Low Temperature Atomic Force Microscopes
Updated: Apr 26
Binning et al. designed the atomic force microscope (AFM) in 1986. The forces operating between a fine tip and a sample are measured by the AFM. The tip of a cantilever is attached to the free end and brought extremely close to a surface. Positive or negative bending of the cantilever is caused by attractive or repulsive forces coming from interactions between the tip and the surface. A laser beam is reflected from the backside of the cantilever and used to detect the bending.
Introduction about Low Temperature Atomic Force Microscopes
Scanning Probe Microscopes (SPMs) are notable for their capacity to observe features at the atomic and molecular level, allowing us to get a better knowledge of how systems operate and open up new avenues for research in a variety of industries. Life science, materials science, electrochemistry, polymer science, biophysics, nanotechnology, biotechnology, and a variety of other fields are among them.
Atomic force microscopy is now used to study metal semiconductors, soft biological samples, conductive and non-conductive materials in a variety of settings (air, liquid, vacuum).
This technology may be used to measure the size of nano-objects or even manipulate them.
The sample preparation methods for AFM are generally straightforward and easy, and there is no need for staining and/or coating. There have been few reports of freeze–fracture surface observations, despite the fact that several research groups have reported imaging of biological materials in air or fluid. This is most likely due to the fact that there are few AFMs suited for biological material, i.e., those that can work below freezing without forming considerable ice.
Various Techniques for Developing Low Temperature
Due to the ability to image insulating surfaces, atomic force microscopy has become an essential technique for researching surfaces at the atomic scale in real space. Many researchers have built low temperature equipment to improve the AFM's stability. Although the construction of such devices is difficult, key benefits such as reduced piezo hysteresis and creep, reduced thermal drift, and lower noise levels make them attractive.
The microscope is constructed in the shape of a copper cylinder with a diameter of just 3 cm and a height of 10 cm in this way. It has two shields to deflect heat radiation and keep the sample and tip clean during cryopumping. A commercial flow cryostat is linked to the shields (type APD Cryogenics LT-3B).
The cryostat uses thermal contact to cool the microscope and can be used with liquid nitrogen or liquid helium. As measured in close proximity to the sample, temperatures as low as 7 K can be obtained. The AFM may be stabilized at temperatures between 7 K and room temperature by simultaneously heating the cryostat.
It's a difficult effort to develop an STM that can attain atomic resolution while working at extremely low temperatures. Minimal-temperature operation provides low thermal drift and low thermal noise, both of which are necessary for high-resolution measurements. On the other hand, mechanical vibrations introduced by extremely low temperature refrigeration systems typically stymie efforts to get high-resolution readings. Furthermore, the physical space within the cryostat is frequently insufficient to allow an efficient cryogenic vibration-isolation stage, especially when a high magnetic field is required.
LN2 Method (Cold Finger Concept)
In this experiment, a commercial AFM was modified, such as an AFM by NanoMagnetics Instruments. For the purpose of this experiment. The laser diode, four-quadrant photodiode, and preamplifier are all included in the AFM head and are all situated inside the vacuum chamber. The optical beam bounce method is used to detect the cantilever's deflection.
Before evacuation, the laser is positioned. During observation, the head and hence the location of the cantilever are kept constant. The scanner is a tube piezo with a scan range of 80-380 m35 m. The XYZ coarse alignment stage is where the scanner is attached. This stage may be modified from outside the vacuum chamber to position the sample under the tip and to find a feature of interest other than the freeze–fractured material.
The sample stage can then be cooled using a heat conductor made of ultra pure copper foils, with one end attached to the sample stage and the other to the cold finger. Copper foils are used to not only transport heat but also to decouple vibrations caused by LN2 boiling. The copper foils are also used for coarse stage movements and scanning with the piezo scanner. A heating resistor can be used to heat the sample stage from its terminal temperature of -175–0 °C. The heating resistor can be used to bring the sample to room temperature. A platinum thin film thermistor can be used to monitor the temperature of the sample stage.
A commercially available proportional integral differential temperature controller that adjusts the power wasted in the resistor can control the temperature of the sample holder in the range of -175 to 0 °C. The temperature adjustment is accurate to 0.1 °C. An epoxy pillar heat shield separates the sample holder from the piezo tube in terms of temperature.
The sample holder is encased in a gold-coated copper shroud. The copper shroud's end is connected to the LN2 cooled cold finger. During cooling, the temperature of the shroud is held at -150 °C.