Scanning Probe Microscopy Techniques

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Questions and Answers

Which advantage of AFM allows it to operate in various environments?

Flexible

Works in any environment (correct)

Can be used on practically any sample

Non-destructive

What is a significant disadvantage of AFM related to image acquisition?

Low-quality images at high speed

Requires a vacuum environment

Slow image capture process (correct)

Limited to conducting samples only

Which option describes a limitation of AFM regarding the samples it can analyze?

Requires conductive surfaces

Can only examine small biological samples

Can only analyze samples under vacuum

Scan area is often limited (correct)

What might cause poor or unreliable images when using AFM?

Damaged or contaminated tip Signup and view all the answers

What consideration must be taken into account to prevent image distortion in AFM?

Mitigating sample drift Signup and view all the answers

What physical property does a scanning tunneling microscope (STM) primarily measure to create images?

Electric current flowing between the probe and the surface Signup and view all the answers

Which component is responsible for precise positioning in a scanning probe microscope?

Piezoelectric actuators Signup and view all the answers

What does the term 'Fermi level' refer to in the context of STM?

The highest occupied crystal orbital in a metal at absolute zero Signup and view all the answers

How does the atomic force microscope (AFM) determine the force value during measurement?

By measuring the deflection of a cantilever due to forces between the tip and sample Signup and view all the answers

What role does the work function play in scanning tunneling microscopy?

It represents the energy needed to remove an electron from the metal Signup and view all the answers

Which interaction is primarily measured in both STM and AFM techniques?

Probe-surface interaction forces Signup and view all the answers

What happens when voltage is applied between the tip and the sample in an STM?

Electrons quantum tunnel between the probe and sample, causing current flow Signup and view all the answers

Which of the following statements about scanning probe techniques is true?

They provide images through the measurement of varying tip-sample interactions. Signup and view all the answers

What is the relationship between the cantilever spring constant and the force causing it to bend according to Hooke's Law?

The force is directly proportional to the displacement. Signup and view all the answers

What happens to the laser beam deflection when the interaction with the surface is stronger?

The laser beam deflection increases. Signup and view all the answers

What is the average radius range of a typical AFM tip?

2 - 50 nm Signup and view all the answers

How does the radius of the AFM tip affect the image obtained?

It convolutes the image of surface features. Signup and view all the answers

In contact mode of AFM, what is kept constant during imaging?

The deflection of the cantilever. Signup and view all the answers

What is a potential drawback of contact mode operation in AFM?

It can scratch softer samples. Signup and view all the answers

What characterizes the tapping mode of AFM?

The cantilever oscillates at its resonant frequency. Signup and view all the answers

What type of interaction shifts the cantilever resonant frequency lower in tapping mode?

Attractive interaction. Signup and view all the answers

What is the main advantage of non-contact mode in AFM?

Reduced contact damage to samples. Signup and view all the answers

How does Chemical Force Microscopy (CFM) differ from traditional AFM?

It functionalizes the tip to study chemical nature. Signup and view all the answers

What is the resulting size of a convoluted feature given by the equation $L = 4(Rr)^{1/2}$?

Always larger than the actual feature size. Signup and view all the answers

What is a key disadvantage of using larger tip radii in AFM imaging?

Worsened lateral resolution. Signup and view all the answers

How is the piezo voltage used in AFM?

To maintain constant deflection or amplitude. Signup and view all the answers

What benefit does phase shift provide in tapping mode AFM?

Gives chemical contrast with topography. Signup and view all the answers

What is needed for electrons to flow between the tip and sample?

They must overcome the potential difference created by the voltage. Signup and view all the answers

What effect does a smaller gap distance have on tunneling current?

It decreases wave function decay. Signup and view all the answers

In scanning tunneling microscopy (STM), what happens as the tunneling current decays?

The tip is repositioned via a feedback control. Signup and view all the answers

What is the purpose of applying a bias voltage in the STM?

To alter the Fermi levels of the tip and the sample. Signup and view all the answers

What does the feedback loop in STM primarily ensure?

Constant tunneling current despite surface fluctuations. Signup and view all the answers

Which component of the STM is key for maintaining the height of the tip?

The constructed piezoelectric crystal. Signup and view all the answers

What enables electrons to tunnel across barriers in quantum mechanics?

Electrons behave like waves. Signup and view all the answers

In STM, which part of the conduction band do electrons come from?

Highest occupied molecular orbital. Signup and view all the answers

How does the atomic force microscope (AFM) measure the tip-sample interaction?

By recording deflections of a cantilever. Signup and view all the answers

What defines the atomic resolution capability in both STM and AFM?

The sharpness of the tip used. Signup and view all the answers

What must be ensured to maintain a constant tunneling current?

The tip constantly adjusts its position. Signup and view all the answers

Which of the following is a basic principle applicable to all scanning probe microscopy techniques?

A scanner is necessary for precise positioning. Signup and view all the answers

What is the role of the applied bias in relation to electron tunneling in STM?

It adjusts the Fermi levels to allow tunneling. Signup and view all the answers

Why is the atomic resolution in AFM considered significant?

It provides surface detail at an atomic level. Signup and view all the answers

Study Notes

Scanning Probe Microscopy (SPM) Techniques

SPMs image surfaces by measuring the interaction between a probe and the surface.
The changing properties of the probe as it's scanned are measured to infer the nature of this interaction.
This allows for image generation, similar to confocal or STED microscopy, line by line.

Scanning Tunneling Microscopy (STM)

STM measures the electric current flowing between a probe tip and the sample surface.
Electrons quantum tunnel between the tip and sample.

Key Components of STM

Tip: Connected to a control unit/computer to measure and record current, and control tip position.
Sample: A voltage (bias) is applied to induce current flow.
Piezoelectric Actuators: Precisely position the tip or sample.

STM Operating Principle

Fermi Level (EF): Highest occupied crystal orbital in a metal; equivalent to HOMO in molecules.
Work Function (Φ): Energy needed to remove the least bound electron from a metal and place it outside the metal.
Tip-Sample Interaction: Closing the tip and sample equalizes Fermi levels. Electrons tunnel across the vacuum barrier to create current flow.
Applied Bias (Voltage): Shifting the relative Fermi levels of tip and sample with bias V, creating a current flow (electrons tunnel for energies above the tip's Fermi level).
Tunneling Current: Controlled by gap distance; smaller gap increases overlap and tunneling probability, leading to a larger current that decays exponentially with increasing distance.
STM Tip: Atomic-sized blob at the end, often ending in a single atom, enabling atomic resolution.

Feedback Loop in STM

Ensures constant proximity to the sample (exponentially decaying current with distance).
Constant Current Feedback Loop: Piezoelectric crystal adjusts tip height to maintain constant current when scanning.
Topography (height map) is generated by piezo voltage variations across the position.

Imaging Molecular Orbitals with STM

Tunneling electrons can originate from HOMO or LUMO (lowest unoccupied molecular orbital).
Specific biases can image unique levels.

Atomic Force Microscopy (AFM)

AFM uses a sharp tip on a cantilever to measure tip-sample interactions, causing cantilever deflection.
Deflection is measured by reflecting a laser beam off the cantilever, converting it to a force value.
Atomic resolution is possible.

AFM Operating Principle

Cantilever Deflection: Sample-tip forces cause predictable cantilever bending, which is measured.
Force Measurement: Hooke's Law (F = -kx) relates force to cantilever deflection. Attractive/repulsive interactions result in corresponding deflection variations.

AFM Tip

Typically sharp (2-50nm radius) for accurate interaction with surface features.
The tip radius can affect the image (convolution).
Larger radii blur the image, and require careful calibration/deconvolution of results.

AFM Modes:
Contact Mode: Constant cantilever deflection is maintained as the tip makes contact.
Tapping Mode: Oscillating cantilever tip at resonant frequency intermittently contacts the surface, maintaining constant amplitude. (Superior for fragile samples)
Non-Contact Mode: Oscillating cantilever tip maintains distance, minimizing sample damage and highlighting surface attractive interactions.

Chemical Force Microscopy (CFM)

A modification of AFM where the tip is functionalized to study specific chemical interactions (beyond van der Waals).
Imaging chemical properties and properties (contrast) of the sample.

Advantages of AFM

Adaptable to diverse samples (e.g. conductive, non-conductive).
Works in various environments.
Non-destructive.
Low cost, versatile and small (equipment-wise)

Disadvantages of AFM

Slower imaging speed.
Limited scan area and height range.
Prone to tip damage and sample drift.

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Description

Explore the principles and key components of scanning probe microscopy, particularly focusing on scanning tunneling microscopy (STM). This quiz covers how STM operates, including the roles of the probe tip, sample, and piezoelectric actuators, as well as the underlying concepts such as Fermi level and work function.