Chapter 8 - Scanning Tunneling Microscopies and their applications for material characterization Flashcards
What are the main principles of STM and EC-STM?
STM: small tip is probed over a conducting surface. A bias is applied, and tunneling of electrons occurs between tip and surface. Measure current. STM tip is controlled by piezoelement.
EC-STM: Electrochemical STM. We apply a bias over the working electrode and the STM tip. Still measure the tunneling current. We control the potential of the working electrode vs the reference electrode using a potentiostat.
What are the typical modes of operation of STM? Analyze advantages and disadvantages of each of them.
Constant current mode and constant height mode.
Constant current: height always adjusted
+ absolute height is available
- low scan speeds
Constant height: \+ fast scan speeds - absolute height not available - not applicable for rough surfaces - technologically difficult to keep a constant height within required position
What are the main technical distinctions between the “normal” STM and STM experiments performed in the presence of liquids under potential control.
In an EC-STM, there is a three electrode set-up. THe substrate works as a working electrode, then there is counter and reference electrodes immersed in the liquid. These are connected through a potentiostat, and allows us to adjust the working electrode potential with respect to the reference electrode, while the current is allowed to flow between the working electrode and the counter electrode.
What is the power of the video-rate scanning probe microscopies? Analyze the pros and cons considering the use of STM and AFM.
The power of the video-rate scanning probe microscopies is that it allows us to study the development of e.g. adsorbate process on a surface.
STM:
+ better resolution
- for heterogenous compounds we get no topological information, and the images can be hard to interpret
- looking at electronic density of states, not the atoms themselves
AFM:
+ gives topological information
- lower resolution
Explain what the differences between the Miller indices approach and the terrace-edge approach in describing single crystal surfaces.
In the terrace-edge approach we use the most basic Miller index-planes, that is (111) and (100) (sometimes also (110)) to describe any number of more complex planes, such as fcc(13,1,1).
We do this by looking at the terraces formed, and counting how many atoms they form until the next edge, n, and then looking at what type of surface we see. Then we see what kind of crystal the step is. If the terrace consists of (100)-planes and has a length of 7, and the edges of (111) planes, we can then denote this plane [7(1 0 0)x(1 1 1)].
If there is a kink in the edge, we can also denote this by putting this in the middle. What kind of plane is the kink, and how many atoms does it consist of?
Why do we need to visualize the surface with atomic resolution?
There are many things we would want to investigate using a STM.
- Investigation and control of 2D/3D atomic layer assembling.
- Monitoring of adsorbate dynamics and surface phase transition.
- Detection of ordered adlayers and identification of their structures.
- Investigation of catalytic activity.
In one example of a STM image, we see a monoatomic step taken in an electrolyte at a Ag-surface. It looks distorted. Why?
Because the Ag in that particular case were very mobile at the steps.
When talking about catalysts, what are some things that are critical to understand?
What the contribution of the different facets to the overall reaction rate is. And also how the electrolyte constituents influence the target electrocatalytic reaction.
How can changing the potential between STM tip and surface tell us something about the dynamics of adsorbates?
We can see which potentials the adsorbate reaction occurs at.
What are important parameters which control the tunneling current in an STM?
The higher bias applied, the higher I.
The higher the tunneling barrier, the lower I.
The broader the barrier, the lower I.
What determines the barrier height and width?
The distance between the sample and tip.
The properties of the medium between the sample and tip. The bias between the sample and tip.
How is the STM tip moved?
It is connected to a piezoelement, that expands and contracts with the appropriate voltages, to precisely control the tip.
How are we able to get high resolution STM images?
Because the tip at the end has one atom that carries the majority of the current. This is due to the strong distance dependence of the tunnel effect.
How should one normally prepare a STM tip?
In many cases, electrochemical etching is the best way to go. Here the etching rate is higher at the meniscus, and thus we get a nice clean cut.
What are normally the tips for EC-STM made of?
W or Pt-Ir.
