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clfu2000

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[资源] SPM简介

What is SPM?
Scanning Probe Microscopes (SPM) allow to see atoms and molecules at surfaces. There exist several types of SPM and all these different types can be used in different environments.
For the invention of the first type (the scanning Tunneling Microscope, STM, in 1981) the Nobel price was awarded in 1986 to the inventors Binnig and Rohrer of IBM in Switzerland.

On this site one can find out what the working principles are of the SPMs, where they can be used for, and what are the practical steps to be taken to get the instruments to work. In addition you can do realistic SPM experiments and image analysis with a SPM simulator.
SPM measurements can be quite easy when the instrument is sitting in the ambient air or rather difficult when, for example, the instument is located in an ultra high vacuum or at extreme low temperatures.

Every different type uses a different probe (tip), which has specific propeties.

SPM short introduction
A Scanning Probe Microscope (SPM) is an instrument to make a height image of a very small part of a surface. This works basically the same as in a geographical map where altitude lines give the local height of the surface of the earth.

How to make a height image.
This is the surface we want to measure.

With the probe, which is for example a very sharp needle, the height is measured. Because the probe can measure the height only at one place at a time, the probe is moved over the surface and measures the height at every point. This is called scanning.

The results of the measurements are put in the height map.
For example in the figure below, the heights are indicated by different shades of gray. A real image consists of much more points, for example 512 x 512.

Different kinds
The probe of the SPM can can be different things:

A very sharp needle:

Scanning Tunneling Microscope (STM)
A kind of tiny diving board with a sharp tip mounted onto it:

Atomic Force Microscope (AFM)
An optical glass fiber with a sharp point:

Scanning Near-field Optical Microscope (SNOM, not only feeling but also seeing!)
A more detailed description you can find in the different types paragraph.
SPM: Different types

All SPM's use a sharp needle to scan the surface of the sample. In the STM the sharp needle "feels" where the sample surface is located by measuring the tiny current which flows from needle to sample when the distance is very small. But there are a number of other ways the needle can probe the sample without toutching it (or touching it only very lightly). All these different methods give a height map of the sample surface. However, because the probing uses different phenomena also different properties of the sample can be observed. Just like there exist different geographical maps as soil type maps (sand, clay, peat), vegetation maps (woodland, heather, agriculture, …) or population density maps, different sample properties can be mapped by using the appopriate SPM probes.

The most important types are described below. By clicking the sub-heads more detailed descriptions are opened.

STM (scanning tunneling microscope).

The STM is the first SPM invented. In this case the probe is a very sharp metallic needle. When the needle appoaches a conducting sample very closely (e.g. 1 nanometer) a small current starts to flow. This "tunnel'' current is very strongly dependent on the sample-needle distance and is used to control the sample-needle distance. With this type of SPM many material properties of the sample can be mapped. Especially properties related to the electronic structure of the sample like magnetism, work function and semiconductor properties.

stearine acid
with a STM(35*35nm)
AFM (atomic force microscope).
In an AFM one uses the force between the needle and the sample surface. The seedle sits on a tiny leaf spring. The degree of bending of the leaf spring indicates the strength of the force. For measuring the bending many different method can be used. Mostly one observes the dispacement of a light beam deflected from the leaf spring. Properties, which can be measured are: Mechanical properties on a nanometer scale like elastic constants, coefficients of friction, and local forces. Advantage: The AFM can be used for non-conducting samples. About all materials can be studied.

virusses observed
by an AFM
400*400nm
MFM (magnetic force microscope).
The working of the MFM is based on the same principles as the AFM but the needle is magnetic. As a consequence the needle reacts also to the magnetism of the sample allowing the mapping of the magnetism. The MFM is often used to study the magnetic bits of hard disks, for example: how to make the bits as small as possible, how close together can one write them without getting to much overlap of the bits, etc. Mapped property: magnetic fringe field of the sample; boundaries between regions with different magnetism (domain boundaries).

Magnetic information
on a hard disk measured
by an AFM
10*10 micrometer
SNOM (scanning near-field optical microscope).
The needle consists of a sharpened optical glass fiber. The tip is so sharp that hardly any light can escape from the tip. However, just as with the STM, we get a kind of tunneling when the tip is extremely close to the sample: tunneling of light particles (photons). Mapped property: With the SNOM we can map the optical properties of the sample. Actually as in an ordinairy microscope but with much higher magnification

molecules measured by a SNOM
SP-STM (spin polarized-STM)
In the SP-STM a magnetic needle is used. The tunneling probability of electrons from needle to sample depends on the direction of the magnetism of the sample. When sample and needle have the same direction then the tunneling probility is larger than when they have opposite direction. This allows to make a map of the magnetic structure of the sample surface. The magnetic resolution is at the atomic scale, just like for the common STM. Comparing with MFM, the resolution can be hundred times better. But the measurements with SP-STM are much more difficult. Only a few laboratories in the world have mastered these measurements, among them the university of Nijmegen. Property mapped: strength and direction of the magnetism of the sample surface.

Magnetic signal of SP-STM

[search]SPM[/search]
http://www.spmlab.science.ru.nl/eng/uitleg/

[ Last edited by clfu2000 on 2007-7-11 at 18:17 ]

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