Lecture room width - 10 meters
Human Hair thickness- ca. 100
microns (0.1 mm)
[mine is 60 microns]
Typical organic molecule - 1 nm
C-C bond - 0.15 nm (1.5Å)
(Room:Hair :: Hair:Molecule :: 100,000:1)
You would probably not consider a hair to be inconceivably thin compared to the lecture room's width. By the same token molecules are not inconceivably small, just very small - 100,000 times smaller in width than a hair. So it is not inconceivable that we could "feel" individual molecules (or even atoms), still it is certainly a point of pride for science that by using SPM we can do so. Before SPM came along in the mid '80s chemists tended to think of normal molecules as inconceivably small, so it came as quite a surprise that a rather simple machine could feel one.
(I) For details on how STM and AFM work click to download a booklet (~900Kb, PDF) describing scanning probe microscopy. (Note: you are not responsible for material in this booklet. It is provided as a reference for the curious. You only have to understand what went on in lecture. )
Think about the lateral and depth scales in comparison to atomic and molecular dimensions in the following pictures.
(II) Routine Resolution AFM
The image to the right shows the surface of a crystal of benzoin (C6H5CHOHCOC6H5) as it dissolves under a liquid of 95% water : 5% n-propanol (as if we're looking at the bottom of a shallow pond.
The 256x256 pixel image spans about 5 microns (1/10th of a hair's breadth). Each benzoin molecule is about 5Å (0.5 nm) wide. How many molecules wide is the picture? How many molecules wide is a pixel ?
The surface is colored as if illuminated by a light from the left (like a satellite picture taken with afternoon sun). Two light, straight lines crossing the whole image are steps separating terraces that differ in height by two molecules. Actually two single-moledule steps join near the top left to make the top step double along most of its length.
The AFM tip has been used to scratch a cross about 10 molecular layers deep near the center of the image.
[This is one frame from a movie showing how the crystal dissolves. The movie was made at Yale by Kraig Steffen Click to see the movie (364K).]
(III) Near-Atomic Resolution STM
Here is an STM image showing some 25 molecules of 12-bromoundecanoic acid [Br(CH2)12COOH] lying down side-by-side in rows to make a layer one molecule thick on the flat surface of graphite. Below the STM image is a model diagram explaining what it shows.
The black bar marks the location of a single long molecule stretching some 1.5 nm (15 Å) from the dark brown COOH group on the lower left to the bright yellow Br atom on the upper right.. A corresponding molecule is outlined in a box at the lower left of the artist's model of the packing shown below. (Note that successive molecules along the row traced by the long black line are anti-parallel, meeting COOH to COOH then Br to Br.)
The molecules are sufficiently thin that electrons can "tunnel" down through them from the sharp(!) metal tip to the electrically conductive graphite (which is sketched as a pattern of hexagons at corners of the model where layer of acid molecules has been removed. The extreme sensitivity of "tunnelling" current to distance, when the tip is almost touching the molecule being imaged, allows clear, single-molecule resolution without damaging the delicate monolayer. The image almost has atomic resolution, since, knowing what to look for, one can trace the zig-zag chain of 11 light brown carbon atoms (green in the model) between the COOH group and the bromine atom, and perhaps see the H atoms as small yellow balls (gray in the model).
[For this image we thank Dalia Yablon and the Columbia University research lab of Professor George Flynn (Yale '60). More images are available on the group's web page.]
The spectacular "Quantum Corral" of 48 iron atoms is shown at this IBM Almaden site.
For the specific image shown in class click here.
(V) Click Veeco, to see actual tips used in AFM (and other kinds of SPMs)
At the top left of the page click on Probes/Modes and Application to see the variety of tips available. Note the size of the tips especially their radius of curvature (how wide the bottom of the tip is).