Pushing the thinness limit of silver films for flexible optoelectronic devices via ion-beam thinning-back process

Abstract

Reducing the silver film to 10 nm theoretically allows higher transparency but in practice leads to degraded transparency and electrical conductivity because the ultrathin film tends to be discontinuous. Herein, we developed a thinning-back process to address this dilemma, in which silver film is first deposited to a larger thickness with high continuity and then thinned back to a reduced thickness with an ultrasmooth surface, both implemented by a flood ion beam. Contributed by the shallow implantation of silver atoms into the substrate during deposition, the thinness of silver films down to 4.5 nm can be obtained, thinner than ever before. The atomic-level surface smooth permits excellent visible transparency, electrical conductivity, and the lowest haze among all existing transparent conductors. Moreover, the ultrathin silver film exhibits the unique robustness of mechanical flexibility. Therefore, the ion-beam thinning-back process presents a promising solution towards the excellent transparent conductor for flexible optoelectronic devices.

Fig. 1. UTAF fabrication.

a 3D schematics showing the ion-beam thinning-back process flow in UTAF fabrication. b Series of schematic diagrams exhibiting the detailed growth process of Ag film using the IBS technique. The inset SEM images from top to bottom present the morphologies at nominal thicknesses of 1, 4, and 9 nm. c Serial schemes showing the thinning-back and polishing process of the Ag film using glancing ion beam etching with a given incident angle referring to sample surface. The inset SEM images from left to right demonstrate the topography evolution of the Ag film with reduced thicknesses of 8, 6 and 4.5 nm. d Photograph of 4.5-nm-thick UTAF on a PET plate (60 mm × 35 mm). Scale bars are 50 nm for all insets in panels (b, c).

Fig. 2. Morphological analysis of a 4.5-nm-thick UTAF.

a Electron micrograph of the surface morphology. Scale bar: 200 nm. b Corresponding AFM topography mapping with a scanning resolution of 512 × 512 pixels in a 1 × 1 μm² area. Scale bar: 200 nm. c Cross-sectional TEM image of a 4.5-nm-thick UTAF. Scale bar: 5 nm. d, e EDS mappings of Ti and Ag elements in the scanning TEM mode, respectively. Scale bars: 2 nm. f Signal analysis for Ag and Ti distributions at the interface. The signals are extracted from the greyscale integration of corresponding EDS mapping images in panels d and e.

Fig. 3. Investigating the effect of Ag implantation.

a Calculated average implantation depth of ejected Ag atoms into TiO_x underlayer as a function of the kinetic energy in SRIM software. The kinetic energy of the Ag atom is in the range of 1–20 eV with an increment of 1 eV. The average implantation depth is the mean value of the vertical projection range from 10⁶ Ag atoms in the single calculation. Inset scheme demonstrates Ag implantation with various kinetic energies during the IBS deposition. b Schematic showing the different behavior of Ag adatoms in UTAF fabrication using direct deposition and thinning-back processes. c, d MD simulation showing the structural evolution of the ordered-arrangement Ag matrix without and with interface implantation at 300 K, respectively.

Fig. 4. Optical and electrical characteristics of UTAFs.

a R_s for UTAFs with changing reduced thicknesses during thinning back. Raw data (hollow dots) of R_s from five random positions are extracted for each sample. The line fit is the plot of R_s of Ag films in the FS-MS model. b Corresponding measured visible transmittance of UTAFs. Solid lines represent continuous UTAFs. The t_Ag values labeled on solid lines is the optical thickness measured by ellipsometry. The dashed line represents the particle-shaped UTAF fabricated by direct deposition using the IBS technique. c Average visible haze of the UTAFs with different thickness. The inset shows the principle and calculation of optical haze for a film. d FoM of UTAFs with changing reduced thickness t_Ag. e Photographs of 9-nm-thick UTAFs on a 12-inch glass wafer (top panel) and a 300 × 300 mm² PET plate (bottom panel).

Fig. 5. Mechanical flexibility and flexible light-emitting device application of UTAFs.

a Static flexibility of a 9-nm-thick UTAF with different folding radii. The inset photograph presents of the flexibility test unit. b Folding stability of the electrical conductance of a 9-nm-thick UTAF and commercial ITO within 10⁵ cycles. Insets show the SEM images and photographs of the ITO and UTAF after 10⁴ cycles and 10⁵ cycles, respectively. Scale bars in insets: 25 μm in left photographs; 100 nm in right SEM images. c Proof-of-concept demonstration of a foldable resistive touch panel. d 3D scheme for the configuration of a foldable ACEL device. e Luminance plot of a foldable ACEL device with changing folding curvature. The ACEL device is driven by a sinusoidal signal (160 V, 16 kHz). The inset picture shows a folded ACEL device with an HNU logo. f Luminance stability of the ACEL device undergoing a cyclic folding test. Inset images present the unfolding and folding states for the working ACEL device.