Tomasz Kowalewski

Transistor Paint: High Mobilities in Small Bandgap Polymer Semiconductor Based on the Strong Acceptor, Diketopyrrolopyrrole and Strong Donor, Dithienopyrrole

Plastic electronics based on organic semiconductors attract considerable attention due to the promise of low cost, lightweight, and fl exible large area electronic devices such as organic fi eld effect transistors (OFETs), [ 1 ] organic photovoltaics (OPVs), [ 2 ] electrochromic devices, [ 3 ] and organic light emitting diodes (OLEDs). [ 4 ] Of the semiconductors, regioregular poly(3alkylthiophenes) rr-P3ATs are the most studied and researched material. [ 5 ] This is predominately due to the development of Grignard Metathesis (GRIM) that provides the facile synthesis of structurally pure rr-P3AT with narrow polydisperities. [ 6 ] The pristine rr-P3ATs are known to form into a nanofi brillar morphology promoted by the pi-stacked polymer backbone and lamellar packing of the alkyl side chain. It is postulated that this morphology is the reason for its high charge mobility ( ∼ 0.1 cm 2 /Vs). [ 7 ] The nanofi brillar structures act as a charge transport pathway where holes can hop from one polymer chain to another down the nanofi bril. Unfortunately, these nanofi brillar structures have some major vulnerabilities that can be detrimental to charge transport such as large interfi brillar grain boundaries that trap charges and variable morphologies that are diffi cult to reproduce, both resulting in reduced charge mobilities. [ 8 ]

Conducting Block Copolymer Nanowires Containing Regioregular Poly(3‐Hexylthiophene) and Polystyrene

Grignard Metathesis polymerization (GRIM) for the synthesis of regioregular poly(3‐alkylthiophenes) proceeds via a “living” chain growth mechanism. Due to the “living” nature of this polymerization regioregular poly(3‐alkylthiophenes) with predetermined molecular weight, narrow molecular weight distributions and desired chain end functionality are now readily available. Allyl terminated poly(3‐hexylthiophene) was successfully used as a precursor for the synthesis of di‐block copolymers containing polystyrene. The addition of “living” poly(styryl)lithium to the allyl terminated regioregular poly(3‐hexylthiophene) generated the di‐block copolymer. Poly(3‐hexylthiophene)‐b‐polystyrene was also synthesized by atom transfer radical polymerization. Integration of poly(3‐hexylthiophene) in di‐block copolymers with polystyrene leads to the formation of nanowire morphology and self‐ordered conducting nanostructured materials.

Thermoresponsive hydrogel scaffolds with tailored hydrophilic pores.

Thermoresponsive hydrogels with efficient water-release channels were prepared by incorporating star-shaped macromolecular pore precursors, with degradable disulfide crosslinked cores and hydrophilic poly(ethylene oxide) (PEO) arms, into the gel network. The gel framework exhibiting lower critical solution temperature (LCST) behavior was synthesized by atom transfer radical polymerization (ATRP) of 2-(2-methoxyethoxy)ethyl methacrylate and ethylene glycol dimethacrylate. The incorporation of degradable star macromolecules (dSM) was facilitated by growing the gel from ATRP initiator sites contained within their cores. Following the formation of the gel, the dSM cores were degraded, yielding uniform pores lined with hydrophilic PEO chains. The effect of hydrophilic pores on thermoresponsive hydrogel performances was studied by comparing hydrogels containing hydrophilic pores with analogous hydrogels with neutral pores or with pore-free controls. Dye absorption/release experiments pointed to the suitability of newly synthesized hydrogels as controlled-release media, for example, for drug delivery. Cell culture experiments confirmed their nontoxicity and biocompatibility (cell viability >98%).

Insights into fluid tapping-mode atomic force microscopy provided by numerical simulations

In an attempt to understand the physics underlying tapping mode atomic force microscopy (TMAFM) operated in fluids, simulations of complete TMAFM experiments were performed based on a cantilever model invoking the damped driven harmonic oscillator with a single degree of freedom with parameters based on real experiments, the most important of which was a low quality factor (Q). Such a low-Q oscillator captures some of the essential features related to operation of TMAFM in fluids, when compared to real experiments. Fluid TMAFM (a low-Q system) is characterized by a highly anharmonic deflection signal when compared to operation in air (a high-Q system). Our model was able to capture this hallmark of fluid TMAFM without the inclusion of more nuanced hydrodynamic effects. Such modeling can aid in the understanding of tip-sample interactions in fluid TMAFM and in the development of techniques to extract meaningful mechanical surface properties from such interactions.

Imaging stability and average tip-sample force in tapping mode atomic force microscopy

In tapping mode atomic force microscopy (AFM), a cantilever is driven near its resonance frequency and intermittently strikes the sample while raster scanned across a surface. The oscillation amplitude is monitored via a feedback loop to construct topography maps of surfaces at the nanoscale. This paper deals with two major limits on scanning rates when operating in air: (1) the slow transient response of the cantilever and (2) instabilities associated with systems with high quality factors (Q). Due to the slow transient response, the AFM has difficulty in instantly responding to steps along the surface, resulting in the need for slower scan rates and higher gains to more accurately track the surface. However, the use of higher gains leads to more pronounced instabilities associated with high Q systems. By driving the cantilever well below its resonance frequency, stability of the system is greatly improved, resulting in better feature tracking and allowing for scanning at higher speeds with larger gains....

Dependence of field-effect mobility and contact resistance on nanostructure in regioregular poly(3-hexylthiophene) thin film transistors

The mobility and contact resistance of transistors based on regioregular poly(3-hexylthiophene) (P3HT) with Ti∕Pt electrodes were investigated as a function of the molecular weight (MW) of P3HT. For an increase in MW from 5.5to11kDa, the mobility increased from 0.04to0.16cm2V−1s−1, whereas the contact resistance decreased from 1.7to0.6MΩ. Further increases in MW yielded an apparent saturation in both the mobility and the contact resistance. A nanofibrilar morphology was observed where the width of the nanofibrils increases with MW. A qualitative model based on polymer chain folding is proposed to explain the electrical results.