Study of the time evolution of the surface morphology of thin asymmetric diblock copolymer films under solvent vapor

Developing Anisotropy in Self-Assembled Block Copolymers: Methods, Properties, and Applications

Block copolymers (BCPs) self-assembly has continually attracted interest as a means to provide bottom-up control over nanostructures. While various methods have been demonstrated for efficiently ordering BCP nanodomains, most of them do not generically afford control of nanostructural orientation. For many applications of BCPs, such as energy storage, microelectronics, and separation membranes, alignment of nanodomains is a key requirement for enabling their practical use or enhancing materials performance. This review focuses on summarizing research progress on the development of anisotropy in BCP systems, covering a variety of topics from established aligning techniques, resultant material properties, and the associated applications. Specifically, the significance of aligning nanostructures and the anisotropic properties of BCPs is discussed and highlighted by demonstrating a few promising applications. Finally, the challenges and outlook are presented to further implement aligned BCPs into practical nanotechnological applications, where exciting opportunities exist.

Exploring the Self-Assembly Capabilities of ABA-Type SBS, SIS, and Their Analogous Hydrogenated Copolymers onto Different Nanostructures Using Atomic Force Microscopy

In this work, the self-assembled morphologies obtained for poly(styrene-b-butadiene-b-styrene) (SBS) and poly(styrene-b-isoprene-b-styrene) (SIS) ABA-type copolymers were investigated before and after hydrogenation of the polydiene block, which led to poly(styrene-b-ethylene)/poly(ethylene-b-styrene) (SEES) and poly(styrene-b-ethylene)/poly(propylene-b-styrene) (SEPS) copolymers, respectively. The evaluation of different morphologies was carried out using atomic force microscopy (AFM), analyzing the effect of various parameters such as the solvent and polymer concentrations employed for film casting (toluene, cyclohexane, or tetrahydrofurane with concentrations of 1 and 3 wt%), together with that of the annealing treatment (thermal annealing at room temperature, and 60, 80, and 100 °C). The effect of these parameters in combination with the chemical nature of the polydiene block led to different morphologies with different topographic aspects affecting the roughness (Ra) of the film.

Development of Ordered, Porous (Sub-25 nm Dimensions) Surface Membrane Structures Using a Block Copolymer Approach

In an effort to develop block copolymer lithography to create high aspect vertical pore arrangements in a substrate surface we have used a microphase separated poly(ethylene oxide) -b- polystyrene (PEO-b-PS) block copolymer (BCP) thin film where (and most unusually) PS not PEO is the cylinder forming phase and PEO is the majority block. Compared to previous work, we can amplify etch contrast by inclusion of hard mask material into the matrix block allowing the cylinder polymer to be removed and the exposed substrate subject to deep etching thereby generating uniform, arranged, sub-25 nm cylindrical nanopore arrays. Briefly, selective metal ion inclusion into the PEO matrix and subsequent processing (etch/modification) was applied for creating iron oxide nanohole arrays. The oxide nanoholes (22 nm diameter) were cylindrical, uniform diameter and mimics the original BCP nanopatterns. The oxide nanohole network is demonstrated as a resistant mask to fabricate ultra dense, well ordered, good sidewall profile silicon nanopore arrays on substrate surface through the pattern transfer approach. The Si nanopores have uniform diameter and smooth sidewalls throughout their depth. The depth of the porous structure can be controlled via the etch process.

Perpendicular Structure Formation of Block Copolymer Thin Films during Thermal Solvent Vapor Annealing: Solvent and Thickness Effects

Solvent vapor annealing of block copolymer (BCP) thin films can produce a range of interesting morphologies, especially when the perpendicular orientation of micro-domains with respect to the substrate plays a role. This, for instance, allows BCP thin films to serve as useful templates for nanolithography and hybrid materials preparation. However, precise control of the arising morphologies is essential, but in most cases difficult to achieve. In this work, we investigated the solvent and thickness effects on the morphology of poly(styrene-b-2 vinyl pyridine) (PS-b-P2VP) thin films with a film thickness range from 0.4 L₀ up to 0.8 L₀. Ordered perpendicular structures were achieved. One of the main merits of our work is that the phase behavior of the ultra-high molecular weight BCP thin films, which hold a 100-nm sized domain distance, can be easily monitored via current available techniques, such as scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope (TEM). Systematic monitoring of the self-assembly behavior during solvent vapor annealing can thus provide an experimental guideline for the optimization of processing conditions of related BCP films systems.

Reduction and control of domain spacing by additive inclusion: morphology and orientation effects of glycols on microphase separated PS-b-PEO

Cylindrical phase polystyrene-b-polyethylene oxide (PS-b-PEO) block copolymer (BCP) was combined with lower molecular weight poly/ethylene glycols at different concentrations and their effect on the microphase separation of BCP thin films were studied. Well-ordered microphase separated, periodic nanostructures were realized using a solvent annealing approach for solution cast thin films. By optimizing solvent exposure time, the nature and concentration of the additives etc. the morphology and orientation of the films can be controlled. The addition of the glycols to PS-b-PEO enables a simple method by which the microdomain spacing of the phase separated BCP can be controlled at dimensions below 50 nm. Most interestingly, the additives results in an expected increase in domain spacing (i.e. pitch size) but in some conditions an unexpected reduction in domain spacing. The pitch size achieved by modification is in the range of 16-31 nm compared to an unmodified BCP system which exhibits a pitch size of 25 nm. The pitch size modification achieved can be explained in terms of chemical structure, solubility parameters, crystallinity and glass transition temperature of the PEO because the additives act as PEO 'stress cracking agents' whereas the PS matrix remains chemically unaffected.

Directed self-assembly of block copolymers: a tutorial review of strategies for enabling nanotechnology with soft matter

Self-assembly of soft materials is broadly considered an attractive means of generating nanoscale structures and patterns over large areas. However, the spontaneous formation of equilibrium nanostructures in response to temperature and concentration changes, for example, must be guided to yield the long-range order and orientation required for utility in a given scenario. In this review we examine directed self-assembly (DSA) of block copolymers (BCPs) as canonical examples of nanostructured soft matter systems which are additionally compelling for creating functional materials and devices. We survey well established and newly emerging DSA methods from a tutorial perspective. Special emphasis is given to exploring underlying physical phenomena, identifying prototypical BCPs that are compatible with different DSA techniques, describing experimental methods and highlighting the attractive functional properties of block copolymers overall. Finally we offer a brief perspective on some unresolved issues and future opportunities in this field.

Fabrication of ordered, large scale, horizontally-aligned si nanowire arrays based on an in situ hard mask block copolymer approach

A simple technique is demonstrated to fabricate horizontal, uniform, and hexagonally arranged Sinanowire arrays with controlled orientation and density at spatially well defined locations on a substrate based on an in situ hard-mask pattern-formation approach by microphase-separated block-copolymer thin films. The technique may have significant application in the manufacture of transistor circuitry.

Nanoscale protein arrays of rich morphologies via self-assembly on chemically treated diblock copolymer surfaces

Well-controlled assembly of proteins on supramolecular templates of block copolymers can be extremely useful for high-throughput biodetection. We report the adsorption and assembly characteristics of a model antibody protein to various polystyrene-block-poly(4-vinylpyridine) templates whose distinctive nanoscale structures are obtained through time-regulated exposure to chloroform vapor. The strong adsorption preference of the protein to the polystyrene segment in the diblock copolymer templates leads to an easily predictable, controllable, rich set of nanoscale protein morphologies through self-assembly. We also demonstrate that the chemical identities of various subareas within individual nanostructures can be readily elucidated by investigating the corresponding protein adsorption behavior on each chemically distinct area of the template. In our approach, a rich set of intricate nanoscale morphologies of protein arrays that cannot be easily attained through other means can be generated straightforwardly via self-assembly of proteins on chemically treated diblock copolymer surfaces, without the use of clean-room-based fabrication tools. Our approach provides much-needed flexibility and versatility for the use of block copolymer-based protein arrays in biodetection. The ease of fabrication in producing well-defined and self-assembled templates can contribute to a high degree of versatility and simplicity in acquiring an intricate nanoscale geometry and spatial distribution of proteins in arrays. These advantages can be extremely beneficial both for fundamental research and biomedical detection, especially in the areas of solid-state-based, high-throughput protein sensing.

Elucidation of novel nanostructures by time-lapse monitoring of polystyrene-block-polyvinylpyridine under chemical treatment

Nanoscale micellar structures of polystyrene-block-polyvinylpyridine (PS-b-PVP) diblock copolymers have proven their effectiveness in lithography and biological detection by serving as a choice material to produce nanoscale guides and delivery systems in a straightforward and rapid manner through self-assembly. Such applications can greatly benefit from having high versatility for the selection of template sizes (pattern repeat spacing) and shapes (pattern geometry), especially when reaching a size regime that conventional top-down fabrication techniques may not readily be able to provide desired feature dimensions. Selective chemical treatments of the diblock copolymers are one of the useful methods yielding a rich set of nanoscale features on PS-b-PVP. Exposure to selective vapor can induce reorganization of the polymeric chains of PS-b-PVP and alter the original micellar nanostructures. In this Article, we identify for the first time a host of new nanostructures formed at different stages of chloroform vapor annealing by performing time-lapse atomic force microscopy measurements. We determine key, time-dependent, topological parameters defining each nanostructure and present the likely scenario of polymeric chain reorganization during the morphological evolution of the diblock polymer nanodomains over time. We also ascertain intermediate morphological states containing the characteristic nanostructures from two consecutive phases as well as transition states appearing for a short time in between two subsequent phases. These research efforts may not only provide insight into the domain evolution steps of the micellar to the cylindrical structures of PS-b-PVP but may also be technologically advantageous for subwavelength mask design in nanolithography and high-density array fabrication in high throughput biodetection.

A simple top-down/bottom-up approach to sectored, ordered arrays of nanoscopic elements using block copolymers

A top-down/bottom-up approach is demonstrated by combining electron-beam (e-beam) lithography and a solvent annealing process. Micellar arrays of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) with a high degree of lateral order can be produced on a surface where sectoring is defined by e-beam patterning. The e-beam is used to crosslink the block copolymer (BCP) film immediately after spin-coating when the BCP is disordered or in a highly ordered solvent-annealed film. Any patterns can be written into the BCP by crosslinking. Upon exposure to a preferential solvent for the minor component block followed by drying, cylindrical nanopores are generated within the nonexposed areas by a surface reconstruction process, while, in the exposed areas, the films remain unchanged. Nickel nanodot arrays can be placed over selected areas on a surface by thermal evaporation and lift-off process.

Periodic porous stripe patterning in a polymer blend film induced by phase separation during spin-casting

A periodic striping pattern with microscale pore size is observed on the surface of thin films prepared by spin-casting from a polystyrene (PS) and polyethylene glycol (PEG) blend solution. The pattern is created by the convection generated by thermal gradients in the solution between the substrate and film solution during solvent evaporation, the radial flow of the spin-coated solution, and the primary and secondary phase separation of the PS and PEG solutions. The formation mechanism of the periodic porous stripe pattern is discussed, wherein the effects of the polymer blend weight ratio, polymer concentration, and drying rate on the formation of the periodic porous striping pattern are investigated using scanning electron and atomic force microscopy.

Ordering of PS-b-P4VP on patterned silicon surfaces

We demonstrate a method to fabricate high-quality patterned micelle arrays using poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) block copolymer. Long-range order of the PS-b-P4VP micelle in hexagonal arrays was induced by topographic grating patterns during solvent annealing. The size and row spacing of block copolymer micelle arrays created in this way were uniform. By controlling the thickness of the polymer on the crests and in the troughs of the grating patterns, we prepared PS-b-P4VP micelle arrays having different sizes.