Sanpon Vantasin

Fabrication of a highly sensitive surface-enhanced Raman scattering substrate for monitoring the catalytic degradation of organic pollutants

In this paper, we demonstrate a simple and reliable two-step strategy based on an electrospinning technique combined with in situ calcination for the fabrication of ZnO nanofibers deposited on a silver foil surface. These nanofibers are used as a novel sensitive surface-enhanced Raman scattering (SERS) substrate. The strong interactions between ZnO nanofibers and silver foil afford continuous delocalized surface plasmons, resulting in localization of the electric field at the gap between the ZnO nanofibers and silver foil; thus, the exciton–plasmon interactions between ZnO nanofibers and the silver foil surface contribute to the enhanced scattering, generating a large electromagnetic field enhancement. In addition, the ZnO nanofibers deposited on the silver foil surface exhibit enhanced photocatalytic activity toward the degradation of organic pollutants because of the charge separation effect and increase in the lifetime of the photogenerated excitons under ultraviolet light irradiation; thus, this new substrate can be used as a SERS substrate for determining the catalytic activity and reaction kinetics during the photodegradation of organic pollutants.

3D SERS Imaging Using Chemically Synthesized Highly Symmetric Nanoporous Silver Microparticles

3D surface-enhanced Raman scattering (SERS) imaging with highly symmetric 3D silver microparticles as a SERS substrate was developed. Although the synthesis method is purely chemical and does not involve lithography, the synthesized nanoporous silver microparticles possess a regular hexapod shape and octahedral symmetry. By using p-aminothiophenol (PATP) as a probe molecule, the 3D enhancement patterns of the particles were shown to be very regular and predictable, resembling the particle shape and exhibiting symmetry. An application to the detection of 3D inhomogeneity in a polymer blend, which relies on the predictable enhancement pattern of the substrate, is presented. 3D SERS imaging using the substrate also provides an improvement in spatial resolution along the Z axis, which is a challenge for Raman measurement in polymers, especially layered polymeric systems.

Nanoporous silver microstructure for single particle surface-enhanced Raman scattering spectroscopy

The potential of a nanoporous Ag microstructure (np-AgMs) for use as a single particle for surface-enhanced Raman scattering spectroscopy (SERS), with the added advantages of being easy to manipulate and reusable, was successfully demonstrated. The np-AgMs with interconnected pore and controllable pore size were fabricated from symmetric hexapod AgCl via a galvanic replacement reaction in NaCl solution with zinc (Zn) as the sacrificed metal. The clean surface of np-AgMs enables rapid surface functionalization with easy handling and sample preparation as no particle aggregation occurs. The SERS acquisition spots on the np-AgMs can be visually selected using a normal Raman microscope. SERS spectra of p-aminothiophenol (PATP) with a concentration range of 10−8–10−3 M can be achieved. The position-dependent enhancement of np-AgMs was expendably evaluated. The signal-position correlation was confirmed by electric filed enhancement obtained from Finite-difference time-domain (FDTD) calculation. In addition, the highly stable substrate showed insignificant loss of the enhanced Raman signal after several cycles of chemical re-generation. Finally, the potential application of np-AgMs in label-free detection of biomolecules including hemoprotein, protein without chromophore and DNA strains at low concentration of 500 μg mL−1 was demonstrated.

Side-illuminated tip-enhanced Raman study of edge phonon in graphene at the electrical breakdown limit

Nanoscale integration of graphene into a circuit requires a stable performance under high current density. However, the effects of the current density that approach the electronic breakdown limit of graphene are not well understood. We explored the effects of a high current density, close to the electronic breakdown limit of 10 A/cm (∼3.0 × 108 A/cm2), on graphene, using tip-enhanced Raman scattering. The results showed that the high current density induces Raman bands at 1456 and 1530 cm−1, which were assigned to edge-phonon modes originating from zigzag and armchair edges. This led us to conclude that C–C bonds are cleaved due to the high current density, leaving edge structures behind, which were detected through the observation of localized phonons.

Graphene nanoridges as a directional plasmon launcher

Launching and control of graphene plasmon are crucial for nanodevice applications. To achieve that, previous studies used foreign object and/or angled illumination to provide plasmon launching and directional control. In this study, we considered graphene nanoridges, which is a defect-free natural structure of graphene to launch plasmon, using analytic method and simulation. The result shows that a single graphene nanoridge can launch plasmon, with an interesting relationship between the SPP amplitude and ridge physical curve length. By using two nanoridges with different size, the interference between SPP wave launch from each ridge result in right-left asymmetric plasmon launching. With the proper size and separation, unidirectional, bidirectional, or wavelength-sorted plasmon launching can be achieved.

Extension of nano-scaled exploration into solution/liquid systems using tip-enhanced Raman scattering

This review shows updated experimental cases of tip-enhanced Raman scattering (TERS) operated in solution/liquid systems. TERS in solution/liquid is still infancy, but very essential and challenging because crucial and complicated biological processes such as photosynthesis, biological electron transfer, and cellular respiration take place and undergo in water, electrolytes, or buffers. The measurements of dry samples do not reflect real activities in those kinds of systems. To deeply understand them, TERS in solution/liquid is needed to be developed. The first TERS experiment in solution/liquid is successfully performed in 2009. After that time, TERS in solution/liquid has gradually been developed. It shows a potential to study structural changes of biomembranes, opening the world of dynamic living cells. TERS is combined with electrochemical techniques, establishing electrochemical TERS (EC-TERS) in 2015. EC-TERS creates an interesting path to fulfil the knowledge about electrochemical-related reactions or processes. TERS tip can be functionalized with sensitive molecules to act as a “surface-enhanced Raman scattering (SERS) at tip” for investigating distinct properties of systems in solution/liquid e.g., pH and electron transfer mechanism. TERS setup is continuously under developing. Versatile geometry of the setup and a guideline of a systematic implementation for a setup of TERS in solution/liquid are proposed. New style of setup is also reported for TERS imaging in solution/liquid. From all of these, TERS in solution/liquid will expand a nano-scaled exploration into solution/liquid systems of various fields e.g., energy storages, catalysts, electronic devices, medicines, alternative energy sources, and build a next step of nanoscience and nanotechnology.

3D SERS imaging based on chemically-synthesized highly-symmetric nanoporous silver microparticles

This study presents the synthesis, SERS properties in three dimensions, and an application of 3D symmetric nanoporous silver microparticles. The particles are synthesized by purely chemical process: controlled precipitation of AgCl to acquire highly symmetric AgCl microparticle, followed by in-place to convert AgCl into nanoporous silver. The particles display highly predictable SERS enhancement pattern in three dimensions, which resembles particle shape and retains symmetry. The highly regular enhancement pattern allows an application in the study of inhomogeneity in two-layer polymer system, by improving spatial resolution in Z axis.

Tip-enhanced Raman spectroscopy of nanostructures on epitaxial graphene and graphene microisland

Despite often illustrated as a perfect two-dimensional sheet, real graphene sample is not always flat. Nanostructures can be occurred on graphene sheet, especially for epitaxial graphene. The nanostructures alter the electrical and mechanical properties of graphene. This is crucial for epitaxial graphene since its main potential is in the electronics and optics application. This study investigates nanostructures on epitaxial graphene by tip-enhanced Raman spectroscopy, which is a technique that can provide Raman spectra with great spatial resolution, exceeding the diffraction limit of light. The results suggest that the compressive strain on nanoridges is weaker compared to neighbor flat area, supporting the ‘ridge as compressive strain relaxation’ mechanism. TERS measurement of nanoridges on epitaxial graphene microisland also indicates that the ‘Si vapor trapping’ mechanism for ridge formation is unlikely to occur.