Spectroscopy with STM Though scanning tunneling microscopes are known mainly for producing images at the atomic scale, they are also good for high-resolution spectroscopic analyses because the data are not spatially averaged. Such studies are useful for looking at surface phenomena, for chemical identification, and for processes such as heterogeneous catalysis. Inelastic electron tunneling spectroscopy (IETS), on the other hand, is the most sensitive technique for measurements of surfaces, requiring as few as 109 molecules to produce a spectrum. However, the molecules studied through IETS are typically buried in a complex environment, which makes characterization more difficult. That is why alternative methods such as electron energy loss spectroscopy and IR spectroscopy, which can be performed on well-ordered surfaces are often used. Now, Wilson Ho and colleagues at Cornell University describe a technique that has the advantages of both. Using an ultra-high vacuum scanning tunneling microscope, the researchers performed IETS on a single molecule on the surface. Rather than using the metal-oxidemetal tunneling junction typical of IETS, they chose a scanning tunneling microscope junction—a sharp metal tip separated from the surface by a vacuum gap. Because the microscope can create images of the surface with atomic resolution the foondinfif environment of a molecule be determined precisely. In addition because both vibrational spectra and inelastic images can be produced the adsorbed molecules can be identified in two ways The authors used this method to look at acetylene and deuterated acetylene on Cu(100) and to obtain an inelastic tunneling spectrum. They suggest that the technique will permit more detailed studies of small molecules in complex environments and of functional groups within a single molecule. (Sciencc 1998,280,1732-35)
eter of 9.7 nm. The authors created a range of modified surfaces—from isolated molecules to near-monolayer coverage. The heights of the isolated adsorbed denDendrimers, or "starburst" polymers, have drimers indicated that they were flattened attracted much attention, but there have more than would be expected from their been few reports of the visualization of indispherical shape in solution. For example, the vidual dendrimers. Richard M. Crooks of Texas A&M University, Antonio JJ Riccc oo height of the individual G8 dendrimers on a naked gold surface ranged from 3.5 to 4.0 nm, Sandia National Laboratories, and their coworkers image generation 4 and 8 (G4, G8) which is —60% less than the ideal sphere. The authors believe that this deviation from polyamidoamine (PAMAM) dendrimers the ideal sphere is the result of Au-amine adsorbed on gold surfaces. G4 dendrimers are soft and deformable with an ideal spheri- bonds at multiple sites. Interestingly, when cal diameter of 4.5 nm, whereas G8 dendrim- the modified surfaces were exposed to hexaers have a harder exterior and an ideal diam- decanethiol solution, the dendrimers changed their conformation from oblate to prolate as more stable fhiol-Au bonds replaced of the amine-Au bonds With near-monolayer coverage the hexadecanethiol caused the dendrimers acereeate and form "Dillars" as hieh as 30 nm (T Am Chem Top-view topographical images of (a) G8 and (d) G4 dendrimers on a
Reach for the starbursts
Au(111) surface. Profiles of the surfaces are shown in b and e. Exposure to hexadecanethiol for 4 h results in the profiles in c and f.
Low-cost oxygen sensor Light-emitting diodes and a ruthenium poly(pyridyl) complex could prove to be the basis of a low-cost, solid-state oxygen sensor to replace electrochemical sensors or other optical sensors. Frank V. Bright and co-workers at the State University of New York at Buffalo, Innovative Scientific Solutions (Dayton, OH), and the University of Virginia describe a sensor in which a ruthenium complex is immobilized in a porous sol-gel-derivedfilmthat is coated directly on a quantum well LED. The ruthenium complex—tris(4,7diphenyl-l,10-phenanthroline)rutheniumll, or [Ru(dpp)3]2+—was selected because it has a strong absorption band in the bluegreen spectral region, a high luminescent quantum yield, a long excited state llfettme, and minimal overlap between the absorption and emission spectra. The output of the LEDs was stable in terms of wavelength and intensity over the voltage range 2-5 V, with a
q,
1QQ8 120
relative standard deviation
