Paul Stavrinou

Abstract:

Metamaterials are a promising new class of materials, in which sub-wavelength physical structures, rather than variations in chemical composition, can be used to modify the nature of their interaction with electromagnetic radiation. Here we show that a metamaterials approach, using a discrete physical geometry (conformation) of the segments of a polymer chain as the vector for a substantial refractive index change, can be used to enable visible wavelength, conjugated polymer photonic elements. In particular, we demonstrate that a novel form of dip-pen nanolithography provides an effective means to pattern the so-called β-phase conformation in poly(9,9-dioctylfluorene) thin films. This can be done on length scales ≤500 nm, as required to fabricate a variety of such elements, two of which are theoretically modelled using complex photonic dispersion calculations.

Abstract:

Copper thiocyanate (CuSCN) is introduced as a hole‐injection/hole‐transport layer (HIL/HTL) for solution‐processed organic light‐emitting diodes (OLEDs). The OLED devices reported here with CuSCN as HIL/HTL perform significantly better than equivalent devices fabricated with a PEDOT:PSS HIL/HTL, and solution‐processed, phosphorescent, small‐molecule, green OLEDs with maximum luminance ≥10 000 cd m^‐2, maximum luminous efficiency ≤50 cd A^‐1, and maximum luminous power efficiency ≤55 lm W^‐1 are demonstrated.

Abstract:

We report a detailed analysis on the use of simultaneous substrate heating in conjunction with scanning thermal lithography (SThL) to dramatically increase the patterning speed of conventional SThL systems. The investigation consists of finite element simulations as well as practical assessments of the speed at which different organic precursors are thermally converted to produce standalone electrically active and passive nanostructures. As a proof of concept the high-speed SThL method was used to pattern semiconducting pentacene nanoribbons, which were subsequently incorporated into functioning transistors. Simultaneous substrate heating was found to allow patterning of functional devices at writing speeds >19 times higher than transistors produced at identical speeds but with the substrate maintained at room temperature. These fast written transistors exhibit 100× higher hole mobility with high on/off current ratio and negligible operating hysteresis. The generality of the proposed high-speed SThL method was further demonstrated with the rapid patterning of conductive nanostructured metal electrodes with excellent spatial resolution employing an appropriate polymer precursor as the chemical resist. It is proposed that these advances further support the case for using SThL systems as rapid prototypers for low micron and nanoscale structures for both direct patterning of precursors and indirect patterning of metals and other materials using suitable chemical resist.

Abstract:

We investigate the surface plasma oxidation of polydimethylsiloxane (PDMS) elastomers and its implication for the morphologies attainable by wrinkling of glassy-elastomer 'bilayers'. The kinetics of glassy skin formation is found to follow a logarithmic dependence with plasma exposure time t and, for various plasma intensities I, the relevant control variable is shown to be dose (≡I × t). We model the mechanism and kinetics of glassy film formation by plasma oxidation with a frontal propagation coarse-grained model, describing the spatio-temporal evolution of a conversion order parameter (ϕ) orthogonal to the film surface. The model is validated by X-ray reflectivity experiments, which confirm the logarithmic growth and quantify the initial growth of a transient, incomplete, skin layer during the early stage of plasma exposure. Three regimes are identified as (I) induction, (II) formation and (III) propagation with a combination of X-ray and wrinkling experiments. The simultaneous increase in thickness and skin mechanical modulus is found to be responsible for an unexpected minimum wavelength λmin attainable, which depends on critical strain εc and is ultimately limited by mechanical failure of the elastomer (λmin ≃ 140 nm is demonstrated at ε = 200%). We conclude by establishing a 1D surface morphology diagram, in terms of wavelength λ and amplitude A, limitations and capabilities for producing highly ordered (sub-)micropatterns over macroscopic areas using plasma oxidised PDMS under uniaxial strain.