Quiz about STM and AFM

An advantage of contact mode AFM compared to tapping mode AFM is:

• A reduction in lateral and normal forces being applied to the sample surface

• Faster scan speeds

• Can be used in ambient or liquid environments

• Better for biological specimens

Answer explanation

- In contact mode the applied forces increases

-They both can be used in ambient or liquid environments

- Contact mode destroy samples easier

- It is actually faster because of the continuous contact

Which statement is correct regarding the lateral resolution limit for AFM?

• The lateral resolution limit is determined by size of the tip

• The lateral resolution limit is determined by the wavelength of the laser

• There is no lateral resolution limit

• The lateral resolution limit is 0,1 microns

Answer explanation

- There must be some limit

- Laser wavelength would matter, if AFM would be based on optical imaging and not on interactions

- Since the tip size typically defines the smallest features that can be resolved, and tip size is usually 5-15 nm << 0,1 microns (100nm)

What has caused the issue with this image? The image is 5 microns x 5 microns.

• A worn tip

• Tip contamination

• Scanner non-linearity

• A double tip

Answer explanation

- Top left: tip contamination, blob-shaped artifacts

- Bottom left: worn tip, oddly shaped artifacts

- Bottom right: non-linearity

- Top right: double tip, which can cause overlapping signals and distortions in the captured data.

How can STM reveal the honeycomb structure and electronic properties of graphene?

• The exponential dependence of the tunneling current on distance enables atomic-resolution imaging, revealing the honeycomb structure

• STM can only measure the thermal properties of graphene, not its electronic states

• STM is not suitable for studying graphene because it is too thin to produce a reliable tunneling signal

• The STM tip can move individual carbon atoms, allowing the honeycomb structure to be reconstructed directly

Answer explanation

- The tunneling signal will be strong enough

- STM can image electron densities of states

- Unlike weakly bound atoms on a metal surface, the carbon atoms in graphene are tightly bonded to their neighbors making direct manipulation with an STM tip not a feasible method for reconstructing its structure

- STM's tunneling current has an exponential dependence on distance, allowing for atomic-resolution imaging. This capability reveals the honeycomb structure of graphene effectively.

What is meant by the term 'drift' in AFM?

• Creep in the voltage applied to the photodiode due to the feedback

• Change in the set-point with time during long scans

• Change in the position of the piezoelectric elements due to non-linearity

• Change in the relative position of the sample compared to the tip due to heating

Answer explanation

- Creep in the voltage... Can't be correct, because drift is typically a mechanical effect

- Change in the set-point...Incorrect because a drifting set-point could occur due to feedback loop instabilities, but this is not the standard definition of drift in AFM

- Change in the position... Incorrect because non-linearity is a separate issue related to the piezo response, not a slow, time-dependent movement like drift

- So it is a change in the relative position of the sample compared to the tip due to heating

What is meant by term 'creep' in AFM?

• Movement of the sample from heating during long scans

• Continued slow movement of the piezoelectric scanner after the initial fast movement following a large change in applied voltage

• Lateral stretching of the sample due to the forces applied by the tip during contact imaging

• Distortion of the image which occurs if the tip and sample have not been cooled to the same temperature before the experiment

Answer explanation

- Movement of... Incorrect because this describes thermal drift

- Tip and sample not in same temperature... Incorrect because this is also related to thermal drift

- Lateral stretching..Incorrect because this refers to sample deformation or mechanical compliance

- Piezoelectric materials exhibit creep, meaning they do not instantly stabilize after a voltage change. Instead, they continue to expand or contract slowly

Which of the following is NOT a type of a scanner artefact?

• Thermal noise

• Hysteresis

• Non-linearity

• Creep

Answer explanation

- Scanner nonlinearity occurs when the piezoelectric scanner does not expand or contract proportionally to the applied voltage

- Piezoelectric creep refers to the slow, time-dependent drift of the scanner due to the piezo material’s relaxation effects after a movement command.

- Piezoelectric hysteresis occurs when the scanner’s response depends on its previous movements, causing non-reproducible positioning and distortions

- Thermal noise originates from random fluctuations of electrons due to temperature but is not a scanner artefact. It is an inherent source of noise in electronic circuits

Which of the following statements regarding tapping mode AFM is NOT correct?

• Tapping mode decreases the forces applied to the sample surface compared to the contact mode

• The tip lightly taps or contacts the sample surface at the bottom of each oscillation

• The cantilever is oscillated at a frequency much greater than the cantilever's resonance frequency

• Tapping mode can be operated in ambient and liquid environments

Answer explanation

- The tip briefly interacts with the surface, which reduces lateral forces

- It is suitable for both air and liquid environments

- A feedback loop adjusts the Z-position of the scanner to maintain a constant oscillation amplitude, which is used as the imaging signal

- The incorrect statement is that the cantilever is oscillated at a frequency much greater than its resonance frequency. In tapping mode, the frequency is typically close to the resonance frequency to achieve optimal tapping.

Which of the following statements is incorrect?

• In tapping mode, changes in the amplitude of the cantilever, vibrating near or at its resonance frequency, are measured

• In contact mode, the force applied can be quite high and hence damage the sample

• Tapping mode has reduced lateral forces compared to contact mode

• In contact mode, the position of the laser on the cantilever moves during the scan to provide the best sensitivity

Answer explanation

The incorrect statement is about contact mode; the laser does not move on the cantilever during scanning. Instead, the laser is fixed at a specific point on the cantilever, and changes in the cantilever’s deflection are measured as it interacts with the surface.

When performing STS with an STM, what is measured, and what physical quantity can be derived from it?

• The tunneling current is measured at different temperatures -> material's heat capacity

• The direction of the tunneling current is measured in a magnetic field

  ->material’s topological properties

• The derivative of the tunneling current as a function of tip-sample distance

  ->electron mobility

• The derivative of the tunneling current as function of voltage is measured -> local density of states (LDOS).

Answer explanation

- The tunneling current...Incorrect because STM/STS does not measure heat capacity

- dI/dd... Incorrect because electron mobility describes how electrons move in response to an electric field

- magnetic field...Incorrect because STS does not measure the direction of current in a way that directly reveals topological properties

- In STS, dI/dV is directly proportional to local density of states at a given position