Quite an impressive explanation Rener thanks for that! So basically it is just a question of the relative size of the atoms. When we start to try and manipulate such fundamental elements of matter the tools used are also made of similar sized material elements. Considering the sensitivity, as described by your entering the same room as the STM, what are the ultimate limiting factos likely to be in this level of nano technology?
Jack Regner Trampedach wrote:
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 because the 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 thereforeonly 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 thesubstrate 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 theone direction than the others. You can find a higher resolution image here: http://www.almaden.ibm.com/vis/stm/images/ibm.tifwhere 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 setof 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 thesubstrate crystal, so both were resolved. I hope that explains things. Regner
