Functionalized Nanomaterials – Characterization and Application

Date

2020

Authors

Yan, Fangzhi

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Abstract

Noble metal nanoclusters (NMNCs), such as silver and gold nanoclusters (AgNCs and AuNCs), have been an intriguing research subject over the past two decades. They have many unique properties due to their size and functionalized ligands compared to larger bare nanoparticles or bulk materials. Mass spectrometry is one of the best ways to characterize NMNCs due to its informative results that can convert directly into the chemical formula and the cure charge. Here we modified the synthesis method to produce mass spectrometry friendly AuNCs and studied its self-assembly process. We found many less abundant/stable AuNCs that having the same eight superatom valence electrons as the well-known Au₂₅ligand₁₈⁻.

We also explored the possible application in nanocluster assisted laser desorption/ionization and found distinctive results compared to organic matrices and bare nanoparticles. We further extended the application study in the bandgap modification of semiconducting two-dimensional materials (S2DM). S2DM, such as monolayer molybdenum disulfide (MoS₂) nanosheets, is one of the promising candidates for the next generation of ultrathin electronic circuits. Modifying the bandgap of S2DM is essential in ultrathin optoelectronic applications. Plasmonic nanoparticles, such as silver nanodisks (AgNDs), have been used to enhance the optoelectrical properties of S2DM. However, utilizing ultrahigh-resolution absorption imaging, Raman, and photoluminescence spectroscopy, we found photoexcited AgNDs exhibit opposing effects on the bandgap of MoS₂ nanosheets make it less effective in tunning the bandgap. In contrast, photoexcited NMNCs can produce a large quantity of energetic hot electrons with a lifetime in the hundreds of picosecond time scale. These hot electrons can inject into the conduction band of an adjacent monolayered MoS₂. Electron doping is an efficient technique to alter the electronic bandgap and change the exciton binding energy of MoS₂, thus modifying the optical bandgap. Introducing a low amount of AgNCs or AuNCs to the surface of MoS₂ nanosheets lowered its optical bandgap. The slight increases in the electron density on monolayered MoS₂ induced Coulomb screening and bandgap renormalization, which shift the optical bandgap to lower energy. Conversely, the optical bandgap of MoS₂ nanosheets was increased when integrated with a high concentration of NMNCs. The high electron doping density on MoS₂ induced the Pauli blocking resulted in the population of the dark state excitation at higher energy. Raman measurement agreed-well with the optical measurement and confirmed that the electron doping efficiency of AgNCs is higher than AuNCs.

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Keywords

Mass spectrometry, Nanocluster, semiconducting two-dimensional materials

Citation

Department

Chemistry