Anthony M. Higgins

Anisotropic spinodal dewetting as a route to self-assembly of patterned surfaces

The ability to pattern surfaces on a microscopic length scale is of importance for technological applications such as the fabrication of microelectronic circuits and digital storage media. Devices fabricated entirely from polymers are now available, opening up the possibility of adapting polymer processing technologies to fabricate cheap, large-area devices using non-lithographic techniques—for example, by exploiting dewetting and phase separation in thin films. But the final pattern adopted by the polymer film using such approaches requires a template printed onto the substrate by optical lithography, microcontact printing or vapour deposition. Here we describe a simple process for patterning surfaces that does not require a template. Our method involves the spinodal dewetting of a polymer surface by a thin polymer film, in which a liquid film breaks up owing to the amplification of thermal fluctuations in film thickness induced by dispersion forces. A preferred orientation is imposed on the dewetting process simply by rubbing the substrate, and this gives rise to patterns of remarkably well-aligned polymer lines. The width of these lines is well-defined, and is controlled by the magnitude of the dispersion forces at the interface, which in turn can be varied by varying the thickness of the polymer substrate. We expect that further work will make it possible to optimize the degree of order in the final morphology.

Surface segregation and self-stratification in blends of spin-cast polyfluorene derivatives

We have used helium-3 nuclear reaction analysis to study the morphology of spin-cast blends of poly(9,-dioctylfluorene) (F8) and poly(9,-dioctylfluorene-alt-benzothiadiazole) as a function of composition, casting solvent, and initial solvent concentration. In blends cast from toluene, a surface segregated layer is observed for a broad range of F8 compositions. The surface excess has a maximum of ?nm at an F8 volume fraction = 0.5, dropping down to ?2?nm for = 0.2 and?0.8. The existence of such surface segregated layers could play an important role in charge transport in optoelectronic devices made from blends of conjugated polymers. Films cast from chloroform show negligible surface segregation. Scanning probe microscopy experiments show that films cast from toluene exhibit significant in-plane structure, in contrast to those cast from chloroform, which show minimal lateral structure.

Interfacial structure in semiconducting polymer devices

We discuss some recent findings relating to the structure of interfaces in semiconducting polymer devices. The structure of three different types of interface is characterized via neutron reflectivity and scanning force microscopy. In the first example we find that enrichment of dopant occurs at the surface of a doped polymeric conductor, and that this enriched layer penetrates several nanometres into the material. Secondly, we find that the interface between a semiconducting polymer and an insulating polymer is not molecularly sharp, but is rather broad with a width typically of the order of 1 nm. Finally we present some initial neutron reflectivity measurements on the interface between two different semiconducting polymers (one of which is deuterated). Again we find that this interface is diffuse, with a typical width of the order of a few nanometres.

Control of roughness at interfaces and the impact on charge mobility in all-polymer field-effect transistors

The interfacial roughness at a buried dielectric–semiconductor interface in an all-polymer field-effect transistor (FET) is characterised by specular neutron reflectivity. Using a mixed solvent (toluene–cyclohexane) to deposit poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) directly on top of poly(methylmethacrylate) (PMMA), we are able to fabricate bilayer FET architectures with systematically controlled roughness at the buried polymer–polymer interface over a range of approximately 1 to 3 nm, simply by changing the solvent ratio. This system has the advantage that these two solvents are completely miscible and the mixture can dissolve F8T2, but the solvent quality of the mixture with respect to PMMA can be continuously varied by changing the ratio of toluene to cyclohexane from 100% toluene (good solvent for PMMA) to 100% cyclohexane (non-solvent for PMMA). By also fabricating F8T2 FETs from the same mixed solvents, but with silicon oxide (SiO2) as the dielectric layer, we are able to compare the effects of solvent quality and interface roughness on field-effect mobility. We find that the solvent ratio strongly affects mobility in F8T2–SiO2 FETs. In contrast charge mobility in F8T2–PMMA FETs shows relatively little dependence on solvent ratio, indicating that, to a first approximation, the effects of solvent quality and interfacial roughness cancel one another out in this system.

A neutron reflectometry study of the interface between poly(9,9-dioctylfluorene) and poly(methyl methacrylate)

Neutron reflectivity was used to study the interface between the semiconducting polymer poly(9,9-dioctylfluorene) (PFO) and the insulating polymer poly(methyl methacrylate) (PMMA). The PFO/PMMA interfacial width was measured in the nematic and crystalline phases of the PFO, both with the PMMA on top of the PFO and vice versa. These interfaces are broad compared to atomic length scales, with measured interfacial widths in the range from 10 to 20 A. We found that the interfacial width was independent of both the chosen geometry and the thermal processing history. The equilibrium interfacial width only depended on temperature, with the width in the nematic phase of the PFO being broader than in the crystalline regime.

All-polymer field-effect transistors using a brush gate dielectric

Interfaces between a poly(3-hexylthiophene) [P3HT] and an end-grafted (brush) layer of poly(methyl methacrylate) [PMMA] are shown using neutron reflectometry to be dependent on heat treatment. Annealing the samples allows part of the brush layer to cross into the P3HT layer creating a very asymmetric interface. We suggest that the P3HT rearrangement occurs, creating space for movement of the brush into the film. This interpenetration was observed with two different molecular weight (17.5 and 28 kg mol−1) P3HT films. Output characteristics of devices made from P3HT layers on PMMA brushes show that different amounts of heat treatment do not significantly change the device performance. Saturated hole mobilities are dependent on heat treatment, with devices made from a smaller molecular weight P3HT (22 kg mol−1) demonstrating larger mobilities than devices created using 48 kg mol−1 P3HT, but only after heat treatment.

Measurement of molecular mixing at a conjugated polymer interface by specular and off-specular neutron scattering

Measurements have been performed on thermally equilibrated conjugated-polymer/insulating-polymer bilayers, using specular and off-specular neutron reflectivity. While specular reflectivity is only sensitive to the structure normal to the sample, off-specular measurements can probe the structure of the buried polymer/polymer interface in the plane of the sample. Systematic analysis of the scattering from a set of samples with varying insulating-polymer-thickness, using the distorted-wave Born approximation (DWBA), has allowed a robust determination of the intrinsic width at the buried polymer/polymer interface. The quantification of this width (12 Å ± 4 Å) allows us to examine aspects of the conjugated polymer conformation at the interface, by appealing to self-consistent field theory (SCFT) predictions for equilibrium polymer/polymer interfaces in the cases of flexible and semi-flexible chains. This analysis enables us to infer that mixing at this particular interface cannot be described in terms of polymer chain segments that adopt conformations similar to a random walk. Instead, a more plausible explanation is that the conjugated polymer chain segments become significantly oriented in the plane of the interface. It is important to point out that we are only able to reach this conclusion following the extensive analysis of reflectivity data, followed by comparison with SCFT predictions. It is not simply the case that conjugated polymers would be expected to adopt this kind of oriented conformation at the interface, because of their relatively high chain stiffness. It is the combination of a high stiffness and a relatively narrow intrinsic interfacial width that results in a deviation from flexible chain behaviour.