Scanning Tunneling microscopes (STM) and their application for material characterisation 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. Here the sample is in a solution.
Why do we need to visualize the surface with atomic resolution.
Get a “conductivity map” of the surface. Indicates the local electronic density of states. Important for understanding the materials chemical reactivities and their facets.
Need to know the contributions of different facets to the reaction rate. Where the reaction takes place, for electrocatalytic reaction. To see the stepped surfaces. How the electron properties look like on the surface, are there some domains, defects, periodic structures.
- 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.
What are the typical modes of operation of STM? Analyze advantages and disadvantages of each of them.
Two modes: Constant current mode and constant height mode.
Constant current: height always adjusted
+ absolute height is available
- low scan speed
Constant height: between 0.3-1nm (measure tunneling current)
+ fast scan speeds
- absolute height not available
- not applicable for rough surfaces
- technologically difficult to keep a constant height within required position (0.001nm)
What are the main technical distinctions between the “normal” STM and STM experiments performed in the presence of liquids under potential control (EC-STM)?
In an EC-STM, there is a three electrode set-up. The substrate works as a working electrode, then there is a counter and reference electrode 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 WE and the CE. In situ monitoring of redox processes on the sample of the surface electrode.
What is the power of the video-rate scanning probe microscopies?
The power of the video-rate scanning probe microscopies is that it allows us to study the development of e.g. absorbate process on surface. (Cl is absorbed on Cu surface).
Analyze the pros and cons considering the use of STM and AFM.
STM:
+ Better resolution
+ faster /can scan in almost real-time
- 2D image for heterogenous compounds (because the atoms have different electron clouds) we get no topological information (x, y dimention), the image can be hard to interpret
- looking at the electronic density of states, not actually the atoms themselves
AFM: \+ 3D image gives topological information (x, y, z dimentions) \+ sample don’t have to be conducting - lower resolution - takes longer time to scan
Explain what are the differences between the Miller indexes approach and the terrace-edge approach in describing single crystal surfaces.
We do this by looking at the terraces formed and counting how many atoms in the terrace 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 edge of (111) planes, we can them denote this single crystal surface as [7(111)x(100)]
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. At the step the atoms are more loosly bonded than at the terrace
When talking about catalysts, what are some things that are critical to understand?
What the contribution of the different facets to the overall reaction 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 barrier, the lower I.
The broader the barrier, the smaller 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 voltage, to precisely control the tip. (x,y direction)
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 tunneling 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, and hopefully 1 atom at the tip.
