In Situ Electrochemical Measurements Using Keysight 7500 AFM

Table of Contents

Introduction
Instrumentation
Examples of EC-SPM Measurement
Oxygen-Free and Controlled Environments
Copper Electrodeposition on Au
Conclusion

Introduction

Electro-chemical scanning probe microscope (EC-SPM) integrates two separate, valuable methods: scanning probe microscopy (SPM) and electrochemistry (EC). A three-electrode cell and a potentiostat are included in the electrochemical unit; the former regulates the electrochemical condition of the working electrode, which often serves as the sample in most experiments. The scanning probe microscope defines the solid electrode surface using an active or a passive probe.

When passive probes similar to those used in ECAFM, are utilized, it is not possible to control the probe potential. The AFM cantilever serves as an inert probe with the ability to track the topographic changes of the electrode surface using standard AFM imaging modes. Such topographic changes are induced by electrochemical processes. However, when active probes like those used in EC-STM, are utilized a bipotentiostat is generally employed to regulate the probe potential as well as the sample potential against the same reference electrode.

As with typical STM imaging, the tunneling current existing between the sample and the tip relies on the possible variations and also on the distance between the two. It is utilized as the control signal for the formation of STM image. In the case of EC-STM, the morphological data about the surface of the electrode under potential control can possibly be recorded in the constant-current mode.“Current vs. voltage” spectroscopy can be used to analyze the changes occurring in the localized electronic condition of the electrode surface with electrochemical potential.

EC-SPM is capable of providing nanoscale resolution of the surface of the electrode in liquid. However, the exact merit of EC-SPM is its ability to manage the experimental conditions such as humidity and temperature, and emulate the real-world environment of the sample being tested. This is imperative for studies relating to corrosion and energy.

Instrumentation

Keysight Technologies have developed a high-performance instrument called the 7500 AFM/SPM microscope. The system delivers high-resolution imaging with built-in environmental control functions. The Keysight 7500 comes with phase imaging, acoustic AC mode and contact mode, and includes a universal scanner that can function in both closed-loop and open-loop modes.

Users can easily and rapidly switch the imaging modes with the 7500 AFM/SPM microscope, thanks to the easy-to-load, interchangeable nose cones of the scanner. All 7500 AFM/SPM microscopes use the lowest-noise closed-loop scanner to provide excellent performance and convenience in imaging, without actually affecting the image quality or resolution.

There is an optional EC glove box that has been specifically designed for use with the Keysight 7500. This has a dual-chamber design that make it easy to prepare samples under environmental control, such as temperature, humidity, reactive gases, etc. in a single area and then shifts to an internal chamber that is integrated just under the head/scanner of the microscope.

A true reference and controls sample potential is realized through an optional mini reference electrode (Ag/AgCl). The Keysight Pico IC isolation chamber can be accommodated with the 7500 head on the glove box. An optional bipotentiostat can also be fitted to the Keysight 7500 microscope that allows a steady electrochemistry control in EC-STM or EC-AFM mode. The bipotentiostat has a set of different sensitivity settings such as 10 nA/V, 1 mA/V, 100 µA/V, 1 µA/V, and 100 nA/V, spanning four orders of magnitude of currents between 10 pA and 10 mA.

The Keysight 7500, when combined with a uniquely designed liquid cell, serves as an ideal imaging system for electrochemistry analysis. This liquid cell enables the use of a mini-reference (Ag/AgCl) electrode for accurate potential control, rather than a quasi Ag wire reference provided by other EC-SPM systems.

Examples of EC-SPM Measurement

Electrochemical SPM has been used to study various systems, such as membrane ion transport, film growth and dissolution, surface adsorption, fuel cell, catalysis, and photovoltaic. The following examples emphasize only the fundamental EC-SPM capabilities of the Keysight 7500 microscope.

Oxygen-Free and Controlled Environments

In many EC-SPM experiments, environmental control is very important, for instance, it is known that the amount of oxygen present in the experimental environment can considerably affect electrochemical processes. As a result, both the cell and the electrolyte in standard electrochemical experiments are deoxygenated by removing inert gases like Ar and N2, either before or during the course of the experiment.

The built-in environmental control system for the Keysight 7500 microscope can control humidity, temperature, liquid exchange, etc. (Figure 1). This level of control is handy for preparing and transporting samples that are often needed for battery analysis. The environmental control system of the 7500 microscope helps to achieve a clean and oxygen-free environment to guarantee appropriate electrochemical experiments. Through N2 purging, the level of oxygen can be decreased from ambient (~21%) to less than 1% in just 5 minutes.

Figure 1. Oxygen level control by glove box. CV of Au in 0.1M H2SO4 recorded after 1 hr in N2 (red) and 30 min after exposure to air (blue).

Copper Electrodeposition on Au

Electrodeposited copper is widely employed in microelectromechanical (MEMS) and microelectronics systems, mostly in chip metallization like interconnects, contacts, as well as in the filling of vias. These devices and processes are constantly pushed toward the nanoscale, so there is an increased need to perform additional studies on the conditions that impact the electrochemical plating process. Electrochemical SPM methods have helped to understand the mechanisms of growth and nucleation processes, and how these processes can be improved or prevented by adsorption and additives.

Here, the basic experiment investigates the impact of overpotential on the film properties and kinetics of the deposition of copper on epitaxial Au(111)/mica surface. 15 mM CuSO4 in 0.1 M H2SO4 was used as the electrolyte, and the potential was regulated against a quasi-reference electrode (Cu/Cu2+). Next, the electrode potential was pulsed for a period of 2 seconds to -0.1, -0.2, -0.3, and -0.4 V in a sequence. Following each pulse, the copper film that was electrochemically deposited was maintained at 0.0 V for AFM imaging for approximately 2 minutes and the same was removed at 0.4 V prior to the next pulse. The films deposited at each different overpotential value and imaged by contact AFM are shown in Figure 2.

Figure 2. Three-dimensional EC-AFM topography image of Cu films deposited at different overpotentials.

The speed of copper deposition highly relies on the applied overpotential, and this impacts the surface properties like the film roughness. Therefore EC-SPM can be used to assess the surface roughness and kinetics of film growth. For instance, it is possible to determine the thickness of the copper films that were deposited during the pulse experiment, from the averaged line profile depicted in Figure 3(A). Figure 3(B) shows the predicted surface roughness and thickness of individual films.

Figure 3. (A) Averaged topography line profile from the AFM image. (B) The estimated thickness and surface roughness of the Cu films deposited at different overpotentials.

Conclusion

EC-SPM is an ideal tool for direct and high-resolution analysis of physicochemical processes, and helps to understand the thermodynamics and kinetics of electrified interfaces at atomic and molecular levels. The use of electrochemical SPM will continue to affect and spur studies in many different disciplines, such as corrosion, energy, and life sciences.

This information has been sourced, reviewed and adapted from materials provided by Keysight Technologies.

For more information on this source, please visit Keysight Technologies.

Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Submit