Dr. Ying-Zhong Ma Profile

Dr. Ying-Zhong Ma

at Oak Ridge National Lab

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Publications (2)

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Proceedings Article | 5 October 2023 Poster + Paper

Probing liquid/liquid interfaces at and away from equilibrium using vibrational sum frequency generation

Benjamin Doughty, Uvinduni Premadasa, Lu Lin, Ying-Zhong Ma, Robert Sacci, Vera Bocharova, Nikki Thiele

Proceedings Volume 12681, 126810D (2023) https://doi.org/10.1117/12.2677555

KEYWORDS: Interfaces, Sum frequency generation, Optical surfaces, Spectroscopy

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The self-assembly and transport of species at oil/aqueous interfaces represent key mechanistic steps in processes spanning from liquid extraction to soft-matter neuromorphic electrical devices. While technologically and fundamentally important, there is a notable lack of chemical understanding regarding these buried interfaces due to challenges in differentiating species located at the surface from the nearby bulk phases. In this work, we will describe some of the challenges in probing chemistry at these liquid/liquid interfaces and detail the approaches we have developed using vibrational sum frequency generation to enable studies of these complex interfaces at and away from equilibrium. From these spectroscopic developments, new insight into solvation, ion pairing, aggregation, complexation, and transport will be discussed in the context of solvent extraction chemistry. Notably, we show how the presence of both oil and aqueous phases provides an environment to drive fundamentally different phenomena from what is observed at model air/aqueous interfaces. This difference arises from key differences in solvation and bulk populations that feedback onto the surface to drive emergent and often functional molecular assemblies.

Proceedings Article | 2 March 2010 Paper

Exciton annihilation and dephasing dynamics in semiconducting single-walled carbon nanotubes

Matthew Graham, Ying-Zhong Ma, Alexander Green, Mark Hersam, Graham Fleming

Proceedings Volume 7600, 76001F (2010) https://doi.org/10.1117/12.841194

KEYWORDS: Excitons, Temperature metrology, Single walled carbon nanotubes, Scattering, Semiconductors, Spectroscopy, Picosecond phenomena, Signal processing, Femtosecond phenomena, Absorption

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Semiconducting single-walled carbon nanotubes (SWNTs) are one of the most intriguing nanomaterials due to their large aspect ratios, size tunable properties, and dominant many body interactions. While the dynamics of exciton population relaxation have been well characterized, optical dephasing processes have only been examined indirectly through steady-state measurements such as single-molecule spectroscopy that can yield highly variable estimates of the homogeneous linewidth. To bring clarity to these conflicting estimates, a time-domain measurement of exciton dephasing at an ensemble level is necessary. Using two-pulse photon echo (2PE) spectroscopy, comparatively long dephasing times approaching 200 fs are extracted for the (6,5) tube species at room temperature. In this contribution, we extend our previous study of 2PE and pump-probe spectroscopy to low temperatures to investigate inelastic exciton-exciton scattering. In contrast to the population kinetics observed upon excitation of the second transition-allowed excitonic state (E₂₂), our one-color pump-probe data instead shows faster relaxation upon cooling to 60 K when the lowest transition-allowed state (E₁₁) is directly excited for the (6,5) tube species. Analysis of the kinetics obtained suggests that the observed acceleration of kinetic decay at low temperature originates from an increasing rate of exciton-exciton annihilation. In order to directly probe exciton-exciton scattering processes, femtosecond 2PE signal is measured as a function of excitation fluence and temperature. Consistent with the observed enhancement of exciton-exciton scattering and annihilation at low temperatures, the dephasing rates show a correlated trend with the temperature dependence of the population lifetimes extracted from one-color pump-probe measurements.

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