The Scanning Tunneling Microscope An Eye at the Nanometer Scale

Document Sample
The Scanning Tunneling Microscope An Eye at the Nanometer Scale Powered By Docstoc
					                                       The Scanning Tunneling Microscope
                                         An Eye at the Nanometer Scale
                                                                                                                                                                                        SECTION DE PHYSIQUE
                                                                                                                                                                                        DÉPARTEMENT DE PHYSIQUE
                                                                                                                                                                                        DE LA MATIÈRE CONDENSÉE




        The scanning tunneling microscope is a powerful instrument which allows to investigate the
     surfaces of conductors with atomic resolution. The instrument can work in extreme environments:
                      Low pressure, low temperature and even high magnetic fields.
               The technique also allows to study the local electronic properties of materials
                                   or to modify surfaces at a nanoscale.

  Materials Nanoscale Imaging
  If a small voltage is applied between the sample to investigate surface and a sharp tip, a tunneling                                                              Carbon atoms on graphite (HOPG)
  current will flow between these electrodes when the gap is reduced to a few atomic diameters. A crucial
  requirement for the success of this technique is an accurate control of the tunneling barrier width.
  Therefore, the tunneling tip is mounted on a three-dimensional piezoelectric drive which allows tip
  postionning with sub-nanoscopic resolution.
  The technique allows real-space imaging of conducting surfaces, by simultaneously scanning the tip
  over the surface at constant heigth while recording the variation of the tunneling current. In another
  safer mode, the vertical tip actuator is connected to an electronic feed-back loop, which continuously
  adjusts the vertical tip position, to maintain a constant tunneling current, i.e. a constant tip to sample
  distance. In this mode, the recording of the feed-back voltage allows to image the surface.
  The instrument was developed at IBM in Rüschlikon near Zürich and the involved scientists were
  awarded in 1986 by the Nobel Prize in Physics.

                                 Piezoelectric
                                    motor
                                                                                                                                                           Atomic resolution on a high temperature superconductor
                                                                                                                                                                              (Bi2Sr2Ca1Cu2O8)

        Current measurement




                                     Tip


   Voltage

                                                                                                                                                                Perovskite thin film surface (SrRu0.37Ti0.63O3)

                                                                                                                When the tip moves over the surface,
                                                                                                          the current varies exponentially with the distance
                                                                                                               between the tip and the sample surface



  In another mode called tunnelling spectroscopy, the instrument allows to
  measure the local electronic properties of the investigated sample. As a result,
  the technique can differentiate a semiconductor, from an ordinary metal or from
  a superconductor.
  This mode is probably the most sophisticated application of the instrument
  showing that the scanning tunneling microscope is not only an eye but also a
  nose at the nanoscopic level!

  Materials Nanoscale Modifications
  The scanning tunneling microscope allows to create various artificial structures in order to study the physical properties of the modified surfaces.

          1µm 2           500nm2




                                                                                                                                                                           Source: D. Eigler, IBM Almaden, USA
                                             2 nm                                                                                                                          www.almaden.ibm.com

                                                                                           8.5nm
         200nm2           37.5nm2                        Source: A. Takagi, PhD thesis, University of Geneva



  Swiss cross etched using STM in an YBa2Cu3O7 crystal. Its size is smaller than 30                                                   Iron atoms arranged one by one on a copper surface.
  nanometers, the trenches are about 2nm deep and wide (one unit cell)!



For more information on Scanning Tunneling Microscopy, contact Olivier Kuffer
DPMC, University of Geneva, 24 quai Ernest-Ansermet, CH-1211 Geneva 4,
Telephone : (022) 379 66 99, Fax : (022) 379 68 69                                                                                                                                          www.manep.ch
E-mail : olivier.kuffer@physics.unige.ch