Scanning tunneling microscope (STM)


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Scanning tunneling microscope (STM)

  1. 1. An Najah National University Faculty of Science Physics Department Scanning Tunneling Microscope Prof: Gassan Saffarini Prepared by: Balsam Ata 2012
  2. 2. List of contents: 1-Introduction……………………………………….….3 2- The basic of STM……………………………………4 3-STM design……………………………………………7 4-STM operation……………………………………..…11 5-Modes of operation……………………………….…..13 5-1-Constant current mode………………………….….13 5-2- Constant Height Mode…………………………….14 6- density of state imaging… ……………………….…15 7-STM applications………………………………….…16 8-STM images……………………………………….…18 9- STM related studies…………………………………21 10-Refrences....................................................................23
  3. 3. List of figures: Fig.1: Rectangular potential barrier and particle wave function Ψ…………………………………………………………….5 Fig.2: Scanning tip……………………………………….…7 Fig.3: Electrochemical Etching……………………………..8 Fig.4 : Scanning Tunneling Microscope schematic………..10 Fig.5: The scanning tunneling microscope…………………11 Fig.6: Voltage biase vs tunneling current………………….12 Fig.7: Constant current mode …. ………………………….13 Fig.8: Constant Height Mode ………………………..……14 Fig.9: STM images show the steps of "quantum corral" formation………………………………………………..…16
  4. 4. 1- Introduction A scanning tunneling microscope (STM)is an instrument for imaging surfaces at the atomic level [1]. It was invented in 1981 by Gred Binnig and Heinrich Rohrer at IBM Zurich. Five years later they were awarded the Nobel prize in physics for its invention [2]. The STM was the first instrument to generate real-space images of surface with atomic resolution [3]. STM has good resolution considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution [4]. STM gives true atomic resolution on some samples even at ambient conditions. Scanning tunneling microscopy can be applied to study conductive surfaces or thin nonconductive films and small objects deposited on conductive substrates [5]. The STM is a non-optical microscope which employs principles of quantum mechanics. A very fine tip is moved over the surface of the material under study, and a voltage is applied between probe and the surface. Depending on the voltage and its characteristics electrons will "tunnel" or jump from the tip to the surface (or vice-versa depending on the polarity), resulting in a weak electric current. The size of this current is exponentially dependent on the distance between tip and the surface . By scanning the tip over the surface and measuring the current, one can thus reconstruct the surface structure of the material under study [6].
  5. 5. 2- The basic of STM. The STM based on the concept of quantum tunneling [7], quantum tunneling is a microscopic phenomenon where a particle can penetrate or pass through a potential barrier. This barrier is assumed to be higher than the kinetic energy of the particle ,therefore such a motion is not allowed by the laws of classical mechanics [8]. To understand the phenomenon, particles attempting to travel between potential barriers can be compared to a ball trying to roll over a hill; quantum mechanics and classical mechanics differ in their treatment of this scenario. Classical mechanics predicts that particles that do not have enough energy to classically surmount a barrier will not be able to reach the other side. Thus, a ball without sufficient energy to surmount the hill would roll back down. Or, lacking the energy to penetrate a wall, it would bounce back (reflection) or in the extreme case, bury itself inside the wall (absorption). In quantum mechanics, these particles can, with a very small probability, tunnel to the other side, thus crossing the barrier [9].The reason for this difference comes from the treatment of matter in quantum mechanics as having properties of waves and particles(wave –particle duality ) [10]. Problems in quantum mechanics center around the analysis of the wave function for a system. Using mathematical formulations of quantum mechanics, such as the Schrödinger equation, the wave function can be solved. This is directly related to the probability density of the particle's position, which describes the probability that the particle is at any given place.
  6. 6. The simplest problems in quantum tunneling are onedimensional such as the rectangular barrier . Fig. 1. Rectangular potential barrier and particle wave function Ψ [11]. The wave function can be found by solving time_independent Schrödinger equation for the system in one dimension Where m is the mass of the particles Planck constant/2 ,V(x) the height of the barrier ,E the energy of the incident particles 1) When X<0 =0( means no forces act on the particle)
  7. 7. 2) When 0<X<L =U0 , 3) When X>L =0 The constant F=0 because there is no barrier to reflect the particle. By applying the boundary conditions 1234- We can find the amplitudes(A,B,C,D,E) of each wave function. The wave function represents the incoming particles moving to the right ,and represents the reflected particles moving to the left ; represents the transmitted particles moving to the right .The wave function represents the particles inside the barrier ,some of which end up in region 3 while the others return to region 1 [12] The transmission probability T for a particle to pass through the barrier is equal to
  8. 8. T= J transmitted /J incident where J(x ,t) the probability current which is equal to : So the approximate transmission probability : T= Where L is the width of the barrier, and K2= [12]. 3- STM design The basic components of the STM : 1-The sample:aclean conducting or semiconducting surface[14]. 2-Scanning tip : The tip is the trickiest part in the STM ,it needs a small curvature to resolve coarse structures . For atomic resolution a mini tip with a one atomic end is necessary. Fig.2 scanning tip [15]. Tips typically are made out of tungsten, platinum or a Pt-Ir wire. A sharp tip can be produced by: Cutting and grinding
  9. 9. Electrochemical etching The tungsten wire is put into a solution of NaOH and kept on a positive potential towards a counter electrode. The etching process takes place predominately on the surface of the solution. When the neck is thin enough the wire fractures due to its weight. Thus actually two tips are produced. The tip has to be cleaned with de ionized water and pure ethanol or methanol. Fig.3 Electrochemical Etching [16]. Most often the tip is covered with an oxide layer and contaminations from the etchant and is also not sharp enough. Thus other treatments to the tip, like annealing or field evaporation are necessary[16]. 3- Piezoelectric tube : piezoelectric controlled height and x, y scanner. The word piezoelectricity means electricity resulting from pressure[17]. A piezoelectric substance is one that produces an electric charge when a mechanical stress is applied (the substance is squeezed or stretched). Conversely, a
  10. 10. mechanical deformation (the substance shrinks or expands) is produced when an electric field is applied[18]. The piezoelectric effect occurs only in non conductive materials. Piezoelectric materials can be divided in 2 main groups: crystals and ceramics. The most well-known piezoelectric material is quartz (SiO2)[19]. 4-Vibration isolation system: due to the extreme sensitivity of tunnel current to height, proper vibration isolation or an extremely rigid STM body is imperative for obtaining usable results. In the first STM by Binnig and Rohrer, magnetic levitation was used to keep the STM free from vibrations; now mechanical spring or gas spring systems are often used[7]. 5-Computer: the computer may also be used for enhancing the image with the help of image processing as well as performing quantitative measurements[20][21].
  11. 11. Fig.4 Scanning Tunneling Microscope schematic [22].
  12. 12. 4- STM Operation: A voltage bias is applied and the tip is brought close to the sample by some 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 when near the sample is typically piezoelectric, maintaining tip-sample separation W typically in the 4-7 Å (0.4-0.7 nm) range, which is the equilibrium position between attractive (3<W<10Å) and repulsive (W<3Å) interactions. Fig.5 The scanning tunneling microscope [2] . In this situation, the voltage bias will cause electrons to tunnel between the tip and sample, creating a current that can be measured .The resulting tunneling current is a function of tip position, applied voltage, and the local density of states (LDOS)
  13. 13. of the sample Information is acquired by monitoring the current as the tip's position scans across the surface, and is usually displayed in image form [9]. It∝Vtexp(-A√Θz) It: tunneling current Vt: tunneling voltage or (voltage bias) Z= Tip-sample separation, typically 4-10 Å Θ: work function, typically 3-5 ev . [23] Fig. 6 Voltage biase vs tunneling current . [23]
  14. 14. 5- Modes of operation. 5-1 Constant Current Mode: In STM bias voltage is applied between a sharp conductive tip and a conductive sample, so when the sample is approached to a few angstroms from the tip, tunneling current occurs, that indicates proximity of the tip to the sample with very high accuracy. In Constant Current mode (CCM) of operation when scanning sample surface the scanner keeps the current constant by feedback circuit[5]. Fig.7 Constant current mode [24]. feedback electronics adjust the height by a voltage to the piezoelectric height control mechanism [25]. 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 [26].
  15. 15. 5-2 Constant Height Mode In 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[26]. Fig.8 Constant Height Mode [24]. In Constant Height mode (CHM) of operation the scanner of STM moves the tip only in plane, so that current between the tip and the sample surface visualizes the sample relief. Because in this mode the adjusting of the surface height is not needed a higher scan speed can be obtained. CHM can only be applied if the sample surface is very flat, because surface corrugations higher than 5-10 A will cause the tip to crash. The weak feedback is still present to maintain a constant average tipsample distance. As the information on the surface structure is obtained via the current, a direct gauging of height differences is no longer possible [27].
  16. 16. 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, than the current change in constant height mode [26]. 6-Density of States imaging As long as measured in STM current is determined by the tunneling processes through tip-sample surface gap its value depends not only on the barrier height but on the electron density of states also. Accordingly obtained in STM images are not simply images of sample surface relief (topography), these images can be hardly affected by the density of electronic states distribution over the sample surface. Good example of Local Density of States (LDOS) influence on the STM image is wellknown image of highly oriented pyrolitic graphite (HOPG) atomic lattice. Only half atoms are visible in STM. Similar case is image of GaAs atomic lattice. LDOS determining can also help to distinguish chemical nature of the surface atoms. LDOS acquisition is provided simultaneously with the STM images obtaining [28].
  17. 17. 7-STM Applications Most of STM applications are in nanotechnology and biology. Determination of surface structures is one of the most important applications of the STM. 7-1 Manipulation of Atoms: One innovative applications of STM recently found is manipulation of atoms. Example, Iron atoms are placed on Cu(111) surface at very low temperature (4K), Iron atoms are first physisorbed on the Cu surface, then the tip is placed directly over a physisorbed atom and lowered to increase the attractive force by increasing the tunneling current, the atom was dragged by the tip and moves across the surface to a desired position. Then, the tip was withdrawn by lowering the tunneling current[29]. Fig.9 STM images show the steps of "quantum corral" formation [29].
  18. 18. 7-2 Scanning tunneling microscope study of iron(II) phthalocyanine growth on metals and insulating surfaces: Due to the broad field of applications metal phthalocyanine (MPc) molecules and their derivatives have attracted intense interest of researchers within previous decades .They are important compounds for optical and organic electronic devices such as organic light-emitting diodes, thin film transistors, and solar cells . The physical properties of MPc molecular films are strongly affected not only by the molecular structure but also by the molecular orientation in thin films as well as by the interface to the hosting carrier. Therefore, molecule–molecule and molecule–substrate interactions are important issues for the formation of highly ordered MPc molecular films and attracted considerable interest in these planar molecular model systems, in experiments and in numerical modeling . The scanning tunneling microscopy (STM) and spectroscopy (STS) are intensively used to determine both, the geometric and the electronic structures of MPc molecules deposited on metallic surfaces. The adsorption configurations of several MPc molecules on various metal substrates have been studied with STM under ultra-high vacuum (UHV) conditions[30]. 7-3 Modification of thin gold films with the scanning tunneling microscope: Thin gold films, which were deposited by sputter deposition onto highly oriented graphite surfaces, were investigated and modified by means of a scanning tunneling microscope. By applying short voltage pulses to the vertical piezoelectric element or to the tunneling tip, hole patterns were generated[31].
  19. 19. 8- STM images: All images that are obtained STM device is the image of gray tones and to obtain a color image, computer programs are used to highlight important features to show it in the picture[32]. 2-D network of 4 nm Au cluster array on Ga As [33]. Atoms of n-type MoS2, a common dry lubricant. The bright spots indicate S atoms, which account for its excellent lubrication properties [33].
  20. 20. STM image, 35 nm x 35 nm, of single substitutional Cr impurities (small bumps) in the Fe(001) surface [34]. STM image, 7 nm x 7 nm, of a single zig-zag chain of Cs atoms (red) on the GaAs(110) surface (blue) [34].
  21. 21. STM image of individual silicon (Si) atoms [35]. Image of DNA [36].
  22. 22. 9-STM related studies: Many other microscopy techniques have been developed based upon STM. 9-1 Photon scanning microscopy (PSTM): which uses photons instead of tunneling electrons to image surface structure[37]. 9-2 Scanning tunneling potentiometry (STP) used to study the spatial variation of the electric potential on thin film surfaces. Topography and potential distribution of the film surface are measured simultaneously[38]. 9-3 Atomic force microscopy (AFM): or scanning force microscopy (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit[39]. The Atomic Force Microscope was developed to overcome a basic drawback with STM - that it can only image conducting or semiconducting surfaces. The AFM, however, has the advantage of imaging almost any type of surface, including polymers, ceramics, composites, glass, and biological samples [40]. 9-4 Spin polarized scanning tunneling microscopy (SPSTM): is a specialized application of scanning tunneling microscopy (STM) that can provide detailed information of magnetic phenomena on the single-atom scale additional to the atomic
  23. 23. topology gained with STM. SP-STM opened a novel approach to static and dynamic magnetic processes as precise investigations of domain walls in ferromagnetic and ant ferromagnetic systems, as well as thermal and current-induced switching of nonmagnetic particles[41]. 9-5 Scanning Tunneling Spectroscopy (STS): is an extension of Scanning Tunneling Microscopy (STM) which is used to provide information about the density of electrons in a sample as a function of their energy [42].
  24. 24. References: [1] e-Binnig-0/ G. Binnig, H. Rohrer (1986). "Scanning tunneling microscopy". IBM Journal of Research and Development 30: 4 [2] www. noble prize .org [3] wnloads/Scanning_Tunneling_Microscopy.pdf [4] e-Binnig-0/ (2000). Scanning tunneling microscopy and its applications. New York: Springer Verlag. ISBN 3-540-65715-0. [5] [6] icroscope.htm [7] e-Binnig-0/ ^ a b c d e f g h i j k l m n o p q r s t u v C. Julian Chen (1993). Introduction to Scanning Tunneling Microscopy. Oxford University Press. ISBN 0-19-507150-6. [8]Mohsen Razavy (2003 ),Quantum theory of tunneling [9] e-Binnig-0/ "Quantum Tunneling Time". ASU.'Quantum%20Tunelling%20Ti me'%20AJP000023.pdf. Retrieved 2012-01-28 [10] te-Binnig-0/ -^ a b c d Nimtz and Haibel, "Zero Time Space", page 1. WILEY-VCH Verlag GmbH & Co. 2008 [11]
  25. 25. [12]Arther Beiser (1981) Concept of modern physics ,six edition [13] David J .Griffiths(1995) ,Introduction to Quantum Mechanics, second edition [14] [15] [16] [17] Harper, Douglas. "piezoelectric". Online Etymology Dictionary. [18] [19] [20] te-Binnig-0/ R. V. Lapshin (1995). "Analytical model for the approximation of hysteresis loop and its application to the scanning tunneling microscope" (PDF). Review of Scientific Instruments 66 (9): 4718–4730. Bibcode 1995RScI...66.4718L. DOI:10.1063/1.1145314. al1995.(Russian translation is available). [21] te-Binnig-0/ ^ R. V. Lapshin (2004). "Feature-oriented scanning methodology for probe microscopy and nanotechnology" (PDF). Nanotechnology 15 (9): 1135–1151. Bibcode 2004Nanot..15.1135L. DOI:10.1088/0957-4484/15/9/006. 004 [22] ScanningTunnelingMicroscope schematic.png [23] [24] ownloads/Scanning_Tunneling_Microscopy.pdf
  26. 26. [25]^ a b c K. Oura, V. G. Lifshits, A. A. Saranin, A. V. Zotov, and M. Katayama (2003). Surface science: an introduction. Berlin: Springer-Verlag. ISBN 3-540-00545-5. [26] te-Binnig-0/ ^ a b c d D. A. Bonnell and B. D. Huey (2001). "Basic principles of scanning probe microscopy". In D. A. Bonnell. Scanning probe microscopy and spectroscopy: Theory, techniques, and applications (2 ed.). New York: Wiley-VCH. ISBN 0-471-24824-X [27] [28] [29] htm [30] [31] [32] [33] [34] [35] [ 36]www. [37] [38] [39] [40] [41] [42]