Probing the Unseen

NANOSCALE ELECTRONICS

Nanometer-scale electronics moved forward on two fronts recently as a pair of research teams reported advances in fabricating and characterizing molecular devices.

In one of the studies, researchers demonstrated a new vibrational spectroscopy technique that can be used to probe molecules hidden in buried interfaces in molecular junctions. In the other investigation, scientists devised a room-temperature method for fabricating complex molecular structures and positioning them on surfaces with atomic-scale control.

With conventional electronics-fabrication methods rapidly approaching the limit of miniaturization, scientists are trying to fashion nanoscale electronic components using small numbers of molecules. The goal is to drastically miniaturize today's circuitry to prepare faster and even more densely packed electronic components.

At the University of Minnesota, chemistry professor Xiaoyang Zhu and graduate student Yongseok Jun developed a procedure for recording infrared spectra from molecules concealed in molecular junctions. Typically, the junctions are formed by trapping molecules between a pair of very fine metal electrodes. The setup is used to probe current-voltage properties of the trapped molecules but provides little information about their structure and conformation.

To sidestep that problem, the Minnesota team sandwiched layers of octadecyltrichlorosilane--and separately, mercaptohexadecanoic acid--between a gold electrode and a silicon crystal designed for attenuated total-reflection Fourier transform infrared spectroscopy [J. Am. Chem. Soc., published online Sept. 24, http://dx.doi.org/10.1021/ja046431p]. By using a lightly doped crystal or by coating the crystal with a very thin layer of gold, the group enabled the semiconductor to function as an IR waveguide and an electrode. Then the team measured changes in the frequency, intensity, and shape of spectral features caused by bringing gold into contact with the molecules. Now the group is working to record spectra while voltage is applied to the samples.

Meanwhile, Mark C. Hersam, Rajiv Basu, and their coworkers in the materials science and engineering department at Northwestern University used a scanning tunneling microscope tip to deposit tetramethylpiperidinyloxy (TEMPO) radicals on opposite ends of a row of silicon-atom dimers on a hydrogen-covered silicon crystal. Then they removed a single hydrogen atom from the same row of dimers--thereby exposing a silicon dangling bond (an unsaturated valency)--and exposed the crystal to styrene. The room-temperature sequence resulted in a precisely positioned chain of styrene molecules bound at both ends by a pair of TEMPO molecules [Appl. Phys. Lett., 85, 2619 (2004)].