Scanning probe microscopy is a class of mapping techniques based on the interaction between a nanosized probe and a surface of interest. Among these are scanning tunneling microscopy and atomic force microscopy, the most well-known and among the first developed (in 1981 and 1986, respectively). As there are many extensive textbooks on scanning probe techniques, we will only briefly provide some background information on atomic force microscopy, used in this work, and its relevance in solid state electrochemical systems.
2.4.1 Atomic Force Microscopy
Atomic force microscopy (AFM) relies on the interaction force between a nanoscale probe and a surface. In the simplest terms, it operates by dragging a sharp tip across a surface while monitoring the tip’s lateral and vertical position; if the size of the tip is smaller than the features on the surface, then one can obtain a well-resolved image of the topography of that surface. AFM tips today can be fabricated with radii below 5 nm, and one need not rely on mechanical contact between tip and sample in order to obtain an image. When a probe is brought close to the sample surface, the atoms on the very tip of the probe and atoms at the sample’s surface exhibit various interaction forces depending on their separation distance (Figure 2.9a). These forces include but are not limited to van der Waals, capillary, electrostatic, and chemical forces and are frequently exploited for mapping surface properties.
AFM tips/probes are usually made from silicon or silicon nitride and vary in shape and size. A probe is usually comprised of a finely etched tip which extends from the apex of a pyramid/cone (Figure 2.9b); the latter, in turn is supported on a beam, commonly a rectangular or triangular cantilever which extends from a silicon chip (Figure 2.9c). The tip radius, cantilever dimensions, and material properties are all design parameters which dictate the imaging capabilities of the probe. Generally, the tip radius controls the image resolution, and the beam characteristics give the spring constant of the probe and therefore the range of forces that can be probed during measurement. The interaction force between the tip and sample, F, is related to the cantilever characteristics via the relation
cantilever
F k x (2.14)
where kcantiliver is the spring constant of the probe and Δx is the deflection of the cantilever. Probes with spring constants ranging from 0.01 to 40 N m–1 are available commercially; the choice of probe depends on the application and imaging mode of interest.
Figure 2.9 AFM probes: (a) interaction of tip and surface atoms, (b) SEM micrograph42 of a cantilever and (c) a sketch of a probe.
Although there are several different operating modes in AFM, we discuss only contact mode operation which is used in this study. For a more extensive review, the reader is referred to the textbook by Bonnell.43 Contact mode imaging is typically carried out by maintaining a constant force between the sample and probe by moving either the sample or probe to control the separation distance between the two. The instrumentation required for an atomic force microscope is relatively simple compared to other high resolution imaging techniques such as electron microscopy (Figure 2.10). As the probe is rastered across the sample surface via a piezoelectric scanner, the deflection of the
tip
sample surface
force
(a)
(c)
Si-based chip
cantilever 10 μm
(b)
cantilever tip
cantilever is monitored by bouncing a laser signal off the top (reflective) surface of the cantilever. The reflected laser signal is measured by a four-quadrant photodiode detector, the signal from which is fed into the scanner to control the height of the sample to maintain a constant tip-sample separation/force. (Although the sample is being moved by the scanner in Figure 2.10, it is equally common to have the probe attached to the scanner and moved across an immobile sample.) The x-y-z movement of the piezoelectric is recorded to yield the final topography image. In order to perform a typical contact mode measurement, a desired force (or cantilever deflection) is specified by the user. Probes used for contact mode measurements are relatively compliant, with spring constants typically 0.1–1 N m–1 to minimize damage to the sample and tip.
Figure 2.10 Operating principle of atomic force microscopy and basic components.
sample detection &
feedback controller
scanner (xyz control)
cantilever/probe
2.4.2 Electrical Atomic Force Microscopy
Electrical (or conducting) AFM is a technique in which a metal-coated AFM probe is used rather than an electrically insulated one. For electrical AFM, the system typically is operated in contact mode so that the probe is in physical contact with the sample surface. Given the proper instrumentation, the spatial variation of both topography and current can be recorded as the tip is rastered across a surface, and the local currents can be correlated to properties of the sample.
In this work, we extend such a technique to study materials which are ionic conductors. In this case, electrochemical reactions must occur at the probe | sample contact in order for current to pass. As such, the metal coating on the probe is one which is catalytically active, and the ionic conductor is fabricated with an active electrode at its underside to complete the circuit. Compared to conventional electrical AFM, measured currents can be significantly lower because the current is limited not only by the size of the probe but also the electrochemical activity of the probe | sample interface. Thus, a challenge of electrochemical AFM is its high system impedance, and thus great care must be taken to minimize spurious contributions to the electrochemical response. This includes reducing electrical and mechanical noise that causes fluctuations in the measured current and accounting for effects of stray capacitance.
Electrical AFM provides many advantages for exploring solid state electrochemical systems. The electrochemical activity of an ionic material at the nanoscale and its spatial heterogeneity can be readily explored. Not only this, AFM studies do not require vacuum, and, therefore, many materials can be examined under appropriate atmospheres and at elevated temperatures. Many metals/catalysts can be
examined readily by simply changing the coating on the AFM probe, and current collection from the nanoscale probe, by design of the microscope, is automatic. Finally, as discussed in the Section 2.5.2 below, the small size of the probe enables electrochemical studies at the metal | solid electrolyte interface, under bias, without contributions from the second electrode (at the opposing side of the electrolyte).