Massively Multiplexed Submicron Particle Patterning in Acoustically Driven Oscillating Nanocavities.

Nanoacoustic fields are a promising method for particle actuation at the nanoscale, though THz frequencies are typically required to create nanoscale wavelengths. In this work, the generation of robust nanoscale force gradients is demonstrated using MHz driving frequencies via acoustic-structure interactions. A structured elastic layer at the interface between a microfluidic channel and a traveling surface acoustic wave (SAW) device results in submicron acoustic traps, each of which can trap individual submicron particles. The acoustically driven deformation of nanocavities gives rise to time-averaged acoustic fields which direct suspended particles toward, and trap them within, the nanocavities. The use of SAWs permits massively multiplexed particle manipulation with deterministic patterning at the single-particle level. In this work, 300 nm diameter particles are acoustically trapped in 500 nm diameter cavities using traveling SAWs with wavelengths in the range of 20-80 µm with one particle per cavity. On-demand generation of nanoscale acoustic force gradients has wide applications in nanoparticle manipulation, including bioparticle enrichment and enhanced catalytic reactions for industrial applications.

Phototriggerable Transient Electronics via Fullerene-Mediated Degradation of Polymer:Fullerene Encapsulation Layer.

Transient electronics is an emerging class of electronics that has attracted a lot of attention because of its potential as an environmental-friendly alternative to the existing end-of-life product disposal or treatments. However, the controlled degradation of transient electronics under environmentally benign conditions remains a challenge. In this work, the tunable degradation of transient electronics including passive resistor devices and active memory devices was realized by photodegradable thin polymer films comprising fullerene derivatives, [6,6]-phenyl-C61-butyric acid methyl esters (PCBM). The photodegradation of polymer:PCBM under an aqueous environment is triggered by ultraviolet (UV) light. Experimental results demonstrate that the addition of PCBM in commodity polymers, including but not limited to polystyrene, results in a catalytic effect on polymer photodegradation when triggered by UV light. The degradation mechanism of transient electronics is ascribed to the photodegradation of polymer:PCBM encapsulation layers caused by the synergistic effect between UV and water exposure. The polymer:PCBM encapsulation system presented herein offers a simple way to achieve the realization of light-triggered device degradation for bioapplication and expands the material options for tailorable degradation of transient electronics.

Spinodal clustering in thin films of nanoparticle-polymer mixtures.

Thin supported polystyrene-C(60) fullerene mixtures annealed above their glass transition temperature develop spinodal surface undulations which depend on film thickness h(20-500 nm), polymer molecular mass M(w), temperature, and time t. The dominant wavelength λ ∼1-10 µm scales linearly with h and coarsening kinetics follow λ∼t{α}, with 0< α(h) < 1/3; the morphology eventually pins at long times. This spinodal surface excitation contrasts with dewetting suppression and film stability observed in low -M(w) polymers and results from the interplay of binary miscibility and fullerene substrate attraction.

Nondestructive three-dimensional analysis of layered polymer structures with chemical imaging.

Three-dimensional (3D) chemical information was obtained by means of a combination of two-dimensional attenuated total reflection Fourier transform infrared (ATR-FT-IR) imaging with a focal plane array detector and variable angle depth profiling. Since the penetration depth of the evanescent wave in ATR spectroscopy is not limited by diffraction, it was possible to resolve thin sandwiched polymer layers nondestructively within a stack of polymer layers. Chemical images were obtained from layers of different thickness of the laminate by moving a custom-made aperture to specific positions on the condenser lens of the ATR accessory. Sequences of absorption images detect the successive appearance of thin, buried layers of polybutylmethacrylate (d(PBMA) = 400 nm) and polycarbonate (d(TMPC) = 300 nm) in different depths of the stack of polymer layers. The depth resolution of variable-angle ATR-FT-IR imaging is sufficiently high to detect surface roughness at the interface between different polymer layers. Two different stacks of polymers with reordered sandwich-layers were imaged simultaneously, demonstrating the potential of variable angle ATR-FT-IR for 3D-imaging of a sample with xyz-heterogeneity, which can be a powerful analytical technique for materials science and biomedical research.

Photoswitchable Solubility of Fullerene-Doped Polymer Thin Films.

Controlling polymer film solubility is of fundamental and practical interest and is typically achieved by synthetically modifying the polymer structure to insert reactive groups. Here, we demonstrate that the addition of fullerenes or its derivatives (C60 or phenyl-C61-butyric acid methyl ester, PCBM) to polymers, followed by ultraviolet (UV) illumination can change the film solubility. Contrary to most synthetic polymers, which dissolve in organic solvents but not in water, the fullerene-doped polymer films (such as polystyrene) can dissolve in water yet remain stable in organic solvents. This photoswitchable solubility effect is not observed in either film constituents individually and is derived from a synergy of photochemistries. First, polymer photooxidation generates macroradicals which cross-link with radical-scavenging PCBM, thereby contributing to the films' insolubility in organic solvents. Second, light exposure enhances polymer photooxidation in the presence of PCBM via the singlet oxygen pathway. This results in polymer backbone scission and formation of photooxidized products which can form hydrogen bonds with water, both contributing to water solubility. Nevertheless, the illuminated doped polymer thin films are mechanically robust, exhibiting significantly increased modulus and density compared to their pristine counterpart, such that they can remain intact even upon sonication in conventional organic solvents. We further demonstrate the application of this solubility-switching effect in dual tone photolithography, via a facile, economical, and environmentally benign solution-processing route made possible by the photoactive nature of polymer-PCBM thin films.

Morphological stability and performance of polymer-fullerene solar cells under thermal stress: the impact of photoinduced PC60BM oligomerization.

We report a general light processing strategy for organic solar cells (OSC) that exploits the propensity of the fullerene derivative PC60BM to photo-oligomerize, which is capable of both stabilizing the polymer:PC60BM active layer morphology and enhancing the device stability under thermal annealing. The observations hold for blends of PC60BM with an array of benchmark donor polymer systems, including P3HT, DPP-TT-T, PTB7, and PCDTBT. The morphology and kinetics of the thermally induced PC60BM crystallization within the blend films are investigated as a function of substrate and temperature. PC60BM nucleation rates on SiOx substrates exhibit a pronounced peak profile with temperature, whose maximum is polymer and blend-composition dependent. Modest illumination (<10 mW/cm(2)) significantly suppresses nucleation, which is quantified as function of dose, but does not affect crystalline shape or growth, in the micrometer range. On PEDOT:PSS substrates, thermally induced PC60BM aggregation is observed on smaller (≈ 100 nm) length scales, depending upon donor polymer, and also suppressed by light exposure. The concurrent thermal dissociation process of PC60BM oligomers in blend films is also investigated and the activation energy of the fullerene-fullerene bond is estimated to be 0.96 ± 0.04 eV. Following light processing, the thermal stability, and thus lifetime, of PCDTBT:PC60BM devices increases for annealing times up to 150 h. In contrast, PCDTBT:PC70BM OSCs are found to be largely light insensitive. The results are rationalized in terms of the suppression of PC60BM micro- and nanoscopic crystallization processes upon thermal annealing caused by photoinduced PC60BM oligomerization.

Patterning polymer-fullerene nanocomposite thin films with light.

The stability and association of polymer-fullerene films upon thermal annealing depends strongly on exposure to light, even at ambient conditions. As a result, dewetting of nanocomposite films can be prevented and the characteristic lengthscales of phase separated morphologies finely tuned. Coupling photopatterning with either self-organization process provides a powerful route for the directed assembly of fullerene-based nanocomposites into functional "circuits".

Performance enhancement of fullerene-based solar cells by light processing.

A key challenge to the commercialization of organic bulk heterojunction solar cells is the achievement of morphological stability, particularly under thermal stress conditions. Here we show that a low-level light exposure processing step during fabrication of blend polymer:PC60BM solar cells can result in a 10-fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerization of PC60BM that effectively hinders their diffusion and crystallization in the blend. We thus suggest that light processing may be a promising, general and cost-effective strategy to optimize fullerene-based solar cell performance. The low level of light exposure required suggests not only that this may be an easily implementable strategy to enhance performance, but also that light-induced PC60BM oligomerization may have inadvertently influenced previous studies of organic solar cell device behaviour.