Scanning Tunneling Microscopy

Transcription

1 Scanning Tunneling Microscopy The wavelike properties of electrons allows them to tunnel beyond the regions of a solid into a region of space forbidden for them to exist in. In this region they can be detected and used to image the surface from which the came from The STM is based on the concept of quantum tunneling. When a conducting tip is brought very near to a metallic or semiconducting surface, a bias between the two can allow electrons to tunnel through the vacuum between them. For low voltages, this tunneling current is a function of the local density of states (LDOS) at the Fermi level, Ef, of the sample Variations in current as the probe passes over the surface are translated into an image. STM can be a challenging technique, as it requires extremely clean surfaces and sharp tips. 6-1

2 QM Tunneling first look Until you know more about quantum mechanics and wavefunctions (see 2 nd part of CHEM 2060) we cannot get too involved. A ball hitting an impenetrable wall will bounce back. Imagine throwing a baseball to a friend on the other side of a mile high brick wall, directly through wall. One would be rightfully astonished if, rather than bouncing back upon impact, the ball were to simply pass through to your friend on the other side of the wall. For objects of very small mass, as is the electron, wavelike nature has a more pronounced effect, so such an event, referred to as tunneling, has some probability[ 6-2

3 classical picture Quantum Tunneling electron electric field in classical physics the electron is repelled by an electric field as long as energy of electron is below energy of the field quantum picture electron wave in quantum mechanics the wave of the electron encounters the electric field but has some finite probability of tunneling through 6-3

4 Electrons in solid do not possess enough energy to leave the material So the only way they can get out is by tunneling STM uses an electronic signal to relay the size of the tunneling current between A tip and sample. Small changes in probe-surface distance Result in large changes in tunneling current. Tunneling current varies exponentially with distance 6-4

5 Tunneling current between two flat plates separated by a distance W in a vacuum is given by I % V exp (-2kW) m : electron rest mass U : Potential Energy across surfaces E: Energy of electron h: Planck s constant Typically E-U = 4eV In practice one must allow for the geometry of the probe tip See eqn 3.40 in text. 6-5

6 Build you own STM for $100: 6-6

7 The tip potential is biased wr.t. the surface If tip is +ve electrons flow from the surface and vice versa Some tips: TEM images TEM Picture of a cut STM Tip (Pt/Ir, 0.008" diameter). The image was taken before the STM experiment 200 kv, Bar length = 5 micrometer The cut tip is not well-shaped and sometimes it produces multiple tips. But it can be very sharp at the end. 6-7

8 Sharp tips work best : no multiple images Electrochemical etching is often used 6-8

9 Vibration Isolation The tip-sample distance must be kept constant within 0.01Å to get good atomic resolution. Therefore it is absolutely necessary to reduce inner vibrations and to isolate the system from external vibrations. Environmental vibrations are caused by: Vibration of the building Hz Running people 2-4Hz Vacuum pumps Sound Commercially Available A few thousand dollars 6-9

10 Imaging Modes constant-current mode, tunneling current is constant. adjusts the height of scanner at each measurement point, e.g. when the system senses a tunneling current increase, it adjusts voltage applied to piezoelectric scanner so that scanner lifts the tip and gives an increase in tip-sample distance. scanner height at each location constitutes topographic image. constantcurrent mode is generally used to acquire surface height data, scan speed limited by feedback response and thus takes longer to image an irregular surface at a larger scan size. constant-height mode, tip scans at a constant height above the sample and the tunneling current changes due to the topography and the local surface electronic properties of the sample. The current image is a result of measured tunneling current at each location on the sample surface. The constant-height mode can acquire data faster because the system doesn t have to move the scanner in the vertical direction, so it is most often used for imaging relatively smooth surfaces. 6-10

11 Scanning Tunneling Spectroscopy: page 152: maybe later : you need CHEM 2070 and Some solid state Physics first Some example Images building a quantum corral atom by atom 6-11

12 More images Scanning tunneling microscopy image of an array of self-organizing Pt nanowires on Ge(001). Note the periodicity doubling of the Pt nanowires along the direction of elongation. The spacing between adjacent Pt nanowires is only 1.6 nm. Sample bias V and tunneling current 0.54 na. 6-12

13 Atomically resolved STM measurement of a carbon nanotube. The chirality is visible. see later in course 6-13

14 STM image, 7 nm x 7 nm of a single zig-zag chain of Cs atoms (red) on the GaAs(110) surface (blue). 6-14