Tengxiang Huang

Probing the Location of Hot Spots by Surface-Enhanced Raman Spectroscopy: Toward Uniform Substrates

Wide applications of surface plasmon resonance rely on the in-depth understanding of the near-field distribution over a metallic nanostructure. However, precisely locating the strongest electric field in a metallic nanostructure still remains a great challenge in experiments because the field strength decays exponentially from the surface. Here, we demonstrate that the hot spot position for gold nanoparticles over a metal film can be precisely located using surface-enhanced Raman spectroscopy (SERS) by rationally choosing the probe molecules and excitation wavelengths. The finite difference time domain simulation verifies the experimental results and further reveals that the enhancement for the above system is sensitive to the distance between nanoparticles and the metal film but insensitive to the distance of nanoparticles. On the basis of this finding, we propose and demonstrate an approach of using a nanoparticles-on-metal film substrate as a uniform SERS substrate. This work provides a convenient way to probe the location of strong near-field enhancement with SERS and will have important implications in both surface analysis and surface plasmonics.

Electrochemical Tip-Enhanced Raman Spectroscopy.

Interfacial properties are highly important to the performance of some energy-related systems. The in-depth understanding of the interface requires highly sensitive in situ techniques that can provide fingerprint molecular information at nanometer resolution. We developed an electrochemical tip-enhanced Raman spectroscopy (EC-TERS) by introduction of the light horizontally to the EC-STM cell to minimize the optical distortion and to keep the TERS measurement under a well-controlled condition. We obtained potential-dependent EC-TERS from the adsorbed aromatic molecule on a Au(111) surface and observed a substantial change in the molecule configuration with potential as a result of the protonation and deprotonation of the molecule. Such a change was not observable in EC-SERS (surface-enhanced), indicating EC-TERS can more faithfully reflect the fine interfacial structure than EC-SERS. This work will open a new era for using EC-TERS as an important nanospectroscopy tool for the molecular level and nanoscale analysis of some important electrochemical systems including solar cells, lithium ion batteries, fuel cells, and corrosion.

Gold-coated AFM tips for tip-enhanced Raman spectroscopy: theoretical calculation and experimental demonstration

The optimal gold-coated atomic force microscopy (AFM) tip-substrate system for tip-enhanced Raman spectroscopy (TERS) was designed theoretically and demonstrated experimentally. By optimizing the tip, excitation laser, and the substrate, the TERS enhancement factor can be tuned to as high as 9 orders of magnitude, and the spatial resolution could be down to 5 nm. Preliminary experimental results for AFM tips coated with gold layer of different thicknesses reveal that the maximum enhancement can be achieved when the thickness is about 60-80 nm, which is in good agreement with the theoretical prediction. Our results not only provide a deep understanding of the underlying physical mechanism of AFM tip-based TERS, but also guide the rational construction of a working AFM-TERS system with a high efficiency.

A Plasmonic Sensor Array with Ultrahigh Figures of Merit and Resonance Linewidths down to 3 nm

Surface plasmon polaritons (SPPs) are extremely sensitive to the surrounding refractive index and have found important applications in ultrasensitive label-free sensing. Reducing the linewidth of an SPP mode is an effective way to improve the figure of merit (FOM) and hence the sensitivity of the plasmonic mode. Many efforts have been devoted to achieving a narrow linewidth by mode coupling, which inevitably results in an asymmetrical lineshape compromising the performance. Instead, the SPP modes are directly narrowed by elaborately engineering periodic plasmonic structures with minimized feature sizes to effectively reduce the radiative losses. A narrow linewidth smaller than 8 nm is achieved over a wide wavelength ranging from 600 to 960 nm and a minimum full width at half maximum of 3 nm at 960 nm. Benefiting from the almost perfect Lorentzian lineshape and the extremely narrow linewidth, a record FOM value of 730 is obtained. The sensor is capable of detecting bovine serum albumin with an ultralow concentration of 10-10 m. The sensor has great potential for practical application for its ultrahigh FOM, broad working wavelength, and ease of high-throughput fabrication.

Electrochemical tip-enhanced Raman spectroscopy (Presentation Recording)

Tip-enhanced Raman spectroscopy (TERS) can not only provide very high sensitivity but also high spatial resolution, and has found applications in various fields, including surface science, materials, and biology. Most of previous TERS studies were performed in air or in the ultrahigh vacuum. If TERS study can be performed in the electrochemical environment, the electronic properties of the surface can be well controlled so that the interaction of the molecules with the substrate and the configuration of the molecules on the surface can also be well controlled. However, the EC-TERS is not just a simple combination of electrochemistry with TERS, or the combination of EC-STM with Raman. It is a merge of STM, electrochemistry and Raman spectroscopy, and the mutual interference among these techniques makes the EC-TERS particularly challenge: the light distortion in EC system, the sensitivity, the tip coating to work under EC-STM and retain the TERS activity and cleanliness. We designed a special spectroelectrochemical cell to eliminate the distortion of the liquid layer to the optical path and obtain TER spectra of reasonably good signal to noise ratio for surface adsorbed molecules under electrochemical potential control. For example, potential dependent TERS signal have been obtained for adsorbed aromatic thiol molecule, and much obvious signal change compared with SERS has been found, manifesting the importance of EC-TERS to reveal the interfacial structure of an electrochemical system. We further extended EC-TERS to electrochemical redox system, and clear dependence of the species during redox reaction can be identified.