That is a good question, Jack, and we do see hints of the nickel (Ni) substrate. First of all, you can see several small bumps and holes in the grey background. These are defects in the Ni crystal, where there is either an extra or a missing
Ni atom. These point-defects are not nearly as large as the Xe atoms becausethe Ni atoms are reactive and easily bonds with the Ni crystal - extra ones gets partially embedded in the crystal, whereas Xe is a inert element, and therefore
only binds weakly, through van der Waals forces - it sits mostly on top.I went back to the original Nature paper (Eigler & Schweizer, 1990, Vol. 344, pp. 524)
to find a few more details.The xenon (Xe) atoms are placed on a rectangular grid determined by the grid of the substrate Ni crystal. The Ni crystal has a 0.25 nm cubic unit cell - a box with all sides being of equal length and one Ni atom at each corner - actually, to form the crystal, you would just have the eight of the atom in each corner (the part contained in the quadrant inside the box) and with the eight corners of the box, you will, all in all, have one atom in your unit cell (hence the name). When you stack the boxes together you form whole atoms at all the corners - just to give you the picture. Now this crystal of regularly spaced atoms is cut at a 45° angle, which means the surface will now consist of a grid where in one direction, the atoms are spaced square root of 2 = 1.41 times further apart than in the other direction. The electrons will easily fill-in the narrow gap, but less so for the wide gap - which means the
substrate will look like a plowed field.The dimensions of the unit cell on the surface is 0.35x0.25 nm (a nano-meter is a billionth of a meter in US parlour). The Xe grid has cells that are 4x5 Ni atoms wide, so between each Xe atom, there are 4 Ni atoms in the horizontal direction and 5 in the "depth" direction. They also say that the Ni substrate is not resolved in this picture - in some of the pictures published in the Nature paper, you can see horizontal lines from the substrate crystal - it would be more readily resolved in the
one direction than the others.
You can find a higher resolution image here:
http://www.almaden.ibm.com/vis/stm/images/ibm.tif
where you can see faint diagonal lines. An STM scans continuously in one
direction
and jumps to progressive scan-lines in the other direction, so my
suspicion is that the
scan direction is perpendicular to those diagonal lines - that is, the
diagonal lines are
just an interference pattern between two sets of regularly spaced lines
- you can try
that yourself with two overhead-slides with finely spaced lines on them.
Place one on
top of the other and change the angle between the lines and you will see
a third set
of lines appear.The picture was not made to resolve the substrate - that had been done before. But I have seen similar pictures made by the Aarhus group (first link in previous post), but I couldn't find them yesterday - the lettering was closer to the scale of the
substrate crystal, so both were resolved.
I hope that explains things.
Regner
Jack Lewis wrote:
Thank you for the images which were quite fascinating. This may seem like a dumb question but shouldn't we be able to see some hint of the nickel atoms that the helium atoms were sitting on?Jack Regner Trampedach wrote:On the contrary, Jack. The scanning tunneling microscope was invented in 1987 and can easily resolve atoms. Have a look at: http://www.phys.au.dk/spm/ There is also a STM picture and movie-gallery in the left-hand-side ofthat page. Each bump in the pictures is an atom. We can tell which element a particular atom is, and we can also remove and deposit single atoms usingthe same technology - first done in 1989 by D. Eigler from IBM:http://www.ieee-virtual-museum.org/collection/event.php?id=3457012&lid=1 <http://www.ieee-virtual-museum.org/collection/event.php?id=3457012&lid=1> I have seen the scanning tunneling microscope in action. It drifted markedlywhen I entered the room, because my body-temperature heated the roomand the microscope, resulting in an expansion of the microscope. I also snapped my fingers, and the sound, traveling through the air and the many stages of suspension designed to isolate it from shaking - resulted in a deep furrow in thepicture.
