Takayuki Umakoshi

Fabrication of Near-Field Plasmonic Tip by Photoreduction for Strong Enhancement in Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) offers one of the best techniques for optical analysis and imaging of samples at nanoscale. The most important point in TERS experiments is to obtain a high signal enhancement through a metallic nanotip. Compared with fully metallized tips, the tips that have only one metallic nanoparticle at the apex show better enhancement. Here, we demonstrate a new and simple way to fabricate metallic nanoparticles selectively at the tip apex through photoreduction. By controlling the nanoparticle size, the plasmon resonance of the tip can be tuned. Finally, we demonstrate that such tips give better enhancement in TERS.

Quantum-dot antibody conjugation visualized at the single-molecule scale with high-speed atomic force microscopy

Conjugates of semiconductor quantum dots (QDs) and antibodies have emerged as a promising bioprobes due to their great combination of QDs efficient fluorescence and the high specificity of antigen-antibody reactions. For further developments in this field, it is essential to understand the molecular conformation of the QD-antibody conjugates at the single-molecule scale. Here, we report on the direct imaging of QD-antibody conjugates at the single-molecule scale by using high-speed atomic force microscopy (HS-AFM). Owing to the high spatiotemporal resolution of HS-AFM, we observed the dynamic splitting of individual antibodies during the conjugation process. QD-antibody conjugates were also clearly visualized at the single-molecule scale details. Several important features were even discovered through dynamic observation of the QD-antibody conjugates. We observed an intermediate state of conjugation, where the antibodies attached and detached to QDs repeatedly. We also revealed that the attached antibodies were not steady but drastically fluctuated in their recognition areas due to the Brownian motion. We also demonstrated that HS-AFM observation is useful for the quantitative analysis of fabricated conjugates.

Metallic tips for efficient plasmon nanofocusing and advanced optical nano-imaging

Plasmon nanofocusing, a phenomenon where plasmons propagate on a tapered metallic structure with compressing its energy into a nanometric volume of the apex to generate localized electric field, holds a great promise for near-field optical imaging techniques due to its background-free nature. Because it does not require to illuminate the tip apex with an incident laser, one can efficiently eliminate scattering background noise by the incident laser, which has been an issue in conventional near-field optical microscopies. To apply plasmon nanofocusing for near-field optical imaging, a tapered metallic tip plays an important role as a base material for plasmon propagation. It is therefore essential to establish an efficient and practical methods of the metallic tip fabrication for plasmon-nanofocusing-based optical imaging techniques. In this study, we propose an optimized tip fabrication for efficient plasmon nanofocusing, which achieved 100% reproducibility in plasmon nanofocusing. Through numerical analysis, we have optimized the tip structure, such as types of material, metal thickness, plasmon coupler structure, etc. Also, the fabrication conditions were well-optimized to obtain smooth metal surface down to 0.5 nm roughness to reduce energy loss of plasmon propagation. Through thorough optimizations, we observed plasmon nanofocusing with 100% reproducibility in more than 20 fabricated metallic tips. Such efficient, reliable and practical tip fabrication opens the doors for many potential scientists working in related fields.

Near-field visible light absorption imaging by Raman-nano-light source

We propose a new nano-imaging technique for intrinsic absorption properties of materials under a platform of conventional aperture-less near-field scanning optical microscopy (NSOM). In aperture-less NSOM, when a silicon nanotip is utilized and illuminated by the visible light instead of a metallic tip, Raman scattering of silicon from the tip apex can be obtained. Since the wavelength of this Raman scattered light is shifted to 520cm-1 from the one of the excitation light, far-field background signal excited by the diffraction limited focus spot of the incident light, which is one of the major problems in aperture-less NSOM, can be avoided. When the silicon nano-tip is on the sample and illuminated, the Raman signal of silicon can be partially absorbed by the sample while passing through it, so that measuring the intensity of the Raman signal of silicon enables us to observe the absorption behavior of the sample at nano-scale. Because the absorbance of light is dependent on the absorption coefficient of the sample as well as its sample topography, it is needed to eliminate the effect of the sample topography from the absorption measurement to technically evaluate the absorption coefficient of the sample. For this purpose, we simultaneously employed two different incident lasers and utilized absorbance ratio between two wavelengths to monitor the absorption coefficient of the sample. As an example, we demonstrated that two types of carbon nanotubes, which have different absorption properties, could be clearly distinguished with nano-scale resolution by our technique.

A new technique for fabrication of better metallic nanotips for nanoimaging through tip-enhanced Raman spectroscopy

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.