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STM ppt

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STM ppt

  1. 1. SCANNING TUNNELING MICROSCOPE MCG 5138 -Advanced Topics in Mechanical Engineering Prepared by : Jishang Tailor - 7462006 Roshan Teli - 7476661 Course Instructor - Prof. M. Yandouzi Department of Mechanical Engineering, University of Ottawa
  2. 2. Working Principle • The principle of scanning tunneling microscopy is in quantum mechanics which is different from classical mechanics. • While classical mechanics deals up to macroscopic level, quantum mechanics deals with microscopic level. • Quantum mechanics explains the wave and particle like behavior of tiny particles like photons and electrons. • The quantum mechanics phenomenon which explains tunneling effect is the working principal of scanning tunneling microscopy.
  3. 3. • Electron tunneling Tunneling Effect
  4. 4. What is tunneling effect? It is a phenomenon where a particle tunnels through a barrier that it classically could not surmount. Tunneling Effect
  5. 5. Tunneling Current  In a metal, the energy levels of the electrons are filled up to a particular energy, known as the ‘Fermi energy’ 𝐸 𝐹. In order for an electron to leave the metal, it needs an additional amount of energy Φ, the so-called ‘work function’.  The electrons need to overcome a barrier Φ to travel from tip to specimen or vice versa.  When a sharp metallic tip is brought very close to the surface of a conductor, an electric current can be detected due to the tunneling of electrons through the air gap. In this case, the air gap is considered as the barrier and is only few 𝐴° thick.  When an electrical voltage V is applied between sample and tip, this tunneling phenomenon results in a net electrical current, the ‘tunneling current’.
  6. 6. Components of STM The main components of STM include scanning tip, Piezoelectric scanner, Distance control and scanning unit, Vibration isolation system and Data processing unit (Computer). 1. Scanning Tip : STM tips are usually made from tungsten metal or a platinum-iridium alloy where at the very end of the tip (called apex) there is one atom of the material. Scanning tip is the most important aspect of the STM as tunneling current is carried by that particular atom. STM tip under 103 x and 105 x magnificationSample and Tip at atomic level
  7. 7. 2. Piezoelectric Scanner :  The scanner tip is attached to a piezoelectric tube scanner. Piezoelectric effect is a phenomenon under which the material changes its length accordingly when put under an electrical voltage.  By adjusting the voltage on the piezoelectric element, the distance between the tip and the surface can be regulated.  Piezoelectric crystals expand and contract very slightly depending on the voltage applied to them and this principle is used to control the horizontal position x, y, and the height z of the scanning tip. Components of STM
  8. 8. 3. Distance Control and Scanning unit :  Position control using piezoelectric means is extremely fine, so a coarse control is needed to position the tip close enough to the sample before the piezoelectric control can take over. 4. Data Processing Unit (Computer) :  The computer records the tunneling current and controls the voltage to the piezoelectric tubes to produce a 3-dimensional map of the sample surface. Components of STM
  9. 9. 5. Vibration Isolation System :  STM deals with extremely fine position measurements so the isolation of any vibrations is very important.  The tip and surface distance must be maintained in 𝐴°(0.1 𝑛𝑚)to get desired atomic resolution.  Due to extremely high sensitivity of tunneling current between tip and sample surface height, it is absolutely necessary to reduce inner vibrations and to isolate the system from external vibration.  Damping can be achieved by : Pneumatic systems Spring system Eddy current system Components of STM
  10. 10. Working of STM
  11. 11. • First, a voltage bias is applied and the tip is brought close to the sample by coarse sample-to-tip control, which is turned off when the tip and sample are sufficiently close. At close range, fine control of the tip in all three dimensions near the sample is typically piezoelectric, maintaining tip- sample separation W typically in the 4-7 𝐴° (0.4-0.7 nm) range. • In this situation, the voltage bias will cause electrons to tunnel between the tip and sample, creating a current that can be measured. Once tunneling is established, the tip's bias and position with respect to the sample can be varied and data are obtained from the resulting changes in current. • If the tip is moved across the sample in the x-y plane, the changes in surface height and density of states cause changes in current. These changes are mapped in images. This change in current with respect to position can be measured itself, or the height, z, of the tip corresponding to a constant current can be measured. • There two modes : Constant height mode Constant current mode Working of STM
  12. 12. Constant Height Mode :  The voltage and height are both held constant while the current changes to keep the voltage from changing; this leads to an image made of current changes over the surface, which can be related to charge density. Working of STM  The benefit to using a constant height mode is that it is faster, as the piezoelectric movements require more time to register the height change in constant current mode.  However, it is applicable only when sample surface is real flat, the corrugation more than 6 to 7 𝐴° will lead tip to crash.  Generally less preferred due to the risk of damaging the tip.
  13. 13. Constant Current Mode :  Feedback electronics adjust the height by a voltage to the piezoelectric height control mechanism.  This leads to a height variation and thus the image comes from the tip topography across the sample and gives a constant charge density surface; this means contrast on the image is due to variations in charge density Working of STM  It is a time consuming method compared to the constant height mode as the feed back control has to adjust the current constant according height as the tip moves along the specimen surface.
  14. 14. What STM measures?  STM images are not direct surface images of the sample as in the case of optical microscopy rather it is measure of the local density of states of a material at it surface as a function of lateral (x-y) position on the sample surface and energy.  Within sample each electron has specific energy level and only certain number of electrons can occupy that level at a time .  The distribution that gives number of electrons allowed per energy level as a function of certain energy level is called the density of states.  So, the grey scale image generated is direct measurement atomic corrugation of the surface. Use of STM
  15. 15. Generation of images  The amount of adjustment done by the feedback loop is recorded and defines the grid value which can be displayed as the grey scale image.  Once the grid values are assigned one can use it to deform it perpendicular to the surface and get 3D image.  Now, different colors patterns are used to color the image and visualize it. Images
  16. 16. Advantages:  STMs are versatile. They can be used in ultra high vacuum, air, water and other liquids and gasses.  STMs give three dimensional profile of a surface, which allows researchers to examine a multitude of characteristics, including roughness, surface defects and molecule size.  Lateral Resolution of 0.1 nm and 0.01 nm of resolution in depth can be achieved. Disadvantages:  It is very expensive.  It need specific training to operate effectively.  STM need very clean surface, excellent vibration control while operation, single atom tip. Merits and Demerits
  17. 17.  If the tip has is not prepared as per standard which contains multipoint and if the tip get contaminated during the operation, it creates multi signal which are resulted as the artifacts in the image.  Sometimes the frequency gained by amplifier is too high which makes the tube to oscillate which gives noisy image.  Sometimes rapid change in voltage are noted due to scanner encounters a large step during scanning. The tube will continue to move even if the voltage remains fixed which results in hazy image. Artifacts in Image
  18. 18. Paper Review :  The selected paper is research on Fabrication of nanoscale alumina on NiAl(1 0 0) surface using scanning tunneling microscope under ultrathin vacuum condition.  In the research, two modes of oxidation were observed under different conditions in formation of the alumina on the surface of NiAl(1 0 0).  The first mode of oxidation observed under the tunneling current greater than 0.4 nA and the power greater than 0.24 nW. Here, Al and O atoms formed alumina on the surface in the proximity of the tip. The width and thickness of the alumina strips growth are controlled by the current and voltage bias. Research Paper
  19. 19.  In the second mode oxidation smaller power and a smaller bias (1.0V) were used which resulted in growth of crystalline alumina along direction [0 0 1] or [0 1 0] of NiAl(1 0 0) surface in the tip scanned area of oxidized surfaces, irrespective of the scanning direction of tip. Direction of crystalline alumina Direction of scanning Research Paper
  20. 20.  The combination of above two oxidation modes were used to fabricate crystalline alumina on NaAl(1 0 0) surface in which bias was kept constant at 0.7 V and current was raised from 0.3 nA to 1.6 nA which created abrupt alumina on the surface.  With same condition of oxidation on the surface keeping bias at 0.7 V and lowering current to 0.3 nA just altered the topography of the surface and it became more structured, indicating the gradual formation of the oxide strips along direction [0 1 0] and [0 0 1] of NiAl (1 0 0)  The STM tip induced oxidation has been widely studied and this is very important break through in research as industrialization of such technology at economical rate can use to fabricate nanosensors, bio materials, nanomedicine.  It helps to change the property of the structure at nano-scale which helps to create strong structure. Research Paper
  21. 21. Thank You . . .

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