Functionalized Gold Nanoparticles with a Cohesion Enhancer for Robust Flexible Electrodes

Abstract

The development of conductive inks is required to enable additive manufacturing of electronic components and devices. A gold nanoparticle (AuNP) ink is of particular interest due to its high electrical conductivity, chemical stability, and biocompatibility. However, a printed AuNP film suffers from thermally induced microcracks and pores that lead to the poor integrity of a printed electronic component and electrical failure under external mechanical deformation, hence limiting its application for flexible electronics. Here, we employ a multifunctional thiol as a cohesion enhancer in the AuNP ink to prevent the formation of microcracks and pores by mediating the cohesion of AuNPs via strong interaction between the thiol groups and the gold surface. The inkjet-printed AuNP electrode exhibits an electrical conductivity of 3.0 × 10⁶ S/m and stable electrical properties under repeated cycles (>1000) of mechanical deformation even for a single printed layer and in a salt-rich phosphate-buffered saline solution, offering exciting potential for applications in flexible and 3D electronics as well as in bioelectronics and healthcare devices.

Figure 1

(a) Schematic of the synthesis of OT-AuNPs and the ink formulation with a multifunctional thiol (TrisSH) as a cohesion enhancer. (b) Representative TEM and (top inset) HRTEM images of OT-AuNPs. (lower inset) Histogram of the size distribution of OT-AuNPs. (c) (left) Photograph of two ink formulations, Au-TrisSH and Ctrl-Au, with and without TrisSH. (right) Schematic of inkjet deposition of the Au-TrisSH ink and (inset) a photograph of an inkjet-printed gold square-planar spiral coil (scale bar is 3 mm). (d) Dependence of the line widths (W) of the Au-TrisSH ink deposited on Si/SiO₂ substrates on DS values. (inset) Optical microscopy image of AuNP lines printed at different DS values (the scale bar is 200 μm). (e) Dependence of electrical resistivity (ρ) of printed Ctrl-Au and Au-TrisSH lines on sintering temperatures (T_sint) for 30 min. The data points represent the mean and standard deviation for at least three independent measurements. (inset) Optical microscopy images of Ctrl-Au and Au-TrisSH lines post-treated at T_sint = 150 °C (the scale bar is 200 μm). The dotted lines are a guide to the eye.

Figure 2

Electrical performance stability study of single printed layers of Ctrl-Au and Au-TrisSH on a PEN substrate (T_sint = 150 °C). [(a) and right inset] Electrical resistance (R) and normalized electrical resistance change (ΔR/R_o) measured over 1000 bending cycles for the Au-TrisSH electrode. (left inset) Photograph of an inkjet-printed Au-TrisSH electrode on PEN. (b) Dependence of the calculated bending strain (ε) and ΔR/R_o on the bending curvature (r) for the Au-TrisSH electrode. (c) Representative ΔR/R_o dependence of Ctrl-Au and Au-TrisSH electrodes on the number of bending cycles (r = 0.6 cm).

Figure 3

(a) Photograph of a single-layer printed Au-TrisSH electrode on a PEN substrate and a test setup in the PBS solution and (b) its electrical resistance (R) monitored for 1 h with ΔR/R_o (%) < 0.057%. (inset) Photograph of the inkjet-printed Au-TrisSH electrode (the scale bar is 1 cm).

Figure 4

Representative AFM images of the surface morphology and microstructure characterization of single-layer printed gold structures (Si/SiO₂ substrate, T_sint = 150 °C) of (a) Ctrl-Au and (b) Au-TrisSH. The left images were acquired using the PeakForce tapping mode (20 μm × 20 μm); the middle images show the HR images over 1 μm × 1 μm (tapping mode); the right graphs show the surface roughness values, R_a, R_q, and R_sa, estimated from 1 μm × 1 μm images.

Figure 5

Chemical composition of single-layer printed gold structures of Ctrl-Au and Au-TrisSH (Si/SiO₂ substrate, T_sint = 150 °C). (a) Normalized OrbiSIMS data showing Au, octanethiolates, Au-octanethiolate clusters, and oxidized ligand species from the surfaces of the printed structures. (b) XPS S 2p core-level spectra of the Ctrl-Au- and Au-TrisSH-printed samples as-deposited at 90 °C (dotted lines) and sintered at 150 °C (solid lines) compared with those of OT-AuNPs. (c) S/Au atomic ratio of the Ctrl-Au- and Au-TrisSH-printed samples.

Figure 6

PCA of ToF-SIMS data on the surfaces of the single-layer printed gold structures of Ctrl-Au and Au-TrisSH (Si/SiO₂ substrate, T_sint = 150 °C). (a) 2D and (b) 3D score scatter plots of principal components PC1, PC2, and PC3. (c) Explained variance per principal component. Score loadings of (d) PC1, (e) PC2, and (f) PC3.