Scanning probe microscopy
In this kind of microscopy a needle with a very sharp is moved across the surface in meanders (line by line) and a computer reconstructs an image from the interaction of the needles tip with the surface. Depending on the type of interaction used (electrical mechanical or other) scanning probe microscopy is further classifies in subcategories.
The two main types of SPM are:
- electrical: STM scanning tunneling microscopy
- mechanical: AFM atomic force microscopy
In both cases STM and AFM one would like to have only one single atom at the tip. But this is wishful thinking. In reality the average tip looks more like a rounded deformed mountain top. Images have been taken (with big transmission electron microscopes) that show that. See external links. Metallic STM tips (made from etched wire) are usually sharper than AFM tips etched from e.g. a silicon nitride wafer.
- 1 Electrical SPM called: "scanning tunneling microscopy STM"
- 2 Mechanical SPM called: "atomic force microscope AFM"
- 3 How the UHV-systems bow up size and slow down everything
- 4 Experimental difficulties
- 5 Related
- 6 External Links
Electrical SPM called: "scanning tunneling microscopy STM"
Sharp onset of tip surface interaction
A very fortunate physical phenomenon makes it so that rather blunt tips are not too critical. When approaching the (electrically conductive) surface with a (conductive) tip that is connected to a different electric potential the current that arises makes a sharp (exponential) increase over a faction of an atomic diameter. This effectively hides all the atoms behind the ones at the very front. The reason for this behavior is that when the electrons (described as waves) seep into a space region where the potential energy is higher than their own energy (the vacuum between tip and surface) the wave becomes takes the shape of an exponentially decaying function. This is the infamous "tunneling effect". No magic here, just math describing physics usefully accurate.
Manipulation instead of just observation
Due to the direct physical contact between tip and surface (which is not present in electron microscopy) one can actually manipulate stuff like molecules and even atoms on that surface beside just merely imaging it.
DIY level base simplicity
The core principle of the electrical SPM (called scanning tunneling microscopy STM) is actually so low tech that that resolving of single atoms has been repeatedly archived by hobbyists in a domestic environment (living-room). No joke. This was limited to the easiest surfaces to image though. No shoving atoms around here. This is reserved for more professional systems.
Crashing tip by coughing loud
Somewhat miraculous seems that even low tech damping systems can in fact suffice to prevent the tip from instantly crashing into the surface and destroying itself. Still one should refrain from coughing too loud when located in the in the same room as the SPM microscope.
Since images are taken line by line and the moving parts are so much larger than the imaged resolution taking an image can take considerable time (minutes to hours). At any time (even without external vibrations) the tip can change its shape and thereby change or degrade the image. A very unnerving situation.
Moving fractions of an atom
To take these images one needs to move a needle just fractions of the diameter of an atom in a decently controlled fashion. Again somewhat miraculous there really are actuators that make this quite easy: "piezo-actuators". In some cigarette-lighters one can find a pieces of quartz. When they are hit and quenched together a very tiny bit they produce a very high voltage (creating the spark for ignition). The piezoelectric effect. Operated in reverse when such a crystal is exposed to just a very tiny voltage it contracts/expands/shears a just a very very tiny bit.
Hovering just above the surface
To keep the tip from crashing into the surface in normal operation one uses a feedback loop (control engineering). When the current rises one pulls back a bit when it sinks one moves closer a bit. How fast and in which character one does that can be set by some control parameters.
When following the height profile of the surface there is a fundamental speed limit though that can not be broken by any choice of control parameters. Problematically this speed limit is quite low since the moving mass of the piezo actuators is gigantic in comparison to the mass of atoms. Imagine moving a beard stubble (representing a molecule) with giant oil tanker (representing just the needle the much bigger piezo-actuators are not even included).
- When moving to fast down an edge one makes a jump creating a "shadow" in the image.
- When moving to fast up an edge one crashes the tip into the edge.
Drastic miniaturization of SPM systems into the microscale (MEMS SPM) could reduce that speed problem.
The smaller scanning probe microscopes are built the faster they can work (See: Scaling laws).
(TODO: Check if there are atomic resolution MEMS SPM's)
Instead of keeping the current constant one can alternatively set a fixed height and then look at the variation of the current while meandering over the surface. At some level of current one should retract though to prevent a crash. Also vertical drift becomes an issue. Some magnetic types of SPM are done this way.
How fat tips change the images
When steps on the surface are sharper than the tip one images the tip with the surface instead of the surface with the tip. This can be identifies by translation symmetric repeating patterns in the image. folding. This especially happens with blunt rough tips with two or more apexes. All the imaged structures are are doubled or multiplied in an other way. The image on the computer is a result of a mathematical folding operation between tip structure and surface structure. If the exact structure of the tip is known one can apply a reverse fold. This does not work for arbitrary blunt tips though. Noise! (TODO: lookup details of the limit here).
There have been attempts to automate the extraction of the tip structure (live?) from scanning over a surface with known structure and comparing the tip surface folding image with an ideal theoretical image of the surface.
Instead of removing the unpredictable shape-changes of tips by mathematical processing there have also been attempts to make tips that are more stable and or more defined. Very popular is picking up a carbon monoxide molecule that sticks down from the rather blunt tip like a needle with a very well known electron density distribution (DOS). Also attempted where attaching nanotubes or wrapping tips in graphene sheets. Modified tips can be very expensive. If not in cost when bought then costly in preparation time when self-made.
Mechanical SPM called: "atomic force microscope AFM"
Many of the things mentioned for STM's hold equally for AFM's.
The tips in AFM's are usually etched from a wafer shaped like a pyramid pointing down from a tuning fork bar. The tips typically quickly swings up and down quickly while slowly meandering over the surface to take the image. This can happen somewhat closer to the surface (tapping mode) or a little farther away (non contact mode). (Note that whether something contacts or not is a bit of a moot point in the soft sub nanoscale area. One can use various definitions for contact. There are VdW radii for atoms, covalent radii for atoms, and many other possibilities.)
The interaction of the tuning fork with the surface changes the oscillation frequency. This change can be used for the feedback loop. The deflection of the tuning fork is usually measured by a laser reflected of its back into a four quadrant photo diode.
Getting atomic resolution with AFM microscopes is harder than with STM microscopes. Since:
- the onset of the interaction is less sharp - (TODO: to very and quantify).
- tips are usually less sharp
To get atomic resolution (which initially was thought to be impossible) one needs to use a tuning fork with a higher stiffness.
How the UHV-systems bow up size and slow down everything
SPM microscopes themselves are quite small from human perspective (about the size of a bean can). This includes the tip/needle, the piezoelectric actuators and the suspension. Amplification electronics add a suitcase sized box.
But SPM microscopes that are featuring atomic resolution on a wide range on surfaces are enclosed in an ultra high vacuum (UHV) system and those usually fills a whole rooms. Optional cryogenic systems add to that.
There have been attempts to miniaturize SPM-systems (TODO: add references) but there has been almost no progress towards miniaturization of UHV-systems (state 2017). The lack of UHV-system miniaturization may be a factor in lack of motivation for SPM miniaturization.
SPM microscopes can be operated in air but there are barely any surfaces that can be imaged with atomic resolution. Notable exceptions are chemically very nonreactive (tech term: inert) surfaces like gold or single crystalline graphite (aka highly ordered pyrolytic graphite HOPG).
Samples inside of UHV system (e.g. Omicrom) are (often/always/sometimes?) purely mechanically tele-manipulated via crude grippers that are fed through stainless steel bellows. There also is rather bad visibility into UHV systems since there are only few view-ports in the system (fixed perspective). One can easily can crash the gripper that holds a sample into some internal obstacles loosing a valuable sample.
To retrieve stuff that fell down the whole system has to be opened. But this lets in water vapor. So after after and while pumping it down again the whole room filling system needs to be heated to oven temperatures over the course of several days!!
- general: scanning probe microscopy; electrical: scanning tunneling microscopy; mechanical: atomic force microscopy; combined: conductive atomic force microscopy
- At "Foresight Technical Conference 2013" Neil Sarkar gave a talk called "microscopic microscopes for the masses"
(TODO: Video recording was online (known!) - find it and add link to this vimeo video - was it removed?)
Now (2016) the microscope is available at a price point of $2,490 USD: (announcement)
- (TODO: Add links to SPM miniaturization attempts)
Images of tips
TEM-images of tungsten STM tips where atomic lattice features are resolved:
- Figure 3: Transmission electron micrographs of recrystallized W tips processed by FDSS.
This figure is part of the paper: Field-directed sputter sharpening for tailored probe materials and atomic-scale lithography
- Image of a tip wrapped with a graphene sheet: