Portland, Ore. - The United States, Italy and China are cooperating to develop superior thin-film technologies that could endow future devices with properties not possible in today's processing.
A group of scientists in the United States and Italy has described transparent organic thin films that use self-assembly to create ultrathin layers with superior electro-optical properties. A cooperating group in the United States and China has demonstrated that superconductivity can be precisely controlled in ultrathin films.
Professors Tobin Marks and Antonio Facchetti at Northwestern University collaborated with professor Giorgio Pagani of the University of Milano-Bicocca to demonstrate a vapor-deposition technique for fabricating transparent organic thin films that self-assemble. The group reports a hundredfold increase in the self-assembled material's electro-optical responses at both the 1.3- and the 1.55-micron communications wavelengths.
By using hydrogen-bonding reactions, the group demonstrated that electro-optical thin films can be fabricated on silicon, glass, ceramic and organic-polymer substrates. The method is said to offer a lower-cost approach that nevertheless improves electro-optical properties.
"Vapor deposition of electro-optical films is still a relatively new technique," said Marks, "but it offers many advantages over conventional processing."
In the lab, Marks' and Pagani's group used hydrogen bonding to create the conditions under which precisely controlled thin films self-assemble. To maximize the electro-optical response of the material, the group ensured that the molecules' dipoles were oriented in the same direction on each layer during deposition, but were reversed from the orientation of adjacent layers. Use of these alternating layers-called asymmetric deposition-results in configurations that unlock the nonlinear optical properties masked by layers of symmetrically oriented dipoles, the team said.
To prove the concept, the group fabricated three compounds, each containing a pyridine ring on one end that acted as a hydrogen-bond acceptor. On the other end, a hydrogen-bond donor was fabricated. When the matched ends of the asymmetrical molecules were linked, the result was conjugated asymmetrical thin films. This head-to-tail alignment reportedly resulted in films with a 100x greater nonlinear response than lithium niobate.
Separately, Qi-Kun Xue of the Chinese Academy of Sciences in Beijing, with researchers from the University of Texas (Austin) and the University of California at Berkeley, showed that thin films can be more easily coaxed into superconductivity with careful deposition techniques. The team deposited crystalline lead onto silicon substrates at every thickness between 10 and 30 monolayers, then measured the superconductivity transition temperature at which the metal lost its resistance to electricity. The lead proved stable when deposited in odd numbers of between 11 and 21 monolayers. Further, the team demonstrated that superconductivity began occurring at lower temperatures for thinner films: 5 Kelvin at 15 monolayers vs. more than 6 K at 25 monolayers.