Optical Probes and Atomic Force Microscopy

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Questions and Answers

What is the wavelength range of visible light used in optical methods?

300 – 400 nm

800 – 1000 nm

200 – 300 nm

400 – 800 nm (correct)

What does the numerical aperture (N.A.) of a lens indicate?

The maximum wavelength it can detect

The total light intensity emitted by the lens

The lens's ability to focus on large objects

The range of angles the lens can accept or emit light (correct)

In the formula for numerical aperture, N.A. = n sin(θ), what does 'n' represent?

The maximum resolution achievable

The refractive index of the medium (correct)

The distance between the lens and the sample

The angle of light incidence

What is the maximum numerical aperture achievable in modern optics?

1.4 Signup and view all the answers

Which of the following angles corresponds to a numerically calculated N.A. of approximately 0.122 when n = 1.0?

7° Signup and view all the answers

Which component is essential in a modern compound microscope to improve light quality?

Condenser Signup and view all the answers

What role does the half-angle θ play in determining the numerical aperture?

It affects the resolution of the microscope Signup and view all the answers

What is a characteristic feature of early compound microscopes compared to modern ones?

Lack of light sources Signup and view all the answers

What is a key advantage of Scanning Electron Microscopy (SEM)?

Good imaging resolution of a few tens of nanometers. Signup and view all the answers

What is the primary function of the cantilever in an Atomic Force Microscope (AFM)?

To maintain a constant height between the probe tip and the sample Signup and view all the answers

Which of the following is a disadvantage of using SEM?

Requires metal coating for non-conducting materials. Signup and view all the answers

How does the operation principle of Transmission Electron Microscopy (TEM) differ from that of SEM?

TEM captures images using transmitted electrons. Signup and view all the answers

Which operation mode of AFM is best suited for soft samples?

Tapping mode Signup and view all the answers

What is the typical accelerating voltage range for SEM?

0.5-30 kV Signup and view all the answers

How does the AFM detect deflection of the cantilever?

Using a photodiode detector that captures the reflected laser beam Signup and view all the answers

What is the typical size of the atomically sharpened probe tip used in AFM?

20-50 nm Signup and view all the answers

Which statement correctly describes TEM's resolution capability?

Can reach resolutions down to 1Å. Signup and view all the answers

Which measurement does the contact mode of AFM provide?

Quantitative data on particle and pore size Signup and view all the answers

What mechanism is used to maintain a constant distance between the probe tip and the sample in AFM?

Feedback control system Signup and view all the answers

In the context of AFM, what does high resolution provide?

Accurate topographic mapping of the sample Signup and view all the answers

What is primarily measured by the split photodiode in AFM?

Deflections of the cantilever Signup and view all the answers

What is necessary for a sample to be compatible with a scanning tunneling microscope (STM)?

The sample must be conductive Signup and view all the answers

What does the feedback loop in an STM primarily help control?

The distance between the tip and the sample Signup and view all the answers

How does STM achieve atomic scale resolution?

By minimizing the distance between the tip and the sample Signup and view all the answers

Which component of the STM is primarily responsible for measuring tunneling current?

Current amplifier Signup and view all the answers

What phenomenon allows STM to map the density of states (DoS) in solid-state materials?

Quantum tunneling Signup and view all the answers

What is one capability of STM beyond imaging surfaces?

Inducing chemical reactions Signup and view all the answers

What type of environmental condition is critical for the operation of an STM?

Ultra high vacuum Signup and view all the answers

In which directions can STM achieve atomic scale resolution?

In x, y, and z directions Signup and view all the answers

What primarily determines the brightness of elements in Scanning Electron Microscopy images?

The atomic number of the elements within the specimen Signup and view all the answers

What type of electrons are referred to as Backscattered Electrons (BSE)?

High-energy electrons that are reflected by specimen atoms Signup and view all the answers

What is the maximum penetration depth of the excitation in the specimen for backscattered electrons?

400 nm Signup and view all the answers

In Scanning Electron Microscopy, what effect is utilized to identify areas with different chemical compositions?

Elastic scattering effect Signup and view all the answers

Which statement correctly describes the interaction of backscattered electrons with specimen atoms?

They undergo both elastic and inelastic scattering Signup and view all the answers

Study Notes

Optical Probes

Visible light is the primary probe used in optical methods, with wavelengths ranging from 400 to 800 nm.
The numerical aperture (N.A.) of a lens signifies its ability to accept or emit light, impacting resolution; N.A. is defined as N.A. = n sin(θ), where n is the refractive index and θ is the half-angle of light entering the lens.
Modern optics can achieve N.A. values up to about 1.4.

Atomic Force Microscope (AFM)

AFM operates using the mechanical deflection of a cantilever, utilizing a sharp probe tip to assess sample surface contours.
The system maintains a fixed distance between the probe and sample via a feedback mechanism to ensure a constant force.
A laser beam reflects off the cantilever onto a photodiode to detect variations in the sample's topography.
Modes of AFM include:
- Contact mode: Continuous contact with the sample, suitable for hard surfaces; offers high resolution for quantitative topographic mapping.

Scanning Tunneling Microscope (STM)

STM uses an electronic signal to provide information about samples through tunneling current between the probe tip and the substrate.
Capable of atomic-scale resolution in three dimensions, STM can also map the density of states in solid materials.
Can induce and orient chemical reactions via the STM probe.

Scanning Electron Microscopy (SEM)

Utilizes backscattered electrons (BSE) for imaging, where high-energy electrons from the beam scatter off specimen atoms.
Heavy elements backscatter electrons more effectively than lighter elements, appearing brighter in the resultant images.
Contrast can be detected between areas with different chemical compositions; excitation depth is less than 400 nm.
Advantages: Works with diverse samples, offers excellent topology and depth of field, and achieves high imaging resolution (tens of nanometers).
Disadvantages: Requires a vacuum, limiting studies of living specimens or liquids; non-conducting materials often need metallic coatings to avoid charge accumulation.

Transmission Electron Microscope (TEM)

Functions similarly to SEM but captures images from electrons transmitted through the specimen, offering higher resolution down to 1 Å.
The accelerating voltage in TEM ranges from 40-1200 kV, which is significantly higher than the 0.5-30 kV used in SEM, promoting its superior resolution.
Awarded the Nobel Prize in Physics in 1986 to Ernst Ruska for contributions to electron microscopy.

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Description

Explore the principles of optical probes, focusing on visible light wavelengths and numerical aperture, essential for resolution in imaging techniques. Additionally, learn about Atomic Force Microscopy (AFM), its operational mechanisms, and various modes, particularly in topography mapping. This quiz encompasses the fundamentals crucial for understanding modern microscopy.