Keysight Technologies In situ Electrochemical Measurement Using 9500 AFM Application Note
Introduction EC-SPM essentially combines two independent techniques together: electrochemistry and SPM. The electrochemical unit includes a potentiosat and a 3-electrode cell that controls the electrochemical state of the working electrode (usually the sample). The SPM characterizes the surface of the solid electrode with either a passive probe or an active probe. With a passive probe in EC-AFM, the potential of the probe is not controlled. The AFM cantilever acts as an inert probe that monitors the topographic changes of the electrode surface caused by electrochemical processes, using standard AFM imaging modes. On the other hand, with an active probe in EC-STM, a bipotentiostat is used to control the potential of the sample and the probe vs. the same reference electrode. Like a regular STM, the tunneling current between the tip and the sample depends on the potential difference and the distance between the two, and is used as the control signal for STM imaging. With EC-STM, the morphological information of the electrode surface under potential control can be imaged in constant current mode, and the changes of localized electronic state of the electrode surface with electrochemical potential can be studied by current vs. voltage spectroscopy. Besides EC-AFM/STM, there is another EC SPM technique called scanning electrochemical microscopy (SECM). SECM combines fundamental microelectrochemical information via the entire palette of electroanalytical techniques with imaging modalities. The fundamental principle of scanning electrochemical microscope is to scan a biased ultra-microelectrode (UME) as an imaging probe across the sample surface, while recording Faradaic (redox) currents at the UME, and optionally, also at the sample. In contrast to EC-AFM and EC-STM, SECM is not limited to conductive or biased samples. In addition, due to thoroughly developed theoretical descriptions and models, SECM readily allows the quantification and prediction of experimental data. Since the introduction of SECM, a multitude of imaging modalities have been developed and experimentally demonstrated based on a wide variety of electrochemical analysis techniques including DC voltammetry, redox competition mode, and alternating current (AC)-SECM imaging. EC-SPM is capable of giving nanometer scale resolution in the study of electrode surface in liquid. The true merit of EC-SPM is, however, the capability to control the experimental conditions such as temperature, humidity, etc. to mimic the real world environment for the sample under test. This is particularly important for energy and corrosion researches. Instrumentation The Keysight 9500 AFM/SPM microscope is a high-performance instrument that delivers high resolution imaging and QuickScan capabilities with integrated environmental control functions. The standard Keysight 9500 AFM/SPM includes contact mode, acoustic AC mode, and phase imaging that comes with one universal scanner operating in both open-loop and closed-loop mode. Switching imaging modes with the Keysight 9500 AFM/SPM microscope is quick and convenient, a result of the scanner s interchangeable, easy-to-load nose cones. All 9500 AFMs come with low noise closed loop position detectors to provide the ultimate convenience and performance in imaging, without sacrificing resolution and image quality. The Keysight 9500 AFM/SPM is equipped with an optional bipotentiostat which enables stable electrochemistry control in either EC-AFM/EC-STM or SECM mode. The bipotentiostat offers a series of different sensitivity settings of 10 na/v, 100 na/v, 1 μa/v, 100 μa/v and 1mA/V covering a wide range of current from 10pA to 10mA. The system also has a specially designed liquid cell that allows one to use a mini-reference (Ag/AgCl) electrode (Figure 1C) for true potential control. For the tip current measurement in EC-STM and SECM modes, preamps of three different sensitivities are available for users to choose: 1nA/V, 10nA/V, and 100nA/V. A custom log-scale preamp is also available on special request.
03 Keysight In situ Electrochemical Measurement Using 9500 AFM - Application Note Examples of EC-SPM Measurement Electrochemical SPM has been applied to study a wide range of systems ranging from surface adsorption, film growth and dissolution, membrane transport, battery, fuel cell and photovoltaic to catalysis. The examples below only show the basic capability of EC-SPM with Keysight 9500 AFM/SPM. Oxygen Free and Controlled Environments Environmental control is critical for many EC-SPM experiments. For example, it has been long recognized that electrochemical processes are significantly affected by the amount of oxygen existing in the experimental environments. Therefore, the electrolyte and the cell in regular electrochemical experiments are often deoxygenated by purging inert gases such as N 2 and Ar before or during the experiment. Integrated environmental control system as shown in Figure 1A for the 9500 system has the capabilities of temperature control, humidity control, liquid exchange, etc., particularly the sample preparation and transportation which is often necessary for battery studies. With the help of environmental systems, clean and oxygen free conditions can be achieved to ensure proper electrochemical experiments. An optional EC glove box (Figure 1B) designed specifically for use with the 9500 features a dual-chamber design that allows samples to be prepared under environmental control (e.g., humidity, temperature, oxygen levels, reactive gases) in one area and then moved to an inner chamber directly underneath the microscope s head/scanner. (A) (B) (C) Figure 1. Keysight 9500 AFM with integrated environmental chamber (A), optional dual-chamber EC glove box for Keysight 9500 AFM system (B), and an EC-AFM cell features a mini Ag/AgCl reference electrode (C).
04 Keysight In situ Electrochemical Measurement Using 9500 AFM - Application Note Copper Electrodeposition on Au by EC-AFM and EC-STM Electrodeposited copper is used extensively in microelectronics and micro-electro-mechanical systems (MEMS), particularly in chip metallization such as contacts and interconnects and the filling of vias. As these processes and devices are continually driven toward the nanoscale, the need for more research into conditions affecting the electrochemical plating process arises. Electrochemical SPM techniques have been extremely beneficial in understanding the dynamics of nucleation and growth processes, and how additives and adsorption can inhibit or enhance these processes. The simple experiment here demonstrates how ECAFM can be used to study the effect of over potential on the kinetics and film properties of Cu deposition on epitaxial Au(111)/Mica surface. The electrolyte used was 15mM CuSO 4 in 0.1M H 2 SO 4 and the potential was controlled against a quasi-reference electrode (Cu/Cu 2+ ). Figure 2 shows the Cu islands deposited at preferred nucleation sites on Au substrate imaged by contact AFM. The electrode potential was held at -0.1V/(Cu/Cu 2+ ) for the image on the top. The image on the bottom reveals the alternating deposition of Cu in real time during the electrode potential sweep from -0.05V to 0.5V and then back to -0.05V, where the deposition of the Cu islands responds with high sensitivity to the applied potential. The electrodeposition of the first monolayer of a metal onto a foreign substrate frequently proceeds at potentials positive with respect to the equilibrium potential of the deposited metal. Such a reaction is then called underpotential deposition (UPD). The structure of the first monolayer has an impact on the deposition of the further layers and therefore on the morphology of the electrodeposited materials. For this reason, the mechanism of UPD and the structure of the underpotentially deposited monolayers are the subjects of intense investigations. The UPD of copper at gold is the example of the most exhaustive studies that employed numerous in situ and ex situ techniques. These studies have shown that copper adatoms coadsorb at Au electrodes with the anions of the electrolyte and that the anions influence the amount of deposited metal atoms and the ordering of the overlayer. Figure 3 shows the formation of a ( 3x 3)R30 o structure at 2/3 ML Cu coverage, which is a honeycomb copper superstructure stabilized by sulfate organized on top of Cu adatoms. This honeycomb structure exists over the entire potential range between the two voltammetric peaks as shown in Figure 3. Figure 2. (Top) EC-AFM topography image of Cu islands deposited on epitaxial Au(111)/Mica surface at -0.1V/(Cu/Cu 2+ ); (Bottom) Real-time deposition of Cu islands under electrode potential sweep in the range of -0.05/(Cu/Cu 2+ ) to 0.5/(Cu/ Cu 2+ ), with a scan size of 5 µm. Figure 3. High resolution EC-STM image revealing the overlayer structure formed by Cu UPD on Au(111) surface. This honeycomb structure exists over the entire potential range between the two voltammetric peaks on the CV.
05 Keysight In situ Electrochemical Measurement Using 9500 AFM - Application Note Electrochemical Imaging by SECM SECM imaging is commonly operated in constant-height mode, where the tip is held at a fixed position above the sample surface. The spatial resolution of SECM depends significantly on the size and geometry of the UME and the substrate-to-tip distance. One major drawback of constant height operation in SECM is the lack of sufficient spatial resolution due to current dependent positioning of the microelectrode and the convolution of topographical and electrochemical information. The probe offered by Keysight provides an innovative solution to this problem, which directly integrates a micro- or nanoelectrode into an AFM probe. exposed electrode non-conductive AFM tip metal insulation layer Figure 4. Microfabricated bifunctional AFM-SECM probe. The probe has a frame-shaped electrode recessed from the AFM tip that governs the distance between the electrode and the imaged surface. This integrated probe (the AFM-SECM probe) maintains the functionality of both AFM and SECM technique, by integrating a sub-microelectrode recessed from the end of the AFM tip. Consequently, the electrode is located at a pre-defined distance to the sample surface, determined by the height of the actual AFM tip (Figure 4). Thus, in-situ (electro) chemical information on a wide range of homogeneous or heterogeneous electron-transfer processes occurring at surfaces and interfaces can be simultaneously investigated with surface morphology. SECM imaging is often performed under the so-called feedback mode. Here, the probe and the sample are immersed in a solution containing a redox active species (e.g., R providing a reversible redox behavior governed by diffusion, R O + ne - ). If an appropriate potential is applied at the probe, R is oxidized to O, thereby resulting in a steady state Faradaic current, which is proportional to the radius, r of the UME and the concentration, c of the redox species following I = 4nFDcr (n = number of transferred electrons, F = Faraday constant, D diffusion constant of the redox active species). A typical CV of the AFM-SECM tip electrode in 1 mm FeMethanol + 0.1 M KCl solution is shown in Figure 5. Figure 5. CV recorded on the AFM-SECM tip electrode in 1 mm FeMethanol + 0.1 M KCl solution vs Ag/AgCl reference (note, some conventions show the current with opposite sign).
06 Keysight In situ Electrochemical Measurement Using 9500 AFM - Application Note As the biased probe (i.e., the UME) is moved toward a sample surface at a distance of several electrode radii, the faradaic current measured at the probe is increasingly affected by the sample surface properties. An insulating sample surface or surface feature leads to a diffusion limitation of electroactive species (here, R) towards the electrode, and hence, results in a reduced faradaic current vs. the steady state current. A conductive sample surface or surface feature results in a locally increased apparent concentration of the redox species, as the oxidized species can be regenerated at the sample surface (here by reduction), which leads to an instantaneously increased faradaic current in comparison to the steady state current. Figure 6 shows typical approach curves obtained on Au and Si using the AFM-SECM tip electrode in 1 mm FeMethanol + 0.1 M KCl solution. Figure 6. SECM approach curves obtained on Au and Si using the AFM-SECM tip electrode in 1 mm FeMethanol + 0.1 M KCl solution. Results from a simultaneously recorded contact mode AFM and feedback mode SECM experiment on a model sample are presented in Figure 7 to demonstrate the functionality of the combined AFM-SECM system. The model sample reveals conductive (gold stripes) and non-conductive (Si wafer) surfaces. This Au/Si sample was imaged in contact mode AFM in a 1 mm FeMethanol + 0.1 M KCl solution with the integrated AFM-SECM electrode biased at 220 mv vs. Ag/AgCl. The simultaneously recorded topography and current images are shown in Figure 7. The SECM image was obtained in the so-called feedback mode, while the sample was not biased during the experiment. Due to the feedback effect, as explained above, the SECM current is smaller on the insulating Si surface, and is larger on the conducting Au surface. Topography SECM current Figure 7. Topography (left) and SECM (right) images of an Au/Si sample recorded in 1 mm FeMethanol solution/0.1 M KCl with a combined AFM-SECM probe biased at 220mV vs. Ag/AgCl. Topography image shows the deposited Au strip on Si substrate, and the SECM image shows a corresponding larger current on the conductive Au surface.
07 Keysight In situ Electrochemical Measurement Using 9500 AFM - Application Note Summary EC-SPM techniques including EC-AFM/EC-STM and SECM have been proven to be indispensable tools for direct high-resolution studies of physicochemical processes, providing an atomic and molecular basis for understanding the kinetics and thermodynamics of electrified interfaces. Application of electrochemical SPM will continue to impact research in many disciplines, including energy, corrosion, and life science. AFM Instrumentation from Keysight Technologies Keysight Technologies offers high precision, modular AFM solutions for research, industry, and education. Exceptional worldwide support is provided by experienced application scientists and technical service personnel. Keysight s leading-edge R&D laboratories are dedicated to the timely introduction and optimization of innovative, easy-to-use AFM technologies. www.keysight.com/find/afm For more information on Keysight Technologies products, applications or services, please contact your local Keysight office. The complete list is available at: www.keysight.com/find/contactus Americas Canada (877) 894 4414 Brazil 55 11 3351 7010 Mexico 001 800 254 2440 United States (800) 829 4444 Asia Pacific Australia 1 800 629 485 China 800 810 0189 Hong Kong 800 938 693 India 1 800 11 2626 Japan 0120 (421) 345 Korea 080 769 0800 Malaysia 1 800 888 848 Singapore 1 800 375 8100 Taiwan 0800 047 866 Other AP Countries (65) 6375 8100 Europe & Middle East Austria 0800 001122 Belgium 0800 58580 Finland 0800 523252 France 0805 980333 Germany 0800 6270999 Ireland 1800 832700 Israel 1 809 343051 Italy 800 599100 Luxembourg +32 800 58580 Netherlands 0800 0233200 Russia 8800 5009286 Spain 800 000154 Sweden 0200 882255 Switzerland 0800 805353 Opt. 1 (DE) Opt. 2 (FR) Opt. 3 (IT) United Kingdom 0800 0260637 For other unlisted countries: www.keysight.com/find/contactus (BP-02-10-16) This information is subject to change without notice. Keysight Technologies, 2016 Published in USA, October 11, 2016 5992-1901EN www.keysight.com