Optical Probes and Atomic Force Microscopy

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.

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.