SPM (Scanning Probe Microscopy)
Principle
A probe is scanned across a sample surface in such a small distance, that an interaction between the sample surface and the tip of the probe takes place. This interaction is measured and the measurement results taken at each single point are put together to construct an image of the surface.
Interactions:
electromagnetic radiation
wave function overlap between surface atoms and probe atoms
mechanical, electrical or magnetic force
Scanning tunneling microscopy (STM)
the interaction between tip and surface can
change the state of the sample surface.
Working principle
high resolution (lateral and vertical)
contains information about the topology and the electronic
properties of the sample surface.
atomic level
A metallic tip is brought near to a (semi)conducting surface, voltage is applied and a current is measured, while moving the tip across the surface. The current measured contains info about the surface structure
For small applied voltage one finds jt ∝ V (in this case DOS is independent of V) --> linear dependece
By applying an external bias voltage the equilibrium is disturbed; electrons from the tip tunnel into empty sample states and a net tunneling current is observed.
The tip is represented by an s – wave function. If the tip is scanned at constant height (cte V at z-piezo) the tunneling current varies depending on the wave function overlap between tip s-states and surface states
To describe the tunneling current --> occupied DOS of our tip and the unoccupied DOS of the sample (the DOS of the ideally tip doesn't change while scanning the surface, so it doesn't have to be taken into account)
The density of states can be deduced by modulation of the bias voltage! Or Current-Imaging tunneling Spectroscopy (CITS)
tunneling current depende from the distance tip – sample (also valid for the 3D)
The wave function decays exponentially within the potential wall, which yields the exponential dependence jt∝e−2a κ of the tunneling current.
Sensitives to vibrations
Piezoelectric effect: Some materials change their length if an electric field is applied. The inverse can also occur (deform the crystal by applying an electric field).
This effect is used to move the tip of over a surface in a controlled way.
Operation Modes
Current constant
Height Constant
varies the z-position (height) of the tip by a corresponding applied voltage
obtains planes of constant DOS
Distance (tip-surface) isn't varied
Work Function Mode
needs flat surfaces
Measures the position dependence of the tunneling current
The distance tip – sample is varied periodically by applying a high frequency voltage at the z-piezo and measuring the tunneling current utilizing a lock-in technique.
tunneling current is measured as a function of tip-surface distance
Spectroscopy Mode
measures the tunneling current as a function of applied voltage
Enables real space images
Images give information about surface unit cell size and symmetry and/or surface electronic structure
obtains info about the LDOS of the sample surface
tip is at a fixed position over the sample surface
info about the local height of the potential barrier
Only works with semiconducting surfaces