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20 September 2013 A new technique for fabrication of better metallic nanotips for nanoimaging through tip-enhanced Raman spectroscopy
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Tip-enhanced Raman spectroscopy (TERS) offers one of the best techniques for analysis and imaging of molecule structures at nanoscale spatial resolution. An important issue in TERS is to improve the detection sensitivity of inherently weak Raman scattering so as to observe varieties of materials. For enhancement of the Raman signal, fully metallized tips are utilized in TERS, which enhance signals through plasmon oscillation at the tip apex. However, length of metal along the tip axis is on the order of a few to a few tens of micrometers, which means the plasmon resonant wavelength is much longer than the wavelength of the visible light used in TERS. From that point, if the tip has a metallic nanostructure on the apex, it would give better enhancement in the visible range compared with fully metallized tips. In this research, we employed photoreduction as a new fabrication method to grow a metallic nanostructure at the tip apex. We found a particular property of photoreduction that it occurs selectively at sharp corners, such as the tip apex of silicon cantilevers. Through this property, we succeeded in growing silver nanoparticles selectively at the tip apex. One of the advantages of the photoreduction is that the size of metal nanostructures is well controlled by optimizing various parameters. We controlled the size of silver nanoparticles from 100 to 400 nm by changing the laser exposure time. Furthermore, we obtained an order of magnitude higher enhancement from our fabricated tip compared with fully metalized tips through TERS measurements.
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Takayuki Umakoshi, Taka-aki Yano, Yuika Saito, and Prabhat Verma "A new technique for fabrication of better metallic nanotips for nanoimaging through tip-enhanced Raman spectroscopy", Proc. SPIE 8815, Nanoimaging and Nanospectroscopy, 88150O (20 September 2013);

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