SCANNING PROBE MICROSCOPY .

SCANNING PROBE MICROSCOPY.BASIC PRINCIPLES ,SCANNING TUNNELING MICROSCOPY .ATOMIC FORCE MICROSCOPY.

1. THE INSTITUE OF SCIENCE . TOPIC : Scanning probe microscopy. -sana shaikh m.sc(physical chemistry) Paper-2.

2. Scanning probe microscopy (SPM), First scanning probe microscope invented in 1981 by Binning and Roher ,for which they received the Nobel prize. The family of SPM uses no lenses ,but rather a probe that interacts with the sample surface. Simple design. Low cost . Easy to handle Automatically resolves images

3. Most used SPM technique.

4. Basic principles of SPM.

5. SPM. For the techniques to provide information on the surface structure at the atomic level: the position of the tip with respect to the surface must be very accurately controlled (to within about 0.1 Å) by moving either the surface or the tip. It is possible to accurately control the relative positions of tip and surface by ensuring good vibrational isolation of the microscope and using sensitive piezoelectric positioning devices. the tip must be very sharp - ideally terminating in just a single atom at its closest point of approach to the surface.

6. STM. It has metal needle that scans sample by moving back and forth over it, gathering information about the curvature of the surface. STM uses sharp conducting tip with bias voltage applied between the tip and the sample. When the tip is brought within about 10A of the sample ,electrons from the sample being to “tunnel” through the 10 A gap into the tip or vice versa depending upon the sign of the bias voltage. The resulting current varies with the tip-to-sample spacing and it is the signal used to create an STM image. STM is designed to scan a sample in either of two modes Constant –height mode Constant –current mode.

7. AFM. The AFM probes the surface of sample with a sharp tip, a couple of microns long often less than 100A in diameter .the tip is located at the free end of the cantilever that is 100 to 200 Um long. Force between tip and the sample surface causes the cantilever to bend or deflect. A detector measures the cantilever deflection allow a computer to generate a map of surface topography . Can be used to study insulators and semi conductors as well as electrical conductors.

8. Advantages . The resolution of the microscopes is not limited by diffraction, only by the size of the probe-sample interaction volume . Hence the ability to measure small local differences in object height (like that of 135 picometres steps on <100> silicon) is unparalleled. Laterally the probe-sample interaction extends only across the tip atom or atoms involved in the interaction. The interaction can be used to modify the sample to create small structures (Scanning probe lithography). Unlike electron microscope methods, specimens do not require a partial vacuum but can be observed in air at standard temperature and pressure or while submerged in a liquid reaction vessel.

9. Disadvantages The detailed shape of the scanning tip is sometimes difficult to determine. Its effect on the resulting data is particularly noticeable if the specimen varies greatly in height over lateral distances of 10 nm or less. The scanning techniques are generally slower in acquiring images , due to the scanning process. As a result, efforts are being made to greatly improve the scanning rate. The maximum image size is generally smaller. Scanning probe microscopy is often not useful for examining buried solid-solid or liquid-liquid interfaces.

10. Reference:- Skoog D A, West D M , Fundamentals of Analytical Chemistry, Thomson Asia Pvt ltd., 8 th Ed, (2004) 2) Skoog, Holler, Nieman, Principles of Instrumental Analysis http://teachers.stanford.edu/activities/SPMReference/SPMReference.pdf http://slideplayer.com/slide/4959102/