Scanning Tunneling Microscope

Introduction to STM, working principle, advantages and disadvantages.

1. 1 SCANNING TUNNELING MICROSCOPE Submitted by : Sumeena Karki Roll no: 05/071 Central Department of Biotechnology Kirtipur, Nepal Date:2071/12/22

2. INTRODUCTION A  scanning tunneling microscope  ( STM ) is an instrument for imaging surfaces at the atomic level . The scanning tunneling microscope (STM) was invented by Binnig and Rohrer and implemented by Binnig, Rohrer, Gerber, and Weibel Nobel prize (at IBM Zurich)in Physics in 1986 Gerber built first STM as well as first Atomic Force Microscope. 2 Binnig (right, holding flowers) and Heinrich Rohrer (left, holding flowers ) holding a soccer ball is Christoph Gerber

3. Working principle 3

4. Working principle For an STM, good resolution is considered to be 0.1  nm  lateral resolution and 0.01 nm depth resolution The STM is based on the concept of quantum tunneling which falls under domain of quantum mechanics Quantum mechanics:Matter has both particle as well as wave nature  wavelike properties of  electrons permit them to “tunnel” beyond the surface of a solid into regions of space that are forbidden to them under the rules of classical  physics Smaller the barrier width more precisely wave can pass through 4

5. Contd … The probability of finding such tunneling electrons decreases exponentially as the distance from the surface increases. The STM makes use of this extreme sensitivity to distance. The sharp tip of a tungsten needle is positioned a few angstroms from the sample surface . A small voltage is applied between the probe tip and the surface, causing electrons to tunnel across the gap. As the probe is scanned over the surface, it registers variations in the tunneling current, and this information can be processed to provide a topographical image of the surface . In metals, electrons appear to be freely moving particles, but this is illusory. In reality, the electrons move from atom to atom by tunneling through the potential barrier between two atomic sites. 5

6. Contd … 6 In metals, electrons appear to be freely moving particles, but this is illusory . In reality, the electrons move from atom to atom by tunneling through the potential barrier between two atomic sites. In a typical case, with the atoms spaced five angstroms apart, there is a finite probability that the electron will penetrate the barrier and move to the adjacent atom. The electrons are in motion around the nucleus, and they approach the barrier with a frequency of 10 17  per second. For each approach to the barrier, the probability of tunneling is 10 −4 , and the electrons cross the barrier at the rate of 10 13 per second. When the tip is moved close to the sample, the spacing between the tip and the surface is reduced to a value comparable to the spacing between neighbouring atoms in the lattice . In this circumstance, the tunneling electron can move either to the adjacent atoms in the lattice or to the atom on the tip of the probe. The tunneling current to the tip measures the density of electrons at the surface of the sample, and this information is displayed in the image

7. P iezoelectricity Greek: piezo means to squeeze or press, and  electric  or  electron   , which means  amber, an ancient source of electric charge the electric charge  that accumulates in certain solid materials in response to applied mechanical stress usually by change of polarisation It provides variation in direction with details about orientation A probe tip, usually made of W or Pt– Ir alloy, is attached to a piezodrive , which consists of three mutually perpendicular piezoelectric transducers: x piezo , y piezo , and z piezo . Upon applying a voltage, a piezoelectric transducer expands or contracts the tip scans on the xy plane The electron wavefunctions in the tip overlap electron wavefunctions in the sample surface. A finite tunneling conductance is generated. By applying a bias voltage between the tip and the sample, a tunneling current is generated 7

8. Two modes of working of STM 1.   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. It is faster, as the piezoelectric movements require more time to register the height change in constant current mode than the current change in constant height mode 2.Constant current mode Contrast on the image is due to variations in charge density.   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 8

9. Sample preparation An STM specimen needs a substrate that is extremely flat, down to the atomic level. If the specimen is uneven, the STM probe will have difficulties in scanning very steep pits or ridges. Graphite One of the most commonly used STM substrates is a special form of graphite (highly oriented pyrolytic graphite or HOPG). It is a naturally layered material that is easy to prepare and relatively inert. A fresh surface can be obtained as easily as pressing a piece of adhesive tape to the surface and peeling away the top layer. The resulting surface will have large flat areas useful for scanning. Other Substrates Other popular materials that provide large, atomically flat surfaces include mica, quartz, and silicon. These materials are insulators, so to be used for STM a thin layer of noble metal (mainly gold or platinum) is deposited on the surface. Annealing (heating and then slowly cooling) the metal layer helps to smooth the surface and produce large flat areas. 9

10. contd … Growing Gold A thin gold film can also be grown while deposited on a flat mica surface and then stripped away. The surface of the gold that was in contact with the mica will be very flat over large areas . 10 Fig: The surface of gold film grown on quartz. Fig: The surface of flame annealed gold film.

11. Advanced STM 11 Atomic force microscopy(AFM) low-energy electron diffraction (LEED)

12. Photoelectron spectroscopy (XPS/UPS/AES)  12 P reparation techniques( molecular evaporators, molecular spray and pulse valves) and  analysing techniques (as X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and low-temperature scanning tunneling microscopy (LT-STM))

13. Result shown by STM 13 (a) STM image of the self-assembled monolayer of cysteine on Au(111)(b) proposed model of Cysteine (c)ball and stick model of cysteine

14. Advantages of STM The STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambients , and at temperatures ranging from near zero kelvin  to a few hundred degrees Celsius STM gives 2D/3D image and better resolution of atoms The STM can be cooled to temperatures less than 4 K (−269 °C, or −452 °F)—the temperature of liquid helium . It can be heated above 973 K (700 °C, or 1,300 °F). The low temperature is used to investigate the properties of superconducting materials, while the high temperature is employed to study the rapid diffusion of atoms across the surface of metals and their corrosion 14

15. Advantages contd … The strong electric field between tip and sample has been utilized to move atoms along the sample surface. It has been used to enhance the etching rates in various gases. In one instance, a voltage of four volts was applied; the field at the tip was strong enough to remove atoms from the tip and deposit them on a substrate . A sequence of STM images showing the assembly of two Chinese characters from single Ag atoms on a Ag(111) surface. The lateral manipulation technique allows the exact placing of single atoms on desired atomic sites. An assembly involves not only the movement of single atoms but requires also many repair and cleaning steps until the final structure is completed 15

16. STMs use highly specialized equipment that is fragile and expensive($30000-$1,50,000) STMs can be difficult to use effectively. I t requires very stable and clean surfaces , excellent vibration control and sharp tips 16 Disadvantages of STM

17. C. Julian Chen “ Introduction to Scanning Tunneling Microscopy ”, Second Edition , Department of Applied Physics and Applied Mathematics , Columbia University, New York Tersoff , J.: Hamann , D. R.: Theory of the scanning tunneling microscope,  Physical Review B 31, 1985, p. 805 – 813 ( https://en.wikipedia.org/wiki/Scanning_tunneling_microscope ) http://www.nobelprize.org/educational/physics/microscopes/scanning/preparation.html https://nanolab.unibas.ch/TemplateReducedQC2/pages/equipment.htm http://www.britannica.com/technology/scanning-tunneling-microscope 17 References