Scanning Tunnelling Microscopy Essay Example

Scanning Tunnelling Microscopy Essay Example Scanning Tunnelling Microscopy Essay Scanning Tunnelling Microscopy Essay The STM with its unmatched combination of high vertical and lateral resolution in a promising new tool that can be operated under ambient conditions, yielding three-dimensional detailed images, (H. Strecker and G. Persch, 1990, p441-445). The scanning tunnelling microscope (STM) is a solid-state microscope based on the principle of quantum mechanical tunnelling of electrons between a sharp tip and a conducting sample. Surfaces can be studied by allowing the individual atoms to be imaged in real space. By scanning the tip across a sample surface it is possible to image directly the three dimensional structure of a surface down to atomic scale resolutions. Prior to the invention of STM, the only way that surface structures could be deduced was by more indirect methods such as low energy electron diffraction (LEED) or medium-energy ion scattering (MEIS). Technique The tip used in STM is very sharp and ideally terminates into a single atom. The tip is mounted onto a system of piezo electric drives, which provide movement in three dimensions. (http://nanowiz.tripod.com/stmbasic/stmbasic.htm, 14/10/03). The movement is controllable with sub-atomic scale accuracy and can be brought within a few Amstroms of the conducting sample surface. The metallic tip and the conducting substrate are in very close proximity but are not in actual physical contact, ( chem.qmw.ac.uk/surfaces/scc/, 7/10/03). At separations as small as this, the outer electron orbitals of the tip and sample overlap. On the application of a bias voltage between the tip and the surface, electrons are able to tunnel through the vacuum barrier via the quantum mechanical tunnelling effect. The direction of current flow is determined by the polarity of the bias. If the sample is biased negative with respect to the tip, then electrons will flow from the surface to the tip as shown above, whilst if the sample is biased positive with respect to the tip, then electrons will flow from the tip to the surface as shown below. (chem.qmw.ac.uk/surfaces/scc/, 7/10/03). The exponential (in vacuum) decay of the electron wavefunctions means that the tunnelling current is extremely sensitive to the tip-sample separation. This provides a very fine resolution of the surface. Quantum Mechanical Tunnelling The infinite potential walled particle in a box theory does not allow any of the wave function to escape the box as it would have to have more than infinity energy to cross the barrier. Allowing the potential energy well to be a finite number has the effect of making it possible for the wave function of a particle that is trapped in this potential well, to partially escape and thus have a presence outside the confines of the box. The wave function can transverse the potential barrier, although it will decay exponentially through the barrier. Assuming that the wave function does not totally decay away before the end of the barrier, the particle can have a physical presence on the other side of the potential barrier. If the potential barrier is long range, then the wave function will decay away exponentially and tend towards zero. Upon reaching the end of the potential barrier, the particle will have an infinite small wave function and zero presence on the other side of the barrier. This property is known as quantum mechanical tunnelling. (chembio.uoguelph.ca/educmat/chm729/STMpage/stmdet.htm,10/10/03). The quantum mechanical phenomenon creates the high degree of sensitivity necessary for atomic scale imaging of surfaces. The quantum mechanical tunnelling current is highly dependent on the tip-surface distance. The distance between tip and surface is usually of the order of 0.3 nm and the tunnelling voltage V ranges from a few mV up to a few V, depending on the conductivity of the surface. The tunnelling current typically varies between 10 pA and 1 nA, (fys.kuleuven.ac.be/vsm /spm/introduction.html12/10/03). The tunnel current decreases to 1/10 of its initial value for every 0.1 nm increase in gap separation, (Kaiser, W. J. Stroscio, J. A., 1993, p78) The essential aspect of STM is the extreme sensitivity of the tunnelling current to the tip sample separation. It is therefore important to realise that the tunnelling current is a quantum phenomenon. In classical physics the current could not flow across a gap. Modes of Operation Constant height mode In this mode the vertical position of the tip is not changed, equivalent to a slow or disabled feedback. The tunnelling current varies depending on topography and the local surface electronic properties of the sample. The current as a function of lateral position represents the surface image. This mode is only appropriate for atomically flat surfaces. If the surface were not flat, the STM tip would crash. An advantage of constant height mode is that it can be used at high scanning frequencies (up to 10 kHz). (http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2002/sm242/stmdesign.htm, 12/10/03). Constant current mode In the constant-current mode, the current is used as the input to a feedback circuit that moves the scanner together with the tip up and down in the height direction. With an applied potential, the tip is brought close to the sample surface until the tunnelling current set point is detected, at which point the constant-current feedback loop is locked. When the tip moves laterally to a new position, any subtle sample-tip distance variation will lead to the fluctuation of the tunnelling current. Consequently, the feedback circuit will move the tip up and down until the current keeps the set-point value. As a result, the moving tip keeps the constant sample-tip distance, tracing the surface topography. The main advantage of constant current mode is that the tip will not crash into a large cluster of atoms at the surface. Constant current mode can measure irregular surfaces with high precision, but the measurement takes more time, (http://std2.fic.uni.lodz. pl/stm.html, 13/10/03). (http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2002/sm242/stmdesign.htm, 12/10/03). Tip etching The construction of the tip is one of the most crucial aspects of STM. The tip must be approximately one atom thick in order for the STM to be carried out effectively. Surface pictures can appear to be distorted due to the presence of more than one sharp protrusion, (Ouseph, P. J. Gossman, M., 1998, 701-704). Some important characteristics of a tip are * Sharp tips which allow high resolution STM observations * Small resonance area * Thick taper to reduce tip oscillation during STM scans (mme.wsu.edu/~reu/poster2000/Ronald2000/ronald/ppframe.htm, 14/10/03) Multiple tips can be formed when suspended particles are picked up from the etching solution, (NaOH, KCN). Multiple tips often lead to the occurrence of shadow effects and ghost images, (physik.tu-berlin.de/institute/IFFP/daehne/ index.htm?/institute/ IFFP/daehne /forsch/rs-tips.htm, 20/10/03). Conditions The STM can operate in many different kinds of environments. A simple STM can be run in ambient air. However, in order to achieve effective results in Surface Science the STM is run in ultra-high vacuum. Vibration Isolation As the tip is oscillating across a very small area, it is important to isolate vibration. Vibration is kept to a minimum by several methods. Placing the STM on a spring/damping table is one way to cut out vibrations travelling through the floor. The first typical isolation system is the coiled spring suspension with magnetic damping and the second is a stack of metal plates with viton dampers between each pair of steel plates, (Kaiser, W. J., 1993, pp58). The basement level of a building is preferred because there are lower level of vibrations. To stop wave propagating through the air, a foam cover can be placed over the instrument, (mme.wsu.edu/~reu/Matt/Matt.htm, (19/10/03).