Department of Chemistry,
University College London,
T: +44 (0)20 7679 1003
Scanned Probe Microscopy
Group SPM Facilities
Our group currently uses three scanning probe microscopes; an Omicron UHV Low Temperature Scanning Tunnelling Microscope (LT-STM), which can operate down to 4K; an Omicron UHV Variable Temperature STM (VT-STM) which operates in the range 50-1100K; and an Omicron UHV Atomic Force Microscope / Scanning Tunnelling Microscope (AFM/STM), which is capable of atomic resolution on insulating samples in its non-contact AFM mode.
Theoretical Background of Scanning Tunnelling Microscopy
Invented in 1982 by Gerd Binnig and Heinrich Rohrer, the scanning tunnelling microscope (STM) has had a massive impact in many diverse scientific disciplines with its ability to image and manipulate individual atoms. Its inventors were awarded the Nobel Prize in Physics in 1986 and STMs are used today in fields such as surface science, condensed matter physics, and especially in the study of nanotechnology.
A schematic of the experimental setup of an STM is shown in the figure below
|Schematic of STM Operation|
Invented in 1982 by Gerd Binnig and Heinrich Rohrer, the scanning tunnelling microscope (STM) has had a massive impact in many diverse scientific disciplines with its ability to image and manipulate individual atoms. Its inventors were awarded the Nobel Prize in Physics in 1986 and STMs are used today in fields such as surface science, condensed matter physics, and especially in the study of nanotechnology.A schematic of the experimental setup of an STM is shown in the figure below:
During operation an atomically sharp conductive tip is brought to within a few angstroms of a surface, and a bias voltage applied between the two. This results in a small tunnelling current (~ few nA) between the tip and sample which has an exponential dependence on the tip-sample separation. The tip is mounted on a piezo-electric transducer which allows it to be positioned with great accuracy, and by rastering the tip over the surface a map can be built up of the topography of the surface with atomic resolution. There are two common modes of operation of an STM;(i) constant height mode, where the height above the sample is kept constant and the current variation provides the contrast for imaging; and (ii) the more common constant current mode where the tunnelling current is kept constant via a feedback loop which adjusts the height of the tip as it scans the surface.
|(i) Constant Height Mode||(ii) Constant Current Mode|
In the animation below we demonstrate a constant current line scan above a typical surface, that of the rutile TiO2 (110) 1x1. It is interesting to note that the tip does not follow the topography of the surface as we are in fact imaging the local density of states (LDOS) of the surface as we scan, and since different atoms have different surface states, they will appear as different heights in the scan.
The technique of STM is very powerful and it is now routine to capture images with atomic resolution, although in order to do this it is normally necessary to perform the experiments under ultra-high vacuum (UHV) conditions to ensure the surfaces are very clean and to employ surface preparation techniques such as argon ion bombardment and high temperature annealing to provide a well ordered sample. It is also possible to perform spectroscopic measurements of surface atoms and adsorbates, as described here, by holding the tip above a single atom, varying the voltage and measuring the current. In this way conductivity measurements can be made, as well as vibrational spectroscopy.
The primary limitation of STM is that it requires the sample to be either conducting or semi-conducting, however in our group we also employ an atomic force microscope (AFM) which is not limited in this way, and can provide atomic resolution on insulating samples as well.
Theoretical Background of Atomic Force Microscopy
Introduced in 1986 by Binnig, Quate and Gerber, the atomic force microscope (AFM) overcame one of the key limitations of the (STM), by alowing atomic resolution imaging on insulating surfaces. The principle behind is demonstrated below; a sharp tip mounted on a cantilever is brought into very close proximity or contact with a surface and there is a deflection of the cantilever due to the interatomic forces between surface and tip. The deflection of this cantilever can then be measured, yielding the force between the tip and the surface.
|Schematic of AFM Operation|
There are two operating modes possible with the Omicron AFM/STM, that of contact (static) AFM, where the tip experiences net repulsive forces from the atoms at the surface; and non-contact (dynamic), where the tip is further away from the surface and experiences net attractive forces. In contact mode the deflection of the cantilever is directly proportional to the force on it; this force is kept constant with a feedback loop and by rastering the tip over the surface a map of the topography can be built up. In the non-contact mode the form of detection used is that of frequency modulation (FM). In FM-AFM the cantilever is driven at its resonant frequency which is shifted as a result of interactions between the tip and the surface; this frequency shift is kept constant with a feedback loop leading to a topographic map of the surface.
Although the setup is relatively simple, the requirement of modelling the numerous interaction forces greatly complicates analysis of AFM images, especially in the non-contact mode which is the only way to achieve true atomic resolution. In the image below, a sample of HOPG (Highly Oriented Pyrolytic Graphite) has been imaged in the lateral force contact mode (image composed by the torsion of the cantilever as it scans, and appears to demonstrate atomic resolution.
|Lateral force contact AFM of HOPG (2.5 nm x 2.5 nm, FN 0.2 nN)|
However, this is not considered to be demonstrative of true atomic resolution; it is not possible to image defects in the surface in this way due to the effect of multiple atoms in the tip and surface contributing to the interaction force. Instead it is necessary to use the non-contact mode to gain true atomic (and even in some cases sub-atomic) resolution, where it is considered that the effect of many atom contributions to the interaction forces are minimised.