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Posted: Oct 28, 2011
Nanowires are more heterogeneous than anticipated
(Nanowerk News) Further progress in the miniaturization of electronic circuitry using conventional approaches will soon reach its limits. Delicate, inorganic semiconducting nanowires offer a possible means of extending these limits, and are of great interest for applications in the areas of optoelectronics and photovoltaics. However, the physical properties of these tiny structures are poorly understood.
"With few exceptions, optical investigations on nanowires have so far been carried out almost exclusively by conventional optical microscopy, which has limited spatial resolution," says Professor Achim Hartschuh of the Faculty of Chemistry and Pharmacy at LMU Munich. He and his group have helped to develop a new method, called tip-enhanced near-field microscopy, which makes use of a tapered metal tip as an optical antenna ("Optical Imaging of CdSe Nanowires with Nanoscale Resolution").
When irradiated with a laser beam, the antenna concentrates the incident light in a minuscule volume around the tip. This confinement effect focuses the light more sharply than is possible using a normal lens. If the tip is positioned a couple of nanometers from the sample, the enhanced optical field also amplifies the weak optical response of single nanostructures – which provides insights into their molecular organization at nanoscale resolution. Using this method, Hartschuh and his colleagues were able to carry out measurements of photoluminescence and Raman scattering on nanowires at a resolution of less than 20 nanometers.
Photoluminescence and Raman scattering both result from interactions of light quanta (photons) with the excited states of the sample, and provide the basis for sensitive optical techniques that can be used to characterize the chemical and electronic properties of materials. Photoluminescence supplies information on quantum effects that are dependent on the diameter of the sample under study.
Raman spectroscopy, on the other hand, is sensitive to the chemical structure of the nanowires. "It was hitherto unclear how these structural and electronic properties vary along the length of nanowires at scales of few nanometers," says Hartschuh, who is also affiliated with the Center for NanoScience (CeNS) at LMU and is a member of the Nanosystems Initiative Munich (NIM), a Cluster of Excellence at LMU. "With the aid of tip-enhanced near-field microscopy, we have been able to close this gap in our knowledge and, to our surprise, we observed significant energy fluctuations along the length of single nanowires," he reports. These variations point to the presence of a higher degree of optical heterogeneity in nanowires than could be visualized using conventional methods.
As Hartschuh emphasizes, "the new findings underline the broad applicability of the tip-enhanced method of near-field microscopy that we have helped to develop." He plans to continue work on the development of innovative high-resolution methods of nanospectroscopy as part of his NEWNANOSPEC project, which is supported by a European Research Council (ERC) Starting Grant.