application, e.g., for electronic devices, storage media, and as a starting base for the generation of more complex structures. To exemplify the latter subject, it has been shown that a periodic surface structure with periodicity of surface energy and topology allows for the alignment of liquid crystals [1] or the consecutive construction of channels or pillars being vertically aligned with respect to the substrate, [2] thus making the application of strong external fields redundant. [3,4] Structures in the range of 45-300 nm are predominantly fabricated by either optical-or electron-beam lithography. To reach the sub-45 nm range, self-assembled block-copolymer mesophases are employed as templates within lithographic processes. For instance, Adamson and co-workers reported on the use of block copolymers in a lithography etching process to generate dense periodic arrays of holes and dots in a silicon nitride coated silicon wafer.[5] However, this process deals only with the structuring of a given surface, i.e., the physical properties of the final material are limited by the choice of the starting material and the restrictions of lithography.Herein, we present a straightforward approach towards a whole range of ultrathin, mesostructured metal oxide layers by sol-gel processing via a modified evaporation-induced selfassembly (EISA) approach introduced by Ogawa et al. and Brinker et al. several years ago.[6] EISA usually provides mesostructured films with a thickness of 50-1000 nm, which therefore consist of several stacked layers (multilayers of micellar aggregates). [7] However, it is challenging to use this well-established procedure to generate corresponding ultrathin films with ordered mesoporosity, although it is usually assumed that such fine control can only be achieved by Langmuir-Blodgett (LB) techniques. Kunitake and co-workers reported the successful preparation of ultrathin, self-supporting films of different oxides by sequential sol-gel processing using polymer underlayers and spin-coating, but the porosity was disordered and the metal oxides amorphous.[8]In the present study, it is shown that a modified EISA procedure allows for the preparation of crack-free, ultrathin, crystalline metal oxide films with highly ordered in-plane mesostructure and a tunable film thickness on the scale of only several nanometers, even achieving the sequential formation of mesostructured metal oxides with exactly one micellar monolayer in height. Different structures (e.g., circular pores or line patterns) of the final surface layer material can be simply adjusted by varying the assembly structure of the template, and by using sufficiently small templates, the periodicity of the patterns can be downsized to about 4-5 nm. In addition, the structure is instantaneously generated on macroscopic length scales (i.e., not serial/slow). This has been beyond the reach of common lithography techniques to date. Furthermore, in contrast to physical methods suc...

Integration of functional complex oxide nanomaterials on silicon

“…The construction of porous and hierarchically structured materials is accomplished by bridging together sol-gel chemistry, multiple templating, and advanced processing, namely Integrative Chemistry Strategy. The main chemical routes and strategies in this direction have been thoroughly described in several review articles [51][52][53].…”

Ordered mesoporous crystalline titania with high thermal stability from comb-like liquid crystal block copolymers

“…The rst case is the nanocasting process by using ordered mesoporous silicas or carbons as hard templates. Poly(ethylene-co-butylene)-b-poly(ethylene oxide) (PHB-b-PEO referred to also as "KLE") was rstly reported to synthesize highly crystalline cubic mesoporous TiO 2 lms through dip-coating by Smarsly and co-workers, but the whole mesostructure collapsed at 700 C. 31,32 Wiesner et al reported an effective method, called combined assembly by so and hard (CASH) chemistries, for highly crystalline mesoporous TiO 2 by using in situ generated sturdy, amorphous carbon derived from amphiphilic BCP polyisoprene-b-poly(ethylene oxide) (PI-b-PEO) as a rigid support to prevent the collapse of mesostructures during the crystallization. 23 The nanocasting method is an effective way to prepare mesoporous TiO 2 with crystalline frameworks.…”

Directing the Self‐Assembly of Mesostructured Hybrid Materials: Effect of Polymer Concentration and Solvent Type

“…[10] Control the formation of mesostructures in thin films using the EISA approach becomes increasingly difficult as the film thickness decreases. [11] However, a number of reports demonstrate the successful preparation of mesoporous structures in ultra-thin films. [11,12] The present work explores the possibility to fabricate and control well-defined nanostructures in ultra-thin films of functional hybrid materials based on amphiphilic diblock copolymers and metallic nanoparticles.…”

“…[11] However, a number of reports demonstrate the successful preparation of mesoporous structures in ultra-thin films. [11,12] The present work explores the possibility to fabricate and control well-defined nanostructures in ultra-thin films of functional hybrid materials based on amphiphilic diblock copolymers and metallic nanoparticles. Additionally the block copolymer not only stabilizes the nanoparticles but allows to control the self-assembly.…”