Hybrid Nanowire Ion-to-Electron Transducers for Integrated Bioelectronic Circuitry

A key task in the emerging field of bioelectronics is the transduction between ionic/protonic and electronic signals at high fidelity. This is a considerable challenge since the two carrier types exhibit intrinsically different physics and are best supported by very different materials typeselectronic signals in inorganic semiconductors and ionic/protonic signals in organic or bio-organic polymers, gels, or electrolytes. Here we demonstrate a new class of organic−inorganic transducing interface featuring semiconducting nanowires electrostatically gated using a solid proton-transporting hygroscopic polymer. This model platform allows us to study the basic transducing mechanisms as well as deliver high fidelity signal conversion by tapping into and drawing together the best candidates from traditionally disparate realms of electronic materials research. By combining complementary $n$- and $p$-type transducers we demonstrate functional logic with significant potential for scaling toward high-density integrated bioelectronic circuitry.This work was funded by the Australian Research Council (ARC), the University of New South Wales, the University of Queensland, Danish National Research Foundation and the Innovation Fund. A.P.M. acknowledges an ARC Future Fellowship (FT0990285) and DJC acknowledges Australian Nanotechnology Network Short Term Visit support. P.M. is an ARC Discovery Outstanding Research Award Fellow and the work at UQ was funded under the ARC Discovery Program (DP140103653). The Centre for Organic Photonics and Electronics is a strategic initiative of the University of Queensland. We thank Helen Rutlidge for conducting the inductively coupled plasma mass spectrometry measurements. This work was performed in part using the NSW and ACT nodes of the Australian National Fabrication Facility (ANFF) and the Mark Wainwright Analytical Centre at UNSW

Hybrid nanowire ion-to-electron transducers for integrated bioelectronic circuitry(Conference Presentation)

A key task in bioelectronics is the transduction between ionic/protonic signals and electronic signals at high fidelity. This is a considerable challenge since the two carrier types exhibit intrinsically different physics. We present our work on a new class of organic-inorganic transducing interface utilising semiconducting InAs and GaAs nanowires directly gated with a proton transporting hygroscopic polymer consisting of undoped polyethylene oxide (PEO) patterned to nanoscale dimensions by a newly developed electron-beam lithography process [1]. Remarkably, we find our undoped PEO polymer electrolyte gate dielectric [2] gives equivalent electrical performance to the more traditionally used LiClO4-doped PEO [3], with an ionic conductivity three orders of magnitude higher than previously reported for undoped PEO [4]. The observed behaviour is consistent with proton conduction in PEO. We attribute our undoped PEO-based devices’ performance to the small external surface and high surface-to-volume ratio of both the nanowire conducting channel and patterned PEO dielectric in our devices, as well as the enhanced hydration afforded by device processing and atmospheric conditions. In addition to studying the basic transducing mechanisms, we also demonstrate high-fidelity ionic to electronic conversion of a.c. signals at frequencies up to 50 Hz. Moreover, by combining complementary n- and p-type transducers we demonstrate functional hybrid ionic-electronic circuits can achieve logic (NOT operation), and with some further engineering of the nanowire contacts, potentially also amplification. Our device structures have significant potential to be scaled towards realising integrated bioelectronic circuitry. [1] D.J. Carrad et al., Nano Letters 14, 94 (2014). [2] D.J. Carrad et al., Manuscript in preparation (2016). [3] S.H. Kim et al., Advanced Materials 25, 1822 (2013). [4] S.K. Fullerton-Shirey et al., Macromolecules 42, 2142 (2009)

Using polymer electrolyte gates to set-and-freeze threshold voltage and local potential in nanowire-based devices and thermoelectrics

The strongly temperature-dependent ionic mobility in polymer electrolytes is used to "freeze in" specific ionic charge environments around a nanowire using a local wrap-gate geometry. This makes it possible to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 220 K. These are characterized in detail by combining conductance and thermovoltage measurements with modeling. The results demonstrate that local polymer electrolyte gates are compatible with nanowire thermoelectrics, where they offer the advantage of a very low thermal conductivity, and hold great potential towards setting the optimal operating point for solid-state cooling applications

Electron-beam patterning of polymer electrolyte films to make multiple nanoscale gates for nanowire transistors

We report an electron-beam based method for the nanoscale patterning of the poly(ethylene oxide)/LiClO4 polymer electrolyte. We use the patterned polymer electrolyte as a high capacitance gate dielectric in single nanowire transistors and obtain subthreshold swings comparable to conventional metal/oxide wrap-gated nanowire transistors. Patterning eliminates gate/contact overlap, which reduces parasitic effects and enables multiple, independently controllable gates. The method's simplicity broadens the scope for using polymer electrolyte gating in studies of nanowires and other nanoscale devices. © 2013 American Chemical Society

Nanoscale polymer electrolytes: Fabrication and applications using nanowire transistors

We present a method to pattern the poly(ethylene oxide)/LiClO4 polymer electrolyte on the nm-scale by electron beam lithography. We use the patterned polymer electrolyte as a high capacitance gate dielectric for InAs nanowire transistors in a 'wrap-gate' geometry. Patterning enabled multiple independently controllable gates on the same device, which exhibit stability and subthreshold characteristics comparable to conventional metal/oxide wrap-gates at room temperature. The strong tuning at room temperature combined with the 'freeze-out' of Li+/ClO4- transport for T < 220 K allows a wide range of charge environments to be set and held for quantum transport measurements. The simplicity of fabrication and excellent gate characteristics broadens the scope for polymer electrolyte gating in studies of nanowires and other nanoscale devices

Epitaxial Pb on InAs nanowires for quantum devices

Semiconductor-superconductor hybrids are used for realizing complex quantum phenomena but are limited in the accessible magnetic field and temperature range. Now, hybrid devices made from InAs nanowires and epitaxially matched, single-crystal, atomically flat Pb films present superior characteristics, doubling the available parameter space. Semiconductor-superconductor hybrids are widely used to realize complex quantum phenomena, such as topological superconductivity and spins coupled to Cooper pairs. Accessing new, exotic regimes at high magnetic fields and increasing operating temperatures beyond the state-of-the-art requires new, epitaxially matched semiconductor-superconductor materials. One challenge is the generation of favourable conditions for heterostructural formation between materials with the desired properties. Here we harness an increased knowledge of metal-on-semiconductor growth to develop InAs nanowires with epitaxially matched, single-crystal, atomically flat Pb films with no axial grain boundaries. These highly ordered heterostructures have a critical temperature of 7 K and a superconducting gap of 1.25 meV, which remains hard at 8.5 T, and therefore they offer a parameter space more than twice as large as those of alternative semiconductor-superconductor hybrids. Additionally, InAs/Pb island devices exhibit magnetic field-driven transitions from a Cooper pair to single-electron charging, a prerequisite for use in topological quantum computation. Semiconductor-Pb hybrids potentially enable access to entirely new regimes for a number of different quantum systems

Surface functionalization of III-V Nanowires

The physical and chemical properties of semiconductor nanowires are significantly influenced by their surface structure and morphology. This can be understood in that surfaces make out a much larger part of the total structure as compared to macroscale objects. An immediate consequence is that the lack of surface control can result in poor performance and reproducibility of any nanowire device. It is clear that bad performance is problematic, but it must be stressed that without performance reproducibility across millions of nanowires they can never become a useful real technology. This is indeed why many promising nanostructures and materials lost interest of both the scientific and commercial communities. However, surface control also can be used to strongly enhance nanowire performance and even introduce new functionality. As a result, surface functionalization is a key issue for nanowire science and technology. In this chapter, we describe in detail how standard surface science techniques such as Scanning Tunneling Microscopy (STM) and X-ray Photoemission Spectroscopy (XPS) can be modified for effective studies of 1D nanowires despite that they have been originally invented only for large and flat 2D surfaces. We go on to give a number of examples on how these techniques have revealed the precise structure–function relationship in particular of III–V semiconductor nanowires and their surfaces. We further discuss, how this can be used to control the structure and chemistry of the wires down to the atomic scale enabling new functionality for (opto)electronics, sensors, and many other device types. While we focus on III–V nanowires, the examples and techniques put forward should be applicable to many other material systems and types of nanostructures