Scanning Tunneling Microscope

A scanning tunneling microscope (STM) is an instrument for imaging surfaces at the atomic level.STM is based on the concept of quantum tunneling.

1. SCANNING TUNNELING MICROSCOPY (STM) HİLAL AYBİKE CAN JEWELERY ENGINEERING ISTANBUL COMMERCE UNIVERSITY December,2016

2. 1. INTRODUCTION The scanning tunneling microscope (STM) is a magnificent microscope ever built . It was generated in 1981 by Gerd Binning and Heinrich Rohrer of IBM’s Zurih Lab i n Zurich,Switzerland . The invention deserved Nobel prize for physics in 1986. On October , 1986, the soccer team of IBM Zurich Laboratory a nd Down Chemical played a game which had been arranged earlier . To everyone’s surprise , a few hours before the game , The S weedish Academy announced the Nobel Prize for Gerd Binning ( right ) and Heinrich Rohrer ( left ).

3. The Scanning Tunneling Microscope is an electron microscope that transmit three-dimensional images of the electron cloud around the nucleus . The STM allows the inspection of the properties of a conductive solid surface at an atomic size. It is a very important technique in determining the atomic structures and electronic states of the surface under investigation. Surfaces can be viewed at the atomic size thanks to the high resolu t ion (0.1 Ǻ) that STM has .

4. Basic components of STM Five basic components : 1. Metal tip , 2. Piezoelectric scanner , 3. Current amplifier ( nA ), 4. Bipotentiostat ( bias ), 5. Feedback loop ( current ). The scanner can be mounted with the tip or the sample stage.

5. 2.TUNNELING EFFECT The basis of STM is the quantum tunneling theory. According to quantum tunneling theory, when the energy of an electron exceeds its total energy, it can penetrate regions which are impossible according to classical physics. So it can tunnel . In other words, if two conductors are brought closer to each other by 10 Ǻ or more and a potential difference is created between them, the electrons are likely to tunnel through the potential barrier between these two conductors.

6. 2.1.GENERATING TUNNELING CURRENT A tunneling current occurs when electrons move through a barrier that they shouldn't be able to move through in classic physic s . In classical terms, if you don't have enough energy to move "over" a barrier, you won't. However, in quantum mechanical world, electrons have wavelike properties. These waves don’t end suddenly at a wall or barrier, but taper off quickly. If the barrier is thin enough, the probability function can expand into the next region, through the barrier. If a very sharp metal tip, called a probe (atomic size), is approached to the surface to be investigated by a mechanical system (1-10 Ǻ) and a potential difference is applied, the wave functions of the type and the surface superimpose and electrons are superimposed on the surface, A current is generated called tunneling current. A probe tip, usually made of W or Pt – Ir alloy. The tip is mechanically connected to the scanner, an XYZ position device realized by means of piezoelectric materials. Distance between tip and surface must be 1Å to 10Å.

7. 2.2. DIRECTION OF TUNNELING CURRENT The direction of flow depends on the sign of the applied potential. If the sample is negatively charged, the electrons will pass through the filled orbital of the sample to the empty orbital of the tip. The flow here is very small and at nA level.

8. 3.WORK PRINCIBLE OF STM When voltage is generated between a specimen and a tip with a very small distance (a few angstroms ), due to the quantum tunneling, there is an electron transition from the specimen to tip or from tip to specimen, which corresponds to a tunnel current at the picoamper stage. This tunneling current, which occurs when scanning the specimen on the specimen surface, is measured and is used to obtain surface tomography, since this current is a function of the distance between the specimen and the tip.

9. In constant-height mode , the tip scan in a horizontal plane above the sample and the tunneling current varies depending on topography and the local surface electronic properties of the sample . The tunneling current measured at each location on the sample surface constitute the data set, creating the topography image in figüre (a) In the constant - current mode , by adjusting the height of the type, the current is held constant by the feedback circuit which keeps the current constant and is moved on the type surface. During this time, changes are recorded in the height of the height of the image is created. Showed as figüre (b)

10. 3.2. DISTANCE BETWEEN TIP AND SPECIMEN The tunneling current varies exponentially depending on the distance between the individual specimens. It = Ve - kd V : potential difference between conductors . k : constant which depends on conductors composition d : distance between the lowest (nearest to the sample) atom on the tip and the highest (nearest) atom on the sample. If the distance increases, the tunneling current ( It ) will decrease exponentially .

11. 4. TIP PROPERTIES The most important part of the scanning tunneling microscope is the t i p s. The tunneling t ip , provides the best images that are limited by a single metal atom of the t i p. The traces were obtained by cutting the platinum / iridium wires or by electrochemically etching the tungsten metal. At the present time, electrochemical etching of Pt / Ir wires yields very clear solubility types Close-up view of a Pt-Ir type with a diameter of 0.25 mm used in STM work .

12. FACTORS EFECTING THE RESOLUTION One of the factors affecting resolution is corrugation , i.e . how much the electron density of surface atoms varies in height above the surface . Graphite has a large corrugation , and is very planar , and thus is one of the easiest materials to image with atomic resolution . ( see next slide for example ) STM does NOT probe the nuclear position directly , but rather it is a probe of the electron density , so STM images do not always show the position of the atoms . STM imaging depends on the nature of the surface and the magnitude and sign of the tunneling current . For example , if you have Cu and Si on the same surface , under the same condition , the current with Cu is much higher .

13. POSITIONING The large distance range the tip has to be controlled on makes it necessary to use two positioners: a coarse and a fine positioner . The fine positioner is also used as a scanner. Every fine positioner/scanner is made out of a piezocrystal or piezoceramic material.

14. 5. PIEZOELECTRIC EFECT The piezoelectric effect was discovered by Pierre Curie in 1880. The effect is created by squeezing the sides of certain crystals, such as quartz .The result creat e s of opposite charges on both sides. The effect can be reversed as well; by applying a voltage across a piezoelectric crystal, it is going to be elongate or compress. The process is based on fundamental structure of a Crystal lattice crystals generally have a charge balance where negative and positive charges precisely nullify eac other out along the rigid planes of the crystal lattice . When this charge balance is disrupted by an external force , such as, applying physical stress to a Crystal, the energy is transferred by electric charge carries , creating a surface charge density , which can be collected via electrodes .

15. STM W ork P rincible

16. STM ADVANTAGES STMs are helpful because they can give researchers a three dimensional profile of surface, which allows researchers to analysis a multitude of characteristics, including roughness, surface defects and determining things about the molecules size and conformation. It is capable of capturing much more detail than other microscopes. This helps researchers better understand the subject of their research on molecular level . STMs are also versatile. They can be used for ultra high vacuum, air, water and other liquids and gasses . They will activate in temperatures as low as zero Kelvin up to a few hundred degrees Celsius . Scanning Tunneling Microscope works faster than Atomic Force Microscope. AFM max sample size is 150x150 µm . On the other hand, STM generates mm size length and width. Lastly , resolution of STM is much better than AFM.

17. STM DISADVANTAGES There are very few disadvantages to using a scanning tunneling microscope. STMs can be difficult to use effectively. There is very specific technique that requires a lot of skill and precision. STM requires very stable and smooth surfaces, excellent vibration control and sharp tips. STMs use highly specialized equipment that is fragile and expensive . Although , STM analysis only conductive materials, AFM uses for conductive and insulator materials. STM requires vacuum atmosphere but AFM can work even in liquid. For that reason AFM can be used for biological materials.

18. REFERENCES 2 . J. Chen , Introduction to Scanning Tunneling Microscopy , New York, Oxford Univ . Press (1993). Gan , Y., Chu , W., & Qiao , L. (2003). STM investigation on interaction between superstructure and grain boundary in graphite . Surface Science , 539(1), 120-128. Marti , O., & Amrein , M. ( Eds .). (2012). STM and SFM in Biology . Academic Press .. Microscopy ( pp . 59-67). Springer Netherlands . http:// jdetrick.blogspot.com.tr/2012/03/scanning-tunneling-microscope-and-3-d.html http ://www.nanoscience.com/technology/scanning-tunneling-microscopy/how-stm-works/

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