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Scanning tunneling microscope (STM)

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  • 1. An Najah National UniversityFaculty of Science Physics DepartmentScanning Tunneling Microscope Prof: Gassan Saffarini Prepared by: Balsam Ata2012

2. List of contents: 1-Introduction..3 2- The basic of STM4 3-STM design7 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 studies21 10-Refrences....................................................................23 3. List of figures: Fig.1: Rectangular potential barrier and particle wave function .5 Fig.2: Scanning tip.7 Fig.3: Electrochemical Etching..8 Fig.4 : Scanning Tunneling Microscope schematic..10 Fig.5: The scanning tunneling microscope11 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. 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. 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 Schrdinger 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. The simplest problems in quantum tunneling are onedimensional such as the rectangular barrier .Fig. 1. Rectangular potential barrier and particle wave function [11].The wave function can be found by solving time_independent Schrdinger equation for the system in one dimensionWhere 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) WhenX

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An Najah National University Faculty of Science Physics Department Scanning Tunneling Microscope
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