Microscopy Home PageThis is one of the most recent approaches to imaging small objects. It is unconventional in its mode of operation and is not a single method, rather a whole raft of closely allied but subtly different techniques.
This new kind of microscopy can image objects of widely ranging types and dimensions. It equals or surpasses the resolution of the electron microscope, but at the same time it avoids the requirement for vacuum. A scanning probe microscope can image a specimen in air, or immersed in a liquid.
Depending on the precise technique, the image contrast may represent the topography of the surface, magnetic properties of the specimen, aspects of surface chemistry, or a host of other factors.
The first instrument to be developed was a scanning tunneling microscope. This was invented in 1980 by two IBM engineers, G Binnig and H Rohrer. In 1986 they shared a Nobel prize for their pioneering work. It wasn't until the mid eighties that instruments were manufactured commercially, but today instruments are widely available and used in many ways.
The early scanning tunneling devices were followed by atomic force microscopes and later by a variety of other allied systems. Some of these are briefly described below. A very brief potted history is available on a page put out by TopoMetrix.
All scanning probe microscopes use a very sharp point as a probe. Ideally the tip of the probe is a single atom, finer tips giving higher resolution.
The probe is brought into close proximity to the surface of the specimen, but is not allowed to touch it. Some property of the tip/specimen distance is measured and used in a feedback control loop. In the scanning tunneling microscope for example, a small electric current is measured. As the tip approaches the specimen within atomic dimensions, the current rapidly increases for a constant voltage, and the tip is driven closer or further from the specimen in order to keep the current constant.
The tip or specimen is moved laterally (scanned) and changes in the vertical position measured. As a raster is built up an image is constructed to reveal the topography of the surface.
A good, illustrated introduction to the process is given by Basel University. An even fuller, more technical tutorial is provided by TopoMetrix. Wolfgang Heckl's group in Munich includes a brief, well-illustrated explanation
Depending on the tip material and the property being measured, maps of various surface properties can be constructed. Some of the approaches are discussed on the Park Scientific Instruments page. This site also includes pictures of the company's microscopes and some beautifully rendered images down to atomic resolution.
The University of Missouri at St.Louis also has a fine selection of images, with more added each month. IBM's STM Image Gallery contains further excellent images.
A particular case-study of a specialised technique is provided by Stanford University's work with the AFM on ultrafast voltage measurement using capacitive forces. An alternating current scanning tunelling microscope (ACSTM) is in use at Penn State University for work on non-conducting materials.
Blaine Stine runs a Web page dedicated to scanning probe microscopy, he offers a range of images, a selection of links to related sites, and some general discussion of the topic.
In the earliest days of a new instrument you must build the device yourself or do without. Many crude (and not so crude) scanning probe microscopes were built from scratch in labs and workshops around the world, mostly in university departments. This pioneering spirit remains today in the form of the 'home brew' scanning tunneling microscope project.
As with most new technologies there are early openings for small start-up companies to carve niches for themselves. The established manufacturers often fail to see the early promise of new methods. That's not to say they fail to innovate, but their effort is often focussed on improvements to existing product lines. Risky but exciting tends to be the preserve of the two-man business run from a garage or garden-shed, at least initially. Once the idea is accepted more widely and shown to have real market potential the older companies begin to move in as well.
BioForce is a good example of a start-up company which has seen a particular niche and seized it. They produce biologically modified tips which enable a microscope to detect and image molecular interactions.
New and established companies alike manufacture these instruments today. The list below is not intended to be exhaustive, it is simply a list of those companies providing Web pages of which I am aware. In alphabetical order, the companies are BioForce, Digital Instruments, Park Scientific Instruments, and TopoMetrix.
Apart from the obvious function of imaging structures at a range of scales, certain scanning probe instruments can be used to modify surfaces. The scanning tunneling microscope in particular is useful for this. By increasing the tip/specimen voltage above that needed for imaging, it is possible to move atoms, or attach other atoms or groups.
The University of Illinois is investigating the use of scanning tunneling microscopy for nanolithography in order to fabricate smaller microcircuits. The process depends on the application of small voltages which trigger chemical reactions between the substrate and adsorbed gas molecules. Taking this process to its logical conclusion, it may one day be possible to build complex nanomachines using scanning probe instruments.
The University of Missouri page discusses some research applications from nuclear track pits in mica, to the oxidation of silicon surfaces, to the study of crystal lattice distortions. At Stanford University the atomic force microscope is used for ultrafast voltage measurements, while at Penn State the Weiss Group is studying the molecular interactions of benzene with solid substrates, working at low temperatures to reduce diffusion rates.
To see how these instruments can be used in biological research, take a look at TopoMetrix' paper on the red blood cell cytoskeleton, or at Wolfgang Heckl's work on DNA molecules.
It's worth taking a look at the other Web sites in the scanning probe microscope list. Taken together they provide a wider view of the uses of these instruments, including images in many cases.
These novel instruments have already made a big impact in many fields of research and study. As they continue to become more widely available they will also be used more and more for routine functions such as quality control, medicine, and measurement.
It's likely that further development will lead to smaller, cheaper designs which will be resistant to vibration. We should also expect to see further growth in the range of phenomena that can be measured by scanned tips of various kinds.
Increasing the resolution still further is problematical. It's hard to conceive of a tip diameter smaller than a single atom and this has already been achieved. But scanning probe instruments have outstripped the resolution of every other kind of microscope, and will continue to reveal fine details of biological and inanimate materails. There's great scope for exciting new results in the years to come.