Bottom-up device fabrication via the seeded growth of polymer-based nanowires

Abstract

The bottom-up assembly of nanoelectronic devices from molecular building blocks is a target of widespread interest. Herein we demonstrate an in situ seeded growth approach to produce a nanowire-based electrical device. This exploits the chemisorption of block terpolymer-based seed fibres with a thiophene-functionalised corona onto metal electrodes as the initial step. We then use these surface-bound seeds to initiate the growth of well-defined one-dimensional fibre-like micelles via the seeded growth method known as "Living crystallisation-driven self-assembly'' and demonstrate that they are capable of spanning an interelectrode gap. Finally, a chemical oxidation step was used to transform the nanofibres into nanowires to generate a two-terminal device. This seeded growth approach of growing well-defined circuit elements provides a useful new design tool for bottom-up device fabrication.

Fig. 1. Growth of fibre-like micelles via the living crystallisation-driven self-assembly of PFDMS block terpolymers. (A) Schematic representation of the formation of cylindrical micelles. The chemical structure of unimers are shown in orange (PFDMS), red (PDMS) and purple (P3OT). (B) Self-assembly of PFDMS₄₄-b-PDMS₂₅₀-b-P3OT₁₇ unimers to form polydisperse micelles. (C) Fragmentation of the micelles using sonication to yield seeds. (D) Chemical adsorption of seeds on a metal (Au or Pt) electrode surface. (E) Growth of a fibre-like micelle from adsorbed seeds to cross the inter-electrode gap of an electrical device. (F and G) AFM images of adsorbed seeds on gold surface after immersion for 2 h in a 0.5 μg ml^–1 solution in decane. The surfaces were then washed by immersion for (F) 2 min and (G) 24 h in pure decane. (H) Image of a seed adsorbed on a gold substrate from a 0.005 μg ml^–1 solution of seeds immersed for 2 h. (I) Image of a micelle grown from an adsorbed seed on gold by immersion in 4 μg ml^–1 solution of unimers for 18 h.

Fig. 2. AFM images of seeds and fibre-like micelles of PFDMS₄₄-b-PDMS₂₅₀-b-P3OT₁₇ on gold surfaces. The gold coated silicon slides were immersed for 2 h in seeds solution (A) 0.02 μg ml^–1, (B) 0.1 μg ml^–1 and (C) 1 μg ml^–1. (D) Variation in number of seeds as function of seeds solution concentration. (E–G) The micelles were grown from 0.02 μg ml^–1 immobilised seeds on Au surfaces by immersion for 1 day in unimer solutions with concentrations (E) 1, (F) 2 and (G) 4 μg ml^–1. (H) Variation in length of micelles after 1 day as a function of unimer solution concentration. (I–K) Slow growth of the micelles from 0.005 μg ml^–1 seeds on gold surfaces and immersion in 0.5 μg ml^–1 of unimers solution for (I) 2, (J) 6 and (K) 10 days. (L) Variation in length of micelles as function of immersion time.

Fig. 3. Fibre-like micelle length distributions against reaction time. Probability mass functions for the length of micelles grown on gold-coated silicon slides in Fig. 2L. The solid black line shows the Poisson distribution at each time point and the x-axis is given in terms of a normalised length described in the text. The micelles were grown from 0.005 μg ml^–1 seeds on gold surfaces and immersion in 0.5 μg ml^–1 of unimers solution for the times stated on the legend. The probabilities were estimated using AFM to measure the length of 50 separate micelles (30 in the case of the 10 day time point); standard deviations on each data point were obtained by applying counting statistics.

Fig. 4. Growth of fibre-like micelles across an electrode gap. (A) A large area scan and (B) a magnified area of AFM height images of seeds were adsorbed on gold electrodes from 0.005 μg ml^–1 solution of seeds. (C) AFM large-scan height image and (D) a 3D magnified area of the AFM height image of micelles were grown up from the adsorbed seeds on gold electrodes by immersion in 1 μg ml^–1 solution of unimers for 5 days. (E) The associated cross-section along the blue line in (D) shows the height of the micelle.

Fig. 5. Fabrication of a simple mesoscopic electrical device. (A) Schematic representation of a simple electrical circuit constructed from bundlesv of fibre-like terpolymer micelles grown on platinum microband electrodes. (B) I–V measurements of the fibre-like micelles before (red) and after (green) doping with NOBF₄. Inset, an optical image of bundles of fibre-like micelles grown from adsorbed seeds on a platinum microband electrode (MBE). Bundles of fibre-like micelles were grown from a high concentration of seeds (0.1 μg ml^–1, 10 min) and unimer (10 μl ml^–1, 7 days); they appear as red spots on the platinum MBE in the optical micrograph.