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Scanning Probe Microscopy
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Scanning Probe Microscopy
Lecture 5
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1
Question
What is Scanning Probe Microscopy (SPM) and what is its main principle?
Answer
SPM techniques use a sharp physical probe to interact with the surface of a sample, employing near-field tip-sample interactions to overcome diffraction limits in resolution, achieving resolutions of 0.1 nm or better, and enabling direct surface topography measurements including the third dimension.
2
Question
How does SPM compare to Optical Microscopy and Scanning Electron Microscopy (SEM) regarding lateral and vertical resolution?
Answer
SPM achieves lateral resolutions between 0.1 - 3.0 nm and vertical resolution as fine as 0.01 nm, significantly better than Optical Microscopy (lateral ~1 μm, no vertical resolution) and surpassing SEM (lateral 1-10 nm, no vertical resolution).
3
Question
Describe operation for Scanning Tunneling Microscopy (STM) and the formula for tunneling current I.
Answer
relies on tunneling current measurements to accurately map the surface topography of materials with atomic scale resolution. V bias: Bias voltage applied between the tip and the sample. e: The base of the natural logarithm (Euler’s number, approximately 2.718). C: A constant related to the properties of the barrier d: The tip-sample distance (gap between the probe tip and the sample surface).
4
Question
What are the main operating modes of STM?
Answer
5
Question
Explain the cause of image distortions in STM and how tip sharpness affects resolution.
Answer
STM image distortions arise due to the finite shape and structure of the tip, which convolutes with the sample surface topography. A less sharp or blunt tip results in a poorer resolution and distorted images because the tunneling current averages over multiple atoms instead of a single atomic site.
6
Question
What kind of vertical image contrast distortions can occur in STM, and what causes them?
Answer
Vertical image distortions in STM are caused by the interplay of local density of states (LDOS) and the variable tunneling gap distance, influenced by the sample height and bias voltage. This can cause features to appear either as bright raised or dark sunken areas depending on these electronic and geometric effects.
7
Question
What types of interaction forces are typically involved between the AFM tip and the sample in air?
Answer
8
Question
Draw the force vs. distance curve in AFM tip-sample interactions and explain the significance of repulsive and attractive regions.
Answer
starts with strong repulsive forces at very short distances due to atomic contact, then rapidly drops to an attractive negative force region as the tip approaches but does not touch the surface
9
Question
What is dynamic AFM mode operation (no interaction)?
Answer
spring constant k, effective mass m
10
Question
What is dynamic AFM mode operation (interactions)?
Answer
11
Question
How does frequency modulation differ from amplitude modulation in dynamic AFM mode operation?
Answer
Frequency modulation tracks changes in the oscillation frequency (Δω) and maintains it at a setpoint, while amplitude modulation tracks changes in amplitude (ΔA) of the oscillation.
12
Question
In AFM dynamic mode, how do attractive and repulsive interactions affect the frequency response curve?
Answer
Attractive interactions shift the frequency response curve to lower frequencies (to the left), while repulsive interactions shift it to higher frequencies (to the right).
13
Question
Radius of different tip shapes ;)
Answer
14
Question
Describe the importance of different AFM tip shapes shown in SEM images (sharp pyramidal, V-shaped, spherical, needle-like).
Answer
Sharp pyramidal or needle-like tips provide higher spatial resolution, while spherical tips may be used for gentler interactions or specific measurement types. V-shaped cantilevers affect bending properties.
15
Question
What is the significance of the angle θ in conical AFM tips?
Answer
The angle θ defines the sharpness of the conical tip; a smaller θ indicates a sharper tip, which can better resolve fine details of the surface features during scanning.
16
Question
How to find tip radius from AFM data and identify incorrect slope
Answer
17
Question
In a PID feedback control system for AFM, what do the terms rise time, peak time, maximum overshoot, and settling time describe?
Answer
- Rise time: Time to go from 10% to 90% of the target output. - Peak time: Time to reach the first maximum overshoot. - Maximum overshoot: The highest amount the output exceeds the target. - Settling time: Time to remain within ±1% of the target value without further deviation.
18
Question
What effects does increasing the integral gain \( K_I \) have on the closed-loop response in AFM control?
Answer
19
Question
What does the graph of 'Height (nm)' vs 'Distance (nm)' obtained from AFM scans represent?
Answer
It shows the surface height profile of the scanned region, displaying variations in height as the AFM tip moves across the sample, including peaks and valleys corresponding to surface features.
20
Question
Why is slope correction necessary in AFM data analysis?
Answer
Slope correction removes artifacts or tilt in the measured height data caused by sample mounting or scanning inconsistencies, allowing accurate interpretation of true surface topography.