Chem 111 Quiz 3 – October 21, 2005 Name: Multiple Choice: For ...

Chem 111Quiz 3 – October 21, 2005Name: _______________________________Multiple Choice: For the following, select from:A. Atomic Force Microscopy (AFM)B. Scanning Tunneling Microscopy (STM)C. Both of the above_C__ 1. Requires an extremely sharp tip for good resolution._B__ 2. Relies on the unusual quantum properties of electrons._A__ 3. Can image the surface of any solid._A__ 4. Probe touches the surface being imaged._B__ 5. Produces images at the atomic level._B__ 6. Surface being imaged must conduct electricity._C__ 7. Can be modified to carefully position nanomaterials in precise locations._A__ 8. Tip can be used to measure magnetic interactions._C__ 9. Probe scans over the surface in two directions._B__10. Relies on the flow of electric current.11. a) Briefly explain why some portions of the STM image of graphite are light in color whileothers are dark. What do the different shades represent? What is the instrument actuallydetecting? (3 points)The instrument is actually recording differences in current flowas the tip moves over the surface.The different shades of white, gray and black represents regionsthat where the tunneling current is different. The white areas arewhere the current is greatest and represent positions where thecarbon atoms are. As the shade transitions to black, the currentis decreasing and means that the tip is farther from atoms onthe surface.

) When 11-bromoundecanol is placed on the graphite, a pattern develops (see below). Weknow that Br produces spots in the STM image and the OH groups tend to be dark. So, whichof the following best shows the way in which adjacent molecules of 11-bromoundecanol align?(1 points)a)b)Br OH Br OHBr OH HOBrWhy did you make this selection? (2 points)The OH’s of different molecules orient towardone another because this arrangement promoteshydrogen bonding between the molecules whichis lower energy than if the OH groups pointtoward a bromine to which they cannot H bond.12. a) STM operates by one of two means. But both allow the imaging of the surface of asubstrate. In the constant current mode, the instrument is set to maintain a constanttunneling current from the tip to the sample. How does the instrument maintain thecurrent and how does this allow imaging of the surface features? (2 points)In constant current, either the tip moves away from the sample or the sample moves away fromthe tip as current increase. So, the image on the computer screen actually depicts the relativedistance between the tip and the sample.b) The sketch below shows the atoms on the surface of a sample. Draw the path of the STM tip(in constant current mode) as it passes over the sample. (2 points)path of STM tipsurface of sampleatoms of sample