Dr. Zhicong Zeng Profile

Dr. Zhicong Zeng

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Proceedings Article | 5 September 2018 Paper

Local heating effect study by photothermal imaging

ZhiCong Zeng, Zachary Schultz

Proceedings Volume 10726, 107260H (2018) https://doi.org/10.1117/12.2321311

KEYWORDS: Photothermal effect, Gold, Amplifiers, Surface enhanced Raman spectroscopy

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Aggregated gold nanoparticles are widely used in surface-enhanced Raman scattering (SERS), however, gold nanoparticles are excellent light absorbers and its local heating effect should be concerned. The optical properties of plasmonic nanoparticles are strongly dependent on interactions with other nanoparticles, which complicates analysis for systems larger than a few particles. In this work we examine heat dissipation in aggregated nanoparticles and its influence on surface-enhanced Raman scattering (SERS) through correlated photothermal heterodyne imaging (PHI). For dimers the per particle absorption cross sections show evidence of interparticle coupling; however, the effects are much smaller than those for the field enhancements that are important for SERS. For larger aggregates the total absorption was observed to be simply proportional to aggregate volume. This observation allows us to model light absorption and heating in the aggregates by assuming that the particles act as independent heating sources. To push the detection limits in PHI of our system, we use the home-built DC-10MHz low noise large area photodiode amplifier and obtain 7 nV/Hz^1/2 noise level which closed to the limitation of SR844 Lock-in amplifier itself. Our work aims to use local heating effects from molecules to improve the spatial resolution and chemical sensitivity of label-free microscopy.

Proceedings Article | 5 October 2015 Presentation

Electrochemical tip-enhanced Raman spectroscopy (Presentation Recording)

Zhicong Zeng, Shengchao Huang, Tengxiang Huang, Maohua Li, Bin Ren

Proceedings Volume 9554, 95540V (2015) https://doi.org/10.1117/12.2190077

KEYWORDS: Raman spectroscopy, Molecules, Molecular interactions, Distortion, Spatial resolution, Surface properties, Biology, Scanning tunneling microscopy, Coating, Liquids

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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.

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