Nanoscale Intelligent Imaging Based on Real-Time Analysis of Approach Curve by Scanning Electrochemical Microscopy

Abstract

Scanning electrochemical microscopy (SECM) enables high-resolution imaging by examining the amperometric response of an ultramicroelectrode tip near a substrate. Spatial resolution, however, is compromised for non-flat substrates, where distances from a tip far exceed the tip size to avoid artifacts caused by the tip-substrate contact. Herein, we propose a new imaging mode of SECM based on real-time analysis of approach curve to actively control nanoscale tip-substrate distances without contact. The power of this software-based method is demonstrated by imaging an insulating substrate with step edges using standard instrumentation without combination of another method for distance measurement, e.g., atomic force microscopy. An ~500 nm-diameter Pt tip approaches down to ~50 nm from upper and lower terraces of a 500 nm-height step edge, which are located by real-time theoretical fitting of experimental approach curve to ensure the lack of electrochemical reactivity. The tip approach to step edge can be terminated at <20 nm prior to the tip-substrate contact as soon as the theory deviates from the tip current, which is analyzed numerically afterward to locate the inert edge. The advantageous local adjustment of tip height and tip current at the final point of tip approach distinguishes the proposed imaging mode from other modes based on standard instrumentation. In addition, the glass sheath of Pt tip is thinned to ~150 nm to rarely contact the step edge, which is unavoidable and instantaneously detected as an abrupt change in the slope of approach curve to prevent the damage of fragile nanotip.

Simultaneous Intelligent Imaging of Nanoscale Reactivity and Topography by Scanning Electrochemical Microscopy

“…A home-built SECM instrument was equipped with a potentiostat (CHI 802D, CH Instruments, Austin, TX) and controlled by using the LabVIEW program based on a custom fuzzy logic algorithm as reported previously . In addition, an SECM stage was accommodated in an isothermal chamber equipped with heat sinks and vacuum insulation plates to minimize thermal drift .…”

“…In addition, an SECM stage was accommodated in an isothermal chamber equipped with heat sinks and vacuum insulation plates to minimize thermal drift . Pt tips were fabricated by laser-assisted pulling, heat annealing, and focused-ion-beam (FIB) milling and characterized by SEM before and after the use for SECM experiments to check the damage to the tips . The tips were protected from electrostatic discharge as described in the Supporting Information.…”

“…Herein, we advance the intelligent mode of nanoscale SECM to simultaneously obtain separate reactivity and topography images of non-flat substrates with both reactive and inert regions. In the intelligent mode, the distance-dependent current response of an SECM nanotip is assessed after every step of the vertical tip approach to the substrate to achieve a short tip–substrate distance, d , without contact, thereby maximizing the spatial resolution based on feedback effects as represented by where h ∞ is the radius of the local substrate surface seen by using a disk-shaped tip with a radius of a .…”

“…In the intelligent mode, the distance-dependent current response of an SECM nanotip is assessed after every step of the vertical tip approach to the substrate to achieve a short tip–substrate distance, d , without contact, thereby maximizing the spatial resolution based on feedback effects as represented by where h ∞ is the radius of the local substrate surface seen by using a disk-shaped tip with a radius of a . Previously, we employed non-flat inert substrates to demonstrate that the intelligent mode enables non-contact topography imaging with a high resolution of h ∞ ≤ 1.45 a with d ≤ 0.3 a in eq in contrast to other imaging modes based on approach curves, that is, intermittent-contact and hopping ( h ∞ = 8.5 a with d = 5 a ) modes. In this study, we reinforce the intelligent mode by the numerical post-analysis of each approach curve to locate the substrate under the tip for quantitative topography imaging and determine the tip current at a constant distance for topography-independent reactivity imaging.…”

“…The adoption of combined SECM techniques, however, has been limited 13 by the requirement of specialized hardware including a bifunctional probe and a dual probe for combination with atomic force microscopy 14 (AFM) and scanning ion-conductance microscopy, 15 respectively. Herein, we advance the intelligent mode of nanoscale SECM 16 to simultaneously obtain separate reactivity and topography images of non-flat substrates with both reactive and inert regions. In the intelligent mode, the distancedependent current response of an SECM nanotip is assessed after every step of the vertical tip approach to the substrate to achieve a short tip−substrate distance, d, without contact, thereby maximizing the spatial resolution based on feedback effects as represented by 17 T h i s c o n t e n t i s where h ∞ is the radius of the local substrate surface seen by using a disk-shaped tip with a radius of a.…”