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A pinboard by
Jingjing Jiang

Graduate Research Assistant, California Institute of Technology

PINBOARD SUMMARY

Atomic force microscopy (AFM) can "see" the surface of samples on a nanometer scale. It can also simultaneously measure electrical and mechanical properties of the sample. The system has a probe and a laser detector. The probe consists of a spring-like cantilever and a sharp tip. The sharp tip scans through the sample surfaces. When the relative displacement of the sample and tip changes, the detector records the deflection of the laser which hits the back of the probe and is reflected by the back to the detector. Since the sharp tip only has a diameter of several to tens of nanometers, AFM can achieve a high resolution, and is very useful in understanding the nanoscale and microscale properties of different materials. AFM has different modes and different types of probes. When the probe is conductive, electrical currents of a conductive sample can be simultaneously captured with the morphology. When the AFM operates in a mode that records the force curve between the tip and the sample, mechanical properties like modulus of a material can be mapped as well.

4 ITEMS PINNED

Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging.

Abstract: Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated. The nanoelectrode probe is coated with dielectric materials and has an exposed conical Pt tip apex of ∼200 nm in height and of ∼25 nm in end-tip radius. These characteristic dimensions permit sub-100 nm spatial resolution for electrochemical imaging. With this nanoelectrode probe we have extended AFM-based nanoelectrical measurements to liquid environments. Experimental data and numerical simulations are used to understand the response of the nanoelectrode probe. With PeakForce SECM, we successfully characterized a surface defect on a highly-oriented pyrolytic graphite electrode showing correlated topographical, electrochemical and nanomechanical information at the highest AFM-SECM resolution. The SECM nanoelectrode also enabled the measurement of heterogeneous electrical conductivity of electrode surfaces in liquid. These studies extend the basic understanding of heterogeneity on graphite/graphene surfaces for electrochemical applications.

Pub.: 01 Feb '17, Pinned: 11 Jul '17

Nanoelectrical and Nanoelectrochemical Imaging of Pt/p-Si and Pt/p+-Si Electrodes.

Abstract: The interfacial properties of electrolessly deposited Pt nanoparticles (Pt-NP) on p-Si and p+-Si electrodes have been resolved on the nanometer scale using a combination of scanning probe methods. Atomic-force microscopy (AFM) showed highly dispersed Pt nanoparticles. Conductive AFM measurements showed that only about half of the particles exhibited measurable contact currents, with a factor of 10^3 difference in current. Local current-voltage measurements revealed a rectifying junction with a resistance of ≥ 10 MΩ at the Pt-NP/p-Si interface, while Pt-NP/p+-Si samples formed an Ohmic junction with a local resistance of ≥ 1 MΩ. The particles were strongly attached to the sample surface in air. However in contact with an electrolyte, the adhesion of the particles to the surface was substantially lower. Scanning electrochemical microscopy (SECM) showed smaller, but more uniform electrochemical currents for the particles relative to the currents observed in conductive AFM measurements. In accord with the conductive AFM measurements, SECM measurements showed conductance through the substrate for only a minority of the particles. These results suggest that the electrochemical performance of the electrolessly deposited Pt nanoparticles on Si is ascribable to: 1) the high resistance of the contact between the particles and the substrate; 2) the low (<50%) fraction of particles that support high currents; and 3) the low adhesion of the particles to the surface in the electrolyte.

Pub.: 22 Jun '17, Pinned: 11 Jul '17