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For decades, thinking big has frequently meant pursuing smaller and smaller goals. Take ultrathin films for instance. Often less than 10-15 nanometers in width, ultrathin films are used in diverse applications, from optoelectronics to biological sensors. (A nanometer is roughly one 100,000th the width of a human hair.)
A central requirement for performing these Lilliputian feats is accurate composition and structural analysis. Yet, 'looking inside' these films, which are often multi-layered, calls for highly sensitive probes. Most available techniques do not provide the depth information essential for evaluating layered structures. (Similarly, X-rays offer a spectacular glimpse into the human body, however determining the relative depth of individual structures is highly difficult.) Techniques devised to solve this problem are generally complicated and frequently damage the sample, distorting the results.
Now, Dr. Hagai Cohen of the Weizmann Institute Chemical Services and Prof. Israel Rubinstein of the Materials and Interfaces Department have developed a novel method for evaluating ultrathin films, specifically, non-conducting films on conducting substrates. Recently appearing in Nature, their study builds upon X-ray Photoelectron Spectroscopy (XPS), a common surface analysis technique.
In XPS, the sample is irradiated with X-rays, causing photoelectrons to be ejected. By measuring the photoelectrons' energy, it is possible to determine the atoms from which they originated. Researchers have routinely used an electron flood gun to neutralize the positive surface charge formed in non-conducting samples as a natural consequence of the photoelectron ejection, since the charging affects the photoelectrons' energy, distorting the measurements.
However, proving that one person's stumbling block may be another's stepping stone, Cohen and Rubinstein realized that the charging effect actually provides structural information - the magnitude of the photoelectron energy change correlates directly with the atoms' depth within the film (the deeper the atom the smaller the change). They decided to turn things around, using the electron gun to flood the sample with low energy electrons, thus negatively charging the surface and causing controlled, easily detectable changes in the energy of the ejected photoelectrons. By measuring these changes, the researchers were able to determine both the atom type and its depth within the film.
To evaluate their approach, the scientists used one of their previous research accomplishments - a highly organized ultrathin film, which they laced with marker atoms at different depths. When tested on this system, the new method provided depth information with a superior resolution of about one nanometer while causing minimal damage to the sample. It also offered a unique side-benefit, yielding information regarding the film's electrical properties.
The Weizmann innovation should prove beneficial in developing a wide range of microelectronic applications as well as in studying various chemical and biological systems.
This research was conducted together with Prof. Abraham Shanzer of the Organic Chemistry Department, Dr. Alexander Vaskevich of the Materials and Interfaces Department, and doctoral students Ilanit Doron-Mor, Anat Hatzor and Tamar van der Boom-Moav.
Prof. Israel Rubinstein's research is supported by the Philip Klutznick Endowed Scientific Research Fund, the Minerva Foundation, the Henri Gutwirth Fund for the Promotion of Research and the Fritz Haber Center for Physical Chemistry.
The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel. Its 2,500 scientists, students and support staff are engaged in more than 1,000 research projects across the spectrum of contemporary science.