Scanning Probe Microscopy
Solid probe scanning the surface to provide images. The different types are :
- scanning tunneling microscope (STM)
- atomic force microscope (AFM)
Scanning tunneling microscope (STM)
- Binnig and Rohrer were awarded the Nobel Prize in Physics in 1986 (IBM, Zurich) for STM, Ernst Ruska for electron microscopy
- STM uses a tunneling current, a phenomenon of quantum mechanics, to examine material surfaces.
- The tunneling current flows through an atomic-scale gap between a sharp metallic tip and conducting surface atoms.
Tunneling – a quantum mechanical phenomena where the particle tunnels through thin barrier
Tunneling - quantum-mechanical effect where a particle crosses through a classically-forbidden potential energy barrier…flow of electrons result in current.
Types of connections:
- sample is biased -ve with respect to the tip, then electrons will flow from the surface to the tip
- sample is biased +ve with respect to the tip, then electrons will flow from the tip to the surface
Imaging in STM
The image is
bright - Tip to atom distance low meaning More tunneling current
dark - Tip to atom distance High meaning Less tunneling current
STM tip made up of tungsten for conductivity purpose
Need to operate at vacuum (possible in air, liquids),
Useful for only conducting samples like metals
Tunneling current is a function of tip position, applied voltage, and the local density of states of the sample
Modes of STM operation
There are two modes of operation. Imaging of the surface topology may then be carried out in one of two ways:
constant height mode - in which the tunnelling current is monitored as the tip is scanned parallel to the surface. A plot of the tunnelling current v/s tip position therefore shows a periodic variation which matches that of the surface structure hence it provides a direct "image" of the surface
constant current mode - in which the tunnelling current is maintained constant as the tip is scanned across the surface. The image is then formed by plotting the tip height (strictly, the voltage applied to the piezo) v/s the lateral tip position. – Preferred to avoid tip damage
Atomic Force Microscopy (AFM)
AFM detects near-field forces between the tip and sample
Operating principle of AFM
Sample is placed on the scanner. The cantilever with the tip is positioned over the sample using microscopic positioning system. Laser light is incident on the cantilever which reflects and is detected by a photo diode which records the position of the laser beam. Which changes as the position of the cantilever changes.
Contact mode The tip is in direct contact with the sample.
Intermittent contact mode Also known as tapping mode, the cantilever is forced to vibrate a little below its resonant frequency and moved towards the surface ….cantilever oscillation amplitude is reduced due to energy loss caused by the tip contacting the surface
- Noncontact mode The cantilever is forced to vibrate a little above its resonant frequency high above the sample surface
Advantages of AFM
Can also be used in imaging non conductive material
Disadvantages of AFM
Difficult to access sidewalls, corners -relatively slow
Confocal Microscopy (Or) Laser scanning Confocal microscopy (LSCM)
An optical imaging technique used to increase the optical resolution and contrast by using a point illumination and a spatial pinhole to eliminate out – of focus light
Confocal vs optical Microscopy -
laser light source
scanning device has better resolution of ~ 200 nm
provides three-dimensional (3D) optical resolution
- Laser beam is focused as an intense spot on a certain focal plane of the specimen by a condenser lens, which is also serves as an objective lens to collect the reflected beam.
- A pinhole aperture is placed at a confocal plane in front of the light detector.
- The reflected beam from the focal plane in a specimen becomes a focused point at the confocal plane.
- The pinhole aperture blocks the reflected light from the out-of-focal plane from entering the detector.
WHY LASER? Since the pinhole aperture can block a large amount of reflected light, high-intensity laser illumination is necessary to ensure that sufficient signals are received by the detector. The detector is commonly a photomultiplier tube (PMT) that converts light signals to electric signals for image processing in a computer.
Raster scanning – to acquire the image in 3D… After finishing one scanning plane, the focal spot is moved in the vertical direction to scan a next parallel plane