Preface for the special issue on Microscopy of Semiconducting Materials 2019

This issue contains selected invited and contributed presentations from the 21st international conference on 'Microscopy of Semiconducting Materials' held at Fitzwilliam College, University of Cambridge, on 9–12 April 2019. The meeting was organised by the Institute of Physics, supported by the Royal Microscopical Society, the European Microscopy Society, attolight (Platinum sponsor), JEOL (Gold sponsor) and ThermoFisher Scientific (Silver sponsor)

Coaxial Nickel Poly(Vinylidene Fluoride Trifluoroethylene) Nanowires for Magnetoelectric Applications

Magnetoelectric (ME) composite materials, in which the coupling between magnetostricitve and piezoelectric effects is achieved, are potential candidates for multifunctional devices where the interplay between electrical, magnetic and mechanical properties of these structures can be fully exploited. Nanostructured composites are particularly interesting due to the enhancement of ME coupling expected at the nanoscale. However, direct studies of ME coupling in nanocomposites by scanning probe techniques are rare due to the complex interplay of forces at play, including those arising from electrostatic, magnetic and electromechanical interactions. In this work, the ME coupling of coaxial nickel - polyvinylidene fluoride trifluoroethylene [Ni-P(VDF-TrFE)] composite nanowires, fabricated by a scalable template-wetting based technique, is studied using a systematic sequence of scanning probe techniques. Individual ME nanowires were subjected to an electric field sufficient for ferroelectric poling in piezo-response force microscopy (PFM) mode, while magnetic force microscopy (MFM) was used to measure localised changes in magnetization as a result of electrical poling. Kelvin probe force microscopy (KPFM) measurements of surface potential were conducted to eliminate for the effect of contact potential differences during these measurements. An inverse, static, magnetoelectric coupling coefficient of ~1 x 10-11 s m-1 was found in our coaxial nanocomposite nanowires, comparable to other types of planar composites studied in this work, despite having an inferior piezoelectric-to-magnetostrictive volume ratio. The efficient ME coupling in our coaxial nanowires is attributed to the larger surface-to-volume interfacial contact between Ni and P(VDF-TrFE), and is promising for future integration into ME composite devices such as magnetic field sensors or energy harvesters

Self-assembly of collagen bundles and enhanced piezoelectricity induced by chemical crosslinking.

The piezoelectricity of collagen is purported to be linked to many biological processes including bone formation and wound healing. Although the piezoelectricity of tissue-derived collagen has been documented across the length scales, little work has been undertaken to characterise the local electromechanical properties of processed collagen, which is used as a base for tissue-engineering implants. In this work, three chemically distinct treatments used to form structurally and mechanically stable scaffolds-EDC-NHS, genipin and tissue transglutaminase-are investigated for their effect on collagen piezolectricity. Crosslinking with EDC-NHS is noted to produce a distinct self-assembly of the fibres into bundles roughly 300 nm in width regardless of the collagen origin. These fibre bundles also show a localised piezoelectric response, with enhanced vertical piezoelectricity of collagen. Such topographical features are not observed with the other two chemical treatments, although the shear piezoelectric response is significantly enhanced upon crosslinking. These observations are reconciled by a proposed effect of the crosslinking mechanisms on the molecular and nanostructure of collagen. These results highlight the ability to modify the electromechanical properties of collagen using chemical crosslinking methods.ERC, Bill and Melinda Gates Foundation, Geistlich Pharma A

Time-resolved open-circuit conductive atomic force microscopy for direct electromechanical characterisation.

Studying nanomaterial piezoelectricity and triboelectricity is attractive for energy and sensing applications. However, quantitative characterisation of electromechanical effects in nanomaterials is challenging due to practical limitations and possible combination of effects, resulting in contradicting reports at times. When it comes to piezoelectricity at the nanoscale, piezoresponse force microscopy (PFM) is the default characterisation tool. In PFM the converse piezoelectric effect is measured - the conversion from electrical signal to mechanical response. However, there is an underlying desire to measure the direct piezoelectric effect - conversion of mechanical deformation to an electrical signal. This corresponds to energy harvesting and sensing. Here we present time-resolved open-circuit conductive atomic force microscopy (cAFM) as a new methodology to carry out direct electromechanical characterisation. We show, both theoretically and experimentally, that the standard short-circuit cAFM mode is inadequate for piezoelectric characterisation, and that resulting measurements are governed by competing mechanisms. We apply the new methodology to nanowires of GaAs, an important semiconductor, with relatively low piezoelectric coefficients. The results suggest that time-resolved operation distinguishes between triboelectric and piezoelectric signals, and that by measuring the open-circuit voltage rather than short-circuit current, the new methodology allows quantitative characterisation of the vertical piezoelectric coefficient. The result for GaAs nanowires, ∼ 1-3 pm V-1, is in good agreement with existing knowledge and theory. This method represents a significant advance in understanding the coexistence of different electromechanical effects, and in quantitative piezoelectric nanoscale characterisation. The easy implementation will enable better understanding of electromechanics at the nanoscale

Poly-L-lactic acid nanotubes as soft piezoelectric interfaces for biology: controlling cell attachment via polymer crystallinity

It has become increasingly evident that the mechanical and electrical environment of a cell is crucial in determining its function and the subsequent behaviour of multicellular systems. Platforms through which cells can directly interface with mechanical and electrical stimuli are therefore of great interest. Piezoelectric materials are attractive in this context due to their ability to inter-convert mechanical and electrical energy, and piezoelectric nanomaterials in particular are ideal candidates for tools within mechanobiology, given their ability to both detect and apply small forces on a length scale that is compatible with cellular dimensions. The choice of piezoelectric material is crucial to ensure compatibility with cells under investigation, both in terms of stiffness and biocompatibility. Here, we show that poly-L-lactic acid nanotubes, grown by a melt-press template wetting technique, can provide a “soft” piezoelectric interface onto which human dermal fibroblasts readily attach. Interestingly, by controlling the crystallinity of the nanotubes, the level of attachment can be regulated. In this work, we provide detailed nanoscale characterization of these nanotubes to show how differences in stiffness, surface potential and piezoelectric activity of these nanotubes result in differences in cellular behaviour

Research data supporting "Time-resolved open-circuit conductive atomic force microscopy for direct electromechanical characterisation"

These files contain the original AFM files and outputs of the experiments described in the paper. As such they were obtained using the Bruker Icon AFM and have either two dimensional scans containing topography and current at different biases when scanning atop nanowires, or current vs. deflection (or height) obtained as the AFM cantilever was driven towards the sample. To be viewed using AFM software or any data software in case of txt files